Resonance in Microwave Spectra of Bilayer Hole Sample

We report microwave spectra of a bilayer hole sample, with nominal interlayer distance d $\approx $ 230 {\AA} and balanced total density p$\approx $7.46 $\times $10$^{10}$ cm$^{-2}$. The wafer was designed to suppress interlayer tunneling; dc transport studies [1] of other pieces of the wafer exhibited the interlayer-coherent quantum Hall effect at total Landau filling factor $\nu $=1, with an insulating phase reentrant around it. In the present study, a clearly identifiable peak in the spectrum (Re($\sigma _{xx})$ vs frequency) appears at 3.2 GHz as $\nu $ is decreased below $\sim $0.9. The peak frequency increases with decreasing $\nu $ to $\sim $ 5 GHz at $\nu $ =0.2. Though quite broad (Q$\sim $1) for $\nu >$0.5, the resonance sharpens drastically as $\nu $ is decreased below that value, reaching Q$\approx $10 at $\nu $=0.2. Particularly for $\nu <$0.5, the resonance can be interpreted analogously to that observed in single layer 2D hole systems [2], as a pinning mode of a Wigner crystal. [1] E. Tutuc et al. , Phys. Rev. Lett. \textbf{91}, 076802 (2003). [2] C. C. Li et al., Phys. Rev. Lett. \textbf{79}, 1353 (1997).