Interlayer exciton valley polarization dynamics in large magnetic fields

Johannes Holler, Malte Selig, Michael Kempf, Jonas Zipfel, Philipp Nagler, Manuel Katzer, Florian Katsch, Mariana V. Ballottin, Anatolie A. Mitioglu, Alexey Chernikov, Peter C. M. Christianen, Christian Schüller, Andreas Knorr, and Tobias Korn

Phys. Rev. B 105, 085303 – Published 9 February 2022

Abstract

In van der Waals heterostructures consisting of stacked ${MoSe}_{2}$ and ${WSe}_{2}$ monolayers, optically bright interlayer excitons can be observed when the constituent layers are crystallographically aligned. The symmetry of the monolayers allows for two different types of alignment, in which the momentum-direct interlayer transitions are either valley conserving ( $R$ -type alignment) or change the valley index ( $H$ -type antialignment). Here, we study the valley polarization dynamics of interlayer excitons in magnetic fields up to 30 T by time-resolved photoluminescence. For all interlayer exciton types, we find a finite initial photoluminescence circular degree of polarization after unpolarized excitation in applied magnetic fields. For interlayer excitons in $H$ -type heterostructures, we observe a systematic increase of the photoluminescence circular degree of polarization with time in applied magnetic fields, which saturates at values close to unity for the largest fields. By contrast, for interlayer excitons in $R$ -type heterostructures, the photoluminescence circular degree of polarization shows a decrease and a zero crossing before saturating with opposite polarization. This unintuitive behavior can be explained by a model considering the different interlayer exciton states in $H$ - and $R$ -type heterostructures and their selection rules coupling photoluminescence helicity and valley polarization.

Received 5 November 2021
Revised 25 January 2022
Accepted 26 January 2022

DOI:https://doi.org/10.1103/PhysRevB.105.085303

Physics Subject Headings (PhySH)

Excitons Valleytronics

Heterostructures Van der Waals systems

Time-resolved photoluminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Johannes Holler ¹, Malte Selig ², Michael Kempf³, Jonas Zipfel^1,4, Philipp Nagler¹, Manuel Katzer ², Florian Katsch², Mariana V. Ballottin⁵, Anatolie A. Mitioglu⁵, Alexey Chernikov^1,6, Peter C. M. Christianen⁵, Christian Schüller ¹, Andreas Knorr², and Tobias Korn ^3,*

¹Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93040 Regensburg, Germany
²Institut für Theoretische Physik, Technische Universität Berlin, 10587 Berlin, Germany
³Institut für Physik, Universität Rostock, 18059 Rostock, Germany
⁴Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁵High Field Magnet Laboratory (HFML-EMFL), Radboud University, 6525 ED Nijmegen, Netherlands
⁶Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Würzburg-Dresden Cluster of Excellence ct.qmat, Technische Universität Dresden, 01062 Dresden, Germany

^*tobias.korn@uni-rostock.de

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Vol. 105, Iss. 8 — 15 February 2022

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Images

Figure 1
Schematic illustration of the exciton dispersion and $g$ factors. (a) Exciton dispersion and relaxation pathways in the aligned structure. (b) Exciton dispersion and relaxation pathways in the antialigned structure. In (a) and (b), spin up (down) bands are indicated in blue (red). The solid dispersions refer to the intralayer bands, and the dashed dispersions account for interlayer bands. The brown arrows indicate tunneling transitions between intra- and interlayer states and transitions between different domains. The dashed brown arrow indicates relaxation of interlayer excitons. The yellow arrows indicate the exchange coupling between $↑ ↑$ and $↓ ↓$ states. The pink bar indicates the energetic region of the pump pulse. $g$ factors and optical selection rules in the aligned structure of (c) the intralayer excitons, (d) the interlayer $R_{h}^{h}$ , and (e) the interlayer $R_{h}^{X}$ exciton. $g$ factors and optical selection rules in the antialigned structure of (f) the ${WSe}_{2}$ intralayer excitons, (g) the ${MoSe}_{2}$ intralayer exciton, and (h) the interlayer $H_{h}^{X}$ exciton. In (c)–(f) blue (red) bands indicate spin up bands. Dispersions are shown by solid (dashed) lines for a positive (zero) magnetic field.
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Figure 2
(a) PL spectra of aligned and antialigned HSs measured without applied magnetic field (gray lines) and helicity-resolved spectra at 20 T applied magnetic field (black and red lines). The vertical dashed lines indicate the peak positions at zero magnetic field and serve as a guide to the eye. (b) Magnetic-field-dependent energy splitting $Δ E_{Z}$ of ILE in aligned (black dots) and antialigned (blue stars) HSs. The solid lines indicate linear fits to the data, which were used to extract the effective $g$ factors. Helicity-resolved ILE PL traces for aligned and antialigned HSs at (c) 0 T and (d) 20 T.
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Figure 3
Degree of polarization of the photoluminescence in the (a) antialigned and (b) aligned structures as a function of time for various magnetic fields. The semitransparent lines represent the data calculated from helicity-resolved PL traces; the solid lines are fits to the data using Eq. (10). Initial (stars) and saturation (dots) values of the DOP for (c) antialigned and (d) aligned HSs extracted from the data in (a) and (b). In (c), the initial DOP is fitted with a linear function, and the saturation value of the DOP is fitted using an exponential saturation function.
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Figure 4
Degree of polarization of the photoluminescence in the (a) antialigned and (b) aligned structures as a function of time for various magnetic fields. Interlayer exciton valley polarization degree in the (c) antialigned and (d) aligned structures as a function of time for various magnetic fields.
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