3,307 research outputs found
Theory of Flux-Flow Resistivity near for s-wave Type-II Superconductors
This paper presents a microscopic calculation of the flux-flow resistivity
for s-wave type-II superconductors with arbitrary impurity
concentrations near the upper critical field . It is found that, as the
mean free path becomes longer, increases gradually from the
dirty-limit result of Thompson [Phys. Rev. B{\bf 1}, 327 (1970)] and Takayama
and Ebisawa [Prog. Theor. Phys. {\bf 44}, 1450 (1970)]. The limiting behaviors
suggest that at low temperatures may change from convex downward
to upward as increases, thus deviating substantially from the linear
dependence predicted by the Bardeen-Stephen theory
[Phys. Rev. {\bf 140}, A1197 (1965)]
Gap Anisotropy and de Haas-van Alphen Effect in Type-II Superconductors
We present a theoretical study on the de Haas-van Alphen (dHvA) oscillation
in the vortex state of type-II superconductors, with a special focus on the
connection between the gap anisotropy and the oscillation damping. Numerical
calculations for three different gap structures clearly indicate that the
average gap along extremal orbits is relevant for the magnitude of the extra
damping, thereby providing a support for experimental efforts to probe gap
anisotropy through the dHvA signal. We also derive an analytic formula for the
extra damping which gives a good fit to the numerical results.Comment: 5 pages, 1 figure, changes in Introductio
Spatially resolved electronic structure of an isovalent nitrogen center in GaAs
Small numbers of nitrogen dopants dramatically modify the electronic
properties of GaAs, generating very large shifts in the conduction-band
energies with nonlinear concentration dependence, and impurity-associated
spatially-localized resonant states within the conduction band. Cross-sectional
scanning tunneling microscopy provides the local electronic structure of single
nitrogen dopants at the (110) GaAs surface, yielding highly anisotropic spatial
shapes when the empty states are imaged. Measurements of the resonant states
relative to the GaAs surface states and their spatial extent allow an
unambiguous assignment of specific features to nitrogen atoms at different
depths below the cleaved (110) surface. Multiband tight binding calculations
around the resonance energy of nitrogen in the conduction band match the imaged
features. The spatial anisotropy is attributed to the tetrahedral symmetry of
the bulk lattice. Additionally, the voltage dependence of the electronic
contrast for two features in the filled state imaging suggest these features
could be related to a locally modified surface state
Properties of Nambu-Goldstone Bosons in a Single-Component Bose-Einstein Condensate
We theoretically study the properties of Nambu-Goldstone bosons in an
interacting single-component Bose-Einstein condensate (BEC). We first point out
that the proofs of Goldstone's theorem by Goldstone, et al. [Phys. Rev. {\bf
127} (1962) 965] may be relevant to distinct massless modes of the BEC: whereas
the first proof deals with the poles of the single-particle Green's function
, the second one concerns those of the two-particle Green's function.
Thus, there may be multiple Nambu-Goldstone bosons even in the single-component
BEC with broken U(1) symmetry. The second mode turns out to have an infinite
lifetime in the long-wavelength limit in agreement with the conventional
viewpoint. In contrast, the first mode from , i.e., the Bogoliubov
mode in the weak-coupling regime, is shown to be a "bubbling" mode fluctuating
temporally out of and back into the condensate. The substantial lifetime
originates from an "improper" structure of the self-energy inherent in the BEC,
which has been overlooked so far and will be elucidated here, and removes
various infrared divergences pointed out previously.Comment: 9 pages, 6 gigure
Critical light scattering in liquids
We compare theoretical results for the characteristic frequency of the
Rayleigh peak calculated in one-loop order within the field theoretical method
of the renormalization group theory with experiments and other theoretical
results. Our expressions describe the non-asymptotic crossover in temperature,
density and wave vector. In addition we discuss the frequency dependent shear
viscosity evaluated within the same model and compare our theoretical results
with recent experiments in microgravity.Comment: 17 pages, 12 figure
Entropy and Spin Susceptibility of s-wave Type-II Superconductors near
A theoretical study is performed on the entropy and the spin
susceptibility near the upper critical field of s-wave
type-II superconductors with arbitrary impurity concentrations. The changes of
these quantities through may be expressed as , for example, where is the average flux density
and denotes entropy in the normal state. It is found that the
slopes and at T=0 are identical, connected
directly with the zero-energy density of states, and vary from 1.72 in the
dirty limit to in the clean limit. This mean-free-path dependence
of and at T=0 is quantitatively the same as that
of the slope for the flux-flow resistivity studied
previously. The result suggests that and near
T=0 are convex downward (upward) in the dirty (clean) limit, deviating
substantially from the linear behavior . The specific-heat
jump at also shows fairly large mean-free-path dependence.Comment: 8 pages, 5 figure
Quasiparticles of d-wave superconductors in finite magnetic fields
We study quasiparticles of d-wave superconductors in the vortex lattice by
self-consistently solving the Bogoliubov-de Gennes equations. It is found for a
pure state that: (i) low-energy quasiparticle bands in the
magnetic Brillouin zone have rather large dispersion even in low magnetic
fields, indicating absense of bound states for an isolated vortex; (ii) in
finite fields with small, the calculated tunneling conductance at
the vortex core shows a double-peak structure near zero bias, as qualitatively
consistent with the STM experiment by Maggio-Aprile et al. [Phys. Rev. Lett.
{\bf 75} (1995) 2754]. We also find that mixing of a - or an s-wave
component, if any, develops gradually without transitions as the field is
increased, having little effect on the tunneling spectra.Comment: 4 pages, 4 figures, LaTe
Self-Consistent Approximations for Superconductivity beyond the Bardeen-Cooper-Schrieffer Theory
We develop a concise self-consistent perturbation expansion for
superconductivity where all the pair processes are naturally incorporated
without drawing "anomalous" Feynman diagrams. This simplification results from
introducing an interaction vertex that is symmetric in the particle-hole
indices besides the ordinary space-spin coordinates. The formalism
automatically satisfies conservation laws, includes the Luttinger-Ward theory
as the normal-state limit, and reproduces the Bardeen-Cooper-Schrieffer theory
as the lowest-order approximation. It enables us to study the thermodynamic,
single-particle, two-particle, and dynamical properties of superconductors with
competing fluctuations based on a single functional of Green's
function in the Nambu space. Specifically, we derive closed equations
in the FLEX-S approximation, i.e., the fluctuation exchange approximation for
superconductivity with all the pair processes, which contains extra terms
besides those in the standard FLEX approximation.Comment: 14 pages, 6 figure
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