Researchers observed Wigner crystal polarons in an atomically thin semiconductor, according to a study published in Nature Physics. The team detected these hybrid light–matter quasiparticles using cryogenic optical reflectance measurements on a charge-tunable tungsten diselenide monolayer.
The optical experiments, conducted at temperatures down to 1.6 Kelvin, demonstrated that the polarons emerge when neutral excitons couple to the collective excitations of an underlying Wigner crystal. The polarons appeared as umklapp replicas of attractive exciton–polarons across singlet and triplet configurations.
Spectral signatures and hybridization
Unlike standard exciton umklapp transitions, the Wigner crystal polarons displayed non-zero energy offsets in the low-electron-density limit. The singlet and triplet polaron branches exhibited energy offsets of approximately 5 millielectronvolts and 6 millielectronvolts, respectively, maintaining a separation of about 7 millielectronvolts. Spectral fits yielded a translational exciton mass of roughly 0.68 times the free electron mass.
Theoretical analysis using the Chevy approximation indicated that the polarons form through interaction-mediated hybridization between bare and Bragg-scattered states. The hybridization strength increased with the magnitude of the many-body Wigner crystal gap, yielding polaron oscillator strengths estimated between 10% and 20% of the main attractive polaron resonances.
Phase stability and spin control
The Wigner crystal phase persisted up to an electron density of approximately 7 × 1011 per square centimetre and survived up to a critical melting temperature of around 30 Kelvin. Thermal fluctuations suppressed the phase boundary above this temperature, matching a dome-shaped phase diagram.
Researchers altered the spin state of the electronic crystal by applying an external 5-tesla magnetic field and by illuminating the sample with circularly polarized light. At 5 tesla, the valley Zeeman effect induced circular polarization in the polaron resonances, mirroring the helicity behavior of the corresponding attractive polarons.
