358 research outputs found
Numerical Studies of the two-leg Hubbard ladder
The Hubbard model on a two-leg ladder structure has been studied by a
combination of series expansions at T=0 and the density-matrix renormalization
group. We report results for the ground state energy and spin-gap
at half-filling, as well as dispersion curves for one and two-hole
excitations. For small both and show a dramatic drop near
, which becomes more gradual for larger . This
represents a crossover from a "band insulator" phase to a strongly correlated
spin liquid. The lowest-lying two-hole state rapidly becomes strongly bound as
increases, indicating the possibility that phase separation may
occur. The various features are collected in a "phase diagram" for the model.Comment: 10 figures, revte
Density matrix renormalisation group for a quantum spin chain at non-zero temperature
We apply a recent adaptation of White's density matrix renormalisation group
(DMRG) method to a simple quantum spin model, the dimerised chain, in
order to assess the applicabilty of the DMRG to quantum systems at non-zero
temperature. We find that very reasonable results can be obtained for the
thermodynamic functions down to low temperatures using a very small basis set.
Low temperature results are found to be most accurate in the case when there is
a substantial energy gap.Comment: 6 pages, Standard Latex File + 7 PostScript figures available on
reques
Quantized Lattice Dynamic Effects on the Spin-Peierls Transition
The density matrix renormalization group method is used to investigate the
spin-Peierls transition for Heisenberg spins coupled to quantized phonons. We
use a phonon spectrum that interpolates between a gapped, dispersionless
(Einstein) limit to a gapless, dispersive (Debye) limit. A variety of
theoretical probes are used to determine the quantum phase transition,
including energy gap crossing, a finite size scaling analysis, bond order
auto-correlation functions, and bipartite quantum entanglement. All these
probes indicate that in the antiadiabatic phonon limit a quantum phase
transition of the Berezinskii-Kosterlitz-Thouless type is observed at a
non-zero spin-phonon coupling, . An extrapolation from the
Einstein limit to the Debye limit is accompanied by an increase in for a fixed optical () phonon gap. We therefore conclude that the
dimerized ground state is more unstable with respect to Debye phonons, with the
introduction of phonon dispersion renormalizing the effective spin-lattice
coupling for the Peierls-active mode. We also show that the staggered spin-spin
and phonon displacement order parameters are unreliable means of determining
the transition.Comment: To be published in Phys. Rev.
Onset of incommensurability in quantum spin chains
In quantum spin chains, it has been observed that the incommensurability
occurs near valence-bond-solid (VBS)-type solvable points, and the correlation
length becomes shortest at VBS-type points. Besides, the correlation function
decays purely exponentially at VBS-type points, in contrast with the
two-dimensional (2D) Ornstein-Zernicke type behavior in the other region with
an excitation gap. We propose a mechanism to explain the onset of the
incommensurability and the shortest correlation length at VBS-like points. This
theory can be applicable for more general cases.Comment: 9 pages, 2 figure
Numerical and approximate analytical results for the frustrated spin-1/2 quantum spin chain
We study the frustrated phase of the quantum spin-
system with nearest-neighbour and next-nearest-neighbour isotropic exchange
known as the Majumdar-Ghosh Hamiltonian. We first apply the coupled-cluster
method of quantum many-body theory based on a spiral model state to obtain the
ground state energy and the pitch angle. These results are compared with
accurate numerical results using the density matrix renormalisation group
method, which also gives the correlation functions. We also investigate the
periodicity of the phase using the Marshall sign criterion. We discuss
particularly the behaviour close to the phase transitions at each end of the
frustrated phase.Comment: 17 pages, Standard Latex File + 7 PostScript figures in separate
file. Figures also can also be requested from [email protected]
- …