A new metallic state in two dimensions

We report the discovery of a new and unexpected kind of metal in two dimensions (2D), which exists in the presence of scattering by local magnetic moments. The experiment was carried out on a 2D electron system in silicon, where the local magnetic moments have been induced by disorder and their number was varied using substrate bias (V_sub). In the new metal, the conductivity decreases as σ(n_s,T)=σ(n_s,T=0)+A(n_s)T² (n_s – carrier density) to a non-zero value as temperature T→0. In three dimensions, this T² dependence is well known, and results from Kondo scattering by local magnetic moments. In 2D, however, the existence of a metal with dσ/dT>0 is very surprising. As the number of local moments is reduced, the range of temperatures [T<T_m(V_sub)] where they dominate transport becomes smaller. For T>T_m, we observe the usual 2D metallic behavior with dσ/dT<0.