Magnetic field induced polarization enhancement in the photoluminescence of MBE-grown WSe$_2$ layers

We report an experimental study of the magnetic-field dependence of the optically pumped valley polarization in an epitaxial tungsten diselenide (WSe$_2$) monolayer grown by molecular-beam epitaxy (MBE) on a hexagonal boron nitride (hBN) substrate. Circularly polarized photoluminescence (PL) measurements reveal that applying a weak out-of-plane magnetic field, on the order of 0.1 T, dramatically increases the effectiveness of the optical orientation of the emission associated with defect-bound localized excitons. We compare the obtained results with the earlier studies on the reference exfoliated monolayers, discussing both qualitative similarity as well as quantitative differences. Our observations are further supplemented by the results of time-resolved PL measurements, which confirm the pseudospin relaxation time of approximately 25 ps, a value significantly shorter than the $\approx$100 ps previously reported for mechanically exfoliated samples.

💡 Research Summary

The authors present a comprehensive experimental investigation of magnetic‑field‑induced valley‑polarization enhancement (FIPE) in monolayer tungsten diselenide (WSe₂) grown by molecular‑beam epitaxy (MBE) on hexagonal‑boron‑nitride (hBN) substrates. Using both continuous‑wave (CW) and time‑resolved photoluminescence (PL) techniques, they demonstrate that an out‑of‑plane magnetic field as low as ~0.1 T dramatically increases the circular polarization degree of the emission associated with defect‑bound localized excitons (LE). The CW measurements record co‑circular and cross‑circular PL spectra for each magnetic field value, from which the polarization degree PD = (I_co – I_cross)/(I_co + I_cross) is extracted. The PD versus magnetic field exhibits a characteristic dip centered at zero field, which is fitted with a Lorentzian function to obtain three parameters: the asymptotic polarization y₀, the relative dip amplitude A_mod, and the half‑width at half‑maximum B₀. For the MBE‑grown samples, B₀ is found to be ≈80 mT, four times larger than the ≈20 mT reported for mechanically exfoliated WSe₂ monolayers. Within a Hanle‑type model, B₀ is directly proportional to the pseudospin depolarization rate γ_dep, implying that the pseudospin relaxation time τ_dep in the MBE samples is reduced from ~100 ps (exfoliated) to ~20–25 ps.

Time‑resolved PL, recorded with a streak camera after 150‑fs pulsed excitation at 700 nm, confirms this rapid depolarization. The co‑ and cross‑polarized transients show multi‑exponential decay, and the derived polarization degree decays with a characteristic time of ≈25 ps, matching the CW‑based estimate. This fast decay indicates that the intermediate dark state, which mediates the FIPE effect, loses its valley information much more quickly in MBE‑grown material.

Temperature‑dependent CW measurements (5–20 K) reveal that while overall PL intensity and the absolute polarization degree decrease with increasing temperature, both the relative dip amplitude A_mod and the half‑width B₀ remain essentially constant. This behavior contrasts with earlier reports on exfoliated WSe₂, where B₀ shrank with temperature, and resembles observations made on WS₂ monolayers. The authors interpret the temperature‑independent B₀ as evidence that the energy‑relaxation rate γ_relax remains much faster than the intrinsic inter‑valley scattering rate γ_inter throughout the examined temperature range, a regime likely promoted by the higher disorder and defect density inherent to the MBE growth process.

In summary, the study confirms that FIPE is a robust phenomenon in high‑quality, epitaxially grown WSe₂ monolayers, but the underlying valley dynamics differ markedly from those of exfoliated flakes. The MBE technique yields samples with excellent optical homogeneity and narrow exciton lines, yet introduces a distinct defect landscape that accelerates pseudospin depolarization by roughly a factor of four. These findings underscore the critical influence of synthesis method on valley‑tronic properties and suggest that optimizing growth conditions will be essential for realizing practical devices that rely on long‑lived valley polarization.