Collective Modes of nu =2 Quantum Hall Bilayers in Tilted Magnetic Field
  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. 
Anna Lopatnikova, Steven H. Simon, Eugene Demler