343 research outputs found
Tc suppression and resistivity in cuprates with out of plane defects
Recent experiments introducing controlled disorder into optimally doped
cuprate superconductors by both electron irradiation and chemical substitution
have found unusual behavior in the rate of suppression of the critical
temperature Tc vs. increase in residual resistivity. We show here that the
unexpected discovery that the rate of Tc suppression vs. resistivity is
stronger for out-of-plane than for in-plane impurities may be explained by
consistent calculation of both Tc and resistivity if the potential scattering
is assumed to be nearly forward in nature. For realistic models of impurity
potentials, we further show that significant deviations from the universal
Abrikosov-Gor'kov Tc suppression behavior may be expected for out of plane
impurities.Comment: 6 pages, 5 figure
Volovik effect in a highly anisotropic multiband superconductor: experiment and theory
We present measurements of the specific heat coefficient \gamma(= C/T) in the
low temperature limit as a function of an applied magnetic field for the
Fe-based superconductor BaFe(AsP). We find both a
linear regime at higher fields and a limiting square root behavior at very
low fields. The crossover from a Volovik-like to a linear field
dependence can be understood from a multiband calculation in the quasiclassical
approximation assuming gaps with different momentum dependence on the hole- and
electron-like Fermi surface sheets.Comment: 11 pages, 8 figures, 1 table, submitted to Phys. Rev.
Structure of BSCCO supermodulation from ab initio calculations
We present results of density functional theory (DFT) calculation of the
structural supermodulation in BSCCO-2212 structure, and show that the
supermodulation is indeed a spontaneous symmetry breaking of the nominal
crystal symmetry, rather than a phenomenon driven by interstitial O dopants.
The structure obtained is in excellent quantitative agreement with recent x-ray
studies, and reproduces several qualitative aspects of scanning tunnelling
microscopy (STM) experiments as well. The primary structural modulation
affecting the CuO_2 plane is found to be a buckling wave of tilted CuO_5
half-octahedra, with maximum tilt angle near the phase of the supermodulation
where recent STM experiments have discovered an enhancement of the
superconducting gap. We argue that the tilting of the half-octahedra and
concommitant planar buckling are directly modulating the superconducting pair
interaction.Comment: 4 pages, 3 figure
Theory of Andreev reflection in a two-orbital model of iron-pnictide superconductors
A recently developed theory for the problem of Andreev reflection between a
normal metal (N) and a multiband superconductor (MBS) assumes that the incident
wave from the normal metal is coherently transmitted through several bands
inside the superconductor. Such splitting of the probability amplitude into
several channels is the analogue of a quantum waveguide. Thus, the appropriate
matching conditions for the wave function at the N/MBS interface are derived
from an extension of quantum waveguide theory. Interference effects between the
transmitted waves inside the superconductor manifest themselves in the
conductance. We provide results for a FeAs superconductor, in the framework of
a recently proposed effective two-band model and two recently proposed gap
symmetries: in the sign-reversed s-wave () scenario
resonant transmission through surface Andreev bound states (ABS) at nonzero
energy is found as well as destructive interference effects that produce zeros
in the conductance; in the extended s-wave ()
scenario no ABS at finite energy are found.Comment: 4 pages, 5 figure
Repulsion and attraction in high Tc superconductors
The influence of repulsion and attraction in high-Tc superconductors to the
gap functions is studied. A systematic method is proposed to compute the gap
functions using the irreducible representations of the point group. It is found
that a pure s-wave superconductivity exists only at very low temperatures, and
attractive potentials on the near shells significantly expand the gap functions
and increase significantly the critical temperature of superconductivity. A
strong on-site repulsion drives the gap into a gap. It is
expected that superconductivity with the symmetry reaches a high
critical temperature due to the cooperation of the on-site and the next-nearest
neighbor attractions.Comment: 4 pages, 5figure
Simple Real-Space Picture of Nodeless and Nodal s-wave Gap Functions in Iron Pnictide Superconductors
We propose a simple way to parameterize the gap function in iron pnictides.
The key idea is to use orbital representation, not band representation, and to
assume real-space short-range pairing. Our parameterization reproduces fairly
well the structure of gap function obtained in microscopic calculation. At the
same time the present parameterization is simple enough to obtain an intuitive
picture and to develop a phenomenological theory. We also discuss
simplification of the treatment of the superconducting state.Comment: 4 page
Single vortex structure in two models of iron pnictide superconductivity
The structure of a single vortex in a FeAs superconductor is studied in the
framework of two formulations of superconductivity for the recently proposed
sign-reversed wave () scenario: {\it (i)} a continuum model taking
into account the existence of an electron and a hole band with a repulsive
local interaction between the two; {\it (ii)} a lattice tight-binding model
with two orbitals per unit cell and a next-nearest-neighbour attractive
interaction. In the first model, the local density of states (LDOS) at the
vortex centre, as a function of energy, exhibits a peak at the Fermi level,
while in the second model such LDOS peak is deviated from the Fermi level and
its energy depends on band filling. An impurity located outside the vortex core
has little effect on the LDOS peak, but an impurity close to the vortex core
can almost suppress it and modify its position.Comment: 17 pages, 15 figures. Accepted for publication in New Journal of
Physic
Time scales of epidemic spread and risk perception on adaptive networks
Incorporating dynamic contact networks and delayed awareness into a contagion
model with memory, we study the spreading patterns of infectious diseases in
connected populations. It is found that the spread of an infectious disease is
not only related to the past exposures of an individual to the infected but
also to the time scales of risk perception reflected in the social network
adaptation. The epidemic threshold is found to decrease with the rise
of the time scale parameter s and the memory length T, they satisfy the
equation .
Both the lifetime of the epidemic and the topological property of the evolved
network are considered. The standard deviation of the degree
distribution increases with the rise of the absorbing time , a power-law
relation is found
Evidence for Nodal superconductivity in SrScFePO
Point contact Andreev reflection spectra have been taken as a function of
temperature and magnetic field on the polycrystalline form of the newly
discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak
which disappears at the superconducting transition temperature, dominates all
of the spectra. Data taken in high magnetic fields show that this feature
survives until 7T at 2K and a flattening of the feature is observed in some
contacts. Here we inspect whether these observations can be interpreted within
a d-wave, or nodal order parameter framework which would be consistent with the
recent theoretical model where the height of the P in the Fe-P-Fe plane is key
to the symmetry of the superconductivity. However, in polycrystalline samples
care must be taken when examining Andreev spectra to eliminate or take into
account artefacts associated with the possible effects of Josephson junctions
and random alignment of grains.Comment: Published versio
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