22 research outputs found
Theoretical Study of Magnetoelectric Effects in Honeycomb Antiferromagnet Co4Nb2O9
The honeycomb antiferromagnet Co4Nb2O9 is known to exhibit an interesting
magnetoelectric effect that the electric polarization rotates at the twice
speed in the opposite direction relative to the rotation of the external
magnetic field applied in the basal ab-plane. The spin-dependent electric
dipole can be an origin of the magnetoelectric effect. It is described by the
product of spin operators at different sites (type-I theory) or at the same
site (type-II theory). We examine the electric polarization for the two cases
on the basis of the symmetry analysis of the crystal structure of Co4Nb2O9, and
conclude that the latter is the origin of the observed result. This paper also
gives a general description of the field-induced electric polarization on
honeycomb lattices with the C3 point group symmetry on the basis of the type-I
theory.Comment: 6 pages, 2 figures, to be published in JPS Conf. Proc. (2020),
Proceedings of International Conference on Strongly Correlated Electron
Systems 2019 (SCES-2019, Okayama, Japan
Orbital Localization and Delocalization Effects in the U 5f^2 Configuration: Impurity Problem
Anderson models, based on quantum chemical studies of the molecule of
U(C_8H_8)_2, are applied to investigate the problem of an U impurity in a
metal. The special point here is that the U 5f-orbitals are divided into two
subsets: an almost completely localized set and a considerably delocalized one.
Due to the crystal field, both localized and delocalized U 5f-orbitals affect
the low-energy physics. A numerical renormalization group study shows that
every fixed point is characterized by a residual local spin and a phase shift.
The latter changes between 0 and \pi/2, which indicates the competition between
two different fixed points. Such a competition between the different local
spins at the fixed points reflects itself in the impurity magnetic
susceptibility at high temperatures. These different features cannot be
obtained if the special characters of U 5f-orbitals are neglected.Comment: 4 pages, REVTeX, email to [email protected]