Nonequilibrium Relaxations and Aging Effects near a Metal-Insulator Transition in Two Dimensions

The relaxations of conductivity $\sigma (t)$ have been studied in the glassy regime of a strongly disordered two-dimensional (2D) electron system in Si after a temporary change of carrier density $n_s$ during the waiting time $t_w$. Two types of response have been observed~[1]: (a) monotonic, where relaxations exhibit aging, \textit{i.e.} dependence on history, determined by $t_w$ and temperature; (b) nonmonotonic, where a memory of the sample history is lost. The conditions that separate the two regimes have been determined. A detailed study of the aging regime~[2] reveals an abrupt change in the nature of the glassy phase at the metal-insulator transition (MIT) before it vanishes entirely at a higher density $n_g$. Our results provide further support to theories describing the 2D MIT as the melting of a Coulomb glass, and put constraints on the models of glassy freezing.\\ \noindent [1] J. Jaroszy\'nski and D. Popovi\'c, Phys. Rev. Lett. {\bf 99}, 046405 (2007).\\ \noindent [2] J. Jaroszy\'nski and D. Popovi\'c, Phys. Rev. Lett. {\bf 99}, 216401 (2007).