University of Minnesota
School of Physics & Astronomy

Condensed Matter Seminar

Wednesday, September 25th 2019
1:25 pm:
Speaker: Scott Crooker, National High Magnetic Field Laboratory, Los Alamos National Lab
Subject: Electrons, holes, and excitons in monolayer semiconductors: Insights from optical spectroscopy in (really) high magnetic fields

Much of the current interest in atomically thin transition-metal dichalcogenide (TMD) semiconductors such as MoS2 and WSe2 derives from the physics of coupled spin & valley degrees of freedom and the potential for new spin/valley-based devices. This talk will discuss recent optical studies that probe the valley-related 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 properties relevant for optoelectronics, such as exciton mass, size, binding energy, and dielectric screening. To date, many of these parameters are still assumed from density functional theory. Historically, magneto-optical spectroscopy has played an essential role in determining these properties in semiconductors; 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. By following the diamagnetic shifts and valley Zeeman splittings of the exciton’s 1s ground state and its excited 2s, 3s, … ns Rydberg states, we determined exciton masses, radii, binding energies, dielectric properties, and free-particle bandgaps. These data allow a quantitative comparison with the popular “Rytova-Keldysh” model of the (non-hydrogenic) attractive electron-hole potential in a 2D material, and provide essential ingredients for the rational design of optoelectronic van der Waals structures [1,2].

In separate studies we used ultrafast optical methods for time-resolved Kerr rotation to measure the coupled spin-valley dynamics of resident electrons and holes in electrostatically-gated TMD monolayers [3,4]. Very long relaxation timescales of order 100 ns are observed for electrons in n-type monolayers, which is many orders of magnitude longer than typical exciton lifetimes. Even longer valley relaxation (2 microseconds) is observed for holes in p-type monolayers, confirming long-standing expectations of strong spin-valley locking in monolayer TMD semiconductors.

*In collaboration with X. Marie & B. Urbaszek (INSA-Toulouse), and X. Xu (U. Washington)

[1] M. Goryca et al., Nature Comm. (in press); arXiv:1904.03238
[2] A. V. Stier et al., Phys. Rev. Lett. 120, 057405 (2018)
[3] M. Goryca et al., Science Advances 5, eaau4899 (2019)
[4] P. Dey et al., Phys. Rev. Lett. 119, 137401 (2017)

Faculty Host: Paul Crowell

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