Influence of Inversion Symmetry on the Metallic Behaviour in a Dilute Two-dimensional Hole System

In the past five years numerous experimental studies of a wide variety of low disorder two-dimensional (2D) semiconductor systems have revealed an unexpectedly large decrease in the resistance as the temperature is lowered from T ~ 1 K, suggesting the existence of a 2D metal. Although numerous theories have been put forward to explain this metallic-like behaviour (which contradicts the expectations of one parameter scaling theory), its origins, and the question of whether it persists to T = 0, are still subjects of great debate. We present a detailed study of the influence of inversion symmetry on the B = 0 metallic behaviour in a low density GaAs hole gas close to the apparent two-dimensional metal—insulator transition. The strength of the metallic behaviour (determined by the size of the drop in resistance as T→ 0) is found to be almost independent of the electric field across the hole gas, and is predominantly determined by the magnitude of k F l at low temperatures (i.e. by the low temperature resistivity). These results suggest that the shape of the potential well and spin—orbit effects alone cannot account for the existence of metallic behaviour in low density, strongly interacting 2D systems.