Low temperature magnetotransport of 2D electron and hole systems in high-mobility Si [sbnd]Si 1 − xGe x heterostructures

Abstract

Low-temperature magnetotransport measurements of 2D electron and hole systems (2DES, 2DHS) in high quality n- and p-type modulation-doped Si Si 1 − x Ge x heterostructures (respectively) have been extended to high magnetic fields (50 T) and low temperatures (30 mK). For the high-mobility 2DES in n-Si, a two-valley system, signatures of the fractional quantum Hall effect (FQHE) in the region v < 1 (one valley occupied, lowest spin state) usually observed in GaAs AlGaAs are replicated out to v = 2/5 at B ≈ 48T. For 1 < v < 2, however, (both valleys occupied, lowest spin state), prominent FQHE states such as v = 5/3 are absent, indicative of the importance of valley occupation. For the 2DHS in p-Si 1 − x Ge x , in addition to the QHE, two low-temperature insulating phases (IP) are identified at v = 1.5 and v ≲ 0.5 (B ≈ 30T), with re-entrance to the QHE regime at v = 1 ( B ≈ 15T). The IP centred at v = 1.5 has the characteristics of a Hall insulator but is unanticipated by the global phase diagram for 2D systems. The important physics associated with the IP is related to an energy degeneracy of adjacent Landau levels of opposite spin.