Small Energy Gap Revealed in CrBr3 by Scanning Tunneling Spectroscopy

TL;DR

This study uses scanning tunneling spectroscopy and DFT calculations to determine that CrBr3 has a much smaller energy gap (~0.57 eV) than previously believed, clarifying longstanding controversy.

Contribution

It provides the first unambiguous measurement of CrBr3's small energy gap, resolving conflicting optical measurement results and enhancing understanding of this 2D magnetic material.

Findings

CrBr3 has a peak-to-peak energy gap of 0.57 eV.

The measured energy gap aligns with optical transition data.

Defect-free crystal structure confirmed by STM topography.

Abstract

CrBr$_{3}$ is a layered van der Waals material with magnetic ordering down to the 2D limit. For decades, based on optical measurements, it is believed that the energy gap of CrBr$_{3}$ is in the range of 1.68-2.1 eV. However, controversial results have indicated that the band gap of CrBr$_{3}$ is possibly smaller than that. An unambiguous determination of the energy gap is critical to the correct interpretations of the experimental results of CrBr$_{3}$. Here, we present the scanning tunneling microscopy and spectroscopy (STM/S) results of CrBr$_{3}$ thin and thick flakes exfoliated onto pyropytic graphite (HOPG) surfaces and density functional theory (DFT) calculations to reveal the small energy gap (peak-to-peak energy gap to be 0.57 eV $\pm$ 0.04 eV; or the onset signal energy gap to be 0.29 $\pm$ 0.05 eV from dI/dV spectra). Atomic resolution topography images show the defect-free crystal structure and the dI/dV spectra exhibit multiple peak features measured at 77 K. The conduction band - valence band peak pairs in the multi-peak dI/dV spectrum agree very well with all reported optical transitions. STM topography images of mono- and bi-layer CrBr$_{3}$ flakes exhibit edge degradation due to short air exposure (~15 min) during sample transfer. The unambiguously determined small energy gap settles the controversy and is the key in better understanding CrBr$_{3}$ and similar materials.