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Generalized boundary conditions for the circuit theory of mesoscopic
transport: The circuit theory of mesoscopic transport provides a unified framework to
describe spin-dependent or superconductivity-related phenomena. We extend this
theory to hybrid systems of normal metals, ferromagnets and superconductors.
Our main result is an expression for the current through an arbitrary contact
between two general isotropic "nodes", which is suitable to describe the
presence of superconducting and ferromagnetic elements in the system, as well
as magnetically active interfaces/contacts. In certain cases (weak ferromagnet
and magnetic tunnel junction) we derive transparent and simple results for the
matrix current. | cond-mat_supr-con |
On the roles of graphene oxide doping for enhanced supercurrent in MgB2
based superconductors: Due to their graphene-like properties after oxygen reduction, incorporation
of graphene oxide (GO) sheets into correlated-electron materials offers a new
pathway for tailoring their properties. Fabricating GO nanocomposites with
polycrystalline MgB2 superconductors leads to an order of magnitude enhancement
of the supercurrent at 5 K/8 T and 20 K/4 T. Herein, we introduce a novel
experimental approach to overcome the formidable challenge of performing
quantitative microscopy and microanalysis of such composites, so as to unveil
how GO doping influences the structure and hence the material properties. Atom
probe microscopy and electron microscopy were used to directly image the GO
within the MgB2, and we combined these data with computational simulations to
derive the property-enhancing mechanisms. Our results reveal synergetic effects
of GO, namely, via localized atomic (carbon and oxygen) doping as well as
texturing of the crystals, which provide both inter and intra granular flux
pinning. This study opens up new insights into how low-dimensional
nanostructures can be integrated into composites to modify the overall
properties, using a methodology amenable to a wide range of applications. | cond-mat_supr-con |
The underdoped-overdoped transition in YBa_2Cu_3O_x: Oxygen doping in metallic YBa_2Cu_3O_x induces quadrupolar "alpha-ortho'',
and breathing "beta-ortho" deformations of the CuO_2 planes. Breathing
beta-ortho deformations favour hybridizations of the pd sigma Cu3d_x^2-y^2 -
O2p_x,y with the pd pi Cu3d_x,z, 3d_y,z-O2p_z bands relaxing the confinement of
the carriers in the overdoped regime, x>6.95. | cond-mat_supr-con |
Magnetization Measurement of a Possible High-Temperature Superconducting
State in Amorphous Carbon Doped with Sulfur: Magnetization M(T,H) measurements performed on thoroughly characterized
commercial amorphous carbon powder doped with sulfur (AC-S), revealed the
occurrence of an inhomogeneous superconductivity (SC) below T_c = 38 K. The
constructed magnetic field-temperature (H-T) phase diagram resembles that of
type-II superconductors. However, AC-S demonstrates a number of anomalies. In
particular, we observed (1) a non-monotonic behavior of the lower critical
field H_c1(T); (2) a pronounced positive curvature of the "upper critical field
boundary" that we associated with the flux lattice melting line Hm(T); (3) a
spontaneous ferromagnetic-like magnetization M0 coexisting with SC. Based on
the analysis of experimental results we propose a nonstandard SC state in AC-S. | cond-mat_supr-con |
Coexistence of orbital degeneracy lifting and superconductivity in
iron-based superconductors: In contrast to conventional superconducting (SC) materials, superconductivity
in high-temperature superconductors (HTCs) usually emerges in the presence of
other fluctuating orders with similar or higher energy scales, thus instigating
debates over their relevance for the SC pairing mechanism. In iron-based
superconductors (IBSCs), local orbital fluctuations have been proposed to be
directly responsible for the structural phase transition and closely related to
the observed giant magnetic anisotropy and electronic nematicity. However,
whether superconductivity can emerge from, or even coexist with orbital
fluctuations, remains unclear. Here we report the angle-resolved photoemission
spectroscopy (ARPES) observation of the lifting of symmetry-protected band
degeneracy, and consequently the breakdown of local tetragonal symmetry in the
SC state of Li(Fe1-xCox)As. Supported by theoretical simulations, we analyse
the doping and temperature dependences of this band-splitting and demonstrate
an intimate connection between ferro-orbital correlations and
superconductivity. | cond-mat_supr-con |
Nonlatching Superconducting Nanowire Single-Photon Detection with
Quasi-Constant-Voltage Bias: Latching is a serious issue in superconducting nanowire single-photon
detector (SNSPD) technology. By extensively studying the electrical
transportation characteristics of SNSPD with different bias schemes, we
conclude that latching is a result of the improper bias to SNSPD. With the
quasi-constant-voltage bias scheme, the intrinsic nonlatching nature of SNSPD
is observed and discussed. The SNSPD working in the nonlatching bias shows a
smaller jitter and a higher pulse amplitude than that in the previous
anti-latching method. The quantum efficiency of SNSPD with the pulsed photon
frequency up to 3 GHz is measured successfully, which further proves the
nonlatching operation of SNSPD. | cond-mat_supr-con |
Anomalous Fano factor as a signature of Bogoliubov Fermi surfaces: Noise spectroscopy is a key technique to investigate the nature and dynamics
of charge carriers in superconductors. The recently discovered superconducting
hybrids with Bogoliubov Fermi surfaces exhibit a particularly intriguing and
rich charge dynamics, as their charge carriers consist of both Cooper pairs and
an extensive number of Bogoliubov quasiparticles. Motivated by this, we compute
the noise spectra of Bogoliubov Fermi surfaces and identify their key
signatures in the differential conductance and the Fano factor. Specifically,
we consider a semiconductor/superconductor hybrid device with an in-plane
magnetic field, which exhibits several Bogoliubov Fermi surfaces. The number
and orientation of the Bogoliubov Fermi surfaces in this device can be readily
controlled by the applied magnetic field, which in turn alters the noise
signal. In particular, we find that the Fano factor exhibits a reduced value,
substantially lower than two, whenever the charge dynamics is governed by a
large number of Bogoliubov quasiparticles. Using experimentally relevant
parameters, we make a number of specific predictions for the noise spectra,
that can be used as direct evidence of Bogoliubov Fermi surfaces. In
particular, we find that the Fano factor as a function of magnetic field and
spin-orbit coupling exhibits characteristic discontinuities at the transition
lines that separate phases with different number of Bogoliubov Fermi surfaces. | cond-mat_supr-con |
Josephson junctions of topological nodal superconductors: Transition metal dichalcogenides (TMDs) offer a unique platform to study
unconventional superconductivity, owing to the presence of strong spin-orbit
coupling and a remarkable stability to an in-plane magnetic field. A recent
study found that when an in-plane field applied to a superconducting monolayer
TMD is increased beyond the Pauli critical limit, a quantum phase transition
occurs into a topological nodal superconducting phase which hosts Majorana flat
bands. We study the current-phase relation of this nodal superconductor in a
Josephson junction geometry. We find that the nodal superconductivity is
associated with an energy-phase relation that depends on the momentum
transverse to the current direction, with a $4\pi$ periodicity in between pairs
of nodal points. We interpret this response as a result of a series of quantum
phase transitions, driven by the transverse momentum, which separate a
topological trivial phase and two distinct topologically non-trivial phases
characterized by different winding invariants. This analysis sheds light on the
stability of the Majorana flat bands to symmetry-breaking perturbations. | cond-mat_supr-con |
High-temperature surface superconductivity in rhombohedral graphite: Surface superconductivity in rhombohedral graphite is a robust phenomenon
which can exist even when higher order hoppings between the layers lift the
topological protection of the surface flat band and introduce a quadratic
dispersion of electrons with a heavy effective mass. We show that for weak
pairing interaction, the flat band character of the surface superconductivity
transforms into a BCS-like relation with high critical temperature
characterized by a higher coupling constant due to a much larger density of
states than in the bulk. Our results offer an explanation for the recent
findings of graphite superconductivity with an unusually high transition
temperature. | cond-mat_supr-con |
Competing Superconducting States in Strong Ferromagnets: We report results from a systematic study of the competition of exotic
triplet pair density wave (PDW) superconductivity (SC) with homogeneous (zero
pair momentum) SC in strongly polarized media such as half metallic systems.
From the two different PDW states allowed by symmetry in this background only
one may dominate or even coexist with homogeneous SC. We propose a direct
experimental identification of PDW SC in this context. Our results suggest that
these exotic states may plausibly emerge in heterostructures involving
proximity of SC with half-metallic CrO$_2$ where induced SC is established in
the half metallic region and in strongly ferromagnetic superconductors. | cond-mat_supr-con |
Thermodynamic signatures of quantum criticality in cuprates: The three central phenomena of cuprate superconductors are linked by a common
doping $p^{\star}$, where the enigmatic pseudogap phase ends, around which the
superconducting phase forms a dome, and at which the resistivity exhibits an
anomalous linear dependence on temperature as $T \to 0$. However, the
fundamental nature of $p^{\star}$ remains unclear, in particular whether it
marks a true quantum phase transition. We have measured the specific heat $C$
of the cuprates Eu-LSCO and Nd-LSCO at low temperature in magnetic fields large
enough to suppress superconductivity, over a wide doping range across
$p^{\star}$. As a function of doping, we find that the electronic term $C_{\rm
el}$ is strongly peaked at $p^{\star}$, where it exhibits a $-T$log$T$
dependence as $T \to 0$. These are the classic signatures of a quantum critical
point, as observed in heavy-fermion and iron-based superconductors where their
antiferromagnetic phase ends. We conclude that the pseudogap phase of cuprates
ends at a quantum critical point, whose associated fluctuations are most likely
involved in the $d$-wave pairing and the anomalous scattering. | cond-mat_supr-con |
Theory of thermally activated vortex bundles flow over the
directional-dependent potential barriers in type-II superocnductors: The thermally activated vortex bundle flow over the directional-dependent
energy barrier in type-II superconductors is investigated. The coherent
oscillation frequency and the mean direction of the random collective pinning
force of the vortex bundles are evaluated by applying the random walk theorem.
The thermally activated vortex bundle flow velocity is obtained.The
temperature- and field-dependent Hall and longitudinal resistivities induced by
the bundle flow for type-II superconducting bulk materials and thin films are
calculated. All the results are in agreement with the experiments. | cond-mat_supr-con |
Bound states and impurity averaging in unconventional superconductors: The question of anomalous transport due to a band of impurity states in
unconventional superconductors is discussed. In general, the bound state
energies are not in midgap, even in the unitarity limit. This implies that,
generically, the states associated with impurities are broad resonances, not
true bound states. There is no impurity band in the usual sense of the phrase.
The wavefunctions of these resonances possess interesting anisotropies in real
space, but this does not result in anomalous hopping between impurities. I
conclude that the system of resonances produces no qualitative modifications to
the T-matrix theory with impurity averaging which is normally used to treat the
low-temperature transport of unconventional superconductors. However, users of
this method often assume a density of states which is symmetric around the
chemical potential. This is not normally the case. It is found that the
non-crossing approximation is not valid in a strictly two-dimensional system. | cond-mat_supr-con |
Existence of the Abrikosov vortex state in two-dimensional type-II
superconductors without pinning: Theory alternative to the vortex lattice melting theories is advertised. The
vortex lattice melting theories are science fiction cond-mat/9811051 because
the Abrikosov state is not the vortex lattice with crystalline long-range
order. Since the fluctuation correction to the Abrikosov solution is infinite
in the thermodynamic limit (K.Maki and H.Takayama, 1972) any fluctuation theory
of the mixed state should consider a superconductor with finite sizes. Such
nonperturbative theory for the easiest case of two-dimensional superconductor
in the lowest Landau level approximation is presented in this work. The
thermodynamic averages of the spatial average order parameter and of the
Abrikosov parameter $\beta_{a}$ are calculated. It is shown that the position
H_{c4} of the transition into the Abrikosov state (i.e. in the mixed state with
long-range phase coherence) depends strongly on sizes of two-dimensional
superconductor. Fluctuations eliminate the Abrikosov vortex state in a wide
region of the mixed state of thin films with real sizes and without pinning
disorders, i.e. H_{c4} << H_{c2}. The latter has experimental corroboration in
Phys.Rev.Lett. 75, 2586 (1995). | cond-mat_supr-con |
Suppression of Tunneling of Superconducting Vortices Caused by a Remote
Gate: Example of an Extended Object Tunneling: We discuss a recent experiment in which the resistance of a superconducting
film has been measured in magnetic field. A strong decrease of the
superconducting film resistance has been observed when a metallic gate is
placed above the film. We study how the magnetic coupling between vortices in a
thin superconducting film and electrons in a remote unbiased gate suppresses
the tunneling rate of the vortices. We examine two general approaches to
analyze tunneling in the presence of slow low-energy degrees of freedom: the
functional-integral and scattering formalisms. In the first one, the response
of the electrons inside the metallic gate to a change in the vortex position is
described by the "tunneling with dissipation". We consider the Eddy current
induced in the gate by the magnetic flux of the vortex as a result of
tunneling. In the second approach, the response is given in terms of scattering
of the electrons by the magnetic flux of the vortex in a way similar to the
Aharonov-Bohm scattering. A sudden change in the vortex position leads to the
Orthogonality Catastrophe that opposes the vortex tunneling. We show that the
magnetic coupling between the vortices and the electrons inside the gate can
lead to a dramatic suppression of the vortex tunneling, restoring the
superconducting property in accord with the experiment. | cond-mat_supr-con |
Role of Sb in the superconducting kagome metal CsV$_3$Sb$_5$ revealed by
its anisotropic compression: Pressure evolution of the superconducting kagome metal CsV$_3$Sb$_5$ is
studied with single-crystal x-ray diffraction and density-functional
band-structure calculations. A highly anisotropic compression observed up to 5
GPa is ascribed to the fast shrinkage of the Cs-Sb distances and suppression of
Cs rattling motion. This prevents Sb displacements required to stabilize the
three-dimensional charge-density-wave (CDW) order and elucidates the
disappearance of the CDW already at 2 GPa despite only minor changes in the
electronic structure of the normal state. At higher pressures, vanadium bands
still change only marginally, whereas antimony bands undergo a major
reconstruction caused by the gradual formation of the interlayer Sb-Sb bonds.
Our results exclude pressure tuning of vanadium kagome bands as the main
mechanism for the non-trivial evolution of superconductivity in real-world
kagome metals. Concurrently, we establish the central role of Sb atoms in the
stabilization of a three-dimensional CDW and Fermi surface reconstruction. | cond-mat_supr-con |
Energy dependence of a vortex line length near a zigzag of pinning
centers: A vortex line, shaped by a zigzag of pinning centers, is described here
through a three-dimensional unit cell containing two pinning centers positioned
symmetrically with respect to its center. The unit cell is a cube of side
$L=12\xi$, the pinning centers are insulating spheres of radius $R$, taken
within the range $0.2\xi$ to $3.0\xi$, $\xi$ being the coherence length. We
calculate the free energy density of these systems in the framework of the
Ginzburg-Landau theory. | cond-mat_supr-con |
Searching for Superconductivity in High Entropy Oxide Ruddlesden-Popper
Cuprate Films: In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper
(La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and
Sr+2 at concentrations known to induce superconductivity in the simple parent
compounds, Nd2CuO4 and La2CuO4. Electron doped
(La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped
(La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be
single crystal, epitaxially strained, and highly uniform. Transport
measurements demonstrate that all as-grown films are insulating regardless of
doping. Annealing studies show that resistivity can be tuned by modifying
oxygen stoichiometry and inducing metallicity but without superconductivity.
These results in turn are connected to extended x-ray absorption fine structure
(EXAFS) results indicating that the lack of superconductivity in the high
entropy cuprates likely originates from a large distortion within the Cu-O
plane ({\sigma}2>0.015 {\AA}2) due to A-site cation size variance, which drives
localization of charge carriers. These findings describe new opportunities for
controlling charge- and orbital-mediated functional responses in
Ruddlesden-Popper crystal structures, driven by balancing of cation size and
charge variances that may be exploited for functionally important behaviors
such as superconductivity, antiferromagnetism, and metal-insulator transitions,
while opening less understood phase spaces hosting doped Mott insulators,
strange metals, quantum criticality, pseudogaps, and ordered charge density
waves. | cond-mat_supr-con |
Tunneling spectroscopy for ferromagnet/superconductor junctions: In tunneling spectroscopy studies of ferromagnet/superconductor (F/S)
junctions, the effects of spin polarization, Fermi wavevector mismatch (FWM)
between the F and S regions, and interfacial resistance play a crucial role. We
study the low bias conductance spectrum of these junctions, governed by Andreev
reflection at the F/S interface. We consider both d- and s-wave superconductors
as well as mixed states of the $d+is$ form. We present results for a range of
values of the relevant parameters and find that a rich variety of features
appears, depending on pairing state and other conditions. We show that in the
presence of FWM, spin polarization can enhance Andreev reflection and give rise
to a zero bias conductance peak for an s-wave superconductor. | cond-mat_supr-con |
Suppression of surface barrier in superconductors by columnar defects: We investigate the influence of columnar defects in layered superconductors
on the thermally activated penetration of pancake vortices through the surface
barrier. Columnar defects, located near the surface, facilitate penetration of
vortices through the surface barrier, by creating ``weak spots'', through which
pancakes can penetrate into the superconductor. Penetration of a pancake
mediated by an isolated column, located near the surface, is a two-stage
process involving hopping from the surface to the column and the detachment
from the column into the bulk; each stage is controlled by its own activation
barrier. The resulting effective energy is equal to the maximum of those two
barriers. For a given external field there exists an optimum location of the
column for which the barriers for the both processes are equal and the
reduction of the effective penetration barrier is maximal. At high fields the
effective penetration field is approximately two times smaller than in
unirradiated samples. We also estimate the suppression of the effective
penetration field by column clusters. This mechanism provides further reduction
of the penetration field at low temperatures. | cond-mat_supr-con |
Detection of Geometric Phases in Flux Qubits with Coherent Pulses: We propose a experimentally feasible scheme to demonstrate the geometric
phase in flux qubits by means of detuning coherent microwave pulse techniques.
Through measuring the probability of the persistent current state in flux
qubits, one can detect the Berry phase that is acquired with system's
Hamiltonian adiabatical circular evolution in the parameter space. Furthermore,
we show that one should choose an appropriate amplitude of pulses in an
experiment to obtain high readout resolution when detuning frequency of pulses
is fixed and controlled phase shift gates can be implemented based on the
geometric phases by inductance coupling two flux qubits. | cond-mat_supr-con |
Probing the Order Parameter of Superconducting LiFeAs using Pb/LiFeAs
and Au/LiFeAs Point-Contact Spectroscopy: We have fabricated c-axis point contact junctions between high-quality LiFeAs
single crystals and Pb or Au tips in order to study the nature of the
superconducting order parameter of LiFeAs, one of the few stoichiometric
iron-based superconductors. The observation of the Josephson current in c-axis
junctions with a conventional s-wave superconductor as the counterelectrode
indicates that the pairing symmetry in LiFeAs is not pure d-wave or pure
spin-triplet p-wave. A superconducting gap is clearly observed in point contact
Andreev reflection measurements performed on both Pb/LiFeAs and Au/LiFeAs
junctions. The conductance spectra can be well described by the
Blonder-Tinkham-Klapwijk model with a lifetime broadening term, resulting in a
gap value of \approx 1.6 meV (2{\Delta}/kBTC \approx 2.2). | cond-mat_supr-con |
Josephson current through a long quantum wire: The dc Josephson current through a long SNS junction receives contributions
from both Andreev bound states localized in the normal region as well as from
scattering states incoming from the superconducting leads. We show that in the
limit of a long junction, this current, at low temperatures, can be expressed
entirely in terms of properties of the Andreev bound states at the Fermi
energy: the normal and Andreev reflection amplitudes at the left-hand and at
the right-hand S-N interface. This has important implications for treating
interactions in such systems. | cond-mat_supr-con |
Electronic structure of new quaternary superconductors LaONiBi and
LaOCuBi from first principles: Based on first-principles FLAPW-GGA calculations, we have investigated the
electronic structure of newly synthesized novel superconductors LaONiBi and
LaOCuBi, the first bismuth-containing compounds from the family of quaternary
oxypnictides which attract now a great deal of interest in search for novel
26-55K superconductors. The band structure, density of states and Fermi
surfaces are discussed. Our results indicate that the bonding inside of the
(La-O) and (Ni(Cu)-Bi) layers is covalent whereas the bonding between the
(La-O) and (Ni(Cu)- Bi) blocks is mostly ionic. For both oxybismuthides, the
DOSs at the Fermi level are formed mainly by the states of the (Ni(Cu)-Bi)
layers, the corresponding Fermi surfaces have a twodimensional character and
the conduction should be strongly anisotropic andhappen only on the (Ni(Cu)-Bi)
layers. As a whole, the new oxybismuthides may be described as low-TC
superconducting non-magnetic ionic metals. | cond-mat_supr-con |
Josephson critical currents in annular superconductors with Pearl
vortices: We investigate the influence of Pearl vortices in the vicinity of an
edge-type Josephson junction for a superconducting thin-film loop in the form
of an annulus, under uniform magnetic field. Specifically, we obtain the exact
analytic formulation that allows to describe the circulating current density
and the gauge invariant phase increment $\Delta\phi$ across the junction. The
main properties of $\Delta\phi$ and their influence on the critical current
pattern $I_c(B)$ are described quantitatively in terms of the loop's width to
radius ratio $W/R$ and of the vortex position within the loop ${\bf r}_v$. It
is shown that narrow loops ($W/R < 0.3$) may be well described by the straight
geometry limit. However, such approximation fails to predict a number of
distinctive features captured by our formulation, as the node lifting effect of
the $I_c(B)$ pattern in wide loops or the actual influence of a vortex pinned
at different positions. | cond-mat_supr-con |
Band-selective clean- and dirty-limit superconductivity with nodeless
gaps in the bilayer iron-based superconductor CsCa$_2$Fe$_4$As$_4$F$_2$: The optical properties of the new iron-based superconductor
CsCa$_2$Fe$_4$As$_4$F$_2$ with $T_c \sim 29$~K have been determined. In the
normal state a good description of the low-frequency response is obtained with
a superposition of two Drude components of which one has a very low scattering
rate (narrow Drude-peak) and the other a rather large one (broad Drude-peak).
Well below $T_c \sim 29$~K, a pronounced gap feature is observed which involves
a complete suppression of the optical conductivity below $\sim$ 110~cm$^{-1}$
and thus is characteristic of a nodeless superconducting state. The optical
response of the broad Drude-component can be described with a dirty-limit
Mattis-Bardeen-type response with a single isotropic gap of $2\Delta \simeq
14$~meV. To the contrary, the response of the narrow Drude-component is in the
ultra-clean-limit and its entire spectral weight is transferred to the
zero-frequency $\delta(\omega)$ function that accounts for the loss-free
response of the condensate. These observations provide clear evidence for a
band-selective coexistence of clean- and dirty-limit superconductivity with
nodeless gaps in CsCa$_2$Fe$_4$As$_4$F$_2$. | cond-mat_supr-con |
Evidence from $^{77}$Se Knight shifts for triplet superconductivity in
(TMTSF)$_2$PF$_6$: The layered quasi-one-dimensional molecular superconductor (TMTSF)$_2$PF$_6$
is a very exotic material with a superconducting order parameter whose ground
state symmetry has remained ill-defined. Here we present a pulsed NMR Knight
shift (K) study of $^{77}$Se measured simultaneously with transport in
pressurized (TMTSF)$_2$PF$_6$. The Knight shift is linearly dependent on the
electron spin susceptibility $\chi_s$, and is therefore a direct measure of the
spin polarization in the superconducting state. For a singlet superconductor,
the spin contribution to the Knight shift, K$_s$, falls rapidly on cooling
through the transition. The present experiments indicate no observable change
in K between the metallic and superconducting states, and thus strongly support
the hypothesis of triplet p-wave superconductivity in (TMTSF)$_2$PF$_6$. | cond-mat_supr-con |
Complete zero-energy flat bands of surface states in fully gapped chiral
noncentrosymmetric superconductors: Noncentrosymmetric superconductors can support flat bands of zero-energy
surface states in part of their surface Brillouin zone. This requires that they
obey time-reversal symmetry and have a sufficiently strong
triplet-to-singlet-pairing ratio to exhibit nodal lines in the bulk. These
bands are protected by a winding number that relies on chiral symmetry, which
is realized as the product of time-reversal and particle-hole symmetry. We
reveal a way to stabilize a flat band in the entire surface Brillouin zone,
while the bulk dispersion is fully gapped. This idea could lead to a robust
platform for quantum computation and represents an alternative route to
strongly correlated flat bands in two dimensions, besides twisted bilayer
graphene. The necessary ingredient is an additional spin-rotation symmetry that
forces the direction of the spin-orbit-coupling vector not to depend on the
momentum component normal to the surface. We define a winding number which
leads to flat zero-energy surface bands due to bulk-boundary correspondence. We
discuss under which conditions this winding number is nonzero in the entire
surface Brillouin zone and verify the occurrence of zero-energy surface states
by exact numerical diagonalization of the Bogoliubov-de Gennes Hamiltonian for
a slab. In addition, we consider how a weak breaking of the additional symmetry
affects the surface band, employing first-order perturbation theory and a
quasiclassical approximation. We find that the surface states and the bulk gap
persist for weak breaking of the additional symmetry but that the band does not
remain perfectly flat. The broadening of the band strongly depends on the
deviation of the spin-orbit-coupling vector from its unperturbed direction as
well as on the spin-orbit-coupling strength and the triplet-pairing amplitude. | cond-mat_supr-con |
Superconductivity in novel BiS2-based layered superconductor
LaO1-xFxBiS2: Layered superconductors have provided some interesting fields in condensed
matter physics owing to the low dimensionality of their electronic states. For
example, the high-Tc (high transition temperature) cuprates and the Fe-based
superconductors possess a layered crystal structure composed of a stacking of
spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes
or Fe-Anion layers. The spacer layers provide carriers to the conduction layers
and induce exotic superconductivity. Recently, we have reported
superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was
found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The
crystal structure is composed of a stacking of BiS2 superconducting layers and
the spacer layers, which resembles those of high-Tc cuprate and the Fe-based
superconductors. Here we report a discovery of a new type of BiS2-based layered
superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K. | cond-mat_supr-con |
Advanced surface characterization of
Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ epitaxial thin films: We report on the systematic characterization of
Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ epitaxial thin films on CaF$_2$ substrate
in view of their possible use for superconducting electronic applications. By
using different and complementary techniques we studied the morphological
characteristics of the surface, the structural properties, the magnetic
response, and the superconducting properties in terms of critical temperature,
critical current, and energy gaps. Particular attention was paid to the
homogeneity of the films and to the comparison of their superconducting
properties with those of single crystals of the same compound. | cond-mat_supr-con |
Superconductivity and abnormal pressure effect in Sr0.5La0.5FBiSe2
superconductor: Through the solid state reaction method, we synthesized a new BiSe2-based
superconductor Sr0:5La0:5FBiSe2 with superconducting transition temperature
Tc?3.8 K. A strong diamagnetic signal below Tc in susceptibility ?(T) is
observed indicating the bulk nature of superconductivity. Different to most
BiS2-based compounds where superconductivity develops from a
semiconducting-like normal state, the present compound exhibits a metallic
behavior down to Tc. Under weak magnetic field or pressure, however, a
remarkable crossover from metallic to insulating behaviors takes place around
Tmin where the resistivity picks up a local minimum. With increasing pressure,
Tc decreases monotonously and Tmin shifts to high temperatures, while the
absolute value of the normal state resistivity at low temperatures first
decreases and then increases with pressure up to 2.5 GPa. These results imply
that the electronic structure of Sr0:5La0:5FBiSe2 may be different to those in
the other BiS2-based systems. | cond-mat_supr-con |
Majorana spintronics: We propose a systematic magnetic-flux-free approach to detect, manipulate and
braid Majorana fermions in a semiconductor nanowire-based topological Josephson
junction by utilizing the Majorana spin degree of freedom. We find an intrinsic
$\pi$-phase difference between spin-triplet pairings enforced by the Majorana
zeros modes (MZMs) at the two ends of a one-dimensional spinful topological
superconductor. This $\pi$-phase is identified to be a spin-dependent
superconducting phase, referred to as the spin-phase, which we show to be
tunable by controlling spin-orbit coupling strength via electric gates. This
electric controllable spin-phase not only affects the coupling energy between
MZMs but also leads to a fractional Josephson effect in the absence of any
applied magnetic flux, which enables the efficient topological qubit readout.
We thus propose an all-electrically controlled superconductor-semiconductor
hybrid circuit to manipulate MZMs and to detect their non-Abelian braiding
statistics properties. Our work on spin properties of topological Josephson
effects potentially opens up a new thrust for spintronic applications with
Majorana-based semiconductor quantum circuits. | cond-mat_supr-con |
Magnetism in SQUIDs at Millikelvin Temperatures: We have characterized the temperature dependence of the flux threading dc
SQUIDs cooled to millikelvin temperatures. The flux increases as 1/T as
temperature is lowered; moreover, the flux change is proportional to the
density of trapped vortices. The data is compatible with the thermal
polarization of surface spins in the trapped fields of the vortices. In the
absence of trapped flux, we observe evidence of spin-glass freezing at low
temperature. These results suggest an explanation for the "universal" 1/f flux
noise in SQUIDs and superconducting qubits. | cond-mat_supr-con |
In-situ fabrication of cobalt-doped SrFe2As2 thin films by using pulsed
laser deposition with excimer laser: The remarkably high superconducting transition temperature and upper critical
field of iron(Fe)-based layered superconductors, despite ferromagnetic material
base, open the prospect for superconducting electronics. However, success in
superconducting electronics has been limited because of difficulties in
fabricating high-quality thin films. We report the growth of high-quality
c-axis-oriented cobalt(Co)-doped SrFe2As2 thin films with bulk
superconductivity by using an in-situ pulsed laser deposition technique with a
248-nm-wavelength KrF excimer laser and an arsenic(As)-rich phase target. The
temperature and field dependences of the magnetization showing strong
diamagnetism and transport critical current density with superior Jc-H
performance are reported. These results provide necessary information for
practical applications of Fe-based superconductors. | cond-mat_supr-con |
Quantum Phase Transitions in Superconducting Arrays with General
Capacitance Matrices: We investigate quantum phase transitions in two-dimensional superconducting
arrays with general capacitance matrices and discrete charge states. We use the
perturbation theory together with the simulated annealing method to obtain the
zero-temperature phase diagrams, which display various lobe-like structures of
insulating solid phases, and examine the possibility of supersolid phase. At
nonzero temperatures, an effective classical Hamiltonian is obtained through
the use of the variational method in the path-integral formalism, and the
corresponding phase diagram is found approximately. The insulating lobes of the
solid phases are shown to exist at sufficiently low temperatures, and results
of Monte Carlo simulations are also presented. | cond-mat_supr-con |
Quasiparticle entropy in superconductor/normal metal/superconductor
proximity junctions in the diffusive limit: We discuss the quasiparticle entropy and heat capacity of a dirty
superconductor-normal metal-superconductor junction. In the case of short
junctions, the inverse proximity effect extending in the superconducting banks
plays a crucial role in determining the thermodynamic quantities. In this case,
commonly used approximations can violate thermodynamic relations between
supercurrent and quasiparticle entropy. We provide analytical and numerical
results as a function of different geometrical parameters. Quantitative
estimates for the heat capacity can be relevant for the design of caloritronic
devices or radiation sensor applications. | cond-mat_supr-con |
Amplitude `Higgs' mode in 2H-NbSe2 Superconductor: We report experimental evidences for the observation of the superconducting
amplitude mode, so-called `Higgs' mode in the charge density wave
superconductor 2H-NbSe2 using Raman scattering. By comparing 2H-NbSe2 and its
iso-structural partner 2H-NbS2 which shows superconductivity but lacks the
charge density wave order, we demonstrate that the superconducting mode in
2H-NbSe2 owes its spectral weight to the presence of the coexisting charge
density wave order. In addition, temperature dependent measurements in 2H-NbSe2
show a full spectral weight transfer from the charge density wave mode to the
superconducting mode upon entering the superconducting phase. Both observations
are fully consistent with a superconducting amplitude mode or Higgs mode. | cond-mat_supr-con |
Colossal thermomagnetic response in the exotic superconductor URu2Si2: When a superconductor is heated above its critical temperature $T_c$,
macroscopic coherence vanishes, leaving behind droplets of thermally
fluctuating Cooper pair. This superconducting fluctuation effect above $T_c$
has been investigated for many decades and its influence on the transport,
thermoelectric and thermodynamic quantities in most superconductors is well
understood by the standard Gaussian fluctuation theories. The transverse
thermoelectric (Nernst) effect is particularly sensitive to the fluctuations,
and the large Nernst signal found in the pseudogap regime of the underdoped
high-$T_c$ cuprates has raised much debate on its connection to the origin of
superconductivity. Here we report on the observation of a colossal Nernst
signal due to the superconducting fluctuations in the heavy-fermion
superconductor URu$_2$Si$_2$. The Nernst coefficient is enhanced by as large as
one million times over the theoretically expected value within the standard
framework of superconducting fluctuations. This, for the first time in any
known material, results in a sizeable thermomagnetic figure of merit
approaching unity. Moreover, contrary to the conventional wisdom, the
enhancement in the Nernst signal is more significant with the reduction of the
impurity scattering rate. This anomalous Nernst effect intimately reflects the
highly unusual superconducting state embedded in the so-called hidden-order
phase of URu$_2$Si$_2$. The results invoke possible chiral or Berry-phase
fluctuations originated from the topological aspect of this superconductor,
which are associated with the effective magnetic field intrinsically induced by
broken time-reversal symmetry of the superconducting order parameter. | cond-mat_supr-con |
Characterizing two-dimensional superconductivity via nanoscale noise
magnetometry with single-spin qubits: We propose nanoscale magnetometry via isolated single-spin qubits as a probe
of superconductivity in two-dimensional materials. We characterize the magnetic
field noise at the qubit location, arising from current and spin fluctuations
in the sample and leading to measurable polarization decay of the qubit. We
show that the noise due to transverse current fluctuations studied as a
function of temperature and sample-probe distance can be used to extract useful
information about the transition to a superconducting phase and the pairing
symmetry of the superconductor. Surprisingly, at low temperatures, the dominant
contribution to the magnetic noise arises from longitudinal current
fluctuations and can be used to probe collective modes such as monolayer
plasmons and bilayer Josephson plasmons. We also characterize the noise due to
spin fluctuations, which allows probing the spin structure of the pairing wave
function. Our results provide a non-invasive route to probe the rich physics of
two-dimensional superconductors. | cond-mat_supr-con |
Common Electronic Origin of Superconductivity in (Li,Fe)OHFeSe Bulk
Superconductor and Single-Layer FeSe/SrTiO3 Films: The mechanism of high temperature superconductivity in the iron-based
superconductors remains an outstanding issue in condensed matter physics. The
electronic structure, in particular the Fermi surface topology, is considered
to play an essential role in dictating the superconductivity. Recent revelation
of distinct electronic structure and possible high temperature
superconductivity with a transition temperature Tc above 65 K in the
single-layer FeSe films grown on the SrTiO3 substrate provides key information
on the roles of Fermi surface topology and interface in inducing or enhancing
superconductivity. Here we report high resolution angle-resolved photoemission
measurement on the electronic structure and superconducting gap of a novel
FeSe-based superconductor, (Li0.84Fe0.16)OHFe0.98Se, with a Tc at 41 K. We find
that this single-phase bulk superconductor shows remarkably similar electronic
behaviors to that of the superconducting single-layer FeSe/SrTiO3 film in terms
of Fermi surface topology, band structure and nearly isotropic superconducting
gap without nodes. These observations provide significant insights in
understanding high temperature superconductivity in the single-layer
FeSe/SrTiO3 film in particular, and the mechanism of superconductivity in the
iron-based superconductors in general. | cond-mat_supr-con |
Multiband superconductivity and a deep gap minimum from the specific
heat in KCa$_2$(Fe$_{1-x}$Ni$_x$)$_4$As$_4$F$_2$ ($x$ = 0, 0.05, 0.13): Specific heat can explore low-energy quasiparticle excitations of
superconductors, so it is a powerful tool for bulk measurement on the
superconducting gap structure and pairing symmetry. Here, we report an in-depth
investigation on the specific heat of the multiband superconductors
KCa$_2$(Fe$_{1-x}$Ni$_x$)$_4$As$_4$F$_2$ ($x$ = 0, 0.05, 0.13) single crystals
and the overdoped non-superconducting one with $x$ = 0.17. Clear specific heat
anomalies can be observed at the superconducting transition temperature of 33.6
K and 28.8 K for the samples with $x$ = 0 and $x$ = 0.05, respectively. For the
two samples, the magnetic field induced specific heat coefficient
$\Delta\gamma(H)$ in the low-temperature limit increases rapidly below 2 T,
then it rises slowly above 2 T. Using the non-superconducting sample with $x$ =
0.17 as a reference, the specific heat of phonon background for various
superconducting samples can be obtained and subtracted, which allows us to
extract the electronic specific heat of the superconducting samples. Through
comparative analyses, it is found that the energy gap structure including two
$s$-wave gaps and an extended $s$-wave gap with large anisotropy can reasonably
describe the electronic specific heat data. According to these results, we
suggest that at least one anisotropic superconducting gap with a deep gap
minimum should exist in this multiband system. With the doping of Ni, the
superconducting transition temperature of the sample decreases along with the
decrease of the large $s$-wave gap, but the extended $s$-wave gap increases due
to the enlarged electron pockets via adding more electrons. Despite these
changes, the general properties of the gap structure remain unchanged versus
doping Ni. | cond-mat_supr-con |
Strain-driven nematicity of the odd-parity superconductivity in
Sr$_x$Bi$_2$Se$_3$: We present a novel experimental evidence for the odd-parity nematic
superconductivity in high-quality single crystals of doped topological
insulator Sr$_x$Bi$_2$Se$_3$. The X-ray diffraction shows that the grown single
crystals are either weakly stretched or compressed uniaxially in the basal
plane along one of the crystal axis. We show that in the superconducting state,
the upper critical magnetic field $H_{c2}$ has a two-fold rotational symmetry
and depends on the sign of the strain: in the stretched samples, the maximum of
$H_{c2}$ is achieved when the in-plane magnetic field is transverse to the
strain axis, while in the compressed samples this maximum is observed when the
field is along the strain direction. This result is naturally explained within
a framework of the odd-parity nematic superconductivity coupled to the strain.
Magnetoresistance in the normal state is independent of the current direction
and also has a two-fold rotational symmetry that demonstrates the nematicity of
the electronic system in the normal state. | cond-mat_supr-con |
Chiral and helical $p$-wave superconductivity in doped bilayer BiH: We investigate the superconductivity (SC) driven by correlation effects in
electron-doped bilayer BiH near a type-II van Hove singularity (vHS). By
functional renormalization group, we find triplet $p$-wave pairing prevails in
the interaction parameter space, except for spin density wave (SDW) closer to
the vHS or when the interaction is too strong. Because of the large atomic
spin-orbital coupling (SOC), the $p$-wave pairing occurs between equal-spin
electrons, and is chiral and two-fold degenerate. The chiral state supports
in-gap edge states, even though the low energy bands in the SC state are
topologically trivial. The absence of mirror symmetry allows Rashba SOC that
couples unequal spins, but we find its effect is of very high order, and can
only drive the chiral $p$-wave into helical $p$-wave deep in the SC state.
Interestingly, there is a six-fold degeneracy in the helical states, reflected
by the relative phase angle $\theta=n\pi/3$ (for integer $n$) between the spin
components of the helical pairing function. The phase angle is shown to be
stable in the vortex state. | cond-mat_supr-con |
Instability of Majorana states in Shiba chains due to leakage into a
topological substrate: We revisit the problem of Majorana states in chains of scalar impurities
deposited on a superconductor with a mixed s-wave and p-wave pairing. We also
study the formation of Majorana states for magnetic impurity chains. We find
that the magnetic impurity chains exhibit well-localized Majorana states when
the substrate is trivial, but these states hybridize and get dissolved in the
bulk when the substrate is topological. Most surprisingly, and contrary to
previous predictions, the scalar impurity chain does not support fully
localized Majorana states except for very small and finely tuned parameter
regimes, mostly for a non-topological substrate close to the topological
transition. Our results indicate that a purely p-wave or a dominant p-wave
substrate are not good candidates to support either magnetic or scalar impurity
topological Shiba chains. | cond-mat_supr-con |
Mixed higher-order topology and nodal and nodeless flat band topological
phases in a superconducting multiorbital model: We investigate the topological phases that appear in an orbital version of
the Benalcazar-Bernevig-Hughes (BBH) model in the presence of conventional
spin-singlet $s$-wave superconductivity and with the possibility of tuning an
in-plane magnetic field. We chart out the phase diagram by considering
different boundary conditions, with the topology of the individual phases
further examined by considering both the Wannier and entanglement spectra, as
well as the Majorana polarization.
For weak to moderate values of magnetic field and superconducting pairing
amplitude, we find a second-order topological superconductor phase with eight
zero-energy corner modes. Further increasing field or pairing, half of the
corner states can be turned into zero-energy edge-localized modes, thus forming
a type of hybrid-order phase. Then, we find two different putative first-order
topological phases, a nodal and a nodeless phase, both with zero-energy flat
bands localized along mirror-symmetric open edges. For the nodal phase, the
flat bands are localized between the nodes in reciprocal space, while in the
nodeless phase, with its a full bulk gap, the zero-energy boundary flat band
spans the whole Brillouin zone. | cond-mat_supr-con |
Spin Transport in Half-Metallic Ferromagnet-Superconductor Junctions: We investigate the charge and spin transport in half-metallic ferromagnet
($F$) and superconductor ($S$) nanojunctions. We utilize a self-consistent
microscopic method that can accommodate the broad range of energy scales
present, and ensures proximity effects that account for the interactions at the
interfaces are accurately determined. Two experimentally relevant half-metallic
junction types are considered: The first is a $F_1 F_2 S$ structure, where a
half-metallic ferromagnet $F_1$ adjoins a weaker conventional ferromagnet
$F_2$. The current is injected through the $F_1$ layer by means of an applied
bias voltage. The second configuration involves a $S F_1 F_2 F_3 S$ Josephson
junction whereby a phase difference $\Delta\varphi$ between the two
superconducting electrodes generates the supercurrent flow. In this case, the
central half-metallic $F_2$ layer is surrounded by two weak ferromagnets $F_1$
and $F_3$. By placing a ferromagnet with a weak exchange field adjacent to an
$S$ layer, we are able to optimize the conversion process in which
opposite-spin triplet pairs are converted into equal-spin triplet pairs that
propagate deep into the half-metallic regions in both junction types. For the
tunnel junctions, we study the bias-induced local magnetization, spin currents,
and spin transfer torques for various orientations of the relative
magnetization angle $\theta$ in the $F$ layers. We find that the bias-induced
equal-spin triplet pairs are maximized in the half-metal for
$\theta\approx90^\circ$ and as part of the conversion process, are
anticorrelated with the opposite-spin pairs. We show that the charge current
density is maximized, corresponding to the occurrence of a large amplitude of
equal-spin triplet pairs, when the exchange interaction of the weak ferromagnet
is about $0.1E_F.$ | cond-mat_supr-con |
Hall coefficient and angle-resolved photoemission in systems with strong
pair fluctuations: We examine the normal-state temperature and doping dependence of the Hall
coefficient in the context of a pair-fluctuation scenario, based on a model
where itinerant electrons are hybridized with localized electron pairs via a
charge exchange term. We show that an anomalous behavior of the Hall effect,
qualitatively similar to that observed in high-Tc superconductors, can be
attributed to the non-Fermi liquid properties of the single-particle spectral
function which exhibits pseudogap features. Our calculations are based on a
dynamical mean-field procedure which relates the transport coefficients to the
single-particle spectral function in an exact way. | cond-mat_supr-con |
From Cooper-pair glass to unconventional superconductivity: a unified
approach to cuprates and pnictides: We report a microscopic model wherein the unconventional superconductivity
emerges from an incoherent `Cooper-pair glass' state. Driven by the pair-pair
interaction, a new type of quasi-Bose phase transition is at work. The
interaction leads to the unconventional coupling of the quasiparticles to
excited pair states, or `super-quasiparticles', with a non-retarded
energy-dependent gap. The model describes quantitatively the quasiparticle
excitation spectra of both cuprates and pnictides, including the universal
`peak-dip-hump' signatures, and for the pseudogap phase above $T_c$. The
results show that instantaneous pair-pair interactions account for the SC
condensation without a collective mode. | cond-mat_supr-con |
Resilience of d-wave superconductivity to nearest-neighbor repulsion: Many theoretical approaches find d-wave superconductivity in the prototypical
one-band Hubbard model for high-temperature superconductors. At strong-coupling
(U > W, where U is the on-site repulsion and W=8t the bandwidth) pairing is
controlled by the exchange energy J=4t^2/U. One may then surmise, ignoring
retardation effects, that near-neighbor Coulomb repulsion V will destroy
superconductivity when it becomes larger than J, a condition that is easily
satisfied in cuprates for example. Using Cellular Dynamical Mean-Field theory
with an exact diagonalization solver for the extended Hubbard model, we show
that pairing, at strong coupling, is preserved even when V>>J, as long as
V<U/2. While at weak coupling V always reduces the spin fluctuations and hence
d-wave pairing, at strong coupling, in the underdoped regime, the increase of
J=4t^2/(U-V) caused by V increases binding at low frequency while the
pair-breaking effect of V is pushed to high frequency. These two effects
compensate in the underdoped regime, in the presence of a pseudogap. While the
pseudogap competes with superconductivity, the proximity to the Mott transition
that leads to the pseudogap, and retardation effects, protect d-wave
superconductivity from V. | cond-mat_supr-con |
Key Ingredients for Superconductivity in Cuprates: Using high resolution angle-resolved photoemission data in conjunction with
that from neutron and other probes, we show that electron-phonon (el-ph)
coupling is strong in cuprates superconductors and it plays an important role
in pairing. In addition to the strong electron correlation, the inclusion of
phonons provides a theoretical framework explaining many important phenomena
that cannot be understood by a strongly correlated electronic model alone.
Especially it is indispensable to explain the difference among materials. The
phonons with the wave number around the (0,qx) and (qx,0) axes create the
d-wave pairing while that near (pi,pi) are pair breaking. Therefore the
half-breathing mode of the oxygen motions helps d-wave superconductivity. | cond-mat_supr-con |
Interplay between magnetism and superconductivity in EuFe(2-x)Co(x)As2
studied by 57Fe and 151Eu Mössbauer spectroscopy: The compound EuFe(2-x)Co(x)As2 was investigated by means of the 57Fe and
151Eu Moessbauer spectroscopy versus temperature (4.2 - 300 K) for x=0
(parent), x=0.34 - 0.39 (superconductor) and x=0.58 (overdoped). It was found
that spin density wave (SDW) is suppressed by Co-substitution, however it
survives in the region of superconductivity, but iron spectra exhibit some
non-magnetic component in the superconducting region. Europium orders
anti-ferromagnetically regardless of the Co concentration with the spin
re-orientation from the a-axis in the parent compound toward c-axis with the
increasing replacement of iron by cobalt. The re-orientation takes place close
to the a-c plane. Some trivalent europium appears in EuFe(2-x)Co(x)As2 versus
substitution due to the chemical pressure induced by Co-atoms and it
experiences some transferred hyperfine field from Eu2+. Iron experiences some
transferred field due to the europium ordering for substituted samples in the
SDW and non-magnetic state both, while the transferred field is undetectable in
the parent compound. Superconductivity coexists with the 4f-europium magnetic
order within the same volume. It seems that superconductivity has some
filamentary character in EuFe(2-x)Co(x)As2 and it is confined to the
non-magnetic component seen by the iron Moessbauer spectroscopy. | cond-mat_supr-con |
Resistive transition in disordered superconductors with varying
intergrain coupling: The effect of disorder is investigated in granular superconductive materials
with strong and weak links. The transition is controlled by the interplay of
the \emph{tunneling} $g$ and \emph{intragrain} $g_{intr}$ conductances, which
depend on the strength of the intergrain coupling. For $g \ll g_{intr}$, the
transition involves first the grain boundary, while for $g \sim g_{intr}$ the
transition occurs into the whole grain. The different intergrain coupling is
considered by modelling the superconducting material as a disordered network of
Josephson junctions. Numerical simulations show that on increasing the
disorder, the resistive transition occurs for lower temperatures and the curve
broadens. These features are enhanced in disordered superconductors with strong
links. The different behaviour is further checked by estimating the average
network resistance for weak and strong links in the framework of the effective
medium approximation theory. These results may be relevant to shed light on
long standing puzzles as: (i) enhancement of the superconducting transition
temperature of many metals in the granular states; (ii) suppression of
superconductivity in homogeneously disordered films compared to standard
granular systems close to the metal-insulator transition; (iii) enhanced
degradation of superconductivity by doping and impurities in strongly linked
materials, such as magnesium diboride, compared to weakly-linked
superconductors, such as cuprates. | cond-mat_supr-con |
Superconductivity of cerium under pressures up to 54GPa: Cerium is a fascinating element due to its diverse physical properties, which
include forming various crystal structures ($\gamma$, $\alpha$, $\alpha^{'}$,
$\alpha^{''}$ and $\epsilon$), mixed valence behavior and superconductivity,
making it an ideal platform for investigating the interplay between different
electronic states. Here, we present a comprehensive transport study of cerium
under hydrostatic pressures up to 54 GPa. Upon applying pressure, cerium
undergoes the $\alpha$ $\rightarrow$ $\alpha^{''}$ transition at around 4.9
GPa, which is accompanied by the appearance of superconductivity with $T_{\rm
c}$ of 0.4 K, and $T_{\rm c}$ slightly increases to 0.5 K at 11.4 GPa. At 14.3
GPa, $T_{\rm c}$ suddenly increases when the $\alpha^{''}$ phase transforms
into the $\epsilon$ phase, reaching a maximum value of 1.25 K at around 17.2
GPa. Upon further increasing the pressure, $T_{\rm c}$ monotonically decreases.
Together with the results of previous studies, our findings suggest that the
evolution of superconductivity in cerium is closely correlated with the
multiple pressure-induced structural transitions and corresponding unusual
electronic structures. | cond-mat_supr-con |
Surface states and local spin susceptibility in doped three-dimensional
topological insulators with odd-parity superconducting pairing symmetry: We investigate characteristic features in the spin response of doped
three-dimensional topological insulators with odd-parity unequal-spin
superconducting pairing, which are predicted to have gapless Majorana surface
modes. These Majorana modes contribute to the local spin susceptibility, giving
rise to a characteristic temperature behavior of the Knight shift and the
spin-lattice relaxation time in magnetic resonance experiments. Because of
their different decay lengths, the Majorana modes can be observed and clearly
distinguished from the Dirac modes of the topological insulator by local probes
which allow for a depth-controlled study of the electron spins on the nanometer
length scale. | cond-mat_supr-con |
Charge distribution in C$_{60}$ crystal doped by electric field: The calculations of the charge distribution in C$_{60}$-based FET structure
are presented. A simple model is proposed to describe the distribution of the
injected electrons or holes between two-dimensional layers. The calculations
show that the relative charge distribution between the layers turns to be
independent on the total amount of injected charges. The charge density is
maximal on the surface layer and drops exponentially with the depth increase.
The relative amounts of injected charge involved in the top layer are 73 and 64
per cent in the case of electron and hole injection, respectively. Thus, the
charge localization on the crystal surface turns to be markedly different from
near complete one that was obtained earlier within tight- binding model for the
charge concentration providing superconductivity. | cond-mat_supr-con |
Half-integer Shapiro-steps in superconducting qubit with a
$π$-Josephson junction: A superconducting quantum interference device (SQUID) comprising 0- and
$\pi$-Josephson junctions (JJs), called $\pi$-SQUID, is studied by the
resistively shunted junction model. The $\pi$-SQUID shows half-integer
Shapiro-steps (SS) under microwave irradiation at the voltage $V$ =
$(\hbar/2e)\Omega (n/2)$, with angular frequency $\Omega$ and half-integer
$n$/2 in addition to integer $n$. We show that the $\pi$-SQUID can be a
$\pi$-qubit with spontaneous loop currents by which the half-integer SS are
induced. Making the 0- and $\pi$-JJs equivalent is a key for the half-integer
SS and realizing the $\pi$-qubit. | cond-mat_supr-con |
Controlling Factors of Tc-Dome Structure in 1111-Type Iron Arsenide
Superconductors: We investigated the effects of phosphorus substitution on the shape of the
Tc(x) dome in 1111-type SmFeAs1-yPyO1-xHx (0 < x < 0.5). Hydride ion
substitution of oxide sites (O2- -> H-) exerts a chemical pressure effect,
i.e., a structural reduction of the Pn-Fe-Pn angle {\alpha} (Pn = P, As) and
also dopes electrons into the FePn layer to induce superconductivity. Isovalent
phosphorus substitution (P3- -> As3-) can induce only a chemical pressure
effect, i.e., an increase of {\alpha} for La-substitution of Sm-sites. As y
increases from 0.0 to 0.5, the single Tc dome gradually splits into two domes,
similar to those of LaFeAsO1-xHx with a Tc valley at x ~ 0.16. We found that
the Tc valley is located around (x, {\alpha}) ~ (0.16, 113{\deg}) for both
SmFeAs1-yPyO1-xHx and LaFeAsO1-xHx series, irrespective of changes in the Pn
anion and Ln cation species. This result suggests that suppression of Tc leads
to the emergence of a Tc valley when both the shape of FePn4 tetrahedra
represented by {\alpha} and electron doping level of x meet the above criterion
in 1111 type iron oxypnictide superconductors. | cond-mat_supr-con |
Properties of a two orbital model for oxypnictide superconductors:
Magnetic order, B_2g spin-singlet pairing channel, and its nodal structure: A two orbital model for the new Fe-based superconductors is studied using the
Lanczos method as well as pairing mean-field approximations. Our main goals are
(i) to provide a comprehensive analysis of this model using numerical
techniques with focus on half-filling and on the state with two more electrons
than half-filling and (ii) to investigate the nodal structure of the mean-field
superconducting state and compare the results with angle-resolved photoemission
data. In particular, we provide evidence that at half-filling spin 'stripes',
as observed experimentally, dominate over competing states.
Depending on parameters, the state with two more electrons added to half
filling is either triplet or singlet. Since experiments suggest spin singlet
pairs, our focus is on this state. Under rotation, it transforms as the B_2g
representation of the D_4h group. We also show that the s+/- pairing operator
transforms as A_1g and becomes dominant only in an unphysical regime of the
model where the undoped state is an insulator. For robust values of the
effective electronic attraction producing the Cooper pairs, assumption
compatible with recent angle-resolved photoemission (ARPES) results that
suggesting small Cooper-pair size, the nodes of the two-orbital model are found
to be located only at the electron pockets. Since recent ARPES efforts have
searched for nodes at the hole pockets or only in a few directions at the
electron pockets, our results for the nodal distribution may help to guide
future experiments. More in general, the investigations reported here aim to
establish several of the properties of the two orbital model. Only a detailed
comparison with experiments will clarify how far this simple model present a
valid description of the Fe pnictides. | cond-mat_supr-con |
Doping dependent optical properties of Bi2201: An experimental study of the in-plane optical conductivity of
(Pb$_{x}$,Bi$_{2-x}$)(La$_{y}$Sr$_{2-y}$)CuO$_{6+\delta}$ (Bi2201) is presented
for a broad doping and temperature range. The in-plane conductivity is analyzed
within a strong coupling formalism. We address the interrelationship between
the optical conductivity ($\sigma(\omega)$), the single particle self energy,
and the electron-boson spectral function. We find that the frequency and
temperature dependence can be well described within this formalism. We present
a universal description of optical, ARPES and tunneling spectra. The full
frequency and temperature dependence of the optical spectra and single particle
self-energy is shown to result from an electron-boson spectral function, which
shows a strong doping dependence and weak temperature dependence. | cond-mat_supr-con |
Proximity of LaOFeAs to a magnetic instability: We investigate the effect of external pressure on the Fe magnetic moment in
undoped LaOFeAs within the framework of density functional theory and show that
this system is close to a magnetic instability: The Fe moment is found to drop
by nearly a factor of 3 within a pressure range of $\pm$ 5 GPa around the
calculated equilibrium volume. While the Fe moments show an unusually strong
sensitivity to the spin arrangement (type of anti-ferromagnetic structure), the
low temperature structural distortion is found to have only a minor influence
on them. Analysis of the Fermi surface topology and nesting features shows that
these properties change very little up to pressures of at least 10 GPa. We
discuss the magnetic instability in terms of the itinerancy of this system. | cond-mat_supr-con |
Local characterization of superconductivity in BaFe2(As1-xPx)2: We use magnetic force microscopy (MFM) to characterize superconductivity
across the superconducting dome in ${\rm {BaFe}_2 \left(As_{1-x}P_x\right)_2}$,
a pnictide with a peak in the penetration depth ($\lambda_{ab}$) at optimal
doping ($\rm x_{opt}$), as shown in sample-wide measurements. Our local
measurements show a peak at $\rm x_{opt}$ and a $T_C$ vs. $\lambda_{ab}^{-2}$
dependence similar on both sides of $\rm x_{opt}$. Near the underdoped edge of
the dome $\lambda_{ab}$ increases sharply suggesting that superconductivity
competes with another phase. Indeed MFM vortex imaging shows correlated defects
parallel to twin boundaries only in underdoped samples and not for $\rm x\ge
x_{opt}$. | cond-mat_supr-con |
Controlling spin supercurrents via nonequilibrium spin injection: We propose a mechanism whereby spin supercurrents can be manipulated in
superconductor/ferromagnet proximity systems via nonequilibrium spin injection.
We find that if a spin supercurrent exists in equilibrium, a nonequilibrium
spin accumulation will exert a torque on the spins transported by this current.
This interaction causes a new spin supercurrent contribution to manifest out of
equilibrium, which is proportional to and polarized perpendicularly to both the
injected spins and equilibrium spin current. This is interesting for several
reasons: as a fundamental physical effect; due to possible applications as a
way to control spin supercurrents; and timeliness in light of recent
experiments on spin injection in proximitized superconductors. | cond-mat_supr-con |
Condensate entanglement and multigap superconductivity in nanoscale
superconductors: A Green functions approach is used to study superconductivity in nanofilms
and nanowires. We show that the superconducting condensate results from the
multimodal entanglement, or internal Josephson coupling, of the subcondensates
associated with the manifold of Fermi surface subparts resulting from
size-quantisation. This entanglement is of critical importance in these
systems, since without it superconductivity would be extremely weak, if not
completely negligible. Further, the multimodal character of the condensate
generally results in multigap superconductivity, with great quantitative
consequence for the value of the critical parameters. Our approach suggests
that these are universal characteristics of confined superconductors. | cond-mat_supr-con |
Determining Pair Interactions from Structural Correlations: We examine metastable configurations of a two-dimensional system of
interacting particles on a quenched random potential landscape and ask how the
configurational pair correlation function is related to the particle
interactions and the statistical properties of the potential landscape.
Understanding this relation facilitates quantitative studies of magnetic flux
line interactions in type II superconductors, using structural information
available from Lorentz microscope images or Bitter decorations.
Previous work by some of us supported the conjecture that the relationship
between pair correlations and interactions in pinned flux line ensembles is
analogous to the corresponding relationship in the theory of simple liquids.
The present paper aims at a more thorough understanding of this relation. We
report the results of numerical simulations and present a theory for the low
density behavior of the pair correlation function which agrees well with our
simulations and captures features observed in experiments. In particular, we
find that the resulting description goes beyond the conjectured classical
liquid type relation and we remark on the differences. | cond-mat_supr-con |
Andreev bound states in rounded corners of d-wave superconductors: Andreev bound states at boundaries of d-wave superconductors are strongly
influenced by the boundary geometry itself. In this work, the zero-energy
spectral weight of the local quasiparticle density of states is presented for
the case of wedge-shaped boundaries with rounded corners. Generally, both
orientation of the d-wave and the specific local reflection properties of the
rounded wedges determine, whether Andreev bound states exist or not. For the
bisecting line of the wedge being parallel to the nodal direction of the d-wave
gap function, strong zero-energy Andreev bound states are expected at the round
part of the boundary. | cond-mat_supr-con |
Magnetic properties of magnetically textured Bi-2212 ceramics: This paper aims at reporting magnetic properties of bulk polycrystalline
Bi2Sr2Ca0.8Dy0.2Cu2O8-y samples textured under a magnetic field. The
microstructure of these materials is highly anisotropic and exhibits particular
features needed to be taken into account in order to interpret their magnetic
and electrical properties. First the AC magnetic susceptibility c = c ' - j c"
has been measured for several magnetic fields (H // ab and H // c) and compared
to the electrical resistivity data. The structure of the c" peak is shown to be
related to the chemical content distribution of the superconducting grains.
Next, the magnetic flux profiles have been extracted from the magnetic
measurements using the Campbell - Rollins procedure. The anisotropy of the flux
profiles and their peculiar curvature behaviour for H // c point out the role
of both grain platelet structure and the presence of secondary phases. From
these results, we conclude that the magnetic properties of such magnetically
textured materials do not allow for a reliable extraction of the critical
current density Jc but essentially probe geometric effects. Such effects have
to be taken into account for improving the manufacture of attractive high-Tc
materials. | cond-mat_supr-con |
Anisotropy of the upper critical fields and the paramagnetic Meissner
effect in La1.85Sr0.15CuO4 single Crystals: Optimally-doped La1.85Sr0.15CuO4 single crystals have been investigated by dc
and ac magnetic measurements. These crystals have rectangular needle-like
shapes with the long needle axis parallel to the crystallographic c axis
(c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the
temperature dependence of the upper critical fields (HC2) and the surface
critical field (HC3) were measured. The H-T phase diagram is presented. Close
to TC =35 K, for the c-crystal, {\gamma}c = / = 1.80(2), whereas for the
a-crystal the {\gamma}a = / =4.0(2) obtained, is much higher than the
theoretical value 1.69. At low applied dc fields, positive field-cooled
branches known as the "paramagnetic Meissner effect" (PME) are observed, their
magnitude is inversely proportional to H. The anisotropic PME is observed in
both a- and c-crystals, only when the applied field is along the basal planes.
It is speculated that the high {\gamma}a and the PME are connected to each
other. | cond-mat_supr-con |
Metastability in Josephson transmission lines: Thermal activation and macroscopic quantum tunneling in current-biased
discrete Josephson transmission lines are studied theoretically. The degrees of
freedom under consideration are the phases across the junctions which are
coupled to each other via the inductances of the system. The resistively
shunted junctions that we investigate constitute a system of N interacting
degrees of freedom with an overdamped dynamics. We calculate the decay rate
within exponential accuracy as a function of temperature and current. Slightly
below the critical current, the decay from the metastable state occurs via a
unique ("rigid") saddlepoint solution of the Euclidean action describing the
simultaneous decay of the phases in all the junctions. When the current is
reduced, a crossover to a regime takes place, where the decay occurs via an
"elastic" saddlepoint solution and the phases across the junctions leave the
metastable state one after another. This leads to an increased decay rate
compared with the rigid case both in the thermal and the quantum regime. The
rigid-to-elastic crossover can be sharp or smooth analogous to first- or
second- order phase transitions, respectively. The various regimes are
summarized in a current-temperature decay diagram. | cond-mat_supr-con |
Deterministic Josephson Vortex Ratchet with a load: We investigate experimentally a deterministic underdamped Josephson vortex
ratchet -- a fluxon-particle moving along a Josephson junction in an asymmetric
periodic potential. By applying a sinusoidal driving current one can compel the
vortex to move in a certain direction, producing average dc voltage across the
junction. Being in such a rectification regime we also load the ratchet, i.e.,
apply an additional dc bias current I_dc (counterforce) which tilts the
potential so that the fluxon climbs uphill due to the ratchet effect. The value
of the bias current at which the fluxon stops climbing up defines the strength
of the ratchet effect and is determined experimentally. This allows us to
estimate the loading capability of the ratchet, the output power and
efficiency. For the quasi-static regime we present a simple model which
delivers simple analytic expressions for the above mentioned figures of merit. | cond-mat_supr-con |
Tunable two-dimensional superconductivity and spin-orbit coupling at the
EuO/KTaO3(110) interface: Unconventional quantum states, most notably the two-dimensional (2D)
superconductivity, have been realized at the interfaces of oxide
heterostructures where they can be effectively tuned by the gate voltage
($V_G$). Here we report that the interface between high-quality EuO (111) thin
film and KTaO3 (KTO) (110) substrate shows superconductivity with onset
transition temperature $T_c^{onset}$ = 1.35 K. The 2D nature of
superconductivity is verified by the large anisotropy of the upper critical
field and the characteristics of a Berezinskii-Kosterlitz-Thouless transition.
By applying $V_G$, $T_c^{onset}$ can be tuned from ~ 1 to 1.7 K; such an
enhancement can be possibly associated with a boosted spin-orbit energy
${\epsilon}_{so}$ = $\hbar$ / ${\tau}_{so}$, where ${\tau}_{so}$ is the
spin-orbit relaxation time. Further analysis of ${\tau}_{so}$ based on the
upper critical field ($H_{c2}$) and magnetoconductance reveals complex nature
of spin-orbit coupling (SOC) at the EuO/KTO(110) interface with different
mechanisms dominate the influence of SOC effects for the superconductivity and
the magnetotransport in the normal state. Our results demonstrate that the SOC
should be considered as an important factor determining the 2D
superconductivity at oxide interfaces. | cond-mat_supr-con |
Superconducting energy gap and c-axis plasma frequency of
(Nd,Sm)O0.82F0.18FeAs superconductors from infrared ellipsometry: We present ellipsometric measurements of the far-infrared dielectric response
of polycrystalline samples of the new pnictide superconductor RO0.82F0.18FeAs
(R=Nd and Sm). We find evidence that the electronic properties are strongly
anisotropic such that the optical spectra are dominated by the weakly
conducting c-axis response similar as in the cuprate high-temperature
superconductors (HTSC). Accordingly, we obtain an upper limit of the c-axis
superconducting plasma frequency of $\omega_{{\rm pl},c}^{\rm SC}\leq 260\cm$
which corresponds to a lower limit of the c-axis magnetic penetration depth of
$\lambda_c\geq6\mum$ and an anisotropy of $\lambda_c/\lambda_{ab}\geq 30$ as
compared to $\lambda_{ab}=185$ nm from muon spin rotation [A. Drew {\it et
al.}, arXiv:0805.1042]. Also in analogy to the cuprate HTSC, our spectra
exhibit the signatures of a gap-like suppression of the conductivity in the
superconducting state with a large gap magnitude of $2\Delta\approx300\cm$ (37
meV) and a ratio of $2\Delta/k_{\rm B}\tc\approx8$ that is suggestive of strong
coupling. | cond-mat_supr-con |
Scaling of the Fano effect of the in-plane Fe-As phonon and the
superconducting critical temperature in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$: By means of infrared spectroscopy we determine the temperature-doping phase
diagram of the Fano effect for the in-plane Fe-As stretching mode in
Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. The Fano parameter $1/q^2$, which is a
measure of the phonon coupling to the electronic particle-hole continuum, shows
a remarkable sensitivity to the magnetic/structural orderings at low
temperatures. More strikingly, at elevated temperatures in the
paramagnetic/tetragonal state we find a linear correlation between $1/q^2$ and
the superconducting critical temperature $T_c$. Based on theoretical
calculations and symmetry considerations, we identify the relevant interband
transitions that are coupled to the Fe-As mode. In particular, we show that a
sizable $xy$ orbital component at the Fermi level is fundamental for the Fano
effect and possibly also for the superconducting pairing. | cond-mat_supr-con |
Ab initio Study of Cross-Interface Electron-Phonon Couplings in FeSe
Thin Films on SrTiO$_3$ and BaTiO$_3$: We study the electron-phonon coupling strength near the interface of
monolayer and bilayer FeSe thin films on SrTiO$_3$, BaTiO$_3$, and
oxygen-vacant SrTiO$_3$ substrates, using ab initio methods. The calculated
total electron-phonon coupling strength $\lambda=0.2\text{--}0.3$ cannot
account for the high $T_c\sim 70$ K observed in these systems through the
conventional phonon-mediated pairing mechanism. In all of these systems,
however, we find that the coupling constant of a polar oxygen branch peaks at
$\mathbf{q}=0$ with negligible coupling elsewhere, while the energy of this
mode coincides with the offset energy of the replica bands measured recently by
angle-resolved photoemission spectroscopy experiments. But the integrated
coupling strength for this mode from our current calculations is still too
small to produce the observed high $T_c$, even through the more efficient
pairing mechanism provided by the forward scattering. We arrive at the same
qualitative conclusion when considering a checkerboard antiferromagnetic
configuration in the Fe layer. In light of the experimental observations of the
replica band feature and the relatively high $T_c$ of FeSe monolayers on polar
substrates, our results point towards a cooperative role for the
electron-phonon interaction, where the cross-interface interaction acts in
conjunction with a purely electronic interaction. We also discuss a few
scenarios where the coupling strength obtained here may be enhanced. | cond-mat_supr-con |
High-Density Superconductive Logic Circuits Utilizing 0 and $π$
Josephson Junctions: Superconductor Electronics (SCE) is a fast and power-efficient technology
with great potential for overcoming conventional CMOS electronics' scaling
limits. Nevertheless, the primary challenge confronting SCE today pertains to
its integration level, which lags several orders of magnitude behind CMOS
circuits. In this study, we have innovated and simulated a novel logic family
grounded in the principles of phase shifts occurring in 0 and $\pi$ Josephson
junctions. The fast phase logic (FPL) eliminates the need for large inductor
loops and shunt resistances by combining the half-flux and phase logic.
Therefore, the Josephson junction (JJ) area only limits the integration
density. The cells designed with this paradigm are fast, and the clock-to-Q
delay is about 4ps while maintaining over 50% parameter margins. This logic is
power efficient and can increase the integration by at least 100$\times$ in the
SCE chips. | cond-mat_supr-con |
Z$_2$ topology and superconductivity from symmetry lowering of a 3D
Dirac Metal Au$_2$Pb: 3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons
and exhibit exotic physical properties It has been suggested that structurally
distorting a DSM can create a Topological Insulator (TI), but this has not yet
been experimentally verified. Furthermore, quasiparticle excitations known as
Majorana Fermions have been theoretically proposed to exist in materials that
exhibit superconductivity and topological surface states. Here we show that the
cubic Laves phase Au$_2$Pb has a bulk Dirac cone above 100 K that gaps out upon
cooling at a structural phase transition to create a topologically non trivial
phase that superconducts below 1.2 K. The nontrivial Z$_2$ = -1 invariant in
the low temperature phase indicates that Au$_2$Pb in its superconducting state
must have topological surface states. These characteristics make Au$_2$Pb a
unique platform for studying the transition between bulk Dirac electrons and
topological surface states as well as studying the interaction of
superconductivity with topological surface states. | cond-mat_supr-con |
Generation of Teraherz Oscillations by Thin Superconducting Film in
Fluctuation Regime: Explicit analytical expressions for conductivity of a superconducting film
above and below critical temperature in an arbitrary electric field are derived
in the frameworks of the time dependent Ginzburg-Landau theory. It is confirmed
that slightly below critical temperature the differential conductivity of
superconducting film can become negative for small enough values of electric
field. This fact may cause generation of electromagnetic oscillations if the
superconducting film is appropriately coupled of with a resonator. Their
maximal frequency is proportional to the value of critical temperature of
superconducting transition. The obtained results can stimulate the development
of Terahertz generators on the basis of high temperature superconducting films. | cond-mat_supr-con |
High Tc superconductivity in heavy Rare Earth Hydrides: correlation
between the presence of the f states on the Fermi surface, nesting and the
value of Tc: Lanthanum hydrides, containing hydrogen framework structures under
compression, display a superconducting state with a high observed critical
temperature. However, this phenomenon has so far only been observed at very
high pressures. Here, we computationally search for superconductors with very
high critical temperatures, but at much lower pressures. We uncover two such
sodalite-type clathrate hydrides, YbH6 and LuH6 (Tc =145 K at P=70 GPa for YbH6
and, especially, Tc =273 K at P=100 GPa for LuH6). These striking properties
are a consequence of the strong interrelationship between the f states present
at the Fermi surface, structural stability and Tc value. For example, TmH6,
with unfilled 4f orbitals, is stable at 50 GPa, while it has a relatively low
value of Tc of 25 K. As for the YbH6 and LuH6 compounds, they have filled
f-shells and the decrease of the f-energy below the Fermi level leads to
formation of the nesting regions on the Fermi surface and, as a result, to
phonon "softening" and an increase in Tc. | cond-mat_supr-con |
Superconductivity at 15.6 K in Calcium-doped Tb_{1-x}Ca_xFeAsO: the
structure requirement for achieving superconductivity in the hole-doped 1111
phase: Superconductivity at about 15.6 K was achieved in Tb_{1-x}Ca_xFeAsO by
partially substituting Tb^{3+} with Ca^{2+} in the nominal doping region x =
0.40 \sim 0.50. A detailed investigation was carried out in a typical sample
with doping level of x = 0.44. The upper critical field of this sample was
estimated to be 77 Tesla from the magnetic field dependent resistivity data.
The domination of hole-like charge carriers in the low-temperature region was
confirmed by Hall effect measurements. The comparison between the calcium-doped
sample Pr_{1-x}Ca_xFeAsO (non-superconductive) and the Strontium-doped sample
Pr_{1-x}Sr_xFeAsO (superconductive) suggests that a lager ion radius of the
doped alkaline-earth element compared with that of the rare-earth element may
be a necessary requirement for achieving superconductivity in the hole-doped
1111 phase. | cond-mat_supr-con |
Robust nodal superconductivity induced by isovalent doping in
Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x$)$_2$: We present the ultra-low-temperature heat transport study of iron-based
superconductors Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ and
BaFe$_2$(As$_{1-x}$P$_x$)$_2$. For optimally doped
Ba(Fe$_{0.64}$Ru$_{0.36}$)$_2$As$_2$, a large residual linear term $\kappa_0/T$
at zero field and a $\sqrt{H}$ dependence of $\kappa_0(H)/T$ are observed,
which provide strong evidences for nodes in the superconducting gap. This
result demonstrates that the isovalent Ru doping can also induce nodal
superconductivity, as P does in BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$.
Furthermore, in underdoped Ba(Fe$_{0.77}$Ru$_{0.23}$)$_2$As$_2$ and heavily
underdoped BaFe$_2$(As$_{0.82}$P$_{0.18}$)$_2$, $\kappa_0/T$ manifests similar
nodal behavior, which shows the robustness of nodal superconductivity in the
underdoped regime and puts constraint on theoretical models. | cond-mat_supr-con |
How to measure the spatial correlations in disordered
Berezinski-Kosterlitz-Thouless transition ?: The Berezinski-Kosterlitz-Thouless transition is a unique two dimensional
phase transition, separating two phases with exponentially and power-law
decaying correlations, respectively. In disordered systems, these correlations
propagate along favorable paths, with the transition marking the point where
global coherence is lost. Here we propose an experimental method to probe
locally these particular paths in superconducting thin films, which exhibit
this transition, and demonstrate theoretically that close to the transition the
coherence propagate along a ramified network, reminiscent of a percolation
transition. We suggest and calculate experimentally accessible quantities that
can shed light on the spatial correlations in the system as it approaches the
critical point. | cond-mat_supr-con |
Percolative nature of the dc paraconductivity in the cuprate
superconductors: We present an investigation of the planar direct-current (dc)
paraconductivity of the model cuprate material HgBa$_2$CuO$_{4+\delta}$ in the
underdoped part of the phase diagram. The simple quadratic
temperature-dependence of the Fermi-liquid normal-state resistivity enables us
to extract the paraconductivity above the macroscopic $T_c$ with great
accuracy. The paraconductivity exhibits unusual exponential temperature
dependence, with a characteristic temperature scale that is distinct from
$T_c$. In the entire temperature range where it is discernable, the
paraconductivity is quantitatively explained by a simple superconducting
percolation model, which implies that underlying gap disorder dominates the
emergence of superconductivity. | cond-mat_supr-con |
Double-helix magnetic order in CrAs with Pnma space group: The transition metal pnictide CrAs exhibits superconductivity in the vicinity
of a helimagnetic phase, where it has been found that the propagation vector is
parallel to the c axis and the magnetic moments lie in the ab plane. Here we
use ab initio calculations to study the magnetic interactions in the material.
Mapping onto a Heisenberg Hamiltonian, we calculate the magnetic exchanges by
using LDA+U calculations and we unveil the origin of the magnetic frustration.
Finally, we reproduce the double helix magnetic order with a propagation vector
Q = (0, 0, 0.456) and we obtain the magnetic transition temperature TN through
Monte-Carlo simulations of the specific heat. Due to the limitations of the use
of the Heisenberg Hamiltonian for itinerant magnetic systems, the theoretical
TN underestimated the experimental value of the pure CrAs. However, our results
are in good agreement with those found for the alloy CrAs0.5Sb0.5 belonging to
the same space group, showing that our result can describe this material class. | cond-mat_supr-con |
Unusual Hole-doping-dependent Electronic Instability and Electron-Phonon
Coupling in Infinite-layer Nickelates: The interplay between superconductivity and charge density waves (CDWs) under
hole doping in cuprates is one of the central phenomena in condensed matter
physics. Recently, CDWs are also observed in CaCuO$_2$-analogous nickelates
RNiO$_2$ (R = La, Nd) but exhibit fundamentally different hole-doping-dependent
behaviors compared to that in cuprates, raising a challenging question on its
origin. In this article, we propose that electronic instability (EI) and
moment-dependent electron-phonon coupling (MEPC), mainly contributed by Ni
3dx2-y2 and R 5dz2, respectively, may be the possible reasons for CDW formation
in RNiO$_2$. Without hole doping, a strong Fermi surface nesting (FSN) induced
by the unique feature of van Hove singularity (VHS) across Fermi level exists
in RNiO$_2$ but not in CaCuO$_2$, and the unusual temperature-insensitive
feature of EI and MEPC could result in rather high temperature CDWs in
RNiO$_2$. Under hole doping, the reduced FSN of Ni 3dx2-y2 by the shift of VHS
and decreased occupation of R 5dz2 largely weaken EI and MEPC in RNiO$_2$,
respectively, suppressing the CDW formation. Our theory not only offers
possible explanations to some puzzling experimental observations, but also
establishes a unified understanding on the hole-doping-dependent EI and MEPC in
nickelates and cuprates. | cond-mat_supr-con |
Optical conductivity of iron-based superconductors: The new family of unconventional iron-based superconductors discovered in
2006 immediately relieved their copper-based high-temperature predecessors as
the most actively studied superconducting compounds in the world. The
experimental and theoretical effort made in order to unravel the mechanism of
superconductivity in these materials has been overwhelming. Although our
understanding of their microscopic properties has been improving steadily, the
pairing mechanism giving rise to superconducting transition temperatures up to
55 K remains elusive. And yet the hope is strong that these materials, which
possess a drastically different electronic structure but similarly high
transition temperatures compared to the copper-based compounds, will shed
essential new light onto the several-decade-old problem of unconventional
superconductivity. In this work we review the current understanding of the
itinerant-charge-carrier dynamics in the iron-based superconductors and parent
compounds largely based on the optical-conductivity data the community has
gleaned over the past seven years using such experimental techniques as
reflectivity, ellipsometry, and terahertz transmission measurements and analyze
the implications of these studies for the microscopic properties of the
iron-based materials as well as the mechanism of superconductivity therein. | cond-mat_supr-con |
Paramagnon dispersion and damping in doped
Na$_{x}$Ca$_{2-x}$CuO$_2$Cl$_2$: Using Resonant Inelastic X-ray Scattering, we measure the paramagnon
dispersion and damping of undoped, antiferromagnetic Ca$_2$CuO$_2$Cl$_2$ as
well as doped, superconducting Na$_{x}$Ca$_{2-x}$CuO$_2$Cl$_2$. Our estimation
of the spin-exchange parameter and width of the paramagnon peak at the zone
boundary $X=(0.5,0)$ confirms that no simple relation can be drawn between
these parameters and the critical temperature $T_\mathrm{c}$. Consistently with
other cuprate compounds, we show that upon doping there is a slight softening
at $(0.25,0)$, but not at the zone boundary $X$. In combination with these
measurements we perform calculations of the dynamical spin structure factor of
the one-band Hubbard model using cluster dynamical mean-field theory. The
calculations are in excellent agreement with the experiment in the undoped
case, both in terms of energy position and width. While the increase in width
is also captured upon doping, the dynamical spin structure factor shows a
sizable softening at $X$, which provides insightful information on the
length-scale of the spin fluctuations in doped cuprates. | cond-mat_supr-con |
Shell Structure and Strengthening of Superconducting Pair Correlation in
Nanoclusters: The existence of shell structure and the accompanying high degeneracy of
electronic levels leads to the possibility of strong superconducting pairing in
metallic nanoclusters with N~100-1000 delocalized electrons. The most favorable
cases correspond to (a) "magic" clusters with strongly degenerate highest
occupied and lowest unoccupied shells and a relatively small energy spacing
between them as well as to (b) clusters with slightly incomplete shells and
small Jahn-Teller splitting. It is shown that realistic sets of parameters lead
to very high values of Tc as well as to a strong alteration of the energy
spectrum. The impact of fluctuations is analyzed. Spectroscopic experiments
aimed at detecting the presence of pair correlations are proposed. The pairing
should also manifest itself via odd-even effects in cluster spectra, similar to
the case of nuclei. | cond-mat_supr-con |
Hidden magnetic excitation in the pseudogap phase of a model cuprate
superconductor: The elucidation of the pseudogap phenomenon of the cuprates, a set of
anomalous physical properties below the characteristic temperature T* and above
the superconducting transition temperature Tc, has been a major challenge in
condensed matter physics for the past two decades. Following initial
indications of broken time-reversal symmetry in photoemission experiments,
recent polarized neutron diffraction work demonstrated the universal existence
of an unusual magnetic order below T*. These findings have the profound
implication that the pseudogap regime constitutes a genuine new phase of matter
rather than a mere crossover phenomenon. They are furthermore consistent with a
particular type of order involving circulating orbital currents, and with the
notion that the phase diagram is controlled by a quantum critical point. Here
we report inelastic neutron scattering results for HgBa2CuO4+x (Hg1201) that
reveal a fundamental collective magnetic mode associated with the unusual
order, and that further support this picture. The mode's intensity rises below
the same temperature T* and its dispersion is weak, as expected for an
Ising-like order parameter. Its energy of 52-56 meV and its enormous integrated
spectral weight render it a new candidate for the hitherto unexplained
ubiquitous electron-boson coupling features observed in spectroscopic studies. | cond-mat_supr-con |
Microwave Surface Impedance Measurements of the Electronic State and
Dissipation of Magnetic Vortices in Superconducting Iron-Based LiFeAs Single
Crystals: LiFeAs is one of the iron-based superconductors having multiple gaps with the
possible sign reversal. To clarify how those novel natures affect the energy
dissipation of magnetic vortices, we investigated the microwave surface
impedance of LiFeAs single crystals under finite magnetic fields. The flux-flow
resistivity enhanced rapidly at low magnetic fields, which is similar to the
case of MgB$_{2}$. This is probably the consequence of the multiple-gap nature
and the gap anisotropy. This suggest that the sign-reversal is not important
for the flux-flow even for multiple-gap superconductors. As for the electronic
state, the vortex core of LiFeAs turned out to be "moderately clean".
Furthermore, the mean free path inside the vortex core was much shorter than
that outside, and was close to the core radius. These results strongly suggest
a process specific to the core boundary is important for a scattering mechanism
inside the vortex core. | cond-mat_supr-con |
Superconductivity at 17 K in (Fe2P2)(Sr4Sc2O6): a new superconducting
layered pnictide oxide with a thick perovskite oxide layer: A new layered oxypnictide (Fe2P2)(Sr4Sc2O6) have been synthesized by
solid-state reaction. This material has an alternating layer stacking structure
of anti-fluorite Fe2P2 and perovskite-based Sr4Sc2O6 oxide layers. Space group
of the material is P4/nmm and lattice constants a and c are 4.016 A and 15.543
A, respectively. The interlayer Fe-Fe distance corresponding to the c-axis
length is the longest ever reported in the iron-based oxypnictide systems. In
both magnetization and resistivity measurements, the present compound exhibited
superconductivity below 17 K, which is much higher than that of LaFePO and the
highest in arsenic-free iron-based oxypnictide systems under ambient pressure. | cond-mat_supr-con |
Direct observation of melting in a 2-D superconducting vortex lattice: Topological defects such as dislocations and disclinations are predicted to
determine the twodimensional (2-D) melting transition. In 2-D superconducting
vortex lattices, macroscopic measurements evidence melting close to the
transition to the normal state. However, the direct observation at the scale of
individual vortices of the melting sequence has never been performed. Here we
provide step by step imaging through scanning tunneling spectroscopy of a 2-D
system of vortices up to the melting transition in a focused-ion-beam
nanodeposited W-based superconducting thin film. We show directly the
transition into an isotropic liquid below the superconducting critical
temperature. Before that, we find a hexatic phase, characterized by the
appearance of free dislocations, and a smectic-like phase, possibly originated
through partial disclination unbinding. These results represent a significant
step in the understanding of melting of 2-D systems, with impact across several
research fields, such as liquid crystal molecules, or lipids in membranes. | cond-mat_supr-con |
New Series of Nickel-Based Pnictide Oxide Superconductors
(Ni2Pn2)(Sr4Sc2O6) (Pn = P, As): We have discovered new nickel-based pnictide oxide superconductors,
(Ni2Pn2)(Sr4Sc2O6) (Pn = P, As). These compounds have a tetragonal unit cell
with a space group of P4/nmm and they consist of alternate stacking of
anti-fluorite Ni2Pn2 layers and K2NiF4-type Sr4Sc2O6 blocking layers. Lattice
parameters were a = 4.044 A and c = 15.23 A for (Ni2P2)(Sr4Sc2O6) and a = 4.078
A and c = 15.41 A for (Ni2As2)(Sr4Sc2O6), indicating their thicker blocking
layers than that of LaNiPO (c ~ 8.1 A). Both (Ni2P2)(Sr4Sc2O6) and
(Ni2As2)(Sr4Sc2O6) exhibited superconductivity with zero resistivity at 3.3 K
and 2.7 K, respectively. The perfect diamagnetism observed in both compounds
guaranteed their bulk superconductivity. | cond-mat_supr-con |
Pressure-Induced Superconductivity and Its Scaling with Doping-Induced
Superconductivity in the Iron Pnictide with Skutterudite Intermediary Layers: The Ca10(PtnAs8)(Fe2As2)5 (n=3,4) compounds are a new type of iron pnictide
superconductors whose structures consist of stacking Ca-PtnAs8-Ca-Fe2As2 layers
in a unit cell. When n=3 (the 10-3-8 phase), the undoped compound is an
antiferromagnetic (AFM) semiconductor, while, when n=4 (the 10-4-8 phase), the
undoped compound is a superconductor with the transition temperature of 26K.
Here we report the results of high-pressure studies on the 10-3-8 compound
obtained through a combination of in-situ resistance, magnetic susceptibility,
and Hall coefficient measurements. We find that its AFM order can be suppressed
completely at 3.5 GPa and then superconducting state appears in the pressure
range of 3.5-7 GPa. The pressure dependence of superconducting transition
temperature displays a dome-like shape. | cond-mat_supr-con |
Possible Dynamic States in Inductively Coupled Intrinsic Josephson
Junctions of Layered High-$T_c$ Superconductors: Based on computer simulations and theoretical analysis, a new dynamic state
is found in inductively coupled intrinsic Josephson junctions in the absence of
an external magnetic field. In this state, the plasma oscillation is uniform
along the c axis and there are $(2m+1)\pi$ phase kinks, with $m$ being an
integer, periodic and thus non-uniform in the $c$ direction. In the IV
characteristics, the state manifests itself as current steps occurring at all
cavity modes. Inside the current steps, the plasma oscillation becomes strong,
which generates several harmonics in frequency spectra at a given voltage. The
recent experiments on terahertz radiations from the mesa of a BSCCO single
crystal can be explained in terms of this state. | cond-mat_supr-con |
Critical currents in superconductors with quasiperiodic pinning arrays:
One-dimensional chains and two-dimensional Penrose lattices: We study the critical depinning current J_c, as a function of the applied
magnetic flux Phi, for quasiperiodic (QP) pinning arrays, including
one-dimensional (1D) chains and two-dimensional (2D) arrays of pinning centers
placed on the nodes of a five-fold Penrose lattice. In 1D QP chains of pinning
sites, the peaks in J_c(Phi) are shown to be determined by a sequence of
harmonics of long and short periods of the chain. This sequence includes as a
subset the sequence of successive Fibonacci numbers. We also analyze the
evolution of J_c(Phi) while a continuous transition occurs from a periodic
lattice of pinning centers to a QP one; the continuous transition is achieved
by varying the ratio gamma = a_S/a_L of lengths of the short a_S and the long
a_L segments, starting from gamma = 1 for a periodic sequence. We find that the
peaks related to the Fibonacci sequence are most pronounced when gamma is equal
to the "golden mean". The critical current J_c(Phi) in QP lattice has a
remarkable self-similarity. This effect is demonstrated both in real space and
in reciprocal k-space. In 2D QP pinning arrays (e.g., Penrose lattices), the
pinning of vortices is related to matching conditions between the vortex
lattice and the QP lattice of pinning centers. Although more subtle to analyze
than in 1D pinning chains, the structure in J_c(Phi) is determined by the
presence of two different kinds of elements forming the 2D QP lattice. Indeed,
we predict analytically and numerically the main features of J_c(Phi) for
Penrose lattices. Comparing the J_c's for QP (Penrose), periodic (triangular)
and random arrays of pinning sites, we have found that the QP lattice provides
an unusually broad critical current J_c(Phi), that could be useful for
practical applications demanding high J_c's over a wide range of fields. | cond-mat_supr-con |
Nonequilibrium transport via spin-induced sub-gap states in
superconductor/quantum dot/normal metal cotunnel junctions: We study low-temperature transport through a Coulomb blockaded quantum dot
(QD) contacted by a normal (N), and a superconducting (S) electrode. Within an
effective cotunneling model the conduction electron self energy is calculated
to leading order in the cotunneling amplitudes and subsequently resummed to
obtain the nonequilibrium T-matrix, from which we obtain the nonlinear
cotunneling conductance. For even occupied dots the system can be conceived as
an effective S/N-cotunnel junction with subgap transport mediated by Andreev
reflections. The net spin of an odd occupied dot, however, leads to the
formation of sub-gap resonances inside the superconducting gap which gives rise
to a characteristic peak-dip structure in the differential conductance, as
observed in recent experiments. | cond-mat_supr-con |
Temperature and polarization dependence of low-energy magnetic
fluctuations in nearly-optimal-doped NaFe$_{0.9785}$Co$_{0.0215}$As: We use unpolarized and polarized neutron scattering to study the temperature
and polarization dependence of low-energy magnetic fluctuations in
nearly-optimal-doped NaFe$_{0.9785}$Co$_{0.0215}$As, with coexisting
superconductivity ($T_{\rm c}\approx19$ K) and weak antiferromagnetic order
($T_{\rm N}\approx30$ K, ordered moment $\approx0.02$ $\mu_{\rm B}$/Fe). A
single spin resonance mode with intensity tracking the superconducting order
parameter is observed, although energy of the mode only softens slightly on
approaching $T_{\rm c}$. Polarized neutron scattering reveals that the single
resonance is mostly isotropic in spin space, similar to overdoped
NaFe$_{0.935}$Co$_{0.045}$As but different from optimal electron-, hole-, and
isovalent-doped BaFe$_2$As$_2$ compounds, all featuring an additional prominent
anisotropic component. Spin anisotropy in NaFe$_{0.9785}$Co$_{0.0215}$As is
instead present at energies below the resonance, which becomes partially gapped
below $T_{\rm c}$, similar to the situation in optimal-doped
YBa$_2$Cu$_3$O$_{6.9}$. Our results indicate that anisotropic spin fluctuations
in NaFe$_{1-x}$Co$_x$As appear in the form of a resonance in the underdoped
regime, become partially gapped below $T_{\rm c}$ near optimal doping and
disappear in overdoped compounds. | cond-mat_supr-con |
Elementary vortex pinning potential in a chiral p-wave superconductor: The elementary vortex pinning potential is studied in a chiral p-wave
superconductor with a pairing d=z(k_x + i k_y) on the basis of the
quasiclassical theory of superconductivity. An analytical investigation and
numerical results are presented to show that the vortex pinning potential is
dependent on whether the vorticity and chirality are parallel or antiparallel.
Mutual cancellation of the vorticity and chirality around a vortex is
physically crucial to the effect of the pinning center inside the vortex core. | cond-mat_supr-con |
The geometric theory of low dimensional superconductivity: The study of the electromagnetic properties of 2D materials is an area of
intensive research. Most of the efforts in the subject have been oriented
towards the understanding and modelling of the microscopic behaviour of the
charge carriers within the medium. Albeit there is a well established manner to
express London's equations in the language of differential geometry, an
effective geometric theory of the macroscopic phenomenon is still lacking. In
this work we concentrate in obtaining the underlying geometric structure of
superconductivity in two dimensional materials. To this effect, we produce an
intrinsic framework which allows us to clearly identify the geometric
assumptions leading to London's equations and the Meissner state. Specifically,
we show that any two-dimensional medium whose response to an externally applied
electromagnetic field results in a divergence free geodesically flowing induced
current must be a superconductor. In this manner, we conclude that the
underlying geometry of this type of media is that of a three dimensional
Lorentzian contact manifold. Moreover, we show that the geometric condition
macroscopically encoding the superconducting phenomena emerges from a
variational principle, exhibiting that the macroscopic hallmark of
superconductivity is expressed as the non-vanishing of the induced current's
helicity. | cond-mat_supr-con |
Further detailing of the Bose-Einstein negative-U state in the high
temperature superconducting cuprates: The cause of the sharp and universal optimization of the HTSC condensation
energy at the hole doping concentration of p = 0.19 is identified within the
context of the boson-fermion negative-U modelling and stripe phase electronic
organization. Some recent structural, optical and positron annihilation
experiments adding further support to this type of modelling of HTSC are
briefly examined. | cond-mat_supr-con |
Melting of the vortex lattice in high $T_{c}$ superconductors: The precise measurements of vortex melting point towards a need to develop a
quantitative theoretical description of thermal fluctuations in vortex matter.
To tackle the difficult problem of melting, the description of both the solid
and the liquid phase should reach the precision level well below 0.1%. Such a
theory in the framework of the Ginzburg - Landau approach is presented. The
melting line location is determined and magnetization, specific heat jumps
along it are calculated. We find that the magnetization in the liquid is larger
than that in the solid by 1.8% regardless of the melting temperature, while the
specific heat jump is about 6% and slowly decreases with temperature. The
magnetization curves agree with experimental results on $YBCO$ and Monte Carlo
simulations. | cond-mat_supr-con |
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