25 research outputs found
Dimensional crossover in SrRuO within slave-boson mean-field theory
Motivated by the anomalous temperature dependence of the c-axis resistivity
of SrRuO, the dimensional crossover from a network of perpendicular
one-dimensional chains to a two-dimensional system due to a weak hybridization
between the perpendicular chains is studied. The corresponding two-orbital
Hubbard model is treated within a slave-boson mean-field theory (SBMFT) to take
correlation effects into account such as the spin-charge separation on the
one-dimensional chains. Using an RPA-like formulation for the Green's function
of collective spinon-holon excitations the emergence of quasiparticles at
low-temperatures is examined. The results are used to discuss the evolution of
the spectral density and the c-axis transport within a tunneling approach. For
the latter a regime change between low- and high-temperature regime is found in
qualitative accordance with experimental data
Effective charging energy for a regular granular metal array
We study the Ambegaokar-Eckern-Sch\"{o}n (AES) model for a regular array of
metallic grains coupled by tunnel junctions of conductance and calculate
both paramagnetic and diamagnetic terms in the Kubo formula for the
conductivity. We find analytically, and confirm by numerical path integral
Monte Carlo methods, that for the conductivity obeys an Arrhenius law
with an effective charging energy
when the temperature is sufficiently low, due to a subtle cancellation between
inelastic-cotunneling contributions in the paramagnetic and diamagnetic
terms. We present numerical results for the effective charging energy and
compare the results with recent theoretical analyses. We discuss the different
ways in which the experimentally observed
law could be attributed to disorder.Comment: 5 pages, 3 figures, ReVTeX; added estimates of effective charging
energies and discussion of effects of disorde
Coulomb blockade and quantum tunnelling in the low-conductivity phase of granular metals
We study the effects of Coulomb interaction and inter-grain quantum
tunnelling in an array of metallic grains using the phase-functional approach
for temperatures well below the charging energy of individual
grains yet large compared to the level spacing in the grains. When the
inter-grain tunnelling conductance , the conductivity in
dimensions decreases logarithmically with temperature
(), while for ,
the conductivity shows simple activated behaviour ().
We show, for bare tunnelling conductance , that the parameter
determines the competition between
charging and tunnelling effects. At low enough temperatures in the regime
, a charge is shared among a finite
number of grains, and we find a soft
activation behaviour of the conductivity, , where is the effective
coordination number of a grain.Comment: 11 pages REVTeX, 3 Figures. Appendix added, replaced with published
versio
Coherent-incoherent transition in the sub-Ohmic spin-boson model
We study the spin-boson model with a sub-Ohmic bath using a variational
method. The transition from coherent dynamics to incoherent tunneling is found
to be abrupt as a function of the coupling strength and to exist for
any power , where the bath coupling is described by . We find non-monotonic temperature dependence of the
two-level gap and a re-entrance regime close to the transition due
to non-adiabatic low-frequency bath modes. Differences between thermodynamic
and dynamic conditions for the transition as well as the limitations of the
simplified bath description are discussed.Comment: 12 pages, 4 figure
Interlayer c-axis transport in the normal state of cuprates
A theoretical model of c-axis transport properties in cuprates is proposed.
Inter-plane and in-plane charge fluctuations make hopping between planes
incoherent and diffusive (the in-plane momentum is not conserved after
tunneling). The non-Drude optical conductivity and the
power-law temperature dependence of the {\it dc} conductivity are generically
explained by the strong fluctuations excited in the process of tunneling.
Several microscopic models of the charge fluctuation spectrum are considered.Comment: 8 page
Sum rule analysis of Umklapp processes and Coulomb energy: application to cuprate superconductivity
The third moment frequency sum rule for the density-density correlation
function is rederived in the presence of Umklapp processes. Upper and lower
bounds on the electron-electron Coulomb energy are derived in two-dimensional
and three-dimensional media, and the Umklapp processes are shown to be crucial
in determining the spectrum of the density fluctuations (especially for the
two-dimensional systems). This and other standard sum rules can be used in
conjunction with experimental spectroscopies (electron-energy loss
spectroscopy, optical ellipsometry, etc.) to analyse changes of the
electron-electron Coulomb energy at the superconducting transition in cuprates
Indications of coherence-incoherence crossover in layered transport
For many layered metals the temperature dependence of the interlayer
resistance has a different behavior than the intralayer resistance. In order to
better understand interlayer transport we consider a concrete model which
exhibits this behavior. A small polaron model is used to illustrate how the
interlayer transport is related to the coherence of quasi-particles within the
layers. Explicit results are given for the electron spectral function,
interlayer optical conductivity and the interlayer magnetoresistance. All these
quantities have two contributions: one coherent (dominant at low temperatures)
and one incoherent (dominant at high temperatures).Comment: 6 pages, 4 figures, REVTEX
Interlayer hopping properties of electrons in layered metals
A formalism is proposed to study the electron tunneling between extended
states, based on the spin-boson Hamiltonian previously used in two-level
systems. It is applied to analyze the out--of--plane tunneling in layered
metals considering different models. By studying the effects of in--plane
interactions on the interlayer tunneling of electrons near the Fermi level, we
establish the relation between departure from Fermi liquid behavior driven by
electron correlations inside the layer and the out of plane coherence. Response
functions, directly comparable with experimental data are obtained