Kristen Kaasbjerg: Symmetry protection against quasiparticle scattering in disordered monolayer transition-metal dichalcogenides

Kristen Kaasbjerg (1), Johannes H. J. Martiny (1), Tony Low (2), and Antti-Pekka Jauho (1) 1) Center for Nanostructured Graphene (CNG), Dept. of Micro- and Nanotechnology, Technical University of Denmark, Denmark 2) Department of Electrical and Computer Engineering, University of Minnesota, USA Two-dimensional (2D) monolayers of transition metal dichalcogenides (TMDs; MX2) are promissing candidates for spin- and valley-based electronic devices. In order to realize their apparent potential for valleytronics application, long valley lifetimes are required. However, atomic vacancies which are among the most common types of atomic disorder in 2D TMDs, are expected to limit the achievable valley lifetimes. Here, we demonstrate that besides the spin-valley coupling which partially protects the valley degree of freedom against intervalley scattering as illustrated in Fig. 1, the symmetry and position of atomic disorder give rise to additional selection rules. As shown in Fig. 1, K, K' scattering by disorder with threefold rotational symmetry (C3), e.g., atomic vacancies, is forbidden by symmetry, except in the conduction band for M-centered disorder. We propose to verify our predictions with Fourier transfrom scanning tunneling spectroscopy (FT-STS) which provides direct access to the available scattering channels in q space. For this purpose, we calculate the FT-STS and QPI spectra using an atomistic DFT approach for two archetypal TMDs: the direct gap, small spin-orbit split MoS2, and the indirect gap, large spin-orbit split WSe2. In both cases, the K, K' conduction-band intervalley FT-STS peak is completely absent for X vacancies while appearing clearly for M vacancies, thus suggesting an alternative explanation for its conspicious absence in experiments [1,2]. 1. H. Liu, J. Chen, H. Yu, F. Yang, L. Jiao, G.-B. Liu, W. Ho, C. Gao, J. Jia, W. Yao, and M. Xie, "Observation of intervalley quantum interference in epitaxial monolayer tungsten diselenide," Nature Commun. 6, 8180 (2015). 2. M. Yankowitz, D. McKenzie, and B. J. LeRoy, "Local spectroscopic characterization of spin and layer polarization in WSe2," Phys. Rev. Lett. 115, 136803 (2015). Kristen Kaasbjerg is a researcher at the Center for Nanostructured Graphene at DTU where he is working on theoretical quantum transport in 2D materials and their nanostructures.