104 research outputs found
Conductance as a Function of the Temperature in the Double Exchange Model
We have used the Kubo formula to calculate the temperature dependence of the
electrical conductance of the double exchange Hamiltonian. We average the
conductance over an statistical ensemble of clusters, which are obtained by
performing Monte Carlo simulations on the classical spin orientation of the
double exchange Hamiltonian. We find that for electron concentrations bigger
than 0.1, the system is metallic at all temperatures. In particular it is not
observed any change in the temperature dependence of the resistivity near the
magnetical critical temperature. The calculated resistivity near is
around ten times smaller than the experimental value. We conclude that the
double exchange model is not able to explain the metal to insulator transition
which experimentally occurs at temperatures near the magnetic critical
temperature.Comment: 6 pages, 5 figures included in the tex
Kondo lattice model: Unitary transformations, spin dynamics, strongly correlated charged modes, and vacuum instability
Using unitary transformations, we express the Kondo lattice Hamiltonian in
terms of fermionic operators that annihilate the ground state of the
interacting system and that represent the best possible approximations to the
actual charged excitations. In this way, we obtain an effective Hamiltonian
which, for small couplings, consists in a kinetic term for conduction electrons
and holes, an RKKY-like term, and a renormalized Kondo interaction. The
physical picture of the system implied by this formalism is that of a vacuum
state consisting in a background of RKKY-induced spin correlations, where two
kinds of elementary modes can be excited: Soft neutral modes associated with
deformations of the spin liquid, which lead to very large low-temperature
values of the heat capacity and magnetic susceptibility, and charged modes
corresponding to the excitation of electrons and holes in the system. Using the
translational and spin rotational symmetries, we construct a simple ansatz to
determine the charged excitations neglecting the effects of the spin
correlations. Apart from the `normal', uncorrelated states, we find strongly
correlated charged modes involving soft electrons (or holes) and spin
fluctuations, which strongly renormalize the low-energy charged spectrum, and
whose energy becomes negative beyond a critical coupling, signaling a vacuum
instability and a transition to a new phase.Comment: 35 pages, revtex 3.
Orbital Polarons in the Metal-Insulator Transition of Manganites
The metal-insulator transition in manganites is strongly influenced by the
concentration of holes present in the system. Based upon an orbitally
degenerate Mott-Hubbard model we analyze two possible localization scenarios to
account for this doping dependence: First, we rule out that the transition is
initiated by a disorder-order crossover in the orbital sector, showing that its
effect on charge mobility is only small. Second, we introduce the idea of
orbital polarons originating from a strong polarization of orbitals in the
vicinity of holes. Considering this direct coupling between charge and orbital
degree of freedom in addition to lattice effects we are able to explain well
the phase diagram of manganites for low and intermediate hole concentrations
Magnetic Coherence in Cuprate Superconductors
Recent inelastic neutron scattering (INS) experiments on
LaSrCuO observed a {\it magnetic coherence effect}, i.e.,
strong frequency and momentum dependent changes of the spin susceptibility,
, in the superconducting phase. We show that this effect is a direct
consequence of changes in the damping of incommensurate antiferromagnetic spin
fluctuations due to the appearance of a d-wave gap in the fermionic spectrum.
Our theoretical results provide a quantitative explanation for the weak
momentum dependence of the observed spin-gap. Moreover, we predict {\bf (a)} a
Fermi surface in LaSrCuO which is closed around up
to optimal doping, and {\bf (b)} similar changes in for all cuprates
with an incommensurate magnetic response.Comment: 5 pages, 4 figures, Fig.3 is in colo
Big, Fast Vortices in the d-RVB theory of High Temperature Superconductivity
The effect of proximity to a Mott insulating phase on the superflow
properties of a d-wave superconductor is studied using the slave boson-U(1)
gauge theory model. The model has two limits corresponding to superconductivity
emerging either out of a 'renormalized fermi liquid' or out of a
non-fermi-liquid regime. Three crucial physical parameters are identified: the
size of the vortex \textit{as determined from the supercurrent it induces;} the
coupling of the superflow to the quasiparticles and the 'nondissipative time
derivative' term. As the Mott phase is approached, the core size as defined
from the supercurrent diverges, the coupling between superflow and
quasiparticles vanishes, and the magnitude of the nondissipative time
derivative dramatically increases. The dissipation due to a moving vortex is
found to vary as the third power of the doping. The upper critical field and
the size of the critical regime in which paraconductivity may be observed are
estimated, and found to be controlled by the supercurrent length scale
Quantum Disordered Regime and Spin Gap in the Cuprate Superconductors
We discuss the crossover from the quantum critical, , to the quantum
disordered regime in high-T materials in relation to the experimental data
on the nuclear relaxation, bulk susceptibility, and inelastic neutron
scattering. In our scenario, the spin excitations develop a gap
well above T, which is supplemented by the
quasiparticle gap below T. The above experiments yield consistent estimates
for the value of the spin gap, which increases as the correlation length
decreases.Comment: 14 pages, REVTeX v3.0, PostScript file for 3 figures is attached,
UIUC-P-93-07-06
Small Fermi surface in the one-dimensional Kondo lattice model
We study the one-dimensional Kondo lattice model through the density matrix
renormalization group (DMRG). Our results for the spin correlation function
indicate the presence of a small Fermi surface in large portions of the phase
diagram, in contrast to some previous studies that used the same technique. We
argue that the discrepancy is due to the open boundary conditions, which
introduce strong charge perturbations that strongly affect the spin Friedel
oscillations.Comment: 5 pages, 7 figure
Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance
We report detailed 17O, 139La, and 63Cu Nuclear Magnetic Resonance (NMR) and
Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered
La1.875Ba0.125CuO4 single crystal and in oriented powder samples of
La1.8-xEu0.2SrxCuO4. We observe a partial wipeout of the 17O NMR intensity and
a simultaneous drop of the 17O electric field gradient (EFG) at low
temperatures where the spin stripe order sets in. In contrast, the 63Cu
intensity is completely wiped out at the same temperature. The drop of the 17O
quadrupole frequency is compatible with a charge stripe order. The 17O spin
lattice relaxation rate shows a peak similar to that of the 139La, which is of
magnetic origin. This peak is doping dependent and is maximal at x ~ 1/8.Comment: submitted to European Physical Journal Special Topic
Average Lattice Symmetry and Nanoscale Structural Correlations in Magnetoresistive Manganites
We report x-ray scattering studies of nanoscale structural correlations in
the paramagnetic phases of the perovskite manganites
La(CaSr)MnO,
LaSrMnO, and NdSrMnO. We find
that these correlations are present in the orthorhombic phase in
La(CaSr)MnO, but they disappear
abruptly at the orthorhombic-to-rhombohedral transition in this compound. The
orthorhombic phase exhibits increased electrical resistivity and reduced
ferromagnetic coupling, in agreement with the association of the nanoscale
correlations with insulating regions. In contrast, the correlations were not
detected in the two other compounds, which exhibit rhombohedral and tetragonal
phases. Based on these results, as well as on previously published work, we
propose that the local structure of the paramagnetic phase correlates strongly
with the average lattice symmetry, and that the nanoscale correlations are an
important factor distinguishing the insulating and the metallic phases in these
compounds.Comment: a note on recent experimental work, and a new reference adde
On the effects of the magnetic field and the isotopic substitution upon the infrared absorption of manganites
Employing a variational approach that takes into account electron-phonon and
magnetic interactions in perovskites with , the
effects of the magnetic field and the oxygen isotope substitution on the phase
diagram, the electron-phonon correlation function and the infrared absorption
at are studied. The lattice displacements show a strong correlation
with the conductivity and the magnetic properties of the system. Then the
conductivity spectra are characterized by a marked sensitivity to the external
parameters near the phase boundary.Comment: 10 figure
- …