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Weber, Chris P.; Schoop, Leslie M.; Parkin, Stuart S. P.; Newby, Robert C.; Nateprov, Alex; Lotsch, Bettina; Mariserla, Bala Murali Krishna; Kim, J. Matthew; Dani, Keshav M.; Bechtel, Hans A.; Arushanov, Ernest; Ali, Mazhar (2018): Directly photoexcited Dirac and Weyl fermions in ZrSiS and NbAs. In: Applied Physics Letters, Vol. 113, No. 22, 221906
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We report ultrafast optical measurements of the Dirac line-node semimetal ZrSiS and the Weyl semimetal NbAs, using mid-infrared pump photons from 86 meV to 500 meV to directly excite Dirac and Weyl fermions within the linearly dispersing bands. In NbAs, the photoexcited Weyl fermions initially form a non-thermal distribution, signified by a brief spike in the differential reflectivity whose sign is controlled by the relative energy of the pump and probe photons. In ZrSiS, electron-electron scattering rapidly thermalizes the electrons, and the spike is not observed. Subsequently, hot carriers in both materials cool within a few picoseconds. This cooling, as seen in the two materials' differential reflectivity, differs in sign, shape, and timescale. Nonetheless, we find that it may be described in a simple model of thermal electrons, without free parameters. The electronic cooling in ZrSiS is particularly fast, which may make the material useful for optoelectronic applications. Published by AIP Publishing.