Neutral and Charged Inter-Valley Biexcitons in Monolayer MoSe$_2$ (1609.02008v1)

Abstract: In atomically thin transition metal dichalcogenides (TMDs), reduced dielectric screening of the Coulomb interaction leads to strongly correlated many-body states, including excitons and trions, that dominate the optical properties. Higher-order states, such as bound biexcitons, are possible but are difficult to identify unambiguously using linear optical spectroscopy methods alone. Here, we implement polarization-resolved two-dimensional coherent spectroscopy to unravel the complex optical response of monolayer MoSe$_2$ and identify multiple higher-order correlated states. Decisive signatures of neutral and charged inter-valley biexcitons appear in cross-polarized two-dimensional spectra as distinct resonances with respective ~20 meV and ~5 meV binding energies--similar to recent calculations using variational and Monte Carlo methods. A theoretical model taking into account the valley-dependent optical selection rules reveals the specific quantum pathways that give rise to these states. Inter-valley biexcitons identified here, comprised of neutral and charged excitons from different valleys, offer new opportunities for creating exotic exciton-polariton condensates and for developing ultrathin biexciton lasers and polarization-entangled photon sources.

An Analysis of Neutral and Charged Inter-Valley Biexcitons in Monolayer MoSe

This paper presents an extensive paper of biexcitons in monolayer molybdenum diselenide (MoSe$_2$), with a focus on the characterization and identification of neutral and charged inter-valley biexcitons using polarization-resolved two-dimensional coherent spectroscopy (2DCS). The findings contribute significantly to the understanding of many-body interactions in two-dimensional transition metal dichalcogenides (TMDs), which have garnered attention due to their unique electronic and optical properties driven by reduced dielectric screening and strong Coulomb interactions.

Key Findings

The researchers successfully leveraged the high sensitivity of 2DCS to higher-order electron-hole correlations, which enabled the differentiation between neutral and charged biexcitons in monolayer MoSe$_2$. Essential observations were made:

1. Neutral Biexcitons: Identified through distinct off-diagonal features in cross-circular polarized 2D spectra, the neutral biexcitons exhibit a binding energy of approximately 20 meV. These states consist of two excitons located in different valleys, namely the K and K' valleys, and manifest unique optical signatures when subjected to cross-polarized excitation sequences.
2. Charged Biexcitons: A five-particle complex, the charged biexciton is a bound state formed between an exciton and a negatively charged trion. The charged biexciton exhibits a binding energy of around 5 meV. The spectral shifts observed in the 2DCS support this characterization, offering spectroscopic isolation of these complex states from trion resonances.

Theoretical Models and Experimental Setup

The findings are supported by theoretical modeling, incorporating the complexity of valley-dependent selection rules and the Coulomb interactions between quasiparticles. The theoretical framework accounts for many-body interactions, modeling the interactions using a density matrix approach that considers possible quantum pathways, which the experimental spectra corroborate.

Experimental methods involved mechanically exfoliated MoSe$_2$ on a sapphire substrate, cooled to 20 K, with 2DCS experiments performed using phase-stabilized, carefully polarized ultrafast laser pulses. The high precision of this approach allowed for the disentanglement of complex optical phenomena not accessible through one-dimensional techniques.

Implications and Future Directions

The identification and characterization of neutral and charged biexcitons offer promising avenues for the development of novel optoelectronic applications, such as ultrathin biexciton lasers or devices based on polarization-entangled photon sources. The insights gained into the higher-order correlated states in TMDs could guide the search for more exotic composite state formations, including exciton-polariton condensates and higher-order quasiparticle assemblies.

In the broader scope, this paper provides a significant foundation for further theoretical and experimental investigations into many-body interactions within atomically thin materials. Future developments may focus on leveraging the unique inter-valley biexciton configurations identified here in advancing quantum information technologies, where control over valley and spin degrees of freedom is crucial.

The comprehensive data and nuanced analysis of quantum pathways highlight the fruitful intersection of advanced spectroscopic techniques and theoretical modeling in elucidating complex quantum phenomena in novel materials.