Photos: Simon Raiber
Our projects are funded by the DFG via individual projects, and the Collaborative Research Center SFB 1277.
In this project, the effect of moiré superlattices (Figs. 1a and b) in van-der-Waals homo- and heterobilayers, based on the semiconducting transition-metal dichalcogenides MoSe2, WSe2, and WS2, are investigated. Moiré superlattices form in van-der-Waals structures due to differences in the lattice constants of the constituent materials and/or due to a nonzero twist angle (Figs. 1a and 2). While selenide-based heterostructures (e.g., MoSe2/WSe2) show atomic reconstruction for untwisted structures and for small twist angles, sulfide-selenide heterobilayers (e.g., MoSe2/WS2) exhibit moiré superlattices even for untwisted structures, due to the larger lattice mismatch. As experimental methods, we employ low-frequency resonant Raman spectroscopy (e.g., Fig. 1c), as well as cw- and time-resolved photoluminescence, using a streak-camera system.
Fig. 1 | (a) Schematic picture of a twisted MoSe2 homobilayer. The green arrows and the green-shaded area mark the moiré supercell, while the red arrows outline the crystallographic supercell. (b) Hexagonal Brillouin zone of layer 1, with reciprocal basis vectors b1 and b2 of the two twisted layers 1 and 2. g is the reciprocal lattice vector of the moiré superlattice. (c) Low-frequency Raman spectra of a series of twisted MoSe2 homobilayers. The small arrows mark moiré phonons.
Fig. 2 | Left: Microscope image of MoSe2/WSe2 heterostructure. Right: Polar plot of second-harmonic generation intensities for the determination of the crystal orientations of the constituent layers. A twist angle of about 5° between the MoSe2 and WSe2 layers can be determined.
Ongoing PhD project:M. Sc. Philipp Parzefall |
We aim to achieve new levels of control over excitons and other electronic quasiparticles in van-der-Waals heterostructures by introducing additional functional layers. The van-der-Waals heterostructures are based on monolayers of the semiconducting transition-metal dichalcogenides MoSe2 or WSe2, and, as functional layer, e.g., the layered antiferromagnet CrSBr (Fig. 1a). Time-resolved experiments are performed with a tunable two-color pump-probe setup, based on a mode-locked Ti:Sapphire laser, and a white-light continuum-generating optical fiber. With this setup, we can perform four different experiments in the same measurement run: Time-resolved Kerr ellipticity (TRKE), transient differential reflectivity (DR), white-light reflectance contrast (RC), and photoluminescence (PL). While we probe the emission and absorption properties of the quasiparticles via PL (Fig. 1b) and RC (Fig. 1c), respectively, the spin and particle dynamics are investigated by TRKE (Fig. 1d) and DR (Fig. 1e) experiments.
(a) Schematic picture of a typical van-der-Waals heterostructure, consisting of monolayer MoSe2, on top of a CrSBr crystal. (b) Photoluminescence, (c) reflectance contrast, (d) time-resolved Kerr ellipticity, and, (e) transient differential reflectivity experiments in the spectral region of the optical bandgap of MoSe2.
Ongoing PhD project:M. Sc. Andreas Beer |
In this project, the spin-, valley-, and pseudospin dynamics in MoSe2 and WSe2 multilayers are explored under external electric and magnetic fields. The project is based on our recent, intriguing finding of pseudospin quantum beats in MoSe2 and WSe2 multilayers, which revealed rather unexpected nonzero in-plane g factors of these materials. Time-resolved Faraday ellipticity, transient differential transmission, and time-resolved photoluminescence experiments are employed to research the origin of these quantum oscillations in depth.
Spectrally-resolved four-wave mixing (FWM) experiment on a GaAs-AlGaAs superlattice, showing coherent Bloch oscillations. |
Time-resolved Faraday ellipticity experiment on a WSe2 multilayer in an in-plane magnetic field, showing pseudospin quantum beats. An artistic view of the multilayer is projected in the background. | |
Ongoing PhD project:M. Sc. Anna Weindl |
We have five laboratories in our group. One lab for the preparation of 2D crystals and their heterostructures via mechanical exfoliation and deterministic transfer (2D Crystal Fab), and four labs for optical spectroscopy.
Experiments:
Preparation of single- and few-layer samples via mechanical exfoliation
Preparation of artificial hetero- or multi-layer structures via a deterministic transfer process
Pictures:
(a) Microscope image of WSe2 layer, encapsulated in hBN layers. (b) Exfoliated MoSe2 sample. The numbers of layers are indicated.
Microscope image of WSe2-MoSe2 heterostructure, prepared by deterministic transfer of single layers.
Experiments:
Experiments:
Time- and spatially resolved two-color Kerr rotation with mode-locked Ti:Sapphire laser and white-light source
Microscope setup for time- and spatially-resolved experiments, temperatures down to 4 K
Second-harmonic generation with mode-locked Ti-Sapphire femtosecond laser
Spectrometer with 0.25 m focal length and CCD camera
Photoluminescence and white-light reflectance
Spectrometer with 0.25 m focal length and CCD camera
Experiments:
Resonant Magneto-Raman Spectroscopy
Triple Raman spectrometer with LN2-cooled CCD detector, tunable cw Ti:Sapphire laser, magnetic fields up to 9 T, temperatures down to 2 K
Resonant Micro-Raman Spectroscopy
Microscope setup for spatially-resolved experiments, tunable cw Ti:Sapphire laser, temperatures down to 4 K
Experiments: