344 research outputs found
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
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
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
Induced Kramer-Pesch-Effect in a Two Gap Superconductor: Application to MgB2
The size of the vortex core in a clean superconductor is strongly temperature
dependent and shrinks with decreasing temperature, decreasing to zero for T ->
0. We study this so-called Kramer-Pesch effect both for a single gap
superconductor and for the case of a two gap superconductor using parameters
appropriate for Magnesium Diboride. Usually, the Kramer-Pesch effect is absent
in the dirty limit. Here, we show that the Kramer-Pesch effect exists in both
bands of a two gap superconductor even if only one of the two bands is in the
clean limit and the other band in the dirty limit, a case appropriate for MgB2.
In this case an induced Kramer-Pesch effect appears in the dirty band. Besides
numerical results we also present an analytical model for the spatial variation
of the pairing potential in the vicinity of the vortex center that allows a
simple calculation of the vortex core radius even in the limit T -> 0.Comment: 12 pages, 12 figure
Fermi surface topology and vortex state in MgB2
Based on a detailed modeling of the Fermi surface topology of MgB2 we
calculated the anisotropy of the upper critical field Bc2 within the two gap
model. The sigma-band is modeled as a distorted cylinder and the pi-band as a
half-torus, with parameters determined from bandstructure calculations. Our
results show that the unusual strong temperature dependence of the Bc2
anisotropy, that has been observed recently, can be understood due to the small
c-axis dispersion of the cylindrical Fermi surface sheets and the small
interband pairing interaction as obtained from bandstructure calculations. We
calculate the magnetic field dependence of the density of states within the
vortex state for field in c-axis direction and compare with recent measurements
of the specific heat on MgB2 single crystals.Comment: 2 pages, 2 figure
Andreev bound states at a cuprate grain boundary junction: A lower bound for the upper critical field
We investigate in-plane quasiparticle tunneling across thin film grain
boundary junctions (GBJs) of the electron-doped cuprate
LaCeCuO in magnetic fields up to T, perpendicular to
the CuO layers. The differential conductance in the superconducting state
shows a zero bias conductance peak (ZBCP) due to zero energy surface Andreev
bound states. With increasing temperature , the ZBCP vanishes at the
critical temperature K if B=0, and at K for B=16 T. As
the ZBCP is related to the macroscopic phase coherence of the superconducting
state, we argue that the disappearance of the ZBCP at a field
must occur below the upper critical field of the superconductor. We
find T which is at least a factor of 2.5 higher than
previous estimates of .Comment: 4 pages, 4 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
Local density of states at polygonal boundaries of d-wave superconductors
Besides the well-known existence of Andreev bound states, the zero-energy
local density of states at the boundary of a d-wave superconductor strongly
depends on the boundary geometry itself. In this work, we examine the influence
of both a simple wedge-shaped boundary geometry and a more complicated
polygonal or faceted boundary structure on the local density of states. For a
wedge-shaped boundary geometry, we find oscillations of the zero-energy density
of states in the corner of the wedge, depending on the opening angle of the
wedge. Furthermore, we study the influence of a single Abrikosov vortex
situated near a boundary, which is of either macroscopic or microscopic
roughness.Comment: 10 pages, 11 figures; submitted to Phys. Rev.
Shadow on the wall cast by an Abrikosov vortex
At the surface of a d-wave superconductor, a zero-energy peak in the
quasiparticle spectrum can be observed. This peak appears due to Andreev bound
states and is maximal if the nodal direction of the d-wave pairing potential is
perpendicular to the boundary. We examine the effect of a single Abrikosov
vortex in front of a reflecting boundary on the zero-energy density of states.
We can clearly see a splitting of the low-energy peak and therefore a
suppression of the zero-energy density of states in a shadow-like region
extending from the vortex to the boundary. This effect is stable for different
models of the single Abrikosov vortex, for different mean free paths and also
for different distances between the vortex center and the boundary. This
observation promises to have also a substantial influence on the differential
conductance and the tunneling characteristics for low excitation energies.Comment: 5 pages, 5 figure
Superconductivity in striped and multi-Fermi-surface Hubbard models: From the cuprates to the pnictides
Single- and multi-band Hubbard models have been found to describe many of the
complex phenomena that are observed in the cuprate and iron-based
high-temperature superconductors. Simulations of these models therefore provide
an ideal framework to study and understand the superconducting properties of
these systems and the mechanisms responsible for them. Here we review recent
dynamic cluster quantum Monte Carlo simulations of these models, which provide
an unbiased view of the leading correlations in the system. In particular, we
discuss what these simulations tell us about superconductivity in the
homogeneous 2D single-orbital Hubbard model, and how charge stripes affect this
behavior. We then describe recent simulations of a bilayer Hubbard model, which
provides a simple model to study the type and nature of pairing in systems with
multiple Fermi surfaces such as the iron-based superconductors.Comment: Published as part of Superstripes 2011 (Rome) conference proceeding
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