Quantum Hall Ferromagnetic-Paramagnetic Transition in p-Si/SiGe/Si Quantum Wells in a Tilted Magnetic Field

Abstract

Magnetoresistance components ρxx and ρxy were measured in two p-Si/SiGe/Si quantum well samples with an anisotropic g-factor in a tilted magnetic field of up to 18T as a function of temperature (20 mK-2 K) and tilt angle. We analyzed dependences of the conductivity, the activation energy ΔE, and the filling factor ν on the tilt angle ⊝. In the sample with density p = 2 × 1011 cm−2 in the vicinity of ν = 2 ΔE(⊝) undergoes a minima at ⊝ ≍ 60o, while ν(⊝) shows a sharp jump. These facts allowed us to conclude that at ⊝ ≍ 60o and ν ≍ 2 a crossing of the Landau levels 0↑ and 1↓ occurs. It leads to the first order ferromagnetic-paramagnetic (F-P) phase transition. A coexistence of two phases at the transition point also supports the idea. However, in another sample, with p = 7.2 × 1010 cm−2, no transition was observed. For both samples we have obtained the dependences of the effective g-factor on the tilt angle, which led us to the conclusion that the F-P transition in the p-Si/SiGe/Si structure in a tilted magnetic field is a result of a violation of the g∗-factor axial symmetry due to disorder.