712 research outputs found
Transfer-matrix approach to multiband Josephson junctions
We study the influence of multiple bands on the properties of Josephson
junctions. In particular we focus on the two gap superconductor magnesium
diboride. We present a formalism to describe tunneling at a point contact
between two MgB2 electrodes generalizing the transfer-matrix approach to
multiple bands. A simple model is presented to determine the effective hopping
amplitudes between the different energy bands as a function of the
misorientation angle of the electrodes. We calculate the critical current and
the current-voltage characteristics for N-I-S and S-I-S contacts with different
orientation for junctions with both high and low transparency. We find that
interband tunneling processes become increasingly important with increasing
misorientation angle. This is reflected in certain features in the differential
tunneling conductance in both the tunneling limit as well as for multiple
Andreev reflections.Comment: 13 pages, 8 figure
Spectrum of low energy excitations in the vortex state: comparison of Doppler shift method to quasiclassical approach
We present a detailed comparison of numerical solutions of the quasiclassical
Eilenberger equations with several approximation schemes for the density of
states of s- and d-wave superconductors in the vortex state, which have been
used recently. In particular, we critically examine the use of the Doppler
shift method, which has been claimed to give good results for d-wave
superconductors. Studying the single vortex case we show that there are
important contributions coming from core states, which extend far from the
vortex cores into the nodal directions and are not present in the Doppler shift
method, but significantly affect the density of states at low energies. This
leads to sizeable corrections to Volovik's law, which we expect to be sensitive
to impurity scattering. For a vortex lattice we also show comparisons with the
method due to Brandt, Pesch, and Tewordt and an approximate analytical method,
generalizing a method due to Pesch. These are high field approximations
strictly valid close to the upper critical field Bc2. At low energies the
approximate analytical method turns out to give impressively good results over
a broad field range and we recommend the use of this method for studies of the
vortex state at not too low magnetic fields.Comment: 11 pages, 11 figures; revised version, error in Fig. 6b remove
Origin of Gap Anisotropy in Spin Fluctuation Models of the Fe-pnictides
We discuss the large gap anisotropy found for the A1g (s-wave) state in RPA
spin-fluctuation and functional renormalization group calculations and show how
the simple arguments leading to isotropic sign-switched s-wave states in these
systems need to be supplemented by a consideration of pair scattering within
Fermi surface sheets and between the individual electron sheets as well. In
addition, accounting for the orbital makeup of the states on the Fermi surface
is found to be crucial.Comment: 6 pages, 7 figure
Local modulations of the spin-fluctuation mediated pairing interaction by impurities in d-wave superconductors
We present a self-consistent real space formulation of spin-fluctuation
mediated d-wave pairing. By calculating all relevant inhomogeneous spin and
charge susceptibilities in real space within the random phase approximation
(RPA), we obtain the effective pairing interaction and study its spatial
dependence near both local potential and hopping impurities. A remarkably large
enhancement of the pairing interaction may be obtained near the impurity site.
We discuss the relevance of our result to inhomogeneities observed by scanning
tunneling spectroscopy on the surface of cuprate superconductors.Comment: 8 pages, 7 figure
Supercurrent through grain boundaries in the presence of strong correlations
Strong correlations are known to severely reduce the mobility of charge
carriers near half-filling and thus have an important influence on the current
carrying properties of grain boundaries in the high- cuprates. In this
work we present an extension of the Gutzwiller projection approach to treat
electronic correlations below as well as above half-filling consistently. We
apply this method to investigate the critical current through grain boundaries
with a wide range of misalignment angles for electron- and hole-doped systems.
For the latter excellent agreement with experimental data is found. We further
provide a detailed comparison to an analogous weak-coupling evaluation.Comment: 4 pages, 3 figure
Tunneling spectroscopy for probing orbital anisotropy in iron pnictides
Using realistic multi-orbital tight-binding Hamiltonians and the T-matrix
formalism, we explore the effects of a non-magnetic impurity on the local
density of states in Fe-based compounds. We show that scanning tunneling
spectroscopy (STS) has very specific anisotropic signatures that track the
evolution of orbital splitting (OS) and antiferromagnetic gaps. Both
anisotropies exhibit two patterns that split in energy with decreasing
temperature, but for OS these two patterns map onto each other under 90 degree
rotation. STS experiments that observe these signatures should expose the
underlying magnetic and orbital order as a function of temperature across
various phase transitions.Comment: 12 pages, 9 figures, replacement with minor changes suggested by
referee
Extinction of impurity resonances in large-gap regions of inhomogeneous d-wave superconductors
Impurity resonances observed by scanning tunneling spectroscopy in the
superconducting state have been used to deduce properties of the underlying
pure state. Here we study a longstanding puzzle associated with these
measurements, the apparent extinction of these resonances for Ni and Zn
impurities in large-gap regions of the inhomogeneous BSCCO superconductor. We
calculate the effect of order parameter and hopping suppression near the
impurity site, and find that these two effects are sufficient to explain the
missing resonances in the case of Ni. There are several possible scenarios for
the extinction of the Zn resonances, which we discuss in turn; in addition, we
propose measurements which could distinguish among them.Comment: 10 pages, 8 figure
Vortex core shrinkage in a two gap superconductor: application to MgB2
As a model for the vortex core in MgB2 we study a two band model with a clean
sigma band and a dirty pi band. We present calculations of the vortex core size
in both bands as a function of temperature and show that there exists a
Kramer-Pesch effect in both bands even though only one of the bands is in the
clean limit. We present calculations for different pi band diffusivities and
coherence lengths.Comment: Submitted to M2S-HTSC-VIII conference proceeding
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
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