679 research outputs found
Optical excitations of Peierls-Mott insulators with bond disorder
The density-matrix renormalization group (DMRG) is employed to calculate
optical properties of the half-filled Hubbard model with nearest-neighbor
interactions. In order to model the optical excitations of oligoenes, a Peierls
dimerization is included whose strength for the single bonds may fluctuate.
Systems with up to 100 electrons are investigated, their wave functions are
analyzed, and relevant length-scales for the low-lying optical excitations are
identified. The presented approach provides a concise picture for the size
dependence of the optical absorption in oligoenes.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
Multi-band Gutzwiller wave functions for general on-site interactions
We introduce Gutzwiller wave functions for multi-band models with general
on-site Coulomb interactions. As these wave functions employ correlators for
the exact atomic eigenstates they are exact both in the non-interacting and in
the atomic limit. We evaluate them in infinite lattice dimensions for all
interaction strengths without any restrictions on the structure of the
Hamiltonian or the symmetry of the ground state. The results for the
ground-state energy allow us to derive an effective one-electron Hamiltonian
for Landau quasi-particles, applicable for finite temperatures and frequencies
within the Fermi-liquid regime. As applications for a two-band model we study
the Brinkman-Rice metal-to-insulator transition at half band-filling, and the
transition to itinerant ferromagnetism for two specific fillings, at and close
to a peak in the density of states of the non-interacting system. Our new
results significantly differ from those for earlier Gutzwiller wave functions
where only density-type interactions were included. When the correct spin
symmetries for the two-electron states are taken into account, the importance
of the Hund's-rule exchange interaction is even more pronounced and leads to
paramagnetic metallic ground states with large local magnetic moments.
Ferromagnetism requires fairly large interaction strengths, and the resulting
ferromagnetic state is a strongly correlated metal.Comment: 37 pages, 10 figures; accepted for publication in Phys. Rev. B 57
(March 15, 1998
Nearly universal crossing point of the specific heat curves of Hubbard models
A nearly universal feature of the specific heat curves C(T,U) vs. T for
different U of a general class of Hubbard models is observed. That is, the
value C_+ of the specific heat curves at their high-temperature crossing point
T_+ is almost independent of lattice structure and spatial dimension d, with
C_+/k_B \approx 0.34. This surprising feature is explained within second order
perturbation theory in U by identifying two small parameters controlling the
value of C_+: the integral over the deviation of the density of states
N(\epsilon) from a constant value, characterized by \delta N=\int d\epsilon
|N(\epsilon)-1/2|, and the inverse dimension, 1/d.Comment: Revtex, 9 pages, 6 figure
Improving the genetic system for Halorubrum lacusprofundi to allow in-frame deletions
Halorubrum lacusprofundi is a cold-adapted halophilic archaeon isolated from Deep Lake, Antarctica. Hrr. lacusprofundi is commonly used to study adaptation to cold environments and thereby a potential source for biotechnological products. Additionally, in contrast to other haloarchaeal model organisms, Hrr. lacusprofundi is also susceptible to a range of different viruses and virus-like elements, making it a great model to study virus-host interactions in a cold-adapted organism. A genetic system has previously been reported for Hrr. lacusprofundi; however, it does not allow in-frame deletions and multiple gene knockouts. Here, we report the successful generation of uracil auxotrophic (pyrE2) mutants of two strains of Hrr. lacusprofundi. Subsequently, we attempted to generate knockout mutants using the auxotrophic marker for selection. However, surprisingly, only the combination of the auxotrophic marker and antibiotic selection allowed the timely and clean in-frame deletion of a target gene. Finally, we show that vectors established for the model organism Haloferax volcanii are deployable for genetic manipulation of Hrr. lacusprofundi, allowing the use of the portfolio of genetic tools available for H. volcanii in Hrr. lacusprofundi
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
Exact results for the optical absorption of strongly correlated electrons in a half-filled Peierls-distorted chain
In this second of three articles on the optical absorption of electrons in a
half-filled Peierls-distorted chain we present exact results for strongly
correlated tight-binding electrons. In the limit of a strong on-site
interaction we map the Hubbard model onto the Harris-Lange model which can
be solved exactly in one dimension in terms of spinless fermions for the charge
excitations. The exact solution allows for an interpretation of the charge
dynamics in terms of parallel Hubbard bands with a free-electron dispersion of
band-width , separated by the Hubbard interaction . The spin degrees of
freedom enter the expressions for the optical absorption only via a momentum
dependent but static ground state expectation value. The remaining spin problem
can be traced out exactly since the eigenstates of the Harris-Lange model are
spin-degenerate. This corresponds to the Hubbard model at temperatures large
compared to the spin exchange energy. Explicit results are given for the
optical absorption in the presence of a lattice distortion and a
nearest-neighbor interaction . We find that the optical absorption for
is dominated by a peak at and broad but weak absorption bands for . For an appreciable nearest-neighbor interaction, ,
almost all spectral weight is transferred to Simpson's exciton band which is
eventually Peierls-split.Comment: 50 pages REVTEX 3.0, 6 postscript figures; hardcopy versions before
May 96 are obsolete; accepted for publication in The Philosophical Magazine
Nonlinear Optical Response of Spin Density Wave Insulators
We calculate the third order nonlinear optical response in the Hubbard model
within the spin density wave (SDW) mean field ansatz in which the gap is due to
onsite Coulomb repulsion. We obtain closed-form analytical results in one
dimension (1D) and two dimension (2D), which show that nonlinear optical
response in SDW insulators in 2D is stronger than both 3D and 1D. We also
calculate the two photon absorption (TPA) arising from the stress tensor term.
We show that in the SDW, the contribution from stress tensor term to the
low-energy peak corresponding to two photon absorption becomes identically zero
if we consider the gauge invariant current properly.Comment: we use \psfrag in figur
Charge-Transfer Excitations in One-Dimensional Dimerized Mott Insulators
We investigate the optical properties of one-dimensional (1D) dimerized Mott
insulators using the 1D dimerized extended Hubbard model. Numerical
calculations and a perturbative analysis from the decoupled-dimer limit clarify
that there are three relevant classes of charge-transfer (CT) states generated
by photoexcitation: interdimer CT unbound states, interdimer CT exciton states,
and intradimer CT exciton states. This classification is applied to
understanding the optical properties of an organic molecular material,
1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its
photoinduced transition from the dimerized spin-singlet phase to the regular
paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is
the interdimer CT exciton state and the second lowest state is the intradimer
CT exciton state.Comment: 6 pages, 6 figures, to be published in J. Phys. Soc. Jp
Time-Dependent Gutzwiller Theory for Multiband Hubbard Models
Based on the variational Gutzwiller theory, we present a method for the
computation of response functions for multiband Hubbard models with general
local Coulomb interactions. The improvement over the conventional random-phase
approximation is exemplified for an infinite-dimensional two-band Hubbard model
where the incorporation of the local multiplet-structure leads to a much larger
sensitivity of ferromagnetism on the Hund coupling. Our method can be
implemented into LDA+Gutzwiller schemes and will therefore be an important tool
for the computation of response functions for strongly correlated materials.Comment: 4 pages, 3 figure
Soft elasticity in biaxial smectic and smectic-C elastomers
Ideal (monodomain) smectic- elastomers crosslinked in the smectic-
phase are simply uniaxial rubbers, provided deformations are small. From these
materials smectic- elastomers are produced by a cooling through the
smectic- to smectic- phase transition. At least in principle, biaxial
smectic elastomers could also be produced via cooling from the smectic- to a
biaxial smectic phase. These phase transitions, respectively from to and from to symmetry, spontaneously
break the rotational symmetry in the smectic planes. We study the above
transitions and the elasticity of the smectic- and biaxial phases in three
different but related models: Landau-like phenomenological models as functions
of the Cauchy--Saint-Laurent strain tensor for both the biaxial and the
smectic- phases and a detailed model, including contributions from the
elastic network, smectic layer compression, and smectic- tilt for the
smectic- phase as a function of both strain and the -director. We show
that the emergent phases exhibit soft elasticity characterized by the vanishing
of certain elastic moduli. We analyze in some detail the role of spontaneous
symmetry breaking as the origin of soft elasticity and we discuss different
manifestations of softness like the absence of restoring forces under certain
shears and extensional strains.Comment: 26 pages, 6 figure
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