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Diamagnetism of real-space pairs above Tc in hole doped cuprates: The nonlinear normal state diamagnetism reported by Lu Li et al. [Phys. Rev.
B 81, 054510 (2010)] is shown to be incompatible with an acclaimed Cooper
pairing and vortex liquid above the resistive critical temperature. Instead it
is perfectly compatible with the normal state Landau diamagnetism of real-space
composed bosons, which describes the nonlinear magnetization curves in less
anisotropic cuprates La-Sr-Cu-O (LSCO) and Y-Ba-Cu-O (YBCO) as well as in
strongly anisotropic bismuth-based cuprates in the whole range of available
magnetic fields. | cond-mat_supr-con |
Shaping graphene superconductivity with nanometer precision: Graphene holds great potential for superconductivity due to its pure
two-dimensional nature, the ability to tune its carrier density through
electrostatic gating, and its unique, relativistic-like electronic properties.
At present, we are still far from controlling and understanding graphene
superconductivity, mainly because the selective introduction of superconducting
properties to graphene is experimentally very challenging. Here, we have
developed a method that enables shaping at will graphene superconductivity
through a precise control of graphene-superconductor junctions. The method
combines the proximity effect with scanning tunnelling microscope (STM)
manipulation capabilities. We first grow Pb nano-islands that locally induce
superconductivity in graphene. Using a STM, Pb nano-islands can be selectively
displaced, over different types of graphene surfaces, with nanometre scale
precision, in any direction, over distances of hundreds of nanometres. This
opens an exciting playground where a large number of predefined
graphene-superconductor hybrid structures can be investigated with atomic scale
precision. To illustrate the potential, we perform a series of experiments,
rationalized by the quasi-classical theory of superconductivity, going from the
fundamental understanding of superconductor-graphene-superconductor
heterostructures to the construction of superconductor nanocorrals, further
used as "portable" experimental probes of local magnetic moments in graphene. | cond-mat_supr-con |
Vortex Interaction with Mesoscopic Irregularities in High Temperature
Superconductors: The conformal mapping method is used to study the problem of flux line
interaction with surface cavities having cylindrical profile and characteristic
size much less than the penetration length, i.e, within mesoscopic scale. It is
shown that the metastable states are achieved when the dimensions of the
surface irregularities do not exceed the coherence length. Our study shows that
the surface barrier may vanished at some weak point at which the surface
irregularities have mesoscopic scales. On the other hand, the surface barrier
is completely disappeared when the surface defects size is more greater than
penetration length. Our results are compared with the available experimental
data and theoretical results. | cond-mat_supr-con |
Magnetic-Field-Induced Localization of Quasiparticles in Underdoped
La$_{2-x}$Sr$_x$CuO$_4$ Single Crystals: Magnetic-field-induced ordering of electrons around vortices is a striking
phenomenon recently found in high-$T_c$ cuprates. To identify its consequence
in the quasiparticle dynamics, the magnetic-field ($H$) dependence of the
low-temperature thermal conductivity $\kappa$ of La$_{2-x}$Sr$_x$CuO$_4$
crystals is studied for a wide doping range. It is found that the behavior of
$\kappa(H)$ in the sub-Kelvin region changes drastically across optimum doping,
and the data for underdoped samples are indicative of unusual
magnetic-field-induced localization of quasiparticles; this localization
phenomenon is probably responsible for the unusual "insulating normal state"
under high magnetic fields. | cond-mat_supr-con |
The dual nature of magnetism in a uranium heavy fermion system: The duality between localized and itinerant nature of magnetism in
$5\textit{f}$ electron systems has been a longstanding puzzle. Here, we report
inelastic neutron scattering measurements, which reveal both local and
itinerant aspects of magnetism in a single crystalline system of
UPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum
of diffuse magnetic scattering with a resonance-like gap of $\approx$ 7 meV,
and surprising absence of coherent spin-waves, suggestive of itinerant
magnetism. While the gap closes above the Neel temperature, strong dynamic spin
correlations persist to high temperature. Nevertheless, the size and
temperature dependence of the total magnetic spectral weight can be well
described by local moment with $J=4$. Furthermore, polarized neutron
measurements reveal that the magnetic fluctuations are mostly transverse, with
little or none of the longitudinal component expected for itinerant moments.
These results suggest that a dual description of local and itinerant magnetism
is required to understand UPt$_{2}$Si$_{2}$, and by extension, other 5$f$
systems in general. | cond-mat_supr-con |
Vortex pattern in a nanoscopic cylinder: A superconducting nanoscopic cylinder, with radius $R = 4.0\xi$ and height $D
= 4.0\xi$ is submitted to an applied field along the cylinder axis. The
Ginzburg-Landau theory is solved in three-dimensions using the simulated
annealing technique to minimize the free energy functional. We obtain different
vortex patterns, some of which are giant vortices and up to twelve vortices are
able to fit inside the cylinder | cond-mat_supr-con |
Thermal Conductivity as a Probe of Quasi-Particles in the Cuprates: In underdoped YBa_2Cu_3O_x (x=6.63), the low-T thermal conductivity Kappa_xx
varies steeply with field B at small B, and saturates to a nearly
field-independent value at high fields. The simple expression [1+p(T)|B|]^(-1)
provides an excellent fit to Kappa_xx(B) over a wide range of fields. From the
fit, we extract the zero-field mean-free-path, and the low temperature behavior
of the QP current. The procedure also allows the QP Hall angle Theta_QP to be
obtained. We find that Theta_QP falls on the 1/T^2 curve extrapolated from the
electrical Hall angle above Tc. Moreover, it shares the same T dependence as
the field scale p(T) extracted from Kappa_xx. We discuss implications of these
results. | cond-mat_supr-con |
Comment on ``Critical current density from magnetization hysteresis data
using the critical-state model'': A recent paper [Physical Review {\bf B 64} 014508 (2001)] claims to present
an exact method to extract the critical current density J$_C$ from the M-H
hysteresis curve. We show that this claim appears unjustified. | cond-mat_supr-con |
Vortex Phase Diagram of weakly pinned YBa$_2$Cu$_3$O$_{7-δ}$ for H
$\parallel$ c: Vortex phase diagram in a weakly pinned crystal of YBCO for H $\parallel$ c
is reviewed in the light of a recent elucidation of the process of `inverse
melting' in a Bismuth cuprate system and the imaging of an interface between
the ordered and the disordered regions across the peak effect in 2H-NbSe$_2$.
In the given YBCO crystal, a clear distinction can be made between the second
magnetization peak (SMP) and the peak effect (PE) between 65 K and 75 K. The
field region between the peak fields of the SMP (H$^m_{smp}$) and the onset
fields of the PE (H$^{on}_{pe}$)is not only continuously connected to the Bragg
glass phase at lower fields but it is also sandwiched between the higher
temperature vortex liquid phase and the lower temperature vortex glass phase.
Thus, an ordered vortex state between H$^m_{smp}$ and H$^{on}_{pe}$ can get
transformed to the (disordered) vortex liquid state on heating as well as to
the (disordered) vortex glass state on cooling, a situation analogous to the
thermal melting and the inverse melting phenomenon seen in a Bismuth cuprate. | cond-mat_supr-con |
Microwave absorption in s- and d-wave disordered superconductors: We model s- and d-wave ceramic superconductors with a three-dimensional
lattice of randomly distributed Josephson junctions with finite
self-inductance. The field and temperature dependences of the microwave
absoption are obtained by solving the corresponding Langevin dynamical
equations. We find that at magnetic field H=0 the microwave absoption of the
s-wave samples, when plotted against the field, has a minimum at any
temperature. In the case of d-wave superconductors one has a peak at H=0 in the
temperature region where the paramagnetic Meissner effect is observable. These
results agree with experiments. The dependence of the microwave absorption on
the screening strength was found to be nontrivial due to the crossover from the
weak to the strong screening regime. | cond-mat_supr-con |
A Trapped Field of 17.6 T in Melt-Processed, Bulk Gd-Ba-Cu-O Reinforced
with Shrink-Fit Steel: The ability of large grain, REBa$_{2}$Cu$_{3}$O$_{7-\delta}$ [(RE)BCO; RE =
rare earth] bulk superconductors to trap magnetic field is determined by their
critical current. With high trapped fields, however, bulk samples are subject
to a relatively large Lorentz force, and their performance is limited primarily
by their tensile strength. Consequently, sample reinforcement is the key to
performance improvement in these technologically important materials. In this
work, we report a trapped field of 17.6 T, the largest reported to date, in a
stack of two, silver-doped GdBCO superconducting bulk samples, each of diameter
25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with
shrink-fit stainless steel. This sample preparation technique has the advantage
of being relatively straightforward and inexpensive to implement and offers the
prospect of easy access to portable, high magnetic fields without any
requirement for a sustaining current source. | cond-mat_supr-con |
Low-temperature high-frequency dynamic magnetic susceptibility of
classical spin-ice Dy$_2$Ti$_2$O$_7$: Radio-frequency (14.6 MHz) AC magnetic susceptibility, $\chi^{\prime}_{AC}$,
of \dytio\ was measured using a self-oscillating tunnel-diode resonator.
Measurements were made with the excitation AC field parallel to the
superimposed DC magnetic field up 5 T in a wide temperature range from 50 mK to
100 K. At 14.6 MHz a known broad peak of $\chi^{\prime}_{AC}(T)$ from kHz -
range audio-frequency measurements around 15~K for both [111] and [110]
directions shifts to 45~K, continuing the Arrhenius activated behavior with the
same activation energy barrier of $E_a \approx 230$~K. Magnetic field
dependence of $\chi^{\prime}_{AC}$ along [111] reproduces previously reported
low-temperature two-in-two-out to three-in-one-out spin configuration
transition at about 1~T, and an intermediate phase between 1 and 1.5~T. The
boundaries of the intermediate phase show reasonable overlap with the
literature data and connect at a critical endpoint of the first-order
transition line, suggesting that these low-temperature features are frequency
independent. An unusual upturn of magnetic susceptibility at $T \to 0$ was
observed in magnetic fields between 1.5~T and 2~T for both magnetic field
directions, before fully polarized configuration sets in above 2~T. | cond-mat_supr-con |
Single particle tunneling spectroscopy and superconducting gaps in
layered iron based superconductor KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$: We perform scanning tunneling microscopy/spectroscopy study on the layered
iron based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with a critical temperature
of about 33.5 K. Two types of terminated surfaces are generally observed after
cleaving the samples in vacuum. On one commonly obtained surface, we observe a
full gap feature with energy gap values close to 4.6 meV. This type of spectrum
shows a clean and uniform full gap in space, which indicates the absence of gap
nodes in this superconductor. Quasiparticle interference patterns have also
been measured, which show no scattering patterns between the hole and tiny
electron pockets, but rather an intra-band scattering pattern is observed
possibly due to the hole-like $\alpha$ pocket. The Fermi energy of this band is
only about $24\pm6$ meV as derived from the energy dispersion result.
Meanwhile, impurity induced bound-state peaks can be observed at about $\pm2.2$
meV on some spectra, and the peak value seems to be independent to magnetic
field. On the second type of surface which is rarely obtained, the fully gapped
feature can still be observed on the tunneling spectra, although multiple gaps
are obtained either from a single spectrum or separate ones, and the gap values
determined from coherence peaks locate mainly in the range from 4 to 8 meV. Our
results clearly indicate multiple and nodeless superconducting gap nature in
this layered superconductor KCa$_2$Fe$_4$As$_4$F$_2$, and the superfluid is
mainly contributed by the hole-like Fermi surfaces near $\Gamma$ point. This
would inspire further consideration on the effect of the shallow and incipient
bands near M point, and help to understand the pairing mechanism in this highly
layered iron-based superconductor. | cond-mat_supr-con |
Pair correlations in the two-orbital Hubbard ladder: Implications on
superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$: Motivated by high-temperature superconductivity in pressurized
La$_3$Ni$_2$O$_7$, we investigate the pair correlations in the two-orbital
Hubbard ladder, which consists of the nearly half-filled and nearly
quarter-filled orbitals. By employing the density matrix renormalization group
method, we demonstrate that the pair correlation exhibits a power-law decay
against the distance while the spin correlation decays exponentially. The decay
exponent of the pair correlation of the nearly half-filled orbital is
comparable to the exponent of the quasi-long-range superconducting correlation
in the doped single-orbital Hubbard ladder, which suggests the importance of
the $d_{3z^2-r^2}$ orbital in La$_3$Ni$_2$O$_7$. | cond-mat_supr-con |
Topological Josephson Heat Engine: The promise of fault-tolerant quantum computing has made topological
superconductors the focus of intense research during the past decade. In this
context, topological Josephson junctions based on nanowires or on topological
insulators provide an alternative route for probing topological
superconductivity. As a hallmark of their topological nature, such junctions
exhibit a ground-state fermion parity that is $4\pi$-periodic in the
superconducting phase difference $\phi$. Finding unambiguous experimental
evidence for this $4\pi$-periodicity still proves a difficult task, however.
Here we propose a topological Josephson heat engine implemented by a
Josephson-Stirling cycle as an alternative thermodynamic approach to test the
ground-state parity. Using a Josephson junction based on a quantum spin Hall
(QSH) insulator, we show how the thermodynamic cycle can be used to test the
$4\pi$-periodicity of the topological ground state and to distinguish between
parity-conserving and non-parity-conserving engines. Interestingly, we find
that parity conservation generally boosts both the efficiency and power of the
topological heat engine with respect to its non-topological counterpart. Our
results, applicable not only to QSH-based junctions but also to any topological
Josephson junction, demonstrate the potential of the intriguing and fruitful
marriage between topology and coherent thermodynamics. | cond-mat_supr-con |
Polar Kerr effect from a time-reversal symmetry breaking unidirectional
charge density wave: We analyze the Hall conductivity $\sigma_{xy}(\omega)$ of a charge ordered
state with momentum $\mathbf{Q}=(0,2Q)$ and calculate the intrinsic
contribution to the Kerr angle $\Theta_K$ using the fully reconstructed
tight-binding band structure for layered cuprates beyond the low energy hot
spots model and particle hole symmetry. We show that such a unidirectional
charge density wave (CDW), which breaks time reversal symmetry as recently put
forward by Wang and Chubukov [Phys. Rev. B {\bf 90}, 035149 (2014)], leads to a
nonzero polar Kerr effect as observed experimentally. In addition, we model a
fluctuating CDW via a large quasiparticle damping of the order of the CDW gap
and discuss possible implications for the pseudogap phase. We can qualitatively
reproduce previous measurements of underdoped cuprates but making quantitative
connections to experiments is hampered by the sensitivity of the polar Kerr
effect with respect to the complex refractive index $n(\omega)$. | cond-mat_supr-con |
A possible cooling effect in high temperature superconductors: We show that an adiabatic increase of the supercurrent along a superconductor
with lines of nodes of the order parameter on the Fermi surface can result in a
cooling effect. The maximum cooling occurs if the supercurrent increases up to
its critical value. The effect can also be observed in a mixed state of a bulk
sample. An estimate of the energy dissipation shows that substantial cooling
can be performed during a reasonable time even in the microkelvin regime. | cond-mat_supr-con |
Spin-pumping from a ferromagnetic insulator to an unconventional
superconductor with interfacial Andreev bound-states: Spin-pumping from a ferromagnetic insulator into a high-$T_c$ superconductor
with a $d$-wave superconducting order parameter has recently been
experimentally observed. Such unconventional superconducting order is known to
produce interfacial bound-states for certain crystallographic orientations.
Here, we present a methodology which can be used to study spin-pumping into
unconventional superconductors, including the role of interfacial bound-states.
As an example, we determine how the crystallographic orientation of the
$d$-wave order parameter relative the interface changes the spin-pumping
effect. We find that the spin-pumping effect is slightly enhanced at low
temperatures for orientations hosting interfacial bound-states compared to
other superconducting states. However, the spin-pumping effect does not show a
coherence peak close to $T_c$ for such orientations, and instead remains
smaller than the normal state value for all $T$. For orientations not hosting
interfacial bound-states, we find that the pumped spin current can be increased
to several times the normal-state spin current at frequencies that are small
compared to the superconducting gap. Our results show that the spin-pumping
dependency on frequency and temperature changes qualitatively depending on the
crystallographic orientation of unconventional superconducting order parameters
relative the interface. | cond-mat_supr-con |
Impurity bands in magnetic superconductors with spin density wave: Magnetic superconductors define a broad class of strongly correlated
materials in which superconductivity may coexist with either localized or
itinerant long-range magnetic order. In this work we consider a multiband model
of a disordered magnetic superconductor which realizes coexistence of
unconventional superconductivity and a spin-density-wave. We derive an exact
$T$-matrix and compute a single particle density of states in this system. In a
purely superconducting state the interband scattering potential leads to an
appearance of the localized Yu-Shiba-Rusinov bound states. Our main finding is
that in the fairly broad swath of the coexistence region superconductivity
remains fully gapped despite the presence of the impurity bands. We also
discuss the effects of spatial inhomogeneities on the density of states in
strongly contaminated superconductors. | cond-mat_supr-con |
Quasi-1-Dimensional Superconductivity in Highly Disordered NbN Nanowires: The topic of superconductivity in strongly disordered materials has attracted
a significant attention. In particular vivid debates are related to the subject
of intrinsic spatial inhomogeneity responsible for non-BCS relation between the
superconducting gap and the pairing potential. Here we report experimental
study of electron transport properties of narrow NbN nanowires with effective
cross sections of the order of the debated inhomogeneity scales. We find that
conventional models based on phase slip concept provide reasonable fits for the
shape of the R(T) transition curve. Temperature dependence of the critical
current follows the text-book Ginzburg-Landau prediction for
quasi-one-dimensional superconducting channel Ic~(1-T/Tc)^3/2. Hence, one may
conclude that the intrinsic electronic inhomogeneity either does not exist in
our structures, or, if exist, does not affect their resistive state properties. | cond-mat_supr-con |
Overscreening in Hubbard electron systems: We show that doping-induced charge fluctuations in strongly correlated
Hubbard electron systems near the 1/2-filled, insulating limit cause
overscreening of the electron-electron Coulomb repulsion. The resulting
attractive screened interaction potential supports d_{x^2-y^2}-pairing with a
strongly peaked, doping dependent pairing strength at lower doping, followed by
s-wave pairing at larger doping levels. | cond-mat_supr-con |
Unconventional dynamical covalency driven superconductivity in Nb doped
SrTiO3: Nb doped SrTiO3, the first discovered two-gap superconductor, is shown to be
the most unconventional one of the known multiband superconductors, since the
smaller of the two superconducting gaps follows a non BCS temperature
dependence. Such a behavior stems from two cooperating effects: an extreme
anisotropy in the frequency dependent interactions, involving one very soft
mode and an almost vanishing interband interaction. In contrast to all other
multiband superconductors, the temperature dependence of the superfluid density
of Nb doped SrTiO3 is predicted to exhibit an inflection point close to Tc and
not close to T=0. | cond-mat_supr-con |
Optimization of parameters of nanostructure for study inverse proximity
effects on "superconductor-ferromagnetic" interface using Polarized Neutron
Reflectometry in enhanced standing wave regime: This work is devoted to experimental study of influence of superconductivity
(S) on ferromagnetism (FM) (inverse proximity effects) with the help of
Polarized Neutron Reflectivity. Combining meausurements of specular and diffuse
intensities it is possible to obtain full picture of magnetization change in
S/FM layered systems like magnetization rotation, domain state formation,
inducing of magnetization in S layer, etc. To increase weak magnetic signal we
propose to use enhanced neutron standing wave regime (e.g. waveguides). Choose
of materials, optimization of thicknesses of layers, estimation of roughnesses
influence is presented in this work. | cond-mat_supr-con |
Evidence for an angular dependent contribution from columnar defects to
the equilibrium magnetization of YBa_2Cu_3O_{7-d}: We have measured an angle-dependent contribution to the equilibrium
magnetization of a YBa_2Cu_3O_{7-d} single crystal with columnar defects
created by 5.8GeV Pb ions. This contribution manifests itself as a jump in the
equilibrium torque signal, when the magnetic field crosses that of the defects.
The magnitude of the signal, which is observed in a narrow temperature interval
less than 2K and for fields up to about twice the irradiation field, is used to
estimate the the energy gained by vortex pinning on the defects. The vanishing
of the effective pinning energy at a temperature below Tc is attributed to its
renormalization by thermal fluctuations. | cond-mat_supr-con |
Ground state angular momentum, spectral asymmetry, and topology in
chiral superfluids and superconductors: Recently it was discovered that the ground state orbital angular momentum in
two-dimensional chiral superfluids with pairing symmetry $(p_x+ip_y)^\nu$
depends on the winding number $\nu$ in a striking manner. The ground state
value for the $\nu=1$ case is $L_z=\hbar N/2$ as expected by counting the
Cooper pairs, while a dramatic cancellation takes place for $\nu>1$. The origin
of the cancellation is associated with the topological edge states that appear
in a finite geometry and give rise to a spectral asymmetry. Here we study the
reduction of orbital angular momentum for different potential profiles and
pairing strengths, showing that the result $L_z=\hbar N/2$ is robust for
$\nu=1$ under all studied circumstances. We study how angular momentum depends
on the gap size $\Delta/E_F$ and obtain the result $L_z=\frac{\hbar\nu}{2}
N(1-\frac{\mu}{E_F})$ for $\nu=2,3$. Thus, the gap-dependence of $L_z$ for
$\nu<4$ enters at most through the chemical potential while $\nu\geq4$ is
qualitatively different. In addition, we generalize the spectral asymmetry
arguments to \emph{total} angular momentum in the ground state of triplet
superfluids where due to a spin-orbit coupling $L_z$ is not a good quantum
number. We find that the ground state total angular momentum also behaves very
differently depending on total angular momentum of the Cooper pairs. | cond-mat_supr-con |
Bifurcation structure and chaos in nanomagnet coupled to Josephson
junction: Recently an irregular easy axis reorientation demonstrating the Kapitza
pendulum features were observed in numerical simulations of nanomagnet coupled
to the Josephson junction. To explain its origin we study the magnetization
bifurcations and chaos which appear in this system due to interplay of
superconductivity and magnetism. The bifurcation structure of the magnetization
under the variation of Josephson to magnetic energy ratio as a control
parameter demonstrates several precessional motions. They are related to
chaotic behavior, bistability, and multiperiodic orbits in the ferromagnetic
resonance region. Effect of external periodic drive on the bifurcation
structure is investigated. The results demonstrate high-frequency modes of
periodic motion and chaotic response near resonance. Far from the ferromagnetic
resonance we observe a quasiperiodic behavior. | cond-mat_supr-con |
Change of the vortex core structure in two-band superconductors at
impurity-scattering-driven $s_\pm/s_{++}$ crossover: We report a nontrivial transition in the core structure of vortices in
two-band superconductors as a function of interband impurity scattering. We
demonstrate that, in addition to singular zeros of the order parameter, the
vortices there can acquire a circular nodal line around the singular point in
one of the superconducting components. It results in the formation of the
peculiar "moat"-like profile in one of the superconducting gaps. The moat-core
vortices occur generically in the vicinity of the impurity-induced crossover
between $s_{\pm}$ and $s_{++}$ states. | cond-mat_supr-con |
Temperature dependence of iron local magnetic moment in phase-separated
superconducting chalcogenide: We have studied local magnetic moment and electronic phase separation in
superconducting K$_{x}$Fe$_{2-y}$Se$_2$ by x-ray emission and absorption
spectroscopy. Detailed temperature dependent measurements at the Fe K-edge have
revealed coexisting electronic phases and their correlation with the transport
properties. By cooling down, the local magnetic moment of Fe shows a sharp drop
across the superconducting transition temperature (T$_c$) and the coexisting
phases exchange spectral weights with the low spin state gaining intensity at
the expense of the higher spin state. After annealing the sample across the
iron-vacancy order temperature, the system does not recover the initial state
and the spectral weight anomaly at T$_c$ as well as superconductivity
disappear. The results clearly underline that the coexistence of the low spin
and high spin phases and the transitions between them provide unusual magnetic
fluctuations and have a fundamental role in the superconducting mechanism of
electronically inhomogeneous K$_{x}$Fe$_{2-y}$Se$_2$ system. | cond-mat_supr-con |
Time relaxation of microwave second order response of superconductors in
the critical state: Relaxation of the microwave second order response of YBa2Cu3O7 and
Ba0.6K0.4BiO3 crystals in the critical state is studied. The samples are
exposed to static and pulsed microwave magnetic fields. The second harmonic
signals decay during the time in which the microwave pulse endures. The decay
times depend on the supercon-ductor investigated and on the way the value of
the static field has been reached. | cond-mat_supr-con |
Paramagnetic Effects in the Vortex Lattice Field Distribution of
Strongly Type-II Superconductors: We present an analysis of the magnetic field distribution in the Abrikosov
lattice of high-$\kappa$ superconductors with singlet pairing in the case where
the critical field is mainly determined by the Pauli limit and the superfluid
currents partly come from the paramagnetic interaction of electron spins with
the local magnetic field. The derivation is performed in the frame of the
generalized Clem variational method which is valid not too close to the
critical field and furthermore with the Abrikosov type theory in the vicinity
of it. The found vortex lattice form factor increases with increasing field and
then falls down at approach of the upper critical field where the
superconducting state is suppressed. | cond-mat_supr-con |
Quasiparticle dispersion and lineshape in a strongly correlated liquid
with the fermion condensate: A model of a strongly correlated electron liquid based on the fermion
condensation (FC) is applied to the consideration of high temperature
superconductors in its superconducting and normal states. Within our model the
appearance of FC presents a boundary, separating the region of strongly
interacting electron liquid from the region of strongly correlated electron
liquid. We show that at temperatures $T<T_c$ the quasiparticle dispersion in
systems with FC can be presented by two straight lines, characterized by
effective masses $M^*_{FC}$ and $M^*_L$, respectively, and intersecting near
the binding energy which is of the order of the superconducting gap. This same
picture persists in the normal state being modified only by the presence of the
quasiparticle width. We argue that this strong change of the quasiparticle
dispersion can be enhanced in underdoped samples due to strengthening the FC
influence. The single particle excitation width and lineshape are also studied. | cond-mat_supr-con |
Stacked Josephson junction SQUID: Operation of a Superconducting Quantum Interference Device (SQUID) made of
stacked Josephson junctions is analyzed numerically for a variety of junction
parameters. Due to a magnetic coupling of junctions in the stack, such a SQUID
has certain advantages as compared to an uncoupled multi-junction SQUID.
Namely, metastability of current-flux modulation can be reduced and a
voltage-flux modulation can be improved if junctions in the stack are
phase-locked. Optimum operation of the SQUID is expected for moderately long,
strongly coupled stacked Josephson junctions. A possibility of making a stacked
Josephson junction SQUID based on intrinsic Josephson junctions in high-Tc
superconductor is discussed. | cond-mat_supr-con |
Depinning of a vortex chain in a disordered flow channel: We study depinning of vortex chains in channels formed by static, disordered
vortex arrays. Depinning is governed either by the barrier for defect
nucleation or for defect motion, depending on whether the chain periodicity is
commensurate or incommensurate with the surrounding arrays. We analyze the
reduction of the gap between these barriers as function of disorder. At large
disorder, commensurability becomes irrelevant and the pinning force is reduced
to a small fraction of the ideal shear strength of ordered channels.
Implications for experiments on channel devices are discussed. | cond-mat_supr-con |
Antiferromagnetically Driven Electronic Correlation in Iron Pnictides
and Cuprates: The iron pnictides and the cuprates represent two families of materials,
where strong antiferromagnetic correlation drives three other distinct ordering
tendencies: (1) superconducting pairing, (2) Fermi surface distortion, and (3)
orbital current order. We propose that (1)-(3) and the antiferromagnetic
correlation are the hallmarks of a class of strongly correlated materials to
which the cuprates and pnictides belong. In this paper we present the results
of the functional renormalization group studies to support the above claim. In
addition, we show that as a function of the interlayer hopping parameter, the
double layer Hubbard model nicely interpolates between the cuprate and the iron
pnictide physics. Finally, as a check, we will present the renormalization
group study of a ladder version of the iron pnictide, and compare the results
to those of the two-dimensional model. | cond-mat_supr-con |
Emergence of Superconductivity from Fully Incoherent Normal State in an
Iron-Based Superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$: In unconventional superconductors, it is generally believed that
understanding the physical properties of the normal state is a pre-requisite
for understanding the superconductivity mechanism. In conventional
superconductors like niobium or lead, the normal state is a Fermi liquid with a
well-defined Fermi surface and well-defined quasipartcles along the Fermi
surface. Superconductivity is realized in this case by the Fermi surface
instability in the superconducting state and the formation and condensation of
the electron pairs (Cooper pairing). The high temperature cuprate
superconductors, on the other hand, represent another extreme case that
superconductivity can be realized in the underdoped region where there is
neither well-defined Fermi surface due to the pseudogap formation nor
quasiparticles near the antinodal regions in the normal state. Here we report a
novel scenario that superconductivity is realized in a system with well-defined
Fermi surface but without quasiparticles along the Fermi surface in the normal
state. High resolution laser-based angle-resolved photoemission measurements
have been performed on an optimally-doped iron-based superconductor
(Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. We find that, while sharp superconducting
coherence peaks emerge in the superconducting state on the hole-like Fermi
surface sheets, no quasiparticle peak is present in the normal state. Its
electronic behaviours deviate strongly from a Fermi liquid system. The
superconducting gap of such a system exhibits an unusual temperature dependence
that it is nearly a constant in the superconducting state and abruptly closes
at T$_c$. These observations have provided a new platform to study
unconventional superconductivity in a non-Fermi liquid system. | cond-mat_supr-con |
Enhancement of the retrapping current of superconducting microbridges of
finite length: We theoretically find that the resistance of a superconducting
microbridge/nanowire {\it decreases} while the retrapping current $I_r$ for the
transition to the superconducting state {\it increases} when one suppresses the
magnitude of the order parameter $|\Delta|$ in the attached superconducting
leads. This effect is a consequence of the increased energy interval for
diffusion of the 'hot' nonequilibrium quasiparticles (induced by the
oscillations of $|\Delta|$ in the center of the microbridge) to the leads. The
effect is absent in short microbridges (with length less than the coherence
length) and it is relatively weak in long microbridges (with length larger than
the inelastic relaxation length of the nonequilibrium distribution function). A
nonmonotonous dependence of $I_r$ on the length of the microbridge is
predicted. Our results are important for the explanation of the enhancement of
the critical current and the appearance of negative magnetoresistance observed
in many recent experiments on superconducting microbridges/nanowires. | cond-mat_supr-con |
Hysteresis in mesoscopic superconducting disks: the Bean-Livingston
barrier: The magnetization behavior of mesoscopic superconducting disks can show
hysteretic behavior which we explain by using the Ginzburg-Landau (GL) theory
and properly taking into account the de-magnetization effects due to
geometrical form factors. In large disks the Bean-Livingston surface barrier is
responsible for the hysteresis. While in small disks a volume barrier is
responsible for this hysteresis. It is shown that although the sample
magnetization is diamagnetic (negative), the measured magnetization can be
positive at certain fields as observed experimentally, which is a consequence
of the de-magnetization effects and the experimental set up. | cond-mat_supr-con |
Superconductivity and Charge Density Wave of CuIr2Te4 by Iodine Doping: Here we report a systematic investigation on the evolution of the structural
and physical properties, including the charge density wave and
superconductivity of the polycrystalline CuIr2Te4-xIx. X-ray diffraction
results indicate that both of a and c lattice parameters increase linearly. The
resistivity measurements indicate that the charge density wave is destabilized
with slight x but reappears when x is large than 0.9. Meanwhile, the
superconducting transition temperature enhances as x raises and reaches a
maximum value of around 2.95 K for the optimal composition CuIr2Te3.9I0.1
followed by a slight decrease with higher iodine doping content. The specific
heat jump for the optimal composition CuIr2Te3.9I0.1 is approximately 1.46,
which is close to the Bardeen Cooper Schrieffer value which is 1.43, indicating
it is a bulk superconductor. The results of thermodynamic heat capacity
measurements under different magnetic fields, magnetization and
magneto-transport measurements further suggest that CuIr2Te4-xIx bulks are type
II superconductors. Finally, an electronic phase diagram for this CuIr2Te4-xIx
system has been constructed. The present study provides a suitable material
platform for further investigation of the interplay of the CDW and
superconductivity. | cond-mat_supr-con |
Quantum phase slip phenomenon in superconducting nanowires with
low-Ohmic environment: In a number of recent experiments it has been demonstrated that in
ultra-narrow superconducting channels quantum fluctuations of the order
parameter, alternatively called quantum phase slips, are responsible for the
finite resistance well below the critical temperature. The acceptable agreement
between those experiments and the models describing quantum fluctuations in
quasi-one-dimensional superconductors has been established. However the very
concept of the phase slip is justified when these fluctuations are the
relatively rare events, meaning that the effective resistance of the system
should be much smaller than the normal state equivalent. In this paper we study
the limit of the strong quantum fluctuations where the existing models are not
applicable. In particular case of ultra-thin titanium nanowires it is
demonstrated that below the expected critical temperature the resistance does
not demonstrate any trend towards the conventional for a superconductor
zero-resistivity state even at negligibly small measuring currents. Application
of a small magnetic field leads to an unusual negative magnetoresistance, which
becomes more pronounced at lower temperatures. The origin of the negative
magnetoresistance effect is not clear. | cond-mat_supr-con |
Doping dependence of electromagnetic response in cuprate superconductors: The study of the electromagnetic response in cuprate superconductors plays a
crucial role in the understanding of the essential physics of these materials.
Here the doping dependence of the electromagnetic response in cuprate
superconductors is studied within the kinetic-energy driven superconducting
mechanism. The kernel of the response function is evaluated based on the linear
response approximation for a purely transverse vector potential, and can be
broken up into its diamagnetic and paramagnetic parts. In particular, this
paramagnetic part exactly cancels the corresponding diamagnetic part in the
normal-state, and then the Meissner effect is obtained within the entire
superconducting phase. Following this kernel of the response function, the
electromagnetic response calculation in terms of the specular reflection model
qualitatively reproduces many of the striking features observed in the
experiments. In particular, the local magnetic-field profile follows an
exponential law, while the superfluid density exhibits the nonlinear
temperature behavior at the lowest temperatures, followed by the linear
temperature dependence extending over the most of the superconducting
temperature range. Moreover, the maximal value of the superfluid density occurs
at around the critical doping $\delta_{\rm critical}\sim 0.16$, and then
decreases in both lower doped and higher doped regimes. The theory also shows
that the nonlinear temperature dependence of the superfluid density at the
lowest temperatures can be attributed to the nonlocal effects induced by the
d-wave gap nodes on the electron Fermi surface. | cond-mat_supr-con |
Critical behavior at superconductor-insulator phase transitions near one
dimension: I argue that the system of interacting bosons at zero temperature and in
random external potential possesses a simple critical point which describes the
proliferation of disorder-induced topological defects in the superfluid ground
state, and which is located at weak disorder close to and above one dimension.
This makes it possible to address the critical behavior at the superfluid-Bose
glass transition in dirty boson systems by expanding around the lower critical
dimension d=1. Within the formulated renormalization procedure near d=1 the
dynamical critical exponent is obtained exactly and the correlation length
exponent is calculated as a Laurent series in the parameter \sqrt{\epsilon},
with \epsilon=d-1: z=d, \nu=1/\sqrt{3\epsilon} for the short range, and z=1,
\nu=\sqrt{2/3\epsilon}, for the long-range Coulomb interaction between bosons.
The identified critical point should be stable against the residual
perturbations in the effective action for the superfluid, at least in
dimensions 1\leq d \leq 2, for both short-range and Coulomb interactions. For
the superfluid-Mott insulator transition in the system in a periodic potential
and at a commensurate density of bosons I find \nu=(1/2\sqrt{\epsilon})+
1/4+O(\sqrt{\epsilon}), which yields a result reasonably close to the known XY
critical exponent in d=2+1. The critical behavior of the superfluid density,
phonon velocity and the compressibility in the system with the short-range
interactions is discussed. | cond-mat_supr-con |
Fractional Josephson Vortices and Braiding of Majorana Zero Modes in
Planar Superconductor-Semiconductor Heterostructures: We consider the one-dimensional (1D) topological superconductor that may form
in a planar superconductor-metal-superconductor Josephson junction in which the
metal is is subjected to spin orbit coupling and to an in-plane magnetic field.
This 1D topological superconductor has been the subject of recent theoretical
and experimental attention. We examine the effect of perpendicular magnetic
field and a supercurrent driven across the junction on the position and
structure of the Majorana zero modes that are associated with the topological
superconductor. In particular, we show that under certain conditions the
Josephson vortices fractionalize to half-vortices, each carrying half of the
superconducting flux quantum and a single Majorana zero mode. Furthemore, we
show that the system allows for a current-controlled braiding of Majorana zero
modes. | cond-mat_supr-con |
Is the anisotropy of the upper critical field of Sr$_2$RuO$_4$
consistent with a helical $p$-wave state?: We calculate the angular and temperature $T$ dependencies of the upper
critical field $H_{c2}(\theta,\phi,T)$ for the $C_{4v}$ point group helical
$p$-wave states, assuming a single uniaxial ellipsoidal Fermi surface, Pauli
limiting, and strong spin-orbit coupling that locks the spin-triplet $\vec{\bf
d}$-vectors onto the layers. Good fits to the Sr$_2$RuO$_4$
$H_{c2,a}(\theta,T)$ data of Kittaka {\it et al.} [Phys. Rev. B {\bf 80},
174514 (2009)] are obtained. Helical states with $\vec{\bf d}(\vec{\bf
k})=k_x\vec{\bf x}-k_y\vec{\bf y}$ and $k_y\vec{\bf x}+k_x\vec{\bf y}$ (or
$k_x\vec{\bf x}+k_y\vec{\bf y}$ and $k_y\vec{\bf x}-k_x\vec{\bf y}$) produce
$H_{c2}(90^{\circ},\phi,T)$ that greatly exceed (or do not exhibit) the
four-fold azimuthal anisotropy magnitudes observed in Sr$_2$RuO$_4$ by Kittaka
{\it et al.} and by Mao {\it et al.} [Phys. Rev. Lett. {\bf 84}, 991 (2000)],
respectively. | cond-mat_supr-con |
Renormalized q-dependent Spin Susceptibility by inverting the Random
Phase Approximation: Implications for quantitative assessment of the role of
spin fluctuations in 2D Ising superconductor NbSe$_{2}$: Accurate determination of the full momentum-dependent spin susceptibility
$\chi(\mathbf{q}) $ is very important for the description of magnetism and
superconductivity. While in principle the formalism for calculating
$\chi(\mathbf{q})$ in the linear response density functional theory (DFT) is
well established, hardly any publicly available code includes this capability.
Here, we describe an alternative way to calculate the static
$\chi(\mathbf{q})$, which can be applied to most common DFT codes without
additional programming. The method combined standard fixed-spin-moment
calculations of $\chi(\mathbf{0}) $ with direct calculations of the energy of
spin spirals stabilized by an artificial Hubbard interaction. From these
calculations, $\chi_{DFT}(\mathbf{q} )$ can be extracted by inverting the RPA
formula. We apply this recipe to the recently discovered Ising
superconductivity in NbSe$_2$ monolayer, one of the most exciting findings in
superconductivity in recent years. It was proposed that spin fluctuations may
strongly affect the parity of the order parameter. Previous estimates suggested
proximity to ferromagnetism, $i.e.$, $\chi(\mathbf{q})$ peaked at
$\mathbf{q}=0$. We find that the structure of spin fluctuations is more
complicated, with the fluctuation spectrum sharply peaked at $\mathbf{q}\approx
(0.2,0)$. Such a spectrum would change the interband pairing interaction and
considerably affect the superconducting state. | cond-mat_supr-con |
Nonlinear Coupling in Nb/NbN Superconducting Microwave Resonators: In this experimental study we show that the coupling between Nb/NbN
superconducting microwave resonators and their feedline can be made amplitude
dependent. We employ this mechanism to tune the resonators into critical
coupling condition, a preferable mode of operation for a wide range of
applications. Moreover we examine the dependence of critical coupling state on
other parameters such as temperature and magnetic field. Possible novel
applications based on such nonlinear coupling are briefly discussed. | cond-mat_supr-con |
Local dynamical lattice instabilities: Prerequisites for resonant
pairing superconductivity: Fluctuating local diamagnetic pairs of electrons, embedded in a Fermi sea,
are candidates for non-phonon-mediated superconductors without the stringent
conditions on Tc which arise in phonon-mediated BCS classical low-Tc
superconductors. The local accumulations of charge, from which such diamagnetic
fluctuations originate, are irrevocably coupled to local dynamical lattice
instabilities and form composite charge-lattice excitations of the system. For
a superconducting phase to be realized, such excitations must be itinerant
spatially phase-coherent modes. This can be achieved by resonant pair tunneling
in and out of polaronic cation-ligand sites. Materials in which
superconductivity driven by such local lattice instability can be expected,
have a Tc which is controlled by the phase stiffness rather than the amplitude
of the diamagnetic pair fluctuations. Above Tc, a pseudogap phase will be
maintained up to a T*, where this pairing amplitude disappears. We discuss the
characteristic local charge and lattice properties which characterize this
pseudogap phase and which form the prerequisites for establishing a
phase-coherent macroscopic superconducting state. | cond-mat_supr-con |
A close look at antiferromagnetism in multidimensional phase diagram of
electron-doped copper oxide: Emergency of superconductivity at the instabilities of antiferromagnetism
(AFM), spin/charge density waves has been widely recognized in unconventional
superconductors. In copper-oxide superconductors, spin fluctuations play a
predominant role in electron pairing with electron dopants yet composite orders
veil the nature of superconductivity for hole-doped family. However, in
electron-doped ones the ending point of AFM is still in controversy for
different probes or its sensitivity to oxygen content. Here, by carefully
tuning the oxygen content, a systematic study of Hall signal and
magnetoresistivity up to 58 Tesla on optimally doped La2-xCexCuO4+-{\delta} (x
= 0.10) thin films identifies two characteristic temperatures at 62.5+-7.5 K
and 25+-5 K. The former is quite robust whereas the latter becomes flexible
with increasing magnetic field, thereby linked to two- and three-dimensional
AFM, evident from the multidimensional phase diagram as a function of oxygen as
well as Ce dopants. Consequently, the observation of extended AFM phase in
contrast to {\mu}SR probe corroborates an elevated critical doping in field,
providing an unambiguous picture to understand the interactions between AFM and
superconductivity. | cond-mat_supr-con |
Sensitivity of the superconducting state and magnetic susceptibility to
key aspects of electronic structure in ferropnictides: Experiments on the iron-pnictide superconductors appear to show some
materials where the ground state is fully gapped, and others where low-energy
excitations dominate, possibly indicative of gap nodes. Within the framework of
a 5-orbital spin fluctuation theory for these systems, we discuss how changes
in the doping, the electronic structure or interaction parameters can tune the
system from a fully gapped to nodal sign-changing gap with s-wave ($A_{1g}$)
symmetry ($s^\pm$). In particular we focus on the role of the hole pocket at
the $(\pi,\pi)$ point of the unfolded Brillouin zone identified as crucial to
the pairing by Kuroki {\it et al.}, and show that its presence leads to
additional nesting of hole and electron pockets which stabilizes the isotropic
$s^\pm$ state. The pocket's contribution to the pairing can be tuned by doping,
surface effects, and by changes in interaction parameters, which we examine.
Analytic expressions for orbital pairing vertices calculated within the RPA
fluctuation exchange approximation allow us to draw connections between aspects
of electronic structure, interaction parameters, and the form of the
superconducting gap. | cond-mat_supr-con |
A new seeding technique for the reliable fabrication of large, SmBCO
single grains containing silver using top seeded melt growth: Silver (Ag) is an established additive for improving the mechanical
properties of single grain, (RE)BCO bulk superconductors (where RE = Sm, Gd and
Y). The presence of Ag in the (RE)BCO bulk composition, however, typically
reduces the melting temperature of the single crystal seed in the top seeded
melt growth (TSMG) process, which complicates significantly the controlled
nucleation and subsequent epitaxial growth of a single grain, which is
essential for high field engineering applications. The reduced reliability of
the seeding process in the presence of Ag is particularly acute for the SmBCO
system, since the melting temperature of SmBCO is very close to that of the
generic NdBCO(MgO) seed. SmBCO has the highest superconducting transition
temperature, Tc, and exhibits the most pronounced "peak" effect at higher
magnetic field of all materials in the family of (RE)BCO bulk superconductors
and, therefore, has the greatest potential for use in practical applications
(compared to GdBCO and YBCO, in particular). Development of an effective
seeding process, therefore, is one of the major challenges of the TSMG process
for the growth of large, high quantity single grain superconductors. In this
paper, we report a novel technique that involves introducing a buffer layer
between the seed crystal and the precursor pellet, primarily to inhibit the
diffusion of Ag from the green body to the seed during melt processing in order
to prevent the melting of the seed. The success rate of the seeding process
using this technique is 100% for relatively small batch samples. The
superconducting properties, Tc, Jc and trapped fields, of the single grains
fabricated using the buffers are reported and the micro-structures in the
vicinity of the buffer of single grains fabricated by the modified technique
are analysed. | cond-mat_supr-con |
Electronic Structure of
$\mathrm{Sr}_{1-y}\mathrm{Ca}_{y}\mathrm{Fe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$
($x$ = 0.25, $y$ = 0.08) Revealed by Angle-Resolved Photoemission
Spectroscopy: We have investigated the electronic structure of
$\mathrm{Sr}_{1-y}\mathrm{Ca}_{y}\mathrm{Fe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$
($x$ = 0.25, $y$ = 0.08) by means of angle-resolved photoemission spectroscopy.
From the comparison with the results of
$\mathrm{BaFe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$, the effects
of smaller structural anisotropy ($c/a$) on the Fermi surfaces (FSs) and the
gap structures are discussed. The observed FSs have three dimensional shapes.
One of the hole FSs is strongly warped between the $\Gamma$ and $Z$ points, and
the innermost FS observed at the $Z$ point disappears at the $\Gamma$ point,
which is similar to the FS features of
$\mathrm{SrFe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$ ($x$ = 0.35).
In the superconducting state, the node like gap-minimum is present for the
$d_{xy}$ electron FS near the $X$ point, while the gaps around the other high
symmetry points are isotropic. Several theoretical models based on the spin
and/or the orbital fluctuation are examined to explain all these experimental
results. | cond-mat_supr-con |
Nonlinear Seebeck Effect in a Model Granular Superconductor: The change of the Josephson supercurrent density of a weakly-connected
granular superconductor in response to externally applied arbitrary thermal
gradient dT/dx (nonlinear Seebeck effect) is considered within a model of 3D
Josephson junction arrays. For dT/dx>(dT/dx)_c, where (dT/dx)_c is estimated to
be of the order of 10^4 K/m for YBCO ceramics with an average grain's size of
10 microns, the weak-links-dominated thermopower S (Seebeck coefficient) is
predicted to become strongly dT/dx-dependent. | cond-mat_supr-con |
Stability of Mixed-Symmetry Superconducting States with Broken
Time-Reversal Symmetry against Lattice Distortions: We examine the stability of mixed-symmetry superconducting states with broken
time-reversal symmetry in spatial-symmetry-broken systems, including chiral
states, on the basis of the free-energy functional derived in the weak-coupling
theory. We consider a generic a_1 + i a_2 wave state, with a_1 and a_2 being
different symmetry indices such as (a_1,a_2) = (d,s), (p_x,p_y), and (d,d').The
time-reversal symmetry of the mixed-symmetry state with the a_1- and a_2-wave
components is broken when the phases of these components differ, and such a
state is called the time-reversal-symmetry breaking (TRSB) state. However,
their phases are equated by Cooper-pair scattering between these components if
it occurs; i.e., when the off-diagonal elements S_{a_1 a_2} = S_{a_2 a_1} of
the scattering matrix are nonzero, they destabilize the TRSB state. Hence, it
has often been believed that the TRSB state is stable only in systems with a
spatial symmetry that guarantees S_{a_1 a_2}=0. We note that, contrary to this
belief, the TRSB state can remain stable in systems without the spatial
symmetry when the relative phase shifts so that S_{a_1 a_2} = 0 is restored,
which results in a distorted TRSB (a_1 + a_2) + i a_2 wave state. Here, note
that the restoration of S_{a_1 a_2} = 0 does not imply that the symmetry of the
quasi-particle energy E_k is recovered. This study shows that such
stabilization of the TRSB state occurs when the distortion is sufficiently
small and \Delta_{a_1} \Delta_{a_2} is sufficiently large, where \Delta_a is
the amplitude of the a-wave component in the TRSB state in the absence of the
distortion. We clarify the manner in which the shift in the relative phase
eliminates S_{a_1 a_2} and prove that such a state yields a free-energy
minimum. We also propose a formula for the upper bound of the degree of lattice
distortion, below which the TRSB state can be stable. | cond-mat_supr-con |
History effects and pinning regimes in solid vortex matter: We propose a phenomenological model that accounts for the history effects
observed in ac susceptibility measurements in YBa2Cu3O7 single crystals [Phys.
Rev. Lett. 84, 4200 (2000) and Phys. Rev. Lett. 86, 504 (2001)]. Central to the
model is the assumption that the penetrating ac magnetic field modifies the
vortex lattice mobility, trapping different robust dynamical states in
different regions of the sample. We discuss in detail on the response of the
superconductor to an ac magnetic field when the vortex lattice mobility is not
uniform inside the sample. We begin with an analytical description for a simple
geometry (slab) and then we perform numerical calculations for a strip in a
transverse magnetic field which include relaxation effects. In calculations,
the vortex system is assumed to coexist in different pinning regimes. The
vortex behavior in the regions where the induced current density j has been
always below a given threshold (j_c^>) is described by an elastic Campbell-like
regime (or a critical state regime with local high critical current density,
j_c^>). When the VS is shaken by symmetrical (e.g. sinusoidal) ac fields, the
critical current density is modified to j_c^< (which is smaller than j_c^>) at
regions where vortices have been forced to oscillate by a current density
larger than j_c^>. Experimentally, an initial state with high critical current
density (j_c^>) can be obtained by zero field cooling, field cooling (with no
applied ac field) or by shaking the vortex lattice with an asymmetrical (e.g.
sawtooth) field. We compare our calculations with experimental ac
susceptibility results in YBa2Cu3O7 single crystals. | cond-mat_supr-con |
Spin-dependent quasiparticle reflection and bound states at interfaces
with itinerant antiferromagnets: We present a formulation of the quasiclassical theory of junctions between
itinerant antiferromagnets (AF) and s-wave (sSC) and d-wave superconductors
(dSC). For the simplest two-sublattice antiferromagnet on a bipartite lattice,
we derive Andreev-type equations and show that their solutions lead to a novel
channel of quasiparticle reflection. In particular, quasiparticles in a normal
metal with energies less than or comparable to the antiferromagnetic gap
experience spin-dependent retroreflection at antiferromagnet-normal metal
(AF/N) transparent (100) and (110) interfaces. A relative phase difference of
pi between up spin and down spin quasiparticle reflection amplitudes is shown
to lead to zero-energy interface bound states on AF/sSC interfaces. For an
sSC/AF/sSC junction, these bound states are found to be split, due to a finite
width of the AF interlayer, and carry the supercurrent. At AF/dSC interfaces we
find no zero-energy bound states for both interface orientations we considered,
in contrast with the case of (110) impenetrable surface of a dSC. | cond-mat_supr-con |
Magnetic dipole induced guided vortex motion: We present evidence of magnetically controlled guided vortex motion in a
hybrid superconductor/ferromagnet nanosystem consisting of an Al film on top of
a square array of permalloy square rings. When the rings are magnetized with an
in-plane external field H, an array of point-like dipoles with moments
antiparallel to H, is formed. The resulting magnetic template generates a
strongly anisotropic pinning potential landscape for vortices in the
superconducting layer. Transport measurements show that this anisotropy is able
to confine the flux motion along the high symmetry axes of the square lattice
of dipoles. This guided vortex motion can be either re-routed by 90 degrees by
simply changing the dipole orientation or even strongly suppressed by inducing
a flux-closure magnetic state with very low stray fields in the rings. | cond-mat_supr-con |
Coherent Modulation of the YBa2Cu3O6+x Atomic Structure by Displacive
Stimulated Ionic Raman Scattering: We discuss the mechanism of coherent phonon generation by Stimulated Ionic
Raman Scattering, a process different from conventional excitation with near
visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling
between a directly excited infrared-active phonon mode and other Raman modes.
We experimentally study the response of YBa2Cu3O6+x to the resonant excitation
of apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown
in the past to enhance the interlayer superconducting coupling. We find
coherent oscillations of four totally symmetric (Ag) Raman modes and make a
critical assessment of the role of these oscillatory motions in the enhancement
of superconductivity. | cond-mat_supr-con |
Theory of the in-plane anisotropy of magnetic excitations in
YBa_{2}Cu_{3}O_{6+y}: A pronounced xy-anisotropy was observed in recent neutron scattering
experiments for magnetic excitations in untwinned YBa_{2}Cu_{3}O_{6+y} (YBCO).
The small anisotropy of the bare band structure due to the orthorhombic crystal
symmetry seems to be enhanced by correlation effects. A natural possibility is
that the system is close to a Pomeranchuk instability associated with a d-wave
Fermi surface deformation (dFSD). We investigate this possibility in the
bilayer t-J model within a self-consistent slave-boson mean-field theory. We
show that the dFSD correlations drive a pronounced xy-anisotropy of magnetic
excitations at low doping and at relatively high temperatures, providing a
scenario for the observed xy-anisotropy in optimally doped as well as
underdoped YBCO, including the pseudogap phase. | cond-mat_supr-con |
Universal Fermi velocity in highly compressed hydride superconductors: Fermi velocity, $v_F$, is one of the primary characteristics of any
conductor, including superconductors. For conductors at ambient pressure
several experimental techniques have been developed to measure $v_F$ and, for
instance, Zhou et al (Nature 423 398 (2003)) reported that high-Tc cuprates
exhibit universal nodal Fermi velocity of $v_F$ = $2.7*10^5$ m/s. However,
there were no experimental techniques applied to measure $v_F$ in
highly-compressed near-room-temperature superconductors (NRTS), due to
experimental challenges. Here to answer a question about the existence of the
universal Fermi velocity in NRTS materials, we analyzed full inventory of the
ground-state upper critical field data, Bc2(0), for these materials and found
that this class of superconductors exhibits universal Fermi velocity of $v_F$ =
(1/1.3)*(2${\Delta}$(0)/(kBTc))*10^5 m/s (where ${\Delta}(0)$ is ground state
amplitude of the energy gap). Due to the ratio of 2${\Delta}$(0)/$({k_B}{T_c})$
is varying within a narrow arrange of 3.2 < 2${\Delta}$(0)/$({k_B}{T_c})$ < 5,
then $v_F$ in NRTS materials is within the same ballpark with its high-Tc
cuprates counterpart. | cond-mat_supr-con |
The Ground-state Inter-plane Superconducting Coherence Length of
La$_{1.875}$Sr$_{0.125}$CuO$_4$ Measured by a "Xiometer": A long excitation coil piercing a superconducting (SC) ring is used to
generate ever increasing persistent current in the ring, until the current
destroys the order parameter. Given that the penetration depth $\lambda$ is
known, this experiment measures, hypothetically, the coherence length $\xi$,
hence the name "Xiometer". We examine various aspects of this theoretically
driven hypothesis by testing niobium rings with different dimensions, and by
comparing the results to the known values of $\xi$. We then apply the method to
two La$_{1.875}$Sr$_{0.125}$CuO$_4$ rings at $T \rightarrow 0$. In one, the
current flows in the CuO$_2$ planes hence it is set by $\xi_{ab}$. In the
other, the current must cross planes and is determined by $\xi_{c}$. We find
that $\xi_{c}=1.3 \pm 0.1$~nm, and $\xi_{ab}<2.3~$nm indicating that at low
temperatures the Cooper pairs are three dimensional. | cond-mat_supr-con |
The influence of the spin-dependent phases of tunneling electrons on the
conductance of a point ferromagnet/isolator/d-wave superconductor contact: The influence of the spin-dependent phase shifts (SDPS) associated to the
electronic reflection and transmission amplitudes acquired by electrons upon
scattering on the potential barrier on the Andreev reflection probability of
electron and hole excitations for a ferromagnet/isolator/d-wave superconductor
(FIS) contact and the charge conductance of the FIS contact is studied. Various
superconductor orientations are considered. It is found that SDPS can suppress
the zero-potential peak and restore finite-potential peaks in the charge
conductance of the F/I/d-wave superconductor contact for the (110) orientation
of the d-wave superconductor and, on the contrary, can restore the
zero-potential peak and suppress finite-potential peaks for the $\{100\}$
orientation of the d-wave superconductor. | cond-mat_supr-con |
Magnetoelectric effects in superconductors due to spin-orbit scattering:
a non-linear $σ$-model description: We suggest a generalization of nonlinear $\sigma$-model for diffusive
superconducting systems to account for magnetoelectric effects due to
spin-orbit scattering. In the leading orders of spin-orbit strength and
gradient expansion it includes two additional terms responsible for the
spin-Hall effect and the spin-current swapping. First, assuming a
delta-correlated disorder we derive the new terms from the Keldysh path
integral representation of the generating functional. Then we argue
phenomenologically that they exhaust all invariants allowed in the effective
action to the leading order in the spin-orbit coupling (SOC). Finally, the
results are confirmed by a direct derivation of the saddle-point (Usadel)
equation from the quantum kinetic equations in the presence of randomly
distributed impurities with SOC. At this point we correct a recent derivation
of the Usadel equation that includes magneto-electric effects and does not
resort to the Born approximation. | cond-mat_supr-con |
Collective mode in the SU(2) theory of cuprates: Recent advances in momentum-resolved electron energy-loss spectroscopy
(MEELS) and resonant inelastic X-ray scattering (RIXS) now allow one to access
the charge response function with unprecedented versatility and accuracy. This
allows for the study of excitations which were inaccessible recently, such as
low-energy and finite momentum collective modes. The SU(2) theory of the
cuprates is based on a composite order parameter with SU(2) symmetry
fluctuating between superconductivity and charge order. The phase where it
fluctuates is a candidate for the pseudogap phase of the cuprates. This theory
has a signature, enabling its strict experimental test, which is the
fluctuation between these two orders, corresponding to a charge 2 spin 0 mode
at the charge ordering wave-vector. Here we derive the influence of this SU(2)
collective mode on the charge susceptibility in both strong and weak coupling
limits, and discuss its relation to MEELS, RIXS and Raman experiments. We find
two peaks in the charge susceptibility at finite energy, whose middle is the
charge ordering wave-vector, and discuss their evolution in the phase diagram. | cond-mat_supr-con |
Exotic Superconducting Phases of Ultracold Atom Mixtures on Triangular
Lattices: We study the phase diagram of two-dimensional Bose-Fermi mixtures of
ultracold atoms on a triangular optical lattice, in the limit when the velocity
of bosonic condensate fluctuations is much larger than the Fermi velocity.
We contrast this work with our previous results for a square lattice system
in Phys. Rev. Lett. {\bf 97}, 030601 (2006).
Using functional renormalization group techniques we show that the phase
diagrams for a triangular lattice contain exotic superconducting phases. For
spin-1/2 fermions on an isotropic lattice we find a competition of $s$-, $p$-,
extended $d$-, and $f$-wave symmetry, as well as antiferromagnetic order. For
an anisotropic lattice, we further find an extended p-wave phase. A Bose-Fermi
mixture with spinless fermions on an isotropic lattice shows a competition
between $p$- and $f$-wave symmetry.
These phases can be traced back to the geometric shapes of the Fermi surfaces
in various regimes, as well as the intrinsic frustration of a triangular
lattice. | cond-mat_supr-con |
Superconductivity in the non-oxide Perovskite MgCNi3: The oxide perovskites are a large family of materials with many important
physical properties. Of particular interest has been the fact that this
structure type provides an excellent structural framework for the existence of
superconductivity. The high Tc copper oxides are the most famous examples of
superconducting perovskites, but there are many others [1]. Intermetallic
compounds have been the source of many superconducting materials in the past,
but they have been eclipsed in recent years by the perovskite oxides. The
recent discovery of superconductivity in MgB2 [2] suggests that intermetallic
compounds with simple structure types are worth serious reconsideration as
sources of new superconducting materials. Here we report the observation of
superconductivity at 8 K in the perovskite structure intermetallic compound
MgCNi3, linking what appear at first sight to be mutually exclusive classes of
superconducting materials. The observation of superconductivity in MgCNi3
indicates that MgB2 will not be the only one of its kind within the chemical
paradigm that it suggests for new superconducting materials. | cond-mat_supr-con |
Theory of Local Density of States of d-Wave Superconducting State Near
the Surfaces of the t-J Model: Spatial dependencies of the pair potential and the local density of states
near the surfaces of $d_{x^{2}-y^{2}}$-wave superconductors are studied
theoretically. The calculation is based on the t-J model within a mean-field
theory with Gutzwiller approximation. Various types of surface geometries are
considered. Similar to our result in the extended Hubbard model, it is found
that the formation of zero-energy states strongly depends on the surface
geometry. In addition to this feature, the zero-energy states give peak
splitting for the (110) surfaces when the super-exchange interaction $J$ is
large. This is due to the induced s-wave component near the surface. The
present result explains the microscopic origin of the spontaneous time-
reversal symmetry breaking at the surfaces of high-$T_{c}$ superconductors. | cond-mat_supr-con |
Superconducting and thermal properties of ex-situ Glidcop sheathed
multifilamentary MgB2 wires: In DC and AC practical applications of MgB2 superconducting wires an
important role is represented by the material sheath which has to provide,
among other things, a suitable electrical and thermal stabilization. A way to
obtain a large enough amount of low resistivity material in to the conductor
architecture is to use it as external sheath. In this paper we study ex-situ
multifilamentary MgB2 wires using oxide-dispersion-strengthened copper
(GlidCop) as external sheath in order to reach a good compromise between
critical current density and thermal properties. We prepared three GlidCop
samples differing by the content of dispersed sub-microscopic Al2O3 particles.
We characterized the superconducting and thermal properties and we showed that
the good thermal conductivity together the good mechanical properties and a
reasonable critical current density make of GlidCop composite wire a useful
conductor for applications where high thermal conductivity is request at
temperature above 30K, such as Superconducting-FCL. | cond-mat_supr-con |
Temperature, RF Field, and Frequency Dependence Performance Evaluation
of Superconducting Niobium Half-Wave Cavity: Recent advancement in superconducting radio frequency cavity processing
techniques, with diffusion of impurities within the RF penetration depth,
resulted in high quality factor with increase in quality factor with increasing
accelerating gradient. The increase in quality factor is the result of a
decrease in the surface resistance as a result of nonmagnetic impurities doping
and change in electronic density of states. The fundamental understanding of
the dependence of surface resistance on frequency and surface preparation is
still an active area of research. Here, we present the result of RF
measurements of the TEM modes in a coaxial half wave niobium cavity resonating
at frequencies between 0.3-1.3 GHz. The temperature dependence of the surface
resistance was measured between 4.2 K and 1.6 K. The field dependence of the
surface resistance was measured at 2.0 K. The baseline measurements were made
after standard surface preparation by buffered chemical polishing. | cond-mat_supr-con |
Electronic structure of CaFe2As2: Contribution of itinerant Fe 3d-states
to the Fermi Level: We present density functional theory (DFT) calculations and a full set of
X-ray spectra (resonant inelastic X-ray scattering and X-ray photoelectron
spectra) measurements of single crystal CaFe2As2. The experimental valence band
spectra are consistent with our DFT calculations. Both theory and experiment
show that the Fe 3d-states dominate the Fermi level and hybridize with Ca
3d-states. The simple shape of Xray photoelectron (XPS) Fe 2p-core level
spectrum (without any satellite structure typical for correlated systems)
suggests itinerant character of the Fe 3d-electrons. Based on the similarity of
the calculated and experimental Fe 3d-states distribution in LaOFeAs and
CaFe2As2 we conclude that superconductivity in the FeAs-systems can be
described within a minimal model, taking into account only Fe 3d-bands close to
the Fermi level. | cond-mat_supr-con |
ARPES experiment in fermiology of quasi-2D metals (Review Article): Angle resolved photoemission spectroscopy (ARPES) enables direct observation
of the Fermi surface and underlying electronic structure of crystals---the
basic concepts to describe all the electronic properties of solids and to
understand the key electronic interactions involved. The method is the most
effective to study quasi-2D metals, to which the subjects of almost all hot
problems in modern condensed matter physics have happened to belong. This has
forced incredibly the development of the ARPES method which we face now. The
aim of this paper is to introduce to the reader the state-of-the-art ARPES,
reviewing the results of its application to such topical problems as high
temperature superconductivity in cuprates and iron based superconductors, and
electronic ordering in the transition metal dichalcogenides and manganites. | cond-mat_supr-con |
Electronic theory for the normal state spin dynamics in Sr$_2$RuO$_4$:
anisotropy due to spin-orbit coupling: Using a three-band Hubbard Hamiltonian we calculate within the
random-phase-approximation the spin susceptibility, $\chi({\bf q},\omega)$, and
NMR spin-lattice relaxation rate, 1/T$_1$, in the normal state of the triplet
superconductor Sr$_2$RuO$_4$ and obtain quantitative agreement with
experimental data. Most importantly, we find that due to spin-orbit coupling
the out-of-plane component of the spin susceptibility $\chi^{zz}$ becomes at
low temperatures two times larger than the in-plane one. As a consequence
strong incommensurate antiferromagnetic fluctuations of the
quasi-one-dimensional $xz$- and $yz$-bands point into the $z$-direction. Our
results provide further evidence for the importance of spin fluctuations for
triplet superconductivity in Sr$_2$RuO$_4$. | cond-mat_supr-con |
DFT+U study of magnetic order in doped La$_2$CuO$_4$ crystals: This article presents the results of several magnetic phases of doped
La$_{2-x}$Sr$_x$CuO$_4$ using density-functional theory with an added Hubbard
term (DFT+U). Doping factors from $x=0$ to 0.25 were examined. We found that a
bond centered stripe is the magnetic ground state for $x=1/8$ and $x=1/4$. No
stable stripe order was found for $x=1/6$. Analysis of the electron density
revealed that apical oxygen atoms, those located above and below the copper
atoms in the CuO$_2$ planes, hold a non negligible part of the holes at large
doping and present a small spin polarization. Finally, the charge
reorganization caused by the magnetic stripe modulation was studied for bond
centered and atom centered stripes. | cond-mat_supr-con |
Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates: We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor,
using high precision torsional oscillator and DC calorimetry techniques. Our
investigation focused on the onset of superfluidity at low temperatures as the
4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel
system were used to determine the superfluid density of films with transition
temperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor
system probed the excitation spectrum of both non-superfluid and superfluid
films for temperatures down to 10 mK. Both sets of measurements suggest that
the critical coverage for the onset of superfluidity corresponds to a mobility
edge in the chemical potential, so that the onset transition is the bosonic
analog of a superconductor-insulator transition. The superfluid density
measurements, however, are not in agreement with the scaling theory of an onset
transition from a gapless, Bose glass phase to a superfluid. The heat capacity
measurements show that the non-superfluid phase is better characterized as an
insulator with a gap. | cond-mat_supr-con |
Controlling the Intrinsic Josephson Junction Number in a
$\mathbf{Bi_2Sr_2CaCu_2O_{8+δ}}$ Mesa: In fabricating $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ intrinsic Josephson
junctions in 4-terminal mesa structures, we modify the conventional fabrication
process by markedly reducing the etching rates of argon ion milling. As a
result, the junction number in a stack can be controlled quite satisfactorily
as long as we carefully adjust those factors such as the etching time and the
thickness of the evaporated layers. The error in the junction number is within
$\pm 1$. By additional ion etching if necessary, we can controllably decrease
the junction number to a rather small value, and even a single intrinsic
Josephson junction can be produced. | cond-mat_supr-con |
Superconducting electronic state in optimally doped YBa2Cu3O7-d observed
with laser-excited angle-resolved photoemission spectroscopy: Low energy electronic structure of optimally doped YBa2Cu3O7-d is
investigated using laser-excited angle-resolved photoemission spectroscopy. The
surface state and the CuO chain band that usually overlap the CuO2 plane
derived bands are not detected, thus enabling a clear observation of the bulk
superconducting state. The observed bilayer splitting of the Fermi surface is
~0.08 angstrom^{-1} along the (0,0)-(pi,pi) direction, significantly larger
than Bi2Sr2CaCu2O8+d. The kink structure of the band dispersion reflecting the
renormalization effect at ~60 meV shows up similarly as in other hole-doped
cuprates. The momentum-dependence of the superconducting gap shows
d_{x^2-y^2}-wave like amplitude, but exhibits a nonzero minimum of ~12 meV
along the (0,0)-(pi,pi) direction. Possible origins of such an unexpected
"nodeless" gap behavior are discussed. | cond-mat_supr-con |
Optical Conductivity Anisotropy in the Undoped Three-Orbital Hubbard
Model for the Pnictides: The resistivity anisotropy unveiled in the study of detwinned single crystals
of the undoped 122 pnictides is here studied using the two-dimensional
three-orbital Hubbard model in the mean-field approximation. Calculating the
Drude weight in the x and y directions at zero temperature for a Q=(\pi,0)
magnetically ordered state, the conductance along the antiferromagnetic
direction is shown to be larger than along the ferromagnetic direction. This
effect is caused by the suppression of the d_{yz} orbital at the Fermi surface,
but additional insight based on the momentum dependence of the transitions
induced by the current operator is provided. It is shown that the effective
suppression of the inter-orbital hopping d_{xy} and d_{yz} along the y
direction is the main cause of the anisotropy. | cond-mat_supr-con |
Probing d-wave pairing correlations in the pseudogap regime of the
cuprate superconductors via low-energy states near impurities: The issue of probing the pseudogap regime of the cuprate superconductors,
specifically with regard to the existence and nature of superconducting pairing
correlations of d-wave symmetry, is explored theoretically. It is shown that if
the d-wave correlations believed to describe the superconducting state persist
into the pseudogap regime, but with pair-potential phase-fluctuations that
destroy their long-range nature, then the low-energy quasiparticle states
observed near extended impurities in the truly superconducting state should
also persist as resonances in the pseudogap regime. The scattering of
quasiparticles by these phase-fluctuations broadens what was (in the
superconducting state) a sharp peak in the single-particle spectral function at
low energy, as we demonstrate within the context of a simple model. This peak
and its broadening is, in principle, accessible via scanning tunneling
spectroscopy near extended impurities in the pseudogap regime. If so, such
experiments would provide a probe of the extent to which d-wave superconducting
correlations persist upon entering the pseudogap regime, thus providing a
stringent diagnostic of the phase-fluctuation scenario. | cond-mat_supr-con |
Hund's coupling stabilized superconductivity in the presence of
spin-orbit interactions: The intraorbital repulsive Hubbard interaction cannot lead to attractive
superconducting pairing states, except through the Kohn-Luttinger mechanism.
This situation may change when we include additional local interactions such as
the interorbital repulsion $U^\prime$ and Hund's interactions $J$. Adding these
local interactions, we study the nature of the superconducting pairs in systems
with tetragonal crystal symmetry including the $d_{xz}$ and $d_{yz}$ orbitals,
and in octahedral systems including all three of $d_{xz}$, $d_{yz}$, and
$d_{xy}$ orbitals. In the tetragonal case, spin-orbit interactions can
stabilize attractive pairing channels containing spin triplet, orbital singlet
character. Depending on the form of spin-orbit coupling, pairing channels
belonging to degenerate, non-trivial irreducible representations may be
stabilized. In the octahedral case, the pairing interactions of superconducting
channels are found to depend critically on the number of bands crossing the
Fermi energy. | cond-mat_supr-con |
Pairing, Pseudogap and Fermi Arcs in Cuprates: We use Angle Resolved Photoemission Spectroscopy (ARPES) to study the
relationship between the pseudogap, pairing and Fermi arcs in cuprates. High
quality data measured over a wide range of dopings reveals a consistent picture
of Fermiology and pairing in these materials. The pseudogap is due to an
ordered state that competes with superconductivity rather then preformed pairs.
Pairing does occur below Tpair~150K and significantly above Tc, but well below
T* and the doping dependence of this temperature scale is distinct from that of
the pseudogap. The d-wave gap is present below Tpair, and its interplay with
strong scattering creates "artificial" Fermi arcs for Tc<T<Tpair. However,
above Tpair, the pseudogap exists only at the antipodal region. This leads to
presence of real, gapless Fermi arcs close to the node. The length of these
arcs remains constant up to T*, where the full Fermi surface is recovered. We
demonstrate that these findings resolve a number of seemingly contradictory
scenarios. | cond-mat_supr-con |
Theoretical study on the correlation between the spin fluctuation and
$T_c$ in the isovalent-doped 1111 iron-based superconductors: Motivated by recent experiments on isovalent-doped 1111 iron-based
superconductors LaFeAs$_{1-{x}}$P$_{x}$O$_{1-{y}}$F$_{y}$ and the theoretical
study that followed, we investigate, within the five orbital model, the
correlation between the spin fluctuation and the superconducting transition
temperature, which exhibits a double dome feature upon varying the Fe-As-Fe
bond angle. Around the first dome with higher $T_c$, the low energy spin
fluctuation and $T_c$ are not tightly correlated because the finite energy spin
fluctuation also contributes to superconductivity. On the other hand, the
strength of the low-energy spin fluctuation originating from the $d_{xz/yz}$
orbital is correlated with $T_c$ in the second dome with lower $T_c$. These
calculation results are consistent with recent NMR study, and hence strongly
suggest that the pairing in the iron-based superconductors is predominantly
caused by the multi-orbital spin fluctuation. | cond-mat_supr-con |
First order $0$ - $π$ phase transitions in
superconductor/ferromagnet/superconductor trilayers: We study the thermodynamics of the diffusive SFS trilayer composed of thin
superconductor (S) and ferromagnet (F) layers. On the basis the self-consistent
solutions of nonlinear Usadel equations in the F and S layers we obtain the
Ginzburg--Landau expansion and compute the condensation free energy and entropy
of the $0$ (even) and $\pi$ (odd) order parameter configurations. The first
order $0-\pi$ transition as a function of temperature $T$ occurs, which is
responsible for a jump of the averaged magnetic field penetration depth
$\lambda(T)$ recently observed on experiments [N.Pompeo, et. al., Phys. Rev. B
90, 064510 (2014)]. The generalized Ginzburg-Landau functional was proposed to
describe SFS trilayer for arbitrary phase difference between the
superconducting order parameters in the S layers. The temperature dependence of
the SFS Josephson junction critical current demonstrates the strong
anharmonicity of the corresponding current--phase relation in the vicinity of
the $0-\pi$ transition. In rf SQUID, coexistence of stable and metastable $0$
and $\pi$ states provides integer and half--integer fluxoid configurations. | cond-mat_supr-con |
Constrain relations for superfluid weight and pairings in a chiral flat
band superconductor: Within ten years, flat band (FB) superconductivity has gained a huge interest
for its remarkable features and connection to quantum geometry. We investigate
the superconductivity in a FB system whose orbitals are inequivalent and in
which the gap and the quantum metric are tunable. The key feature of the
present theoretical study is to show a unique and simple constrain relation
that pairings obey. Furthermore, pairings and superfluid weight in partially
filled FB are shown to be controlled by those of the half-filled lattice. We
argue that the geometry of the lattice or the complexity of the hopping terms
have no impact on the features revealed in this work as far as the system is
bipartite. | cond-mat_supr-con |
Non-London electrodynamics in a multiband London model:
anisotropy-induced non-localities and multiple magnetic field penetration
lengths: The London model describes strongly type-2 superconductors as massive vector
field theories, where the magnetic field decays exponentially at the length
scale of the London penetration length. This also holds for isotropic
multi-band extensions, where the presence of multiple bands merely renormalises
the London penetration length. We show that, by contrast, the magnetic
properties of anisotropic multi-band London models are not this simple, and the
anisotropy leads to the inter-band phase differences becoming coupled to the
magnetic field. This results in the magnetic field in such systems having N+1
penetration lengths, where N is the number of field components or bands. That
is, in a given direction, the magnetic field decay is described by N+1 modes
with different amplitudes and different decay length scales. For certain
anisotropies we obtain magnetic modes with complex masses. That means that
magnetic field decay is not described by a monotonic exponential increment set
by a real penetration length but instead is oscillating. Some of the
penetration lengths are shown to diverge away from the superconducting phase
transition when the mass of the phase-difference mode vanishes. Finally the
anisotropy-driven hybridization of the London mode with the Leggett modes can
provide an effectively non-local magnetic response in the nominally local
London model. Focusing on the two-component model, we discuss the magnetic
field inversion that results from the effective non-locality, both near the
surface of the superconductor and around vortices. In the regime where the
magnetic field decay becomes non-monotonic, the multiband London superconductor
is shown to form weakly-bound states of vortices. | cond-mat_supr-con |
Ferromagnetic-superconducting hybrid films and their possible
applications: A direct study in a model combinatorial film: Model combinatorial films (CFs) which host a pure superconductor adjacent to
a ferromagneticsuperconducting hybrid film (HF) are manufactured for the study
of the influence of ferromagnetic nanoparticles (FNs) on the nucleation of
superconductivity. Careful resistance measurements were performed
simultaneously on two different sites of the CFs. Enhancement of
superconductivity and magnetic memory effects were observed only on the hybrid
site of the CFs but were absent on their purely superconducting part. Our
results give direct proof that the FNs modulate the superconducting order
parameter in an efficient and controlled way giving us the possibility of
miscellaneous practical applications. | cond-mat_supr-con |
The resonance peak in the electron-doped cuprate superconductors: We study the emergence of a magnetic resonance in the superconducting state
of the electron-doped cuprate superconductors. We show that the recently
observed resonance peak in the electron-doped superconductor
Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$ is consistent with an overdamped spin
exciton located near the particle-hole continuum. We present predictions for
the magnetic-field dependence of the resonance mode as well as its temperature
evolution in those parts of the phase diagram where $d_{x^2-y^2}$-wave
superconductivity may coexist with an antiferromagnetic spin-density wave. | cond-mat_supr-con |
Comment on "Large energy gaps in CaC6 from tunneling spectroscopy:
possible evidence of strong-coupling superconductivity": Comment on "Large energy gaps in CaC6 from tunneling spectroscopy: possible
evidence of strong-coupling superconductivity" | cond-mat_supr-con |
In-gap states of magnetic impurity in quantum spin Hall insulator
proximitized to a superconductor: We study in-gap states of a single magnetic impurity embedded in a honeycomb
monolayer which is deposited on superconducting substrate. The intrinsic
spin-orbit coupling induces the quantum spin Hall insulating (QSHI) phase
gapped around the Fermi energy. Under such circumstances we consider the
emergence of Shiba-like bound states driven by the superconducting proximity
effect. We investigate their topography, spin-polarization and signatures of
the quantum phase transition manifested by reversal of the local currents
circulating around the magnetic impurity. These phenomena might be important
for more exotic in-gap quasiparticles in such complex nanostructures as
magnetic nanowires or islands, where the spin-orbit interaction along with the
proximity induced electron pairing give rise to topological phases hosting the
protected boundary modes. | cond-mat_supr-con |
Nonlinear features of the superconductor--ferromagnet--superconductor
$\varphi_0$ Josephson junction in ferromagnetic resonance region: We demonstrate the manifestations of the nonlinear features in magnetic
dynamics and IV-characteristics of the $\varphi_0$ Josephson junction in the
ferromagnetic resonance region. We show that at small values of system
parameters, namely, damping, spin-orbit interaction, and Josephson to magnetic
energy ratio, the magnetic dynamics is reduced to the dynamics of the scalar
Duffing oscillator, driven by the Josephson oscillations. The role of
increasing superconducting current in the resonance region is clarified.
Shifting of the ferromagnetic resonant frequency and the reversal of its
damping dependence due to nonlinearity are demonstrated by the full
Landau-Lifshitz-Gilbert-Josephson system of equations, and in its different
approximations. Finally, we demonstrate the negative differential resistance in
the IV--characteristics, and its correlation with the foldover effect. | cond-mat_supr-con |
Diamagnetic susceptibility obtained from the six-vertex model and its
implications for the high-temperature diamagnetic state of cuprate
superconductors: We study the diamagnetism of the 6-vertex model with the arrows as directed
bond currents. To our knowledge, this is the first study of the diamagnetism of
this model. A special version of this model, called F model, describes the
thermal disordering transition of an orbital antiferromagnet, known as
d-density wave (DDW), a proposed state for the pseudogap phase of the high-Tc
cuprates. We find that the F model is strongly diamagnetic and the
susceptibility may diverge in the high temperature critical phase with power
law arrow correlations. These results may explain the surprising recent
observation of a diverging low-field diamagnetic susceptibility seen in some
optimally doped cuprates within the DDW model of the pseudogap phase. | cond-mat_supr-con |
Evidence of zero point fluctuation of vortices in a very weakly pinned
a-MoGe thin film: In a Type II superconductor, the vortex core behaves like a normal metal.
Consequently, the single-particle density of states in the vortex core of a
conventional Type II superconductor remains either flat or (for very clean
single crystals) exhibits a peak at zero bias due to the formation of Caroli-de
Gennes-Matricon bound state inside the core. Here we report an unusual
observation from scanning tunneling spectroscopy measurements in a weakly
pinned thin film of the conventional s-wave superconductor a-MoGe, namely, that
a soft gap in the local density of states continues to exist even at the center
of the vortex core. We ascribe this observation to rapid fluctuation of
vortices about their mean position that blurs the boundary between the gapless
normal core and the gapped superconducting region outside. Analyzing the data
as a function of magnetic field we show that the variation of fluctuation
amplitude as a function of magnetic field is consistent with quantum zero-point
motion of vortices. | cond-mat_supr-con |
Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures: Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x
mesa structures leads to persistent doping of the mesas. Here we employ this
effect for analysis of the doping dependence of the electronic spectra of
Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are
able to controllably and reversibly change the doping state of the same single
crystal from underdoped to overdoped state, without changing its chemical
composition. It is observed that such physical doping is affecting
superconductivity in Bi-2212 similar to chemical doping by oxygen impurities:
with overdoping the critical temperature and the superconducting gap decrease,
with underdoping the c-axis critical current rapidly decreases due to
progressively more incoherent interlayer tunneling and the pseudogap rapidly
increases, indicative for the presence of the critical doping point. We
distinguish two main mechanisms of persistent electric doping: (i) even in
voltage contribution, attributed to a charge transfer effect, and (ii) odd in
voltage contribution, attributed to reordering of oxygen impurities. | cond-mat_supr-con |
The Topological Relation Between Bulk Gap Nodes and Surface Bound States
: Application to Iron-based Superconductors: In the past few years materials with protected gapless surface (edge) states
have risen to the central stage of condensed matter physics. Almost all
discussions centered around topological insulators and superconductors, which
possess full quasiparticle gaps in the bulk. In this paper we argue systems
with topological stable bulk nodes offer another class of materials with robust
gapless surface states. Moreover the location of the bulk nodes determines the
Miller index of the surfaces that show (or not show) such states. Measuring the
spectroscopic signature of these zero modes allows a phase-sensitive
determination of the nodal structures of unconventional superconductors when
other phase-sensitive techniques are not applicable. We apply this idea to
gapless iron based superconductors and show how to distinguish accidental from
symmetry dictated nodes. We shall argue the same idea leads to a method for
detecting a class of the elusive spin liquids. | cond-mat_supr-con |
Unconventional superconductivity induced in Nb films by adsorbed chiral
molecules: Motivated by recent observations of chiral-induced magnetization and
spin-selective transport we studied the effect of chiral molecules on
conventional BCS superconductors. By applying scanning tunneling spectroscopy,
we demonstrate that the singlet-pairing s-wave order parameter of Nb is
significantly altered upon adsorption of chiral polyalanine alpha-helix
molecules on its surface. The tunneling spectra exhibit zero-bias conductance
peaks embedded inside gaps or gap-like features, suggesting the emergence of
unconventional triplet-pairing components with either d-wave or p-wave
symmetry, as corroborated by simulations. These results may open a way for
realizing simple superconducting spintroinics devices. | cond-mat_supr-con |
Comment on "Microwave vortex dissipation of superconducting Nd-Ce-Cu-O
epitaxial films in high magnetic fields": While the electrodynamic treatment of a superconducting thin film by Yeh et
al. [Phys. Rev. B 48, 9861 (1993)] ostensibly has application for the
"thin-film limit," their work is shown to employ bulk-like instead of thin-film
electrodynamics, and as such obtains an ostensible violation of the first law
of thermodynamics and a surface resistance independent of the resistivity.
Other theoretical and experimental difficulties are briefly noted. | cond-mat_supr-con |
Cooper-pair size and binding energy for unconventional superconducting
systems: The main proposal of this paper is to analyze the size of the Cooper pairs
composed by unbalanced mass fermions from different electronic bands along the
BCS-BEC crossover and study the binding energy of the pairs. We are considering
an interaction between fermions with different masses leading to an inter-band
pairing. In addiction to the attractive interaction we have an hybridization
term to couple both bands, which in general acts unfavorable for the pairing
between the electrons. We get first order phase transitions as the
hybridization break the Cooper pairs for the the $s$-wave symmetry of the gap
amplitude. The results show the dependence of the Cooper-pair size as a
function of the hybridization for $T=0$. We also propose the structure of the
binding energy of the inter-band system as a function of the two-bands
quasi-particle energies. | cond-mat_supr-con |
Upper limit on spontaneous supercurrents in Sr$_2$RuO$_4$: It is widely believed that the perovskite Sr$_2$RuO$_4$ is an unconventional
superconductor with broken time reversal symmetry. It has been predicted that
superconductors with broken time reversal symmetry should have spontaneously
generated supercurrents at edges and domain walls. We have done careful imaging
of the magnetic fields above Sr$_2$RuO$_4$ single crystals using scanning Hall
bar and SQUID microscopies, and see no evidence for such spontaneously
generated supercurrents. We use the results from our magnetic imaging to place
upper limits on the spontaneously generated supercurrents at edges and domain
walls as a function of domain size. For a single domain, this upper limit is
below the predicted signal by two orders of magnitude. We speculate on the
causes and implications of the lack of large spontaneous supercurrents in this
very interesting superconducting system. | cond-mat_supr-con |
Magnetism and Superconductivity in Ferromagnetic Heavy Fermion System
UCoGe under In-plane Magnetic Fields: We study the ferromagnetic superconductor UCoGe at ambient pressure under
$ab$-plane magnetic fields $\vec{H}$ which are perpendicular to the
ferromagnetic easy axis. It is shown that, by taking into account the
Dyaloshinskii-Moriya interaction arising from the zigzag chain crystal
structure of UCoGe, we can qualitatively explain the experimentally observed
in-plane anisotropy for critical magnetic fields of the paramagnetic
transition. Because of this strong dependence on the magnetic field direction,
upper critical fields of superconductivity, which is mediated by ferromagnetic
spin fluctuations, also become strongly anisotropic. The experimental
observation of "S-shaped" $H_{c2}\parallel b$-axis is qualitatively explained
as a result of enhancement of the spin fluctuations due to decreased Curie
temperature by the $b$-axis magnetic field. We also show that the S-shaped
$H_{c2}$ is accompanied by a rotation of the $d$-vector, which would be a key
to understand the experiments not only at ambient pressure but also under
pressure. | cond-mat_supr-con |
Mössbauer spectroscopy study of magnetic fluctuations in
superconducting RbGd$_2$Fe$_4$As$_4$O$_2$: $^{57}$Fe M\"ossbauer spectra were measured at different temperatures between
5.9 K and 300 K on the recently discovered self-doped superconducting
RbGd$_2$Fe$_4$As$_4$O$_2$ with T$_c$ as high as 35 K. Singlet pattern was
observed down to the lowest temperature measured in this work, indicating the
absence of static magnetic order on the Fe site. The intermediate isomer shift
in comparison with that of the samples RbFe$_2$As$_2$ and GdFeAsO confirms the
self doping induced local electronic structure change. Surprisingly, we observe
two magnetic fluctuation induced spectral broadenings below $\sim$15 K and
$\sim$100 K which are believed to be originated from the transferred magnetic
fluctuations of the Gd$^{3+}$ moments and that of the magnetic fluctuations of
the Fe atoms, respectively. | cond-mat_supr-con |
Three-band superconductivity and the order parameter that breaks
time-reversal symmetry: We consider a model of multiband superconductivity, inspired by iron
pnictides, in which three bands are connected via repulsive pair-scattering
terms. Generically, three distinct superconducting states arise within such a
model. Two of them are straightforward generalizations of the two-gap order
parameter while the third one corresponds to a time-reversal symmetry breaking
order parameter, altogether absent within the two-band model. Potential
observation of such a genuinely frustrated state would be a particularly vivid
manifestation of the repulsive interactions being at the root of iron-based
high temperature superconductivity. We construct the phase diagram of this
model and discuss its relevance to the iron pnictides family of high
temperature superconductors. We also study the case of the Josephson coupling
between a two-band s' (or extended s-wave) superconductor and a single-gap
s-wave superconductor, and the associated phase diagram. | cond-mat_supr-con |
Pressure Driven Fractionalization of Ionic Spins Results in Cupratelike
High-$T_c$ Superconductivity in La$_3$Ni$_2$O$_7$: Beyond 14GPa of pressure, bi-layered La$_3$Ni$_2$O$_7$ was recently found to
develop strong superconductivity above the liquid nitrogen boiling temperature.
An immediate essential question is the pressure-induced qualitative change of
electronic structure that enables the exciting high-temperature
superconductivity. We investigate this timely question via a numerical
multi-scale derivation of effective many-body physics. At the atomic scale, we
first clarify that the system has a strong charge transfer nature with
itinerant carriers residing mainly in the in-plane oxygen between spin-1
Ni$^{2+}$ ions. We then elucidate in eV- and sub-eV-scale the key physical
effect of the applied pressure: It induces a cupratelike electronic structure
through partially screening the Ni spin from 1 to 1/2. This suggests a
high-temperature superconductivity in La$_3$Ni$_2$O$_7$ with microscopic
mechanism and ($d$-wave) symmetry similar to that in the cuprates. | cond-mat_supr-con |
Observation of Higher-Order Sideband Transitions and First-Order
Sideband Rabi Oscillations in a Superconducting Flux Qubit Coupled to a SQUID
Plasma Mode: We report results of spectroscopic measurements and time-domain measurements
of a superconducting flux qubit. The dc superconducting quantum interference
device (SQUID), used for readout of the qubit, and a shunt capacitor formed an
LC resonator generating a SQUID plasma mode. Higher-order red and blue
sidebands were observed in a simple measurement scheme because the resonant
energy of the resonator, 600 MHz, was comparable to the thermal energy. We also
observed Rabi oscillations on the carrier transition and the first-order
sideband transitions. Because the qubit was coupled to a single arm of the dc
SQUID, the qubit-SQUID coupling was significant at zero bias current, where
these phenomena were observed. The ratios between the Rabi periods for the
carrier transition and the sideband transitions are compared with those
estimated from the coupling constant, which was separately determined. The
result may be explained by assuming initial excitation of the resonator. | cond-mat_supr-con |
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