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

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.