Optical Investigation of Monolayer and Bulk Tungsten Diselenide (WSe<sub>2</sub>) in High Magnetic Fields

Optical spectroscopy in high magnetic fields <i>B</i> ≤ 65 T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe<sub>2</sub>, the exciton emission shifts linearly with the magnetic field and exhibits a splitting that originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac Fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe<sub>2</sub> where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model.