117 research outputs found
Green-Function-Based Monte Carlo Method for Classical Fields Coupled to Fermions
Microscopic models of classical degrees of freedom coupled to non-interacting
fermions occur in many different contexts. Prominent examples from solid state
physics are descriptions of colossal magnetoresistance manganites and diluted
magnetic semiconductors, or auxiliary field methods for correlated electron
systems. Monte Carlo simulations are vital for an understanding of such
systems, but notorious for requiring the solution of the fermion problem with
each change in the classical field configuration. We present an efficient,
truncation-free O(N) method on the basis of Chebyshev expanded local Green
functions, which allows us to simulate systems of unprecedented size N.Comment: 4 pages, 3 figure
Bose-Einstein condensation of magnons in TlCuCl
A quantitative study of the field-induced magnetic ordering in TlCuCl in
terms of a Bose-Einstein condensation (BEC) of magnons is presented. It is
shown that the hitherto proposed simple BEC scenario is in quantitative and
qualitative disagreement with experiment. It is further shown that even very
small Dzyaloshinsky-Moriya interactions or a staggered tensor component of
a certain type can change the BEC picture qualitatively. Such terms lead to a
nonzero condensate density for all temperatures and a gapped quasiparticle
spectrum. Including this type of interaction allows us to obtain good agreement
with experimental data.Comment: 2 pages, 2 figures, submitted to SCES'0
Bipolarons from long range interactions: Singlet and triplet pairs in the screened Hubbard-Froehlich model on the chain
We present details of a continuous-time quantum Monte-Carlo algorithm for the
screened Hubbard-Froehlich bipolaron. We simulate the bipolaron in one
dimension with arbitrary interaction range in the presence of Coulomb
repulsion, computing the effective mass, binding energy, total number of
phonons associated with the bipolaron, mass isotope exponent and bipolaron
radius in a comprehensive survey of the parameter space. We discuss the role of
the range of the electron-phonon interaction, demonstrating the evolution from
Holstein to Froehlich bipolarons and we compare the properties of bipolarons
with singlet and triplet pairing. Finally, we present simulations of the
bipolaron dispersion. The band width of the Froehlich bipolaron is found to be
broad, and the decrease in bandwidth as the two polarons bind into a bipolaron
is found to be far less rapid than in the case of the Holstein interaction. The
properties of bipolarons formed from long range electron-phonon interactions,
such as light strongly bound bipolarons and intersite pairing when Coulomb
repulsion is large, are found to be robust against screening, with qualitative
differences between Holstein and screened Froehlich bipolarons found even for
interactions screened within a single lattice site.Comment: 20 pages, 17 figure
On the absorption of microwaves by the one-dimensional spin-1/2 Heisenberg-Ising magnet
We analyze the absorption of microwaves by the Heisenberg-Ising chain
combining exact calculations, based on the integrability of the model, with
numerical calculations. Within linear response theory the absorbed intensity is
determined by the imaginary part of the dynamical susceptibility. The moments
of the normalized intensity can be used to define the shift of the resonance
frequency induced by the interactions and the line width independently of the
shape of the spectral line. These moments can be calculated exactly as
functions of temperature and strength of an external magnetic field, as long as
the field is directed along the symmetry axis of the chain. This allows us to
discuss the line width and the resonance shift for a given magnetic field in
the full range of possible anisotropy parameters. For the interpretation of
these data we need a qualitative knowledge of the line shape which we obtain
from fully numerical calculations for finite chains. Exact analytical results
on the line shape are out of reach of current theories. From our numerical work
we could extract, however, an empirical parameter-free model of the line shape
at high temperatures which is extremely accurate over a wide range of
anisotropy parameters and is exact at the free fermion point and at the
isotropic point. Another prediction of the line shape can be made in the
zero-temperature and zero magnetic field limit, where the sufficiently
anisotropic model shows strong absorption. For anisotropy parameters in the
massive phase we derive the exact two-spinon contribution to the spectral line.
From the intensity sum rule it can be estimated that this contribution accounts
for more than 80% of the spectral weight if the anisotropy parameter is
moderately above its value at the isotropic point.Comment: 23 pages, revtex4-1, 11 figure
Density-Matrix Algorithm for Phonon Hilbert Space Reduction in the Numerical Diagonalization of Quantum Many-Body Systems
Combining density-matrix and Lanczos algorithms we propose a new optimized
phonon approach for finite-cluster diagonalizations of interacting
electron-phonon systems. To illustrate the efficiency and reliability of our
method, we investigate the problem of bipolaron band formation in the extended
Holstein Hubbard model.Comment: 14 pages, 6 figures, Workshop on High Performance Computing in
Science and Engineering, Stuttgart 200
Microscopic modelling of doped manganites
Colossal magneto-resistance manganites are characterised by a complex
interplay of charge, spin, orbital and lattice degrees of freedom. Formulating
microscopic models for these compounds aims at meeting to conflicting
objectives: sufficient simplification without excessive restrictions on the
phase space. We give a detailed introduction to the electronic structure of
manganites and derive a microscopic model for their low energy physics.
Focussing on short range electron-lattice and spin-orbital correlations we
supplement the modelling with numerical simulations.Comment: 20 pages, 10 figs, accepted for publ. in New J. Phys., Focus issue on
Orbital Physic
Quantum lattice fluctuations in a frustrated Heisenberg spin-Peierls chain
As a simple model for spin-Peierls systems we study a frustrated Heisenberg
chain coupled to optical phonons. In view of the anorganic spin-Peierls
compound CuGeO3 we consider two different mechanisms of spin-phonon coupling.
Combining variational concepts in the adiabatic regime and perturbation theory
in the anti-adiabatic regime we derive effective spin Hamiltonians which cover
the dynamical effect of phonons in an approximate way. Ground-state phase
diagrams of these models are determined, and the effect of frustration is
discussed. Comparing the properties of the ground state and of low-lying
excitations with exact diagonalization data for the full quantum spin phonon
models, good agreement is found especially in the anti-adiabatic regime.Comment: 9 pages, 7 figures included, submitted to Phys. Rev.
Considerations on the quantum double-exchange Hamiltonian
Schwinger bosons allow for an advantageous representation of quantum
double-exchange. We review this subject, comment on previous results, and
address the transition to the semiclassical limit. We derive an effective
fermionic Hamiltonian for the spin-dependent hopping of holes interacting with
a background of local spins, which is used in a related publication within a
two-phase description of colossal magnetoresistant manganites.Comment: 7 pages, 3 figure
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