Weak localisation and the “destruction” of thetwo-dimensional metallic behaviour by a parallelmagnetic field

摘要

Metallic behaviour, defined as a drop in resistivity with decreasing temperature, insome low-density two-dimensional (2D) systems appears to contradict the predictionthat non-interacting electrons are always localised in 2D. The rise and saturation, in aparallel magnetic field B‖, of the resistivity of such a system has been interpretedby some authors as showing a transition from this anomalous “metallic” state toa fundamentally different insulating ground state. We demonstrate, using a highmobilitySi-MOSFET, that this insulator at large B‖ is due to coherent backscattering(weak localisation). At B‖ = 0, this effect also exists but is weak, at experimentaltemperatures, compared to “metallic behaviour”. At large B‖, it is stronger. Thissuggests that the difference in the low-B‖ and the high-B‖ systems may only bequantitative (and not qualitative), and that in the limit of zero temperature, bothare insulating. Instead of “destroying” the metal, B‖ only facilitates localisation.