A first-principles Quantum Monte Carlo study of two-dimensional (2D) GaSe

Area of Science:

• Materials Science
• Condensed Matter Physics
• Computational Chemistry

Background:

• Two-dimensional post-transition metal chalcogenides (2D PTMCs) show promise for electronic and optical devices.
• Gallium Selenide (GaSe) is a well-synthesized 2D PTMC, but theoretical calculations show significant variations.
• Existing methods like Density Functional Theory (DFT) and GW-BSE calculations are sensitive to initial wavefunctions, leading to discrepancies.

Purpose of the Study:

• To establish a reliable theoretical benchmark for pristine monolayer Gallium Selenide (GaSe).
• To address discrepancies in calculated electronic and structural properties of 2D GaSe.
• To evaluate the suitability of the Diffusion Monte Carlo (DMC) method for 2D PTMCs.

Main Methods:

• Employed the many-body Diffusion Monte Carlo (DMC) method to calculate ground and excited state properties of GaSe.
• Benchmarked DMC results against experimental data and various Density Functional Theory (DFT) functionals (LDA, PBE, SCAN, HSE06).
• Compared DMC calculations with GW-BSE results obtained using PBE and SCAN wavefunctions.

Main Results:

• Monolayer GaSe is confirmed as an indirect gap semiconductor (Γ-M).
• The calculated quasiparticle electronic gap closely matches experimental values.
• DMC results show low exciton binding energy and agree well with experimental data for lattice parameters and cohesive energy.

Conclusions:

• The Diffusion Monte Carlo (DMC) method provides a robust and accurate approach for calculating properties of 2D GaSe, with weaker dependence on the trial wavefunction.
• This study presents a definitive theoretical benchmark for pristine monolayer GaSe.
• The findings will facilitate further investigations into 2D PTMCs using DMC methods.