264 research outputs found
Spectral properties of the 2D Holstein t-J model
Employing the Lanczos algorithm in combination with a kernel polynomial
moment expansion (KPM) and the maximum entropy method (MEM), we show a way of
calculating charge and spin excitations in the Holstein t-J model, including
the full quantum nature of phonons. To analyze polaron band formation we
evaluate the hole spectral function for a wide range of electron-phonon
coupling strengths. For the first time, we present results for the optical
conductivity of the 2D Holstein t-J model.Comment: 2 pages, Latex. Submitted to Physica C, Proc. Int. Conf. on M2HTSC
Polaronic effects in strongly coupled electron-phonon systems: Exact diagonalization results for the 2D Holstein t-J model
Ground-state and dynamical properties of the 2D Holstein t-J model are
examined by means of direct Lanczos diagonalization, using a truncation method
of the phononic Hilbert space. The single-hole spectral function shows the
formation of a narrow hole-polaron band as the electron-phonon coupling
increases, where the polaronic band collapse is favoured by strong Coulomb
correlations. In the two-hole sector, the hole-hole correlations unambiguously
indicate the existence of inter-site bipolaronic states. At quarter-filling, a
polaronic superlattice is formed in the adiabatic strong-coupling regime.Comment: 3 pages, LaTeX, 6 Postscript figures, Proc. Int. Conf. on Strongly
Correlated Electron Systems, Zuerich, August 1996, accepted for publication
in Physica
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
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
Magnetic and lattice polaron in Holstein-t-J model
We investigate the interplay between the formation of lattice and magnetic
polaron in the case of a single hole in the antiferromagnetic background. We
present an exact analytical solution of the Holstein-t-J model in infinite
dimensions. Ground state energy, electron-lattice correlation function, spin
bag dimension as well as spectral properties are calculated. The magnetic and
hole-lattice correlations sustain each other, i.e. the presence of
antiferromagnetic correlations favors the formation of the lattice polaron at
lower value of the electron-phonon coupling while the polaronic effect
contributes to reduce the number of spin defects in the antiferromagnetic
background. The crossover towards a spin-lattice small polaron region of the
phase diagram becomes a discontinuous transition in the adiabatic limit.Comment: revtex, 8 eps figures included NEW version. Appendix with a full
proof include
Quantum lattice dynamical effects on the single-particle excitations in 1D Mott and Peierls insulators
As a generic model describing quasi-one-dimensional Mott and Peierls
insulators, we investigate the Holstein-Hubbard model for half-filled bands
using numerical techniques. Combining Lanczos diagonalization with Chebyshev
moment expansion we calculate exactly the photoemission and inverse
photoemission spectra and use these to establish the phase diagram of the
model. While polaronic features emerge only at strong electron-phonon
couplings, pronounced phonon signatures, such as multi-quanta band states, can
be found in the Mott insulating regime as well. In order to corroborate the
Mott to Peierls transition scenario, we determine the spin and charge
excitation gaps by a finite-size scaling analysis based on density-matrix
renormalization group calculations.Comment: 5 pages, 5 figure
Peierls Dimerization with Non-Adiabatic Spin-Phonon Coupling
We study the magnetic properties of a frustrated Heisenberg spin chain with a
dynamic spin-phonon interaction. By Lanczos diagonalization, preserving the
full lattice dynamics, we explore the non-adiabatic regime with phonon
frequencies comparable to the exchange coupling energy which is e.g. the
relevant limit for the spin-Peierls compound . When compared to the
static limit of an alternating spin chain the magnetic properties are strongly
renormalized due to the coupled dynamics of spin and lattice degrees of
freedom. The magnitude of the spin triplet excitation gap changes from a strong
to a weak dimerization dependence with increasing phonon frequencies implying
the necessity to include dynamic effects in an attempt for a quantitative
description of the spin-Peierls state.Comment: 4 pages, 5 figure
Berry phases and pairing symmetry in Holstein-Hubbard polaron systems
We study the tunneling dynamics of dopant-induced hole polarons which are
self-localized by electron-phonon coupling in a two-dimensional antiferro-
magnet. Our treatment is based on a path integral formulation of the adia-
batic approximation, combined with many-body tight-binding, instanton, con-
strained lattice dynamics, and many-body exact diagonalization techniques. Our
results are mainly based on the Holstein- and, for comparison, on the
Holstein-Hubbard model. We also study effects of 2nd neighbor hopping and
long-range electron-electron Coulomb repulsion. The polaron tunneling dynamics
is mapped onto an effective low-energy Hamiltonian which takes the form of a
fermion tight-binding model with occupancy dependent, predominant- ly 2nd and
3rd neighbor tunneling matrix elements, excluded double occupan- cy, and an
effective intersite charge interactions. Antiferromagnetic spin correlations in
the original many-electron Hamiltonian are reflected by an attractive
contribution to the 1st neighbor charge interaction and by Berry phase factors
which determine the signs of effective polaron tunneling ma- trix elements. In
the two-polaron case, these phase factors lead to polaron pair wave functions
of either -wave symmetry or p-wave symme- try with zero and
nonzero total pair momentum, respectively. Implications for the doping
dependent isotope effect, pseudo-gap and Tc of a superconduc- ting polaron pair
condensate are discussed/compared to observed in cuprates.Comment: 23 pages, revtex, 13 ps figure
Optical absorption and single-particle excitations in the 2D Holstein t-J model
To discuss the interplay of electronic and lattice degrees of freedom in
systems with strong Coulomb correlations we have performed an extensive
numerical study of the two-dimensional Holstein t-J model. The model describes
the interaction of holes, doped in a quantum antiferromagnet, with a
dispersionsless optical phonon mode. We apply finite-lattice Lanczos
diagonalization, combined with a well-controlled phonon Hilbert space
truncation, to the Hamiltonian. The focus is on the dynamical properties. In
particular we have evaluated the single-particle spectral function and the
optical conductivity for characteristic hole-phonon couplings, spin exchange
interactions and phonon frequencies. The results are used to analyze the
formation of hole polarons in great detail. Links with experiments on layered
perovskites are made. Supplementary we compare the Chebyshev recursion and
maximum entropy algorithms, used for calculating spectral functions, with
standard Lanczos methods.Comment: 32 pages, 12 figures, submitted to Phys. Rev.
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