A central tenet in the theory of quantum phase transitions (QPTs) is that a nonanalyticity in a ground-state energy implies a QPT. Here we report on a finding that challenges this assertion. As a case study we take a phase diagram of a one-dimensional band insulator with spin-orbit coupled electrons, supporting trivial, and topological gapped phases separated by intersecting critical surfaces. The intersections define multicritical lines across which the ground-state energy becomes nonanalytical, concurrent with a closing of the band gap, but with no phase transition taking place.
We report valley dynamics of spin-singlet (IX S ) and spin-triplet (IX T ) interlayer excitons in MoS 2 /WS 2 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 IX T , which is crucial for valley pseudospin manipulation. Furthermore, the exciton g-factors (g IXd S 13.4 and g IXd T 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 IX S and IX T , and phonon-assisted intervalley scattering significantly enhance photoluminescence intensity (up to 325% for IX S and 1075% for IX T ) in the α-valley, while suppressing it in the β-valley. Consequently, the valley polarization of IX S and IX T emissions abruptly converges to unity at low temperatures. At a critical exchange field, valley-selective switching of interlayer exciton states, from IX T to IX S in the β-valley, occurs. Under such a condition, valley polarization retains very high values, above 90% for IX T and 85% for IX S at room temperature. These attractive characteristics make interlayer excitons in MoS 2 /WS 2 heterostructures promising for quantum applications.
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