Polaronic Electron Crystals in 2D Moiré Materials

Project Title: Polaronic Electron Crystals in 2D Moiré Materials Principal Investigator: Xiaoyang Zhu; Affiliation: Columbia University

Understanding and controlling quantum phases of matter are at the frontiers of materials research. Such quantum phases as exemplified by superconductivity may revolutionize a broad range of technologies. The groundbreaking discoveries of superconductivity and related quantum phenomena in twisted bilayer graphene in 2018 has opened the door to unprecedented opportunities to realizing quantum matter. This is because the formation of moiré pattern from the stacking of two-dimensional crystals allows the easy tuning of key physical parameters for the formation of quantum phases of electrons.

The PI aims to understand why such quantum phases form and, more importantly, how such quantum phases may be stabilized to high temperatures. If such quantum phases of electrons can be stabilized at close to room temperature, opportunities for applications are greatly increased. The PI will use the model system of hetero-bilayers of two-dimensional semiconductors called transition metal dichalcogenides that are known to form a range of quantum phases of electrons. Specifically, the proposed research will explore how the electrons couple to lattice vibrations called phonons to achieve extra stability. To probe electron-phonon interaction, the PI will measure the vibrations of the ordered electrons and determine the temperature dependence of these electron phases. In the former, a short laser pulse will perturb the ordered electron states to launch the oscillation of the electrons. The oscillation will be detected from the optical response by a time-delayed probe laser pulse. This experiment is akin to listening in on the ringing of a wine glass after it has been hit by a spoon. In the second experiment, the PI will detect signatures of the quantum phases as a function of temperature. Why a normal quantum phase electrons should decay with increasing temperature, their coupling to phonons may result in unique temperatures. These experiments will provide quantitative insight into the nature of correlated electrons, the potential energy landscape for ordering, and their interactions with phonons. Such understanding will guide the search of quantum mater, particularly those that are stable up to room temperature.