Dispersion relations and speeds of sound in special sectors for the integrable chain with alternating spins

Based on our previous analysis \cite{doerfel3} of the anisotropic integrable chain consisting of spins $s=1/2$ and $s=1$ we compare the dispersion relations for the sectors with infinite Fermi zones. Further we calculate the speeds of sound for regions close to sector borders, where the Fermi radii either vanish or diverge, and compare the results.Comment: 11 pages, LaTeX2e, uses iopart.cls,graphicx.sty and psfrag.sty, 2 figure

Complete phase diagram for the integrable chain with alternating spins in the sectors with competing interactions

We investigate the anisotropic integrable spin chain consisting of spins $s={1/2}$ and $s=1$ by means of thermodynamic Bethe ansatz for the anisotropy $\gamma>\pi/3$, where the analysis of the Takahashi conditions leads to a more complicated string picture. We give the phase diagram with respect to the two real coupling constants $\bar{c}$ and $\tilde{c}$, which contains a new region where the ground state is formed by strings with infinite Fermi zones. In this region the velocities of sound for the two physical excitations have been calculated from the dressed energies. This leads to an additional line of conformal invariance not known before.Comment: 13 pages, LaTeX, uses ioplppt.sty and epsfig.sty, figure 3 correcte

Non-commutative Euclidean structures in compact spaces

Based on results for real deformation parameter q we introduce a compact non- commutative structure covariant under the quantum group SOq(3) for q being a root of unity. To match the algebra of the q-deformed operators with necesarry conjugation properties it is helpful to define a module over the algebra genera- ted by the powers of q. In a representation where X is diagonal we show how P can be calculated. To manifest some typical properties an example of a one-di- mensional q-deformed Heisenberg algebra is also considered and compared with non-compact case.Comment: Changed conten

Existence of the magnetization plateau in a class of exactly solvable Ising-Heisenberg chains

The mapping transformation technique is applied to obtain exact results for the spin-1/2 and spin-S (S=1/2,1) Ising-Heisenberg antiferromagnetic chain in the presence of an external magnetic field. Within this scheme, a field-induced first-order metamagnetic transition resulting in multiplateau magnetization curves, is investigated in detail. It is found that the scenario of the plateau formation depends fundamentally on the ratio between Ising and Heisenbrg interaction constants, as well as on the anisotropy strength of the XXZ Heisenberg interaction.Comment: 16 pages, 10 figures, submitted to J. Phys: Condens. Matte

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 $s=1$ and $s=\frac{1}{2}$, started in our paper \cite{meissner}. The thermodynamic Bethe ansatz is analysed especially for the case, when the signs of the two couplings $\bar{c}$ and $\tilde{c}$ differ. For the conformally invariant model ($\bar{c}=\tilde{c}$) 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 $T=0$).Comment: 22 pages, LaTeX, uses ioplppt.sty and PicTeX macro

Mixed Heisenberg Chains. I. The Ground State Problem

We consider a mechanism for competing interactions in alternating Heisenberg spin chains due to the formation of local spin-singlet pairs. The competition of spin-1 and spin-0 states reveals hidden Ising symmetry of such alternating chains.Comment: 7 pages, RevTeX, 4 embedded eps figures, final versio

Ground state and low excitations of an integrable chain with alternating spins

An anisotropic integrable spin chain, consisting of spins $s=1$ and $s=\frac{1}{2}$, is investigated \cite{devega}. It is characterized by two real parameters $\bar{c}$ and $\tilde{c}$, the coupling constants of the spin interactions. For the case $\bar{c}<0$ and $\tilde{c}<0$ 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

Thermodynamics and conformal properties of XXZ chains with alternating spins

The quantum periodic XXZ chain with alternating spins is studied. The properties of the related R-matrix and Hamiltonians are discussed. A compact expression for the ground state energy is obtained. The corresponding conformal anomaly is found via the finite-size computations and also by means of the Bethe ansatz method. In the presence of an external magnetic field, the magnetic susceptibility is derived. The results are also generalized to the case of a chain containing several different spins.Comment: 28 pages, LaTeX2