# The Nielsen-Ninomiya theorem, PT-invariant non-Hermiticity and single 8-shaped Dirac cone

## TL;DR

This paper explores how relaxing Hermiticity in lattice models while maintaining PT symmetry can lead to unconventional fermion spectra, including non-equal chiral fermions and robust massless excitations with complex energy branches.

## Contribution

It introduces a PT-invariant non-Hermitian lattice model that exhibits a single 8-shaped Dirac cone and breaks the Nielsen-Ninomiya theorem constraints.

## Key findings

- The model shows a skewed 8-shaped energy spectrum.
- PT symmetry is spontaneously broken, leading to complex energy branches.
- Massless excitations are robust against local disorder.

## Abstract

The Nielsen-Ninomiya theorem implies that any local, Hermitian and translationally invariant lattice action in even-dimensional spacetime possess an equal number of left- and right-handed chiral fermions. We argue that if one sacrifices the property of Hermiticity while keeping the locality and translation invariance, while imposing invariance of the action under the space-time (PT) reversal symmetry, then the excitation spectrum of the theory may contain a non-equal number of left- and right-handed massless fermions with real-valued dispersion. We illustrate our statement in a simple 1+1 dimensional lattice model which exhibits a skewed 8-figure patterns in its energy spectrum. A drawback of the model is that the PT symmetry of the Hamiltonian is spontaneously broken implying that the energy spectrum contains complex branches. We also demonstrate that the Dirac cone is robust against local disorder so that the massless excitations in this PT-invariant model are not gapped by random space-dependent perturbations in the couplings.