Spin and valley degrees of freedom in a bilayer graphene quantum point contact: Zeeman splitting and interaction effects

TL;DR

This study investigates how in-plane magnetic fields lift degeneracy in bilayer graphene quantum point contacts, revealing Zeeman spin splitting, interaction effects, and a 0.7 conductance anomaly explained by a phenomenological model.

Contribution

It demonstrates the combined effects of magnetic fields and interactions on spin and valley degeneracies in bilayer graphene quantum point contacts, introducing a phenomenological model for the 0.7 anomaly.

Findings

Zeeman spin splitting observed in the first three subbands.

Enhanced Landé g-factors due to confinement and interactions.

Identification of a 0.7 anomaly linked to interaction-induced degeneracy lifting.

Abstract

We present a study on the lifting of degeneracy of the size-quantized energy levels in an electrostatically defined quantum point contact in bilayer graphene by the application of in-plane magnetic fields. We observe a Zeeman spin splitting of the first three subbands, characterized by effective Land\'{e} $g$-factors that are enhanced by confinement and interactions. In the gate-voltage dependence of the conductance, a shoulder-like feature below the lowest subband appears, which we identify as a $0.7$ anomaly stemming from the interaction-induced lifting of the band degeneracy. We employ a phenomenological model of the $0.7$ anomaly to the gate-defined channel in bilayer graphene subject to in-plane magnetic field. Based on the qualitative theoretical predictions for the conductance evolution with increasing magnetic field, we conclude that the assumption of an effective spontaneous spin splitting is capable of describing our observations, while the valley degree of freedom remains degenerate.