Giant spin-orbit-induced spin splitting in two-dimensional transition-metal dichalcogenide semiconductors

Abstract

Fully relativistic first-principles calculations based on density functional theory are performed to study the spin-orbit-induced spin splitting in monolayer systems of the transition-metal dichalcogenides MoS 2 , MoSe 2 , WS 2 , and WSe 2 . All these systems are identified as direct-band-gap semiconductors. Giant spin splittings of 148-456 meV result from missing inversion symmetry. Full out-of-plane spin polarization is due to the two-dimensional nature of the electron motion and the potential gradient asymmetry. By suppression of the Dyakonov-Perel spin relaxation, spin lifetimes are expected to be very long. Because of the giant spin splittings, the studied materials have great potential in spintronics applications.

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides

“…The lack of inversion symmetry, confinement of electron motion in plane and high mass of the elements in the MX 2 materials lead to a very strong spin-orbit splitting, with the valence-band splittings ranging between 0.15 and 0.45 eV (ref. 154). This is in contrast to graphene, which has very weak spin-orbit interaction primarily owing to the low mass of carbon.…”

“…In the group-IV semiconducting dichalcogenides (MoS 2 , MoSe 2 , WS 2 and WSe 2 ), a unique set of conditions gives rise to both strong spin-orbit-induced electronic band splitting 154 and spin-valley coupling 153 . Monolayer TMDCs such as MoS 2 lack inversion symmetry, as seen in the structural diagrams in Fig.…”

Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides

“…1.9 eV) at these two valley points allow for valley-polarization by optical pumping [163][164][165]. Owing to the broken inversion symmetry, spin-orbit interactions split the valence bands by approximately 160 meV and the spin projection, Sz, is well defined along the c-axis of the crystal with the two bands being of spin down (E↓) and spin up (E↑) in character [62,164,166]. This broken spin degeneracy together with timereversal symmetry leads to inherent coupling of the valley and the spin of the valence bands in monolayer MoS 2 leading to the valley-dependent optical selection rule [163,165].…”

“…This indirect-to-direct gap crossover from bulk to monolayer is also evident from a sudden increase in photoconductivity at approximately 1.8 eV in the monolayer MoS 2 as a consequence of the latter being a direct-band gap semiconductor [60]. Monolayer MoS 2 (space group, P-6 m2) has time-reversal symmetry [E ↑ (k) = E ↑ (−k)] due to which the spin-orbit coupling (SOC) lifts the spin degeneracy of otherwise twofold degenerate valence bands at the K point into two bands with spin-up and spin-down character (figure 14a) [61,62]. Thus two direct excitonic transitions, namely A1 and B1, are allowed at the K point (figure 6a, inset).…”

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

“…1À3 In many senses, the most unique property of the TMD family of materials consisting of combinations of Mo, W, S, and Se is the strong spinÀorbit coupling. 4 This originates from the d-orbitals of the heavy elements (Mo and W) 5 and offers exciting opportunities for device applications, leveraging spin and valley degrees of freedom. 6À8 Compared to the more studied monolayer TMDs, bilayer structures offer an added layer degree of freedom.…”