Spin-valley coupling in 2-dimensional transition metal dichalcogenides

Abstract

Atomically thin layers of group VI transition metal dichalcogenides (TMDCs) have been recognized as a promising material for ultimately thin electronic devices and emerging electronics, particularly the conceptual valley based electronics (Valleytronics). The family of TMDCs formulated by MX2 (M = Mo, W; X = S, Se) has a common structure of X-M-X covalently bonded hexagonal quasi-2D network weakly stacked by weak Van der Waals forces. Bulk TMDCs crystals are indirect band gap semiconductors. As the TMDC is reduced to single layer (monolayer), the band gap becomes a direct one located at the corners of the 2D hexagonal Brillouin zone denoted as K and -K valleys. These two energetically degenerate but inequivalent valleys offering a new degree of freedom for low energy carriers can be conceptually treated as a potential information carrier, in analogy to the charge and spin degree of freedom. Recent advances reveal intriguing valley-dependent properties arising in monolayers, for example valley-dependent optical selection rule, nonzero but contrasting Berry curvature at the two valleys, and spin-valley coupling, etc. These exotic properties make atomically thin TMDCs crystals an unprecedented platform for physics study as well as emerging electronics.

The study in this thesis elaborates an experimental demonstration of spin polarization via controlling of the polarization field of the optical pumping in monolayer WS2. The valley polarization was realized by controlling the polarization field of interband optical excitations and the spin polarization was simultaneously generated via spin-valley locking in monolayer WS2. The spin polarization was electrically detected by a lateral spin-valve structure consisting of a tunneling barrier of Al2O3 and superlattice structured Cobalt-Palladium (Co/Pd) ferromagnetic electrodes with perpendicular magnetic anisotropy (PMA). A near-unit spin polarization was observed as well as long spin lifetime and spin free path for holes were demonstrated, which gave a direct evidence on spin-valley locking in monolayer TMDCs. The study demonstrates the potential application of monolayer TMDCs in the field of semiconductor based spintronics and valleytronics.