Metallic state in a strongly interacting spinless two-valley electron system in two dimensions

We have studied the strongly interacting, two-valley two-dimensional (2D) electron system in ultrahigh mobility SiGe/Si/SiGe quantum wells in parallel magnetic fields strong enough to completely polarize the electron spins thus making the electron system “spinless.” It occurs that the metallic temperature dependence of the resistivity, although weaker than that in the absence of magnetic field, still remains strong even when the spin degree of freedom is removed. Several independent methods have been used to establish the existence of the genuine MIT in the spinless two-valley 2D system. This is in contrast to the previous results obtained on more disordered silicon samples, where the polarizing magnetic field causes a complete quench of the metallic temperature behavior.

Figure 1

Resistivity of an electron system in a SiGe/Si/SiGe quantum well placed in the spin-polarizing magnetic field $B^{*}$ as a function of temperature for different electron densities. The magnetic fields used are spanned in the range between approximately 1 and 2 T. The critical region around the MIT is color gradated. The inset shows a closeup view of $\rho \left(T\right)$ for $n_{\text{s}}=2.09\times {10}^{10}{\mathrm{cm}}^{-2}$.

Figure 2

Main panel: activation energy $E_{\text{a}}$ and square root of the threshold voltage $V_{\text{c}}^{1/2}$ vs electron density. Solid lines correspond to the best linear fits. Upper inset: a typical $I\text{−}V$ dependence on the insulating side of the MIT at $T=30$ mK. Lower inset: Arrhenius plots of the temperature dependence of the resistivity for two electron densities on the insulating side.

Figure 3

Critical electron density for the MIT as a function of the magnetic field parallel (squares) and perpendicular (circles) to the 2D plane. The cutoff resistivity for the MIT was chosen at $\rho =200\mathrm{k}\mathrm{\Omega }$ at a temperature of 30 mK (see text).