Ipsita Das, Dmitri K. Efetov, ICFO Spain and Benjamin A. Piot, LNCMI Grenoble.
Different interacting phases such as correlated insulators, superconductors, magnetic states, Chern insulators, strange metals, anomalous quantum Hall effect etc. have been extensively studied in the flat band of magic-angle twisted bilayer graphene (MATBG) in the last few years. However, the high-magnetic-field Hofstadter spectrum has remained unexplored so far.
In this work, an international team of ICFO (Spain), Princeton University (USA), NIMS (Japan), and LNCMI Grenoble (France) has studied the detailed magneto-transport behavior of a MATBG device with twist angle θ ~1.12° in an external magnetic field up to 31 T, corresponding to one magnetic flux quantum per moiré unit cell Φ0. We resolve the continuous evolution of the Hofstadter spectrum from zero field to Φ0. At Φ0, we observe reentrant-correlated insulators at certain integer fillings of the flat band and interaction-driven Fermi-surface reconstructions, evidenced by the emergence of new sets of Landau levels (LLs). Additionally, we study the rich interaction reconstructed Hofstadter spectrum in higher-energy passive bands.
The exact theoretical study of the Bistritzer-MacDonald (BM) Hofstadter spectrum in MATBG has predicted a set of eight well-isolated low-energy flat bands at Φ0, with comparable bandwidth but with different symmetry and topology than at zero field. Our experimental observation of enhanced resistance peaks at the band filling ν = +2 and +3 close to Φ0 verifies this prediction. New reconstructed Fermi surfaces from different integer fillings have a fully lifted degeneracy, giving rise to LLs with the sequence νl = +1, +2, +3 from ν = +1, νl = ±2, ±3, ±4, ±5 from νl = +2 and νl = ±1, ±2, ±3, ±4 from ν = +3. This suggests that both spin and valley degeneracies have been lifted for all the integer fillings at Φ0 in contrast to zero-field.
To access the higher energy bands, we further tuned the carrier density beyond ν > 4 up to Φ0. The strongest LLs observed in this regime match with the largest gaps calculated from our gaugeinvariant single-particle Hofstadter spectrum verifying the reentrant flat bands at Φ0.
Figure: (a) Color plot of Rxx as a function of B and v for the full
phase space from B = 0 to B = 31 T showing all the states. (b)
Line-cut of Rxx vs v verifying the reentrance of resistive peaks
at v = +2, +3 at B = 30 T. (c) Temperature dependence of these
resistive states at B = 28 T.
Observation of reentrant correlated insulators and interaction driven Fermi surface reconstructions at one magnetic flux quantum
per moiré unit cell in magic-angle twisted bilayer graphene, I. Das, C. Shen, A. Jaoui, J. HerzogArbeitman, A. Chew, C.-W. Cho, K. Watanabe, T. Taniguchi,
B. A. Piot, B. A. Bernevig, and D. K. Efetov, arXiv:2111.11341v1
(accepted for publication in Physical Review Letters).
https://arxiv.org/abs/2111.11341
Contact: f.stefani@hzdr.de , t.herrmannsdoerfer@hzdr.de
