1,779 research outputs found
Geometric Constructions Underlying Relativistic Description of Spin on the Base of Non-Grassmann Vector-Like Variable
Basic notions of Dirac theory of constrained systems have their analogs in
differential geometry. Combination of the two approaches gives more clear
understanding of both classical and quantum mechanics, when we deal with a
model with complicated structure of constraints. In this work we describe and
discuss the spin fiber bundle which appeared in various mechanical models where
spin is described by vector-like variable
Frenkel electron on an arbitrary electromagnetic background and magnetic Zitterbewegung
We present Lagrangian which implies both necessary constraints and dynamical
equations for position and spin of relativistic spin one-half particle. The
model is consistent for any value of magnetic moment and for arbitrary
electromagnetic background. Our equations coincide with those of Frenkel in the
approximation in which the latter have been obtained by Frenkel. Transition
from approximate to exact equations yields two structural modifications of the
theory. First, Frenkel condition on spin-tensor turns into the Pirani
condition. Second, canonical momentum is no more proportional to velocity. Due
to this, even when (Frenkel case), the complete and approximate
equations predict different behavior of particle. The difference between
momentum and velocity means extra contribution into spin-orbit interaction. To
estimate the contribution, we found exact solution to complete equations for
the case of uniform magnetic field. While Frenkel electron moves around the
circle, our particle experiences magnetic {\it Zitterbewegung}, that is
oscillates in the direction of magnetic field with amplitude of order of
Compton wavelength for the fast particle. Besides, the particle has dipole
electric moment.Comment: 20 pages, 1 figure, close to published versio
Ab initio calculations of the physical properties of transition metal carbides and nitrides and possible routes to high-Tc
Ab initio linear-response calculations are reported of the phonon spectra and
the electron-phonon interaction for several transition metal carbides and
nitrides in a NaCl-type structure. For NbC, the kinetic, optical, and
superconducting properties are calculated in detail at various pressures and
the normal-pressure results are found to well agree with the experiment.
Factors accounting for the relatively low critical temperatures Tc in
transition metal compounds with light elements are considered and the possible
ways of increasing Tc are discussed.Comment: 19 pages, 7 figure
Bosonic Spectral Function and The Electron-Phonon Interaction in HTSC Cuprates
In Part I we discuss accumulating experimental evidence related to the
structure and origin of the bosonic spectral function in high-temperature
superconducting (HTSC) cuprates at and near optimal doping. Some global
properties of the spectral function, such as number and positions of peaks, are
extracted by combining optics, neutron scattering, ARPES and tunnelling
measurements. These methods give convincing evidence for strong electron-phonon
interaction (EPI) with the coupling constant between 1-3 in cuprates near
optimal doping. Here we clarify how these results are in favor of the
Eliashberg-like theory for HTSC cuprates near optimal doping. In Part II we
discuss some theoretical ingredients - such as strong EPI, strong correlations
- which are necessary to explain the experimental results related to the
mechanism of d-wave pairing in optimally doped cuprates. These comprise the
Migdal-Eliashberg theory for EPI in strongly correlated systems which give rise
to the forward scattering peak. The latter is further supported by the weakly
screened Madelung interaction in the ionic-metallic structure of layered
cuprates. In this approach EPI is responsible for the strength of pairing while
the residual Coulomb interaction (by including spin fluctuations) triggers the
d-wave pairing.Comment: 59 pages, 38 figures, review articl
Field-Dependent Critical Current in Type-II Superconducting Strips: Combined Effect of Bulk Pinning and Geometrical Edge Barrier
Recent theoretical and experimental research on low-bulk-pinning
superconducting strips has revealed striking dome-like magnetic-field
distributions due to geometrical edge barriers. The observed magnetic-flux
profiles differ strongly from those in strips in which bulk pinning is
dominant. In this paper we theoretically describe the current and field
distributions of a superconducting strip under the combined influence of both a
geometrical edge barrier and bulk pinning at the strip's critical current Ic,
where a longitudinal voltage first appears. We calculate Ic and find its
dependence upon a perpendicular applied magnetic field Ha. The behavior is
governed by a parameter p, defined as the ratio of the bulk-pinning critical
current Ip to the geometrical-barrier critical current Is0. We find that when p
> 2/pi and Ip is field-independent, Ic vs Ha exhibits a plateau for small Ha,
followed by the dependence Ic-Ip ~ 1/Ha in higher magnetic fields.Comment: 4 pages, 2 figures, Fig. 1 revised, submitted to Phys. Rev.
- …