Multigap superconductivity and strong electron-boson coupling in Fe-based superconductors: a point-contact Andreev-reflection study of Ba(Fe(1-x)Co(x))2As2 single crystals

Directional point-contact Andreev-reflection measurements in Ba(Fe(1-x)Co(x))2As2 single crystals (T(c) = 24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The point-contact Andreev-reflection spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Ω(b)(T) is obtained, very similar to the spin-resonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band s ± Eliashberg model by using an electron-boson spectral function peaked at Ω(0) = 12 meV ? Ω(b)(0).     

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu(0.2)Sr(0.8)(Fe(0.86)Co(0.14))(2)As(2) single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with T(c) is approximately equal to 20 K and an antiferromagnetic (AFM) ordering of Eu(2+) moments at T(N) is approximately equal to 2.0 K in zero field. (75)As NMR experiments have been performed in the two external field directions (H is parallel to ab) and (H is parallel to c). (75)As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T(1)) follow a T(3) law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s(±) type of superconductivity. In the temperature range 160-18 K 1/T(1)T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations.     

Preparation of NiO and CoO nanoparticles using M2+-oleate (M = Ni,Co) as precursor

The preparation of NiO and CoO nanoparticles was reported. The dot-like NiO and flower-like CoO nanoparticles were obtained using M²⁺-oleate (M = Ni, Co) as precursor via thermal decomposition method. Transmission electron microscopic (TEM) images monitored the growth of NiO and CoO nanoparticles. When the reaction complex including M²⁺-oleate (M = Ni, Co) precursor, oleic acid and 1-octadecene was heated to the refluxing temperature (320 °C), the formed NiO and CoO nanoparticles were needle-like and very small, indicating low growth speed. However, when the reaction complex was kept refluxing for 30 min, dot-like NiO and flower-like CoO nanoparticles were observed, suggesting the accelerated growth at this refluxing stage. The difference of the morphology of the resultant NiO and CoO nanoparticles resulted from the difference of their growth mode. Selected-area electron diffraction (SAED) patterns showed the face-centered cubic structures of NiO and CoO nanoparticles. The magnetic property of the nanoparticles was studied using vibrating sample magnetometer (VSM).     

Ni2X2 (X = pnictide,chalcogenide, or B) based superconductors

We review the properties of Ni-based superconductors which contain Ni₂X₂ (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi₂As₂. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron–phonon mediated superconductors. However, the increase in T_c in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure–property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.     

Two-band BCS mechanism of superconductivity in a Ba(Fe0.9Co0.1)2As2 high-temperature superconductor

Terahertz and infrared spectra of the conductivity, σ(ν), and dielectric constant, ?(ν), of a Ba(Fe_0.9Co_0.1)₂As₂ film (T _c = 20 K) have been analyzed together with previous specific-heat and angular resolved photoelectron spectroscopy data. It has been shown that the spectra σ(ν) and ?(ν) of Ba(Fe_0.9Co_0.1)₂As₂ in the superconducting phase at T = 5 K, as well as the magnetic field penetration depth, can be described well using the standard Bardeen-Cooper-Schrieffer (BCS) model with an additive contribution of electron and hole bands. It has been found that the measured temperature dependence of the magnetic field penetration depth in a wide temperature range 5 K < T < T _c can be described only with the introduction of interband pairing interaction. The coupling constant of electron and hole bands, λ_{1, 2} = 0.1, as well as the temperature dependences of superconducting gaps in the electron and hole subsystems, has been determined using the model of two-band superconductivity developed earlier for MgB₂.     

Magnetic irreversibility lines and critical currents of Bi(2212) single crystals doped by Fe, Ni, Co and Zn

We report measurements of the AC-susceptibility and the irreversible magnetization of Bi(2212) single crystals doped with up to 2 at.% of Fe, Ni, Co and Zn on the Cu position. Low concentrations of the doping elements, typically below 1 at.%, definitely increase the pinning force density and shift the magnetic irreversibility lines towards higher fields. In contrast, higher concentrations strongly suppress the critical current density and the irreversibility fields. A special situation occurs in the regime of low temperatures below about 10 K where we find that even very low concentrations of the doping elements are deteriorative for the critical current density.     

Magnetic torque analysis including vortex core contributions for Ba(Fe,Co)2As2 single crystal

Superconducting torque measurements for a Ba(Fe,Co)₂As₂ iron arsenic superconductor have been carried out to obtain anisotropy in 1 kG at 10 K as well as in 10 kG at 22 K. We found that the anisotropy in penetration depth γ_λ and the anisotropy in coherence length γ_ξ in the multi-band Kogan analysis are modest or almost isotropic. Theoretical calculation suggests the anisotropy of carrier mass γ_λ is approximated as 3 in BaFe₂As₂ system. The γ_λ value changes from 1 to 3 obtained by the torque analyses, and is in good agreement with the theoretical prediction. The torque curve was also analyzed by our new torque theory, which takes into account the vortex core contribution properly. We conclude that our Ba(Fe,Co)₂As₂ single crystal is almost isotropic in marked contrast with high-T_c cuprates.     

X-ray photoelectron spectroscopy of LiM0.05Mn1.95O4 (M = Ni,Fe and Ti)

LiMn₂O₄ and LiM_0.05Mn_1.95O₄ (M = Ni, Fe and Ti) were synthesized by using solid-state reactions and their surface stoichiometries were confirmed by XPS data. The crystal and electronic structures were investigated by using X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). XRD data suggested that LiM_0.05Mn_1.95O₄ possesses nearly no any variations in lattice parameters compared with LiMn₂O₄ for slight substitution of Ni, Fe and Ti; the substituted Ni, Fe and Ti ions were located on the 16d octahedral sites in the spinel crystal lattice. The XPS results suggested that Fe and Ti ions were at + 3 and + 4 oxidation states, respectively; while Ni ions are mixed with + 2 and + 3 oxidation states. The normal oxidation state of Mn ions in the above four materials is almost the same and calculated as + 3.55 according to the splitting energies of Mn3s states.     

Optical and scintillation properties of Sr3BGa3Si2O14 (B = Nb,Ta) single crystals

Optical and scintillation properties of Sr₃NbGa₃Si₂O₁₄ [SNGS] and Sr₃TaGa₃Si₂O₁₄ [STGS] single crystals with the langasite-type crystal structure were investigated as a novel scintillator materials. In the transmittance spectra of the SNGS and STGS polished specimens, absorption peaks around 380 and 505 nm were observed and the absorptions are considered to be attributable to the excess oxygen in the crystals. An emission peak around 420 nm was observed in the X-ray radioluminescence spectrum of the SNGS crystal. On the other hand, there was an emission peak around 335 nm in the X-ray radioluminescence spectrum of the STGS crystal.     

Effect of Mn substitution on superconductivity in iron selenide(Li,Fe)OHFeSe single crystals

We synthesize a series of Mn substituted(Li, Fe)OHFeSe superconductor single crystals via a modified ion-exchange method, with the Mn concentration z(the atomic ratio of Mn:Se) ranging from 0 to 0.07. The distribution homogeneity of the Mn element incorporated into the lattice of(Li, Fe)OHFeSe is checked by combined measurements of high-angleannular-dark-field(HAADF) imaging and electron energy-loss spectroscopy(EELS). Interestingly, we find that the superconducting transition temperature T_c and unit cell parameter c of the Mn-doped(Li, Fe)OHFeSe samples display similar V-shaped evolutions with the increasing dopant concentration z. We propose that, with increasing doping level, the Mn dopant first occupies the tetrahedral sites in the(Li, Fe)OH layers before starting to substitute the Fe element in the superconducting Fe Se layers, which accounts for the V-shaped change in cell parameter c. The observed positive correlation between the T_c and lattice parameter c, regardless of the Mn doping level z, indicates that a larger interlayer separation, or a weaker interlayer coupling, is essential for the high-T_c superconductivity in(Li, Fe)OHFeSe. This agrees with our previous observations on powder, single crystal, and film samples of(Li, Fe)OHFeSe superconductors.     

High temperature properties of the n = 2 Ruddlesden–Popper phases (La,Sr)3(Fe,Ni)2O7−δ

The crystal chemistry and mixed conductor properties of the n = 2 member of the Ruddlesden–Popper (R–P) phases Sr_3−xLa_xFe_2−yNi_yO_7−δ with 0 ≤ x ≤ 0.3 and 0 ≤ y ≤ 1.0 have been studied at high temperature. High-temperature X-ray diffraction and thermogravimetric measurements of the equilibrium pO₂ (10^− 5 ≤ pO₂ ≤ 1 atm) in the temperature range 400 ≤ T ≤ 1000 °C indicate that the Sr₃FeNiO_7−δ phase is able to accommodate a large oxygen non-stoichiometry (δ ∼ 1.5) without structural transformations. The electrical conductivity and oxygen permeability increase with the substitution of Ni for Fe in the range 550 ≤ T ≤ 1000 °C. The electrical transport of the Sr₃FeNiO_7−δ phase is thermally activated and the activation energy decreases with the substitution of Ni for Fe for a given oxygen content. The increase in the oxygen permeation flux with increasing Ni content is due to an increasing oxygen non-stoichiometry and a lower activation energy for permeation.     

Dimensionality crossover upon magnetic saturation in Fe, Ni and Co

The magnetic saturation process of iron, nickel and cobalt single-crystal spheres is studied using neutron scattering in a vertical magnetic field. It is observed that upon magnetic saturation, the scattering intensities decrease instead of increasing. This indicates a decreasing coherent scattering with field. The spin precession around the field axis therefore can be assumed to be incoherent along directions transverse to the field. Comparison of the temperature dependence of the spontaneous magnetization measured by zero field NMR on the one hand and by the macroscopic magnetization on the other hand shows that Fe, Ni and Co are three-dimensional (3D) in the zero field ground state but one dimensional (1D) in the magnetically saturated state. The observed decrease in neutron scattering intensity is consistent with this conclusion. The change in dimensionality is associated with a crossover. Our neutron scattering study shows that the crossover occurs at a field that is smaller than the demagnetization field. The dimensionality crossover, therefore, is driven not by the field but by the associated forced magnetostriction.     

Preparation,thermal expansion,chemical compatibility,electrical conductivity and polarization of A2−αA′αMO4 (A = Pr,Sm; A′ = Sr; M = Mn,Ni; α = 0.3, 0.6) as a new cathode for SOFC

A_2−αA′_αMO₄ (A = Pr, Sm, A′ = Sr, M = Ni, Mn) with K₂NiF₄-type structure were synthesized by solid reaction. Their chemical stability, electrical conductivity and thermal expansion behavior as well as cathodic polarization were investigated in relation to the cathode of SOFC. The results showed that A_2−αA′_αMO₄ exhibited a low reactivity with yttria stabilized zirconia (YSZ) electrolyte. The thermal expansion coefficient (TEC) values were changed with the ionic radius of A. The specific conductivities of the nickelates were higher than those of manganites. While the nickelates showed a lower cathodic polarization in comparison with manganites.     

Measurement of a sign-changing two-gap superconducting phase in electron-doped Ba(Fe(1-x)Co(x))2As2 single crystals using scanning tunneling spectroscopy

Scanning tunneling spectroscopic studies of Ba(Fe(1-x)Co(x))(2)As(2) (x=0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish above the superconducting transition T(c). The two-gap nature and the slightly doping- and energy-dependent quasiparticle scattering interferences near the wave vectors (±π, 0) and (0, ±π) are consistent with sign-changing s-wave superconductivity. The excess zero-bias conductance and the large gap-to-T(c) ratios suggest dominant unitary impurity scattering.     

Constricted hysteresis loops in Fe and Ni single crystals

Magnetic hysteresis loops reflect the variety of magnetic domain structures and have been considered to have normal rectangular or leaf-like shapes in standard ferromagnets such as Fe and Ni metals. We report on observations of constricted hysteresis loops in Fe and Ni single crystals with very low defect densities. The constricted loops were observed below T=150 K and in a medium temperature range from 150 to 430 K in Fe and Ni single crystals, respectively. These constricted loops disappear by weak plastic deformation for both single crystals. The origin of constricted hysteresis loops was explained by eddy current effects under less domain wall pinning due to dislocations.     

Microscopic coexistence of superconductivity and magnetism in Ba(1-x)K(x)Fe2As2

It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba(1-x)K(x)Fe(2)As(2) exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume-sensitive muon spin rotation study of Ba(1-x)K(x)Fe(2)As(2) showing that this paradigm does not hold true in the underdoped region of the phase diagram (0≤x≤0.25). Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.     

Raman spectroscopy of CaM2+Ge2O6 (M2+ = Mg,Mn, Fe,Co, Ni,Zn) clinopyroxenes

The Raman spectra of Ge‐clinopyroxenes CaM²⁺Ge₂O₆ (M²⁺ = Mg, Mn, Fe, Co, Ni, Zn), general formula M2M1T₂O₆, are reported for the first time. Their spectral features are discussed in comparison with corresponding Si‐pyroxenes. The vibrational wavenumbers of germanates may be roughly obtained by a scale factor of about ~0.8 by those of the corresponding silicates, due to the Ge‐Si mass difference. The main peaks in the germanate Raman spectra at ~850 and ~540 cm⁻¹ may be related to Ge‐O tetrahedral stretching and chain bending, respectively; minor peaks between 200 and 400 cm⁻¹ are ascribed to bending and stretching of the non‐tetrahedral cations. Within Ge‐pyroxenes, possible correlations between crystallographic parameters and the vibrational wavenumbers are investigated. The main stretching mode at ~850 cm⁻¹ shows wavenumber changes with M²⁺ substitutions, but no simple correlation can be found with M²⁺ cation mass or size. On the other hand, the chain bending wavenumber linearly decreases with increasing ionic radius of the M²⁺ cation: the expansion of the M1 polyhedron reduces the chain kinking angle and the Ge‐Ge distances correspondingly increase. Copyright © 2015 John Wiley & Sons, Ltd.     

Oxygen adsorption and surface segregation in (2 1 1) surfaces of Pt(shell)/M(core) and Pt3M (M = Co,Ir) alloys

Density functional theory is used to study oxygen adsorption and its effect on surface segregation in (2 1 1) surfaces of Pt(shell)/M(core) and Pt₃M (M = Co, Ir) alloys. It is found that the most energetically favorable oxygen adsorption site is the bridge site over and parallel to the (1 0 0) step. Surface segregation phenomena is observed in Pt₃Co, Pt₃Ir and Pt/Co(core) systems. The Pt/Ir(core) structure was the only one, among the studied systems, that showed antisegregation behavior even in presence of oxygen adsorbed.     

BIMEVOX as dense membrane in catalytic reactor (ME = Co,Cu, Ta)

A catalytic dense membrane reactor allows to physically separate the oxygen feed from the reactant (hydrocarbon) feed with a catalytic membrane chosen among oxide ion conducting materials. The membrane plays a double role, it provides the oxygen needed for selective oxidation and acts as a catalyst. The catalytic properties of BIMEVOX (ME = Co, Cu, Ta) membranes were examined in the mild oxidation of propene and of propane. During the complex transient state observed when the surface is rough, the nature and distribution of products are different from those obtained with mirror-polished membranes in a former work. In particular, syngas is formed with propene as well as with propane, and it precedes the production of hydrogen and coke. The complex behaviour differs according to ME and seems to be related to the different nature of electron semi-conduction induced by each dopant.