Symmetry-broken states at half-integer moire fillings in twisted bilayer graphene-WSe2 heterostructures

Magic-angle twisted bilayer graphene (TBG) has recently become a playground for exploring many correlated phases such as superconductivity, correlated insulators, orbital ferromagnetism, Chern insulators, and nematicity hosted by strongly interacting electrons within the low energy flat bands of the system. The rapid progress in this field has enabled the fine-tuning of several experimental knobs to access the low energy degrees of freedom of the band structure, which favor exotic phases at or near integer number of carriers per moire unit cell. However, experimental demonstration of ordered states at fractional moire band-fillings at zero applied magnetic field B is a challenging pursuit. Engineering the dielectric environment of TBG results in a highly tunable platform to investigate the strong correlations. In this work, we have investigated such a moire heterostructure assembled by placing a tungsten diselenide (WSe2) layer on top of a TBG. We observe symmetry-broken states at half-integer band-fillings of v = 0.5 and 3.5 at B ≈ 0. Further, we observe reset of charge carriers at v = 2 and 3. In addition to magnetotransport, we employ thermoelectricity as a tool to probe the system at B = 0. Our band structure calculations are consistent with a spontaneously broken translational symmetry supercell with twice the area of the original TBG moire cell.  The combined effects of a commensurate density wave potential and spin-orbit coupling terms lead to degeneracy-lifted, zone-folded moire bands with spin-valley isospin ordering anisotropy that describe the states at half-integer fillings observed experimentally. Our results suggest the existence of a spin-charge density wave ground state at half-integer fillings of TBG in the zero B-field limit.