Michael Ma

SU(4) Theory for Spin Systems with Orbital Degeneracy

The isotropic limit of spin systems with orbital degeneracy has global SU(4) symmetry. On many 2D lattices, the ground state does not possess long-range order, which may explain the observed spin liquid properties of LiNiO{sub 2} . In the SU(4) N{acute e}el-ordered state, spin-spin correlations can be antiferromagnetic between two neighboring sites with parallel magnetic moments. {copyright} {ital 1998} {ital The American Physical Society }

Resonant spin Hall conductance in two-dimensional electron systems with a Rashba interaction in a perpendicular magnetic field.

We study transport properties of a two-dimensional electron system with Rashba spin-orbit coupling in a perpendicular magnetic field. The spin-orbit coupling competes with Zeeman splitting to introduce additional degeneracies between different Landau levels at certain magnetic fields. This degeneracy, if occurring at the Fermi level, gives rise to a resonant spin Hall conductance, whose height is divergent as 1/T and whose weight is divergent as -ln(T at low temperatures. The Hall conductance is unaffected by the Rashba coupling.

Orbitally Degenerate Spin-1 Model for Insulating V{sub 2}O{sub 3}

Motivated by recent neutron, x-ray absorption, and resonant scattering experiments, we revisit the electronic structure of V2O3. We propose a model in which S = 1 V3+ ions are coupled in the vertical V-V pairs forming twofold orbitally degenerate configurations with S = 2. Ferro-orbital ordering of the V-V pairs gives a description which is consistent with all experiments in the antiferromagnetic insulating phase.

Directional Ordering of Fluctuations in a Two-Dimensional Compass Model

In the Mott insulating phase of the transition metal oxides, the effective orbital-orbital interaction is directional both in orbital space and in real space. We discuss a classical realization of directional coupling in two dimensions. Despite extensive degeneracy of the ground state, the model exhibits partial orbital ordering in the form of directional ordering of fluctuations at low temperatures stabilized by an entropy gap. Transition to the disordered phase is shown to be in the Ising universality class through exact mapping and multicanonical Monte Carlo simulations.

Resonant spin Hall conductance in quantum Hall systems lacking bulk and structural inversion symmetry

A previous work [Shen, Ma, Xie, and Zhang, Phys. Rev. Lett. 92, 256603 (2004)] on two-dimensional quantum wells with Rashba type spin-orbit interaction under a strong perpendicular magnetic field is generalized to include the Dresselhaus coupling. The Rashba coupling and the Dresselhaus coupling interplay with the Zeeman effect in opposing ways. The former tends to produce a resonant spin Hall effect at certain magnetic fields while the latter suppresses it. Due to the resonant spin Hall effect, the spin Hall current is highly nonohmic at low temperatures. The condition for the resonant spin Hall conductance in the presence of both Rashba and Dresselhaus couplings is derived using a perturbation method. In the presence of disorder, we argue that the resonant spin Hall conductance occurs when the two Zeeman split extended states near the Fermi level become degenerate due to the Rashba coupling, and that the quantized charge Hall conductance changes by 2e(2)/h instead of e(2)/h as the magnetic field changes through the resonant field.

Theory for supersolid {sup 4}He: Vacancy condensation facilitated by a low-energy bound state of a vacancy and an interstitial

Although both vacancies and interstitials have relatively high activation energies in the normal solid, we propose that a lower energy bound state of a vacancy and an interstitial may facilitate vacancy condensation to give supersolidity in

Plaquette ordering in SU(4) antiferromagnets

^{4}\mathrm{He}

Ground state and excitations of a four-component fermion model

. We use a phenomenological two-band boson lattice model to demonstrate this new mechanism and discuss the possible relevance to the recently observed superfluidlike, nonclassical rotational inertial experiments of Kim and Chan in solid

Pathology of Schwinger boson mean-field theory for Heisenberg spin models

^{4}\mathrm{He}

Mott insulator-superfluid transitions in a two-band model at finite temperature and possible application to supersolid {sup 4}He

. Some of our results may also be applicable to trapped bosons in optical lattices.