Zeeman-Induced Gapless Superconductivity with Partial Fermi Surface

We show that an in-plane magnetic field can drive two-dimensional spin-orbit-coupled systems under superconducting proximity effect into a gapless phase where parts of the normal state Fermi surface are gapped, and the ungapped parts are reconstructed into a small Fermi surface of Bogoliubov quasiparticles at zero energy. Charge distribution, spin texture, and density of states of such "partial Fermi surface" are discussed. Material platforms for its physical realization are proposed.Comment: 5 pages, 2 figure

Edelstein effect and supercurrent diode effect

We self-consistently calculate the supercurrent diode effect from microscopic models of quasi one- and two-dimensional clean superconductors with spin-orbit coupling under external Zeeman fields, and show that the Edelstein effect is responsible for the supercurrent diode effect. In turn, the supercurrent diode effect may serve as a direct measurement of the Edelstein effect as its application.Comment: 4 pages+ 1 page references, 1 figur

Unconventional Superconductivity and Density Waves in Twisted Bilayer Graphene

We study electronic ordering instabilities of twisted bilayer graphene with $n=2$ electrons per supercell, where correlated insulator state and superconductivity are recently observed. Motivated by the Fermi surface nesting and the proximity to Van Hove singularity, we introduce a hot-spot model to study the effect of various electron interactions systematically. Using renormalization group method, we find $d$/$p$-wave superconductivity and charge/spin density wave emerge as the two types of leading instabilities driven by Coulomb repulsion. The density wave state has a gapped energy spectrum at $n=2$ and yields a single doubly-degenerate pocket upon doping to $n>2$. The intertwinement of density wave and superconductivity and the quasiparticle spectrum in the density wave state are consistent with experimental observations.Comment: 15 pages, 12 figures; updated discussion and analysis on density wave state

Magic of high order van Hove singularity

We introduce a new type of van Hove singularity in two dimensions, where a saddle point in momentum space is changed from second-order to high-order. Correspondingly, the density of states near such ``high-order van Hove singularity'' is significantly enhanced from logarithmic to power-law divergence, which promises stronger electron correlation effects. High-order van Hove singularity can be generally achieved by tuning the band structure with a single parameter in moir\'e superlattices, such as twisted bilayer graphene by tuning twist angle or applying pressure, and trilayer graphene by applying vertical electric field.Comment: 8 pages and 4 figures, figs. 1, 2 and 3 updated and Supplemental Material adde