Stable topological phases in a family of two-dimensional fermion models

TL;DR

This paper demonstrates that a broad class of two-dimensional spinless fermion models inherently favor topological superconducting phases with non-zero Chern numbers, driven by symmetry considerations and attractive interactions.

Contribution

It proves a theorem showing that any 2D representation of the symmetry group leads to a topological superconductor as the ground state, and applies this to specific lattice models.

Findings

Square lattice model exhibits only a topological p+ip state.

Triangular lattice model transitions from p+ip to f-wave superconductor with doping.

Ground states are always topological superconductors under attraction in at least one channel.

Abstract

We show that a large class of two-dimensional spinless fermion models exhibit topological superconducting phases characterized by a non-zero Chern number. More specifically, we consider a generic one-band Hamiltonian of spinless fermions that is invariant under both time-reversal, $\mathbb{T}$, and a group of rotations and reflections, $\mathbb{G}$, which is either the dihedral point-symmetry group of an underlying lattice, $\mathbb{G}=D_n$, or the orthogonal group of rotations in continuum, $\mathbb{G}={\rm O}(2)$. Pairing symmetries are classified according to the irreducible representations of $ \mathbb{T} \otimes \mathbb{G}$. We prove a theorem that for any two-dimensional representation of this group, a time-reversal symmetry breaking paired state is energetically favorable. This implies that the ground state of any spinless fermion Hamiltonian in continuum or on a square lattice with a singly-connected Fermi surface is always a topological superconductor in the presence of attraction in at least one channel. Motivated by this discovery, we examine phase diagrams of two specific lattice models with nearest-neighbor hopping and attraction on a square lattice and a triangular lattice. In accordance with the general theorem, the former model exhibits only a topological $(p + ip)$-wave state, while the latter shows a doping-tuned quantum phase transition from such state to a non-topological, but still exotic $f$-wave superconductor.