Scaling behavior of metal–insulator transitions in a Si/SiGe two dimensional hole gas

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

“…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.…”

“…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. Recently, a scaling analysis of the magnetoconductivity measurements [1,2,6] demonstrated that the MITs at 1:5 and in the quantum limit both scale with the same critical exponent, supporting the idea that both MITs are driven by the same mechanism. However, although scaling behavior is a signature of a phase transition, it does not reveal the origin of the insulating state at 1:5.…”

“…1 at approximately 4:7T and 6:2T occur at 1:25 and 1.75 and correspond to the ''critical magnetic fields,'' where xx is temperature independent. They separate the metallic phases ( < 1:25 and > 1:75) from the insulating phase (1:25 < < 1:75) [6]. In the resistivity shown in the inset of Fig.…”

“…In the resistivity shown in the inset of Fig. 1, this is particularly clear at 1:25, but at 1:75 it is blurred by the proximity of a separate integer quantum Hall effect transition [1,6], and a sharp crossing point can be resolved only at lower temperatures. Thus, the transition from the metal to the insulating state is directly seen in S xx , but in a completely different manner from xx .…”

“…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.…”