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Two-component uniform spin susceptibility in superconducting
HgBa$_{2}$CuO$_{4+δ}$ single crystals determined with $^{63}$Cu and
$^{199}$Hg NMR: $^{63}$Cu and $^{199}$Hg NMR shifts for an optimally and underdoped
HgBa$_{2}$CuO$_{4+\delta}$ single crystal are reported, and the temperature
dependence dictates a two-component description of the uniform spin
susceptibility. The first component, associated with the pseudogap phenomenon
in the NMR shifts, decreases already at room temperature and continues to drop
as the temperature is lowered, without a drastic change at the transition
temperature into the superconducting state. The second component is temperature
independent above the superconducting transition temperature and vanishes
rapidly below it. It increases with doping and is a substantial part of the
total spin susceptibility measured at both nuclei. | cond-mat_supr-con |
Fifty years of Hubbard and Anderson lattice models: from magnetism to
unconventional superconductivity - A brief overview: We briefly overview the importance of Hubbard and Anderson-lattice models as
applied to explanation of high-temperature and heavy-fermion superconductivity.
Application of the models during the last two decades provided an explanation
of the paired states in correlated fermion systems and thus extended
essentially their earlier usage to the description of itinerant magnetism,
fluctuating valence, and the metal-insulator transition. In second part, we
also present some of the new results concerning the unconventional
superconductivity and obtained very recently in our group. A comparison with
experiment is also discussed, but the main emphasis is put on rationalization
of the superconducting properties of those materials within the real-space
pairing mechanism based on either kinetic exchange and/or Kondo-type
interaction combined with the electron correlation effects. | cond-mat_supr-con |
Doping Dependence of Anisotropic Resistivities in Trilayered
Superconductor Bi2Sr2Ca2Cu3O10+delta (Bi-2223): The doping dependence of the themopower, in-plane resistivity rho_ab(T),
out-of-plane resistivity rho_c(T), and susceptibility has been systematically
measured for high-quality single crystal Bi2Sr2Ca2Cu3O10+delta. We found that
the transition temperature Tc and pseudogap formation temperature T_rho_c*,
below which rho_c shows a typical upturn, do not change from their optimum
values in the "overdoped" region, even though doping actually proceeds. This
suggests that, in overdoped region, the bulk $T_c$ is determined by the always
underdoped inner plane, which have a large superconducting gap, while the
carriers are mostly doped in the outer planes, which have a large phase
stiffness. | cond-mat_supr-con |
Comparative study on the thermoelectric effect of parent oxypnictides
La$T$AsO ($T$ = Fe, Ni): The thermopower and Nernst effect were investigated for undoped parent
compounds LaFeAsO and LaNiAsO. Both thermopower and Nernst signal in iron-based
LaFeAsO are significantly larger than those in nickel-based LaNiAsO.
Furthermore, abrupt changes in both thermopower and Nernst effect are observed
below the structural phase transition temperature and spin-density wave (SDW)
type antiferromagnetic (AFM) order temperature in Fe-based LaFeAsO. On the
other hand, Nernst effect is very small in the Ni-based LaNiAsO and it is
weakly temperature-dependent, reminiscent of the case in normal metals. We
suggest that the effect of SDW order on the spin scattering rate should play an
important role in the anomalous temperature dependence of Hall effect and
Nernst effect in LaFeAsO. The contrast behavior between the LaFeAsO and LaNiAsO
systems implies that the LaFeAsO system is fundamentally different from the
LaNiAsO system and this may provide clues to the mechanism of high $T_c$
superconductivity in the Fe-based systems. | cond-mat_supr-con |
Structure and Composition of the 200 K-Superconducting Phase of H2S
under Ultrahigh Pressure: The Perovskite (SH-)(H3S+): H2S is converted under ultrahigh pressure (> 110 GPa) to a metallic phase
that becomes superconducting with a record Tc of 200 K. It has been proposed
that the superconducting phase is body-centered cubic H3S ( Im3m , a = 3.089
{\AA}) resulting from a decomposition reaction 3H2S --> 2H3S + S. The analogy
of H2S and H2O leads us to a very different conclusion. The well-known
dissociation of water into H3O+ and OH- increases by orders of magnitude under
pressure. An equivalent behavior of H2S is anticipated under pressure with the
dissociation, 2H2S --> H3S+ + SH- forming a perovskite structure (SH-)(H3S+),
which consists of corner-sharing SH6 octahedra with SH- at each A-site (i.e.,
the center of each S8 cube). Our DFT calculations show that the perovskite
(SH-)(H3S+) is thermodynamically more stable than the Im3m structure of H3S,
and suggest that the A-site H atoms are most likely fluxional even at Tc. | cond-mat_supr-con |
Superconductivity in boron-doped carbon nanotube networks: By using the five Angstrom diameter pores of calcined zeolite as the
template, we have fabricated boron doped carbon nanotube networks via the
chemical vapor deposition method. Raman data indicate the network to comprise
segments of interconnected carbon nano tubes. Transport measurements showed a
superconducting transition initiating at 40K, with a sharp downturn around 20K
to a low resistance state at 2K, accompanied by a low resistance plateau in the
current voltage characteristic, fluctuating around zero resistance. Magnetic
measurements exhibited the Meissner effect characteristic of thin
superconducting wire networks in which the superconducting wire radius is much
smaller than the London penetration length. At low magnetic field, the negative
diamagnetic susceptibility was observed to persist beyond 200K. The transport
and magnetic data are reconciled on the basis of a physical model based on weak
links comprising short, one-dimensional superconducting nano tubes, that govern
the global transport behavior. | cond-mat_supr-con |
Quantum phase-slips in Josephson junction rings: We study quantum phase-slip (QPS) processes in a superconducting ring
containing N Josephson junctions and threaded by an external static magnetic
flux. In a such system, a QPS consists of a quantum tunneling event connecting
two distinct classical states of the phases with different persistent currents
[K. A. Matveev et al., Phys. Rev. Lett. 89, 096802 (2002)]. When the Josephson
coupling energy EJ of the junctions is larger than the charging energy EC =
e2/2C where C is the junction capacitance, the quantum amplitude for the QPS
process is exponentially small in the ratio EJ/EC. At given magnetic flux each
QPS can be described as the tunneling of the phase difference of a single
junction of almost 2pi, accompanied by a small harmonic displacement of the
phase difference of the other N-1 junctions. As a consequence the total QPS
amplitude nu is a global property of the ring. Here we study the dependence of
nu on the ring size N taking into account the effect of a finite capacitance C0
to ground which leads to the appearance of low-frequency dispersive modes.
Josephson and charging effects compete and lead to a nonmonotonic dependence of
the ring critical current on N. For N=infty, the system converges either
towards a superconducting or an insulating state, depending on the ratio
between the charging energy E0 = e2/2C0 and the Josephson coupling energy EJ. | cond-mat_supr-con |
Characterization of the superconducting phase in tellurium hydride at
high pressure: At present, hydrogen-based compounds constitute one of the most promising
classes of materials for applications as a phonon-mediated high-temperature
superconductors. Herein, the behavior of the superconducting phase in tellurium
hydride (HTe) at high pressure ($p=300$ GPa) is analyzed in details, by using
the isotropic Migdal-Eliashberg equations. The chosen pressure conditions are
considered here as a case study which corresponds to the highest critical
temperature value ($T_{c}$) in the analyzed material, as determined within
recent density functional theory simulations. It is found that the
Migdal-Eliashberg formalism, which constitutes a strong-coupling generalization
of the Bardeen-Cooper-Schrieffer (BCS) theory, predicts that the critical
temperature value ($T_{c}=52.73$ K) is higher than previous estimates of the
McMillan formula. Further investigations show that the characteristic
dimensionless ratios for the the thermodynamic critical field, the specific
heat for the superconducting state, and the superconducting band gap exceeds
the limits of the BCS theory. In this context, also the effective electron mass
is not equal to the bare electron mass as provided by the BCS theory. On the
basis of these findings it is predicted that the strong-coupling and
retardation effects play pivotal role in the superconducting phase of HTe at
300 GPa, in agreement with similar theoretical estimates for the sibling
hydrogen and hydrogen-based compounds. Hence, it is suggested that the
superconducting state in HTe cannot be properly described within the mean-field
picture of the BCS theory. | cond-mat_supr-con |
Probing pair-breaking mechanisms in proximity-induced hybrid
superconducting interfaces: Understanding depairing effects in a hybrid-superconducting interface
utilizing high spin-orbit materials such as topological insulators or 1D
semiconducting nanowires is becoming an important research topic in the study
of proximity-induced superconductivity. Experimentally, proximity-induced
superconductivity is found to suppress at much lower magnetic fields compared
to the superconducting layer without a good understanding of its cause. Here,
we provide a phenomenological tool to characterize different pair-breaking
mechanisms, the ones that break or preserve time reversal symmetry, and show
how they affect the differential tunneling conductance response. Importantly,
we probe the properties of the SC layer at the hybrid interface and observe
conductance peak pinning at zero bias in a larger field range with eventual
signs of weak peak splitting. Further, the effect of varying the spin-orbit
scattering and the Lande g-factor in tuning the conductance peaks show
interesting trends. | cond-mat_supr-con |
Study of Local Nonlinear Properties Using a Near-Field Microwave
Microscope: We have developed a near-field microwave microscope to locally apply
microwave frequency currents and fields to superconductors, and dielectric
substrates, and measure the locally generated 2nd and 3rd harmonic responses.
We measure the local nonlinear response of a Tl_2Ba_2CaCu_2O_y film grown on an
MgO substrate, and observe a large response due to the enhanced current density
near the edge. We also study the local nonlinear response of a YBa_2Cu_3O_7-d
thin film grown on a bi-crystal SrTiO_3 (STO) substrate, and spatially identify
the grain boundary through higher harmonic measurements. The spatial resolution
is determined by the magnetic loop probe size. A scaling current density JNL is
extracted to quantify the magnitude of the nonlinearity of the superconductor.
Preliminary results on the nonlinear properties of some commonly used
substrates, e.g. MgO and STO, have also been obtained | cond-mat_supr-con |
Superconductivity from site-selective Ru doping studies in Zr$_5$Ge$_3$
compound: Systematical doping studies have been carried out to search for the possible
superconductivity in the transition metal doped Zr$_5$Ge$_3$ system.
Superconductivity up to 5.7K is discovered in the Ru-doped
Zr$_5$Ge$_{2.5}$Ru$_{0.5}$ sample. Interestingly, with the same Ru-doping,
superconductivity is only induced with doping at the Ge site, but remains
absent down to 1.8K with doping at the Zr site or interstitial site. Both
magnetic and transport studies have revealed the bulk superconductivity nature
for Ru-doped Zr$_5$Ge$_{2.5}$Ru$_{0.5}$ sample. The high upper critical field,
enhanced electron correlation, and extremely small electron-phonon coupling,
have indicated possible unconventional superconductivity in this system, which
warrants further detailed theoretical and experimental studies. | cond-mat_supr-con |
Extracting phase information about the superconducting order parameter
from defect bound states: Impurity bound states and quasi-particle scattering from these can serve as
sensitive probes for identifying the pairing state of a superconducting
condensate. We introduce and discuss defect bound state quasi-particle
interference (DBS-QPI) imaging as a tool to extract information about the
symmetry of the order parameter from spatial maps of the density of states
around magnetic and non-magnetic impurities. We show that the phase information
contained in the scattering patterns around impurities can provide valuable
information beyond what is obtained through conventional QPI imaging. Keeping
track of phase, rather than just magnitudes, in the Fourier transforms is
achieved through phase-referenced Fourier transforms that preserve both real
and imaginary parts of the QPI images. We further compare DBS-QPI to other
approaches which have been proposed to use either QPI or defect scattering to
distinguish different symmetries of the order parameter. | cond-mat_supr-con |
Effective medium approximation and the complex optical properties of the
inhomogeneous superconductor K_{0.8}Fe_{2-y}Se_2: The in-plane optical properties of the inhomogeneous iron-chalcogenide
superconductor K_{0.8}Fe_{2-y}Se_2 with a critical temperature Tc = 31 K have
been modeled in the normal state using the Bruggeman effective medium
approximation for metallic inclusions in an insulating matrix. The volume
fraction for the inclusions is estimated to be ~ 10%; however, they appear to
be highly distorted, suggesting a filamentary network of conducting regions
joined through weak links. The value for the Drude plasma frequency in the
inclusions is much larger than the volume average, which when considered with
the reasonably low values for the scattering rate, suggests that the transport
in the grains is always metallic. Estimates for the dc conductivity and the
superfluid density in the grains places the inclusions on the universal scaling
line close to the other homogeneous iron-based superconductors. | cond-mat_supr-con |
Flatband-Induced Itinerant Ferromagnetism in RbCo$_2$Se$_2$: $A$Co$_2$Se$_2$ ($A$=K,Rb,Cs) is a homologue of the iron-based
superconductor, $A$Fe$_2$Se$_2$. From a comprehensive study of RbCo$_2$Se$_2$
via measurements of magnetization, transport, neutron diffraction,
angle-resolved photoemission spectroscopy, and first-principle calculations, we
identify a ferromagnetic order accompanied by an orbital-dependent
spin-splitting of the electronic dispersions. Furthermore, we identify the
ordered moment to be dominated by a $d_{x^2-y^2}$ flatband near the Fermi
level, which exhibits the largest spin splitting across the ferromagnetic
transition, suggesting an itinerant origin of the ferromagnetism. In the
broader context of the iron-based superconductors, we find this $d_{x^2-y^2}$
flatband to be a common feature in the band structures of both
iron-chalcogenides and iron-pnictides, accessible via heavy electron doping. | cond-mat_supr-con |
Hidden charge-conjugation, parity, and time-reversal symmetries and
massive Goldstone (Higgs) modes in superconductors: A massive Goldstone (MG) mode, often referred to as a Higgs amplitude mode,
is a collective excitation that arises in a system involving spontaneous
breaking of a continuous symmetry, along with a gapless Nambu-Goldstone mode.
It has been known in the previous studies that a pure amplitude MG mode emerges
in superconductors if the dispersion of fermions exhibits the particle-hole
(p-h) symmetry. However, clear understanding of the relation between the
symmetry of the Hamiltonian and the MG modes has not been reached. Here we
reveal the fundamental connection between the discrete symmetry of the
Hamiltonian and the emergence of pure amplitude MG modes. To this end, we
introduce nontrivial charge-conjugation ($\mathcal C$), parity ($\mathcal P$),
and time-reversal ($\mathcal T$) operations that involve the swapping of pairs
of wave vectors symmetrical with respect to the Fermi surface. The product of
$\mathcal{CPT}$ (or its permutations) represents an exact symmetry analogous to
the CPT theorem in the relativistic field theory. It is shown that a fermionic
Hamiltonian with a p-h symmetric dispersion exhibits the discrete symmetries
under $\mathcal C$, $\mathcal P$, $\mathcal T$, and $\mathcal{CPT}$. We find
that in the superconducting ground state, $\mathcal T$ and $\mathcal P$ are
spontaneously broken simultaneously with the U(1) symmetry. Moreover, we
rigorously show that amplitude and phase fluctuations of the gap function are
uncoupled due to the unbroken $\mathcal C$. In the normal phase, the MG and NG
modes become degenerate, and they have opposite parity under $\mathcal T$.
Therefore, we conclude that the lifting of the degeneracy in the
superconducting phase and the resulting emergence of the pure amplitude MG mode
can be identified as a consequence of the the spontaneous breaking of $\mathcal
T$ symmetry but not of $\mathcal P$ or U(1). | cond-mat_supr-con |
Quasiparticle conductance in Spin Valve Josephson Structures: We study the quasiparticle current in clean ferromagnetic Josephson
structures of the form $S_1/F_1/N/F_2/S_2$, where $S$, $F$, and $N$ denote
superconducting, ferromagnetic or normal layers respectively. Our focus is on
the structure of the conductance $G$ as a function of bias $V$, emphasizing the
subgap region. We use a fully self consistent numerical method, coupled to a
transfer matrix procedure to extract $G(V)$. We choose material parameters
appropriate to experimentally realized Co Cu Nb structures. We find a resonance
peak structure as a function of the intermediate layer thickness and of the
misalignement angle $\phi$ between $F$ layers. To understand this resonance
structure, we develop an approximate analytic method. For experimentally
relevant thicknesses, the conductance has multiple subgap peaks which oscillate
in position between low and critical bias positions. These oscillations occur
in both $\phi$ and the layer thicknesses. We compare our results with those
obtained for the spin valve structures $(F_1/N/F_2/S_2)$ and discuss the
implications of our results for the fabrication of spin Josephson devices. | cond-mat_supr-con |
Reverse Monte Carlo study of apical Cu-O bond distortions in
YBa2Cu3O6.93: A combination of neutron total scattering measurement and reverse Monte Carlo
(RMC) refinement is applied to the study of apical Cu-O bond distortions in the
high-Tc superconductor YBa2Cu3O6.93. We show that the average structure is not
consistent with a split-site model for the corresponding Cu and O positions,
but that the local structure nevertheless reveals the existence of two separate
apical Cu-O bond lengths. Using G(r) data obtained from a variety of Qmax
values we show that this result is independent of the data treatment
methodology. We also find that the resulting 'short' and 'long' Cu-O bond
lengths agree well with the results of previous EXAFS studies. The existence of
bimodal apical Cu-O bond distributions in the context of a single-site average
structure model is interpreted in terms of correlated displacements of the Cu
and O atoms. We find evidence also for the clustering of short apical Cu-O
bonds within our RMC configurations. | cond-mat_supr-con |
Evolution of vortex pinning in the FeSe$_{1-x}$S$_x$ system: We present a comprehensive study of vortex matter and pinning evolution in
the FeSe$_{1-x}$S$_x$ system with various doping degree. The influence of
sulphur substitution on vortex pinning and peak effect occurrence is studied.
We show that there is a complex interplay among various pinning contributions
in the FeSe$_{1-x}$S$_x$ system. Additionally, we study a possible vortex
liquid-vortex glass/lattice transition and find an evidence that the vortex
liquid-vortex glass phase transition in FeSe has a quasi two-dimensional
nature. We investigate the upper critical field behaviour in FeSe$_{1-x}$S$_x$
system, and found that the upper critical field is higher than that predicted
by the Werthamer-Helfand-Hohenberg (WHH) model, whereas its temperature
dependence could be fitted within a two-band framework. Finally, a detailed H-T
phase diagram is presented. | cond-mat_supr-con |
Elevated Tc levels in YBa2Cu3O6.5 modeled on a 3-dimensional doped bond
structure of chain and plane pairs: The complex phenomenology of shot quenched YBa2Cu3O6.5 with Tc=100K and 200K
levels is compared with laser pulsed analogs with an eye on explaining the
presumed Tc=552K of the latter. Shot quenching can produce metastable states
with pronounced increases in plane metric and cell volume, accompanied by a
rough doubling of Tc to a 100K level of an orthorhombic with 3-fold O
coordinated chain Cu (O3 type). These states decay over a non-superconducting
transition range to the conventional Tc=50K level of O24. We consider the plane
expanded laser pulsed materials to contain aspects of O42 plane n-doped
counterparts of the O3 n-doped version of shot quench preparations. In
addition, we assume that highly charged p-doped chains of 4-fold O coordination
form hole pairs at trijugate position, allowing close approach of the apical O
to the electron-doped planes. They are now capable of participating in the
bonding with the plane pairs at corresponding 3a0/2 location. The overall pair
number is therefore multiplied, and the coupling strengthened, by limited 3-D
effects within the Plane-Chain-Plane sandwich. The latter can be seen as an
extended chemical bonding system that has the potential to equilibrate
contractive and expansive pairs and so obviate the need for distinction of
doping type as it may exchange it dynamically. It is argued that indications
for a Tc=200K level on shot quenching has a related origin and represents one
in series of predicted Tc levels based on bond order principles. Predictions
are made where similar effects can be expected in other compound classes. | cond-mat_supr-con |
Phases of Mott-Hubbard bilayers: A phase diagram of two Mott-Hubbard planes interacting with a short-range
Coulomb repulsion is presented. Considering the case of equal amount of doping
by holes in one layer as electrons in the other, a holon-doublon inter-layer
exciton formation is shown to be a natural consequence of Coulomb attraction.
Quasiparticle spectrum is gapped and incoherent below a critical doping
$\delta_c$ due to the formation of excitons. A spin liquid insulator (SLI)
phase is thus realized without the lattice frustration. The critical value
$\delta_{c}$ sensitively depends on the inter-layer interaction strength. In
the $tJ$ model description of each layer with the d-wave pairing, $\delta_{c}$
marks the crossover between SLI and d-wave superconductor. The SLI phase,
despite being non-superconducting and charge-gapped, still shows
electromagnetic response similar to that of a superfluid due to the exciton
transport. Including antiferromagnetic order in the $tJ$ model introduces
magnetically ordered phases at low doping and pushes the spin liquid phase to a
larger inter-layer interaction strength and higher doping concentrations. | cond-mat_supr-con |
Properties of the phonon-induced pairing interaction in
YBa$_2$Cu$_3$O$_7$ within the local density approximation: The properties of the phonon-induced interaction between electrons are
studied using the local density approximation (LDA). Restricting the electron
momenta to the Fermi surface we find generally that this interaction has a
pronounced peak for large momentum transfers and that the interband
contributions between bonding and antibonding band are of the same magnitude as
the intraband ones. Results are given for various symmetry averages of this
interaction over the Fermi surface. In particular, we find that the
dimensionless coupling constant in the d-wave channel $\lambda^d$, relevant for
superconductivity, is only 0.022, i.e., even about ten times smaller than the
small value of the s-wave channel. Similarly, the LDA contribution to the
resistivity is about a factor 10 times smaller than the observed resistivity
suggesting that phonons are not the important low-energy excitations in
high-T$_c$ oxides. | cond-mat_supr-con |
Pressure induced metallization and possible unconventional
superconductivity in spin liquid $NaYbSe_{2}$: Beyond the conventional electron pairing mediated by phonons,
high-temperature superconductivity in cuprates is believed to stem from quantum
spin liquid (QSL). The unconventional superconductivity by doping a spin
liquid/Mott insulator, is a long-sought goal but a principal challenge in
condensed matter physics because of the lack of an ideal QSL platform. Here we
report the pressure induced metallization and possible unconventional
superconductivity in $NaYbSe_{2}$, which belongs to a large and ideal family of
triangular lattice spin liquid we revealed recently and is evidenced to possess
a QSL ground state. The charge gap of NaYbSe2 is gradually reduced by applying
pressures, and at ~20 GPa the crystal jumps into a superconducting (SC) phase
with Tc ~ 5.8 K even before the insulating gap is completely closed. The
metallization is confirmed by further high-pressure experiments but the sign of
superconductivity is not well repeated. No symmetry breaking accompanies the SC
transition, as indicated by X-ray diffraction and low-temperature Raman
experiments under high pressures. This intrinsically connects QSL and SC
phases, and suggests an unconventional superconductivity developed from QSL. We
further observed the magnetic-field-tuned superconductor-insulator transition
which is analogous to that found in the underdoped cuprate superconductor
$La_{2-x}Sr_{x}CuO_{4}$. The study is expected to inspire interest in exploring
new types of superconductors and sheds light into the intriguing physics from a
spin liquid/Mott insulator to a superconductor. | cond-mat_supr-con |
Weakly correlated electrons on a square lattice: a renormalization group
theory: We study the weakly interacting Hubbard model on the square lattice using a
one-loop renormalization group approach. The transition temperature T_c between
the metallic and (nearly) ordered states is found. In the parquet regime, (T_c
>> |mu|), the dominant correlations at temperatures below T_c are
antiferromagnetic while in the BCS regime (T_c << |mu|) at T_c the d-wave
singlet pairing susceptibility is most divergent. | cond-mat_supr-con |
Superconducting d-wave stripes in cuprates: Valence bond order
coexisting with nodal quasiparticles: We point out that unidirectional bond-centered charge-density-wave states in
cuprates involve electronic order in both s- and d-wave channels, with
non-local Coulomb repulsion suppressing the s-wave component. The resulting
bond-charge-density wave, coexisting with superconductivity, is compatible with
recent photoemission and tunneling data and as well as neutron-scattering
measurements, once long-range order is destroyed by slow fluctuations or glassy
disorder. In particular, the real-space structure of d-wave stripes is
consistent with the scanning-tunneling-microscopy measurements on both
underdoped Bi2Sr2CaCu2O8+x and Ca2-xNaxCuO2Cl2 of Kohsaka et al. [Science 315,
1380 (2007), arXiv:cond-mat/0703309]. | cond-mat_supr-con |
Andreev reflection observation in Nb/FeSi/Nb and Nb/FeSi/Si/Nb Josephson
junctions: Electrical properties of Josephson junctions Nb/FeSi/Nb with
superconductor/ferromagnet (S/F)interfaces are presented. Due to Andreev
reflection the nearly exact quadruple enhancement of the tunnel junction
differential conductance compared with that of the normal state was achieved.
The transparency of the S/F interfaces in our junctions was estimated to be
close to unity. This almost ideal value is obtained due to the use of a very
smooth amorphous magnetic FeSi alloy for the barrier preparation. The real
structure of the Nb/FeSi/Nb tunnel junction is described as a S/F/I/F/S
junction. Also Nb/FeSi/Si/Nb Josephson junctions were investigated and the
results found on these junctions confirm the effects observed in Nb/FeSi/Nb. | cond-mat_supr-con |
Time-Reversal-Symmetry-Broken Superconductivity Induced by Frustrated
Inter-Component Couplings: Superconductivity is associated with spontaneously broken gauge symmetry. In
some exotic superconductors the time-reversal symmetry is broken as well,
accompanied with internal magnetic field. A time-reversal symmetry broken
(TRSB) superconductivity without internal magnetic field involved can be
induced by frustrated inter-component couplings, which becomes a realistic
issue recently due to the discovery of iron-pnictide superconductors. Here we
derive stability condition for this novel TRSB state using the Ginzburg-Landau
(GL) theory. We find that there are multiple divergent coherence lengths, and
that this novel superconductivity cannot be categorized by the GL number into
type I or type II. We reveal that the critical Josephson current of a
constriction junction between two bulk superconductors of different chiralities
is suppressed significantly from that for same chirality. This effect provides
a unique way to verify experimentally this brand new superconductivity. | cond-mat_supr-con |
Topological phase separation in 2D hard-core Bose-Hubbard system away
from half-filling: We suppose that the doping of the 2D hard-core boson system away from
half-filling may result in the formation of multi-center topological defect
such as charge order (CO) bubble domain(s) with Bose superfluid (BS) and extra
bosons both localized in domain wall(s), or a {\it topological} CO+BS {\it
phase separation}, rather than an uniform mixed CO+BS supersolid phase.
Starting from the classical model we predict the properties of the respective
quantum system. The long-wavelength behavior of the system is believed to
remind that of granular superconductors, CDW materials, Wigner crystals, and
multi-skyrmion system akin in a quantum Hall ferromagnetic state of a 2D
electron gas. | cond-mat_supr-con |
Dislocation Majorana zero modes in perovskite oxide 2DEG: Much of the current experimental efforts for detecting Majorana zero modes
have been centered on probing the boundary of quantum wires with strong
spin-orbit coupling. The same type of Majorana zero mode can also be realized
at crystalline dislocations in 2D superconductors with the nontrivial weak
topological indices. Unlike at an Abrikosov vortex, at such a dislocation,
there are no other low-lying midgap states than the Majorana zero mode so that
it avoids usual complications encountered in experimental detections such as
scanning tunneling microscope (STM) measurements. We will show that, using the
anisotropic dispersion of the $t_{2g}$ orbitals of Ti or Ta atoms, such a weak
topological superconductivity can be realized when the surface 2DEG of
SrTiO$_3$ or KTaO$_3$ becomes superconducting, which can occur through either
intrinsic pairing or proximity to existing s-wave superconductors. | cond-mat_supr-con |
Dispersion of the odd magnetic resonant mode in near-optimally doped
Bi2Sr2CaCu2O8+d: We report a neutron scattering study of the spin excitation spectrum in the
superconducting state of slightly overdoped Bi2Sr2CaCu2O8+d system (Tc=87 K).
We focus on the dispersion of the resonance peak in the superconducting state
that is due to a S=1 collective mode. The measured spin excitation spectrum
bears a strong similarity to the spectrum of the YBa2Cu3O6+x system for a
similar doping level i.e. x= 0.95-1), which consists of intersecting upward-
and downward-dispersing branches. A close comparison of the threshold of the
electron-hole spin flip continuum, deduced from angle resolved photo-emission
measurements in the same system, indicates that the magnetic response in the
superconducting state is confined, in both energy and momentum, below the
gapped Stoner continuum. In contrast to YBa2Cu3O6+x, the spin excitation
spectrum is broader than the experimental resolution. In the framework of an
itinerant-electron model, we quantitatively relate this intrinsic energy width
to the superconducting gap distribution observed in scanning tunnelling
microscopy experiments. Our study further suggests a significant in-plane
anisotropy of the magnetic response. | cond-mat_supr-con |
Mo$_{3}$ReRuC: A noncentrosymmetric superconductor formed in the
MoReRu-Mo$_{2}$C system: A quaternary compound with the composition Mo$_{3}$ReRuC is obtained in a
previously unexplored MoReRu-Mo$_{2}$C system. According to x-ray structural
analysis, Mo$_{3}$ReRuC crystallizes in the noncentrosymmetric space group
$P$4$_{1}$32 (cubic $\beta$-Mn type structure, $a$ = 6.8107(1) {\AA}). Below
7.7 K, Mo$_{3}$ReRuC becomes a bulk type-II superconductor with an upper
critical field close to the Pauli paramagnetic limit. The specific heat data
gives a large normalized jump $\Delta$$C_{\rm p}$/$\gamma$$T_{\rm c}$ = 2.3 at
$T_{\rm c}$, which points to a strongly coupled superconducting state. First
principles calculations show that its electronic states at the Fermi level are
mainly contributed by Mo, Re and Ru atoms and strongly increased by the
spin-orbit coupling. Our finding suggests that the intermediate phase between
alloys and carbides may be a good place to look for $\beta$-Mn type
noncentrosymmetric superconductors. | cond-mat_supr-con |
Superconducting energy gap in $\rm Ba_{1-x}K_xBiO_3$: Temperature
dependence: The superconducting energy gap of $\rm Ba_{1-x}K_xBiO_3$ has been measured by
tunneling. Despite the fact that the sample was macroscopically single phase
with very sharp superconducting transition $T_c$ at 32~$K$, some of the
measured tunnel junctions made by point contacts between silver tip and single
crystal of $\rm Ba_{1-x}K_xBiO_3$ had lower transition at 20~$K$. Local
variation of the potassium concentration as well as oxygen deficiency in $\rm
Ba_{1-x}K_xBiO_3$ at the place where the point contact is made can account for
the change of $T_c$. The conductance curves of the tunnel junctions reveal the
BCS behavior with a small broadening of the superconducting-gap structure. A
value of the energy gap scales with $T_c$. The reduced gap amounts to
$2\Delta/kT_c = 4\div 4.3$ indicating a medium coupling strength. Temperature
dependence of the energy gap follows the BCS prediction. | cond-mat_supr-con |
Stability Conditions in Gapless Superconductors: Gapless superconductivity can arise when pairing occurs between fermion
species with different Fermi surface sizes, provided there is a sufficiently
large mismatch between Fermi surfaces and/or at sufficiently large coupling
constant. In gapless states, secondary Fermi surfaces appear where
quasiparticle excitation energy vanishes. This work focuses on homogeneous and
isotropic superfluids in the s-wave channel, with either zero (conventional
superconductor), one, or two spherical Fermi surfaces. The stability conditions
for these candidate phases are analyzed. It is found that gapless states with
one Fermi surface are stable in the BEC region, while gapless states with two
Fermi surfaces are unstable in all parameter space. The results can be applied
to ultracold fermionic atom systems. | cond-mat_supr-con |
Aharonov-Bohm Effect for Quasiparticles around a Vortex Line in a D-wave
Superconductor: On the basis of the Bogolubov-de Gennes theory we develop an analytical
description of low-energy extended quasiparticle states around an isolated flux
line in a superconductor with gap nodes. The wavefunctions of these excitations
and the corresponding density of states are shown to be strongly influenced by
the interaction with a pure gauge potential due to the Aharonov-Bohm scenario. | cond-mat_supr-con |
Magnetic resonance in a singlet-triplet Josephson junction: We study a singlet-triplet Josephson junction between a conventional s-wave
superconductor and an unconventional p$_{\rm x}$-wave superconductor. The
Andreev spectrum of the junction yields a spontaneous magnetization in
equilibrium. This allows manipulating the occupation of the Andreev levels
using an ac Zeeman field. The induced Rabi oscillations manifest themselves as
a resonance in the current-phase relation. For a circularly polarized magnetic
field, we find a spin selection rule, yielding Rabi oscillations only in a
certain interval of the superconducting phase difference. | cond-mat_supr-con |
Anomalous lattice expansion of RuSr2Eu1.5Ce0.5Cu2O10(Ru-1222) magneto
superconductor: A low temperature X-ray diffraction study: This is the first report of the observation of the onset of excess volume and
also of the strain along the a-axis near the magnetic ordering temperature in
Ru-1222 superconductor, and indicates a coupling between the lattice and the
magnetism in this system. Magnetization, magneto transport and thermoelectric
power measurements being carried out on the same sample are also reported. | cond-mat_supr-con |
Thin Spectrum States in Superconductors: We show that finite size superconductors have a spectrum of states at
extremely low energy, i.e. inside the superconducting gap. The presence of this
{\it thin spectrum} is a generic feature and related to the fact that in a
superconductor the global phase rotation symmetry is spontaneously broken. For
a strong coupling superconductor we find the spectrum by exactly solving the
Lieb-Mattis type Hamiltonian onto which the problem maps. One of the physical
consequences of the presence of thin states is that they cause quantum
decoherence in superconducting qubits of finite extent. | cond-mat_supr-con |
Multiorbital effects on the transport and the superconducting
fluctuations in LiFeAs: The resistivity, Hall effect and transverse magnetoresistance (MR) have been
measured in low residual resistivity single crystals of LiFeAs. A comparison
with angle resolved photoemission spectroscopy and quantum oscillation data
implies that four carrier bands unevenly contribute to the transport. However
the scattering rates of the carriers all display the T^2 behavior expected for
a Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2
variation permit us to extract the superconducting fluctuation contribution to
the conductivity. Though below Tc the anisotropy of superconductivity is rather
small, the superconducting fluctuations display a quasi ideal two-dimensional
behavior which persists up to 1.4 Tc. These results call for a refined
theoretical understanding of the multiband behavior of superconductivity in
this pnictide. | cond-mat_supr-con |
Temperature and field dependence of thermally activated flux flow
resistance in Bi2Sr2CaCu2O8+δ superconductor: We study the temperature dependence of the resistivity as a function of
magnetic field in superconducting transition (Tconset - TcR=0) region for
different Bi2Sr2CaCu2O8+{\delta} superconducting samples being synthesized
using sol-gel method. The superconducting transition temperature (TcR=0) of the
studied samples is increased from 32 K to 82K by simply increasing the final
sintering temperature with an improved grains morphology. On the other hand,
broadening of transition is increased substantially with decrease in sintering
temperature; this is because Tconset is not affected much with grains
morphology. Further broadening of the superconducting transition is seen under
magnetic field, which is being explained on the basis of thermally activated
flux flow (TAFF) below superconducting transition temperature (Tc). TAFF
activation energy (U0) is calculated using the resistive broadening of samples
in the presence of magnetic field. Temperature dependence of TAFF activation
energy revealed linear temperature dependence for all the samples. Further,
magnetic field dependence is found to obey power law for all the samples and
the negative exponent is increased with increase in sintering temperature or
the improved grains morphology for different Bi-2212 samples. We believe that
the sintering temperature and the ensuing role of grain morphology is yet a key
issue to be addressed in case of cuprate superconductors. | cond-mat_supr-con |
Continuous and reversible tuning of the disorder-driven
superconductor-insulator transition in bilayer graphene: The influence of static disorder on a quantum phase transition (QPT) is a
fundamental issue in condensed matter physics. As a prototypical example of a
disorder-tuned QPT, the superconductor-insulator transition (SIT) has been
investigated intensively over the past three decades, but as yet without a
general consensus on its nature. A key element is good control of disorder.
Here, we present an experimental study of the SIT based on precise in-situ
tuning of disorder in dual-gated bilayer graphene proximity-coupled to two
superconducting electrodes through electrical and reversible control of the
band gap and the charge carrier density. In the presence of a static disorder
potential, Andreev-paired carriers formed close to the Fermi level in bilayer
graphene constitute a randomly distributed network of proximity-induced
superconducting puddles. The landscape of the network was easily tuned by
electrical gating to induce percolative clusters at the onset of
superconductivity. This is evidenced by scaling behavior consistent with the
classical percolation in transport measurements. At lower temperatures, the
solely electrical tuning of the disorder-induced landscape enables us to
observe, for the first time, a crossover from classical to quantum percolation
in a single device, which elucidates how thermal dephasing engages in
separating the two regimes. | cond-mat_supr-con |
The London moment: what a rotating superconductor reveals about
superconductivity: The London moment is the magnetic moment acquired by a rotating
superconductor. We propose that the London moment reveals the following
fundamental properties of the superconducting state: (i) superconductors
(unlike normal metals) know the $sign$ of the charge carriers, (ii) the
superconducting charge carriers are $free$ electrons, (iii) electrons are
expelled from the interior to the surface in the transition to the
superconducting state, (iv) superfluid electrons occupy orbits of radius
$2\lambda_L$ ($\lambda_L=$London penetration depth), and (v) a spin current
exists in the ground state of superconductors. These properties are consistent
with the Meissner effect, however the Meissner effect does not $directly$
reveal the sign of the charge carriers nor the fact that the carrier's mass is
the free electron mass nor the fact that a spin current exists in
superconductors. Note also that within the BCS theory of superconductivity none
of the key properties of superconductors listed above are predicted. Instead,
these properties are predicted by the theory of hole superconductivity. | cond-mat_supr-con |
Superconductivity of Cobalt in Thin Films: Due to competing long range ferromagnetic order, the transition metals Fe, Co
and Ni are not superconductors at ambient pressure. While superconductivity was
observed in a non-magnetic phase of Fe, stabilized under pressure, it is yet to
be discovered in Co and Ni under any experimental conditions. Here, we report
emergence of superconductivity in the recently discovered high-density
nonmagnetic face centered cubic phase in Co thin films below a transition
temperature (Tc) of ~5.4 K, as revealed in experiments based on point-contact
spectroscopy and resistance, and four-probe measurements of resistance at
ambient pressure. We confirm the non-magnetic nature of the dense fcc phase of
Co within first-principles density functional theory, and show that its
superconductivity below 5 K originates from anomalous softening of
zone-boundary phonons and their enhanced coupling with electrons upon biaxial
strain. | cond-mat_supr-con |
Exact solution of the Lawrence-Doniach model in parallel magnetic fields: For the first time, we obtain the complete and exact analytical solution of
the Lawrence-Doniach model for layered superconductors in external parallel
magnetic fields. By solving a nontrivial mathematical problem of exact
minimization of the free-energy functional, we derive a closed, self-consistent
system of mean-field equations involving only two variables. Exact solutions to
these equations prove simultaneous penetration of Josephson vortices into all
the barriers, yield a completely new expression for the lower critical field,
refute the concept of a triangular Josephson vortex lattice and clarify the
physics of Fraunhofer oscillations of the total critical Josephson current.
PACS numbers: 74.80.Dm, 74.20.De, 74.50.+r | cond-mat_supr-con |
Emergent phenomena in multicomponent superconductivity: an introduction
to the focus issue: Multicomponent superconductivity is a novel quantum phenomenon in many
different superconducting materials, such as multiband ones in which different
superconducting gaps open in different Fermi surfaces, films engineered at the
atomic scale to enter the quantum confined regime, multilayers, two-dimensional
electron gases at the oxide interfaces, and complex materials in which
different electronic orbitals or different carriers participate in the
formation of the superconducting condensate. In all these systems the increased
number of degrees of freedom of the multicomponent superconducting
wave-function allows for emergent quantum effects that are otherwise
unattainable in single-component superconductors. In this editorial paper we
introduce the present focus issue, exploring the complex but fascinating
physics of multicomponent superconductivity. | cond-mat_supr-con |
Reduction of electron repulsion and enhancement of $T_{c}$ in small
diffusive superconducting grains: The superconducting properties of small metallic grains has been a topic of
active research for half a century now. Early experiments demonstrated a
remarkable rise in the critical temperature, $T_{c}$, with reducing grain size
in a variety of materials. In two dimensional diffusive superconductors,
$T_{c}$ is decreased due to enhanced Coulomb repulsion. We propose that in
finite size grains, the diffusive enhancement of the Coulomb repulsion is
weakened and leads ultimately to an increase in $T_{c}$ in isolated, disordered
two dimensional grains. Our mechanism is superimposed on the possible
enhancement in $T_{c}$ due to the change in the density of states of finite
size systems. | cond-mat_supr-con |
Doping evoluton of antiferromagnetic order and structural distortion in
LaFeAsO$_{1-x}$F$_x$: We use neutron scattering to study the structural distortion and
antiferromagnetic (AFM) order in LaFeAsO$_{1-x}$F$_{x}$ as the system is doped
with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO
exhibits a structural distortion, changing the symmetry from tetragonal (space
group $P4/nmm$) to orthorhombic (space group $Cmma$) at 155 K, and then
followed by an AFM order at 137 K. Doping the system with F gradually decreases
the structural distortion temperature, but suppresses the long range AFM order
before the emergence of superconductivity. Therefore, while superconductivity
in these Fe oxypnictides can survive in either the tetragonal or the
orthorhombic crystal structure, it competes directly with static AFM order. | cond-mat_supr-con |
The Berry curvature of the Bogoliubov quasiparticle Bloch states in the
unconventional superconductor Sr$_2$RuO$_4$: We will extend the concept of electron band Berry curvatures to
superconducting materials. We show that this can be defined for the
Bogoliubov-de Gennes equation describing the superconducting state in a
periodic crystal. In addition, the concept is exploited to understand the
driving mechanism for the optical Kerr effect in time reversal symmetry
breaking superconductors. Finally, we establish a sum rule analogue to the
normal state Hall sum rule making quantitative contact between the imaginary
part of the optical conductivity and the Berry curvature. The general theory
will be applied and tested against the drosophila of the p-wave paired
materials Sr$_2$RuO$_4$. | cond-mat_supr-con |
Field-driven transition in the Ba$_{1-x}$K$_x$Fe$_2$As$_2$
superconductor with splayed columnar defects: Through 2.6 GeV U irradiations, we have induced bimodal splayed columnar
defects in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals with splay angles, $\pm
5 ^\circ$, $\pm 10 ^\circ$, $\pm 15 ^\circ$, and $\pm 20 ^\circ$. Critical
current densities through magnetization measurements were carefully evaluated,
where a splay angle of $\pm 5 ^\circ$ brought about the highest $J_\mathrm{c}$.
Mageto-optical images close to $T_\mathrm{c}$ indicates highly anisotropic
discontinuity lines in the remnant state, and with anisotropy increasing with
greater splay angles. Moreover, amongst those with splayed columnar defects,
anomalous non-monotonic field dependences of $J_\mathrm{c}$ and $S$ with an
extrema at some fraction of the matching field are observed. We discuss that
such $J_\mathrm{c}$ enhancement arises from a field-driven coupling transition
in which intervortex interactions reorganize the vortex structure to be
accommodated into columnar defects, thereby increasing pinning at higher
fields. | cond-mat_supr-con |
Ginzburg-Landau theory for the time-dependent phase field in a
two-dimensional d-wave superconductor: We derive a finite temperature time-dependent effective theory for the phase
$\theta$ of the pairing field, which is appropriate for a 2D conducting
electron system with non-retarded d-wave attraction. As for s-wave pairing the
effective action contains terms with Landau damping, but their structure
appears to be different from the s-wave case due to the fact that the Landau
damping is determined by the quasiparticle group velocity $v_g$, which for the
d-wave pairing does not have the same direction as the non-interacting Fermi
velocity $v_F$. We show that for the d-wave pairing the Landau terms have a
linear low temperature dependence and in contrast to the s-wave case are
important for all finite temperatures. | cond-mat_supr-con |
Even Parity, Orbital Singlet and Spin Triplet Pairing for
Superconducting $La(O_{1-x}F_x)FeAs$: In the present paper, we propose the parity even,orbital singlet and spin
triplet pairing state as the ground state of the newly discovered
super-conductor $LaO_{1-x}F_xFeAs$.The pairing mechanism involves both the
special shape of the electron fermi surface and the strong ferromagnetic
fluctuation induced by Hund's rule coupling.The special behavior of the
Bogoliubov quasi-particle spectrum may leads to "Fermi arc" like anisotropy
super-conducting gap, which can be detected by angle resolved photo
emission(ARPES).The impurity effects are also discussed. | cond-mat_supr-con |
Critical currents and vortex-unbinding transitions in quench-condensed
ultrathin films of Bismuth and Tin: We have investigated the I-V characteristics of strongly disordered
ultra-thin films of {\it Bi} and {\it Sn} produced by quench-condensation. Our
results show that both these sytems can be visualized as strongly disordered
arrays of Josephson junctions. The experimentally observed I-V characteristics
of these films is hysteretic, when the injected current is ramped from zero to
critical current and back. These are remarkably similar to the hysteretic I-V
of an underdamped single junction. We show by computer simulations that
hysteresis can persist in a very strongly disordered array. It is also possible
to estimate the individual junction parameters ($R$, $C$ and $I_c$) from the
experimental I-Vs of the film using this model. The films studied are in a
regime where the Josephson-coupling energy is larger than the charging energy.
We find that a simple relation $I_c(T)=I_c(0)(1-(T/T_c)^4)$ describes the
temperature dependence of the critical current quite accurately for films with
sheet resistance $\sim$ 500$\Omega$ or lower. We also find evidence of a
vortex-unbindi | cond-mat_supr-con |
Comment on `Strong Vortex Liquid Correlation' from Multiterminal
Measurements on Untwinned YBa$_2$Cu$_3$O$_{7-δ}$ Single Crystals': A.Rydh and \"O.Rapp [Phys. Rev. Lett. {\bf 86}, 1873 (2001).] claim that the
vortex liquid in untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ crystals is correlated
above the melting transition, in striking contrast to previous work [D.L\'opez
{\it et al.}, Phys. Rev. Lett. {\bf 76}, 4034 (1996).]. In this Comment we
present new measurements using the same experimental technique on twinned and
untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ crystals with similar overall
characteristics as those reported by Rydh and Rapp . The comparison of the
vortex correlation response in both cases indicates that the central conclusion
of their work is not correct. Our results reconfirm the work by L\'opez {\it et
al.} and points on the origin of the misinterpretation in the work of Rydh and
Rapp. | cond-mat_supr-con |
Spin Excitation in d-wave Superconductors : A Fermi Liquid Picture: A detailed study of the Inelastic Neutron Scattering (INS) spectra of the
high-$T_c$ cuprates based on the Fermi liquid (FL) picture is given. We focus
on the issue of the transformation between the commensurate and incommensurate
(IC) excitation driven by frequency or $temperature$. For
La$_{2-x}$Sr$_x$CuO$_4$ (LSCO), the condition of small $\Delta(0)/v_F a$ (where
$a$ is the lattice constant, and henceforth will be set to 1) can
simultaneously reproduces the always existing IC peaks in the superconducting
(SC) and normal state, and the always fixed location at temperature or
frequency change. For YBa$_2$Cu$_3$O$_{6+x}$ (YBCO), a moderate $\Delta(0)/v_F
a$ and proximity of the van Hove singularity (vHS) at ${\bar M}=(0,\pi)$ to the
Fermi level can reproduce the frequency- and temperature-driven shifting IC
peaks in the SC state, and the vanishing of the IC peak in the normal state.
The commensurate peak is found to be more appropriately described as a random
phase approximation (RPA) effect. We address the conditional peak shifting
behavior to a refined consideration on the nesting effect which is previously
overlook. As a result, both the data on LSCO and the recent data on YBCO (on
YBa$_2$Cu$_3$O$_{6.7}$ by Arai $et$ $al.$ and YBa$_2$Cu$_3$O$_{6.85}$ by
Bourges $et$ $al.$) can be reasonably reconciled within a FL picture. We also
point out that the one-dimensional-like data by Mook $et$ $al.$ on a detwinned
and more underdoped sample YBa$_2$Cu$_3$O$_{6.6}$ could be due to a gap
anisotropy effect discussed by Rendell and Carbotte, and we proceed to suggest
a way of clarifying it. | cond-mat_supr-con |
Cooper Pairing in A Doped 2D Antiferromagnet with Spin-Orbit Coupling: We study the two-dimensional Hubbard model with the Rashba type spin-orbit
coupling within and beyond the mean-field theory. The antiferromagnetic ground
state for the model at half-filling and the Cooper pairing induced by
antiferromagnetic spin fluctuations near half-filling are examined based on the
random-phase approximation. We show that the antiferromagnetic order is
suppressed and the magnetic susceptibility turns out to be anisotropic in the
presence of the spin-orbit coupling. Energy spectrums of transverse spin
fluctuations are obtained and the effective interactions between holes mediated
by antiferromagnetic spin fluctuations are deduced in the case of low hole
doping. It seems that the spin-orbit coupling tends to form s+p-wave Cooper
pairs, while the s+d-wave pairing is dominant when the spin-orbit coupling is
absent. | cond-mat_supr-con |
Visualization of Electron Nematicity and Unidirectional Antiferroic
Fluctuations at High Temperatures in NaFeAs: The driving forces behind electronic nematicity in the iron pnictides remain
hotly debated. We use atomic-resolution variable-temperature scanning tunneling
spectroscopy to provide the first direct visual evidence that local electronic
nematicity and unidirectional antiferroic (stripe) fluctuations persist to
temperatures almost twice the nominal structural ordering temperature in the
parent pnictide NaFeAs. Low-temperature spectroscopic imaging of
nematically-ordered NaFeAs shows anisotropic electronic features that are not
observed for isostructural, non-nematic LiFeAs. The local electronic features
are shown to arise from scattering interference around crystalline defects in
NaFeAs, and their spatial anisotropy is a direct consequence of the structural
and stripe-magnetic order present at low temperature. We show that the
anisotropic features persist up to high temperatures in the nominally
tetragonal phase of the crystal. The spatial distribution and energy dependence
of the anisotropy at high temperatures is explained by the persistence of large
amplitude, short-range, unidirectional, antiferroic (stripe) fluctuations,
indicating that strong density wave fluctuations exist and couple to near-Fermi
surface electrons even far from the structural and density wave phase
boundaries. | cond-mat_supr-con |
Revisiting Anderson-Higgs mechanism: application of Lieb-Schultz-Mattis
theorem: We consider an electron model of superconductivity on a three-dimensional
lattice where there are on-site attractive Hubbard interaction and long-range
repulsive Coulomb interaction. It is claimed that fully gapped $s$-wave
superconductivity within this model, if present, exhibits spontaneous
translation symmetry breaking possibly related to a charge order. Our
discussions are based on an application of the Lieb-Schultz-Mattis theorem
under some physical assumptions. The inconsistency between the proposed
supersolid and experiments can impose some constraints on a reasonable choice
of a theoretical model. | cond-mat_supr-con |
Unveiling the hidden nematicity and spin subsystem in FeSe: The nematic order (nematicity) is considered one of the essential ingredients
to understand the mechanism of Fe-based superconductivity. In most Fe-based
superconductors (pnictides), nematic order is reasonably close to the
antiferromagnetic order. In FeSe, in contrast, a nematic order emerges below
the structure phase transition at T_s = 90 K with no magnetic order. The case
of FeSe is of paramount importance to a universal picture of Fe-based
superconductors. The polarized ultrafast spectroscopy provides a tool to probe
simultaneously the electronic structure and the magnetic interactions through
quasiparticle dynamics. Here we show that this approach reveals both the
electronic and magnetic nematicity below and, surprisingly, its fluctuations
far above Ts to at least 200 K. The quantitative pump-probe data clearly
identify a correlation between the topology of the Fermi surface (FS) and the
magnetism in all temperature regimes, thus providing profound insight into the
driving factors of nematicity in FeSe and the origin of its uniqueness. | cond-mat_supr-con |
Impurity effects on the spin excitation spectra in a d-wave
superconductor: The effects of nonmagnetic impurity on the spin excitation spectra in a
$d_{x^2-y^2}$-wave superconductor are examined, using the self-consistent
$t$-matrix approximation. It is shown that the impurity self-energy acts to
shift the position of the resonance peak to low frequencies and broaden the
peak. While the impurity vertex correction causes a broad spectral weight in
the spin gap at the impurity concentrations where no clear resonance peak is
observed. The gaplike feature still remains in low frequency region upon the
introduction of impurities. Incorporating these two effects, we find that the
result is in qualitative agreement with experiments on
YBa_{2}(Cu_{1-x}Zn_{x})_{3}O_{6+y}. | cond-mat_supr-con |
Deviations from the extended London model at high magnetic fields in
YBa$_2$Cu$_3$O$_7$: We report on the evolution with magnetic field and temperature of the vortex
lattice (VL) in fully-oxygenated YBa2Cu3O7 as studied by time-of-flight small
angle neutron scattering. Using the HFM/EXED beamline, we have obtained data up
to 25.9 T - much higher than that available previously. Our VL structure
results indicate the progressive suppression by field of the superconductivity
along the crystallographic b (CuO chain) direction. The intensity of the
diffracted signal reveals the spatial variation of magnetization caused by the
VL (the "form factor"). Instead of a rapid fall-off with field, as seen in
superconductors with smaller upper critical fields, we find that the form
factor is almost constant with field above about 12 T. We speculate that this
is due to Pauli paramagnetic moments, which increase at high fields due to
alignment of the spins of quasiparticles in the vortex cores. | cond-mat_supr-con |
Organic Superconductors: when correlations and magnetism walk in: This survey provides a brief account for the start of organic
superconductivity motivated by the quest for high Tc superconductors and its
development since the eighties'. Besides superconductivity found in 1D organics
in 1980, progresses in this field of research have contributed to better
understand the physics of low dimensional conductors highlighted by the wealth
of new remarkable properties. Correlations conspire to govern the low
temperature properties of the metallic phase. The contribution of
antiferromagnetic fluctuations to the interchain Cooper pairing proposed by the
theory is borne out by experimental investigations and supports
supercondutivity emerging from a non Fermi liquid background. Quasi one
dimensional organic superconductors can therefore be considered as simple
prototype systems for the more complex high Tc materials. | cond-mat_supr-con |
Normal State Nernst Effect in Electron-doped Pr2-xCexCuO4:
Superconducting Fluctuations and Two-band Transport: We report a systematic study of normal state Nernst effect in the
electron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_{4-\delta}$ over a wide range of
doping (0.05$\leq x \leq$0.21) and temperature. At low temperatures, we
observed a notable vortex Nernst signal above T$_c$ in the underdoped films,
but no such normal state vortex Nernst signal is found in the overdoped region.
The superconducting fluctuations in the underdoped region are most likely
incoherent phase fluctuations as found in hole-doped cuprates. At high
temperatures, a large normal state Nernst signal is found at dopings from
slightly underdoped to highly overdoped. Combined with normal state
thermoelectric power, Hall effect and magnetoresistance measurements, the large
Nernst effect is compatible with two-band model. For the highly overdoped
films, the large Nernst effect is anomalous and not explainable with a simple
hole-like Fermi surface seen in photoemission experiments. | cond-mat_supr-con |
An elastic lattice in a random potential: Using Monte Carlo simulations, we study the properties of an elastic
triangular lattice subject to a random background potential. As the cooling
rate is reduced, we observe a rather sudden crossover between two different
glass phases, one with exponential decay of correlations, the other with
power-law decay. Contrary to predictions derived from continuum models, no
evidence of a crossover in the mean-square displacement, B(r), from quadratic
growth at small r, to logarithmic growth at large r is found. | cond-mat_supr-con |
High-pressure flux growth, structural, and superconducting properties of
LnFeAsO (Ln = Pr, Nd, Sm) single crystals: Single crystals of the LnFeAsO (Ln1111, Ln = Pr, Nd, and Sm) family with
lateral dimensions up to 1 mm were grown from NaAs and KAs flux at high
pressure. The crystals are of good structural quality and become
superconducting when O is partially substituted by F (PrFeAsO1-xFx and
NdFeAsO1-xFx) or when Fe is substituted by Co (SmFe1-xCoxAsO). From
magnetization measurements, we estimate the temperature dependence and
anisotropy of the upper critical field and the critical current density of
underdoped PrFeAsO0.7F0.3 crystal with Tc = 25 K. Single crystals of
SmFe1-xCoxAsO with maximal Tc up to 16.3 K for x = 0.08 were grown for the
first time. From transport and magnetic measurements we estimate the critical
fields and their anisotropy, and find these superconducting properties to be
quite comparable to the ones in SmFeAsO1-xFx with a much higher Tc of = 50 K.
The magnetically measured critical current densities are as high as 109 A/m2 at
2 K up to 7 T, with indication of the usual fishtail effect. The upper critical
field estimated from resistivity measurements is anisotropic with slopes of
-8.7 T/K (H // ab-plane) and -1.7 T/K (H // c-axis). This anisotropy (= 5) is
similar to that in other Ln1111 crystals with various higher Tc s. | cond-mat_supr-con |
Absence of Cooper-type bound states in three- and few-electron systems: It is shown that the appearance of a fixed-point singularity in the kernel of
the two-electron Cooper problem is responsible for the formation of the Cooper
pair for an arbitrarily weak attractive interaction between two electrons. This
singularity is absent in the problem of three and few superconducting electrons
at zero temperature on the full Fermi sea. Consequently, such three- and
few-electron systems on the full Fermi sea do not form Cooper-type bound states
for an arbitrarily weak attractive pair interaction. | cond-mat_supr-con |
Observation of the Nernst signal generated by fluctuating Cooper pairs: Long-range order is destroyed in a superconductor warmed above its critical
temperature (Tc). However, amplitude fluctuations of the superconducting order
parameter survive and lead to a number of well established phenomena such as
paraconductivity : an excess of charge conductivity due to the presence of
short-lived Cooper pairs in the normal state. According to an untested theory,
these pairs generate a transverse thermoelectric (Nernst) signal. In amorphous
superconducting films, the lifetime of Cooper pairs exceeds the elastic
lifetime of quasi-particles in a wide temperature range above Tc; consequently,
the Cooper pairs Nernst signal dominate the response of the normal electrons
well above Tc. In two dimensions, the magnitude of the expected signal depends
only on universal constants and the superconducting coherence length, so the
theory can be unambiguously tested. Here, we report on the observation of a
Nernst signal in such a superconductor traced deep into the normal state. Since
the amplitude of this signal is in excellent agreement with the theoretical
prediction, the result provides the first unambiguous case for a Nernst effect
produced by short-lived Cooper pairs. | cond-mat_supr-con |
AC losses in macroscopic thin-walled superconducting niobium cylinders: Measurements of the ac response represent a widely-used method for probing
the properties of superconductors. In the surface superconducting state (SSS),
increase of the current beyond the surface critical current $I_c$ leads to
breakdown of SSS and penetration of external magnetic field into the sample
bulk. An interesting free-of-bulk system in SSS is offered by thin-walled
superconducting cylinders. The critical state model (CSM) asserts the ac
susceptibility $\chi$ to exhibit jumps as a function of the external ac field
amplitude $H_{ac}$, because of the periodic destruction and restoration of SSS
in the cylinder wall. Here, we investigate experimentally the low-frequency
(128-8192\,Hz) ac response of thin-walled superconducting cylinders in
superimposed dc and ac magnetic fields applied parallel to the cylinder axis.
Distinct from the CSM predictions, experiments reveal that $\chi$ is a smooth
function of $H_{ac}$. For the explanation of our observations we propose a
phenomenological model of partial penetration of magnetic flux (PPMF). The PPMF
model implies that after a restoration of the superconducting state, the
magnetic fields inside and outside the cylinder are not equal, and the value of
the penetrating flux is random for each penetration. This model fits very well
to the experimental data on the temperature dependence of the first-harmonic
$\chi_1$ at any $H_{ac}$ and dc field magnitude. However, in a certain
temperature range the values of physical parameters deduced within the
framework of the PPMF model are questionable. | cond-mat_supr-con |
Columnar defects acting as passive internal field detectors: We have studied the angular dependence of the irreversible magnetization of
several YBa$_2$Cu$_3$O$_7$ and 2H-NbSe$_2$ single crystals with columnar
defects tilted off the c-axis. At high magnetic fields, the irreversible
magnetization $M_i(\Theta_H)$ exhibits a well known maximum when the applied
field is parallel to the tracks. As the field is decreased below $H \sim 0.02
H_{c2}$, the peak shifts away from the tracks' direction toward either the
c-axis or the ab-planes. We demonstrate that this shift results from the
misalignment between the external and internal field directions due to the
competition between anisotropy and geometry effects. | cond-mat_supr-con |
Roles of anisotropic and unequal gaps in the quasiparticle interference
of superconducting iron pnictides: We investigate the role of gap characteristics such as anisotropy and
inequality of the gaps in the quasiparticle interferences of iron pnictides
using a five-orbital tight-binding model. We examine how the difference in the
sensitivities exhibited by the sign-changing and -preserving $s$-wave
superconductivity in an annular region around ($\pi, 0$), which can be used to
determine the sign change of the superconducting gap, gets affected when the
gaps are unequal on the electron and hole pocket. In addition, we also discuss
how robust these differentiating features are on changing the quasiparticle
energy or when the gap is anisotropic. | cond-mat_supr-con |
Possible realization of an antiferromagnetic Griffiths phase in
Ba[Fe(1-x)Mn(x)](2)As(2): We investigate magnetic ordering in metallic Ba[Fe(1-x)Mn(x)](2)As(2) and
discuss the unusual magnetic phase, which was recently discovered for Mn
concentrations x > 10%. We argue that it can be understood as a Griffiths-type
phase that forms above the quantum critical point associated with the
suppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in
BaFe2As2 by the randomly introduced localized Mn moments acting as strong
magnetic impurities. While the SDW transition at x = 0, 2.5% and 5% remains
equally sharp, in the x = 12% sample we observe an abrupt smearing of the
antiferromagnetic transition in temperature and a considerable suppression of
the spin gap in the magnetic excitation spectrum. According to our
muon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data,
antiferromagnetically ordered rare regions start forming in the x = 12% sample
significantly above the N\'eel temperature of the parent compound. Upon
cooling, their volume grows continuously, leading to an increase in the
magnetic Bragg intensity and to the gradual opening of a partial spin gap in
the magnetic excitation spectrum. Using neutron Larmor diffraction, we also
demonstrate that the magnetically ordered volume is characterized by a finite
orthorhombic distortion, which could not be resolved in previous diffraction
studies most probably due to its coexistence with the tetragonal phase and a
microstrain-induced broadening of the Bragg reflections. We argue that
Ba[Fe(1-x)Mn(x)](2)As(2) could represent an interesting model spin-glass
system, in which localized magnetic moments are randomly embedded into a SDW
metal with Fermi surface nesting. | cond-mat_supr-con |
Superconductivity in High-Entropy-Alloy Telluride AgInSnPbBiTe5: A polycrystalline sample of the high-entropy-alloy-type telluride
AgInSnPbBiTe5 was synthesized using high-pressure synthesis. Superconductivity
with a transition temperature (Tc) of 2.6 K was observed in AgInSnPbBiTe5.
Elemental and structural analyses revealed that five metals are mixed in a
metal site of an NaCl-type structure. Since AgInSnPbBiTe5 has a cation site
(Ag, In, Sn, Pb, and Bi) and an anion site (Te), this is the first example of a
high-entropy-alloy (HEA) pseudo-binary superconductor. | cond-mat_supr-con |
Anomalous doping dependence of the fluctuation-induced diamagnetism in
superconductors of YBCO family: SQUID magnetization measurements in oriented powders of Y$_{1-x}$Ca$_{x}$Ba$%
_{2}$Cu$_{3}$O$_{y}$, with $x$ ranging from 0 to 0.2, for $y\approx 6.1$ and
$y\approx 6.97$, have been performed in order to study the doping dependence of
the fluctuating diamagnetism above the superconducting transition temperature
$T_{c}$. While for optimally doped compounds the diamagnetic susceptibility and
the magnetization curves $-M_{fl}(T=const$) vs. $H$ are rather well justified
on the basis of an anisotropic Ginzburg-Landau (GL) functional, in underdoped
and overdoped regimes an anomalous diamagnetism is observed, with a large
enhancement with respect to the GL scenario. Furthermore the shape of
magnetization curves differs strongly from the one derived in that scheme. The
anomalies are discussed in terms of phase fluctuations of the order parameter
in a layered system of vortices and in the assumption of charge inhomogeneities
inducing local, non percolating, superconducting regions with $T_{c}^{(loc)}$
higher than the resistive transition temperature $T_{c}$. The susceptibility
displays activated temperature behavior, a mark characteristic of the
vortex-antivortex description, while history dependent magnetization, with
relaxation after zero-field cooling, is consistent with the hypothesis of
superconducting droplets in the normal state. Thus the theoretical picture
consistently accounts for most experimental findings. | cond-mat_supr-con |
The Field Perturbation Theory of Pseudogaps in HTSC: Here I establish the field perturbation theory of pseudogaps in HTSC. The
proposed ground state suggests an internal particle-hole field, which is normal
to nesting surfaces, and having twice the Fermi wave-number. It is proved that
the system violates momentum conservation by the wave-vector of this internal
field. This violation applies to the quasi-particle propagators, as well as to
the interactions. Interaction vertices via the Pauli matrix- are established.
This, in turn, establishes the validity of the pseudogap Hartree self-energy. | cond-mat_supr-con |
Effect of superlattice modulation of electronic parameters on
superconducting density of states in cuprate superconductors: Recent scanning tunneling microscopy on BSCCO 2212 has revealed a substantial
spatial supermodulation of the energy gap in the superconducting state. We
propose that this gap modulation is due to the superlattice modulations of the
atoms in the structure, and hence the parameters in a microscopic model of the
CuO2 plane. The gap modulation is estimated using renormalized mean field
theory for a t-t'-J model on a superlattice. The results compare well with
experiment. | cond-mat_supr-con |
Evidence for Pauli-limiting behaviour at high fields and enhanced upper
critical fields near T_c in several disordered FeAs based Superconductors: We report resistivity and upper critical field B_c2(T) data for disordered
(As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field
range up to 60 T. These samples exhibit a slightly enhanced superconducting
transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4
T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K
above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0)
approximately 63 T. We compare our results with B_c2(T)-data from the
literature for clean and disordered samples. Whereas clean samples show almost
no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced
flattening of B_c2(T) for applied fields H II ab observed for several
disordered closely related systems is interpreted also as a manifestation of
PLB. Consequences are discussed in terms of disorder effects within the frames
of (un)conventional superconductivity, respectively. | cond-mat_supr-con |
Microstructural and transport properties of superconducting
FeTe0.65Se0.35 crystals: The issue concerning the nature and the role of microstructural
inhomogeneities in iron chalcogenide superconducting crystals of FeTe0.65Se0.35
and their correlation with transport properties of this system was addressed.
Presented data demonstrate that chemical disorder originating from the kinetics
of the crystal growth process significantly influences the superconducting
properties of an Fe-Te-Se system. Transport measurements of the transition
temperature and critical current density performed for microscopic bridges
allow us to deduce the local properties of a superconductor with
microstructural inhomogeneities, and significant differences were noted. The
variances observed in the local properties were explained as a consequence of
weak superconducting links existing in the studied crystals. The results
confirm that inhomogeneous spatial distribution of ions and small hexagonal
symmetry nanoscale regions with nanoscale phase separation also seem to enhance
the superconductivity in this system with respect to the values of the critical
current density. Magnetic measurements confirm the conclusions drawn from the
transport measurements. | cond-mat_supr-con |
Periodicity of magnetization reversals in $\varphi_0$ Josephson junction: The magnetization reversal in ${\varphi_0}$-Josephson junction with direct
coupling between magnetic moment and Josephson current has been studied. By
adding pulse signal, the dynamics of magnetic moment components have been
simulated and the full magnetization reversal at different parameters of the
junction has been demonstrated. We obtain a detailed pictures representing the
intervals of the damping parameter $\alpha$, Josephson to magnetic energy
relation $G$ and spin-orbit coupling parameter $r$ with full magnetization
reversal. A periodicity in the appearance of magnetization reversal intervals
with increase in Josephson to magnetic energy relation is found. The obtained
results might be used in different fields of superconducting spintronics. | cond-mat_supr-con |
Quantum vortex fluctuations in cuprate superconductors: We study the effects of quantum vortex fluctuations in two-dimensional
superconductors using a dual theory of vortices, and investigate the relevance
to underdoped cuprates where the superconductor-insulator transition (SIT) is
possibly driven by quantum vortex proliferation. We find that a broad enough
phase fluctuation regime may exist for experimental observation of the quantum
vortex fluctuations near SIT in underdoped cuprates. We propose that this
scenario can be tested via pair-tunneling experiments which measure the
characteristic resonances in the zero-temperature pair-field susceptibility in
the vortex-proliferated insulating phase. | cond-mat_supr-con |
Energy gaps in high-$T_c$ superconductors: BCS after all?: A major impediment to solving the problem of high-$T_c$ superconductivity is
the ongoing confusion about the magnitude, structure and doping dependence of
the superconducting gap, $\Delta_0$, and of the mysterious pseudogap found in
underdoped samples\cite{TallonLoram}. The pseudogap opens around the ($\pi$,0)
antinodes below a temperature $T^*$ leaving Fermi arcs across the remnant Fermi
surface\cite{Kanigel} on which the superconducting gap forms at $T_c$. One
thing that seems agreed is that the ratio $2\Delta_0/k_BT_c$ well exceeds the
BCS value and grows with underdoping\cite{Miyakawa1,Miyakawa2}, suggesting
unconventional, non-BCS superconductivity. Here we re-examine data from many
spectroscopies, especially Raman $B_{1g}$ and $B_{2g}$
scattering\cite{Sacuto,Guyard}, and reconcile them all within a two-gap
scenario showing that the points of disagreement are an artefact of
spectral-weight loss arising from the pseudogap. Crucially, we find that
$\Delta_0(p)$, or more generally the order parameter, now scales with the
mean-field $T_c$ value, adopting the weak-coupling BCS ratio across the entire
phase diagram. | cond-mat_supr-con |
Stretched exponential spin relaxation in organic superconductors: Proton NMR measurements on the organic superconductor
$\kappa-(ET)_2Cu[N(CN)_2]Br$ ($T_C = 11.6$ K) exhibit stretched exponential
spin-lattice relaxation below $T\approx 25$ K, suggestive of an inhomogeneous
magnetic phase that develops in the normal state and coexists with
superconductivity. The onset of this phase coincides approximately with a large
normal state Nernst signal reported previously. By contrast, the closely
related superconductor $\kappa-(ET)_2Cu[(NCS)_2]Br$ ($T_C = 10.5$ K) shows
single exponential spin-lattice relaxation and a conventional Nernst effect.
The temperature range $T_C < T < 30$ K encompasses several phenomena in the
$\kappa-(ET)_2X$ conductors, including changes in conduction electron spin
resonance, electronic phase separation and the onset of antiferromagnetic
order. Analogous behavior in $La_{2-x}Sr_xCuO_4$ suggests that a density wave
may develop in $\kappa-(ET)_2Cu[N(CN)_2]Br$. | cond-mat_supr-con |
Tunneling Studies of Pseudogaps: a Comment: Recent tunneling observations of a pseudogap in Bi_2Sr_2CaCu_2O_{8+delta} are
not necessarily evidence that the pseudogap is associated with superconducting
fluctuations. The data are here analyzed as a competition between
superconductivity and a density wave (taken as a simplified model for a striped
phase). | cond-mat_supr-con |
Hole-Doped Room-Temperature Superconductivity in H$_{3}$S$_{1-x}$Z$_x$
(Z=C, Si): We examine the effects of the low-level substitution of S atoms by C and Si
atoms on the superconductivity of H$_3$S with the $Im\bar{3}m$ structure at
megabar pressure. The hole doping can fine-tune the Fermi energy to reach the
electronic density-of-states peak maximizing the electron-phonon coupling. This
can boost the critical temperature from the original 203 K to 289 K and 283 K,
respectively, for H$_3$S$_{0.962}$C$_{0.038}$ at 260 GPa and
H$_3$S$_{0.960}$Si$_{0.040}$ at 230 GPa. The former may provide an explanation
for the recent experimental observation of room-temperature superconductivity
in a highly compressed C-S-H system [Nature 586, 373-377 (2020)]. Our work
opens a new avenue for substantially raising the critical temperatures of
hydrogen-rich materials. | cond-mat_supr-con |
The Superfluid Glass Phase of 3He-A: It is established theoretically that an ordered state with continuous
symmetry is inherently unstable to arbitrarily small amounts of disorder [1,
2]. This principle is of central importance in a wide variety of condensed
systems including superconducting vortices [3, 4], Ising spin models [5] and
their dynamics [6], and liquid crystals in porous media [7, 8], where some
degree of disorder is ubiquitous, although its experimental observation has
been elusive. Based on these ideas it was predicted [9] that 3He in high
porosity aerogel would become a superfluid glass. We report here our nuclear
magnetic resonance measurements on 3He in aerogel demonstrating destruction of
long range orientational order of the intrinsic superfluid orbital angular
momentum, confirming the existence of a superfluid glass. In contrast, 3He-A
generated by warming from superfluid 3He-B has perfect long-range orientational
order providing a mechanism for switching off this effect. | cond-mat_supr-con |
Effective Vortex Pinning in MgB2 thin films: We discuss pinning properties of MgB2 thin films grown by pulsed-laser
deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are
identified that contribute most effectively to the pinning of vortices in
randomly oriented films. The EB process produces low defected crystallites with
small grain size providing enhanced pinning at grain boundaries without
degradation of Tc. The PLD process produces films with structural disorder on a
scale less that the coherence length that further improves pinning, but also
depresses Tc. | cond-mat_supr-con |
Electronic structure, lattice dynamics and magnetism of new ThXAsN
(X=Fe,Co,Ni) superconductors: A First Principles Study: In this work, we present a comparative first principles study of mechanical
properties, electronic structure, phonon dispersion relation, electron-phonon
coupling and magnetism in three isostructural superconductors, namely, ThFeAsN,
ThCoAsN and ThNiAsN. Experimentally, ThFeAsN and ThNiAsN show superconducting
properties, while ThCoAsN has not been synthesized. Our calculated elastic
constants show that all these systems are mechanically stable. Significant
differences in the electronic structures of these three compounds in terms of
density of states, band structures and Fermi surfaces, are found. Our phonon
calculations reveal that all the systems including ThCoAsN, are dynamically
stable. Phonon dispersion relations indicate that the optical modes of all the
three systems are almost the same while there are significant variations in the
low frequency manifold consisting of mixed modes. The electron-phonon coupling
constants and superconducting transition temperatures calculated based on the
Eliashberg formalism, predict a rather high $T_c$ of 6.4 K for ThCoAsN and also
a $T_c$ of 3.4 K for ThNiAsN which agrees well with the experimental value of
4.3 K. Nevertheless, we find a $T_c$ of 0.05 K for ThFeAsN, which is much
smaller than the experimental $T_c$ of $\sim$30 K. However, a simple analysis
considering the amplifying effects of spin density wave order and out-of-plane
soft phonon modes suggests that the $T_c$ could be increased considerably to
$\sim$10 K. Finally, we also discuss the effect of anion As height on the
electronic structures and study possible magnetic states in these three
compounds. | cond-mat_supr-con |
2D orbital-like magnetic order in ${\rm La_{2-x}Sr_xCuO_4}$: In high temperature copper oxides superconductors, a novel magnetic order
associated with the pseudogap phase has been identified in two different
cuprate families over a wide region of temperature and doping. We here report
the observation below 120 K of a similar magnetic ordering in the archetypal
cuprate ${\rm La_{2-x}Sr_xCuO_4}$ (LSCO) system for x=0.085. In contrast to the
previous reports, the magnetic ordering in LSCO is {\it\bf only} short range
with an in-plane correlation length of $\sim$ 10 \AA\ and is bidimensional
(2D). Such a less pronounced order suggests an interaction with other
electronic instabilities. In particular, LSCO also exhibits a strong tendency
towards stripes ordering at the expense of the superconducting state. | cond-mat_supr-con |
Tunable nodal kagome superconductivity in charge ordered RbV3Sb5: Unconventional superconductors often feature competing orders, small
superfluid density, and nodal electronic pairing. While unusual
superconductivity has been proposed in the kagome metals AV3Sb5, key
spectroscopic evidence has remained elusive. Here we utilize pressure-tuned (up
to 1.85 GPa) and ultra-low temperature (down to 18 mK) muon spin spectroscopy
to uncover the unconventional nature of superconductivity in RbV3Sb5. At
ambient pressure, we detect an enhancement of the width of the internal
magnetic field distribution sensed by the muon ensemble, indicative of
time-reversal symmetry breaking charge order. Remarkably, the superconducting
state displays nodal energy gap and a reduced superfluid density, which can be
attributed to the competition with the novel charge order. Upon applying
pressure, the charge-order transitions are suppressed, the superfluid density
increases, and the superconducting state progressively evolves from nodal to
nodeless. Once charge order is eliminated, we find a superconducting pairing
state that is not only fully gapped, but also spontaneously breaks
time-reversal symmetry. Our results point to unprecedented tunable nodal kagome
superconductivity competing with time-reversal symmetry-breaking charge order
and offer unique insights into the nature of the pairing state. | cond-mat_supr-con |
The decrease of the critical current of coated conductors when a
perpendicular magnetic field is applied : a Josephson effect point of view: A large decrease is observed in the critical current density of YBCO coated
conductors (CC) and related compounds when a strong perpendicular magnetic
field is applied. While measurements are generally carried out at 77K only,
here we present a magnetic technique permitting to determine the critical
current per unit width of conductor (Icr/w) in a large temperature range. We
report measurements carried out on various CCs that show that, in addition to
the reduction in the critical temperature that can be attributed to the low
value of the irreversibility field near Tc, the field application results in a
large decrease of Icr/w at all temperatures. We ascribe this reduction to the
Josephson behaviour of the twin boundaries included in the YBCO layer. | cond-mat_supr-con |
In-Plane and Out-of-Plane Optical Properties of NdBa2Cu3Ox Single
Crystals close to x = 7.0: We present results of reflectivity measurements with E parallel to c and E
perpendicular to c on NdBa2Cu3Ox (Nd123) single crystals close to full oxygen
doping. Along the c-axis the optical conductivity shows a well developed
absorption band around 450 cm^-1 at all temperatures. The in-plane optical
properties are dominated by crystal-field excitations at low energies, a
prominent step at 400 cm^-1 and a weaker feature between 500 - 550 cm^-1. A
comparison of the c-axis optical conductivity and the in-plane scattering rate
with neutron scattering derived spin susceptibility spectra suggests that the
in-plane and out-of-plane anomalies are caused by the same mechanism, probably
electron--spin scattering. | cond-mat_supr-con |
Nesting, spin-fluctuations, and odd-gap superconductivity in NaxCoO2
yH2O: We have calculated the one-electron susceptibility of hydrated NaxCoO2 and
find strong nesting nearly commensurate with a 2X2 superstructure. The nesting
involves about 70% of all electrons at the Fermi level and is robust with
respect to doping. This nesting creates a tendency to a charge density wave
compatible with the charge order often seen at x approx 0.5, which is usually
ascribed to electrostatic repulsion of Na ions. In the spin channel, it gives
rise to strong spin-fluctuations, which should be important for
superconductivity. The superconducting state most compatible with this nesting
structure is an odd-gap triplet s-wave state. | cond-mat_supr-con |
Probing the electron-phonon interaction in correlated systems with
coherent lattice fluctuation spectroscopy: Tailoring the properties of correlated oxides is accomplished by chemical
doping, pressure, temperature or magnetic field. Photoexcitation is a valid
alternative to reach out-of-equilibrium states otherwise inaccessible. Here, we
quantitatively estimate the coupling between a lattice distortion and the
charge-transfer excitation in (La$_2$CuO$_{4+\delta}$). We photoinduce a
coherent La ion vibration and monitor the response of the optical constants in
a broad energy range, providing quantitative information on the electron-phonon
matrix element that can be compared to theoretical models. We propose the same
methodology to probe electron-electron interactions in other materials. | cond-mat_supr-con |
Superconductivity and unusual magnetic behavior in amorphous carbon: Traces of superconductivity (SC) at elevated temperatures (up to 65 K) were
observed by magnetic measurements in three different inhomogeneous sulfur doped
amorphous carbon (a-C) systems: (a) in commercial and (b) synthesized powders
and (c) in a-C thin films. (a) Studies performed on commercial (a-C) powder
which contains 0.21% of sulfur, revealed traces of non-percolated
superconducting phases below Tc = 65 K. The SC volume fraction is enhanced by
the sulfur doping. (b) a-C powder obtained by pyrolytic decomposition of
sucrose did not show any sign for SC above 5 K. This powder was mixed with
sulfur and synthesized at 400 C (a-CS). The inhomogeneous products obtained,
show traces of SC phases at TC= 17 and 42 K. (c) Non-superconducting composite
a-C-W thin films were grown by electron-beam induced deposition. SC emerged at
Tc = 34.4 K only after heat treatment with sulfur. Other parts of the pyrolytic
a-CS powder, show unusual magnetic features. (i) Pronounced irreversible peaks
around 55-75 K appear in the first zero-field-cooled (ZFC) sweep only. Their
origin is not known. (ii) Unexpectedly these peaks are totally suppressed in
the second ZFC runs measured a few minutes later. (iii) Around the peak
position the field-cooled (FC) curves cross the ZFC plots (ZFC>FC). These
peculiar magnetic observations also ascribed to a-CS powder prepared from the
commercial a-C powder and are connected to each other. All SC and magnetic
phenomena observed are intrinsic properties of the sulfur doped a-C materials.
It is proposed that the a-CS systems behave similarly to well known high TC
curates and/or pnictides in which SC emerges from magnetic states. | cond-mat_supr-con |
Nature of 45 degree vortex lattice reorientation in tetragonal
superconductors: The transformation of the vortex lattice in a tetragonal superconductor which
consists of its 45 degree reorientation relative to the crystal axes is studied
using the nonlocal London model. It is shown that the reorientation occurs as
two successive second order (continuous) phase transitions. The transition
magnetic fields are calculated for a range of parameters relevant for
borocarbide superconductors in which the reorientation has been observed. | cond-mat_supr-con |
Possible Triplet Superconducting Order in Magnetic Superconducting Phase
induced by Paramagnetic Pair-Breaking: Motivated by recent thermal conductivity measurements in the superconductor
CeCoIn5, we theoretically examine a possible staggered spin-triplet
superconducting order to be induced by the coupled spin-density-wave (SDW) and
d-wave superconducting (SC) orders in the high field and low temperature (HFLT)
SC phase peculiar to this material with strong paramagnetic pair-breaking
(PPB). It is shown that one type of the $\pi$-triplet order is consistent with
the thermal conductivity data and can naturally be incorporated in the picture
that the Q-phase is a consequence of the strong PPB effect inducing the SDW
order and the FFLO spatial modulation parallel to the applied magnetic field. | cond-mat_supr-con |
Half-quantum vortex and d-soliton in Sr$_2$RuO$_4$: Assuming that the superconductivity in Sr$_2$RuO$_4$ is described by a planar
p-wave order parameter, we consider possible topological defects in
Sr$_2$RuO$_4$. In particular, it is shown that both of the ${\hat d}$-soliton
and half-quantum vortex can be created in the presence of the magnetic field
parallel to the $a$-$b$ plane. We discuss how one can detect the ${\hat
d}$-soliton and half-quantum vortex experimentally. | cond-mat_supr-con |
Uniaxial "nematic-like" electronic structure and Fermi surface of
untwinned CaFe2As2: Obtaining the electronic structure of the newly discovered iron-based
superconductors is the key to understanding the mechanism of their
high-temperature superconductivity. We used angle-resolved photoemission
spectroscopy (ARPES) to make direct measurements of the electronic structure
and Fermi surface (FS) of the untwinned uniaxial state of CaFe2As2, the parent
compound of iron-based superconductors. We observed unequal dispersions and FS
geometries along the orthogonal Fe-Fe bond directions. More importantly,
unidirectional straight and flat FS segments are observed near the zone center,
which indicates the existence of a unidirectional nematic charge density wave
order, strengthening the case for a quantum electronic liquid crystalline
"nematic" phase. Further, the doping dependence extrapolates to a possible
quantum critical point of the disappearance of this order in the heavily
overdoped regime of these materials. | cond-mat_supr-con |
Theory of the superconductivity of $UGe_2$ revisited: We present a unified theory of magnetism and superconductivity of $UGe_2$. To
this end, we consider part of $5f$ uranium electrons as mostly itinerant and
other ones as mostly localized. The main feature that distinguishes the
localized from the itinerant electrons is the effect of the pressure on them.
The pressure strongly screens the itinerant electrons while the localized ones
are almost unaffected. The screening of itinerant electrons leads to decreasing
of their Coulomb repulsion, therefore to formation of doubly occupied and empty
states. These states are spin-singlet and the effective spin of itinerant
electrons, the zero-temperature magnetization in units of Bohr magneton,
decreases. We obtain an effective two-spin Heisenberg model, which explains the
magnetization-temperature diagram of $UGe_2$. It is shown that the
experimentally observed characteristic temperature $T_x$, is a partial order
transition temperature. Below the Curie temperature $(T_x<T_C)$ the system
undergoes a transition from high temperature phase, were only localized
electrons contribute the magnetization, to the low temperature one, where both
itinerant and localized electrons contribute the magnetization. The
characteristic temperature decreases when pressure increases. At the quantum
partial order point $T_x=0$, the Zeeman splitting of the itinerant electrons is
zero. This permits formation of Cooper pairs and an onset of superconductivity
induced by the transversal fluctuations of the localized electrons. Small
deviation from the quantum partial ordered state leads to suppression of
superconductivity. This explains the dome form of the superconducting
transition temperature. The very low superconducting critical temperature is a
consequence of the Ising ferromagnetism. | cond-mat_supr-con |
Fractal superconductivity near localization threshold: We develop a semi-quantitative theory of electron pairing and resulting
superconductivity in bulk "poor conductors" in which Fermi energy $E_F$ is
located in the region of localized states not so far from the Anderson mobility
edge $E_c$. We review the existing theories and experimental data and argue
that a large class of disordered films is described by this model.
Our theoretical analysis is based on the analytical treatment of pairing
correlations, described in the basis of the exact single-particle eigenstates
of the 3D Anderson model, which we combine with numerical data on eigenfunction
correlations. Fractal nature of critical wavefunction's correlations is shown
to be crucial for the physics of these systems.
We identify three distinct phases: 'critical' superconductive state formed at
$E_F=E_c$, superconducting state with a strong pseudogap, realized due to
pairing of weakly localized electrons and insulating state realized at $E_F$
still deeper inside localized band. The 'critical' superconducting phase is
characterized by the enhancement of the transition temperature with respect to
BCS result, by the inhomogeneous spatial distribution of superconductive order
parameter and local density of states. The major new feature of the
pseudo-gaped state is the presence of two independent energy scales:
superconducting gap $\Delta$, that is due to many-body correlations and a new
"pseudogap" energy scale $\Delta_P$ which characterizes typical binding energy
of localized electron pairs and leads to the insulating behavior of the
resistivity as a function of temperature above superconductive $T_c$. Two gap
nature of the "pseudo-gaped superconductor" is shown to lead to a number of
unusual physical properties. | cond-mat_supr-con |
Structural phase transition and superlattice misfit strain of RFeAsO (R
= La, Pr, Nd and Sm): The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO
(La-1111) and SmFeAsO (Sm-1111) single crystals measured by high resolution
x-ray diffraction is found to be sharp while the RFeAsO (R=La, Nd, Pr, Sm)
polycrystalline samples show a broad continuous SPT. Comparing the
polycrystalline and the single crystal 1111 samples, the critical exponents of
the SPT are found to be the same while the correlation length critical
exponents are found to be very different. These results imply that the lattice
fluctuations in 1111 systems change in samples with different surface to volume
ratio that is assigned to the relieve of the temperature dependent superlattice
misfit strain between active iron layers and the spacer layers in 1111 systems.
This phenomenon that is missing in the AFe2As2 (A=Ca, Sr, Ba) "122" systems,
with the same electronic structure but different for the thickness and the
elastic constant of the spacer layers, is related with the different maximum
superconducting transition temperature in the 1111 (55 K) versus 122 (35 K)
systems and implies the surface reconstruction in 1111 single crystals. | cond-mat_supr-con |
Quantum mechanics of superconducting nanowires: In a short superconducting nanowire connected to bulk superconducting leads,
quantum phase slips behave as a system of linearly (as opposed to
logarithmically) interacting charges. This system maps onto quantum mechanics
of a particle in a periodic potential. We show that, while the state with a
high density of phase slips is not a true insulator (a consequence of Josephson
tunneling between the leads), for a range of parameters it behaves as such down
to unobservably small temperatures. We also show that quantum phase slips give
rise to multiple branches (bands) in the energy-current relation and to an
interband ("exciton") mode. | cond-mat_supr-con |
Nematic state stabilized by off-site Coulomb interaction in iron-based
superconductors: Using a variational Monte Carlo method, we investigate the nematic state in
iron-base superconductors based on a three-band Hubbard model. Our results
demonstrate that the nematic state, formed by introducing an anisotropic
hopping order into the projected wave function, can arise in the underdoped
regime when a realistic off-site Coulomb interaction $V$ is considered. {\color
{red} We demonstrate that the off-site Coulomb interaction $V$, which is
neglected so far in the analysis of iron-base superconductors, make a dominant
contribution to the stabilization of nematic state. We calculate the doping
dependencies of the anisotropic properties such as the unequal occupation of
$d_{xz}$ and $d_{yz}$ orbitals, anisotropies of kinetic energy and spin
correlations, and show that they are all suppressed upon electron doping, which
are consistent with the intrinsic anisotropies observed by optical spectrum
measurement and ARPES experiments. | cond-mat_supr-con |
Transition Temperature and Upper Critical Field in SmFeAsO1-xFx
Synthesized at Low Heating Temperatures: Low-temperature synthesis is a promising and potentially effective method for
improving superconducting properties. We report on the fabrication of
polycrystalline samples of SmFeAsO1-xFx with nominal x content varying in a
wide range of x = 0-0.35 synthesized at 900 deg C. This synthesis temperature
is around 300 deg C lower than the conventional synthesis temperature. The
variation in the lattice parameters and transition temperature (Tc) of various
F-doped samples indicates that reduction of the unit cell volume (V) seems to
be the main reason for the rise of Tc up to 57.8 K. Magnetoresistance
measurements showed that the upper critical field slope (dHc2/dT) increased
with increasing F concentration up to x = 0.2, where it reached a maximum value
of -8 T/K corresponding to a coherence length of 10 angstrom. At still higher F
doping levels, dHc2/dT and the low field Jc decreased; above 0.5 T, however, Jc
had almost the same value. Compared with previous reports, the present
synthesis route with low synthesis temperatures and commonly available FeF2 as
the source of F is more effective at introducing F into the SmFeAsO system and
thereby resulting in improved superconducting properties for the system. In
addition, this new sample preparation method also reduces unnecessary problems
such as the evaporation of F and reaction between the crucible and
superconductor during the solid-state reaction. | cond-mat_supr-con |
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