Orbital order in bilayer graphene at filling factor $\nu =-1 $

TL;DR

This paper investigates the orbital order in bilayer graphene at filling factor  = -1, revealing a series of electric potential-driven phase transitions between interlayer-coherent and orbitally coherent states with distinct microwave absorption signatures.

Contribution

It introduces a detailed analysis of how electric potential differences induce transitions between different orbital pseudospin ordered states in bilayer graphene at  = -1.

Findings

Electric potential difference drives transitions from interlayer-coherent to orbitally coherent states.

Orbital pseudospins in the OCS carry electric dipoles with ordered orientations.

Microwave spectra sharply distinguish between different ordered states.

Abstract

In a graphene bilayer with Bernal stacking both $n=0$ and $n=1$ orbital Landau levels have zero kinetic energy. An electronic state in the N=0 Landau level consequently has three quantum numbers in addition to its guiding center label: its spin, its valley index $K$ or $K^{\prime}$, and an orbital quantum number $n=0,1.$ The two-dimensional electron gas (2DEG) in the bilayer supports a wide variety of broken-symmetry states in which the pseudospins associated these three quantum numbers order in a manner that is dependent on both filling factor $\nu $ and the electric potential difference between the layers. In this paper, we study the case of $\nu =-1$ in an external field strong enough to freeze electronic spins. We show that an electric potential difference between layers drives a series of transitions, starting from interlayer-coherent states (ICS) at small potentials and leading to orbitally coherent states (OCS) that are polarized in a single layer. Orbital pseudospins carry electric dipoles with orientations that are ordered in the OCS and have Dzyaloshinskii-Moriya interactions that can lead to spiral instabilities. We show that the microwave absorption spectra of ICSs, OCSs, and the mixed states that occur at intermediate potentials are sharply distinct.