Structural Phase Transitions in SrRh2As2

SrRh2As2 exhibits structural phase transitions reminiscent to those of BaFe2As2, but crystallizes with three polymorphs derived from the tetragonal ThCr2Si2-type structure. The structure of \alpha-SrRh2As2 is monoclinic with a = 421.2(1) pm, b = 1105.6(2) pm, c = 843.0(1) pm and \beta = 95{\deg} and was refined as a partially pseudo meroedric twin in the space group P21/c with R1 = 0.0928. \beta-SrRh2As2 crystallizes with a modulated structure in the (3+1) dimensional superspace group Fmmm(10\gamma)\sigma 00 with the unit cell parameters a = 1114.4(3) pm, b = 574.4(2) pm and c = 611.5(2) pm and an incommensurable modulation vector q = (1, 0, 0.3311(4)). High temperature single crystal diffraction experiments confirm the tetragonal ThCr2Si2-type structure for \gamma-SrRh2As2 above 350{\deg}C. Electronic band structure calculations indicate that the structural distortion in alpha-SrRh2As2 is caused by strong Rh-Rh bonding interactions and has no magnetic origin as suggested for isotypic BaFe2As2.Comment: 16 pages, 10 Figure

Role of heat and mechanical treatments in the fabrication of superconducting Ba0.6K0.4Fe2As2 ex-situ Powder-In-Tube tapes

Among the recently discovered Fe-based superconducting compounds, the (K,Ba)Fe2As2 phase is attracting large interest within the scientific community interested in conductor developments. In fact, after some years of development, critical current densities Jc of about 105 A/cm2 at fields up to more than 10 T have been obtained in powder in tube (PIT) processed wires and tapes. Here we explore the crucial points in the wire/tape fabrication by means of the ex-situ PIT method. We focus on scaling up processes which are crucial for the industrial fabrication. We analyzed the effects on the microstructure of the different heat and mechanical treatments. By an extensive microstructural analysis correlated with the transport properties we addressed the issues concerning the phase purity, the internal porosity and crack formation in the superconducting core region. Our best conductors with a filling factor of about 30 heat treated at 800 C exhibited Tc = 38 K the highest value measured in such kind of superconducting tape. The microstructure analysis shows clean and well connected grain boundaries but rather poor density: The measured Jc of about 3 x 10^4 A/cm2 in self-field is suppressed by less than a factor 7 at 7 T. Such not yet optimized Jc values can be accounted for by the reduced density while the moderate in-field suppression and a rather high n-factor confirm the high homogeneity and uniformity of these tapes

Ab initio lattice dynamics simulations and inelastic neutron scattering spectra for studying phonons in BaFe2As2: Effect of structural phase transition, structural relaxation and magnetic ordering

We have performed extensive ab initio calculations to investigate phonon dynamics and their possible role in superconductivity in BaFe2As2 and related systems. The calculations are compared to inelastic neutron scattering data that offer improved resolution over published data [Mittal et al., PRB 78 104514 (2008)], in particular at low frequencies. Effects of structural phase transition and full/partial structural relaxation, with and without magnetic ordering, on the calculated vibrational density of states are reported. Phonons are best reproduced using either the relaxed magnetic structures or the experimental cell. Several phonon branches are affected by the subtle structural changes associated with the transition from the tetragonal to the orthorhombic phase. Effects of phonon induced distortions on the electronic and spin structure have been investigated. It is found that for some vibrational modes, there is a significant change of the electronic distribution and spin populations around the Fermi level. A peak at 20 meV in the experimental data falls into the pseudo-gap region of the calculation. This was also the case reported in our recent work combined with an empirical parametric calculation [Mittal et al., PRB 78 104514 (2008)]. The combined evidence for the coupling of electronic and spin degrees of freedom with phonons is relevant to the current interest in superconductivity in BaFe2As2 and related systems

Why TcT_c of (CaFeAs)10_{10}Pt3.58_{3.58}As8_8 is twice as high as (CaFe0.95_{0.95}Pt0.05_{0.05}As)10_{10}Pt3_3As8_8

Recently discovered (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$ and (CaFeAs)$_{10}$Pt$_{4-y}$As$_8$ superconductors are very similar materials having the same elemental composition and structurally similar superconducting FeAs slabs. Yet the maximal critical temperature achieved by changing Pt concentration is approximately twice higher in the latter. Using angle-resolved photoemission spectroscopy(ARPES) we compare the electronic structure of their optimally doped compounds and find drastic differences. Our results highlight the sensitivity of critical temperature to the details of fermiology and point to the decisive role of band-edge singularities in the mechanism of high-$T_c$ superconductivity

Transition from Mott insulator to superconductor in GaNb4_{4}Se8_{8} and GaTa4_{4}Se8_{8} under high pressure

Electronic conduction in GaM$_{4}$Se$_{8}$ (M=Nb;Ta) compounds with the fcc GaMo$_{4}$S$_{8}$-type structure originates from hopping of localized unpaired electrons (S=1/2) among widely separated tetrahedral M$_{4}$ metal clusters. We show that under pressure these systems transform from Mott insulators to a metallic and superconducting state with T$_{C}$=2.9 and 5.8K at 13 and 11.5GPa for GaNb$_{4}$Se$_{8}$ and GaTa$_{4}$Se$_{8}$, respectively. The occurrence of superconductivity is shown to be connected with a pressure-induced decrease of the MSe$_{6}$ octahedral distortion and simultaneous softening of the phonon associated with MSe-bonds.Comment: 10 pages, 5 figure

Extended Magnetic Dome Induced by Low Pressures in Superconducting FeSe1-x_\mathrm{1\text{-}x}Sx_\mathrm{x}

We report muon spin rotation ($\mu$SR) and magnetization measurements under pressure on Fe$_{1+\delta}$Se$_\mathrm{1\text{-}x}$S$_\mathrm{x}$ with x $\approx 0.11$.Above $p\approx0.6$ GPa we find microscopic coexistence of superconductivity with an extended dome of long range magnetic order that spans a pressure range between previously reported separated magnetic phases. The magnetism initially competes on an atomic scale with the coexisting superconductivity leading to a local maximum and minimum of the superconducting $T_\mathrm{c}(p)$. The maximum of $T_\mathrm{c}$ corresponds to the onset of magnetism while the minimum coincides with the pressure of strongest competition. A shift of the maximum of $T_\mathrm{c}(p)$ for a series of single crystals with x up to 0.14 roughly extrapolates to a putative magnetic and superconducting state at ambient pressure for x $\geq0.2$.Comment: 10 pages, 6 figures, including supplemental materia

Correlation effects in Ni 3d states of LaNiPO

The electronic structure of the new superconducting material LaNiPO experimentally probed by soft X-ray spectroscopy and theoretically calculated by the combination of local density approximation with Dynamical Mean-Field Theory (LDA+DMFT) are compared herein. We have measured the Ni L2,3 X-ray emission (XES) and absorption (XAS) spectra which probe the occupied and unoccupied the Ni 3d states, respectively. In LaNiPO, the Ni 3d states are strongly renormalized by dynamical correlations and shifted about 1.5 eV lower in the valence band than the corresponding Fe 3d states in LaFeAsO. We further obtain a lower Hubbard band at -9 eV below the Fermi level in LaNiPO which bears striking resemblance to the lower Hubbard band in the correlated oxide NiO, while no such band is observed in LaFeAsO. These results are also supported by the intensity ratio between the transition metal L2 and L3 bands measured experimentally to be higher in LaNiPO than in LaFeAsO, indicating the presence of the stronger electron correlations in the Ni 3d states in LaNiPO in comparison with the Fe 3d states in LaFeAsO. These findings are in accordance with resonantly excited transition metal L3 X-ray emission spectra which probe occupied metal 3d-states and show the appearance of the lower Hubbard band in LaNiPO and NiO and its absence in LaFeAsO.Comment: 6 pages, 5 figure