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Correlated trends of coexisting magnetism and superconductivity in
optimally electron-doped oxy-pnictides: We report on the recovery of the short-range static magnetic order and on the
concomitant degradation of the superconducting state in optimally F-doped
SmFe_(1-x)Ru_(x)AsO_0.85F_0.15 for 0.1< x<0.6. The two reduced order parameters
coexist within nanometer-size domains in the FeAs layers and finally disappear
around a common critical threshold x_c=0.6. Superconductivity and magnetism are
shown to be closely related to two distinct well-defined local electronic
environments of the FeAs layers. The two transition temperatures, controlled by
the isoelectronic and diamagnetic Ru substitution, scale with the volume
fraction of the corresponding environments. This fact indicates that
superconductivity is assisted by magnetic fluctuations, which are frozen
whenever a short-range static order appears, and totally vanish above the
magnetic dilution threshold x_c. | cond-mat_supr-con |
Fractional Flux Plateau in Magnetization Curve of Multicomponent
Superconductor Loop: Time-reversal symmetry (TRS) may be broken in superconductors with three or
more condensates interacting repulsively, yielding two degenerate states
specified by chirality of gap functions. We consider a loop of such
superconductor with two halves occupied by the two states with opposite
chiralities. Fractional flux plateaus are found in magnetization curve
associated with free-energy minima, where the two domain walls between the two
halves accommodate different inter-component phase kinks leading to finite
winding numbers in a part of the whole condensates around the loop. Fractional
flux plateaus form pairs with their heights related to the flux quantum {\Phi}0
= hc/2e. This phenomenon is a clear evidence of time-reversal symmetry broken
(TRSB) superconductivity, which in a general point of view provides a novel
chance to explore relative phase difference, phase kink and soliton in
ubiquitous multi-component superconductivity such as that in iron pnicitides. | cond-mat_supr-con |
Quantum detector of noise based on a system of asymmetric Al
superconducting rings: The quantum rectification effect observed on asymmetric superconducting loops
is proposed to use as a basic of a noise detector with maximum sensitivity. The
measurements show that a critical amplitude of noise or ac current decreases
down to zero near superconducting transition, Tc, just as superconducting
critical current. Therefore any how weak noise, right down to the equilibrium
one, can induce the quantum oscillations of the dc voltage near Tc. The
transformation of the power of random noise into the dc power observed on
asymmetric superconducting loops gives very important advantage: even very weak
noise can give enough high output power in a system with large number of loops. | cond-mat_supr-con |
Upper bounds on the superfluid stiffness and superconducting $T_c$:
Applications to twisted-bilayer graphene and ultra-cold Fermi gases: Understanding the material parameters that control the superconducting
transition temperature $T_c$ is a problem of fundamental importance. In many
novel superconductors, phase fluctuations determine $T_c$, rather than the
collapse of the pairing amplitude. We derive rigorous upper bounds on the
superfluid phase stiffness for multi-band systems, valid in any dimension. This
in turn leads to an upper bound on $T_c$ in two dimensions (2D), which holds
irrespective of pairing mechanism, interaction strength, or order-parameter
symmetry. Our bound is particularly useful for the strongly correlated regime
of low-density and narrow-band systems, where mean field theory fails. For a
simple parabolic band in 2D with Fermi energy $E_F$, we find that $k_BT_c \leq
E_F/8$, an exact result that has direct implications for the 2D BCS-BEC
crossover in ultra-cold Fermi gases. Applying our multi-band bound to
magic-angle twisted bilayer graphene (MA-TBG), we find that band structure
results constrain the maximum $T_c$ to be close to the experimentally observed
value. Finally, we discuss the question of deriving rigorous upper bounds on
$T_c$ in 3D. | cond-mat_supr-con |
Spin flip scattering engendered quantum spin torque in a Josephson
junction: We examine a Josephson junction with two Ferromagnets and a spin flipper
sandwiched between two superconductors. In such Ferromagnetic Josephson
junctions, equilibrium spin torque exists only when Ferromagnets are
misaligned. This is explained via the "conventional" mechanism of spin transfer
torque, which owes its origin to the misalignment of two Ferromagnets. However,
we see surprisingly when the magnetic moments of the Ferromagnets are aligned
parallel or antiparallel, there is a finite equilibrium spin torque due to the
quantum mechanism of spin-flip scattering. We explore the properties of this
unique spin-flip scattering induced equilibrium quantum spin torque, especially
its tunability via exchange coupling and phase difference across the
superconductors. | cond-mat_supr-con |
Continuum dual theory of the transition in 3D lattice superconductor: A recently proposed form of dual theory for the three dimensional
superconductor is rederived starting from the lattice electrodynamics and
studied by renormalization group. The superfluid density below and close to the
transition vanishes as inverse of the correlation length of the disorder field.
The corresponding universal amplitude is given by the fixed point value of the
dual charge, and it is calculated to the leading order. The continuum dual
theory predicts the divergence of the magnetic field penetration depth with the
XY exponent, in contradiction to the results obtained from the Ginzburg-Landau
theory for the superconducting order-parameter. Possible reasons for this
difference are discussed. | cond-mat_supr-con |
Fermi surface reconstruction due to hidden rotating antiferromagnetism
in n and p-type high-$T_C$ cuprates: The Fermi surface calculated within the rotating antiferromagentism theory
undergoes a topological change when doping changes from p-type to n-type, in
qualitative agreement with experimental data for n-type cuprate
Nd$_{2-x}$Ce$_x$CuO$_4$ and p-type La$_{2-x}$Sr$_x$CuO$_4$. Also, the
reconstruction of the Fermi surface observed experimentally close to optimal
doing in p-type cuprates, and slightly higher than optimal doping in the
overdoped regime for this n-type high-$T_C$ cuprate is well accounted for in
this theory, and is a consequence of quantum criticality caused by the
disappearance of rotating antiferromagnetism. The present results are in
qualitative agreement with the recently observed quantum oscillations in some
high-$T_C$ cuprates regarding the change in the size of the Fermi surface as
doping evolves and the location of its reconstruction. This paper presents new
results about the application of the rotating antiferromagnetism theory to the
study of electronic structure for n-type materials. | cond-mat_supr-con |
Giant Triplet Proximity Effect in $π$-biased Josephson Junctions with
Spin-Orbit Coupling: In diffusive Josephson junctions the phase-difference $\phi$ between the
superconductors strongly influences the spectroscopic features of the layer
separating them. The observation of a uniform minigap and its phase modulation
were only recently experimentally reported, demonstrating agreement with
theoretical predictions up to now - a vanishing minigap at $\phi=\pi$.
Remarkably, we find that in the presence of intrinsic spin-orbit coupling a
giant proximity effect due to spin-triplet Cooper pairs can develop at
$\phi=\pi$, in complete contrast to the suppressed proximity effect without
spin-orbit coupling. We here report a combined numerical and analytical study
of this effect, proving its presence solely based on symmetry arguments, which
makes it independent of the specific parameters used in experiments. We show
that the spectroscopic signature of the triplets is present throughout the
entire ferromagnetic layer. Our finding offers a new way to artificially
create, control and isolate spin-triplet superconductivity. | cond-mat_supr-con |
Fixed Number and Quantum Size Effects in Nanoscale Superconductors: In recent experiments on nanoscale Al particles, whose electron number was
fixed by charging effects, a ``negative gap'' was observed in particles with an
odd number of electrons. This observation has called into question the use of a
grand canonical ensemble in describing superconductivity in such ultrasmall
particles.
We have studied the effects of fixed electron number and finite size in
nanoscale superconductors, by applying the canonical BCS theory for the
attractive Hubbard model. The ground state energy and the energy gap are
compared with the conventional and parity-projected grand canonical BCS
results, and in one dimension also with the exact solutions by the Bethe
ansatz. The crossover from the bulk to quantum limit is studied for various
regimes of electron density and coupling strength. The effects of boundary
conditions and different lattice structures are also examined.
A ``negative gap'' for odd electron number emerges most naturally in the
canonical scheme. For even electron number, the gap is particularly large for
``magic numbers'' of electrons for a given system size or of atoms for a fixed
electron density. These features are in accordance with the exact solutions,
but are essentially missed in the grand canonical results. | cond-mat_supr-con |
The dc-Josephson effect with more than four superconducting leads: By definition, the $p$-terminal dc-Josephson current is sensitive to the
superconducting phase variables of $p$ terminals. In the paper, we establish
protocol for direct detection of the $p$-terminal dc-Josephson effect with
$p\ge 3$ in a device containing $N$ superconducting leads
$S_1,\,S_2,\,...,\,S_N$ having the phase variables
$\varphi_1,\,\varphi_2,\,...,\,\varphi_N$. The calculated signal ${\chi}^{(N)}$
can be probed in microwave experiments, and it corresponds to the higher-order
nonlocal inverse inductance obtained from differentiating the current $I_1$
through $S_1$ with respect to the remaining $N-2$ independent phase differences
$\varphi_2-\varphi_N,\,\varphi_3-\varphi_N,\, ...,\varphi_{N-1}-\varphi_N$. We
find that the values $p\le N-2$ do not contribute to ${\chi}^{(N)}$, and that
${\chi}^{(N)}\ne 0$ implies evidence for the $p=N-1$ or the $p=N$-terminal
dc-Josephson currents. For $N=4$ superconducting leads, we demonstrate that
${\chi}^{(4)}\ne 0$ implies evidence for the $p=3$ or $p=4$ dc-Josephson
effect, irrespective of the $p=2$-terminal dc-Josephson current. Thus, we
provide a way to demonstrate the dc-Josephson effect with more than three
terminals (i.e. with $p\ge 3$) in a device containing more than four
superconducting leads (i.e. with $N\ge 4$). The predicted ${\chi}^{(4)}$ is
"yes or no" answer to the $p\ge 3$ dc-Josephson effect, i.e. for $N=4$,
nonvanishingly small $\chi^{(4)}\ne 0$ implies the $p=3$ or $p=4$-terminal
dc-Josephson effect and vanishingly small $\chi^{(4)}=0$ implies absence of the
$p=3$ and $p=4$-terminal dc-Josephson effect. The paper can be viewed as
generalizing the recently considered $\varphi$-junctions in Andreev molecules
to arbitrary number $N$ of the superconducting leads, and it relies on basic
properties of the dc-Josephson effect that are not directly related to
nontrivial topology and Weyl point singularities. | cond-mat_supr-con |
Dynamical Structure Factor of Fulde-Ferrell-Larkin-Ovchinnikov
Superconductors: Superconductor with a spatially-modulated order parameter is known as
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor. Using the
time-dependent Ginzburg-Landau (TDGL) formalism we have theoretically studied
the temporal behaviour of the equal-time correlation function, or the structure
factor, of a FFLO superconductor following a sudden quench from the unpaired,
or normal, state to the FFLO state. We find that quenching into the ordered
FFLO phase can reveal the existence of a line in the mean-field phase diagram
which cannot be accessed by static properties. | cond-mat_supr-con |
Identifying the pairing mechanism in Fe-As based superconductors: gaps
and isotope effects: The temperature dependencies of the coupled superconducting gaps, observed in
Fe-As based superconducting compounds is calculated and a universal temperature
scaling observed which is only present if the coupled order parameters both
have s-wave symmetry. Predictions for possible isotope effects on the
transition temperature are made if phonons are involved in the pairing or
polaronic effects are of importance. Comparison to experimental data is given
where these are available. | cond-mat_supr-con |
Controllable tunnelling of single flux-quanta mediated by quantum
phase-slip in disordered superconducting loops: Quantum phase-slip (QPS) is the exact dual to the well-known Josephson
effect. Although there are numerous proposals for applications of QPS devices,
experimental work to develop these remains in the relatively early stages.
Significant barriers to exploiting QPS nanowires for useful technologies still
exist, such as establishing robust nanowire fabrication methods that allow
coupling to low-loss circuits, and demonstrating control over the QPS process
with an experimenter-controlled external bias. Here we report experiments which
show that both of these barriers have been overcome. We present measurements at
300 mK of NbN coplanar waveguide (CPW) resonators embedded with nanowires
fabricated using a neon focused ion-beam. The internal quality factor exceeds
$2\times10^{4}$ -- significantly higher than previously reported in comparable
experiments. The resonator frequency tunes periodically with an applied
magnetic field, revealing tunnelling of the order parameter that always occurs
at half-integer values of the applied flux. In contrast to previous studies of
single QPS, the order-parameter tunnelling is shown to be adiabatic,
demonstrating improved control over energy dissipation in nanowire QPS
circuits. Our results highlight a promising pathway towards realising low-loss
nanowire-based QPS devices. | cond-mat_supr-con |
Chiral CP^2 skyrmions in three-band superconductors: It is shown that under certain conditions, three-component superconductors
(and in particular three-band systems) allow stable topological defects
different from vortices. We demonstrate the existence of these excitations,
characterized by a $CP^2$ topological invariant, in models for three-component
superconductors with broken time reversal symmetry. We term these topological
defects "chiral $GL^{(3)}$ skyrmions", where "chiral" refers to the fact that
due to broken time reversal symmetry, these defects come in inequivalent left-
and right-handed versions. In certain cases these objects are energetically
cheaper than vortices and should be induced by an applied magnetic field. In
other situations these skyrmions are metastable states, which can be produced
by a quench. Observation of these defects can signal broken time reversal
symmetry in three-band superconductors or in Josephson-coupled bilayers of
$s_\pm$ and s-wave superconductors. | cond-mat_supr-con |
Improved Single-Crystal Growth of Sr$_2$RuO$_4$: High-quality single crystals are essentially needed for the investigation of
the novel bulk properties of unconventional superconductors. The availability
of such crystals grown by the floating-zone method has helped to unveil the
unconventional superconductivity of the layered perovskite Sr$_2$RuO$_4$, which
is considered as a strong candidate of a topological spin-triplet
superconductor. Yet, recent progress of investigations urges further efforts to
obtain ultimately high-quality crystalline samples. In this paper, we focus on
the method of preparation of feed rods for the floating-zone melting and report
on the improvements of the crystal growth. We present details of the improved
methods used to obtain crystals with superconducting transition temperatures
$T_\mathrm{c}$ that are consistently as high as 1.4 K, as well as the
properties of these crystals. | cond-mat_supr-con |
Pressure tuning of Fermi surface topology of optimally doped
BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$: The superconducting, transport, and structural properties of optimally
electron-doped BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ are investigated by combining the
electrical resistance and synchrotron X-ray diffraction measurements at high
pressures. The superconducting transition temperature of this system is found
to decrease in a similar way of the axial ratio of $c/a$ with increasing
pressure but vanishing at a critical pressure of 7.5 GPa where $c/a$ has a dip
and an isostructural transformation from a tetragonal to a collapsed tetragonal
phase takes place. The resistance is found to obey a linear temperature
dependence, evidencing the antiferromagnetic spin-fluctuations transport
mechanism. The pressure effects are interpreted within the framework of
pressure-induced Fermi surface topology modification in which pressure
suppresses both the quasiparticle effective mass and the strength of the
antiferromagnetic spin fluctuations leading to the reduction of
superconductivity, accordingly. The absent superconductivity in the collapsed
tetragonal phase is suggested to result from the complete suppression of the
antiferromagnetic spin fluctuations. | cond-mat_supr-con |
Nature of carrier doping in T'-La1.8-xEu0.2SrxCuO4 studied by X-Ray
Photoemission and Absorption Spectroscopy: Recently, hole-doped superconducting cuprates with the T'-structure
La1.8-xEu0.2SrxCuO4 (LESCO) have attracted a lot of attention. We have
performed x-ray photoemission and absorption spectroscopy measurements on
as-grown and reduced T0-LESCO. Results show that electrons and holes were doped
by reduction annealing and Sr substitution, respectively. However, it is shown
that the system remains on the electron-doped side of the Mott insulator or
that the charge-transfer gap is collapsed in the parent compound. | cond-mat_supr-con |
Substrate mediated nitridation of niobium into superconducting Nb2N thin
films for phase slip study: Here we report a novel nitridation technique for transforming niobium into
hexagonal Nb2N which appears to be superconducting below 1K. The nitridation is
achieved by high temperature annealing of Nb films grown on Si3N4/Si (100)
substrate under high vacuum. The structural characterization directs the
formation of a majority Nb2N phase while the morphology shows granular nature
of the films. The temperature dependent resistance measurements reveal a wide
metal-to-superconductor transition featuring two distinct transition regions.
The region close to the normal state varies strongly with the film thickness,
whereas, the second region in the vicinity of the superconducting state remains
almost unaltered but exhibiting resistive tailing. The current-voltage
characteristics also display wide transition embedded with intermediate
resistive states originated by phase slip lines. The transition width in
current and the number of resistive steps depend on film thickness and they
both increase with decrease in thickness. The broadening in transition width is
explained by progressive establishment of superconductivity through proximity
coupled superconducting nano-grains while finite size effects and quantum
fluctuation may lead to the resistive tailing. Finally, by comparing with Nb
control samples, we emphasize that Nb2N offers unconventional superconductivity
with promises in the field of phase slip based device applications. | cond-mat_supr-con |
Generic nodeless Larkin Ovchinnikov states due to singlet-triplet mixing: Larkin-Ovchinnikov (LO) states typically have a singlet-gap that vanishes
along real-space lines. These real-space nodes lead to Andreev midgap states
which can serve as a signature of LO pairing. We show that at these nodes, an
odd-parity, spin-triplet component is always induced, leading to a nodeless LO
phase. We find the two-dimensional weak coupling, clean limit s-wave phase
diagram when this spin-triplet part is included. The triplet component is large
and increases the stability of the FFLO phase. We also show that the
spin-triplet contribution pushes the midgap states away from zero energy.
Finally, we show how our results can be explained phenomenologically though
Lifshitz invariants. These invariants provide a simple approach to understand
the role of unconventional pairing states, spin-orbit coupling, and
inhomogeneous mixed singlet-triplet states that are not due to a FFLO
instability. We discuss our results in the context of organic superconductors. | cond-mat_supr-con |
Upper critical magnetic field of $Ln$O$_{0.5}$F$_{0.5}$BiS$_2$ ($Ln$ =
La, Nd) superconductors at ambient and high pressure: The upper critical field $H_{c2}$ of polycrystalline samples of
$Ln$O$_{0.5}$F$_{0.5}$BiS$_{2}$ ($Ln$ = La, Nd) at ambient pressure (tetragonal
structure) and high pressure (HP) (monoclinic structure) have been investigated
via electrical resistivity measurements at various magnetic fields up to 8.5 T.
The $H_{c2}$($T$) curves for all the samples show an uncharacteristic concave
upward curvature at temperatures below $T_c$, which cannot be described by the
conventional one-band Werthamer-Helfand-Hohenberg theory. For the
LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ sample under HP, as temperature is decreased, the
upper critical field $H_{onset}$, estimated from the onset of the
superconducting transitions, increases slowly between 4.9 and 5.8 T compared
with the slope of $H_{onset}$($T$) below 4.9 T and above 5.8 T. This anomalous
behavior reveals a remarkable similarity in superconductivity between
LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ samples measured under HP and synthesized under
HP, although the crystal structures of the two samples were reported to be
different. The experimental results support the idea that local atomic
environment, which can be tuned by applying external pressure and can be
quenched to ambient pressure via high temperature-pressure annealing, is
possibly more essential to the enhancement of $T_c$ for BiS$_2$-based
superconductors than the structural phase transition. On the other hand, such
anomalous behavior is very subtle in the case of NdO$_{0.5}$F$_{0.5}$BiS$_{2}$
under HP, suggesting that the anisotropy of the upper critical field in the
$ab$-plane and the possible lattice deformation induced by external pressure is
weak. This explains why the pressure-induced enhancement of $T_c$ for
NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ is not as large as that for
LaO$_{0.5}$F$_{0.5}$BiS$_{2}$. | cond-mat_supr-con |
Fermi surface reconstruction in electron-doped cuprates without
antiferromagnetic long-range order: Fermi surface (FS) topology is a fundamental property of metals and
superconductors. In electron-doped cuprate Nd2-xCexCuO4 (NCCO), an unexpected
FS reconstruction has been observed in optimal- and over-doped regime
(x=0.15-0.17) by quantum oscillation measurements (QOM). This is all the more
puzzling because neutron scattering suggests that the antiferromagnetic (AFM)
long-range order, which is believed to reconstruct the FS, vanishes before
x=0.14. To reconcile the conflict, a widely discussed external magnetic
field-induced AFM long-range order in QOM explains the FS reconstruction as an
extrinsic property. Here, we report angle-resolved photoemission (ARPES)
evidence of FS reconstruction in optimal- and over-doped NCCO. The observed FSs
are in quantitative agreement with QOM, suggesting an intrinsic FS
reconstruction without field. This reconstructed FS, despite its importance as
a basis to understand electron-doped cuprates, cannot be explained under the
traditional scheme. Furthermore, the energy gap of the reconstruction decreases
rapidly near x=0.17 like an order parameter, echoing the quantum critical
doping in transport. The totality of the data points to a mysterious order
between x=0.14 and 0.17, whose appearance favors the FS reconstruction and
disappearance defines the quantum critical doping. A recent topological
proposal provides an ansatz for its origin. | cond-mat_supr-con |
Tuning competing orders in La2-xSrxCuO4 cuprate superconductors by the
application of an external magnetic field: We report the results of a combined muon spin rotation and neutron scattering
study on La2-xSrxCuO4 in the vicinity of the so-called 1/8-anomaly. Application
of a magnetic field drives the system towards a magnetically ordered
spin-density-wave state, which is fully developed at 1/8 doping. The results
are discussed in terms of competition between antiferromagnetic and
superconducting order parameters. | cond-mat_supr-con |
Bosonic Spectral Function in HTSC Cuprates: Part I - Experimental
Evidence for Strong Electron-Phonon Interaction: In Part I we discuss accumulating experimental evidence related to the
structure and origin of the bosonic spectral function \alpha ^{2}F in
high-temperature superconducting (HTSC) cuprates near optimal doping. Some
global properties of \alpha ^{2}F such as number and positions of peaks, are
extracted by combining optics, neutron scattering, ARPES and tunnelling
measurements. These methods give convincing evidence for strong electron-phonon
interaction (EPI) with 1<\lambda \lesssim 3 in cuprates near optimal doping.
Here we clarify how these results are in favor of the Eliashberg-like theory
for HTSC cuprates near optimal doping.We argue that the neglect of EPI in some
previous studies of HTSC was based on a number of deceptive prejudices related
to the strength of EPI, on some physical misconceptions and misleading
interpretation of experimental results. | cond-mat_supr-con |
Effect of doping on the magnetostructural ordered phase of iron
arsenides: A comparative study of the resistivity anisotropy in the doped
BaFe$_2$As$_2$ with doping into three different sites: In order to unravel a role of doping in the iron-based superconductors, we
investigated the in-plane resistivity for BaFe$_2$As$_2$ doped at either of the
three different lattice sites, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$,
BaFe$_2$(As$_{1-x}$P$_x$)$_2$, and Ba$_{1-x}$K$_x$Fe$_2$As$_2$, focusing on the
doping effect in the low-temperature antiferromagnetic/orthorhombic (AFO)
phase. A major role of doping in the high-temperature paramagnetic/tetragonal
(PT) phase is known to change the Fermi surface by supplying charge carriers or
by exerting chemical pressure. In the AFO phase, we found a clear correlation
between the magnitude of residual resistivity and resistivity anisotropy. This
indicates that the resistivity anisotropy originates from the anisotropic
impurity scattering from dopant atoms. The magnitude of residual resistivity is
also found to be a parameter controlling the suppression rate of AFO ordering
temperature $T_s$. Therefore, the dominant role of doping in the AFO phase is
to introduce disorder to the system, distinct from that in the PT phase. | cond-mat_supr-con |
Transport and magnetic properties of La-doped CaFe$_2$As$_2$: We measured the transport properties and susceptibility of single crystals
Ca$_{1-x}$La$_x$Fe$_2$As$_2$(x=0, 0.05, 0.1, 0.15, 0.19 and 0.25). Large
in-plane resistivity anisotropy similar to that in Co-doped 122 iron-pnictides
is observed although no transition metals were introduced in the FeAs-plane.
The in-plane resistivity anisotropy gradually increases with La doping below
T$_{SDW}$, being different from the hole-doped 122 superconductors. The
susceptibilities of the samples show that La doping leads to suppression of SDW
and induces a Curie-Weiss-like behavior at low temperature, which is much
stronger than the other 122 iron-based superconductors. | cond-mat_supr-con |
Domes of $T_c$ in single-band and multiband superconductors with
finite-range attractive interactions: The rise and fall of the superconducting transition temperature $T_c$ upon
tuning carrier density or external parameters, such as pressure or magnetic
field, is ubiquitously observed in a wide range of quantum materials. In order
to investigate such domes of $T_c$, we go beyond the prototypical attractive
Hubbard model, and consider a lattice model of electrons coupled via
instantaneous, spatially extended, attractive interactions. By numerically
solving the mean-field equations, as well as going beyond mean field theory
using a functional renormalization group approach, we find that for a
characteristic interaction range $\ell$, there exists a dome in $T_c$ around
$k_F \ell \! \sim \! {\mathcal{O}}(1)$. For multiband systems, our mean field
theory shows the presence of additional domes in the vicinity of Lifshitz
transitions. Our results hold in both two and three dimensions and can be
intuitively understood from the geometric relation between the Fermi surface
and the interaction range. Our model may be relevant for domes of $T_c$ in
dilute weakly coupled superconductors or in engineered cold atom systems. | cond-mat_supr-con |
Numerical Study of Energy Loss by a Nanomechanical Oscillator Coupled to
a Cooper Pair Box: We calculate the dynamics of a nanomechanical oscillator (NMO) coupled
capacitively to a Cooper pair box (CPB), by solving a stochastic Schrodinger
equation with two Lindblad operators. Both the NMO and the CPB are assumed
dissipative, and the coupling is treated within the rotating wave
approximation. We show numerically that, if the CPB decay time is smaller than
the NMO decay time, the coupled NMO will lose energy faster, and the coupled
CPB more slowly, than do the uncoupled NMO and CPB. The results show that the
efficiency of energy loss by an NMO can be substantially increased if the NMO
is coupled to a CPB. | cond-mat_supr-con |
High-energy ball milling and synthesi temperature study to improve
superconducting properties of MgB2 ex-situ tapes and wires: MgB2 monofilamentary nickel-sheated tapes and wires were fabricated by means
of the ex-situ powder-in-tube method using either high-energy ball milled and
low temperature synthesized powders. All sample were sintered at 920 C in Ar
flow. The milling time and the revolution speed were tuned in order to maximize
the critical current density in field (Jc): the maximum Jc value of 6 x 10e4
A/cm2 at 5 K and 4 T was obtained corresponding to the tape prepared with
powders milled for 144h at 180rpm. Vorious synthesis temperature were also
investigated (730-900 C) finding a best Jc value for the wire prepared with
powders synthesized at 745 C. We speculate that this optimal temperature is due
to the fluidifying effect of unreacted magnesium content before the sintering
process which could better connect the grains. | cond-mat_supr-con |
Spectral signatures of modulated d-wave superconducting phases: We calculate within a mean-field theory the spectral signatures of various
striped d-wave superconducting phases. We consider both in-phase and anti-phase
modulations of the superconducting order across a stripe boundary, and the
effects of coexisting inhomogeneous orders, including spin stripes, charge
stripes, and modulated d-density-wave. We find that the anti-phase modulated
d-wave superconductor exhibits zero-energy spectral weight, primarily along
extended arcs in momentum space. Concomitantly, a Fermi surface appears and
typically includes both open segments and closed pockets. When weak homogeneous
superconductivity is also present the Fermi surface collapses onto nodal
points. Among them are the nodal points of the homogeneous d-wave
superconductor, but others typically exist at positions which depend on the
details of the modulation and the band structure. Upon increasing the amplitude
of the constant component these additional points move towards the edges of the
reduced Brillouin zone where they eventually disappear. The above signatures
are also manifested in the density of states of the clean, and the disordered
system. While the presence of coexisting orders changes some details of the
spectral function, we find that the evolution of the Fermi-surface and the
distribution of the low-energy spectral weight are largely unaffected by them. | cond-mat_supr-con |
Sublattice-sensitive Majorana Modes: For two- and three-dimensional topological insulators whose unit cells
consist of multiple sublattices, the boundary terminating at which type of
sublattice can affect the time-reversal invariant momentum at which the Dirac
points of helical boundary states are located. Through a general theory and a
representative model, we reveal that this interesting property allows the
realization of Majorana modes at sublattice domain walls forming on the
boundary when the boundary Dirac points of the topological insulator are gapped
by an unconventional superconductor in proximity. Intriguingly, we find that
the sensitive sublattice-dependence of the Majorana modes allows their
positions to be precisely manipulated by locally controlling the terminating
sublattices or boundary potential. Our work reveals that the common sublattice
degrees of freedom in materials open a new route to realize and manipulate
Majorana modes. | cond-mat_supr-con |
On a possibility of effective electron attraction without "a glue": With the means of Schrodinger equation a two electron problem is considered.
It is shown that electrons in a periodic potential might experience effective
attraction without any mediating agents; which means electrons in HTSC might
have effective attraction potential. | cond-mat_supr-con |
High-energy magnetic excitations in overdoped
La$_{2-x}$Sr$_{x}$CuO$_{4}$ studied by neutron and resonant inelastic X-ray
scattering: We have performed neutron inelastic scattering and resonant inelastic X-ray
scattering (RIXS) at the Cu-$L_3$ edge to study high-energy magnetic
excitations at energy transfers of more than 100 meV for overdoped
La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.25$ ($T_c=15$ K) and $x=0.30$
(non-superconducting) using identical single crystal samples for the two
techniques. From constant-energy slices of neutron scattering cross-sections,
we have identified magnetic excitations up to ~250 meV for $x=0.25$. Although
the width in the momentum direction is large, the peak positions along the (pi,
pi) direction agree with the dispersion relation of the spin-wave in the
non-doped La$_{2}$CuO$_{4}$ (LCO), which is consistent with the previous RIXS
results of cuprate superconductors. Using RIXS at the Cu-$L_3$ edge, we have
measured the dispersion relations of the so-called paramagnon mode along both
(pi, pi) and (pi, 0) directions. Although in both directions the neutron and
RIXS data connect with each other and the paramagnon along (pi, 0) agrees well
with the LCO spin-wave dispersion, the paramagnon in the (pi, pi) direction
probed by RIXS appears to be less dispersive and the excitation energy is lower
than the spin-wave of LCO near (pi/2, pi/2). Thus, our results indicate
consistency between neutron inelastic scattering and RIXS, and elucidate the
entire magnetic excitation in the (pi, pi) direction by the complementary use
of two probes. The polarization dependence of the RIXS profiles indicates that
appreciable charge excitations exist in the same energy range of magnetic
excitations, reflecting the itinerant character of the overdoped sample. A
possible anisotropy in the charge excitation intensity might explain the
apparent differences in the paramagnon dispersion in the (pi, pi) direction as
detected by the X-ray scattering. | cond-mat_supr-con |
Analysis of the paraconductivity in underdoped $La_{2-x}Sr_xCuO_4$ thin
films using high magnetic fields: The contribution of superconducting fluctuations to the conductivity, or
paraconductivity is studied in the underdoped regime of $La_{2-x}Sr_xCuO_4$
cuprates. A perpendicular magnetic field up to 50 T is applied to suppress the
superconductivity and obtain the normal state resistivity which is then used to
calculate the paraconductivity. Surprisingly enough, it is consistent with a
two-dimensional Aslamazov-Larkin (AL) regime of Gaussian fluctuations close to
the critical temperature. At higher temperature, the paraconductivity shows a
power-law decrease in temperature (as $T^{-\alpha}$) as was previously shown
for underdoped $YBa_2Cu_3O_{7-\delta}$ and $Bi_2Sr_2CaCu_2O_{8+\delta}$
samples. Our observations are not consistent with the existence of
Kosterlitz-Thouless fluctuations. This tends to indicate that the
superconducting pair amplitude is not already defined above $T_C$ in the
pseudogap state. | cond-mat_supr-con |
Evidence for unusual superconducting correlations coexisting with stripe
order in La(1.875)Ba(0.125)CuO(4): We present new x-ray and neutron scattering measurements of stripe order in
La(1.875)Ba(0.125)CuO(4), along with low-field susceptibility, thermal
conductivity, and specific heat data. We compare these with previously reported
results for resistivity and thermopower. Temperature-dependent features
indicating transitions (or crossovers) are correlated among the various
experimental quantities. Taking into account recent spectroscopic studies, we
argue that the most likely interpretation of the complete collection of results
is that an unusual form of two-dimensional superconducting correlations appears
together with the onset of spin-stripe order. Recent theoretical proposals for
a sinusoidally-modulated superconducting state compatible with stripe order
provide an intriguing explanation of our results and motivate further
experimental tests. We also discuss evidence for one-dimensional pairing
correlations that appear together with the charge order. With regard to the
overall phenomenology, we consider the degree to which similar behavior may
have been observed in other cuprates, and describe possible connections to
various puzzling phenomena in cuprate superconductors. | cond-mat_supr-con |
Supercurrent induced domain wall motion: We study the dynamics of a magnetic domain wall, inserted in, or juxtaposed
to, a conventional superconductor, via the passage of a spin polarized current
through a FSF junction. Solving the Landau-Lifshitz-Gilbert equation of motion
for the magnetic moments we calculate the velocity of the domain wall and
compare it with the case of a FNF junction. We find that in several regimes the
domain wall velocity is larger when it is driven by a supercurrent. | cond-mat_supr-con |
Simulation of some quantum gates, with decoherence: Methods and results for numerical simulations of one and two interacting
rf-Squid systems suitable for adiabatic quantum gates are presented. These are
based on high accuracy numerical solutions to the static and time dependent
Schroedinger equation for the full Squid Hamiltonian in one and two variables.
Among the points examined in the static analysis is the range of validity of
the effective two-state or ``spin 1/2'' picture. A range of parameters is
determined where the picture holds to good accuracy as the energy levels
undergo gate manipulations. Some general points are presented concerning the
relations between device parameters and ``good'' quantum mechanical state
spaces.
The time dependent simulations allow the examination of suitable conditions
for adiabatic behavior, and permits the introduction of a random noise to
simulate the effects of decoherence. A formula is derived and tested relating
the random noise to the decoherence rate. Sensitivity to device and operating
parameters for the logical gates NOT and CNOT are examined, with particular
attention to values of the tunnel parameter beta slightly above one. It appears
that with values of beta close to one, a quantum CNOT gate is possible even
with rather short decoherence times.
Many of the methods and results will apply to coupled double-potential well
systems in general. | cond-mat_supr-con |
Switching of intra-orbital spin excitations in electron-doped iron
pnictide superconductors: We investigate the doping dependence of the magnetic excitations in
two-superconducting-dome-system LaFeAsO1-xDx. Using inelastic neutron
scattering, spin fluctuations at different wavenumbers were observed under both
superconducting domes around x = 0.1 and 0.4, but vanished at x = 0.2
corresponding to the Tc valley. Theoretical calculations indicate that the
characteristic doping dependence of spin fluctuations is rationally explained
as a consequence of the switching of the two intra-orbital nestings within
Fe-3dYZ, ZX and 3dX2-Y2 by electron doping. The present results imply that the
multi-orbital nature plays an important role in the doping and / or material
dependence of the Tc of the iron pnictide superconductors. | cond-mat_supr-con |
Topological superconducting transition driven by time-reversal-symmetry
breaking: Three-dimensional line-nodal superconductors exhibit nontrivial topology,
which is protected by the time-reversal symmetry. Here we investigate four
types of short-range interaction between the gapless line-nodal fermionic
quasiparticles by carrying renormalization group analysis. We find that such
interactions can induce the dynamical breaking of time-reversal symmetry, which
alters the topology and might lead to six possible distinct superconducting
states, distinguished by the group representations. After computing the
susceptibilities for all the possible phase-transition instabilities, we
establish that the superconducting pairing characterized by $id_{xz}$-wave gap
symmetry is the leading instability in noncentrosymmetric superconductors.
Appropriate extension of this approach is promising to pick out the most
favorable superconducting pairing during similar topology-changing transition
in the polar phase of $^3$He. | cond-mat_supr-con |
Superfluid transition in disordered dipolar Fermi gases: We consider a weakly interacting two-component Fermi gas of dipolar particles
(magnetic atoms or polar molecules) in the two-dimensional geometry. The
dipole-dipole interaction (together with the short-range interaction at
Feshbach resonances) for dipoles perpendicular to the plane of translational
motion may provide a superfluid transition. The dipole-dipole scattering
amplitude is momentum dependent, which violates the Anderson theorem claiming
the independence of the transition temperature on the presence of weak
disorder. We have shown that the disorder can strongly increase the critical
temperature (up to 10 nK at realistic densities). This opens wide possibilities
for the studies of the superfluid regime in weakly interacting Fermi gases,
which was not observed so far. | cond-mat_supr-con |
Tunneling spectra for ($d_{x^2-y^2}+is$)-wave superconductors versus
tunneling spectra for ($d_{x^2-y^2}+id_{xy}$)-wave superconductors: The tunneling conductance spectra of a normal metal / insulator / singlet
superconductor is calculated from the reflection amplitudes using the
Blonder-Tinkham-Klapwijk (BTK) formulation. The pairing symmetry of the
superconductor is assumed to be $d_{x^2-y^2}+is$, or $d_{x^2-y^2}+id_{xy}$. It
is found that in the ($d_{x^2-y^2}+is$)-wave case there is a well defined
conductance peak in the conductance spectra, in the amplitude of the secondary
s-wave component. In the ($d_{x^2-y^2}+id_{xy}$)-wave case the tunneling
conductance has residual values within the gap, due to the formation of bound
states. The bound state energies depend on the angle of the incident
quasiparticles, and also on the boundary orientation. On the basis of this
observation an electron focusing experiment is proposed to probe the
($d_{x^2-y^2}+id_{xy}$)-wave state. | cond-mat_supr-con |
Ab initio calculations of the electronic structure of cuprates using
large scale cluster techniques: The local electronic structures of La2CuO4, three members of the
Yttrium-family (YBa2Cu3O6, YBa2Cu3O7, and YBa2Cu4O8), and to some extent of
Nd2CuO4 have been determined using all-electron ab-initio cluster calculations
for clusters comprising up to thirteen planar copper atoms associated with
their nearest planar and apical oxygen atoms. Spin-polarized calculations in
the framework of density functional theory have enabled an estimation of the
superexchange couplings J. Electric field gradients at the planar copper sites
are determined and their dependence on the occupation of the various atomic
orbitals are investigated in detail. The changes of the electronic field
gradient and of the occupation of orbitals upon doping are studied and
discussed. Furthermore, magnetic hyperfine fields are evaluated and
disentangled into on-site and transferred contributions, and the chemical
shifts at the copper nucleus are calculated. In general the results are in good
agreement with values deduced from experiments except for the value of the
chemical shift with applied field perpendicular to the CuO2-plane. | cond-mat_supr-con |
Disorder-induced time effect in the antiferromagnetic domain state of
Fe1+yTe: We report on temperature-dependent soft X-ray absorption spectroscopy (XAS)
measurements utilizing linearly polarized synchrotron radiation to probe
magnetic phase transitions in iron-rich Fe1+yTe. X-ray magnetic linear
dichroism (XMLD) signals, which sense magnetic ordering processes at surfaces,
start to increase monotonically below the N\'eel temperature TN = 57 K. This
increase is due to a progressive bicollinear antiferromagnetic (AFM) alignment
of Fe spins of the monoclinic Fe1+yTe parent phase. This AFM alignment was
achieved by a [100]-oriented biasing field favoring a single-domain state
during cooling across TN. Our specific heat and magnetization measurements
confirm the bulk character of this AFM phase transition. On longer time scales,
however, we observe that the field-biased AFM state is highly unstable even at
the lowest temperature of T = 3 K. After switching off the biasing field, the
XMLD signal decays exponentially with a time constant {\tau} = 1506 s. The
initial XMLD signal is restored only upon repeating a cycle consisting of
heating and field-cooling through TN. We explain the time effect by a gradual
formation of a multi-domain state with 90 deg rotated AFM domains, promoted by
structural disorder, facilitating the motion of twin-domains. Significant
disorder in our Fe1+yTe sample is evident from our X-ray diffraction and
specific heat data. The stability of magnetic phases in Fe-chalcogenides is an
important material property, since the Fe(Te1-xSex) phase diagram shows
magnetism intimately connected with superconductivity. | cond-mat_supr-con |
Thermal Conductivity in 3D NJL Model Under External Magnetic Field: The thermal conductivity of the (2+1)-dimensional NJL model in the presence
of a constant magnetic field is calculated in the mean-field approximation and
its different asymptotic regimes are analyzed. Taking into account the
dynamical generation of a fermion mass due to the magnetic catalysis
phenomenon, it is shown that for certain relations among the theory's
parameters (particle width, temperature and magnetic field), the profile of the
thermal conductivity versus the applied field exhibits kink- and plateau-like
behaviors. We point out possible applications to planar condensed matter. | cond-mat_supr-con |
Quasiparticle Thermal Conductivities in a Type-II Superconductor at High
Magnetic Field: We present a calculation of the quasiparticle contribution to the
longitudinal thermal conductivities as well as transverse (Hall) thermal
conductivity of an extreme type-II superconductor in a high magnetic field and
at low temperatures. In the limit of frequency and temperature approaching
zero, both longitudinal and transverse conductivities upon entering the
superconducting state undergo a reduction from their respective normal state
values by the factor $(\Gamma /\Delta)^2$, which measures the size of the
region at the Fermi surface containing gapless quasiparticle excitations. We
use our theory to numerically compute the longitudinal transport coefficient in
borocarbide and A-15 superconductors. The agreement with recent experimental
data on LuNi_2B_2C is very good. | cond-mat_supr-con |
Anomalous Dimension of Dirac's Gauge-Invariant Nonlocal Order Parameter
in Ginzburg-Landau Field Theory: In a Ginzburg-Landau theory with $n$ fields, the anomalous dimension of the
gauge-invariant nonlocal order parameter defined by the long-distance limit of
Dirac's gauge-invariant two-point function is calculated. The result is exact
for all $n$ to first order in $\epsilon \equiv 4-d$, and for all $d\in (2,4)$
to first order in $1/n$, and coincides with the previously calculated
gauge-dependent exponent in the Landau gauge. | cond-mat_supr-con |
Carbonaceous sulfur hydride system: the strong-coupled room-temperature
superconductor with a low value of Ginzburg-Landau parameter: The superconducting state in Carbonaceous Sulfur Hydride (C-S-H) system is
characterized by the record-high critical temperature of $288$~K experimentally
observed at $\sim$267 GPa. Herein, we determined the properties of the
\mbox{C-S-H} superconducting phase within the scope of both classical
Eliashberg equations (CEE) and the Eliashberg equations with vertex corrections
(VCEE). We took into account the scenarios pertinent to either the intermediate
or the high value of electron-phonon coupling constant ($\lambda\sim 0.75$ or
$\lambda\sim 3.3$, respectively). The scenario for the intermediate value,
however, cannot be actually realized due to the anomally high value of
logarithmic phonon frequency ($\omega_{\rm ln}/k_{B}=7150$~K) it would require.
On the other hand, we found it possible to reproduce correctly the value of
$T_{C}$ and other thermodynamic quantities in the case of strong coupling.
However, the vertex corrections lower the order parameter values within the
range from $\sim 50$~K to $\sim275$~K. For the upper critical field $H_{C2}\sim
27$~T, the Ginzburg-Landau parameter $\kappa$ is of the order of $1.7$. This
correlates well with the sharp drop of resistance observed by Hirsch and
Marsiglio at the critical temperature. The strong-coupling scenario for C-S-H
system is also suggested by the high values of $\lambda$ estimated for ${\rm
H_{3}S}$ ($\lambda\sim 2.1$, $\kappa\sim 1.5$), ${\rm LaH_{10}}$ ($\lambda\sim
2.8$-$3.9$, $\kappa\sim 1.6$), and ${\rm YH_{6}}$ ($\lambda\sim 1.7$,
$\kappa\sim 1.3$) compounds. | cond-mat_supr-con |
Flux growth at ambient pressure of millimeter-sized single crystals of
LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO: Millimeter-sized single crystals of LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO
were grown in NaAs flux at ambient pressure. The detailed growth procedure and
crystal characterizations are reported. The as-grown crystals have typical
dimensions of 3 * 4 * 0.05-0.3 mm3 with the crystallographic c-axis
perpendicular to the plane of the plate-like single crystals. Some crystals
manifest linear dimensions as large as 4-5 mm. X-ray and neutron single crystal
scattering confirmed that LaFeAsO crystals exhibit a structural phase
transition at Ts ~ 154 K and a magnetic phase transition at TSDW ~ 140 K. The
transition temperatures agree with those determined by anisotropic
magnetization, in-plane electrical resistivity and specific heat measurements
and are consistent with previous reports on polycrystalline samples. Co and F
were successfully introduced into the lattice leading to superconducting
LaFe1-xCoxAsO and LaFeAsO1-xFx single crystals, respectively. This growth
protocol has been successfully employed to grow single crystals of NdFeAsO.
Thus it is expected to be broadly applicable to grow other RMAsO (R = rare
earth, M = transition metal) compounds. These large crystals will facilitate
the efforts of unraveling the underlying physics of iron pniticide
superconductors. | cond-mat_supr-con |
Temperature-doping phase diagrams for Ba(Fe1-xTMx)2As2(TM=Ni, Cu, Cu /
Co) single crystals: Microscopic, structural, transport and thermodynamic measurements of single
crystalline Ba(Fe1-xTMx)2As2 (TM = Ni and Cu) series, as well as two mixed TM =
Cu / Co series, are reported. All the transport and thermodynamic measurements
indicate that the structural and magnetic phase transitions at 134 K in pure
BaFe2As2 are monotonically suppressed and increasingly separated in a similar
manner by these dopants. In the Ba(Fe1-xNix)2As2 (x =< 0.072),
superconductivity, with Tc up to 19 K, is stabilized for 0.024 =< x =< 0.072.
In the Ba(Fe1-xCux)2As2 (x =< 0.356) series, although the structural and
magnetic transitions are suppressed, there is only a very limited region of
superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found
only for the x = 0.044 samples. In the Ba(Fe1-x-yCoxCuy)2As2 series,
superconductivity, with Tc values up to 12 K (x ~ 0.022 series) and 20 K (x ~
0.047 series), is stabilized. Quantitative analysis of the detailed
temperature-dopant concentration (T-x) and temperature-extra electrons (T-e)
phase diagrams of these series shows that there exists a limited range of the
number of extra electrons added, inside which the superconductivity can be
stabilized if the structural and magnetic phase transitions are suppressed
enough. Moreover, comparison with pressure-temperature phase diagram data, for
samples spanning the whole doping range, further reenforces the conclusion that
suppression of the structural / magnetic phase transition temperature enhances
Tc on the underdoped side, but for the overdoped side Tcmax is determined by e.
Therefore, by choosing the combination of dopants that are used, we can adjust
the relative positions of the upper phase lines (structural and magnetic phase
transitions) and the superconducting dome to control the occurrence and
disappearance of the superconductivity in transition metal, electron-doped
BaFe2As2. | cond-mat_supr-con |
Pairing Mechanism of the Heavily Electron Doped FeSe Systems: Dynamical
Tuning of the Pairing Cutoff Energy: We studied pairing mechanism of the heavily electron doped FeSe (HEDIS)
systems, which commonly have one incipient hole band -- a band top below the
Fermi level by a finite energy distance $\epsilon_b$ -- at $\Gamma$ point and
ordinary electron bands at $M$ points in Brillouin zone (BZ). We found that the
system allows two degenerate superconducting solutions with the exactly same
$T_c$ in clean limit: the incipient $s^{\pm}_{he}$-gap ($\Delta_h^{-} \neq 0$,
$\Delta_e^{+} \neq 0$) and $s_{ee}^{++}$-gap ($\Delta_h =0$, $\Delta_e^{+} \neq
0$) solutions with different pairing cutoffs, $\Lambda_{sf}$ (spin fluctuation
energy) and $\epsilon_b$, respectively. The $s_{ee}^{++}$-gap solution, in
which the system dynamically renormalizes the original pairing cutoff
$\Lambda_{sf}$ to $\Lambda_{phys}=\epsilon_b$ ($< \Lambda_{sf}$), therefore
actively eliminates the incipient hole band from forming Cooper pairs, but
without loss of $T_c$, becomes immune to the impurity pair-breaking. As a
result, the HEDIS systems, by dynamically tuning the pairing cutoff and
selecting the $s_{ee}^{++}$-pairing state, can always achieve the maximum $T_c$
-- the $T_c$ of the degenerate $s^{\pm}_{he}$ solution in the ideal clean limit
-- latent in the original pairing interactions, even in dirty limit. | cond-mat_supr-con |
Structure Re-determination and Superconductivity Observation of Bulk 1T
MoS2: 2H MoS2 has been intensively studied because of layer-dependent electronic
structures and novel physical properties. Though the metastable 1T MoS2 with
the [MoS6] octahedron was observed from the microscopic area, the true crystal
structure of 1T phase has not been determined strictly. Moreover, the true
physical properties have not been demonstrated from experiments due to the
challenge for the preparation of pure 1T MoS2 crystals. Here, we successfully
synthesized the 1T MoS2 single crystals and re-determined the crystal structure
of 1T MoS2 from single-crystal X-ray diffraction. 1T MoS2 crystalizes in space
group P-3m1 with a cell of a = b = 3.190(3) {\AA} and c = 5.945(6) {\AA}. The
individual MoS2 layer consists of MoS6 octahedron sharing edge with each other.
More surprisingly, the bulk 1T MoS2 crystals undergo a superconducting
transition of Tc = 4 K, which is the first observation of superconductivity in
pure 1T MoS2 phase. | cond-mat_supr-con |
$^{23}$Na NMR Study of Layered Superconductor $Na_xCoO_2- yH_2O$: We measured $^{23}$Na-NMR spectra and nuclear spin-lattice relaxation rates
$^{23}(1/T_1)$ of superconducting and non-superconducting bilayer hydrate
$Na_xCoO_2-yH_2O$ (x ~ 0.3, y ~ 1.3). The central resonance frequency shows a
small but various shift due to the difference in the shielding effect by
intercalated $H_2O$ molecules. The different shielding effect also gives a
large difference in the magnitude of $^{23}(1/T_1)$. | cond-mat_supr-con |
Multi-component strongly attractive Fermi gas: a color superconductor in
a one-dimensional harmonic trap: Recent advances in ultra-cold atomic Fermi gases make it possible to achieve
a fermionic superfluid with multiple spin components. In this context, any
mean-field description is expected to fail, owing to the presence of tightly
bound clusters or molecules that consist of more than two particles. Here we
present a detailed study of a strongly interacting multi-component Fermi gas in
a highly elongated or quasi-one-dimensional harmonic trap, which could be
readily obtained in experiment. By using the exact Bethe ansatz solution and a
local density approximation treatment of the harmonic trap, we investigate the
equation of state of the multi-component Fermi gas in both a homogeneous and
trapped environment, as well as the density profiles and low-energy collective
modes. The binding energy of multi-component bound clusters is also given. We
show that there is a peak in the collective mode frequency at the critical
density for a deconfining transition to a many-body state that is analogous to
the quark color superconductor state expected in neutron stars. | cond-mat_supr-con |
Resistivity tensor of vortex-lattice states in Josephson junction arrays: Two-dimensional Josephson junction arrays frustrated by a perpendicular
magnetic field are predicted to form a cascade of distinct vortex lattice
states. Here, we show that the resistivity tensor provides both structural and
dynamical information on the vortex-lattice states and intervening phase
transitions, which allows for experimental identification of these
symmetry-breaking ground states. We illustrate our general approach by a
microscopic theory of the resistivity tensor for a range of magnetic fields
exhibiting a rich set of vortex lattices as well as transitions to
liquid-crystalline vortex states. | cond-mat_supr-con |
Quantum Superconductor-Metal Transition in a 2D Proximity-Coupled Array: We construct a theory of quantum fluctuatons in a regular array of small
superconductive islands connected via low-resistance tunnel contacts to a dirty
thin metal film.
Electron-electron interaction in the film is assumed to be repulsive. The
system is macroscopically superconductive when the distance between
neighbouring islands is short enough. The zero-temperature phase transition
from superconductive to normal-conductive state is shown to occur with the
increase of distance between superconductive islands; the logarithm of the
critical distance is proportional to the low-frequency zero-voltage Andreev
conductance between the SC island and the film. This critical distance is
always much less the than the two-dimensional localization length, so the
considered effect develops when weak-localization corrections are still small.
The dependence of the critical temperature on the film conductance and
inter-island distance is found. | cond-mat_supr-con |
Spectroscopy of Magnetic Excitations in Magnetic Superconductors Using
Vortex Motion: In magnetic superconductors a moving vortex lattice is accompanied by an ac
magnetic field which leads to the generation of spin waves. At resonance
conditions the dynamics of vortices in magnetic superconductors changes
drastically, resulting in strong peaks in the dc I-V characteristics at
voltages at which the washboard frequency of vortex lattice matches the spin
wave frequency $\omega_s({\bf g})$, where ${\bf g}$ are the reciprocal vortex
lattice vectors. We show that if washboard frequency lies above the magnetic
gap, peaks in the I-V characteristics in borocarbides and cuprate layered
magnetic superconductors are strong enough to be observed over the background
determined by the quasiparticles. | cond-mat_supr-con |
Electronic Structure of Superconducting Infinite-Layer Lanthanum
Nickelates: Revealing the momentum-resolved electronic structure of infinite-layer
nickelates is essential for understanding this new class of unconventional
superconductors, but has been hindered by the formidable challenges in
improving the sample quality. In this work, we report for the first time the
angle-resolved photoemission spectroscopy of superconducting
La$_{0.8}$Sr$_{0.2}$NiO$_{2}$ films prepared by molecular beam epitaxy and
${\mathrm{\textit{in situ}}}$ atomic-hydrogen reduction. The measured Fermi
topology closely matches theoretical calculations, showing a large
Ni-$d_{x^2-y^2}$ derived Fermi sheet that evolves from hole-like to
electron-like along $k_{z}$, and a three-dimensional (3D) electron pocket
centered at Brillouin zone corner. The Ni-$d_{x^2-y^2}$ derived bands show a
mass enhancement ($m^*/m_{\rm{DFT}}$) of 2-3,while the 3D electron band shows
negligible band renormalization. Moreover, the Ni-$d_{x^2-y^2}$ derived states
also display a band dispersion anomaly at higher binding energy, reminiscent of
the waterfall feature and kinks observed in cuprates. | cond-mat_supr-con |
Coexistence of Two Sharp-Mode Couplings and Their Unusual Momentum
Dependence in the Superconducting State of Bi2Sr2CaCu2O8+d Superconductor
Revealed by Laser-Based Angle-Resolved Photoemission: Super-high resolution laser-based angle-resolved photoemission measurements
have been carried out on Bi2Sr2CaCu2O8+d (Bi2212) superconductors to
investigate momentum dependence of electron coupling with collective
excitations (modes). Two coexisting energy scales are clearly revealed over a
large momentum space for the first time in the superconducting state of an
overdoped Bi2212 superconductor. These two energy scales exhibit distinct
momentum dependence: one keeps its energy near 78 meV over a large momentum
space while the other changes its energy from $\sim$40 meV near the antinodal
region to $\sim$70 meV near the nodal region. These observations provide a new
picture on momentum evolution of electron-boson coupling in Bi2212 that
electrons are coupled with two sharp modes simultaneously over a large momentum
space in the superconducting states. Their unusual momentum dependence poses a
challenge to our current understanding of electron-mode-coupling and its role
for high temperature superconductivity in cuprate superconductors. | cond-mat_supr-con |
Intrinsic superconducting diode effects in tilted Weyl and Dirac
semimetals: We explore Weyl and Dirac semimetals with tilted nodes as platforms for
realizing an intrinsic superconducting diode effect. Although tilting breaks
sufficient spatial and time-reversal symmetries, we prove that -- at least for
conventional $s$-wave singlet pairing -- the effect is forbidden by an emergent
particle-hole symmetry at low energies if the Fermi level is tuned to the
nodes. Then, as a stepping stone to the three-dimensional semimetals, we
analyze a minimal one-dimensional model with a tilted helical node using
Ginzburg-Landau theory. While one might naively expect a drastic enhancement of
the effect when the node turns from type-I to type-II, we find that the
presence of multiple Fermi pockets is more important as it enables multiple
pairing amplitudes with indepedent contributions to supercurrents in opposite
directions. Equipped with this insight, we construct minimal lattice models of
Weyl and Dirac semimetals and study the superconducting diode effect in them.
Once again, we see a substantial enhancement when the normal state has multiple
Fermi pockets per node that can accommodate more than one pairing channel. In
summary, this study sheds light on the key factors governing the intrinsic
superconducting diode effect in systems with asymmetric band structures and
paves the way for realizing it in topological semimetals. | cond-mat_supr-con |
Superconductivity in the doped topological insulator Cu$_x$Bi$_2$Se$_3$
under high pressure: We report a high-pressure single crystal study of the topological
superconductor Cu$_x$Bi$_2$Se$_3$. Resistivity measurements under pressure show
superconductivity is depressed smoothly. At the same time the metallic behavior
is gradually lost. The upper critical field data $B_{c2}(T)$ under pressure
collapse onto a universal curve. The absence of Pauli limiting and the
comparison of $B_{c2}(T)$ to a polar state function point to spin-triplet
superconductivity, but an anisotropic spin-singlet state cannot be discarded
completely. | cond-mat_supr-con |
Single-crystal growth and extremely high H_c2 of 12442-type Fe-based
superconductor KCa_2Fe_4As_4F_2: Millimeter sized single crystals of KCa_2Fe_4As_4F_2 were grown using a
self-flux method. The chemical compositions and crystal structure were
characterized carefully. Superconductivity with the critical transition T_c =
33.5 K was confirmed by both the resistivity and magnetic susceptibility
measurements. Moreover, the upper critical field H_c2 was studied by the
resistivity measurements under different magnetic fields. A rather steep
increase for the in-plane H_c2^ab with cooling, d\mu_0H_c2^a/dT|T_c = -50.9
T/K, was observed, indicating an extremely high upper critical field. Possible
origins for this behavior were discussed. The findings in our work is a great
promotion both for understanding the physical properties and applications of
12442-type Fe-based superconductors. | cond-mat_supr-con |
Complex superconductivity in the noncentrosymmetric compound Re$_{6}$Zr: We report the electronic structure, synthesis, and measurements of the
magnetic, transport, and thermal properties of the polycrystalline
noncentrosymmetric compound Re$_{6}$Zr. We observed a bulk superconducting
transition at temperature $T_{c}$ $\sim$ 6.7 K, and measured the resistivity,
heat capacity, thermal conductivity, and the London penetration depth below the
transition, as well as performed doping and pressure studies. From these
measurements we extracted the critical field, and the superconducting
parameters of Re$_{6}$Zr. Our measurements indicate a relatively weak to
moderate contribution from a triplet component to the order parameter, and
favor a full superconducting gap, although we cannot exclude the existence of
point nodes based on our data. | cond-mat_supr-con |
Superconducting Gap Structure of LaFePO Studied by Thermal Conductivity: The superconducting gap structure of LaFePO ($T_c=7.4 $K) is studied by
thermal conductivity ($\kappa$) at low temperatures in fields $H$ parallel and
perpendicular to the c axis. A clear two-step field dependence of $\kappa(H)$
with a characteristic field $H_s(\sim 350$ Oe) much lower than the upper
critical field $H_{c2}$ is observed. In spite of large anisotropy of $H_{c2}$,
$\kappa(H)$ in both $H$-directions is nearly identical below $H_s$. Above
$H_s$, $\kappa(H)$ grows gradually with $H$ with a convex curvature, followed
by a steep increase with strong upward curvature near $H_{c2}$. These results
indicate the multigap superconductivity with active two-dimensional (2D) and
passive 3D bands having contrasting gap values. Together with the recent
penetration depth results, we suggest that the 2D bands consist of nodal and
nodeless ones, consistent with the extended s-wave symmetry. | cond-mat_supr-con |
Raman scattering from a superconductivity-induced bound state in $MgB_2$: It is shown that the sharp peak in the $E_{2g}$ Raman spectrum of
superconducting $MgB_2$ is due to a bound state caused by the electron-phonon
coupling. Our theory explains why this peak appears only in the spectra with
$E_{2g}$ symmetry and only in the $\sigma$ but not $\pi$ bands. The properties
of the bound state and the Raman spectrum are investigated, also in the
presence of impurity scattering. | cond-mat_supr-con |
Stripe Disordering Transition: We have recently begun Monte Carlo simulations of the dynamics of stripe
phases in the cuprates. A simple model of spinodal decomposition of the holes
allows us to incorporate Coulomb repulsion and coherency strains. We find
evidence for a possible stripe disordering transition, at a temperature below
the pseudogap onset. Experimental searches for such a transition can provide
constraints for models of stripe formation. | cond-mat_supr-con |
Correcting 100 years of misunderstanding: electric fields in
superconductors, hole superconductivity, and the Meissner effect: From the outset of superconductivity research it was assumed that no
electrostatic fields could exist inside superconductors, and this assumption
was incorporated into conventional London electrodynamics. Yet the London
brothers themselves initially (in 1935) had proposed an electrodynamic theory
of superconductors that allowed for static electric fields in their interior,
which they unfortunately discarded a year later. I argue that the Meissner
effect in superconductors necessitates the existence of an electrostatic field
in their interior, originating in the expulsion of negative charge from the
interior to the surface when a metal becomes superconducting. The theory of
hole superconductivity predicts this physics, and associated with it a
macroscopic spin current in the ground state of superconductors ("Spin Meissner
effect"), qualitatively different from what is predicted by conventional
BCS-London theory. A new London-like electrodynamic description of
superconductors is proposed to describe this physics. Within this theory
superconductivity is driven by lowering of quantum kinetic energy, the fact
that the Coulomb repulsion strongly depends on the character of the charge
carriers, namely whether electron- or hole-like, and the spin-orbit
interaction. The electron-phonon interaction does not play a significant role,
yet the existence of an isotope effect in many superconductors is easily
understood. In the strong coupling regime the theory appears to favor local
charge inhomogeneity. The theory is proposed to apply to all superconducting
materials, from the elements to the high $T_c$ cuprates and pnictides, is
highly falsifiable, and explains a wide variety of experimental observations. | cond-mat_supr-con |
Long-range spin and charge accumulation in mesoscopic superconductors
with Zeeman splitting: We describe the far from equilibrium non-local transport in a diffusive
superconducting wire with a Zeeman splitting, taking into account the different
spin relaxation mechanisms. We demonstrate that due to the Zeeman splitting an
injection of a current in a superconducting wire creates a spin accumulation
that can only relax via thermalization. In addition the Zeeman splitting also
causes a suppression of the spin-orbit and spin-flip scattering rates. These
two effects lead to long-range spin and charge accumulations detectable in the
non-local signal. Our model explains the main qualitative features of recent
experimental results in terms of realistic parameters and predicts a strong
dependence of the non-local signal on the orbital depairing effect from an
induced magnetic field. | cond-mat_supr-con |
Suppression of superconductivity at nematic critical point in underdoped
cuprates: A nematic quantum critical point is anticipated to exist in the
superconducting dome of some high-temperature superconductors. The nematic
order competes with the superconducting order and hence reduces the
superconducting condensate at $T = 0$. Moreover, the critical fluctuations of
nematic order can excite more nodal quasiparticles out of the condensate. We
address these two effects within an effective field theory and show that
superfluid density $\rho^s(T)$ and superconducting temperature $T_c$ are both
suppressed strongly by the critical fluctuations. The strong suppression of
superconductivity provides a possible way to determine the nematic quantum
critical point. | cond-mat_supr-con |
Orbital selective spin excitations and their impact on superconductivity
of LiFe1-xCoxAs: We use neutron scattering to study spin excitations in single crystals of
LiFe$_{0.88}$Co$_{0.12}$As, which is located near the boundary of the
superconducting phase of LiFe$_{1-x}$Co$_{x}$As and exhibits non-Fermi-liquid
behavior indicative of a quantum critical point. By comparing spin excitations
of LiFe$_{0.88}$Co$_{0.12}$As with a combined density functional theory (DFT)
and dynamical mean field theory (DMFT) calculation, we conclude that
wave-vector correlated low energy spin excitations are mostly from the $d_{xy}$
orbitals, while high-energy spin excitations arise from the $d_{yz}$ and
$d_{xz}$ orbitals. Unlike most iron pnictides, the strong orbital selective
spin excitations in LiFeAs family cannot be described by anisotropic Heisenberg
Hamiltonian. While the evolution of low-energy spin excitations of
LiFe$_{1-x}$Co$_x$As are consistent with electron-hole Fermi surface nesting
condition for the $d_{xy}$ orbital, the reduced superconductivity in
LiFe$_{0.88}$Co$_{0.12}$As suggests that Fermi surface nesting conditions for
the $d_{yz}$ and $d_{xz}$ orbitals are also important for superconductivity in
iron pnictides. | cond-mat_supr-con |
Critical Temperature Enhancement of Topological Superconductors: A
Dynamical Mean Field Study: We show that a critical temperature Tc for spin-singlet two-dimensional
superconductivity is enhanced by a cooperation between the Zeeman magnetic
field and the Rashba spin-orbit coupling, where a superconductivity becomes
topologically non-trivial below Tc. The dynamical mean field theory (DMFT) with
the segment-based hybridization-expansion continuous-time quantum Monte Carlo
impurity solver (ct-HYB) is used for accurately evaluating a critical
temperature, without any Fermion sign problem. A strong-coupling approach shows
that spin-flip driven local pair hopping leads to part of this enhancement,
especially effects of the magnetic field. We propose physical settings suitable
for verifying the present calculations, one-atom-layer system on Si(111) and
ionic-liquid based electric double-layer transistors (EDLTs). | cond-mat_supr-con |
Tunnel and thermal c-axis transport in BSCCO in the normal and pseudogap
state: We consider the problem of c-axis transport in double-layered cuprates, in
particular with reference to Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$
compounds. We exploit the effect of the two barriers on the thermal and tunnel
transport. The resulting model is able to describe accurately the normal state
c-axis resistivity in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$, from the
underdoped side up to the strongly overdoped. We extend the model, without
introducing additional parameters, in order to allow for the decrease of the
barrier when an external voltage bias is applied. The extended model is found
to describe properly the c-axis resistivity for small voltage bias above the
pseudogap temperature $T^{*}$, the c-axis resistivity for large voltage bias
even below $T_c$, and the differential $dI/dV$ curves taken in mesa structures. | cond-mat_supr-con |
Nuclear ferromagnetism induced Fulde-Ferell-Larkin-Ovchinnikov state: We present a theoretical study of the influence of the nuclear ferromagnetism
on superconductivity in the presence of the electron-nuclear spin interaction.
It is demonstrated that in some metals, e.g. Rh, W, the BCS condensate imbedded
in a matrix of ferromagneticaly ordered nuclear spins should manifest the FFLO
(Fulde-Ferel-Larkin-Ovchinniov) state. We outline that the optimal experimental
conditions for observation of FFLO could be achieved by creation, via adiabatic
nuclear demagnetization, of the negative nuclear spin temperatures. In this
case the nuclear polarization points in the opposite to the external magnetic
field direction and the electromagnetic part of the nuclear spin magnetization
compensates the external magnetic field, while the exchange part creates the
nonhomogeneous superconducting order parameter. | cond-mat_supr-con |
Fermiological Interpretation of Superconductivity/Non-superconductivity
of FeTe_{1-x}Se_{x} Thin Crystal Determined by Quantum Oscillation
Measurement: We have successfully observed quantum oscillation (QO) for FeTe_{1-x}Se_{x}.
QO measurements were performed using non-superconducting and superconducting
thin crystals of FeTe_{0.65}Se_{0.35} fabricated by the scotch-tape method. We
show that the Fermi surfaces (FS) of the non-superconducting crystal are in
good agreement with the rigid band shift model based on electron doping by
excess Fe while that of the superconducting crystal is in good agreement with
the calculated FS with no shift. From the FS comparison of both crystals, we
demonstrate the change of the cross-sectional area of the FS, suggesting that
the suppression of the FS nesting with the vector Q_{s} = (\pi, \pi) due to
excess Fe results in the disappearance of the superconductivity. | cond-mat_supr-con |
Accurate Measurements of the Intrinsic Surface Impedance of Thin YBCO
Films Using a Modified Two-tone Resonator Method: We propose a modified two-tone method that could be used for sensitive
measurements of the intrinsic microwave surface impedance (Zs) of thin
superconductor films and the tand of a low-loss dielectric. An open-gap
resonator scheme is used to measure the penetration depth of thin
superconductor films and extract the intrinsic Zs of the superconductor films
from its measured effective surface resistance and penetration depth. We use a
very small gap of 10 um between the top plate and the rest parts of the
resonator. The loss tangent of rutile in the low 10^-7 range and the dielectric
constant as high as ~110 are observed at temperatures below 10 K at ~15.2 GHz,
which enable to measure the Rs of the 10 mm-in-diameter YBCO films as low as
\~100 micro-ohm at the same frequency (f). The discrepancy between the
effective surface resistance at ~15.2 GHz and that at ~8.5 GHz scaled to ~15.2
GHz appears less than 6 % when the relations of Rs ~ f^2 and loss tangent ~ f
are used. We describe usefulness of our measurement method for measuring the
intrinsic microwave properties of various superconductor samples. | cond-mat_supr-con |
Carrier relaxation dynamics in intra-gap states: the case of the
superconductor Y Ba_2 Cu_3O_7-delta and the charge-density-wave semiconductor
K_0.3MoO_3: The unusual slow carrier relaxation dynamics - observed in femtosecond
pump-probe experiments on high-temperature superconductors and recently also in
a charge-density-wave system - is analyzed in terms of a model for relaxation
of carriers in intra-gap states. The data on YBa_2Cu_3O_7-delta near optimum
doping and K_0.3$MoO_3 are found to be described very well with the model using
a BCS-like gap which closes at T_c. From the analysis of the data we conclude
that a significant intra-gap density of localized states exists in these
materials, which can be clearly distinguished from quasiparticle states by the
time-resolved optical experiments because of the different time- and
temperature-dependences of the photoinduced transmission or reflection.
Localized charges are suggested to be the most likely origin of the intra-gap
states, while the similarity of the response in the two materials appear to
exclude spin and vortex excitations. | cond-mat_supr-con |
Possible origin of the pseudogap end point in the high-T$_{c}$ cuprates: Recent experiments find that the pseudogap phase of the high-T$_{c}$ cuprates
ends suddenly at an electron doping $x^{*}$ when the Fermi surface change its
shape from hole-like to electron-like. In this short note, we argue that the
antiferromagnetic(AF) spin correlation of the system should drop abruptly at
the same doping. At the same time, we argue that the critical behavior observed
at $x^{*}$ in the specific heat measurement should be attributed to the strong
renormalization of the quasiparticle excitation in the anti-nodal region by the
critical AF spin fluctuation. This picture also predicts that any
pseudogap-like spectral feature related to AF scattering in the electron-doped
cuprates should terminate at the doping level when the Fermi surface becomes
tangent to the boundary of AF Brillouin zone. Recent ARPES measurement on the
electron-doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ seems to be consistent with
such a prediction. | cond-mat_supr-con |
Type-II Ising Pairing in Few-Layer Stanene: Spin-orbit coupling has proven indispensable in realizing topological
materials and more recently Ising pairing in two-dimensional superconductors.
This pairing mechanism relies on inversion symmetry breaking and sustains
anomalously large in-plane polarizing magnetic fields whose upper limit is
expected to diverge at low temperatures, although experimental demonstration of
this has remained elusive due to the required fields. In this work, the
recently discovered superconductor few-layer stanene, i.e. epitaxially strained
$\alpha$-Sn, is shown to exhibit a new type of Ising pairing between carriers
residing in bands with different orbital indices near the $\Gamma$-point. The
bands are split as a result of spin-orbit locking without the participation of
inversion symmetry breaking. The in-plane upper critical field is strongly
enhanced at ultra-low temperature and reveals the sought for upturn. | cond-mat_supr-con |
Reentrant topological transitions with Majorana end states in 1D
superconductors by lattice modulation: The possibility to observe and manipulate Majorana fermions as end states of
one-dimensional topological superconductors has been actively discussed
recently. In a quantum wire with strong spin-orbit coupling placed in proximity
to a bulk superconductor, a topological superconductor has been expected to be
realized when the band energy is split by the application of a magnetic field.
When a periodic lattice modulation is applied multiple topological
superconductor phases appear in the phase diagram. Some of them occur for
higher filling factors compared to the case without the modulation. We study
the effects of phase jumps and argue that the topologically nontrivial state of
the whole system is retained even if they are present. We also study the effect
of the spatial modulation in the hopping parameter. | cond-mat_supr-con |
Charge-4e supercurrent in a two-dimensional InAs-Al
superconductor-semiconductor heterostructure: Superconducting qubits with intrinsic noise protection offer a promising
approach to improve the coherence of quantum information. Crucial to such
protected qubits is the encoding of the logical quantum states into
wavefunctions with disjoint support. Such encoding can be achieved by a
Josephson element with an unusual charge-4e supercurrent emerging from the
coherent transfer of pairs of Cooper-pairs. In this work, we demonstrate the
controlled conversion of a conventional charge-2e dominated to a charge-4e
dominated supercurrent in a superconducting quantum interference device (SQUID)
consisting of gate-tunable planar Josephson junctions (JJs). We investigate the
ac Josephson effect of the SQUID and measure a dominant photon emission at
twice the fundamental Josephson frequency together with a doubling of the
number of Shapiro steps, both consistent with the appearance of charge-4e
supercurrent. Our results present a step towards novel protected
superconducting qubits based on superconductor-semiconductor hybrid materials. | cond-mat_supr-con |
Quantum anomalous vortex and Majorana zero mode in iron-based
superconductor Fe(Te,Se): In topological insulators doped with magnetic ions, spin-orbit coupling and
ferromagnetism give rise to the quantum anomalous Hall effect. Here we show
that in s-wave superconductors with strong spin-orbit coupling, magnetic
impurity ions can generate topological vortices in the absence of external
magnetic fields. Such vortices, dubbed quantum anomalous vortices, support
robust Majorana zero-energy modes when superconductivity is induced in the
topological surface states. We demonstrate that the zero-energy bound states
observed in Fe(Te,Se) superconductors are possible realizations of the Majorana
zero modes in quantum anomalous vortices produced by the interstitial magnetic
Fe. The quantum anomalous vortex matter not only advances fundamental
understandings of topological defect excitations of Cooper pairing, but also
provides new and advantageous platforms for manipulating Majorana zero modes in
quantum computing. | cond-mat_supr-con |
Superconductivity in the orthorhombic phase of thermoelectric
CsPbxBi4-xTe6 with 0.3=<x=<1.0: Experimental measurements clearly reveal the presence of bulk
superconductivity in the CsPbxBi4-xTe6 (0.3=<x=<1.0) materials, i.e. the first
member of the thermoelectric series of Cs[PbmBi3Te5+m], these materials have
the layered orthorhombic structure containing infinite anionic [PbBi3Te6]-
slabs separated with Cs+ cations. Temperature dependences of electrical
resistivity, magnetic susceptibility, and specific heat have consistently
demonstrated that the superconducting transition in CsPb0.3Bi3.7Te6 occurs at
Tc=3.1K, with a superconducting volume fraction close to 100% at 1.8 K.
Structural study using aberration-corrected STEM/TEM reveals a rich variety of
microstructural phenomena in correlation with the Pb-ordering and chemical
inhomogeneity. The superconducting material CsPb0.3Bi3.7Te6 with the highest Tc
shows a clear ordered structure with a modulation wave vector of q=a*/2+
c*/1.35 on the a-c plane. Our study evidently demonstrates that
superconductivity deriving upon doping of narrow-gap semiconductor is a viable
approach for exploration of novel superconductors. | cond-mat_supr-con |
Anomalous Josephson effect in p-wave dirty junctions: The Josephson effect in p-wave superconductor / diffusive normal metal /
p-wave superconductor junctions is studied theoretically. Amplitudes of
Josephson currents are several orders of magnitude larger than those in s-wave
junctions. Current-phase (J-\phi) relations in low temperatures are close to
those in ballistic junctions such as J\propto\sin(\phi/2) and J\propto\phi even
in the presence of random impurity potentials. A cooperative effect between the
midgap Andreev resonant states and the proximity effect causes such anomalous
properties and is a character of the spin-triplet superconductor junctions. | cond-mat_supr-con |
Vortex configurations and metastability in mesoscopic superconductors: The vortex dynamics in mesoscopic superconducting cylinders with rectangular
cross section under an axially applied magnetic field is investigated in the
multivortex London regime. The rectangles considered range from a square up to
an infinite slab. The flux distribution and total flux carried by a vortex
placed in an arbitrary position of the sample is calculated analytically by
assuming Clem's solution for the vortex core. The Bean-Livingston energy
barrier is also analytically calculated in this framework. A Langevin algorithm
simulates the flux penetration and dynamical evolution of the vortices as the
external field is slowly cycled. The simulated magnetization process is
governed by metastable states. The magnetization curves are hysteretic, with
paramagnetic response in part of the downward branch, and present a series of
peaks corresponding to the entry or expulsion of a single vortex. For elongated
rectangles, the vortices arrange themselves into parallel vortex chains and an
additional modulation of the magnetization, corresponding to creation or
destruction of a vortex chain, comes out. | cond-mat_supr-con |
Proximity effect induced intriguing superconductivity in van der Waals
heterostructure of magnetic topological insulator and conventional
superconductor: Nontrivial topological superconductivity has received enormous research
attentions due to its potential for diverse applications in topological quantum
computing. The intrinsic issue concerning the correlation between a topological
insulator and a superconductor is, however, still widely open. Here, we
systemically report an emergent superconductivity in a cross-junction composed
of a magnetic topological insulator MnBi2Te4 and a conventional superconductor
NbSe2. Remarkably, the interface indicates existence of a reduced
superconductivity at surface of NbSe2 and a proximity-effectinduced
superconductivity at surface of MnBi2Te4. Furthermore, the in-plane
angular-dependent magnetoresistance measurements reveal the fingerprints of the
paring symmetry behaviors for these superconducting gaps as a unconventional
nature. Our findings extend our views and ideas of topological
superconductivity in the superconducting heterostructures with time-reversal
symmetry breaking, offering an exciting opportunity to elucidate the
cooperative effects on the surface state of a topological insulator aligning a
superconductor. | cond-mat_supr-con |
Hole Localization in Underdoped Superconducting Cuprates Near 1/8th
Doping: Measurements of thermal conductivity versus temperature over a broad range of
doping in YBa$_2$Cu$_3$O$_{6+x}$ and HgBa$_2$Ca$_{n-1}$Cu$_n$O$_{2n+2+\delta}$
($n$=1,2,3) suggest that small domains of localized holes develop for hole
concentrations near $p$=1/8. The data imply a mechanism for localization that
is intrinsic to the CuO$_2$-planes and is enhanced via pinning associated with
oxygen-vacancy clusters. | cond-mat_supr-con |
Shapiro Step on the rc-Branch in the IV-characteristic of a Josephson
Junction with an LC-shunting: The effect of external radiation on the phase dynamics of Josephson junctions
shunted by an LC circuit is examined. It is shown that additional resonant
circuit (rc) branches appear in the current voltage characteristics of the
junctions when the Josephson frequency is equal to the frequency of the formed
resonance circuit. We show that the amplitude dependence of the Shapiro step
width crucially changes when the Shapiro step is on the rc-branch in comparison
to the case of Josephson junction without shunt. The experimental
implementation of these effects might give very important advantages for
existing methods and technologies. | cond-mat_supr-con |
Nonequilibrium Josephson effect in mesoscopic ballistic multiterminal
SNS junctions: We present a detailed study of nonequilibrium Josephson currents and
conductance in ballistic multiterminal SNS-devices. Nonequilibrium is created
by means of quasiparticle injection from a normal reservoir connected to the
normal part of the junction. By applying a voltage at the normal reservoir the
Josephson current can be suppressed or the direction of the current can be
reversed. For a junction longer than the thermal length, $L\gg\xi_T$, the
nonequilibrium current increases linearly with applied voltage, saturating at a
value equal to the equilibrium current of a short junction. The conductance
exhibits a finite bias anomaly around $eV \sim \hbar v_F/L$. For symmetric
injection, the conductance oscillates $2\pi$-periodically with the phase
difference $\phi$ between the superconductors, with position of the minimum
($\phi=0$ or $\pi$) dependent on applied voltage and temperature. For
asymmetric injection, both the nonequilibrium Josephson current and the
conductance becomes $\pi$-periodic in phase difference. Inclusion of barriers
at the NS-interfaces gives rise to a resonant behavior of the total Josephson
current with respect to junction length with a period $\sim \lambda_F$. Both
three and four terminal junctions are studied. | cond-mat_supr-con |
Magnetic Exciton Mediated Superconductivity in the Hidden-Order Phase of
URu2Si2: We propose the magnetic exciton mediated superconductivity occurring in the
enigmatic hidden-order phase of URu2Si2. The characteristic of the massive
collective excitation observed only in the hidden-order phase is well
reproduced by the antiferro hexadecapole ordering model as the trace of the
dispersive crystalline-electric-field excitation. The disappearance of the
superconductivity in the high-pressure antiferro magnetic phase can naturally
be understood by the sudden suppression of the magnetic-exciton intensity. The
analysis of the momentum dependence of the magnetic-exciton mode leads to the
exotic chiral d-wave singlet pairing in the Eg symmetry. The Ising-like
magnetic-field response of the mode yields the strong anisotropy observed in
the upper critical field even for the rather isotropic 3-dimensional Fermi
surfaces of this compound. | cond-mat_supr-con |
Direct penetration of spin-triplet superconductivity into a ferromagnet
in Au/SrRuO3/Sr2RuO4 junctions: Efforts have been ongoing to establish superconducting spintronics utilizing
ferromagnet/superconductor heterostructures1. Previously reported devices are
based on spin-singlet superconductors (SSCs), where the spin degree of freedom
is lost. Spin-polarized supercurrent induction in ferromagnetic metals (FMs) is
achieved even with SSCs, but only with the aid of interfacial complex magnetic
structures, which severely affect information imprinted to the electron spin.
Use of spin-triplet superconductors (TSCs) with active spins potentially
overcomes this difficulty and further leads to novel functionalities. Here, we
report spin-triplet superconductivity induction into a FM SrRuO3 from a leading
TSC candidate Sr2RuO4, by fabricating microscopic devices using an epitaxial
SrRuO3/Sr2RuO4 hybrid. The differential conductance, exhibiting
Andreev-reflection features with multiple energy scales up to around half
tesla, indicates the penetration of superconductivity over a considerable
distance of 15 nm across the SrRuO3 layer without help of interfacial complex
magnetism. This demonstrates the first FM/TSC device exhibiting the
spin-triplet proximity effect. | cond-mat_supr-con |
Improved superconducting properties in nanocrystalline bulk MgB_2: Highly dense nanocrystalline MgB_2 bulk superconductors with distinctly
improved pinning were prepared by mechanical alloying of Mg and B powders and
hot compaction at ambient temperatures. The nanocrystalline samples reveal high
critical current densities of 105 A/cm2 at 20 K and 1 T together with a
strongly shifted irreversibility line towards higher fields resulting in
H_irr(T) ~ 0.8 H_c2(T), whereas typically H_irr(T) ~ 0.5 H_c2(T) is observed
for bulk untextured samples. These values exceed that of all other so far
reported bulk samples and are in the range of the values of thin films. The
improved pinning of this material which mainly consists of spherical grains of
about 40-100 nm in size is attributed to the large number of grain boundaries. | cond-mat_supr-con |
Alignment of Carbon Nanotube Additives for Improved Performance of
Magnesium Diboride Superconductors: The rapid progress on MgB2 superconductor since its discovery[1] has made
this material a strong competitor to low and high temperature superconductors
(HTS) for applications with a great potential to catch the niche market such as
in magnetic resonant imaging (MRI). Thanks to the lack of weak links and the
two-gap superconductivity of MgB2 [2,3] a number of additives have been
successfully used to enhance the critical current density, Jc and the upper
critical field, Hc2.[4-12] Carbon nanotubes (CNTs) have unusually electrical,
mechanical and thermal properties[13-16] and hence is an ideal component to
fabricate composites for improving their performance. To take advantages of the
extraordinary properties of CNTs it is important to align CNTs in the
composites. Here we report a method of alignment of CNTs in the CNT/MgB2
superconductor composite wires through a readily scalable drawing technique.
The aligned CNT doped MgB2 wires show an enhancement in magnetic Jc(H) by more
than an order of magnitude in high magnetic fields, compared to the undoped
ones. The CNTs have also significantly enhanced the heat transfer and
dissipation. CNTs have been used mainly in structural materials, but here the
advantage of their use in functional composites is shown and this has wider
ramifications for other functional materials. | cond-mat_supr-con |
d-wave superconductivity and its coexistence with antiferromagnetism in
t-J-U model: Statistically consistent Gutzwiller approach: We discuss the coexistence of antiferromagnetism and d-wave superconductivity
within the so-called statistically-consistent Gutzwiller approximation (SGA)
applied to the t-J-U model. In this approach, the averages calculated in a
self-consistent manner coincide with those determined variationally. Such
consistency is not guaranteed within the standard renormalized mean field
theory. With the help of SGA, we show that for the typical value J/|t| = 1/3,
coexistence of antiferromagnetism (AF) and superconductivity (SC) appears only
for U/|t| > 10.6 and in a very narrow range of doping (\delta < 0.006) in the
vicinity of the Mott insulating state, in contrast to some previous reports. In
the coexistent AF+SC phase, a staggered spin-triplet component of the
superconducting gap appears also naturally; its value is very small. | cond-mat_supr-con |
Magnetization-dependent $T_c$ shift in F/S/F trilayers with a strong
ferromagnet: We have measured the superconducting transition temperature $T_c$ of Ni/Nb/Ni
trilayers when the magnetizations of the two outer Ni layers are parallel (P)
and antiparallel (AP). The largest difference in $T_c$ occurs when the Nb
thickness is just above the critical thickness at which superconductivity
disappears completely. We have observed a difference in $T_c$ between the P and
AP states as large as 41 mK - a significant increase over earlier results in
samples with higher $T_c$ and with a CuNi alloy in place of the Ni. Our result
also demonstrates that strong elemental ferromagnets are promising candidates
for future investigations of ferromagnet/superconductor heterostructures. | cond-mat_supr-con |
Structural Evolution of One-dimensional Spin Ladder Compounds
Sr14-xCaxCu24O41 with Ca doping and Related Hole Redistribution Evidence: Incommensurate crystal structures of spin ladder series Sr14-xCaxCu24O41
(x=3, 7, 11, 12.2) were characterized by powder neutron scattering method and
refined using the superspace group Xmmm(00{\gamma})ss0 (equivalent to
superspace group Fmmm(0,0,1+{\gamma})ss0); X stands for non-standard centering
(0,0,0,0), (0,1/2,1/2,1/2), (1/2,1/2,0,0), (1/2,0,1/2,1/2)) with a modulated
structure model. The Ca doping effects on the lattice parameters, atomic
displacement, Cu-O distances, Cu-O bond angles and Cu bond valence sum were
characterized. The refined results show that the CuO4 planar units in both
chain and ladder sublattices become closer to square shape with an increase of
Ca doping. The Cu bond valence sum calculation provided new evidence for the
charge transfer from the chains to ladders (approximately 0.16 holes per Cu
from x=0 to 12.2). The charge transfer was attributed to two different
mechanisms: (a) the Cu-O bond distance shrinkage on the ladder; (b) increase of
the interaction between two sublattices, resulting in Cu-O bonding between the
chains and ladders. The low temperature structural refinement resulted in the
similar conclusion, with a slight charge backflow to the chains. | cond-mat_supr-con |
A chemist's view of chemical bonding in the mechanism of high
temperature superconductivity: Through a 2-dimensionnal tight-binding crystal orbital approach and a (CuO2)2
square unit cell of parameter a, we show that a Cu2+/O2- -> Cu+/O- charge
transfer is likely to occur at the M($\pi/a$, $\pi/a$) point of the Brillouin
zone, for O4 groups with antibonding b1g symmetry. This approach emphasises the
role of oxygen-oxygen interactions in avoiding the nesting of the Fermi surface
and agrees with its observed topology. At the M point of the Brillouin zone,
oxygen atoms are strongly dissymmetric ("Janus" atoms) and link copper atoms
with different environments (a1g vs. b1g symmetries). Further hole doping
generates two situations: two holes (S=0) and/or a single hole (S=+/- 1/2) in
the O4 b1g groups ($\sigma$ or $\pi$), with a possible equilibrium between
them; the former can be considered as a "hole lone pair" by analogy with
electron lone pairs. Mulliken-Jaffe electronegativity considerations justify
the nearly zero U Hubbard parameter value. The mixing of the pair-occupied with
pair-iunoccupied wave functions is realised via an electronic hamiltonian in
place of the electron-phonon coupling of the pristine BCS theory. | cond-mat_supr-con |
Theoretical explanation of electric field-induced superconductive
critical temperature shifts in Indium thin films: We calculate the effect of a static electric field on the superconductive
critical temperature of Indium thin films in the framework of proximity effect
Eliashberg theory, in order to explain 60 years old experimental data. Since in
the theoretical model we employ all quantities of interest can be computed
ab-initio (i.e. electronic densities of states, Fermi energy shifts and
Eliashberg spectral functions), the only free parameter is in general the
thickness of the surface layer where the electric field acts. However, in the
weak electrostatic field limit Thomas-Fermi approximation is still valid and
therefore no free parameters are left, as this perturbed layer is known to have
a thickness of the order of the Thomas-Fermi screening length. We show that the
theoretical model can reproduce experimental data, even when the magnitude of
the induced charge densities are so small to be usually neglected. | cond-mat_supr-con |
Magnetic field effect on Fe-induced short-range magnetic correlation and
electrical conductivity in
Bi$_{1.75}$Pb$_{0.35}$Sr$_{1.90}$Cu$_{0.91}$Fe$_{0.09}$O$_{6+y}$: We report electrical resistivity measurements and neutron diffraction studies
under magnetic fields of
Bi$_{1.75}$Pb$_{0.35}$Sr$_{1.90}$Cu$_{0.91}$Fe$_{0.09}$O$_{6+y}$, in which hole
carriers are overdoped. This compound shows short-range incommensurate magnetic
correlation with incommensurability $\delta=0.21$, whereas a Fe-free compound
shows no magnetic correlation. Resistivity shows an up turn at low temperature
in the form of $ln(1/T)$ and shows no superconductivity. We observe reduction
of resistivity by applying magnetic fields (i.e., a negative magnetoresistive
effect) at temperatures below the onset of short-range magnetic correlation.
Application of magnetic fields also suppresses the Fe induced incommensurate
magnetic correlation. We compare and contrast these observations with two
different models: 1) stripe order, and 2) dilute magnetic moments in a metallic
alloy, with associated Kondo behavior. The latter picture appears to be more
relevant to the present results. | cond-mat_supr-con |
Electron Phonon Interaction and Strong Correlations in High-Temperature
Superconductors: One can not avoid unavoidable: The important role of the electron-phonon interaction (EPI) in explaining the
properties of the normal state and pairing mechanism in high-T$_{c}$
superconductors (HTSC) is discussed. A number of experimental results are
analyzed such as: dynamical conductivity, Raman scattering, neutron scattering,
ARPES, tunnelling measurements, isotope effect and etc. They give convincing
evidence that the EPI is strong and dominantly contributes to pairing in HTSC
oxides. It is argued that strong electronic correlations in conjunction with
the pronounced (in relatively weakly screened materials) EPI are unavoidable
ingredients for the microscopic theory of pairing in HTSC oxides. I present the
well defined and controllable theory of strong correlations and the EPI. It is
shown that strong correlations give rise to the pronounced \textit{forward
scattering peak} in the EPI - the FSP theory. The FSP theory explains in a
consistent way several (crucial) puzzles such as much smaller transport
coupling constant than the pairing one ($\lambda_{tr}\ll \lambda$), which are
present if one interprets the results in HTSC oxides by the old
Migdal-Eliashberg theory for the EPI. The ARPES shift puzzle where the nodal
kink at 70 meV is unshifted in the superconducting state, while the anti-nodal
one at 40 meV is shifted can be explained at present only by the FSP theory. A
number of other interesting predictions of the FSP theory are also discussed. | cond-mat_supr-con |
Magic Doping Fractions in High-Temperature Superconductors: We report hole-doping dependence of the in-plane resistivity \rho_{ab} in a
cuprate superconductor La_{2-x}Sr_{x}CuO_{4}, carefully examined using a series
of high-quality single crystals. Our detailed measurements find a tendency
towards charge ordering at particular rational hole doping fractions of 1/16,
3/32, 1/8, and 3/16. This observation appears to suggest a specific form of
charge order and is most consistent with the recent theoretical prediction of
the checkerboard-type ordering of the Cooper pairs at rational doping fractions
x = (2m+1)/2^n, with integers m and n. | cond-mat_supr-con |
Three-dimensional energy gap and origin of charge-density wave in kagome
superconductor KV3Sb5: Kagome lattices offer a fertile ground to explore exotic quantum phenomena
associated with electron correlation and band topology. The recent discovery of
superconductivity coexisting with charge-density wave (CDW) in the kagome
metals KV3Sb5, RbV3Sb5, and CsV3Sb5 suggests an intriguing entanglement of
electronic order and superconductivity. However, the microscopic origin of CDW,
a key to understanding the superconducting mechanism and its possible
topological nature, remains elusive. Here, we report angle-resolved
photoemission spectroscopy of KV3Sb5 and demonstrate a substantial
reconstruction of Fermi surface in the CDW state that accompanies the formation
of small three-dimensional pockets. The CDW gap exhibits a periodicity of
undistorted Brillouin zone along the out-of-plane wave vector, signifying a
dominant role of the in-plane inter-saddle-point scattering to the mechanism of
CDW. The characteristics of experimental band dispersion can be captured by
first-principles calculations with the inverse star-of-David structural
distortion. The present result indicates a direct link between the low-energy
excitations and CDW, and puts constraints on the microscopic theory of
superconductivity in alkali-metal kagome lattices. | cond-mat_supr-con |
Depth profile of London length induced by nonuniform scattering rate
maximizing breakdown magnetic field in type II superconductors: The breakdown magnetic field is the key parameter which determines the
performance of superconducting radio-frequency cavities. This is the maximum
field up to which the Meissner state remains stable and in uniform material, it
is approximately given by the thermodynamic field. There are several recent
suggestions to use nonuniform structures to enhance the breakdown field. One of
possible realizations of such structure is depth profile of the scattering rate
which, in the first approximation, modifies the London penetration depth but
does not change the thermodynamic field. In this paper, we evaluate the optimal
profile of the London penetration depth for which the screening current density
reaches the local depairing value \emph{simultaneously at every point within
finite-size region}. Such profile is realized for close-to-linear decrease of
the London penetration depth within the length scale proportional to its value
at the surface. Achieving noticeable enhancement of the breakdown field,
however, requires strong enhancement of the London length within large region
without affecting the thermodynamic field. | cond-mat_supr-con |
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