Pinning mode of 2D electron stripe phase around 9/2 Landau level filling

We present finite frequency conductivity measurements on ultra high mobility two-dimensional electron systems in GaAs/AlGaAs. At low temperature $T\leq 150$ mK, strong anisotropy of the DC resistivities [1] for higher Landau levels (LL) near half filling is present, and is taken to be due to a striped phase, in which the charge density is anisotropically modulated. In our AC studies, we observe a sharp resonance at $f_{pk}\approx 100 $ MHz in the real part of the diagonal conductivity, in range of LL filling $\nu$ between 4.4 and 4.6, when the AC electric field is polarized along the ``hard" direction, nominally perpendicular to the stripes. The resonance vanishes for $T\geq$ 120 mK. No resonance is observed with the electric field along the easy direction, parallel to the stripes. Resonances are present [2] in both orientations, in the bubble phases found at higher and lower $\nu $ than the stripe phase.\\ 1. M. P. Lilly {\em et al.}, Phys. Rev. Lett. {\bf 82,} 394 (1999);\\ R. R. Du {\em et al.}, Solid State Comm. {\bf 109}, 389 (1999).\\ 2. R. M. Lewis {\em et al.}, Phys. Rev. Lett. {\bf 89}, 136804 (2002).