Quantum Materials - 04

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) can be a platform for valleytronics [1] because they can host excitons with the valley degrees of freedom. 2D TMDs have two independent valleys, and these valleys can be selected by circularly polarized light excitation due to inversion symmetry breaking and strong spin-orbit coupling. Moreover, coherent superpositions of valleys can also be created using linearly polarized light excitation. This optical manipulation can pave the way for future quantum technology based on the valley degrees of freedom. However, the rapid decoherence of the valley superposition [2] poses a challenge for further development. In our previous study, we investigated valley coherence in a moiré superlattice made of WSe₂ and WS₂, aiming to extend the valley coherence time through polarization-resolved PL spectroscopy. However, we did not observe a significant increase in the degree of linear polarization, which is a key measure of valley coherence. This finding suggests that the valley coherence time is considerably shorter than the lifetime of moiré excitons [3].

In this study, we aimed to utilize the Stark effect to enhance valley coherence. Additionally, to isolate coherence time independent of exciton lifetime, we conducted polarization- and time-resolved PL measurements. The stark effect is expected to suppress valley exchange interactions, which are one of the main causes of decoherence. Figures 1a and b present a 2D map illustrating the PL intensities of moiré excitons plotted against the top and bottom gate voltages, along with typical PL spectra measured with and without displacement fields. The significant changes in PL intensity and peak position clearly demonstrate the successful modulation of PL by the Stark effect. In my poster presentation, I will discuss in detail how the Stark effect influences valley coherence.