Charge and spin density wave orders in field-biased Bernal bilayer graphene

TL;DR

This study investigates the charge and spin density wave orders in biased Bernal bilayer graphene, revealing a strong-coupling mechanism near a quantum critical point that explains unusual transport phenomena at phase boundaries.

Contribution

It introduces a strong-coupling theoretical framework to explain charge/spin density wave orders in bilayer graphene near quantum criticality, challenging standard weak-coupling models.

Findings

Observation of nonlinear transport characteristic of CDW/SDW states.

Identification of a phase diagram incompatible with weak-coupling theories.

Development of a strong-coupling model explaining collective modes and phase behavior.

Abstract

This paper aims to clarify the nature of a surprising ordered phase recently reported in biased Bernal bilayer graphene that occurs at the phase boundary between the isospin-polarized and unpolarized phases. Strong nonlinearity of transport at abnormally small currents, with $dI/dV$ vs. $I$ sharply rising and then falling back, is typical for a charge/spin-density-wave state (CDW or SDW) sliding transport. Here, however, it is observed at an isospin-order phase boundary, prompting a question about the CDW/SDW mechanism and its relation to the quantum critical point. We argue that the observed phase diagram cannot be understood within a standard weak-coupling picture. Rather, it points to a mechanism that relies on an effective interaction enhancement at a quantum critical point. We develop a detailed strong-coupling framework accounting for the soft collective modes that explain these observations.