577 research outputs found
Charge Distribution Near Oxygen Vacancies in Reduced Ceria
Understanding the electronic charge distribution around oxygen vacancies in
transition metal and rare earth oxides is a scientific challenge of
considerable technological importance. We show how significant information
about the charge distribution around vacancies in cerium oxide can be gained
from a study of high resolution crystal structures of higher order oxides which
exhibit ordering of oxygen vacancies. Specifically, we consider the
implications of a bond valence sum analysis of CeO and
CeO. To illuminate our analysis we show alternative
representations of the crystal structures in terms of orderly arrays of
co-ordination defects and in terms of flourite-type modules. We found that in
CeO, the excess charge resulting from removal of an oxygen atom
delocalizes among all three triclinic Ce sites closest to the O vacancy. In
CeO, the charge localizes on the next nearest neighbour Ce atoms.
Our main result is that the charge prefers to distribute itself so that it is
farthest away from the O vacancies. This contradicts \emph{the standard picture
of charge localisation} which assumes that each of the two excess electrons
localises on one of the cerium ions nearest to the vacancy. This standard
picture is assumed in most calculations based on density functional theory
(DFT). Based on the known crystal structure of PrO, we also
predict that the charge in CeO will be found in the second
coordination shell of the O vacancy. Although this review focuses on bulk
cerium oxides our approach to characterising electronic properties of oxygen
vacancies and the physical insights gained should also be relevant to surface
defects and to other rare earth and transition metal oxides.Comment: 20 pages, 23 figures. The replacement file has a new format for the
figures are the document layout but no change in content. v3 has the
following main changes: 1. The abstract and introduction were extensively
revised. 2. Sec. IV was removed. 3. The Conclusion was rewritte
Electronic and magnetic properties of the ionic Hubbard model on the striped triangular lattice at 3/4 filling
We report a detailed study of a model Hamiltonian which exhibits a rich
interplay of geometrical spin frustration, strong electronic correlations, and
charge ordering. The character of the insulating phase depends on the magnitude
of Delta/|t| and on the sign of t. We find a Mott insulator for Delta >> U >>
|t|; a charge transfer insulator for U >> \Delta >> |t|; and a correlated
covalent insulator for U >> \Delta ~ |t|. The charge transfer insulating state
is investigated using a strong coupling expansion. The frustration of the
triangular lattice can lead to antiferromagnetism or ferromagnetism depending
on the sign of the hopping matrix element, t. We identify the "ring" exchange
process around a triangular plaquette which determines the sign of the magnetic
interactions. Exact diagonalization calculations are performed on the model for
a wide range of parameters and compared to the strong coupling expansion. The
regime U >> \Delta ~ |t| and t<0 is relevant to Na05CoO2. The calculated
optical conductivity and the spectral density are discussed in the light of
recent experiments on Na05CoO2.Comment: 15 pages, 15 figure
Antiferromagnetic Spin Fluctuations in the Metallic Phase of Quasi-Two-Dimensional Organic Superconductors
We give a quantitative analysis of the previously published nuclear magnetic
resonance (NMR) experiments in the k-(ET)2X family of organic charge transfer
salts by using the phenomenological spin fluctuation model of Moriya, and
Millis, Monien and Pines (M-MMP). For temperatures above T_nmr ~ 50 K, the
model gives a good quantitative description of the data in the metallic phases
of several k-(ET)2X materials. These materials display antiferromagnetic
correlation lengths which increase with decreasing temperature and grow to
several lattice constants by T_nmr. It is shown that the fact that the
dimensionless Korringa ratio is much larger than unity is inconsistent with a
broad class of theoretical models (such as dynamical mean-field theory) which
neglects spatial correlations and/or vertex corrections. For materials close to
the Mott insulating phase the nuclear spin relaxation rate, the Knight shift
and the Korringa ratio all decrease significantly with decreasing temperature
below T_nmr. This cannot be described by the M-MMP model and the most natural
explanation is that a pseudogap, similar to that observed in the underdoped
cuprate superconductors, opens up in the density of states below T_nmr. Such a
pseudogap has recently been predicted to occur in the dimerised organic charge
transfer salts materials by the resonating valence bond (RVB) theory. We
propose specific new experiments on organic superconductors to elucidate these
issues. For example, measurements to see if high magnetic fields or high
pressures can be used to close the pseudogap would be extremely valuable.Comment: 11 pages, 2 figures. Accepted for publication in Phys. Rev.
Fermi surface of underdoped cuprate superconductors from interlayer magnetoresistance: closed pockets versus open arcs
An outstanding question about the underdoped cuprates concerns the true nature of their Fermi surface which appears as a set of disconnected arcs. Theoretical models have proposed two distinct possibilities: (1) each arc is the observable part of a partially hidden closed pocket and (2) each arc is open, truncated at its apparent ends. We show that measurements of the variation in the interlayer resistance with the direction of a magnetic field parallel to the layers can qualitatively distinguish closed pockets from open arcs. This is possible because the field can be oriented such that all electrons on arcs encounter a large Lorentz force and resulting magnetoresistance whereas some electrons on pockets escape the effect by moving parallel to the field. © 2010 The American Physical Society
Apparent Violation of the Wiedemann-Franz law near a magnetic field tuned metal-antiferromagnetic quantum critical point
The temperature dependence of the interlayer electrical and thermal
resistivity in a layered metal are calculated for Fermi liquid quasiparticles
which are scattered inelastically by two-dimensional antiferromagnetic spin
fluctuations. Both resistivities have a linear temperature dependence over a
broad temperature range. Extrapolations to zero temperature made from this
linear- range give values that appear to violate the Wiedemann-Franz law.
However, below a low-temperature scale, which becomes small close to the
critical point, a recovery of this law occurs. Our results describe recent
measurements on CeCoIn near a magnetic field-induced quantum phase
transition. Hence, the experiments do not necessarily imply a non-Fermi liquid
ground state.Comment: 4 pages, 2 figures; accepted to Phys. Rev. Let
Quasiparticles at the verge of localization near the Mott metal-insulator transition in a two-dimensional material
The dynamics of charge carriers close to the Mott transition is explored
theoretically and experimentally in the quasi two-dimensional organic
charge-transfer salt -(BEDT-TTF)Cu[N(CN)]BrCl, with
varying Br content. The frequency dependence of the conductivity deviates
significantly from simple Drude model behavior: there is a strong
redistribution of spectral weight as the Mott transition is approached and with
temperature. The effective mass of the quasiparticles increases considerably
when coming close to the insulating phase. A dynamical mean-field-theory
treatment of the relevant Hubbard model gives a good quantitative description
of the experimental data.Comment: 5 pages, 4 figure
- …