10 research outputs found
Mixed Heisenberg Chains. II. Thermodynamics
We consider thermodynamic properties, e.g. specific heat, magnetic
susceptibility, of alternating Heisenberg spin chains. Due to a hidden Ising
symmetry these chains can be decomposed into a set of finite chain fragments.
The problem of finding the thermodynamic quantities is effectively separated
into two parts. First we deal with finite objects, secondly we can incorporate
the fragments into a statistical ensemble. As functions of the coupling
constants, the models exhibit special features in the thermodynamic quantities,
e.g. the specific heat displays double peaks at low enough temperatures. These
features stem from first order quantum phase transitions at zero temperature,
which have been investigated in the first part of this work.Comment: 12 pages, RevTeX, 12 embedded eps figures, cf. cond-mat/9703206,
minor modification
Crystal Field, Magnetic Anisotropy and Excitations in Rare-Earth Hexaborides
We clarify the role of crystalline electric field (CEF) induced magnetic
anisotropy in the ground state and spin-wave spectrum of cubic rare-earth
materials with dominating isotropic magnetic exchange interactions. In
particular we study the hexaboride NdB_6 which is shown to exhibit strong
spin-quadrupolar coupling. The CEF scheme is analyzed and a non-collinear
magnetization response is found. The spin orientation in the
antiferromagnetically ordered ground-state is identified. Moreover, the spin
excitations are evaluated and in agreement with inelastic neutron scattering a
suppression of one of the two magnetic modes in the strong-coupling regime is
predicted.Comment: 4 pages, 1 eps-figur
Concerning Order and Disorder in the Ensemble of Cu-O Chain Fragments in Oxygen Deficient Planes of Y-Ba-Cu-O
In connection with numerous X-ray and neutron investigations of some high
temperature superconductors (YBaCuO and related compounds) a
non-trivial part of the structure factor, coming from partly disordered
Cu-O--O-Cu chain fragments, situated within basal planes, CuO, can
be a subject of theoretical interest. Closely connected to such a diffusive
part of the structure factor are the correlation lengths, which are also
available in neutron and X-ray diffraction studies and depend on a degree of
oxygen disorder in a basal plane. The quantitative measure of such a disorder
can be associated with temperature of a sample anneal, , at which oxygen
in a basal plane remains frozen-in high temperature equilibrium after a fast
quench of a sample to room or lower temperature. The structure factor evolution
with is vizualized in figures after the numerical calculations. The
theoretical approach employed in the paper has been developed for the
orthorhombic state of YBCO.Comment: Revtex, 27 pages, 14 PostScript figures upon request, ITP/GU/94/0
Model for the low-temperature magnetic phases observed in doped YBa_2Cu_3O_{6+x}
A classical statistical model for the antiferromagnetic (AFM) ordering of the
Cu-spins in the CuO_2 planes of reduced YBa_2Cu_3O_{6+x} type materials is
presented. The magnetic phases considered are the experimentally observed
high-temperature AFI phase with ordering vector Q_I=(1/2,1/2,0), and the
low-temperature phases: AFII with Q_II=(1/2,1/2,1/2) and intermediate TA (Turn
Angle) phases TAI, TAII and TAIII with components of both ordering vectors. It
is shown that the AFII and TA phases result from an effective ferromagnetic
(FM) type coupling mediated by free spins in the CuO_x basal plane. Good
agreement with experimental data is obtained for realistic model parameters.Comment: 11 pages, 2 Postscript figures, Submitted to Phys.Rev.Let
Quadrupolar and magnetic ordering in CeB6
The quadrupolar ordering in CeB_6 is explained in terms of the electrostatic
interaction of quadrupolar moments arranged into a simple cubic lattice. The
representation of magnetic and quadrupolar moments by means of quasispins of
two kinds is employed. A linear increase of the quadrupolar transition
temperature T_Q(H) with applied magnetic field and its further re-entrance are
described using a generalized spherical model which is well adjusted to a
particular problem of the quadrupolar ordering in CeB_6. The theory naturally
explains the growing specific heat jump at T_Q(H) with increasing magnetic
field. The role of the quadrupolar ordering in the formation of the magnetic
ordering, as well as the possible critical experiments and applications to
other rare-earth compounds, are discussed.Comment: 40 pages, 9 Postscript figures, to appear in Phys.Rev.
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