1,139 research outputs found
Orbital dynamics: The origin of the anomalous optical spectra in ferromagnetic manganites
We discuss the role of orbital degeneracy in the transport properties of
perovskite manganites, focusing in particular on the optical conductivity in
the metallic ferromagnetic phase at low temperatures. Orbital degeneracy and
strong correlations are described by an orbital t-J model which we treat in a
slave-boson approach. Employing the memory-function formalism we calculate the
optical conductivity, which is found to exhibit a broad incoherent component
extending up to bare bandwidth accompanied by a strong suppression of the Drude
weight. Further, we calculate the constant of T-linear specific heat. Our
results are in overall agreement with experiment and suggest low-energy orbital
fluctuations as the origin of the strongly correlated nature of the metallic
phase of manganites.Comment: To appear in: Phys. Rev. B 58 (Rapid Communications), 1 November 199
Superconductor-to-Spin-Density-Wave Transition in Quasi-One-Dimensional Metals with Ising Anisotropy
We study a mechanism for superconductivity in quasi-one-dimensional materials
with Ising anisotropy. In an isolated chain Ising anisotropy opens a spin gap;
if inter-chain coupling is sufficiently weak, single particle hopping is
suppressed and the physics of coupled chains is controlled by a competition
between pair hopping and exchange interaction. Spin density wave and triplet
superconductivity phases are found separated by a first order phase transition.
For particular parameter values a second order transition described by SO(4)
symmetry is found.Comment: 18 pages, 1 figur
Spin Gaps and Bilayer Coupling in YBaCuO and YBaCuO
We investigate the relevance to the physics of underdoped
YBaCuO and YBaCuO of the quantum critical point
which occurs in a model of two antiferromagnetically coupled planes of
antiferromagnetically correlated spins. We use a Schwinger boson mean field
theory and a scaling analysis to obtain the phase diagram of the model and the
temperature and frequency dependence of various susceptibilities and relaxation
rates. We distinguish between a low coupled-planes regime in which
the optic spin excitations are frozen out and a high
decoupled-planes regime in which the two planes fluctuate independently. In the
coupled-planes regime the yttrium nuclear relaxation rate at low temperatures
is larger relative to the copper and oxygen rates than would be naively
expected in a model of uncorrelated planes. Available data suggest that in
YBaCuO the crossover from the coupled to the decoupled planes
regime occurs at or . The predicted correlation length is
of order 6 lattice constants at . Experimental data related to the
antiferromagnetic susceptibility of YBaCuO may be made consistent
with the theory, but available data for the uniform susceptibility are
inconsistent with the theory.Comment: RevTex 3.
Double exchange magnets: Spin-dynamics in the paramagnetic phase
The electronic structure of perovskite manganese oxides is investigated in
terms of a Kondo lattice model with ferromagnetic Hund coupling and
antiferromagnetic exchange between -spins using a finite temperature
diagonalization technique. Results for the dynamic structure factor are
consistent with recent neutron scattering experiments for the bilayer manganite
LaSrMnO . The susceptibility shows Curie-Weiss
behaviour and is used to derive a phase diagram. In the paramagnetic phase
carriers are characterized as ferromagnetic polarons in an antiferromagnetic
spin liquid.Comment: Revtex, 4 pages with 5 postscript figures include
On the Fermi Liquid to Polaron Crossover II: Double Exchange and the Physics of "Colossal" Magnetoresistance
We use the dynamical mean field method to study a model of electrons
Jahn-Teller coupled to localized classical oscillators and ferromagnetically
coupled to ``core spins'', which, we argue, contains the essential physics of
the ``colossal magnetoresistance'' manganites . We
determine the different regimes of the model and present results for the
temperature and frequency dependence of the conductivity, the electron spectral
function and the root mean square lattice parameter fluctuations. We compare
our results to data, and give a qualitative discussion of important physics not
included in the calculation. Extensive use is made of results from a companion
paper titled: ``On the Fermi Liquid to Polaron Crossover I: General Results''.Comment: 34 pages, 10 figures. Depends on previous paper titled "On the Fermi
Liquid to Poalron Crossover I: General Result
The quantum phase transition of itinerant helimagnets
We investigate the quantum phase transition of itinerant electrons from a
paramagnet to a state which displays long-period helical structures due to a
Dzyaloshinskii instability of the ferromagnetic state. In particular, we study
how the self-generated effective long-range interaction recently identified in
itinerant quantum ferromagnets is cut-off by the helical ordering. We find that
for a sufficiently strong Dzyaloshinskii instability the helimagnetic quantum
phase transition is of second order with mean-field exponents. In contrast, for
a weak Dzyaloshinskii instability the transition is analogous to that in
itinerant quantum ferromagnets, i.e. it is of first order, as has been observed
in MnSi.Comment: 5 pages RevTe
Non-Equilibrium Quantum Dissipation
Dissipative processes in non-equilibrium many-body systems are fundamentally
different than their equilibrium counterparts. Such processes are of great
importance for the understanding of relaxation in single molecule devices. As a
detailed case study, we investigate here a generic spin-fermion model, where a
two-level system couples to two metallic leads with different chemical
potentials. We present results for the spin relaxation rate in the nonadiabatic
limit for an arbitrary coupling to the leads, using both analytical and exact
numerical methods. The non-equilibrium dynamics is reflected by an exponential
relaxation at long times and via complex phase shifts, leading in some cases to
an "anti-orthogonality" effect. In the limit of strong system-lead coupling at
zero temperature we demonstrate the onset of a Marcus-like Gaussian decay with
{\it voltage difference} activation. This is analogous to the equilibrium
spin-boson model, where at strong coupling and high temperatures the spin
excitation rate manifests temperature activated Gaussian behavior. We find that
there is no simple linear relationship between the role of the temperature in
the bosonic system and a voltage drop in a non-equilibrium electronic case. The
two models also differ by the orthogonality-catastrophe factor existing in a
fermionic system, which modifies the resulting lineshapes. Implications for
current characteristics are discussed. We demonstrate the violation of
pair-wise Coulomb gas behavior for strong coupling to the leads. The results
presented in this paper form the basis of an exact, non-perturbative
description of steady-state quantum dissipative systems
Optical-conductivity sum rule in cuprates and unconventional charge density waves: a short review
We begin with an overview of the experimental results for the temperature and
doping dependences of the optical-conductivity spectral weight in cuprate
superconductors across the whole phase diagram. Then we discuss recent attempts
to explain the observed behavior of the spectral weight using reduced and full
models with unconventional charge-density waves.Comment: 17 pages, RevTeX4, 4 EPS figures; Invited paper for a special issue
of Low Temperature Physics dedicated to the 20th anniversary of HTS
On the Fermi Liquid to Polaron Crossover I: General Results
We use analytic techniques and the dynamical mean field method to study the
crossover from fermi liquid to polaron behavior in models of electrons
interacting with dispersionless classical phonons.Comment: 42 pages, 13 figure
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