Zero Magnetic Field Brightens Hidden Signals in Novel Material

Xinyun Wang and colleagues from National University of Singapore and Quantum Innovation Centre and Nanyang Technological University and National University of Singapore and Institute of Materials Research and Engineering have activated typically inaccessible ‘dark’ excitons in monolayer WSe2 without external magnetic fields. By using a ferroelectric hybrid perovskite heterostructure, they successfully broke the rotational symmetry of the WSe2, effectively ‘brightening’ these spin-forbidden dark excitons. The electrically reconfigurable approach, supported by a tight-binding model revealing an asymmetric intersublattice interaction and induced spin-orbit coupling, represents a key advancement towards practical spin exciton control and opens new avenues for information processing technologies.

Ferroelectric perovskites unlock control of dark excitons in tungsten diselenide heterostructures

The technique central to this work relies on the ferroelectric proximity effect, a phenomenon analogous to influencing a compass with a nearby magnet without direct contact. Layering tungsten diselenide (WSe2) with a ferroelectric perovskite material creates a heterostructure where the perovskite’s internal electric field extends into the adjacent WSe2 monolayer. This field breaks the rotational symmetry within the WSe2, a crucial step in activating typically inaccessible dark excitons. Dark excitons, unlike their ‘bright’ counterparts, are spin-forbidden, meaning they do not readily absorb or emit light under normal circumstances. This characteristic makes them attractive for information storage and processing, as they are less susceptible to environmental noise, but also presents a significant challenge for their observation and manipulation. These dark excitons represent a hidden pathway for electrons to carry information, normally invisible but now made accessible through this innovative approach. The ability to control these excitons without external magnetic fields is particularly significant, as magnetic fields can be cumbersome and energy-intensive to maintain.

Scientists fabricated a vertical heterostructure combining monolayer tungsten diselenide with the perovskite material, TMA3Sb2Cl9, to induce this effect. TMA3Sb2Cl9 was chosen for its relatively high dielectric constant and its optical transparency in the visible spectrum, allowing for efficient excitation of the WSe2 monolayer. A 6.7° twist angle was determined for one sample using second-harmonic generation (SHG) measurements, carefully controlling the coupling strength and resulting polarization. SHG is a nonlinear optical process highly sensitive to symmetry; changes in the SHG signal directly indicate alterations in the symmetry of the heterostructure. The heterostructure benefits from a clean interface and the perovskite’s optical transparency, both important for efficient field transfer and minimal interference during light excitation. Precise control over the interface quality is achieved through careful material preparation and deposition techniques, ensuring optimal proximity effect coupling. The choice of TMA3Sb2Cl9 also minimises unwanted absorption or emission that could mask the excitonic signals from the WSe2.

Ferroelectric Coupling and Rotational Symmetry Breaking Brighten Dark Excitons in Twisted WSe2

A 6.7° twist angle between tungsten diselenide and a perovskite crystal proved key, a threshold previously requiring strong 8 Tesla magnetic fields for comparable results. This breakthrough splits the dark exciton manifold into strictly dark and grey excitons, offering a pathway towards electrically reconfigurable spin and valley control in two-dimensional semiconductors. The twist angle demonstrably breaks the rotational symmetry of the WSe2 monolayer, evidenced by the emergence of previously forbidden Raman modes, typically only seen in strained WSe2, and a modified second-harmonic generation pattern inconsistent with either material’s individual symmetry. Raman spectroscopy provides information about the vibrational modes of the material, and the appearance of new modes confirms the structural changes induced by the perovskite. Magneto-photoluminescence measurements revealed that applying an 8 Tesla magnetic field brightened newly observed excitonic peaks in the heterostructure, confirming their spin-forbidden, or ‘dark’, nature. This served as a benchmark to validate the effectiveness of the ferroelectric approach in achieving a similar effect without the need for a magnetic field. Currently, however, data on long-term stability or scalability beyond small sample sizes are lacking, hindering immediate translation into functional devices. Further research is needed to address these challenges and explore the potential for large-scale fabrication and reliable operation under realistic conditions. Understanding the degradation mechanisms and developing encapsulation strategies will be crucial for practical applications.

Revealing previously hidden exciton states via ferroelectric brightening

Controlling the spin of electrons holds immense promise for next-generation computing and data storage, offering the potential for faster, more energy-efficient devices. Conventional electronic devices rely on charge, but spin-based devices, or spintronics, leverage the intrinsic angular momentum of electrons. This work successfully brightens ‘dark excitons’, quasiparticles that can carry spin information, without relying on bulky, power-hungry magnets, a longstanding challenge in the field. Excitons are bound electron-hole pairs, and their spin state can be manipulated for information processing. It is important to acknowledge that this work presently demonstrates visibility rather than definitive control over exciton spin, representing a vital first step towards realising genuinely steerable spin-based devices. The ability to observe these dark excitons opens the door to exploring methods for actively controlling their spin and valley degrees of freedom, potentially leading to novel spintronic devices.

A new method for manipulating spin excitons, quasiparticles with the potential to carry information, has been established within two-dimensional materials. Tungsten diselenide combined with a perovskite crystal bypassed the need for strong magnetic fields traditionally required to activate these ‘dark excitons’, normally hidden from optical detection. The interaction strength and exciton polarization are influenced by the angle between these layered materials. This electrically reconfigurable control mechanism, achieved through the ferroelectric proximity effect, offers a significant advantage over traditional methods relying on magnetic fields. The potential applications extend beyond computing and data storage, encompassing areas such as quantum information processing and optoelectronics. Further investigation into the underlying physics and materials optimisation will be essential to fully unlock the potential of this promising approach.

The research successfully brightened dark excitons in monolayer tungsten diselenide without the use of external magnetic fields. This is important because it demonstrates a new electrically reconfigurable method for accessing and potentially controlling the spin of these quasiparticles, which could be used for information processing. By combining the tungsten diselenide with a perovskite crystal, researchers leveraged the ferroelectric proximity effect to break symmetry and reveal the normally hidden excitons. The study indicates that the angle between the two materials influences the strength of this interaction and the resulting exciton polarization.