Magneto-optics of monolayer tungsten disulfide

Researchers of the University of Regensburg, the University of Münster and the High Field Magnet Laboratory Nijmegen have observed some remarkable optical properties of monolayer WS2. The consortium has reported those observations in a trilogy of papers in Physical Review Letters, Nature Communications and Nano Letters.

Single layer transition-metal dichalcogenides, such as MoS2, MoSe2, WS2, WSe2, are two dimensional semiconductors, with a honeycomb lattice. Their bandstructures show a pair of inequivalent valleys (local extrema) at the +K and -K points of the Brillouin zone. The valleys in the conduction and valence bands are separated by a direct band-gap in the visible spectral range, resulting in efficient light absorption and emission. The existence of valleys results in charge carriers that exhibit, in addition to their real spin, an extra property called pseudospin, accompanied by a magnetic moment.

In this project the magneto-optical properties of monolayer WS2 have been determined. The photoluminescence emission is dominated by neutral and charged electron-hole pairs (excitons). Two distinct types of charged excitons (trions), singlets (X-s) and triplets (X-t), have been observed, just below the emission line of the neutral exciton X (Figure 1). For all types of excitons the g-factors have been determined, while the observation of the diamagnetic shifts of the excitons gives insight into the real-space extension of these quasiparticles. The magnetic field induced valley polarization effects shed light onto the exciton and trion dispersion relations in reciprocal space.


Figure 1. Left (σ, left panel) and right (σ+, right panel) circularly polarized emission from monolayer WS2 at 4.2 K and different magnetic fields. Neutral excitons (X) as well as different charged excitons, singlets (X-s) and triplets (X-t), can be distinguished.

A remarkable magnetic-field-induced rotation of the polarized light emission of neutral excitons has been observed (figure 2). A field-induced valley Zeeman splitting causes a rotation of the emission polarization with respect to the excitation by up to 35° and reduces the linear polarization degree by up to 16%. From these results it is deduced that coherent light emission from the valleys decays with a time constant of 260 fs.


Figure 2. a) Measured normalized photoluminescence intensity (solid circles) for monolayer WS2 as a function of the analyzer angle, under linearly polarized excitation for 0 and 25 T. The blue and orange lines indicate the polarization patterns obtained from different models. b) Relative rotation angle between the excitation and emission polarization for different magnetic fields. c) Linear polarization degree of the emission as a function of the magnetic field. The orange lines show the global fit to the data using a model taking into account exciton valley coherence.

These remarkable properties pave the way to study and utilize valley-dependent phenomena (“valleytronics”) by optical means, which is very promising for novel opto-electronic applications.

Related publications:

Magnetic-Field-Induced Rotation of Polarized Light Emission from Monolayer WS2, R. Schmidt, A. Arora, G. Plechinger, P. Nagler, A. Granados del Águila, M.V. Ballottin, P. C. M. Christianen, S. Michaelis de Vasconcellos, C. Schüller, T. Korn and R. Bratschitsch, Physical Review Letters 117, 077402 (2016)

Trion fine structure and coupled spin–valley dynamics in monolayer tungsten disulfide, G. Plechinger, P. Nagler, A. Arora, R. Schmidt, A. Chernikov, A. Granados del Águila, P. C. M. Christianen, R. Bratschitsch, C. Schüller and T. Korn, Nature Communications 7, 12715 (2016)

Excitonic valley effects in monolayer WS2 under high magnetic fields, G. Plechinger, P. Nagler, A. Arora, A. Granados del Águila, M.V. Ballottin, T. Frank, P. Steinleitner, M. Gmitra, Jaroslav Fabian, P. C. M. Christianen, R. Bratschitsch, C. Schüller and T. Korn, Nano Letters 16, 7899-7904 (2016)