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Two-dimensional (2D) materials have proved to be a promising platform for studying exotic quasiparticles, such as excitons. Excitons are bound states that emerge when an electron in a material absorbs energy and rises to a higher energy level, leaving a hole (i.e., the absence of an electron) at the site that it used to occupy.

Researchers at Heriot-Watt University and other institutes recently observed two distinct exciton states in bilayer molybdenum diselenide (MoSeâ‚‚) with a 2H-stacked configuration, which involves the alignment of two monolayers with a characteristic rotational symmetry. Their paper, in Â鶹ÒùÔºical Review Letters, reports the observation of one of these states known as quadrupolar excitons in 2H-MoSeâ‚‚.

"Our work was inspired by the ongoing effort to explore and control excitonic phenomena in atomically thin semiconductor materials, which are rich platforms for studying complex quantum states," Mauro Brotons-Gisbert, senior author of the paper, told Â鶹ÒùÔº. "In particular, bilayer transition metal dichalcogenides (TMDs) like MoSeâ‚‚ naturally host interlayer excitons with a dipolar character—bound states of electrons and holes residing in adjacent layers."

An interesting feature of dipolar interlayer excitons is that they can be manipulated using external and magnetic fields. Past physics studies have predicted and demonstrated that these excitons can give rise to a wide range of exotic collective states.

"In this study, our objective was to investigate the less-understood exciton states that emerge from the strong hybridization of dipolar interlayer excitons in naturally stacked bilayer MoSe₂," said Brian Gerardot, another senior author of the paper. "We were particularly interested in the spin, valley, and layer degrees of freedom of these excitonic quasiparticles and their unusual optical responses—and that led us to the observation of quadrupolar excitons."

As part of their recent study, Brotons-Gisbert and his colleagues specifically studied a sample of high-quality bilayer MoSeâ‚‚, which was encapsulated in hexagonal boron nitride and equipped with dual electric gates. These gates allowed them to precisely control the electric field across the material's layers.

Using a technique known as helicity-resolved reflectance contrast spectroscopy, the researchers measured changes in the material's optical absorption that took place when they applied different electric and magnetic fields to their sample. Their measurements ultimately allowed them to uncover various interlayer exciton states.

"One of these states behaved very differently: its energy shifted quadratically (not linearly) with the applied electric field and its optical response didn't match that of typical dipolar excitons," explained Shun Feng, first author of the paper. "This distinct behavior matched our theoretical model for a quadrupolar exciton, formed by the hybridization of dipolar exciton states with equal and opposite dipole moments."

The most notable result of the team's experiments was the identification of quadrupolar excitons in 2H-MoSeâ‚‚ as well as their origin. A key feature of these excitons is that while they have no net dipolar moment, they still respond to external fields and can thus be controlled experimentally.

"From a scientific perspective, this expands our understanding of excitonic interactions in 2D materials, particularly in regimes where hybridization and symmetry breaking play a key role," said Feng. "From a practical standpoint, quadrupolar excitons could offer new ways to simulate complex quantum systems using engineered exciton interactions."

This research group's recent work offers direct evidence that quadrupolar excitons exist in naturally stacked bilayer MoSeâ‚‚, demonstrating that these excitons can be tuned using electric fields. In the future, it could pave the way for deeper explorations of many-body interactions in 2D materials, while also potentially informing the future development of optoelectronics and quantum technologies based on bilayer MoSeâ‚‚.

"We plan to build on our current results by further exploring exciton complexes in bilayer TMDs, particularly their potential for simulating many-body quantum phenomena," added Brotons-Gisbert. "One of our key goals is to understand how to control and tune these complexes using external fields, strain, or stacking configurations. By studying their interactions in detail, we hope to use them as a platform to emulate strongly correlated systems—a key step toward exciton-based quantum simulators and devices."

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