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May 16 2025
2Scientists at Heriot-Watt University, along with collaborators from other institutions, have reported the observation of two distinct exciton states in bilayer molybdenum diselenide (MoSe₂) with a 2H stacking configuration. This work, recently published in Physical Review Letters, highlights the identification of a novel excitonic state known as the quadrupolar exciton—offering new insights into the behavior of quasiparticles in atomically thin materials.
Excitons are quasiparticles formed when an electron in a material absorbs energy and jumps to a higher energy level, leaving behind a positively charged hole. The electron and hole are then bound together by Coulombic attraction, creating an exciton. Two-dimensional materials like MoSe₂ serve as excellent platforms for investigating such quantum phenomena due to their unique optical and electronic properties.
"Two-dimensional semiconductors, especially bilayer transition metal dichalcogenides such as MoSe₂, offer fertile ground for studying complex quantum states," explained Mauro Brotons-Gisbert, a senior author of the study. "Our goal was to probe deeper into the lesser-understood exciton states that result from strong hybridization in naturally stacked 2H-MoSe₂."
The research team focused on interlayer excitons—states where the electron and hole reside in adjacent layers of the bilayer structure. These dipolar excitons, which carry a net electric dipole moment, can be manipulated using external electric and magnetic fields, making them ideal for exploring exotic many-body states.
In their experiments, the researchers worked with high-quality bilayer MoSe₂ samples encapsulated in hexagonal boron nitride. The devices were equipped with dual electric gates, enabling precise control of the electric field across the layers. Using helicity-resolved reflectance contrast spectroscopy, they observed the optical response of the system under varying electric and magnetic fields.
"Among the excitonic states we observed, one displayed an unexpected quadratic energy shift in response to an electric field—unlike the typical linear shift seen in dipolar excitons," noted Shun Feng, the study's lead author. "This behavior was consistent with our theoretical model of a quadrupolar exciton, formed through the hybridization of oppositely oriented dipolar exciton states."
Quadrupolar excitons, while lacking a net dipole moment, still respond to external fields due to their internal charge distribution. This makes them both scientifically intriguing and practically useful for engineered quantum systems.
"This finding not only expands our understanding of excitonic physics in 2D materials, particularly where symmetry and hybridization are crucial, but also introduces a potential pathway for simulating complex quantum systems," said Feng.
Looking ahead, the research team aims to explore further exciton complexes in bilayer TMDs, with a focus on tuning these states via external fields, mechanical strain, or layer configuration. "Our long-term objective is to use these excitonic systems to emulate strongly correlated quantum states, potentially leading to the development of exciton-based quantum simulators and next-generation optoelectronic devices," added Brotons-Gisbert.
Source:https://phys.org/news/2025-05-distinct-exciton-states-2h-stacked.html
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