B08 - Ultrafast spin dynamics at the interface between TMDCs and metals
One of the overarching goals of the entire TRR is to establish monolayer transition metal dichalcogenides (TMDCs, e.g. monolayer MoS2 or monolayer WS2) as new materials for ultrafast spintronics. The potential advantages of TMDCs for spintronics include the possibility of optical manipulation of spin/valley degrees of freedom and facile tunability of spin properties via surface modification. Recent experiments on TMDCs already confirmed coupled spin/valley carrier dynamics, strong quantum-confined excitonic effects, and long spin relaxation times. At the same time, while multiple groups probed spin dynamics in pristine TMDC materials, virtually nothing is known regarding the ultrafast evolution of spin currents at metal/TMDC interfaces. The understanding of such interfaces is crucially important for operation of any realistic device. The presence of built-in strain, built-in electric fields, and proximal magnetic fields at the TMDC/metal interface are expected to dramatically alter the spin relaxation time and affect the spin injection efficiency. The behavior of spins at the TMDC/metal interface is equally interesting from the fundamental perspective. Among predicted yet mostly unstudied phenomena are the appearance of strong strain-induced pseudomagnetic fields, proximity-induced ferromagnetism, and new excitonic phenomena. Therefore, the main goal of this project is to controllably investigate the phenomena affecting the dynamics of spins at the interface between TMDCs and metals.
Specifically, we will create model systems to study the effects of the following interfacial phenomena on the spin dynamics:
- Mechanical strain. Near metallic contacts, TMDCs are inevitably strained. The strain is expected to have dramatic effects on spin dynamics. At a moderate strain of 0.5–1%, the bandgap character transforms from direct to indirect, decreasing the lifetime of direct excitons. Larger strain effectively breaks the rotational symmetry producing effects similar to that of a magnetic field. Both phenomena are expected to strongly affect the spin relaxation and diffusion.
- Spin tunneling. Spin-polarized carriers can tunnel between TMDCs and proximal metals at a rate dependent on material separation. The dynamics of tunneling is made especially interesting and different from that of a conventional metal by the presence of strongly bound excitons in TMDCs.
- Built-in electric fields. Strong electric fields develop at the Schottky junction between metals and TMDCs. This field is predicted to break apart TMDC excitons thereby affecting the charge and spin relaxation.
- Proximity-induced magnetism. For TMDCs in atomic proximity of ferromagnets, strong “exchange” fields of order of several Tesla act on spins in the TMDCs. Such fields should break the spin degeneracy, affect spin relaxation, and cause spin precession.
To probe these effects, we will develop time-resolved photocurrent spectroscopy (trPC), a new optoelectronic technique for probing spin transport. In trPC, the photocurrent produced in a TMDC device is measured as a function of time delay between two ultrashort light pulses exciting the sample. This optoelectronic technique is expected to allow probing devices much smaller than the diffraction limit, to be sensitive to carrier transport/diffusion, and to enable separation of the effects in TMDCs from those in proximal materials. The trPC will be complemented by time-resolved Kerr rotation microscopy (trKR). This combination of techniques allows probing the spin dynamics of excitons and free carriers in device geometries with vertical or lateral interfaces.
Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS2
M. G. Harats, J. N. Kirchhof, M. Qiao, K. Greben, and K. I. Bolotin
Nat. Photonics 14, 324 (2020) - DOI: 10.1038/s41566-019-0581-5