New Class of Magnetoresistance Oscillations: Interaction of a Two-Dimensional Electron Gas with Leaky Interface Phonons

We report on a new class of magnetoresistance oscillations observed in a high-mobility two-dimensional electron gas (2DEG) in $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterostructures. Appearing in a weak magnetic field ( $B<\mathrm{}0.3\mathrm{}\mathrm{T}$) and only in a narrow temperature range ( $2\mathrm{K}<T<\mathrm{}9\mathrm{}\mathrm{K}$), these oscillations are periodic in $1/B$ with a frequency proportional to the electron Fermi wave vector, $k_F$. We interpret the effect as a magnetophonon resonance of the 2DEG with leaky interface-acoustic phonon modes carrying a wave vector ${q=2k}_F$. Calculations show a few branches of such modes existing on the $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ interface, and their velocities are in quantitative agreement with the observation.