968 research outputs found
Microwave Response of V3Si Single Crystals: Evidence for Two-Gap Superconductivity
The investigation of the temperature dependences of microwave surface
impedance and complex conductivity of V3Si single crystals with different
stoichiometry allowed to observe a number of peculiarities which are in
remarkable contradiction with single-gap Bardeen-Cooper-Schrieffer theory. At
the same time, they can be well described by two-band model of
superconductivity, thus strongly evidencing the existence of two distinct
energy gaps with zero-temperature values Delta1~1.8Tc and Delta2~0.95Tc in
V3Si.Comment: Submitted to Europhysics Letter
Is the term "type-1.5 superconductivity" warranted by Ginzburg-Landau theory?
It is shown that within the Ginzburg-Landau (GL) approximation the order
parameters Delta1(r, T) and Delta2(r, T) in two-band superconductors vary on
the same length scale, the difference in the zero-T coherence lengths xi0_i
~vF_i/Delta_i(0), i = 1, 2 notwithstanding. This amounts to a single physical
GL parameter kappa and the classic GL dichotomy: kappa < 1/sqrt(2) for type-I
and kappa > 1/sqrt(2) for type-II.Comment: 5 pages, revised and extended version; previous title "Two-band
superconductors near Tc" change
Revivals, collapses and magnetic-pulse generation in quantum rings
Using a microscopic theory based on the density matrix formalism we
investigate quantum revivals and collapses of the charge polarization and
charge current dynamics in mesoscopic rings driven by short asymmetric
electromagnetic pulses. The collapsed state is utilized for sub-picosecond
switching of the current and associated magnetization, enabling thus the
generation of pulsed magnetic fields with a tunable time structure and shape
asymmetry which provides a new tool to study ultrafast spin-dynamics and
ratchet-based effects.Comment: 4 pages, 2 figure
Variational discrete variable representation for excitons on a lattice
We construct numerical basis function sets on a lattice, whose spatial
extension is scalable from single lattice sites to the continuum limit. They
allow us to compute small and large bound states with comparable, moderate
effort. Adopting concepts of discrete variable representations, a diagonal form
of the potential term is achieved through a unitary transformation to Gaussian
quadrature points. Thereby the computational effort in three dimensions scales
as the fourth instead of the sixth power of the number of basis functions along
each axis, such that it is reduced by two orders of magnitude in realistic
examples. As an improvement over standard discrete variable representations,
our construction preserves the variational principle. It allows for the
calculation of binding energies, wave functions, and excitation spectra. We use
this technique to study central-cell corrections for excitons beyond the
continuum approximation. A discussion of the mass and spectrum of the yellow
exciton series in the cuprous oxide, which does not follow the hydrogenic
Rydberg series of Mott-Wannier excitons, is given on the basis of a simple
lattice model.Comment: 12 pages, 7 figures. Final version as publishe
PAMELA's cosmic positron from decaying LSP in SO(10) SUSY GUT
We propose two viable scenarios explaining the recent observations on cosmic
positron excess. In both scenarios, the present relic density in the Universe
is assumed to be still supported by thermally produced WIMP or LSP (\chi). One
of the scenarios is based on two dark matter (DM) components (\chi,X) scenario,
and the other is on SO(10) SUSY GUT. In the two DM components scenario,
extremely small amount of non-thermally produced meta-stable DM component
[O(10^{-10}) < n_X /n_\chi] explains the cosmic positron excess. In the SO(10)
model, extremely small R-parity violation for LSP decay to e^\pm is naturally
achieved with a non-zero VEV of the superpartner of one right-handed neutrino
(\tilde{\nu}^c) and a global symmetry.Comment: 6 pages, Talks presented in PASCOS, SUSY, and COSMO/CosPA in 201
Strong interaction of correlated electrons with phonons: Exchange of phonon clouds by polarons
We investigate the interaction of strongly correlated electrons with phonons
in the frame of the Hubbard-Holstein model. The electron-phonon interaction is
considered to be strong and is an important parameter of the model besides the
Coulomb repulsion of electrons and band filling. This interaction with the
nondispersive optical phonons has been transformed to the problem of mobile
polarons by using the canonical transformation of Lang and Firsov. We discuss
in particular the case for which the on-site Coulomb repulsion is exactly
cancelled by the phonon-mediated attractive interaction and suggest that
polarons exchanging phonon clouds can lead to polaron pairing and
superconductivity. It is then the frequency of the collective mode of phonon
clouds being larger than the bare frequency, which determines the
superconducting transition temperature.Comment: 23 pages, Submitted to Phys. Rev.
Effect of disorder on the NMR relaxation rate in two-band superconductors
We calculate the effect of nonmagnetic impurity scattering on the
spin-lattice relaxation rate in two-band superconductors with the s-wave
pairing symmetry. It is found that for the interaction parameters appropriate
for MgB2 the Hebel-Slichter peak is suppressed by disorder in the limit of
small interband impurity scattering rate. In the limit of strong impurity
scattering, when the gap functions in the two bands become nearly equal, the
single-band behavior is recovered with a well-defined coherence peak just below
the transition temperature.Comment: 6 pages, 4 figure
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