425 research outputs found
Optical conductivity of the half-filled Hubbard chain
We combine well-controlled analytical and numerical methods to determine the
optical conductivity of the one-dimensional Mott-Hubbard insulator at zero
temperature. A dynamical density-matrix renormalization group method provides
the entire absorption spectrum for all but very small coupling strengths. In
this limit we calculate the conductivity analytically using exact
field-theoretical methods. Above the Lieb-Wu gap the conductivity exhibits a
characteristic square-root increase. For small to moderate interactions, a
sharp maximum occurs just above the gap. For larger interactions, another weak
feature becomes visible around the middle of the absorption band.Comment: 4 pages with 3 eps figures, published version (changes in text and
references
Excitons in one-dimensional Mott insulators
We employ dynamical density-matrix renormalization group (DDMRG) and
field-theory methods to determine the frequency-dependent optical conductivity
in one-dimensional extended, half-filled Hubbard models. The field-theory
approach is applicable to the regime of `small' Mott gaps which is the most
difficult to access by DDMRG. For very large Mott gaps the DDMRG recovers
analytical results obtained previously by means of strong-coupling techniques.
We focus on exciton formation at energies below the onset of the absorption
continuum. As a consequence of spin-charge separation, these Mott-Hubbard
excitons are bound states of spinless, charged excitations (`holon-antiholon'
pairs). We also determine exciton binding energies and sizes. In contrast to
simple band insulators, we observe that excitons exist in the Mott-insulating
phase only for a sufficiently strong intersite Coulomb repulsion. Furthermore,
our results show that the exciton binding energy and size are not related in a
simple way to the strength of the Coulomb interaction.Comment: 15 pages, 6 eps figures, corrected typos in labels of figures 4,5,
and
Optimization of Gutzwiller Wavefunctions in Quantum Monte Carlo
Gutzwiller functions are popular variational wavefunctions for correlated
electrons in Hubbard models. Following the variational principle, we are
interested in the Gutzwiller parameters that minimize e.g. the expectation
value of the energy. Rewriting the expectation value as a rational function in
the Gutzwiller parameters, we find a very efficient way for performing that
minimization. The method can be used to optimize general Gutzwiller-type
wavefunctions both, in variational and in fixed-node diffusion Monte Carlo.Comment: 9 pages RevTeX with 10 eps figure
Electronic Structure of Paramagnetic V_2O_3: Strongly Correlated Metallic and Mott Insulating Phase
LDA+DMFT, the computation scheme merging the local density approximation and
the dynamical mean-field theory, is employed to calculate spectra both below
and above the Fermi energy and spin and orbital occupations in the correlated
paramagnetic metallic and Mott insulating phase of V_2O_3. The self-consistent
DMFT equations are solved by quantum Monte Carlo simulations. Room temperature
calculations provide direct comparison with experiment. They show a significant
increase of the quasiparticle height in comparison with the results at 1160 K.
We also obtain new insights into the nature of the Mott-Hubbard transition in
V_2O_3. Namely, it is found to be strikingly different from that in the
one-band Hubbard model due to the orbital degrees of freedom. Furthermore we
resolve the puzzle of the unexpectedly small Mott gap in Cr-doped V_2O_3.Comment: 14 pages, 22 figure
Band-Insulator-Metal-Mott-Insulator transition in the half--filled ionic-Hubbard chain
We investigate the ground state phase diagram of the half-filled
repulsive Hubbard model in the presence of a staggered ionic
potential , using the continuum-limit bosonization approach. We find,
that with increasing on-site-repulsion , depending on the value of the
next-nearest-hopping amplitude , the model shows three different
versions of the ground state phase diagram. For , the ground state phase diagram consists of the following
three insulating phases: Band-Insulator at , Ferroelectric Insulator
at . For
there is only one transition from a spin gapped
metallic phase at .
Finally, for intermediate values of the next-nearest-hopping amplitude
we find that with increasing
on-site repulsion, at the model undergoes a second-order
commensurate-incommensurate type transition from a band insulator into a
metallic state and at larger there is a Kosterlitz-Thouless type
transition from a metal into a ferroelectric insulator.Comment: 9 pages 3 figure
Density-matrix renormalisation group approach to quantum impurity problems
A dynamic density-matrix renormalisation group approach to the spectral
properties of quantum impurity problems is presented. The method is
demonstrated on the spectral density of the flat-band symmetric single-impurity
Anderson model. We show that this approach provides the impurity spectral
density for all frequencies and coupling strengths. In particular, Hubbard
satellites at high energy can be obtained with a good resolution. The main
difficulties are the necessary discretisation of the host band hybridised with
the impurity and the resolution of sharp spectral features such as the
Abrikosov-Suhl resonance.Comment: 16 pages, 6 figures, submitted to Journal of Physics: Condensed
Matte
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