Tuning gaps and phases of a two-subband system in a quantizing magnetic field

In this work we study the properties of a two-subband quasi-two-dimensional electron system in a strong magnetic field when the electron filling factor is equal to 4. When the cyclotron energy is close to the intersubband splitting the system can be mapped onto a four-level electron system with an effective filling factor of 2. The ground state is either a ferromagnetic state or a spin-singlet state, depending on the values of the interlevel splitting and Zeeman energy. The boundaries between these phases are strongly influenced by the interelectron interaction. A significant exchange-mediated enhancement of the excitation gap results in the suppression of the electron-phonon interaction. The rate of absorption of nonequilibrium phonons is calculated as a function of Zeeman energy and intersubband splitting. The phonon absorption rate has two peaks as a function of intersubband splitting and has a steplike structure as a function of Zeeman energy.