Fermion-induced quantum critical points

TL;DR

This paper introduces fermion-induced quantum critical points (FIQCP), where gapless fermions enable continuous phase transitions that are traditionally expected to be first-order, supported by RG analysis and quantum Monte Carlo simulations.

Contribution

The study reveals that gapless fermions can induce second-order quantum phase transitions at Landau-forbidden points, supported by theoretical RG analysis and sign-problem-free Monte Carlo simulations.

Findings

Evidence of FIQCP in SU(N) fermion models for N=2 to 6

RG analysis supports the existence of FIQCPs

Potential experimental realization in graphene-like materials

Abstract

A unified theory of quantum critical points beyond the conventional Landau-Ginzburg-Wilson paradigm remains unknown. According to Landau cubic criterion, phase transitions should be first-order when cubic terms of order parameters are allowed by symmetry in the Landau-Ginzburg free energy. Here, from renormalization group (RG) analysis we show that second-order quantum phase transitions can occur at such putatively first-order transitions in interacting two-dimensional Dirac semimetals. As such type of Landau-forbidden quantum critical points are induced by gapless fermions, we call them fermion-induced quantum critical points (FIQCP). We further introduce a microscopic model of SU(N) fermions on the honeycomb lattice featuring a transition between Dirac semimetals and Kekule valence bond solids. Remarkably, our large-scale sign-problem-free Majorana quantum Monte Carlo simulations show convincing evidences of a FIQCP for N=2,3,4,5,6, consistent with the RG analysis. We finally discuss possible experimental realizations of the FIQCP in graphene and graphene-like materials.