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Point-contact spectroscopy of superconductors in the nonequilibrium
state: A phase transition of the region of the superconductor near the point contact
into a new nonequi- librium state at the critical density of
nonequilibriumquasiparticles is observed. | cond-mat_supr-con |
Oxygen adsorption induced superconductivity in ultrathin FeTe film on
SrTiO3(001): The phenomenon of oxygen incorporation induced superconductivity in iron
telluride (Fe1+yTe, with antiferromagnetic (AFM) orders) is intriguing and
quite different from the case of FeSe. Until now, the microscopic origin of the
induced superconductivity and the role of oxygen are far from clear. Here, by
combining in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and
x-ray photoemission spectroscopy (XPS) on oxygenated FeTe, we found physically
adsorbed O2 molecules crystallized into c(2/3x2) structure as an oxygen
overlayer at low temperature, which was vital for superconductivity. The O2
overlayer were not epitaxial on the FeTe lattice, which implied weak O2-FeTe
interaction but strong molecular interactions. Energy shift observed in the STS
and XPS measurements indicated hole doping effect from the O2 overlayer to the
FeTe layer, leading to a superconducting gap of 4.5 meV opened across the Fermi
level. Our direct microscopic probe clarified the role of oxygen on FeTe and
emphasized the importance of charge transfer effect to induce superconductivity
in iron-chalcogenide thin films. | cond-mat_supr-con |
Prediction of superconducting properties of materials based on machine
learning models: The application of superconducting materials is becoming more and more
widespread. Traditionally, the discovery of new superconducting materials
relies on the experience of experts and a large number of "trial and error"
experiments, which not only increases the cost of experiments but also prolongs
the period of discovering new superconducting materials. In recent years,
machine learning has been increasingly applied to materials science. Based on
this, this manuscript proposes the use of XGBoost model to identify
superconductors; the first application of deep forest model to predict the
critical temperature of superconductors; the first application of deep forest
to predict the band gap of materials; and application of a new sub-network
model to predict the Fermi energy level of materials. Compared with our known
similar literature, all the above algorithms reach state-of-the-art. Finally,
this manuscript uses the above models to search the COD public dataset and
identify 50 candidate superconducting materials with possible critical
temperature greater than 90 K. | cond-mat_supr-con |
Field-angle dependence reveals odd-parity superconductivity in
CeRh$_2$As$_2$: CeRh$_2$As$_2$ is an unconventional superconductor with multiple
superconducting phases and $T_\mathrm{c} = 0.26$ K. When $H\parallel c$, it
shows a field-induced transition at $\mu_0H^* = 4$ T from a low-field
superconducting state SC1 to a high-field state SC2 with a large critical field
of $\mu_0H_\mathrm{c2} = 14$ T. In contrast, for $H\perp c$, only the SC1 with
$\mu_0H_\mathrm{c2} = 2$ T is observed. A simple model based on the crystal
symmetry was able to reproduce the phase-diagrams and their anisotropy,
identifying SC1 and SC2 with even and odd parity superconducting states,
respectively. However, additional orders were observed in the normal state
which might have an influence on the change of the superconducting state at
$H^*$. Here, we present a comprehensive study of the angle dependence of the
upper critical fields using magnetic ac-susceptibility, specific heat and
torque on single crystals of CeRh$_2$As$_2$. The experiments show that the
state SC2 is strongly suppressed when rotating the magnetic field away from the
$c$ axis and it disappears for an angle of 35$^{\circ}$. This behavior agrees
perfectly with our extended model of a pseudospin triplet state with $\vec{d}$
vector in the plane and hence allows to nail down that SC2 is indeed the
suggested odd-parity state. | cond-mat_supr-con |
Robust topological superconductivity in spin-orbit coupled systems at
higher-order van Hove filling: Van Hove singularities (VHSs) in proximity to the Fermi level promote
electronic interactions and generate diverse competing instabilities. It is
also known that a nontrivial Berry phase derived from spin-orbit coupling (SOC)
can introduce an intriguing decoration into the interactions and thus alter
correlated phenomena. However, it is unclear how and what type of new physics
can emerge in a system featured by the interplay between VHSs and the Berry
phase. Here, based on a general Rashba model on the square lattice, we
comprehensively explore such an interplay and its significant influence on the
competing electronic instabilities by performing a parquet renormalization
group analysis. Despite the existence of a variety of comparable fluctuations
in the particle-particle and particle-hole channels associated with
higher-order VHSs, we find that the chiral $p \pm ip$ pairings emerge as two
stable fixed trajectories within the generic interaction parameter space,
namely the system becomes a robust topological superconductor. The chiral
pairings stem from the hopping interaction induced by the nontrivial Berry
phase. The possible experimental realization and implications are discussed.
Our work sheds new light on the correlated states in quantum materials with
strong SOC and offers fresh insights into the exploration of topological
superconductivity. | cond-mat_supr-con |
Microwave spectroscopy of Majorana vortex modes: The observation of zero-bias conductance peaks in vortex cores of certain
Fe-based superconductors has sparked renewed interest in vortex-bound Majorana
states. These materials are believed to be intrinsically topological in their
bulk phase, thus avoiding potentially problematic interface physics encountered
in superconductor-semiconductor heterostructures. However, progress toward a
vortex-based topological qubit is hindered by our inability to measure the
topological quantum state of a non-local vortex Majorana state, i.e., the
charge of a vortex pair. In this paper, we theoretically propose a
microwave-based charge parity readout of the Majorana vortex pair charge. A
microwave resonator above the vortices can couple to the charge allowing for a
dispersive readout of the Majorana parity. Our technique may also be used in
vortices in conventional superconductors and allows one to probe the lifetime
of vortex-bound quasiparticles, which is currently beyond existing scanning
tunneling microscopy capabilities. | cond-mat_supr-con |
Microscopic derivation of Ginzburg-Landau equations for coexistent
states of superconductivity and magnetism: Ginzburg-Landau (GL) equations for the coexistent states of superconductivity
and magnetism are derived microscopically from the extended Hubbard model with
on-site repulsive and nearest-neighbor attractive interactions. In the derived
GL free energy a cubic term that couples the spin-singlet and spin-triplet
components of superconducting order parameters (SCOP) with magnetization
exists. This term gives rise to a spin-triplet SCOP near the interface between
a spin-singlet superconductor and a ferromagnet, consistent with previous
theoretical studies based on the Bogoliubov de Gennes method and the
quasiclassical Green's function theory. In coexistent states of singlet
superconductivity and antiferromagnetism it leads to the occurrence of
pi-triplet SCOPs. | cond-mat_supr-con |
Induced unconventional superconductivity on the surface states of
Bi$_2$Te$_3$ topological insulator: Topological superconductivity is central to a variety of novel phenomena
involving the interplay between topologically ordered phases and
broken-symmetry states. The key ingredient is an unconventional order
parameter, with an orbital component containing a chiral $p_x$ + i$p_y$ wave
term. Here we present phase-sensitive measurements, based on the quantum
interference in nanoscale Josephson junctions, realized by using Bi$_2$Te$_3$
topological insulator. We demonstrate that the induced superconductivity is
unconventional and consistent with a sign-changing order parameter, such as a
chiral $p_x$ + i$p_y$ component. The magnetic field pattern of the junctions
shows a dip at zero externally applied magnetic field, which is an
incontrovertible signature of the simultaneous existence of 0 and $\pi$
coupling within the junction, inherent to a non trivial order parameter phase.
The nano-textured morphology of the Bi$_2$Te$_3$ flakes, and the dramatic role
played by thermal strain are the surprising key factors for the display of an
unconventional induced order parameter. | cond-mat_supr-con |
Noise Performance of Lumped Element Direct Current Superconducting
Quantum Interference Device Amplifiers in the 4 GHz-8 GHz Range: We report on the noise of a lumped element Direct Current Superconducting
Quantum Interference Device amplifier. We show that the noise temperature in
the 4 GHz-8 GHz range over ranges of 10's of MHz is below 1 kelvin (three
photons of added noise), characterize the overall behavior of the noise as a
function of bias parameters, and discuss potential mechanisms which determine
the noise performance in this amplifier. We show that this device can provide
more than a factor of 10 improvement in practical system noise over existing
phase-preserving microwave measurement systems in this frequency band. | cond-mat_supr-con |
Ambient Temperature Growth and Superconducting Properties of Ti-V Alloy
Thin Films: A study on the optimization of ambient temperature growth and superconducting
properties of Ti-V alloy thin films grown on SiO2-coated Si substrate is
reported here. These films have been synthesized by co-sputtering of Ti and V
targets, and films having different Ti concentrations were deposited to get the
optimized critical temperature (TC) of thin films close to the bulk value. The
maximum TC of 5.2 K has been obtained in the Ti40V60 composition, which is
further increased to 6.2 K when a 10 nm thick Ti underlayer is added below the
Ti-V film. GIXRD measurements confirm the formation of Ti-V alloys in the
desired crystal structure. The upper critical field (HC2) of the thin films has
been estimated with the help of magnetotransport measurements. The utility of
Ti-V alloy thin films in superconducting radiation detection applications is
ascertained. | cond-mat_supr-con |
Impact of nonlocal electrodynamics on flux noise and inductance of
superconducting wires: We present exact numerical calculations of supercurrent density, inductance,
and impurity-induced flux noise of cylindrical superconducting wires in the
nonlocal Pippard regime, which occurs when the Pippard coherence length is
larger than the London penetration depth. In this regime the supercurrent
density displays a peak away from the surface, and changes sign inside the
superconductor, signalling a breakdown of the usual approximation of local
London electrodynamics with a renormalized penetration depth. Our calculations
show that the internal inductance and the bulk flux noise power is enhanced in
nonlocal superconductors. In contrast, the kinetic inductance is reduced and
the surface flux noise remains the same. As a result, impurity spins in the
bulk may dominate the flux noise in superconducting qubits in the Pippard
regime, such as the ones using aluminum superconductors with large electron
mean free path. | cond-mat_supr-con |
Imaging Local Sources of Intermodulation in Superconducting Microwave
Devices: This work presents new experimental results on low-temperature (LT)
characterization of local rf properties of passive superconducting (SC)
microwave devices using a novel Laser Scanning Microscope (LSM). In this
technique, a modulated laser beam is focused onto and scanned over the surface
of a resonant SC device to probe the spatial distribution of rf current. The
highly localized photo-induced change of the kinetic inductance of the SC
device produces both a shift of the resonant frequency f_0 and change of the
quality factor Q. An image of these changes is recorded as the laser spot is
scanned over the device. We present the first measurements of spatially
resolved intermodulation response in a High Temperature Superconducting (HTS)
co-planar waveguide resonator, opening up a new window into the local origins
of nonlinearity in the HTS materials. | cond-mat_supr-con |
Room temperature Peierls distortion in small radius nanotubes: By means of {\it ab initio} simulations, we investigate the phonon band
structure and electron-phonon coupling in small 4-\AA diameter nanotubes. We
show that both the C(5,0) and C(3,3) tubes undergo above room temperature a
Peierls transition mediated by an acoustical long-wavelength and an optical
$q=2k_F$ phonons respectively. In the armchair geometry, we verify that the
electron-phonon coupling parameter $\lambda$ originates mainly from phonons at
$q=2k_F$ and is strongly enhanced when the diameter decreases. These results
question the origin of superconductivity in small diameter nanotubes. | cond-mat_supr-con |
A new kind of vortex pinning in superconductor / ferromagnet
nanocomposites: This paper reports the observation of hysteresis in the vortex pinning in a
superconductor / ferromagnetic epitaxial nanocomposite consisting of fcc Gd
particles incorporated in a Nb matrix. We show that this hysteretic pinning is
associated with magnetic reversal losses in the Gd particles and is
fundamentally different in origin to pinning interactions previously observed
for ferromagnetic particles or other microstructural features. | cond-mat_supr-con |
The effects of Zn Impurity on the Properties of Doped Cuprates in the
Normal State: We study the interplay of quantum impurity, and collective spinon and holon
dynamics in Zn doped high-T$_c$ cuprates in the normal state. The
two-dimensional t-t$^{\prime}$-J models with one and a small amount of Zn
impurity are investigated within a numerical method based on the double-time
Green function theory. We study the inhomogeneities of holon density and
antiferromagnetic correlation background in cases with different Zn
concentrations, and obtain that doped holes tend to assemble around the Zn
impurity with their mobility being reduced. Therefore a bound state of holon is
formed around the nonmagnetic Zn impurity with the effect helping Zn to
introduce local antiferromagnetism around itself. The incommensurate peaks we
obtained in the spin structure factor indicate that Zn impurities have effects
on mixing the q=($\pi$, $\pi$) and q=0 components in spin excitations. | cond-mat_supr-con |
Odd triplet superconductivity in clean and moderately disordered SFFS
junctions: We study the Josephson effect and pairing correlations in SFFS junctions that
consist of conventional superconductors (S) connected through two metallic
monodomain ferromagnets (F) with transparent and spin inactive interfaces. We
solve the Eilenberger equations for arbitrary relative orientation of
magnetizations of the two F layers in the clean limit and for moderate disorder
in ferromagnets. Spatial variation of pair amplitudes, singlet $f_s$, and odd
in frequency triplet $f_{t0}$ and $f_{t1}$, with 0 and $\pm1$ spin projections,
as well as the Josephson current-phase relations are calculated for different
values of the ferromagnetic layers thickness and angle $\alpha$ between
in-plane magnetizations. In contrast to the dirty limit case, we find that for
$0<\alpha<\pi$ both spin singlet and triplet pair amplitudes in F layers
power-law decay in the same oscillatory manner with distance from the FS
interfaces. This decay gets faster as the impurity-scattering rate in
ferromagnets is increased. The computed triplet amplitude $f_{t1}$ has opposite
signs in the two magnet regions, penetrates into the superconductors and
monotonically decays over the same distance, which is the superconducting
coherence length, as the singlet amplitude $f_s$ saturates to the bulk value.
We point out that influence of misorientation of magnetizations on the
Josephson current can not be attributed directly to the appearance of odd
triplet correlations. | cond-mat_supr-con |
Double-dome superconductivity under pressure in the V-based Kagome
metals AV3Sb5 (A = Rb and K): We present high-pressure electrical transport measurements on the newly
discovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have
an ideal Kagome lattice of vanadium. Two superconducting domes under pressure
are observed in both compounds, as previously observed in their sister compound
CsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient
pressure to the maximum of 4.15 K at 0.38 GPa in the first dome. The second
superconducting dome has the highest $T_c$ of 1.57 K at 28.8 GPa. KV$_3$Sb$_5$
displays a similar double-dome phase diagram, however, its two maximum $T_c$s
are lower, and the $T_c$ drops faster in the second dome than RbV$_3$Sb$_5$. An
integrated temperature-pressure phase diagram of $A$V$_3$Sb$_5$ ($A$ = Cs, Rb
and K) is constructed, showing that the ionic radius of the intercalated
alkali-metal atoms has a significant effect. Our work demonstrates that
double-dome superconductivity under pressure is a common feature of these
V-based Kagome metals. | cond-mat_supr-con |
Numerical extension of the power law Jc(B) to zero field in thin
superconducting films: Numerical simulations of the current and field distribution in thin
superconducting films are carried out for a given material law Jc(B) and as a
function of the applied field H, taking the sample's self-field into account.
The dependence of the critical current density on the applied field Jc(H) is
computed for comparison with experiment, considering the geometry of transport
measurements.
We show that extrapolating the high field power law Jc \propto B^{-\alpha} to
the lowest fields results in a finite critical current at zero applied field
Jc(H=0), despite the singularity of Jc(B). Moreover, particular features of the
experiment, such as a low field plateau in Jc(H), are reproduced and found to
be determined by the self-field. | cond-mat_supr-con |
Impact of the transport supercurrent on the Josephson effect: We study the weak link between current-carrying superconductors, both
conventional and d-wave. The state of the system is controlled by two
parameters: the order parameter phase difference $\phi$ and the superfluid
velocity $v_{s}$, which parameterizes the parallel to the boundary transport
supercurrent which is injected externally. The low-temperature current-phase
relations are derived. We consider two models of weak links: a constriction
between two conventional superconductors and a plane boundary between two
differently orientated d-wave superconductors. We show that for some relation
between $\phi $ and $v_{s}$ quasiparticles create the current along the
boundary which flows in the direction opposite to the transport supercurrent. | cond-mat_supr-con |
An attempt at a resonating mean-field theoretical description of thermal
behavior of two-gap superconductivity: The resonating mean-field theory (Res-MFT) has been applied and shown to
effectively describe two-gap superconductivity (SC). Particularly at T=0 using
a suitable chemical potential, the two-gap SC in MgB2 has been well described
by the Res-Hartree-Bogoliubov theory (Res-HBT). The Res-HB ground state
generated with HB wave functions almost exhausts the ground-state correlation
energy in all the correlation regimes. In this paper we make an attempt at a
Res-MF theoretical description of thermal behavior of the two-gap SC. In an
energy-gap case we find a new formula leading to a higher Tc than the Tc of the
usual HB formula. | cond-mat_supr-con |
Diboride compounds doped with transition metals$\unicode{x2013}$a route
to superconductivity through structure stabilization as well as defects: Recent investigations into MoB$_{2}$ have unveiled a direct connection
between a pressure-induced structural transition to a P6/mmm space group
structure and the emergence of superconductivity, producing critical
temperatures up to 32 K at 100 GPa. This pressure-induced superconducting state
underscores the potential of doped MoB$_{2}$ as a possible candidate for
metastable superconductivity at ambient pressure. In this work, we demonstrate
that doping by Zr, Hf, or Ta stabilizes the P6/mmm structure at ambient
pressure and results in the realization of a superconducting state with
critical temperatures ranging from 2.4 up to 8.5 K depending on the specific
doping. We estimate the electron-phonon coupling $\lambda$ and the density of
states based on resistivity and specific heat data, finding that $\lambda$
ranges from 0.4 - 0.6 for these compounds. Finally, to investigate the role of
possible metastable defect structures on the critical temperature, we analyze
MoB$_{2}$, MoB$_{2.5}$, and Nb/Zr-doped MoB$_{2}$ using rapid cooling
techniques. Notably, splat-quenching produces samples with higher critical
temperatures and even retains superconductivity in MoB$_{2}$ at ambient
pressure, achieving a critical temperature of 4.5 K. | cond-mat_supr-con |
Phase Separation, Competition, and Volume Fraction Control in
NaFe$_{1-x}$Co$_x$As: We report a detailed nuclear magnetic resonance (NMR) study by combined
$^{23}$Na and $^{75}$As measurements over a broad range of doping to map the
phase diagram of NaFe$_{1-x}$Co$_x$As. In the underdoped regime ($x \le$
0.017), we find a magnetic phase with robust antiferromagnetic (AFM) order,
which we denote the {\it s}-AFM phase, cohabiting with a phase of weak and
possibly proximity-induced AFM order ({\it w}-AFM) whose volume fraction $V
\simeq 8$\% is approximately constant. Near optimal doping, at $x = 0.0175$, we
observe a phase separation between static antiferromagnetism related to the
{\it s}-AFM phase and a paramagnetic (PM) phase related to {\it w}-AFM. The
volume fraction of AFM phase increases upon cooling, but both the N{\'e}el
temperature and the volume fraction can be suppressed systematically by
applying a $c$-axis magnetic field. On cooling below $T_c$, superconductivity
occupies the PM region and its volume fraction grows at the expense of the AFM
phase, demonstrating a phase separation of the two types of order based on
volume exclusion. At higher dopings, static antiferromagnetism and even
critical AFM fluctuations are completely suppressed by superconductivity. Thus
the phase diagram we establish contains two distinct types of phase separation
and reflects a strong competition between AFM and superconducting phases both
in real space and in momentum space. We suggest that both this strict mutual
exclusion and the robustness of superconductivity against magnetism are
consequences of the extreme two-dimensionality of NaFeAs. | cond-mat_supr-con |
Topological superconducting phases and Josephson effect in curved
time-reversal-invariant superconductors: We consider a Rashba spin-orbit coupled nanowire with anisotropic
spin-singlet superconducting pairing and time-reversal-invariant symmetry. We
explore the evolution of the topological superconducting phases of this system
due to geometric deformations for the representative case of a wire bent in a
semielliptical shape. We find that when the system is in its topological
superconducting phase, strong inhomogeneities in the profile curvature can
produce a pair of localized eigenmodes, which can be attributed to a nonuniform
topological phase. The curved geometric profile also allows to tune the spin
correlations of the superconducting state via the induced inhomogeneity of the
spin-orbit coupling (SOC). The geometric control of the superconducting pair
correlations allows to manipulate the critical current in Josephson junctions
made up of two time reversal invariant topological superconductors separated by
a spin-orbit coupled normal metal. In particular, we find that the curvature
inhomogeneity can be exploited for amplifying the current intensity, but also
to generate a $0-\pi$ transition, and a second harmonic contribution, which
generates, for some specific geometric configurations, a $\varphi$-junction
behavior. | cond-mat_supr-con |
Pressure effects on the electronic structure and magnetic properties of
infinite-layer nickelates: Motivated by the discovery of superconductivity in infinite-layer nickelates
RNiO$_2$ (R= rare-earth), and the subsequent enhancement of their T$_c$ with
pressure, we investigate the evolution of the electronic structure and magnetic
properties of this family of materials via first-principles calculations
employing hydrostatic and chemical pressure as tuning knobs. Overall, our
analysis shows that pressure tends to increase the R-$5d$ self-doping effect,
as well as the Ni-$d _{x^{2}-y^{2}}$ bandwidth, the $e_g$ energy splitting, the
charge transfer energy, and the superexchange ($J$). Using the energy scale of
$J$ as a predictor of superconducting tendencies, we anticipate that pressure
can indeed be a feasible means to further increase the T$_c$ in this family of
materials. | cond-mat_supr-con |
Interplay between atomic fluctuations and charge density waves in
La$_{2-x}$Sr$_{x}$CuO$_{4}$: In the cuprate superconductors, the spatial coherence of the charge density
wave (CDW) state grows rapidly below a characteristic temperature
$T_\mathrm{CDW}$, the nature of which is debated. We have combined a set of
x-ray scattering techniques to study La$_{1.88}$Sr$_{0.12}$CuO$_{4}$
($T_\mathrm{CDW}$~$\approx$~80\,K) to shed light on this discussion. We observe
the emergence of a crystal structure, which is consistent with the CDW
modulation in symmetry, well above $T_\mathrm{CDW}$. This global structural
change also induces strong fluctuations of local atomic disorder in the
intermediate temperature region. At $T_\mathrm{CDW}$, the temperature
dependence of this structure develops a kink, while the atomic disorder is
minimized. We find that the atomic relaxation dynamics cross over from a
cooperative to an incoherent response at $T_\mathrm{CDW}$. These results reveal
a rich interplay between the CDWs and atomic fluctuations of distinct
spatio-temporal scales. For example, the CDW coherence is enhanced on
quasi-elastic timescales by incoherent atomic relaxation. | cond-mat_supr-con |
Compartmentalizing the cuprate strange metal: It has long been recognized that the key to unlocking the mystery of cuprate
high-Tc superconductivity lies in understanding the anomalous normal state from
which pairs form and condense. While many of its defining properties have been
identified, they are often considered either at a singular doping level or as
an isolated phenomenon as a function of doping. As a result, their relation to
each other and to the pseudogap (PG), strange metal (SM) and
non-superconducting (non-SC) regimes that define the cuprate phase diagram has
yet to be elucidated. Here, we report a high-field in-plane MR study on several
cuprate families spanning all 3 regimes that reveal a complex yet nonetheless
systematic evolution of the form of the MR, with each regime possessing its own
distinct scaling behavior. In the PG regime, the MR exhibits pure H/T^2 scaling
at low fields and H-linearity at the highest field strengths. While the
H-linearity persists inside the SM regime, the scaling changes abruptly to H/T.
The size of the H-linear slope, meanwhile, is found to be correlated with both
the T-linear resistivity coefficient and Tc, strengthening the characterization
of the SM regime as a quantum critical phase. We interpret the omnipresence of
H-linear MR across both regimes as a signature of highly anisotropic, possibly
discontinuous features on the Fermi surface. Finally, within the non-SC,
Fermi-liquid regime, we observe a recovery of conventional Kohler scaling. This
comprehensive study establishes the distinct nature of the magnetotransport
within each regime and identifies power-law scaling of the normal state MR as a
defining feature of SC hole-doped cuprates. The incompatibility of such
power-law scaling with any known variant of Boltzmann transport theory
motivates the quest for an altogether new theoretical framework, one in which
the MR is entirely decoupled from elastic impurity scattering. | cond-mat_supr-con |
Violation of Kohler rule in Ta2PdTe6 and absence of same in Nb2PdS5- A
high field magneto transport study: Here, we present the comparative study of magnetotransport properties of
recently discovered Ta2PdTe6 and Nb2PdS5 superconductors. The XRD and
magnetotransport measurements are performed on these samples to investigate
structure and superconducting properties as well as normal state transport
properties of these compounds. Both the compounds are crystallized in
monoclinic structure within space group C2m. Here, we observe superconductivity
in both the compounds Ta2PdTe6 (Tc =4.4 K) and Nb2PdS5 (Tc =6.6 K). We see a
linear magnetoresistance in Ta2PdTe6 as well as violation of Kohler rule in
same compound. On the other hand, we find the absence of same in Nb2PdS5
compound. | cond-mat_supr-con |
Why the lowest Landau level approximation works in strongly type II
superconductors: Higher than the lowest Landau level contributions to magnetization and
specific heat of superconductors are calculated using Ginzburg - Landau
equations approach. Corrections to the excitation spectrum around solution of
these equations (treated perturbatively) are found. Due to symmetries of the
problem leading to numerous cancellations the range of validity of the LLL
approximation in mean field is much wider then a naive range and extends all
the way down to $H = {H_{c2}(T)}/13$. Moreover the contribution of higher
Landau levels is significantly smaller compared to LLL than expected naively.
We show that like the LLL part the lattice excitation spectrum at small
quasimomenta is softer than that of usual acoustic phonons. This enhanses the
effect of fluctuations. The mean field calculation extends to third order,
while the fluctuation contribution due to HLL is to one loop. This complements
the earlier calculation of the LLL part to two loop order. | cond-mat_supr-con |
A Model to Describe Transport Properties in
$Bi_2Sr_2(Ca_zPr_{1-z})Cu_2O_{8+y}$: A pseudo-spin model is proposed, as a means to describe some transport
properties (resistivity and Hall mobility) in
$Bi_2Sr_2(Ca_zPr_{1-z})Cu_2O_{8+y}$. Our model is based in a double-well
potential where tunneling in a given site and interaction between different
lattice sites are allowed only through the excited states. Doping of the pure
system by the addition of $Pr$ increases the ratio between the activation
energy and the tunneling constant. The model Hamiltonian displays some features
which are present in the hydrogen-bonded ferroelectrics. Its dynamics is
treated in the random phase approximation and the characteristic frequency
(time) is used in a Drude formula in order to obtain some transport properties
of the system, namely the electric resistivity and the Hall mobility. The
quantities calculated in this work are compared with the experimental data of
B. Beschoten, S. Sadewasser, G. G\"{u}ntherodt and C. Quitmann [Phys. Rev.
Lett.77, 1837(1996)]. | cond-mat_supr-con |
Laser beam focusing by layered superconductor tuned by DC magnetic field: We develop a theory of the propagation and focusing of the THz Gaussian laser
beam through the layered superconductor slab of finite thickness in the
presence of an external DC magnetic field in a nonlinear regime. We show that,
in this case, focusing of radiation, which results from the specific
nonlinearity of the medium, can be flexibly tuned by the external magnetic
field providing a new way to control THz waves. We analytically study the main
characteristics of the Gaussian beam, its waist, and the focusing distance as
the functions of wave frequency, amplitude, and the external magnetic field.
The results of analytic calculations are supported by the numerical simulation
of the electromagnetic field distribution in the slab. | cond-mat_supr-con |
Strongly correlated s-wave pairing in the n-type infinite-layer cuprate: Quasiparticle tunneling spectra of the electron-doped (n-type) infinite-layer
cuprate Sr_{0.9}La_{0.1}CuO_2 reveal characteristics that counter a number of
common phenomena in the hole-doped (p-type) cuprates. The optimally doped
Sr_{0.9}La_{0.1}CuO_2 with T_c = 43 K exhibits a momentum-independent
superconducting gap \Delta = 13.0 +- 1.0 meV that substantially exceeds the BCS
value, and the spectral characteristics indicate insignificant quasiparticle
damping by spin fluctuations and the absence of pseudogap. The response to
quantum impurities in the Cu-sites also differs fundamentally from that of the
p-type cuprates with d_{x^2-y^2}-wave pairing symmetry. | cond-mat_supr-con |
Corrections to the Higgs Mode Masses in Superfluid 3He: Superfluid 3He has a rich spectrum of collective modes with both massive and
massless excitations. The masses of these modes can be precisely measured using
acoustic spectroscopy and fit to theoretical models. Prior comparisons of the
experimental results with theory did not include strong-coupling effects beyond
the weak-coupling-plus BCS model, so-called non-trivial strong-coupling
corrections. In this work we utilize recent strong-coupling calculations to
determine the Higgs masses and find consistency between experiments that relate
them to a sub-dominant $f$-wave pairing strength. | cond-mat_supr-con |
Superconductor strip with transport current: Magneto-optical study of
current distribution and its relaxation: The dynamics of magnetic flux distributions across a YBaCuO strip carrying
transport current is measured using magneto-optical imaging at 20 K. The
current is applied in pulses of 40-5000 ms duration and magnitude close to the
critical one, 5.5 A. During the pulse some extra flux usually penetrates the
strip, so the local field increases in magnitude. When the strip is initially
penetrated by flux, the local field either increases or decreases depending
both on the spatial coordinate and the current magnitude. Meanwhile, the
current density always tends to redistribute more uniformly. Despite the
relaxation, all distributions remain qualitatively similar to the Bean model
predictions. | cond-mat_supr-con |
Quantum liquid crystals in imbalanced Fermi gas: fluctuations and
fractional vortices in Larkin-Ovchinnikov states: We develop a low-energy model of a unidirectional Larkin-Ovchinnikov (LO)
state. Because the underlying rotational and translational symmetries are
broken spontaneously, this gapless superfluid is a smectic liquid crystal, that
exhibits fluctuations that are qualitatively stronger than in a conventional
superfluid, thus requiring a fully nonlinear description of its Goldstone
modes. Consequently, at nonzero temperature the LO superfluid is an algebraic
phase even in 3d. It exhibits half-integer vortex-dislocation defects, whose
unbinding leads to transitions to a superfluid nematic and other phases. In 2d
at nonzero temperature, the LO state is always unstable to a charge-4 nematic
superfluid. We expect this superfluid liquid-crystal phenomenology to be
realizable in imbalanced resonant Fermi gases trapped isotropically. | cond-mat_supr-con |
Effects of Magnetic and non-Magnetic Impurities on the Superconducting
State of YBa$_{2}$Cu$_{3}$O$_{7-δ}$: We report Electronic Raman Scattering measurements on optimally doped Zn and
Ni substituted YBCO in A$_{1g}$ and B$_{1g}$ channels. We show that the
superconducting gap energy is independant of magnetic Ni and non-magnetic Zn
substitutions. On the contrary the collective A$_{1g}$ mode follows the
critical temperature $T_c$ with two distinct slopes for Ni and Zn, and tracks
the magnetic resonance seen in inelastic neutron scattering. We explain the
unconventionnal energy dependence of the superconducting gap and discuss the
behaviour of the A$_{1g}$ mode within magnetic and non-magnetic impurities. | cond-mat_supr-con |
Low-Temperature Rapid Synthesis and Superconductivity of Fe-Based
Oxypnictide Superconductors: we were able to develop a novel method to synthesize Fe-based oxypnictide
superconductors. By using LnAs and FeO as the starting materials and a
ball-milling process prior to solid-state sintering, Tc as high as 50.7 K was
obtained with the sample of Sm 0.85Nd0.15FeAsO0.85F0.15 prepared by sintering
at temperatures as low as 1173 K for times as short as 20 min. | cond-mat_supr-con |
Robust creation of entangled states of two coupled flux qubits via
dynamic control of the transition frequencies: Coherent control and the creation of entangled states are discussed in a
system of two superconducting flux qubits interacting with each other through
their mutual inductance and identically coupling to a reservoir of harmonic
oscillators. We present different schemes using continuous-wave control fields
or Stark-chirped rapid adiabatic passages, both of which rely on a dynamic
control of the qubit transition frequencies via the external bias flux in order
to maximize the fidelity of the target states. For comparison, also special
area pulse schemes are discussed. The qubits are operated around the optimum
point, and decoherence is modelled via a bath of harmonic oscillators. As our
main result, we achieve controlled robust creation of different Bell states
consisting of the collective ground and excited state of the two-qubit system. | cond-mat_supr-con |
Experimental study of intrinsic multiple Andreev reflections effect in
GdO(F)FeAs superconductor array junctions: We report the first observation of the intrinsic multiple Andreev reflections
effect (IMARE) in S-n-S-...-S-arrays (S = superconductor, n = normal metal)
formed by "break-junction" technique in GdO(F)FeAs superconductor (Tc = 48 - 53
K). We show that superconducting gap peculiarities at dI/dV-spectra sharpen
dramatically in the arrays as compared with that in the single-contact spectra;
this enables to improve significantly accuracy of the bulk superconducting
parameters determination. Using IMARE, we determined the large and the small
gap values \Delta_L = 11 +- 1.1 meV and \Delta_S = 2.6 +- 0.4 meV. The
BCS-ratio 2\Delta_L/kTc^{local} = 5.0 - 5.9 > 3.52 (Tc^{local} is the contact
area critical temperature) evidences for a strong electron-boson coupling. The
results obtained agree well with our previous data by Andreev spectroscopy for
single SnS-contacts. | cond-mat_supr-con |
Non-Fermi liquid behavior due to U(1) gauge field in two dimensions: We study the damping rate of massless Dirac fermions due to the U(1) gauge
field in (2+1)-dimensional quantum electrodynamics. In the absence of a Maxwell
term for the gauge field, the fermion damping rate
$\mathrm{Im}\Sigma(\omega,T)$ is found to diverge in both perturbative and
self-consistent results. In the presence of a Maxwell term, there is still
divergence in the perturbative results for $\mathrm{Im}\Sigma(\omega,T)$. Once
the Maxwell term is included into the self-consistent equations for fermion
self-energy and vacuum polarization functions, the fermion damping rate is free
of divergence and exhibits non-Fermi liquid behavior:
$\mathrm{Im}\Sigma(\omega,T) \propto \mathrm{max}(\sqrt{\omega},\sqrt{T})$. | cond-mat_supr-con |
Observation of diamagnetic strange-metal phase in sulfur-copper codoped
lead apatite: By codoping sulfur and copper into lead apatite, the crystal grains are
directionally stacked and the room-temperature resistivity is reduced from
insulating to $2\times10^{-5}~\Omega\cdot$m. The resistance-temperature curve
exhibits a nearly linear relationship suggesting the presence of strange-metal
phase. A possible Meissner effect is observed in dc magnetic measurements. An
even-in-field transverse voltage is found indicating the possible contribution
of individual vortex dynamics. A clear pathway towards superconductivity in
this material is subsequently benchmarked. | cond-mat_supr-con |
Ac$_3$Ni$_2$O$_7$ and La$_2$$Ae$Ni$_2$O$_6$F ($Ae$ = Sr, Ba): Benchmark
Materials for Bilayer Nickelate Superconductivity: We theoretically propose Ac$_3$Ni$_2$O$_7$, La$_2$BaNi$_2$O$_6$F, and
La$_2$SrNi$_2$O$_6$F compounds to be benchmark materials for bilayer nickelate
superconductivity. The stable phase of Ac$_3$Ni$_2$O$_7$ and
La$_2$BaNi$_2$O$_6$F are found to be $I4/mmm$ without the lattice distortion
caused by octahedra rotation at ambient pressure, where as the lattice
distortion in La$_2$SrNi$_2$O$_6$F can be suppressed with relatively small
external pressure of 4 GPa. The magnetism, electronic structure and spin
susceptibilities of Ac$_3$Ni$_2$O$_7$ are extremely close to those of
La$_3$Ni$_2$O$_7$ at 30 GPa. The ground state of La$_2$BaNi$_2$O$_6$F and
La$_2$SrNi$_2$O$_6$F are antiferromagnetically coupled checkerboard bilayer
with sizable magnetic moment on Ni. In addition, the inter-layer coupling
$J_{\perp}$ between Ni-bilayers in La$_2$BaNi$_2$O$_6$F or La$_2$SrNi$_2$O$_6$F
is only $\sim$ 1/10 of that in Ac$_3$Ni$_2$O$_7$ or La$_3$Ni$_2$O$_7$ at 30
GPa. We argue that these compounds may serve as superconducting candidates at
ambient pressure and can be employed to testify theoretical proposals for
bilayer nickelate superconductivity. | cond-mat_supr-con |
Majorana bound states in vortex lattices on iron-based superconductors: Majorana quasi-particles may arise as zero-energy bound states in vortices on
the surface of a topological insulator that is proximitized by a conventional
superconductor. Such a system finds its natural realization in the iron-based
superconductor FeTe$_{0.55}$Se$_{0.45}$ that combines bulk $s$-wave pairing
with spin helical Dirac surface states, and which thus comprises the
ingredients for Majorana modes in absence of an additional proximitizing
superconductor. In this work, we investigate the emergence of Majorana vortex
modes and lattices in such materials depending on parameters like the magnetic
field strength and vortex lattice disorder. A simple 2D square lattice model
here allows us to capture the basic physics of the underlying materials system.
To address the problem of disordered vortex lattice, which occurs in real
systems, we adopt the technique of the singular gauge transformation which we
modify such that it can be used in a system with periodic boundary conditions.
This approach allows us to go to larger vortex lattices than otherwise
accessible, and is successful in replicating several experimental observations
of Majorana vortex bound states in the FeTe$_{0.55}$Se$_{0.45}$ platform.
Finally it can be related to a simple disordered Majorana lattice model that
should be useful for further investigations on the role of interactions, and
towards topological quantum computation. | cond-mat_supr-con |
The Distribution of the Energy Gap and Josephson IcRn Product in Bi2212
by Tunneling Spectroscopy: We present direct measurements of the density of states by tunneling
spectroscopy on slightly overdoped Bi2212 single crystals at low temperature
using break-junction and point-contact techniques. We find that (i) the
variation of the gap magnitude, Delta, between 20 and 36 meV is likely to be
intrinsic to the Bi2212, and (ii) there is a correlation between the maximum
value of the Josephson IcRn product and the gap magnitude: IcRn decreases with
the increase of Delta. The maximum IcRn value of 26 mV is observed at Delta =
20.5 meV. For Delta = 36.5 meV, the maximum measured value of IcRn is 7.3 mV.
We conclude that (i) the distribution of the Josephson IcRn product as a
function of gap magnitude can not be explained by the presence of a single
energy gap in Bi2212, and (ii) the coherence energy scale in Bi2212 has the
maximum Josephson strength. | cond-mat_supr-con |
Chiral-Flux-Phase-Based Topological Superconductivity in Kagome Systems
with Mixed Edge Chiralities: Recent studies have attracted intense attention on the quasi-2D kagome
superconductors $ A\text{V}_3\text{Sb}_5 $ ($ A = $ K, Rb, and Cs) where the
unexpected chiral flux phase (CFP) associates with the spontaneous
time-reversal symmetry breaking in charge density wave (CDW) states. Here,
commencing from the 2-by-2 CDW phases, we bridge the gap between topological
superconductivity (TSC) and time-reversal asymmetric CFP in kagome systems.
Several chiral TSC states featuring distinct Chern numbers emerge for an s-wave
or a d-wave superconducting pairing symmetry. Importantly, these CFP-based TSC
phases possess unique gapless edge modes with mixed chiralities (i.e., both
positive and negative chiralities), but with the net chiralities consistent
with the Bogoliubov-de Gennes Chern numbers. We further study the transport
properties of a two-terminal junction, using Chern insulator or normal metal
leads via atomic Green's function method with Landauer-B\"uttiker formalism. In
both cases, the normal electron tunneling and the crossed Andreev reflection
oscillate as the chemical potential changes, but together contribute to plateau
transmissions (1 and 3/2, respectively). These behaviors can be regarded as the
signature of a topological superconductor hosting edge states with mixed
chiralities. | cond-mat_supr-con |
Superconductivity and multiple pressure-induced phases in BaPt$_2$As$_2$: The newly discovered BaPt$_2$As$_2$ shows a structural distortion at around
275~K, followed by the emergence of superconductivity at lower temperatures.
Here we identify the presence of charge density wave (CDW) order at room
temperature and ambient pressure using single crystal x-ray diffraction, with
both a superlattice and an incommensurate modulation, where there is a change
of the superlattice structure below $\simeq$ 275~K. Upon applying pressure,
BaPt$_2$As$_2$ shows a rich temperature-pressure phase diagram with multiple
pressure-induced transitions at high temperatures, the emergence or
disappearance of which are correlated with sudden changes in the
superconducting transition temperature $T_c$. These findings demonstrate that
BaPt$_2$As$_2$ is a promising new system for studying competing interactions
and the relationship between high-temperature electronic instabilities and
superconductivity. | cond-mat_supr-con |
Remote Sensing and Control of Phase Qubits: We demonstrate a remote sensing design of phase qubits by separating the
control and readout circuits from the qubit loop. This design improves
measurement reliability because the control readout chip can be fabricated
using more robust materials and can be reused to test different qubit chips.
Typical qubit measurements such as Rabi oscillations, spectroscopy, and
excited-state energy relaxation are presented. | cond-mat_supr-con |
Particle-hole symmetry breaking in the pseudogap state of
Pb0.55Bi1.5Sr1.6La0.4CuO6+d: A quantum-chemical perspective: Two Bi2201 model systems are employed to demonstrate how, beside the Cu-O
\sigma-band, a second band of purely O2p\pi character can be made to cross the
Fermi level owing to its sensitivity to the local crystal field. This result is
employed to explain the particle-hole symmetry breaking across the pseudo-gap
recently reported by Shen and co-workers, see M. Hashimoto et al., Nature
Physics 6, (2010) 414. Support for a two-bands-on-a-checkerboard candidate
mechanism for High-Tc superconductivity is claimed. | cond-mat_supr-con |
Dynamic spin-triplet order induced by alternating electric fields in
superconductor-ferromagnet-superconductor Josephson junctions: Dynamic states offer extended possibilities to control the properties of
quantum matter. Recent efforts are focused on studying the ordered states which
appear exclusively under the time-dependent drives. Here we demonstrate a class
of systems which feature dynamic spin-triplet superconducting order stimulated
by the alternating electric field. The effect is based on the interplay of
ferromagnetism, interfacial spin-orbital coupling (SOC) and the condensate
motion driven by the field, which converts hidden static p-wave order, produced
by the joint action of the ferromagnetism and the SOC, into dynamical s-wave
equal-spin triplet correlations. We demonstrate that the critical current of
Josephson junctions hosting these states is proportional to the electromagnetic
power, supplied either by the external irradiation or by the ac current source.
Based on these unusual properties we propose the scheme of a Josephson
transistor which can be switched by the ac voltage and demonstrates an
even-numbered sequence of Shapiro steps. Combining the photo-active Josephson
junctions with recently discovered Josephson phase batteries we find
photo-magnetic SQUID devices which can generate spontaneous magnetic fields
while being exposed to irradiation. | cond-mat_supr-con |
Bound States of Defects in Superconducting LiFeAs Studied by Scanning
Tunneling Spectroscopy: Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and
spectroscopy (STS). Topographic images of the five predominant defects allow
the identification of their position within the lattice. The most commonly
observed defect is associated with an Fe site and does not break the local
lattice symmetry, exhibiting a bound state near the edge of the smaller gap in
this multi-gap superconductor. Three other common defects, including one also
on an Fe site, are observed to break local lattice symmetry and are
pair-breaking indicated by clear in-gap bound states, in addition to states
near the smaller gap edge. STS maps reveal complex, extended real-space bound
state patterns, including one with a chiral distribution of the local density
of states (LDOS). The multiple bound state resonances observed within the gaps
and at the inner gap edge are consistent with theoretical predictions for
s$^{\pm}$ gap symmetry proposed for LiFeAs and other iron pnictides. | cond-mat_supr-con |
Anomalous Electronic Susceptibility in Bi2sr2cuo6+d: We report magnetic susceptibility performed on overdoped Bi2Sr2CuO6+d powders
as a function of oxygen doping d and temperature T. The decrease of the spin
susceptibility chis with increasing T is confirmed. At sufficient high
temperature, chis presents an unusual linear temperature dependence chis=chis0
-chi1T . Moreover, a linear correlation between chi1 and chis0 for increasing
hole concentration has been displayed. These non conventional metal features
will be discussed in terms of a singular narrow-band structures. | 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 strain on the electronic structure, superconductivity, and
nematicity in FeSe studied by angle-resolved photoemission spectroscopy: One of central issues in iron-based superconductors is the role of structural
change to the superconducting transition temperature (T_c). It was found in
FeSe that the lattice strain leads to a drastic increase in T_c, accompanied by
suppression of nematic order. By angle-resolved photoemission spectroscopy on
tensile- or compressive-strained and strain-free FeSe, we experimentally show
that the in-plane strain causes a marked change in the energy overlap
(DeltaE_{h-e}) between the hole and electron pockets in the normal state. The
change in DeltaE_{h-e} modifies the Fermi-surface volume, leading to a change
in T_c. Furthermore, the strength of nematicity is also found to be
characterized by DeltaE_{h-e}. These results suggest that the key to
understanding the phase diagram is the fermiology and interactions linked to
the semimetallic band overlap. | cond-mat_supr-con |
Vortex-antivortex nucleation in magnetically nanotextured
superconductors: Magnetic-field-driven and thermal scenarios: Within the Ginzburg-Landau formalism, we predict two novel mechanisms of
vortex-antivortex nucleation in a magnetically nanostructured superconductor.
Although counterintuitive, nucleation of vortex-antivortex pairs can be
activated in a superconducting (SC) film covered by arrays of submicron
ferromagnets (FMs) when exposed to an external homogeneous magnetic field. In
another scenario, we predict the thermal induction of vortex-antivortex
configurations in SC/FM samples. This phenomenon leads to a new type of
Little-Parks oscillations of the FM magnetization-temperature phase boundary of
the superconducting film. | cond-mat_supr-con |
Enhanced Superconductivity on the Tetragonal Lattice in FeSe under
Hydrostatic Pressure: Superconductivity under pressure in FeSe ($T_{\rm c}$$\sim$7.5 K) has been
investigated using single-crystal specimens through the measurements of DC
magnetization and electrical resistivity. A characteristic three-step increase
in $T_{\rm c}$ has been found under hydrostatic pressure up to $\sim$34 K above
7 GPa. The structural transition from a tetragonal phase to an orthorhombic
phase ($T_{\rm s}$$\sim$87 K) is found to disappear at $P$$\sim$2.3 GPa, above
which $T_{\rm c}$ increases rapidly, suggesting that the superconductivity is
enhanced by the tetragonal environment. Under non-hydrostatic pressure, the
increase in $T_{\rm c}$ is suppressed and the superconductive volume fraction
is considerably reduced above 2 GPa, probably owing to the breaking of the
tetragonal lattice symmetry by the uniaxial stress. The intimate correlation
between the enhanced (suppressed) superconductivity and the tetragonality
(orthorhombicity) in the phase diagram is a common feature of FeSe and other
iron-pnictide superconductors. | cond-mat_supr-con |
Triplanar Model for the Gap and Penetration Depth in YBCO: YBaCuO_7 is a trilayer material with a unit cell consisting of a CuO_2
bilayer with a CuO plane of chains in between. Starting with a model of
isolated planes coupled through a transverse matrix element, we consider the
possibility of intra as well as interplane pairing within a nearly
antiferromagnetic Fermi liquid model. Solutions of a set of three coupled BCS
equations for the gap exhibit orthorhombic symmetry with s- as well as d-wave
contributions. The temperature dependence and a-b in plane anisotropy of the
resulting penetration depth is discussed and compared with experiment. | cond-mat_supr-con |
Reduction of the electrodynamics of superconductors to those for
conductors with the incorporation of spatial dispersion: We derive general frequency dependencies of the surface impedance modulus for
conductors without the dc dissipation, i. e. for superconductors or perfect
conductors. The frequency-dependent surface impedance was applied for the
solutions corresponding to the spatially dispersive eigenvalues of the
permittivity operator for conductors. We demonstrate that appropriately taken
into account effects of the spatial dispersion can give the general frequency
dependence of the surface impedance for the obtained solutions including that
for superconductor. It is shown that an incorporation of the spatial dispersion
leads to an appearance of the Meissner effect in perfect conductors in the same
manner as in superconductors. | cond-mat_supr-con |
The maximal superconductivity in proximity to charge density wave
quantum critical point in Cu$_x$TiSe$_2$: Superconductivity emerges in $1T$-TiSe$_2$ when its charge density wave (CDW)
order is suppressed by Cu intercalation or pressure. Since the CDW state is
thought to be an excitonic insulator, an interesting question is whether the
superconductivity is also mediated by the excitonic fluctuations. We
investigated this question as to the nature of doping induced superconductivity
in Cu$_x$TiSe$_2$ by asking if it is consistent with the phonon-mediated
pairing. We employed the {\it ab initio} density functional theory and density
functional perturbation theory to compute the electron-phonon coupling
Eliashberg function from which to calculate the superconducting (SC) critical
temperature $T_c$. The calculated $T_c $ as a function of the doping
concentration $x$ exhibits a dome shape with the maximum $T_c$ of $2-6$ K at $x
\approx 0.05$ for the Coulomb pseudopotential $0 \leq \mu^* \leq 0.1$. The
maximal $T_c$ was found to be pinned to the quantum critical point at which the
CDW is completely suppressed and the corresponding phonon mode becomes soft.
Underlying physics is that the reduced phonon frequency enhances the
electron-phonon coupling constant $\lambda$ which overcompensates the frequency
decrease to produce a net increase of $T_c$. The doping induced
superconductivity in Cu$_x$TiSe$_2$ seems to be consistent with the
phonon-mediated pairing. Comparative discussion was made with the pressure
induced superconductivity in TiSe$_2$. | cond-mat_supr-con |
Pressure-induced magnetic collapse and metallization of
$\mathrm{TlF}{\mathrm{e}}_{1.6}\mathrm{S}{\mathrm{e}}_{2}$: The crystal structure, magnetic ordering, and electrical resistivity of
TlFe1.6Se2 were studied at high pressures. Below ~7 GPa, TlFe1.6Se2 is an
antiferromagnetically ordered semiconductor with a ThCr2Si2-type structure. The
insulator-to-metal transformation observed at a pressure of ~ 7 GPa is
accompanied by a loss of magnetic ordering and an isostructural phase
transition. In the pressure range ~ 7.5 - 11 GPa a remarkable downturn in
resistivity, which resembles a superconducting transition, is observed below 15
K. We discuss this feature as the possible onset of superconductivity
originating from a phase separation in a small fraction of the sample in the
vicinity of the magnetic transition. | cond-mat_supr-con |
Hidden symmetry and knot solitons in a charged two-condensate Bose
system: We show that a charged two-condensate Ginzburg-Landau model or equivalently a
Gross-Pitaevskii functional for two charged Bose condensates, can be mapped
onto a version of the nonlinear O(3) $\sigma$-model. This implies in particular
that such a system possesses a hidden O(3) symmetry and allows for the
formation of stable knotted solitons. The results, in particular, should be
relevant to the superconducting MgB_2. | cond-mat_supr-con |
Effects of the Spin-Orbit Coupling and the Superconductivity in
simple-cubic alpha-Polonium: We have investigated the mechanism of stabilizing the simple-cubic (SC)
structure in polonium (alpha- Po), based on the phonon dispersion calculations
using the first-principles all-electron band method. We have demonstrated that
the stable SC structure results from the suppression of the Peierls instability
due to the strong spin-orbit coupling (SOC) in alpha-Po. Further, we have
explored the possible superconductivity in alpha-Po, and predicted that it
becomes a superconductor with Tc ~ 4 K. The transverse soft phonon mode at q ~
2/3 R, which is greatly influenced by the SOC, plays an important role both in
the structural stability and the superconductivity in alpha-Po. We have
discussed effects of the SOC and the volume variation on the phonon dispersions
and superconducting properties of alpha-Po. | cond-mat_supr-con |
Evidence for quasi-two-dimensional superconductivity in infinite-layer
nickelates: After being expected as a promising analogue to cuprates for decades,
superconductivity was recently discovered in infinite-layer nickelates,
providing new opportunities to explore mechanisms of high-temperature
superconductivity. However, in sharp contrast to the single-band
quasi-two-dimensional superconductivity in cuprates, nickelates exhibit a
multi-band electronic structure and an unexpected isotropic superconductivity
as reported recently, which challenges the cuprate-like picture in nickelates.
Here, we show the superconductivity in nickelates is actually anisotropic and
quasi-two-dimensional in nature, as that in cuprates. By synthesizing
high-quality lanthanide nickelate films with enhanced crystallinity and
superconductivity ($T_{c}^{onset}$ = 18.8 K, $T_{c}^{zero}$ = 16.5 K), strong
anisotropic magnetotransport behaviors have been observed. The
quasi-two-dimensional nature is further confirmed by the existence of a
cusp-like peak of the angle-dependent $T_{c}$, and a
Berezinskii-Kosterlitz-Thouless transition near $T_{c}$. Our work thus suggests
a quasi-two-dimensional superconductivity in infinite-layer nickelates,
implying a single-3$d_{x^2-y^2}$-band cuprate-like picture may remain valid in
these compounds. | cond-mat_supr-con |
Electronic structure studies of BaFe2As2 by angle-resolved photoemission
spectroscopy: We report high resolution angle-resolved photoemission spectroscopy (ARPES)
studies of the electronic structure of BaFe$_2$As$_2$, which is one of the
parent compounds of the Fe-pnictide superconductors. ARPES measurements have
been performed at 20 K and 300 K, corresponding to the orthorhombic
antiferromagnetic phase and the tetragonal paramagnetic phase, respectively.
Photon energies between 30 and 175 eV and polarizations parallel and
perpendicular to the scattering plane have been used. Measurements of the Fermi
surface yield two hole pockets at the $\Gamma$-point and an electron pocket at
each of the X-points. The topology of the pockets has been concluded from the
dispersion of the spectral weight as a function of binding energy. Changes in
the spectral weight at the Fermi level upon variation of the polarization of
the incident photons yield important information on the orbital character of
the states near the Fermi level. No differences in the electronic structure
between 20 and 300 K could be resolved. The results are compared with density
functional theory band structure calculations for the tetragonal paramagnetic
phase. | cond-mat_supr-con |
Correlation of Fe-based Superconductivity and Electron-Phonon Coupling
in an FeAs/Oxide Heterostructure: Interfacial phonons between iron-based superconductors (FeSCs) and perovskite
substrates have received considerable attention due to the possibility of
enhancing preexisting superconductivity. Using scanning tunneling spectroscopy,
we studied the correlation between superconductivity and e-ph interaction with
interfacial-phonons in an iron-based superconductor Sr$_2$VO$_3$FeAs ($T_c
\approx$ 33 K) made of alternating FeSC and oxide layers. The quasiparticle
interference measurement over regions with systematically different average
superconducting gaps due to the e-ph coupling locally modulated by O vacancies
in VO$_2$ layer, and supporting self-consistent momentum-dependent Eliashberg
calculations provide a unique real-space evidence of the forward-scattering
interfacial phonon contribution to the total superconducting pairing. | cond-mat_supr-con |
Fragmentation of Fast Josephson Vortices and Breakdown of Ordered States
by Moving Topological Defects: Topological defects such as vortices, dislocations or domain walls define
many important effects in superconductivity, superfluidity, magnetism, liquid
crystals, and plasticity of solids. Here we address the breakdown of the
topologically-protected stability of such defects driven by strong external
forces. We focus on Josephson vortices that appear at planar weak links of
suppressed superconductivity which have attracted much attention for electronic
applications, new sources of THz radiation, and low-dissipative computing. Our
numerical simulations show that a rapidly moving vortex driven by a constant
current becomes unstable with respect to generation of vortex-antivortex pairs
caused by Cherenkov radiation. As a result, vortices and antivortices become
spatially separated and accumulate continuously on the opposite sides of an
expanding dissipative domain. This effect is most pronounced in thin film edge
Josephson junctions at low temperatures where a single vortex can switch the
whole junction into a resistive state at currents well below the Josephson
critical current. Our work gives a new insight into instability of a moving
topological defect which destroys global long-range order in a way that is
remarkably similar to the crack propagation in solids. | cond-mat_supr-con |
Persistent Charge Density Wave Memory in a Cuprate Superconductor: Although charge density wave (CDW) correlations appear to be a ubiquitous
feature of the superconducting cuprates, their disparate properties suggest a
crucial role for coupling or pinning of the CDW to lattice deformations and
disorder. While diffraction intensities can demonstrate the occurrence of CDW
domain formation, the lack of scattering phase information has limited our
understanding of this process. Here, we report coherent resonant x-ray speckle
correlation analysis, which directly determines the reproducibility of CDW
domain patterns in La1.875Ba0.125CuO4 (LBCO 1/8) with thermal cycling. While
CDW order is only observed below 54 K, where a structural phase transition
results in equivalent Cu-O bonds, we discover remarkably reproducible CDW
domain memory upon repeated cycling to temperatures well above that transition.
That memory is only lost on cycling across the transition at 240(3) K that
restores the four-fold symmetry of the copper-oxide planes. We infer that the
structural-domain twinning pattern that develops below 240 K determines the CDW
pinning landscape below 54 K. These results open a new view into the complex
coupling between charge and lattice degrees of freedom in superconducting
cuprates. | cond-mat_supr-con |
Intrinsic Properties of Stoichiometric LaOFeP: DC and ac magnetization, resistivity, specific heat, and neutron diffraction
data reveal that stoichiometric LaOFeP is metallic and non-superconducting
above T = 0.35 K, with gamma = 12.5 mJ/mol*K. Neutron diffraction data at room
temperature and T = 10 K are well described by the stoichiometric, tetragonal
ZrCuSiAs structure and show no signs of structural distortions or long range
magnetic ordering, to an estimated detectability limit of 0.07 uB/Fe. We
propose a model, based on the shape of the iron-pnictide tetrahedron, that
explains the differences between LaOFeP and LaOFeAs, the parent compound of the
recently discovered high-Tc oxyarsenides, which, in contrast, shows both
structural and spin density wave (SDW) transitions. | cond-mat_supr-con |
Striking Zn impurity effect on the Fe-based superconductor
BaFe1.87Co0.13As2: Nonmagnetic impurity effect was studied on the n-type Fe-based superconductor
BaFe1.87Co0.13As2 (Tc = 25 K) by a successful Zn substitution for Fe up to 7
at.%. Magnetic susceptibility, electrical resistivity, specific heat, and Hall
coefficient measurements indicated that Tc linearly decreases with the Zn
concentration and disappears at 7 at.%. The result is quantitatively comparable
with what was observed for YBCO, while it disagrees with a recent report for
the p-type Ba0.5K0.5Fe2As2. Fragile SC against a nonmagnetic impurity was first
confirmed for the n-type 122 Fe-based superconductor. | cond-mat_supr-con |
Huge magnetostriction in superconducting single-crystalline
BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$: The performance of iron-based superconductors in high magnetic fields plays
an important role for their practical application. In this work, we measured
the magnetostriction and magnetization of BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$
single crystals using pulsed magnetic fields up to 60 T and static magnetic
fields up to 33 T, respectively. A huge longitudinal magnetostriction (of the
order of 10$ ^{-4} $) was observed in the direction of the twin boundaries. The
magnetization measurements evidence a high critical-current density due to
strong bulk pinning. By using magnetization data with an exponential
flux-pinning model, we can reproduce the magnetostriction curves qualitatively.
This result shows that the magnetostriction of
BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$ can be well explained by a
flux-pinning-induced mechanism. | cond-mat_supr-con |
Manifesto for a higher Tc -- lessons from pnictides and cuprates: We explore energy scales, features in the normal state transport, relevant
interactions and constraints for the pairing mechanisms in the high-Tc cuprates
and Fe-pnictides. Based on this analysis we attempt to identify a number of
attributes of superconductors with a higher T_c. Expanded version of the
article published in Nature Physics, 7, 271 (2011). | cond-mat_supr-con |
Magnetic and structural properties of the iron silicide superconductor
LaFeSiH: The magnetic and structural properties of the recently discovered
pnictogen/chalcogen-free superconductor LaFeSiH ($T_c\simeq10$~K) have been
investigated by $^{57}$Fe synchrotron M{\"o}ssbauer source (SMS) spectroscopy,
x-ray and neutron powder diffraction and $^{29}$Si nuclear magnetic resonance
spectroscopy (NMR). No sign of long range magnetic order or local moments has
been detected in any of the measurements and LaFeSiH remains tetragonal down to
2 K. The activated temperature dependence of both the NMR Knight shift and the
relaxation rate $1/T_1$ is analogous to that observed in strongly overdoped
Fe-based superconductors. These results, together with the
temperature-independent NMR linewidth, show that LaFeSiH is an homogeneous
metal, far from any magnetic or nematic instability, and with similar Fermi
surface properties as strongly overdoped iron pnictides. This raises the
prospect of enhancing the $T_c$ of LaFeSiH by reducing its carrier
concentration through appropriate chemical substitutions. Additional SMS
spectroscopy measurements under hydrostatic pressure up to 18.8~GPa found no
measurable hyperfine field. | cond-mat_supr-con |
Interplay between collective modes in hybrid electron gas-superconductor
structures: We study hybridization of collective plasmon and
Carlson-Goldman-Artemenko-Volkov modes in a hybrid system, consisting of a
two-dimensional layers of electron gas in the normal state and superconductor,
coupled by long-range Coulomb forces. The interaction between these collective
modes is not possible in a regular single-layer two-dimensional system since
they exist in non-overlapping domains of dimensionless parameter $\omega\tau$,
where $\omega$ is the external electromagnetic field frequency and $\tau$ is
electron scattering time. Thus, in a single-layer structure, these modes are
mutually exclusive. However, the coupling may become possible in a hybrid
system consisting of two separated in space materials with different
properties, in particular, the electron scattering time. We investigate the
electromagnetic power absorption by the hybrid system and reveal the conditions
necessary for the hybridization of collective modes. | cond-mat_supr-con |
Non-thermal origin of nonlinear transport across magnetically induced
superconductor-metal-insulator transition: We have studied the effect of perpendicular magnetic fields and temperatures
on the nonlinear electronic transport in amorphous Ta superconducting thin
films. The films exhibit a magnetic field induced metallic behavior intervening
the superconductor-insulator transition in the zero temperature limit. We show
that the nonlinear transport in the superconducting and metallic phase is of
non-thermal origin and accompanies an extraordinarily long voltage response
time. | cond-mat_supr-con |
Modification of the effective action approach for the Leggett mode: The modified procedure to get the effective action is discussed. Careful
observation of phase of electrons leads to the conclusion that
Hubbard-Stratonovich field should be pair wavefunction, instead of gap. The
modified theory is valid for arbitrary strong interband coupling while the
earlier approach only deals with weak interband coupling. This paper also
discusses experimental observation of the Leggett mode: Raman spectroscopy. The
spectral peak which corresponds to the Leggett mode is calculated in earlier
the earlier and the modified theory, which shows that they are much different. | cond-mat_supr-con |
Collision Dynamics of Two Bose-Einstein Condensates in the Presence of
Raman Coupling: A collision of two-component Bose-Einstein condensates in the presence of
Raman coupling is proposed and studied by numerical simulations. Raman
transitions are found to be able to reduce collision-produced irregular
excitations by forming a time-averaged attractive optical potential. Raman
transitions also support a kind of dark soliton pairs in two-component
Bose-Einstein condensates. Soliton pairs and their remnant single solitons are
shown to be controllable by adjusting the initial relative phase between the
two colliding condensates or the two-photon detuning of Raman transitions. | cond-mat_supr-con |
Hall-Lorenz ratio of YBa2Cu3O7 using Ionization energy based Fermi-Dirac
statistics and charge-spin separation: The temperature dependent properties of heat capacity, heat conductivity and
Hall-Lorenz ratio have been solved numerically after taking the previously
proposed ionization energy based Fermi-Dirac statistics and the coexistence of
Fermi and charge-spin separated liquid into account. The
thermo-magneto-electronic properties are entirely for spin and charge carriers,
hence the phonon contribution has been neglected. A linear dependence between
the Hall-Lorenz ratio and temperature ($T$) is also obtained in accordance with
the experimental results for overdoped YBa$_2$Cu$_3$O$_{7-\delta}$, if these
conditions, $E_I$ $<$ $T_c$ with respect to resistivity and there are no spinon
pairings ($T^*$ = 0) are satisfied. Heat conductivity based on both pure and
electron-contaminated charge-spin separated liquid in $ab$-planes above $T_c$
are found to increase with decreasing $T$ as a consequence of inverse
proportionality with $T$. The $T$-dependence of heat capacity are also
highlighted, which qualitatively complies with the experimental findings. | cond-mat_supr-con |
Comment on "Enhanced two-dimensional properties of the four-layered
cuprate high-Tc superconductor TlBa-2Ca-3Cu-4O-y": We reanalyze published magnetization data and demonstrate that the conclusion
of the original authors, claiming enhanced two-dimensional properties of the
cuprate superconductor Tl-1234, is not supported by the experimental results.
Our analysis shows that the magnetic field dependence of the mixed-state
magnetization for this particular sample is amazingly close to the results of
numerical calculations by E. H. Brandt for an ideal vortex lattice without
fluctuations. This good agreement between experiment and theory allows for the
evaluation of the absolute values of the upper critical field. | cond-mat_supr-con |
Effect of biaxial strain on the phase transitions of Ca(Fe1-xCox)2As2: We study the effect of applied strain as a physical control parameter for the
phase transitions of Ca(Fe1-xCox)2As2 using resistivity, magnetization, x-ray
diffraction and 57Fe M\"ossbauer spectroscopy. Biaxial strain, namely
compression of the basal plane of the tetragonal unit cell, is created through
firm bonding of samples to a rigid substrate, via differential thermal
expansion. This strain is shown to induce a magneto-structural phase transition
in originally paramagnetic samples; and superconductivity in previously
non-superconducting ones. The magneto-structural transition is gradual as a
consequence of using strain instead of pressure or stress as a tuning
parameter. | cond-mat_supr-con |
Optical sum increase due to electron undressing: For a system with a fixed number of electrons, the total optical sum is a
constant, independent of many-body interactions, of impurity scattering and of
temperature. For a single band in a metal, such a sum rule is no longer
independent of the interactions or temperature, when the dispersion and/or
finite bandwidth is accounted for. We adopt such a model, with electrons
coupled to a single Einstein oscillator of frequency $\omega_{E}$, and study
the optical spectral weight. The optical sum depends on both the strength of
the coupling and on the characteristic phonon frequency, $\omega_{E}$. A
hardening of $\omega_{E}$, due, for example, to a phase transition, leads to
electron undressing and translates into a decrease in the electron kinetic
energy and an increase in the total optical sum, as observed in recent
experiments in the cuprate superconductors. | cond-mat_supr-con |
Superconductivity in layered CeO0.5F0.5BiS2: We report appearance of superconductivity in CeO0.5F0.5BiS2. The bulk
polycrystalline samples CeOBiS2 and CeO0.5F0.5BiS2 are synthesized by
conventional solid state reaction route via vacuum encapsulation technique.
Detailed structural analysis showed that the studied CeO0.5F0.5BiS2 compound is
crystallized in tetragonal P4/nmm space group with lattice parameters a =
4.016(3) A, c = 13.604(2) A. DC magnetization measurement (MT-curve) shows the
ferromagnetic signal at the low temperature region. The superconductivity is
established in CeO0.5F0.5BiS2 at Tconset = 2.5K by electrical transport
measurement. Under applied magnetic field both Tc onset and Tc ({\rho} =0)
decrease to lower temperatures and an upper critical field [Hc2(0)] above
1.2Tesla is estimated. The results suggest coexistence of ferromagnetism and
superconductivity for the CeO0.5F0.5BiS2 sample. | cond-mat_supr-con |
Negative in-plane and out-of-plane magnetoresistivities in optimally
doped Bi2Sr2Ca0.8Y0.2Cu2O8+d single crystal: Both the in-plane and out-of-plane magnetoresistivities have been measured in
the normal state of an optimally doped Bi2Sr2Ca0.8Y0.2Cu2O8+d single crystal
with a magnetic field applied parallel and perpendicular to the CuO2 planes.
Whatever the magnetic field and the current directions are, a negative
magnetoresistivity is obtained over a wide range of temperature above the
critical temperature Tc. For the in-plane and out-of-plane measurements, the
non-dominant orbital contribution to magnetoresistivity suggests the
substantial role played by the spin degrees of freedom. | cond-mat_supr-con |
Superconductivity of the New Medium-Entropy Alloy V4Ti2W with a
Body-Centered Cubic Structure: Medium- and high-entropy alloy (MEA and HEA) superconductors have attracted
considerable interest since their discovery. This paper reports the
superconducting properties of ternary tungsten-containing MEA V4Ti2W for the
first time. V4Ti2W is a type II superconductor with a body-centered cubic (BCC)
structure. Experimental results of resistivity, magnetization, and heat
capacity indicate that the superconducting transition temperature of the MEA
V4Ti2W is roughly 5.0 K. The critical magnetic fields at the upper and lower
ends are 9.93(2) T and 40.7(3) mT, respectively. Interestingly, few BCC MEA
superconductors with VEC greater than 4.8 have been found. The addition of
tungsten leads to a VEC of 4.83 e/a for V4Ti2W, which is rarely higher than the
4.8 value. Adding tungsten element expands the variety of MEA alloys, which may
improve the microstructure and mechanical properties of materials and even
superconducting properties. This material could potentially offer a new
platform for the investigation of innovative MEA and HEA superconductors. | cond-mat_supr-con |
Spectrum of low energy excitations in the vortex state: comparison of
Doppler shift method to quasiclassical approach: We present a detailed comparison of numerical solutions of the quasiclassical
Eilenberger equations with several approximation schemes for the density of
states of s- and d-wave superconductors in the vortex state, which have been
used recently. In particular, we critically examine the use of the Doppler
shift method, which has been claimed to give good results for d-wave
superconductors. Studying the single vortex case we show that there are
important contributions coming from core states, which extend far from the
vortex cores into the nodal directions and are not present in the Doppler shift
method, but significantly affect the density of states at low energies. This
leads to sizeable corrections to Volovik's law, which we expect to be sensitive
to impurity scattering. For a vortex lattice we also show comparisons with the
method due to Brandt, Pesch, and Tewordt and an approximate analytical method,
generalizing a method due to Pesch. These are high field approximations
strictly valid close to the upper critical field Bc2. At low energies the
approximate analytical method turns out to give impressively good results over
a broad field range and we recommend the use of this method for studies of the
vortex state at not too low magnetic fields. | cond-mat_supr-con |
Phase diagram of nickelate superconductors calculated by dynamical
vertex approximation: We review the electronic structure of nickelate superconductors with and
without effects of electronic correlations. As a minimal model we identify the
one-band Hubbard model for the Ni 3$d_{x^2-y^2}$ orbital plus a pocket around
the $A$-momentum. The latter however merely acts as a decoupled electron
reservoir. This reservoir makes a careful translation from {nominal} Sr-doping
to the doping of the one-band Hubbard model mandatory. Our dynamical mean-field
theory calculations, in part already supported by experiment, indicate that the
$\Gamma$ pocket, Nd 4$f$ orbitals, oxygen 2$p$ and {the} other Ni 3$d$ orbitals
are not relevant in the superconducting doping regime. The physics is
completely different if topotactic hydrogen is present or the oxygen reduction
is incomplete. Then, a two-band physics hosted by the Ni 3$d_{x^2-y^2}$ and
3$d_{3z^2-r^2}$ orbitals emerges. Based on our minimal modeling we calculated
the superconducting $T_c$ vs. Sr-doping $x$ phase diagram prior to experiment
using the dynamical vertex approximation. For such a notoriously difficult to
determine quantity as $T_c$, the agreement with experiment is astonishingly
good. The prediction that $T_c$ is enhanced with pressure or compressive
strain, has been confirmed experimentally as well. This supports that the
one-band Hubbard model plus an electron reservoir is the appropriate minimal
model. | cond-mat_supr-con |
Thermodynamic evidence for nematic superconductivity in
Cu$_x$Bi$_2$Se$_3$: Unconventional superconductivity is characterized by the spontaneous symmetry
breaking of the macroscopic superconducting wavefunction in addition to the
gauge symmetry breaking, such as rotational-symmetry breaking with respect to
the underlying crystal-lattice symmetry. Particularly, superconductivity with
spontaneous rotational-symmetry breaking in the wavefunction amplitude and thus
in bulk properties, not yet reported previously, is intriguing and can be
termed "nematic" superconductivity in analogy to nematic liquid-crystal phases.
Here, based on specific-heat measurements of the single-crystalline
Cu$_x$Bi$_2$Se$_3$ under accurate magnetic-field-direction control, we report
thermodynamic evidence for nematic superconductivity, namely, clear
two-fold-symmetric behavior in a trigonal lattice. The results indicate
realization of an "odd-parity nematic" state, feasible only by macroscopic
quantum condensates and distinct from nematic states in liquid crystals. The
results also confirm topologically non-trivial superconductivity in
Cu$_x$Bi$_2$Se$_3$. | cond-mat_supr-con |
Magnetic hysteresis of a superconducting microstrip resonator with a
high edge barrier: We investigate the magnetic hysteresis of a superconducting microstrip
resonator with a high edge barrier. We measure the magnetic hysteresis while
either sweeping a magnetic field or tuning the edge barrier by high microwave
current. We show that the magnetic hysteresis of such a device is qualitatively
different from that of one without an edge barrier and can be understood based
on the generalized critical-state model. In particular, we propose and
demonstrate a simple and intuitive method that relies on a plot of the quality
factor versus the resonance frequency for revealing the physical processes
behind those hysteretic behaviors. Based on this, we find that the interplay
between the Meisser current and vortex pinning is essential for understanding
the magnetic hysteresis of such a device. | cond-mat_supr-con |
Thermally assisted quantum vortex tunneling in the Hall and dissipative
regime: Quantum vortex tunneling is studied for the case where the Hall and the
dissipative dynamics are simultaneously present. For a given temperature, the
magnetization relaxation rate is calculated as a function of the external
current and the quasiparticle scattering time. The relaxation rate is solved
analytically at zero temperature and obtained numerically at finite
temperatures by the variational method. In the moderately clean samples, we
have found that a minimum in the relaxation rate exists at zero temperature,
which tends to disappear with increase in the temperature. | cond-mat_supr-con |
Transport signatures of fragile-glass dynamics in the melting of the
two-dimensional vortex lattice: In two-dimensional (2D) systems, the melting from a solid to an isotropic
liquid can occur via an intermediate phase that retains orientational order.
However, in 2D superconducting vortex lattices, the effect of orientational
correlations on transport, and their interplay with disorder remain open
questions. Here we study a 2D weakly pinned vortex system in amorphous MoGe
films over an extensive range of temperatures ($\bm{T}$) and perpendicular
magnetic fields ($\bm{H}$) using linear and nonlinear transport measurements.
We find that, at low fields, the resistivity obeys the Vogel-Fulcher-Tamman
(VFT) form, $\bm{\rho(T)\propto\exp[-{W}(H)/(T-T_0(H))]}$, characteristic of
fragile glasses. As $\bm{H}$ increases, $\bm{T_0(H)}$ is suppressed to zero,
and a standard vortex liquid behavior consistent with a $\bm{T=0}$
superconducting transition is observed. Our findings, supported also by
simulations, suggest that the presence of orientational correlations gives rise
to a heterogeneous dynamics responsible for the VFT behavior. The effects of
quenched disorder become dominant at high $\bm{H}$, where a crossover to a
strong-glass behavior is observed. This is a new insight into the dynamics of
melting in 2D systems with competing orders. | cond-mat_supr-con |
Superconducting mechanism for the cuprate Ba$_2$CuO$_{3+δ}$ based
on a multiorbital Lieb lattice model: For the recently discovered cuprate superconductor
$\mathrm{Ba_{2}CuO_{3+\delta}}$, we propose a lattice structure which resembles
the model considered by Lieb to represent the vastly oxygen-deficient material.
We first investigate the stability of the Lieb-lattice structure, and then
construct a multiorbital Hubbard model based on first-principles calculation.
By applying the fluctuation-exchange approximation to the model and solving the
linearized Eliashberg equation, we show that $s$-wave and $d$-wave pairings
closely compete with each other, and, more interestingly, that the
intra-orbital and inter-orbital pairings coexist. We further show that, if the
energy of the $d_{3z^2-r^2}$ band is raised to make it "incipient" with the
lower edge of the band close to the Fermi level within a realistic band filling
regime, $s\pm$-wave superconductivity is strongly enhanced. We reveal an
intriguing relation between the Lieb model and the two-orbital model for the
usual K$_2$NiF$_4$ structure where a close competition between $s-$ and
$d-$wave pairings is known to occur. The enhanced superconductivity in the
present model is further shown to be related to an enhancement found previously
in the bilayer Hubbard model with an incipient band. | cond-mat_supr-con |
Helium-Three in Aerogel: Liquid 3He confined in silica aerogel provides us with a unique system to
study the effects of quenched disorder on the properties of a strongly
correlated quantum liquid. The superfluid phases display interplay between
disorder and complex symmetry-breaking. | cond-mat_supr-con |
Pressure effects on the unconventional superconductivity of the
noncentrosymmetric LaNiC2: The unconventional superconductivity in the noncentrosymmetric LaNiC$_2$, and
its evolution with pressure, is analyzed basing on the {\it ab initio}
computations and the full Eliashberg formalism. First principles calculations
of the electronic structure, phonons and the electron-phonon coupling are
reported in the pressure range 0-15 GPa. The thermodynamic properties of the
superconducting state were determined numerically solving the Eliashberg
equations. We found that already at $p=0$ GPa, the superconducting parameters
deviate from the BCS-type, and a large value of the Coulomb pseudopotential
$\mu^{\star}=0.22$ is required to get the critical temperature $T_c = 2.8$~K
consistent with experiment. If such $\mu^{\star}$ is used, the Eliashberg
formalism reproduces also the experimentally observed values of the
superconducting order parameter, the electronic specific heat jump at the
critical temperature, and the change of the London penetration depth with
temperature. This shows, that deviation of the above-mentioned parameters from
the BCS predictions do not prejudge on the triplet or multiple gap nature of
the superconductivity in this compound. Under the external pressure,
calculations predict continuous increase of the electron-phonon coupling
constant in the whole pressure range 0-15~GPa, consistent with the
experimentally observed increase in $T_c$ for the pressure range 0-4~GPa, but
inconsistent with the drop of $T_c$ above 4~GPa and the disappearance of the
superconductivity above 7~GPa, reported experimentally. The disappearance of
superconductivity may be accounted for by increasing the $\mu^{\star}$ to 0.36
at 7~GPa, which supports the hypothesis of the formation of a new high-pressure
electronic phase, which competes with the superconductivity. | cond-mat_supr-con |
NMR evidence for an intimate relationship between antiferromagnetic spin
fluctuations and extended s-wave superconductivity in mono-crystalline
SrFe2(As_{1-x}P_{x})2: We report systematic 31P-NMR study on iron (Fe)-based superconductors
SrFe2(As_{1-x}P_{x})2 (Sr122AsP), in which a superconducting (SC) transition
temperature Tc at x=0.35 increases from Tc=26 K up to 33 K by annealing an
as-grown mono-crystalline sample. The present NMR study has unraveled that Tc
reaches a highest value of 33 K at x=0.35 around a quantum critical point at
which antiferromagnetic (AFM) order disappears. When noting that the SC
transition disappears at x=0.6 where the AFM spin fluctuations (SFs) are no
longer present, we remark that the onset and increase of Tc are apparently
associated with the emergence and enhancement of AFM-SFs, respectively. In the
SC state, the residual density of state (RDOS) at the Fermi energy EF in the SC
state becomes much smaller for the annealed sample than for the as-grown one,
suggesting that some inhomogeneity and/or imperfection for the latter increases
RDOS as expected for unconventional SC state with nodal gap. These findings in
Sr122AsP are consistent with the unconventional s(+-)-wave Cooper pairing state
that is mediated by AFM-SFs. We also discuss other key-ingredients besides the
AFM-SFs to increase Tc further. | cond-mat_supr-con |
Origin of Topological Surface Superconductivity in
FeSe$_{0.45}$Te$_{0.55}$: The engineering of Majorana zero modes in topological superconductors, a new
paradigm for the realization of topological quantum computing and
topology-based devices, has been hampered by the absence of materials with
sufficiently large superconducting gaps. Recent experiments, however, have
provided enthralling evidence for the existence of topological surface
superconductivity in the iron-based superconductor FeSe$_{0.45}$Te$_{0.55}$
possessing a full $s_\pm$-wave gap of a few meV. Here, we propose a mechanism
for the emergence of topological superconductivity on the surface of
FeSe$_{0.45}$Te$_{0.55}$ by demonstrating that the interplay between the
$s_\pm$-wave symmetry of the superconducting gap, recently observed surface
magnetism, and a Rashba spin-orbit interaction gives rise to several
topological superconducting phases. Moreover, the proposed mechanism explains a
series of experimentally observed hallmarks of topological superconductivity,
such as the emergence of Majorana zero modes in the center of vortex cores and
at the end of line defects, as well as of chiral Majorana edge modes along
certain types of domain walls. We also propose that the spatial distribution of
supercurrents near a domain wall is a characteristic signature measurable via a
scanning superconducting quantum interference device that can distinguish
between chiral Majorana edge modes and trivial in-gap states. | cond-mat_supr-con |
Probing the topological band structure of diffusive multiterminal
Josephson junction devices with conductance measurements: The energy of an Andreev bound state in a clean normal metal in contact with
two superconductors disperses with the difference $\Delta \phi$ in the
superconducting phase between the superconductors in much the same way as the
energies of electrons in a one-dimensional crystal disperse with the crystal
momentum $k$ of the electrons. A normal metal with $n$ superconductors maps on
to a $n-1$ dimensional crystal, each dimension corresponding to the phase
difference $\phi_i$ between a specific pair of superconductors. The resulting
band structure as a function of the phase differences $\{\Delta \phi_i\}$ has
been proposed to have a topological nature, with gapped regions characterized
by different Chern numbers separated by regions where the gap in the
quasiparticle spectrum closes. A similar complex evolution of the quasiparticle
spectrum with $\{\Delta \phi_i\}$ has also been predicted for diffusive normal
metals in contact with multiple superconductors. Here we show that the
variation of the density of states at the Fermi energy of such a system can be
directly probed by relatively simple conductance measurements, allowing rapid
characterization of the energy spectrum. | cond-mat_supr-con |
Controlling superconductivity of CeIrIn$_5$ microstructures by substrate
selection: Superconductor/metal interfaces are usually fabricated in heterostructures
that join these dissimilar materials. A conceptually different approach has
recently exploited the strain sensitivity of heavy-fermion superconductors,
selectively transforming regions of the crystal into the metallic state by
strain gradients. The strain is generated by differential thermal contraction
between the sample and the substrate. Here, we present an improved
finite-element model that reliably predicts the superconducting transition
temperature in CeIrIn$_5$ even in complex structures. Different substrates are
employed to tailor the strain field into the desired shapes. Using this
approach, both highly complex and strained as well as strain-free
microstructures are fabricated to validate the model. This enables full control
over the microscopic strain fields, and forms the basis for more advanced
structuring of superconductors as in Josephson junctions. | cond-mat_supr-con |
Generic Finite Size Enhancement of Pairing in Mesoscopic Fermi Systems: The finite size dependent enhancement of pairing in mesoscopic Fermi systems
is studied under the assumption that the BCS approach is valid and that the two
body force is size independent. Different systems are investigated such as
superconducting metallic grains and films as well atomic nuclei. It is shown
that the finite size enhancement of pairing in these systems is in part due to
the presence of a surface which accounts quite well for the data of nuclei and
explains a good fraction of the enhancement in Al grains. | cond-mat_supr-con |
Pair Density Waves from Local Band Geometry: A band-projection formalism is developed for calculating the superfluid
weight in two-dimensional multi-orbital superconductors with an
orbital-dependent pairing. It is discovered that, in this case, the band
geometric superfluid stiffness tensor can be locally non-positive-definite in
some regions of the Brillouin zone. When these regions are large enough or
include nodal singularities, the total superfluid weight becomes
non-positive-definite due to pairing fluctuations, resulting in the transition
of a BCS state to a pair-density wave (PDW). This geometric BCS-PDW transition
is studied in the context of two-orbital superconductors, and proof of the
existence of a geometric BCS-PDW transition in a generic topological flat band
is established. | cond-mat_supr-con |
Electron-Phonon Superconductivity in LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$: We report density functional calculations of the electronic structure, Fermi
surface, phonon spectrum and electron--phonon coupling for newly discovered
superconductor LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$. Significant similarity between
LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ and LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is found, i.e.
there is a strong Fermi surface nesting at ($\pi $,$\pi $,0), which results in
unstable phonon branches. Combining the frozen phonon total energy calculations
and an anharmonic oscillator model, we find that the quantum fluctuation
prevents the appearance of static long--range order. The calculation shows that
LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is highly anisotropic, and same as
LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, this compound is also a conventional
electron-phonon coupling induced superconductor. | cond-mat_supr-con |
Critical Velocity in the Presence of Surface Bound States in Superfluid
$^3$He-B: A microelectromechanical oscillator with a gap of 1.25 $\mu$m was immersed in
superfluid $^3$He-B and cooled below 250 $\mu$K at various pressures.
Mechanical resonances of its shear motion were measured at various levels of
driving force. The oscillator enters into a nonlinear regime above a certain
threshold velocity. The damping increases rapidly in the nonlinear region and
eventually prevents the velocity of the oscillator from increasing beyond the
critical velocity which is much lower than the Landau critical velocity. We
propose that this peculiar nonlinear behavior stems from the escape of
quasiparticles from the surface bound states into the bulk fluid. | cond-mat_supr-con |
Reinterpretation of the equilibrium magnetization of a Tl-based single
crystal. Another phase transition in the mixed state of high-Tc
superconductors?: We apply a recently developed scaling procedure for the analysis of the
equilibrium magnetization M that was measured on a Tl-based single crystal and
was recently reported in the literature. The results of our analysis are
distinctly different from those obtained in the original publication where the
Hao-Clem model served to analyze the magnetization data. We argue that the
Hao-Clem model is not adequate for a quantitative description of the mixed
state in high-Tc superconductors especially in high magnetic fields. The scaled
equilibrium magnetization data reveal a pronounced kink in the M(H) dependence
that might be indicative of a phase transition in the mixed state. | cond-mat_supr-con |
Finite-size effects in hyperuniform vortex matter: Novel hyperuniform materials are emerging as an active field of applied and
basic research since they can be designed to have exceptional physical
properties. This ubiquitous state of matter presents a hidden order that is
characterized by the density of constituents of the system being uniform at
large scales, as in a perfect crystal, although they can be isotropic and
disordered like a liquid. In the quest for synthesizing hyperuniform materials
in experimental conditions, the impact of finite-size effects remains as an
open question to be addressed. We use vortex matter in type-II superconductors
as a toy model system to study this issue. We previously reported that vortex
matter nucleated in samples with point disorder is effectively hyperuniform and
thus presents the interesting physical properties inherent to hyperuniform
systems. In this work we present experimental evidence that on decreasing the
thickness of the vortex system its hyperuniform order is depleted. By means of
hydrodynamic arguments we show that the experimentally observed depletion can
be associated to two crossovers that we describe within a hydrodynamic
approximation. The first crossover length is thickness-dependent and separates
a class-II hyperuniform regime at intermediate lengthscales from a regime that
can become asymptotically non-hyperuniform for large wavelengths in very thin
samples. The second crossover takes place at smaller lengthscales and marks the
onset of a faster increase of density fluctuations due to the dispersivity of
the elastic constants. Our work points to a novel mechanism of emerging
hyperuniformity controlled by the thickness of the host sample, an issue that
has to be taken into account when growing hyperuniform structures for
technological applications. | cond-mat_supr-con |
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