7,872 research outputs found
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
Charge Transport in Manganites: Hopping Conduction, the Anomalous Hall Effect and Universal Scaling
The low-temperature Hall resistivity \rho_{xy} of La_{2/3}A_{1/3}MnO_3 single
crystals (where A stands for Ca, Pb and Ca, or Sr) can be separated into
Ordinary and Anomalous contributions, giving rise to Ordinary and Anomalous
Hall effects, respectively. However, no such decomposition is possible near the
Curie temperature which, in these systems, is close to metal-to-insulator
transition. Rather, for all of these compounds and to a good approximation, the
\rho_{xy} data at various temperatures and magnetic fields collapse (up to an
overall scale), on to a single function of the reduced magnetization
m=M/M_{sat}, the extremum of this function lying at m~0.4. A new mechanism for
the Anomalous Hall Effect in the inelastic hopping regime, which reproduces
these scaling curves, is identified. This mechanism, which is an extension of
Holstein's model for the Ordinary Hall effect in the hopping regime, arises
from the combined effects of the double-exchange-induced quantal phase in
triads of Mn ions and spin-orbit interactions. We identify processes that lead
to the Anomalous Hall Effect for localized carriers and, along the way, analyze
issues of quantum interference in the presence of phonon-assisted hopping. Our
results suggest that, near the ferromagnet-to-paramagnet transition, it is
appropriate to describe transport in manganites in terms of carrier hopping
between states that are localized due to combined effect of magnetic and
non-magnetic disorder. We attribute the qualitative variations in resistivity
characteristics across manganite compounds to the differing strengths of their
carrier self-trapping, and conclude that both disorder-induced localization and
self-trapping effects are important for transport.Comment: 29 pages, 20 figure
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