Peter Christianen, HFML Nijmegen

Single-layer transition-metal dichalcogenides, such as MoS2, MoSe2, WS2, WSe2, are two-dimensional semiconductors with a honeycomb lattice. Their band structures 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 charge carriers in these valleys have, in addition to their real spin, an extra property called pseudospin, accompanied by a magnetic moment. In a collaboration between the Universities of Regensburg and Münster and the High Field Magnet Laboratory Nijmegen 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 (XS) and triplets (XT), have been observed, just below the emission line of the neutral exciton X (Figure 1). The magnetic field-induced valley polarization effects shed light onto the exciton and trion dispersion relations in reciprocal space. A remarkable magnetic-field-induced rotation of the polarized light emission of neutral excitons has been observed (Figure 2). A fieldinduced 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 isdeduced that coherent light emission from the valleys decays with a time constant of 260 fs. 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.

Reference:
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,
Phys. Rev. Lett. 117, 077402 (2016).

Reference:
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,
Nat. Commun. 7, 12715 (2016).

Reference:
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 (2016).