Superconductivity, pseudogap, and phase separation in topological flat bands

Nodeless kagome superconductivity in LaRu3Si2

“…This band structure reveals a pair of Dirac points similar to those found in graphene, and a dispersionless, flat band that originates from the kinetic frustration associated with the geometry of the kagome lattice. Flat bands are exciting because the associated high density of electronic states hints at possible correlated electronic phases when found close to the Fermi level [8][9][10]. The possibility of accessing flat bands and their influence on the physical properties of the system has been studied for about three decades [10][11][12][13][14][15][16][17].…”

“…Recently, superconductivity was discovered in bilayer graphene [18] when its individual layers are twisted with respect to each other by a specific angle, giving rise to a flat band. Recent Monte Carlo calculations on a two-dimensional system [8] demonstrate that the ground state is a superconductor and find a broad pseudogap regime that exhibits strong pairing fluctuations and even a tendency towards electronic phase separation. Moreover, a square-octagon lattice was theoretically studied.…”

“…Two perfectly flat bands were found [9] and the calculated superconducting phase diagram was found to have two superconducting domes [9], as observed in several types of unconventional superconductors. Thus, there is a resurgence of interest from the experimental front in flat bands as a means to explore unconventional superconductivity [8,19,20].…”

Flat-band-induced non-Fermi-liquid behavior of multicomponent fermions

“…Quasiflat bands, whose bandwidth is comparable or smaller than the typical energy scale of interactions, seem to explain why the critical temperature of the superconducting state recently observed in magic-angle twisted bilayer graphene is large compared to the Fermi energy [6][7][8][9]. Also the normal states above the critical temperature of ordered phases are expected to be nontrivial: since a noninteracting flat band system does not have a Fermi surface and is an insulator at any filling, a Landau-Fermi liquid is generally not expected [10,11].…”

Multicellularity of Delicate Topological Insulators

“…The multicellular Hopf insulator is already known to manifest higher-order topology, quantized surface magnetism [62], and quantized magnetoelectric polarizability [43]; it would be interesting to investigate if these properties extend to other multicellular and/or delicate topological insulators. Beyond band theory, we expect multicellularity to add a new chapter to the interplay between nonunicellular WFs, generalized Hubbard models, and exotic correlated phases [63][64][65].…”