Symmetry-breaking and spin-blockage effects on carrier dynamics in single-layer tungsten diselenide

TL;DR

This study reveals how reducing WSe2 to a single layer alters carrier decay dynamics due to symmetry breaking and spin effects, with implications for ultrafast optoelectronic device design.

Contribution

It provides the first comparative analysis of carrier dynamics in single-layer versus bulk WSe2, highlighting the impact of symmetry-breaking on decay channels.

Findings

Single-layer WSe2 shows slower carrier decay than bulk.

Symmetry breaking lifts spin degeneracy, affecting decay pathways.

Distinct decay timescales are observed in monolayer WSe2.

Abstract

Understanding carrier creation and evolution in materials initiated by pulsed optical excitation is central to developing ultrafast optoelectronics. We demonstrate herein that the dynamic response of a system can be drastically modified when its physical dimension is reduced to the atomic scale, the ultimate limit of device miniaturization. A comparative study of single-layer (SL) tungsten diselenide(WSe2) relative to bulk WSe2 shows substantial differences in the transient response as measured by time- and angle-resolved photoemission spectroscopy (TRARPES). The conduction-band minimum in bulk WSe2, populated by optical pumping, decays promptly. The corresponding decay for SL WSe2 is much slower and exhibits two time constants. The results indicate the presence of two distinct decay channels in the SL that are correlated with the breaking of space inversion symmetry in the two-dimensional limit. This symmetry breaking lifts the spin degeneracy of the bands, which in turn causes the blockage of decay for one spin channel. The stark contrast between the single layer and the bulk illustrates the basic carrier scattering processes operating at different timescales that can be substantially modified by dimensional and symmetry-reduction effects.