No abstract

No abstract

Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like (α and β) and two electron-like (η and δ) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along kz. However, a third hole-like band (γ) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be the most vulnerable to defects, its absence in our data is likely due to defect scattering -and yet superconductivity can exist without coherent quasiparticles in the γ band. This may help resolve the current controversy on the superconducting gap structure of FeS. Moreover, by comparing the β bandwidths of various iron chalcogenides, including FeS, FeSe1−xSx, FeSe, and FeSe1−xTex, we find that the β bandwidth of FeS is the broadest. However, the band renormalization factor of FeS is still quite large, when compared with the band calculations, which indicates sizable electron correlations. This explains why the unconventional superconductivity can persist over such a broad range of isovalent substitution in FeSe1−xTex and FeSe1−xSx.

Based on the complexity and difficult understanding on low-temperature resistivity minimum in manganites, the effect of ferroelectric-poling-induced strain on Kondo-type transport behavior was systemically investigated as a function of magnetic field for La 0.7 Ca 0.15 Sr 0.15 MnO 3 manganite thin films grown on ferroelectric 0.67Pb͑Mg 1/3 Nb 2/3 ͒O 3 -0.33PbTiO 3 ͑PMN-PT͒ single-crystal substrates. The results show that the lowtemperature resistivity upturn is mainly caused from quantum correction effects driven by electron-electron interaction and inelastic scattering. Whether the PMN-PT substrate is in unpoled or poled state, the temperature where the resistivity shows an upturn near 15 K shifts to a higher temperature under magnetic field. The ferroelectric poling induces a reduction in the in-plane tensile strain and thus the lattice distortion of the film, which suppresses the resistivity upturn. These prove that the local lattice distortion relevant to the strain of the film is one of the main disorders that influence the resistivity upturn. The present results will be meaning to understand the physical mechanism of Kondo-type behavior at low temperature in colossal magnetoresistance manganites.

1 μm thick Si solar cells based on nanocone grating (NCG) with height of 100-800 nm and period of 100, 500, and 800 nm are numerically investigated through reflectivities, absorption enhancement factors, absorption spectra, optical generation rates, ultimate efficiencies, and diffraction angles. Compared with the planar Si solar cell, absorption enhancement are observed in any solar cells with NCG surface. Their absorption enhancement mechanism varies with the incident wavelength range. When incident wavelength λ < 500 nm, antireflection of their front surface dominates the absorption enhancement behavior due to their stronger absorption coefficients. When 600 nm > λ > 500 nm, even though the absorption enhancement is still dominated by antireflection of the front surface, cavity-resonance effect and guided-mode excitation induced by high order diffraction start to make contribution. When λ > 600 nm, the contribution of guided-mode excitation induced by lower-order diffraction becomes larger and larger once the diffraction angle is larger than its critical angle. For the structure with P = 100 nm, high-order diffraction cut-off at the longer wavelength range is the main reason of its lower absorption enhancement and ultimate conversion efficiency. For P = 800 nm, the lower absorption enhancement and ultimate efficiency is also observed due to the high reflection loss and mode leakage induced by 1st order diffraction where its diffraction angle is lower than its critical angle. Higher absorption and ultimate conversion efficiencies are achieved in P = 500 nm due to the good balance between antireflection performance and guide-mode excitation induced by the high order diffraction is achieved. Moreover, such absorption enhancement is closely related with its height of NCG gratings. Reflection loss reduction, the interaction volume reduction between the incident light and Si material, and higher photon density in NCG structure coexists with H increasing, which results in absorption enhancement in P = 500 nm and P = 800 nm, but absorption reduction in P = 100 nm where high order diffraction cut-off. Based on these analysis, we do believe that high absorption and ultimate conversion efficiency should be achieved in NCG-based solar cells where both the lower reflection in short wavelength domain and guide-mode excitation induced by 1st and 2nd diffraction in longer wavelength domain can be achieved. According to this rule, the optimized structure is NCG with P = 559 nm and H = 500 nm, by which, the highest optical generation rate of 536.57 × 104 W/cm3 and ultimate efficiency of 28.132% are achieved. Such analysis should benefit the design of the thin film solar cells with nano-structured diffraction gratings.