Daijiro Yoshioka

Spontaneous interlayer coherence in double-layer quantum Hall systems: Charged vortices and Kosterlitz-Thouless phase transitions

At strong magnetic fields double-layer two-dimensional-electron-gas systems can form an unusual broken symmetry state with spontaneous inter-layer phase coherence. In this paper we explore the rich variety of quantum and finite-temperature phase transitions associated with this broken symmetry. We describe the system using a pseudospin language in which the layer degree-of-freedom is mapped to a fictional spin 1/2 degree-of-freedom. With this mapping the spontaneous symmetry breaking is equivalent to that of a spin 1/2 easy-plane ferromagnet. In this language spin-textures can carry a charge. In particular, vortices carry e/2 electrical charge and vortex-antivortex pairs can be neutral or carry charge e. We derive an effective low-energy action and use it to discuss the charged and collective neutral excitations of the system. We have obtained the parameters of the Landau-Ginzburg functional from first-principles estimates and from finite-size exact diagonalization studies. We use these results to estimate the dependence of the critical temperature for the Kosterlitz-Thouless phase transition on layer separation.

Double Quantum Well Electron-Hole Systems in Strong Magnetic Fields

Two-dimensional electron-hole systems with long excitonic life-times can be realized in double quantum well systems by applying an electric field which localizes electrons and holes in opposite wells. The ground state of such a system is investigated theoretically and numerically in the strong magnetic field limit. It is shown that when the separation between the electron layer and the hole layer, d , is small, an excitonic condensate state is realized. On the other hand, when d is large, either independent fractional quantum Hall states for electrons and holes or coupled Wigner crystal (dipole density wave) states may be realized depending on the filling factor. The difference in photoluminescence spectra between these states is discussed.

QUANTUM FERROMAGNETISM AND PHASE TRANSITIONS IN DOUBLE-LAYER QUANTUM HALL SYSTEMS

Double layer quantum Hall systems have interesting properties associated with interlayer correlations. At

Mean Field Theory of the Square Lattice Antiferromagnetic Heisenberg Model

\nu =1/m

Electronic Phase Transition of Graphite in a Strong Magnetic Field

where

SUPPRESSION OF PERSISTENT CURRENTS IN ONE-DIMENSIONAL DISORDERED RINGS BY THE COULOMB INTERACTION

m

Excitation energies of the fractional quantum Hall effect

is an odd integer they exhibit spontaneous symmetry breaking equivalent to that of spin

Ground-state phase diagram of 2D electrons in a high Landau level: a density-matrix renormalization group study.

1/2

Charge Density Wave State of Two-Dimensional Electrons in Strong Magnetic Fields

easy-plane ferromagnets, with the layer degree of freedom playing the role of spin. We explore the rich variety of quantum and finite temperature phase transitions in these systems. In particular, we show that a magnetic field oriented parallel to the layers induces a highly collective commensurate-incommensurate phase transition in the magnetic order.

Boson Mean Field Theory of the Square Lattice Heisenberg Model

A nearly half-filled two-dimensional Hubbard model has been considered as a model for a high transition temperature oxide superconductor. A new scheme to treat this model, the slave-fermion method, is proposed. The method is applied to the model in the limit of large U at half-filled, where the model reduces to the antiferromagnetic Heisenberg model. It is shown that the ground state energy becomes much lower in the slave-fermion scheme than that in the slave-boson scheme. The ground state shows antiferromagnetic long range order at T =0.