217 research outputs found
Real-time dynamics at finite temperature by DMRG: A path-integral approach
We propose a path-integral variant of the DMRG method to calculate real-time
correlation functions at arbitrary finite temperatures. To illustrate the
method we study the longitudinal autocorrelation function of the -chain.
By comparison with exact results at the free fermion point we show that our
method yields accurate results up to a limiting time which is determined by the
spectrum of the reduced density matrix.Comment: 5 pages, 4 figure
Effects of an external magnetic field on the gaps and quantum corrections in an ordered Heisenberg antiferromagnet with Dzyaloshinskii-Moriya anisotropy
We study the effects of external magnetic field on the properties of an
ordered Heisenberg antiferromagnet with the Dzyaloshinskii-Moriya (DM)
interaction. Using the spin-wave theory quantum correction to the energy,
on-site magnetization, and uniform magnetization are calculated as a function
of the field H and the DM anisotropy constant D. It is shown that the spin-wave
excitations exhibit an unusual field-evolution of the gaps. This leads to
various non-analytic dependencies of the quantum corrections on H and D. It is
also demonstrated that, quite generally, the DM interaction suppresses quantum
fluctuations, thus driving the system to a more classical ground state. Most of
the discussion is devoted to the spin-S, two-dimensional square lattice
antiferromagnet, whose S=1/2 case is closely realized in K2V3O8 where at H=0
the DM anisotropy is hidden by the easy-axis anisotropy but is revealed in a
finite field. The theoretical results for the field-dependence of the
spin-excitation gaps in this material are presented and the implications for
other systems are discussed.Comment: 15+ pages, 14 Figure
Temperature driven crossover phenomena in the correlation lengths of the one-dimensional t-J model
We describe a modified transfer matrix renormalization group (TMRG) algorithm
and apply it to calculate thermodynamic properties of the one-dimensional t-J
model. At the supersymmetric point we compare with Bethe ansatz results and
make direct connection to conformal field theory (CFT). In particular we study
the crossover from the non-universal high T lattice into the quantum critical
regime by calculating various correlation lengths and static correlation
functions. Finally, the existence of a spin-gap phase is confirmed.Comment: 7 pages, 7 figure
Ground-state properties of two-dimensional dimerized Heisenberg models
The purpose of this paper is to investigate the ground-state properties of
two-dimensional Heisenberg models on a square lattice with a given
dimerization. Our aim is threefold: First, we want to investigate the
dimensional transition from two to one dimension for three models consisting of
weakly coupled chains for large dimerizations. Simple scaling arguments show
that the interchain coupling is always relevant. The ground states of two of
these models therefore have one-dimensional nature only at the decoupling
point. The third considered model is more complicated, because it contains
additional relevant intrachain couplings leading to a gap as shown by scaling
arguments and numerical investigations. Second, we investigate at which point
the dimerization destroys the N\'eel ordered ground state of the isotropic
model. Within a mapping to a nonlinear sigma-model and linear spinwave theory
(LSWT) we conclude that the stability of the N\'eel ordered state depends on
the microscopic details of the model. Third, the considered models also can be
regarded as effective models for a spin system with spin-phonon coupling. This
leads to the question if a spin-Peierls transition, i.e. a gain of total energy
due to lattice distortion, is possible. LSWT shows that such a transition is
possible under certain conditions leading to a coexistence of long-range order
and spin-Peierls dimerization. We also find that the gain of magnetic energy is
largest for a stair-like distortion of the lattice.Comment: 13 pages, 11 figures, revte
NMR Response in quasi one-dimensional Spin-1/2 Antiferromagnets
Non-magnetic impurities break a quantum spin chain into finite segments and
induce Friedel-like oscillations in the local susceptibility near the edges.
The signature of these oscillations has been observed in Knight shift
experiments on the high-temperature superconductor YBaCuO and
on the spin-chain compound SrCuO. Here we analytically calculate NMR
spectra, compare with the available experimental data for SrCuO, and
show that the interchain coupling is responsible for the complicated and so far
unexplained lineshape. Our results are based on a parameter-free formula for
the local susceptibility of a finite spin chain obtained by bosonization which
is checked by comparing with quantum Monte Carlo and density-matrix
renormalization group calculations.Comment: final versio
Doping a Mott insulator with orbital degrees of freedom
We study the effects of hole doping on one-dimensional Mott insulators with
orbital degrees of freedom. We describe the system in terms of a generalized
t-J model. At a specific point in parameter space the model becomes integrable
in analogy to the one-band supersymmetric t-J model. We use the Bethe ansatz to
derive a set of nonlinear integral equations which allow us to study the
thermodynamics exactly. Moving away from this special point in parameter space
we use the density-matrix renormalization group applied to transfer matrices to
study the evolution of various phases of the undoped system with doping and
temperature. Finally, we study a one-dimensional version of a realistic model
for cubic titanates which includes the anisotropy of the orbital sector due to
Hund's coupling. We find a transition from a phase with antiferromagnetically
correlated spins to a phase where the spins are fully ferromagnetically
polarized, a strong tendency towards phase separation at large Hund's coupling,
as well as the possibility of an instability towards triplet superconductivity
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