Electronic properties of van der Waals crystals under hydrostatic pressure

Electronic properties of layered van der Waals crystals can be effectively tuned by means of external and configurational factors. It allows for the investigation of the fundamental material properties that are valuable for technological applications. Here we show, how Density Functional Theory (DFT) calculations allow to interpret the experimental results on quantitative level.
We present experimental and DFT studies of the electronic band structure of MoTe2 at high hydrostatic pressures. Modulated photoreflectance measurements allowed determination of the pressure coefficients of six direct transitions, with positive and negative values, which can be attributed to a strong splitting of the conduction bands with increasing pressure and the presence of hidden spin-polarized electronic states. These results prove that the spin-valley locking effect takes place in centrosymmetric transition metal dichalcogenides [1].
We also report experimental and theoretical study of the electronic band structure of orthorhombic GeS crystals under hydrostatic pressure. Polarization-resolved photoreflectance measurements allowed to extract the energies, optical dichroic ratios, and pressure coefficients of the direct optical transitions. These findings are discussed in view of DFT calculations, which predict that nondegenerate states in different valleys can be individually selected through linearly polarized light. Based on this, an assignation of the direct optical transitions to the electronic band structure is provided. These results provide evidence that GeS is a strong candidate for valleytronic applications in nondegenerate systems [2, 3].
Finally, we combined calculations within DFT and the effective Bethe-Salpeter equation, with high-pressure optical measurements in order to thoroughly describe the effect of strain and dielectric environment onto the electronic band structure and optical properties of a few-layered WS2. Our results show that WS2 remains fully adhered to the substrate at least up to a 0.6% in-plane compressive strain for a wide range of substrate materials. We provide a useful model to describe effect of strain on the optical properties on general strain conditions. Within this model, exceptionally large compressive uniaxial and biaxial in-plane gauge factors were obtained, which confirm transition metal dichalcogenides as very promising candidates for flexible functionalities [4].

[1] R. Oliva, T. Woźniak, F. Dybała, J. Kopaczek, P. Scharoch, R. Kudrawiec, Mater. Res. Lett. 8, 75 (2020).
[2] A. Tołłoczko, R. Oliva, T. Woźniak, J. Kopaczek, P. Scharoch, R. Kudrawiec, Mater. Advances 6, 1886 (2020).
[3] R. Oliva, T. Woźniak, F. Dybała, A. Tołłoczko, J. Kopaczek, P. Scharoch, R. Kudrawiec, Phys. Rev. B 101, 235205 (2020).
[4] R. Oliva, T. Woźniak, P. E. Faria Junior, F. Dybała, J. Kopaczek, J. Fabian, P. Scharoch, R. Kudrawiec, arXiv:2202.08551 (2022).