13 research outputs found
Optical excitations in a one-dimensional Mott insulator
The density-matrix renormalization-group (DMRG) method is used to investigate
optical excitations in the Mott insulating phase of a one-dimensional extended
Hubbard model. The linear optical conductivity is calculated using the
dynamical DMRG method and the nature of the lowest optically excited states is
investigated using a symmetrized DMRG approach. The numerical calculations
agree perfectly with field-theoretical predictions for a small Mott gap and
analytical results for a large Mott gap obtained with a strong-coupling
analysis. Is is shown that four types of optical excitations exist in this Mott
insulator: pairs of unbound charge excitations, excitons, excitonic strings,
and charge-density-wave (CDW) droplets. Each type of excitations dominates the
low-energy optical spectrum in some region of the interaction parameter space
and corresponds to distinct spectral features: a continuum starting at the Mott
gap (unbound charge excitations), a single peak or several isolated peaks below
the Mott gap (excitons and excitonic strings, respectively), and a continuum
below the Mott gap (CDW droplets).Comment: 12 pages (REVTEX 4), 12 figures (in 14 eps files), 1 tabl
Material-Specific Investigations of Correlated Electron Systems
We present the results of numerical studies for selected materials with
strongly correlated electrons using a combination of the local-density
approximation and dynamical mean-field theory (DMFT). For the solution of the
DMFT equations a continuous-time quantum Monte-Carlo algorithm was employed.
All simulations were performed on the supercomputer HLRB II at the Leibniz
Rechenzentrum in Munich. Specifically we have analyzed the pressure induced
metal-insulator transitions in Fe2O3 and NiS2, the charge susceptibility of the
fluctuating-valence elemental metal Yb, and the spectral properties of a
covalent band-insulator model which includes local electronic correlations.Comment: 14 pages, 7 figures, to appear in "High Performance Computing in
Science and Engineering, Garching 2009" (Springer
From Gapped Excitons to Gapless Triplons in One Dimension
Often, exotic phases appear in the phase diagrams between conventional
phases. Their elementary excitations are of particular interest. Here, we
consider the example of the ionic Hubbard model in one dimension. This model is
a band insulator (BI) for weak interaction and a Mott insulator (MI) for strong
interaction. Inbetween, a spontaneously dimerized insulator (SDI) occurs which
is governed by energetically low-lying charge and spin degrees of freedom.
Applying a systematically controlled version of the continuous unitary
transformations (CUTs) we are able to determine the dispersions of the
elementary charge and spin excitations and of their most relevant bound states
on equal footing. The key idea is to start from an externally dimerized system
using the relative weak interdimer coupling as small expansion parameter which
finally is set to unity to recover the original model.Comment: 18 pages, 10 figure
Two-Particle-Self-Consistent Approach for the Hubbard Model
Even at weak to intermediate coupling, the Hubbard model poses a formidable
challenge. In two dimensions in particular, standard methods such as the Random
Phase Approximation are no longer valid since they predict a finite temperature
antiferromagnetic phase transition prohibited by the Mermin-Wagner theorem. The
Two-Particle-Self-Consistent (TPSC) approach satisfies that theorem as well as
particle conservation, the Pauli principle, the local moment and local charge
sum rules. The self-energy formula does not assume a Migdal theorem. There is
consistency between one- and two-particle quantities. Internal accuracy checks
allow one to test the limits of validity of TPSC. Here I present a pedagogical
review of TPSC along with a short summary of existing results and two case
studies: a) the opening of a pseudogap in two dimensions when the correlation
length is larger than the thermal de Broglie wavelength, and b) the conditions
for the appearance of d-wave superconductivity in the two-dimensional Hubbard
model.Comment: Chapter in "Theoretical methods for Strongly Correlated Systems",
Edited by A. Avella and F. Mancini, Springer Verlag, (2011) 55 pages.
Misprint in Eq.(23) corrected (thanks D. Bergeron
Multi-orbital physics in lithium-molybdenum purple-bronze: going beyond paradigm
We investigate the role of inter-orbital fluctuations in the low energy
physics of a quasi-1D material - lithium molybdenum purple bronze (LMO). It is
an exceptional material that may provide us a long sought realization of a
Tomonaga-Luttinger liquid (TLL) physics, but its behaviour at temperatures of
the order of K remains puzzling despite numerous efforts. Here
we make a conjecture that the physics around is dominated by
multi-orbital excitations. Their properties can be captured using an excitonic
picture. Using this relatively simple model we compute fermionic Green's
function in the presence of excitons. We find that the spectral function is
broadened with a Gaussian and its temperature dependence acquires an extra
factor. Both effects are in perfect agreement with experimental findings.
We also compute the resistivity for temperatures above and below critical
temperature . We explain an upturn of the resistivity at 28K and interpret
the suppression of this extra component of resistivity when a magnetic field is
applied along the conducting axis. Furthermore, in the framework of our model,
we qualitatively discuss and consistently explain other experimentally detected
peculiarities of purple bronze: the breaking of Wiedmann-Franz law and the
magnetochromatic behaviour