Correlated phases in hole- and electron-doped Bernal bilayer graphene

by Dr Anna Seiler (Georg-August-Universität Göttingen)

Wednesday 6 Dec 2023, 13:15 → 14:45 Europe/Berlin

Hybrid seminar at Ruhr-Universität Bochum and online on Zoom

https://ruhr-uni-bochum.zoom.us/j/2026263704?pwd=WWl1ckF5RnNrT0d4bFNqU1FWbjBYZz09

Abstract:

Diverging density of states can lead to correlated phases in low-dimensional systems. This includes the naturally occurring Bernal bilayer graphene that exhibits a complex low-energy band structure consisting of four linearly-dispersed Dirac cones, Lifshitz transitions and concomitant van Hove singularities.

Here, we present the observation of experimental signatures consistent with various interaction-driven phases in biased Bernal bilayer graphene. At hole doping, trigonal warping is very pronounced, leading to regions with annular Fermi surfaces [1]. Our observations include the fractional metals of Stoner type. More prominently, we have found competing nontrivial insulating and metallic phases that exhibit intriguing temperature dependences and nonlinear I-V characteristics at zero magnetic fields. 

In addition, we report a novel interaction-driven behaviour in the Stoner phases in the electron-doped regime of biased Bernal bilayer graphene in which the energy bands are even flatter and the region of the annular Fermi surface is neglectable [2]. Specifically, we find that the spin- and valley-polarized Stoner phases in this regime exhibit an insulator-like temperature dependence on the conductance. This unexpected behaviour challenges the conventional picture of metallic Stoner magnetism in this system. Furthermore, these Stoner phases feature a nonlinear transport behaviour that is sensitive to the onsets of the Stoner orders. Our results suggest the emergence of exotic correlated orders beyond Stoner ferromagnetism, such as charge density waves or Wigner crystal states.

 

[1] Anna M. Seiler, Fabian R. Geisenhof, Felix Winterer, Kenji Watanabe, Takashi Taniguchi, Tianyi Xu, Fan Zhang, R. Thomas Weitz. Quantum cascade of correlated phases in trigonally warped bilayer graphene. Nature 608, 298-302 (2022)

[2] Anna M. Seiler, Martin Statz, Isabell Weimer, Nils Jacobsen, Kenji Watanabe, Takashi Taniguchi, Zhiyu Dong, Leonid S. Levitov, R. Thomas Weitz. Interaction-driven (quasi-) insulating ground states of gapped electron-doped bilayer graphene. arXiv:2308.00827 (2023)