3,013 research outputs found
Robust superconductivity and transport properties in (Li1-xFex)OHFeSe single crystals
The recently discovered (LiFe)OHFeSe superconductor with
about 40K provides a good platform for investigating the magnetization
and electrical transport properties of FeSe-based superconductors. By using a
hydrothermal ion-exchange method, we have successfully grown crystals of
(LiFe)OHFeSe. X-ray diffraction on the sample shows the single
crystalline PbO-type structure with the c-axis preferential orientation.
Magnetic susceptibility and resistive measurements show an onset
superconducting transition at around =38.3K. Using the magnetization
hysteresis loops and Bean critical state model, a large critical current
is observed in low temperature region. The critical current density is
suppressed exponentially with increasing magnetic field. Temperature
dependencies of resistivity under various currents and fields are measured,
revealing a robust superconducting current density and bulk superconductivity.Comment: 5 pages, 5 figure
Phase Diagram of a Spin-Orbit Coupled Fermi Gases in a Bilayer Optical Lattice
We investigate the stability of helical superfluid phase in a spin-orbit
coupled Fermi gas loaded in a bilayer optical lattice. The phase diagram of the
system is constructed in the mean field framework. We investigate the
topological properties of the superfluid phases by a nontrivial application of
the Fermi surface topological invariant to our time-reversal invariant system
with degeneracies on the Fermi surface. We find that there is a first-order
phase boundary in the phase diagram of half filling case and the superfluid
phases are all topological trivial. The superfluid phase is topological
nontrivial when the filling fraction deviates from the half filling. In the
topological nontrivial superfluid phase, a full pairing gap exists in the bulk
and gapless helical Majorana edge states exist at the boundary.Comment: 6 pages, 3 figure
Vortex lattice and vortex bound states in CsFeAs investigated by scanning tunneling microscopy/spectroscopy
We investigate the vortex lattice and vortex bound states in CsFeAs
single crystals by scanning tunneling microscopy/spectroscopy (STM/STS) under
various magnetic fields. A possible structural transition or crossover of
vortex lattice is observed with the increase of magnetic field, i.e., the
vortex lattice changes from a distorted hexagonal lattice to a distorted
tetragonal one at the magnetic field near 0.5 T. It is found that a mixture of
stripelike hexagonal and square vortex lattices emerges in the crossover
region. The vortex bound state is also observed in the vortex center. The
tunneling spectra crossing a vortex show that the bound-state peak position
holds near zero bias with STM tip moving away from the vortex core center. The
Fermi energy estimated from the vortex bound state energy is very small. Our
investigations provide experimental information to both the vortex lattice and
the vortex bound states in this iron-based superconductor.Comment: 7 pages, 5 figure
Sign reversal superconducting gaps revealed by phase referenced quasi-particle interference of impurity induced bound states in (LiFe)OHFeZnSe
By measuring the spatial distribution of differential conductance near
impurities on Fe sites, we have obtained the quasi-particle interference (QPI)
patterns in the (LiFe)OHFeZnSe superconductor with only
electron Fermi surfaces. By taking the Fourier transform on these patterns, we
investigate the scattering features between the two circles of electron pockets
formed by folding or hybridization. We treat the data by using the recent
theoretical approach [arXiv:1710.09089] which is specially designed for the
impurity induced bound states. It is found that the superconducting gap sign is
reversed on the two electron pockets, which can be directly visualized by the
phase-referenced QPI technique, indicating that the Cooper pairing is induced
by the repulsive interaction. Our results further show that this method is also
applicable for data measured for multiple impurities, which provides an easy
and feasible way for detecting the gap function of unconventional
superconductors
Possible superconducting fluctuation and pseudogap state above in CsFeAs
Resistive, magnetization, torque, specific heat and scanning tunneling
microscopy measurements are carried out on the hole heavily doped
CsFeAs single crystals. A characteristic temperature K,
which is several times higher than the superconducting transition temperature
K, is observed and possibly related to the superconducting
fluctuation or the pseudogap state. A diamagnetic signal detected by torque
measurements starts from the superconducting state, keeps finite and vanishes
gradually until a temperature near . Temperature dependent resistivity and
specific heat also show kinks near . An asymmetric gap-like feature with
the energy of 8.4 meV and a symmetric superconducting related gap of 2.2 meV on
the scanning tunneling spectra are detected, and these pseudogap-related
features disappear at temperatures up to at least 9 K. These observations by
different experimental tools suggest the possible existence of superconducting
fluctuation or pseudogap state in the temperature range up to 4 - 6 times of
in CsFeAs.Comment: 6 pages, 4 figure
Possible Triplet Superconductivity in Graphene at Low Filling
We study the Hubbard model on the honeycomb lattice with nearest-neighbor
hopping () and next-nearest-neighbor one (). When , the
single-particle spectrum is featured by the continuously distributed Van-Hove
saddle points at the band bottom, where the density of states diverges in
power-law. We investigate possible unconventional superconductivity in such
system with Fermi level close to the band bottom by employing both random phase
approximation and determinant quantum Monte-Carlo approaches. Our study reveals
a possible triplet superconductivity in this system with appropriate
interactions. Our results might provide a possible route to look for triplet
superconductivity with relatively-high transition temperature in a low-filled
graphene and other similar systems.Comment: 5.1pages, 9 figure
Nodal Superconducting Gap in -FeS
Low temperature specific heat has been measured in superconductor -FeS
with T = 4.55 K. It is found that the low temperature electronic specific
heat C/T can be fitted to a linear relation in the low temperature region,
but fails to be described by an exponential relation as expected by an s-wave
gap. We try fittings to the data with different gap structures and find that a
model with one or two nodal gaps can fit the data. Under a magnetic field, the
field induced specific heat =[C(H)-C(0)]/T shows the
Volovik relation , suggesting the presence
of nodal gap(s) in this material.Comment: 6 pages, 5 figure
Topological conditions of scale-free networks for cooperation to evolve
Evolutionary game theory is employed to study topological conditions of
scale-free networks for the evolution of cooperation. We show that Apollonian
Networks (ANs) are perfect scale-free networks, on which cooperation can spread
to all individuals, even though there are initially only 3 or 4 hubs occupied
by cooperators and all the others by defectors. Local topological features such
as degree, clustering coefficient, gradient as well as topology potential are
adopted to analyze the advantages of ANs in cooperation enhancement.
Furthermore, a degree-skeleton underlying ANs is uncovered for understanding
the cooperation diffusion. Constructing this kind degree-skeleton for random
scale-free networks promotes cooperation level close to that of
Barab\'asi-Albert networks, which gives deeper insights into the origin of the
latter on organization and further promotion of cooperation.Comment: 4 pages, 4 figure
Strong coupling superconductivity revealed by scanning tunneling microscope in tetragonal FeS
We investigate the electronic properties of the tetragonal FeS superconductor
by using scanning tunneling microscope/spectroscopy. It is found that the
typical tunneling spectrum on the top layer of sulfur can be nicely fitted with
an anisotropic s-wave or a combination of two superconducting components in
which one may have a highly anisotropic or nodal like superconducting gap. The
fittings lead to the maximum superconducting gap
0.90meV, which yields a ratio of 2 4.65. This
value is larger than that of the predicted value 3.53 by the BCS theory in the
weak coupling limit, indicating a strong coupling superconductivity. Two kinds
of defects are observed on the surface, which can be assigned to the defects on
the S sites (four-fold image) and Fe sites (dumbbell shape). Impurity induced
resonance states are found only for the former defects and stay at zero-bias
energy.Comment: 6 pages, 5 figure
Scrutinizing the double superconducting gaps and strong coupling pairing in (Li1-xFexOH)FeSe
In the iron based superconductors, one of the on-going frontier studies is
about the pairing mechanism. The recent interest concerns the high temperature
superconductivity and its intimate reason in the monolayer FeSe thin films. The
challenge here is how the double superconducting gaps seen by the scanning
tunnelling spectroscopy (STS) associate however to only one set of Fermi
pockets seen by the angle resolved photoemission spectroscopy (ARPES). The
recently discovered (Li1-xFexOH)FeSe phase with Tc=40 K provides a good
platform to check the fundamental problems. Here we report the STS study on the
(Li1-xFexOH)FeSe single crystals. The STS spectrum clearly indicates the
presence of double anisotropic gaps with maximum magnitudes of Delta_1=14.3 meV
and Delta_2=8.6 meV, and mimics that of the monolayer FeSe thin film. Further
analysis based on the quasiparticle interference (QPI) allows us to rule out
the d-wave gap, and for the first time assign the larger (smaller) gap to the
outer (inner) hybridized Fermi pockets associating with the dxy (dxz/dyz)
orbitals, respectively. The huge value Delta_1/k_BT_c = 8.7 discovered here
undoubtedly proves the strong coupling mechanism in the present superconducting
system.Comment: 24 pages, 10 figures, with the Supplementary Information. The second
version supersedes the original version with detailed QPI data and analysi
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