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Triplet superconductivity and spin density wave in biased AB bilayer
graphene: We examine spin density wave and triplet superconductivity as possible ground
states of the Bernal bilayer graphene. The spin density wave is stable for the
unbiased and undoped bilayer. Both the doping and the applied bias voltage
destroy this phase. We show that, when biased and slightly doped, bilayer can
host a triplet superconducting phase. The mechanisms for both ordered phases
rely on the renormalized Coulomb interaction. Consistency of our theoretical
conclusions with recent experimental results are discussed. | cond-mat_supr-con |
Interpretation of Inelastic Neutron Scattering Data Using the Phase
Diagram of Hole-Doped Cuprates: Inelastic Neutron Scattering (INS) data in LSCO, YBCO and Bi2212 are
discussed. In the literature, the INS spectra remain far from being
comprehensively understood. We show that local (Q-integrated) susceptibility
data and the energy dependence of the spin susceptibility at antiferromagnetic
vector, Q = (p,p), can be interpreted by using the phase diagram for hole-doped
cuprates. We analyze also the origin of the resonant peak which relates to the
order parameter for long-range phase coherence in hole-doped cuprates. Thus, we
present an interpretation of neutron data in LSCO, YBCO and Bi2212 published so
far. | cond-mat_supr-con |
Superconductivity in iron-based F-doped layered quaternary compound
Nd[O1-xFx]FeAs: The recently discovered quaternary arsenide oxide superconductor
La[O1-xFx]FeAs with the superconducting critical transition temperature (Tc) of
26 K [1], has been quickly expanded to another high-Tc superconducting system
beyond copper oxides by the replacement of La with other rare earth elements,
such as Sm, Ce, and Pr etc. [2-4], and the Pr[O1-xFx]FeAs has become to be the
first non-cuprate superconductor that holding a Tc above 50 K. All these
arsenide (including phosphide) superconductors formed in a same tetragonal
layered structure with the space group P4/nmm which has an alternant stacked
Fe-As layer and RO (R = rare earth metals) layer. Here we report the discovery
of another superconductor in this system, the neodymium-arsenide Nd[O1-xFx]FeAs
with an resistivity onset Tc of 51.9 K, which is the second non-cuprate
compound that superconducts above 50 K. | cond-mat_supr-con |
Spectroscopy of electron-phonon interaction of superconducting point
contacts: experimental aspects: The recovering procedure of the electron-phonon interaction (EPI) functions
from the additional nonlinearities of the current-voltage curve ($I-V$ curve)
of point contacts associated with an excess current is considered. The approach
proposed takes into account both inelastic scattering, which causes suppression
of the excess current in the reabsorption of nonequilibrium phonons by
electrons undergoing Andreev reflection (Andreev electrons), and elastic
processes associated with the electron-phonon renormalization of the energy
spectrum in a superconductor. The results obtained are systematically expounded
for both the ballistic contacts, wherein the second derivatives of the $I-V$
curve in the normal state are proportional to the EPI functions, and
inhomogeneous contacts (with dirty constrictions and clean banks), whose second
derivatives in the normal state are either free of phonon singularities or
weakly pronounced. | cond-mat_supr-con |
Muon-Spin Rotation Spectra in the Mixed Phase of High-T_c
Superconductors : Thermal Fluctuations and Disorder Effects: We study muon-spin rotation (muSR) spectra in the mixed phase of highly
anisotropic layered superconductors, specifically Bi_2+xSr_2-xCaCu_2O_8+delta
(BSCCO), by modeling the fluid and solid phases of pancake vortices using
liquid-state and density functional methods. The role of thermal fluctuations
in causing motional narrowing of muSR lineshapes is quantified in terms of a
first-principles theory of the flux-lattice melting transition. The effects of
random point pinning are investigated using a replica treatment of liquid state
correlations and a replicated density functional theory. Our results indicate
that motional narrowing in the pure system, although substantial, cannot
account for the remarkably small linewidths obtained experimentally at
relatively high fields and low temperatures. We find that satisfactory
agreement with the muSR data for BSCCO in this regime can be obtained through
the ansatz that this ``phase'' is characterized by frozen short-range
positional correlations reflecting the structure of the liquid just above the
melting transition. This proposal is consistent with recent suggestions of a
``pinned liquid'' or ``glassy'' state of pancake vortices in the presence of
pinning disorder. Our results for the high-temperature liquid phase indicate
that measurable linewidths may be obtained in this phase as a consequence of
density inhomogeneities induced by the pinning disorder. The results presented
here comprise a unified, first-principles theoretical treatment of muSR spectra
in highly anisotropic layered superconductors in terms of a controlled set of
approximations. | cond-mat_supr-con |
Tunable mechanically-induced hysteresis in suspended Josephson junctions: The coupling of superconducting systems to mechanical resonators is an
emerging field, with wide reaching implications including high precision
sensing and metrology. Experimental signatures of this coupling have so far
been small, seldom and often reliant on high frequency AC electronics. To
overcome this limitation, in this work we consider a mechanical resonator
suspended between two superconducting contacts to form a suspended Josephson
junction in which the electronic normal- and super-currents can be coupled to
mechanical motion via the Lorentz force due to an external magnetic field. We
show both analytically and numerically that this electro-mechanical coupling
produces unprecedented mechanically-induced hysteresis loops in the junction's
DC I-V characteristic. Firstly, we unveil how this new hysteresis may be
exploited to access a huge mechanically-induced Shapiro-like voltage plateau,
extending over a current range comparable with the junction's critical current.
We then investigate a sudden mechanically-induced retrapping that occurs at
strong coupling. Our analytical treatment provides a clear explanation for the
effects above and allows us to derive simple relationships between the features
in the DC I-V characteristic and the resonance frequency and quality factor of
the mechanical resonator. We stress that our setup requires only DC current
bias and voltage measurements, allowing the activation and detection of
high-frequency mechanical oscillations in state of the art devices and without
the need of any AC equipment. | cond-mat_supr-con |
Strong local moment antiferromagnetic spin fluctuations in V-doped
LiFeAs: We use neutron scattering to study vanadium (hole)-doped LiFe$_{1-x}$V$_x$As.
In the undoped state, LiFeAs exhibits superconductivity at $T_c=18$ K and
transverse incommensurate spin excitations similar to electron overdoped iron
pnictides. Upon vanadium-doping to form LiFe$_{0.955}$V$_{0.045}$, the
transverse incommensurate spin excitations in LiFeAs transform into
longitudinally elongated in a similar fashion as that of potassium (hole) doped
Ba$_{0.7}$K$_{0.3}$Fe$_2$As$_2$, but with dramatically enhanced magnetic
scattering and elimination of superconductivity. This is different from the
suppression of the overall magnetic excitations in hole doped BaFe$_2$As$_2$
and the enhancement of superconductivity near optimal hole doping. These
results are consistent with density function theory plus dynamic mean field
theory calculations, suggesting that vanadium-doping in LiFeAs may induce an
enlarged effective magnetic moment $S_{eff}$ with a spin crossover ground state
arising from the inter-orbital scattering of itinerant electrons. | cond-mat_supr-con |
Crossover of the high-energy spin fluctuations from collective triplons
to localized magnetic excitations in doped Sr14-xCaxCu24O41 cuprate ladders: We studied the magnetic excitations in the quasi-one-dimensional (q-1D)
ladder subsystem of Sr_(14-x) Ca_x Cu_24 O_41(SCCO) using Cu L_3-edge resonant
inelastic X-ray scattering (RIXS). By comparing momentum-resolved RIXS spectra
with (x=12.2) and without (x=0) high Ca content, we track the evolution of the
magnetic excitations from collective two-triplon (2T) excitations (x=0) to
weakly-dispersive gapped modes at an energy of 280 meV (x=12.2). Density matrix
renormalization group (DMRG) calculations of the RIXS response in the doped
ladders suggest that the flat magnetic dispersion and damped excitation profile
observed at x=12.2 originates from enhanced hole localization. This
interpretation is supported by polarization-dependent RIXS measurements, where
we disentangle the spin-conserving {\Delta}S=0 scattering from the predominant
{\Delta}S=1 spin-flip signal in the RIXS spectra. The results show that the
low-energy weight in the {\Delta}S=0 channel is depleted when Sr is replaced by
Ca, consistent with a reduced carrier mobility. Our results demonstrate that
off-ladder impurities can affect both the low-energy magnetic excitations and
superconducting correlations in the CuO_4 plaquettes. Finally, our study
characterizes the magnetic and charge fluctuations in the phase from which
superconductivity emerges in SCCO at elevated pressures. | cond-mat_supr-con |
MgB2 single crystals: high pressure growth and anisotropic properties: Single crystals of MgB2 with a size up to 1.5x0.9x0.2 mm3 have been grown
with a high pressure cubic anvil technique. The crystal growth process is very
peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of
BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9.
Magnetic measurements in fields of 1-5 Oe show sharp transitions to the
superconducting state at 37-38.6 K with width of ~0.5 K. The high quality of
the crystals allowed the accurate determination of magnetic, transport and
optical properties as well as scanning tunnelling spectroscopy (STS) and
decoration studies. Investigations of crystals with torque magnetometry show
that Hc2//c is very low (24 kOe at 15 K), while Hc2//ab increases up to 140 kOe
at 15 K. The upper critical field anisotropy gamma = Hc2//ab/ Hc2//c was found
to be temperature dependent (decreasing from 6 at 15 K to 2.8 at 35 K). The
effective anisotropy gamma_eff, as calculated from reversible torque data near
Tc, is field dependent (increasing roughly linearly from 2 in zero field to 3.7
in 10 kOe). The temperature and field dependence of the anisotropy can be
related to the double gap structure of MgB2 with a large two-dimensional gap
and small three-dimensional gap, the latter being rapidly suppressed in a
magnetic field. Torque magnetometry investigations show a pronounced peak
effect, indicating an order-disorder transition of vortex matter. Decoration
experiments and STS visualise a hexagonal vortex lattice. STS spectra evidence
two gaps (3 meV/6 meV) with direction dependent weight. Magneto-optic
investigations with H//c show a clear signature of the smaller of the two gaps,
disappearing in fields higher than Hc2//c. | cond-mat_supr-con |
Multi-Dimensional Coherent Spectroscopy of Light-Driven States and their
Collective Modes in Multi-Band Superconductors: We present a comprehensive theory of light-controlled multi-band
superconductivity and apply it to predict distinctive signatures of
light-driven superconducting (SC) states in terahertz multi-dimensional
coherent spectroscopy (THz-MDCS) experiments. We first derive gauge-invariant
Maxwell-Bloch equations for multi-band BCS superconductors. For this, we go
beyond previously considered Anderson pseudo-spin precession models to include
quantum transport effects. By calculating the THz-MDCS spectra measured
experimentally, we then identify unique signatures of finite-momentum
Cooper-pairing states that live longer than the laser pulse. These
non-equilibrium SC states are characterized by long-lived canting of Anderson
pseudo-spins. The pseudo-spin oscillators that describe the properties of these
SC states are parametrically driven by both finite-momentum Cooper pairing and
by time oscillations of the order parameter relative phase. We show that such
strong parametric driving leads to drastic changes in the THz-MDCS spectral
shape from the predictions of third-order nonlinear susceptibility
calculations. These spectral changes strongly depend on the
interband-to-intraband interaction ratio and on the collective modes of the
light-driven state. | cond-mat_supr-con |
Electron-phonon superconductivity in $A$Pt$_3$P compounds: from weak to
strong coupling: We study the newly discovered Pt phosphides $A$Pt$_3$P ($A$=Sr, Ca, La) [ T.
Takayama et al. Phys. Rev. Lett. 108, 237001 (2012)] using first-principles
calculations and Migdal-Eliashberg theory. Given the remarkable agreement with
the experiment, we exclude the charge-density wave scenario proposed by
previous first-principles calculations, and give conclusive answers concerning
the superconducting state in these materials. The pairing increases from La to
Ca and Sr due to changes in the electron-phonon matrix elements and
low-frequency phonons. Although we find that all three compounds are well
described by conventional s-wave superconductivity and spin-orbit coupling of
Pt plays a marginal role, we show that it could be possible to tune the
structure from centrosymmetric to noncentrosymmetric opening new perspectives
towards the understanding of unconventional superconductivity. | cond-mat_supr-con |
Giant anisotropic magnetoresistance in Ising superconductor-magnetic
insulator tunnel junctions: Superconductivity and magnetism are generally incompatible because of the
opposing requirement on electron spin alignment. When combined, they produce a
multitude of fascinating phenomena, including unconventional superconductivity
and topological superconductivity. The emergence of two-dimensional (2D)layered
superconducting and magnetic materials that can form nanoscale junctions with
atomically sharp interfaces presents an ideal laboratory to explore new
phenomena from coexisting superconductivity and magnetic ordering. Here we
report tunneling spectroscopy under an in-plane magnetic field of
superconductor-ferromagnet-superconductor (S/F/S) tunnel junctions that are
made of 2D Ising superconductor NbSe2 and ferromagnetic insulator CrBr3. We
observe nearly 100% tunneling anisotropic magnetoresistance (AMR), that is,
difference in tunnel resistance upon changing magnetization direction from
out-of-plane to inplane. The giant tunneling AMR is induced by
superconductivity, particularly, a result of interfacial magnetic exchange
coupling and spin-dependent quasiparticle scattering. We also observe an
intriguing magnetic hysteresis effect in superconducting gap energy and
quasiparticle scattering rate with a critical temperature that is 2 K below the
superconducting transition temperature. Our study paves the path for exploring
superconducting spintronic and unconventional superconductivity in van der
Waals heterostructures. | cond-mat_supr-con |
Properties of a diagonal 2-orbital ladder model of the Fe-pnictide
superconductors: We study a diagonal 2-orbital ladder model of the Fe based superconductors
using the density matrix renormalization group method. At half filling, we find
a close competition between a "spin-striped" state and a non-collinear
"spin-checkerboard" state, as well as significant nematic correlations. Upon
finite hole or electron doping, the dominant pairing correlations are found to
have A$_{1,g}$ ($S-$wave) symmetry. | cond-mat_supr-con |
Shaping graphene superconductivity with nanometer precision: Graphene holds great potential for superconductivity due to its pure
two-dimensional nature, the ability to tune its carrier density through
electrostatic gating, and its unique, relativistic-like electronic properties.
At present, we are still far from controlling and understanding graphene
superconductivity, mainly because the selective introduction of superconducting
properties to graphene is experimentally very challenging. Here, we have
developed a method that enables shaping at will graphene superconductivity
through a precise control of graphene-superconductor junctions. The method
combines the proximity effect with scanning tunnelling microscope (STM)
manipulation capabilities. We first grow Pb nano-islands that locally induce
superconductivity in graphene. Using a STM, Pb nano-islands can be selectively
displaced, over different types of graphene surfaces, with nanometre scale
precision, in any direction, over distances of hundreds of nanometres. This
opens an exciting playground where a large number of predefined
graphene-superconductor hybrid structures can be investigated with atomic scale
precision. To illustrate the potential, we perform a series of experiments,
rationalized by the quasi-classical theory of superconductivity, going from the
fundamental understanding of superconductor-graphene-superconductor
heterostructures to the construction of superconductor nanocorrals, further
used as "portable" experimental probes of local magnetic moments in graphene. | cond-mat_supr-con |
Percolative nature of the transition from 60 K to 90 K phase in
YBa2Cu3O6+d: We have measured the heat capacity of YBa2Cu3O6+d for 0.7<d<0.8 between 1.8
and 300K. It was found that doping dependences of specific heat jump and
temperature of heat capacity jump contradict to the assumption of spatially
homogeneous electronic density. The results suggest that the transition from
60K to 90K phase has a percolative nature and the structure of underdoped 60K
phase can be considered as array of superconducting nanoclusters embedded in
the insulating matrix. | cond-mat_supr-con |
Spin Fluctuation-Induced Superconductivity in κ-BEDT-TTF Compounds: Spin fluctuation-induced superconductivity in quasi-two dimensional organic
compounds, \kappa-BEDT-TTF salts, is investigated within a fluctuation exchange
(FLEX) approximation using a half-filled Hubbard model with a right-angled
isosceles triangular lattice (transfer matrices -\tau, -\tau^\prime), extending
a previous work above T_c. An energy gap of A_2 or (x^2-y^2)-type develops with
decreasing temperature below T_c more rapidly than in the BCS model. The
calculated dynamical susceptibilities enough below T_c show sharp resonance
peaks like those in certain cuprates superconductors. The calculated nuclear
spin-lattice relaxation rate 1/T_1 shows a T^3 behavior below T_c in accordance
with experiment. Estimated values of 1/T_1 are roughly consistent with
experimental results. A prediction is made for the doping concentration
dependence of T_c and the antiferromagnetic and superconductive instability
points are calculated in the U/\tau vs. \tau^\prime/\tau plane. | cond-mat_supr-con |
Controlling the superconducting transition by spin-orbit coupling: Whereas there exists considerable evidence for the conversion of singlet
Cooper pairs into triplet Cooper pairs in the presence of inhomogeneous
magnetic fields, recent theoretical proposals have suggested an alternative way
to exert control over triplet generation: intrinsic spin-orbit coupling in a
homogeneous ferromagnet coupled to a superconductor. Here, we proximity-couple
Nb to an asymmetric Pt/Co/Pt trilayer, which acts as an effective spin-orbit
coupled ferromagnet owing to structural inversion asymmetry. Unconventional
modulation of the superconducting critical temperature as a function of
in-plane and out-of- plane applied magnetic fields suggests the presence of
triplets that can be controlled by the magnetic orientation of a single
homogeneous ferromagnet. Our studies demonstrate for the first time an active
role of spin-orbit coupling in controlling the triplets -- an important step
towards the realization of novel superconducting spintronic devices. | cond-mat_supr-con |
Superconducting density of states and bandstructure at the surface of
the candidate topological superconductor Au2Pb: The electronic bandstructure of Au$_2$Pb has a Dirac cone which gaps when
undergoing a structural transition into a low temperature superconducting
phase. This suggests that the superconducting phase ($T_c=1.1$ K) might hold
topological properties at the surface. Here we make Scanning Tunneling
Microscopy experiments on the surface of superconducting Au$_2$Pb. We measure
the superconducting gap and find a sizeable superconducting density of states
at the Fermi level. We discuss possible origins for this finding in terms of
superconductivity induced into surface states. | cond-mat_supr-con |
Dynamic Phase Transitions in Superconductivity: In this Letter, the dynamic phase transitions of the time-dependent
Ginzburg-Landau equations are analyzed using a newly developed dynamic
transition theory and a new classification scheme of dynamics phase
transitions. First, we demonstrate that there are two type of dynamic
transitions, jump and continuous, dictated by the sign of a nondimensional
parameter R. This parameter is computable, and depends on the material
property, the applied field, and the geometry of domain that the sample
occupies. Second, using the parameter R, precise analytical formulas for
critical domain size, and for critical magnetic fields are derived. | cond-mat_supr-con |
Observation of enhanced coherence in Josephson SQUID cavities using a
hybrid fabrication approach: We study the coherence of flux-tunable Josephson junction resonators made
with two different fabrication processes. In the first process, devices are
made using a single step of evaporation in which the resonator and the
junctions of the SQUID are made at the same time. In the second process,
devices are made with an identical geometry, but in which the resonators are
made from a MoRe superconding layer to which an the junctions are added later
in a second step. To characterize the coherence of the two types of SQUID
cavities, we observe and analyze the quality factor of their resonances as a
function of flux and photon number. Despite a detailed cleaning process applied
during fabrication, the single-step Al devices show significantly worse quality
factor than the hybrid devices, and conclude that a the hybrid technique
provides a much more reliable approach for fabricating high-Q flux-tunable
resonators. | cond-mat_supr-con |
Pre-formed Cooper pairs and Bose-Einstein condensation in cuprate
superconductors: A two-dimensional (2D) assembly of noninteracting, temperature-dependent,
pre-formed Cooper pairs in chemical/thermal equilibrium with unpaired fermions
is examined in a binary boson-fermion statistical model as the Bose-Einstein
condensation (BEC) singularity temperature $T_{c}$ is approached from above.
Compared with BCS theory (which is {\it not} a BEC theory) substantially higher
$T_{c}$'s are obtained without any adjustable parameters, that fall roughly
within the range of empirical $T_{c}$'s for quasi-2D cuprate superconductors. | cond-mat_supr-con |
Magnetic Field Effect on the Supercurrent of an SNS junction: In this paper we study the effect of a Zeeman field on the supercurrent of a
mesoscopic SNS junction. It is shown that the supercurrent suppression is due
to a redistribution of current-carrying states in energy space. A dramatic
consequence is that (part of the) the suppressed supercurrent can be recovered
with a suitable non-equilibrium distribution of quasiparticles. | cond-mat_supr-con |
Spin-excitation anisotropy in the nematic state of detwinned FeSe: The origin of the electronic nematicity in FeSe is one of the most important
unresolved puzzles in the study of iron-based superconductors. In both spin-
and orbital-nematic models, the intrinsic magnetic excitations at
$\mathbf{Q}_1=(1, 0)$ and $\mathbf{Q}_2=(0, 1)$ of twin-free FeSe are expected
to provide decisive criteria for clarifying this issue. Although a
spin-fluctuation anisotropy below 10 meV between $\mathbf{Q}_1$ and
$\mathbf{Q}_2$ has been observed by inelastic neutron scattering around
$T_c\sim 9$ K ($<<T_s\sim 90$ K), it remains unclear whether such an anisotropy
also persists at higher energies and associates with the nematic transition
$T_{\rm s}$. Here we use resonant inelastic x-ray scattering (RIXS) to probe
the high-energy magnetic excitations of uniaxial-strain detwinned FeSe and
{\BFA}. A prominent anisotropy between the magnetic excitations along the $H$
and $K$ directions is found to persist to $\sim200$ meV in FeSe, which is even
more pronounced than the anisotropy of spin waves in {\BFA}. This anisotropy
decreases gradually with increasing temperature and finally vanishes at a
temperature around the nematic transition temperature $T_{\rm s}$. Our results
reveal an unprecedented strong spin-excitation anisotropy with a large energy
scale well above the $d_{xz}/d_{yz}$ orbital splitting, suggesting that the
nematic phase transition is primarily spin-driven. Moreover, the measured
high-energy spin excitations are dispersive and underdamped, which can be
understood from a local-moment perspective. Our findings provide the
much-needed understanding of the mechanism for the nematicity of FeSe and
points to a unified description of the correlation physics across seemingly
distinct classes of Fe-based superconductors. | cond-mat_supr-con |
Self-Consistent Vertex Correction Analysis for Iron-Based
Superconductors: Mechanism of Coulomb-Interaction-Driven Orbital Fluctuations: We study the mechanism of orbital/spin fluctuations due to multiorbital
Coulomb interaction in iron-based superconductors, going beyond the
random-phase-approximation. For this purpose, we develop a self-consistent
vertex correction (SC-VC) method, and find that multiple orbital fluctuations
in addition to spin fluctuations are mutually emphasized by the "multimode
interference effect" described by the VC. Then, both the antiferro-orbital and
ferro-orbital (=nematic) fluctuations simultaneously develop for $J/U \sim
0.1$, both of which contribute to the s-wave superconductivity. Especially, the
ferro-orbital fluctuations give the orthorhombic structure transition as well
as the softening of shear modulus $C_{66}$. | cond-mat_supr-con |
Magnetic flux pinning in superconductors with hyperbolic-tesselation
arrays of pinning sites: We study magnetic flux interacting with arrays of pinning sites (APS) placed
on vertices of hyperbolic tesselations (HT). We show that, due to the gradient
in the density of pinning sites, HT APS are capable of trapping vortices for a
broad range of applied magnetic fluxes. Thus, the penetration of magnetic field
in HT APS is essentially different from the usual scenario predicted by the
Bean model. We demonstrate that, due to the enhanced asymmetry of the surface
barrier for vortex entry and exit, this HT APS could be used as a "capacitor"
to store magnetic flux. | cond-mat_supr-con |
Topological invariants beyond symmetry indicators: Boundary diagnostics
for twofold rotationally symmetric superconductors: Topological crystalline superconductors are known to have possible
higher-order topology, which results in Majorana modes on $d-2$ or
lower-dimensional boundaries. Given the rich possibilities of boundary
signatures, it is desirable to have topological invariants that can predict the
type of Majorana modes from band structures. Although symmetry indicators, a
type of invariant that depends only on the band data at high-symmetry points,
have been proposed for certain crystalline superconductors, there exist
symmetry classes in which symmetry indicators fail to distinguish
superconductors with different Majorana boundaries. Here, we systematically
obtain topological invariants for an example of this kind, two-dimensional
time-reversal symmetric superconductors with twofold rotational symmetry $C_2$.
First, we show that the nontrivial topology is independent of band data on the
high-symmetry points by conducting a momentum-space classification study. Then,
from the resulting K groups, we derive calculable expressions for four
$\mathbb{Z}_2$ invariants defined on high-symmetry lines or general points in
the Brillouin zone. Finally, together with a real-space classification study,
we establish the bulk-boundary correspondence and show that the four
$\mathbb{Z}_2$ invariants can predict Majorana boundary types from band
structures. Our proposed invariants can fuel practical material searches for
$C_2$-symmetric topological superconductors featuring Majorana edge and corner
modes. | cond-mat_supr-con |
Evolution of correlation strength in KxFe(2-y)Se2 superconductor doped
with S: We report the evolution of thermal transport properties of iron-based
superconductor K$_x$Fe$_{2-y}$Se$_2$ with sulfur substitution at Se sites.
Sulfur doping suppresses the superconducting $T_c$ as well as the Seebeck
coefficient. The Seebeck coefficient of all crystals in the low temperature
range can be described very well by diffusive thermoelectric response model.
The zero-temperature extrapolated value of Seebeck coefficient divided by
temperature $S/T$ gradually decreases from $-0.48 \mu V/K^2$ to a very small
value $\sim$ 0.03 $\mu$V/K$^2$ where $T_c$ is completely suppressed. The normal
state electron Sommerfeld term ($\gamma_n$) of specific heat also decreases
with the increase of sulfur content. The dcrease of $S/T$ and $\gamma_n$
reflects a suppression of the density of states at the Fermi energy, or a
change in the Fermi surface that would induce the suppression of correlation
strength. | cond-mat_supr-con |
Superconducting transition temperatures of the elements related to
elastic constants: For a given crystal structure, say body-centred-cubic, the many-body
Hamiltonian in which nuclear and electron motions are to be treated from the
outset on the same footing, has parameters, for the elements, which can be
classified as (i) atomic mass M, (ii) atomic number Z, characterizing the
external potential in which electrons move, and (iii) bcc lattice spacing, or
equivalently one can utilize atomic volume, Omega. Since the thermodynamic
quantities can be determined from H, we conclude that Tc, the superconducting
transition temperature, when it is non-zero, may be formally expressed as Tc =
Tc^(M) (Z, Omega). One piece of evidence in support is that, in an atomic
number vs atomic volume graph, the superconducting elements lie in a well
defined region. Two other relevant points are that (a) Tc is related by BCS
theory, though not simply, to the Debye temperature, which in turn is
calculable from the elastic constants C_{11}, C_{12}, and C_{44}, the atomic
weight and the atomic volume, and (b) Tc for five bcc transition metals is
linear in the Cauchy deviation C* = (C_{12} - C_{44})/(C_{12} + C_{44}).
Finally, via elastic constants, mass density and atomic volume, a correlation
between C* and the Debye temperature is established for the five bcc transition
elements. | cond-mat_supr-con |
Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions: We report magnetic and electrical measurements of Nb Josephson junctions with
strongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these
materials show multiple oscillations of critical current with barrier thickness
implying repeated 0-pi phase-transitions in the superconducting order
parameter. We show in particular that the Co barrier devices can be accurately
modelled using existing clean limit theories and so that, despite the high
exchange energy (309 meV), the large IcRN value in the pi-state means Co
barriers are ideally suited to the practical development of superconducting
pi-shift devices. | cond-mat_supr-con |
Topological states in normal and superconducting $p$-wave chains: We study a two-band model of fermions in a 1d chain with an antisymmetric
hybridization that breaks inversion symmetry. We find that for certain values
of its parameters, the $sp$-chain maps formally into a $p$-wave superconducting
chain, the archetypical 1d system exhibiting Majorana fermions. The
eigenspectra, including the existence of zero energy modes in the topological
phase, agree for both models. The end states too share several similarities in
both models, such as the behavior of the localization length, the non-trivial
topological index and robustness to disorder. However, we show by mapping the
$s$- and $p$- fermions to two copies of Majoranas, that the excitations in the
ends of a finite $sp$ chain are indeed conventional fermions though endowed
with protected topological properties. Our results are obtained by a scattering
approach in a semi-infinite chain with an edge defect treated within the
$T$-matrix approximation. We augment the analytical results with exact
numerical diagonalization that allow us to extend our results to arbitrary
parameters and also to disordered systems. | cond-mat_supr-con |
Studies on Fabrication of Ag/HgBaCaCuO/CdSe Heterostructures by
Pulse-Electrodeposition Route: Metal/superconductor/semiconductor (Ag/HgBaCaCuO/CdSe) heterostructures have
been successfully fabricated using pulse-electrodeposition technique. The
electrochemical parameters are optimized and diffusion free growth of CdSe onto
Ag/HgBaCaCuO was obtained by employing under-potential deposition and by
studying nucleation and growth mechanism during deposition. The
heterostructures are characterized by X-ray diffraction (XRD), full-width at
half-maximum (FWHM), scanning electron microscopy (SEM) studies and low
temperature four probe electrical resistivity measurements. After the
deposition of CdSe the critical transition temperature of HgBaCaCuO films was
found be increased from 115 K with Jc = 1.7 x 103 A/cm2 to 117.2 K with Jc =
1.91 x 103 A/cm2. When the heterostructure was irradiated with red He-Ne laser
(2 mW), the Tc was further enhanced to 120.3 K with Jc = 3.7 x 103 A/cm2. This
increase in superconducting parameters of HgBaCaCuO in Ag/ HgBaCaCuO/CdSe
heterostructure has been explained at length in this paper.
Keywords. Electrodeposition; Hg-based cuprate; semiconductor;
heterostructures; electrical properties.
PACS Nos 81.15.Pq; 74.72.Gr; 78.40.Fy; 84.37; 73.40 *E-mail:
[email protected], [email protected] | cond-mat_supr-con |
Effects of Disorder on Superconductivity of Systems with Coexisting
Itinerant Electrons and Local Pairs: We study the influence of diagonal disorder (random site energy) of local
pair (LP) site energies on the superconducting properties of a system of
coexisting local pairs and itinerant electrons described by the (hard-core)
boson-fermion model. Our analysis shows that the properties of such a model
with s-wave pairing can be very strongly affected by the diagonal disorder in
LP subsystem (the randomness of the LP site energies). This is in contrast with
the conventional s-wave BCS superconductors, which according to the Anderson's
theorem are rather insensitive to the diagonal disorder (i.e. to nonmagnetic
impurities). It has been found that the disorder effects depend in a crucial
way on the total particle concentration n and the LP level position DELTA_o and
depending on the parameters the system can exhibit various types of
superconducting behaviour, including the LP-like, intermediate (MIXED)and the
'BCS'-like. In the extended range of {n,DELTA_o} the superconducting ordering
is suppressed by the randomness of the LP site energies and the increasing
disorder induces a changeover from the MIXEDlike behaviour to the BCS-like one,
connected with abrupt reduction of T_c and energy gap to zero. However, there
also exist a definite range of {n,DELTA_o} in which the increasing disorder has
a quite different effect: namely it can substantially enhance T_c or even lead
to the phenomenon which can be called disorder induced superconductivity.
Another interesting effect is a possibility of a disorder induced bound pair
formation of itinerant electrons, connected with the change-over to the LP-like
regime. | cond-mat_supr-con |
Thermodynamic phase diagram and phase competition in BaFe2(As1-xPx)2
studied by thermal expansion: High-resolution thermal-expansion and specific-heat measurements were
performed on single crystalline BaFe2(As1-xPx)2 (0 < x < 0.33, x = 1). The
observation of clear anomalies allows to establish the thermodynamic phase
diagram which features a small coexistence region of SDW and superconductivity
with a steep rise of Tc on the underdoped side. Samples that undergo the
tetragonal-orthorhombic structural transition are detwinned in situ, and the
response of the sample length to the magneto-structural and superconducting
transitions is studied for all three crystallographic directions. It is shown
that a reduction of the magnetic order by superconductivity is reflected in all
lattice parameters. On the overdoped side, superconductivity affects the
lattice parameters in much the same way as the SDW on the underdoped side,
suggesting an intimate relation between the two types of order. Moreover, the
uniaxial pressure derivatives of Tc are calculated using the Ehrenfest relation
and are found to be large and anisotropic. A correspondence between
substitution and uniaxial pressure is established, i.e., uniaxial pressure
along the b-axis (c-axis) corresponds to a decrease (increase) of the P
content. By studying the electronic contribution to the thermal expansion we
find evidence for a maximum of the electronic density of states at optimal
doping. | cond-mat_supr-con |
Pairing competition in a quasi-one-dimensional model of organic
superconductors (TMTSF)$_{2}X$ in magnetic field: We microscopically study the effect of the magnetic field (Zeeman splitting)
on the superconducting state in a model for quasi-one-dimensional organic
superconductors (TMTSF)$_{2}X$. We investigate the competition between spin
singlet and spin triplet pairings and the
Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) state by random phase approximation.
While we studied the competition by comparison with the eigenvalue of the gap
equation at a fixed temperature in our previous study (Phys. Rev. Lett.
\textbf{102} (2009) 016403), here we obtain both the $T_c$ for each pairing
state and a phase diagram in the $T$(temperature)-$h_z$(field)-$V_y$(strength
of the charge fluctuation) space. The phase diagram shows that consecutive
transitions from singlet pairing to the FFLO state and further to $S_z=1$
triplet pairing can occur upon increasing the magnetic field when $2k_{F}$
charge fluctuations coexist with $2k_{F}$ spin fluctuations. In the FFLO state,
the singlet d-wave and $S_{z}=0$ triplet $f$-wave components are strongly mixed
especially when the charge fluctuations are strong. | cond-mat_supr-con |
Two-dimensional topological superconductivity candidate in van der Waals
layered material: Two-dimensional (2D) topological superconductors are highly desired because
they not only offer opportunities for exploring novel exotic quantum physics,
but also possesses potential applications in quantum computation. However,
there are few reports on 2D superconductors, let alone topological
superconductors. Here, we find a 2D monolayer W$_2$N$_3$, which can be
exfoliated from its real van der Waals bulk material with much lower
exfoliation energy than MoS$_2$, to be a topological metal with exotic
topological states at different energy level. Due to the Van Hove
singularities, the density of states near Fermi level are high, making the
monolayer a compensate metal. Moreover, the monolayer W$_2$N$_3$ is unveiled to
be a superconductor with the superconducting transition temperature Tc $\sim$
22 K and a superconducting gap of about 5 meV based on the anisotropic
Migdal-Eliashberg formalism, arising from the strong electron-phonon coupling
around the $\Gamma$ point. Because of the strong electron and lattice coupling,
the monolayer displays a non-Fermi liquid behavior in its normal states at
temperatures lower than 80 K, where the specific heat exhibit T$^3$ behavior
and the Wiedemann-Franz law dramatically violates. Our findings not only
provide the platform to study the emergent phenomena in 2D topological
superconductors, but also open a door to discover more 2D high-temperature
topological superconductors in van der Waals materials. | cond-mat_supr-con |
Asymmetric Little-Parks Oscillations in Full Shell Double Nanowires: Little-Parks oscillations of a hollow superconducting cylinder are of
interest for flux-driven topological superconductivity in single Rashba
nanowires. The oscillations are typically symmetric in the orientation of the
applied magnetic flux. Using double InAs nanowires coated by an epitaxial
superconducting Al shell which, despite the non-centro-symmetric geometry,
behaves effectively as one hollow cylinder, we demonstrate that a small
misalignment of the applied parallel field with respect to the axis of the
nanowires can produce field-asymmetric Little-Parks oscillations. These are
revealed by the simultaneous application of a magnetic field perpendicular to
the misaligned parallel field direction. The asymmetry occurs in both the
destructive regime, in which superconductivity is destroyed for half-integer
quanta of flux through the shell, and in the non-destructive regime, where
superconductivity is depressed but not fully destroyed at these flux values. | cond-mat_supr-con |
Detection of the phase shift from a single quantized superconducting
vortex: An Abrikosov vortex in a superconductor carries a flux quantum, Phi_0 =
hc/2e, localized at its center, but induces a global 2pi phase rotation in the
superconducting condensate. This long-range gauge field outside the area
pierced by a magnetic field is due to the Aharonov-Bohm effect, which is a
non-classical phenomenon that illustrates the significance of potentials rather
than forces in quantum mechanics. In the London gauge, the phase of the
condensate is given by the polar angle around the vortex. Here we raise the
question whether this phase shift could be detected by means of Cooper pair
interferometry using Josephson junctions as phase-sensitive detectors. We
introduce a single Abrikosov vortex into a superconducting lead with a detector
junction made at the edge of the lead. We observe that the vortex induces a
Josephson phase shift equal to the polar angle of the vortex within the
junction length. When the vortex is close to the junction it induces a pi-step
in the Josephson phase difference, leading to a controllable and reversible
switching of the junction into the 0 - pi state. This in turn results in an
unusual Phi_0/2 quantization of the flux in the junction. The vortex may hence
act as a tunable "phase battery" for quantum electronics. | cond-mat_supr-con |
Superconducting fluctuations in the reversible magnetization of the
iron-pnictide $Ba_{1-x}K_xFe_2As_2$: We report on isofield magnetization curves obtained as a function of
temperature in two single crystals of $Ba_{1-x}K_xFe_2As_2$ with
superconducting transition temperature $T_c$=28K and 32.7 K. Results obtained
for fields above 20 kOe show a well defined rounding effect on the reversible
region extending 1-3 K above $T_c(H)$ masking the transition. This rounding
appears to be due to three-dimensional critical fluctuations, as the higher
field curves obey a well know scaling law for this type of critical
fluctuations. We also analysed the asymptotic behavior of $\sqrt M$vs.T curves
in the reversible region which probes the shape of the gap near $T_c(H)$.
Results of the analysis suggests that phase fluctuations are important in
$Ba_{1-x}K_xFe_2As_2$ which is consistent with nodes in the gap. | cond-mat_supr-con |
Synthesis and physical characterization of superconductivity-magnetism
crossover compound RuSr2EuCeCu2O10: We carefully studied the nonsuperconducting sample of the
magneto-superconducting RuSr2(Eu1-xCex)Cu2O10 series with composition
RuSr2EuCeCu2O10. This compound seems to exhibit a complex magnetic state as
revealed by host of techniques like resistivity, thermopower, magnetic
susceptibility and MR measurements. | cond-mat_supr-con |
Electron and Superconducting Properties of the AFeAs (A= Li, Na) Family
Alkali-Metal Pnictides: Current Stage of the Research (mini-review): The review is focused on one of the most exotic families of iron-based
superconductors belonging to the AFeAs structural type, where A is alkali
metal. We briefly concern physical and electron properties of the typical
members of this family, LiFeAs and NaFeAs, discuss the theoretical models
describing the multiple-gap superconducting state, and the experimental data
available in literature. As well, we specify the main unsolved problems, that
seem crucial for both the AFeAs family and for iron-based superconductors in
general. | cond-mat_supr-con |
The role of domain walls on the vortex creep dynamics in unconventional
superconducors: We investigate the influence of domain walls on the vortex dynamics in
superconductors with multi-component order parameters. We show that, due to
their complex structure domain walls can carry vortices with fractional flux
quanta. The decay of conventional vortices into fractional ones on domain walls
is examined. This decay presents an extraordinarily strong pinning mechanism
for vortices and turns domain walls occupied with pinned fractional vortices
into efficient barriers for the vortex motion. Therefore, domain walls can act
as fences for the flux flow, preventing the decay of the remnant magnetic flux
enclosed by them. Furthermore, the consequences of this property of domain
walls on the vortex dynamics are discussed in connection with observed noise in
the hysteresis cycle, using the Bean model of the critical vortex state. Based
on this picture experimental data in the unconventional superconductors
UPt$_3$, U$_{1-x}$Th$_x$Be$_{13}$ and Sr$_2$RuO$_4$ are interpreted. | cond-mat_supr-con |
Computational Materials Design for High Critical Temperature
Superconductivity in Hole-Doped Delafossite CuAlO$_2$: Transparent
Superconductors: We have calculated the superconducting critical temperature $T_c$ of
hole-doped delafossite CuAlO$_2$ based on the first-principles calculations.
According our calculation, $0.2\sim0.3$ hole-doped CuAlO$_2$ can become a
phonon-mediated high-$T_c$ superconductor with $T_c\sim50$ K. In the hole-doped
CuAlO$_2$, the A$_1$L$_1$ phonon mode that stretches O-Cu-O dumbbell has a
strong electron-phonon interaction with electrons of the flat band in Cu
3d$_{3z^2-r^2}$ and the O 2p$_z$ anti-bonding $\pi$-band. | cond-mat_supr-con |
The spin exchange interaction effect on Tc equation of anisotropic
impure superconductors: We study the influence of spin exchange interaction of impurity scattering on
critical temperature of anisotropic impure superconductors. The model of random
nonmagnetic and magnetic impurity are revised to cover the effect of spin
exchange interaction . The sign of magnitude of the second-order Born
scattering have been changed after consideration the spin exchange interaction
effect that also effect to form of Tc equation. We can get the general Tc
equation that can be described anisotropic impure superconductors well and
cover all model done before. | cond-mat_supr-con |
Vortex dynamics and losses due to pinning: Dissipation from trapped
magnetic flux in resonant superconducting radio-frequency cavities: We use a model of vortex dynamics and collective weak pinning theory to study
the residual dissipation due to trapped magnetic flux in a dirty
superconductor. Using simple estimates, approximate analytical calculations,
and numerical simulations, we make predictions and comparisons with experiments
performed in CERN and Cornell on resonant superconducting radio-frequency NbCu,
doped-Nb and Nb$_3$Sn cavities. We invoke hysteretic losses originating in a
rugged pinning potential landscape to explain the linear behavior of the
sensitivity of the residual resistance to trapped magnetic flux as a function
of the amplitude of the radio-frequency field. Our calculations also predict
and describe the crossover from hysteretic-dominated to viscous-dominated
regimes of dissipation. We propose simple formulas describing power losses and
crossover behavior, which can be used to guide the tuning of material
parameters to optimize cavity performance. | cond-mat_supr-con |
Energy dependence of the spin excitation anisotropy in uniaxial-strained
BaFe1.9Ni0.1As2: We use inelastic neutron scattering to study the temperature and energy
dependence of the spin excitation anisotropy in uniaxial-strained
electron-doped iron pnictide BaFe$_{1.9}$Ni$_{0.1}$As$_2$ near optimal
superconductivity ($T_c=20$ K). Our work has been motivated by the observation
of in-plane resistivity anisotropy in the paramagnetic tetragonal phase of
electron-underdoped iron pnictides under uniaxial pressure, which has been
attributed to a spin-driven Ising-nematic state or orbital ordering. Here we
show that the spin excitation anisotropy, a signature of the spin-driven
Ising-nematic phase, exists for energies below $\sim$60 meV in
uniaxial-strained BaFe$_{1.9}$Ni$_{0.1}$As$_2$. Since this energy scale is
considerably larger than the energy splitting of the $d_{xz}$ and $d_{yz}$
bands of uniaxial-strained Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ near optimal
superconductivity, spin Ising-nematic correlations is likely the driving force
for the resistivity anisotropy and associated electronic nematic correlations. | cond-mat_supr-con |
Dynamic cluster quantum Monte Carlo simulations of a two-dimensional
Hubbard model with stripe-like charge density wave modulations: Interplay
between inhomogeneity and superconductivity: Using dynamic cluster quantum Monte Carlo simulations, we study the
superconducting behavior of a 1/8 doped two-dimensional Hubbard model with
imposed uni-directional stripe-like charge density wave modulation. We find a
significant increase of the pairing correlations and critical temperature
relative to the homogeneous system when the modulation length-scale is
sufficiently large. With a separable form of the irreducible particle-particle
vertex, we show that optimized superconductivity is obtained for moderate
modulation strength due to a delicate balance between the modulation enhanced
pairing interaction, and a concomitant suppression of the bare
particle-particle excitations by a modulation reduction of the quasi-particle
weight. | cond-mat_supr-con |
Scanning SQUID study of ferromagnetism and superconductivity in
infinite-layer nickelates: Infinite-layer nickelates $R_{1-x}$Sr$_{x}$NiO$_{2}$ ($R$ = La, Pr, Nd) are a
class of superconductors with structural similarities to cuprates. Although
long-range antiferromagnetic order has not been observed for these materials,
magnetic effects such as antiferromagnetic spin fluctuations and spin-glass
behavior have been reported. Different experiments have drawn different
conclusions about whether the pairing symmetry is $s$- or $d$ wave. In this
paper, we applied a scanning superconducting quantum interference device
(SQUID) to probe the magnetic behavior of film samples of three infinite-layer
nickelates (La$_{0.85}$Sr$_{0.15}$NiO$_2$, Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, and
Nd$_{0.775}$Sr$_{0.225}$NiO$_2$) grown on SrTiO$_3$ (STO), each with a nominal
thickness of 20 unit cells. In all three films, we observed a ferromagnetic
background. We also measured the magnetic susceptibility above the
superconducting critical temperature in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ and
La$_{0.85}$Sr$_{0.15}$NiO$_2$ and identified a non-Curie-Weiss dynamic
susceptibility. Both magnetic features are likely due to NiO$_x$ nanoparticles.
Additionally, we investigated superconductivity in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$
and Nd$_{0.775}$Sr$_{0.225}$NiO$_2$, which exhibited inhomogeneous diamagnetic
screening. The superfluid density inferred from the diamagnetic susceptibility
in relatively homogeneous regions shows $T$-linear behavior in both samples.
Finally, we observed superconducting vortices in
Nd$_{0.775}$Sr$_{0.225}$NiO$_2$. We determined a Pearl length of 330 $\upmu$m
for Nd$_{0.775}$Sr$_{0.225}$NiO$_2$ at 300 mK both from the strength of the
diamagnetism and from the size and shape of the vortices. These results
highlight the importance of considering NiO$_x$ particles when interpreting
experimental results for these films. | cond-mat_supr-con |
Comparative Study of Dense Bulk MgB$_2$ Materials Prepared by Different
Methods: We report on the results of a comparative investigation of highly dense bulk
MgB$_2$ samples prepared by three methods: (i) hot deformation; (ii) high
pressure sintering; and (iii) mechanical alloying of Mg and B powders with
subsequent hot compaction. All types of samples were studied by
ac-susceptibility, dc-magnetization and resistivity measurements in magnetic
fields up to $\mu_0H=160$ kOe. A small but distinct anisotropy of the upper
critical field $H_{c2}^{a,b}/H_{c2}^{c}\sim1.2$ connected with some texture of
MgB$_2$ grains was found for the hot deformed samples. The samples prepared by
high pressure sintering as well as by mechanical alloying show improved
superconducting properties, including high upper critical fields $H_{c2}$
($\mu_0H_{c2}(0)\sim23$ T), irreversibility fields $H_{irr}$ which are strongly
shifted towards higher values $H_{irr}(T)\sim0.8H_{c2}(T)$ and high critical
current $J_c$ ($J_c=10^5$ A/cm$^2$ at 20 K and 1 T). | cond-mat_supr-con |
Generating Giant Spin Currents Using Nodal Topological Superconductors: In this work, we show that a giant spin current can be injected into a nodal
topological superconductor, using a normal paramagnetic lead, through a large
number of zero energy Majorana fermions at the superconductor edge. The giant
spin current is caused by the selective equal spin Andreev reflections (SESAR)
induced by Majorana fermions. In each SESAR event, a pair of electrons with
certain spin polarization are injected into the nodal topological
superconductor, even though the pairing in the bulk of the nodal superconductor
is spin-singlet s-wave. We further explain the origin of the spin current by
showing that the pairing correlation at the edge of a nodal topological
superconductor is predominantly equal spin-triplet at zero energy. The
experimental consequences of SESAR in nodal topological superconductors are
discussed. | cond-mat_supr-con |
Characterization of superconducting nanowire single-photon detector with
artificial constrictions: Statistical studies on the performance of different superconducting nanowire
single-photon detectors (SNSPDs) on one chip suggested that random
constrictions existed in the nanowire that were barely registered by scanning
electron microscopy. With the aid of advanced e-beam lithography, artificial
geometric constrictions were fabricated on SNSPDs as well as single nanowires.
In this way, we studied the influence of artificial constrictions on SNSPDs in
a straight forward manner. By introducing artificial constrictions with
different wire widths in single nanowires, we concluded that the dark counts of
SNSPDs originate from a single constriction. Further introducing artificial
constrictions in SNSPDs, we studied the relationship between detection
efficiency and kinetic inductance and the bias current, confirming the
hypothesis that constrictions exist in SNSPDs. | cond-mat_supr-con |
Angular Position of Nodes in the Superconducting Gap of Quasi-2D
Heavy-Fermion Superconductor CeCoIn_5: The thermal conductivity of the heavy-fermion superconductor CeCoIn_5 has
been studied in a magnetic field rotating within the 2D planes. A clear
fourfold symmetry of the thermal conductivity which is characteristic of a
superconducting gap with nodes along the (+-pi,+-pi)-directions is resolved.
The thermal conductivity measurement also reveals a first order transition at
H_c2, indicating a Pauli limited superconducting state. These results indicate
that the symmetry most likely belongs to d_{x^2-y^2}, implying that the
anisotropic antiferromagnetic fluctuation is relevant to the superconductivity. | cond-mat_supr-con |
A model for critical current effects in point-contact Andreev-reflection
spectroscopy: It is well known that point-contact Andreev reflection spectroscopy provides
reliable measurements of the energy gap(s) in a superconductor when the contact
is in the ballistic or nearly-ballistic regime. However, especially when the
mean free path of the material under study is small, obtaining ballistic
contacts can be a major challenge. One of the signatures of a Maxwell
contribution to the contact resistance is the presence of "dips" in the
differential conductance, associated to the sudden appearance of a Maxwell
term, in turn due to the attainment of the critical current of the material in
the contact region. Here we show that, using a proper model for the $R(I)$ of
the material under study, it is possible to fit the experimental curves
(without the need of normalization) obtaining the correct values of the gap
amplitudes even in the presence of such dips, as well as the temperature
dependence of the critical current in the contact. We present a test of the
procedure in the case of Andreev-reflection spectra in
Mg$_{0.85}$Al$_{0.15}$B$_2$ single crystals. | cond-mat_supr-con |
Third harmonic generation from collective modes in disordered
superconductors: Recent experiments with strong THz fields in both conventional and
unconventional superconductors have clearly evidenced a marked third-harmonic
generation below the superconducting temperature $T_c$. Its interpretation
challenged substantial theoretical work aimed at establishing the relative
efficiency of quasiparticle excitations and collective modes in triggering such
a resonant response. Here we compute the non-linear current by implementing a
time-dependent Bogoljubov de-Gennes approach, with the twofold aim to account
non-perturbatively for the effect of local disorder, and to include the
contribution of all collective modes, i.e. superconducting amplitude (Higgs)
and phase fluctuations, and charge fluctuations. We show that, in agreement
with previous work, already at small disorder the quasiparticle response is
dominated by paramagnetic effects. We further demonstrate that paramagnetic
processes mediate also the response of all collective modes, with a substantial
contribution of charge/phase fluctuations. These processes, which have been
overlooked so far, turn out to dominate the third-order current at strong
disorder. In addition, we show that disorder strongly influences the
polarization dependence of the non-linear response, with a marked difference
between the clean and the disordered case. Our results are particularly
relevant for recent experiments in cuprates, whose band structure is in a first
approximation reproduced by our lattice model. | cond-mat_supr-con |
Effect of La Doping on the Crystal Structure, Electric, Magnetic and
Morphologic Properties of the BSCCO System: Studies of the doping process can provide a better understanding of the
superconducting mechanisms in cuprous oxide materials. In this work, we studied
the doping effects on the crystal structure, electric, morphologic and magnetic
properties of the BSCCO system with the nominal composition
$Bi_{1.6}Pb_{0.4}Sr_{(2-x)}RE_xCa_2Cu_3O_{(10-{\delta})}$. Here, the rare earth
element (RE) was replaced by La in the sites of Sr. The x was ranged from 0.0
to 2.0, in steps of 0.5. The samples were prepared based on Pechini's method.
The resulting powder was pressed at room temperature and the pellets were
submitted to several heat treatments. The characterizations confirm the La in
the sites of Sr however, the superconducting properties of the sample were not
improved. | cond-mat_supr-con |
Coupling between dynamic magnetic and charge-order correlations in the
cuprate superconductor Nd$_{2-x}$Ce$_{x}$CuO$_4$: Charge order has now been observed in several cuprate high-temperature
superconductors. We report a resonant inelastic x-ray scattering experiment on
the electron-doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_4$ that demonstrates the
existence of dynamic correlations at the charge order wave vector. Upon cooling
we observe a softening in the electronic response, which has been predicted to
occur for a d-wave charge order in electron-doped cuprates. At low
temperatures, the energy range of these excitations coincides with that of the
dispersive magnetic modes known as paramagnons. Furthermore, measurements where
the polarization of the scattered photon is resolved indicate that the dynamic
response at the charge order wave vector primarily involves spin-flip
excitations. Overall, our findings indicate a coupling between dynamic magnetic
and charge-order correlations in the cuprates. | cond-mat_supr-con |
Pair correlations of the hybridized orbitals in a ladder model for the
bilayer nickelate La$_3$Ni$_2$O$_7$: To clarify the nature of high-temperature superconductivity in the bilayer
nickelate La$_3$Ni$_2$O$_7$ under pressure, we investigate, using the
density-matrix renormalization group method, the pair correlations in the
two-orbital $t$-$J$ ladder model. While the interchain-intraorbital pair
correlations exhibit a slow power-law decay in both orbitals, the interorbital
pair correlation also develops due to the hybridization via interorbital
hopping. These intra and interorbital pair correlations are enhanced by Hund's
coupling, but more importantly, the interorbital pairing correlation develops
even without Hund's coupling, where apparent interorbital pairing glue is
absent. Our finding suggests that the hybridized two-orbital picture precisely
describes superconductivity in the bilayer nickelate. | cond-mat_supr-con |
Pairing state in the rutheno-cuprate superconductor RuSr2GdCu2O8: A
point contact Andreev Reflection Spectroscopy study: The results of Point Contact Andreev Reflection
Spectroscopy on polycrystalline RuSr$_2$GdCu$_2$O$_8$ pellets are presented.
The wide variety of the measured spectra are all explained in terms of a
modified BTK model considering a \emph{d-wave} symmetry of the superconducting
order parameter. Remarkably low values of the energy gap $\Delta=(2.8\pm
0.2)meV$ and of the $2\Delta/k_BT_c\simeq 2$ ratio are inferred. From the
temperature evolution of the $dI/dV$ vs $V$ characteristics we extract a
sublinear temperature dependence of the superconducting energy gap. The
magnetic field dependence of the conductance spectra at low temperatures is
also reported. From the $\Delta$ vs $H$ evolution, a critical magnetic field
$H_{c_2}\simeq 30 T$ is inferred. To properly explain the curves showing
gap-like features at higher voltages, we consider the formation of a Josephson
junction in series with the Point Contact junction, as a consequence of the
granularity of the sample. | cond-mat_supr-con |
Epitaxial growth and electronic structure of Ruddlesden-Popper
nickelates ($ \mathrm{La}_{n+1}\mathrm{Ni}_{n}\mathrm{O}_{3n+1}, n=1-5 $): We report the epitaxial growth of Ruddlesden-Popper nickelates, $
\mathrm{La}_{n+1}\mathrm{Ni}_{n}\mathrm{O}_{3n+1} $, with $ n $ up to 5 by
reactive molecular beam epitaxy (MBE). X-ray diffractions indicate high
crystalline quality of these films and transport measurements show strong
dependence on the $ n $ values. Angle-resolved photoemission spectroscopy
(ARPES) reveals the electronic structure of $
\mathrm{La}_{5}\mathrm{Ni}_{4}\mathrm{O}_{13} $, showing a large hole-like
pocket centered around the Brillouin zone corner with a $ (\pi, \pi) $
back-folded copy. | cond-mat_supr-con |
Phase segregation of superconductivity and ferromagnetism at
LaAlO$_3$/SrTiO$_3$ interface: The highly conductive two-dimensional electron gas formed at the interface
between insulating SrTiO$_3$ and LaAlO$_3$ shows low-temperature
superconductivity coexisting with inhomogeneous ferromagnetism. The Rashba
spin-orbit interaction with in-plane Zeeman field of the system favors $p_x \pm
i p_y$-wave superconductivity at finite momentum. Owing to the intrinsic
disorder at the interface, the role of spatial inhomogeneity on the
superconducting and ferromagnetic states becomes important. We find that for
strong disorder, the system breaks up into mutually excluded regions of
superconductivity and ferromagnetism. This inhomogeneity-driven electronic
phase separation accounts for the unusual coexistence of superconductivity and
ferromagnetism observed at the interface. | cond-mat_supr-con |
A splitting integrator for the BCS equations of superconductivity: The BCS equations are the centerpiece of the microscopic description of
superconductivity. Their space discretization yields a system of coupled
ordinary differential equations. In this work, we come up with fast time
evolution schemes based on a splitting approach. One of the schemes only
requires basic operations. For the physically important case of the BCS
equations for a contact interaction potential, the computational cost of the
schemes increases only linearly with the dimension of the space discretization.
Their accuracy is demonstrated in extensive numerical experiments. These
experiments also show that the physical energy of the system is preserve up to
very small errors. | cond-mat_supr-con |
Anomalous optical absorption in overdoped cuprates near the
charge-ordering instability: We propose an interpretation for the hump observed in the optical
conductivity at or below a few hundreds of cm$^{-1}$, in overdoped cuprates
like the electron-doped Nd_{2-x}Ce_xCuO_{4-y} at x\gtrsim 0.15 and the
hole-doped Bi_2Sr_2CuO_6 and La_{2-x}Sr_xCuO_4. This interpretation is based on
the direct excitation of charge collective modes, which become nearly critical
in the proximity to a charge-ordering instability. The nearly critical
character of these excitations entails a peculiar temperature dependence and a
pseudo-scaling form of the lineshapes, which are in agreement with the
experimental data. | cond-mat_supr-con |
The excitation spectrum of mesoscopic proximity structures: We investigate one aspect of the proximity effect, viz., the local density of
states of a superconductor-normal metal sandwich. In contrast to earlier work,
we allow for the presence of an arbitrary concentration of impurities in the
structure. The superconductor induces a gap in the normal metal spectrum that
is proportional to the inverse of the elastic mean free path l_N for rather
clean systems. For a mean free path much shorter than the thickness of the
normal metal, we find a gap size proportional to l_N that approaches the
behavior predicted by the Usadel equation (diffusive limit). We also discuss
the influence of interface and surface roughness, the consequences of a
non-ideal transmittivity of the interface, and the dependence of our results on
the choice of the model of impurity scattering. | cond-mat_supr-con |
Flux growth and physical properties of Mo3Sb7 single crystals: Millimeter sized single crystals of Mo3Sb7 are grown using the self-flux
technique and a thorough characterization of their structural, magnetic,
thermal and transport properties is reported. The structure parameters for the
high-temperature cubic phase and the low-temperature tetragonal phase were, for
the first time, determined with neutron single crystal diffraction. Both X-ray
powder diffraction and neutron single crystal diffraction at room temperature
confirmed that Mo3Sb7 crystallizes in Ir3Ge7-type cubic structure with space
group Im-3m. The cubic-tetragonal structure transition at 53K is verified by
the peak splitting of (4 0 0) reflection observed by X-ray single crystal
diffraction and the dramatic intensity change of (12 0 0) peak observed by
neutron single crystal diffraction. The structural transition is accompanied by
a sharp drop in magnetic susceptibility, electrical resistivity, and
thermopower while cooling. A weak lambda anomaly was also observed around 53K
in the temperature dependence of specific heat and the entropy change across
the transition is estimated to be 1.80J/molMoK. The temperature dependence of
magnetic susceptibility was measured up to 750K and it follows a Curie-Weiss
behavior above room temperature. Analysis of the low-temperature magnetic
susceptibility suggests a spin gap of 110K around 53K. A typical phonon thermal
conductivity was observed in the low temperature tetragonal phase. A glassy
phonon thermal conductivity above 53K suggests a structural instability in a
wide temperature range. Superconductivity was observed at 2.35K in the as-grown
crystals and the dimensionless specific heat jump was determined to be 1.49,
which is slightly larger than the BCS value of 1.43 for the weak-coupling
limit. | cond-mat_supr-con |
Possible "Magnéli" phases and self-alloying in the superconducting
sulfur hydride: We theoretically give an infinite number of metastable crystal structures for
the superconducting sulfur hydride H$_{x}$S under pressure. It has been thought
that theoretically predicted structures of H$_{2}$S and H$_{3}$S exhibit low
and high $T_{\rm c}$ in the experiment, respectively. The newly found
structures are long-period modulated crystals where slab-like H$_{2}$S and
H$_{3}$S regions intergrow in a microscopic scale. The extremely small
formation enthalpy for the H$_{2}$S--H$_{3}$S boundary indicated with the
first-principles calculations suggests possible alloying of these phases
through formation of local H$_{3}$S regions. The modulated structures and
gradual alloying transformations between them not only explain the peculiar
pressure dependence of $T_{\rm c}$ in sulfur hydride observed experimentally,
but also could prevail in the experimental samples under various compression
schemes. | cond-mat_supr-con |
Measurements of surface impedance of superconductors as a function of
frequency in microwave range: We report measurements of the complex resistivity in YBCO and MgB$_2$ thin
films over a continuous frequency spectrum in the microwave range, making use
of a Corbino disk geometry. The paper mainly focuses on the extraction of the
resistivity from raw data, displaying data analysis procedure and its limits of
validity. We obtain and show resistivity curves as a function of frequency and
temperature denoting a frequency dependent widening of the superconducting
transition. | cond-mat_supr-con |
Consequences of the peculiar intrinsic properties of MgB2 on its
macroscopic current flow: The influence of two important features of magnesium diboride on the
macroscopic transport properties of polycrystalline MgB2 is discussed in the
framework of a percolation model. While two band superconductivity does not
have significant consequences in the field and temperature range of possible
power applications, the opposite is true for the anisotropy of the upper
critical field. The field dependence of the critical current densities strongly
increases and the macroscopic supercurrents disappear well below the apparent
upper critical field. The common scaling laws for the field dependence of the
volume pinning force are altered and Kramer's plot is no longer linear,
although grain boundary pinning dominates in nearly all polycrystalline MgB2
conductors. In contrast to the conventional superconductors NbTi and Nb3Sn, a
significant critical current anisotropy can be induced by the preparation
technique of MgB2 tapes. | cond-mat_supr-con |
Anomalous Surface Impedance due to Odd-frequency Cooper Pairs: We discuss dynamical response of odd-frequency Cooper pairs to
electromagnetic field. By using the quasiclassical Green function method, we
calculate the surface impedance (Z=R-iX) of a normal metal thin film which
covers a superconductor. In contrast to the standard relation (i.e., R << X),
the surface impedance in spin-triplet proximity structures shows anomalous
behavior (i.e., R>X) at low temperatures. This unusual relation is a result of
the penetration of odd-frequency pairs into the normal metal and reflects the
negative Cooper pair density. | cond-mat_supr-con |
Phase-sensitive determination of nodal $\mathbf{d}$-wave order parameter
in single-band and multiband superconductors: Determining the exact pairing symmetry of the superconducting order parameter
in candidate unconventional superconductors remains an important challenge.
Recently a new method, based on phase sensitive quasiparticle interference
measurements, was developed to identify gap sign changes in isotropic multiband
systems. Here we extend this approach to the single-band and multiband nodal
$d$-wave superconducting cases relevant, respectively, for the cuprates and
likely for the infinite-layer nickelate superconductors. Combining analytical
and numerical calculations, we show that the antisymmetrized correction to the
tunneling density of states due to nonmagnetic impurities in the Born and
intermediate-scattering limits shows characteristic features for sign-changing
and sign-preserving scattering wavevectors, as well as for the
momentum-integrated quantity. Furthermore, using a realistic approach
accounting for the Wannier orbitals, we model scanning tunneling microscopy
data of $\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_{8+\delta}$, which should
allow the comparison of our theory with experimental data. | cond-mat_supr-con |
Inversion symmetry of Josephson current as test of chiral domain wall
motion in Sr$_{2}$RuO$_{4}$: Clarifying the chiral domains structure of superconducting Sr$_{2}$RuO$_{4}$
has been a long-standing issue in identifying its peculiar topological
superconducting state. We evaluated the critical current $I_{c}$ versus the
magnetic field $H$ of Nb/Sr$_{2}$RuO$_{4}$ Josephson junctions, changing the
junction dimension in expectation of that the number of domains in the junction
is controlled. $I_{c}(H)$ exhibits a recovery from inversion symmetry breaking
to invariance when the dimension is reduced to several microns. This inversion
invariant behavior indicates the disappearance of domain walls; thus, the size
of a single domain is estimated at approximately several microns. | cond-mat_supr-con |
Field-Induced Boson Insulating States in a 2D Superconducting Electron
Gas with Strong Spin-Orbit Scatterings: The phenomenon of field-induced superconductor-to-insulator transitions
observed experimentally in electron-doped SrTiO$_{3}$/LaAlO$_{3}$ interfaces,
analyzed recently by means of 2D superconducting fluctuations theory (Phys.
Rev. B \textbf{104}, 054503 (2021)), is revisited with new insights associating
it with the appearance at low temperatures of field-induced boson insulating
states. Within the framework of the time-dependent Ginzburg-Landau functional
approach, we pinpoint the origin of these states in field-induced extreme
softening of fluctuation modes over a large region in momentum space, upon
diminishing temperature, which drives Cooper-pair fluctuations to condense into
mesoscopic puddles in real space. Dynamical quantum tunneling of Cooper-pair
fluctuations out of these puddles, introduced within a phenomenological
approach, which break into mobile single-electron states, contains the
high-field resistance onset predicted by the exclusive boson theory. | cond-mat_supr-con |
Divergent Vortex Mass in a Superconducting Film in Proximity to a Metal: We consider a moving vortex in a two dimensional superconductor located at a
distance $d$ from a metallic overlayer. Starting from the microscopic imaginary
time action we integrate out the electronic degrees of freedom to obtain a low
energy, long wavelength effective action for the vortex. We focus our attention
on the vortex kinetic energy and derive a general expression for the vortex
mass. We find that in the limit $d\to \infty$ the Coulomb screening of the
density fluctuations, associated with vortex motion, results in a very small
vortex mass as has been obtained in earlier studies. In the opposite limit of
$d\ll \xi $ where $\xi$ is the coherence length of the superconductor we find
that the vortex mass diverges logarithmically with the size of the system as
the proximity to the metal makes the screening processes, that usually make the
mass small, ineffective. We comment on the relevance of our results to recent
experiments which show a dramatic fall in resistance when a metallic gate is
placed near a supeconducting film in a magnetic field at low temperature. | cond-mat_supr-con |
Emergence of Superconductivity from Fully Incoherent Normal State in an
Iron-Based Superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$: In unconventional superconductors, it is generally believed that
understanding the physical properties of the normal state is a pre-requisite
for understanding the superconductivity mechanism. In conventional
superconductors like niobium or lead, the normal state is a Fermi liquid with a
well-defined Fermi surface and well-defined quasipartcles along the Fermi
surface. Superconductivity is realized in this case by the Fermi surface
instability in the superconducting state and the formation and condensation of
the electron pairs (Cooper pairing). The high temperature cuprate
superconductors, on the other hand, represent another extreme case that
superconductivity can be realized in the underdoped region where there is
neither well-defined Fermi surface due to the pseudogap formation nor
quasiparticles near the antinodal regions in the normal state. Here we report a
novel scenario that superconductivity is realized in a system with well-defined
Fermi surface but without quasiparticles along the Fermi surface in the normal
state. High resolution laser-based angle-resolved photoemission measurements
have been performed on an optimally-doped iron-based superconductor
(Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. We find that, while sharp superconducting
coherence peaks emerge in the superconducting state on the hole-like Fermi
surface sheets, no quasiparticle peak is present in the normal state. Its
electronic behaviours deviate strongly from a Fermi liquid system. The
superconducting gap of such a system exhibits an unusual temperature dependence
that it is nearly a constant in the superconducting state and abruptly closes
at T$_c$. These observations have provided a new platform to study
unconventional superconductivity in a non-Fermi liquid system. | cond-mat_supr-con |
Pre-pairing and the "Filling" Gap in the Cuprates From the Tomographic
Density of States: We use the tomographic density of states (TDoS), which is a measure of the
density of states for a single slice through the band structure of a solid, to
study the temperature evolution of the superconducting gap in the cuprates. The
TDoS provides unprecedented accuracy in determining both the superconducting
pair-forming strength, $\Delta$, and the pair-breaking rate, $\Gamma$. In both
optimally- and under-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, we find the
near-nodal $\Delta$ smoothly evolves through the superconducting transition
temperature - clear evidence for the existence of pre-formed pairs.
Additionally, we find the long observed `filling' of the superconducting gap in
the cuprates is due to the strongly temperature dependent $\Gamma$. | cond-mat_supr-con |
Emergent superconductivity in van der Waals Kagome material Pd3P2S8
under high pressure: Kagome lattice systems have been proposed to host rich physics, which provide
an excellent platform to explore unusual quantum states. Here, we report on the
discovery of superconductivity in van der Waals material Pd3P2S8 under
pressure. The superconductivity is observed in Pd3P2S8 for those pressures
where the temperature dependence of the resistivity changes from a
semiconducting-like behavior to that of a normal metal. The superconducting
transition temperature Tc increases with applied pressure and reaches ~ 6.83 K
at 79.5 GPa. Combining high-pressure XRD, Raman spectroscopy and theoretical
calculations, our results demonstrate that the observed superconductivity
induced by high pressure in Pd3P2S8 is closely related to the formation of
amorphous phase, which results from the structural instability due to the
enhanced coupling between interlayer Pd and S atoms upon compression. | cond-mat_supr-con |
Character of ground state of an aperiodic frustrated Josephson junction
array: We study the energy spectrum for an aperiodic Josephson junction ladder, as a
function of frustration. Frustration is brought about by application of a
transverse magnetic field, and aperiodicity is imposed by the arrangement of
plaquettes with two incommensurate areas. We study the effect of the
incommensurate plaquette areas in conjunction with that of the aperiodicity.
The structure of the energy spectrum at deep minima is shown to be described by
a model that treats the plaquettes independently. The energy spectrum is a
quasiperiodic function of frustration; short range correlations in the
arrangement of plaquettes have a small effect on the energy power spectrum. | cond-mat_supr-con |
Antiferromagnetic Correlation and the Pairing Mechanism of the Cuprates
and Iron Pnictides : a View From the Functional Renormalization Group Studies: We compare the one-loop functional renormalization group results for the
cuprates and the iron pnictides. Interestingly a coherent picture suggesting
that antiferromagnetic correlation causes pairing for both materials emerges. | cond-mat_supr-con |
Approximate scaling relation for the anharmonic electron-phonon problem: An approximate scaling relation is found for the transition temperature to a
charge-density-wave instability in the anharmonic electron-phonon problem,
which maps a wide range of interaction strengths, anharmonicities, and phonon
frequencies onto a common functional form. The relation employs the
wave-function renormalization parameter and is valid even for systems that are
not Fermi liquids. | cond-mat_supr-con |
Temperature dependence of the spectral weight in p- and n-type cuprates:
a study of normal state partial gaps and electronic kinetic energy: The optical conductivity of CuO2 (copper-oxygen) planes in p- and n-type
cuprates thin films at various doping levels is deduced from highly accurate
reflectivity data. The temperature dependence of the real part sigma1(omega) of
this optical conductivity and the corresponding spectral weight allow to track
the opening of a partial gap in the normal state of n-type Pr{2-x}Ce(x)CuO4
(PCCO), but not of p-type Bi2Sr2CaCu2O(8+delta} (BSCCO) cuprates. This is a
clear difference between these two families of cuprates, which we briefly
discuss. In BSCCO, the change of the electronic kinetic energy Ekin - deduced
from the spectral weight- at the superconducting transition is found to cross
over from a conventional BCS behavior (increase of Ekin below Tc to an
unconventional behavior (decrease of Ekin below Tc) as the free carrier density
decreases. This behavior appears to be linked to the energy scale over which
spectral weight is lost and goes into the superfluid condensate, hence may be
related to Mott physics. | cond-mat_supr-con |
Stabilization of an ambient pressure, collapsed tetragonal phase in
CaFe2As2 and tuning of the orthorhombic / antiferromagnetic transition
temperature by over 70 K by control of nano-precipitates: We have found a remarkably large response of the transition temperature of
CaFe2As2 single crystals grown out of excess FeAs to annealing / quenching
temperature. Whereas crystals that are annealed at 400 C exhibit a first order
phase transition from a high temperature tetragonal to a low temperature
orthorhombic and antiferromagnetic state near 170 K, crystals that have been
quenched from 960 C exhibit a transition from a high temperature tetragonal
phase to a low temperature, non-magnetic, collapsed tetragonal phase below 100
K. By use of temperature dependent electrical resistivity, magnetic
susceptibility, X-ray diffraction, Mossbauer spectroscopy and nuclear magnetic
resonance measurements we have been able to demonstrate that the transition
temperature can be reduced in a monotonic fashion by varying the annealing /
quenching temperature from 400 to 850 C with the low temperature state
remaining antiferromagnetic for transition temperatures larger than 100 K and
becoming collapsed tetragonal / non-magnetic for transition temperatures below
90 K. This suppression of the orthorhombic / antiferromagnetic phase transition
and its ultimate replacement with the collapsed tetragonal / non-magnetic phase
is similar to what has been observed for CaFe2As2 under hydrostatic pressure.
Transmission electron microscopy studies indicate that there is a temperature
dependent, width of formation of CaFe2As2 with a decreasing amount of excess Fe
and As being soluble in the single crystal at lower annealing temperatures. For
samples quenched from 960 C there is a fine (of order 10 nm), semi-uniform
distribution of precipitate that can be associated with an average strain field
whereas for samples annealed at 400 C the excess Fe and As form mesoscopic
grains that induce little strain throughout the CaFe2As2 lattice. | cond-mat_supr-con |
Phase fluctuations and the pseudogap in YBa2Cu3Ox: The thermodynamics of the superconducting transition is studied as a function
of doping using high-resolution expansivity data of YBa2Cu3Ox single crystals
and Monte-Carlo simulations of the anisotropic 3D-XY model. We directly show
that Tc of underdoped YBa2Cu3Ox is strongly suppressed from its mean-field
value (Tc-MF) by phase fluctuations of the superconducting order parameter. For
overdoped YBa2Cu3Ox fluctuation effects are greatly reduced and Tc ~ Tc-MF . We
find that Tc-MF exhibits a similar doping dependence as the pseudogap energy,
naturally suggesting that the pseudogap arises from phase-incoherent Cooper
pairing. | cond-mat_supr-con |
Surface density of states of s+-wave Cooper pairs in a two-band model: We calculate surface density of state (SDOS) of s+-wave Cooper pair in
two-band superconductor model, where gap functions have different signs between
two bands. We find that Andreev bound state appears at surface due to the sign
change in the gap function in the interband quasiparticle scattering. However,
we do not obtain the zero-energy peak of SDOS in contrast to the d-wave case.
The tunneling spectroscopy of s+-wave is much more complex as compared to the
d-wave case realized in high-Tc cuprates. | cond-mat_supr-con |
The effect of {11}^B substitution on the superconductivity in MgCNi3: The crystal structure of boron doped superconducting
MgC_{1-x}{11}^B_{x}Ni_{3}, studied by powder neutron diffraction, is reported.
The solubility limit of boron is determined to be approximately x=0.16. The
unit cell expands from a = 3.81089(2) to 3.81966(2) Angstroms as x increases
from x=0 to x=0.155. Boron ({11}^B) doping decreases Tc with increasing x: from
7.09K (x=0) to 6.44K (x=0.155). | cond-mat_supr-con |
First-Order Reorientation of the Flux-Line Lattice in CaAlSi: The flux line lattice in CaAlSi has been studied by small angle neutron
scattering. A well defined hexagonal flux line lattice is seen just above Hc1
in an applied field of only 54 Oe. A 30 degree reorientation of this vortex
lattice has been observed in a very low field of 200 Oe. This reorientation
transition appears to be of first-order and could be explained by non-local
effects. The magnetic field dependence of the form factor is well described by
a single penetration depth of 1496(1) angstroms and a single coherence length
of 307(1) angstroms at 2 K. At 1.5 K the penetration depth anisotropy is 2.7(1)
with the field applied perpendicular to the c axis and agrees with the
coherence length anisotropy determined from critical field measurements. | cond-mat_supr-con |
Superconductivity in cuprates governed by topological constraints: The remarkable universality of the cuprate $T_c$ dome suggests a very
fundamental unifying principle. Moreover, the superconducting gap is known to
persist above $T_c$ in the pseudogap phase of all cuprates. So, contrary to
BCS, the gap cannot be the order parameter of the transition.
In this work, we show that both the $T_c$-dome and the pseudogap line
$T^*(p)$ arise from a unique and identifiable principle: the interaction of
localized `pairons' on an antiferromagnetic square lattice. The topological
constraints on such preformed pairons give rise to both the $T_c$ dome and the
pairing energy {\it simultaneously}. It also provides a natural explanation for
the critical doping points of the phase diagram.
The model matches perfectly both the $T^*$ and $T_c$ experimental lines, with
only one adjustable parameter. | cond-mat_supr-con |
Optical phonons along the c axis of YBa_2Cu_3O_{6+x}, for x=0.5 -> 0.95: The c-axis polarized phonon spectra of single crystals of YBCO_{6+x} have
been measured for the doping range x=0.5 -> 0.95, between 10 K and 300 K. The
low background electronic conductivity, determined by Kramers-Kronig analysis
of the reflectance, leads to a rich phonon structure. With decreased doping the
five normally-active B_{1u} modes broaden and the high-frequency apical oxygen
mode splits into two components. We associate the higher of these with the
two-fold coordinated copper-oxygen sticks. The 155 cm^{-1} low-frequency mode,
which involves the apical and chain-oxygens, splits into at least three
components with decreasing doping. Some phonon anomalies which occur near T_c
in the highly-doped material occur well above T_c in the oxygen-reduced
systems. An unusual broad phonon band develops in the normal state at approx
400 cm^{-1}, which becomes more intense at low doping and low temperatures,
borrowing oscillator strength from apical and plane oxygen modes resulting in a
major transformation of the phonon spectrum below approx 150 K. | cond-mat_supr-con |
Phase transition in a chain of quantum vortices: We consider interacting vortices in a quasi-one-dimensional array of
Josephson junctions with small capacitance. If the charging energy of a
junction is of the order of the Josephson energy, the fluctuations of the
superconducting order parameter in the system are considerable, and the
vortices behave as quantum particles. Their density may be tuned by an external
magnetic field, and therefore one can control the commensurability of the
one-dimensional vortex lattice with the lattice of Josephson junctions. We show
that the interplay between the quantum nature of a vortex, and the long-range
interaction between the vortices leads to the existence of a specific
commensurate-incommensurate transition in a one-dimensional vortex lattice. In
the commensurate phase an elementary excitation is a soliton, with energy
separated from the ground state by a finite gap. This gap vanishes in the
incommensurate phase. Each soliton carries a fraction of a flux quantum; the
propagation of solitons leads to a finite resistance of the array. We find the
dependence of the resistance activation energy on the magnetic field and
parameters of the Josephson array. This energy consists of the above-mentioned
gap, and also of a boundary pinning term, which is different in the
commensurate and incommensurate phases. The developed theory allows us to
explain quantitatively the available experimental data. | cond-mat_supr-con |
Multiple optical gaps and laser with magnonic pumping in 2D Ising
superconductors: Ising superconductivity has been recently discovered in 2D transition metal
dichalcogenides. We report that such superconductors have unusual optical
properties controlled by the in-plane Zeeman field. First, we find several
optical gaps visible as peaks of the conductivity and the Raman susceptibility.
Moreover, we find that the Ising spin splitting in the spectrum of Bogolubov
quasiparticles enables strong population inversion generated by the
time-dependent Zeeman field. Ultimately this leads to the possibility of the
superconducting laser with magnonic pumping which can be realized in the van
der Waals structures consisting of the Ising superconductor and the
ferromagnetic insulator layers. | cond-mat_supr-con |
Eliashberg theory for spin-fluctuations mediated superconductivity --
Application to bulk and monolayer FeSe: We present a novel method for embedding spin and charge fluctuations in an
anisotropic, multi-band and full-bandwidth Eliashberg treatment of
superconductivity. Our analytical framework, based on the random phase
approximation, allows for a selfconsistent calculation of material specific
characteristics in the interacting, and more specifically, the superconducting
state. We apply this approach to bulk FeSe as representative for the iron-based
superconductors and successfully solve for the superconducting transition
temperature $T_c$, the gap symmetry and the gap magnitude. We obtain $T_c
\approx 6$ K, consistent with experiment ($T_c \approx 8$ K), as well as other
quantities in good agreement with experimental observations, thus supporting
spin fluctuations mediated pairing in bulk FeSe. On the contrary, applying our
approach to monolayer FeSe on SrTiO$_3$ we find that spin fluctuations within
the full Eliashberg framework give a $d$-wave gap with $T_c\le 11$ K and
therefore cannot provide an explanation for a critical temperature as high as
observed experimentally ($T_c \approx 70$ K). Our results hence point towards
interfacial electron-phonon coupling as the dominant Cooper pairing mediator in
this system. | cond-mat_supr-con |
Microwave properties of YBa$_2$Cu$_3$O$_{7-δ}$ films with BaZrO$_3$
nanoinclusions: We present measurements of the microwave complex surface impedance at 47.7
GHz in YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) films deposited by pulsed laser
deposition with the explicit goal to introduce BaZrO$_3$ (BZO) nanoinclusions.
Composite targets obtained by addition of BZO powder in molar percents ranging
from 2.5 to 7 mol.% have been prepared and characterized. Measurements of the
microwave surface impedance indicate a broadened transition in zero field,
however compensated by a very large pinning frequency, indicating that while
intergrain properties are still to be optimized the effect of nanometric
inclusions substantially enhances the intragrain vortex pinning. | cond-mat_supr-con |
Pseudogap and Amplitude Fluctuations in High Temperature Superconductors: Amplitude fluctuations of the pairing field are responsible together with
phase fluctuations for the pseudogap phenomena in high temperature
superconductors. Here we present the more detailed theory of the amplitude and
phase fluctuations approach in the framework of a fermionic pairing model. New
experimental comparisons are presented for the specific heat of the curprate
LSCO confirming the generality of this phenomenological approach. The strong
decrease of amplitude fluctuations near optimal doping induces the illusion of
a "quantum critical point", which in fact does not exist since the pseudogap
energy scale is always different from zero even in the overdoped regime. | cond-mat_supr-con |
Soliton induced critical current oscillations in two-band
superconducting bridges: Using time-dependent Ginzburg-Landau theory we find oscillations of critical
current density $j_c$ as a function of the length $L$ of the bridge formed from
two-band superconductor. We explain this effect by appearance of the phase
solitons in the bridge at $j<j_c$, those number changes with change of $L$. In
case of sufficiently strong interband coupling oscillations of $j_c$ disappear. | cond-mat_supr-con |
Evidence for superconducting gap nodes in the zone-centered hole bands
of KFe2As2 from magnetic penetration-depth measurements: Among the iron-based pnictide superconductors the material KFe$_2$As$_2$ is
unusual in that its Fermi surface does not consist of quasi-nested electron and
hole pockets. Here we report measurements of the temperature dependent London
penetration depth of very clean crystals of this compound with residual
resistivity ratio $>1200$. We show that the superfluid density at low
temperatures exhibits a strong linear-in-temperature dependence which implies
that there are line nodes in the energy gap on the large zone-centered hole
sheets. The results indicate that KFe$_2$As$_2$ is an unconventional
superconductor with strong electron correlations. | cond-mat_supr-con |
Superconductivity with angular dependent coupling: stripes, Coulomb
repulsion and enhanced Tc: We have analysed the effect of intrinsic doping inhomogeneity and the
presence of stripes in high-T_c superconductors on coupling \lambda by using a
simple analytically solvable model with an angular dependent \lambda (\phi)
represented by a square-well form. We have found that the introduction of the
Coulomb repulsion \lambda_C, increasing the ''contrast'' | \lambda | + |
\lambda_C |, or the depth of the angular modulation of \lambda (\phi), leads to
a remarkable enhancement of T_c. This effect can be optimized by combining
attractive (\lambda <0) and repulsive (\lambda >0) interactions along stripes
and perpendicular to them. | cond-mat_supr-con |
The Fulde-Ferrell-Larkin-Ovchinnikov state in pnictides: Fe-based superconductors (FeSC) exhibit all the properties of systems that
allow the formation of a superconducting phase with oscillating order
parameter, called the Fulde--Ferrell--Larkin--Ovchinnikov (FFLO) phase. By the
analysis of the Cooper pair susceptibility in two-band FeSC, such systems are
shown to support the existence of a FFLO phase, regardless of the exhibited
order parameter symmetry. We also show the state with nonzero Cooper pair
momentum, in superconducting FeSC with $\sim \cos(k_{x}) \cdot \cos (k_{y})$
symmetry, to be the ground state of the system in a certain parameter range. | cond-mat_supr-con |
Optical determination of the superconducting energy gap in
electron-doped Pr_{1.85}Ce_{0.15}CuO_4: The optical properties of single crystal Pr_{1.85}Ce_{0.15}CuO_4 have been
measured over a wide frequency range above and below the critical temperature
(T_c \simeq 20 K). In the normal state the coherent part of the conductivity is
described by the Drude model, from which the scattering rate just above T_c is
determined to be 1/\tau \simeq 80 cm^{-1}. The condition that \hbar/\tau
\approx 2k_B T near T_c appears to be a general result in many of the cuprate
superconductors. Below T_c the formation of a superconducting energy gap is
clearly visible in the reflectance, from which the gap maximum is estimated to
be \Delta_0 \simeq 35 cm^{-1} (4.3 meV). The ability to observe the
superconducting energy gap in the optical properties favors the nonmonotonic
over the monotonic description of the d-wave gap. The penetration depth for
T\ll T_c is \lambda \simeq 2000 \AA, which when taken with the estimated value
for the dc conductivity just above T_c of \sigma_{dc} \simeq 35 \times 10^3
\Omega^{-1}cm^{-1} places this material on the general scaling line for the
cuprates defined by 1/\lambda^2 \propto \sigma_{dc}(T\simeq T_c) \times T_c.
This result is consistent with the observation that 1/\tau \approx 2\Delta_0,
which implies that the material is not in the clean limit. | cond-mat_supr-con |
Evidence of Strong Correlations and Coherence-Incoherence Crossover in
the Iron Pnictide Superconductor KFe2As2: Using resistivity, heat-capacity, thermal-expansion, and susceptibility
measurements we study the normal-state behavior of KFe2As2. We find that both
the Sommerfeld coefficient gamma = 103 mJ mol-1 K-2 and the Pauli
susceptibility chi = 4x10-4 are strongly enhanced, which confirm the existence
of heavy quasiparticles inferred from previous de Haas-van Alphen and ARPES
experiments. We discuss this large enhancement using a Gutzwiller slave-boson
mean-field calculation, which reveals the proximity of KFe2As2 to an
orbital-selective Mott transition. The temperature dependence of the magnetic
susceptibility and the thermal expansion provide strong experimental evidence
for the existence of a coherence-incoherence crossover, similar to what is
found in heavy fermion and ruthenate compounds, due to Hund's coupling between
orbitals. | cond-mat_supr-con |
Magnetoresistance and Hall effect in e-doped superconducting SrLaCuO
thin films: We have epitaxially grown c-axis oriented SrxLa1-xCuO2 thin films by rf
sputtering on KTaO3 substrates with x = 0.12. The as-grown deposits are
insulating and a series of superconducting films with various Tc(R=0) up to 26
K have been obtained by in-situ oxygen reduction. Transport measurements in the
ab plane of these samples have been undertaken. We report original results on
the temperature dependence of the Hall effect and on the anisotropic
magnetoresistance (T > Tc). We discuss the magnitude of upper critical fields
and anisotropy, the Hall effect, which presents changes of sign indicative of
the existence of two types of carriers, the normal state magnetoresistance,
negative in parallel magnetic field, a possible signature of spin scattering.
These properties are compared to those of hole-doped cuprates, such as
BiSr(La)CuO with comparable Tc. | cond-mat_supr-con |
d-Wave superconductivity on the checkerboard Hubbard model at weak and
strong coupling: It has been argued that inhomogeneity generally can enhance superconductivity
in the cuprate high-Tc materials. To check the validity of this claim, we study
d-wave superconductivity on the checkerboard Hubbard model on a square lattice
using the Cellular Dynamical Mean Field theory method with an exact
diagonalization solver at zero temperature. The d-wave order parameter is
computed for various inhomogeneity levels over the entire doping range of
interest in both strong and weak coupling regimes. At a given doping, the size
of the d-wave order parameter manifests itself directly in the height of the
coherence peaks and hence is an appropriate measure of the strength of
superconductivity. The weak coupling results reveal a suppression of the order
parameter in the presence of inhomogeneity for small to intermediate hole
dopings, while it is enhanced for large dopings. In contrast, for strong
coupling there is a monotonic decrease in the maximum amplitude of the
superconducting order parameter with inhomogeneity over the entire doping range
of interest. Furthermore, at moderately high inhomogeneity, the system
undergoes a first-order transition from the superconducting to the normal state
in the underdoped regime. In the overdoped regime, the change in the value of
the superconducting order parameter correlates with the height of the lowest
energy peak in the spectral weight of antiferromagnetic spin fluctuations,
confirming the connection between antiferromagnetic fluctuations and d-wave
superconductivity found in earlier studies on the homogeneous case. Our results
are benchmarked by comparisons with numerically exact results on the
checkerboard Hubbard ladder. | cond-mat_supr-con |
Imaging Flux Vortices in MgB2 using Transmission Electron Microscopy: We report the successful imaging of flux vortices in single crystal MgB2
using transmission electron microscopy. The specimen was thinned to electron
transparency (350 nm thickness) by focussed ion beam milling. An artefact of
the thinning process was the production of longitudinal thickness undulations
of height 1-2 nm in the sample which acted as pinning sites due to the energy
required for the vortices to cross them. These had a profound effect on the
patterns of vortex order observed which we examine here.
Supplementary information can be downloaded from
http://www-hrem.msm.cam.ac.uk/people/loudon/#publications | cond-mat_supr-con |
Iron-based superconductors: teenage, complex, challenging: The advent of iron-based superconductors in 2008 came as a complete surprise
to the condensed matter community. Now 15 years later, they are beginning to
impart some of their new-found wisdom on a slew of emerging superconductors
that boast similar traits. | cond-mat_supr-con |
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