Circular dichroism in two-dimensional BC$_6$N and B$_3$C$_2$N$_3$ in absence of intervalley excitonic coupling

TL;DR

This study investigates valley excitonic properties of two 2D noncentrosymmetric materials, BC$_6$N and B$_3$C$_2$N$_3$, revealing their potential for valley polarization and optoelectronic applications due to strong excitonic effects and minimal intervalley scattering.

Contribution

The paper provides first-principles calculations demonstrating valley polarization in BC$_6$N and B$_3$C$_2$N$_3$ without intervalley excitonic coupling, highlighting their suitability for optical devices.

Findings

Significant excitonic binding energies of 0.74 eV and 1.31 eV.

Ability to exhibit valley polarization under circularly polarized light.

Dark triplet excitons are more stable than singlet excitons.

Abstract

Two-dimensional (2D) noncentrosymmetric systems offer potential opportunities for exploiting the valley degrees of freedom for advanced information processing, owing to non-zero Berry curvature. However, such valley polarization in 2D materials is crucially governed by the intervalley excitonic scattering in momentum space due to reduced electronic degrees of freedom and consequent enhanced electronic correlation. Here, we study the valley excitonic properties of two 2D noncentrosymmetric complementary structures, namely, BC$_6$N and B$_3$C$_2$N$_3$ using first principles-based GW calculations combined with the Bethe-Salpeter equation (BSE), that brings the many-body interactions among the quasiparticles. The \textbf{k}-resolved oscillator strength of their first bright exciton indicates their ability to exhibit valley polarization under the irradiation of circularly polarized light of different chiralities. Both the systems show significant singlet excitonic binding energies of 0.74 eV and 1.31 eV, respectively. Higher stability of dark triplet excitons as compared to the singlet one can lead to higher quantum efficiency in both the systems. The combination of large excitonic binding energies and the valley polarization ability with minimal intervalley scattering make them promising candidates for applications in advanced optical devices and information storage technologies.