Control of spin-valley state in a charged WSe₂ quantum dot by optical helicity

We present low temperature (~4 K) photoluminescence study of positively charged quantum dots (QDs) in an ambipolar WSe₂ field effect transistor. At a negative gate voltage when the monolayer sample is lightly hole-doped, a single peak (S-peak) and a doublet (D-peak) appear simultaneously and spectrally wander in an identical manner. We thus assign S- and D-peaks originate from the same QD. D-peaks are the neutral (X⁰) QD with fine structure splitting of ~600 µeV resulting from the e-h exchange interaction. While the exchange interaction in a singly, positively charged (X⁺) QD is expected to vanish due to holes forming a singlet and Pauli blocking. We assign S-peaks to be X⁺ QD which has a binding energy of -10 meV with respect to X⁰. X⁺ QD is doubly degenerate at B = 0 T. The degeneracy of X⁺ is lifted in finite B field, allowing us to spectrally distinguish between the spin-valley states (with excess hole spin up in -K valley or spin down in K valley). By controlling the helicity of the excitation laser, we observe selective initialization of spin-valley of excess hole in X⁺ QD under small B field. Our results show that spin-valley degree is robust in charged WSe₂ QDs and enables valleytronics on single localized charge carriers.