Wigner Polarons Reveal Zero-Field Wigner Crystal Dynamics in Monolayer Semiconductors

The behaviour of electrons confined within materials can give rise to exotic states of matter, and researchers are continually seeking ways to understand and control these phenomena, particularly in two-dimensional materials. Lifu Zhang from the University of Maryland, College Park, Liuxin Gu, and Haydn S Adlong from ETH Zürich, alongside Arthur Christianen from ETH Zürich, Eugen Dizer from Universität Heidelberg, and Ruihao Ni, now report a significant advance in this field, revealing the internal dynamics of Wigner crystals, lattices formed solely by interacting electrons. They demonstrate that monolayer tungsten diselenide provides an ideal platform to host these crystals and, crucially, that exciton spectroscopy allows direct observation of both their static structure and dynamic behaviour, uncovering striking optical resonances they identify as ‘Wigner polarons’. This breakthrough not only provides a new way to probe correlated electron systems, but also enables all-optical control of spins within the crystal, paving the way for ultrafast manipulation of interaction-driven phase transitions.

Researchers demonstrate that monolayer tungsten diselenide provides an ideal environment to host these crystals and, crucially, that exciton spectroscopy allows direct observation of both their structure and movement. This breakthrough not only provides a new way to investigate correlated electron systems, but also enables all-optical control of spins within the crystal, paving the way for ultrafast manipulation of interaction-driven phase transitions.,.

Wigner Crystals Formed and Probed in WSe2

Scientists have established monolayer tungsten diselenide as a novel platform to host Wigner crystals, utilizing exciton spectroscopy to directly probe both their static and dynamic properties. The study fabricated devices consisting of monolayer WSe2 encapsulated within hexagonal boron nitride, with graphite serving as a gate electrode, enabling precise control over electron density. Reflectance contrast measurements, performed at very low temperatures, revealed key features indicative of Wigner crystal formation and behaviour. In the charge-neutral state, sharp exciton resonances were observed, transitioning to repulsive and attractive polaron branches when the sample was electron-doped, revealing the initial response of the material to applied voltage.

To detect Wigner crystals, researchers focused on umklapp scattering, where the periodic potential created by the crystal folds high-momentum excitons into detectable light. Voltage derivatives of the reflectance contrast spectra clearly revealed a secondary resonance, confirming the formation of a Wigner crystal below a specific electron density, a first-time observation in WSe2. Remarkably, the team also observed features originating from Wigner crystals above the attractive polaron states, identifying these as Wigner polarons, new quasiparticles arising from the dynamic properties of the crystal’s vibrational modes. These Wigner polarons exhibited a distinct energy shift with increasing electron density, distinguishing them from the static umklapp resonance.

Quantitative analysis involved fitting reflectance-contrast spectra to extract energies of the repulsive polaron, umklapp resonance, and Wigner polarons, yielding consistent results across multiple measurement techniques. The extracted repulsive polaron-umklapp splitting increased linearly with electron density, allowing scientists to determine an exciton mass consistent with prior measurements. By observing the disappearance of the umklapp resonance, the study estimated the conditions under which the Wigner crystal melts, suggesting an enhanced stability in WSe2 compared to theoretical predictions and other materials. The team further found that the energy splitting between Wigner polarons and attractive polarons scales with the fourth root of electron density, reflecting the density-dependence of characteristic Wigner crystal phonon energies.,.

Wigner Crystals and Exciton-Polaron Interactions

This research investigates the formation and properties of Wigner crystals in monolayer tungsten diselenide, focusing on the interaction between excitons, electrons in the Wigner crystal, and the resulting polarons. The study uses differential reflectance spectroscopy to probe these interactions and identify phase transitions within the system. Key findings include confirmation of Wigner crystal formation at low electron densities, characterization of different polarons, exciton-polarons and Wigner-polarons, and observation of a significant energy splitting between them, attributed to the localization of electrons in the Wigner crystal. This energy splitting is density-dependent and exhibits a significant zero-density intercept, suggesting a strong intrinsic interaction.

The Wigner crystal undergoes a thermal phase transition at around 25-30 Kelvin, with the Umklapp signature disappearing at this temperature. Spin polarization of the Wigner crystal has minimal effect on the Umklapp scattering, and high laser pump powers can reduce the energy of the Wigner polaron, potentially due to heating or other excitation mechanisms. Detailed analysis demonstrates the process of extracting the Umklapp peak energy, comparing different methods for determining the energy splitting, and presenting the density dependence of the energy splitting. The research provides strong evidence for the formation of Wigner crystals in monolayer tungsten diselenide and the interaction between excitons and electrons within these crystals. The observed energy splitting between the exciton-polaron and Wigner-polaron is a key signature of this interaction, and the temperature dependence of the Umklapp signature confirms the thermal phase transition of the Wigner crystal.,.

Wigner Crystals Controlled and Probed Optically

This research establishes monolayer tungsten diselenide as a platform to host zero-field Wigner crystals, demonstrating a new method to probe both their static and dynamic properties using exciton spectroscopy. The team directly observed Wigner polarons, quasiparticles formed by the interaction between excitons and the electron lattice, providing critical insight into the internal dynamics of these correlated electron systems. Furthermore, they achieved all-optical control of spins within the Wigner crystal, successfully probing valley-dependent Wigner polaron scattering without the need for external magnetic fields. The study also demonstrates optical melting of the Wigner crystal, revealing differing responses between static and dynamic resonances to optical excitation, which suggests a complex interplay of interactions. While acknowledging that further theoretical work is needed to fully understand these observations, the researchers highlight the potential for exciton-assisted optical melting to reveal new aspects of quantum criticality and enable ultrafast control of quantum phase transitions. Future studies will focus on elucidating the interplay between exciton-electron and electron-electron interactions, potentially leading to novel optoelectronic and quantum devices that leverage the unique magnetic susceptibility near the quantum melting of Wigner crystals.