In the Dirac semimetal BaNiS 2 , the Dirac nodes are located along the Γ−M symmetry line of the Brillouin zone, instead of being pinned at fixed high-symmetry points. We take advantage of this peculiar feature to demonstrate the possibility of moving the Dirac bands along the Γ−M symmetry line in reciprocal space by varying the concentration of K atoms adsorbed onto the surface of cleaved BaNiS 2 single crystals. By means of first-principles calculations, we give a full account of this observation by considering the effect of the electrons donated by the K atom on the charge transfer gap, which establishes a promising tool for engineering Dirac states at surfaces, interfaces, and heterostructures.
Time-resolved ARPES makes it possible to directly visualize the band dispersion of photoexcited solids, as well as to study its time evolution on the femtosecond time scale. In this article, we show how this technique can be used to monitor the ultrafast hot carrier dynamics and the conduction band dispersion in two typical monochalcogenide semiconductors: direct band gap, n-type indium selenide and indirect band gap, p-type germanium selenide. With this approach, one can directly estimate the effective electron masses of these semiconductors. Moreover, the dynamics of hot electrons in the two semiconductors are analyzed and compared. Our findings provide valuable information for the use of monochalcogenide semiconductors in future optoelectronic devices.
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