Xatu

Official Resources

• Source Code: https://github.com/xatu-code/xatu
• Paper: Efficient computation of optical excitations in two-dimensional materials with the Xatu code (Computer Physics Communications)
• License: Ask authors / See Repository

Overview

Xatu (eXcitons from ATomistic calcUlations) is a specialized program and library designed to solve the Bethe-Salpeter Equation (BSE) for solids to obtain the exciton spectrum. It is particularly optimized for two-dimensional (2D) materials and operates as a post-processing tool taking electronic band structures from Tight-Binding models or DFT calculations (based on local orbitals) as input.

Theoretical Methods

• Bethe-Salpeter Equation (BSE): Full solution for excitonic spectra.
• Tight-Binding / LCAO: Uses local orbital basis sets (from codes like SIESTA or TB models).
• Dielectric Screening: Implements various screening models suitable for 2D and 3D systems.
• Optical Properties: Calculates excitonic absorption spectra and wavefunctions.

Capabilities

• Exciton Spectrum: Diagonalization of the BSE Hamiltonian.
• Dimensionality: Specialized for 2D materials (e.g., hBN, MoS2) but applicable to general solids.
• Input Interfaces: Compatible with local orbital band structures (e.g., Wannier90, Tight-Binding).
• Analysis: Characterization of exciton binding energies, wavefunctions, and optical strengths.

Implementation & Tech Stack

• Languages: C++, Fortran, Python.
• Libraries: Built upon Armadillo C++ algebra library (BLAS, LAPACK, ARPACK backend).
• Parallelization: Designed for efficiency on modern architectures.

Workflow

1. DFT/TB Calculation: Generate electronic structure (Hamiltonian in local basis).
2. Screening: Compute or model the static dielectric function.
3. BSE Construction: Xatu builds the electron-hole interaction kernel.
4. Diagonalization: Solves for exciton eigenstates and eigenvalues.
5. Post-processing: Analysis of optical absorption and exciton character.

Application Areas

• 2D Materials: Transition metal dichalcogenides (TMDs), hBN, graphene derivatives.
• Excitonics: Study of bound electron-hole pairs in reduced dimensions.
• Optical Materials: Designing materials for optoelectronics.

Citation

If you use Xatu, please cite:

A. J. Uría-Álvarez et al., "Efficient computation of optical excitations in two-dimensional materials with the Xatu code", Computer Physics Communications (2023).