Optical Studies of Spin in 2D Crystals

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****Technical Abstract****Two-dimensional (2D) crystals are a fascinating new class of materials that exhibit novel electronic, spintronic, and optical properties. In this project, we will use optical techniques to investigate the spin-dependent properties of two types of 2D crystals: MoS2 (and related metal dichalcogenides) and graphene. Monolayer MoS2 is a direct gap semiconductor with a giant spin splitting of the valence band at the K/K' valleys. Interestingly, this is predicted to suppress most types of spin relaxation and generate long spin lifetimes for holes. Further, the giant spin splitting makes MoS2 a promising candidate for realizing the intrinsic spin Hall effect. Our aim is to observe these novel properties using time-resolved Kerr microscopy to directly measure the spin polarization and spin dynamics in MoS2. Graphene is a promising material for spintronics due to its long spin diffusion length at room temperature. The forefront of the field of graphene spintronics concerns the nature of spin relaxation and induced magnetism. Nearly all studies are based on spin transport, but this approach has its limitations such as spin relaxation induced by ferromagnetic contacts and reliance on Hanle analysis. We will utilize time-resolved optical techniques to overcome these limitations. Together, these studies on MoS2 and graphene lie at the forefront of spin-dependent physics in 2D crystals. This project will support the education of two PhD students, who will receive excellent training for careers in industry and academia.****Non-Technical Abstract****Two-dimensional (2D) crystals are a remarkable class of materials that exhibit fascinating new properties and have the potential to revolutionize electronics beyond conventional silicon technologies. In particular for spintronic devices, 2D crystals are exhibiting much better performance than their three-dimensional counterparts and special spin-dependent properties related to their 2D structure are predicted. Why does 2D perform better, can it be further improved, and can the predicted new properties be demonstrated? To answer such key questions, we will use powerful imaging techniques combining ultrafast pulsed lasers and optical microscopes to directly visualize the motion and rotation of electron spins in 2D crystals. The spin can be thought of as a tiny magnet that is attached to an electron as is flows through a device. In a spintronic device, the direction of an electron's magnetic poles ("north" and "south") is used to carry information throughout the device, and this information can be manipulated by rotating the direction of the poles. By using lasers and optics to image the motion and rotation of these magnetic poles (i.e. spin), we can investigate how they hold information, how they lose information, and how the direction of the poles is related to the electron's motion. Understanding these issues will enable the development of advanced spintronic devices for electronics beyond silicon. This project will support the education of two PhD students, who will receive excellent training for careers in industry and academia.