The valley Zeeman effect in inter- and intra-valley trions in monolayer WSe2

T P Lyons,F Withers,M Brooks,T Taniguchi,A I Tartakovskii,G Burkard,S Dufferwiel,K S Novoselov,K Watanabe

doi:10.1038/s41467-019-10228-7

Abstract

Monolayer transition metal dichalcogenides (TMDs) hold great promise for future information processing applications utilizing a combination of electron spin and valley pseudospin. This unique spin system has led to observation of the valley Zeeman effect in neutral and charged excitonic resonances under applied magnetic fields. However, reported values of the trion valley Zeeman splitting remain highly inconsistent across studies. Here, we utilize high quality hBN encapsulated monolayer WSe2 to enable simultaneous measurement of both intervalley and intravalley trion photoluminescence. We find the valley Zeeman splitting of each trion state to be describable only by a combination of three distinct g-factors, one arising from the exciton-like valley Zeeman effect, the other two, trion specific, g-factors associated with recoil of the excess electron. This complex picture goes significantly beyond the valley Zeeman effect reported for neutral excitons, and eliminates the ambiguity surrounding the magneto-optical response of trions in tungsten based TMD monolayers.

Highlights

We reveal the complexities of trion magneto-PL, demonstrating that the trion energy splitting in PL does not reflect the underlying valley Zeeman splitting of the initial state
The process of trion radiative recombination itself modifies the emitted photon energy, via electron recoil to valley polarized conduction band states, having the effect of enhancing the singlet-singlet splitting, and diminishing the triplet-triplet splitting. On top of this asymmetry, we observe the action of a third process, associated with the electron recoil, which we attribute to Landau level quantization of both initial and final states
We arrive at the significant conclusion that any measurement of the trion valley splitting when treating it as a single resonance cannot yield an accurate measurement of the true valley Zeeman effect, as the measured value will depend on the relative contributions of the four fine structure components, each of which have different rates of shift, as shown in Fig. 3d, alongside variable relative PL intensities, as shown in Figs. 2b and 3b

Summary

Results

The initial state is subject to energy shifts arising from the atomic orbital and Berry curvature associated magnetic moments inherent to monolayer WSe2, much like the neutral exciton[20,21,22,25]. This is approximately double the value expected from purely atomic orbital contributions in the valence band (−4 μBB), implying a large Berry curvature associated magnetic moment of all trion states before recombination, present in the initial state but absent in the final state, in agreement with previous suggestions[21,25,29]. The contribution to gz from the Berry curvature may be expressed as me 2h2

(Supplementary

Discussion

Methods