{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"205721d2e9c06aec381d3e5fc4df212668d76cc64a3fa7bb5d5c0b4a02c874fd","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":["ai:gpt-5.5"],"kind":"seed"},"parents":["792b55f23b024a76d80f7ae565abd9f06195c3da5298ad13d180a00441145d8e","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"9ed6c0b4bce4ea0efff8ce846ed17917da01916001eb006027c4f72a95c427fe","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":["ai:gpt-5.5"],"external_id":"cm.mit-2d-electrons.parallel-field-suppression","kind":"phenomenon","literature_status":"open","n":"2","parents":[],"plain":"A magnetic field along the sheet, which aligns electron spins but does not bend their paths, raises the resistance many times and in silicon transistors removes the metal-like behavior. Why spin alignment has such a large effect, and why some ultra-clean samples stay metallic even with all spins aligned, is not understood.","posed_since":"","precise":"In Si-MOSFETs near $n_c$ an in-plane field $B_{\\mathrm{parallel}}$ raises $\\rho$ by one to two orders of magnitude and saturates at $B_c(n)$, where the electrons become fully spin polarized; above $B_c$ the metallic $\\mathrm{d}\\rho/\\mathrm{d}T > 0$ is lost in Si-MOSFETs, while in ultra-clean SiGe/Si/SiGe wells a metallic state and a metal-insulator transition survive in the fully polarized (spinless, two-valley) system (Melnikov et al. 2020). Explain the magnitude of the positive magnetoresistance and its saturation at $B_c$, explain why the spinless two-valley system stays metallic while the Si-MOSFET does not, and give the ratio $n_c(B > B_c)/n_c(0)$.","problem_ref":"792b55f23b024a76d80f7ae565abd9f06195c3da5298ad13d180a00441145d8e","references":"","settled_by":"A theory that predicts $\\rho(B_{\\mathrm{parallel}}, T, n)$ including $B_{c}(n)$ with parameters fixed independently, plus transport at $B > B_{c}$ down to $T << 0.01 E_{F}$ in both Si-MOSFETs and SiGe/Si/SiGe wells.","status_note":"A metallic state of fully spin-polarized electrons was reported in ultra-clean SiGe/Si/SiGe wells (Melnikov et al., Physical Review B 101, 045302, 2020); no theory accounts for the material dependence.","title":"Why does an in-plane field suppress the 2D metallic 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