Miłosz Rybak

Two-dimensional van der Waals materials display rich nonequilibrium physics due to their reduced dimensionality and strong light–matter interactions. Layered metal phosphorus trichalcogenides (MPX3) feature bandgaps spanning the visible to ultraviolet range and exhibit a complex interplay between excitons, phonons, and defects. Here we use static and time-resolved photoluminescence and femtosecond broadband transient reflectivity to investigate the temperature-dependent ultrafast carrier dynamics in layered HgPSe3, a semiconductor with a quasi-direct bandgap near 2 eV. We identify distinct sub-bandgap photoluminescence features that become prominent at low temperature. Their temperature and fluence dependence strongly indicate radiative recombination involving localized states. The transient reflectivity map displays three features associated with interband optical transitions in the visible range, as corroborated by density functional theory band structure calculations. These features exhibit fluence-dependent decay dynamics, indicating a competition between carrier localization into sub-bandgap states, state-filling effects that limit the available electronic phase space for hot-carrier relaxation, and phonon-bottleneck-limited carrier cooling. Our results provide new insights into the ultrafast carrier dynamics of HgPSe3 and establish a framework for understanding the role of sub-bandgap states in MPX3 semiconductors, with potential implications for broadband optoelectronic and photodetection applications.

High-pressure spectroscopic and diffraction studies on HgPSe ₃ uncover a phase transition at ≈3.6 GPa from a monoclinic to a triclinic phase, accompanied by a quasi-direct-to-indirect bandgap crossover.

We explore symmetry-breaking phenomena in MnPS3/TMDC (MoS2, WS2, MoSe2, WSe2) heterostructures using first-principles calculations, considering two high-symmetry stacking configurations, S1 and S2, which differ not only by their interfacial registry but also by a 30° twist between the layers. Depending on the stacking geometry, the systems exhibit two distinct types of nonrelativistic spin splitting (NRSS): S2 hosts altermagnetic-like band crossings, while S1 shows global spin splitting characteristic of symmetry-breaking NRSS. Magnetic exchange and anisotropy parameters indicate that the intrinsic magnetic properties of MnPS3 are largely preserved upon interfacing. Including spin-orbit coupling, we find tunable conduction-valley splitting controlled by the MnPS3 spin orientation. Our results identify MnPS3 as a symmetry-tunable antiferromagnetic substrate capable of inducing and controlling spin and valley effects in 2D heterostructures without relying on net magnetization or strong SOC, offering a route toward nonvolatile valleytronic functionalities.

Van der Waals crystals offer opportunities to engineer material properties by stacking diverse 2D layers into heterostructures. Charge redistribution at these interfaces, governed by band alignment and Fermi levels, enables control over optical, electronic, and magnetic behavior. In this study, we investigated the band alignment and excitation spectra of exfoliated MPS₃ (M = Mn, Fe, Co, Ni) using X-ray and UV photoelectron spectroscopy, optical absorption, and DFT + U calculations. Ionization potentials from 5.4 eV (FePS₃) to 6.2 eV (NiPS₃) were determined. The resulting band diagrams differentiate localized d states from hybridized p-d states, offering insights for designing functional heterostructures. Selective occupation of unoccupied 3d states provides a pathway to tune magnetic order. Theoretical results show that MnPS₃ is well-aligned for both hydrogen and oxygen evolution reactions (HER and OER), while other MPS₃ are promising for OER. MnPS₃/NiPS₃ heterostructure exhibits optimal band alignment for efficient water splitting across a broad pH range.

The unusual magnetic properties of van der Waals‐type antiferromagnetic semiconductors such as transition metal thiophosphates make them highly attractive for spintronics and optoelectronics. However, a link between the magnetic and optical properties of these materials, required for practical applications, has not yet been established. A combined experimental and theoretical study of magnetic, optical, and structural properties of CrPS ₄ samples is reported. It is found that the magnetic‐field‐dependent circular polarization degree of the photoluminescence is a direct measure of the net magnetization of CrPS ₄ . Complementary Raman scattering measured as a function of magnetic field and temperature enables the determination of the magnetic susceptibility curve of the material. Experimental results are supported by density functional theory calculations that take as input the lattice parameters determined from temperature‐dependent X‐ray diffraction measurements. This allows the impact of spin ordering on the spectral position of Raman transitions in CrPS ₄ to be explained, as well as the anomalous temperature shifts of some of them. The presented method for all‐optical determination of magnetic properties is highly promising for studies of spin ordering and magnetic phase transitions in single‐ or few‐layer samples of magnetic layered materials, for which a poor signal‐to‐noise ratio precludes reliable neutron scattering or magnetometry measurements.