RQS Seminar: Wigner crystal polarons & quantum anomalous Hall states

Dr. You Zhou will be presenting on two papers, "Spectroscopy of Wigner crystal polarons in an atomically thin semiconductor" and "Probing Quantum Anomalous Hall States in Twisted Bilayer WSe₂ via Attractive Polaron Spectroscopy."

Abstract for "Spectroscopy of Wigner crystal polarons in an atomically thin semiconductor": Strongly interacting electrons in two-dimensional systems can spontaneously break translational symmetry, forming a periodic Wigner crystal. Although these crystals have been realized in several platforms, experimental studies of their collective many-body excitations in the absence of a magnetic field have not been demonstrated. Here we access this regime optically by uncovering Wigner crystal polarons, which are hybrid light–matter quasiparticles that arise from the dressing of excitons by the collective excitations of the Wigner crystal. These polarons manifest as optical resonances in the cryogenic reflectance spectra of a charge-tunable WSe₂ monolayer, appearing concurrently with previously identified exciton umklapp transitions. In contrast to the latter, the energies of Wigner crystal polarons are governed not only by the electronic lattice constant but also by their hybridization with attractive exciton–polarons, whose strength is controlled by electronic interactions. These many-body excitations provide an optical interface to the spin state of the Wigner crystal that, as we demonstrate, can be controlled both magnetically and optically. Our work establishes layered materials as a platform for exploring dynamical impurity dressing by strongly correlated electronic orders. 

Abstract for "Probing Quantum Anomalous Hall States in Twisted Bilayer WSe₂ via Attractive Polaron Spectroscopy": Moiré superlattices in semiconductors exhibit a rich variety of interaction-induced topological states, including quantum anomalous Hall (QAH) effects [1–8]. A recent study hinted that twisted WSe₂ homobilayer (tWSe₂) could host a QAH state but lacked direct evidence of ferromagnetism, a key hallmark of this phase [9]. Here, we report the first direct evidence of QAH states in tWSe₂ with spontaneous ferromagnetism. Specifically, we employ polarization-resolved attractive polaron spectroscopy on a dual-gated, 2° tWSe₂ and observe direct signatures of spontaneous time-reversal symmetry breaking at hole filling 𝜈 =1. Together with a Chern number (𝐶) measurement via Streda formula analysis, we identify this magnetized state as a topological state, characterized by 𝐶 =1. Furthermore, we demonstrate that these topological and magnetic properties are tunable via a finite displacement field, between a QAH ferromagnetic state and an antiferromagnetic state. Our findings position tWSe₂ as a highly versatile, stable, and optically addressable platform for investigating topological order and strong correlations in two-dimensional landscapes.