Quantum critical behavior of two-dimensional Fermi systems with quadratic band touching

TL;DR

This paper investigates the quantum critical behavior of two-dimensional Fermi systems with quadratic band touching, revealing a quantum critical point and phase transitions characterized by emergent Lorentz invariance and a crossover in dynamic critical exponents.

Contribution

It demonstrates the existence of a quantum critical point in 2D Fermi systems with quadratic band touching, classifies its universality, and analyzes the interaction-induced splitting into Dirac cones.

Findings

Existence of a quantum critical point separating disordered and ordered phases.

Interaction-induced splitting of quadratic band touching into Dirac cones.

Quantum critical point belongs to the Gross-Neveu universality class.

Abstract

We consider two-dimensional Fermi systems with quadratic band touching and $C_3$ symmetry, as realizable in Bernal-stacked honeycomb bilayers. Within a renormalization-group analysis, we demonstrate the existence of a quantum critical point at a finite value of the density-density interactions, separating a semimetallic disordered phase at weak coupling from a gapped ordered phase at strong coupling. The latter may be characterized by, for instance, antiferromagnetic, quantum anomalous Hall, or charge density wave order. In the semimetallic phase, each point of quadratic band touching splits into four Dirac cones as a consequence of the nontrivial interaction-induced self-energy correction, which we compute to the two-loop order. We show that the quantum critical point is in the $(2+1)$-dimensional Gross-Neveu universality class characterized by emergent Lorentz invariance and a dynamic critical exponent $z=1$. At finite temperatures, $T > 0$, we hence conjecture a crossover between $z=2$ at intermediate $T$ and $z=1$ at low $T$, and we construct the resulting nontrivial phase diagram as function of coupling strength and temperature.