Collective Modes of nu =2 Quantum Hall Bilayers in Tilted Magnetic Field

TL;DR

This study investigates the collective excitation spectra of nu=2 quantum Hall bilayers in tilted magnetic fields, revealing mode behaviors near phase transitions and potential experimental signatures.

Contribution

It provides the first detailed analysis of collective modes across different phases, including the behavior near phase transitions, using time-dependent Hartree Fock approximation.

Findings

Softening of magnetoroton minimum signals first order transition

Collective mode features are experimentally accessible

Phase diagram includes symmetric, ferromagnetic, and canted phases

Abstract

We use time-dependent Hartree Fock approximation to study the collective mode spectra of nu=2 quantum Hall bilayers in tilted magnetic field 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 a 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.