Junctions and Superconducting Symmetry in Twisted Bilayer Graphene

A controllable interlayer shielding effect in twisted multilayer graphene quantum dots

When multilayer graphene (MLG) is subjected to a vertical electric field, electrons traverse across the layers and its interlayer electrical conductance and shielding effect exhibit remarkable diversity, leading to exotic phenomena and diverse applications in photoelectric devices. The rearrangement of electrons induced by this external field is aptly described by polarizability, which quantifies the electronic response to the applied field. In this work, we have developed a polarizability decomposition scheme based on field-induced electron density variations computed using a first-principles approach. This scheme allows us to isolate the inter- and intra-layer contributions from the total polarizability of twisted multilayer graphene (TMG) quantum dots. The inter- and intra-layer counterparts reflect the charge transfer (CT) and field shielding effects among the layers, respectively. While the strongest shield effect is observed between the outermost two layers, the largest CT change is noted in the outermost layers, but small or nearly zero CT changes in the inner layers. Significant CT and shielding effects are observed not only in strongly coupled Bernal stacking, but also in the structures misaligned from full-(AA)N stacking by a small and size-dependent twist angle. The dielectric behaviors of the TMG quantum dots of a few layers are layer-dependent and different from those of typical dielectrics. Moreover, both the shielding and CT effects exhibit variation with thickness, twist angle and disc size, suggesting controllable conductive/dielectric conversion in the vertical direction. Considering the inter- and intralayer polarizability, our study addresses the precise modulation of interlayer conductance and shielding effect for TMG quantum dots, essential for microstructure design and performance manipulation of MLG-based photoelectric devices.

Vestigial singlet pairing in a fluctuating magnetic triplet superconductor and its implications for graphene superlattices

Stacking and twisting graphene layers allows to create and control a two-dimensional electron liquid with strong correlations. Experiments indicate that these systems exhibit strong tendencies towards both magnetism and triplet superconductivity. Motivated by this phenomenology, we study a 2D model of fluctuating triplet pairing and spin magnetism. Individually, their respective order parameters, d and N, cannot order at finite temperature. Nonetheless, the model exhibits a variety of vestigial phases, including charge-4e superconductivity and broken time-reversal symmetry. Our main focus is on a phase characterized by finite d ⋅ N, which has the same symmetries as the BCS state, a Meissner effect, and metastable supercurrents, yet rather different spectral properties: most notably, the suppression of the electronic density of states at the Fermi level can resemble that of either a fully gapped or nodal superconductor, depending on parameters. This provides a possible explanation for recent tunneling experiments in the superconducting phase of graphene moiré systems.

Gate-Defined Topological Josephson Junctions in Bernal Bilayer Graphene

Recent experiments on Bernal bilayer graphene (BLG) deposited on monolayer WSe_{2} revealed robust, ultraclean superconductivity coexisting with sizable induced spin-orbit coupling. Here, we propose BLG/WSe_{2} as a platform to engineer gate-defined planar topological Josephson junctions, where the normal and superconducting regions descend from a common material. More precisely, we show that if superconductivity in BLG/WSe_{2} is gapped and emerges from a parent state with intervalley coherence, then Majorana zero-energy modes can form in the barrier region upon applying weak in-plane magnetic fields. Our results spotlight a potential pathway for "internally engineered" topological superconductivity that minimizes detrimental disorder and orbital-magnetic-field effects.