Excitons Across Boundaries
Researchers of the University of Regensburg and the High Field Magnet Laboratory Nijmegen have observed remarkable magnetic properties of interlayer excitons in a WSe2/MoSe2 heterostructure. The excitons exhibit a giant valley Zeeman splitting, with a g-factor of about -15, which stems from the specific stacking of the WSe2 and MoSe2 layers. The study highlights the potential for highly effective magnetic manipulation of interlayer excitons in van der Waals heterostructures and illustrates their capability for valleytronic devices. The results have been published on Nov 16th 2017 in Nature Communications.
Single layer transition-metal dichalcogenides (TMDs), such as MoSe2 and 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.
Stacking monolayer TMDs on top of each other (left panel Figure 1) gives the possibility to realize novel physical properties. For instance, Type II band alignment of the heterostructure leads to the formation of interlayer excitons, which stem from spatially separated electron-hole pairs, where the electron resides in one material and the hole in the other (right panel Figure 1). These interlayer excitons are promising for future valleytronic devices since they combine ultra-long lifetimes with the peculiar spin-valley physics of the constituent monolayers.
Figure 1. Left panel: Photograph of the WSe2/MoSe2 van der Waals heterostructure. Scale bar is 25 mm. Right panel: Optical emission spectrum of the heterostructure at 4 K. The emission from the interlayer excitons is spectrally well separated from the intralayer emission. The inset schematically depicts the type II band alignment leading to a spatial separation of electrons and holes.
In the paper it is shown that the specific stacking alignment of WSe2 and MoSe2 leads to a strong enhancement of the magnetic coupling of electronic transitions. The interlayer exciton emission exhibits a giant magnetic valley splitting with an effective g factor of about −15 (Figure 2) that exceeds typical values for both TMD monolayers and more conventional nonmagnetic semiconductor heterostructures by far. The resulting field-induced valley polarization of the long-lived charge carriers approaches unity, even though both valleys in the two constituent materials are initially equally populated. It emerges from the specific arrangement of the individual layers of the heterostructure in momentum space (Inset right panel Figure 2), which is not attainable in TMD monolayers, enabling momentum allowed optical transitions between valleys of different index.
Figure 2. Left panel: Comparison of the interlayer exciton emission at 0 T and 30 T. At 0 T, both circular polarizations (s+ and s-) show the same energy and intensity. At 30 T, the energy degeneracy is fully lifted and the emission stems almost exclusively from the σ+ transition. Right panel: Corresponding valley-selective splitting of the interlayer exciton. The solid line corresponds to a linear fit of the data, yielding an effective g factor of −15.1 ± 0.1. Inset: alignment of the K+ and K- valleys of the constituent monolayers in momentum space.
The demonstrated stability of the spin–valley polarization through the complete lifting of valley degeneracy in artificial heterostructures provides a highly promising route towards the implementation of the spin–valley degree of freedom for future applications in the fields of quantum computation and nanophotonics.
Giant magnetic splitting inducing near-unity valley polarization in van der Waals heterostructures, P. Nagler, M. V. Ballottin, A. A. Mitioglu, F. Mooshammer, N. Paradiso, C. Strunk, R. Huber, A. Chernikov, P. C. M. Christianen, C. Schüller and T. Korn, Nature Communications 8, 1551 (2017)