Magnetic brightening and control of dark excitons in monolayer WSe2 (1612.03558v1)

Abstract: Monolayer transition metal dichalcogenide (TMDC) crystals, as direct-gap materials with unusually strong light-matter interaction, have attracted much recent attention. In contrast to the initial understanding, the minima of the conduction band are predicted to be spin split. Because of this splitting and the spin-polarized character of the valence bands, the lowest-lying excitonic states in WX2 (X=S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark band-edge excitons, which strongly influence the light emission properties of the material. Here we show how an in-plane magnetic field can brighten the dark excitonic states and allow their properties to be revealed experimentally in monolayer WSe2. In particular, precise energy levels for both the neutral and charged dark excitons were obtained and compared with ab-initio calculations using the GW-BSE approach. Greatly increased emission and valley lifetimes were observed for the brightened dark states as a result of their spin configuration. These studies directly probe the excitonic spin manifold and provide a new route to tune the optical and valley properties of these prototypical two-dimensional semiconductors.

• The paper demonstrates that applying in-plane magnetic fields brightens dark exciton states in monolayer WSe₂, allowing experimental probing of previously inaccessible states.
• The study employs photoluminescence spectroscopy and ab-initio GW-BSE calculations to quantify energy splitting and enhanced valley lifetimes of excitonic states.
• The results open pathways for controlling valley and spin transport in TMDC devices, advancing applications in valleytronics and opto-spintronics.

Magnetic Brightening and Control of Dark Excitons in Monolayer WSe$_2$

The research paper presented explores the intriguing phenomenon of magnetic brightening of dark excitonic states in monolayer tungsten diselenide (WSe$_2$). Transition metal dichalcogenide (TMDC) monolayers like WSe$_2$ are highly regarded due to their direct-bandgap properties and robust light-matter interactions. Of particular interest are the spin-forbidden "dark" excitonic states, which until now, have evaded direct experimental observation. The paper elucidates how in-plane magnetic fields can be employed to "brighten" these dark exciton states, thereby enabling their properties to be experimentally probed. This approach represents a significant advancement in understanding fundamental excitonic interactions and their electronic properties.

Methodology and Findings

By applying an in-plane magnetic field, the paper demonstrates how optical selection rules can be relaxed, leading to an observable photoluminescence (PL) from what were previously inaccessible dark exciton states. The experimental observations when compared with ab-initio calculations using the GW-BSE formalism, reveal a strong agreement regarding the energy levels of both neutral and charged dark excitons. The results bring forth several fascinating conclusions. Increased valley lifetimes of the brightened states were notably longer due to their spin configuration, enhanced by the presence of the magnetic field.

Furthermore, spectroscopic analysis disclosed that the energy splitting between bright and dark trion states is distinct from that of the neutral excitons. This divergence underscores the impact of exciton binding energies and provides quantitative insights into the conduction band structure and many-body interactions characteristic of these materials. The paper details how such measurements can lead to estimations of the splitting in conduction bands, crucial for valley and spin transport manipulation in TMDC-based systems.

Implications and Future Prospective

The implications of this paper are both far-reaching and multi-faceted. From a theoretical standpoint, the ability to brighten dark excitonic states using magnetic fields provides a novel framework for controlling optical and valley properties in two-dimensional semiconductors. This control holds significant promise for applications in valleytronics and opto-spintronics, where manipulation of valley degrees of freedom--often coupled with spin--is paramount.

Practically, the findings suggest exciting possibilities for the utilization of TMDC monolayers in designing optoelectronic devices with tunable properties. Particularly, the ability to access and control long-lived excitonic states could lead to the development of more efficient light-emitting devices and quantum state explorations such as Bose-Einstein condensates in TMDCs.

Conclusion

The paper presents robust numerical results supporting the hypothesis that in-plane magnetic fields afford a compelling technique for accessing dark excitonic states in monolayer WSe$_2$. By bridging experimental observations with theoretical predictions, this work enhances our understanding of excitonic interactions in TMDCs. It opens avenues for ongoing studies into the dynamic regulation of exciton radiative lifetimes and the manipulation of valley and spin characteristics, fostering advancements in quantum material research and device engineering. The methodology delineated in this paper sets a precedent for future research concerned with magnetic field-induced phenomena in two-dimensional semiconductors.