Steve Simon

Abstract:

A novel optically induced, in-plane separation of electrons and holes in modulation doped quantum wells leads to a long-lifetime supply of dense gas of thermalized cold excitons, forming ring emission patterns with large diameters. © 2003 Optical Society of America.

Abstract:

We use the time-dependent Hartree Fock approximation to study the collective-mode spectra of ν=2 quantum Hall bilayers in tilted magnetic fields, allowing for charge imbalance as well as tunneling between the two layers. In a previous companion paper to this work, we studied the zero-temperature global phase diagram of this system, which was found to include symmetric and ferromagnetic phases as well as a first-order transition between two canted phases with spontaneously broken U(1) symmetry. We further found that this first-order transition line ends in a quantum critical point within the canted region. In the current work, we study the excitation spectra of all of these phases and pay particular attention to the behavior of the collective modes near the phase transitions. We find, most interestingly, that the first-order transition between the two canted phases is signaled by a near softening of a magnetoroton minimum. Many of the collective-mode features explored here should be accessible experimentally in light-scattering experiments.

Abstract:

We consider a bilayer quantum Hall system at total filling fraction ν=2 in tilted magnetic field allowing for charge imbalance as well as tunneling between the two layers. Using an "unrestricted Hartree Fock," previously discussed by Burkov and MacDonald [Phys. Rev. B 66, 115323 (2002)], we examine the zero-temperature global phase diagrams that would be accessed experimentally by changing the in-plane field and the bias, voltage between the layers while keeping the tunneling between the two layers fixed. In accordance with previous work, we find symmetric and ferromagnetic phases as well as a first-order transition between two canted phases with spontaneously broken U(1) symmetry. We find that these two canted phases are topologically connected in the phase diagram and, reminiscent of a first-order liquid-gas transition, the first-order transition line between these two phases ends in a quantum critical point. We develop a physical picture of these two phases and describe in detail the physics of the transition.