Spectroscopy of a tunable moiré system with a correlated and topological flat band

Abstract

Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor.

Spontaneous time-reversal symmetry breaking in twisted double bilayer graphene

“…For gate voltages that place the Fermi energy within such a flat and isolated moiré band, transport measurements typically report regions of higher resistivity sometimes referred to as ‘haloes’. Theoretical predictions 9 , 10 , 20 and experimental data 19 , 21 – 28 indicate that, within the halo regions, the moiré band is topologically non-trivial and interactions lead to broken spin and valley symmetries, similar to the symmetry breaking observed in quantum Hall ferromagnetism 29 , 30 .

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Isospin competitions and valley polarized correlated insulators in twisted double bilayer graphene

“…The symmetry breaking instability occurs at the boundary between spin-polarized and unpolarized states in TDBG, and it give rise to quantum critical behaviors 22 . Besides, TDBG might, in principle, host valley-polarized states as the ground states since the perpendicular magnetic field could induce a competition between spin and valley by spin and orbital Zeeman effects 27 , thus resulting to an isospin transition from one to the other.…”

Inventory of high-quality flat-band van der Waals materials

“…Flat bands, characterized by the high density of states and quenched electronic kinetic energy, are considered as desired paradigm to explore fascinating physical phenomena such as superconductivity [1][2][3][4][5][6][7][8][9], charge-density-wave state [10][11][12][13], and nontrivial band topology [14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The recently discovered three-dimensional (3D) compounds [28][29][30][31] with kagome lattice composed of transition-metal ions surprisingly show the coexistence of all of the intriguing phenomena, which has attracted broad interest in condensed-matter physics and material science.…”