We investigate polarization-dependent ultrafast photocurrents in the Weyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient photocurrents are generated along the non-centrosymmetric c-axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the ab-plane. This is consistent with earlier static photocurrent experiments, and demonstrates on the basis of both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay that optically induced photocurrents in TaAs are inherent to the underlying crystal symmetry of the transition metal monopnictide family of Weyl semimetals.In this letter, we demonstrate the generation of both helicity-dependent and helicity-independent ultra-arXiv:1811.02723v2 [cond-mat.str-el]
Electronic structure of the chiral helimagnet and 3d -intercalated transition metal dichalcogenide C r1/3Nb S2 The electronic structure of the chiral helimagnet Cr 1/3 NbS 2 has been studied with core level and angle-resolved photoemission spectroscopy (ARPES). Intercalated Cr atoms are found to be effective in donating electrons to the NbS 2 layers but also cause significant modifications of the electronic structure of the host NbS 2 material. In particular, the data provide evidence that a description of the electronic structure of Cr 1/3 NbS 2 on the basis of a simple rigid band picture is untenable. The data also reveal substantial inconsistencies with the predictions of standard density functional theory. The relevance of these results to the attainment of a correct description of the electronic structure of chiral helimagnets, magnetic thin films/multilayers, and transition metal dichalcogenides intercalated with 3d magnetic elements is discussed.
The chiral helimagnet Cr 1/3 NbS 2 hosts exotic spin textures, whose influence on the magnetotransport properties make this material an ideal candidate for future spintronic applications. To date, the interplay between macroscopic magnetic and transport degrees of freedom is believed to result from a reduction in carrier scattering following spin order. Here, we present electronic structure measurements across the helimagnetic transition temperature T C that challenges this view. We show that the Fermi surface is comprised of strongly hybridized Nband Cr-derived electronic states, and that spectral weight close to the Fermi level increases anomalously as the temperature is lowered below T C. These findings are rationalized on the basis of first principle density functional theory calculations, which reveal a large nearestneighbor exchange energy, suggesting the interaction between local spin moments and hybridized Nb-and Cr-derived itinerant states to go beyond the perturbative interaction of Ruderman-Kittel-Kasuya-Yosida, suggesting instead a mechanism rooted in a Hund's exchange interaction.
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