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Pressure-induced superconductivity in twisted bilayer graphene

December 11, 2018 - 11:00am
Cory Dean
Columbia University

Twisted bilayer graphene (tBLG) with rotational mismatch of ~1.1°, referred to as the “magic angle,” has emerged as an exciting new platform to host strongly correlated electronic states due to its very flat low-energy bands [1-2].  The electronic bandwidth – and consequentially the strength of the correlated states – is determined by an interplay between the separation of the Dirac cones of the two graphene layers in momentum space (set by the twist angle) and the strength of the interlayer electronic coupling (set by the layer spacing). For fixed interlayer coupling, the twist angle 1.1° corresponds to the angle at which the bandwidth becomes a minimum and correlations are therefore expected to be strongest.  In my talk I will present recent experimental efforts in which we find that instead of varying the twist angle, we can modify the band structure at fixed angle by using hydrostatic pressure to tune the interlayer coupling. For angles larger than the native magic angle,  we show that under applied pressure we can  induce robust superconductivity for both hole and electron-type carriers, as well as insulating states at half- and three-quarters band filling. The ability to recover the flat band condition at arbitrary twist angle provides a possible path to both understanding the origin of the superconducting phase and potentially increasing its energy scale.  The role of disorder in these systems and its impact on the observed correlated phases will also be discussed. 

2205 John S. Toll Physics Building (CMTC Conference Room)