Fractionality and $\cal{PT}$- symmetry in a square lattice

TL;DR

This paper analyzes how fractional Laplacians and $ ext{PT}$ symmetry affect the spectral stability of a 2D discrete Schrödinger equation on a square lattice, revealing conditions for real spectra and $ ext{PT}$-symmetry breaking.

Contribution

It provides a closed-form spectrum analysis of a fractional $ ext{PT}$-symmetric lattice, showing how fractional exponents influence spectral phases and stability.

Findings

Increasing gain/loss leads to earlier $ ext{PT}$ symmetry breaking.

Reducing the fractional exponent can restore real spectra.

A $ ext{PT}$-symmetric phase exists at small fractional exponents despite finite gain/loss.

Abstract

We study the spectral stability of a 2D discrete Schr\"{o}dinger equation on a square lattice, in the simultaneous presence of a fractional Laplacian and $\cal{PT}$ symmetry. For that purpose, we compute the plane-wave spectrum in closed form, as a function of the gain/loss parameter and the fractional exponent. Examination of the spectrum reveals that an increase of the gain/loss parameter favors the early appearance of complex eigenvalues, thus is, the onset of a broken ${\cal PT}$ symmetry. On the other hand, as the fractional exponent decreases from unity, at a critical value a gap opens up separating the upper and lower bands, and the spectrum becomes real. Further decrease of the exponent increases the width of the gap and the system remains in the $\cal{PT}$-symmetric phase down to a vanishing value of the fractional exponent. Examination of the density of states and the participation ratio reinforce these observations and lead one to conclude that, unlike the standard, non-fractional case where the binary lattice is always in the broken $\cal{PT}$ phase, for the fractional case it is possible to have a symmetric $\cal{P}{\cal T}$ phase in the presence of a finite gain/loss parameter and a small enough fractional exponent.