Metallic and Deconfined Quantum Criticality in Dirac Systems

TL;DR

This paper investigates quantum phase transitions in a model of interacting Dirac fermions on a bilayer honeycomb lattice, revealing potential new deconfined quantum critical points with gapless fermions through large-scale QMC simulations.

Contribution

It identifies and characterizes two quantum phase transitions in a Dirac fermion system, proposing a novel deconfined quantum critical point involving gapless fermions.

Findings

First transition: semimetal to symmetry-broken semimetal with massive Dirac cones.

Second transition: symmetry restoration and U(1) symmetry breaking, possibly continuous.

QMC results suggest a new deconfined quantum critical point with gapless fermionic excitations.

Abstract

Motivated by the physics of spin-orbital liquids, we study a model of interacting Dirac fermions on a bilayer honeycomb lattice at half filling, featuring an explicit global SO(3)$\times$U(1) symmetry. Using large-scale auxiliary-field quantum Monte Carlo (QMC) simulations, we locate two zero-temperature phase transitions as function of increasing interaction strength. First, we observe a continuous transition from the weakly-interacting semimetal to a different semimetallic phase in which the SO(3) symmetry is spontaneously broken and where two out of three Dirac cones acquire a mass gap. The associated quantum critical point can be understood in terms of a Gross-Neveu-SO(3) theory. Second, we subsequently observe a transition towards an insulating phase in which the SO(3) symmetry is restored and the U(1) symmetry is spontaneously broken. While strongly first order at the mean-field level, the QMC data is consistent with a direct and continuous transition. It is thus a candidate for a new type of deconfined quantum critical point that features gapless fermionic degrees of freedom.