Masking by weak localization of metallic behavior in a two-dimensional electron system in strong parallel magnetic fields

The rise and saturation, in a parallel magnetic field $B_{\Vert },$ of the resistivity of two-dimensional electron systems on the metallic side of the apparent metal-insulator transition (MIT) can be interpreted as the destruction of a metallic state. It is also compatible, however, with the reduction of screening of charged impurities, resulting from the breaking of the spin degeneracy, in a traditional Fermi liquid. We demonstrate, using a Si-MOSFET, that electrons in a strong magnetic field $B_{\Vert }$ parallel to the Si-${\mathrm{SiO}}_2$ interface may exhibit a metalliclike behavior, analogous to the effect observed at $B_{\Vert }=0,$ provided that weak-localization corrections are suppressed. Conventionally, this suppression is achieved by applying a perpendicular magnetic field, but it appears that this also happens when a strong electric field is applied between the source and drain. Both methods are used in this paper. At $B_{\Vert }=0,$ weak-localization corrections are also visible but the metalliclike contribution to the resistivity is greater. These results suggest that spin polarization may simply reduce the strength of mechanisms, such as screening, which contribute a metalliclike temperature dependence to the resistivity, relative to weak-localization corrections. The polarized system undergoes a transition from weak to strong localization at a lower critical resistivity and at a larger density than the unpolarized system.