Universal absorption of two-dimensional systems

TL;DR

This paper explores the optical conductivity of various 2D semiconducting systems, revealing a universal quantum near the band-gap and how it varies with degeneracy and system composition.

Contribution

It introduces a universal optical conductivity quantum for 2D systems and analyzes how degeneracy and mixing of carrier types affect optical responses.

Findings

Universal optical conductivity quantum of σ₀ = (1/16)(e²/ħ) near the band-gap

Optical conductivity depends on degeneracy factors g_s and g_v, and curvature ν

Mixed systems exhibit non-universal optical conductivity

Abstract

We discuss the optical conductivity of several non-interacting two-dimensional (2D) semiconducting systems focusing on gapped Dirac and Schr\"odinger fermions as well as on a system mixing these two types. Close to the band-gap, we can define a universal optical conductivity quantum of $\sigma_0=\frac{1}{16}\frac{e^2}{\hbar}$ for the pure systems. The effective optical conductivity then depends on the degeneracy factors $g_s$ (spin) and $g_v$ (valley) and on the curvature around the band-gap $\nu$, i.e., it generally reads $\sigma=g_sg_v\nu\sigma_0$. For a system composed of both types of carriers, the optical conductivity becomes non-universal.