English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

When two graphene sheets are twisted relative to each other by a small angle,enhanced correlations lead to superconductivity whose origin remains underdebate. Here, we derive some general constraints on superconductivity intwisted bilayer graphene (TBG), independent of its underlying mechanism.Neglecting weak coupling between valleys, the global symmetry group of TBGconsists of independent spin rotations in each valley in addition to valleycharge rotations, $ {\rm SU}(2) \times {\rm SU}(2) \times {\rm U}_V(1) $. Thissymmetry is further enhanced to a full ${\rm SU}(4)$ in the idealized chirallimit. In both cases, we show that any charge $2e$ pairing must break theglobal symmetry. Additionally, if the pairing is unitary the resultingsuperconductor admits fractional vortices. This leads to the following generalstatement: Any superconducting condensate in either symmetry class has tosatisfy one of three possibilities: (i) the superconducting pairing isnon-unitary, (ii) the superconducting condensate has charge $2e$ but admits atleast half quantum vortices which carry a flux of $h/4e$, or (iii) thesuperconducting condensate has charge $2me$, $m>1$, with vortices carrying$h/2me$ flux. The latter possibility can be realized by a symmetric charge $4e$superconductor ($m=2$). Non-unitary pairing (i) is expected in TBG forsuperconductors observed in the vicinity of flavor polarized states. On theother hand, in the absence of flavor polarization, e.g. in the vicinity ofcharge neutrality, one of the two exotic possibilities (ii) and (iii) isexpected. We sketch how all three scenarios can be realized in different limitswithin a strong coupling theory of superconductivity based on skyrmions.Finally we discuss the effect of symmetry lowering anisotropies andexperimental implications of these scenarios.

Symmetry constraints on superconductivity in twisted bilayer graphene: Fractional vortices, $4e$ condensates or non-unitary pairing