Low-Field Metal-Insulator Transition in AB-Stacked Bilayer Graphene

Abstract

We investigate the interplay of in-plane magnetic and transverse electric fields in AB-stacked bilayer graphene. In prior work neglecting trigonal warping, we demonstrated that this configuration induces an insulator-metal transition purely via orbital effects, albeit requiring impractically large magnetic fields ($>100$ T). Here, we extend the analysis to the ultra-low-energy regime by incorporating interlayer skew couplings. In a restricted region of momentum space, trigonal warping produces a fine splitting of Dirac cones leading to a compensated semimetallic state at zero external field. Application of a transverse electric field above a small threshold ($V_c\sim0.6$ meV) reinstates an insulating gap. When an in-plane magnetic field is applied, the orbital gauge vector immediately breaks the $C_3$ spatial symmetry of the lattice, and we uncover two sequential, field-driven transitions separated by an order of magnitude in scale. First, at fields ($B \approx 1$ T), the Zeeman effect drives an indirect insulator-to-semimetal transition at the ungated charge neutrality point; the cross-spin gap closes, generating distinct electron and hole pockets separated in momentum space. Second, as the field increases to $B \approx 10$--$25$ T, the orbital coupling closes the same-spin gap at an energy well away from the charge-neutral Fermi level. By electrostatically tuning the Fermi level to this specific gap-closing energy, we reveal the emergence of $C_3$-broken same-spin Fermi pockets, accompanied by a distinct step-like onset in the density of states. This dual-transition regime provides a highly sensitive platform for tunable, spin-selective transport.