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Flat-band systems are a promising platform for realizing exotic collective ground states with spontaneously broken symmetry because the electron-electron interactions dominate the kinetic energy. A state of particular interest would be the chased after interlayer-phase-coherent exciton condensate but the conventional treatments of the effect of thermal and quantum fluctuations suggest that this state might be either unstable or fragile. In this work, using double twisted bilayer graphene heterostructures as an example, we show that the quantum metric of the Bloch wave functions can strongly stabilize the exciton condensate and reverse the conclusion that one would draw using a conventional approach. The quantum metric contribution arises from interband terms, and flat-bands are most commonly realized by engineering multiband systems. Our results therefore suggest that in many practical situations the quantum metric can play a critical role in determining the stability of exciton condensates in double layers formed by two-dimensional systems with flat-bands.
We show that the recently observed superconductivity in twisted bilayer graphene (TBG) can be explained as a consequence of the Kohn-Luttinger (KL) instability which leads to an effective attraction between electrons with originally repulsive interac
Flatbands with extremely narrow bandwidths on the order of a few mili-electron volts can appear in twisted multilayer graphene systems for appropriate system parameters. Here we investigate the electronic structure of a twisted bi-bilayer graphene, o
When twisted to angles near 1{deg}, graphene multilayers provide a new window on electron correlation physics by hosting gate-tuneable strongly-correlated states, including insulators, superconductors, and unusual magnets. Here we report the discover
Topological insulators realized in materials with strong spin-orbit interactions challenged the long-held view that electronic materials are classified as either conductors or insulators. The emergence of controlled, two-dimensional moire patterns ha
We have studied the dielectric screening of electric field which is induced by a gate voltage in twisted double bilayer graphene by using a sample with a mismatch angle of about 5 degrees. In low temperature magnetotransport measurements, quantum osc