8,280 research outputs found
The metal insulator transition in cluster dynamical mean field theory: intersite correlation, cluster size, interaction strength, and the location of the transition line
To gain insight into the physics of the metal insulator transition and the
effectiveness of cluster dynamical mean field theory (DMFT) we have used one,
two and four site dynamical mean field theory to solve a polaron model of
electrons coupled to a classical phonon field. The cluster size dependence of
the metal to polaronic insulator phase boundary is determined along with
electron spectral functions and cluster correlation functions. Pronounced
cluster size effects start to occur in the intermediate coupling region in
which the cluster calculation leads to a gap and the single-site approximation
does not. Differences (in particular a sharper band edge) persist in the strong
coupling regime. A partial density of states is defined encoding a generalized
nesting property of the band structure; variations in this density of states
account for differences between the dynamical cluster approximation and the
cellular-DMFT implementations of cluster DMFT, and for differences in behavior
between the single band models appropriate for cuprates and the multiband
models appropriate for manganites. A pole or strong resonance in the self
energy is associated with insulating states; the momentum dependence of the
pole is found to distinguish between Slater-like and Mott-like mechanisms for
metal insulator transition. Implications for the theoretical treatment of doped
manganites are discussed.Comment: 28 pages (single column, double space) 15 figure
Implications of the Low-Temperature Instability of Dynamical Mean Theory for Double Exchange Systems
The single-site dynamical mean field theory approximation to the double
exchange model is found to exhibit a previously unnoticed instability, in which
a well-defined ground state which is stable against small perturbations is
found to be unstable to large-amplitude but purely local fluctuations. The
instability is shown to arise either from phase separation or, in a narrow
parameter regime, from the presence of a competing phase. The instability is
therefore suggested as a computationally inexpensive means of locating regimes
of parameter space in which phase separation occurs.Comment: 5 pages 5 figure
Two-particle response in Cluster Dynamical Mean-Field Theory: Formalism and application to the Raman Response of High-temperature Superconductors
A method is presented for the unbiased numerical computation of two-particle
response functions of correlated electron materials via a solution of the
dynamical mean-field equations in the presence of a perturbing field. The power
of the method is demonstrated via a computation of the Raman and
scattering intensities of the two dimensional Hubbard model, in
parameter regimes believed to be relevant to high-temperature
superconductivity. The theory reproduces the `two-magnon' peak characteristic
of the Raman intensity of the insulating parent compounds of high- copper
oxide superconductors and shows how it evolves to a quasiparticle response as
carriers are added. The method can be applied in any situation where a solution
of the equilibrium dynamical mean-field equations is feasible
Structural distortions and model Hamiltonian parameters: from LSDA to a tight-binding description of LaMnO_3
The physics of manganites is often described within an effective two-band
tight-binding (TB) model for the Mn e_g electrons, which apart from the kinetic
energy includes also a local "Hund's rule" coupling to the t_{2g} core spin and
a local coupling to the Jahn-Teller (JT) distortion of the oxygen octahedra. We
test the validity of this model by comparing the energy dispersion calculated
for the TB model with the full Kohn-Sham band-structure calculated within the
local spin-density approximation (LSDA) to density functional theory. We
analyze the effect of magnetic order, JT distortions, and "GdFeO_3-type"
tilt-rotations of the oxygen octahedra. We show that the hopping amplitudes are
independent of magnetic order and JT distortions, and that both effects can be
described with a consistent set of model parameters if hopping between both
nearest and next-nearest neighbors is taken into account. We determine a full
set of model parameters from the density functional theory calculations, and we
show that both JT distortions and Hund's rule coupling are required to obtain
an insulating ground state within LSDA. Furthermore, our calculations show that
the "GdFeO_3-type" rotations of the oxygen octahedra lead to a substantial
reduction of the hopping amplitudes but to no significant deviation from the
simple TB model.Comment: replaced with final (published) version with improved presentatio
Theoretical Description of Pseudocubic Manganites
A comprehensive theoretical model for the bulk manganite system
La(Ca,Sr)MnO is presented. The model includes local and
cooperative Jahn-Teller distortions and the on-site Coulomb and exchange
interaction. The model is is solved in the single-site dynamical mean field
approximation using a solver based on the semiclassical approximation. The
model semi-quantitatively reproduces the observed phase diagram for the doping
and implies that the manganites are in the strong coupling
region but close to Mott insulator/metal phase boundary. The results establish
a formalism for use in a broader range of calculations, for example on
heterostructures.Comment: 14 figures, 34 pages, using the bandwidth obtained from LDA with the
observed structure, and resulting in a better fit to data
Theory of Manganite Superlattice
A theoretical model is proposed for the (0,0,1) superlattice manganite system
(LaMnO)(SrMnO). The model includes the electron-electron,
electron-phonon, and cooperative Jahn-Teller interactions. It is solved using a
version of single-site the dynamical mean field approximation generalized to
incorporate the cooperative Jahn-Teller effect. The phase diagram and
conductivities are calculated. The behavior of the superlattice is found to a
good approximation to be an average over the density-dependent properties of
individual layers, with the density of each layer fixed by electrostatics.Comment: 9 figures, 22 page
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