Spin and valley effects on the quantum phase transition in two dimensions

TL;DR

This study investigates the metal-insulator transition in two-dimensional electron systems, revealing that the transition persists under spin polarization and highlighting the role of spin effects in the transition's characteristics.

Contribution

It demonstrates the persistence of the metal-insulator transition in spin-polarized conditions and shows that existing theories cannot fully explain the observed temperature behavior.

Findings

The transition occurs in both spin-unpolarized and spin-polarized systems.

The temperature $T_{max}$ near the transition is close to the renormalized Fermi temperature.

The behavior of $T_{max}$ with magnetic field challenges current theories.

Abstract

Using several independent methods, we find that the metal-insulator transition occurs in the strongly-interacting two-valley two-dimensional electron system in ultra-high mobility SiGe/Si/SiGe quantum wells in zero magnetic field. The transition survives in this system in parallel magnetic fields strong enough to completely polarize the electrons' spins, thus making the electron system "spinless". In both cases, the resistivity on the metallic side near the transition increases with decreasing temperature, reaches a maximum at a temperature $T_{\text{max}}$, and then decreases. The decrease reaches more than an order of magnitude in zero magnetic field. The value of $T_{\text{max}}$ in zero magnetic field is found to be close to the renormalized Fermi temperature. However, rather than increasing along with the Fermi temperature, the value $T_{\text{max}}$ decreases appreciably for spinless electrons in spin-polarizing magnetic fields. The observed behavior of $T_{\text{max}}$ cannot be described by existing theories. The results indicate the spin-related origin of the effect. At the same time, the low-temperature resistivity drop in both spin-unpolarized and spinless electron systems is described quantitatively by the dynamical mean-field theory.