197 research outputs found
Absence of Dipole Transitions in Vortices of Type II Superconductors
The response of a single vortex to a time dependent field is examined
microscopically and an equation of motion for vortex motion at non-zero
frequencies is derived. Of interest are frequencies near ,
where is the bulk energy gap and is the fermi energy. The low
temperature, clean, extreme type II limit and maintaining of equilibrium with
the lattice are assumed. A simplification occurs for large planar mass
anisotropy. Thus the results may be pertinent to materials such as and
high temperature superconductors. The expected dipole transition between core
states is hidden because of the self consistent nature of the vortex potential.
Instead the vortex itself moves and has a resonance at the frequency of the
transition.Comment: 12 pages, no figure
Local density of states and scanning tunneling currents in graphene
We present exact analytical calculations of scanning tunneling currents in
locally disordered graphene using a multimode description of the microscope
tip. Analytical expressions for the local density of states (LDOS) are given
for energies beyond the Dirac cone approximation. We show that the LDOS at the
and sublattices of graphene are out of phase by implying that the
averaged LDOS, as one moves away from the impurity, shows no trace of the
(with the Fermi momentum) Friedel modulation. This means that a
STM experiment lacking atomic resolution at the sublattice level will not be
able of detecting the presence of the Friedel oscillations [this seems to be
the case in the experiments reported in Phys. Rev. Lett. {\bf 101}, 206802
(2008)]. The momentum maps of the LDOS for different types of impurities are
given. In the case of the vacancy, features are seen in these maps. In
all momentum space maps, and features are seen. The
features are different from what is seen around zero momentum. An
interpretation for these features is given. The calculations reported here are
valid for chemical substitution impurities, such as boron and nitrogen atoms,
as well as for vacancies. It is shown that the density of states close to the
impurity is very sensitive to type of disorder: diagonal, non-diagonal, or
vacancies. In the case of weakly coupled (to the carbon atoms) impurities, the
local density of states presents strong resonances at finite energies, which
leads to steps in the scanning tunneling currents and to suppression of the
Fano factor.Comment: 21 pages. Figures 6 and 7 are correctly displayed in this new versio
Quasiparticle Density of States of Clean and Dirty s-Wave Superconductors in the Vortex State
The quasiparticle density of states (DOS) in the vortex state has been probed
by specific heat measurements under magnetic fields (H) for clean and dirty
s-wave superconductors, Y(Ni1-xPtx)2B2C and Nb1-xTaxSe2. We find that the
quasiparticle DOS per vortex is appreciably H-dependent in the clean-limit
superconductors, while it is H-independent in the dirty superconductors as
expected from a conventional rigid normal electron core picture. We discuss
possible origins for our observations in terms of the shrinking of the vortex
core radius with increasing H.Comment: 5 pages, 4 figures, to appear in J. Phys. Soc. Jpn. Vol. 68 No.
Collective Behavior of Asperities in Dry Friction at Small Velocities
We investigate a simple model of dry friction based on extremal dynamics of
asperities. At small velocities, correlations develop between the asperities,
whose range becomes infinite in the limit of infinitely slow driving, where the
system is self-organized critical. This collective phenomenon leads to
effective aging of the asperities and results in velocity dependence of the
friction force in the form .Comment: 7 pages, 8 figures, revtex, submitted to Phys. Rev.
Local density of states in the vortex lattice in a type II superconductor
Local density of states (LDOS) in the triangular vortex lattice is
investigated based on the quasi-classical Eilenberger theory. We consider the
case of an isotropic s-wave superconductor with the material parameter
appropriate to NbSe_2. At a weak magnetic field, the spatial variation of the
LDOS shows cylindrical structure around a vortex core. On the other hand, at a
high field where the core regions substantially overlap each other, the LDOS is
sixfold star-shaped structure due to the vortex lattice effect. The orientation
of the star coincides with the experimental data of the scanning tunneling
microscopy. That is, the ray of the star extends toward the nearest-neighbor
(next nearest-neighbor) vortex direction at higher (lower) energy.Comment: 10 pages, RevTex, 32 figure
Time-Dependent Partition-Free Approach in Resonant Tunneling Systems
An extended Keldysh formalism, well suited to properly take into account the
initial correlations, is used in order to deal with the time-dependent current
response of a resonant tunneling system. We use a \textit{partition-free}
approach by Cini in which the whole system is in equilibrium before an external
bias is switched on. No fictitious partitions are used. Besides the
steady-state responses one can also calculate physical dynamical responses. In
the noninteracting case we clarify under what circumstances a steady-state
current develops and compare our result with the one obtained in the
partitioned scheme. We prove a Theorem of asymptotic Equivalence between the
two schemes for arbitrary time-dependent disturbances. We also show that the
steady-state current is independent of the history of the external perturbation
(Memory Loss Theorem). In the so called wide-band limit an analytic result for
the time-dependent current is obtained. In the interacting case we propose an
exact non-equilibrium Green function approach based on Time Dependent Density
Functional Theory. The equations are no more difficult than an ordinary Mean
Field treatment. We show how the scattering-state scheme by Lang follows from
our formulation. An exact formula for the steady-state current of an arbitrary
interacting resonant tunneling system is obtained. As an example the
time-dependent current response is calculated in the Random Phase
Approximation.Comment: final version, 18 pages, 9 figure
The Current Carried by Bound States of a Superconducting Vortex
We investigate the spectrum of quasiparticle excitations in the core of
isolated pancake vortices in clean layered superconductors. Analysis of the
spectral current density shows that both the circular current around the vortex
center as well as any transport current through the vortex core is carried by
localized states bound to the core by Andreev scattering. Hence the physical
properties of the core are governed in clean high- superconductors
(e.g. the cuprate superconductors) by the Andreev bound states, and not by
normal electrons as it is the case for traditional (dirty) high-
superconductors.Comment: 17 pages in a RevTex (3.0) file plus 5 Figures in PostScript.
Submitted to Physical Review
Transmission Through Carbon Nanotubes With Polyhedral Caps
We study electron transport between capped carbon nanotubes and a substrate,
and relate the transmission probability to the local density of states in the
cap. Our results show that the transmission probability mimics the behavior of
the density of states at all energies except those that correspond to localized
states in the cap. Close proximity of a substrate causes hybridization of the
localized state. As a result, new transmission paths open from the substrate to
nanotube continuum states via the localized states in the cap. Interference
between various transmission paths gives rise to antiresonances in the
transmission probability, with the minimum transmission equal to zero at
energies of the localized states. Defects in the nanotube that are placed close
to the cap cause resonances in the transmission probability, instead of
antiresonances, near the localized energy levels. Depending on the spatial
position of defects, these resonant states are capable of carrying a large
current. These results are relevant to carbon nanotube based studies of
molecular electronics and probe tip applications
Surmounting Oscillating Barriers
Thermally activated escape over a potential barrier in the presence of
periodic driving is considered. By means of novel time-dependent path-integral
methods we derive asymptotically exact weak-noise expressions for both the
instantaneous and the time-averaged escape rate. The agreement with accurate
numerical results is excellent over a wide range of driving strengths and
driving frequencies.Comment: 4 pages, 4 figure
Cluster-based density-functional approach to quantum transport through molecular and atomic contacts
We present a cluster-based density-functional approach to model charge
transport through molecular and atomic contacts. The electronic structure of
the contacts is determined in the framework of density functional theory, and
the parameters needed to describe transport are extracted from finite clusters.
A similar procedure, restricted to nearest-neighbor interactions in the
electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)].
Here, we show how to systematically improve the description of the electrodes
by extracting bulk parameters from sufficiently large metal clusters. In this
way we avoid problems arising from the use of nonorthogonal basis functions.
For demonstration we apply our method to electron transport through Au contacts
with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure
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