22 research outputs found
Ground State Structure and Low Temperature Behaviour of an Integrable Chain with Alternating Spins
In this paper we continue the investigation of an anisotropic integrable spin
chain, consisting of spins and , started in our paper
\cite{meissner}. The thermodynamic Bethe ansatz is analysed especially for the
case, when the signs of the two couplings and differ. For
the conformally invariant model () we have calculated heat
capacity and magnetic susceptibility at low temperature. In the isotropic limit
our analysis is carried out further and susceptibilities are calculated near
phase transition lines (at ).Comment: 22 pages, LaTeX, uses ioplppt.sty and PicTeX macro
Ground state and low excitations of an integrable chain with alternating spins
An anisotropic integrable spin chain, consisting of spins and
, is investigated \cite{devega}. It is characterized by two real
parameters and , the coupling constants of the spin
interactions. For the case and the ground state
configuration is obtained by means of thermodynamic Bethe ansatz. Furthermore
the low excitations are calculated. It turns out, that apart from free magnon
states being the holes in the ground state rapidity distribution, there exist
bound states given by special string solutions of Bethe ansatz equations (BAE)
in analogy to \cite{babelon}. The dispersion law of these excitations is
calculated numerically.Comment: 16 pages, LaTeX, uses ioplppt.sty and PicTeX macro
Integrable models of coupled Heisenberg chains
We show that the solutions of the Yang--Baxter equation invariant under the
action of the Yangian lead to inhomogenous vertex models. Starting
from a four dimensional representation of we obtain an integrable
family of coupled Heisenberg spin- chains. Some thermodynamical
properties of this model are studied by means of the algebraic Bethe Ansatz.Comment: 10 pages, latex, 5 postscript figure
Excitations and S-matrix for su(3) spin chain combining and ${3^{*}}
The associated Hamiltonian for a su(3) spin chain combining and
representations is calculated. The ansatz equations for this chain
are obtained and solved in the thermodynamic limit, and the ground state and
excitations are described. Thus, relations between the number of roots and the
number of holes in each level have been found . The excited states are
characterized by means of these quantum numbers. Finally, the exact S matrix
for a state with two holes is found.Comment: 17 pages, plaintex, harvmac (to be published in J. of Phys. A
Thermodynamical limit of general gl(N) spin chains: vacuum state and densities
We study the vacuum state of spin chains where each site carry an arbitrary
representation. We prove that the string hypothesis, usually used to solve the
Bethe ansatz equations, is valid for representations characterized by
rectangular Young tableaux. In these cases, we obtain the density of the center
of the strings for the vacuum. We work out different examples and, in
particular, the spin chains with periodic array of impurities.Comment: Latex file, 27 pages, 5 figures (.eps) A more detailed study of the
representations allowing string hypothesis has added. A simpler formula for
the densities is given. References added and misprint correcte
The thermal conductivity of alternating spin chains
We study a class of integrable alternating (S1,S2) quantum spin chains with
critical ground state properties. Our main result is the description of the
thermal Drude weight of the one-dimensional alternating spin chain as a
function of temperature. We have identified the thermal current of the model
with alternating spins as one of the conserved currents underlying the
integrability. This allows for the derivation of a finite set of non-linear
integral equations for the thermal conductivity. Numerical solutions to the
integral equations are presented for specific cases of the spins S1 and S2. In
the low-temperature limit a universal picture evolves where the thermal Drude
weight is proportional to temperature T and central charge c.Comment: 15 pages, 1 figur
Mixing of Ground States in Vertex Models
We consider the analogue of the 6-vertex model constructed from alternating
spin n/2 and spin m/2 lines, where . We identify the transfer matrix
and the space on which it acts in terms of the representation theory of
. We diagonalise the transfer matrix and compute the S-matrix. We
give a trace formula for local correlation functions. When n=1, the 1-point
function of a spin m/2 local variable for the alternating lattice with a
particular ground state is given as a linear combination of the 1-point
functions of the pure spin m/2 model with different ground states. The mixing
ratios are calculated exactly and are expressed in terms of irreducible
characters of and the deformed Virasoro algebra.Comment: 12 pages, LaTeX, typos correcte
Elementary Excitations of Heisenberg Ferrimagnetic Spin Chains
We numerically investigate elementary excitations of the Heisenberg
alternating-spin chains with two kinds of spins 1 and 1/2 antiferromagnetically
coupled to each other. Employing a recently developed efficient Monte Carlo
technique as well as an exact diagonalization method, we verify the spin-wave
argument that the model exhibits two distinct excitations from the ground state
which are gapless and gapped. The gapless branch shows a quadratic dispersion
in the small-momentum region, which is of ferromagnetic type. With the
intention of elucidating the physical mechanism of both excitations, we make a
perturbation approach from the decoupled-dimer limit. The gapless branch is
directly related to spin 1's, while the gapped branch originates from
cooperation of the two kinds of spins.Comment: 7 pages, 7 Postscript figures, RevTe
Critical Behaviour of integrable mixed spins chains
We construct a mixed spin 1/2 and integrable model and investigate its
finite size properties. For a certain conformal invariant mixed spin system the
central charge can be decomposed in terms of the conformal anomaly of two
single integrable models of spin 1/2 and spin . We also compute the
ground state energy and the sound velocity in the thermodynamic limit.Comment: This was the first correct calculation of the central charge in mixed
integrable spin chains. For effects of a magnetic field see
J.Phys.A:Math.Gen. 26 (1993) 730
Combination of Ferromagnetic and Antiferromagnetic Features in Heisenberg Ferrimagnets
We investigate the thermodynamic properties of Heisenberg ferrimagnetic
mixed-spin chains both numerically and analytically with particular emphasis on
the combination of ferromagnetic and antiferromagnetic features. Employing a
new density-matrix renormalization-group technique as well as a quantum Monte
Carlo method, we reveal the overall thermal behavior: At very low temperatures,
the specific heat and the magnetic susceptibility times temperature behave like
and , respectively, whereas at intermediate temperatures,
they exhibit a Schottky-like peak and a minimum, respectively. Developing the
modified spin-wave theory, we complement the numerical findings and give a
precise estimate of the low-temperature behavior.Comment: 9 pages, 9 postscript figures, RevTe