Beats of quantum oscillations of the resistance in two-subband electron systems in tilted magnetic fields

Electron transport in single GaAs quantum wells of widths from $22$ to $46$ nm with two populated quantum-confinement subbands $E^S$ and $E^{AS}$ is investigated at a temperature of $T = 4.2$ K in tilted magnetic fields $B < 2$ T. The angle $\alpha$ between the applied magnetic field and the normal to the plane of the structure under study is varied from $0^\circ$ to $90^\circ$. In a perpendicular magnetic field ($\alpha = 0$), magnetointersubband oscillations with a period determined by the relation $\Delta_{SAS} = E^{AS} - E^S = j\hbar\omega_c$, where $\omega_c$ is the cyclotron frequency and $j$ is a positive integer, are observed in all investigated quantum wells. In tilted fields, the peaks of magnetointersubband oscillations are shifted toward higher fields $B\cos \alpha$. This shift is explained by an increase in the energy splitting $\Delta_{SAS}$ with increasing component $B\sin \alpha$. In $46$- and $36$-nm-wide quantum wells, beats of magnetointersubband oscillations are observed at angles $\alpha > 72^\circ$ and $\alpha > 85^\circ$, respectively. The origin of this unexpected behavior of magnetointersubband oscillations in tilted magnetic fields is discussed.