We present the c-axis optical reflectance measurement on single crystals of BaFe2As2 and SrFe2As2, the parent compounds of FeAs based superconductors. Different from the ab-plane optical response where two distinct energy gaps were observed in the SDW state, only the smaller energy gap could be seen clearly for E c-axis. The very pronounced energy gap structure seen at a higher energy scale for E ab-plane is almost invisible. We propose a novel picture for the band structure evolution across the SDW transition and suggest different driving mechanisms for the formation of the two energy gaps.PACS numbers: 74.25. Gz, 74.70.Xa, 75.30.Fv For quasi-two dimensional layered materials, striking differences could exist in the in-plane and out-ofplane charge transport and dynamics. For example, in some high-T c cuprates, metallic in-plane charge transport coexists with nonmetallic conductivity along the caxis [1,2]. The contrasting behavior violates the conventional concept of band electron transport, and has been the subject of intensive study. Fe-pnictide superconducting materials also crystalize in the layered structure with Fe-As layers separated by alkaline metal ions or other insulator-like layers. Band structure calculations based on the local-density approximation (LDA) or generalized gradient approximations (GGA) indicate dominantly two-dimensional (2D) cylinder-like Fermi surfaces (FSs) along the c-axis [3][4][5]. It is important to see whether or not the Fe-pnictides share similar anisotropic charge dynamical properties with cuprates.Optical spectroscopy is a powerful technique to investigate charge dynamics and band structure of a material as it probes both free carriers and interband excitations. In particular, it yields direct information about the energy gap formation in the broken symmetry state. Optical spectroscopy studies on the ab-plane properties of different Fe-pnictides and chalcogenides systems have been reported by several groups. [6][7][8][9][10][11][12][13][14][15][16][17][18] For the parent compounds of Fe-pnictides, the measurements provide clear evidence for the formation of the partial energy gaps in the magnetic phase, supporting the itinerant picture that the energy gain for the antiferromagnetic ground state is achieved through the opening of a spin-density-wave (SDW) gap on the FSs. [7,10,[14][15][16] For the superconducting samples, the superconducting pairing gaps were also detected by the technique. [6,17,18] However, optical investigations have not been carefully done on the c-axis response of Fe-pnictide materials. There is only one work in the literature containing optical data along the c-axis.[10] Unfortunately, the data were limited to the high frequencies, above 700 cm −1 . Because of this limitation, neither the free-carrier response nor any feature related to the SDW gap were observed. In fact, the reported reflectance data appear to have extraordinarily low values. As information about the anisotropic charge dynamics is extremely important for understanding the materials,...
In this paper, phase-locking dynamics of 2D VCSEL hexagonal array with an integrated Talbot cavity are numerically investigated based on rate equations aiming at achieving high brightness output. The processes of wavelength synchronization and phase locking under different fill factors ff and fractional Talbot cavity lengths L were addressed comprehensively. Different supermodes of phase-locked VCSEL array were then analyzed from both near-field and far-field pattern, and proved to be well matched with the results of coupled-mode theory. With appropriate configuration the Talbot-VCSEL system can operate in a full in-phase mode eventually, which is beneficial for determining the parameter interval corresponding to the most expected single narrow-lobe far-field pattern. Furthermore, the simulation results also indicate that, considering the parametric interactions the distribution of optical feedback from the fractional Talbot cavity should be consistent as much as possible to facilitate the realization of phase-locked state. Our study could provide a theoretical support to obtain the full in-phase coupled VCSEL array with high performance.
Bio‐tillage has recently been proposed as a measure to alleviate soil compaction through biopores created by cover crop roots. The objective of this study was to determine the effect of different cover crops on soil physical properties and the succeeding maize (Zea mays L.) growth in compacted soil. Four treatments, including no cover crop as a control (Con), alfalfa (Medicago sativa L.), oilseed rape (Brassica napus L.), and radish and hairy vetch mixture (Raphanus sativus L. and Vicia villosa Roth), were carried out under both compacted and noncompacted soil conditions. Soil physical properties, such as the volumetric soil water content (SWC), bulk density, saturated hydraulic conductivity (Ks) and air permeability at water potential of −60 hPa (Ka60), and maize root characteristics and yield were measured. The cover crops did not affect the soil bulk density but significantly decreased the SWC in both the compacted and noncompacted soils relative to the Con treatment. The alfalfa treatment presented significantly higher Ks in the noncompacted soil and Ka60 in both the compacted and noncompacted soils than the Con treatment in the soil layer depth of 20–50 cm. The three cover crop treatments improved the maize root biomass density (173.2% for 2018 and 35.6% for 2019) and root length density (50.9% for 2018 and 51.8% for 2019) relative to the Con treatment in the soil layer depth of 10–70 cm in 2018 and soil layer depth of 10–50 cm in 2019 in the compacted soil rather than in the noncompacted soil. Compared with the Con treatment, the radish mixed with hairy vetch treatment in 2018 and the oilseed rape treatment in 2019 significantly enhanced the maize yield in the compacted soil. Our results suggest that alfalfa is the best crop for improving air permeability; however, the oilseed rape and mixture of radish and hairy vetch lead to better maize growth in the compacted soil. Bio‐tillage using cover crops is effective in alleviating soil compaction.
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