3 research outputs found
Energy of bond defects in quantum spin chains obtained from local approximations and from exact diagonalization
We study the influence of ferromagnetic and antiferromagnetic bond defects on
the ground-state energy of antiferromagnetic spin chains. In the absence of
translational invariance, the energy spectrum of the full Hamiltonian is
obtained numerically, by an iterative modification of the power algorithm. In
parallel, approximate analytical energies are obtained from a local-bond
approximation, proposed here. This approximation results in significant
improvement upon the mean-field approximation, at negligible extra
computational effort.Comment: 3 pages, 2 figures. Manuscript accepted by Journal of Magnetism and
Magnetic Materials, special issue for LAWMMM 2007 conferenc
Octupolar ordering of Gamma8 ions in magnetic field
We study f-electron lattice models which are capable of supporting octupolar,
as well dipolar and quadrupolar, order. Analyzing the properties of the Gamma8
ground state quartet, we find that (111)-type combinations of the Gamma5
octupoles Tbeta(111)=Tbeta(x)+Tbeta(y)+Tbeta(z) are the best candidates for
octupolar order parameters. Octupolar ordering induces Gamma5-type quadrupoles
as secondary order parameter. Octupolar order is to some extent assisted, but
in its basic nature unchanged, by allowing for the presence of quadrupolar
interactions. In the absence of an external magnetic field, equivalent results
hold antiferro-octupolar ordering on the fcc lattice. In this sense, the choice
of our model is motivated by the recent suggestion of octupolar ordering in
NpO2.
The bulk of our paper is devoted to a study of the effect of an external
magnetic field on ferro-octupolar ordering. We found that octupolar order
survives up to a critical magnetic field if the field is lying in specific
directions, while for general field directions, the underlying symmetry of the
model is destroyed and therefore the phase transition suppressed even in weak
fields. Field-induced multipoles and field-induced couplings between various
order parameters are discussed on the basis of a group theoretical analysis of
the Helmholtz potential. We also studied the effect of octupolar ordering on
the non-linear magnetic susceptibility which satisfies Ehrenfest-type relations
at continuous octupolar transitions.Comment: 29 pages, 10 figures LaTeX In its contents, the present version
agrees with the published one (see Journal Reference below). Essential
additions to the text in Sec. III, otherwise some change of wording, and
minor correction