Parallel Magnetic Field Effect in the Insulating Phase of 2D Metal-Insulator Transition in p-GaAs with High r$_{\mathrm{s}}$

We present magnetotransport measurements on the insulating side of the 2D metal-insulator transition in p-type GaAs quantum wells with 10 nm width (critical density p$_{\mathrm{c}}\sim $ 0.8*10$^{10}$/cm$^{\mathrm{2}}$, r$_{\mathrm{s}}\sim $ 36). Before entering the disorder dominated regime (p* \textless\ p \textless\ p$_{\mathrm{c}})$ (p* $\sim $ 0.5*10$^{10}$/cm$^{2})$, the conductance of the insulating phase follows a power-law like temperature dependence that is different from the well known thermally activated or variable range hopping behavior for insulators. In this unconventional insulating regime, a strong in-plane magnetic field (B$_{\mathrm{\vert \vert}}$ \textgreater\ B$_{\mathrm{c}} \sim $ 1-2 Tesla) drives the insulating phase into a ``normal'' insulating state which shows the variable range hopping behavior with Coulomb gap. Moreover, with the presence of a strong in-plane magnetic field in the hopping transport regime, large negative magnetoresistance ($\rho $ can decrease by a factor of 5) is observed when increasing the B$_{\mathrm{\bot}}$ component. The authors thank the NSF (DMR-0906415, DMR-0819860) and the Gordon and Betty Moore Foundation for funding support.