45 research outputs found
Dynamics of Tunneling Centers in Metallic Systems
Dynamics of tunneling centers (TC) in metallic systems is studied, using the
technique of bosonization. The interaction of the TC with the conduction
electrons of the metal involves two processes, namely, the screening of the TC
by electrons, and the so-called electron assisted tunneling. The presence of
the latter process leads to a different form of the renormalized tunneling
frequency of the TC, and the tunneling motion is damped with a temperature
dependent relaxation rate. As the temperature is lowered, the relaxation rate
per temperature shows a steep rise as opposed to that in the absence of
electron assisted process. It is expected that this behavior should be observed
at very low temperatures in a careful experiment. The present work thus tries
to go beyond the existing work on the {\it dynamics} of a two-level system in
metals, by treating the electron assisted process.Comment: REVTeX twocolumn format, 5 pages, two PostScript figures available on
request. Preprint # : imsc 94/3
Macroscopic Quantum Tunneling of a Domain Wall in a Ferromagnetic Metal
The macroscopic quantum tunneling of a planar domain wall in a ferromagnetic
metal is studied based on the Hubbard model. It is found that the ohmic
dissipation is present even at zero temperature due to the gapless Stoner
excitation, which is the crucial difference from the case of the insulating
magnet. The dissipative effect is calculated as a function of width of the wall
and is shown to be effective in a thin wall and in a weak ferromagnet. The
results are discussed in the light of recent experiments on ferromagnets with
strong anisotropy. PACS numbers:75.60.Ch, 03.65.Sq, 75.10.LpComment: 13page
Theory of Orbital Kondo Effect with Assisted Hopping in Strongly Correlated Electron Systems: Parquet Equations, Superconductivity and Mass Enhancement
Orbital Kondo effect is treated in a model, where additional to the
conduction band there are localized orbitals close to the Fermi energy. If the
hopping between the conduction band and the localized heavy orbitals depends on
the occupation of the atomic orbitals in the conduction band then orbital Kondo
correlation occurs. The noncommutative nature of the coupling required for the
Kondo effect is formally due to the form factors associated with the assisted
hopping which in the momentum representation depends on the momenta of the
conduction electrons involved. The leading logarithmic vertex corrections are
due to the local Coulomb interaction between the electrons on the heavy orbital
and in the conduction band. The renormalized vertex functions are obtained as a
solution of a closed set of differential equations and they show power
behavior. The amplitude of large renormalization is determined by an infrared
cutoff due to finite energy and dispersion of the heavy particles. The enhanced
assisted hopping rate results in mass enhancement and attractive interaction in
the conduction band. The superconductivity transition temperature calculated is
largest for intermediate mass enhancement, . For larger mass
enhancement the small one particle weight () in the Green's function reduces
the transition temperature which may be characteristic for otherComment: 32 pages, RevTeX 3.0, figures on reques
Macroscopic Quantum Tunneling and Dissipation of Domain Wall in Ferromagnetic Metals
The depinning of a domain wall in ferromagentic metal via macroscopic quantum
tunneling is studied based on the Hubbard model. The dynamics of the
magnetization verctor is shown to be governed by an effective action of
Heisenberg model with a term non-local in time that describes the dissipation
due to the conduction electron. Due to the existence of the Fermi surface there
exists Ohmic dissipation even at zero temperature, which is crucially different
from the case of the insulator. Taking into account the effect of pinning and
the external magnetic field the action is rewritten in terms of a collective
coordinate, the position of the wall, . The tunneling rate for is
calculated by use of the instanton method. It is found that the reduction of
the tunneling rate due to the dissipation is very large for a thin domain wall
with thickness of a few times the lattice spacing, but is negligible for a
thick domain wall. Dissipation due to eddy current is shown to be negligible
for a wall of mesoscopic size.Comment: of pages 26, to appear in "Quantum Tunneling of Magnetization, ed. B.
Barbara and L. Gunther (Kluwer Academic Pub.), Figures available by FAX
(81-48-462-4649