Researchers at ETH Zurich have explored how strong Coulomb interactions can be enhanced through engineered periodic potentials at the nanoscale. By creating a moiré pattern of ferroelectric domains using two layers of hexagonal boron nitride (hBN), they have generated a powerful electric field that affects electrons in an adjacent Transition Metal Dichalcogenide (TMD) layer while leaving excitons (electron-hole pairs) unaffected.
This unique experimental design allows for a clean optical probe of electron states through their interactions with excitons. Using resonant optical spectroscopy, they investigated how varying electron density influences the system, revealing compelling evidence of long-range Coulomb interactions. At specific filling factors of the moiré potential, they observed ordered states forming, indicating strong electron correlations. Furthermore, at higher doping levels, they detected significant changes in the optical excitation spectrum due to on-site electron interactions.
The purely electrostatic nature of this moiré potential represents a significant advantage over conventional TMD moiré materials, where both carriers and excitons are typically subjected to complex potentials arising from lattice reconstruction, polarization fields, and interlayer tunneling. In this system, since excitons remain unaffected by the moiré potential, any modifications in the optical spectrum can be attributed directly to electronic correlations and exciton-electron interactions.
Through differential reflection spectroscopy, they have successfully measured the modulation depth of the moiré potential by analyzing the trion binding energy. These measurements revealed a clear redshift of the trion and associated attractive polaron resonance. Additionally, they identified charge-ordered Mott-Wigner states at filling factors of 1/3 and 2/3 through the appearance of excitonic umklapp resonances.
This work demonstrates a versatile approach for creating electronic superlattices in virtually any target material. The method opens exciting possibilities for exploring exotic quantum phenomena, including chiral layer-pseudospin liquids and kinetic magnetism in TMD monolayers under electrostatic moiré patterns. By separating two identical TMD monolayers with a thin tunneling barrier and exposing them to identical periodic electrostatic potentials, the team can realize moiré electrons or holes with a layer pseudo-spin degree of freedom displaying near SU(2) symmetry, potentially enabling phenomena such as electrically controlled Feshbach resonances.
These findings provide deeper insight into electronic correlations in periodic potentials and their interactions with excitons, advancing fundamental understanding of quantum materials with promising applications in next-generation technologies.
Reference: “Confined Trions and Mott-Wigner States in a Purely Electrostatic Moiré Potential” by Natasha Kiper, Haydn S. Adlong, Arthur Christianen, Martin Kroner, Kenji Watanabe, Takashi Taniguchi and Atac İmamoğlu, 5 March 2025, Physical Review X. DOI: 10.1103/PhysRevX.15.011049
