# Excitonic complexes in anisotropic atomically thin two-dimensional materials: black phosphorus and TiS$_3$

## TL;DR

This study investigates how anisotropy affects the binding energies and structures of excitonic complexes in black phosphorus and TiS$_3$, revealing significant differences in anisotropy and binding energies between the two materials.

## Contribution

It provides the first detailed comparison of excitonic complexes in anisotropic 2D materials black phosphorus and TiS$_3$ using the stochastic variational method.

## Key findings

- Binding energies are larger in black phosphorus than in TiS$_3$.
- Excitonic complexes in black phosphorus are strongly anisotropic.
- In TiS$_3$, excitonic complexes are nearly isotropic despite anisotropic energy spectra.

## Abstract

The effect of anisotropy in the energy spectrum on the binding energy and structural properties of excitons, trions, and biexcitons is investigated. To this end we employ the stochastic variational method with a correlated Gaussian basis. We present results for the binding energy of different excitonic complexes in black phosphorus (bP) and TiS$_3$ and compare them with recent results in the literature when available, for which we find good agreement. The binding energies of excitonic complexes in bP are larger than those in TiS$_3$. We calculate the different average interparticle distances in bP and TiS$_3$ and show that excitonic complexes in bP are strongly anisotropic whereas in TiS$_3$ they are almost isotropic, even though the constituent particles have an anisotropic energy spectrum. This is also confirmed by the correlation functions.