Recently, high-transition-temperature (high-Tc) superconductivity was discovered in the iron pnictide RFeAsO(1-x)F(x) (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO(1-x)F(x) as the system is tuned from a semimetal to a high-Tc superconductor through fluorine (F) doping, x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a collinear antiferromagnetic order with decreasing temperature. With increasing fluorine doping, the structural phase transition decreases gradually and vanishes within the superconductivity dome near x=0.10, whereas the antiferromagnetic order is suppressed before the appearance of superconductivity for x>0.06, resulting in an electronic phase diagram remarkably similar to that of the high-Tc copper oxides. Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-Tc superconductivity in these Fe pnictides.

We performed an angle-resolved photoemission spectroscopy study of BaFe2As2, which is the parent compound of the so-called 122 phase of the iron-pnictide high-temperature superconductors. We reveal the existence of a Dirac cone in the electronic structure of this material below the spin-density-wave temperature, which is responsible for small spots of high photoemission intensity at the Fermi level. Our analysis suggests that the cone is slightly anisotropic and its apex is located very near the Fermi level, leading to tiny Fermi surface pockets. The bands forming the cone show an anisotropic leading edge gap away from the cone that suggests a nodal spin-density-wave description.

Solar energy is a renewable resource that can supply our energy needs in the long term. A semiconductor photocatalysis that is capable of utilizing solar energy has appealed to considerable interests for recent decades, owing to the ability to aim at environmental problems and produce renewal energy. Much effort has been put into the synthesis of a highly efficient semiconductor photocatalyst to promote its real application potential. Hence, we reviewed the most advanced methods and strategies in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites, and (iii) control of the electron−hole (e − -h + ) recombination, while these three processes could be influenced by remodeling the crystal lattice, surface, and interface. Additionally, we individually examined their current applications in energy conversion (i.e., hydrogen evolution, CO 2 reduction, nitrogen fixation, and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment). Overall, in this review, we particularly focused on advanced photocatalytic activity with simultaneous wastewater decontamination and energy conversion and further enriched the mechanism by proposing the electron flow and substance conversion. Finally, this review offers the prospects of semiconductor photocatalysts in the following three vital (distinct) aspects: (i) the large-scale preparation of highly efficient photocatalysts, (ii) the development of sustainable photocatalysis systems, and (iii) the optimization of the photocatalytic process for practical application.

The metallic 1T-MoS has attracted considerable attention as an effective catalyst for hydrogen evolution reactions (HERs). However, the fundamental mechanism about the catalytic activity of 1T-MoS and the associated phase evolution remain elusive and controversial. Herein, we prepared the most stable 1T-MoS by hydrothermal exfoliation of MoS nanosheets vertically rooted into rigid one-dimensional TiO nanofibers. The 1T-MoS can keep highly stable over one year, presenting an ideal model system for investigating the HER catalytic activities as a function of the phase evolution. Both experimental studies and theoretical calculations suggest that 1T phase can be irreversibly transformed into a more active 1T' phase as true active sites in photocatalytic HERs, resulting in a "catalytic site self-optimization". Hydrogen atom adsorption is the major driving force for this phase transition.

In Ba(Fe0.95Co0.05)2As2 all of the 75 As NMR intensity at the paramagnetic resonance position vanishes abruptly below T onset SDW = 56 K, indicating that magnetic (spin density wave) order is present in all of the sample volume, despite bulk superconductivity below Tc=15 K. The two phases thus coexist homogeneously at the microscopic scale. In Ba0.6K0.4Fe2As2, on the other hand, the signal loss below T onset SDW ≃75 K is not complete, revealing that magnetic order is bound to finite-size areas of the sample, while the remaining NMR signal shows a clear superconducting response below Tc = 37 K. Thus, the two phases are not homogeneously mixed, at least for this potassium concentration. For both samples, spatial electronic and/or magnetic inhomogeneity is shown to characterize the NMR properties in the normal state.

We have successfully grown high-quality single crystals of SrFe 2 As 2 and A 0.6 K 0.4 Fe 2 As 2 ͑A = Sr, Ba͒ using flux method. The resistivity, specific heat, and Hall coefficient have been measured. For parent compound SrFe 2 As 2 , an anisotropic resistivity with c / ab as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivities due to the spin-density-wave ͑SDW͒ instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows twofold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K-doped A 0.6 K 0.4 Fe 2 As 2 ͑A = Sr, Ba͒, the SDW instability is suppressed and the superconductivity appears with T c above 35 K. The rather low anisotropy in upper critical field between H ʈ ab and H ʈ c indicates that interplane coupling plays an important role in hole-doped Fe-based superconductors.The recent discovery of superconductivity with transition temperature T c ϳ 26 K in LaFeAsO 1−x F x has generated tremendous interest in scientific community. 1 Shortly after this discovery, the T c was raised to 41-55 K by replacing La by rare-earth Ce, Sm, Pr, Nd, etc., making those systems with T c exceeding 50 K. 2-5 The undoped quaternary compounds crystallize in a tetragonal ZrCuSiAs-type structure, which consists of alternate stacking of edge-sharing Fe 2 As 2 tetrahedral layers and La 2 O 2 tetrahedral layers along c axis. Very recently, superconductivity with T c of up to 38 K was discovered in AFe 2 As 2 ͑A = Ba, Sr, Ca͒ upon K or Na doping. 6-10 AFe 2 As 2 compounds crystallize in a tetragonal ThCr 2 Si 2 -type structure with identical Fe 2 As 2 tetrahedral layers as in LaFeAsO, but separated by single elemental A layers. These compounds contain no oxygen in A layers. The simpler structure of AFe 2 As 2 system makes it more suitable for research of intrinsic physical properties of Fe-based compounds.Except for a relatively high transition temperature, the system displays many interesting properties. The existence of a spin-density-wave ͑SDW͒ instability in parent LaFeAsO ͑Ref. 11͒ was indicated by specific heat, optical measurements, and first-principles calculations, and subsequently confirmed by neutron-scattering, 12 NMR, 13 sR, 14 and Mössbauer 15 spectroscopic measurements. The superconductivity only appears when SDW instability was suppressed by doping carriers or applying pressure. The competition between superconductivity and SDW instability was identified in other rare-earth substituted systems. 2,16,17 Besides the SDW instability, structural distortions from tetragonal to monoclinic were also observed for both ReFeAsO ͑Re = rare earth͒ and AFe 2 As 2 ͑A = Ba, Sr, Ca͒. 18-23 The structural transition temperatures were found to occur at slightly higher than SDW transition temperature in LaFeAsO, 12 but the two transitions occur simultaneously in AFe 2 As 2 ͑A = Ba, Sr, Ca͒. 18,19,24 The band-structure calculation and neutronscattering exp...

In this work, we report a systematic study of the relationship between photocatalytic properties of hydrogen evolution and structures and morphologies of g-C 3 N 4 prepared by different precursors (urea, melamine and dicyandiamide). The photocatalytic performances of H 2 production are affected by the method and degree of polymerization, the degree of protonation, and the morphology of g-C 3 N 4 prepared

No abstract