Figure 2: Valley Zeeman effect and diamagnetic shift in monolayer WS₂.

(a) Reflection spectrum of monolayer WS₂ at B=0 T and T=4 K. The A and B exciton resonances are labelled. (b) Normalized reflection spectra (1−R/R₀) at the A exciton resonance using σ⁺ polarized light. The dashed black trace was acquired at B=0 T. The red trace was acquired at +65 T, and corresponds to the σ⁺ transition in the K valley. The blue trace was acquired at −65 T, which is equivalent (by time-reversal symmetry) to the σ⁻ transition in the K′ valley at +65 T. The valley Zeeman splitting between these two peaks is clearly resolved. (c) Similar reflection spectra and valley Zeeman splitting of the B exciton. (d) Energies (E⁺ and E⁻) of the field-split A exciton versus magnetic field. (e) Same, but for the B exciton. (f) The measured valley Zeeman splitting (E⁺−E⁻) versus magnetic field, for both A and B excitons. The dotted line corresponds to a splitting of −4μ_B (−232 μeV T⁻¹). (g) The average energy, (E⁺ +E⁻)/2, for both the A and B excitons (the zero-field offset has been subtracted). A small quadratic diamagnetic shift is revealed. The dotted lines show quadratic fits to the data (ΔE_dia=σB²), where σ is the diamagnetic shift coefficient. We find that σ_A=0.32±0.02 μeV T⁻² and σ_B=0.11±0.02 μeV T⁻² for the A and B excitons, respectively.