Collective modes of ν=2 quantum Hall bilayers in tilted magnetic fields
Physical Review B Condensed Matter and Materials Physics 70:11 (2004) 1-115326
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.Global phase diagram of ν=2 quantum Hall bilayers in tilted magnetic fields
Physical Review B Condensed Matter and Materials Physics 70:11 (2004) 1-115325
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.Mechanism of exciton emission ring pattern in doped quantum wells
Physica Status Solidi C: Conferences 1:6 (2004) 655-660
MIMO capacity through correlated channels in the presence of correlated interferers and noise: A (not so) large N analysis
IEEE Transactions on Information Theory 49:10 (2003) 2545-2561
Abstract:
The use of multiple-antenna arrays in both transmission and reception promises huge increases in the throughput of wireless communication systems. It is therefore important to analyze the capacities of such systems in realistic situations, which may include spatially correlated channels and correlated noise, as well as correlated interferers with known channel at the receiver. Here, we present an approach that provides analytic expressions for the statistics, i.e., the moments of the distribution, of the mutual information of multiple-antenna systems with arbitrary correlations, interferers, and noise. We assume that the channels of the signal and the interference are Gaussian with arbitrary covariance. Although this method is valid formally for large antenna numbers, it produces extremely accurate results even for arrays with as few as two or three antennas. We also develop a method to analytically optimize over the input signal covariance, which enables us to calculate analytic capacities when the transmitter has knowledge of the statistics of the channel (i.e., the channel covariance). In many cases of interest, this capacity is very close to the full closed-loop capacity, in which the transmitter has instantaneous channel knowledge. We apply this analytic approach to a number of examples and we compare our results with simulations to establish the validity of this approach. This method provides a simple tool to analyze the statistics of throughput for arrays of any size. The emphasis of this paper is on elucidating the novel mathematical methods used.Optimizing multiple-input single-output (MISO) communication systems with general Gaussian channels: Nontrivial covariance and nonzero mean
IEEE Transactions on Information Theory 49:10 (2003) 2770-2780