Phonon-Fostered Valley Polarization of Interlayer Excitons in van der Waals Heterostructures

We report valley dynamics of spin-singlet (IXS) and spin-triplet (IXT) interlayer excitons in MoS2/WS2 heterostructures subjected to an exchange field. Because of their interlayer character, they possess long valley coherence time in addition to long radiative lifetimes, especially for the IXT, which is crucial for valley pseudospin manipulation. Furthermore, the exciton g-factors (gIXS 13.4 and gIXT 17.6) that are more than three times larger than that of intralayer exciton lead to giant valley Zeeman splitting (0.78–1.02 meV/T). Subjected to an exchange field, the competitive interplay among radiative relaxation, thermal excitation between IXS and IXT, and phonon-assisted intervalley scattering significantly enhance photoluminescence intensity (up to 325% for IXS and 1075% for IXT) in the α-valley, while suppressing it in the β-valley. Consequently, the valley polarization of IXS and IXT emissions abruptly converges to unity at low temperatures. At a critical exchange field, valley-selective switching of interlayer exciton states, from IXT to IXS in the β-valley, occurs. Under such a condition, valley polarization retains very high values, above 90% for IXT and 85% for IXS at room temperature. These attractive characteristics make interlayer excitons in MoS2/WS2 heterostructures promising for quantum applications.