246 research outputs found
The Effect of Surfaces on the Tunneling Density of States of an Anisotropically Paired Superconductor
We present calculations of the tunneling density of states in an
anisotropically paired superconductor for two different sample geometries: a
semi-infinite system with a single specular wall, and a slab of finite
thickness and infinite lateral extent. In both cases we are interested in the
effects of surface pair breaking on the tunneling spectrum. We take the stable
bulk phase to be of symmetry. Our calculations are performed
within two different band structure environments: an isotropic cylindrical
Fermi surface with a bulk order parameter of the form ,
and a nontrivial tight-binding Fermi surface with the order parameter structure
coming from an anti-ferromagnetic spin-fluctuation model. In each case we find
additional structures in the energy spectrum coming from the surface layer.
These structures are sensitive to the orientation of the surface with respect
to the crystal lattice, and have their origins in the detailed form of the
momentum and spatial dependence of the order parameter. By means of tunneling
spectroscopy, one can obtain information on both the anisotropy of the energy
gap, |\Delta(\p)|, as well as on the phase of the order parameter,
\Delta(\p) = |\Delta(\p)|e^{i\varphi(\p)}.Comment: 14 pages of revtex text with 11 compressed and encoded figures. To
appear in J. Low Temp. Phys., December, 199
Avalanche statistics of sand heaps
Large scale computer simulations are presented to investigate the avalanche
statistics of sand piles using molecular dynamics. We could show that different
methods of measurement lead to contradicting conclusions, presumably due to
avalanches not reaching the end of the experimental table.Comment: 6 pages, 4 figure
Odd-frequency pairing in normal metal/superconductor junctions
We study the induced odd-frequency pairing states in ballistic normal
metal/superconductor (N/S) junctions where a superconductor has even-frequency
symmetry in the bulk and a normal metal layer has an arbitrary length. Using
the quasiclassical Green's function formalism, we demonstrate that, quite
generally, the pair amplitude in the junction has an admixture of an
odd-frequency component due to the breakdown of translational invariance near
the N/S interface where the pair potential acquires spatial dependence. If a
superconductor has even-parity pair potential (spin-singlet s-wave state), the
odd-frequency pairing component with odd-parity is induced near the N/S
interface, while in the case of odd-parity pair potential (spin-triplet
-wave or spin-singlet -wave) the odd-frequency component with
even-parity is generated. We show that in conventional s-wave junctions, the
amplitude of the odd-frequency pairing state is enhanced at energies
corresponding to the peaks in the local density of states (LDOS). In - and
-wave junctions, the amplitude of the odd-frequency component on the S
side of the N/S interface is enhanced at zero energy where the midgap Andreev
resonant state (MARS) appears due to the sign change of the pair potential. The
odd-frequency component extends into the N region and exceeds the
even-frequency component at energies corresponding to the LDOS peak positions,
including the MARS.Comment: 27 pages, 12 figure
Exact microscopic analysis of a thermal Brownian motor
We study a genuine Brownian motor by hard disk molecular dynamics and
calculate analytically its properties, including its drift speed and thermal
conductivity, from microscopic theory.Comment: 4 pages, 5 figure
Low-energy quasiparticle states near extended scatterers in d-wave superconductors and their connection with SUSY quantum mechanics
Low-energy quasiparticle states, arising from scattering by single-particle
potentials in d-wave superconductors, are addressed. Via a natural extension of
the Andreev approximation, the idea that sign-variations in the superconducting
pair-potential lead to such states is extended beyond its original setting of
boundary scattering to the broader context of scattering by general
single-particle potentials, such as those due to impurities. The
index-theoretic origin of these states is exhibited via a simple connection
with Witten's supersymmetric quantum-mechanical model.Comment: 5 page
Tunneling into Current-Carrying Surface States of High T Superconductors
Theoretical results for the ab-plane tunneling conductance in the d-wave
model for high Tc superconductors are presented. The d-wave model predicts
surface bound states below the maximum gap. A sub-dominant order parameter,
stabilized by the surface, leads to a splitting of the zero-bias conductance
peak (ZBCP) in zero external field and to spontaneous surface currents. In a
magnetic field screening currents shift the quasiparticle bound state spectrum
and lead to a voltage splitting of the ZBCP that is linear in H at low fields,
and saturates at a pairbreaking critical field of order 3 Tesla. Comparisons
with recent experimental results on Cu/YBCO junctions are presented.Comment: 4 pages in a RevTex (3.0) file plus 3 Figures in PostScript. To
appear in Phys. Rev. Let
Thermodynamic properties of thin films of superfluid 3He-A
The pairing correlations in superfluid He-3 are strongly modified by
quasiparticle scattering off a surface or an interface. We present theoretical
results and predictions for the order parameter, the quasiparticle excitation
spectrum and the free energy for thin films of superfluid He-3. Both specular
and diffuse scattering by a substrate are considered, while the free surface is
assumed to be a perfectly reflecting specular boundary. The results are based
on self-consistent calculations of the order parameter and quasiparticle
excitation spectrum at zero pressure. We obtain new results for the phase
diagram, free energy, entropy and specific heat of thin films of superfluid
He-3.Comment: Replaced with an updated versio
D-Wave Superconductors near Surfaces and Interfaces: A Scattering Matrix Approach within the Quasiclassical Technique
A recently developed method [A. Shelankov and M. Ozana, Phys. Rev. B 61, 7077
(2000)] is applied to investigate d-wave superconductors in the vicinity of
(rough) surfaces. While this method allows the incorporation of arbitrary
interfaces into the quasiclassical technique, we discuss, as examples,
diffusive surfaces and boundaries with small tilted mirrors (facets). The
properties of the surface enter via the scattering matrix in the boundary
condition for the quasiclassical Green's function. The diffusive surface is
described by an ensemble of random scattering matrices. We find that the
fluctuations of the density of states around the average are small; the zero
bias conductance peak broadens with increasing disorder. The faceted surface is
described in the model where the scattering matrix couples m in- and m
out-trajectories (m>=2). No zero bias conductance peak is found for [100]
surfaces; the relation to the model of Fogelstrom et al. [Phys. Rev. Lett. 79,
281 (1997)] is discussed.Comment: RevTeX, 19 pages, 18 figure
Two phase transitions in (s+id)-wave Bardeen-Cooper-Schrieffer superconductivity
We establish universal behavior in temperature dependencies of some
observables in -wave BCS superconductivity in the presence of a weak
wave. There also could appear a second second-order phase transition. As
temperature is lowered past the usual critical temperature , a less
ordered superconducting phase is created in wave, which changes to a more
ordered phase in wave at (). The presence of two phase
transitions manifest in two jumps in specific heat at and . The
temperature dependencies of susceptibility, penetration depth, and thermal
conductivity also confirm the new phase transition.Comment: 6 pages, 5 post-script figures
Thermodynamics of a d-wave Superconductor Near a Surface
We study the properties of an anisotropically paired superconductor in the
presence of a specularly reflecting surface. The bulk stable phase of the
superconducting order parameter is taken to have symmetry.
Contributions by order parameter components of different symmetries vanish in
the bulk, but may enter in the vicinity of a wall. We calculate the
self-consistent order parameter and surface free energy within the
quasiclassical formulation of superconductivity. We discuss, in particular, the
dependence of these quantities on the degree of order parameter mixing and the
surface to lattice orientation. Knowledge of the thermodynamically stable order
parameter near a surface is a necessary precondition for calculating measurable
surface properties which we present in a companion paper.Comment: 12 pages of revtex text with 12 compressed and encoded figures. To
appear in J. Low Temp. Phys., December, 199
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