Optical Investigation of Monolayer and Bulk Tungsten Diselenide (WSe₂) in High Magnetic Fields

Abstract

Optical spectroscopy in high magnetic fields B ≤ 65 T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe2, 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 WSe2 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.

Magnetoexcitons in large area CVD-grown monolayerMoS2andMoSe2on sapphire

“…In contrast to photoluminescence measurements, which generally probe only the A exciton, absorption provides easy access to the higher energy B exciton which arises due to the large spin-orbit splitting of the valence band in TMDs. At low temperatures (T 2 K) and in a magnetic field, both excitons exhibit a large splitting of the σ ± transitions with an effective valley g factor g v −4 in agreement with previous magneto-optical investigations of the A exciton in exfoliated single layer TMDs [20,23,24,26]. The similar values for the valley g factor of the A and B excitons is in line with band structure calculations [11,29].…”

“…In a simple two-band model, the masses of the valence and the conduction band are identical so that the intercellular valley magnetic moment is the same for the valence and conduction bands. Thus, there is no intercellular contribution to the valley splitting which arises solely from the μ k = ±2μ B angular momentum of the valence d orbitals giving a valley g factor g v = −4, close to the reported values from photoluminescence (PL) studies in transition metal diselenides [20,[22][23][24][25][26].…”

“…The existence of a valley-contrasting magnetic moment opens a possibility of controlling the valley pseudospin with an external magnetic field [15][16][17][18][19][20]. The application of a magnetic field, perpendicular to the layer, lifts the valley degeneracy splitting the exciton transitions.…”

Magnetoreflection spectroscopy of monolayer transition-metal dichalcogenide semiconductors in pulsed magnetic fields

“…The expected symmetry between positive and negative magnetic fields is confirmed (blue and red data traces, respectively). The spectra reveal a well-resolved splitting of the A and B exciton of ∼14 meV at 60 T, and the derived g-factors of approximately −4 agree well with our recently published results [19] and are in reasonable agreement with recent reports of the valley Zeeman effect in the monolayer transition-metal diselenides WSe 2 and MoSe 2 [11][12][13][14][15][16].…”

“…Owing to strong spinorbit coupling and their lack of structural inversion symmetry, spin and valley degrees of freedom are coupled and valley-specific optical selection rules exist for rightand left-circularly polarized light [5,6]. Consequently, a number of interesting optical and magneto-optical studies of these TMDs have been performed in recent years, in which both spin and valley physics were explored [7][8][9][10][11][12][13][14][15][16][17][18][19].…”