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Thermal and electrical conductivity of Fermi pocket models of underdoped
cuprates: Several models of the electronic spectrum in the pseudogap state of
underdoped cuprates have been proposed to explain ARPES and STM measurements,
which reveal only truncated Fermi pockets instead of a full metallic Fermi
surface. We consider the transport properties expected of four physically
distinct models, and calculate the thermal and electrical conductivity of the
electronic quasiparticles. By proposing transport currents that reflect the
close correspondence between quasiparticles on the Fermi pockets in the
pseudogap and those near nodes in the superconducting state, we show that
measurable transport coefficients provide stringent tests of pseudogap models. | cond-mat_supr-con |
Non-Abelian Anyons and Non-Abelian Vortices in Topological
Superconductors: Anyons are particles obeying statistics of neither bosons nor fermions.
Non-Abelian anyons, whose exchanges are described by a non-Abelian group acting
on a set of wave functions, are attracting a great attention because of
possible applications to topological quantum computations. Braiding of
non-Abelian anyons corresponds to quantum computations. The simplest
non-Abelian anyons are Ising anyons which can be realized by Majorana fermions
hosted by vortices or edges of topological superconductors, $\nu =5/2$ quantum
Hall states, spin liquids, and dense quark matter. While Ising anyons are
insufficient for universal quantum computations, Fibonacci anyons present in
$\nu =12/5$ quantum Hall states can be used for universal quantum computations.
Yang-Lee anyons are non-unitary counterparts of Fibonacci anyons. Another
possibility of non-Abelian anyons (of bosonic origin) is given by vortex
anyons, which are constructed from non-Abelian vortices supported by a
non-Abelian first homotopy group, relevant for certain nematic liquid crystals,
superfluid $^3$He, spinor Bose-Einstein condensates, and high density quark
matter. Finally, there is a unique system admitting two types of non-Abelian
anyons, Majorana fermions (Ising anyons) and non-Abelian vortex anyons. That is
$^3P_2$ superfluids (spin-triplet, $p$-wave paring of neutrons), expected to
exist in neutron star interiors as the largest topological quantum matter in
our universe. | cond-mat_supr-con |
Double path interference and magnetic oscillations in Cooper pair
transport through a single nanowire: We show that the critical current of the Josephson junction consisting of
superconducting electrodes coupled through a nanowire with two conductive
channels can reveal the multi-periodic magnetic oscillations. The
multi-periodicity originates from the quantum mechanical interference between
the channels affected by both the strong spin-orbit coupling and Zeeman
interaction. This minimal two-channel model is shown to explain the complicated
interference phenomena observed recently in Josephson transport through Bi
nanowires. | cond-mat_supr-con |
Ballistic magnon transport and phonon scattering in the antiferromagnet
Nd$_2$CuO$_4$: The thermal conductivity of the antiferromagnet Nd$_2$CuO$_4$ was measured
down to 50 mK. Using the spin-flop transition to switch on and off the acoustic
Nd magnons, we can reliably separate the magnon and phonon contributions to
heat transport. We find that magnons travel ballistically below 0.5 K, with a
thermal conductivity growing as $T^3$, from which we extract their velocity. We
show that the rate of scattering of acoustic magnons by phonons grows as $T^3$,
and the scattering of phonons by magnons peaks at twice the average Nd magnon
frequency. | cond-mat_supr-con |
First Order Premelting Transition of Vortex Lattices: Vortex lattices in the high temperature superconductors undergo a first order
phase transition which has thus far been regarded as melting from a solid to a
liquid. We point out an alternative possibility of a two step process in which
there is a first order transition from an ordinary vortex lattice to a soft
vortex solid followed by another first order melting transition from the soft
vortex solid to a vortex liquid. We focus on the first step. This premelting
transition is induced by vacancy and interstitial vortex lines. We obtain good
agreement with the experimental transition temperature versus field, latent
heat, and magnetization jumps for YBCO and BSCCO. | cond-mat_supr-con |
Predicted formation of superconducting platinum-hydride crystals under
pressure in the presence of molecular hydrogen: Noble metals adopt close-packed structures at ambient pressure and rarely
undergo structural transformation at high pressures. Platinum (Pt), in
particular, is normally considered to be unreactive and is therefore not
expected to form hydrides under pressure. We predict that platinum hydride
(PtH) has a lower enthalpy than its constituents solid Pt and molecular
hydrogen at pressures above 21.5 GPa. We have calculated structural phase
transitions from tetragonal to hexagonal close-packed or face-centered cubic
(fcc) PtH between 70 and 80 GPa. Linear response calculations indicate that PtH
is a superconductor at these pressures with a critical temperature of about
10--25 K. These findings help to shed light on recent observations of
pressure-induced metallization and superconductivity in hydrogen-rich
materials. We show that formation of fcc metal hydrides under pressure is
common among noble metal hydrides and examine the possibility of
superconductivity in these materials. | cond-mat_supr-con |
Periodicity in Al/Ti superconducting single electron transistors: We present experiments on single Cooper-pair transistors made of two
different superconducting materials. We chose Ti and Al to create an energy gap
profile such that the island has a higher gap than the leads, thereby acting as
a barrier to quasiparticle tunneling. Our transport measurements demonstrate
that quasiparticle poisoning is suppressed in all our TiAlTi structures (higher
gap for the island) with clear 2e periodicity observed, whereas full
quasiparticle poisoning is observed in all AlTiAl devices (higher gap for the
leads) with e periodicity. | cond-mat_supr-con |
A Fully Quantum Mechanical Model of a SQUID Ring Coupled to an
Electromagnetic Field: A quantum system comprising of a monochromatic electromagnetic field coupled
to a SQUID ring with sinusoidal non-linearity, is studied. A magnetostatic flux
$\Phi_{x}$ is also threading the SQUID ring, and is used to control the
coupling between the two systems. It is shown that for special values of
$\Phi_{x}$ the system is strongly coupled. The time evolution of the system is
studied. It is shown that exchange of energy takes place between the two modes
and that the system becomes entangled. A second quasi-classical model that
treats the electromagnetic field classically is also studied. A comparison
between the fully quantum mechanical model with the electromagnetic field
initially in a coherent state and the quasi-classical model, is made. | cond-mat_supr-con |
Vortices and quasiparticles near the "superconductor-insulator"
transition in thin films: We study the low temperature behavior of an amorphous superconducting film
driven normal by a perpendicular magnetic field (B). For this purpose we
introduce a new two-fluid formulation consisting of fermionized field induced
vortices and electrically neutralized Bogoliubov quasiparticles (spinons)
interacting via a long-ranged statistical interaction. This approach allows us
to access a novel non-Fermi liquid phase which naturally interpolates between
the low B superconductor and the high B normal metal. We discuss the transport,
thermodynamic, and tunneling properties of the resulting "vortex metal" phase. | cond-mat_supr-con |
Study of grain boundary transparency in (Yb1-xCax)Ba2Cu3O bi-crystal
thin films over a wide temperature, field and field orientation range: The residual low angle grain boundary (GB) network is still the most
important current-limiting mechanism operating in bi-axially textured rare
earth barium copper oxide (REBCO) coated conductors. While Ca-doping is well
established to improve super-current flow across low angle GBs in weak fields
at high temperatures, Ca-doping also depresses Tc, making it so far impractical
for high temperature applications of REBCO coated conductors. On the other
hand, high field magnet applications of REBCO require low temperatures. Here we
systematically evaluate the effectiveness of Ca-doping in improving the GB
transparency, r$^{GB}$= Jc$^{GB}$/Jc$^{grain}$ , of low angle Yb1-xCaxBaCuO
[001] tilt bi-crystal films down to 10K and with magnetic fields perpendicular
and parallel to the film surfaces, while varying the Ca and oxygen doping
level. Using Low Temperature Scanning Laser Microscopy (LTSLM) and
Magneto-Optical Imaging (MOI), we found rGB to strongly depend on the angle
between magnetic field and the GB plane and clearly identified regimes in which
Jc$^{GB}$ can exceed Jc$^{grain}$ (r$^{GB}$>1) where the GB pinning is
optimized by the field being parallel to the GB dislocations. However, even in
this favorable situation, we found that r$^{GB}$ became much smaller at lower
temperatures. Calculations of the GB Ca segregation profile predict that the
high Jc channels between the GB dislocation cores are almost Ca-free. It may be
therefore that the positive effects of Ca doping seen by many authors near Tc
are partly a consequence of the higher Tc of these Ca-free channels. | cond-mat_supr-con |
Electron-phonon coupling in cuprate and iron-based superconductors
revealed by Raman scattering: Electron-phonon coupling (EPC) is one of the most common and fundamental
interactions in solids. It not only dominates many basic dynamic processes like
resistivity, thermal conductivity etc, but also provides the pairing glue in
conventional superconductors. But in high-temperature superconductors (HTSC),
it is still controversial whether or not EPC is in favor of paring. Despite the
controversies, many experiments have provided clear evidence for EPC in HTSC.
In this paper, we briefly review EPC in cuprate and iron-based superconducting
systems revealed by Raman scattering. We introduce how to extract the coupling
information through phonon lineshape. Then we discuss the strength of EPC in
different HTSC systems and possible factors affecting the strength. The
comparative study between Raman phonon theories and experiments allows us to
gain insight into some crucial electronic properties, especially
superconductivity. Finally we summarize and compare EPC in the two existing
HTSC systems, and discuss what role it may play in HTSC. | cond-mat_supr-con |
Novel phase diagram for antiferromagnetism and superconductivity in
pressure-induced heavy-fermion superconductor Ce$_2$RhIn$_8$ probed by In-NQR: We present a novel phase diagram for the antiferromagnetism and
superconductivity in Ce$_2$RhIn$_8$ probed by In-NQR studies under pressure
($P$). The quasi-2D character of antiferromagnetic spin fluctuations in the
paramagnetic state at $P$ = 0 evolves into a 3D character because of the
suppression of antiferromagnetic order for $P > P_{\rm QCP}\sim$ 1.36 GPa (QCP:
antiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate
$1/T_1$ measurements revealed that the superconducting order occurs in the $P$
range 1.36 -- 1.84 GPa, with maximum $T_c\sim$ 0.9 K around $P_{\rm QCP}\sim$
1.36 GPa. | cond-mat_supr-con |
Magnetic Excitations in Strained Infinite-layer Nickelate PrNiO2: Strongly correlated materials often respond sensitively to the external
perturbations. In the recently discovered superconducting infinite-layer
nickelates, the superconducting transition temperature can be dramatically
enhanced via only ~1% compressive strain-tuning enabled by substrate design.
However, the root of such enhancement remains elusive. While the
superconducting pairing mechanism is still not settled, magnetic Cooper pairing
- similar to the cuprates has been proposed. Using resonant inelastic x-ray
scattering, we investigate the magnetic excitations in infinite-layer PrNiO2
thin films for different strain conditions. The magnon bandwidth of PrNiO2
shows only marginal response to strain-tuning, in sharp contrast to the
striking enhancement of the superconducting transition temperature Tc in the
doped superconducting samples. These results suggest the enhancement of Tc is
not mediated by spin excitations and thus provide important empirics for the
understanding of superconductivity in infinite-layer nickelates. | cond-mat_supr-con |
Response to comment on "Broken translational and rotational symmetry via
charge stripe order in underdoped YBa2Cu3O6+y": Fine questions our interpretation of unidirectional-stripes over
bidirectional-checkerboard, and illustrates his criticism by simulating a
momentum space structure consistent with our data and corresponding to a
checkerboard-looking real space density. Here we use a local
rotational-symmetry analysis to demonstrate that the simulated image is in
actuality composed of locally unidirectional modulations of the charge density,
consistent with our original conclusions. | cond-mat_supr-con |
Charge-Stripe Order and Superconductivity in $\mathrm{Ir_{1-x}Pt_xTe_2}$: A combined resistivity and hard x-ray diffraction study of superconductivity
and charge ordering in $\mathrm{Ir_{1-x}Pt_xTe_2}$, as a function of Pt
substitution and externally applied hydrostatic pressure, is presented.
Experiments are focused on samples near the critical composition $x_c\sim
0.045$ where competition and switching between charge order and
superconductivity is established. We show that charge order as a function of
pressure in $\mathrm{Ir_{0.95}Pt_{0.05}Te_{2}}$ is preempted - and hence
triggered - by a structural transition. Charge ordering appears uniaxially
along the short crystallographic (1,0,1) domain axis with a
$\mathrm{(\frac{1}{5},0,\frac{1}{5})}$ modulation. Based on these results we
draw a charge-order phase diagram and discuss the relation between stripe
ordering and superconductivity. | cond-mat_supr-con |
Optical conductivity of CuO_2 infinite-layer films: The infrared conductivity of CaCuO_2, SrCuO_{2-y}, and
Sr_{0.85}Nd_{0.15}CuO_2 infinite-layer films is obtained from reflectivity
measurements by taking into account the substrate contribution. SrCuO_{2-y} and
Sr_{0.85}Nd_{0.15}CuO_2 exhibit extra-phonon modes and structured bands in the
midinfrared, not found in stoichiometric CaCuO_2. These features mirror those
observed in the perovskitic cuprates, thus showing that the polaronic
properties of high-T_c superconductors are intrinsic to the CuO_2 planes. | cond-mat_supr-con |
Annealing, acid, and alcoholic beverage effects on Fe1+yTe0.6Se0.4: We have systematically investigated and compared different methods to induce
superconductivity in iron chalcogenide Fe1+yTe0.6Se0.4 including annealing in
vacuum, N2, O2, I2 atmosphere, and immersing samples into acid and alcoholic
beverages. Vacuum and N2 annealing are proved to be ineffective to induce
superconductivity in Fe1+yTe0.6Se0.4 single crystal. O2 and I2 annealing, acid
and alcoholic beverages can induce superconductivity by oxidizing the excess Fe
in the sample. Superconductivity in O2 annealed sample is in bulk nature, while
I2, acid and alcoholic beverages can only induce superconductivity near the
surface. By comparing different effects of O2, I2, acid and alcoholic
beverages, we propose a scenario to explain how the superconductivity is
induced in the non-superconducting as-grown Fe1+yTe0.6Se0.4. | cond-mat_supr-con |
Multiple nodeless superconducting gaps in noncentrosymmetric
superconductor PbTaSe2 with topological bulk nodal lines: Low-temperature thermal conductivity measurements were performed on single
crystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological
bulk nodal lines in the electronic band structure. It is found that the
residual linear term $\kappa_0/T$ is negligible in zero magnetic field.
Furthermore, the field dependence of $\kappa_0/T$ exhibits a clear "$S$"-shape
curve. These results suggest that PbTaSe$_2$ has multiple nodeless
superconducting gaps. Therefore, the spin-triplet state with gap nodes does not
play an important role in this noncentrosymmetric superconductor with strong
spin-orbital coupling. The fully gapped superconducting state also meets the
requirement of a topological superconductor, if PbTaSe$_2$ is indeed the case. | cond-mat_supr-con |
Tunneling studies in a homogeneously disordered s-wave superconductor:
NbN: We report the evolution of superconducting properties as a function of
disorder in homogeneously disordered epitaxial NbN thin films grown on (100)
MgO substrates, studied through a combination of electrical transport, Hall
Effect and tunneling measurements. The thickness of all our films are >50nm
much larger than the coherence length ~5nm. The effective disorder in different
films encompasses a large range, with the Ioffe-Regel parameter varying in the
range kFl~1.38-8.77. Tunneling measurements on films with different disorder
reveals that for films with large disorder the bulk superconducting transition
temperature (Tc) is not associated with a vanishing of the superconducting
energy gap, but rather a large broadening of the superconducting density of
states. Our results provide strong evidence of the loss of superconductivity
via phase-fluctuations in a disordered s-wave superconductor. | cond-mat_supr-con |
Electronic Structure, Electron-Phonon Coupling, and Multiband Effects in
MgB2: We review the current situation in the theory of superconducting and
transport properties of MgB2. First principle calculations of of the electronic
structure and electron-phonon coupling are discussed and compared with the
experiment. We also present a brief description of the multiband effects in
superconductivity and transport, and how these manifest themselves in MgB2. We
also mention some yet open questions. | cond-mat_supr-con |
Odd-frequency pairing and Ising spin susceptibility in time-reversal
invariant superfluids and superconductors: We here examine the relation between odd-frequency spin-triple even-parity
(OTE) Cooper pairs and anomalous surface magnetic response in time-reversal
invariant (TRI) spin-triplet superfluids and superconductors. The spin
susceptibility generally consists of two contributions: Even-frequency
odd-parity pair amplitudes and odd-frequency even-parity pair amplitudes. The
OTE pair amplitudes are absent in the bulk region, but ubiquitously exist in
the surface and interface region as Andreev bound states. We here clarify that
additional discrete symmetries, originating from the internal symmetry and
point group symmetry, impose strong constraint on the OTE pair amplitudes
emergent in the surface of TRI superfluids and superconductors. As a result of
the symmetry constraint, the magnetic response of the OTE pairs yields
Ising-like anisotropy. For the topological phase of the $^3$He-B in a
restricted geometry, the coupling of the OTE pair amplitudes to an applied
field is prohibited by an additional discrete symmetry. Once the discrete
symmetry is broken, however, the OTE pairs start to couple to the applied
field, which anomalously enhances surface spin susceptibility. Furthermore, we
extend this theory to TRI superconductors, where the corresponding discrete
symmetry is the mirror reflection symmetry. | cond-mat_supr-con |
On the Excitations of a Balian-Werthamer Superconductor: My contribution to this collection of articles in honor of David Lee and John
Reppy on their 90th birthdays is a reflection on the remarkable phenomenology
of the excitation spectra of superfluid $^3$He, in particular the B-phase which
was identified by NMR and acoustic spectroscopy as Balian-Werthamer state shown
in 1963 to be the ground state of a spin-triplet, p-wave superconductor within
weak-coupling BCS theory. The superfluid phases of $^3$He provide paradigms for
electronic superconductors with broken space-time symmetries and non-trivial
ground-state topology. Indeed broken spin- and orbital rotation symmetries lead
to a rich spectrum of collective modes of the order parameter that can be
detected using NMR, acoustic and microwave spectroscopies. The topology of the
BW state implies its low-temperature, low-energy transport properties are
dominated by gapless Majorana modes confined on boundaries or interfaces. Given
the central role the BW state played I discuss the acoustic and electromagnetic
signatures of the BW state, the latter being relevant if an electronic analog
of superfluid $^3$He-B is realized. | cond-mat_supr-con |
Fully dense MgB_2 superconductor textured by hot deformation: Bulk textured MgB_2 material of nearly full density showing a weak c-axis
alignment of the hexagonal MgB_2 grains parallel to the pressure direction was
obtained by hot deformation of a stoichiometric MgB_2 pellet prepared by a
gas-solid reaction. The texture of the material was verified by comparing the
x-ray diffraction patterns of the hot deformed material with isotropic MgB_2
powder. A small, but distinct anisotropy of the upper critical field up to
Hc2^{a,b}/Hc2^{c}~1.2 depending on degree of texture was found by resistance
and susceptibility measurements. No anisotropy of the critical current density
determined from magnetization measurements was found for the textured material. | cond-mat_supr-con |
Critical Current Peaks at $3B_Φ$ in Superconductors with Columnar
Defects: Recrystalizing the Interstitial Glass: The role of commensurability and the interplay of correlated disorder and
interactions on vortex dynamics in the presence of columnar pins is studied via
molecular dynamics simulations. Simulations of dynamics reveal substantial
caging effects and a non-monotonic dependence of the critical current with
enhancements near integer values of the matching field $B_{\phi}$ and
$3B_{\phi}$ in agreement with experiments on the cuprates. We find qualitative
differences in the phase diagram for small and large values of the matching
field. | cond-mat_supr-con |
The Garber Current Pattern: An Additional Contribution to AC Losses in
Helical HTS Cables?: Conductors made of high-temperature (HTS) wires helically wound in one or
more layers on round tubes (CORT) are compact, flexible, and can carry a large
amount of current. Although these conductors were initially developed for DC
applications, e.g. in magnets, it is worth considering their use for AC, e.g.
in underground cables for medium voltage grids and with currents in the
kA-range. In these cases, the major challenge is reducing AC losses. In
contrast to a straight superconducting wire, in a helical arrangement, due to
superconducting shielding, the current does not follow the direction of the
wires, but takes a non-trivial zig-zag path within the individual HTS wires
(Garber pattern). This includes current components across the thickness of the
superconducting layers, so that the often used thin-shell approximation does
not hold. In this contribution, we studied a one-layer three-wire CORT by means
of fully three-dimensional simulations, based on the H-formulation of Maxwell's
equations implemented in the commercial software package COMSOL Multiphysics.
As a result of our simulations, the peculiar current profiles were confirmed.
In addition, the influence of current, pitch angle, and frequency on the AC
losses was studied. We found an optimum for the pitch angle and that the
current profiles strongly depend on frequency. | cond-mat_supr-con |
The Thermodynamics and the Inverse Isotope Effect of superconducting PdH
and PdD under pressure: We present in this paper the thermodynamics of superconducting PdH and PdD
under pressure. We make use of a general method to calculate the thermodynamics
under pressure within the Migdal-Eliashberg theory. We have considered the
crystal lattice to be zincblende taking into account the experimental evidence
for both PdH and PdD at temperatures below 55 K. We have studied, in
particular, the changes induced by pressure in the critical temperature, $T_c$,
in the specific heat jump at $T_c$, in the energy gap at $T=0K$, in the
deviation function $D(t)$ and in the isotope effect coefficient, $\alpha$. We
get a very good agreement with experiment where this data exist. This method
represents a basis on which the thermodynamics of other hydrides under pressure
can be calculated. | cond-mat_supr-con |
Encapsulating high-temperature superconducting twisted van der Waals
heterostructures blocks detrimental effects of disorder: High-temperature cuprate superconductors-based van der Waals (vdW)
heterostructures hold high technological promise. One of the obstacles
hindering progress is the detrimental effect of disorder on the properties of
Josephson junctions (JJ) realized by the vdW devices. Here we report the new
method of fabricating twisted vdW heterostructures made of Bi2Sr2CuCa2O8+d,
crucially improving the JJ characteristics, pushing them up to those of the
intrinsic JJs in bulk samples. The method combines a cryogenic stacking using a
solvent-free stencil mask technique and covering interface by the insulating
hexagonal boron nitride crystals. Despite the high-vacuum condition down to
10-6 mbar in the evaporation chamber, the interface appears to be protected
from water molecules during the in-situ metal deposition only when fully
encapsulated. Comparing the current-voltage curves of encapsulated and
unencapsulated interfaces, we reveal that the encapsulated interfaces'
characteristics are crucially improved so that the corresponding JJs
demonstrate high critical currents and sharpness of the superconducting
transition comparable to those of the intrinsic JJs. Finally, we show that the
encapsulated heterostructures are more stable in time. | cond-mat_supr-con |
Ab-initio Studies of (Li$_{0.8}$Fe$_{0.2}$)OHFeSe Superconductors:
Revealing the Dual Roles of Fe$_{0.2}$ in Structural Stability and Charge
Transfer: The recently discovered (Li$_{0.8}$Fe$_{0.2}$)OHFeSe superconductor provides
a new platform for exploiting the microscopic mechanisms of high-$T_c$
superconductivity in FeSe-derived systems. Using density functional theory
calculations, we first show that substitution of Li by Fe not only
significantly strengthens the attraction between the (Li$_{0.8}$Fe$_{0.2}$)OH
spacing layers and the FeSe superconducting layers along the \emph{c} axis, but
also minimizes the lattice mismatch between the two in the \emph{ab} plane,
both favorable for stabilizing the overall structure. Next we explore the
electron injection into FeSe from the spacing layers, and unambiguously
identify the Fe$_{0.2}$ components to be the dominant atomic origin of the
dramatically enhanced interlayer charge transfer. We further reveal that the
system strongly favors collinear antiferromagnetic ordering in the FeSe layers,
but the spacing layers can be either antiferromagnetic or ferromagnetic
depending on the Fe$_{0.2}$ spatial distribution. Based on these
understandings, we also predict (Li$_{0.8}$Co$_{0.2}$)OHFeSe to be structurally
stable with even larger electron injection and potentially higher $T_c$. | cond-mat_supr-con |
$η$ collective mode as A$_{1g}$ Raman resonance in cuprate
superconductors: We discuss the possible existence a spin singlet excitation with charge
$\pm2$ ($\eta$-mode) originating the $A_{1g}$ Raman resonance in cuprate
superconductors. This $\eta$-mode relates the $d$-wave superconducting singlet
pairing channel to a $d$-wave charge channel. We show that the $\eta$ boson
forms a particle-particle bound state below the $2\Delta$ threshold of the
particle-hole continuum where $\Delta$ is the maximum $d$-wave gap. Within a
generalized random phase approximation and Bethe-Salpether approximation study,
we find that this mode has energies similar to the resonance observed by
Inelastic Neutron Scattering (INS) below the superconducting (SC) coherent peak
at $2\Delta$ in various SC cuprates compounds. We show that it is a very good
candidate for the resonance observed in Raman scattering below the $2\Delta$
peak in the $A_{1g}$ symmetry. Since the $\eta$-mode sits in the $S=0$ channel,
it may be observable via Raman, X -ray or Electron Energy Loss Spectroscopy
probes. | cond-mat_supr-con |
Anomalous metallic state above the upper critical field of the
conventional three-dimensional superconductor AgSnSe2 with strong intrinsic
disorder: We report superconducting properties of AgSnSe2 which is a conventional
type-II superconductor in the very dirty limit due to intrinsically strong
electron scatterings. While this material is an isotropic three-dimensional
(3D) superconductor with a not-so-short coherence length where strong vortex
fluctuations are NOT expected, we found that the magnetic-field-induced
resistive transition at fixed temperatures becomes increasingly broader toward
zero temperature and, surprisingly, that this broadened transition is taking
place largely ABOVE the upper critical field determined thermodynamically from
the specific heat. This result points to the existence of an anomalous metallic
state possibly caused by quantum phase fluctuations in a strongly-disordered 3D
superconductor. | cond-mat_supr-con |
Induced ferromagnetic moment at the interface between epitaxial
SrRuO$_3$ film and Sr$_2$RuO$_4$ single crystal: SrRuO$_3$ (SRO113) is an important material for device physics particularly
as one of the best metallic oxide electrodes for ferroelectric devices. This
oxide has moderate electron correlations with novel properties including
ferromagnetic ordering, which can be utilized in future to spintronics and
superconducting spintronics devices. Recently, we observed strongly enhanced
magnetization of SRO113 thin films grown on single crystals of the spin-triplet
superconductor Sr$_2$RuO$_4$ (SRO214). To clarify the origin of such an
enhancement, we conducted systematic investigations of magnetic properties of
SRO113 films deposited on a variety of oxide substrates. We carefully
subtracted the substrate contributions and found that the enhanced 2
magnetization occurs only for SRO113/SRO214 films. We further found that
neither strain nor metallicity of the substrate plays any significant roles in
the enhancement. The X-ray magnetic circular dichroism reveals that the
substrate-induced strain does not switch the Ru4+ state from the low-spin to
high-spin states. The film-thickness dependence of the magnetization of
SRO113/SRO214 films strongly suggest that the additional magnetization arises
due to the induction of magnetic moment into the SRO214 substrate over 20-nm
depth. Our results imply new magnetic functionality that can trigger studies
searching for yet unknown physical phenomena in magnetic ruthenates. | cond-mat_supr-con |
Superconducting Quantum Interference at the Atomic Scale: A single spin in a Josephson junction can reverse the flow of the
supercurrent. At mesoscopic length scales, such $\pi$-junctions are employed in
various instances from finding the pairing symmetry to quantum computing. In
Yu-Shiba-Rusinov (YSR) states, the atomic scale counterpart of a single spin in
a superconducting tunnel junction, the supercurrent reversal so far has
remained elusive. Using scanning tunneling microscopy (STM), we demonstrate
such a 0 to $\pi$ transition of a Josephson junction through a YSR state as we
continuously change the impurity-superconductor coupling. We detect the sign
change in the critical current by exploiting a second transport channel as
reference in analogy to a superconducting quantum interference device (SQUID),
which provides the STM with the required phase sensitivity. The measured change
in the Josephson current is a signature of the quantum phase transition and
allows its characterization with unprecedented resolution. | cond-mat_supr-con |
Comment on "Quantum Melting of the Quasi-Two-Dimensional Vortex Lattice
in $κ-(ET)_2Cu(NCS)_2$": In a recent Letter Mola et al. \cite{mola} reported magnetization
measurements $M(H,\theta)$ performed on the organic superconductor
$\kappa-$(ET)$_2$Cu(NCS)$_2 (T_c = 9.1$ K) as a function of the magnetic field
$H$ applied at different angles $\theta$ with respect to the $a$-axis
direction. The results \cite{mola} demonstrate:
(a) the occurrence of pronounced irreversible magnetization $M_{\rm irr}(H)$
jumps and (b) their sudden cessation for $H \ge H_m(T,\theta)$. The boundary
line $H_m(T)$ has been interpreted by Mola et al. as the Q2D vortex lattice
(VL) quantum melting phase transition line \cite{mola}. The purpose of this
comment is to show that the results can be understood in a simple way without
invoking "quantum melting phase transition". | cond-mat_supr-con |
Exactly solvable pairing models in two dimensions: The BCS theory models electron correlations with pure zero-momentum pairs.
Here we consider a family of pairing Hamiltonians, where the electron
correlations are modelled with pure arbitrary-momentum pairs. We find all
models in the family are exactly solvable, and present these solutions. It is
interesting to note that the $\eta$ pair or the $d$ -wave pair condensate in
$T_{c}$ superconductivity can be the ground state of a Hamiltonian in the
family. These models are two-dimensional because only the z-component of the
total electron spin $S_z$ is conserved. Significantly, for the $\eta$ pair or
$d$ -wave pairing model in the family we find an analytical expression of
energy and an abrupt ground state change from independent particle state to the
$d$ -wave pair condensate, suggesting a quantum phase transition. | cond-mat_supr-con |
Single Crystal Growth of Cuprate Superconductor
(Lu$_{0.8}$Nd$_{0.2}$)Ba$_2$Cu$_4$O$_8$ by KOH Flux Method: Single crystals of Nd-substituted LuBa$_2$Cu$_4$O$_8$ were successfully grown
by the KOH flux method. The single phase of Lu$_{1-x}$Nd$_x$Ba$_2$Cu$_4$O$_8$
[(Lu,Nd)124] formed at $x = 0.2$. The compound crystallized as orthorhombic
Ammm structure with lattice constants of a = 3.835(3) {\AA}, b = 3.879(2) {\AA}
and c = 27.195(6) {\AA}. Single crystal structural analysis demonstrated that
the Nd ion partially occupied the Lu site. The (Lu,Nd)124 exhibited the
superconducting transition at ~75 K in the magnetic susceptibility and
resistivity. The $T_c$ and the c-axis of the (Lu,Nd)124 corresponded to Tm124.
These results indicate that RE124 equivalent to the one composed of single RE
element is obtained by combination of two RE elements. | cond-mat_supr-con |
Observation of novel gapped phases in potassium doped single layer
p-terphenyl on Au (111): Recently, superconductivity in potassium (K) doped p-terphenyl (C18H14) has
been suggested by the possible observation of the Meissner effect and
subsequent photoemission spectroscopy measurements, but the detailed lattice
structure and more-direct evidence are still lacking. Here we report a low
temperature scanning tunneling microscopy/spectroscopy (STM/STS) study on
K-doped single layer p-terphenyl films grown on Au (111). We observe several
ordered phases with different morphologies and electronic behaviors, in two of
which a sharp and symmetric low-energy gap of about 11 meV opens below 50 K. In
particular, the gap shows no obvious response to a magnetic field up to 11
Tesla, which would caution against superconductivity as an interpretation in
previous reports of K-doped p-terphenyl materials. Such gapped phases are
rarely (if ever) observed in single layer hydrocarbon molecular crystals. Our
work also paves the way for fabricating doped two-dimensional (2D) hydrocarbon
materials, which will provide a platform to search for novel emergent
phenomena. | cond-mat_supr-con |
The Effects of d_{x^2-y^2}-d_{xy} Mixing on Vortex Structures and
Magnetization: The structure of an isolated single vortex and the vortex lattice, and the
magnetization in a $d$-wave superconductor are investigated within a
phenomenological Ginzburg-Landau (GL) model including the mixture of the
$d_{x^2-y^2}$-wave and $d_{xy}$-wave symmetry. The isolated single vortex
structure in a week magnetic field is studied both numerically and
asymptotically. Near the upper critical field $H_{c2}$, the vortex lattice
structure and the magnetization are calculated analytically. | cond-mat_supr-con |
Interplay between kinetic inductance, non-linearity and quasiparticle
dynamics in granular aluminum MKIDs: Microwave kinetic inductance detectors (MKIDs) are thin film, cryogenic,
superconducting resonators. Incident Cooper pair-breaking radiation increases
their kinetic inductance, thereby measurably lowering their resonant frequency.
For a given resonant frequency, the highest MKID responsivity is obtained by
maximizing the kinetic inductance fraction $\alpha$. However, in circuits with
$\alpha$ close to unity, the low supercurrent density reduces the maximum
number of readout photons before bifurcation due to self-Kerr non-linearity,
therefore setting a bound for the maximum $\alpha$ before the noise equivalent
power (NEP) starts to increase. By fabricating granular aluminum MKIDs with
different resistivities, we effectively sweep their kinetic inductance from
tens to several hundreds of pH per square. We find a NEP minimum in the range
of $25\; \text{aW}/\sqrt{\text{Hz}}$ at $\alpha \approx 0.9$, which results
from a tradeoff between the onset of non-linearity and a non-monotonic
dependence of the noise spectral density vs. resistivity. | cond-mat_supr-con |
Multimodal synchrotron X-ray diffraction across the superconducting
transition of Sr$_{0.1}$Bi$_2$Se$_3$: In the doped topological insulator Sr$_x$Bi$_2$Se$_3$, a pronounced in-plane
two-fold symmetry is observed in electronic properties below the
superconducting transition temperature $T_c \sim$ 3 K, despite the three-fold
symmetry of the observed $R\bar{3}m$ space group. The axis of two-fold symmetry
is nominally pinned to one of three rotational equivalent directions and
crystallographic strain has been proposed to be the origin of this pinning. We
carried out multimodal synchrotron diffraction and resistivity measurements
down to $\sim$0.68 K and in magnetic fields up to 45 kG on a single crystal of
Sr$_{0.1}$Bi$_2$Se$_3$ to probe the effect of superconductivity on the
crystallographic distortion. Our results indicate that there is no in-plane
crystallographic distortion at the level of $1x10^{-5}$ associated with the
superconducting transition. These results further support the model that the
large two-fold in-plane anisotropy of superconducting properties of
Sr$_x$Bi$_2$Sr$_3$ is not structural in origin but electronic, namely it is
caused by a nematic superconducting order parameter of Eu symmetry. | cond-mat_supr-con |
Superconductivity-insensitive order at $q$~1/4 in electron doped
cuprates: One of the central questions in the cuprate research is the nature of the
"normal state" which develops into high temperature superconductivity (HTSC).
In the normal state of hole-doped cuprates, the existence of charge density
wave (CDW) is expected to shed light on the mechanism of HTSC. With evidence
emerging for CDW order in the electron-doped cuprates, the CDW would be thought
to be a universal phenomenon in high-$T_c$ cuprates. However, the CDW phenomena
in electron-doped cuprate are quite different than those in hole-doped
cuprates. Here we study the nature of the putative CDW in an electron-doped
cuprate through direct comparisons between as-grown and post-annealed
Nd$_{1.86}$Ce$_{0.14}$CuO$_4$ (NCCO) single crystals using Cu $L_3$-edge
resonant soft x-ray scattering (RSXS) and angle resolved photoemission
spectroscopy (ARPES). The RSXS result reveals that the non-superconducting NCCO
shows the same reflections at the wavevector (~1/4, 0, $l$) as like the
reported superconducting NCCO. This superconductivity-insensitive signal is
quite different with the characteristics of the CDW reflection in hole-doped
cuprates. Moreover, the ARPES result suggests that the fermiology cannot
account for such wavevector. These results call into question the universality
of CDW phenomenon in the cuprates. | cond-mat_supr-con |
The c axis optical conductivity of layered systems in the
superconducting state: In this paper, we discuss the c axis optical conductivity Re [sigma_c(omega)]
in the high T_c superconductors, in the superconducting state. The basic
premise of this work is that electrons travelling along the c axis between
adjacent CuO_2 layers must pass through several intervening layers. In earlier
work we found that, for weak inter-layer coupling, it is preferable for
electrons to travel along the c axis by making a series of interband
transitions rather than to stay within a single (and very narrow) band.
Moreover, we found that many of the properties of the normal state optical
conductivity, including the pseudogap could be explained by interband
transitions. In this work we examine the effect of superconductivity on the
interband conductivity. We find that, while the onset of superconductivity is
clearly evident in the spectrum, there is no clear signature of the symmetry of
the superconducting order parameter. | cond-mat_supr-con |
Group-9 Transition Metal Suboxides Adopting the Filled-Ti$_2$Ni
Structure: A Class of Superconductors Exhibiting Exceptionally High Upper
Critical Fields: The Ti$_2$Ni and the related $\eta$-carbide structure are known to exhibit
various intriguing physical properties. The Ti$_2$Ni structure with the cubic
space group $Fd\bar{3}m$ is surprisingly complex, consisting of a unit cell
with 96 metal atoms. The related $\eta$-carbide compounds correspond to a
filled version of the Ti$_2$Ni structure. Here, we report on the structure and
superconductivity in the $\eta$-carbide type suboxides Ti$_4$M$_2$O with M =
Co, Rh, Ir. We have successfully synthesized all three compounds in single
phase form. We find all three compounds to be type-II bulk superconductors with
transition temperatures of $T_{\rm c}$ = 2.7, 2.8, and 5.4 K, and with
normalized specific heat jumps of $\Delta C/\gamma T_{\rm c}$ = 1.65, 1.28, and
1.80 for Ti$_4$Co$_2$O, Ti$_4$Rh$_2$O, and Ti$_4$Ir$_2$O, respectively. We find
that all three superconductors, exhibit high upper-critical fields.
Particularly noteworthy is Ti$_4$Ir$_2$O with an upper critical field of $\mu_0
H_{\rm c2}{\rm (0)}$ =~16.06~T, which exceeds by far the weak-coupling Pauli
limit of 9.86~T. The role of the void filling light atom X has so far been
uncertain for the overall physical properties of these materials. Herein, we
have successfully grown single crystals of Ti$_2$Co. In contrast to the
metallic $\eta$-carbide type suboxides Ti$_4$M$_2$O, we find that Ti$_2$Co
displays a semimetallic behavior. Hence, the octahedral void-filling oxygen
plays a crucial role for the overall physical properties, even though its
effect on the crystal structure is small. Our results indicate that the design
of new superconductors by incorporation of electron-acceptor atoms may in the
Ti$_2$Ni-type structures and other materials with crystallographic void
position be a promising future approach. The remarkably high upper critical
fields, in this family of compounds, may furthermore spark significant future
interest. | cond-mat_supr-con |
Materials Informatics Reveals Unexplored Structure Space in Cuprate
Superconductors: High-temperature superconducting cuprates have the potential to be
transformative in a wide range of energy applications. In this work we analyse
the corpus of historical data about cuprates using materials informatics and
re-examine how their structures are related to their critical temperatures
(Tc). The available data is highly clustered and no single database contains
all the features of interest to properly examine trends. To work around these
issues we employ a linear calibration approach that allows us to utilise
multiple data sources -- combining fine resolution data for which the Tc is
unknown with coarse resolution data where it is known. The hybrid data set
constructed enables us to explore the trends in Tc with the apical and in-plane
copper-oxygen distances. We show that large regions of the materials space have
yet to be explored and highlight how novel experiments relying on
nano-engineering of the crystal structure may enable us to explore such new
regions. Based on the trends identified we propose that single layer Bi-based
cuprates are good candidate systems for such experiments. | cond-mat_supr-con |
High-Temperature Superconductivity Mechanism for Cuprates: Egorov and March plotted the product of resistivity and the copper
spin-lattice relaxation time vs. temperature for yttrium barium copper oxide
finding a minimum at temperature T greater than the superconducting
temperature, heralding an electronic phase change which can be interpreted as
the formation of a "preformed" pair. In this context we propose a
superconducting mechanism based on the notion that the preformed pair is a
"soft" boson (a localized, different type of Cooper pair) which dissociates
above the classical BKT transition temperature, resulting in two circular
charge density waves. The model suggests explanations for considerable
experimental work and offers a physical explanation for the basis of the
Uemura-Homes law. | cond-mat_supr-con |
STM/STS study on large pseudogap and nodal superconducting gap in
Bi2201(La) and Bi2212: In the present work, scanning tunneling microscopy/spectroscopy (STM/STS)
measurements were carried out on underdoped $\rm Bi_2Sr_{2-{\it x}}La_{\it
x}CuO_{6+\delta}$ and $\rm Bi_2Sr_2CaCu_2O_{8+\delta}$ to clarify the origin of
the pseudogap, in particular, the inhomogeneous large pseudogap. The nodal part
of a d-wave pairing gap, which is under no influence of the inhomogeneous large
pseudogap, was also examined by relating the homogeneous bottom part of the STS
gap to a nodal d-wave gap in momentum space. We report that the inhomogeneous
large pseudogap in the antinodal region links to a two-dimensional electronic
charge order, and that the gap size of the nodal d-wave part $\rm
{\Delta}_{sc}$ scales with the superconducting critical temperature $\rm {\it
T}_c$ in the pseudogap regime. | cond-mat_supr-con |
Ginzburg-Landau expansion and the upper critical field in disordered
attractive Hubbard model: We present a short review of our studies of disorder influence upon Ginzburg
- Landau expansion coefficients in Anderson - Hubbard model with attraction in
the framework of the generalized DMFT+$\Sigma$ approximation. A wide range of
attractive potentials $U$ is considered - from weak coupling limit, where
superconductivity is described by BCS model, to the limit of very strong
coupling, where superconducting transition is related to Bose - Einstein
condensation (BEC) of compact Cooper pairs, which are formed at temperatures
significantly higher than the temperature of superconducting transition, as
well as the wide range of disorders - from weak to strong, when the system is
in the vicinity of Anderson transition. For the same range of parameters we
study in detail the temperature behavior of orbital and paramagnetic upper
critical field $H_{c2}(T)$, which demonstrates the anomalies both due to the
growth of attractive potential and the effects of strong disordering. | cond-mat_supr-con |
Pairing mechanism in Fe pnictide superconductors: By applying an exact unitary transformation to a two-band hamiltonian which
also includes the effects due to large pnictogen polarizabilities, we show that
an attractive spin-mediated Hubbard term appears in the $d_{xz}$, $d_{yz}$
nearest-neighbour channel. This pairing mechanism implies a singlet
superconducting order parameter in iron pnictides. | cond-mat_supr-con |
Comment on the paper by D. Efremov and Yu.N. Ovchinnikov "Singular
ground state of multiband inhomogeneous superconductors", Phys. Rev. B 99,
224508 (2019): We show that the conclusion reported in Ref. 1, that there are no spontaneous
magnetic fields in multiband superconductors that break time reversal symmetry,
is incorrect. We demonstrate that the state proposed in Ref. 1 is not a
solution of the Ginzburg-Landau equations for the considered model. The reason
is that in Ref. 1 one of the Ginzburg-Landau equations is neglected and
substituted by the spurious zero current restriction. This restriction together
with all of the Ginzburg-Landau equations leads to an overdetermined system
which does not have a solution. This inconsistency invalidates all the results
of the paper. | cond-mat_supr-con |
Signatures of a pair density wave at high magnetic fields in cuprates
with charge and spin orders: In underdoped cuprates, the interplay of the pseudogap, superconductivity,
and charge and spin ordering can give rise to exotic quantum states, including
the pair density wave (PDW), in which the superconducting (SC) order parameter
is oscillatory in space. However, the evidence for a PDW state remains
inconclusive and its broader relevance to cuprate physics is an open question.
To test the interlayer frustration, the crucial component of the PDW picture,
we performed transport measurements on La$_{1.7}$Eu$_{0.2}$Sr$_{0.1}$CuO$_{4}$
and La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$, cuprates with "striped" spin and
charge orders, in perpendicular magnetic fields ($H_\perp$), and also with an
additional field applied parallel to CuO$_2$ layers ($H_\parallel$). We
detected several phenomena predicted to arise from the existence of a PDW,
including an enhancement of interlayer SC phase coherence with increasing
$H_\parallel$. Our findings are consistent with the presence of local, PDW
pairing correlations that compete with the uniform SC order at $T_{c}^{0}<
T<(2-6) T_{c}^{0}$, where $T_{c}^{0}$ is the $H=0$ SC transition temperature,
and become dominant at intermediate $H_\perp$ as $T\rightarrow 0$. These data
also provide much-needed transport signatures of the PDW in the regime where
superconductivity is destroyed by quantum phase fluctuations. | cond-mat_supr-con |
Measuring the Localization Length through the superconductor-insulator
transition in ultrathin amorphous beryllium films: Electron transport and tunneling across the superconductor-insulator (SI)
transition have been measured simultaneously for quench-condensed ultrathin
amorphous beryllium films. The anomalous negative magnetoresistance previously
observed in insulating films disappears when Mn impurities are introduced to
the films, restoring a rather clean Efros-Shklovskii type hopping behavior. The
combination of transport and tunneling data allows us to determine,
independently and up to a constant on the order of unity, the localization
length, \xi_{L}, and the dielectric constant, \kappa, for the films. As the
normal-state sheet resistance of the films at 20 K is reduced with increasing
film thickness, \xi_{L} increases exponentially. The SI transition occurs when
\xi_{L} crosses the Ginzburg-Landau coherence length, \xi_{S}. | cond-mat_supr-con |
Magnetic resonance peak and nonmagnetic impurities: Nonmagnetic Zn impurities are known to strongly suppress superconductivity.
We review their effects on the spin excitation spectrum in $\rm
YBa_2Cu_3O_{7}$, as investigated by inelastic neutron scattering measurements. | cond-mat_supr-con |
Pressure effects on the electronic structure and superconductivity of
(TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ high entropy alloy: Effects of pressure on the electronic structure, electron-phonon interaction,
and superconductivity of the high entropy alloy
(TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ are studied in the pressure range 0 - 100 GPa.
The electronic structure is calculated using the Korringa-Kohn-Rostoker method
with the coherent potential approximation. Effects of pressure on the lattice
dynamics are simulated using the Debye-Gr\"{u}neisen model and the
Gr\"{u}neisen parameter at ambient conditions. In addition, the Debye
temperature and Sommerfeld electronic heat capacity coefficient were
experimentally determined. The electron-phonon coupling parameter $\lambda$ is
calculated using the McMillan-Hopfield parameters and computed within the rigid
muffin tin approximation. We find, that the system undergoes the Lifshitz
transition, as one of the bands crosses the Fermi level at elevated pressures.
The electron-phonon coupling parameter $\lambda$ decreases above 10 GPa. The
calculated superconducting $T_c$ increases up to 40 - 50 GPa and, later, is
stabilized at the larger value than for the ambient conditions, in agreement
with the experimental findings. Our results show that the experimentally
observed evolution of $T_c$ with pressure in (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$
can be well explained by the classical electron-phonon mechanism. | cond-mat_supr-con |
Positive current cross-correlations in a highly transparent
normal-superconducting beam splitter due to synchronized Andreev and inverse
Andreev reflections: Predictions are established for linear differential current-current
cross-correlations dSab/dV in a symmetrically biased three-terminal normal
metal-superconductor-normal metal (NSN) device. Highly transparent contacts
turn out to be especially interesting because they feature positive dSab/dV. At
high transparency, processes based on Crossed Andreev Reflection (CAR)
contribute only negligibly to the current and to dSab/dV. Under these
circumstances, current-current cross-correlations can be plausibly interpreted
as a coherent coupling between the two NS interfaces in the form of
synchronized Andreev and inverse Andreev reflections, corresponding to the
process where a pair of electron-like quasi-particles and a pair of hole-like
quasi-particles arrive from the normal electrodes and annihilate in the
superconductor. Hence, positive dSab/dV does not automatically imply CAR. For
tunnel contacts, dSab/dV is positive because of CAR. In between these two
extremities, at intermediate transparencies, dSab/dV is negative because both
processes which cause positive correlations, occur only with small amplitude.
We use scattering theory to obtain analytic expressions for current and noise,
and microscopic calculation using a tight binding model in order to obtain a
clear interpretation of the physical processes. | cond-mat_supr-con |
Bosonic spectral density of epitaxial thin-film La1.83Sr0.17CuO4
superconductors from infrared conductivity measurements: We use optical spectroscopy to investigate the excitations responsible for
the structure in the optical self-energy of thin epitaxial films of
La1.83Sr0.17CuO4. Using Eliashberg formalism to invert the optical spectra we
extract the electron-boson spectral function and find that at low temperature
it has a two component structure closely matching the spin excitation spectrum
recently measured by magnetic neutron scattering. We contrast the temperature
evolution of the spectral density and the two-peak behavior in La2-xSrxCuO4
with another high temperature superconductor Bi2Sr2CaCu2O8+d. The bosonic
spectral functions of the two materials account for the low Tc of LSCO as
compared to Bi-2212. | cond-mat_supr-con |
Theory of heterotic SIS Josephson junctions between single- and
multi-gap superconductors: Using the functional integral method, we construct a theory of heterotic SIS
Josephson junctions between single- and two-gap superconductors. The theory
predicts the presence of in-phase and out-of-phase collective oscillation modes
of superconducting phases. The former corresponds to the Josephson plasma mode
whose frequency is drastically reduced for $\pm$ s-wave symmetry, and the
latter is a counterpart of Leggett's mode in Josephson junctions. We also
reveal that the critical current and the Fraunhofer pattern strongly depend on
the symmetry type of the two-gap superconductor. | cond-mat_supr-con |
Bimagnon studies in cuprates with Resonant Inelastic X-ray Scattering at
the O K edge. I - An assessment on La2CuO4 and a comparison with the
excitation at Cu L3 and Cu K edges: We assess the capabilities of magnetic Resonant Inelastic X-ray Scattering
(RIXS) at the O $K$ edge in undoped cuprates by taking La_{2}CuO_{4} as a
benchmark case, based on a series of RIXS measurements that we present here. By
combining the experimental results with basic theory we point out the
fingerprints of bimagnon in the O $K$ edge RIXS spectra. These are a dominant
peak around 450 meV, the almost complete absence of dispersion both with $\pi$
and $\sigma$ polarization and the almost constant intensity vs. the transferred
momentum with $\sigma$ polarization. This behavior is quite different from Cu
$L_3$ edge RIXS giving a strongly dispersing bimagnon tending to zero at the
center of the Brillouin zone. This is clearly shown by RIXS measurements at the
Cu $L_3$ edge that we present. The Cu $L_3$ bimagnon spectra and those at Cu
$K$ edge - both from the literature and from our data - however, have the same
shape. These similarities and differences are understood in terms of different
sampling of the bimagnon continuum. This panorama points out the unique
possibilities offered by O $K$ RIXS in the study of magnetic excitations in
cuprates near the center of the BZ. | cond-mat_supr-con |
Single vortex fluctuations in a superconducting chip as generating
dephasing and spin flips in cold atom traps: We study trapping of a cold atom by a single vortex line in an extreme type
II superconducting chip, allowing for pinning and friction. We evaluate the
atom's spin flip rate and its dephasing due to the vortex fluctuations in
equilibrium and find that they decay rapidly when the distance to the vortex
exceeds the magnetic penetration length. We find that there are special spin
orientations, depending on the spin location relative to the vortex, at which
spin dephasing is considerably reduced while perpendicular directions have a
reduced spin flip rate. We also show that the vortex must be perpendicular to
the surface for a general shape vortex. | cond-mat_supr-con |
Hidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell Superconducting
Phases in a Magnetic Field in (TMTSF)$_2$ClO$_4$: We solve a long-standing problem about a theoretical description of the upper
critical magnetic field, parallel to conducting layers and perpendicular to
conducting chains, in (TMTSF)$_2$ClO$_4$ superconductor. In particular, we
explain why the experimental upper critical field, $H^{b'}_{c2} \simeq 6 \ T$,
is higher than both the quasi-classical upper critical field and Clogston
paramagnetic limit. We show that this property is due to the coexistence of the
hidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell phases in a magnetic
field in a form of three plane waves with non-zero momenta of the Cooper pairs.
Our results are in good qualitative and quantitative agreement with the recent
experimental measurements of $H^{b'}_{c2}$ and support a singlet d-wave like
scenario of superconductivity in (TMTSF)$_2$ClO$_4$. | cond-mat_supr-con |
Phase stiffness in an antiferromagnetic superconductor: We analyze the suppression of the phase stiffness in a superconductor by
antiferromagnetic order. The analysis is based on a general expression for the
phase stiffness in a mean-field state with coexisting spin-singlet
superconductivity and spiral magnetism. Neel order is included as a special
case. Close to half-filling, where the pairing gap is much smaller than the
magnetic gap, a simple formula for the phase stiffness in terms of magnetic
quasi-particle bands is derived. The phase stiffness is determined by charge
carriers in small electron or hole pockets in this regime. The general analysis
is complemented by a numerical calculation for the two-dimensional Hubbard
model with nearest and next-to-nearest neighbor hopping amplitudes at a
moderate interaction strength. The resulting phase stiffness exhibits a
striking electron-hole asymmetry. In the ground state, it is larger than the
pairing gap on the hole-doped side, and smaller for electron doping. Hence, in
the hole-doped regime near half-filling the ground state pairing gap sets the
scale for the Kosterlitz-Thouless temperature T_c^{KT}, while in the slightly
electron-doped regime T_c^{KT} is determined essentially by the ground state
phase stiffness. | cond-mat_supr-con |
Inhomogeneity of charge density wave order and quenched disorder in a
high Tc superconductor: It has recently been established that the high temperature (high-Tc)
superconducting state coexists with short-range charge-density-wave order and
quenched disorder arising from dopants and strain. This complex, multiscale
phase separation invites the development of theories of high temperature
superconductivity that include complexity. The nature of the spatial interplay
between charge and dopant order that provides a basis for nanoscale phase
separation remains a key open question, because experiments have yet to probe
the unknown spatial distribution at both the nanoscale and mescoscale (between
atomic and macroscopic scale). Here we report micro X-ray diffraction imaging
of the spatial distribution of both the charge-density-wave puddles (domains
with only a few wavelengths) and quenched disorder in HgBa2CuO4+y, the single
layer cuprate with the highest Tc, 95 kelvin. We found that the
charge-density-wave puddles, like the steam bubbles in boiling water, have a
fat-tailed size distribution that is typical of self-organization near a
critical point. However, the quenched disorder, which arises from oxygen
interstitials, has a distribution that is contrary to the usual assumed random,
uncorrelated distribution. The interstitials-oxygen-rich domains are spatially
anti-correlated with the charge-density-wave domains, leading to a complex
emergent geometry of the spatial landscape for superconductivity. | cond-mat_supr-con |
Evidence for a universal length scale of dynamic charge inhomogeneity in
cuprate superconductors: Time-resolved optical experiments can give unique information on the
characteristic length scales of dynamic charge inhomogeneity on femtosecond
timescales. From data on the effective quasiparticle relaxation time r
in La2-xSrxCuO4 and Nd2-xCexCuO4 we derive the temperature- and doping-
dependence of the intrinsic phonon escape length le, which, under certain
circumstances, can be shown to be a direct measure of charge inhomogeneity.
Remarkably, a common feature of both p and n-type cuprates - which has
important consequences for superconductivity - is that as T  Tc from
above, the escape length approaches the zero-temperature superconducting
coherence length, le  s(0). In close vicinity of Tc, le appears
to follow the critical behaviour of the Ginsburg-Landau coherence length,
GL(T). In the normal state le is found to be in excellent agreement
with the mean free path lm obtained from the resistivity data. The data on le
also agree well with the data on structural coherence lengths ls obtained from
neutron scattering experiments, implying the existence of complex intrinsic
textures on different length scales which may have a profound effect on the
functional properties of these materials. | cond-mat_supr-con |
Josephson squelch filter for quantum nanocircuits: We fabricated and tested a squelch circuit consisting of a copper powder
filter with an embedded Josephson junction connected to ground. For small
signals (squelch-ON), the small junction inductance attenuates strongly from DC
to at least 1 GHz, while for higher frequencies dissipation in the copper
powder increases the attenuation exponentially with frequency. For large
signals (squelch-OFF) the circuit behaves as a regular metal powder filter. The
measured ON/OFF ratio is larger than 50dB up to 50 MHz. This squelch can be
applied in low temperature measurement and control circuitry for quantum
nanostructures such as superconducting qubits and quantum dots. | cond-mat_supr-con |
Quantum phase slip junctions: In this paper we demonstrate that, if it exists, coherent quantum phase slip
is the exact dual to Josephson tunneling. We use the duality to predict kinetic
capacitance and a sharp resonance in narrow wires. Biased resistively and
driven at high frequency, quantum phase slip junctions should exhibit current
plateaus of interest for a fundamental standard. | cond-mat_supr-con |
Type I superconductivity in the Dirac semimetal PdTe2: The superconductor PdTe$_2$ was recently classified as a Type II Dirac
semimetal, and advocated to be an improved platform for topological
superconductivity. Here we report magnetic and transport measurements conducted
to determine the nature of the superconducting phase. Surprisingly, we find
that PdTe$_2$ is a Type I superconductor with $T_c = 1.64$ K and a critical
field $\mu_0 H_c (0) = 13.6$ mT. Our crystals also exhibit the intermediate
state as demonstrated by the differential paramagnetic effect. For $H > H_c$ we
observe superconductivity of the surface sheath. This calls for a close
examination of superconductivity in PdTe$_2$ in view of the presence of
topological surface states. | cond-mat_supr-con |
Collective excitations in two-band superconductors: We investigate the eigen oscillations of internal degrees of freedom (Higgs
mode and Goldstone mode) of two-band superconductors using the extended
time-dependent Ginzburg-Landau theory, formulated in a work Grigorishin (2021)
\cite{grig2}, for the case of two coupled order parameters by both the internal
proximity effect and the drag effect. It is demonstrated, that the Goldstone
mode splits into two branches: common mode oscillations with the acoustic
spectrum, which is absorbed by the gauge field, and anti-phase oscillations
with the energy gap (mass) in the spectrum determined by the interband
coupling, which can be associated with the Leggett mode. The Higgs mode splits
into two branches also: a massive one, whose energy gap vanishes at the
critical temperature $T_{c}$, another massive one, whose energy gap does not
vanish at $T_{c}$. It is demonstrated, that the second branch of the Higgs mode
is nonphysical, and it, together with the Leggett mode, can be removed by the
special choice of coefficient at the "drag" term in the Lagrangian. In the same
time, such a choice leaves only one coherence length, thereby prohibiting
so-called type-1.5 superconductors. We analyze experimental data about the
Josephson effect between two-band superconductors. In particular, it is
demonstrated, that the resonant enhancement of the DC current through a
Josephson junction at a resonant bias voltage $V_{\mathrm{res}}$, when the
Josephson frequency matches the frequency of some internal oscillation mode in
two-band superconductors (banks), can be explained with the coupling between AC
Josephson current and Higgs oscillations. Thus, explanation of this effect does
not need the Leggett mode. | cond-mat_supr-con |
Dynamics of spin transport in voltage-biased Josephson junctions: We investigate spin transport in voltage-biased spin-active Josephson
junctions. The interplay of spin filtering, spin mixing, and multiple Andreev
reflection leads to nonlinear voltage dependence of the dc and ac spin current.
We compute the voltage characteristics of the spin current (I_S) for
superconductor-ferromagnet-superconductor (SFS) Josephson junctions. The
sub-harmonic gap structure of I_S(V) is shown to be sensitive to the degree of
spin mixing generated by the ferromagnetic interface, and exhibits a pronounced
even-odd effect associated with spin transport during multiple Andreev
reflection processes. For strong spin mixing both the magnitude and the
direction of the dc spin current can be sensitively controlled by the bias
voltage. | cond-mat_supr-con |
On superconducting mechanism in the iron-based layered superconductors: It is demonstrated that SC mechanism of doped Fe-based compounds is
characteristic for itinerant electron systems with coexistence of both (e-e)-
and (e-h)-pairing arising due to electron-phonon and Coulomb interactions,
respectively. The higher Tc of the SC transition here (as compared with
conventional (LTSC) BCS-systems without (e-h)-pairing) is a natural consequence
of (e-e)-pairing at the background of high density of states which arises in
the narrow energy range near dielectric-(SDW)-gap (pseudogap) edges due to
removing of electronic states from the energy region of dielectric (SDW) gap
(pseudogap) (already formed at the part of the Fermi surface in the normal
state with onset temperature T* (near the same for corresponding structural
transition) due to (e-h)-pairing). Below Tc the system enters the coexistence
(SC+SDW) state. The SDW formed is incommensurate with lattice and dynamic in
character. The phase diagram for such system is determined: the doping
dependence of the SC gap (and Tc) has a maximum (optimal doping) while the
dielectric (SDW) gap (pseudogap) (and T*) is a decreasing function of doping.
These conclusions follow from detailed analysis of available resistivity and
another data for Fe-based superconductors on the basis of model with partial
dielectrization of electron energy spectra. The picture obtained and
manifestation of two order parameters (SC and SDW) in experiments, first of
all, in threshold phenomena are discussed. The comparison with the case of
cuprates is performed. | cond-mat_supr-con |
High-energy magnetic excitations in overdoped
La$_{2-x}$Sr$_{x}$CuO$_{4}$ studied by neutron and resonant inelastic X-ray
scattering: We have performed neutron inelastic scattering and resonant inelastic X-ray
scattering (RIXS) at the Cu-$L_3$ edge to study high-energy magnetic
excitations at energy transfers of more than 100 meV for overdoped
La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.25$ ($T_c=15$ K) and $x=0.30$
(non-superconducting) using identical single crystal samples for the two
techniques. From constant-energy slices of neutron scattering cross-sections,
we have identified magnetic excitations up to ~250 meV for $x=0.25$. Although
the width in the momentum direction is large, the peak positions along the (pi,
pi) direction agree with the dispersion relation of the spin-wave in the
non-doped La$_{2}$CuO$_{4}$ (LCO), which is consistent with the previous RIXS
results of cuprate superconductors. Using RIXS at the Cu-$L_3$ edge, we have
measured the dispersion relations of the so-called paramagnon mode along both
(pi, pi) and (pi, 0) directions. Although in both directions the neutron and
RIXS data connect with each other and the paramagnon along (pi, 0) agrees well
with the LCO spin-wave dispersion, the paramagnon in the (pi, pi) direction
probed by RIXS appears to be less dispersive and the excitation energy is lower
than the spin-wave of LCO near (pi/2, pi/2). Thus, our results indicate
consistency between neutron inelastic scattering and RIXS, and elucidate the
entire magnetic excitation in the (pi, pi) direction by the complementary use
of two probes. The polarization dependence of the RIXS profiles indicates that
appreciable charge excitations exist in the same energy range of magnetic
excitations, reflecting the itinerant character of the overdoped sample. A
possible anisotropy in the charge excitation intensity might explain the
apparent differences in the paramagnon dispersion in the (pi, pi) direction as
detected by the X-ray scattering. | cond-mat_supr-con |
Pairing symmetry in BiS$_{2}-$based superconductors: The possible pairing symmetries for BiS$_{2}-$based superconductors is
investigated by using a minimal two-orbital model with onsite and
nearest-neighbor intraorbital attractions $V_{0}$ and $V_{1}$, respectively. By
using the mean-field approximation and solving the self-consistent equations,
the phase diagram of the pairing symmetry is obtained. It is shown that the
model allows three possible pairing symmetries, depending on the values of
$V_{0}$ and $V_{1}$: the isotopic $s-$wave pairing
[$\Delta_{\mathbf{k}}=\Delta_{s}$], the anisotropic $s-$wave pairing
[$\Delta_{\mathbf{k}}=\Delta_{s}+\frac{\Delta_{xs}}{2}(\cos k_{x}+\cos k_{y})$]
and the $d-$wave pairing [$\Delta_{\mathbf{k}}=\frac{\Delta_{d}}{2}(\cos
k_{x}-\cos k_{y})$]. Furthermore the density of states for these pairing
symmetries exhibit different behaviors which can be used to distinguish them. | cond-mat_supr-con |
Gap and pseudogap evolution within the charge-ordering scenario for
superconducting cuprates: We describe the spectral properties of underdoped cuprates as resulting from
a momentum-dependent pseudogap in the normal state spectrum. Such a model
accounts, within a BCS approach, for the doping dependence of the critical
temperature and for the two-parameter leading-edge shift observed in the
cuprates. By introducing a phenomenological temperature dependence of the
pseudogap, which finds a natural interpretation within the stripe
quantum-critical-point scenario for high-T_c superconductors, we reproduce also
the T_c-T^* bifurcation near optimum doping. Finally, we briefly discuss the
different role of the gap and the pseudogap in determining the spectral and
thermodynamical properties of the model at low temperatures. | cond-mat_supr-con |
Topological Anomalous Skin Effect in Weyl Superconductors: We show that a Weyl superconductor can absorb light via a novel
surface-to-bulk mechanism, which we dub the topological anomalous skin effect.
This occurs even in the absence of disorder for a single-band superconductor,
and is facilitated by the topological splitting of the Hilbert space into bulk
and chiral surface Majorana states. In the clean limit, the effect manifests as
a characteristic absorption peak due to surface-bulk transitions. We also
consider the effects of bulk disorder, using the Keldysh response theory. For
weak disorder, the bulk response is reminiscent of the Mattis-Bardeen result
for $s$-wave superconductors, with strongly suppressed spectral weight below
twice the pairing energy, despite the presence of gapless Weyl points. For
stronger disorder, the bulk response becomes more Drude-like and the $p$-wave
features disappear. We show that the surface-bulk signal survives when combined
with the bulk in the presence of weak disorder. The topological anomalous skin
effect can therefore serve as a fingerprint for Weyl superconductivity. We also
compute the Meissner response in the slab geometry, incorporating the effect of
the surface states. | cond-mat_supr-con |
Is spontaneous vortex generation in superconducting 4Hb-TaS$_2$ from
vison-vortex nucleation with $\mathbb{Z}_2$ topological order?: We propose the superconducting van der Waals material 4Hb-TaS$_2$ to realize
the $\mathbb{Z}_2$ topological order and interpret the recent discovery of the
spontaneous vortex generation in 4Hb-TaS$_2$ as the vison-vortex nucleation.
For the alternating stacking of metallic/superconducting and Mott insulating
layers in 4Hb-TaS$_2$, we expect the local moments in the Mott insulating
1T-TaS$_2$ layer to form the $\mathbb{Z}_2$ topological order. The spontaneous
vortex generation in 4Hb-TaS$_2$ is interpreted from the transition or
nucleation between the superconducting vortex and the $\mathbb{Z}_2$ vison in
different phase regimes. Differing from the single vison-vortex nucleation in
the original Senthil-Fisher's cuprate proposal, we consider such nucleation
process between the superconducting vortex lattice and the vison crystal. We
further propose experiments to distinguish this proposal with the
$\mathbb{Z}_2$ topological order from the chiral spin liquid scenarios. | cond-mat_supr-con |
Frustrated Magnetic Interactions, Giant Magneto-Elastic Coupling, and
Magnetic Phonons in Iron-Pnictides: We present a detailed first principles study of Fe-pnictides with particular
emphasis on competing magnetic interactions, structural phase transition, giant
magneto-elastic coupling and its effect on phonons. The exchange interactions
$J_{i,j}(R)$ are calculated up to $\approx 12 $\AA $. We find that $J_{i,j}(R)$
has an oscillatory character with an envelop decaying as $1/R^3$ along the
stripe-direction while it is very short range along the diagonal direction and
antiferromagnetic. A brief discussion of the neutron scattering determination
of these exchange constants from a single crystal sample with orthorhombic
twinning is given. The lattice parameter dependence of the exchange constants,
$dJ_{i,j}/da$ are calculated for a simple spin-Peierls like model to explain
the fine details of the tetragonal-orthorhombic phase transition. We then
discuss giant magneto-elastic effects in these systems. We show that when the
Fe-spin is turned off the optimized c-values are shorter than experimetnal
values by 1.4 \AA $ $ for CaFe$_2$As$_2$, by 0.4 \AA $ $ for BaFe$_2$As$_2$,
and by 0.13 \AA $ $ for LaOFeAs. Finally, we show that Fe-spin is also required
to obtain the right phonon energies, in particular As c-polarized and Fe-Fe
in-plane modes. Since treating iron as magnetic ion always gives much better
results than non-magnetic ones and since there is no large c-axis reduction
during the normal to superconducting phase transition, the iron magnetic moment
should be present in Fe-pnictides at all times. We discuss the implications of
our results on the mechanism of superconductivity in these fascinating
Fe-pnictide systems. | cond-mat_supr-con |
Design and implementation of a micro-coil induction magnetometer: We present a micron-sized induction magnetometer designed to measure the
magnetic response of superconducting mesoscopic samples. The device is
manufactured using the Memscap PolyMUMPs process and consists of two octagonal
planar parallel micro-coils covering an area of 240 microns X 240 microns,
which are separated by only 2.75 microns. We show that this design is
sufficiently sensitive to detect the Meissner transition at zero dc field, of a
high-Tc superconductor Bi2Sr2CaCu2O8 disk of 40 microns in diameter and 1
micron in thicknesses. | cond-mat_supr-con |
SU(4) Dynamical Symmetry and the Origin of Pseudogaps: This paper has been withdrawn by the author due to incomplete interpretation
for the results. | cond-mat_supr-con |
A Terraced Scanning Superconducting Quantum Interference Device
Susceptometer with Sub-Micron Pickup Loops: Superconducting Quantum Interference Devices (SQUIDs) can have excellent spin
sensitivity depending on their magnetic flux noise, pick-up loop diameter, and
distance from the sample. We report a family of scanning SQUID susceptometers
with terraced tips that position the pick-up loops 300 nm from the sample. The
600 nm - 2 um pickup loops, defined by focused ion beam, are integrated into a
12-layer optical lithography process allowing flux-locked feedback, in situ
background subtraction and optimized flux noise. These features enable a
sensitivity of ~70 electron spins per root Hertz at 4K. | cond-mat_supr-con |
Spin-excitation anisotropy in the nematic state of detwinned FeSe: The origin of the electronic nematicity in FeSe is one of the most important
unresolved puzzles in the study of iron-based superconductors. In both spin-
and orbital-nematic models, the intrinsic magnetic excitations at
$\mathbf{Q}_1=(1, 0)$ and $\mathbf{Q}_2=(0, 1)$ of twin-free FeSe are expected
to provide decisive criteria for clarifying this issue. Although a
spin-fluctuation anisotropy below 10 meV between $\mathbf{Q}_1$ and
$\mathbf{Q}_2$ has been observed by inelastic neutron scattering around
$T_c\sim 9$ K ($<<T_s\sim 90$ K), it remains unclear whether such an anisotropy
also persists at higher energies and associates with the nematic transition
$T_{\rm s}$. Here we use resonant inelastic x-ray scattering (RIXS) to probe
the high-energy magnetic excitations of uniaxial-strain detwinned FeSe and
{\BFA}. A prominent anisotropy between the magnetic excitations along the $H$
and $K$ directions is found to persist to $\sim200$ meV in FeSe, which is even
more pronounced than the anisotropy of spin waves in {\BFA}. This anisotropy
decreases gradually with increasing temperature and finally vanishes at a
temperature around the nematic transition temperature $T_{\rm s}$. Our results
reveal an unprecedented strong spin-excitation anisotropy with a large energy
scale well above the $d_{xz}/d_{yz}$ orbital splitting, suggesting that the
nematic phase transition is primarily spin-driven. Moreover, the measured
high-energy spin excitations are dispersive and underdamped, which can be
understood from a local-moment perspective. Our findings provide the
much-needed understanding of the mechanism for the nematicity of FeSe and
points to a unified description of the correlation physics across seemingly
distinct classes of Fe-based superconductors. | cond-mat_supr-con |
Exploring the vortex phase diagram of Bogoliubov-de Gennes disordered
superconductors: We study the interplay of vortices and disorder in a two-dimensional
disordered superconductor at zero temperature described by the Bogoliubov-de
Gennes (BdG) self-consistent formalism for lattices of sizes up to
$100\times100$ where the magnetic flux is introduced by the Peierls's
substitution. The substantial larger size than in previous approaches ($\leq
36\times 36$) has allowed us to identify a rich phase diagram as a function of
the magnetic flux and the disorder strength. For sufficiently weak disorder,
and not too strong magnetic flux, we observe a slightly distorted Abrikosov
triangular vortex lattice. An increase in the magnetic flux leads to an
unexpected rectangular vortex lattice. A further increase in disorder, or flux
gradually destroy the lattice symmetry though strong vortex repulsion persists.
An even stronger disorder leads to deformed single vortices with an
inhomogeneous core. As number of vortices increases, vortices overlap becomes
more frequent. Finally, we show that global phase coherence is a feature of all
these phases and that disorder enhances substantially the critical magnetic
flux with respect to the clean limit with a maximum on the metallic side of the
insulating transition. | cond-mat_supr-con |
Physical mechanisms of timing jitter in photon detection by current
carrying superconducting nanowires: We studied timing jitter in the appearance of photon counts in meandering
nanowires with different fractional amount of bends. Timing jitter, which is
the probability density of the random time delay between photon absorption in
current-carrying superconducting nanowire and appearance of the normal domain,
reveals two different underlying physical scenarios. In the deterministic
regime, which is realized at large currents and photon energies, jitter is
controlled by position dependent detection threshold in straight parts of
meanders and decreases with the current. At small photon energies, jitter
increases and its current dependence disappears. In this probabilistic regime
jitter is controlled by Poisson process in that magnetic vortices jump randomly
across the wire in areas adjacent to the bends. | cond-mat_supr-con |
1D Bose Gases in an Optical Lattice: We report on the study of the momentum distribution of a one-dimensional Bose
gas in an optical lattice. From the momentum distribution we extract the
condensed fraction of the gas and thereby measure the depletion of the
condensate and compare it with a theorical estimate. We have measured the
coherence length of the gas for systems with average occupation $\bar{n}>1$ and
$\bar{n}<1$ per lattice site. | cond-mat_supr-con |
Band-mixing-mediated Andreev reflection of semiconductor holes: We have investigated Andreev-reflection processes occurring at a clean
interface between a $p$-type semiconductor and a conventional superconductor.
Our calculations are performed within a generalized Bogoliubov-de Gennes
formalism where the details of the semiconductor band structure are described
by a $6\times 6$ Kane model. It is found that Andreev reflection of light-hole
and heavy-hole valence-band carriers is generally possible and that the two
valence-band hole types can be converted into each other in the process. The
normal-reflection and Andreev-reflection amplitudes depend strongly on the
semiconductor's carrier concentration and on the angle of injection. In the
special case of perpendicular incidence, Andreev reflection of heavy holes does
not occur. Moreover, we find conversion-less Andreev reflection to be
impossible above some critical angle, and another critical angle exists above
which the conversion of a heavy hole into a light hole cannot occur. | cond-mat_supr-con |
Extraordinary magnetoresistance in graphite: experimental evidence for
the time-reversal symmetry breaking: The ordinary magnetoresistance (MR) of doped semiconductors is positive and
quadratic in a low magnetic field, B, as it should be in the framework of the
Boltzmann kinetic theory or in the conventional hopping regime. We observe an
unusual highly-anisotropic in-plane MR in graphite, which is neither quadratic
nor always positive. In a certain current direction MR is negative and linear
in B in fields below a few tens of mT with a crossover to a positive MR at
higher fields, while in a perpendicular current direction we observe a giant
super-linear and positive MR. These extraordinary MRs are respectively
explained by a hopping magneto-conductance via non-zero angular momentum
orbitals, and by the magneto-conductance of inhomogeneous media. The linear
orbital NMR is a unique signature of the broken time-reversal symmetry (TRS) in
graphite. While some local paramagnetic centers could be responsible for the
broken TRS, the observed large diamagnetism suggests a more intriguing
mechanism of this breaking, involving superconducting clusters with
unconventional (chiral) order parameters and spontaneously generated
normal-state current loops in graphite. | cond-mat_supr-con |
Thermal evolution of antiferromagnetic correlations and tetrahedral bond
angles in superconducting FeTe$_{1-x}$Se$_x$: It has recently been demonstrated that dynamical magnetic correlations
measured by neutron scattering in iron chalcogenides can be described with
models of short-range correlations characterized by particular {choices of
four-spin plaquettes, where the appropriate choice changes as the} parent
material is doped towards superconductivity. Here we apply such models to
describe measured maps of magnetic scattering as a function of two-dimensional
wave vectors obtained for optimally superconducting crystals of
FeTe$_{1-x}$Se$_x$. We show that the characteristic antiferromagnetic wave
vector evolves from that of the bicollinear structure found in underdoped
chalcogenides (at high temperature) to that associated with the stripe
structure of antiferromagnetic iron arsenides (at low temperature); {these can
both be described with the same local plaquette, but with different
inter-plaquette correlations}. While the magnitude of the low-energy magnetic
spectral weight is substantial at all temperatures, it actually weakens
somewhat at low temperature, where the charge carriers become more itinerant.
The observed change in spin correlations is correlated with the dramatic drop
in the electronic scattering rate and the growth of the bulk nematic response
on cooling. Finally, we also present powder neutron diffraction results for
lattice parameters in FeTe$_{1-x}$Se$_x$ indicating that the tetrahedral bond
angle tends to increase towards the ideal value on cooling, in agreement with
the increased screening of the crystal field by more itinerant electrons and
the correspondingly smaller splitting of the Fe $3d$ orbitals. | cond-mat_supr-con |
Electronic structure and Fermi surface of iron-based superconductors
R2Fe3Si5 (R = Lu;Y;Sc) from first principles: Electronic structures of three superconducting rare-earth iron silicides
(Lu;Y;Sc)2Fe3Si5 and non-superconducting Lu2Ru3Si5, adopting a tetragonal
crystal structure (P4/mnc), have been calculated employing the full-potential
local-orbital method within the density functional theory. The investigations
were focused particularly on the band structures and Fermi surfaces, existing
in four bands and containing rather three-dimensional electronlike and holelike
sheets. They support an idea of unconventional multi-band superconductivity in
these ternaries, proposed earlier by other authors for Lu2Fe3Si5, based on
heat-capacity, resistivity, electromagnetic and muon spin rotation
measurements. Finally, a discussion on differences in the electronic structures
between the investigated here and other common families of iron-based
superconductors is carried out. | cond-mat_supr-con |
Field dependence of the vortex core size in a multi-band superconductor: The magnetic field dependence of the vortex core size in the multi-band
superconductor NbSe2 has been determined from muon spin rotation measurements.
The spatially extended nature of the quasiparticle core states associated with
the smaller gap leads to a rapid field-induced shrinkage of the core size at
low fields, while the more tightly bound nature of the states associated with
the larger gap leads to a field-independent core size for fields greater than 4
kOe. A simple model is proposed for the density of delocalized core states that
establishes a direct relationship between the field-induced reduction of the
vortex core size and the corresponding enhancement of the electronic thermal
conductivity. We show that this model accurately describes both NbSe2 and the
single-band superconductor V3Si. | cond-mat_supr-con |
Electron Correlations in the High Tc-Compounds: Ab-initio correlation results for an idealized high Tc-compound are compared
to density functional (DF) calculations for the same system. It is shown that
and why the DF-charge distribution is wrong. The largest deficiency arises for
the Cu-d(x2-y2)-occupation, originating from strong atomic correlations but
mostly from anomalous neighbor Cu-spin correlations. Both features are beyound
the range of the homogeneous electron gas approximation underlying the
DF-schemes. The ab-initio results also exclude a description of the real system
in a Mott-Hubbard scenario, that is mostly chosen in theory. | cond-mat_supr-con |
Structural and Magnetic Instabilities of La$_{2-x}$Sr$_x$CaCu$_2$O$_6$: A neutron scattering study of nonsuperconducting
La$_{2-x}$Sr$_x$CaCu$_2$O$_6$ (x=0 and 0.2), a bilayer copper oxide without CuO
chains, has revealed an unexpected tetragonal-to-orthorhombic transition with a
doping dependent transition temperature. The predominant structural
modification below the transition is an in-plane shift of the apical oxygen. In
the doped sample, the orthorhombic superstructure is strongly disordered, and a
glassy state involving both magnetic and structural degrees of freedom develops
at low temperature. The spin correlations are commensurate. | cond-mat_supr-con |
Staggered Pairing Phenomenology for UPd_2Al_3 and UNi_2Al_3: We apply the staggered-pairing Ginzburg-Landau phenomenology to describe
superconductivity in UPd_2Al_3 and UNi_2Al_3. The phenomenology was applied
successfully to UPt_3 so it explains why these materials have qualitatively
different superconducting phase diagrams although they have the same
point-group symmetry. UPd_2Al_3 and UNi_2Al_3 have a two-component
superconducting order parameter transforming as an H-point irreducible
representation of the space group. Staggered superconductivity can induce
charge-density waves characterized by new Bragg peaks suggesting experimental
tests of the phenomenology. | cond-mat_supr-con |
Superconductivity in silicon: a review: Silicon, one of the most abundant elements found on Earth, has been an
excellent choice of the semiconductor industry for ages. Despite it's
remarkable applications in modern semiconductor-based electronic devices, the
potential of cubic silicon in superconducting electronics remained a challenge
because even heavily doped silicon crystals do not superconduct under normal
conditions. It is apparent that if superconductivity can be introduced in cubic
silicon, that will bring a breakthrough in low-dissipation electronic
circuitry. Motivated by this, attempts have been made by several research
groups to induce superconductivity in silicon through a number of different
routes. Some of the other structural phases of silicon like $\beta$-Sn and
simple hexagonal are, however, known to display superconductivity. In the
present review article, various theoretical and experimental aspects of
superconductivity in silicon are discussed. Superconductivity in different
phases and different structural forms of silicon are also reviewed. We also
highlight the potential of superconducting phases of silicon for technological
applications in super-conducting nano-electronics. | cond-mat_supr-con |
Field Dependence of the Superconducting Basal Plane Anisotropy of
TmNi2B2C: The superconductor TmNi2B2C possesses a significant four-fold basal plane
anisotropy, leading to a square Vortex Lattice (VL) at intermediate fields.
However, unlike other members of the borocarbide superconductors, the
anisotropy in TmNi2B2C appears to decrease with increasing field, evident by a
reentrance of the square VL phase. We have used Small Angle Neutron Scattering
measurements of the VL to study the field dependence of the anisotropy. Our
results provide a direct, quantitative measurement of the decreasing
anisotropy. We attribute this reduction of the basal plane anisotropy to the
strong Pauli paramagnetic effects observed in TmNi2B2C and the resulting
expansion of vortex cores near Hc2. | cond-mat_supr-con |
High-pressure synthesis and superconductivity of the novel Laves phase
BaIr2: Superconductors comprising 5d transition metals of Ir and Pt have been widely
explored because they have the potential of unique superconductivity caused by
the strong spin-orbit interaction (SOI). We successfully synthesized BaIr2, the
last Laves phase remaining unsynthesized in the MgCu2-type AM2 (A = Ca, Sr, Ba;
M = Rh, Pd, Ir, Pt). BaIr2 was crystallized at 925 C under a pressure of 3.3
GPa via a solid-state reaction between Ba and Ir powders; it was found to have
the longest a-lattice constant of 8.038(1) A among AM2. BaIr2 exhibited bulk
superconductivity at a transition temperature (Tc) of 2.7 K. BaIr2 was found to
have a type-II superconductor with an upper critical field of 67.7 kOe, which
was above the Pauli paramagnetic limit (50 kOe). The electron-phonon coupling
constant and normalized specific heat jump were measured to be 0.63 and 1.2,
respectively, indicating that BaIr2 is a weak-coupling superconductor. The
electronic-structure calculations for BaIr2 revealed that the Ir-5d states are
dominant at the Fermi energy (EF) and the density of states at the EF is
strongly affected by SOI as in the case of CaIr2 and SrIr2. | cond-mat_supr-con |
Minimal timing jitter in superconducting nanowire single photon
detectors: Using two-temperature model coupled with modified time-dependent
Ginzburg-Landau equation we calculate the delay time $\tau_d$ in appearance of
growing normal domain in the current-biased superconducting strip after
absorption of the single photon. We demonstrate that $\tau_d$ depends on the
place in the strip where photon is absorbed and monotonically decreases with
increasing of the current. We argue, that the variation of $\tau_d$ (timing
jitter), connected either with position-dependent response or Fano fluctuations
could be as small as the lowest relaxation time of the superconducting order
parameter $\sim \hbar/k_BT_c$ ($T_c$ is the critical temperature of the
superconductor) when the current approaches the depairing current. | cond-mat_supr-con |
Switching current distributions and subgap structures of underdoped
(Hg,Re)Ba2Ca2Cu3O8+d intrinsic Josephson junctions: We have investigated the intrinsic Josephson properties in slightly
underdoped (Hg,Re)Ba2Ca2Cu3Oy [Hg(Re)1223] intrinsic Josephson junctions (IJJs)
with a dimension of 1.0x1.5x0.11 um^3. The current-voltage characteristics of
the IJJs exhibit clear multiple branches with subgap structures similar to
those of other cuprate superconductors. The switching current distributions
P(I) from the zero-voltage to the nonzero-voltage state in the current-biased
IJJs agree well with the theoretical curves of the thermally assisted escape
model at temperatures above ~5 K. The plasma frequency fp of the IJJs is
estimated to be 1.3 THz from the fluctuation-free critical current density of
2.0x10^5 A/cm2, which is one of the highest among cuprate superconductors,
reflecting the high Tc and a relatively low anisotropy of the Re doped Hg
system. The P(I) gradually becomes independent of temperature below ~5 K, which
suggests a crossover of the escape process from thermal activation to quantum
tunneling at such a high temperature. | cond-mat_supr-con |
Superuid density in overdoped cuprates: thin films versus bulk samples: Recent study of overdoped La$_{2-x}$Sr$_x$CuO$_4$ cuprate superconductor thin
films by Bo\v{z}ovi\'{c} {\it et al.} has revealed several unexpected findings,
most notably the violation of the BCS description which was believed to
adequately describe overdoped cuprates. In particular, it was found that the
superfluid density in La$_{2-x}$Sr$_x$CuO$_4$ films decreases on the overdoped
side as a linear function of critical temperature T$_c$, which was taken as
evidence for the violation of the Homes' law. We show explicitly that the law
is indeed violated, and as the main reason for violation we find that the
superfluid density in Bo\v{z}ovi\'{c}'s films is suppressed more strongly than
in bulk samples. Based on the existing literature data, we show that the
superfluid density in bulk cuprate samples does not decrease with doping, but
instead tends to saturate on the overdoped side. The result is also supported
by our recent measurement of a heavily overdoped bulk La$_{2-x}$Sr$_x$CuO$_4$
sample. Moreover, this saturation of superfluid density might not be limited to
cuprates, as we find evidence for similar behavior in two pnictide
superconductor families. We argue that quantum phase fluctuations play an
important role in suppressing the superfluid density in thin films. | cond-mat_supr-con |
Electronic structure in one-Fe Brillouin zone of iron-pnictide
superconductor CsFe$_2$As$_2$: The multiband nature of iron-pnictide superconductors is one of the keys to
the understanding of their intriguing behavior. The electronic and magnetic
properties heavily rely on the multiband interactions between different
electron and hole pockets near the Fermi level. At the fundamental level,
though many theoretical models were constructed on the basis of the so-called
1-Fe Brillouin zone (BZ) with an emphasis of the basic square lattice of iron
atoms, most electronic structure measurements were interpreted in the 2-Fe BZ.
Whether the 1-Fe BZ is valid in a real system is still an open question. Using
angle-resolved photoemission spectroscopy (ARPES), here we show in an extremely
hole-doped iron-pnictide superconductor CsFe$_2$As$_2$ that the distribution of
electronic spectral weight follows the 1-Fe BZ, and that the emerging band
structure bears some features qualitatively different from theoretical band
structures of the 1-Fe BZ. Our analysis suggests that the interlayer separation
is an important tuning factor for the physics of FeAs layers, the increase of
which can reduce the coupling between Fe and As and lead to the emergence of
the electronic structure in accord with the 1-Fe symmetry of the Fe square
lattice. Our finding puts strong constraints on the theoretical models
constructed on the basis of the 1-Fe BZ. | cond-mat_supr-con |
High-temperature superconductivity in one-unit-cell FeSe films: Since the dramatic interface enhancement of superconducting transition
temperature (Tc) was reported in one unit-cell FeSe film grown on SrTiO3
substrate (1-UC FeSe/STO) by molecular beam epitaxy (MBE), related research on
this system has become a new frontier in condensed matter physics. In this
paper, we present a brief review on this rapidly developing field, mainly
focusing on the superconducting properties of 1-UC FeSe/STO. Experimental
evidences for the high-temperature superconductivity in 1-UC FeSe/STO,
including the direct evidences revealed by transport and diamagnetic
measurements, and other evidences from scanning tunneling microscope (STM) and
angle-resolved photoemission spectroscopy (ARPES), are overviewed. Potential
mechanisms of the enhanced superconductivity are discussed. There are
accumulating arguments suggesting that the strengthened Cooper pairing in 1-UC
FeSe/STO originates from the interface effects, specifically charge transfer
and coupling to phonon modes in TiO2 plane. The study of superconductivity in
1-UC FeSe/STO not only sheds a new light on the mechanism of high-temperature
superconductors with layered structures, but also provides the insight to
explore new superconductors by interface engineering. | cond-mat_supr-con |
Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron-Pnictide
Superconductor BaFe$_{1.8}$Co$_{0.2}$As$_2$: We present an atomic resolution scanning tunneling spectroscopy study of
superconducting BaFe$_{1.8}$Co$_{0.2}$As$_2$ single crystals in magnetic fields
up to $9 \text{Tesla}$. At zero field, a single gap with coherence peaks at
$\overline{\Delta}=6.25 \text{meV}$ is observed in the density of states. At $9
\text{T}$ and $6 \text{T}$, we image a disordered vortex lattice, consistent
with isotropic, single flux quantum vortices. Vortex locations are uncorrelated
with strong scattering surface impurities, demonstrating bulk pinning. The
vortex-induced sub-gap density of states fits an exponential decay from the
vortex center, from which we extract a coherence length $\xi=27.6\pm 2.9
\text{\AA}$, corresponding to an upper critical field $H_{c2}=43 \text{T}$. | cond-mat_supr-con |
Electromagnetic response of superconductors and optical sum rule: The interrelation between the condensation energy and the optical sum rules
has been investigated. It has been shown that the so called 'partial' sum rule
violation is related mainly to a temperature dependence of the relaxation rate
rather than to the appearance of superconductivity itself. Moreover, we
demonstrate that the experimental data on the temperature dependence of the
optical sum rule can be explained rather well by an account of strong
electron-phonon interaction. | cond-mat_supr-con |
Gapless helical superconductivity on the surface of a three-dimensional
topological insulator: Recent angle-resolved photoemission experiments have observed a
proximity-induced superconducting gap in the helical surface states of a thin
film of the 3D topological insulator Bi$_2$Se$_3$ grown on a superconducting
NbSe$_2$ substrate. The superconducting coherence peaks in the electronic
density of states are strongly suppressed when the topological insulator is
doped with magnetic Mn impurities, which was interpreted as the complete
destruction of helical superconductivity in the topological surface states.
Motivated by these experiments, we explore a different possibility: gapless
helical superconductivity, where a gapless electronic density of states
coexists with a nonzero helical superconducting order parameter. We study a
model of superconducting Dirac fermions coupled to random magnetic impurities
within the Abrikosov-Gor'kov framework, and find finite regions of gapless
helical superconductivity in the phase diagram of the system for both
proximity-induced and intrinsic superconductivity. For the latter, we derive
universal rates of supression of the superconducting transition temperature due
to magnetic scattering and, for a Fermi level at the Dirac point, a universal
rate of increase of the quantum critical attraction strength. | cond-mat_supr-con |
Microscopic mechanism for fluctuating pair density wave: In weakly coupled BCS superconductors, only electrons within a tiny energy
window around the Fermi energy, $E_F$, form Cooper pairs. This may not be the
case in strong coupling superconductors such as cuprates, FeSe, SrTiO$_3$ or
cold atom condensates where the pairing scale, $E_B$, becomes comparable or
even larger than $E_F$. In cuprates, for example, a plausible candidate for the
pseudogap state at low doping is a fluctuating pair density wave, but no
microscopic model has yet been found which supports such a state. In this work,
we write an analytically solvable model to examine pairing phases in the
strongly coupled regime and in the presence of anisotropic interactions.
Already for moderate coupling we find an unusual finite temperature phase,
below an instability temperature $T_i$, where local pair correlations have
non-zero center-of-mass momentum but lack long-range order. At low temperature,
this fluctuating pair density wave can condense either to a uniform $d$-wave
superconductor or the widely postulated pair-density wave phase depending on
the interaction strength. Our minimal model offers a unified microscopic
framework to understand the emergence of both fluctuating and long range pair
density waves in realistic systems. | cond-mat_supr-con |
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