text
stringlengths 89
2.49k
| category
stringclasses 19
values |
|---|---|
Stability of Elastic Glass Phases in Random Field XY Magnets and Vortex
Lattices in Type II Superconductors: A description of a dislocation-free elastic glass phase in terms of domain
walls is developed and used as the basis of a renormalization group analysis of
the energetics of dislocation loops added to the system. It is found that even
after optimizing over possible paths of large dislocation loops, their energy
is still very likely to be positive when the dislocation core energy is large.
This implies the existence of an equilibrium elastic glass phase in three
dimensional random field X-Y magnets, and a dislocation free,
bond-orientationally ordered ``Bragg glass'' phase of vortices in dirty Type II
superconductors. | cond-mat_supr-con |
Signatures of Stripe Phases in Hole Doped $La_2NiO_4$: We study nickelate-centered and oxygen-centered stripe phases in doped
La$_{2}$NiO$_{4}$ materials. We use an inhomogeneous Hartree-Fock and
random-phase approximation approach including both electron-electron and
electron-lattice(e-l) coupling for a layer of La$_{2}$NiO$_{4}$. We find that
whether the ground state after commensurate hole doping comprises Ni-centered
or O-centered charge-localized stripes depends sensitively on the e-l
interaction. With increasing e-l interaction strength, a continuous transition
from an O-centered stripe phase to a Ni-centered one is found. Various low- and
high-energy signatures of these two kinds of stripe phases are predicted, which
can clearly distinguish them. These signatures reflect the strongly correlated
spin-charge-lattice features in the vicinity of Ni-centered or O-centered
stripe domains. The importance of e-l interaction for recent experiments on
stripe phases is discussed. | cond-mat_supr-con |
Effects of Disorder in Mg_{1-x}Ta_{x}B_2 Alloys using Coherent-Potential
Approximation: Using Korringa-Kohn-Rostoker coherent-potential approximation in the
atomic-sphere approximation (KKR-ASA CPA) method for taking into account the
effects of disorder, Gaspari-Gyorffy formalism for calculating the
electron-phonon coupling constant $\lambda$, and Allen-Dynes equation for
calculating T_c, we have studied the variation of T_c in Mg_{1-x}Ta_{x}B_2
alloys as a function of Ta) concentration. Our results show that the T_c
decreases with the addition of Ta for upto 40 at% and remains essentially zero
from 60 at% to 80 at% of Ta. We also find TaB_2 to be superconducting, albeit
at a lower temperature. Our analysis shows that the variation in T_c in
Mg_{1-x}Ta_{x}B_2 is mostly dictated by the changes in the B p density of
states with the addition of Ta. | cond-mat_supr-con |
Nanoscale ferromagnet-superconductor-ferromagnet switches controlled by
magnetization orientation: We study clean ferromagnet-superconductor-ferromagnet (FSF) nanostructures in
which the magnetization of the F layers can be parallel (P) or antiparallel
(AP). We consider the case where the thickness of the S layer is of order of
the coherence length, with thinner F layers. We find that reversing the
direction of the magnetization in one of the F layers leads in general to
drastic changes in the superconductor's state. Under a wide variety of
conditions, the AP geometry favors superconductivity. Magnetization reversal in
one of the F layers can lead to the superconductivity turning on and off, or to
switching between different states. Our results are obtained via self
consistent solution of the Bogoliubov-de Gennes equations and evaluation of the
condensation energies of the system. | cond-mat_supr-con |
Pseudogap Phenomena and Phase Diagram in the 2-Band Hubbard Model: High-Tc superconducting materials (HTSC) have anomalous properties such as
the pseudo-gap or spin-gap etc., in Hall coefficient, 1/T1T and the density of
states etc. First including effects of strong on-site repulsion between
d-electrons at Cu-sites, we obtain quasi-particles with super-exchange
interaction Js, whose band width tends to zero, i.e., the system goes to
insulator, as the hole-doping rate tends to zero. The quasi-particles
correspond to Zhang-Rice singlet states. Js larger than the band width combined
with the 2-dimensional character of the system induces strong
antiferro-magnetic (AF) and superconducting (SC) fluctuations. We treat effects
of the AF ones in the FLEX approximation and those of the SC ones in the
self-consistent t-matrix approximation to show that both fluctuations in the
under-doped region start to increase at T0 as T decreases from T>>Tc, the AF
ones dominate the SC ones at T>Tsg, while SC ones dominate at T<Tsg. This
cross-over of the fluctuations causes the anomalous phenomena in the
under-doped region. We also obtain the phase diagram of HTSC consistent to one
observed in experiments. | cond-mat_supr-con |
Phonon linewidth due to electron-phonon interactions with strong forward
scattering in FeSe thin films on oxide substrates: The discovery of an enhanced superconducting transition temperature $T_c$ in
monolayers of FeSe grown on several oxide substrates has opened a new route to
high-$T_c$ superconductivity through interface engineering. One proposal for
the origin of the observed enhancement is an electron-phonon (e-ph) interaction
across the interface that peaked at small momentum transfers. In this paper, we
examine the implications of such a coupling on the phononic properties of the
system. We show that a strong forward scattering leads to a sizable broadening
of phonon lineshape, which may result in charge instabilities at
long-wavelengths. However, we further find that the inclusion of Coulombic
screening significantly reduces the phonon broadening. Our results show that
one might not expect anomalously broad phonon linewidths in the FeSe interface
systems, despite the fact that the e-ph interaction has a strong peak in the
forward scattering (small $q$) direction. | cond-mat_supr-con |
Lattice dynamics of the cluster chain compounds M2Mo6Se6: The lattice dynamics of members of the M2Mo6Se6 family of materials with
guest ions M = K, Rb, Cs, In, and Tl has been studied using inelastic x-ray
scattering and Raman spectroscopy at room temperature, as well as by ab-initio
calculations. We find a good match between calculations and experiment, both
for structure factors (Eigenvectors) and for the calculated phonon frequencies.
The observed lattice dynamics for Tl2Mo6Se6 show no signs of anharmonicity or
absence of avoided dispersion crossings, thus ruling out previously
hypothesised rattling phonon modes. The reduced mode energies for In2Mo6Se6 are
identified as only partially responsible for the lower superconducting
transition temperature Tc in this material when compared to Tl2Mo6Se6. | cond-mat_supr-con |
Multipole superconductivity in nonsymmorphic Sr$_2$IrO$_4$: Discoveries of marked similarities to high-$T_{\text{c}}$ cuprate
superconductors point to the realization of superconductivity in the doped
$J_{\text{eff}} = 1 / 2$ Mott insulator Sr$_2$IrO$_4$. Contrary to the mother
compound of cuprate superconductors, several stacking patterns of in-plane
canted antiferromagnetic moments have been reported, which are distinguished by
the ferromagnetic components as $-++-$, $++++$, and $-+-+$. In this paper, we
clarify unconventional features of the superconductivity coexisting with $-++-$
and $-+-+$ structures. Combining the group theoretical analysis and numerical
calculations for an effective $J_{\text{eff}} = 1 / 2$ model, we show unusual
superconducting gap structures in the $-++-$ state protected by nonsymmorphic
magnetic space group symmetry. Furthermore, our calculation shows that the
Fulde-Ferrell-Larkin-Ovchinnikov superconductivity is inevitably stabilized in
the $-+-+$ state since the odd-parity magnetic $-+-+$ order makes the band
structure asymmetric by cooperating with spin-orbit coupling. These unusual
superconducting properties are signatures of magnetic multipole order in
nonsymmorphic crystal. | cond-mat_supr-con |
Intertype superconductivity evoked by the interplay of disorder and
multiple bands: Nonmagnetic impurity scattering is known to shift up the Ginzburg-Landau
parameter $\kappa$ of a superconductor. In this case, when the system is
initially in type I, it can change its magnetic response, crossing the
intertype domain with $\kappa \sim 1$ between the two standard
superconductivity types and arriving at type II. In the present work we
demonstrate that the impact of disorder can be much more profound in the
presence of the multiband structure of the charge carrier states. In
particular, when the band diffusivities differ from each other, the intertype
domain tends to expand significantly, including points with $\kappa \gg 1$ that
belong to deep type-II in conventional single-band superconductors. Our finding
sheds light on the nontrivial disorder effect and significantly complements
earlier results on the enlargement of the intertype domain in clean multiband
superconductors. | cond-mat_supr-con |
Stochastic resonance in an RF SQUID with shunted ScS junction: Using a point (superconductor-constriction-superconductor, ScS) contact in a
single-Josephson-junction superconducting quantum interference device (RF
SQUID) provides stochastic resonance conditions at any arbitrary small value of
loop inductance and contact critical current, unlike SQUIDs with more
traditional tunnel (superconductor-insulator-superconductor, SIS) junctions.
This is due to the unusual potential energy of the ScS RF SQUID which always
has a barrier between two wells thus making the device bistable. This paper
presents the results of a numerical simulation of the stochastic dynamics of
the magnetic flux in an ScS RF SQUID loop affected by band-limited white
Gaussian noise and low-frequency sine signals of small and moderate amplitudes.
The difference in stochastic amplification of RF SQUID loops incorporating ScS
and SIS junctions is discussed. | cond-mat_supr-con |
Preparation of electron-doped La(2-x)CexCuO4 thin films with various Ce
doping by dc magnetron sputtering: A series of c-axis oriented electron-doped high-Tc superconducting
La(2-x)CexCuO4 thin films, from heavily underdoped x=0.06 to heavily overdoped
x=0.19, have been synthesized by dc magnetron sputtering technique on (100)
SrTiO3 substrates. The influence of various fabrication conditions, such as the
deposition temperature and the deposition rate, etc., on the quality of the
thin films has been scrutinized. We find that the quality of the films is less
sensitive to the deposition temperature in the overdoped region than that in
the underdoped region. In the phase diagram of Tc(x), the superconducting dome
indicates that the optimally doping level is at the point x=0.105 with the
transition temperature Tc0 = 26.5 K. Further more, both the disappearance of
the upturn in the $\rho_{xx}$(T) curve at low temperature under H=10 T and the
positive differential Hall coefficient, $R_H'=d \rho_{xy}/dH$, are observed
around x = 0.15, implying a possible rearrangement of Fermi surface at this
doping level. | cond-mat_supr-con |
Spontaneous Long-range Vortex-antivortex Pair in the Two-band 2D
Superconductor: The relaxation iterative method is used to minimize Ginzburg-Landau model for
the two-band superconductor with Josephson-coupling. A stable spontaneous
vortex-antivortex pair with long range order has been revealed. Our result
appears due to the Josephson-coupling effect, which leads to a
linearly-dependence of total free energy on the distance between
vortex-antivortex pair, hence accounts for this phenomenon. | cond-mat_supr-con |
Superconductivity with High Upper Critical Field in the Cubic
Centrosymmetric $η$-Carbide Nb$_4$Rh$_2$C$_{1-δ}$: The upper critical field is a fundamental measure of the strength of
superconductivity in a material. It is also a cornerstone for the realization
of superconducting magnet applications. The critical field arises because of
the Copper pair breaking at a limiting field, which is due to the Pauli
paramagnetism of the electrons. The maximal possible magnetic field strength
for this effect is commonly known as the Pauli paramagnetic limit given as
$\mu_0 H_{\rm Pauli} \approx 1.86{\rm [T/K]} \cdot T_{\rm c}$ for a
weak-coupling BCS superconductor. The violation of this limit is only rarely
observed. Exceptions include some low-temperature heavy fermion and some
strongly anisotropic superconductors. Here, we report on the superconductivity
at 9.75 K in the centrosymmetric, cubic $\eta$-carbide-type compound
Nb$_4$Rh$_2$C$_{1-\delta}$, with a normalized specific heat jump of $\Delta
C/\gamma T_{\rm c} =$ 1.64. We find that this material has a remarkably high
upper critical field of $\mu_0 H_{\rm c2}{\rm (0)}$ =~28.5~T, which is
exceeding by far its weak-coupling BCS Pauli paramagnetic limit of $\mu_0
H_{\rm Pauli}$~=~18.1 T. Determination of the origin and consequences of this
effect will represent a significant new direction in the study of critical
fields in superconductors. | cond-mat_supr-con |
Hidden Pseudogap and Excitation Spectra in a Strongly Coupled Two-Band
Superfluid/Superconductor: We investigate single-particle excitation properties in the normal state of a
two-band superconductor or superfluid throughout the Bardeen-Cooper-Schrieffer
(BCS) to Bose-Einstein-condensation (BEC) crossover, within the many-body
T-matrix approximation for multi-channel pairing fluctuations. We address the
single-particle density of states and the spectral functions consisting of two
contributions associated with a waekly interacting deep band and a strongly
interacting shallow band, relevant for iron-based multiband superconductors and
multicomponent fermionic superfluids. We show how the pseudogap state in the
shallow band is hidden by the deep band contribution throughout the two-band
BCS-BEC crossover. Our results could explain the missing pseudogap in recent
scanning tunneling microscopy experiments in FeSe superconductors. | cond-mat_supr-con |
Visualization of vortex bound states in polarized Fermi gases at
unitarity: We analyse theoretically a single vortex in 3D trapped atomic Fermi gases
with population polarization near a broad Feshbach resonance. Above a critical
polarization the Andreev-like bound states inside the core become occupied for
the majority component. As a result, the local density difference at the core
center acquires a sudden rise at low temperautres. This provides a
visualization of the lowest bound state within the absorption imaging
technique. As the polarization increases, the core expands gradually, and
correspondingly, the energy of the lowest bound state decreases. | cond-mat_supr-con |
Measurement of the dynamic charge response of materials using
low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS): One of the most fundamental properties of an interacting electron system is
its frequency- and wave-vector-dependent density response function, $\chi({\bf
q},\omega)$. The imaginary part, $\chi''({\bf q},\omega)$, defines the
fundamental bosonic charge excitations of the system, exhibiting peaks wherever
collective modes are present. $\chi$ quantifies the electronic compressibility
of a material, its response to external fields, its ability to screen charge,
and its tendency to form charge density waves. Unfortunately, there has never
been a fully momentum-resolved means to measure $\chi({\bf q},\omega)$ at the
meV energy scale relevant to modern elecronic materials. Here, we demonstrate a
way to measure $\chi$ with quantitative momentum resolution by applying
alignment techniques from x-ray and neutron scattering to surface
high-resolution electron energy-loss spectroscopy (HR-EELS). This approach,
which we refer to here as "M-EELS," allows direct measurement of $\chi''({\bf
q},\omega)$ with meV resolution while controlling the momentum with an accuracy
better than a percent of a typical Brillouin zone. We apply this technique to
finite-q excitations in the optimally-doped high temperature superconductor,
Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi2212), which exhibits several phonons
potentially relevant to dispersion anomalies observed in ARPES and STM
experiments. Our study defines a path to studying the long-sought collective
charge modes in quantum materials at the meV scale and with full momentum
control. | cond-mat_supr-con |
Elastic properties and inter-atomic bonding in new superconductor
KFe2Se2 from first principles calculations: Very recently (November, 2010, PRB, 82, 180520R) the first 122-like ternary
superconductor KxFe2Se2 with enhanced TC ~ 31K has been discovered. This
finding has stimulated much activity in search of related materials and
triggered the intense studies of their properties. Indeed already in 2010-2011
the superconductivity (TC ~ 27-33K) was also found in the series of new
synthesized 122 phases such as CsxFe2Se2, RbxFe2Se2, (TlK)xFeySe2 etc. which
have formed today the new family of superconducting iron-based materials
without toxic As. Here, using the ab initio FLAPW-GGA method we have predicted
for the first time the elastic properties for KFe2Se2 and discussed their
interplay with inter-atomic bonding for this system. Our data reveal that the
examined phase is relatively soft material. In addition, this system is
mechanically stable, adopts considerable elastic anisotropy, and demonstrates
brittleness. These conclusions agree with the bonding picture for KFe2Se2,
where the inter-atomic bonding is highly anisotropic and includes ionic,
covalent and metallic contributions. | cond-mat_supr-con |
Competition of electron-phonon mediated superconductivity and Stoner
magnetism on a flat band: The effective attractive interaction between electrons, mediated by
electron-phonon coupling, is a well-established mechanism of conventional
superconductivity. In metals exhibiting a Fermi surface, the critical
temperature of superconductivity is exponentially smaller than the
characteristic phonon energy. Therefore such superconductors are found only at
temperatures below a few Kelvin. Systems with flat energy bands have been
suggested to cure the problem and provide a route to room-temperature
superconductivity, but previous studies are limited to only BCS models with an
effective attractive interaction. Here we generalize Eliashberg's theory of
strong-coupling superconductivity to systems with flat bands and relate the
mean-field critical temperature to the microscopic parameters describing
electron-phonon and electron-electron interaction. We also analyze the
strong-coupling corrections to the BCS results, and construct the phase diagram
exhibiting superconductivity and magnetic phases on an equal footing. Our
results are especially relevant for novel quantum materials where electronic
dispersion and interaction strength are controllable. | cond-mat_supr-con |
Hollow carbon spheres as an efficient dopant for enhancing critical
current density of MgB2 based tapes: A significant enhancement of Jc and Hirr in MgB2 tapes has been achieved by
the in situ powder-in-tube method utilizing hollow carbon spheres (HCS) as
dopants. At 4.2 K, the transport Jc for the 850C sintered samples reached
3.1x10^4, and 1.4x10^4 A/cm^2 at 10 and 12 T, respectively, and were better
than those of optimal nano-SiC doped tapes. Furthermore, the Hirr for doped
sample was raised up to 16.8 T at 10 K due to the carbon substitution effect.
The results demonstrate that HCS is one of the most promising dopants besides
nano-carbon and SiC for the enhancement of current capacity for MgB2 in high
fields. | cond-mat_supr-con |
A tunable rf SQUID manipulated as flux and phase qubit: We report on two different manipulation procedures of a tunable rf SQUID.
First, we operate this system as a flux qubit, where the coherent evolution
between the two flux states is induced by a rapid change of the energy
potential, turning it from a double well into a single well. The measured
coherent Larmor-like oscillation of the retrapping probability in one of the
wells has a frequency ranging from 6 to 20 GHz, with a theoretically expected
upper limit of 40 GHz. Furthermore, here we also report a manipulation of the
same device as a phase qubit. In the phase regime, the manipulation of the
energy states is realized by applying a resonant microwave drive. In spite of
the conceptual difference between these two manipulation procedures, the
measured decay times of Larmor oscillation and microwave-driven Rabi
oscillation are rather similar. Due to the higher frequency of the Larmor
oscillations, the microwave-free qubit manipulation allows for much faster
coherent operations. | cond-mat_supr-con |
NMR Investigation of the Quasi One-dimensional Superconductor
K$_{2}$Cr$_{3}$As$_{3}$: We report $^{75}$As NMR measurements on the new quasi one-dimensional
superconductor K$_{2}$Cr$_{3}$As$_{3}$ ($T_{c} \sim 6.1$~K) [J.\ K.\ Bao et
al., Phys. Rev. X {\bf 5}, 011013 (2015)]. We found evidence for strong
enhancement of Cr spin fluctuations above $T_c$ in the
[Cr$_{3}$As$_{3}$]$_{\infty}$ double-walled subnano-tubes based on the nuclear
spin-lattice relaxation rate $1/T_{1}$. The power law temperature dependence,
$1/T_{1}T \sim T^{-\gamma}$ ($\gamma \sim 0.25$), is consistent with the
Tomonaga-Luttinger liquid. Moreover, absence of the Hebel-Slichter coherence
peak of $1/T_{1}$ just below $T_{c}$ suggests unconventional nature of
superconductivity. | cond-mat_supr-con |
Effects of Iodine Annealing on Fe1+yTe0.6Se0.4: Effects of iodine annealing to induce bulk superconductivity in
Fe1+yTe0.6Se0.4 have been systematically studied by changing the molar ratio of
iodine to the sample and annealing temperature. The optimal condition to induce
bulk superconductivity with Tc ~14.5 K and self-field Jc(2 K) ~ 5x10^5 A/cm2 is
found to be a molar ratio of iodine of 5-7 % at the annealing temperature of
400 C. Furthermore, the fact that no compounds containing iodine are detected
in the crystal and a significant amount of FeTe2 is produced after the iodine
annealing strongly indicate that the excess iron is consumed to form FeTe2 and
iodine works as a catalyst in this process. | cond-mat_supr-con |
ARPES in the normal state of the cuprates: comparing the marginal Fermi
liquid and spin fluctuation scenarios: We address the issue whether ARPES measurements of the spectral function $A_k
(\omega)$ near the Fermi surface in the normal state of near optimally doped
cuprates can distinguish between the marginal Fermi liquid scenario and the
spin-fluctuation scenario. We argue that the data for momenta near the Fermi
surface are equally well described by both theories, but this agreement is
nearly meaningless as in both cases one has to add to $\Sigma^{\prime \prime}
(\omega)$ a large constant of yet unknown origin. We show that the data can be
well fitted by keeping only this constant term in the self-energy. To
distinguish between the two scenarios, one has to analyze the data away from
the Fermi surface, when the intrinsic piece in $\Sigma (\omega)$ becomes
dominant. | cond-mat_supr-con |
Effect of damage by 2-MeV He ions on the normal and superconducting
properties of magnesium diboride: We have studied the effect of damage induced by 2-MeV alpha particles on the
critical temperature, Tc, and resistivity of MgB2 thin films. This technique
allows defects to be controllably introduced into MgB2 in small successive
steps. Tc decreases linearly as the intragrain resistivity at 40 K increases,
while the intergrain connectivity is not changed. Tc is ultimately reduced to
less than 7 K and we see no evidence for a saturation of Tc at about 20 K,
contrary to the predictions of the Tc of MgB2 in the dirty limit of interband
scattering. | cond-mat_supr-con |
Antiferromagnetic Phases in the Fulde-Ferrell-Larkin-Ovchinnikov State
of CeCoIn_5: The antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) superconducting state is analyzed on the basis of a Ginzburg-Landau
theory. To examine the possible AFM-FFLO state in CeCoIn_5, we focus on the
incommensurate AFM order characterized by the wave vector Q = Q_{0} \pm q_inc
with Q_0 =(\pi,\pi,\pi) and q_inc \parallel [110] or [1-10] in the tetragonal
crystal structure. We formulate the two component Ginzburg-Landau theory and
investigate the two degenerate incommensurate AFM order. We show that the
pinning of AFM moment due to the FFLO nodal planes leads to multiple phases in
magnetic fields along [100] or [010]. The phase diagrams for various coupling
constants between the two order parameters are shown for the comparison with
CeCoIn_5. Experimental results of the NMR and neutron scattering measurements
are discussed. | cond-mat_supr-con |
Localization-delocalization dichotomy: Inherent spectral properties of
the cuprates: We consider hole pairing in the pseudopgap phase of High T_c cuprates, as
arising from resonant scattering on dynamically deformable molecular units. As
a result, localized and delocalized features coexist in the one-particle
spectra: the pseudogap and propagating diffusive Bogoliubov modes. Due to the
anisotropy of the electron dispersion and pairing interaction, these two
manifestations have different impact in the different regions of the Brillouin
zone. We illustrate that for k-vectors crossing the arc, determined by the
chemical potential, joining the anti-nodal and the nodal point. | cond-mat_supr-con |
On the generation of multipartite entangled states in Josephson
architectures: We propose and analyze a scheme for the generation of multipartite entangled
states in a system of inductively coupled Josephson flux qubits. The qubits
have fixed eigenfrequencies during the whole process in order to minimize
decoherence effects and their inductive coupling can be turned on and off at
will by tuning an external control flux. Within this framework, we will show
that a W state in a system of three or more qubits can be generated by
exploiting the sequential one by one coupling of the qubits with one of them
playing the role of an entanglement mediator. | cond-mat_supr-con |
Picosecond Trajectory of Two-dimensional Vortex Motion in
FeSe$_{0.5}$Te$_{0.5}$ Visualized by Terahertz Second Harmonic Generation: We have investigated the vortex dynamics in a thin film of an iron-based
superconductor FeSe$_{0.5}$Te$_{0.5}$ by observing second-harmonic generation
(SHG) in the THz frequency range. We visualized the picosecond trajectory of
two-dimensional vortex motion in a pinning potential tilted by Meissner
shielding current. The SHG perpendicular to the driving field is observed,
corresponding to the nonreciprocal nonlinear Hall effect under the
current-induced inversion symmetry breaking, whereas the linear Hall effect is
negligible. The estimated vortex mass, as light as a bare electron, suggests
that the vortex core moves independently from quasiparticles at such a high
frequency and large velocity $\approx$300 km/s. | cond-mat_supr-con |
Superconducting properties of SmO1-xFxFeAs wires with Tc = 52 K prepared
by the powder-in-tube method: We demonstrate that Ta sheathed SmO1-xFxFeAs wires were successfully
fabricated by the powder-in-tube (PIT) method for the first time. Structural
analysis by mean of x-ray diffraction shows that the main phase of SmO1-xFxFeAs
was obtained by this synthesis method. The transition temperature of the
SmO0.65F0.35FeAs wires was confirmed to be as high as 52 K. Based on
magnetization measurements, it is found that a globe current can flow on
macroscopic sample dimensions with Jc of ~3.9x10^3 A/cm^2 at 5 K and self
field, while a high Jc about 2x10^5 A/cm^2 is observed within the grains,
suggesting that a significant improvement in the globle Jc is possible. It
should be noted that the Jc exhibits a very weak field dependence behavior.
Furthermore, the upper critical fields (Hc2) determined according to the
Werthamer-Helfand-Hohenberg formula are (T= 0 K) = 120 T, indicating a very
encouraging application of the new superconductors. | cond-mat_supr-con |
$^{11}B$ NMR and Relaxation in $MgB_2$ Superconductor: $^{11}B$ NMR and nuclear spin-lattice relaxation rate (NSLR) are reported at
7.2 Tesla and 1.4 Tesla in powder samples of the intermetallic compound $MgB_2$
with superconducting transition temperature in zero field $T_c$ = 39.2 K. From
the first order quadrupole perturbed NMR specrum a quadrupole coupling
frequency of 835 $\pm$ 5 kHz is obtained. The Knight shift is very small and it
decreases to zero in the superconducting phase. The NSLR follows a linear law
with $T_1T$ = 165 $\pm$ 10 (sec K) . The results in the normal phase indicate a
negligible $s$-character of the wave function of the conduction electrons at
the Fermi level. Below $T_c$ the NSLR is strongly field dependent indicating
the presence of an important contribution related to the density and the
thermal motion of flux lines. No coherence peak is observed at the lower field
investigated (1.4 T). | cond-mat_supr-con |
Microscopic evidence for anisotropic multigap superconductivity in the
CsV$_3$Sb$_5$ kagome superconductor: The recently discovered kagome superconductor CsV$_3$Sb$_5$ ($T_c \simeq 2.5$
K) has been found to host charge order as well as a non-trivial band topology,
encompassing multiple Dirac points and probable surface states. Such a complex
and phenomenologically rich system is, therefore, an ideal playground for
observing unusual electronic phases. Here, we report on microscopic studies of
its anisotropic superconducting properties by means of transverse-field muon
spin rotation ($\mu$SR) experiments. The temperature dependences of the
in-plane and out-of-plane components of the magnetic penetration depth
$\lambda_{ab}^{-2}(T)$ and $\lambda_{c}^{-2}(T)$ indicate that the
superconducting order parameter exhibits a two-gap ($s+s$)-wave symmetry,
reflecting the multiple Fermi surfaces of CsV3Sb5. The multiband nature of its
superconductivity is further validated by the different temperature dependences
of the anisotropic magnetic penetration depth $\gamma_\lambda(T)$ and upper
critical field $\gamma_{\rm B_{c2}}(T)$, both in close analogy with the well
known two-gap superconductor MgB$_2$. Remarkably, the high value of the
$T_c/\lambda^{-2}(0)$ ratio in both field orientations strongly suggests the
unconventional nature of superconductivity. The relaxation rates obtained from
zero field $\mu$SR experiments do not show noticeable change across the
superconducting transition, indicating that superconductivity does not break
time reversal symmetry. | cond-mat_supr-con |
Type-1.5 superconductivity in multicomponent systems: In general a superconducting state breaks multiple symmetries and, therefore,
is characterized by several different coherence lengths $\xi_i$, $i=1,...,N$.
Moreover in multiband material even superconducting states that break only a
single symmetry are nonetheless described, under certain conditions by
multi-component theories with multiple coherence lengths. As a result of that
there can appear a state where some coherence lengths are larger and some are
smaller than the magnetic field penetration length $\lambda$: $\xi_1\leq
\xi_2... < \sqrt{2}\lambda<\xi_M\leq...\xi_N$. That state was recently termed
"type-1.5" superconductivity. This breakdown of type-1/type-2 dichotomy is
rather generic near a phase transition between superconducting states with
different symmetries. The examples include the transitions between $U(1)$ and
$U(1)\times U(1)$ states or between $U(1)$ and $U(1)\times Z_2$ states. The
later example is realized in systems that feature transition between s-wave and
$s+is$ states. The extra fundamental length scales have many physical
consequences. In particular in these regimes vortices can attract one another
at long range but repel at shorter ranges. Such a system can form vortex
clusters in low magnetic fields. The vortex clustering in the type-1.5 regime
gives rise to many physical effects, ranging from macroscopic phase separation
in domains of different broken symmetries, to unusual transport properties. | cond-mat_supr-con |
Structural and electronic response upon hole-doping of rare-earth iron
oxyarsenides Nd1-xSrxFeAsO (0 < x < 0.2): Hole-doping of NdFeAsO via partial replacement of Nd3+ by Sr2+ is a
successful route to obtain superconducting phases (Tc = 13.5 K for a Sr2+
content of 20%); however, the structural and electronic response with doping is
different from and non-symmetric to that in the electron-doped side of the
phase diagram. | cond-mat_supr-con |
Growth, Characterization, Vortex Pinning and Vortex Flow Properties of
Single Crystals of Iron Chalcogenide Superconductor FeCr$_{0.02}$Se: We report the growth and characterization of single crystals of iron
chalcogenide superconductor FeCr$_{0.02}$Se. There is an enhancement of the
superconducting transition temperature (T$_{\rm c}$) as compared to the T$_{\rm
c}$ of the single crystals of the parent compound Fe$_{1+x}$Se by about 25%.
The superconducting parameters such as the critical fields, coherence length,
penetration depth and the Ginzburg-Landau parameter have been estimated for
these single crystals. Analysis of the critical current data suggests a
fluctuation in electronic mean free path induced ($\delta l$) pinning mechanism
in this material. Thermally activated transport across the superconducting
transition in the presence of external magnetic fields suggests a crossover
from a single vortex pinning regime at low fields to a collective flux creep
regime at higher magnetic fields. The nature of charge carriers in the normal
state estimated from the Hall effect and thermal transport measurements could
provide crucial information on the mechanism of superconductivity in Fe-based
materials. | cond-mat_supr-con |
Dynamics of metastable vortex states in weakly pinned superconductors: A
phenomenological model: We present a phenomenological model for the vortex dynamics in the peak
effect region of weakly pinned superconductors. We explain the history
dependent dynamic response of the metastable vortex states subjected to a
transport current and the hysteretic voltage-current characteristics observed
in the vicinity of peak effect in weakly pinned superconductors. A strong
variation in voltage current characteristics with the current sweep rate and
the anomalous dependence of critical current density $J_c$ on the magnetic
field sweep rate have also been accounted for by this model. | cond-mat_supr-con |
Interplay between unconventional superconductivity and heavy-fermion
quantum criticality: CeCu$_2$Si$_2$ versus YbRh$_2$Si$_2$: In this paper the low-temperature properties of two isostructural canonical
heavy-fermion compounds are contrasted with regards to the interplay between
antiferromagnetic (AF) quantum criticality and superconductivity. For
CeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of
an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum
critical point (QCP). Inelastic neutron scattering results highlight that both
quantum critical SDW fluctuations as well as Mott-type fluctuations of local
magnetic moments contribute to the formation of Cooper pairs in CeCu$_2$Si$_2$.
In YbRh$_2$Si$_2$, superconductivity appears to be suppressed at $T\gtrsim~10$
mK by AF order ($T_N$ = 70 mK). Ultra-low temperature measurements reveal a
hybrid order between nuclear and 4f-electronic spins, which is dominated by the
Yb-derived nuclear spins, to develop at $T_A$ slightly above 2 mK. The hybrid
order turns out to strongly compete with the primary 4f-electronic order and to
push the material towards its QCP. Apparently, this paves the way for
heavy-fermion superconductivity to form at $T_c$ = 2 mK. Like the pressure -
induced QCP in CeRhIn$_5$, the magnetic field - induced one in YbRh$_2$Si$_2$
is of the local Kondo-destroying variety which corresponds to a Mott-type
transition at zero temperature. Therefore, these materials form the link
between the large family of about fifty low-$T$ unconventional heavy - fermion
superconductors and other families of unconventional superconductors with
higher $T_c$s, notably the doped Mott insulators of the cuprates, organic
charge-transfer salts and some of the Fe-based superconductors. Our study
suggests that heavy-fermion superconductivity near an AF QCP is a robust
phenomenon. | cond-mat_supr-con |
Josephson Junctions with a synthetic antiferromagnetic interlayer: We report measurements of the critical current vs. Co thickness in
Nb/Cu/Co/Ru/Co/Cu/Nb Josephson junctions, where the inner Co/Ru/Co trilayer is
a "synthetic antiferromagnet" with the magnetizations of the two Co layers
coupled antiparallel to each other via the 0.6 nm-thick Ru layer. Due to the
antiparallel magnetization alignment, the net intrinsic magnetic flux in the
junction is nearly zero, and such junctions exhibit excellent Fraunhofer
patterns in the critical current vs. applied magnetic field, even with total Co
thicknesses as large as 23 nm. There are no apparent oscillations in the
critical current vs. Co thickness, consistent with theoretical expectations for
this situation. The critical current of the junctions decays over 4 orders of
magnitude as the total Co thickness increases from 3 to 23 nm. These junctions
may serve as useful templates for future explorations of spin-triplet
superconducting correlations, which are predicted to occur in supercon-
ducting/ferromagnetic hybrid systems in the presence of certain types of
magnetic inhomogeneity. | cond-mat_supr-con |
Majorana bands, Berry curvature, and thermal Hall conductivity in the
vortex state of a chiral p-wave superconductor: Majorana quasiparticles localized in vortex cores of a chiral p-wave
superconductor hybridize with one another to form bands in a vortex lattice. We
begin by solving a fully microscopic theory describing all quasiparticle bands
in a chiral p-wave superconductor in magnetic field, then use this solution to
build localized Wannier wavefunctions corresponding to Majorana quasiparticles.
A low-energy tight-binding theory describing the intervortex hopping of these
is then derived, and its topological properties---which depend crucially on the
signs of the imaginary intervortex hopping parameters---are studied. We show
that the energy gap between the Majorana bands may be either topologically
trivial or nontrivial, depending on whether the Chern number contributions from
the Majorana bands and those from the background superconducting condensate add
constructively or destructively. This topology directly affects the
temperature-dependent thermal Hall conductivity, which we also calculate. | cond-mat_supr-con |
Asymmetrical solutions and role of thermal fluctuations in dc current
driven extended Josephson junction: Extended Josephson junction driven by dc bias current is studied numerically.
Two types of solutions, symmetrical and asymmetrical, are found. The
current-voltage characteristic (IVC) is calculated. The symmetrical solutions
form main histeretic IVC and asymmetrical one create an additional branch.
Depending on the bias current value periodic, quasiperiodic and chaotic modes
of the junction motion was observed. Dynamics of the junction affected by
thermal fluctuations was analyzed. Stability of different states of the
junction is discussed. | cond-mat_supr-con |
A Unified Description of Cuprate and Iron Arsenide Superconductors: We propose a unified description of cuprate and iron-based superconductivity.
Consistency with magnetic structure inferred from neutron scattering implies
significant constraints on the symmetry of the pairing gap for the iron-based
superconductors. We find that this unification requires the orbital pairing
formfactors for the iron arsenides to differ fundamentally from those for
cuprates at the microscopic level. | cond-mat_supr-con |
Similarity of slow stripe fluctations between Sr-doped cuprates and
oxygen-doped nickelates: Stripe fluctuations in La2NiO4.17 have been studied by 139La NMR using the
field and temperature dependence of the linewidth and relaxation rates. In the
formation process of the stripes the NMR line intensity is maximal below 230K,
starts to diminish around 140K, disappears around 50K and recovers at 4K. These
results are shown to be consistent with, but completely complementary to
neutron measurements, and to be generic for oxygen doped nickelates and
underdoped cuprates. | cond-mat_supr-con |
Doping dependence of heat transport in the iron-arsenide superconductor
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$: from isotropic to strongly $k$-dependent gap
structure: The temperature and magnetic field dependence of the in-plane thermal
conductivity $\kappa$ of the iron-arsenide superconductor
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ was measured down to $T \simeq 50$ mK and up to
$H = 15$ T as a function of Co concentration $x$ in the range 0.048 $ \leq x
\leq $ 0.114. In zero magnetic field, a negligible residual linear term in
$\kappa/T$ as $T \to 0$ at all $x$ shows that there are no zero-energy
quasiparticles and hence the superconducting gap has no nodes in the $ab$-plane
anywhere in the phase diagram. However, the field dependence of $\kappa$
reveals a systematic evolution of the superconducting gap with doping $x$, from
large everywhere on the Fermi surface in the underdoped regime, as evidenced by
a flat $\kappa (H)$ at $T \to 0$, to strongly $k$-dependent in the overdoped
regime, where a small magnetic field can induce a large residual linear term,
indicative of a deep minimum in the gap magnitude somewhere on the Fermi
surface. This shows that the superconducting gap structure has a strongly
$k$-dependent amplitude around the Fermi surface only outside the
antiferromagnetic/orthorhombic phase. | cond-mat_supr-con |
Coulomb blockade and Bloch oscillations in superconducting Ti nanowires: Quantum fluctuations in quasi-one-dimensional superconducting channels
leading to spontaneous changes of the phase of the order parameter by $2\pi$,
alternatively called quantum phase slips (QPS), manifest themselves as the
finite resistance well below the critical temperature of thin superconducting
nanowires and the suppression of persistent currents in tiny superconducting
nanorings. Here we report the experimental evidence that in a current-biased
superconducting nanowire the same QPS process is responsible for the insulating
state -- the Coulomb blockade. When exposed to RF radiation, the internal Bloch
oscillations can be synchronized with the external RF drive leading to
formation of quantized current steps on the I-V characteristic. The effects
originate from the fundamental quantum duality of a Josephson junction and a
superconducting nanowire governed by QPS -- the QPS junction (QPSJ). | cond-mat_supr-con |
Multiband behavior and non-metallic low-temperature state of
K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$: We report evidence for multiband transport and an insulating low-temperature
normal state in superconducting K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ with
$T_{c}\approx 20$ K. The temperature-dependent upper critical field, $H_{c2}$,
is well described by a two-band BCS model. The normal-state resistance,
accessible at low temperatures only in pulsed magnetic fields, shows an
insulating logarithmic temperature dependence as $T \rightarrow 0$ after
superconductivity is suppressed. This is similar as for high-$T_{c}$ copper
oxides and granular type-I superconductors, suggesting that the
superconductor-insulator transition observed in high magnetic fields is related
to intrinsic nanoscale phase separation. | cond-mat_supr-con |
Switchable resonant coupling of flux qubits: We propose a coupling scheme, where two or more flux qubits with different
eigenfrequencies share Josephson junctions with a coupler loop devoid of its
own quantum dynamics. Switchable two-qubit coupling is realized by tuning the
frequency of the AC magnetic flux through the coupler to a combination
frequency of two of the qubits. The coupling allows any or all of the qubits to
be simultaneously at the degeneracy point and can change sign. | cond-mat_supr-con |
Odd-parity superconductors with two-component order parameters: nematic
and chiral, full gap and Majorana node: Motivated by the recent experiment indicating that superconductivity in the
doped topological insulator Cu$_x$Bi$_2$Se$_3$ has an odd-parity pairing
symmetry with rotational symmetry breaking, we study the general class of
odd-parity superconductors with two-component order parameters in trigonal and
hexagonal crystal systems. In the presence of strong spin-orbit interaction, we
find two possible superconducting phases below $T_c$, a time-reversal-breaking
(i.e., chiral) phase and an anisotropic (i.e., nematic) phase, and determine
their relative energetics from the gap function in momentum space. The nematic
superconductor generally has a full quasi-particle gap, whereas the chiral
superconductor with a three-dimensional (3D) Fermi surface has point nodes with
lifted spin degeneracy, resulting in itinerant Majorana fermions in the bulk
and topological Majorana arcs on the surface. | cond-mat_supr-con |
Fluctuation conductivity in superconductors in strong electric fields: We study the effect of a strong electric field on the fluctuation
conductivity within the time-dependent Ginzburg-Landau theory for the case of
arbitrary dimension. Our results are based on the analytical derivation of the
velocity distribution law for the fluctuation Cooper pairs, from the Boltzmann
equation. Special attention is drawn to the case of small nonlinearity of
conductivity, which can be investigated experimentally. We obtain a general
relation between the nonlinear conductivity and the temperature derivative of
the linear Aslamazov-Larkin conductivity, applicable to any superconductor. For
the important case of layered superconductors we derive an analogous relation
between the small nonlinear correction for the conductivity and the
fluctuational magnetoconductivity. On the basis of these relations we provide
new experimental methods for determining both the lifetime constant of
metastable Cooper pairs above T_c and the coherence length. A systematic
investigation of the 3rd harmonic of the electric field generated by a harmonic
current can serve as an alternative method for the examination of the
metastable Cooper-pair relaxation time. | cond-mat_supr-con |
Structure and superconductivity of LiFeAs: The lithium ions in Lithium iron arsenide phases with compositions close to
LiFeAs have been located using powder neutron diffraction. These phases exhibit
superconductivity at temperatures at least as high as 16 K demonstrating that
superconductivity in compounds with [FeAs]- anti-PbO-type anionic layers occurs
in compounds with at least three different structure types and occurs for a
wide range of As-Fe-As bond angles. | cond-mat_supr-con |
Effect of Cu doping on superconductivity in TaSe$_3$: Relationship
between superconductivity and induced charge density wave: By measuring the temperature dependence of the resistance, we investigated
the effect of Cu doping on superconductivity (SC) in Cu-doped TaSe$_3$ in which
the charge density wave (CDW) transition is induced by Cu doping. We observed
an emergence of a region where the SC transition temperature ($T_\mathrm{C}$)
decreased in samples with higher Cu concentrations and found that the region
tended to expand with increasing Cu concentration. In addition, the temperature
dependence of the upper critical field ($H_\mathrm{C2}$) of Cu-doped TaSe$_3$
was found to differ from that of pure TaSe$_3$. Based on these experimental
results and the fact that the SC of TaSe$_3$ is filamentary, we conclude that
SC is suppressed locally by Cu doping and competes with the CDW in Cu-doped
TaSe$_3$. The resistance anomaly due to the CDW transition was extremely small
and the size of the anomaly was enhanced with increasing Cu concentration but
the temperature at which the anomaly appeared hardly changed. This result of
the anomaly and the local suppression of SC imply that the induced CDWs are
short-range order in the vicinity of Cu atoms. We also discuss the effect of
the pinning of CDWs on the relationship between SC and short-range order CDWs. | cond-mat_supr-con |
What are the elementary excitations of the BCS model in the canonical
ensemble?: We have found the elementary excitations of the exactly solvable BCS model
for a fixed number of particles. These turn out to have a peculiar dispersion
relation, some of them with no counterpart in the Bogoliubov picture, and
unusual counting properties related to an old conjecture made by Gaudin. We
give an algorithm to count the number of excitations for each excited state and
a graphical interpretation in terms of paths and Young diagrams. For large
systems the excitations are described by an effective Gaudin model, which
accounts for the finite size corrections to BCS. | cond-mat_supr-con |
Positive Seebeck coefficient in highly doped La$_{2-x}$Sr$_x$CuO$_4$
($x$=0.33); its origin and implication: We present a study of the thermoelectric (Seebeck and Nernst) response in
heavily overdoped, non-superconducting La$_{1.67}$Sr$_{0.33}$CuO$_4$. In spite
of the electron-like curvature of the Fermi surface, the Seebeck coefficient is
positive at low temperatures. Such a feature, previously observed in copper,
silver, gold and lithium, is caused by a non-trivial energy dependence of the
scattering time. We argue that this feature implies a strong asymmetry between
the lifetime of occupied and unoccupied states along the zone diagonals and
such an electron-hole asymmetry impedes formation of Cooper pairs along the
nodal direction in the superconducting ground state emerging at lower doping
levels. | cond-mat_supr-con |
Unifying Magnons and Triplons in Stripe-Ordered Cuprate Superconductors: Based on a two-dimensional model of coupled two-leg spin ladders, we derive a
unified picture of recent neutron scattering data of stripe-ordered
La_(15/8)Ba_(1/8)CuO_4, namely of the low-energy magnons around the
superstructure satellites and of the triplon excitations at higher energies.
The resonance peak at the antiferromagnetic wave vector Q_AF in the
stripe-ordered phase corresponds to a saddle point in the dispersion of the
magnetic excitations. Quantitative agreement with the neutron data is obtained
for J= 130-160 meV and J_cyc/J = 0.2-0.25. | cond-mat_supr-con |
Vortex Glass is a Metal: Unified Theory of the Magnetic Field and
Disorder-Tuned Bose Metals: We consider the disordered quantum rotor model in the presence of a magnetic
field. We analyze the transport properties in the vicinity of the multicritical
point between the superconductor, phase glass and paramagnetic phases. We find
that the magnetic field leaves metallic transport of bosons in the glassy phase
in tact. In the vicinity of the vicinity of the superconductivity-to-Bose metal
transition, the resistitivy turns on as $(H-H_c)^{2}$ with $H_c$. This
functional form is in excellent agreement with the experimentally observed
turn-on of the resistivity in the metallic state in MoGe, namely $R\approx
R_c(H-H_c)^\mu$, $1<\mu<3$. The metallic state is also shown to presist in
three spatial dimensions. In addition, we also show that the metallic state
remains intact in the presence of Ohmic dissipation in spite of recent claims
to the contrary. As the phase glass in $d=3$ is identical to the vortex glass,
we conclude that the vortex glass is, in actuality, a metal rather than a
superconductor at T=0. Our analysis unifies the recent experiments on vortex
glass systems in which the linear resistivity remained non-zero below the
putative vortex glass transition and the experiments on thin films in which a
metallic phase has been observed to disrupt the direct transition from a
superconductor to an insulator. | cond-mat_supr-con |
Probability of the resistive state formation caused by absorption of a
single-photon in current-carrying superconducting nano-strips: We have studied supercurrent-assisted formation of the resistive state in
nano-structured Nb and NbN superconducting films after absorption of a single
photon. In amorphous narrow NbN strips the probability of the resistive state
formation has a pronounced spectral cut-off. The corresponding threshold photon
energy decreases with the bias current. Analysis of the experimental data in
the framework of the generalized hot-spot model suggests that the quantum yield
for near-infrared photons increases faster than the photon nergy. Relaxation of
the resistive state depends on the photon energy making the phenomenon feasible
for the development of energy resolving single-photon detectors. | cond-mat_supr-con |
Superconducting topological Dirac semimetals: $P6/m$-Si$_6$ and
$P6/m$-NaSi$_6$: We theoretically propose that hexagonal silicon-based crystals, $P6/m$-Si$_6$
and $P6/m$-NaSi$_6$, are topological Dirac semimetals with superconducting
critical temperatures of 12 K and 13 K, respectively, at ambient pressure. Band
inversion occurs with the Fu-Kane topological invariant $\mathbb{Z}_2=1$, even
in the absence of spin-orbit coupling. The Dirac nodes protected by $C_6$
crystal rotational symmetry remain gapless with spin-orbit coupling. Using
first-principles calculations, we find pressure-induced topological phase
transitions for $P6/m$-Si$_6$ and $P6/m$-NaSi$_6$ with critical external
pressures of 11.5 GPa and 14.9 GPa, respectively. Above the critical pressures,
the Dirac bands are gapped with $\mathbb{Z}_2=0$, while the superconducting
states and the crystal symmetries are retained.Our results may shed light into
a search for silicon-based topological materials with superconductivity. | cond-mat_supr-con |
Quantum decay of the persistent current in a Josephson junction ring: We study the persistent current in a ring consisting of N >> 1 Josephson
junctions threaded by the magnetic flux. When the dynamics of the ring is
dominated by the capacitances of the superconducting islands the system is
equivalent to the xy spin system in 1+1 dimensions at the effective temperature
T*=(2JU)^(1/2), with J being the Josephson energy of the junction and U being
the charging energy of the superconducting island. The numerical problem is
challenging due to the absence of thermodynamic limit and slow dynamics of the
Kosterlitz-Thouless transition. It is investigated on lattices containing up to
one million sites. At T << J the quantum phase slips are frozen. The low-T*
dependence of the persistent current computed numerically agrees quantitatively
with the analytical formula provided by the spin-wave approximation. The high-
T* behavior depends strongly on the magnetic flux and on the number of
superconducting islands N. Depending on the flux, the persistent current gets
destroyed by the phase slips and/or by the superconductor-insulator transition
on increasing T*. | cond-mat_supr-con |
Pressure-induced Superconductivity and Topological Quantum Phase
Transitions in the Topological Semimetal ZrTe2: Topological transition metal dichalcogenides (TMDCs) have attracted much
attention due to its potential applications in spintronics and quantum
computations. In this work, we systematically investigate the structural and
electronic properties of topological TMDCs candidate ZrTe2 under high pressure.
A pressure-induced Lifshitz transition is evidenced by the change of charge
carrier type as well as the Fermi surface. Superconductivity was observed at
around 8.3 GPa without structural phase transition. A typical dome-shape phase
diagram is obtained with the maximum Tc of 5.6 K for ZrTe2. Furthermore, our
theoretical calculations suggest the presence of multiple pressure-induced
topological quantum phase transitions, which coexists with emergence of
superconductivity. The results demonstrate that ZrTe2 with nontrivial topology
of electronic states display new ground states upon compression. | cond-mat_supr-con |
Quantum Pairing Time Orders: We propose the concept of the time-independent correlators for the even- and
odd-frequency pairing states that can be defined for both bosonic and fermionic
quasiparticles. These correlators explicitly capture the existence of two
distinct classes of pairing states and provide a direct probe of the hidden
Berezinskii order. This concept is illustrated in the cases of pairings for
Majorana fermions and quasiparticles in Dirac semimetals. It is shown that the
time-independent correlator is able to effectively capture the energy scale
relevant for pairing. | cond-mat_supr-con |
Type-1.5 Superconductors: We demonstrate the existence of a novel superconducting state in high quality
two-component MgB2 single crystalline superconductors where a unique
combination of both type-1 (kappa_1 < 0.707) and type-2 (kappa_2 > 0.707)
superconductor conditions is realized for the two components of the order
parameter. This condition leads to a vortex-vortex interaction attractive at
long distances and repulsive at short distances, which stabilizes
unconventional stripe- and gossamer-like vortex patterns that we have
visualized in this type-1.5 superconductor using Bitter decoration and also
reproduced in numerical simulations. | cond-mat_supr-con |
Fermi-liquid state in $T$*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$
revealed via element substitution effects on magnetism: Despite its unique structural features, the magnetism of single-layered
cuprate with five oxygen coordination ($T$*-type structure) has not been
investigated thus far. Here, we report the results of muon spin relaxation and
magnetic susceptibility measurements to elucidate the magnetism of $T$*-type
La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) via magnetic Fe- and non-magnetic
Zn-substitution. We clarified the inducement of the spin-glass (SG)-like
magnetically ordered state in
La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$Cu$_y$Fe$_{1-y}$O$_4$ with $x = 0.24 + y$, and
the non-magnetic state in La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$Cu$_y$Zn$_{1-y}$O$_4$
with $x$ = 0.24 after the suppression of superconductivity for $y$ $\geq$
0.025. The SG state lies below $\sim$7 K in a wide Sr concentration range
between 0.19 and 0.34 in 5$\%$ Fe-substituted LESCO. The short-range SG state
is consistent with that originating from the Ruderman-Kittel-Kasuya-Yosida
interaction in a metallic state. Thus, the results provide the first evidence
for Fermi liquid (FL) state in the pristine $T$*-type LESCO. Taking into
account the results of an oxygen $K$-edge X-ray absorption spectroscopy
measurement $[$J. Phys. Soc. Jpn. 89, 075002 (2020)$]$ reporting the actual
hole concentrations in LESCO, our results demonstrate the existence of the FL
state in a lower hole-concentration region, compared to that in $T$-type
La$_{2-x}$Sr$_x$CuO$_4$. The emergence of the FL state in a lower
hole-concentration region is possibly associated with a smaller charge transfer
gap energy in the parent material with five oxygen coordination. | cond-mat_supr-con |
Capturing Complex Behaviour in Josephson Travelling Wave Parametric
Amplifiers: We present an analysis of wave-mixing in recently developed Josephson
Travelling Wave Parametric Amplifiers (JTWPAs). Circuit simulations performed
using WRspice show the full behaviour of the JTWPA allowing propagation of all
tones. The Coupled Mode Equations (CMEs) containing only pump, signal, and
idler propagation are shown to be insufficient to completely capture complex
mixing behaviour in the JTWPA. Extension of the CMEs through additional state
vectors in the analytic solutions allows closer agreement with WRspice. We
consider an ordered framework for the systematic inclusion of extended
eigenmodes and make a qualitative comparison with WRspice at each step. The
agreement between the two methods validates both approaches and provides
insight into the operation of the JTWPA. We show that care should be taken when
using the CMEs and propose that WRspice should be used as a design tool for
non-linear superconducting circuits such as the JTWPA. | cond-mat_supr-con |
Possible coexistence of double-Q magnetic order and chequerboard charge
order in the re-entrant tetragonal phase of Ba0.76K0.24Fe2As2: We investigate the re-entrant tetragonal phase in the iron-based
superconductor Ba0 .76K0.24Fe2As2 by DC magnetization and thermoelectrical
measurements. The reversible magnetization confirms by a thermodynamic method
that the spin alignment in the re-entrant C4 phase is out-of-plane, in
agreement with an itinerant double-Q magnetic order [Allred et al., Nat. Phys.
12, 493 (2016)]. The Nernst coefficient shows the typical unusually large
negative value in the stripe-type spin density wave (SDW) state owing to the
Fermi surface reconstruction associated with SDW and nematic order. At the
transition into the re-entrant C4 tetragonal phase it hardly changes, which
could indicate that instead of a complete vanishing of the associated charge
order, the spin reorientation could trigger a redistribution of the charges to
form a secondary charge order, e.g. in form of a chequerboard-like pattern that
no longer breaks the rotational C4 symmetry. | cond-mat_supr-con |
Stable Thermomagnetic Waves in Hard Superconductors: The problem of the stability of a nonlinear thermomagnetic wave with respect
to small thermal and electromagnetic perturbations in hard superconductors was
studied. It is shown that spatially bounded solutions may correspond only to
the perturbations decaying with time, which implies stability of the nonlinear
thermomagnetic wave. | cond-mat_supr-con |
Role of interactions in the energy of the spin resonance peak in
Fe-based superconductors: We consider the spin response within the five-orbital model for iron-based
superconductors and study two cases: equal and unequal gaps in different bands.
In the first case, the spin resonance peak in the superconducting state appears
below the characteristic energy scale determined by the gap magnitude,
$2\Delta_L$. In the second case, the energy scale corresponds to the sum of
smaller and larger gap magnitudes, $\Delta_L + \Delta_S$. Increasing the values
of the Hubbard interaction and the Hund's exchange, we observe a shift of the
spin resonance energy to lower frequencies. | cond-mat_supr-con |
Four-legged starfish-shaped Cooper pairs with ultrashort antinodal
length scales in cuprate superconductors: Cooper pairs of mutually attracting electrons form the building blocks of
superconductivity. Thirty years after the discovery of high-temperature
superconductivity in cuprates, many details of the pairs remain unknown,
including their size and shape. Here we apply brand new ARPES-based methods
that allow us to reconstruct the shape and size of the pairs in
Bi$_2$Sr$_2$CaCu$_2$O$_{8+{\delta}}$. The pairs are seen to form a
characteristic starfish shape that is very long (>50{\AA}) in the near-nodal
direction but extremely short (~4.5{\AA}) in the antinodal (Cu-O) direction. We
find that this ultrashort antinodal length scale, which is of order a lattice
constant, is approximately constant over a wide range of doping levels even as
many other parameters including the pairing strength change. This suggests that
this new length scale, along with the pair shape, is one of the most
fundamental characteristics of the pairs. Further, the shape and ultrashort
length scale should make the pairs create or intertwine with variations in
charge and pair density, center on various types of lattice positions, and
potentially explain aspects of the nematic order in these materials. | cond-mat_supr-con |
Pressure-induced Superconductivity in Noncentrosymmetric Weyl Semimetals
LaAlX (X = Si and Ge): In topological materials, Dirac fermions can split into two Weyl fermions
with opposite chiralities due to the breaking of space inversion symmetry,
while in non-centrosymmetric superconductors, novel superconducting electron
pairing mechanisms arise because of the antisymmetric spin-orbit coupling. In
this work, we report the pressure-introduced superconductivity in a typical
noncentrosymmetric Weyl semimetal LaAlX (X=Si and Ge). Superconductivity was
observed at around 65 GPa without structural phase transition. A typical
dome-shape phase diagram is obtained with the maximum Tc of 2.5 K (2.1 K) for
LaAlSi (LaAlGe). Furthermore, the application of pressure does not destroy the
nontrivial band topology of LaAlSi up to 80.4 GPa, making such materials as
potential candidates for realizing topological superconductivity. Our discovery
of superconductivity in LaAlX (X=Si and Ge) will provide critical insight in
noncentrosymmetric superconductors and stimulate further study on
superconductivity in Weyl semimetals. | cond-mat_supr-con |
Three-Dimensional Phase-Kink State in Thick Stack of Josephson Junctions
and Terahertz Radiation: The dynamics of superconductivity phase in thick stack of Josephson junctions
with strong inductive coupling, such as the one realized in layered high-$T_c$
cuprates and possibly the recently discovered FeAs-based superconductors, is
investigated under a c-axis bias voltage and in the absence of an external
magnetic field. The kink state found previously by the present authors is
extended to three dimensions for both rectangular and cylindrical geometries.
The IV characteristics are calculated and the distributions of electromagnetic
field inside the samples are clarified. The solution for a cylindrical mesa
exhibits a higher resonating frequency than that of a square mesa with the same
linear size by a factor of $\sim 2.4$. More importantly, from the radius
dependence of the resonance frequency for the cylinder geometry it is possible
to confirm directly the kink state, and thus to reveal the mechanism of the
strong radiation discovered in recent experiments. | cond-mat_supr-con |
Interplay between superconductivity and itinerant magnetism in
underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($x=$ 0.2) probed by the response to
controlled point-like disorder: The response of superconductors to controlled introduction of point-like
disorder is an important tool to probe their microscopic electronic collective
behavior. In the case of iron-based superconductors (IBS), magnetic
fluctuations presumably play an important role in inducing high temperature
superconductivity. In some cases, these two seemingly incompatible orders
coexist microscopically. Therefore, understanding how this unique coexistence
state is affected by disorder can provide important information about the
microscopic mechanisms involved. In one of the most studied pnictide family,
hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (BaK122), this coexistence occurs over a
wide range of doping levels, 0.16~$\lesssim x \lesssim $~0.25. We used
relativistic 2.5 MeV electrons to induce vacancy-interstitial (Frenkel) pairs
that act as efficient point-like scattering centers. Upon increasing dose of
irradiation, the superconducting transition temperature $T_c$ decreases
dramatically. In the absence of nodes in the order parameter this provides a
strong support for a sign-changing $s_{\pm}$ pairing. Simultaneously, in the
normal state, there is a strong violation of the Matthiessen's rule and a
decrease (surprisingly, at the same rate as $T_c$) of the magnetic transition
temperature $T_{sm}$, which indicates the itinerant nature of the long-range
magnetic order. Comparison of the hole-doped BaK122 with electron-doped
Ba(Fe$_x$Co$_{1-x}$)$_2$As$_2$ (FeCo122) with similar $T_{sm}\sim$110~K,
$x=$0.02, reveals significant differences in the normal states, with no
apparent Matthiessen's rule violation above $T_{sm}$ on the electron-doped
side. We interpret these results in terms of the distinct impact of impurity
scattering on the competing itinerant antiferromagnetic and $s_{\pm}$
superconducting orders. | cond-mat_supr-con |
Optical symmetries and anisotropic transport in high-Tc superconductors: A simple symmetry analysis of in-plane and out-of-plane transport in a family
of high temperature superconductors is presented. It is shown that generalized
scaling relations exist between the low frequency electronic Raman response and
the low frequency in-plane and out-of-plane conductivities in both the normal
and superconducting states of the cuprates. Specifically, for both the normal
and superconducting state, the temperature dependence of the low frequency
$B_{1g}$ Raman slope scales with the $c-$axis conductivity, while the $B_{2g}$
Raman slope scales with the in-plane conductivity. Comparison with experiments
in the normal state of Bi-2212 and Y-123 imply that the nodal transport is
largely doping independent and metallic, while transport near the BZ axes is
governed by a quantum critical point near doping $p\sim 0.22$ holes per
CuO$_{2}$ plaquette. Important differences for La-214 are discussed. It is also
shown that the $c-$ axis conductivity rise for $T\ll T_{c}$ is a consequence of
partial conservation of in-plane momentum for out-of-plane transport. | cond-mat_supr-con |
Numerical simulation of thermal noise in Josephson circuits: We present a method to numerically add thermal noise to the equations of
motion for a circuit of Josephson junctions. A new noise term, which we call
"linearly interpolated Gaussian noise," replaces the usual white noise process.
It consists of random noise values spaced at a chosen time interval and
linearly interpolated in-between. This method can be used with variable time
step solvers, allowing more precise control over the error while ensuring that
fast dynamics are not missed by the solver. We derive the spectral density of
such a noise term and compare it to a white noise process. Then we demonstrate
the technique by computing the switching dynamics of a circuit of two Josephson
junctions and comparing the results to the traditional method. | cond-mat_supr-con |
Bi-directional ultrafast electric-field gating of interlayer transport
in a cuprate superconductor: In cuprate superconductors, tunneling between planes makes possible
three-dimensional coherent transport. However, the interlayer tunnelling
amplitude is reduced when an order-parameter phase gradient between planes is
established. As such, c-axis superconductivity can be weakened if a strong
electric field is applied along the c axis. We use high-field single-cycle
terahertz pulses to gate interlayer coupling in La1.84Sr0.16CuO4. We induce
ultrafast oscillations between superconducting and resistive states and switch
the plasmon response on and off, without reducing the density of Cooper pairs.
Indeed, in-plane superconductivity remains unperturbed throughout, revealing a
non-equilibrium state in which the dimensionality of the superconductor is time
dependent. The gating frequency is determined by the electric field strength,
in the spirit of the ac Josephson effect. Non-dissipative, bi-directional
gating of superconductive coupling is of interest for device applications in
ultrafast nanoelectronics. It is also a new example of nonlinear terahertz
physics, applicable to nanoplasmonics and active metamaterials. | cond-mat_supr-con |
Properties of High-Tc Single Crystals as Natural Interferometers in the
THz Frequency Range: We consider oblique incidence of (p)TM-polarized wave on the anisotropic
superconducting slab, immersed on a dielectric media, such that its uniaxial
(c) axis is perpendicular to the surfaces. The below and above plasma frequency
transmissivity patterns are studied and several of its properties determined,
within the context of the Maxwell-London theory. Below, the regime is
attenuated for any incident angle, and there is a transmissivity maximum, quite
pronounced in case of a very high external dielectric constant. Above, a
propagative regime exists where the superconductor is a natural optical
resonator, and we show here that the minimum of the transmission peaks are
modulated by an envelope function associated to the Brewster condition. We
propose this set-up to obtain light with an extremely small transverse
wavelength inside the superconductor. | cond-mat_supr-con |
A Comment on ``Superconducting-Normal Phase Transition in (Ba1-xKx)BiO3,
x = 0.40, 0.47'' by B. F. Woodfield, D. A. Wright, R. A. Fisher, N. E.
Phillips and H. Y. Tang, Phys. Rev. Lett. 83, 4622 (1999): This comment addresses criticisms of our PRL 82 (1999) p. 4532-4535
(cond-mat/9904288) raised by Woodfield et al. in their recent PRL. We find that
the critical fields in single crystals of Ba1-xKxBiO3 (x = 0.4) have anomalous
temperature dependecies which suggests that the superconducting phase
transition in this material is not of order II. | cond-mat_supr-con |
Design and testing of high-speed interconnects for Superconducting
multi-chip modules: Superconducting single flux quantum (SFQ) circuits can process information at
extremely high speeds, in the range of hundreds of GHz. SFQ circuits are based
on Josephson junction cells for switching logic and ballistic transmission for
transferring SFQ pulses. Multi-chip modules (MCM) are often used to implement
larger complex designs, which cannot be fit onto a single chip. We have
optimized the design of wideband interconnects for transferring signals and SFQ
pulses between chips in flip-chip MCMs and evaluated the importance of several
design parameters such as the geometry of bump pads on chips, length of passive
micro-strip lines (MSL)s, number of corners in MSLs as well as flux trapping
and fabrication effects on the operating margins of the MCMs. Several test
circuits have been designed to evaluate the above mentioned features and
fabricated in the framework of 4.5-kA/cm2 HYPRES process. The MCMs bumps for
electrical connections have been deposited using the waferlevel electroplating
process. We have found that, at the optimized configuration, the maximum
operating frequency of the MCM test circuit, a ring oscillator with
chip-to-chip connections, approaches 100 GHz and is not noticeably affected by
the presence of MCM interconnects, decreasing only about 3% with respect to the
same circuit with no inter-chip connections. | cond-mat_supr-con |
Quasiparticle scattering in a superconductor near a nematic critical
point: resonance mode and multiple attractive channels: We analyze the scattering rate for 2D fermions interacting via soft nematic
fluctuations. The ground state is an s-wave superconductor, but other pairing
channels are almost equally attractive. This strongly alters the scattering
rate: At energies beyond the pairing gap $\Delta$, it is renormalized by
contributions from all pairing channels. At energies of order $\Delta$, it is
determined by the competition between scattering into a gapped continuum and
dispersing nematic resonance. The outcome is a "peak-peak-dip-hump" spectrum,
similar, but not identical, to the "peak-dip-hump" structure in the cuprates. | cond-mat_supr-con |
Boron Spectral Density and Disorder Broadening in B-doped Diamond: Comparison of periodic B dopants with a random alloy of substitional boron in
diamond is carried out using several supercells and the coherent potential
approximation (CPA) for the random alloy case. The main peak in the B local
density of states is shifted to lower binding energy compared to the
corresponding C peak in intrinsic diamond. In supercells, this shows up as
strongly B-character bands split from bulk C bands away from the zone center,in
an energy region around -1 eV. Even for a 4*4*4 supercell (BC$_{127}$), effects
of the dopant order are evident in the form of primarily B-character bands just
below the Fermi level at the supercell zone boundary. The bands resulting from
the CPA are of continuous mixed C-B character. They resemble virtual crystal
bands, but broadened somewhat reflecting the disorder-induced lifetime, and are
consistent with angle-resolved photoemission band maps. The B character is 1.7
times larger than for C (per atom) near the top of the valence bands for CPA,
and roughly the same for supercells. CPA results are particularly useful since
they characterize the wavevector and energy dependence of disorder broadening. | cond-mat_supr-con |
Upper critical field Hc2 in Bechgaard salts (TMTSF)2PF6: The symmetry of the superconductivity in Bechgaard salts is still unknown,
though the triplet pairing has been established by Hc2 and NMR for (TMTSF)2PF6.
The large upper critical field at T = 0K (Hc2 ~ 5 Tesla) both for H || a and H
|| b also indicates strongly the triplet pairing.
Here we start with a low energy effective Hamiltonian and study the
temperature dependence of the corresponding Hc2(T)'s.
The present analysis suggests that one chiral f-wave superconductor should be
the most likely candidate near the upper critical field. | cond-mat_supr-con |
Possible Superconductivity in Fe-Sb Based Materials: Density Functional
Study of LiFeSb: We investigate the electronic and other properties of the hypothetical
compound LiFeSb in relation to superconducting LiFeAs and FeSe using density
functional calculations. The results show that LiFeSb in the LiFeAs structure
would be dynamically stable in the sense of having no unstable phonon modes,
and would have very similar electronic and magnetic properties to the layered
Fe based superconductors. Importantly, a very similar structure for the Fermi
surface and a spin density wave related to but stronger than that in the
corresponding As compound is found. These results are indicative of possible
superconductivity analogous to the Fe-As based compounds if the spin density
wave can be suppressed by doping or other means. Prospects for synthesizing
this material in pure form or in solid solution with FeTe are discussed. | cond-mat_supr-con |
Ultrasensitive interplay between ferromagnetism and superconductivity in
NbGd composite thin films: A model binary hybrid system composed of a randomly distributed rare-earth
ferromagnetic (Gd) part embedded in an s-wave superconducting (Nb) matrix is
being manufactured to study the interplay between competing superconducting and
ferromagnetic order parameters. The normal metallic to superconducting phase
transition appears to be very sensitive to the magnetic counterpart and the
modulation of the superconducing properties follow closely to the
Abrikosov-Gorkov (AG) theory of magnetic impurity induced pair breaking
mechanism. A critical concentration of Gd is obtained for the studied NbGd
based composite films (CFs) above which superconductivity disappears. Besides,
a magnetic ordering resembling the paramagnetic Meissner effect (PME) appears
in DC magnetization measurements at temperatures close to the superconducting
transition temperature. The positive magnetization related to the PME emerges
upon doping Nb with Gd. The temperature dependent resistance measurements
evolve in a similar fashion with the concentration of Gd as that with an
external magnetic field and in both the cases, the transition curves accompany
several intermediate features indicating the traces of magnetism originated
either from Gd or from the external field. Finally, the signatures of magnetism
appear evidently in the magnetization and transport measurements for the CFs
with very low (less than 1 at. %) doping of Gd. | cond-mat_supr-con |
Superconducting electronic state in optimally doped YBa2Cu3O7-d observed
with laser-excited angle-resolved photoemission spectroscopy: Low energy electronic structure of optimally doped YBa2Cu3O7-d is
investigated using laser-excited angle-resolved photoemission spectroscopy. The
surface state and the CuO chain band that usually overlap the CuO2 plane
derived bands are not detected, thus enabling a clear observation of the bulk
superconducting state. The observed bilayer splitting of the Fermi surface is
~0.08 angstrom^{-1} along the (0,0)-(pi,pi) direction, significantly larger
than Bi2Sr2CaCu2O8+d. The kink structure of the band dispersion reflecting the
renormalization effect at ~60 meV shows up similarly as in other hole-doped
cuprates. The momentum-dependence of the superconducting gap shows
d_{x^2-y^2}-wave like amplitude, but exhibits a nonzero minimum of ~12 meV
along the (0,0)-(pi,pi) direction. Possible origins of such an unexpected
"nodeless" gap behavior are discussed. | cond-mat_supr-con |
Long-lived, radiation-suppressed superconducting quantum bit in a planar
geometry: We present a superconducting qubit design that is fabricated in a 2D geometry
over a superconducting ground plane to enhance the lifetime. The qubit is
coupled to a microstrip resonator for readout. The circuit is fabricated on a
silicon substrate using low loss, stoichiometric titanium nitride for capacitor
pads and small, shadow-evaporated aluminum/aluminum-oxide junctions. We observe
qubit relaxation and coherence times ($T_1$ and $T_2$) of 11.7 $\pm$ 0.2 $\mu$s
and 8.7 $\pm$ 0.3 $\mu$s, respectively. Calculations show that the proximity of
the superconducting plane suppresses the otherwise high radiation loss of the
qubit. A significant increase in $T_1$ is projected for a reduced
qubit-to-superconducting plane separation. | cond-mat_supr-con |
Impurity-induced broadening of the transition to a
Fulde-Ferrell-Larkin-Ovchinnikov phase: Recent study on doping effects in the heavy fermion superconductor CeCoIn$_5$
has shown that a small amount of doping induces unexpectedly large broadening
of the second order transition into the high field and low temperature (HFLT)
phase of this material. Motivated by this observation, effects of quenched
disorder on the second order transition into a longitudinal
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state are examined. The observed large
broadening of the transition is naturally explained as a consequence of
softness of each FFLO nodal plane. The present results strongly support the
scenario identifying the HFLT phase of CeCoIn$_5$ with a longitudinal FFLO
vortex state. | cond-mat_supr-con |
Impurity bound states in fully gapped $d$-wave superconductors with
subdominant order parameters: Impurities in superconductors and their induced bound states are important
both for engineering novel states such as Majorana zero-energy modes and for
probing bulk properties of the superconducting state. The high-temperature
cuprates offer a clear advantage in a much larger superconducting order
parameter, but the nodal energy spectrum of a pure $d$-wave superconductor only
allows virtual bound states. Fully gapped $d$-wave superconducting states have
however been proposed in several cuprate systems thanks to subdominant order
parameters producing $d+is$- or $d+id'$-wave superconducting states. Here we
study both magnetic and potential impurities in these fully gapped $d$-wave
superconductors. Using analytical T-matrix and complementary numerical
tight-binding lattice calculations, we show that magnetic and potential
impurities behave fundamentally different in $d+is$- and $d+id'$-wave
superconductors. In a $d+is$-wave superconductor, there are no bound states for
potential impurities, while a magnetic impurity produces one pair of bound
states, with a zero-energy level crossing at a finite scattering strength. On
the other hand, a $d+id'$-wave symmetry always give rise to two pairs of bound
states and only produce a reachable zero-energy level crossing if the normal
state has a strong particle-hole asymmetry. | cond-mat_supr-con |
Observation of an isotropic superconducting gap at the Brillouin zone
center of Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$: We performed a high-resolution angle-resolved photoemission spectroscopy
study on superconducting (SC) Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$ ($T_c=29$
K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic $\sim$
8.2 meV 2-dimensional (2D) SC gap ($2\Delta/k_BT_c\sim7$) on quasi-2D electron
Fermi surfaces (FSs) located around M$(\pi,0,0)$-A$(\pi,0,\pi)$, we observe a
$\sim 6.2$ meV isotropic SC gap ($2\Delta/k_BT_c\sim5$) on the Z-centered
electron FS that rules out any d-wave pairing symmetry and rather favors an
s-wave symmetry. All isotropic SC gap amplitudes can be fit by a single gap
function derived from a local strong coupling approach suggesting an
enhancement of the next-next neighbor exchange interaction in the
ferrochalcogenide superconductors. | cond-mat_supr-con |
Size of stripe domains in a superconducting ferromagnet: In a superconducting ferromagnet, the superconducting state appears in the
ferromagnetic phase where usually a domain structure has already developed. We
study the influence of the superconducting screening currents on a stripe
structure with out-of-plane magnetization, in a film of arbitrary thickness. We
find that superconductivity always induces a shrinkage of the domains, and
there is a critical value of penetration depth below which a mono-domain
structure is more stable than the periodic one. Furthermore we investigate the
possible different effects of singlet and triplet superconductivity on the
domain width, as well as the conditions for the existence of vortices in the
domains. The obtained results are then discussed in light of the experimental
data of superconducting ferromagnets URhGe, UGe2, and UCoGe. | cond-mat_supr-con |
Andreev reflection and spin polarization measurement of Co/YBCO junction: We report temperature dependent Andreev reflection measurements of Co/
Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ (YBCO) heterostructure samples with
junction areas of 1 $\mu$m diameter. Modelling of the 5-70 K conductivity data
according to a modified Blonder-Tinkham-Klapwijk theory yields a spin
polarization in Co film amounting to 34% which is almost constant up to 70 K.
The YBCO films have been grown by pulsed laser deposition on sapphire
substrates. The Co films are deposited by thermal evaporation on YBCO. The film
is characterized by powder X-ray diffraction measurements which shows YBCO is
grown in (001) direction.The critical current density, 5 x 10$^{6}$ A/cm$^{2}$,
in YBCO remains nearly constant after deposition of Co at zero field and 77 K. | cond-mat_supr-con |
Magnetism and superconductivity in Ru(1-x)Sr2RECu(2+x)O(8-d) (RE=Gd, Eu)
and RuSr2Gd(1-y)Ce(y)Cu2O8 compounds: We discuss the properties of new superconducting compositions of
Ru(1-x)Sr2RECu(2+x)O(8-d) (RE=Gd, Eu) ruthenocuprates that were synthesized at
600 atm. of oxygen at 1080 C. By changing ratio between the Ru and Cu, the
temperature of superconducting transition (Tc) raises up to Tc max=72 K for
x=0.3, 0.4. The hole doping achieved along the series increases with Cu->Ru
substitution. For x different than x=0, Tc can be subsequently tuned between Tc
max and 0 K by changing oxygen content in the compounds. The magnetic
characteristics of the RE=Gd and Eu based compounds are interpreted as
indicative of constrained dimensionality of the superconducting phase. Muon
spin rotation experiments reveal the presence of the magnetic transitions at
low temperatures (Tm=14-2 K for x=0.1-0.4) that can originate in the response
of Ru/Cu sublattices. RuSr2Gd(1-y)Ce(1-y)Cu2O8 (0<y<0.1) compounds show the
simultaneous increase of TN and decrease of Tc with y. The effect should be
explained by the electron doping that occurs with Ce->Gd substitution.
Properties of these two series allow us to propose phase diagram for 1212-type
ruthenocuprates that links their properties to the hole doping achieved in the
systems. Non-superconducting single-phase RuSr2GdCu2O8 and RuSr2EuCu2O8 are
reported and discussed in the context of the properties of substituted
compounds. | cond-mat_supr-con |
Superconductivity in dilute SrTiO$_3$: a review: Doped SrTiO$_3$, one of the most dilute bulk systems to display
superconductivity, is perhaps the first example of an unconventional
superconductor, as it does not fit into the standard BCS paradigm. More than
five decades of research has revealed a rich temperature-carrier concentration
phase diagram that showcases a superconducting dome, proximity to a putative
quantum critical point, Lifshitz transitions, a multi-gap pairing state and
unusual normal-state transport properties. Research has also extended beyond
bulk SrTiO$_3$, ushering the new field of SrTiO$_3$-based heterostructures.
Because many of these themes are also featured in other quantum materials of
contemporary interest, recent years have seen renewed interest in SrTiO$_3$.
Here, we review the challenges and recent progress in elucidating the
superconducting state of this model system. At the same time that its extreme
dilution requires to revisit several of the approximations that constitute the
successful Migdal-Eliashberg description of electron-phonon superconductivity,
including the suppression of the Coulomb repulsion via the
Tolmachev-Anderson-Morel mechanism, it opens interesting routes for alternative
pairing mechanisms whose applicability remains under debate. For instance,
pairing mechanisms involving longitudinal optical phonons have to overcome the
hurdles created by the anti-adiabatic nature of the pairing interaction,
whereas mechanisms that rely on the soft transverse optical phonons associated
with incipient ferroelectricity face challenges related to the nature of the
electron-phonon coupling. Proposals in which pairing is mediated by plasmons or
promoted locally by defects are also discussed. We finish by surveying the
existing evidence for multi-band superconductivity and outlining promising
directions that can potentially shed new light on the rich problem of
superconductivity in SrTiO$_3$. | cond-mat_supr-con |
Electronic phase separation in iron selenide (Li, Fe)OHFeSe
superconductor system: The phenomenon of phase separation into antiferromagnetic (AFM) and
superconducting (SC) or normal-state regions has great implication for the
origin of high-temperature (high-Tc) superconductivity. However, the occurrence
of an intrinsic antiferromagnetism above the Tc of (Li, Fe)OHFeSe
superconductor is questioned. Here we report a systematic study on a series of
(Li, Fe)OHFeSe single crystal samples with Tc up to ~41 K. We observe an
evident drop in the static magnetization at Tafm ~125 K, in some of the SC (Tc
< ~38 K, cell parameter c < ~9.27 {\AA}) and non-SC samples. We verify that
this AFM signal is intrinsic to (Li, Fe)OHFeSe. Thus, our observations indicate
mesoscopic-to-macroscopic coexistence of an AFM state with the normal (below
Tafm) or SC (below Tc) state in (Li, Fe)OHFeSe. We explain such coexistence by
electronic phase separation, similar to that in high-Tc cuprates and iron
arsenides. However, such an AFM signal can be absent in some other samples of
(Li, Fe)OHFeSe, particularly it is never observed in the SC samples of Tc > ~38
K, owing to a spatial scale of the phase separation too small for the
macroscopic magnetic probe. For this case, we propose a microscopic electronic
phase separation. It is suggested that the microscopic static phase separation
reaches vanishing point in high-Tc (Li, Fe)OHFeSe, by the occurrence of
two-dimensional AFM spin fluctuations below nearly the same temperature as Tafm
reported previously for a (Li, Fe)OHFeSe (Tc ~42 K) single crystal. A complete
phase diagram is thus established. Our study provides key information of the
underlying physics for high-Tc superconductivity. | cond-mat_supr-con |
Theoretical investigation of magnetic order in ReOFeAs, Re = Ce, Pr: Density functional theory (DFT) calculations are carried out on ReOFeAs, Re =
Ce, Pr, the parent compounds of the high-T$_c$ superconductors
ReO$_{1-x}$F$_{x}$FeAs, in order to determine the magnetic order of the ground
state. It is found that the magnetic moments on the Fe sites adopt a collinear
antiferromagnetic order, similar to the case of LaOFeAs. Within the generalized
gradient approximation along with Coulomb onsite repulsion (GGA+U), we show
that the Re magnetic moments also adopt an antiferromagnetic order for which,
within the ReO layer, same spin Re sites lie along a zigzag line perpendicular
to the Fe spin stripes. While within GGA the Re 4f band crosses the Fermi
level, upon inclusion of onsite Coulomb interaction the 4f band splits and
moves away from the Fermi level, making ReOFeAs a Mott insulator. | cond-mat_supr-con |
Hole doping dependences of the magnetic penetration depth and vortex
core size in YBa2Cu3Oy: Evidence for stripe correlations near 1/8 hole doping: We report on muon spin rotation measurements of the internal magnetic field
distribution n(B) in the vortex solid phase of YBa2Cu3Oy (YBCO) single
crystals, from which we have simultaneously determined the hole doping
dependences of the in-plane Ginzburg-Landau (GL) length scales in the
underdoped regime. We find that Tc has a sublinear dependence on
1/lambda_{ab}^2, where lambda_{ab} is the in-plane magnetic penetration depth
in the extrapolated limits T -> 0 and H -> 0. The power coefficient of the
sublinear dependence is close to that determined in severely underdoped YBCO
thin films, indicating that the same relationship between Tc and the superfluid
density is maintained throughout the underdoped regime. The in-plane GL
coherence length (vortex core size) is found to increase with decreasing hole
doping concentration, and exhibit a field dependence that is explained by
proximity-induced superconductivity on the CuO chains. Both the magnetic
penetration depth and the vortex core size are enhanced near 1/8 hole doping,
supporting the belief by some that stripe correlations are a universal property
of high-Tc cuprates. | cond-mat_supr-con |
Effect of Coulomb Interaction and Disorder on Density of States in
Conventional Superconductors: The density of states of the disordered s-wave superconductor is calculated
perturbatively. The effect of Coulomb interaction on diffusively moving
electrons in the normal state has been known before, but in the superconducting
state both diffuson and the screened Coulomb interaction are modified.
Therefore, the correction to the density of states in the superconducting state
exhibits an energy dependence different from that of the normal state. There is
a dip structure in the correction part because the interaction has a peak at
twice the energy of the superconducting gap. The Coulomb interaction and the
superconducting fluctuation cannot be treated separately because the density
fluctuation is coupled to the phase fluctuation in the superconducting state.
This coupling results in the absence of divergence around the gap edge in the
correction part, which suggests the validity of this perturbation calculation. | cond-mat_supr-con |
Hall Anomaly and Vortex-Lattice Melting in Superconducting Single
Crystal YBa2Cu3O7-d: Sub-nanovolt resolution longitudinal and Hall voltages are measured in an
ultra pure YBa2Cu3O7-d single crystal. The Hall anomaly and the first-order
vortex-lattice melting transition are observed simultaneously. Changes in the
dynamic behavior of the vortex solid and liquid are correlated with features of
the Hall conductivity sxy. With the magnetic field oriented at an angle from
the twin-boundaries, the Hall conductivity sharply decreases toward large
negative values at the vortex-lattice melting transition. | cond-mat_supr-con |
Nernst effect anisotropy as a sensitive probe of Fermi surface
distortions from electron-nematic order: We analyze the thermoelectric response in layered metals with spontaneously
broken rotation symmetry. We identify the anisotropy of the quasiparticle
Nernst signal as an extremely sensitive probe of Fermi surface distortions
characteristic of the ordered state. This is due to a subtle interplay of
different transport anisotropies which become additionally enhanced near
van-Hove singularities. Applied to recent experiments, our results reinforce
the proposal that the underdoped cuprate superconductor YBCO displays such
``electron-nematic'' order in the pseudogap regime. | cond-mat_supr-con |
Experimental Evidences for Static Charge Density Waves in Iron
Oxy-pnictides: In this Letter we report high-resolution synchrotron X-ray powder diffraction
and transmission electron microscope analysis of Mn-substituted LaFeAsO
samples, demonstrating that a static incommensurate modulated structure
develops across the low-temperature orthorhombic phase, whose modulation
wave-vector depends on the Mn content. The incommensurate structural distortion
is likely originating from a charge-density-wave instability, a periodic
modulation of the density of conduction electrons associated with a modulation
of the atomic positions. Our results add a new component in the physics of
Fe-based superconductors, indicating that the density wave ordering is
charge-driven. | cond-mat_supr-con |
The pseudogap in Bi2212 single crystals from tunneling measurements: a
possible evidence for the Cooper pairs above Tc: We present electron-tunneling spectroscopy of slightly overdoped Bi2212
single crystals with Tc = 87 - 90 K in a temperature range between 14 K and 290
K using a break-junction technique. The pseudogap which has been detected above
Tc appears at T* = 280 K. The analysis of the spectra shows that there is a
contribution to the pseudogap above Tc, which disappears approximately at 110 -
115 K. We associate this contribution with the presence of incoherent Cooper
pairs. | cond-mat_supr-con |
Macroscale three-dimensional proximity effect in disordered
normal/superconductor nanocomposites: Recently, interest in Superconductor (S)-Normal (N) interfaces was renewed by
the observation of exotic proximity effects in various systems, including
S/semiconductor, S/ferromagnet, and S/topological insulator. In general, the
proximity effect is enhanced in transparent weak links where coherent Andreev
reflection is possible. Also, it is a common knowledge that the proximity
effect is, by definition, is a localized phenomenon that can only be active in
each S/N interface region. However, here we show that a three-dimensional (3D)
macroscale proximity effect is realized in few-micrometer-thick MgO/Mg2Si/MgB2
nanocomposite layers with atomically smooth and clean heterointerfaces. We
found from scanning superconducting quantum interference device (SQUID)
microscopy measurements that a normal region of more than 100x100 square
micrometers totally undergoes transition into a bulk-like superconducting state
although the normal host originally contains less than ~10 vol % of
superconducting MgB2 nanograins in a dispersed manner. In the proximity-induced
superconducting region, vortex formation and annihilation processes as well as
vortex-free Meissner regions were observed with respect to applied fields in a
similar manner as Abrikosov vortices in type-II superconductors. Furthermore,
we found that the induced superconducting layers exhibit an anisotropic
magnetization behavior, in consistent with the formation of the large-scale
superconducting coherence. This unusually extended proximity effect suggests
that disorder-induced interaction and coupling of Andreev bound states, which
are coherent superposition of time reversed electron hole pairs, is realized in
the nanocomposite. Thus, the present results not only expand the limit of the
proximity effect to bulk scales, but also provides a new route to obtain a
proximity-induced superconducting state from disordered systems. | cond-mat_supr-con |
Transport properties of Lix(NH3)yFe2(TezSe1-z)2 single crystals in the
mixed state: We study the electric transport properties of Lix(NH3)yFe2(TezSe1-z)2 single
crystals with z= 0 and 0.6 in the mixed state. Thermally-activated flux-flow,
vortex glass and flux-flow Hall effect (FFHE) behaviors are observed.
Experimental results show that there are rich vortex phases existing in these
systems and the vortex liquid states occupy broad regions of phase diagrams.
Further analysis suggests that thermal fluctuation plays an important role in
the vortex phase diagrams of Lix(NH3)yFe2(TezSe1-z)2. Moreover, for
Lix(NH3)yFe2Se2, there is no sigh reversal of FFHE in the mixed state and a
scaling behavior |rhoxy(mu0H)|=Arhoxx(mu0H)^beta with beta ~ 2.0 is observed. | cond-mat_supr-con |
On the Mechanism of Superconductivity in HTSC from Tunneling
Spectroscopy Measurements on Bi-2212 Single Crystals: The paper has been withdrawn by the author because the observed effect has a
different origin. | cond-mat_supr-con |
Anomalous thermodynamic power laws near topological transitions in nodal
superconductors: Unconventional superconductors are most frequently identified by the
observation of power-law behaviour on low-temperature thermodynamic or
transport properties, such as specific heat. Here we show that, in addition to
the usual point and line nodes, a much wider class of different nodal types can
occur. These new types of nodes typically occur when there are transitions
between different types of gap node topology, for example when point or line
nodes first appear as a function of some physical parameter. We identify
anomalous, non-integer thermodynamic power laws associated with these new nodal
types, and give physical examples of superconductors in which they might be
observed experimentally, including the noncentrosymmetric superconductor
Li$_2$Pd$_{3-x}$Pt$_x$B. | cond-mat_supr-con |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.