Probing valley-valley drag interactions in a sixfold valley-degenerate 2DES on H-Si(111)

In 2D electron systems with multiple anisotropic valleys, multi-valley effects can significantly decrease the Hall coefficient $R_H$ relative to its classical value in the $B\to$0 limit [1]. Valley-valley drag interactions tend to suppress this behavior, making $R_H$ a particularly sensitive probe of electron-electron interactions in such systems. We report systematic measurements of this effect on H-terminated Si(111) surfaces with sixfold valley degeneracy and find that the temperature and density dependence (for 0.07 K $< T <$ 9 K and $2 < n_s < 7\times10^{11}$/cm$^2$) of the damping rate due to drag agrees well with Fermi liquid theory and possible weak disorder effects. However, we consistently observe a negative drag effect in the $T\to$0 limit that is not explained by these models. \\[4pt] [1] R.N. McFarland, T.M. Kott, L. Sun, K. Eng, \& B.E. Kane, Phys. Rev. B 80, 161310(R) (2009).