Charge-tuneable biexciton complexes in monolayer WSe$_{2}$ (1805.04950v1)

Abstract: Multi-exciton states such as biexcitons, albeit theoretically predicted, have remained challenging to identify in atomically thin transition metal dichalcogenides so far. Here, we use excitation-power, electric-field and magnetic-field dependence of photoluminescence to report direct experimental evidence of two biexciton complexes in monolayer tungsten diselenide: the neutral and the negatively charged biexciton. We demonstrate bias-controlled switching between these two states, we determine their internal structure and we resolve a fine-structure splitting of 2.5 meV for the neutral biexciton. Our results unveil multi-particle exciton complexes in transition metal dichalcogenides and offer direct routes to their deterministic control in many-body quantum phenomena.

• The paper demonstrates the detection and characterization of both neutral and negatively charged biexcitons, resolving a 2.5 meV fine-structure splitting via low-temperature PL spectroscopy.
• The paper employs a layered heterostructure design with electric and magnetic field modulation to achieve precise charge-tuneable control of biexciton emissions.
• The paper advances understanding of many-body excitonic interactions in TMDs, paving the way for quantum optoelectronic device applications.

Charge-Tuneable Biexciton Complexes in Monolayer WSe₂

The paper focuses on the experimental identification and characterization of biexciton complexes in monolayer tungsten diselenide (1L-WSe₂), an area of significant interest in the paper of transition metal dichalcogenides (TMDs). The authors present evidence for two distinct biexciton states: a neutral biexciton and a negatively charged biexciton. The paper effectively leverages advances in material processing techniques to mitigate optical quality issues that have historically obscured the detection of such complexes in TMDs.

Key Experimental Findings and Methodologies

1. Detection and Characterization:
   • The researchers employed photoluminescence (PL) spectra analysis at low temperatures to detect the biexciton complexes. A fine-structure splitting of 2.5 meV was resolved for the neutral biexciton, highlighting the paper's meticulous approach in analyzing the internal structure of these complexes.
   • The paper utilized magnetic and electric fields to manipulate and differentiate between neutral and charged exciton states, thus achieving bias-controlled switching between these states. This methodology signifies a robust control over biexciton configurations.
2. Charge Modulation in Devices:
   • A charge-tuneable device was crafted using a layered material heterostructure (LMH) approach, involving the encapsulation of 1L-WSe₂ between hexagonal boron nitride layers and the incorporation of graphene to suppress charge-trap effects. This setup allowed a detailed investigation into the effect of intrinsic and applied charges on biexciton emission.
3. Magnetic Field Dependence:
   • The paper meticulously examined the Zeeman shift in PL spectra under varying magnetic field strengths. The calculated Landé g factors for different species confirmed the fine-structure's origin, critical for future theoretical modeling.

Theoretical Implications

The experimental observations provide crucial insights into the nature of many-body interactions in TMDs. The differentiation of biexciton complexes through fine-structure analysis and charge modulation contributes to understanding the complex energy landscapes that these quantum systems possess. This work sets a foundation for studying coherent many-body phenomena such as exciton condensation and superfluidity in 1L-TMDs.

Future Directions

The findings open pathways for further research into the deterministic control of biexciton complexes, which is vital for developing TMD-based quantum optoelectronic devices. The paper suggests potential exploration into cascaded emission processes and the generation of entangled photon states, promoting advancements in quantum photonic applications.

The paper makes a significant contribution to the field by providing clear experimental evidence for the existence of charge-tuneable biexciton states in monolayer WSe₂, thus paving the way for enhanced control and understanding of excitonic effects in two-dimensional materials.