46 research outputs found
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
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
Bound States of Conical Singularities in Graphene-Based Topological Insulators
We investigate the electronic structure induced by wedge-disclinations
(conical singularities) in a honeycomb lattice model realizing Chern numbers
. We establish a correspondence between the bound state of (i) an
isolated -flux, (ii) an isolated pentagon or heptagon
defect with an external flux of magnitude through
the center and (iii) an isolated square or octagon defect without external
flux, where is the flux quantum. Due to the above correspondence,
the existence of isolated electronic states bound to the disclinations is
robust against various perturbations. These results are also generalized to
graphene-based time-reversal invariant topological insulators.Comment: 5+4 pages, 4+3 figures, revised introduction and Fig.
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