Condensed and Living Matter Seminar: "Twisting nodal superconductors"

Jed Pixley, Rutgers
- | David Rittenhouse Laboratory, A8

The realization of correlated insulators and superconductivity in twisted van der Waals heterostructures has brought forth twisting as a new control knob in condensed matter laboratories. In this talk we will show how twisting can be used to control the low energy physics of bilayers of nodal superconductors. Focusing in the vicinity of the Dirac nodes in their Bogolioubov de Gennes (BdG) spectrum we derive an effective model, which is surprisingly simpler then in twisted bilayer graphene. As a result, we demonstrate that in the limit of small twists the BdG velocity vanishes at a magic-angle, which we estimate for a few nodal superconductors that are available in monolayer form. At the magic-angle the effects of the interactions between the BdG quasiparticles are greatly enhanced leading to a secondary, time reversal breaking, superconducting instability. In addition, we demonstrate quite generally that applying an interlayer current to twisted nodal superconductors at any (small) twist angle induces a topological superconducting state. Preliminary results on the interlayer tunneling of Bi2Sr2CaCu2O8+y (BSCCO) to more precisely estimate the magic-angle at low twist angles will be discussed. Last, recent experiments have successfully twisted thin slabs of BSCCO while maintaining the high temperature superconducting state. The results on the critical current and related Josephson effects for a wide range of twist angles will be presented in conjunction with our theoretical analysis that demonstrates an observation of the second harmonic in the current phase relationship for a twist of 45 degrees.