“Forbidden” transitions between quantum Hall and insulating phases in p-SiGe heterostructures

We show that in dilute metallic p-SiGe heterostructures, magnetic field can cause multiple quantum Hall-insulator-quantum Hall transitions. The insulating states are observed between quantum Hall states with filling factors $\nu =1\mathrm{}$ and 2 and, to the best of our knowledge for the first time, between $\nu =2\mathrm{}$ and 3 and between $\nu =4\mathrm{}$ and 6. The latter are in contradiction with the original global phase diagram for the quantum Hall effect. We suggest that the application of a (perpendicular) magnetic field induces insulating behavior in metallic p-SiGe heterostructures in the same way as in Si metal-oxide-semiconductor field-effect transistors (MOSFETs). This insulator is then in competition with, and interrupted by, integer quantum Hall states leading to the multiple re-entrant transitions. The phase diagram which accounts for these transitions is similar to that previously obtained in Si MOSFETs thus confirming its universal character.