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Exciton Superfluid and Ferromagnetic Superconductivity in Graphene

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Superfluid and superconductors are two prototypical examples of quantum condensates of bosonic particles. By controlling the interaction between two fermionic particles, a composite boson can be formed by pairing fermions. A crossover behavior from weak coupling superconducting Bardeen-Cooper-Schrieffer (BCS) pairing to a superfluid Bose-Einstein condensate (BEC) of tightly bound pairs has been expected as a function of the attractive interaction in Fermi systems. In this talk, we will discuss two such examples realized in graphene heterostructures. In the first part of the presentation, we will discuss an experimental demonstration of magnetoexciton condensation. Employing two layers of graphene separated by an atomically thin insulator, we realize a superfluid condensation of magnetic-field-induced excitons across the double layers of graphene probed by Coulomb drag. Here, we observe dissipationless exciton motion in this system across the BEC -BCS phase boundary controlled by the magnetic field. In the second part of the presentation, we will discuss the recent development of unconventional superconductivity appeared in twisted double graphene bilayers with small twisting angles. We observed that a ferromagnetic correlated insulating state appears by controlling the flatness of the bilayer graphene band using the perpendicular electric field applied by the gate. Upon doping this ferromagnetic insulator, we obtain the superconductivity, whose transition temperature can be controlled by electric fields. Remarkably, we find that increasing in-plane magnetic field increases superconducting transition temperature, suggesting unconventional superconductivity with spin-polarized cooper pairs.

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