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Possible superconductivity in multi-layer-graphene by application of a
gate voltage: The carrier density in tens of nanometers thick graphite samples
(multi-layer-graphene, MLG) has been modified by applying a gate voltage
($V_g$) perpendicular to the graphene planes. Surface potential microscopy
shows inhomogeneities in the carrier density ($n$) in the sample near surface
region and under different values of $V_g$ at room temperature. Transport
measurements on different MLG samples reveal that under a large enough applied
electric field these regions undergo a superconducting-like transition at $T
\lesssim 17$ K. A magnetic field applied parallel or normal to the graphene
layers suppresses the transition without changing appreciably the transition
temperature. | cond-mat_supr-con |
Magneto-elastic coupling model of deformable anisotropic superconductors: We develop a magneto-elastic (ME) coupling model for the interaction between
the vortex lattice and crystal elasticity. The theory extends the Kogan-Clem's
anisotropic Ginzburg-Landau (GL) model to include the elasticity effect. The
anisotropies in superconductivity and elasticity are simultaneously considered
in the GL theory frame. We compare the field and angular dependences of the
magnetization to the relevant experiments. The contribution of the ME
interaction to the magnetization is comparable to the vortex-lattice energy, in
materials with relatively strong pressure dependence of the critical
temperature. The theory can give the appropriate slope of the field dependence
of magnetization near the upper critical field. The magnetization ratio along
different vortex frame axes is independent with the ME interaction. The
theoretical description of the magnetization ratio is applicable only if the
applied field moderately close to the upper critical field. | cond-mat_supr-con |
Temperature Dependence of Superconducting Gaps in Mg(Al)B2 System
Investigated by SnS-Andreev Spectroscopy: Detailed temperature dependence of both superconducting gaps was obtained
directly by means of SnS-Andreev spectroscopy. The \Delta \sigma,\pi(T) -curves
were shown to be deviated from standard BCS-like behavior, due to k-space
proximity effect between \sigma - and \pi - condensates, which could give a key
to experimental determination of interband electron-phonon coupling constants.
For the first time, an excellent qualitative agreement with theoretical
predictions of Nicol and Carbotte, and Moskalenko and Suhl was shown.
dI(V)/dV-spectra of SnS-Andreev contacts based on MgB2 samples (with defects of
crystal structure), and Mg(Al)B2 polycrystalline samples (with the local
critical temperatures Tc variation 10 K < Tc < 37 K) were studied by means of
the "break-junction" technique within the temperature range 4.2 K < T < Tc. | cond-mat_supr-con |
Insulating regime of an underdamped current-biased Josephson junction
supporting $\mathbb{Z}_3$ and $\mathbb{Z}_4$ parafermions: We study analytically a current-biased topological Josephson junction
supporting $\mathbb{Z}_n$ parafermions. First, we show that in an infinite-size
system a pair of parafermions on the junction can be in $n$ different states;
the $2\pi{n}$ periodicity of the phase potential of the junction results in a
significant suppression of the maximal current $I_m$ for an insulating regime
of the underdamped junction. Second, we study the behaviour of a realistic
finite-size system with avoided level crossings characterized by splitting
$\delta$. We consider two limiting cases: when the phase evolution may be
considered adiabatic, which results in decreased periodicity of the effective
potential, and the opposite case, when Landau-Zener transitions restore the
$2\pi{n}$ periodicity of the phase potential. The resulting current $I_m$ is
exponentially different in the opposite limits, which allows us to propose a
new detection method to establish the appearance of parafermions in the system
experimentally, based on measuring $I_m$ at different values of the splitting
$\delta$. | cond-mat_supr-con |
Evidence for field-induced excitations in low-temperature thermal
conductivity of Bi_2Sr_2CaCu_2O_8: The thermal conductivity ,$\kappa$, of Bi_2Sr_2CaCu_2O_8 was studied as a
function of magnetic field. Above 5 K, after an initial decrease, $\kappa(H)$
presents a kink followed by a plateau, as recently reported by Krishana et al..
By contrast, below 1K, the thermal conductivity was found to \emph{increase}
with increasing field. This behavior is indicative of a finite density of
states and is not compatible with the existence of a field-induced fully gapped
$d_{x^{2}-y^{2}}+id_{xy}$ state which was recently proposed to describe the
plateau regime. Our low-temperature results are in agreement with recent works
predicting a field-induced enhancement of thermal conductivity by Doppler shift
of quasi-particle spectrum. | cond-mat_supr-con |
Quantum phase transitions in two-dimensional superconductors: a review
on recent experimental progress: Superconductor-insulator/metal transition (SIT/SMT) as a paradigm of quantum
phase transition has been a research highlight over the last three decades.
Benefit from recent developments in the fabrication and measurements of 2D
superconducting films and nanodevices, unprecedented quantum phenomena have
been revealed in the quantum phase transitions of 2D superconductors. In this
review, we introduce the recent progress on quantum phase transitions in 2D
superconductors, focusing on the quantum Griffiths singularity (QGS) and
anomalous metal state. Characterized by a divergent critical exponent when
approaching zero temperature, QGS of SMT is discovered in ultrathin crystalline
Ga films and subsequently detected in various 2D superconductors. The
universality of QGS indicates the profound influence of quenched disorder on
quantum phase transitions. Besides, in a 2D superconducting system, whether a
metallic ground state can exist is a long-sought mystery. Recently, the
charge-2e quantum oscillations are observed in nanopatterned superconducting
films, indicating the bosonic nature of the anomalous metal state and ending
the debate on whether bosons can exist as a metal. The evidences of the
anomalous metal states have also been reported in crystalline epitaxial thin
films and exfoliated nanoflakes, as well as granular composite films. High
quality filters are used in these works to exclude the influence of external
high frequency noises in ultralow temperature measurements. The observations of
QGS and metallic ground states in 2D superconductors not only reveal the
prominent role of quantum fluctuations and dissipations but also provide new
perspective to explore quantum phase transitions in superconducting systems. | cond-mat_supr-con |
Magnetic field dependence of pairing interaction in ferromagnetic
superconductors with triplet pairing: It is developed a microscopic description of superconductivity in
ferromagnetic materials with triplet pairing triggered by the exchange of
magnetic fluctuations. Instead widely used paramagnon model we work with
phenomenological spectrum of fluctuations in the orthorhombic ferromagnet with
strong magnetic anisotropy. Depending of the field orientation parallel or
perpendicular to the direction of spontaneous magnetization the effective
amplitude of pairing interaction proves to be decreasing or increasing function
of magnetic field that allows to explain the drastic difference in magnitudes
of upper critical field in these directions. | cond-mat_supr-con |
The Microstrain-Doping Phase Diagram of the Iron Pnictides
Heterostructures at Atomic Limit: The 3D phase diagram of iron pnictides where the critical temperature depends
on charge density and microstrain in the active FeAs layers is proposed. The
iron pnictides superconductors are shown to be a practical realization of a
heterostructure at the atomic limit made of a superlattice of FeAs layers
intercalated by spacer layers. We have focussed our interest on the A 1-x
BxFe2As2 (122) families and we show that FeAs layers have a tensile microstrain
due to the misfit strain between the active layers and the spacers. We have
identified the critical range of doping and microstrain where the critical
temperature gets amplified to its maximum value. | cond-mat_supr-con |
The pseudogap and doping dependent magnetic properties of
La2-xSrxCu1-yZnyO4: The effects of planar hole content, p (= x), on the static magnetic
susceptibility, chi(T), of polycrystalline La2-xSrxCu1-yZnyO4 compounds were
investigated over a wide range of Sr (x) and Zn (y) contents. The magnetic
behavior caused by Zn was found to depend strongly on the hole content. The
apparent magnetic moment induced by Zn was larger in underdoped
La2-xSrxCu1-yZnyO4, decreased quite sharply around p ~ 0.19, and did not change
much for further overdoping. This is interpreted in terms of the effect of the
pseudogap on the Zn-induced magnetic behavior, as there is growing evidence
that the pseudogap vanishes quite abruptly at p ~ 0.19 +/- 0.01. From a
detailed analysis of chi(T) data the Zn-induced magnetic contribution was found
to be rather complex and showed non-Curie-like features over a wide range of
temperature. The observed behavior was scrutinized in terms of two scenarios
(a) that of independent localized-moments and (b) low energy quasiparticle
resonances associated with each Zn atom. Our study points towards the latter
scenario and more generally suggests that there is a re-distribution of
quasiparticle spectral weight due to Zn substitution, the features of which are
greatly influenced by the presence and magnitude of the pseudogap. | cond-mat_supr-con |
Minimal timing jitter in superconducting nanowire single photon
detectors: Using two-temperature model coupled with modified time-dependent
Ginzburg-Landau equation we calculate the delay time $\tau_d$ in appearance of
growing normal domain in the current-biased superconducting strip after
absorption of the single photon. We demonstrate that $\tau_d$ depends on the
place in the strip where photon is absorbed and monotonically decreases with
increasing of the current. We argue, that the variation of $\tau_d$ (timing
jitter), connected either with position-dependent response or Fano fluctuations
could be as small as the lowest relaxation time of the superconducting order
parameter $\sim \hbar/k_BT_c$ ($T_c$ is the critical temperature of the
superconductor) when the current approaches the depairing current. | cond-mat_supr-con |
Quasiparticle spectrum near the gap node directions in the mixed state
of d-wave superconductors: We report on a calculation of the quantized energy spectrum and quasiparticle
eigenfunctions for low lying excitations in the mixed state of clean d-wave
superconductors. Our study is based on an approximate analytical solution of
the Bogolubov-de Gennes equations for both rectangular and triangular flux
lattices with one of the primitive translations chosen parallel to the gap node
direction. For excitations with momenta close to a certain gap node we have
obtained a set of eigenfunctions which appear to be extended along the chosen
gap node direction and localized along the perpendicular one on a scale
determined by the intervortex distance. The periodic superfluid velocity field
induces a band structure in the spectrum, which depends essentially on the
vortex lattice geometry. | cond-mat_supr-con |
Nickelate superconductors: an ongoing dialog between theory and
experiments: After decades of fundamental research, unconventional superconductivity has
recently been demonstrated in rare-earth infinite-layer nickelates. The current
view depicts these systems as a new category of superconducting materials, as
they appear to be correlated metals with distinct multiband features in their
phase diagram. Here, we provide an overview of the state of the art in this
rapidly evolving topic. | cond-mat_supr-con |
Theory of the Transition at 0.2 K in Ni-doped Bi2Sr2CaCu2O8: A theory is put forward that the electronic phase transition at 0.2 K in
Ni-doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ is result of the formation of a spin
density wave in the system of Ni impurities. The driving force for the
transition is the exchange interaction between the impurity spins and the spins
of the conduction electrons. This creates a small gap at two of the four nodes
of the superconducting gap. The effect is to reduce the thermal conductivity by
a factor of two, as observed. | cond-mat_supr-con |
Topological Phase Transition in Superconductors with Mirror Symmetry: We provide analytical and numerical evidence that the attractive
two-dimensional Kitaev model on a lattice with mirror symmetry demonstrates
unusual 'intrinsic' phase at half filling. This phase emerges in the phase
diagram at the boundary separating two topological superconductors with
opposite Chern numbers and exists due to condensation of non-zero momentum
Cooper pairs. Unlike Fulde-Ferrell-Larkin-Ovchinnikov superconductivity, the
Cooper pairs momenta are lying along two lines in the Brillouin zone meaning
simultaneous condensation of a continuum of Cooper pairs. | cond-mat_supr-con |
Pseudogap phase of cuprate superconductors confined by Fermi surface
topology: The properties of cuprate high-temperature superconductors are largely shaped
by competing phases whose nature is often a mystery. Chiefly among them is the
pseudogap phase, which sets in at a doping $p^*$ that is material-dependent.
What determines $p^*$ is currently an open question. Here we show that the
pseudogap cannot open on an electron-like Fermi surface, and can only exist
below the doping $p_{FS}$ at which the large Fermi surface goes from hole-like
to electron-like, so that $p^*$ $\leq$ $p_{FS}$. We derive this result from
high-magnetic-field transport measurements in
La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ under pressure, which reveal a large and
unexpected shift of $p^*$ with pressure, driven by a corresponding shift in
$p_{FS}$. This necessary condition for pseudogap formation, imposed by details
of the Fermi surface, is a strong constraint for theories of the pseudogap
phase. Our finding that $p^*$ can be tuned with a modest pressure opens a new
route for experimental studies of the pseudogap. | cond-mat_supr-con |
Enhanced low-energy magnetic excitations evidencing the Cu-induced
localization in an Fe-based superconductor Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$: We have performed inelastic neutron scattering measurements on
optimally-doped Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$ and 10% Cu-doped
Fe$_{0.88}$Cu$_{0.1}$Te$_{0.5}$Se$_{0.5}$ to investigate the substitution
effects on the spin excitations in the whole energy range up to 300 meV. It is
found that substitution of Cu for Fe enhances the low-energy spin excitations
($\le$ 100 meV), especially around the (0.5, 0.5) point, and leaves the
high-energy magnetic excitations intact. In contrast to the expectation that Cu
with spin 1/2 will dilute the magnetic moments contributed by Fe with a larger
spin, we find that the 10% Cu doping enlarges the effective fluctuating moment
from 2.85 to 3.13 $\mu_{\rm B}$/Fe, although there is no long- or short-range
magnetic order around (0.5, 0.5) and (0.5, 0). The presence of enhanced
magnetic excitations in the 10% Cu doped sample which is in the insulating
state indicates that the magnetic excitations must have some contributions from
the local moments, reflecting the dual nature of the magnetism in iron-based
superconductors. We attribute the substitution effects to the localization of
the itinerant electrons induced by Cu dopants. These results also indicate that
the Cu doping does not act as electron donor as in a rigid-band shift model,
but more as scattering centers that localize the system. | cond-mat_supr-con |
Calculation of Leggett-Takagi relaxation in vortices of superfluid 3He-B: We calculate the relaxation of Brinkman-Smith mode via Leggett-Takagi
relaxation in the presence of an isolated vortex in superfluid 3He-B. The
calculation is based on an analytical solution of the order parameter far from
the vortex axis. We obtain an expression for the dissipated power per vortex
length as a function of the tipping angle of the magnetization and the
orientation of the static magnetic field with respect to the vortex. | cond-mat_supr-con |
Structure and Magnetization of Two-Dimensional Vortex Arrays in the
Presence of Periodic Pinning: Ground-state properties of a two-dimensional system of superconducting
vortices in the presence of a periodic array of strong pinning centers are
studied analytically and numerically. The ground states of the vortex system at
different filling ratios are found using a simple geometric argument under the
assumption that the penetration depth is much smaller than the spacing of the
pin lattice. The results of this calculation are confirmed by numerical studies
in which simulated annealing is used to locate the ground states of the vortex
system. The zero-temperature equilibrium magnetization as a function of the
applied field is obtained by numerically calculating the energy of the ground
state for a large number of closely spaced filling ratios. The results show
interesting commensurability effects such as plateaus in the B-H diagram at
simple fractional filling ratios. | cond-mat_supr-con |
Transport properties of the new Fe-based superconductor KxFe2Se2 (Tc =
33 K): We synthesized the new Fe-based superconductor K0.8Fe2Se2 single crystals.
The obtained single crystal exhibited a sharp superconducting transition, and
the onset and zero-resistivity temperature was estimated to be 33 and 31.8 K,
respectively. A high upper critical field of 192 T was obtained. Anisotropy of
superconductivity of K0.8Fe2Se2 was ~3.6. Both the high upper critical field
and comparably low anisotropy are advantageous for the application under high
magnetic field. | cond-mat_supr-con |
The crystal structure of FeSe0.44Te0.56: The crystal structure of the superconductor FeSe0.44Te0.56 was redetermined
by high-resolution X-ray single crystal diffraction at 173 K (anti-PbO-type,
P4/nmm, a=3.7996(2), c=5.9895(6) A, R1=0.022, wR2=0.041, 173 F^2).
Significantly different z-coordinates of tellurium and selenium at the 2c site
are clearly discernible and were refined to z_Te=0.2868(3) and z_Se=0.2468(7).
Thus the chalcogen heights differ by 0.24 A and the Fe-Se bonds are by 0.154 A
shorter than the Fe-Te bonds, while three independent (Te,Se)-Fe-(Te,Se) bond
angles occur. An elevated U33 displacement parameter of the iron atom is
suggestive of a slightly puckered Fe layer resulting from different
combinations of Se or Te neighbors. Such strong disorder underlines the
robustness of superconductivity against structural randomness and has not yet
been considered in theoretical studies of this system. | cond-mat_supr-con |
Hydrogen-induced high-temperature superconductivity in two-dimensional
materials: Example of hydrogenated monolayer MgB$_2$: Hydrogen-based compounds under ultra-high pressure, such as the polyhydrides
H$_3$S and LaH$_{10}$, superconduct through the conventional electron-phonon
coupling mechanism to attain the record critical temperatures known to date. We
demonstrate here that the intrinsic advantages of hydrogen for phonon-mediated
superconductivity can be exploited in a completely different system, namely
two-dimensional (2D) materials. We find that hydrogen adatoms can strongly
enhance superconductivity in 2D materials due to flatband states originating
from atomic-like hydrogen orbitals, with a resulting high density of states,
and due to the emergence of high-frequency hydrogen-related phonon modes that
boost the electron-phonon coupling. As a concrete example, we investigate the
effect of hydrogen adatoms on the superconducting properties of monolayer
MgB$_2$, by solving the fully anisotropic Eliashberg equations, in conjunction
with a first-principles description of the electronic and vibrational states,
and the coupling between them. We show that hydrogenation leads to a high
critical temperature of 67 K, which can be boosted to over 100 K by biaxial
tensile strain. | cond-mat_supr-con |
Charge-density wave transition in magnetic topological semimetal
EuAl$_4$: The interplay among topology, charge-density wave (CDW), and magnetism can
give rise to a plethora of exotic quantum phenomena. Recently, a group of
magnetic topological semimetals with tetragonal lattices and CDW order were
found to exhibit anomalous magnetic instability, helical spin ordering, and the
presence of skyrmions. However, the underlying mechanism responsible for these
observations remains unclear. Here, we conducted a comprehensive investigation
into the impact of CDW on the topological and magnetic properties of EuAl$_4$
using optical spectroscopy and the first-principles calculations. Through
optical spectroscopy, we observed a partial gap (60~meV) on the Fermi surface
and an enhanced mid-infrared absorption around 0.4~eV after the CDW transition.
Magneto-optical spectroscopy and the first-principles calculations proved that,
by affecting the band structure, the CDW order frustrates the antiferromagnetic
interactions but strengthened the ferromagnetic ones, which can destabilize the
magnetism. With lower symmetry in the CDW ordered state, carriers from the Weyl
bands will mediate the anisotropic magnetic interactions promoting the
formation of chiral spin textures. Conversely, without the CDW order, the
counterpart EuGa$_4$ shows robust collinear antiferromagnetic order. Our
findings uncover the pivotal role played by CDW order in arousing intricate
magnetism in topological materials and provide valuable insights into
controlling topological and magnetic properties through the manipulation of CDW
orders. | cond-mat_supr-con |
Superconductivity in undoped CaFe2As2 single crystals: Single crystals of undoped CaFe2As2 were grown by a FeAs self-flux method,
and the crystals were quenched in ice-water rapidly after high temperature
growth. The quenched crystal undergoes a collapsed tetragonal structural phase
transition around 80 K revealed by the temperature dependent X-ray diffraction
measurements. Superconductivity below 25 K was observed in the collapsed phase
by resistivity and magnetization measurements. The isothermal magnetization
curve measured at 2 K indicates that this is a typical type-II superconductor.
For comparison, we systematically characterized the properties of the furnace
cooled, quenched, and post-annealed single crystals, and found strong internal
crystallographic strain existing in the quenched samples, which is the key for
the occurrence of superconductivity in the undoped CaFe2As2 single crystals. | cond-mat_supr-con |
Unveiling Mechanisms of Electric Field Effects on Superconductors by
Magnetic Field Response: We demonstrate that superconducting aluminium nano-bridges can be driven into
a state with complete suppression of the critical supercurrent via
electrostatic gating. Probing both in- and out-of-plane magnetic field
responses in the presence of electrostatic gating can unveil the mechanisms
that primarily cause the superconducting electric field effects. Remarkably, we
find that a magnetic field, independently of its orientation, has only a weak
influence on the critical electric field that identifies the transition from
the superconducting state to a phase with vanishing critical supercurrent. This
observation points to the absence of a direct coupling between the electric
field and the amplitude of the superconducting order parameter or $2\pi$-phase
slips via vortex generation. The magnetic field effect observed in the presence
of electrostatic gating is described within a microscopic model where a
spatially uniform inter-band $\pi$-phase is stabilized by the electric field.
Such an intrinsic superconducting phase rearrangement can account for the
suppression of the supercurrent, as well as for the weak dependence of the
critical magnetic fields on the electric field. | cond-mat_supr-con |
Theory of superconductivity due to Ngai's mechanism in lightly doped
SrTiO3: We develop a theory of superconducting pairing in low-density Strontium
titanate due to quadratic coupling of electron density to soft transverse
optical phonons. It leads to static attractive potential between electrons
which decay length scales inversely with soft optical gap. For low electron
densities attraction between electrons is local and transition temperature Tc
was found. The Tc(n) dependence in agreement with experimental data for low
doping was calculated. Next, we show that suppression of Tc by hydrostatic
pressure and strong increase of Tc due to isotop substitution are explained
within our theory. | cond-mat_supr-con |
Macroscopic character of composite high temperature superconducting
wires: The "d-wave" symmetry of the superconducting order in the cuprate high
temperature superconductors is a well established fact, and one which
identifies them as "unconventional." However, in macroscopic contexts --
including many potential applications ({\it i.e.} superconducting "wires") --
the material is a composite of randomly oriented superconducting grains in a
metallic matrix, in which Josephson coupling between grains mediates the onset
of long-range phase coherence. Here, we analyze the physics at length scales
large compared to the size of such grains, and in particular the macroscopic
character of the long-range order that emerges. While XY-glass order and
macroscopic d-wave superconductivity may be possible, we show that under many
circumstances -- especially when the d-wave superconducting grains are embedded
in a metallic matrix -- the most likely order has global s-wave symmetry. | cond-mat_supr-con |
Two energy scales and close relationship between the pseudogap and
superconductivity in underdoped cuprate superconductors: By measuring the low temperature specific heat, the low energy quasi-particle
excitation has been derived and analyzed in systematically doped
La$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals. The Volovik's relation predicted
for a d-wave superconductor has been well demonstrated in wide doping regime,
showing a robust evidence for the d-wave pairing symmetry. Furthermore the
nodal gap slope $v_\Delta$ of the superconducting gap is derived and is found
to follow the same doping dependence of the pseudogap obtained from ARPES and
tunnelling measurement. This strongly suggests a close relationship between the
pseudogap and superconductivity. Taking the entropy conservation into account,
we argue that the ground state of the pseudogap phase should have Fermi arcs
with finite density of states at zero K, and the transport data show that it
behaves like an insulator due to probably weak localization. A nodal metal
picture for the pseudogap phase cannot interpret the data. Based on the Fermi
arc picture for the pseudogap phase it is found that the superconducting energy
scale or $T_c$ in underdoped regime is governed by both the maximum gap and the
spectral weight from the Fermi arcs. This suggests that there are two energy
scales: superconducting energy scale and the pseudogap. The superconductivity
may be formed by the condensation of Fermi arc quasiparticles through pairing
by exchanging virtue bosons. | cond-mat_supr-con |
Synthesis and superconductivity of new BiS2 based superconductor
PrO0.5F0.5BiS2: We report synthesis and superconductivity at 3.7K in PrO0.5F0.5BiS2. The
newly discovered material belongs to the layered sulfide based REO0.5F0.5BiS2
compounds having ZrCuSiAs type structure. The bulk polycrystalline compound is
synthesized by vacuum encapsulation technique at 7800C in single step. Detailed
structural analysis has shown that the as synthesized PrO0.5F0.5BiS2 is
crystallized in tetragonal P4/nmm space group with lattice parameters a =
4.015(5) {\AA}, c = 13.362(4) {\AA}. Bulk superconductivity is observed in
PrO0.5F0.5BiS2 below 4K from magnetic and transport measurements. Electrical
transport measurements showed superconducting transition temperature (Tc) onset
at 3.7K and Tc ({\rho}=0) at 3.1K. Hump at Tc related to superconducting
transition is not observed in heat capacity measurement and rather a
Schottky-type anomaly is observed at below ~6K. The compound is slightly
semiconducting in normal state. Isothermal magnetization (MH) exhibited typical
type II behavior with lower critical field (Hc1) of around 8Oe. | cond-mat_supr-con |
Parity effect in a small superconducting grain: A rigorous result: The parity effect in an ultra-small superconducting grain is examined. By
applying a generalized version of Lieb's spin-reflection positivity technique,
we show rigorously that the parity parameter $\Delta_P$ is nonvanishing in such
a system. A positive lower bound for $\Delta_P$ is derived. | cond-mat_supr-con |
Magnetic characterization of sintered MgB2 samples: effect of the
substitution or doping with Li, Al and Si: Powdered and sintered MgB2 samples have been characterized through magnetic
measurements performed from T = 5 K up to few degrees above the transition
temperature of about 39 K. We found that the sintered samples behave as
well-connected bodies, showing no trace of granularity. In order to obtain the
critical current density value Jc the Critical State Model has been therefore
employed in a straightforward way. With the aim either to decrease the electron
mean free path or to increase its Jc we have attempted to introduce defects in
the MgB2 structure by different procedures: substitution of Lithium on the
Magnesium site and doping of the precursor Boron powders with Aluminum and
Silicon. The best result in terms of Jc has been achieved by Silicon doping
that, moreover, does not significantly affect the transition temperature. | cond-mat_supr-con |
What does the Josephson effect tell us about the superconducting state
of the cuprates?: We review the recent measurements of the current-phase relation in cuprate
Josephson junctions. Special attention is paid to 45 degree grain boundary
junctions and to c-axis junctions between YBCO and Nb. It is shown that the
anomalous properties of such junctions which have been found experimentally
(large second harmonic of the current phase relation, sign change of the first
harmonic as a function of temperature) are qualitatively consistent with the
d-wave symmetry of pairing in the cuprates. It is argued that in order to
describe the data quantitatively, quantum fluctuations of the order parameter
need to be taken into account. It is pointed out that the Josephson effect is
an ideal tool for measuring the reduced phase stiffness in the cuprates. | cond-mat_supr-con |
High Temperature Superconductivity: the explanation: Soon after the discovery of the first high temperature superconductor by
Georg Bednorz and Alex Mueller in 1986 the late Sir Nevill Mott answering his
own question "Is there an explanation?" [Nature v 327 (1987) 185] expressed a
view that the Bose-Einstein condensation (BEC) of small bipolarons, predicted
by us in 1981, could be the one. Several authors then contemplated BEC of real
space tightly bound pairs, but with a purely electronic mechanism of pairing
rather than with the electron-phonon interaction (EPI). However, a number of
other researchers criticized the bipolaron (or any real-space pairing) scenario
as incompatible with some angle-resolved photoemission spectra (ARPES), with
experimentally determined effective masses of carriers and unconventional
symmetry of the superconducting order parameter in cuprates. Since then the
controversial issue of whether the electron-phonon interaction (EPI) is crucial
for high-temperature superconductivity or weak and inessential has been one of
the most challenging problems of contemporary condensed matter physics. Here I
outline some developments in the bipolaron theory suggesting that the true
origin of high-temperature superconductivity is found in a proper combination
of strong electron-electron correlations with a significant finite-range
(Froehlich) EPI, and that the theory is fully compatible with the key
experiments. | cond-mat_supr-con |
Phenomenological study of spin-triplet ferromagnetic superconductors: Unconventional superconductivity with spin-triplet Cooper pairing is reviewed
on the basis of the quasi-phenomenological Ginzburg-Landau theory. The
superconductivity, in particular, the mixed phase of coexistence of
ferromagnetism and unconventional superconductivity is triggered by the
spontaneous magnetization. The mixed phase is stable whereas the other
superconducting phases that usually exist in unconventional superconductors are
either unstable, or, for particular values of the parameters of the theory,
some of these phases are metastable at relatively low temperatures in a quite
narrow domain of the phase diagram. The phase transitions from the normal phase
to the phase of coexistence is of first order while the phase transition from
the ferromagnetic phase to the coexistence phase can be either of first or
second order depending on the concrete substance. The Cooper pair and crystal
anisotropy are relevant to a more precise outline of the phase diagram shape
and reduce the degeneration of the ground states of the system but they do not
drastically influence the phase stability domains and the thermodynamic
properties of the respective phases. | cond-mat_supr-con |
On the Experimental Evidence for Possible Superconductivity in LK99: The desire to create an energy efficient world is bound to be incomplete
without the discovery of a room temperature superconductor at ambient pressure.
A recent report on the room-temperature ambient-pressure superconductor has
inspired scientists to study the Cu doped Lead apatite named as LK-99. Here, we
have synthesized Cu doped LK-99 and Ni-doped LK-99 compounds and studied their
temperature dependent transport and magnetization behavior. In spite of the
presence of impurity phase Cu$_2$S, the temperature dependent resistance shows
an insulating nature of the sample. The radio frequency penetration depth
measurement unveils the absence of diamagnetic flux expulsion in this sample.
The temperature dependent ac susceptibility measurements reveal the
paramagnetic nature of the Ni doped LK-99. | cond-mat_supr-con |
ARPES kink is a "smoking gun" for the theory of high-Tc superconductors:
dominance of the electron-phonon interaction with forward scattering peak: The ARPES spectra in high-Tc superconductors show four distinctive features
in the quasiparticle self-energy. All of them can be explained consistently by
the theory in which the electron phonon interaction (EPI) with the forward
scattering peak dominates over the Coulomb scattering. In particular, this
theory explains why there is no shift of the nodal kink at 70 meV in the
superconducting state, contrary to the clear shift of the anti-nodal
singularity at 40 meV. The theory predicts a ``knee''-like structure of the
imaginary part of the self-energy, which is phonon dominated for $\omega
\approx \omega^{(70)}_{ph}$, and shows linear behavior for $\omega >
\omega^{(70)}_{ph}$ - due to the Coulomb scattering. Recent ARPES spectra give
that the EPI coupling constant is much larger than the Coulomb one. The
dip-hump structure in the spectral function comes out naturally from the
proposed theory. | cond-mat_supr-con |
Psuedo-isotropic upper critical field in cobalt-doped SrFe2As2 epitaxial
films: The temperature and angular dependence of the upper critical field (Hc2) is
reported for cobalt-doped SrFe2As2 epitaxial films between Tc and 0.5 K in
pulsed magnetic fields up to 50 T. For H parallel c, Hc2 is close to a linear
function of temperature, while in the perpendicular direction there is
significant downward curvature that results in an Hc2 ratio (gamma =
Hc2(perpendicular)/Hc2(parallel) that decreases nearly linearly with
temperature, approaching gamma = 1 at low temperature with Hc2(0) = 47 T. We
measure the complete upper-critical field phase diagram including angular
dependence and model the data using a two band theory allowing us to determine
the anisotropy of both bands, their relative diffusivities, and the
relationship between BCS coupling constant matrix elements. We find an unusual
relationship between the diffusivities of the two bands, with two anisotropic
and opposite bands. This relationship is supported by the observation of a
local maximum for Hc2(parallel) at low temperature. | cond-mat_supr-con |
Proximity Eliashberg theory of electrostatic field-effect-doping in
superconducting films: We calculate the effect of a static electric field on the critical
temperature of a s-wave one band superconductor in the framework of proximity
effect Eliashberg theory. In the weak electrostatic field limit the theory has
no free parameters while, in general, the only free parameter is the thickness
of the surface layer where the electric field acts. We conclude that the best
situation for increasing the critical temperature is to have a very thin film
of a superconducting material with a strong increase of electron-phonon (boson)
constant upon charging. | cond-mat_supr-con |
Nernst effect of the new iron-based superconductor
LaO$_{1-x}$F$_{x}$FeAs: We report the first Nernst effect measurement on the new iron-based
superconductor LaO$_{1-x}$F$_{x}$FeAs $(x=0.1)$. In the normal state, the
Nernst signal is negative and very small. Below $T_{c}$ a large positive peak
caused by vortex motion is observed. The flux flowing regime is quite large
compared to conventional type-II superconductors. However, a clear deviation of
the Nernst signal from normal state background and an anomalous depression of
off-diagonal thermoelectric current in the normal state between $T_{c}$ and 50
K are observed. We propose that this anomaly in the normal state Nernst effect
could correlate with the SDW fluctuations. | cond-mat_supr-con |
Transport evidence for the surface state and spin-phonon interaction in
FeTe$_{0.5}$Se$_{0.5}$: The iron chalcogenides have been proved to be intrinsic topological
superconductors to implement quantum computation because of their unique
electronic structures. The topologically nontrivial surface states of
FeTe$_{0.5}$Se$_{0.5}$ have been predicted by several calculations and then
confirmed by high-resolution photoemission and scanning tunneling experiments.
However, so far, the shreds of the electrical transport evidence for
topological surface states are still in absence. By carrying out electrical
transport experiments, we observe a topological transition with a nonlinear
Hall conductivity and simultaneous linear magnetoresistance near the
superconducting transition temperature. Furthermore, we observe a sign reversal
of the Hall coefficient accompanied by a concurrently softening of the
${A}_{1g}$ phonon mode at about 40 K, indicating a nematic transition. The
synchronized phonon softening with nematicity manifests an enhanced fluctuation
state through spin-phonon interaction. Our results solidly corroborate the
topological surface states of FeTe$_{0.5}$Se$_{0.5}$ and provide an
understanding of the mechanism of the superconductivity in iron chalcogenides. | cond-mat_supr-con |
Screening magnetic fields by a superconducting disk: a simple model: We introduce a simple approach to evaluate the magnetic field distribution
around superconducting samples, based on the London equations; the elementary
variable is the vector potential. This procedure has no adjustable parameters,
only the sample geometry and the London length, $\lambda$, determine the
solution. The calculated field reproduces quantitatively the measured induction
field above MgB$_2$ disks of different diameters, at 20K and for applied fields
lower than 0.4T. The model can be applied if the flux line penetration inside
the sample can be neglected when calculating the induction field distribution
outside the superconductor. Finally we show on a cup-shape geometry how one can
design a magnetic shield satisfying a specific constraint. | cond-mat_supr-con |
AC Loss in Striped (Filamentary) YBCO Coated Conductors Leading to
Designs for High Frequencies and Field-Sweep Amplitudes: AC losses of YBCO coated conductors are investigated by calculation and
experiment for the higher frequency regime. Previous research using YBCO film
deposited onto single crystal substrates demonstrated the effectiveness of
filamentary subdivision as a technique for AC loss reduction. As a result of
these studies the idea of subdividing YBCO coated conductors (both YBCO,
overlayer, and even underlayer) into such stripes suggested itself. The
suggestion was implemented by burning grooves into samples of coated conductor
using laser micromachining. Various machining parameters were investigated, and
the striping and slicing characteristics are presented. Loss measurements were
performed on unstriped as well as striped samples by the pick-up coil technique
at frequencies of from 50-200 Hz at field sweep amplitudes of up to 150 mT. The
effect of soft ferromagnetic Fe shielding was also investigated. The results of
the experiments form a starting point for a more general study of reduced loss
coated conductor design (including hysteretic, coupling, normal eddy current,
and transport losses) projected into higher ranges of frequency and field sweep
amplitude with transformer and all cryogenic motor/generator applications in
mind. | cond-mat_supr-con |
Non-equilibrium phenomena in superconductors probed by femtosecond
time-domain spectroscopy: Development of ultrafast lasers and non-linear optical techniques over the
last two decades provides tools to access real-time dynamics of low energy
excitations in superconductors. For example, time-resolved THz spectroscopy and
time- and angular-resolved photoemission spectroscopy provide access to the
real-time dynamics of the superconducting gap amplitude. Such studies enable
determination of microscopic parameters like quasi-particle recombination
rates, pair-breaking rates and electron-boson coupling constants. Recently,
intense THz pulses have been used to probe the non-linear dynamics, including
observation of collective modes. Moreover, using low frequency electromagnetic
pulses, there are several reports of amplification of superconductivity in both
conventional and unconventional superconductors. Starting with a brief
historical overview of the pioneering work, where non-equilibrium phenomena in
superconductors were investigated using quasi-continuous excitation, we review
some of the insights that are provided by using real-time approaches. We focus
on conventional BCS superconductors, whose ground state is reasonably well
understood, and address similarities and open questions related to the
corresponding studies in high-T$_{c}$ superconductors. | cond-mat_supr-con |
Carbon substitution effect in MgB$_2$: We investigated carbon substitution effect on boron plane of superconducting
MgB$_2$. MgB$_2$ and MgB$_{1.8}$C$_{0.2}$ samples are synthesized under high
pressure furnace. MgB$_{1.8}$C$_{0.2}$ are characterized as AlB$_2$-type single
phase with smaller B-B distance. During the superconducting transition, two
distinct onset temperatures are observed in MgB$_{1.8}$C$_{0.2}$. | cond-mat_supr-con |
Why an ac magnetic field shifts the irreversibility line in type-II
superconductors: We show that for a thin superconducting strip placed in a transverse dc
magnetic field - the typical geometry of experiments with high-Tc
superconductors - the application of a weak ac magnetic field perpendicular to
the dc field generates a dc voltage in the strip. This voltage leads to the
decay of the critical currents circulating in the strip, and eventually the
equilibrium state of the superconductor is established. This relaxation is not
due to thermally activated flux creep but to the "walking" motion of vortices
in the two-dimensional critical state of the strip with in-plane ac field. Our
theory explains the shaking effect that was used for detecting phase
transitions of the vortex lattice in superconductors with pinning. Some recent
experiments on this subject are discussed. | cond-mat_supr-con |
Reply to "Comment on Yerin et al., Phys. Rev. Lett. 121, 077002 (2018),
and Mironov et al., Phys. Rev. Lett. 109, 237002 (2012)" by A. F. Volkov, F.
S. Bergeret, and K. B. Efetov: Here we reply to the comment by A. F. Volkov, F. S. Bergeret, and K. B.
Efetov. | cond-mat_supr-con |
Lithium Beryllium Phosphide (LiBeP): A possible MgB2-like Superconductor: The search for materials similar to magnesium diboride, MgB2, based on
structure and electronic similarity did not produce close enough
superconductors in terms of Tc. Changing the search to iso-electronic and
iso-atomic number equivalents opened new doors to very many possible MgB2-like
superconductors. Here we present LiBeP which meets these new conditions. We
estimate its Tc to be 34.5K if two-gapped or 17.2K if single gapped. | cond-mat_supr-con |
Electronic Phase Separation Transition as the Origin of the
Superconductivity and the Pseudogap Phase of Cuprates: We propose a new phase of matter, an electronic phase separation transition
that starts near the upper pseudogap and segregates the holes into high and low
density domains. The Cahn-Hilliard approach is used to follow quantitatively
this second order transition. The resulting grain boundary potential confines
the charge in domains and favors the development of intragrain superconducting
amplitudes. The zero resistivity transition arises only when the intergrain
Josephson coupling $E_J$ is of the order of the thermal energy and phase
locking among the superconducting grains takes place. We show that this
approach explains the pseudogap and superconducting phases in a natural way and
reproduces some recent scanning tunneling microscopy data | cond-mat_supr-con |
Electronic band structure and chemical bonding in the novel
antiperovskite ZnCNi3 as compared with 8-K superconductor MgCNi3: Energy band structure of the discovered ternary perovskite-like compound
ZnCNi3 reported by Park et al (2004) as a non-superconducting paramagnetic
metal was investigated using the FLMTO-GGA method. The electronic bands,
density of states, Fermi surface, charge density and electron localization
function distributions for ZnCNi3 are obtained and analyzed in comparison with
the isoelectronic and isostructural 8K superconductor MgCNi3. The effect of
external pressure on the electronic states of ZnCNi3 and MgCNi3 is studied. | cond-mat_supr-con |
Superconductivity in $β$-pyrochlore superconductor KOs$_{2}$O$_{6}$:
treatment within strong-coupling Eliashberg theory: We study the influence of the rattling phonons on superconductivity in
$\beta$-pyrochlore KOs$_{2}$O$_{6}$ compound based on the strong-coupling
Eliashberg approach. In particular, analyzing the specific heat data we find
that the rattling phonon frequency changes discontinuously at the critical
temperature of the first order phase transition. Solving the strong-coupling
Eliashberg equations with temperature dependent $\alpha^{2}F(\omega)$, we
investigate the consequence of this first order phase transition for the
anomalous temperature dependence of the superconducting gap. We discuss our
results in context of the recent experimental data. | cond-mat_supr-con |
KFe_2Se_2 is the parent compound of K-doped iron selenide
superconductors: We elucidate the existing controversies in the newly discovered K-doped iron
selenide (KxFe2-ySe2-z) superconductors. The stoichiometric KFe2Se2 with
\surd2\times\surd2 charge ordering was identified as the parent compound of
KxFe2-ySe2-z superconductor using scanning tunneling microscopy and
spectroscopy. The superconductivity is induced in KFe2Se2 by either Se
vacancies or interacting with the anti-ferromagnetic K2Fe4Se5 compound. Totally
four phases were found to exist in KxFe2-ySe2-z: parent compound KFe2Se2,
superconducting KFe2Se2 with \surd2\times\surd5 charge ordering,
superconducting KFe2Se2-z with Se vacancies and insulating K2Fe4Se5 with
\surd5\times\surd5 Fe vacancy order. The phase separation takes place at the
mesoscopic scale under standard molecular beam epitaxy condition. | cond-mat_supr-con |
The Gradient Expansion for the Free-Energy of a Clean Superconductor: We describe a novel method for obtaining the gradient expansion for the free
energy of a clean BCS superconductor. We present explicit results up to fourth
order in the gradients of the order parameter. | cond-mat_supr-con |
Zurek-Kibble domain structures: The Dynamics of Spontaneous Vortex
formation in Annular Josephson Tunnel Junctions: Phase transitions executed in a finite time show a domain structure with
defects, that has been argued by Zurek and Kibble to depend in a characteristic
way on the quench rate. In this letter we present an experiment to measure the
Zurek-Kibble scaling exponent sigma. Using symmetric and long Josephson Tunnel
Junctions, for which the predicted index is sigma = 0.25, we find sigma = 0.27
+/- 0.05. Further, there is agreement with the ZK prediction for the overall
normalisation. | cond-mat_supr-con |
Superconductivity of the New Medium-Entropy Alloy V4Ti2W with a
Body-Centered Cubic Structure: Medium- and high-entropy alloy (MEA and HEA) superconductors have attracted
considerable interest since their discovery. This paper reports the
superconducting properties of ternary tungsten-containing MEA V4Ti2W for the
first time. V4Ti2W is a type II superconductor with a body-centered cubic (BCC)
structure. Experimental results of resistivity, magnetization, and heat
capacity indicate that the superconducting transition temperature of the MEA
V4Ti2W is roughly 5.0 K. The critical magnetic fields at the upper and lower
ends are 9.93(2) T and 40.7(3) mT, respectively. Interestingly, few BCC MEA
superconductors with VEC greater than 4.8 have been found. The addition of
tungsten leads to a VEC of 4.83 e/a for V4Ti2W, which is rarely higher than the
4.8 value. Adding tungsten element expands the variety of MEA alloys, which may
improve the microstructure and mechanical properties of materials and even
superconducting properties. This material could potentially offer a new
platform for the investigation of innovative MEA and HEA superconductors. | cond-mat_supr-con |
Elliptical vortex and oblique vortex lattice in the FeSe superconductor
based on the nematicity and mixed superconducting orders: The electronic nematic phase is characterized as an ordered state of matter
with rotational symmetry breaking, and has been well studied in the quantum
Hall system and the high-$T_c$ superconductors, regardless of cuprate or
pnictide family. The nematic state in high-$T_c$ systems often relates to the
structural transition or electronic instability in the normal phase.
Nevertheless, the electronic states below the superconducting transition
temperature is still an open question. With high-resolution scanning tunneling
microscope measurements, direct observation of vortex core in FeSe thin films
revealed the nematic superconducting state by Song \emph{et al}. Here,
motivated by the experiment, we construct the extended Ginzburg-Landau free
energy to describe the elliptical vortex, where a mixed \emph{s}-wave and
\emph{d}-wave superconducting order is coupled to the nematic order. The
nematic order induces the mixture of two superconducting orders and enhances
the anisotropic interaction between the two superconducting orders, resulting
in a symmetry breaking from $C_4$ to $C_2$. Consequently, the vortex cores are
stretched into an elliptical shape. In the equilibrium state, the elliptical
vortices assemble a lozenge-like vortex lattice, being well consistent with
experimental results. | cond-mat_supr-con |
New Superconductivity Dome in LaFeAsO$_{1-x}$F$_{x}$ Accompanied by
Structural Transition: High temperature superconductivity is often found in the vicinity of
antiferromagnetism. This is also true in LaFeAsO$_{1-x}$F$_{x}$ ($x \leq$ 0.2)
and many other iron-based superconductors, which leads to proposals that
superconductivity is mediated by fluctuations associated with the nearby
magnetism. Here we report the discovery of a new superconductivity dome without
low-energy magnetic fluctuations in LaFeAsO$_{1-x}$F$_{x}$ with 0.25$\leq x
\leq$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ =
0.5$\sim$0.55 is even higher than that at $x \leq$ 0.2. By nuclear magnetic
resonance and Transmission Electron Microscopy, we show that a C4 rotation
symmetry-breaking structural transition takes place for $x>$ 0.5 above $T_c$.
Our results point to a new paradigm of high temperature superconductivity. | cond-mat_supr-con |
Two-dimensional magnetism in the pnictide superconductor parent material
SrFeAsF probed by muon-spin relaxation: We report muon-spin relaxation measurements on SrFeAsF, which is the parent
compound of a newly discovered iron-arsenic-fluoride based series of
superconducting materials. We find that this material has very similar magnetic
properties to LaFeAsO, such as separated magnetic and structural transitions
(TN = 120 K, Ts = 175 K), contrasting with SrFe2As2 where they are coincident.
The muon oscillation frequencies fall away very sharply at TN, which suggests
that the magnetic exchange between the layers is weaker than in comparable
oxypnictide compounds. This is consistent with our specific heat measurements,
which find that the entropy change S = 0.05 J/mol/K largely occurs at the
structural transition and there is no anomaly at TN. | cond-mat_supr-con |
Flux Pinning by Nano Particles Embedded in Polycrystalline Y-123
Superconductors: Bulk superconductor samples of YBa2Cu3-xZnxO7-{\delta} with x = 0, 0.01, 0.03
are synthesized by solid-state reaction route. The structural characterisation
of all samples has been carried out by x-ray-diffraction (XRD) and transmission
electron microscopy (TEM) techniques. The x-ray diffraction patterns indicate
that the gross structure/phase of YBa2Cu3-xZnxO7-{\delta} do not change with
the substitution of Zn up to x=0.03. In TEM investigations of Zn-doped Y-based
cuprates a number of ZnO nano-flower and nano-rod of Y-211 phase are found
dispersed in regular YBa2Cu3-xZnxO7 matrix. These dispersed nano-flowers of ZnO
and nano-rods of Y-211 phase may serve as flux-pinning centers. These pinning
centers enhance critical current density (Jc) value of these HTSC samples. | cond-mat_supr-con |
Microwave transmissions through superconducting coplanar waveguide
resonators with different coupling configurations: We design and fabricate two types of superconducting niobium coplanar
waveguide microwave resonators with different coupling capacitors on high
purity Si substrates. Their microwave transmissions are measured at the
temperatures of 20 mK. It is found that these two types of resonators possess
significantly-different loaded quality factors; one is $5.6\times{10}^{3}$, and
the other is $4.0\times{10}^{4}$. The measured data are fitted well by
classical ABCD matrix approach. We found that the transmission peak deviates
from the standard Lorentizian with a frequency broadening. | cond-mat_supr-con |
Chemically gated electronic structure of a superconducting doped
topological insulator system: Angle resolved photoemission spectroscopy is used to observe changes in the
electronic structure of bulk-doped topological insulator Cu$_x$Bi$_2$Se$_3$ as
additional copper atoms are deposited onto the cleaved crystal surface. Carrier
density and surface-normal electrical field strength near the crystal surface
are estimated to consider the effect of chemical surface gating on atypical
superconducting properties associated with topological insulator order, such as
the dynamics of theoretically predicted Majorana Fermion vortices. | cond-mat_supr-con |
Vortex dynamics in two-dimensional Josephson junction arrays: The dynamic response of unfrustrated two-dimensional Josephson junction
arrays close to, but above the Kosterlitz-Thouless($KT$) transition temperature
is described in terms of the vortex dielectric function $\epsilon(\omega)$. The
latter is calculated by considering separately the contribution of
$\ll$free$\gg$ vortices interacting by a screened Coulomb potential, and the
$\ll$pair motion$\gg$ of vortices that are closer to each other than the $KT$
correlation length. This procedure allows to understand various anomalous
features in $\epsilon(\omega)$ and in the flux noise spectra that have been
observed experimentally and in dynamic simulations. | cond-mat_supr-con |
Impurity-induced quasiparticle interference in the parent compounds of
iron-pnictide superconductors: The impurity-induced quasiparticle interference(QPI) in the parent compounds
of iron-pnictide superconductors is investigated based on a phenomenological
two-orbital four-band model and T-matrix method. We find the QPI is sensitive
to the value of the magnetic order which may vary from one compound to another.
For small value of the magnetic order, the pattern of oscillation in the local
density of states (LDOS) induced by the QPI exhibits two-dimensional
characteristics, consistent with the standing wave state observed in the 1111
compound. For larger value of the magnetic order, the main feature of the
spatial modulation of the LDOS is the existence of one-dimensional stripe
structure which is in agreement with the nematic structure in the parent
compound of the 122 system. In both cases the system shows $C_2$ symmetry and
only in the larger magnetic order case, there exist in-gap bound states. The
corresponding QPI in $q$-space is also presented. The patterns of modulation in
the LDOS at nonzero energies are attributed to the interplay between the
underlying band structure and Fermi surfaces. | cond-mat_supr-con |
Neutron Scattering Studies of spin excitations in hole-doped
Ba0.67K0.33Fe2As2 superconductor: We report inelastic neutron scattering experiments on single crystals of
superconducting Ba0.67K0.33Fe2As2 (Tc = 38 K). In addition to confirming the
resonance previously found in powder samples, we find that spin excitations in
the normal state form longitudinally elongated ellipses along the QAFM
direction in momentum space, consistent with density functional theory
predictions. On cooling below Tc, while the resonance preserves its momentum
anisotropy as expected, spin excitations at energies below the resonance become
essentially isotropic in the in-plane momentum space and dramatically increase
their correlation length. These results suggest that the superconducting gap
structures in Ba0.67Ka0.33Fe2As2 are more complicated than those suggested from
angle resolved photoemission experiments. | cond-mat_supr-con |
Scanning tunnelling spectroscopy of the vortex state in NbSe2 using a
superconducting tip: The vortex electronic structure in the multiband superconductor NbSe2 is
studied by means of Scanning Tunneling Spectroscopy (STS) using a
superconducting tip. The use of a superconducting tip (Pb) as a probe provides
an enhancement of the different features related to the DOS of NbSe2 in the
tunneling conductance curves. This use allows the observation of rich patterns
of electronic states in the conductance images around the vortex cores in a
wide range of temperature, as well as the simultaneous acquisition of Josephson
current images in the vortex state. | cond-mat_supr-con |
Superconductivity without inversion and time-reversal symmetries: The traditional symmetries that protect superconductivity are time-reversal
and inversion. Here, we examine the minimal symmetries protecting
superconductivity in two dimensions and find that time-reversal symmetry and
inversion symmetry are not required, and having a combination of either
symmetry with a mirror operation on the basal plane is sufficient. We classify
superconducting states stabilized by these two symmetries, when time-reversal
and inversion symmetries are not present, and provide realistic minimal models
as examples. Interestingly, several experimentally realized systems, such as
transition metal dichalcogenides and the two-dimensional Rashba system belong
to this category, when subject to an applied magnetic field. | cond-mat_supr-con |
Effect of metal vacancies on the electronic band structure of hexagonal
Nb, Zr and Y diborides: Energy band structures of metal-deficient hexagonal diborides M$_{0.75}$B$_2$
(M = Nb, Zr and Y) were calculated using the full-potential LMTO method. The
metal vacancies change the density of states near the Fermi level and this
effect is quite different for III-V group transition metal diborides.
Contradictory data on superconductivity in diborides may be supposed to be
connected with nonstoichiometry of samples. Vacancy formation energies are
estimated and analyzed. | cond-mat_supr-con |
Trilayer multi-orbital models of $\mathrm{La_{4}Ni_{3}O_{10}}$: Recently, the discovery of superconductivity in Ruddlesden-Popper (RP)
$\mathrm{La_4Ni_3O_{10}}$ under pressure has further expanded the realm of
nickelate-based superconductor family. In this paper, we performed a
first-principle study of $\mathrm{La_4Ni_3O_{10}}$ for both $P2_1/a$ phase at
ambient pressure and $I4/mmm$ phase at high pressure, with $U$=0, 3.5 eV. Our
results confirmed the characteristic upward shift of Ni-$d_{z^2}$ bonding band
under pressure. Moreover, our analysis of electronic spectrum and orbital
occupancy unveil the dynamical mechanism of electronic reconstructions under
pressure, embedded in a critical dual effect. Based on our DFT results, we
further proposed a trilayer two-orbital model by performing Wannier downfolding
on Ni-$e_g$ orbitals. Our model reveals four Fermi surface sheets with
$\alpha,\beta,\beta^\prime,\gamma$ pockets, bearing resemblance to that of
bilayer $\mathrm{La_3Ni_2O_7}$. According to the model, our calculated spin
susceptibility under random phase approximation predicts an analogous magnetic
signal at ${\rm q}=(\frac{\pi}{2},\frac{\pi}{2})$, which is more associated
with nesting within $\beta,\beta^\prime$ pockets. Finally, a high energy
sixteen-orbital model with direct $dp,pp$ hoppings is proposed, which implies
that $\mathrm{La_4Ni_3O_{10}}$ also lies in charge-transfer picture within
Zaanen-Sawatzky-Allen scheme. Our exposition of electronic reconstructions and
multi-orbital models shed light on theoretical electronic correlation study and
experimental exploration for lower pressure in RP series. | cond-mat_supr-con |
Route to Observable Fulde-Ferrell-Larkin-Ovchinnikov Phases in 3D
Spin-Orbit Coupled Degenerate Fermi Gases: The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase was first predicted in 2D
superconductors about 50 years ago, but so far unambiguous experimental
evidences are still lacked. The recently experimentally realized
spin-imbalanced Fermi gases may potentially unveil this elusive state, but
require very stringent experimental conditions. In this Letter, we show that
FFLO phases may be observed even in a 3D degenerate Fermi gas with spin-orbit
coupling and in-plane Zeeman field. The FFLO phase is driven by the interplay
between asymmetry of Fermi surface and superfluid order, instead of the
interplay between magnetic and superconducting order in solid materials. The
predicted FFLO phase exists in a giant parameter region, possesses a stable
long-range superfluid order due to the 3D geometry, and can be observed with
experimentally already achieved temperature ($T\sim 0.05E_{F}$), thus opens a
new fascinating avenue for exploring FFLO physics. | cond-mat_supr-con |
Pseudogap-like phase in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ revealed by
$^{75}$As NQR: We report $^{75}$As NQR measurements on single crystalline
Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0\leq x \leq 0.09$). The nuclear spin-lattice
relaxation rate $T_1^{-1}$ as a function of temperature $T$ and Co dopant
concentration $x$ reveals a normal-state pseudogap-like phase below a crossover
temperature $T^*$ in the under- and optimally-doped region. The resulting
$x$-$T$ phase diagram shows that, after suppression of the spin-density-wave
order, $T^*$ intersects $T_c$ falling to zero rapidly near the optimal doping
regime. Possible origins of the pseudogap behavior are discussed. | cond-mat_supr-con |
Enhancement of functional properties of V$_{0.6}$Ti$_{0.4}$ alloy
superconductor by the addition of yttrium: We show here that the yttrium is immiscible and precipitates with various
sizes in the body centred cubic V$_{0.6}$Ti$_{0.4}$ alloy superconductor. The
number and size of the precipitates are found to depend on the amount of
yttrium added. Precipitates with various sizes up to 30~$\mu$m are found in the
V$_{0.6}$Ti$_{0.4}$ alloy containing 5 at.\% yttrium. The large amount of line
disorders generated by the addition of yttrium in this alloy are found to be
effective in pinning the magnetic flux lines. While the superconducting
transition temperature increases with the increasing amount of yttrium in the
V$_{0.6}$Ti$_{0.4}$ alloy, the critical current density is maximum for the
alloy containing 2 at. \% yttrium, where it is more than 7.5 times the parent
alloy in fields higher than 1~T. We found that the effectiveness of each type
of defect in pinning the flux lines is dependent on the temperature and the
applied magnetic filed. | cond-mat_supr-con |
High-$T$$_\textrm{C}$ Superconductivity in Hydrogen Clathrates Mediated
by Coulomb Interactions between Hydrogen and Central-Atom Electrons: The uniquely characteristic macrostructures of binary hydrogen-clathrate
compounds $M$H$_\textrm{n}$ formed at high pressure, a cage of hydrogens
surrounding a central-atom host, is theoretically predicted in various studies
to include structurally stable phonon-mediated superconductors. High
superconductive transition temperatures $T$$_\textrm{C}$ have thus far been
measured for syntheses with $M$ = La, Y, and Th. In compressed
LaH$_\textrm{10}$, independent studies report $T$$_\textrm{C}$ of 250 K and
over 260 K, a maximum in $T$$_\textrm{C}$ with pressure $P$, and normal-state
resistance scaling with temperature (suggesting unconventional pairing).
According to reported band structure calculations of $Fm$$\bar{3}$$m$-phase
LaH$_\textrm{10}$, the La is anionic, with the chemical valence electrons
appearing evenly split between La and H$_\textrm{10}$. Thus, compressed
LaH$_\textrm{10}$ contains the combination of structure, charge separation, and
optimal balanced allocation of valence electrons for supporting unconventional
high-$T$$_\textrm{C}$ superconductivity mediated by Coulomb interactions
between electronic charges associated with La and H$_\textrm{10}$. A general
expression for the optimal superconducting transition temperature for
$M$H$_\textrm{n}$ clathrates is derived as $T$$_\textrm{C0}$ =
$k$$_\textrm{B}$$^{-1}$$\Lambda$[(n + $v$)/2$A$]$^{1/2}$$e$$^{2}$/$\zeta$,
where $\Lambda$ is a universal constant, (n + $v$) is the chemical valence sum
per formula unit, taking unity for H and $v$ for atom $M$, $A$ is the surface
area of the H-polyhedron cage, and $\zeta$ is the mean distance between the $M$
site and the centroids of the polyhedron faces. Applied to $Fm$$\bar{3}$$m$
LaH$_\textrm{10}$, $T$$_\textrm{C0}$ values of 249.8(1.3) K and 260.7(2.0) K
are found for the two experiments. Associated attributes of charge allocation,
structure, effective Coulomb potential, . . . | cond-mat_supr-con |
Optimal interlayer hopping and high temperature Bose-Einstein
condensation of local pairs in quasi 2D superconductors: Both FeSe and cuprate superconductors are quasi 2D materials with high
transition temperatures and local fermion pairs. Motivated by such systems, we
investigate real space pairing of fermions in an anisotropic lattice model with
intersite attraction, V, and strong local Coulomb repulsion, U, leading to a
determination of the optimal conditions for superconductivity from
Bose-Einstein condensation. Our aim is to gain insight as to why high
temperature superconductors tend to be quasi 2D. We make both analytically and
numerically exact solutions for two body local pairing applicable to
intermediate and strong V. We find that the Bose Einstein condensation
temperature of such local pairs is maximal when hopping between layers is
intermediate relative to in-plane hopping, indicating that the quasi 2D nature
of unconventional superconductors has an important contribution to their high
transition temperatures. | cond-mat_supr-con |
Reversible transitions in high-$T_c$ cuprates based point contacts: The influence of electric felds and currents has been investigated in the
high-$T_c$ superconductors $YBaCuO$ and $BiSrCaCuO$ using a point-contact
geometry with $Ag$ as the counterelectrode, which reveal switching transitions
between states of a different resistance. The origin of this effect in point
contacts is associated with electromigration of the oxygen, driven by the
electric feld as well as by the currentinduced "electron wind". The switching
effect preserves its basic features at elevated temperatures up to room
temperature and in high magnetic felds up to 10 T. | cond-mat_supr-con |
Finite energy spin fluctuation as a pairing glue in systems with
coexisting electron and hole bands: We study, within the fluctuation exchange approximation, the
spin-fluctuation-mediated superconductivity in Hubbard-type models possessing
electron and hole bands, and compare them with a model on a square lattice with
a large Fermi surface. In the square lattice model, superconductivity is more
enhanced for better nesting for a fixed band filling. By contrast, in the
models with electron and hole bands, superconductivity is optimized when the
Fermi surface nesting is degraded to some extent, where finite energy spin
fluctuation around the nesting vector develops. The difference lies in the
robustness of the nesting vector, namely, in models with electron and hole
bands, the wave vector at which the spin susceptibility is maximized is fixed
even when the nesting is degraded, whereas when the Fermi surface is large, the
nesting vector varies with the deformation of the Fermi surface. We also
discuss the possibility of realizing in actual materials the bilayer Hubbard
model, which is a simple model with electron and hole bands, and is expected to
have a very high T_c. | cond-mat_supr-con |
Cold atoms near superconductors: Atomic spin coherence beyond the
Johnson noise limit: We report on the measurement of atomic spin coherence near the surface of a
superconducting niobium wire. As compared to normal conducting metal surfaces,
the atomic spin coherence is maintained for time periods beyond the Johnson
noise limit. The result provides experimental evidence that magnetic near field
noise near the superconductor is strongly suppressed. Such long atomic spin
coherence times near superconductors open the way towards the development of
coherently coupled cold atom / solid state hybrid quantum systems with
potential applications in quantum information processing and precision force
sensing. | cond-mat_supr-con |
Majorana-assisted nonlocal electron transport through a floating
topological superconductor: The nonlocal nature of the fermionic mode spanned by a pair of Majorana bound
states in a one-dimensional topological superconductor has inspired many
proposals aiming at demonstrating this property in transport. In particular,
transport through the mode from a lead attached to the left bound state to a
lead attached to the right will result in current cross-correlations. For ideal
zero modes on a grounded superconductor, the cross-correlations are however
completely suppressed in favor of purely local Andreev reflection. In order to
obtain a non-vanishing cross-correlation, previous studies have required the
presence of an additional global charging energy. Adding nonlocal terms in the
form of a global charging energy to the Hamiltonian when testing the intrinsic
nonlocality of the Majorana modes seems to be conceptually troublesome. Here,
we show that a floating superconductor allows to observe nonlocal current
correlations in the absence of charging energy. We show that the
non-interacting and the Coulomb-blockade regime have the same peak conductance
$e^2/h$ but different shot-noise power; while the shot noise is sub-Poissonian
in the Coulomb-blockade regime in the large bias limit, Poissonian shot noise
is generically obtained in the non-interacting case. | cond-mat_supr-con |
Single crystal growth and optical conductivity of SrPt$_2$As$_2$
superconductors: SrPt$_2$As$_2$ single crystals with CaBe$_2$Ge$_2$-type structure were
synthesized by self-melting technique. X-ray diffraction, transmission electron
microscopy, electrical resistivity, specific heat and optical spectroscopy
measurements were conducted to elucidate the properties of SrPt$_2$As$_2$.
SrPt$_2$As$_2$ single crystals exhibit a superconducting transition at 5.2K,
experiencing a structural phase transition well above room temperature (about
455K). The superconducting and structural phase transition temperatures are
both reduced by 6% Iridium doping. Both pure SrPt$_2$As$_2$ and the doped
single crystals are revealed to be highly metallic with rather high plasma
frequencies. In particular, the optical spectroscopy measurement revealed two
gap-like suppression features. We elaborated that the one at higher energy
scale could be ascribed to the correlation effect, while the other one at lower
energy scale is related to the structural phase transition, leading to the
removal of a small portion of the Fermi surfaces. | cond-mat_supr-con |
S-shaped suppression of the superconducting transition temperature in
Cu-intercalated NbSe2: 2H-NbSe2 is the prototype and most frequently studied of the well-known
transition metal dichalcogenide (TMDC) superconductors. Widely acknowledged to
be a conventional superconductor, its transition temperature to the
superconducting state (Tc) is 7.3 K - a Tc that is substantially higher than
those seen for the majority of TMDCs, where Tcs between 2 and 4 K are the norm.
Here we report the intercalation of Cu into 2H-NbSe2 to make CuxNbSe2. As is
typically found when chemically altering an optimal superconductor, Tc
decreases with increasing x, but the way that Tc is suppressed in this case is
unusual - an S-shaped character is observed, with an inflection point near x =
0.03 and, at higher x, a leveling off of the Tc near 3 K - down to the usual
value for a layered TMDC. Electronic characterization reveals corresponding
S-like behavior for many of the materials parameters that influence Tc. To
illustrate its character, the superconducting phase diagram for CuxNbSe2 is
contrasted to those of FexNbSe2 and NbSe2-xSx. | cond-mat_supr-con |
Superconductivity and topological behavior in gallenene: Among the large variety of two-dimensional (2D) materials discovered to date,
elemental monolayers that host superconductivity are very rare. Using ab initio
calculations we show that recently synthesized gallium monolayers, coined
gallenene, are intrinsically superconducting through electron-phonon coupling.
We reveal that Ga-100 gallenene, a planar monolayer isostructural with
graphene, is the structurally simplest 2D superconductor to date, furthermore
hosting topological edge states due to its honeycomb structure. Our anisotropic
Eliashberg calculations show distinctly three-gap superconductivity in Ga-100,
in contrast to the alternative buckled Ga-010 gallenene which presents a single
anisotropic superconducting gap. Strikingly, the critical temperature ($T_c$)
of gallenene is in the range of $7-10$ K, exceeding the $T_c$ of bulk gallium
from which it is exfoliated. Finally we explore chemical functionalization of
gallenene with hydrogen, and report induced multigap superconductivity with an
enhanced $T_c$ in the resulting gallenane compound. | cond-mat_supr-con |
Superconductivity at 4K in Pd deficient layered Ta_2Pd_xS_6: Here in this short note, we report on the low dimensional 4d and 5d
transition metals-chalcogenide based compounds i.e., Ta2PdxS6, showing
semiconducting to superconducting transition around 4K with upper critical
field outside Pauli paramagnetic limit. It seems couple of different
superconducting phases do exist in these new set of compounds. Our short note
in this regards is thought provoking, asking to explore various unearthed
possible superconducting phases in (Nb/Ta)2Pdx(S/Se/Te)y systems. | cond-mat_supr-con |
A-15 type superconducting hydride $La_4H_{23}$: Nanograined structure
with low strain, strong electron-phonon interaction, and moderate level of
nonadiabaticity: For seven decades by A-15 superconductors we meant metallic $A_3B$ alloys
(where A is a transition metal, and B is groups IIIB and IVB element)
discovered by Hardy and Hulm (Phys. Rev. 89, 884 (1953)). Nb3Ge exhibited the
highest superconducting transition temperature, $T_c = 23 K$, among these
alloys. One of these alloys, $Nb_3Sn$, is primary material in modern applied
superconductivity. Recently Guo et al (arXiv:2307.13067) extended the family of
superconductors where the metallic ions arranged in the beta tungsten (A-15)
sublattice by observation of $T_{c,zero} = 81 K$ in $La_4H_{23}$ phase
compressed at $P = 118 GPa$. Despite the $La_4H_{23}$ has much lower $T_c$ in
comparison with near-room-temperature superconducting $LaH_{10}$ phase
($T_{c,zero} = 250 K$ at $P = 200 GPa$) discovered by Drozdov et al (Nature
569, 531 (2019)), the $La_4H_{23}$ holds the record high $T_c$ within A-15
family. Cross et al (Phys. Rev. B 109, L020503 (2024)) confirmed the
high-temperature superconductivity in the compressed $La_4H_{23}$. In this
paper, we analyzed available experimental data measured in $La_4H_{23}$ and
found that this superconductor exhibits nanograined structure, 6 nm < D < 27
nm, low crystalline strain < 0.005, high electron-phonon coupling constant, 1.5
< $\lambda_{e-ph}$ < 2.7, and moderate level of the nonadiabaticity
$\Theta_{D}/T_{F}$. We found that derived $\Theta_{D}/T_{F}$ and $T_c/T_F$
values for the $La_4H_{23}$ phase are similar to the ones in cuprates,
pnictides, and near-room-temperature superconductors $H_3S$ and $LaH_{10}$,
which implies that the $La_4H_{23}$ phase falls to unconventional
superconductors band in the Uemura plot. | cond-mat_supr-con |
Doping dependence of the coupling of electrons to bosonic modes in the
single-layer high-temperature Bi2Sr2CuO6 superconductor: A recent highlight in the study of high-Tc superconductors is the observation
of band renormalization / self-energy effects on the quasiparticles. This is
seen in the form of kinks in the quasiparticle dispersions as measured by
photoemission and interpreted as signatures of collective bosonic modes
coupling to the electrons. Here we compare for the first time the self-energies
in an optimally doped and strongly overdoped, non-superconducting single-layer
Bi-cuprate (Bi2Sr2CuO6). Besides the appearance of a strong overall weakening,
we also find that weight of the self-energy in the overdoped system shifts to
higher energies. We present evidence that this is related to a change in the
coupling to c-axis phonons due to the rapid change of the c-axis screening in
this doping range. | cond-mat_supr-con |
Tuning the static spin-stripe phase and superconductivity in
La_(2-x)Ba_xCuO_4 (x = 1/8) by hydrostatic pressure: Magnetization and muon spin rotation experiments were performed in
La_(2-x)Ba_xCuO_4 (x = 1/8) as a function of hydrostatic pressure up to p = 2.2
GPa. It was found that the magnetic volume fraction of the static stripe phase
strongly decreases linearly with pressure, while the superconducting volume
fraction increases by the same amount. This demonstrates competition between
bulk superconductivity and static magnetic order in the stripe phase of
La_1.875Ba_0.125CuO_4 and that these phenomena occur in mutually exclusive
spatial regions. The present results also reveal that the static spin-stripe
phase still exists at pressures, where the long-range low-temperature
tetragonal (LTT) structure is completely suppressed. This indicates that the
long-range LTT structure is not necessary for stabilizing the static spin order
in La_1.875Ba_0.125CuO_4. | cond-mat_supr-con |
Strong charge fluctuations manifested in the high-temperature Hall
coefficient of high-T_c cuprates: By measuring the Hall coefficient R_H up to 1000 K in La_2CuO_4,
Pr_{1.3}La_{0.7}CuO_4, and La_{2-x}Sr_xCuO_4 (LSCO), we found that the
temperature (T) dependence of R_H in LSCO for x = 0 - 0.05 at high temperature
undoubtedly signifies a gap over which the charge carriers are thermally
activated, which in turn indicates that in lightly-doped cuprates strong charge
fluctuations are present at high temperature and the carrier number is not a
constant. At higher doping (x = 0.08 - 0.21), the high-temperature R_H(T)
behavior is found to be qualitatively the same, albeit with a weakened
temperature dependence, and we attempt to understand its behavior in terms of a
phenomenological two-carrier model where the thermal activation is considered
for one of the two species. Despite the crude nature of the model, our analysis
gives a reasonable account of R_H both at high temperature and at 0 K for a
wide range of doping, suggesting that charge fluctuations over a gap remain
important at high temperature in LSCO deep into the superconducting doping
regime. Moreover, our model gives a perspective to understand the seemingly
contradicting high-temperature behavior of R_H and the in-plane resistivity
near optimum doping in a consistent manner. Finally, we discuss possible
implications of our results on such issues as the scattering-time separation
and the large pseudogap. | cond-mat_supr-con |
Number theory, periodic orbits and superconductivity in nano-cubes: We study superconductivity in isolated superconducting nano-cubes and
nano-squares of size $L$ in the limit of negligible disorder, $\delta/\Delta_0
\ll 1$ and $k_F L \gg 1$ for which mean-field theory and semiclassical
techniques are applicable, with $k_F$ the Fermi wave vector, $\delta$ the mean
level spacing and $\Delta_0$ the bulk gap. By using periodic orbit theory and
number theory we find explicit analytical expressions for the size dependence
of the superconducting order parameter. Our formalism takes into account
contributions from both the spectral density and the interaction matrix
elements in a basis of one-body eigenstates. The leading size dependence of the
energy gap in three dimensions seems to be universal as it agrees with the
result for chaotic grains. In the region of parameters corresponding to
conventional metallic superconductors, and for sizes $L \gtrsim 10$nm, the
contribution to the superconducting gap from the matrix elements is substantial
($\sim 20\%$). Deviations from the bulk limit are still clearly observed even
for comparatively large grains $L \sim 50$nm. These analytical results are in
excellent agreement with the numerical solution of the mean-field gap equation. | cond-mat_supr-con |
Probing itinerant antiferromagnetism with $d$-wave Andreev reflection
spectroscopy: To study how Andreev reflection (AR) is affected by itinerant
antiferromagnetism, we perform $d$-wave AR spectroscopy with superconducting
YBa$_2$Cu$_3$O$_{7-\delta}$ on TiAu and on variously-oxidized Nb (NbO$_x$)
samples. X-ray photoelectron spectroscopy is also used on the latter to measure
their surface oxide composition. Below the N\'eel temperatures ($T_N$) of both
TiAu and NbO$_x$, the conductance spectra show a dip-like structure instead of
a zero-bias peak within the superconducting energy gap; for NbO$_x$,
higher-oxidized samples show a stronger spectral dip at zero bias. These
observations indicate that itinerant antiferromagnetic order suppresses the AR
process. Interestingly, the spectral dip persists above $T_N$ for both TiAu and
NbO$_x$, implying that spin fluctuations can also suppress AR. Our results
suggest that $d$-wave AR spectroscopy may be used to probe the degree of spin
ordering in itinerant antiferromagnets. | cond-mat_supr-con |
Reentrant AC magnetic susceptibility in Josephson-junction arrays: An
alternative explanation for the paramagnetic Meissner effect: The paramagnetic Meissner effect (PME) measured in high $T_{C}$ granular
superconductors has been attributed to the presence of $\pi$-junctions between
the grains. Here we present measurements of complex AC magnetic susceptibility
from two-dimensional arrays of conventional (non $\pi$) Nb/Al/AlOx/Nb Josephson
junctions. We measured the susceptibility as a function of the temperature $T$,
the AC amplitude of the excitation field, $h_{AC}$, and the external magnetic
field, $H_{DC}$. The experiments show a strong paramagnetic contribution from
the multi-junction loops, which manifests itself as a reentrant screening at
low temperature, for values of $h_{AC}$ higher than 50 mOe. A highly simplified
model, based on a single loop containing four junction, accounts for this
paramagnetic contribution and the range of parameters in which it appears. This
model offers an alternative explanation of PME which does not involve
$\pi$-junctions. | cond-mat_supr-con |
Suppression of superconductivity by Neel-type magnetic fluctuations in
the iron pnictides: Motivated by recent experimental detection of Neel-type ($(\pi,\pi)$)
magnetic fluctuations in some iron pnictides, we study the impact of competing
$(\pi,\pi)$ and $(\pi,0)$ spin fluctuations on the superconductivity of these
materials. We show that, counter-intuitively, even short-range, weak Neel
fluctuations strongly suppress the $s^{+-}$ state, with the main effect arising
from a repulsive contribution to the $s^{+-}$ pairing interaction, complemented
by low frequency inelastic scattering. Further increasing the strength of the
Neel fluctuations leads to a low-$T_{c}$ d-wave state, with a possible
intermediate $s+id$ phase. The results suggest that the absence of
superconductivity in a series of hole-doped pnictides is due to the combination
of short-range Neel fluctuations and pair-breaking impurity scattering, and
also that $T_{c}$ of optimally doped pnictides could be further increased if
residual $(\pi,\pi)$ fluctuations were reduced. | cond-mat_supr-con |
Strong electron-lattice coupling and orbital fluctuations in iron
pnictide superconductor Ba(Fe1-xCox)2As2: This paper has been withdrawn by the author due to some experimental
mistakes. In this paper, we reported that C66, C44 and (C11-C12)/2 show
remarkable softening toward the structural transition temperature TS. The data
reported in this paper were acquired using the ultrasonic frequency lower than
25 MHz. Recently, we performed high-frequency measurements for the same system.
We found that the anomaly of C44 and (C11-C12)/2 tend to disappear rapidly with
increasing the frequency. On the other hand, C66 anomaly is still there at high
frequencies. Therefore, we concluded that the observed anomalies in C44 and
(C11-C12)/2 are not true. They would be ascribed to certain influence by the
large softening of C66. So, we have checked our data through careful
measurements by using ultrasonic frequency higher than 60 MHz, so far. Then, it
has been found that C66 shows still nice softening toward TS, but that its
temperature dependence is slightly different from the results of this paper. We
have accumulated reliable data now. They will be reported in near future. | cond-mat_supr-con |
Comparison of the fluctuation influence on the resistive properties of
the mixed state of BiSrCaCuO and of thin films of conventional superconductor: The resistive properties of layered HTSC BiSrCaCuO in the mixed state are
compared with those of thin films of conventional superconductors with weak
disorders (amorphous Nb_{1-x}0_{x} films) and with strong disorders
(Nb_{1-x}O_{x} films with small grain structure). The excess conductivity is
considered as a function of superconducting electron density and phase
coherence length. It is shown that the transition to the Abrikosov state
differs from the ideal case both in BiSrCaCuO and Nb_{1-x}O_{x} films, i.e. the
appearance of long-range phase coherence is continuous transition in both
cases. The quantitative difference between thin films with weak and strong
disorders is greater than the one between layered HTSC and conventional
superconductors, showing that the dimensionality of the system, rather than the
critical temperature, is the key factor ruling fluctuation effects | cond-mat_supr-con |
Magnetic field induced polarization effects in intrinsically granular
superconductors: Based on the previously suggested model of nanoscale dislocations induced
Josephson junctions and their arrays, we study the magnetic field induced
electric polarization effects in intrinsically granular superconductors. In
addition to a new phenomenon of chemomagnetoelectricity, the model predicts
also a few other interesting effects, including charge analogues of Meissner
paramagnetism (at low fields) and "fishtail" anomaly (at high fields). The
conditions under which these effects can be experimentally measured in
non-stoichiometric high-T_c superconductors are discussed. | cond-mat_supr-con |
Justification of the canonical quantization of the Josephson effect: Quantum devices based on Josephson effect in superconductors are usually
described by a Hamiltonian obtained by commonly used canonical quantization.
However, this recipe has not been yet rigorously justified. We show that this
approach is indeed correct in certain range of parameters. We find the
condition of the validity of such quantization and the lowest corrections to
the Josephson energy. | cond-mat_supr-con |
Geometrical Probability Distribution Functions for Cable-in-Conduit
Conductors with Simply and Multiply Connected Cross-Sections: A geometrical method is presented for the calculation of the strand
distribution functions for cable-in-conduit superconductors with simple and
multiply connected cross-sections. The method is illustrated on different cable
designs with simply and multiply connected structures. | cond-mat_supr-con |
Clocking the Onset of Bilayer Coherence in a High-$\mathrm{T_C}$ Cuprate: In cuprates, a precursor state of superconductivity is speculated to exist
above the critical temperature $\mathrm{T_C}$. Here we show via a combination
of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that
signatures of such a state can be obtained via three independent observables in
an underdoped sample of NdBa$_2$Cu$_3$O$_{6+\delta}$. The pseudogap
correlations were disentangled from the response of laser-broken pairs by
clocking their characteristic time-scales. The onset of a superconducting
precursor state was found at a temperature $\mathrm{T_{ONS}}$ $>$
$\mathrm{T_C}$, consistent with the temperature scale identified via static
optical spectroscopy. Furthermore, the temperature evolution of the coherent
vibration of the Ba ion, strongly renormalized by the onset of
superconductivity, revealed a pronounced anomaly at the same temperature
$\mathrm{T_{ONS}}$. The microscopic nature of such a precursor state is
discussed in terms of pre-formed pairs and enhanced bilayer coherence. | cond-mat_supr-con |
A first principles study on FeAs single layers: FeAs- single layer is tested as a simple model for LaFeAsO and BaFe2As2 based
on first-principles calculations using generalized gradient approximation (GGA)
and GGA+U. The calculated single- layer geometric and electronic structures are
inconsistent with that of bulk materials. The bulk collinear antiferromagnetic
ground state is failed to be obtained in the FeAs- single layer. The monotonous
behavior of the Fe-As distance in z direction upon electron or hole doping is
also in contrast with bulk materials. Our results indicate that, in LaFeAsO and
BaFe2As2, interactions between FeAs layer and other layers beyond simple charge
doping are important, and a single FeAs layer may not represent a good model
for Fe based superconducting materials. | cond-mat_supr-con |
Roles of Antiferromagnetic Fluctuation in High Field Phase Diagram of
Superconductors with Strong Paramagnetic Depairing: The high field phase diagram and magnetic properties of CeCoIn_5 below
H_c2(0) are examined from the picture regarding the high field and low
temperature (HFLT) phase as a possible Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)
vortex lattice. Crucial roles of antiferromagnetic (AFM) fluctuations enhanced
close to H_c2(0) are stressed. The FFLO vortex lattice with a longitudinal
modulation parallel to the field is stabilized compared with those with lateral
modulations as a consequence of the presence of AFM fluctuations. Further, an
unusual field-induced enhancement of the flux distribution is argued to be a
consequence of interplay between the paramagnetic depairing and AFM
fluctuation, both of which are enhanced with increasing field. | cond-mat_supr-con |
Where are holes in Y_{1-x}Pr_xBa2Cu3O7 ?: Recent experiments by Mertz et al (Phys. Rev. B55, 9160, 1997) demonstrated
that the loss of superconductivity upon Pr doping is associated with the change
of the character of the oxygen holes from $p_{\sigma}$ to $p_{\pi}.$ This
experiment sheds new light onto the long-standing problem of $T_{c}$
suppression by Pr, and helps to rule out a number of theoretical models,
leaving only those which predict such a transfer of the O holes. To distinguish
between the two models that do predict such an effect, one has to access the
ratio of the planar and axial character of the holes in both models. We do so
in this paper. | cond-mat_supr-con |
Orientation-dependent Josephson effect in spin-singlet
superconductor/altermagnet/spin-triplet superconductor junctions: We study the Josephson effect in the spin-singlet
superconductor/altermagnet/spin-triplet superconductor junctions using the
Green's function method. The current-phase difference relationships in the
junctions strongly depend on the orientation of altermagnet and the types of
the Cooper pairs. For the orientation angle equal to odd multiples of $\pi/4$,
the current-phase difference relationships are of the $\sin{2\phi}$ type, which
are irrespective of the pairing wave functions in superconductors. For the
other orientation angles, the emergence of the lowest order current becomes
possible and its form, $\sin\phi$ or $\cos\phi$, depends on the pairing wave
functions in superconductors. The $\phi_{0}$ phase and the $0$-$\pi$ transition
can be realized in our junctions due to the appearance of the lowest order
current. The selection rules for the lowest order current are presented. The
symmetric relations satisfied by the current-phase difference relationships are
analyzed through considering the transformations of the junctions under the
mirror reflection, the time-reversal and the spin rotation operations. Our
results not only provide a method to detect the intrinsic spin-triplet
superconductivity but also possess application values in the design of the
field-free quantum devices. | cond-mat_supr-con |
On the coupling of magnetic moments to superconducting quantum
interference devices: We investigate the coupling factor $\phi_\mu$ that quantifies the magnetic
flux $\Phi$ per magnetic moment $\mu$ of a point-like magnetic dipole that
couples to a superconducting quantum interference device (SQUID). Representing
the dipole by a current-carrying loop, the reciprocity of mutual inductances of
SQUID and loop provides a way of calculating $\phi_\mu(\vec{r}, \vec{e}_\mu)$
vs.~position $\vec{r}$ and orientation $\vec{e}_\mu$ of the dipole anywhere in
space from the magnetic field $B(\vec{r})$ produced by a supercurrent
circulating in the SQUID loop. We use numerical simulations based on London and
Ginzburg-Landau theory to calculate $\phi_\mu$ from the supercurrent density
distributions in various SQUID geometries. We treat the far-field regime
($r\gtrsim a=$ inner size of the SQUID loop) with the dipole placed on the
symmetry axis of circular or square shaped loops. We compare expressions for
$\phi_\mu$ from filamentary loop models with simulation results for loops with
finite width $w$ (outer size $A>a$), thickness $d$ and London penetration depth
$\lambda_L$ and show that for thin ($d\ll a$) and narrow ($w < a$) loops the
introduction of an effective loop size $a_{\rm eff}$ in the filamentary
loop-model expressions results in agreement with simulations. For a dipole
placed in the center of the loop, simulations provide an expression
$\phi_\mu(a,A,d,\lambda_L)$ that covers a wide parameter range. In the
near-field regime (dipole centered at small distance $z$ above one SQUID arm)
only coupling to a single strip representing the SQUID arm has to be
considered. Here, we compare simulations with an analytical expression derived
for a homogeneous current density distribution, which yields excellent
agreement for $\lambda_L>w,d$. Moreover, we analyze $\phi_\mu$ provided by the
introduction of a constriction in the SQUID arm below the magnetic dipole. | cond-mat_supr-con |
Conventional mechanisms for "exotic" superconductivity: We consider the pairing state due to the usual BCS mechanism in substances of
cubic and hexagonal symmetry where the Fermi surface forms pockets around
several points of high symmetry. We find that the symmetry imposed on the
multiple pocket positions could give rise to a multidimensional nontrivial
superconducting order parameter. The time reversal symmetry in the pairing
state is broken. We suggest several candidate substances where such ordering
may appear, and discuss means by which such a phase may be identified. | cond-mat_supr-con |
Anisotropic Superconductivity in the Induced Pairing Model: The model of local electron pairs and itinerant fermions coupled via charge
exchange mechanism, which mutually induces superconductivity in both subsystems
is studied for anisotropic pairing symmetry. The phase diagram is presented and
the phase fluctuations effects are analyzed within the Kosterlitz-Thouless
scenario. | cond-mat_supr-con |
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