Quantum Criticality of Type-I and Critically Tilted Dirac Semimetals

TL;DR

This paper studies the quantum critical behavior of tilted Dirac semimetals, revealing a line of fixed points with restored Lorentz invariance at the transition and distinct critical exponents at the Lifshitz point.

Contribution

It provides a detailed renormalisation-group analysis of the universality class of phase transitions in tilted Dirac semimetals, including the Lifshitz transition.

Findings

Criticality of tilted type-I fermions is governed by a line of fixed points.

Lorentz invariance is restored at the phase transition, matching conventional Dirac systems.

Critical exponents at the Lifshitz point differ and Lorentz invariance is broken.

Abstract

We investigate the universality of an Ising symmetry breaking phase transition of tilted two-dimensional Dirac fermions, in the type-I phase as well as at the Lifshitz transition between a type-I and a type-II semimetal, where the Fermi surface changes from point-like to one with electron and hole pockets that touch at the overtilted Dirac cones. We compute the Landau damping of long-wavelength order parameter fluctuations by tilted Dirac fermions and use the resulting IR propagator as input for a renormalisation-group analysis of the resulting Gross-Neveu-Yukawa field theory. We first demonstrate that the criticality of tilted type-I fermions is controlled by a line of fixed points along which the poles of the renormalised Green function correspond to an untilted Dirac spectrum with varying anisotropy of Fermi velocities. At the phase transition the Lorentz invariance is restored, resulting in the same critical exponents as for conventional Dirac systems. The multicritical point is given by the endpoint of the fixed-point line. It can be approached along any path in parameter space that avoids the fixed point line of the critical type-I semimetal. We show that the critical exponents at the Lifshitz point are different and that Lorentz invariance is broken.