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

Abstract

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 ν = 1 and 2 and, for the first time, between ν = 2 and 3 and between ν = 4 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 behaviour in metallic p-SiGe heterostructures in the same way as in Si 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 transition is similar to that previously obtained in Si MOSFETs thus confirming its universal character.

Diffusion Thermopower of a Two-Dimensional Hole Gas in SiGe in a Quantum Hall Insulating State

“…In a perpendicular magnetic field, two-dimensional systems characterized by short-range carrier scattering frequently exhibit a reentrant metal-insulator transition where an insulating phase appears between two integer quantum Hall states [1][2][3]. Such insulating states have been observed in p-type Si=SiGe heterostructures [1] and in Si metal-oxide-semiconductor field-effect transistors [4], while in GaAs heterostructures, characterized by long-range scattering, an insulating phase appears only between fractional Hall states [5].…”

“…The thermopower reflects the metal-insulator transition previously probed by magnetoresistance measurements [1][2][3][5][6][7], but in a totally different manner. The combined data on the resistivity and thermopower at 1:5 suggest that the insulating state is not due to an opening of an energy gap at the Fermi energy, but it is caused by a mobility gap.…”

“…Such insulating states have been observed in p-type Si=SiGe heterostructures [1] and in Si metal-oxide-semiconductor field-effect transistors [4], while in GaAs heterostructures, characterized by long-range scattering, an insulating phase appears only between fractional Hall states [5]. In p-type Si=SiGe, this reentrant metal-insulator transition (MIT) at a filling factor 1:5 is followed at higher field by a second MIT which occurs in the quantum Hall limit ( < 1) where the Fermi energy, E F , lies within the lowest Landau level [1][2][3]6,7]. The origin of the MIT at 1:5, i.e., an insulating phase in the presence of extended states below E F , is not understood.…”

“…This observation may be related to the peculiar many-level structure of a p-type Si=SiGe heterostructure in a magnetic field, when the Zeeman splitting may exceed the cyclotron energy and the state at 2 therefore becomes ferromagnetically polarized. The degeneracy and crossing of Landau levels has been adduced as one of the reasons for the appearance of the insulating state at 1:5 [1,3,10]. The present experiments were performed on a strained, asymmetrically doped Si=Si 0:88 Ge 0:12 quantum well in which the carriers were confined in a triangular potential well (the sample used in this paper was fabricated from the same wafer as CVD191 in Ref.…”

Magnetoresistance in dilute p-Si/SiGe in parallel and tilted magnetic fields

“…One of the interesting phenomena observed in this object was the discovery of the so-called "reentrant Metalto-Insulator transition" in a magnetic field at filling factor ν=3/2 [2][3][4][5]. In Ref.…”