Novel interaction-induced magneto-oscillations in ac conductivity of 2D electron gas

We demonstrate that electron-electron interactions in a high-mobility 2D electron gas give rise to the {\em oscillatory} correction, $\delta\sigma^{int}(\omega)$, to the ac magnetoconductivity, $\sigma(\omega)$. Similarly to the conventional single-particle harmonics of the cyclotron resonance, the oscillating correction is periodic in $\omega_c^{-1}$, where $\omega_c$ is the cyclotron frequency. However, unlike the single-particle oscillations, which are periodic with $\omega$, the interaction correction is periodic with $\omega^{3/2}$. Oscillatory behavior of the interaction-induced magnetoconductivity develops at very low magnetic fields, $\omega_c\ll\omega$; at such fields the conventional harmonics are suppressed by the disorder. The underlying physical process of the new effect is {\em double} backscattering of an electron from the impurity-induced Friedel oscillations. Unlike the case of single-particle oscillations, the electron travels only a {\em small portion} of the Larmour circle during the time $\sim\omega^{-1}$ between the two backscattering events.