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

TL;DR

This paper reports the discovery of new correlated insulators and topological phases in twisted double bilayer graphene, driven by displacement field-induced competition between spin and valley degrees of freedom, revealing controllable isospin polarization.

Contribution

It demonstrates the control of isospin polarization and the emergence of valley polarized correlated and topological insulators in TDBG through displacement field tuning, with experimental and theoretical validation.

Findings

Observation of valley polarized correlated insulators at high displacement fields.

Detection of a valley polarized Chern insulator with C=2 and quantized Hall conductance.

Identification of a valley polarized Fermi surface with topologically trivial Landau levels.

Abstract

New phase of matter usually emerges when a given symmetry breaks spontaneously, which can involve charge, spin, and valley degree of freedoms. Here, we report an observation of new correlated insulators evolved from spin polarized states to valley polarized states in AB-BA stacked twisted double bilayer graphene (TDBG). The transition of the isospin polarization is a result of the competition between spin and valley, driven by the displacement field (D). At a high field |D| > 0.7 V/nm, we observe valley polarized correlated insulators with a big Zeeman g factor of ~10, both at v = 2 in the moir\'e conduction band and more surprisingly at v = -2 in the moir\'e valence band. At a medium field |D| < 0.6 V/nm, by contrast, it is a conventional spin polarized correlated insulator at v = 2 and a featureless metal at v = -2. Moreover, we observe a valley polarized Chern insulator with C = 2 emanating at v = 2 in the electron side and a valley polarized Fermi surface around v = -2 in the hole side. The valley Chern insulator with C = 2 is evident from a well quantized Hall conductance plateau at 2e^2/h and correspondingly a vanishing longitudinal component. The valley polarized Fermi surface is topologically trivial with C = 0, and it shows a series of quantized Landau levels with v_LL = 0, 1, 2, 3, 4 and others. These observations are in good agreements with our band and topology calculations. Our results demonstrate a feasible way to realize isospin control and to obtain new phases of matter in TDBG by the displacement field, and might benefit other twisted or non-twisted multilayer systems.