Exciton dynamics and high-temperature excitonic superfluidity in S-doped graphyne

TL;DR

This study explores the exciton properties and potential for high-temperature excitonic superfluidity in S-doped graphyne, revealing strongly bound excitons, their dynamics, and a feasible BKT transition temperature around 143 K.

Contribution

It provides the first detailed analysis of exciton behavior and superfluidity prospects in S-doped graphyne using advanced many-body calculations.

Findings

Quasiparticle corrections increase the band gap to 1.95 eV.

The lowest bright exciton has a binding energy of 0.72 eV.

Estimated BKT transition temperature is approximately 143 K.

Abstract

S-doped graphyne (S-GY) is a recently synthesized two-dimensional graphyne-based carbon allotrope that provides a promising platform for exciton engineering and coherent many-body phases. Here, we investigate the quasiparticle electronic structure, optical response, and exciton dynamics of monolayer S-GY using the G$_0$W$_0$ approximation and the Bethe--Salpeter equation (BSE). Quasiparticle corrections increase the fundamental band gap from $0.88\,\text{eV}$ (PBE) to $1.95\,\text{eV}$, while slightly reducing the carrier effective masses. The BSE optical response reveals strongly bound excitons, with the lowest bright exciton exhibiting a binding energy of $0.72\,\text{eV}$, as well as a nearly degenerate dark exciton within the thermal energy scale. Analysis of exciton wavefunctions in reciprocal space confirms a hydrogenic Rydberg series with well-defined angular-momentum character, and radiative lifetimes in the nanosecond range at room temperature, comparable to those in transition-metal dichalcogenide monolayers. Finally, we construct the excitonic phase diagram and estimate a crossover density of $\sim6 \times10^{12}~\text{cm}^{-2}$, below which the exciton gas behaves as a dilute Bose system, and the Berezinskii--Kosterlitz--Thouless (BKT) superfluid phase becomes accessible. We estimate a maximum BKT transition temperature of $\sim 143\,\text{K}$ in the freestanding limit for the 1s exciton, indicating that monolayer S-GY may provide favorable conditions for high-temperature excitonic superfluidity in graphyne-based materials.