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Electrons, holes, and excitons in monolayer semiconductors: Insights from optical spectroscopy in (really) high magnetic fields
Speaker: Scott Crooker (Los Alamos National Laboratory)
Much of the current interest in atomically-thin ‘transition metal dichalcogenide’ (TMD) semiconductors such as MoS2 and WSe2 derives from their promise for optoelectronics, and the potential for new spin/valley-based devices. This talk will discuss recent optical studies that probe the physics of electrons, holes, and excitons in monolayer TMD semiconductors, as well as the crucial role played by the surrounding dielectric environment.
Our first studies focused on revealing fundamental material properties relevant for optoelectronics, such as exciton mass, size, binding energy, and dielectric screening. To date, many of these parameters are still assumed from theory. Historically, magneto-optical spectroscopy played an essential role in determining these semiconductors properties; however, for TMD monolayers the relevant field scale is substantial –of order 100 tesla!– due to heavy carrier masses and huge exciton binding energies. Fortunately, modern pulsed magnets can achieve this scale. Using exfoliated monolayers affixed to single-mode optical fibers, we performed low-temperature magneto-absorption spectroscopy up to ~90T of all members of the monolayer TMD family. Following the diamagnetic shifts of the exciton’s 1s ground state and its ex
cited 2s, 3s,… ns Rydberg states revealed exciton masses, radii, binding energies, dielectric properties, and free-particle bandgaps  -- essential ingredients for the rational design of optoelectronic van der Waals structures. Further, high-field studies of electrostatically-gated monolayers reveal spontaneous valley polarization of mobile carriers .
Separately, we also developed an entirely passive, noise-based approach for exploring the intrinsic spin & valley dynamics of electrons and holes in monolayer TMD semiconductors . Under conditions of strict thermal equilibrium, we use light to “listen” to the thermodynamic fluctuations of the valley polarization in a Fermi sea of resident carriers, due to their spontaneous scattering between the K and K’ valleys of the Brillouin zone. The spectra of this ‘valley noise’ reveals encouragingly long valley relaxation timescales (up to microseconds), and provides a viable route toward quantitative studies of intrinsic dynamics, free from any external perturbation, pumping, or excitation.
*In collaboration with Nathan Wilson & Xiaodong Xu (U. Washington), and Cedric Robert, Bernhard Urbaszek, & Xavier Marie (INSA-Toulouse)
 M. Goryca et al., Nat. Commun. 10, 4172 (2019).
 J. Li et al., Phys. Rev. Lett. 125, 147602 (2020).
 M. Goryca et al., Science Advances 5, eaau4899 (2019).
Short bio: Scott Crooker received a B.A. in physics from Cornell (1992) and a Ph.D. in physics from UCSB (1997). Following a postdoc at the NHMFL at Los Alamos National Lab (LANL), he remained as a member of the scientific staff. Research interests include the development of sensitive magneto-optical spectroscopies to probe the static and dynamic behavior of spins and magnetism in novel semiconductor and magnetic materials. He is a LANL fellow, and a fellow of the APS, AAAS, and OSA.
Host: Sang-Wook Cheong