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Correlated states in magic angle twisted bilayer graphene under the optical conductivity scrutiny

Leni Bascones

LPS, bât 510, Moyen amphi

Moiré systems displaying flat bands have emerged as novel platforms to study correlated electron phenomena. A moiré pattern emerges when there is a small mismatch between two overlapping grids, as it happens in twisted bilayer graphene (TBG) with one layer rotated by a relative angle q. Very weakly dispersing bands are found in the so-called Magic Angle TBG with q 1.1deg favoring correlation effects. A plethora of insulating and superconducting states appear as the system is doped and there is evidence of correlation induced effects at the charge neutrality point (CNP) which could originate from spontaneous symmetry breaking.
After introducing some of the correlation effects which have been observed experimentally, I will discuss how optical conductivity measurements can help distinguish different symmetry breaking states, and reveal the nature of the correlated states. Main emphasis will be on the states proposed to explain the experimental signatures of undoped systems : nematic order, which breaks the discrete rotational symmetry of the lattice, and C2T symmetry breaking states. Unexpectedly, we find that a nematic order can induce Fermi pockets, and hence a finite DC conductivity, at the CNP. We also show that the sign of the DC conductivity anisotropy induced by a nematic order depends on the degree of lattice relaxation, the doping and the nature of the symmetry breaking [1].

[1] M.J. Calderón and E. Bascones, arXiv:1912.09935


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