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Structural Modulation in LaO0.9F0.1BiSe2 Single Crystals Revealed by
Scanning Tunneling Microscopy/Spectroscopy: We present scanning tunneling microscopy and spectroscopy measurements on a
cleaved surface of the LaO0.9F0.1BiSe2 single crystals. Tunneling spectra show
a finite local density of states at EF, which is consistent with metallic
conductivity in bulk. In addition, the existence of the supermodulation running
along the diagonal directions of Bi square lattice was revealed. The period of
the supermodulation was about 3 to 5 times the length of the lattice constant.
This period is close to that observed in LaO0.5F0.5BiSe2. | cond-mat_supr-con |
Charge creation and nucleation of longitudinal plasma wave in coupled
Josephson junctions: We study the phase dynamics in coupled Josephson junctions describing by
system of nonlinear differential equations. Results of detailed numerical
simulations of charge creation in the superconducting layers and the
longitudinal plasma wave (LPW) nucleation are presented. We demonstrate the
different time stages in the development of the LPW and present results of FFT
analysis at different values of bias current. The correspondence between the
breakpoint position on the outermost branch of current voltage characteristics
(CVC) and the growing region in time dependence of the electric charge in the
superconducting layer is established. The effects of noise in the bias current
and the external microwave radiation on the charge dynamics of the coupled
Josephson junctions are found. These effects introduce a way to regulate the
process of LPW nucleation in the stack of IJJ. | cond-mat_supr-con |
Zero-crossing Shapiro steps in focused-ion-beam-tailored high-$T_c$
superconducting microstructures: Microwave response of S-shaped Bi$_2$Sr$_2$CaCu2O$_{8+x}$ (Bi-2212)
micron-scale samples, in which the supercurrent was forced to flow
perpendicular to the crystal layers, was investigated. A treatment with a
focused ion beam allowed us to reduce the plasma frequency down to $f_p$$\sim$5
GHz at $T$=0.3 K in naturally stacked Josephson junctions in a crystal. We
observed Shapiro steps at frequencies as low as $\sim$5 GHz. Well-developed
zero-crossing Shapiro steps were observed at frequencies as low as $\sim$10
GHz. They appeared as constant-voltage plateaus with a non-zero voltage
occurring at zero bias current. We confirmed that zero-crossing Shapiro steps
in the Bi-2212 stacked junctions can be observed when the irradiated frequency
is sufficiently larger than $f_p$. The observed high-order fractional steps in
the microwave responses indicate that the interlayer-coupled Bi-2212 Josephson
junctions have nonsinusoidal current-phase relation. Based on the temperature
dependence of the steps we also showed that the finite slope of the steps is
due to the enhancement of the phase diffusion effect. | cond-mat_supr-con |
The circulation radius and critical current density in type-II
superconductors: A method is proposed for estimating the length scale of currents circulating
in superconductors. The estimated circulation radius is used to determine the
critical current density on the basis of magnetic measurements. The obtained
formulas are applicable to samples with negligibly small demagnetizing factors
and to polycrystalline superconductors. The proposed method has been verified
using experimental magnetization loops measured for polycrystalline
YBa$_2$Cu$_3$O$_{7-d}$ and Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$
superconductors. | cond-mat_supr-con |
Giant shot noise from Majorana zero modes in topological trijunctions: The clear-cut experimental identification of Majorana bound states in
transport measurements still poses experimental challenges. We here show that
the zero-energy Majorana state formed at a junction of three topological
superconductor wires is directly responsible for giant shot noise amplitudes,
in particular at low voltages and for small contact transparency. The only
intrinsic noise limitation comes from the current-induced dephasing rate due to
multiple Andreev reflection processes. | cond-mat_supr-con |
Cooper pairs localization in tree-like networks of superconducting
islands: We study inhomogeneous Cooper pairs distribution and localization effects in
tree-like networks of superconducting islands coupled via Josephson weak links.
Using a generalized Feynman's approach, reminiscent of the Bose-Hubbard model,
we demonstrate that the Cooper pairs fraction which localizes on a specific
network's island is limited by the network topology and, if present, by the
repulsive interaction. These findings contribute to clarify the interplay
between confinement effects induced by the network's topology and interaction
and shed some light on recent experiments dealing with networks of Josephson
junctions. | cond-mat_supr-con |
Pinning properties of FeSeTe thin film through multifrequency
measurements of the surface impedance: We present high frequency measurements of the vortex dynamics of a
FeSe$_x$Te$_{1-x}$ ($x=0.5$) thin film grown on a CaF$_2$ substrate and with a
critical temperature $T_c\simeq18\;$K, performed by means of a dual frequency
dielectric resonator at 16.4 GHz and 26.6 GHz. We extract and discuss various
important vortex parameters related to the pinning properties of the sample,
such as the characteristic frequency $\nu_c$, the pinning constant $k_p$ and
the pinning barrier height $U$ relevant for creep phenomena. We find that the
vortex system is in the single-vortex regime, and that pinning attains
relatively high values in terms of $k_p$, indicating significant pinning at the
high frequencies here studied. The pinning barrier energy $U$ is quite small
and exhibits a non-monotonous temperature dependence with a maximum near 12 K.
This result is discussed in terms of core pinning of small portion of vortices
of size $\propto\xi^3$ jumping out of the pinning wells over very small
distances, a process which is favoured in the high frequency, short ranged
vortex oscillations here explored. | cond-mat_supr-con |
Nonequilibrium transport via spin-induced sub-gap states in
superconductor/quantum dot/normal metal cotunnel junctions: We study low-temperature transport through a Coulomb blockaded quantum dot
(QD) contacted by a normal (N), and a superconducting (S) electrode. Within an
effective cotunneling model the conduction electron self energy is calculated
to leading order in the cotunneling amplitudes and subsequently resummed to
obtain the nonequilibrium T-matrix, from which we obtain the nonlinear
cotunneling conductance. For even occupied dots the system can be conceived as
an effective S/N-cotunnel junction with subgap transport mediated by Andreev
reflections. The net spin of an odd occupied dot, however, leads to the
formation of sub-gap resonances inside the superconducting gap which gives rise
to a characteristic peak-dip structure in the differential conductance, as
observed in recent experiments. | cond-mat_supr-con |
Some Global Properties of the Attractive Hubbard Model in the
Superconducting Phase: T-Matrix Approximation in 2D: We have applied the Fast Fourier transform (FFT), which allows to compute
efficiently convolution sums, to solve the set of self-consistent T-matrix
equations to get the Green function of the two dimensional attractive-U Hubbard
modelbelow $T_c$, extending previous calculations of the same authors. Using a
constant order parameter $\Delta(T)$, we calculated $T_c$ as a function of
electron density and interaction strength $U$. These global results deviate
from the BCS behavior remarkably. | cond-mat_supr-con |
Unusual Relationship between Magnetism and Superconductivity in
FeTe$_{0.5}$Se$_{0.5}$: We use neutron scattering, to study magnetic excitations in crystals near the
ideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of
excitations are found, a resonance at (0.5, 0.5, 0) and incommensurate
fluctuations on either side of this position. We show that the two sets of
magnetic excitations behave differently with doping, with the resonance being
fixed in position while the incommensurate excitations move as the doping is
changed. These unusual results show that a common behavior of the low energy
magnetic excitations is not necessary for pairing in these materials. | cond-mat_supr-con |
Control of Correlations in Sr4V2O6Fe2As2 by Chemical Stoichiometry: We show using a combination of powder X-ray and neutron diffraction, first
principles calculations, temperature- and field-dependent magnetization, heat
capacity and resistivity data that the superconducting behavior of
`Sr$_4$V$_2$O$_6$Fe$_2$As$_2$' is dependent on synthesis conditions,
particularly, heating profiles result in unintentional chemical doping. This
compound can be tuned from a state in which the vanadium electrons are
itinerant with a high electronic density of states, to a state where the
vanadium-oxide layers are insulating and presumably antiferromagnetic. | cond-mat_supr-con |
Small Fermi energy, zero point fluctuations and nonadiabaticity in
MgB$_2$: Small Fermi energy effects are induced in MgB$_2$ by the low hole doping in
the $\sigma$ bands which are characterized by a Fermi energy $E_{\rm F}^\sigma
\sim 0.5$ eV. We show that, due to the particularly strong deformation
potential relative to the $E_{2g}$ phonon mode, lattice fluctuations are
reflected in strong fluctuations in the electronic band structure. Quantum
fluctuations associated to the zero-point lattice motion are responsible for an
uncertainty of the Fermi energy of the order of the Fermi energy itself,
leading to the breakdown of the adiabatic principle underlying the
Born-Oppenheimer approximation in MgB$_2$ even if $\omega_{\rm ph}/E_{\rm F}
\sim 0.1-0.2$, where $\omega_{\rm ph}$ are the characteristic phonon
frequencies. This amounts to a new nonadiabatic regime, which could be relevant
to other unconventional superconductors. | cond-mat_supr-con |
Superconductivity in WO2.6F0.4 synthesized by reaction of WO3 with
Teflon: WO3-xFx (x < 0.45) perovskite-like oxyfluorides were prepared by a chemically
reducing fluorination route using the polymer polytetrafluoroethylene (Teflon).
The symmetry of the crystal structures of WO3-xFx changes from monoclinic to
tetragonal to cubic as the fluorine content increases. Fluorine doping changes
insulating WO3 to a metallic conductor, and superconductivity (Tc = 0.4 K) was
discovered in the samples with fluorine contents of 0.41 < x < 0.45. This easy
fluorination method may be applicable to other systems and presents an
opportunity for finding new oxyfluoride superconductors. | cond-mat_supr-con |
Observation of pseudogap-like feature above Tc in LiFeAs and
(Ba0.6K0.4)Fe2As2 by ultrafast optical measurement: We utilize ultrafast optical measurement to study the quasiparticle
relaxation in stoichiometric LiFeAs and nearly optimally doped (BaK)Fe2As2
crystals. According to our temperature-dependent studies of LiFeAs, we have
observed pseudogap-like feature at onset temperature of ~ 55 K, which is above
Tc = 15 K. In addition, the onset temperature of pseudogap ~90K was also
observed in Ba0.6K0.4Fe2As2 (Tc = 36 K). Our findings seem implying that the
pseudogap feature, which is due to antiferromagnetic fluctuations, is universal
for the largely studied 11, 111, 122, and 1111 iron-based superconductors. | cond-mat_supr-con |
Ring Oscillators for Clocking Reversible Superconducting Circuits and
Fabrication Process Benchmarking: Existing concepts of reversible superconducting circuits as well as
demonstrated adiabatic circuits require three-phase bias/clock signals
generated by room temperature sources. A while ago, we suggested that a
multi-phase bias/clock could be provided by a local Josephson junction-based
generator. The generator looks like a long annular Josephson junction, only
composed of discreet elements - junctions and inductors, and closed into a ring
via a flux pump to inject any required number of vortices into the ring. A
steady motion of the vortices forced by a uniformly distributed dc bias current
applied to the ring is accompanied by a nearly harmonic ac currents flowing via
the Josephson junctions (JJs) connected in series with small inductors. These
ac currents serve as multi-phase bias/clock for nSQUID-based circuitry. To
verify this concept and compare the dissipated energy with kBTln2 threshold, we
developed a ring composed of 256 unshunted JJs with 20 {\mu}A target critical
current, Ic. We investigated the behavior of the ring oscillator at each vortex
count from 0 to 256. The measured critical current of the ring with vortices
was about 0.1 {\mu}A per one JJ, which can be explained by unavoidable
nonuniformity of the ring components and the influence of fluxes frozen near
the ring. The corresponding energy dissipation, about 10kBT per passage of one
vortex through one JJ, should be reduced further for prospective experiments
with reversible circuits. However, obtained I-V characteristics could be of
interest for scientists working with long Josephson junctions. Superiority of
the fabrication process used in this work is demonstrated by the obtained about
200 times reduction of Ic of the ring with vortices with respect to a single
comprising JJ, much larger than in any previously described case. | cond-mat_supr-con |
Topological superconductivity on the surface of Fe-based superconductors: As one of the simplest systems for realizing Majorana fermions, topological
superconductor plays an important role in both condensed matter physics and
quantum computations. Based on \emph{ab~initio} calculations and the analysis
of an effective 8-band model with the superconducting pairing, we demonstrate
that the three dimensional extended $s$-wave Fe-based superconductors such as
Fe$_{1+\text{y}}$Se$_{0.5}$Te$_{0.5}$ have a metallic topologically nontrivial
band structure, and exhibit a normal-topological-normal superconductivity phase
transition on the ($001$) surface by tuning the bulk carrier doping level. In
the topological superconductivity (TSC) phase, a Majorana zero mode is trapped
at the end of a magnetic vortex line. We further show that, the surface TSC
phase only exists up to a certain bulk pairing gap, and there is a
normal-topological phase transition driven by the temperature, which has not
been discussed before. These results pave an effective way to realize the TSC
and Majorana fermions in a large class of superconductors. | cond-mat_supr-con |
Mechanism of Cooper-pairing in layered high temperature superconductors: In this study, the pairing mechanism for layered HTS materials based on
attraction between electrons from adjacent layers is proposed. Initially, each
layer has expanded Fermi sphere owing to ridged geometry. When the two layers
are close enough for tunneling, it becomes energetically advantageous to form
correlated quantum states (CQS), reducing the Fermi sphere volume. Cooper
pairs, comprising inter-tunneling electrons, occupy the CQS. The image force is
responsible for the electron-electron attraction. Pair-binding energy and the
corresponding effective mass vary in a wide range. At T>0, some heavy pairs do
not condense. Such pairs are responsible for pseudogap. Light pairs get Bose
condensed and are responsible for superconductivity. The proposed mechanism
provides clarification of superconductivity in cuprates, iron based
superconductors and LSCO/LCO interfaces. It provides explanation of two energy
gaps and two characteristic temperatures in layered superconducting materials.
It also provides clarification on the Fermi surface pockets, anisotropy of
charge transport in pseudogap state, and other properties of HTS materials. The
pseudogap, estimated within the model, fits the experimental values for the
two-layer cuprates, such as YBCO, Bi2212, Tl2212, and Hg1212. | cond-mat_supr-con |
Scenario of Superconducting Transition for quasi-2D HTS: We discuss the scenario of superconducing transition for quasi two dimension
HTS with spin fluctuation pairing mechanism. At mean field temperature of 2D
superconducting transition the interaction of fluctuation spin waves with holes
in copper-oxigen planes leads to the pairing of holes and to the fluctuation
generation of superconducting regions, and also to the essential temperature
dependence of the strength of the interlayer coupling. At decreasing
temperature and sufficiently small interlayer coupling the transition of the
sample to coherent superconducting state occurs. | cond-mat_supr-con |
Connection between the semiconductor--superconductor transition and the
spin-polarized superconducting phase in the honeycomb lattice: The band structure of noninteracting fermions in the honeycomb lattice
exhibits the Dirac cones at the corners of the Brillouin zone. As a
consequence, fermions in this lattice manifest a semiconducting behavior below
some critical value of the onsite attraction, $U_{c}$. However, above $U_{c}$,
the superconducting phase can occur. We discuss an interplay between the
semiconductor--superconductor transition and the possibility of realization of
the spin-polarized superconductivity (the so-called Sarma phase). We show that
the critical interaction can be tuned by the next-nearest-neighbor (NNN)
hopping in the absence of the magnetic field. Moreover, a critical value of the
NNN hopping exists, defining a range of parameters for which the semiconducting
phase can emerge. In the weak coupling limit case, this quantum phase
transition occurs for the absolute value of the NNN hopping equal to one third
of the hopping between the nearest neighbors. Similarly, in the presence of the
magnetic field, the Sarma phase can appear, but only in a range of parameters
for which initially the semiconducting state is observed. Both of these aspects
are attributed to the Lifshitz transition, which is induced by the NNN hopping
as well as the external magnetic field. | cond-mat_supr-con |
Stability conditions for a large anharmonic bipolaron: A large polaron is a quasiparticle that consists of a nearly free electron
interacting with the phonons of a material, whose lattice parameters are much
smaller than the polaron scale. The electron-phonon interaction also leads to
an attractive interaction between electrons, which can allow two polarons to
pair up and form a bipolaron. It has been shown that large bipolarons can form
in theory due to strong 1-electron-1-phonon coupling, but they have not been
seen in real materials because the critical value of the required
electron-phonon interaction is too large. Here, we investigate the effect of
1-electron-2-phonon coupling on the large bipolaron problem.
Starting from a generalization of the Fr\"ohlich Hamiltonian that includes
both the standard 1-electron-1-phonon interaction as well as an anharmonic
1-electron-2-phonon interaction, we use the path integral method to find a
semi-analytical upper bound for the bipolaron energy that is valid at all
values of the Fr\"ohlich coupling strength $\alpha$. We find the bipolaron
phase diagram and conditions for the bipolaron stability by comparing the
bipolaron energy to the energy of two free polarons. The critical value of the
Fr\"ohlich coupling strength $\alpha_{\text{crit}}$ is calculated as a function
of the strength of the 1-electron-2-phonon interaction. The results suggest
that large bipolaron formation is more likely in materials with significant
1-electron-2-phonon interaction as well as strong 1-electron-1-phonon
interaction, such as strontium titanate. | cond-mat_supr-con |
Universal suppression of superfluid weight by disorder independent of
quantum geometry and band dispersion: Motivated by the experimental progress in controlling the properties of the
energy bands in superconductors, significant theoretical efforts have been
devoted to study the effect of the quantum geometry and the flatness of the
dispersion on the superfluid weight. In conventional superconductors, where the
energy bands are wide and the Fermi energy is large, the contribution due to
the quantum geometry is negligible, but in the opposite limit of flat-band
superconductors the superfluid weight originates purely from the quantum
geometry of Bloch wave functions. Here, we study how the energy band dispersion
and the quantum geometry affect the disorder-induced suppression of the
superfluid weight. Surprisingly, we find that the disorder-dependence of the
superfluid weight is universal across a variety of models, and independent of
the quantum geometry and the flatness of the dispersion. Our results suggest
that a flat-band superconductor is as resilient to disorder as a conventional
superconductor. | cond-mat_supr-con |
Tunneling conductance in Superconductor/Ferromagnet junctions: a self
consistent approach: We evaluate the tunneling conductance of clean Ferromomagnet/Superconductor
junctions via a fully self-consistent numerical solution of the microscopic
Bogoliubov-DeGennes equations. We present results for a relevant range of
values of the Fermi wavevector mismatch (FWM), the spin polarization, and the
interfacial scattering strength. For nonzero spin polarization, the conductance
curves vary nonmonotonically with FWM. The FWM dependence of the
self-consistent results is stronger than that previously found in
non-self-consistent calculations, since, in the self-consistent case, the
effective scattering potential near the interface depends on the FWM. The
dependence on interfacial scattering is monotonic. These results confirm that
it is impossible to characterize both the the FWM and the interfacial
scattering by a single effective parameter and that analysis of experimental
data via the use of such one-parameter models is unreliable. | cond-mat_supr-con |
Bias current dependence of superconducting transition temperature in
superconducting spin valve nanowires: Competition between superconducting and ferromagnetic ordering at interfaces
between ferromagnets (F) and superconductors (S) gives rise to several
proximity effects such as odd-triplet superconductivity and spin-polarized
supercurrents. A prominent example of an S/F proximity effect is the spin
switch effect (SSE) observed in S/F/N/F superconducting spin-valve multilayers,
in which the superconducting transition temperature T$_c$ is controlled by the
angle $\phi$ between the magnetic moments of the F layers separated by a
nonmagnetic metallic spacer N. Here we present an experimental study of SSE in
Nb/Co/Cu/Co/CoO$_x$ nanowires measured as a function of bias current flowing in
the plane of the layers. These measurements reveal an unexpected dependence of
T$_c(\phi)$ on the bias current: T$_c(\pi)$--T$_c(0)$ changes sign with
increasing current bias. We attribute the origin of this bias dependence of the
SSE to a spin Hall current flowing perpendicular to the plane of the
multilayer, which suppresses T$_c$ of the multilayer. The bias dependence of
SSE can be important for hybrid F/S devices such as those used in cryogenic
memory for superconducting computers as device dimensions are scaled down to
the nanometer length scale. | cond-mat_supr-con |
Doping evolution of itinerant magnetic excitations in Fe-based
oxypnictides: Employing the four-band tight-binding model we study theoretically the doping
dependence of the spin response in the normal state of novel Fe-based pnictide
superconductors. We show that the commensurate spin density wave (SDW)
transition that arises due to interband scattering between the hole
$\alpha$-pockets and the electron $\beta$-pockets disappears already at the
doping concentration $x \approx 0.04$ reflecting the evolution of the Fermi
surfaces. Correspondingly, with further increase of the doping the
antiferromagnetic fluctuations are suppressed for $x > 0.1$ and the
Im$\chi({\bf Q_{AFM}},\omega)$ becomes nearly temperature independent. At the
same time, we observe that the uniform susceptibility deviates from the
Pauli-like behavior and is increasing with increasing temperature reflecting
the activation processes for the $\alpha$-Fermi surfaces up to temperatures of
about T=800K. With increase of the doping the absolute value of the uniform
susceptibility lowers and its temperature dependence changes. In particular, it
is a constant at low temperatures and then decreases with increasing
temperature. We discuss our results in a context of recent experimental data. | cond-mat_supr-con |
Superconductivity with hard-core repulsion: BCS-Bose crossover and
s-/d-wave competition: We consider fermions on a 2D lattice interacting repulsively on the same site
and attractively on the nearest neighbor sites. The model is relevant, for
instance, to study the competition between antiferromagnetism and
superconductivity in a Kondo lattice. We first solve the two-body problem to
show that in the dilute and strong coupling limit the s-wave Bose condensed
state is always the ground state. We then consider the many-body problem and
treat it at mean-field level by solving exactly the usual gap equation. This
guarantees that the superconducting wave-function correctly vanishes when the
two fermions (with antiparallel spin) sit on the same site. This fact has
important consequences on the superconducting state that are somewhat unusual.
In particular this implies a radial node-line for the gap function. When a next
neighbor hopping t' is present we find that the s-wave state may develop nodes
on the Fermi surface. | cond-mat_supr-con |
Large gap, a pseudogap and proximity effect in the Bi2Te3/Fe1+yTe
interfacial superconductor: We report directional point-contact spectroscopy data on the novel
Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple
layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically
sharp interface. Our data show a very large superconducting twin-gap structure
with an energy scale exceeding that of bulk FeSe or FeSe1-xTex by a factor of
4. While the larger gap is isotropic and attributed to a thin FeTe layer in
proximity of the interface, the smaller gap has a pronounced anisotropy and is
associated with proximity-induced superconductivity in the topological
insulator Bi2Te3. Zero resistance is lost above 8 K, but superconducting
fluctuations are visible up to at least 12 K and the large gap is replaced by a
pseudogap that persists up to 40 K. The spectra show a pronounced zero-bias
conductance peak in the superconducting state, which may be a signature of an
unconventional pairing mechanism. | cond-mat_supr-con |
Anisotropy of the upper critical field in MgB2: the two-gap
Ginzburg-Landau theory: The upper critical field in MgB2 is investigated in the framework of the
two-gap Ginzburg-Landau theory. A variational solution of linearized
Ginzburg-Landau equations agrees well with the Landau level expansion and
demonstrates that spatial distributions of the gap functions are different in
the two bands and change with temperature. The temperature variation of the
ratio of two gaps is responsible for the upward temperature dependence of
in-plane Hc2 as well as for the deviation of its out-of-plane behavior from the
standard angular dependence. The hexagonal in-plane modulations of Hc2 can
change sign with decreasing temperature. | cond-mat_supr-con |
Anisotropic angle-dependent Andreev reflection at the
ferromagnet/superconductor junction on the surface of topological insulators: We theoretically demonstrate that a ferromagnetic/superconductor junction on
the surface of three-dimensional topological insulators (3D TIs) has an
anisotropic angle-dependent Andreev reflection when the in-plane magnetization
has a component perpendicular to the junction. In the presence of in-plane
magnetization, the Dirac cone's location adjusts in the $k$-space, whereas its
out-of-plane component induces a gap. This movement leads to the anisotropic
angle-dependent Andreev reflection and creates transverse conductance flows
parallel to the interface. Also, an indirect gap induces in the junction, which
removes the transport signatures of Majorana bound states. Because of the full
spin-momentum locking of Dirac fermions on the surface of 3DTIs, a torque that
called \textit{Andreev Transfer Torque} (ATT) imposes on the junction.
Moreover, we propose a setup to detect them experimentally. | cond-mat_supr-con |
Symmetry protected line nodes in non-symmorphic magnetic space groups:
Applications to UCoGe and UPd$_2$Al$_3$: We present the group-thoretical classification of gap functions in
superconductors coexisting with some magnetic order in non-symmorphic magnetic
space groups. Based on the weak-coupling BCS theory, we show that UCoGe-type
ferromagnetic superconductors must have horizontal line nodes on either $k_z=0$
or $\pm\pi/c$ plane. Moreover, it is likely that additional Weyl point nodes
exist at the axial point. On the other hand, in UPd$_2$Al$_3$-type
antiferromagnetic superconductors, gap functions with $A_g$ symmetry possess
horizontal line nodes in antiferromagnetic Brillouin zone boundary
perpendicular to $c$-axis. In other words, the conventional fully-gapped
$s$-wave superconductivity is forbidden in this type of antiferromagnetic
superconductors, irrelevant to the pairing mechanism, as long as the Fermi
surface crosses a zone boundary. UCoGe and UPd$_2$Al$_3$ are candidates for
unconventional superconductors possessing hidden symmetry-protected line nodes,
peculiar to non-symmorphic magnetic space groups. | cond-mat_supr-con |
Emergence of an incipient ordering mode in FeSe: The structurally simplest Fe-based superconductor FeSe with a critical
temperature $T_{c}\approx$ 8.5 K displays a breaking of the four-fold
rotational symmetry at a temperature $T_{s}\approx 87$ K. We investigated the
electronic properties of FeSe using scanning tunneling microscopy/spectroscopy
(STM/S), magnetization, and electrical transport measurements. The results
indicated two new energy scales (i) $T^{*} \approx$ 75 K denoted by an onset of
electron-hole asymmetry in STS, enhanced spin fluctuations, and increased
positive magnetoresistance; (ii) $T^{**} \approx$ 22 - 30 K, marked by opening
up of a partial gap of about 8 meV in STS and a recovery of Kohler's rule. Our
results reveal onset of an incipient ordering mode at $T^{*}$ and its
nucleation below $T^{**}$. The ordering mode observed here, both in spin as
well as charge channels, suggests a coupling between the spins with charge,
orbital or pocket degrees of freedom. | cond-mat_supr-con |
Engineering Quantum Interference: A model for describing interference and diffraction of wave functions of
one-dimensional Josephson array interferometers is presented. The derived
expression for critical current modulations accounts for an arbitrary number of
square junctions, variable distance between these, and variable size of their
area. Predictions are tested on real arrays containing up to 20 equally spaced
and identical junctions and on arrays shaped with peculiar geometries. Very
good agreement with the modulations predicted by the model and the experimental
results is obtained for all the tested configurations. It is shown that
specific designs of the arrays generate significant differences in their static
and dynamical (non-zero voltage) properties. The results demonstrate that the
magnetic field dependence of Josephson supercurrents shows how interference and
diffraction of macroscopic quantum wavefunctions can be manipulated and
controlled. | cond-mat_supr-con |
Phase transitions in a three dimensional $U(1) \times U(1)$ lattice
London superconductor: We consider a three-dimensional lattice $U(1) \times U(1)$ superconductor in
the London limit, with two individually conserved condensates. The problem,
generically, has two types of intercomponent interactions of different
characters. First, the condensates are interacting via a minimal coupling to
the same fluctuating gauge field. A second type of coupling is the direct
dissipationless drag represented by a local intercomponent current-current
coupling term in the free energy functional. The interplay between these two
types of interactions produces a number of physical effects not present in
previously investigated $U(1)\times U(1)$ models with only one kind of
intercomponent interaction. In this work, we present a study of the phase
diagram of a $U(1) \times U(1)$ superconductor which includes both of these
interactions. We study phase transitions and two types of competing paired
phases which occur in this general model: (i) a metallic superfluid phase
(where there is order only in the gauge invariant phase difference of the order
parameters), (ii) a composite superconducting phase where there is order in the
phase sum of the order parameters which has many properties of a
single-component superconductor but with a doubled value of electric charge. We
investigate the phase diagram with particular focus on what we call "preemptive
phase transitions". These are phase transitions {\it unique to multicomponent
condensates with competing topological objects}. A sudden proliferation of one
kind of topological defects may come about due to a fluctuating background of
topological defects in other sectors of the theory. | cond-mat_supr-con |
Quantum Coherence of Electrons Field-Emitted from a Superconductor:
Correlations and Entanglement: The correlations of the electrons field-emitted from a superconductor are
fully analyzed, both in space and time. It is proposed that a coincidence
experiment would reveal a positive correlation between the electrons emitted in
opposite directions. The electrons can be entangled and can even violate Bell's
inequality. The crucial role played by Andreev's process is scrutinized,
analytical formulas are derived for the correlations, and the physics behind
the phenomenon is clarified. | cond-mat_supr-con |
Using Josephson junctions to determine the pairing state of
superconductors without crystal inversion symmetry: Theoretical studies of a planar tunnel junction between two superconductors
with antisymmetric spin-orbit coupling are presented. The half-space Green's
function for such a superconductor is determined. This is then used to derive
expressions for the dissipative current and the Josephson current of the
junction. Numerical results are presented in the case of the Rashba spin-orbit
coupling, relevant to the much studied compound CePt$_3$Si. Current-voltage
diagrams, differential conductance and the critical Josephson current are
presented for different crystallographic orientations and different weights of
singlet and triplet components of the pairing state. The main conclusion is
that Josephson junctions with different crystallographic orientations may
provide a direct connection between unconventional pairing in superconductors
of this kind and the absence of inversion symmetry in the crystal. | cond-mat_supr-con |
Interplay between spin density wave and superconductivity in '122' iron
pnictides: 57Fe Mössbauer study: Iron-based superconductors Ba0.7Rb0.3Fe2As2 and CaFe1.92Co0.08As2 of the
'122' family have been investigated by means of the 14.41-keV Moessbauer
transition in 57Fe versus temperature ranging from the room temperature till
4.2 K. A comparison is made with the previously investigated parent compounds
BaFe2As2 and CaFe2As2. It has been found that Moessbauer spectra of these
superconductors are composed of the magnetically split component due to
development of spin density wave (SDW) and non-magnetic component surviving
even at lowest temperatures. The latter component is responsible for
superconductivity. Hence, the superconductivity occurs in the part of the
sample despite the sample is single phase. This phenomenon is caused by the
slight variation of the dopant concentration across the sample (crystal). | cond-mat_supr-con |
Itinerant approach to magnetic neutron scattering of FeSe: effect of
orbital selectivity: Recent STM experiments and theoretical considerations have highlighted the
role of interaction-driven orbital selectivity in FeSe, and its role in
generating the extremely anisotropic superconducting gap structure in this
material. We study the magnetic excitation spectrum resulting from the coherent
quasiparticles within the same renormalized random phase approximation approach
used to explain the STM experiments, and show that it agrees well with the
low-energy momentum and energy dependent response measured by inelastic neutron
scattering experiments. We find a correlation-induced suppression of
$(\pi,\pi)$ scattering due to a small quasiparticle weight of states of
$d_{xy}$ character. We compare predictions for twinned and untwinned crystals,
and predict in particular a strongly $(\pi,0)$-dominated response at low
energies in untwinned systems, in contrast to previous itinerant theories. | cond-mat_supr-con |
The Nernst effect in high-$T_c$ superconductors: The observation of a large Nernst signal $e_N$ in an extended region above
the critical temperature $T_c$ in hole-doped cuprates provides evidence that
vortex excitations survive above $T_c$. The results support the scenario that
superfluidity vanishes because long-range phase coherence is destroyed by
thermally-created vortices (in zero field), and that the pair condensate
extends high into the pseudogap state in the underdoped (UD) regime. We present
a series of measurements to high fields $H$ which provide strong evidence for
this phase-disordering scenario. | cond-mat_supr-con |
Absence of structural transition in TM0.5IrTe2 (TM=Mn, Fe, Co, Ni): TM-doped IrTe2(TM=Mn, Fe, Co, Ni) compounds were synthesized by solid state
reaction. Single crystal x-ray diffraction experiments indicate that part of
the doped TM ions (TM=Fe, Co, and Ni) substitute for Ir, and the rest
intercalate into the octahedral interstitial sites located in between IrTe2
layers. Due to the lattice mismatch between MnTe2 and IrTe2, Mn has limited
solubility in IrTe2 lattice. The trigonal structure is stable in the whole
temperature range 1.80<T<300K for all doped compositions. No long range
magnetic order or superconductivity was observed in any doped compositions
above 1.80K. A spin glass behavior below 10K was observed in Fe-doped IrTe2
from the temperature dependence of magnetization, electrical resistivity, and
specific heat. The low temperature specific heat data suggest the electron
density of states is enhanced in Fe- and Co-doped compositions but reduced in
Ni-doped IrTe2. With the 3d transition metal doping the trigonal a-lattice
parameters increases but the c-lattice parameter decreases. Detailed analysis
of the single crystal x-ray diffraction data shows that interlayer Te-Te
distance increases despite a reduced c-lattice. The importance of the Te-Te,
Te-Ir, and Ir-Ir bonding is discussed. | cond-mat_supr-con |
Magnetic properties of undoped and 15%F doped SmFeAsO compounds: In this paper the magnetic behaviour of SmFeAsO and SmFeAs(O0.85F0.15)
samples is presented and discussed. Molar susceptibility of SmFeAsO exhibits a
local peak at T around 140K due to the establishment of a long range
antiferromagnetic ordering of the Fe moments in Fe-As layers. This feature has
already been observed with different techniques, and frequently ascribed to the
onset of a Spin Density Wave (SDW). At TN around 6K another peak, which we
attribute to the establishment of antiferromagnetic ordering of Sm ion
sublattice, is observed. Furthermore, a temperature independent signal (Pauli
paramagnetism, Landau and core diamagnetism...) is also present in the magnetic
behaviour of this sample. In SmFeAs(O0.85F0.15) the antiferromagnetic ordering
in Fe-As plane is suppressed and superconductivity occurs at T = 52 K, whereas
the antiferromagnetic ordering of Sm ions at low temperature persists, leading
to the coexistence and competition between superconducting and magnetic
orderings. Above the transition temperature, after the subtraction of the Sm
ion sublattice paramagnetic contribution and of the temperature independent
contribution to the experimental susceptibility data, a Curie-Weiss behaviour
for Fe is observed, with a magnetic moment of 1.4 Bohr magneton. | cond-mat_supr-con |
Bulk properties and electronic structure of PuFeAsO: Here we present bulk property measurements and electronic structure
calculations for PuFeAsO, an actinide analogue of the iron-based rare-earth
superconductors RFeAsO. Magnetic susceptibility and heat capacity data suggest
the occurrence of an antiferromagnetic transition at TN=50 K. No further
anomalies have been observed down to 2 K, the minimum temperature that we have
been able to achieve. Structural measurements indicate that PuFeAsO, with its
more localized 5f electrons, bears a stronger resemblance to the RFeAsO
compounds with larger R ions, than NpFeAsO does. | cond-mat_supr-con |
Pseudogap from ARPES experiment: three gaps in cuprates and topological
superconductivity: A term first coined by Mott back in 1968 a `pseudogap' is the depletion of
the electronic density of states at the Fermi level, and pseudogaps have been
observed in many systems. However, since the discovery of the high temperature
superconductors (HTSC) in 1986, the central role attributed to the pseudogap in
these systems has meant that by many researchers now associate the term
pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a
lot of new attention with the rediscovery of two distinct energy scales
(`two-gap scenario') and charge density waves patterns in the cuprates. Despite
many excellent reviews on the pseudogap phenomenon in HTSC, published from its
very discovery up to now, the mechanism of the pseudogap and its relation to
superconductivity are still open questions. The present review represents a
contribution dealing with the pseudogap, focusing on results from angle
resolved photoemission spectroscopy (ARPES) and ends up with the conclusion
that the pseudogap in cuprates is a complex phenomenon which includes at least
three different `intertwined' orders: spin and charge density waves and
preformed pairs, which appears in different parts of the phase diagram. The
density waves in cuprates are competing to superconductivity for the electronic
states but, on the other hand, should drive the electronic structure to
vicinity of Lifshitz transition, that could be a key similarity between the
superconducting cuprates and iron based superconductors. One may also note that
since the pseudogap in cuprates has multiple origins there is no need to recoin
the term suggested by Mott. | cond-mat_supr-con |
Superconductivity driven by the screening of long-distance Coulomb
interaction: The pair-fluctuation contribution reduces the electrostatic screening length
in superconductivity as compared to the normal state. When a conductor
possesses a static background charge distribution, superconductivity arises
even in the absence of an explicit pairing interaction, such that the Coulomb
repulsion is reduced and the total energy is lowered. We demonstrate that the
superconducting gap increases with increased background charge at first, after
which the mixing of the Higgs and plasma modes suppresses superconductivity in
the pseudogap phase. This indicates that the mechanism may be relevant to the
cuprates and iron pnictides. When the background charge is identified with the
incoherent component of optical conductivity in the cuprates, our results
reproduce the shape, size and position of the superconducting dome with zero
free parameters. A superconducting critical temperature of about 1000 K is
possible in ion-doped conductors. | cond-mat_supr-con |
Microwave emission from superconducting vortices in Mo/Si superlattices: Most of superconductors in a magnetic field are penetrated by a lattice of
quantized flux vortices. In the presence of a transport current causing the
vortices to cross sample edges, emission of electromagnetic waves is expected
due to the continuity of tangential components of the fields at the surface.
Yet, such a radiation has not been observed so far due to low radiated power
levels and lacking coherence in the vortex motion. Here, we report emission of
electromagnetic waves from vortices crossing the layers of a
superconductor/insulator Mo/Si superlattice. The emission spectra consist of
narrow harmonically related peaks which can be finely tuned in the GHz range by
the dc bias current and, coarsely, by the in-plane magnetic field value. Our
findings show that superconductor/insulator superlattices can act as dc-tunable
microwave generators bridging the frequency gap between conventional
radiofrequency oscillators and (sub-)terahertz generators relying upon the
Josephson effect. | cond-mat_supr-con |
Combined effects of pressure and Ru substitution on BaFe2As2: The ab-plane resistivity of Ba(Fe1-xRux)2As2 (x = 0.00, 0.09, 0.16, 0.21, and
0.28) was studied under nearly hydrostatic pressures, up to 7.4 GPa, in order
to explore the T-P phase diagram and to compare the combined effects of
iso-electronic Ru substitution and pressure. The parent compound BaFe2As2
exhibits a structural/magnetic phase transition near 134 K. At ambient
pressure, progressively increasing Ru concentration suppresses this phase
transition to lower temperatures at the approximate rate of ~5 K/% Ru and is
correlated with the emergence of superconductivity. By applying pressure to
this system, a similar behavior is seen for each concentration: the
structural/magnetic phase transition is further suppressed and
superconductivity induced and ultimately, for larger x Ru and P, suppressed. A
detailed comparison of the T-P phase diagrams for all Ru concentrations shows
that 3 GPa of pressure is roughly equivalent to 10% Ru substitution.
Furthermore, due to the sensitivity of Ba(Fe1-xRux)2As2 to pressure conditions,
the melting of the liquid media, 4 : 6 light mineral oil : n-pentane and 1 : 1
iso-pentane : n-pentane, used in this study could be readily seen in the
resistivity measurements. This feature was used to determine the freezing
curves for these media and infer their room temperature, hydrostatic limits:
3.5 and 6.5 GPa, respectively. | cond-mat_supr-con |
Visualizing supercurrents in ferromagnetic Josephson junctions with
various arrangements of 0 and πsegments: Josephson junctions with ferromagnetic barrier can have positive or negative
critical current depending on the thickness $d_F$ of the ferromagnetic layer.
Accordingly, the Josephson phase in the ground state is equal to 0 (a
conventional or 0 junction) or to $\pi$ ($\pi$ junction). When 0 and $\pi$
segments are joined to form a "0-$\pi$ junction", spontaneous supercurrents
around the 0-$\pi$ boundary can appear. Here we report on the visualization of
supercurrents in superconductor-insulator-ferromagnet-superconductor (SIFS)
junctions by low-temperature scanning electron microscopy (LTSEM). We discuss
data for rectangular 0, $\pi$, 0-$\pi$, 0-$\pi$-0 and 20 \times 0-$\pi$
junctions, disk-shaped junctions where the 0-$\pi$ boundary forms a ring, and
an annular junction with two 0-$\pi$ boundaries. Within each 0 or $\pi$ segment
the critical current density is fairly homogeneous, as indicated both by
measurements of the magnetic field dependence of the critical current and by
LTSEM. The $\pi$ parts have critical current densities $j_c^\pi$ up to
$35\units{A/cm^2}$ at $T = 4.2\units{K}$, which is a record value for SIFS
junctions with a NiCu F-layer so far. We also demonstrate that SIFS technology
is capable to produce Josephson devices with a unique topology of the 0-$\pi$
boundary. | cond-mat_supr-con |
Multiple Andreev Reflections in Weak Links of Superfluid 3He-B: We calculate the current-pressure characteristics of a ballistic pinhole
aperture between two volumes of B-phase superfluid 3He. The most important
mechanism contributing to dissipative currents in weak links of this type is
the process of multiple Andreev reflections. At low biases this process is
significantly affected by relaxation due to inelastic
quasiparticle-quasiparticle collisions. In the numerical calculations,
suppression of the superfluid order parameter at surfaces is taken into account
self-consistently. When this effect is neglected, the theory may be developed
analytically like in the case of s-wave superconductors. A comparison with
experimental results is presented. | cond-mat_supr-con |
Magnetic properties of magnetically textured Bi-2212 ceramics: This paper aims at reporting magnetic properties of bulk polycrystalline
Bi2Sr2Ca0.8Dy0.2Cu2O8-y samples textured under a magnetic field. The
microstructure of these materials is highly anisotropic and exhibits particular
features needed to be taken into account in order to interpret their magnetic
and electrical properties. First the AC magnetic susceptibility c = c ' - j c"
has been measured for several magnetic fields (H // ab and H // c) and compared
to the electrical resistivity data. The structure of the c" peak is shown to be
related to the chemical content distribution of the superconducting grains.
Next, the magnetic flux profiles have been extracted from the magnetic
measurements using the Campbell - Rollins procedure. The anisotropy of the flux
profiles and their peculiar curvature behaviour for H // c point out the role
of both grain platelet structure and the presence of secondary phases. From
these results, we conclude that the magnetic properties of such magnetically
textured materials do not allow for a reliable extraction of the critical
current density Jc but essentially probe geometric effects. Such effects have
to be taken into account for improving the manufacture of attractive high-Tc
materials. | cond-mat_supr-con |
Tunable artificial vortex ice in nanostructured superconductors with
frustrated kagome lattice of paired antidots: Theoretical proposals for spin ice analogs based on nanostructured
superconductors have suggested larger flexibility for probing the effects of
fluctuations and disorder than in the magnetic systems. In this work, we unveil
the particularities of a vortex ice system by direct observation of the vortex
distribution in a kagome lattice of paired antidots using scanning Hall probe
microscopy. The theoretically suggested vortex ice distribution, lacking long
range order, is observed at half matching field (H_{1}/2). Moreover, the vortex
ice state formed by the pinned vortices is still preserved at 2H_{1}/3. This
unexpected result is attributed to the introduction of interstitial vortices at
these magnetic field values. Although the interstitial vortices increase the
number of possible vortex configurations, it is clearly shown that the vortex
ice state observed at 2H_{1}/3 is less prone to defects than at $H_{1}/2$. In
addition, the non-monotonic variations of the vortex ice quality on the lattice
spacing indicates that a highly ordered vortex ice state cannot be attained by
simply reducing the lattice spacing. The optimal design to observe defect free
vortex ice is discussed based on the experimental statistics. The direct
observations of a tunable vortex ice state provides new opportunities to
explore the order-disorder transition in artificial ice systems. | cond-mat_supr-con |
Evidence for interfacial superconductivity in a bi-collinear
antiferromagnetically ordered FeTe monolayer on a topological insulator: The discovery of high-temperature superconductivity in Fe-based compounds
[1,2] has triggered numerous investigations on the interplay between
superconductivity and magnetism [3] and, more recently, on the enhancement of
transition temperatures through interface effects [4]. It is widely believed
that the emergence of optimal superconductivity is intimately linked to the
suppression of long-range antiferromagnetic (AFM) order, although the exact
microscopic picture of this relationship remains elusive [1] due to the lack of
data with atomic spatial resolution [5-7]. Here, we present a spin-polarized
scanning tunneling spectroscopy (SP-STS) study of ultrathin FeTe$_{1-x}$Se$_x$
(x = 0, 0.5) films grown on prototypical Bi-based bulk topological insulators.
Surprisingly, we find an energy gap at the Fermi level indicating
superconducting correlations up to Tc ~ 6 K for one unit cell thin FeTe layers
grown on Bi2Te3 substrates, in contrast to the non-superconducting FeTe bulk
compound [8]. Moreover, SP-STS reveals that the energy gap spatially coexists
with bicollinear AFM order. This finding opens novel perspectives for
theoretical studies of competing orders in Fe-based superconductors as well as
for experimental investigations of exotic phases in heterostructures of
topological insulators and superconducting layers. | cond-mat_supr-con |
Vortex Dipole in a Trapped Atomic Bose-Einstein Condensate: We calculate the angular momentum and energy of a vortex dipole in a trapped
atomic Bose-Einstein condensate. Fully analytic expressions are obtained. We
apply the results to understand a novel phenomenon in the MIT group experiment,
an excellent agreement is achieved, and further experimental investigation is
proposed to confirm this vortex dipole mechanism. We then suggest an effective
generation and detection of vortex dipole for experimental realization.
Application of the sum rule to calculate collective mode frequency splitting is
also discussed. | cond-mat_supr-con |
Tuning orbital-selective correlation effects in superconducting
Rb$_{0.75}$Fe$_{1.6}$Se$_{2-z}$S$_z$: We report on terahertz time-domain spectroscopy on superconducting and
metallic iron chalcogenides Rb$_{0.75}$Fe$_{1.6}$Se$_{2-z}$S$_z$. The
superconducting transition is reduced from $T_c=$ 32 K ($z=0$) to 22 K
($z=1.0$), and finally suppressed ($z=1.4$) by isoelectronic substitution of Se
with S. Dielectric constant and optical conductivity exhibit a
metal-to-insulator transition associated with an orbital-selective Mott phase.
This orbital-selective Mott transition appears at higher temperature $T_{met}$
with increasing sulfur content, identifying sulfur substitution as an efficient
parameter to tune orbital-dependent correlation effects in iron-chalcogenide
superconductors. The reduced correlations of the $d_{xy}$ charge carriers can
account for the suppression of the superconductivity and the pseudogap-like
feature between $T_c$ and $T_{met}$ that was observed for $z=0$. | cond-mat_supr-con |
Unexpected impact of magnetic disorder on multiband superconductivity: We analyze how the magnetic disorder affects the properties of the two-band
$s_\pm$ and $s_{++}$ models, which are subject of hot discussions regarding
iron-based superconductors and other multiband systems like MgB$_2$. We show
that there are several cases when the transition temperature $T_c$ is not fully
suppressed by magnetic impurities in contrast to the Abrikosov-Gor'kov theory,
but a saturation of $T_c$ takes place in the regime of strong disorder. These
cases are: (1) the purely interband impurity scattering, (2) the unitary
scattering limit. We show that in the former case the $s_\pm$ gap is preserved,
while the $s_{++}$ state transforms into the $s_\pm$ state with increasing
magnetic disorder. For the case (2), the gap structure remains intact. | cond-mat_supr-con |
Electrodynamic response of MgB2 sintered pellets and thin films: We present a study of the electrodynamic response of MgB2 pellets and thin
film samples exhibiting critical temperatures ranging between 26 and 38 K. We
have performed accurate measurements of the surface impedance ZS =RS+iXS as a
function of the temperature and of the magnitude of the electromagnetic field.
The temperature variation and the field dependence of ZS was measured by a
dielectric resonator cavity technique in the microwave region. In particular,
the temperature variation of the magnetic penetration depth was also determined
in the RF region by a single coil mutual inductance method. In the case of the
films, for T<TC/2 a clear exponential behavior of the penetration depth is
observed, which can be explained by a simple BCS s-wave model with a reduced
value of the energy gap. On the contrary, pellets show no evidence of
saturation, and the experimental results strictly follow a quadratic dependence
down to the lowest temperatures. This behavior can be induced by the presence
of metallic Mg inclusions that may locally depress the gap. The analysis of the
field dependence of the surface impedance in the microwave region confirms that
the electrodynamic response of MgB2 is dominated by different sources of
dissipation, depending on the sample history, likely to be ascribed to the
predominance of grain boundaries or normal regions on its surface. | cond-mat_supr-con |
On the extraction of paramagnon excitations from resonant inelastic
X-ray scattering experiments: Resonant X-ray scattering experiments on high-temperature superconductors and
related cuprates have revealed the presence of intense paramagnon scattering at
high excitation energies, of the order of several hundred meV. The excitation
energies appear to show very similar behavior across all compounds, ranging
from magnetically ordered, via superconductors, to heavy fermion systems.
However, we argue that this apparent behavior has been inferred from the data
through model fitting which implicitly imposes such similarities. Using model
fitting that is free from such restrictions, we show that the paramagnons are
not nearly as well-defined as has been asserted previously, and that some
paramagnons might not represent propagating excitations at all. Our work
indicates that the data published previously in the literature will need to be
re-analyzed with proper models. | cond-mat_supr-con |
On the interpretation of the equilibrium magnetization in the mixed
state of high-Tc superconductors: We apply a recently developed scaling procedure to the analysis of
equilibrium magnetization M(H) data that were obtained for T-2212 and
Bi-2212single crystals and were reported in the literature. The temperature
dependencies of the upper critical field and the magnetic field penetration
depth resulting from our analysis are distinctly different from those obtained
in the original publications. We argue that theoretical models, which are
usually employed for analyses of the equilibrium magnetization in the mixed
state of type-II superconductors are not adequate for a quantitative
description of high-Tc superconductors. In addition, we demonstrate that the
scaled equilibrium magnetization M(H) curve for a Tl-2212 sample reveals a
pronounced kink, suggesting a phase transition in the mixed state. | cond-mat_supr-con |
Vortex dynamic, pinning and irreversibility field investigation in
EuRbFe4As4 superconductor: We performed systematic AC susceptibility and magnetic moment measurements to
investigate the vortex dynamics and pinning in the $EuRbFe_4As_4$ single
crystal as a function of temperature, frequency, and DC magnetic field. The
vortex solid-liquid line was determined and it fits well with
$H(T_p)=H_0(1-t_p)^\beta$ using $\beta$=1.74-1.91, for $H\parallel ab$. It
indicates a rather high pinning strength of the vortex system. The activation
energy $U_0$ was determined from thermally activated flux creep theory and
reached 6700 K at low fields, suggesting strong vortex pinning. A field
dependence of $U_0(H\parallel ab)\sim H^a$ with $a=0.47$ suggests thermally
activated plastic pinning or caused by planar defects. Magnetic moment
measurements also confirmed strong pinning in a $EuRbFe_4As_4$ superconductor
and the superconducting response gives the main contribution to the $M(H)$
hysteresis. Additionally, we found evidence of long-range magnetic interactions
in $Eu^{2+}$ sublattice and the FM-like nature of $Eu^{2+}$ atoms ordering. | cond-mat_supr-con |
Imaging the Meissner effect and flux trapping in a hydride
superconductor at megabar pressures using a nanoscale quantum sensor: By directly altering microscopic interactions, pressure provides a powerful
tuning knob for the exploration of condensed phases and geophysical phenomena.
The megabar regime represents an exciting frontier, where recent discoveries
include novel high-temperature superconductors, as well as structural and
valence phase transitions. However, at such high pressures, many conventional
measurement techniques fail. Here, we demonstrate the ability to perform local
magnetometry inside of a diamond anvil cell with sub-micron spatial resolution
at megabar pressures. Our approach utilizes a shallow layer of Nitrogen-Vacancy
(NV) color centers implanted directly within the anvil; crucially, we choose a
crystal cut compatible with the intrinsic symmetries of the NV center to enable
functionality at megabar pressures. We apply our technique to characterize a
recently discovered hydride superconductor, CeH$_9$. By performing simultaneous
magnetometry and electrical transport measurements, we observe the dual
signatures of superconductivity: local diamagnetism characteristic of the
Meissner effect and a sharp drop of the resistance to near zero. By locally
mapping the Meissner effect and flux trapping, we directly image the geometry
of superconducting regions, revealing significant inhomogeneities at the micron
scale. Our work brings quantum sensing to the megabar frontier and enables the
closed loop optimization of superhydride materials synthesis. | cond-mat_supr-con |
Appearance of fluctuating stripes at the onset of the pseudogap in the
high-Tc Superconductor Bi2Sr2CaCu2O8+x: Doped Mott insulators have been shown to have a strong propensity to form
patterns of holes and spins often referred to as stripes. In copper-oxides,
doping also gives rise to the pseudogap state, which transforms into a high
temperature superconductor with sufficient doping or by reducing the
temperature. A long standing question has been the interplay between pseudogap,
which is generic to all hole-doped cuprates, and stripes, whose static form
occurs in only one family of cuprates over a narrow range of the phase diagram.
Here we examine the spatial reorganization of electronic states with the onset
of the pseudogap state at T* in the high-temperature superconductor
Bi2Sr2CaCu2O8+x using spectroscopic mapping with the scanning tunneling
microscope (STM). We find that the onset of the pseudogap phase coincides with
the appearance of electronic patterns that have the predicted characteristics
of fluctuating stripes. As expected, the stripe patterns are strongest when the
hole concentration in the CuO2 planes is close to 1/8 (per Cu). While
demonstrating that the fluctuating stripes emerge with the onset of the
pseudogap state and occur over a large part of the cuprate phase diagram, our
experiments indicate that they are a consequence of pseudogap behavior rather
than its cause. | cond-mat_supr-con |
Doping-Dependent and Orbital-Dependent Band Renormalization in
Ba(Fe_1-xCo_x)_2As_2 Superconductors: Angle resolved photoemission spectroscopy of Ba(Fe1-xCox)2As2 (x = 0.06,
0.14, and 0.24) shows that the width of the Fe 3d yz/zx hole band depends on
the doping level. In contrast, the Fe 3d x^2-y^2 and 3z^2-r^2 bands are rigid
and shifted by the Co doping. The Fe 3d yz/zx hole band is flattened at the
optimal doping level x = 0.06, indicating that the band renormalization of the
Fe 3d yz/zx band correlates with the enhancement of the superconducting
transition temperature. The orbital-dependent and doping-dependent band
renormalization indicates that the fluctuations responsible for the
superconductivity is deeply related to the Fe 3d orbital degeneracy. | cond-mat_supr-con |
Groove-rolling as an alternative process to fabricate Bi-2212 wires for
practical applications: Bi2Sr2CaCu2O8+x (Bi-2212) superconducting long-length wires are mainly
limited in obtaining high critical currents densities (JC) by the internal gas
pressure generated during the heat treatment, which expands the wire diameter
and dedensifies the superconducting filaments. Several ways have been developed
to increase the density of the superconducting filaments and therefore
decreasing the bubble density: much higher critical currents have been reached
always acting on the final as-drawn wires. We here try to pursue the same goal
of having a denser wire by acting on the deformation technique, through a
partial use of the groove-rolling at different wire processing stages. Such
technique has a larger powders compaction power, is straightforwardly adaptable
to long length samples, and allows the fabrication of samples with round,
square or rectangular shape depending on the application requirements. In this
paper we demonstrate the capability of this technique to increase the density
in Bi-2212 wires which leads to a three-fold increase in Jc with respect to
drawn wires, making this approach very promising for fabricating Bi-2212 wires
for high magnetic field magnets, i.e. above 25 T. | cond-mat_supr-con |
Determining the absolute value of magnetic penetration depth in
small-sized superconducting films: In the previous four decades, a two-coil mutual inductance (MI) technique has
been widely employed in characterizing magnetic penetration depth, $\lambda$,
of superconducting films. However, the conventional methods used to obtain
$\lambda$ are not applicable to small-sized films with common shapes, which
limits the application of the MI technique in superconductivity research. Here,
we first employed the fast wavelet collocation (FWC) method to a two-coil
system and then proposed the possibility of directly obtaining the absolute
$\lambda$ of polygonal superconducting films with arbitrary sizes. To verify
its accuracy and advantages, we extracted the $\lambda$ values of square NbN
films with different sizes using the FWC and conventional flux leakage
subtraction (FLS) methods. Notably, the FLS method fails for a $5\times 5 \,
\rm mm^2$ film, which is attributed to the significant current peak at the film
edge. In contrast, the absolute $\lambda$ extracted using the FWC method was
independent of the film size. Finally, we established the applicability of the
FWC method to large coil spacings, which may pave the way for integrating
high-accuracy $\lambda$ measurements with the ionic liquid gating technique. | cond-mat_supr-con |
Type-II Ising superconductivity in two-dimensional materials with strong
spin-orbit coupling: Recent discovery of Ising superconductivity protected against in-plane
magnetic field by spin-orbit coupling (SOC) has stimulated intensive research
interests. The effect, however, was only expected to appear in two-dimensional
(2D) noncentrosymmetric materials with spin-valley locking. In this work, we
proposed a new type of Ising superconductivity in 2D materials with $C_{nz}$
rotational symmetry ($n=3,4,6$). This mechanism, dubbed as type-II Ising
superconductivity, is applicable for centrosymmetric materials. Type-II Ising
superconductivity relies on the SOC-induced spin-orbital locking characterized
by Ising-type Zeeman-like fields displaying opposite signs for opposing
orbitals. We found that type-II Ising superconductivity are most prominent
around time-reversal invariant momenta and is not sensitive to inversion
symmetry breaking. By performing high-throughput first-principles calculations,
about one hundred candidate materials were identified. Our work significantly
enriches the physics and materials of Ising superconductor, opening new
opportunities for fundamental research and practical applications of 2D
materials. | cond-mat_supr-con |
Plain s-wave superconductivity in single-layer FeSe on SrTiO3 probed by
scanning tunneling microscopy: Single-layer FeSe film on SrTiO3(001) was recently found to be the champion
of interfacial superconducting systems, with a much enhanced superconductivity
than the bulk iron-based superconductors. Its superconducting mechanism is of
great interest. Although the film has a simple Fermi surface topology, its
pairing symmetry is unsettled. Here by using low-temperature scanning tunneling
microscopy (STM), we systematically investigated the superconductivity of
single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling
spectrum and magnetic vortex lattice in the film. Quasi-particle interference
(QPI) patterns reveal scatterings between and within the electron pockets, and
put constraints on possible pairing symmetries. By introducing impurity atoms
onto the sample, we show that the magnetic impurities (Cr, Mn) can locally
suppress the superconductivity but the non-magnetic impurities (Zn, Ag and K)
cannot. Our results indicate that single-layer FeSe/SrTiO3 has a plain s-wave
paring symmetry whose order parameter has the same phase on all Fermi surface
sections. | cond-mat_supr-con |
Surface Specific Heat of $^{3}$He and Andreev Bound States: High resolution measurements of the specific heat of liquid $^{3}$He in the
presence of a silver surface have been performed at temperatures near the
superfluid transition in the pressure range of 1 to 29 bar. The surface
contribution to the heat capacity is identified with Andreev bound states of
$^{3}$He quasiparticles that have a range of half a coherence length. | cond-mat_supr-con |
Universal scaling in BCS superconductivity in two dimensions in non-s
waves: The solutions of a renormalized BCS model are studied in two space dimensions
in $s$, $p$ and $d$ waves for finite-range separable potentials. The gap
parameter, the critical temperature $T_c$, the coherence length $\xi$ and the
jump in specific heat at $T_c$ as a function of zero-temperature condensation
energy exhibit universal scalings. In the weak-coupling limit, the present
model yields a small $\xi$ and large $T_c$ appropriate to those for high-$T_c$
cuprates. The specific heat, penetration depth and thermal conductivity as a
function of temperature show universal scaling in $p$ and $d$ waves. | cond-mat_supr-con |
Temperature dependence of clusters with attracting vortices in
superconducting Niobium studied by neutron scattering: We have investigated the intermediate mixed state of a superconducting
niobium sample by Very Small Angle Neutron Scattering.We show that this state
is stabilized through a sequence where a regular vortex lattice appears, which
then coexists with vortex clusters before vanishing at low temperature.
Vortices in clusters have a constant periodicity regardless of the applied
field, exhibit a temperature dependence close to the one of the penetration
depth. The clusters disappear in the high temperature limit. All the results
agree with an explanation in terms of vortex attraction due non local effects,
and indicate a negligible role of pinning. Phase coexistence between Abrikosov
vortex lattice and vortex clusters is reported showing the first order nature
of the boundary line. | cond-mat_supr-con |
Superfluid density of Ba(Fe$_{1-x}M_x$)$_2$As$_2$ from optical
experiments: The temperature dependence of the $ab$-plane optical reflectivity of
Ba(Fe$_{0.92}$Co$_{0.08})_2$As$_{2}$ and Ba(Fe$_{0.95}$Ni$_{0.05})_2$As$_{2}$
single crystals is measured in a wide spectral range. Upon entering the
superconducting regime, the reflectivity in both compounds increases
considerably at low frequency, leading to a clear gap in the optical
conductivity below 100 cm$^{-1}$. From the analysis of the complex conductivity
spectra we obtain the penetration depth $\lambda(T)=(3500\pm 350)$ \AA for
Ba(Fe$_{0.92}$Co$_{0.08})_2$As$_{2}$ and $(3000\pm 300)$ \AA for
Ba(Fe$_{0.95}$Ni$_{0.05})_2$As$_{2}$. The calculated superfluid density
$\rho_s$ of both compounds nicely fits the scaling relation $\rho_s=(125\pm
25)\sigma_{dc}T_c$. | cond-mat_supr-con |
Enhanced superconductivity and superconductor to insulator transition in
nano-crystalline molybdenum thin films: Disorder driven superconductor to insulator transition via intermediate
metallic regime is reported in nano-crystalline thin films of molybdenum. The
nano-structured thin films have been deposited at room temperature using DC
magnetron sputtering at different argon pressures. The grain size has been
tuned using deposition pressure as the sole control parameter. A variation of
particle sizes, room temperature resistivity and superconducting transition has
been studied as a function of deposition pressure. The nano-crystalline
molybdenum thin films are found to have large carrier concentration but very
low mobility and electronic mean free path. Hall and conductivity measurements
have been used to understand the effect of disorder on the carrier density and
mobilities. Ioffe-Regel parameter is shown to correlate with the continuous
metal-insulator transition in our samples. | cond-mat_supr-con |
Imbalanced Superfluid Phase of a Trapped Fermi Gas in the BCS-BEC
Crossover Regime: We theoretically investigate the ground state of trapped neutral fermions
with population imbalance in the BCS-BEC crossover regime. On the basis of the
single-channel Hamiltonian, we perform full numerical calculations of the
Bogoliubov-de Gennes equation coupled with the regularized gap and number
equations. The zero-temperature phase diagram in the crossover regime is
presented, where the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing state
governs the weak-coupling BCS region of a resonance. It is found that the FFLO
oscillation vanishes in the BEC side, in which the system under population
imbalance turns into a phase separation (PS) between locally binding superfluid
and fully polarized spin domains. We also demonstrate numerical calculations
with a large particle number O(10^5), comparable to that observed in recent
experiments. The resulting density profile on a resonance yields the PS, which
is in good agreement with the recent experiments, while the FFLO modulation
exists in the pairing field. It is also proposed that the most favorable
location for the detection of the FFLO oscillation is in the vicinity of the
critical population imbalance in the weak coupling BCS regime, where the
oscillation periodicity becomes much larger than the interparticle spacing.
Finally, we analyze the radio-frequency (RF) spectroscopy in the imbalanced
system. The clear difference in the RF spectroscopy between BCS and BEC sides
reveals the structure of the pairing field and local ``magnetization''. | cond-mat_supr-con |
Turning insulators into superconductors: The recent observation by Batlogg and colleagues of superconductivity in an
organic field-effect transistor is reviewed. | cond-mat_supr-con |
Momentum-Resolved Electronic Relaxation Dynamics in D-wave
Superconductors: Motivated by recent development in time-resolved angle-resolved photoemission
spectroscopy (trARPES) for d-wave superconductors, we analyze the
non-equilibrium relaxation dynamics of the laser pulse excited sample within
the scenario of two-temperature model. It is shown that the main features
reported in the trARPES technique may be understood within this
phenomenological picture. The momentum dependence of the excited quasiparticle
density and the relaxation rate is associated with the dynamics of the nodal
d-wave superconducting gap, and the fluence dependence of the relaxation rate
is related to the recombination process of quasiparticles into Cooper pairs. | cond-mat_supr-con |
Increasing d-wave superconductivity by on site repulsion: We study by Variational Monte Carlo an extended Hubbard model away from half
filled band density which contains two competing nearest-neighbor interactions:
a superexchange $J$ favoring d-wave superconductivity and a repulsion $V$
opposing against it. We find that the on-site repulsion $U$ effectively
enhances the strength of $J$ meanwhile suppressing that of $V$, thus favoring
superconductivity. This result shows that attractions which do not involve
charge fluctuations are very well equipped against strong electron-electron
repulsion so much to get advantage from it. | cond-mat_supr-con |
Metal-to-Insulator Crossover in YBa_{2}Cu_{3}O_{y} Probed by
Low-Temperature Quasiparticle Heat Transport: It was recently demonstrated that in La_{2-x}Sr_{x}CuO_{4} the magnetic-field
(H) dependence of the low-temperature thermal conductivity \kappa up to 16 T
reflects whether the normal state is a metal or an insulator. We measure the H
dependence of \kappa in YBa_{2}Cu_{3}O_{y} (YBCO) at subkelvin temperatures for
a wide doping range, and find that at low doping the \kappa(H) behavior
signifies the change in the ground state in this system as well. Surprisingly,
the critical doping is found to be located deeply inside the underdoped region,
about the hole doping of 0.07 hole/Cu; this critical doping is apparently
related to the stripe correlations as revealed by the in-plane resistivity
anisotropy. | cond-mat_supr-con |
Impurity states in multiband s-wave superconductors: analysis of iron
pnictides: We examine the effect of a single, non-magnetic impurity in a multiband,
extended s-wave superconductor allowing for anisotropy of the gaps on the Fermi
surfaces. We derive analytic expressions for the Green's functions in the
continuum and analyse the conditions for the existence of sharp
impurity-induced resonant states. Underlying band structure is more relevant
for the multiband than for single band case, and mismatch between the bands
generically makes the formation of the impurity states less likely in the
physical regime of parameters. We confirm these conclusions by numerically
solving the impurity problem in a tight-binding parameterization of the bands
relevant to pnictide superconductors. | cond-mat_supr-con |
Electronic Structure, Localization and Spin-State Transition in
Cu-substituted FeSe: Fe$_{1-x}$Cu$_x$Se: We report density functional studies of the Fe$_{1-x}$Cu$_x$Se alloy done
using supercell and coherent potential approximation methods. Magnetic behavior
was investigated using the disordered local moment approach. We find that Cu
occurs in a nominal $d^{10}$ configuration and is highly disruptive to the
electronic structure of the Fe sheets. This would be consistent with a metal
insulator transition due to Anderson localization. We further find a strong
cross over from a weak moment itinerant system to a local moment magnet at $x
\approx 0.12$. We associate this with the experimentally observed jump near
this concentration. Our results are consistent with the characterization of
this concentration dependent jump as a transition to a spin-glass. | cond-mat_supr-con |
Implementation of Grover search algorithm with Josephson charge qubits: A scheme of implementing the Grover search algorithm based on Josephson
charge qubits has been proposed, which would be a key step to scale more
complex quantum algorithms and very important for constructing a real quantum
computer via Josephson charge qubits. The present scheme is simple but fairly
efficient, and easily manipulated because any two-charge-qubit can be
selectively and effectively coupled by a common inductance. More manipulations
can be carried out before decoherence sets in. Our scheme can be realized
within the current technology. | cond-mat_supr-con |
Implications evinced by the phase diagram, anisotropy, magnetic
penetration depths, isotope effects and conductivities of cuprate
superconductors: Anisotropy, thermal and quantum fluctuations and their dependence on dopant
concentration appear to be present in all cuprate superconductors, interwoven
with the microscopic mechanisms responsible for superconductivity. Here we
review anisotropy, in-plane and c-axis penetration depths, isotope effect and
conductivity measurements to reassess the universal behavior of cuprates as
revealed by the doping dependence of these phenomena and of the transition
temperature. | cond-mat_supr-con |
Distinct Pairing Symmetries in $Nd_{1.85}Ce_{0.15}CuO_{4-y}$ and
$La_{1.89}Sr_{0.11}CuO_{4}$ Single Crystals: Evidence from Comparative
Tunnelling Measurements: We used point-contact tunnelling spectroscopy to study the superconducting
pairing symmetry of electron-doped $Nd_{1.85}Ce_{0.15}CuO_{4-y}$ (NCCO) and
hole-doped $La_{1.89}Sr_{0.11}CuO_{4}$ (LSCO). Nearly identical spectra without
zero bias conductance peak (ZBCP) were obtained on the (110) and (100) oriented
surfaces (the so-called nodal and anti-nodal directions) of NCCO. In contrast,
LSCO showed a remarkable ZBCP in the nodal direction as expected from a d-wave
superconductor. Detailed analysis reveals an s-wave component in the pairing
symmetry of the NCCO sample with $\Delta/k_BT_c=1.66$, a value remarkable close
to that of a weakly coupled BCS superconductor. We argue that this s-wave
component is formed at the Fermi surface pockets centered at ($\pm\pi$,0) and
(0,$\pm\pi$) although a d-wave component may also exist. | cond-mat_supr-con |
Possible observation of energy level quantization in an intrinsic
Josephson junction: Energy level quantization (ELQ) is studied to clarify the macroscopic quantum
dynamics of the d-wave Josephson junction (JJ). The influences of the nodal
quasiparticles of d-wave superconductivity on the damping effect are
numerically evaluated on the basis of a phenomenological model. The
calculation, based on realistic parameters for a Bi2Sr2CaCu2O8+d (Bi2212)
intrinsic JJ, shows that the observation of ELQ is possible when the sweep rate
of the bias current exceeds 10 A/sec. High-sweep- rate measurements (121A/sec)
performed on a Bi2212 intrinsic JJ result in the appearance of multiple peaks
in the switching current distribution suggesting the realization of ELQ in the
d-wave JJ. | cond-mat_supr-con |
s+is State with Broken Time Reversal Symmetry in Fe-Based
Superconductors: We analyze the evolution of the superconducting gap structure in strongly
hole doped Ba_{1-x}K_xFe_2As_2 between x=1 and x ~ 0.4 (optimal doping). In the
latter case, the pairing state is most likely s+-, with different gap signs on
hole and electron pockets, but with the same signs of the gap on the two
Gamma-centered hole pockets (a ++ state on hole pockets). In a pure KFe_2As_2
(x=1), which has only hole pockets, laser ARPES data suggested another s+-
state, in which the gap changes sign between hole pockets (a +- state). We
analyze how ++ gap transforms into a +- gap as x -> 1. We found that this
transformation occurs via an intermediate s+is, state in which the gaps on the
two hole pockets differ in phase by `phi', which gradually involves from `phi'
= pi (the +- state) to phi =0 (the ++ state). This state breaks time-reversal
symmetry and has huge potential for applications. We compute the dispersion of
collective excitations and show that two different Leggett-type phase modes
soften at the two end points of TRSB state. | cond-mat_supr-con |
Supergap and subgap enhanced currents in asymmetric {S_1FS_2} Josephson
junctions: We have theoretically studied the supercurrent profiles in three-dimensional
normal metal and ferromagnetic Josephson configurations, where the magnitude of
the superconducting gaps in the superconducting leads are unequal, i.e.,
$\Delta_1\neq \Delta_2$, creating asymmetric $S_1NS_2$ and $S_1FS_2$ systems.
Our results reveal that by increasing the ratio of the superconducting gaps
$\Delta_2/\Delta_1$, the critical supercurrent in a ballistic $S_1NS_2$ system
can be enhanced by more than $100\%$, and reaches a saturation point, or decays
away, depending on the junction thickness, magnetization strength, and chemical
potential. The total critical current in a diffusive $S_1NS_2$ system was found
to be enhanced by more than $50\%$ parabolically, and reaches saturation by
increasing one of the superconducting gaps. In a uniform ferromagnetic
junction, the supercurrent undergoes reversal by increasing
$\Delta_2/\Delta_1>1$. Through decomposing the total supercurrent into its
supergap and subgap components, our results illustrate their crucial relative
contributions to the Josephson current flow. It was found that the competition
of subgap and supergap currents in a $S_1FS_2$ junction results in the
emergence of second harmonics in the current-phase relation. In contrast to a
diffusive asymmetric Josephson configuration, the behavior of the supercurrent
in a ballistic system with $\Delta_2/\Delta_1=1$ can be properly described by
the subgap current component only, in a wide range of parameter sets, including
Fermi level mismatch, magnetization strength, and junction thickness.
Interestingly, when $\Delta_2/\Delta_1>1$, our results have found multiple
parameter sets where the total supercurrent is driven by the supergap
component. Therefore, our comprehensive study highlights the importance of
subgap and supergap supercurrent components in both the ballistic and diffusive
regimes. | cond-mat_supr-con |
Fluctuation, insulation and superconductivity: the pressure-dependent
phase-diagram of Rb$_2$Mo$_6$Se$_6$: The quasi-one-dimensional (q1D) material Rb$_2$Mo$_6$Se$_6$ has been proposed
to display a nontrivial combination of low-dimensional fluctuations and a
dynamical charge density wave (CDW) at ambient pressure. This may lead to a
progressive metal to insulator cross over at low temperature. To explore the
link between the crystal dimensionality and this insulating instability, we
have performed hydrostatic pressure-dependent electrical transport measurements
on single crystals of Rb$_2$Mo$_6$Se$_6$. At low pressure, we observe
thermally-activated behaviour consistent with a temperature-dependent gap
$E_g(T)$ opening below a characteristic temperature $T_{Rmin}$. Upon increasing
the pressure $T_{Rmin}$ initially rises, indicating a reinforcement of the low
temperature insulating state despite a continuous reduction in $E_g(P)$. We
interpret this as a signature of suppressed fluctuations as the dimensionality
of the electronic structure rises. However, $T_{Rmin}$ drops above 8.8 GPa and
superconductivity emerges at 12 GPa. Between 12-24.2 GPa the superconducting
and insulating instabilities coexist, with superconductivity surviving up to
the maximum attained pressure (52.8 GPa). Analysis of the magneto-transport
reveals two distinct regions: at high pressures the anisotropy gradually falls
and the superconducting state appears unremarkable. In contrast, coexistence
with the gapped insulating phase creates a superconducting dome. The emergence
of a peak in the critical temperature Tc despite the depleted density of states
is indicative of enhanced coupling. Our journey from the extreme 1D to 3D
limits in this prototypical q1D metal reveals an intriguing relationship
between superconducting and insulating ground states which is simultaneously
competitive and symbiotic. | cond-mat_supr-con |
Physical Properties of Metallic Antiferromagnetic CaCo{1.86}As2 Single
Crystals: We report studies of CaCo{1.86}As2 single crystals. The electronic structure
is probed by angle-resolved photoemission spectroscopy (ARPES) measurements of
CaCo{1.86}As2 and by full-potential linearized augmented-plane-wave
calculations for the supercell Ca8Co15As16 (CaCo{1.88}As2). Our XRD crystal
structure refinement is consistent with the previous combined refinement of
x-ray and neutron powder diffraction data showing a collapsed-tetragonal
ThCr2Si2-type structure with 7(1)% vacancies on the Co sites corresponding to
the composition CaCo{1.86}As2 [D. G. Quirinale et al., Phys. Rev. B 88, 174420
(2013)]. The anisotropic magnetic susceptibility chi(T) data are consistent
with the magnetic neutron diffraction data of Quirianale et al. that
demonstrate the presence of A-type collinear antiferromagnetic order below the
Neel temperature TN = 52(1) K with the easy axis being the tetragonal c axis.
However, no clear evidence from the resistivity rho(T) and heat capacity Cp(T)
data for a magnetic transition at TN is observed. A metallic ground state is
demonstrated from band calculations and the rho(T), Cp(T) and ARPES data, and
spin-polarized calculations indicate a competition between the A-type AFM and
FM ground states. The Cp(T) data exhibit a large Sommerfield electronic
coefficient reflecting a large density of states at the Fermi energy D(EF),
consistent with the band structure calculations which also indicate a large
D(EF) arising from Co 3d bands. At 1.8 K the M(H) data for H|| c exhibit a
well-defined first-order spin-flop transition at an applied field of 3.5 T. The
small ordered moment of 0.3 muB/Co obtained from the M(H) data at low T, the
large exchange enhancement of chi and the lack of a self-consistent
interpretation of the chi(T) and M(H,T) data in terms of a local moment
Heisenberg model together indicate that the magnetism of CaCo{1.86}As2 is
itinerant. | cond-mat_supr-con |
Finite energy spin fluctuation as a pairing glue in systems with
coexisting electron and hole bands: We study, within the fluctuation exchange approximation, the
spin-fluctuation-mediated superconductivity in Hubbard-type models possessing
electron and hole bands, and compare them with a model on a square lattice with
a large Fermi surface. In the square lattice model, superconductivity is more
enhanced for better nesting for a fixed band filling. By contrast, in the
models with electron and hole bands, superconductivity is optimized when the
Fermi surface nesting is degraded to some extent, where finite energy spin
fluctuation around the nesting vector develops. The difference lies in the
robustness of the nesting vector, namely, in models with electron and hole
bands, the wave vector at which the spin susceptibility is maximized is fixed
even when the nesting is degraded, whereas when the Fermi surface is large, the
nesting vector varies with the deformation of the Fermi surface. We also
discuss the possibility of realizing in actual materials the bilayer Hubbard
model, which is a simple model with electron and hole bands, and is expected to
have a very high T_c. | cond-mat_supr-con |
High-Density Superconductive Logic Circuits Utilizing 0 and $π$
Josephson Junctions: Superconductor Electronics (SCE) is a fast and power-efficient technology
with great potential for overcoming conventional CMOS electronics' scaling
limits. Nevertheless, the primary challenge confronting SCE today pertains to
its integration level, which lags several orders of magnitude behind CMOS
circuits. In this study, we have innovated and simulated a novel logic family
grounded in the principles of phase shifts occurring in 0 and $\pi$ Josephson
junctions. The fast phase logic (FPL) eliminates the need for large inductor
loops and shunt resistances by combining the half-flux and phase logic.
Therefore, the Josephson junction (JJ) area only limits the integration
density. The cells designed with this paradigm are fast, and the clock-to-Q
delay is about 4ps while maintaining over 50% parameter margins. This logic is
power efficient and can increase the integration by at least 100$\times$ in the
SCE chips. | cond-mat_supr-con |
The direct Cu NQR Study of the Stripe Phase in the Lanthanum Cuprates: Using Cu NQR in Eu-doped La_(2-x)Sr_xCuO_4 we find the evidence of the pinned
stripe phase at 1.3K for 0.08<x<0.18. The pinned fraction increases by one
order of magnitude near hole doping x=1/8. The NQR lineshape reveals three
inequivalent Cu positions. A dramatic change of the NQR signal for x > 0.18
correlating with the onset of bulk superconductivity corresponds to the
depinning of the stripe phase. | cond-mat_supr-con |
Non-monotonic behaviour of the superconducting order parameter in
Nb/PdNi bilayers observed through point contact spectroscopy: Point contact spectroscopy measurements have been performed on Nb/PdNi
bilayers in which the thickness of the Nb layer, dNb, was kept constant to 40
nm while the thickness of PdNi, dPdNi, was changed from 2 nm to 9 nm. Features
related to the superconducting gap induced in the ferromagnet have been
observed in the dV/dI versus V curves. These structures show a non-monotonic
behaviour as a function of dPdNi as a consequence of the damped oscillatory
behaviour of the superconducting order parameter in the ferromagnetic layer. | cond-mat_supr-con |
Approaching ideal rectification in superconducting diodes through
multiple Andreev reflections: We analyze the rectification properties of voltage-biased Josephson junctions
exhibiting the superconducting diode effect. Taking into account multiple
Andreev reflection (MAR) processes in our scattering theory, we consider a
short weak link of arbitrary transparency between two superconductors with
finite Cooper pair momentum $2q$. In equilibrium, the diode efficiency is
bounded from above in this model, with maximal efficiency $\eta_0\approx 0.4$.
Out of equilibrium, we find a rich subharmonic structure in the current-voltage
curve. For high transparency and low bias voltage $V$, the rectification
efficiency $\eta(V)$ approaches the ideal value $\eta=1$ for $q\xi\to 1$ (with
coherence length $\xi$). | cond-mat_supr-con |
Size driven phase transitions in pinned vortex systems: We model a tridimensional vortex system in a sample with square superficial
pinning in the top surface and obtain the ground state structures as a function
of the sample thickness. Using a simple Frenkel-Kontorova like model and no
adjustable parameters, we reproduce the experimental vortex configurations seen
in the bottom surface and their range of stability. We find three phases with
two transitions between them, including a continuous one from square to
distorted hexagonal structure and a discontinuous one from distorted hexagonal
to hexagonal structure. | cond-mat_supr-con |
Epitaxial Growth of NdFeAsO Thin Films by Molecular Beam Epitaxy: Epitaxial films of NdFeAsO were grown on GaAs substrates by molecular beam
epitaxy (MBE). All elements including oxygen were supplied from solid sources
using Knudsen cells. The x-ray diffraction pattern of the film prepared with
the optimum growth condition showed no indication of impurity phases. Only
(00l) peaks were observed, indicating that NdFeAsO was grown with the c-axis
perpendicular to the substrate. The window of optimum growth condition was very
narrow, but the NdFeAsO phase was grown with a very good reproducibility.
Despite the absence of any appreciable secondary phase, the resistivity showed
an increase with decreasing temperature. | cond-mat_supr-con |
Absence of the Pauli-Paramagnetic Limit in a Superconducting U$_6$Co: We performed $^{59}$Co nuclear magnetic resonance (NMR) measurements of
single-crystalline U$_6$Co. There is a small decrease in the Knight shift in
the superconducting (SC) state, but this change mainly arises from the SC
diamagnetic effect. The negligible change of the spin part of the Knight shift,
together with the absence of the Pauli-paramagnetic effect in the SC U$_6$Co,
is understood as a consequence of the small spin susceptibility. The nuclear
spin-lattice relaxation rate $1/T_1$ is also measured in the SC state under the
magnetic field, and exhibits a tiny Hebel-Slichter peak just below the SC
transition temperature and exponential behavior at lower temperatures. These
behaviors are in agreement with the full-gap s-wave pairing in U$_6$Co. | cond-mat_supr-con |
Theory of the Resistive Transition in Overdoped $Tl_2Ba_2CuO_{6+x}$:
Implications for the angular dependence of the quasiparticle scattering rate
in High-$T_c$ superconductors: We show that recent measurements of the magnetic field dependence of the
magnetization, specific heat and resistivity of overdoped $T_c \sim 17K$
$Tl_{2}Ba_{2}CuO_{6+\delta}$ in the vicinity of the superconducting $H_{c2}$
imply that the vortex viscosity is anomalously small and that the material
studied is inhomogeneous with small, a few hundred $\AA$, regions in which the
local $T_{c}$ is much higher than the bulk $T_{c}$. The anomalously small
vortex viscosity can be derived from a microscopic model in which the
quasiparticle lifetime varies dramatically around the Fermi surface, being
small everywhere except along the zone diagonal (``cold spot''). We propose
experimental tests of our results. | cond-mat_supr-con |
Ultimate on-chip quantum amplifier: We report amplification of electromagnetic waves by a single artificial atom
in open 1D space. Our three-level artificial atom -- a superconducting quantum
circuit -- coupled to a transmission line presents an analog of a natural atom
in open space. The system is the most fundamental quantum amplifier whose gain
is limited by a spontaneous emission mechanism. The noise performance is
determined by the quantum noise revealed in the spectrum of spontaneous
emission, also characterized in our experiments. | cond-mat_supr-con |
Highly responsive Y-Ba-Cu-O thin film THz detectors with picosecond time
resolution: High-temperature superconducting YBa2Cu3O7-d (YBCO) thin-film detectors with
improved responsivities were developed for fast time-domain measurements in the
THz frequency range. YBCO thin films of 30 nm thickness were patterned to
micro- and nanobridges and embedded into planar log-spiral THz antennas. The
YBCO thin-film detectors were characterized with continuous wave radiation at
0.65 THz. Responsivity values as high as 710 V/W were found for the YBCO
nanobridges. Pulsed measurements in the THz frequency range were performed at
the electron storage ring ANKA from the Karlsruhe Institute of Technology
(KIT). Due to the high responsivities of the nanobridges no biasing was
required for the detection of the coherent synchrotron radiation pulses
achieving very good agreement between the measured pulse shapes and
simulations. | cond-mat_supr-con |
Stripe-like Inhomogeneities, Coherence, and the Physics of the High Tc
Cuprates: The carriers in the high-Tc cuprates are found to be polaron-like "stripons"
carrying charge and located in stripe-like inhomogeneities, "quasi-electrons"
carrying charge and spin, and "svivons" carrying spin and some lattice
distortion. The anomalous spectroscopic and transport properties of the
cuprates are understood. The stripe-like inhomogeneities result from the Bose
condensation of the svivon field, and the speed of their dynamics is determined
by the width of the double-svivon neutron-resonance peak. The connection of
this peak to the peak-dip-hump gap structure observed below Tc emerges
naturally. Pairing results from transitions between pair states of stripons and
quasi-electrons through the exchange of svivons. The pairing symmetry is of the
d_{x^2-y^2} type; however, sign reversal through the charged stripes results in
features not characteristic of this symmetry. The phase diagram is determined
by pairing and coherence lines within the regime of a Mott transition.
Coherence without pairing results in a Fermi-liquid state, and incoherent
pairing results in the pseudogap state where localized electron and electron
pair states exist within the Hubbard gap. A metal-insulator-transition quantum
critical point occurs between these two states at T=0 when the superconducting
state is suppressed. An intrinsic heterogeneity is expected of superconducting
and pseudogap nanoscale regions. | cond-mat_supr-con |
A second phase transition and superconductivity in the beta-pyrochlore
oxide KOs2O6: Another phase transition that is probably of first order is found in the
beta-pyrochlore oxide superconductor KOs2O6 with a superconducting transition
temperature Tc of 9.6 K. It takes place at Tp=7.5 K in the superconducting
state in a zero magnetic field. By applying magnetic fields of up to 140 kOe,
the Tc gradually decreased to 5.2 K, while Tp changed little, eventually
breaking through the Hc2 line at approximately 65 kOe in the H-T diagram. Both
the normal-state resistivity and Hc2 change slightly but significantly across
the second phase transition. It is suggested that the transition is associated
with the rattling of potassium ions located in an oversized cage of osmium and
oxide ions. | cond-mat_supr-con |
One-electron scattering rate and normal-state linear-$T$ resistivity of
the cuprates: Here we use a description of the electronic correlations contained in the
Hubbard model on the square-lattice perturbed by very weak three-dimensional
uniaxial anisotropy in terms of the residual interactions of charge $c$
fermions and spin-neutral composite two-spinon $s1$ fermions. Excellent
quantitative agreement with the anisotropic linear-$\omega$ one-electron
scattering rate and normal-state linear-$T$ resistivity observed in experiments
on hole-doped cuprates with critical concentrations $x_c\approx 0.05$ and
$x_*\approx 0.27$ is achieved. Our results provide strong evidence that the
normal-state linear-$T$ resistivity is a manifestation of low-temperature
scale-invariant physics. | cond-mat_supr-con |
Weyl superconductor phases in a Weyl-semimetal/superconductor multilayer: Topologically nontrivial superconducting phases have been engineered in
topological materials by the proximity effect in contact with conventional
superconductors. In this paper, by using the method of the Kronig-Penney model,
we study the superconducting proximity effect in the bulk electronic states of
Weyl semimetals by considering a multilayer structure consisting of
Weyl-semimetal and superconductor layers. Due to the proximity effect, two Weyl
nodes are decoupled into four nodes of Majorana fermions resulting in
Weyl-superconductor phases or three-dimensional extension of
topological-superconductor phases. We find that mismatch of the Fermi velocity
and potential barriers at the interface gap out Majorana nodes, thus turn
Weyl-superconductor phases with four Majorana nodes into Weyl-superconductor
phases with half of Majorana nodes and topological-superconductor phases with
odd integer Chern numbers. | cond-mat_supr-con |
Nodal band-off-diagonal superconductivity in twisted graphene
superlattices: The superconducting state and mechanism are among the least understood
phenomena in twisted graphene systems. For instance, recent tunneling
experiments indicate a transition between nodal and gapped pairing with
electron filling, which is not naturally understood within current theory. We
demonstrate that the coexistence of superconductivity and flavor polarization
leads to pairing channels that are guaranteed by symmetry to be entirely
band-off-diagonal, with a variety of unusual consequences: most notably, the
pairing invariant under all symmetries can have protected nodal lines or be
fully gapped, depending on parameters, and the band-off-diagonal chiral d-wave
state exhibits transitions between gapped and nodal regions upon varying the
chemical potential. We demonstrate that nodal band-off-diagonal pairing can be
the leading state when only phonons are considered, and is also uniquely
favored by fluctuations of a time-reversal-symmetric intervalley-coherent order
motivated by recent experiments. Consequently, band-off-diagonal
superconductivity allows for the reconciliation of several key experimental
observations in graphene moir\'e systems. | cond-mat_supr-con |
Critical Fields and Anisotropy of NdO0.82F0.18FeAs Single Crystals: The newly discovered iron-based superconductors have stimulated enormous
interests in the field of superconductivity. Since the new superconductor is a
layered system, the anisotropy is a parameter with the first priority to know.
Meanwhile any relevant message about the critical fields (upper critical field
and irreversibility line) are essentially important. By using flux method, we
have successfully grown the single crystals NdO0.82F0.18FeAs at ambient
pressure. Resistive measurements reveal a surprising discovery that the
anisotropy \Gamma = (mc/mab)^{1/2} is below 5, which is much smaller than the
theoretically calculated results. The data measured up to 400 K show a
continuing curved feature which prevents a conjectured linear behavior for an
unconventional metal. The upper critical fields determined based on the
Werthamer-Helfand-Hohenberg formula are H_{c2}^{H||ab}(T=0 K) = 304 T and
H_{c2}^{H||c}(T=0 K)=62-70 T, indicating a very encouraging application of the
new superconductors. | cond-mat_supr-con |
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