32 research outputs found
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
Thermodynamics of multiferroic spin chains
The minimal model to describe many spin chain materials with ferroelectric
properties is the Heisenberg model with ferromagnetic nearest neighbor coupling
J1 and antiferromagnetic next-nearest neighbor coupling J2. Here we study the
thermodynamics of this model using a density-matrix algorithm applied to
transfer matrices. We find that the incommensurate spin-spin correlations -
crucial for the ferroelectric properties and the analogue of the classical
spiral pitch angle - depend not only on the ratio J2/|J1| but also strongly on
temperature. We study small easy-plane anisotropies which can stabilize a
vector chiral order as well as the finite-temperature signatures of multipolar
phases, stable at finite magnetic field. Furthermore, we fit the
susceptibilities of LiCuVO4, LiCu2O2, and Li2ZrCuO4. Contrary to the
literature, we find that for LiCuVO4 the best fit is obtained with J2 ~ 90 K
and J2/|J1| ~ 0.5 and show that these values are consistent with the observed
spin incommensurability. Finally, we discuss our findings concerning the
incommensurate spin-spin correlations and multipolar orders in relation to
future experiments on these compounds.Comment: 10 pages, Fig. 1 modified + other minor change
Effects of Two Energy Scales in Weakly Dimerized Antiferromagnetic Quantum Spin Chains
By means of thermal expansion and specific heat measurements on the
high-pressure phase of (VO)PO, the effects of two energy scales of
the weakly dimerized antiferromagnetic = 1/2 Heisenberg chain are explored.
The low energy scale, given by the spin gap , is found to manifest
itself in a pronounced thermal expansion anomaly. A quantitative analysis,
employing T-DMRG calculations, shows that this feature originates from changes
in the magnetic entropy with respect to , . This term, inaccessible by specific heat, is visible only in the
weak-dimerization limit where it reflects peculiarities of the excitation
spectrum and its sensitivity to variations in .Comment: 4 pages, 4 figures now identical with finally published versio
Dynamical correlation functions of the XXZ model at finite temperature
Combining a lattice path integral formulation for thermodynamics with the
solution of the quantum inverse scattering problem for local spin operators, we
derive a multiple integral representation for the time-dependent longitudinal
correlation function of the spin-1/2 Heisenberg XXZ chain at finite temperature
and in an external magnetic field. Our formula reproduces the previous results
in the following three limits: the static, the zero-temperature and the XY
limits.Comment: 22 pages, v4: typos corrected, published versio