Research

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.

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.

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.

	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.

• Preparation of single- and few-layer samples via mechanical exfoliation

• Preparation of artificial hetero- or multi-layer structures via a deterministic transfer process

(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.

• Time-resolved photoluminescence with streak-camera system
  2 ps maximum time-resolution, microscope setup for spatially-resolved experiments, temperatures down to 4 K
• Pump-probe spectroscopy, e.g., time-resolved Faraday or Kerr rotation, four-wave mixing, with mode-locked Ti-Sapphire femtosecond laser
  Pulse length 100 fs - 600 fs, wavelength range 700 nm - 830 nm, magnetic fields up to 11.5 T, temperatures down to 0.4 K
• cw photoluminescence spectroscopy
  Spectrometer with 0.5 m focal length and CCD camera, various laser sources, magnetic fields up to 11.5 T, temperatures down to 0.4 K

• 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

• 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

• Micro-Raman Spectroscopy and Mapping
  Microscope setup with automated mapping option, spectrometer with 0.5 m focal length and CCD camera,?Bragg filter sets for stray-light reduction, various laser sources?
• White-light reflectance experiments
  white-light source, spectrometer with 0.5 m focal length and CCD camera