RF resonance of two dimensional electron systems in higher Landau levels: Effect of in-plane magnetic fields

We report measurements of the RF diagonal conductivity, Re$[\sigma _{xx} (f)]$, of ultrahigh mobility 2D electron systems with Landau level (LL) filling factor $4<\nu <6$, in the presence of an in-plane magnetic field, $B_{//}$. For nearly half integer filled higher LL's, such samples are thought to show electron stripe phases, which can be reoriented by application of $B_{//}$ [1,2]. With $B_{//} =0$, and $\nu \approx$ 9/2 and 11/2, a resonance around 100 MHz is observed when the microwave electric field $E_{m}$ is along $<1\bar{1}0>$, nominally perpendicular to the stripes, but no resonance is seen when $E_{m}$ is along $<110>$ [3]. $B_{//}$ can switch the resonance on and off depending on the orientations of $B_{//}$ and $E_{m}$ relative to the crystal. However, in the presence of $B_{//}$, a simple comparison with the DC results [1,2] is not possible, since the resonance can be present even for $E_{m}$ along lower DC resistivity direction, which would be thought to be parallel to the stripes. For example, around $6^{o}$ tilting of the magnetic field along $<1\bar{1}0>$, $B_{//} \approx$ 0.28 T, resonances are observed with $E_{m}$ along both orientations. [1] W. Pan {\it et al}., PRL {\bf 83}, 820 (1999). [2] M. P. Lily {\it et al}., PRL {\bf 83}, 824 (1999). [3] G. Sambandamurthy {\it et al}., this conference.