Valley-dependent spin polarization in bulk MoS2 with broken inversion symmetry

Abstract

The valley degree of freedom of electrons is attracting growing interest as a carrier of information in various materials, including graphene, diamond and monolayer transition-metal dichalcogenides. The monolayer transition-metal dichalcogenides are semiconducting and are unique due to the coupling between the spin and valley degrees of freedom originating from the relativistic spin-orbit interaction. Here, we report the direct observation of valley-dependent out-of-plane spin polarization in an archetypal transition-metal dichalcogenide--MoS2--using spin- and angle-resolved photoemission spectroscopy. The result is in fair agreement with a first-principles theoretical prediction. This was made possible by choosing a 3R polytype crystal, which has a non-centrosymmetric structure, rather than the conventional centrosymmetric 2H form. We also confirm robust valley polarization in the 3R form by means of circularly polarized photoluminescence spectroscopy. Non-centrosymmetric transition-metal dichalcogenide crystals may provide a firm basis for the development of magnetic and electric manipulation of spin/valley degrees of freedom.

Spin-resolved photoemission study of epitaxially grown MoSe2and WSe2thin films

“…There has been only a few SARPES studies on MX 2 [37,38,39] despite the large interest on the material class, mainly due to the difficulty in preparing large area samples suitable for measurements in ultra-high vacuum environment, as current SARPES setups at the synchrotron source are bound to large beam spot size to allow maximum photon flux. Moreover, the previous SARPES works are still essentially on bulk samples of MX 2 , such as bulk inversion symmetry broken 3R phase of MoS 2 [37] or spin polarization induced by the space charge residing in the overlayer [38]. Of particular interest, Ref.…”

Spectroscopic Signatures of AA′ and AB Stacking of Chemical Vapor Deposited Bilayer MoS2

“…4,11 Robust valley polarization is assured by the lack of centrosymmetry in all monolayer TMDs as well as bilayer ones with AB stacking. 6 Furthermore, interlayer hopping has been shown to be significantly suppressed by the strong coupling between spin and valley pseudospin as well as layer pseudospin, 12À14 allowing for the signatures of different spin, valley, and layer states to be experimentally observed. Finally, the coherent lengths of a given symmetry are determined by material imperfections with some being intrinsic to the method of fabrication, including chemical vapor deposition (CVD) 15 and exfoliation.…”

Optically and Electrically Controllable Adatom Spin–orbital Dynamics in Transition Metal Dichalcogenides

“…Therefore, the adatom spin is written optically into a |↑ state, while the recombination of electron-hole pairs in MoS 2 [34] will lead finally to an equilibrium without free carriers. We note that the same optical spin orientation mechanism also remains effective if the notoriously strong excitonic effects in materials like MoS 2 [24,25] are accounted for.…”

“…Due to the SOC in MoS 2 , there are valley specific spin splittings of the CB in the Σ valley and VB in the K/K valley [21] usually referred as spin-valley coupling [23][24][25]. The variation of spin lifetimes obtained by including SOC also for the electrons of MoS 2 is shown as the red curve in the left panel of Fig.…”

“…Given the energy separation between the minima/maxima in CBs/VBs, charge doping thus selects the orbital character of carriers at the Fermi level in MX 2 . Due to the C 3v symmetry of adsorption sites, the coupling d m orbitals from M atoms and TM adatoms need to follow an [23][24][25]. This optically determined spin state can be further coupled to the adatom spin via spin-flip scattering, providing a mechanism of optical orientation of the adatom spin.…”