Recent Australian-led research has yielded the world’s first measurement of interactions between Fermi polarons in an atomically thin 2D semiconductor, using ultrafast spectroscopy capable of examining complex quantum materials.

Researchers at Swinburne University of Technology found the signatures of interactions between exciton polarons in experiments on the 2D semiconductor monolayer tungsten disulfide.

FLEET staff from Monash University and RMIT developed a theoretical model to explain the experimental signals. They found that long-range repulsive interactions are mediated by a phase-space-filling effect, while short-range attractive interactions lead to the formation of a cooperatively bound exciton-exciton electron state.

The material

Tungsten disulfide (WS2) is from the family of semiconducting transition metal dichalcogenides (TMDCs). When the bulk material is exfoliated into a single atomic monolayer (less than 1 nanometer thick), the physics of these 2D materials becomes really interesting and controllable.

Much of the intriguing physics is described by the origin and interactions of quasiparticles*. Excitons are such quasiparticles and they dominate the optical response of monolayer WS2. Excitons are formed when electrons from the valence band are excited in the conduction band. The void left behind (a hole) can then bind to the excited electron via Coulomb forces and thus form the exciton.

“This picture becomes more complex when there are excess electrons in the monolayer,” explains lead author Jack Muir. “These ‘reserve’ electrons can be in the conduction bands and do not interact directly with light. The exciton can then bind to these excess electrons and form trions.”

Fermi Polarons

But what happens if the density of the doping is increased? There is no longer just one electron for each exciton, but five, ten, hundreds… At this point, the exciton can be thought of as a defect in a sea of ​​electrons. Interactions between an exciton and the Fermi sea of ​​electrons lead to the formation of new quasiparticles – polarons.

As Monash Professor Meera Parish points out, “Having a defect in a Fermi Sea is a universal problem beyond 2D semiconductors. Polaron quasiparticles play an important role in a range of systems, including cold atomic gases and even the innermost crust of neutron stars.”

The experiment

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Multidimensional coherent spectroscopy (MDCS) uses four precisely controlled ultrafast laser pulses to reveal and quantify interactions.

“Most spectroscopic techniques, such as photoluminescence, are unable to separate interactions from single-particle response. MDCS is optimized for this,” explains corresponding author Professor Jeffrey Davis.

Varying the polarization of the different pulses revealed an interesting observation: there are interactions between Fermi polarons only when they are coupled to the same Fermi Sea.

“It was exciting, something like this had never been seen before in these systems and the physics behind it was new,” says Jack.

Fill stage space

The mechanism of these interactions was identified as coming from a phase-space-filling effect:

When an electron in the Fermi Sea interacts with one exciton as part of a polaron, it cannot be involved in the formation of another polaron. This results in a repulsive force between polarons, and explains the selective interaction we observe with the experiment.

Attractive interactions between polarons were also identified, leading to the formation of bipolarons. The remarkably large binding energy of these bipolarons is believed to be unique to tungsten-based TMDCs and is the result of the specific band arrangement in WS2

.

The identification of both repulsive and attractive interactions, and the underlying mechanisms, is an important step to fully understand Fermi polarons and quasiparticle interactions in general. With this input, we are one step closer to unraveling the rich physics of complex materials and mastering their remarkable macroscopic properties.

Highly bound bipolaron

Interactions between Fermi polarons in monolayer WS2 was published in nature communication in October 2022 (DOI 10.1038/s41467-022-33811-x). The work was funded by the Australian Research Council Center of Excellence in Future Low-Energy Electronics Technologies (FLEET).