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.
Controlling
the photoexcited properties and behavior of hybrid
perovskites by halide doping has the potential to impact a wide range
of emerging technologies, including solar cells and radiation detectors.
Crystalline samples of methylammonium lead bromide substituted with
chlorine (MAPbBr3–x
Cl
x
) were examined by transient reflectivity spectroscopy
and nonadiabatic molecular dynamics simulations. At picosecond time
scales, the addition of chlorine to the perovskite crystal increased
the observed rate of hot carrier cooling and the calculated electron–phonon
coupling constants. Chlorine-doped samples also exhibit a slower surface
recombination velocity and a smaller ambipolar mobility.
We report x-ray absorption and photoemission spectroscopy of the electronic structure in the normal state of metallic YFe2Ge2. The data reveal evidence for large fluctuating spin moments on the Fe sites, as indicated by exchange multiplets appearing in the Fe 3s core level photoemission spectra, even though the compound does not show magnetic order. The magnitude of the multiplet splitting is comparable to that observed in the normal state of the Fe-pnictide superconductors. This shows a connection between YFe2Ge2 and the Fe-based superconductors even though it contains neither pnictogens nor chalcogens. The implication is that the chemical range of compounds showing at least one of the characteristic magnetic signatures of the Fe-based superconductors is broader than previously thought.
We differentiate the effect of strain induced by lattice-mismatched growth from strain induced by mechanical deformation on cubic nonradiative Auger recombination in narrow-gap GaInAsSb/GaSb quantum well (QW) heterostructures. The typical reduction in the Auger coefficient observed with lattice-mismatched growth appears to be due to the concomitant compositional change rather than the addition of strain, with implications for mid-IR semiconductor laser design. We induced a range of internal compressive strain in five samples from −0.90% to −2.07% by varying the composition during the growth and mechanically induced a similar range of internal strain in analogous quantum well membrane samples. We performed time-resolved photoluminescence and differential reflectivity measurements to extract the carrier recombination dynamics, taken at 300 K with carrier densities from 2.7×1018 cm−3 to 1.4×1019 cm−3. We observed no change with strain in the cubic Auger coefficient of samples that were strained mechanically, but we did observe a trend with strain in samples that were strained by the QW alloy composition. Measured Auger coefficients ranged from 3.0×10−29 cm6 s−1 to 3.0×10−28 cm6 s−1.
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