Spin-polarized superconductivity in semimetallic rhombohedral graphene: Part III

A magnetic field usually suppresses superconductivity, especially when the Zeeman energy exceeds the pairing gap, unless mitigated by mechanisms such as unconventional pairing, spin-orbit coupling (SOC), or intrinsic magnetism. Several graphene platforms exhibit superconductivity that remains robust against magnetic fields via such mechanisms. Moreover, in Bernal bilayer graphene an in-plane magnetic field appears to suppress a competing ordered states and thereby triggers superconductivity.  In Part II of this talk, we reported superconductivity in heptalayer rhombohedral graphene that is induced by an in-plane magnetic field (B_||). Here, we examine this behavior in detail. At zero field, an extended sharp resistive feature connects two small superconducting pockets with an integer quantum anomalous Hall (IQAH) state. With B_||, the two pockets merge into a single state the extends over a wide range of top- and bottom-gate voltages. This superconducting state violates the Pauli paramagnetic limit by at least an order of magnitude, consistent with spin-triplet pairing. Zero-field superconductivity and the nonmonotonic dependence of T_c on B_|| are highly gate sensitive, indicating important roles for isospin polarization, intrinsic SOC, and dual-surface charge polarization. We also observe a weak superconducting diode effect in several regions of the superconducting phase, including near the IQAH, suggesting a small valley imbalance. We present a phenomenological model that incorporates the interplay among intrinsic SOC, Hund’s exchange, long-range Coulomb interactions, and valley imbalance to account for these observations.