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Excitons, bound states between an electron (i.e., a negatively charged particle) and a hole (i.e., the absence of an electron) in materials, are a key focus of condensed matter physics studies. These bound states can give rise to interesting and uncommon quantum physical effects, which could be leveraged to develop optoelectronic and quantum technologies.

Over the past few years, physicists have observed a particular type of excitons, known as interlayer excitons, in various materials with two layers (i.e., bilayer materials). An interlayer exciton is a bound state between an electron and a hole that reside in two different layers of a material.

Researchers at Harvard University and other institutes recently observed an unconventional hybridization between interlayer excitons in a bilayer semiconductor, comprised of two layers of molybdenum disulfide (MoSâ‚‚).

Their paper, in Nature Â鶹ÒùÔºics, could offer indirect experimental evidence of a many-body state that has long been theorized, but had not yet been observed experimentally.

"In this field of two-dimensional semiconductors, particularly those based on transition metal dichalcogenides, researchers have pursued two major research directions," Pavel E. Dolgirev, co-author of the paper, told Â鶹ÒùÔº.

"The first is driven by the fact that these are direct band-gap materials hosting optical excitons (an exciton is a bound state of an electron and a hole, much like an atom), which makes them highly promising for electro-optical devices. The second is that their dielectric environment and accessible charge densities place these materials in the strongly interacting two-dimensional regime—a regime we understand very little about theoretically but can nonetheless probe experimentally."

Interlayer excitons in bilayer structures carry a large dipolar moment, meaning that positive and negative charges in these materials are separated by a relatively large distance. As a result of this large dipolar moment, the excitons are highly sensitive and responsive to applied electric fields and noise in the form of electric fields.

"An especially intriguing idea is that an equal-weight superposition of such opposing dipoles would form a state that no longer couples to electric fields at all," said Dolgirev.

"With the aim of understanding and controlling the coherence properties of indirect excitons, we studied the Stark effect and discovered very anomalous behavior that emerges once the sample is doped. This observation underpins our main conclusion about interlayer electron coherence."

As part of their experiments, Dolgirev and his colleagues set out to detect the coherence between interlayer electrons in the bilayer semiconductor MoSâ‚‚ using an optical technique. Specifically, they illuminated a bilayer MoSâ‚‚ sample using broadband white light and measured the reflected signal, all while tuning the electron density via a gate voltage.

"This optical approach is particularly powerful: it allows us to selectively probe specific spin and valley states through well-defined optical transitions," explained Nadine Leisgang, co-author of the paper.

"Moreover, interlayer excitons—where the electron and hole reside in different layers—are highly sensitive to electric fields and their electronic environment. By carefully analyzing how the excitonic features in the reflected spectra evolved with electron density, temperature, and magnetic field, we were able to identify clear signatures of interlayer electron coherence."

The team's experimental observations suggest that interlayer excitons, which are typically uncoupled, did in fact hybridize once their sample was doped. This essentially means that two exciton states "mixed together," producing new and shared states. The hybridization they observed is highly unusual, yet it could open new possibilities for the indirect electronic manipulation of the coherence between excitons.

"We also found indirect signatures of interlayer electron coherence," said Dolgirev. "This is significant not only because detecting such a state without an applied magnetic field has long been a longstanding challenge, but also because we observe these signatures (corresponding to the so-called stochastic anti-crossing) at temperatures as high as 75 K. This raises the intriguing possibility of superfluid-like behavior at rather high temperatures."

The exciton hybridization observed by Dolgirev and his colleagues could be a precursor of a so-called exciton condensation, a long-theorized collective quantum state involving several bound electron-hole pairs.

They are now trying to determine whether a similar hybridization can also occur between so-called quadrupolar excitons in trilayer materials, which also hold promise for the development of optoelectronic devices.

"Additionally, we are also exploring whether this hybridization can be made fully coherent, rather than stochastic, by twisting the layers relative to one another," added Dolgirev.

"According to our conclusions, such a twist should stabilize the order-parameter phase, thereby reducing stochasticity in the hybridization. Concurrently, we also envision experiments that can shed more light on the underlying electronic many-body state."

The synthesis of increasingly cleaner semiconducting materials that exhibit a broader range of carrier transport properties could also help to validate the team's findings in the future. For instance, so-called counterflow experiments with cleaner materials could yield a direct signature of the interlayer electron coherence that they indirectly observed as part of their recent study.