Joanna Jadczak

We report an anomalous valley Zeeman effect in a monolayer with induced by in‐plane magnetic fields. Polarization‐resolved photoluminescence at K reveals linear valley splittings with effective ‐factors of (trion), (intravalley A exciton), and (intervalley exciton) in Voigt geometry. These values exceed both the out‐of‐plane response () and the expected zero splitting for in‐plane fields in binary transition‐metal dichalcogenides. Crucially, resonant inelastic laser‐light scattering yields much smaller ‐factors of 1.16–1.67, consistent with single‐particle Zeeman splitting. The factor‐of‐four ‐factor discrepancy, with near‐constant enhancement across excitonic complexes, indicates that the anomalous valley splitting arises from many‐body interactions. We attribute this effect to anisotropic tensor exchange coupling between excitons and the spin‐polarized Fermi sea. The bright exciton carries valley‐antisymmetric total spin, while the in‐plane field induces Pauli paramagnetic polarization in the Fermi sea. Isotropic exchange cannot couple these orthogonal spin components, but spin‐orbit‐induced anisotropic exchange allows for the coupling and produces valley‐dependent energy shifts. The effect is enhanced because the conduction‐band spin‐orbit splitting nearly vanishes at , unlocking the Fermi sea's in‐plane susceptibility. Our findings establish TMDC alloys near the spin‐orbit crossover as a platform for many‐body‐enhanced valley manipulation using in‐plane magnetic fields.

Atomic defects associated with vacancies in two-dimensional transition metal dichalcogenide monolayers efficiently trap charged carriers and strongly localize excitons. Defects in semiconducting monolayers are seldomly utilized for enhancing optical phenomena, although they may provide resonant intermediate states within the energy band gap for applications with multiphoton excitations, like highly efficient and thermally robust photon upconversion. In an MoS₂ monolayer encapsulated by hBN with high defect and resident electron densities, we observe an upconversion of localized exciton (X_L) emission with a huge energy gain of up to 290 meV. The upconverted X_L emission is robust up to temperatures of about 120 K and exhibits a sublinear or a nearly linear laser power dependence for the energy gain of about 100 meV and above 200 meV, respectively. The upconversion mechanism is explained by a cooperative energy transfer process between the photocreated and resident electrons, in which hybridized pairs of single sulfur vacancies likely act as real intermediate states. Additionally, we find a weak upconversion of the neutral exciton photoluminescence with an energy gain of about 350 meV for quasi-resonant excitation of the X_L exciton. It is attributed to a two-step, two-photon absorption.

Transition metal dichalcogenide monolayers represent unique platforms for studying both electronic and phononic interactions as well as intra- and intervalley exciton complexes. Here, we investigate the upconversion of exciton photoluminescence in MoSe₂ monolayers. Within the nominal transparency window of MoSe₂ the exciton emission is enhanced for resonantly addressing the spin-singlet negative trion and neutral biexciton at a few tens of meV below the neutral exciton transition. We identify that the A'₁ optical phonon at the K valley provides the energy gain in the upconversion process at the trion resonance, while ZA(K) phonons with their spin- and valley-switching properties support the biexciton driven upconversion of the exciton emission. Interestingly, the latter upconversion process yields unpolarized exciton photoluminescence, while the former also leads to circularly polarized emission. Our study highlights high-order exciton complexes interacting with optical and acoustic K-valley phonons and upconverting light into the bright exciton.

Germanium monosulfide with an anisotropic puckered crystalline structure has recently attracted much attention due to its unique optical and electronic properties; however, exciton-phonon interactions were only superficially elucidated. We study the resonant Raman scattering and the photo-luminescence of the optically active Gamma-exciton in layered GeS flakes and evaluate the exciton and phonon responses on variations in the excitation energy, laser-light and emission polarizations, temperature, and laser power. A double-resonance mechanism allows for observing Raman forbidden (dark) first-and second -order longitudinal-optical phonon modes whose symmetries and energies are moreover calculated by density functional perturba-tion theory. For (quasi)-resonant exciton excitation, the selection rules become relaxed so that a fourth-order Fro''hlich intraband process is mediated by the scattering of the electron with a longitudinal-optical and an acoustic phonon. Our results demonstrate considerable coupling between phonons and photogenerated carriers in GeS flakes and the high efficiency of multiorder scattering in optical processes.

Semiconducting layered group-IV monochalcogenides such as black phosphorous and germanium monosulfide with an anisotropic puckered crystalline structure in each layer have recently attracted much attention due to their unique optical and electronic properties. However, exciton-phonon interactions were only superficially elucidated, although they tremendously affect the opto-electronic operation principles and performance. We study the resonant Raman scattering and the photoluminescence of the optically active Γ-exciton in layered GeS flakes and evaluate the exciton and phonon responses on variations in the excitation energy, laser-light and emission polarizations, temperature, and laser power. The resonant Raman scattering leads to the observation of dark first- and second-order optical phonon modes whose symmetries and energies are calculated by means of a density functional perturbation theory. We reveal a double-resonance mechanism activating the Raman forbidden (dark) longitudinal-optical scattering processes: For (quasi)-resonantly exciting excitons in the GeS flakes the selection rules become relaxed so that a fourth-order Fröhlich intraband process is mediated by the scattering of the electron with a longitudinal-optical and an acoustic phonon. Our experiments demonstrate considerable coupling between phonons and photogenerated carriers in GeS flakes and the high efficiency of multi-order scattering in optical processes, and outline that layered GeS as direct band-gap semiconductor provides a promising material system for opto-electronic applications.

Photoluminescence from bulk HfS₂ grown by the chemical vapor transport method is reported. A series of emission lines is apparent at low temperature in the energy range of 1.4–1.5 eV. Two groups of the observed excitonic transitions followed by their replicas involving acoustic and optical phonons are distinguished using classical intensity correlation analysis. The emission is attributed to the recombination of excitons bound to iodine (I₂) molecules intercalated between layers of HfS₂. The I₂ molecules are introduced to the crystal during the growth as halogen transport agents in the growth process. Their presence in the crystal is confirmed by secondary ion mass spectroscopy.

We perform a polarization-resolved magnetoluminescence study of excitons in ReS ₂ . We observe that two linearly polarized Rydberg series of excitons are accompanied by two other Rydberg series of dark excitons, brightened by an in-plane magnetic field. All series extrapolate to the same single-electron bandgap, indicating that the observed excitons originate either from the same valley or from two valleys related by the inversion symmetry, and are split by exchange interaction. To interpret our observations of the magnetic brightening, we have to assume the dominant spin–orbit coupling to be Ising-like, which hints at an approximate symmetry of the electronic states in ReS ₂ which is higher than the crystal symmetry C _i .