Seminarium Fizyki Ciała Stałego

The recent discovery of unconventional superconductivity in a stacked, twisted pair of graphene sheets is a famous consequence of what is known as a flat energy band, in which the kinetic energy of the electrons becomes negligible and their mutual interactions dominate. Materials with flat energy bands give rise to enhanced correlation effects, exotic phases of matter, and unexpected properties. In this talk, I will present our recent results on flat bands in two and three dimensions.I will start with the celebrated example of magic-angle twisted bilayer graphene. This material exhibits flat low-energy bands with Van Hove singularities close to the Fermi level. In our study, we compute four-terminal conductance in mesoscopic, ballistic samples of small-angle twisted bilayer graphene with up to one million lattice sites. We establish a correspondence between features in the wide-junction conductance and the presence of Van Hove singularities in the density of states. Moreover, we identify additional transport features, such as a large, pressure-tunable minimal conductance and conductance peaks coinciding with non-singular band crossings. Our results suggest that twisted bilayer graphene close the magic angle is a unique system featuring simultaneously large conductance due to the quasi-flat bands, strong quantum nonlinearity due to the Van Hove singularities and high sensitivity to external parameters [1].In the second part of my talk, I will lift the study of electronic flat bands into the third dimension. I will show theoretically how to use strain engineering to generate flat three-dimensional energy bands in topological nodal-line semimetals, which are materials whose valence and conduction bands cross to form closed loops. I will unravel the underlying mechanism and present the competition of the arising superconducting and magnetic orders. The required strain profile can be realized, for instance, by bending the sample, which allows for in situ tuning of the emerging correlated phases and the transition temperatures. Finally, I will introduce rhombohedral graphite and CaAgP as promising material candidates to realize this proposal [2].[1] A. S. Ciepielewski, J. Tworzydło, T. Hyart, and A. Lau, Phys. Rev. Research 4, 043145 (2022)[2] A. Lau, T. Hyart, C. Autieri, A. Chen, and D. I. Pikulin, Phys. Rev. X 11, 031017 (2021),UwagaSeminarium w trybie HybrydowymFaculty of Physics room 0.06link to remote mode:https://zoom.us/j/7218838148szczegóły patrz instrukcja :instrukcja: (pdf file)AttentionThe seminar in the Hybrid modeFaculty of Physics room 0.06for details see instruction :instruction: (pdf file)