Optical Spin and Valley Excitations


2D semiconductors provide opto-electronic and opto-spintronic functionality to 2D materials and van der Waals heterostructures. Transition metal dichalcogenides MX2 (with M being Mo or W, and X being S or Se) have very interesting properties. In the monolayer limit, these are direct gap semiconductors with the gap located at the K and -K points of the Brillouin zone. Notably, excitons in the K and -K valleys can be generated selectively in each valley using circularly polarized light to promote electrons from the valence band to the conduction band, leading to a valley polarization that depends on the helicity of the light. In addition, due to a large spin-orbit coupling, the valence band splits into spin up and spin down bands (spins perpendicular to the 2D sheet) with large splittings of a few hundred meV. The conduction band is also spin-split with smaller magnitude of a few tens of meV. Since the spin splittings have opposite sign for the K and -K valleys, this produces a strong spin-valley coupling in the band structure. Thus, circularly polarized optical excitation can be used to excite both spin and valley degrees of freedom of the electrons and holes.

Highlights


Opto-Valleytronic Spin Injection in Monolayer MoS2/Few-Layer Graphene Hybrid Spin Valves

Two-dimensional (2D) materials provide a unique platform for spintronics and valleytronics due to the ability to combine vastly different functionalities into one vertically stacked heterostructure, where the strengths of each of the constituent materials can compensate for the weaknesses of the others. Graphene has been demonstrated to be an exceptional material for spin transport at room temperature; however, it lacks a coupling of the spin and optical degrees of freedom. In contrast, spin/valley polarization can be efficiently generated in monolayer transition metal dichalcogenides (TMD) such as MoS2 via absorption of circularly polarized photons, but lateral spin or valley transport has not been realized at room temperature. In this study, we fabricated monolayer MoS2/few-layer graphene hybrid spin valves and demonstrated, for the first time, the opto-valleytronic spin injection across a TMD/graphene interface. As shown in the picture above, spin-polarized electrons were generated in the MoS2 layer using circularly polarized photon. These electrons subsequently transferred into the graphene layers and diffused toward a ferromagnetic spin detector. We observed that the magnitude and direction of spin polarization could be controlled by both helicity and photon energy. In addition, Hanle spin precession measurements (shown in the figure above) confirmed optical spin injection, spin transport, and electrical detection up to room temperature. These results demonstrate a 2D spintronic/valleytronic system that achieves optical spin injection and lateral spin transport at room temperature in a single device, which paves the way for multifunctional 2D spintronic devices. For more information, see Luo et al., “Opto-Valleytronic Spin Injection in Monolayer MoS2/Few-Layer Graphene Hybrid Spin Valves,” Nano Letters 17, 3877−3883 (2017)


Valley Spin Dynamics in Monolayer WS2

Monolayer transition metal dichalcogenides (TMD) have immense potential for future spintronic and valleytronic applications due to their 2D nature and long spin/valley lifetimes. In this study, we investigated the origin of these long-lived states in n-type WS2 using time-resolved magneto-optic Kerr rotation (TR-MOKE) microscopy and photoluminescence microscopy with ~1 μm spatial resolution. For TR-MOKE, a circularly pump pulse tuned to the absorption edge of the monolayer WS2 generates a valley polarized exciton, and a time-delayed linearly polarized probe pulse exhibits a Kerr rotation of its linear polarization, which is related to the spin and valley polarization. A typical time-delay scan is shown to the right. Next, by moving the sample underneath the overlapped pump and probe spots, we are able to map out the variation of the time-delay signal as a function of position. Typical image maps taken at different pump-probe time delays are shown in the figure to the left. In comparing the spatial dependence of the Kerr rotation signal and the photoluminescence, we observed a correlation of the Kerr rotation with neutral exciton emission and an anticorrelation with trion emission, which provides evidence for the presence of long-lived spin/valley polarized dark trions. For further information, see McCormick et al., “Imaging spin dynamics in monolayer WS2 by time-resolved Kerr rotation microscopy,” 2D Materials 5, 011010 (2018).


Time-Resolved Angle-Resolved Photoemission Spectroscopy (tr-ARPES) of monolayer TMDs and heterostructures

While the TR-MOKE method discussed above is powerful for measuring spin and valley dynamics in monolayer TMDs, the main limitation is that the Kerr rotation measures a combination of the spin and valley polarization and the relative contributions are not always clear. The advantage of ARPES is that the measured electron distribution is momentum-resolved and energy-resolved. Therefore, in a time-resolved measurement, one can directly measure the dynamics of valley polarization of electrons and holes. For such experiments, a circularly polarized pump pulse at the bandgap energy will generate valley-polarized excitons in the monolayer TMD. A time-delayed XUV probe pulse (energy from 15 – 100 eV) then causes the electrons to be emitted from the surface into vacuum, and measuring the energy and angular distribution of the photoelectrons tells us about their energy and momentum distribution in the solid prior to photoemission. By varying the time delay, we are thus able to map out how the electrons move throughout the band structure with 300 fs or 40 fs temporal resolution (depending on measurement mode).

The newest  generation of tr-ARPES systems have a front end that is a photoemission electron microscope (PEEM), which utilizes electron optics to image the sample surface with a spatial resolution of about 50 nm in “real space” mode. In “momentum microscope” mode, the tr-ARPES can be performed with a spatial resolution as good as ~2 micrometers. Therefore, the time-resolved microARPES allows the investigation of electron and valley dynamics on small, exfoliated 2D materials and van der Waals heterostructures. Currently, there are only a few such tr-microARPES systems in the world, including one at Stony Brook University built by Prof. Tom Allison. We have been involved in experiments at Stony Brook which are the first to report direct momentum-resolved measurement of valley polarization dynamics in a monolayer TMD semiconductor. In this study, exfoliated monolayer WS2 on hBN is probed by tr-ARPES for different helicities of the pump pulse, and high-harmonic-generated XUV pulses for the time-delayed probe. Read more about this in Kunin et al., “Momentum-Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS2

We are working with Prof. Allison to bring this capability to the Ohio State University as part of the NSF NeXUS user facility, where it will be coupled with a kilowatt-class laser for wider XUV photon energy range. The image above shows the recently installed Specs Kreios PEEM-ARPES endstation that will be used for the experiments at Ohio State.

 


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