Crystal structure, superconductivity and magnetic properties of the superconducting ferromagnets Gd1.4−xDyxCe0.6Sr2RuCu2O10 (x=0–0.6)

The structural, electrical and magnetic properties of the superconducting ferromagnets, Gd_1.4−xDy_xCe_0.6Sr₂RuCu₂O₁₀ (x=0–0.6) are systematically investigated as a function of Dy doping and temperature. These compounds are characterized by high temperature superconductivity (T_c ranging from 20 to 40 K depending upon the Dy content) co-existing with weak ferromagnetism with two magnetic transitions (T_M2 ranging from 95 to 106 K and T_M1 around 120 K). Doping with Dy gives no significant structural changes except for a minor change in the c/a ratio. However the superconducting transition temperature is significantly suppressed and magnetic ordering temperature enhanced on Dy doping. These effects are described and discussed.     

Superconductivity, Ca content and oxygen deficiency of Ga(Sr,Ca)2(Yb,Ca)Cu2O7

Superconducting transition temperature (T_c), Ca content and oxygen deficiency are studied on GaSr_1.8Ca_0.2Yb_1−xCa_xCu₂O₇ (x≤0.35). Superconducting samples with T_c=52 K are prepared after the annealing at 20 MPa of oxygen. The T_c is reduced through a slight oxygen loss accompanied by annealing in air above 650°C. The oxygen loss suggests the presence of short Cu–O chains in the GaO₄ slab. The formal valence of planar Cu required for the appearance of superconductivity depends on oxygen and Ca contents. The critical formal Cu valences are 2.105 and 2.125 for the samples annealed in air at 600°C and at 835°C, respectively. The values are higher than those of usual high-T_c superconductors. This can be explained by a high concentration of localized holes in the CuO₅ slab.     

Type of phase transitions in a mesoscopic superconducting disc

The Ginzburg–Landau equations coupled with the three-dimensional Maxwell equations are solved for the disc geometry in order to explain recent magnetization experiments. In order to explain the experimental results on a 0.5 m radius Al disc, we have to assume that the superconducting state stays in the lowest angular momentum giant-vortex state even in regions where it is not the lowest energy state.     

Raman spectroscopic study of carbon substitution in MgB2

We study the effect of carbon doping on the Raman spectrum of the MgB₂ superconductor. Out data show that significant changes in the Raman spectra of the MgB_2−xC_x compounds occur for carbon concentrations x>0.04. The E_2g mode at ∼580 cm⁻¹ hardens only moderately upon increased doping despite direct carbon substitution in the boron layers, while the dependence of its full width at half maximum (FWHM) on x reveals the competing effects of reduced electron-phonon coupling and increased disorder. The results are discussed in the framework of anisotropic lattice contraction and the position of the σ sub-bands with respect to the Fermi energy level. The relative intensity of the phonon peak at ∼770 cm⁻¹, associated with a peak in the phonon density of states, increases considerably and dominates for x=0.08. Additionally, it hardens and broadens with increasing x in the range 0.04-0.08. These changes can be associated with carbon substitution-induced disorder in the investigated samples.     

Symmetry and Intervalent Charge Transfer in Copper-Oxide Superconducting Precursors

A vibrationally coupled intervalent charge transfer theory is proposed for the copper oxide-superconductors. The proposed B_1g-vibrational distortion exactly interchanges the chemical environment of the neighboring copper atoms. A quadruple cell is constructed to accommodate the non-stoichiometry and a double copper-oxide-conducting chain with distorted symmetry is postulated. The distorted structure, prone to switching of the copper sites by vibration, is postulated to be the superconducting precursor. The left over-oxygen, due to oxygen deficiency, is allowed to serve as a bridge between the chains and 10 oscillate with the vibration. Symmetry of the system, before and after distortion, is used to explain the spin-angular momentum change, needed to arrive at Cooper-paired electron/holes. The O^? and O⁼ atoms are postulated to be the bridge for superexchange and charge flow between the Cu⁺¹/Cu⁺³ and Cu⁺¹/Cu⁺² pairs. The same theory is used to interpret the mechanisms for the following three types of copper oxide-superconductors: YBa₂Cu₃O_T-x; La_2-xSr_xCuO₄ and the newly discovered electron superconductor Nd_2-xCe_xCuO_4-y. The bridge oxygen oscillation may be chosen not to come from the breathing mode but from the bending deformation mode that involves the relative motion of the Cu atoms (to the oxygen) giving rise to smaller oxygen isotope effect.     

发展超导问题的局部耗散性和吸引子

本文得到了在Lorentz规范下一类发展超导问题解的整体存在性、局部耗散 性和吸引子的存在性.     

Hole doping into Co-12s2 copper oxides with s fluorite-structured layers between CuO2 planes

In this work, the first three members (s=1, 2, 3) of the Co-12s2 homologous series of multi-layered copper oxides are gradually doped with holes through high-pressure oxygenation (HPO). The phases differ from each other only by thickness of the fluorite-structured layer block, (Ce,Y,Ca)-[O₂-(Ce,Y)]_s−1, between two identical CuO₂ planes. High-resolution transmission-electron microscopy (HRTEM) and electron diffraction (ED) analyses together with both synchrotron X-ray and neutron powder diffraction data, reveal that as a consequence of HPO the charge-reservoir CoO₄-tetrahedra chains get broken and the lattice symmetry of the Co-12s2 phases changes from orthorhombic to tetragonal. Oxygen contents are analyzed for the samples with wet-chemical and thermogravimetric techniques. The valence state of copper in the CuO₂ plane is determined from Cu L-edge X-ray absorption near-edge structure (XANES) spectra to be compared with the values estimated through bond-valence-sum (BVS) calculations from the crystal structure data. The positive charge induced by oxygen loading (or aliovalent Ca^II-for-Y^III substitution in CoSr₂YCu₂O_7+δ) is found not to be completely accommodated in the CuO₂ planes but be rather effectively trapped at the charge-reservoir Co atoms. Superconductivity appears in the Co-1212 (CoSr₂YCu₂O_7+δ) samples with the copper valence of 2.13 or higher, whereas in the Co-1222 (CoSr₂(Ce_0.25Y_0.75)₂Cu₂O_9+δ) and Co-1232 (CoSr₂(Ce_0.67Y_0.33)₃Cu₂O_11+δ) samples Cu valence does not increase high enough to induce superconductivity.     

添加Cu2S对Bi-Pb-Sr-Ca-Cu-O体系超导性的影响

本文对Cu_2S添加的Bi-Pb-Sr-Ca-Cu-O体系进行了低温电阻、X射线衍射和XPS研究．实验结果表明，加入少量的Cu_2S能够降低高T_c相的形成温度，改善超导性能，加入Cu_2S引起了Cu 2p_(3/2)谱的主峰向低结合能方向移动，并减少其卫星峰的面积．这表明Cu_2S的加入减少了Cu~(2+)的量，即导致空穴载流子浓度的减小．     

Superconductivity in Li3Ca2C6 intercalated graphite

In this paper, we report the discovery of superconductivity in Li₃Ca₂C₆. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors [T. Enoki, S. Masatsugu, E. Morinobu, Graphite Intercalation Compounds and Applications, Oxford University Press, Oxford, 2003]. It is probably not astonishing, since it is generally admitted that low dimensionality promotes high superconducting transition temperatures. Superconductivity is lacking in pristine graphite, but after charging the graphene planes by intercalation, its electronic properties change considerably and superconducting behaviour can appear. Li₃Ca₂C₆ is a ternary GIC [S. Pruvost, C. Hérold, A. Hérold, P. Lagrange, Eur. J. Inorg. Chem. 8 (2004) 1661-1667], for which the intercalated sheets are very thick and poly layered (five lithium layers and two calcium ones). It contains a great amount of metal (five metallic atoms for six carbon ones). Its critical temperature of 11.15 K is very close to that of CaC₆ GIC [T.E. Weller, M. Ellerby, S.S. Saxena, R.P. Smith, N.T. Skipper, Nat. Phys. 1 (2005) 39-41; N. Emery, C. Hérold, M. d’Astuto, V. Garcia, Ch. Bellin, J.F. Marêché, P. Lagrange, G. Loupias, Phys. Rev. Lett. 95 (2005) 087003] (11.5 K). Both CaC₆ and Li₃Ca₂C₆ GICs possess currently the highest transition temperatures among all the GICs.     

NbOsSi and TaOsSi – Two new superconducting ternary osmium silicides

The new equiatomic silicides NbOsSi and TaOsSi as well as ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) were prepared from the elements by arc-melting. These silicides crystallize with the orthorhombic TiNiSi type structure, space group Pnma. Irregularly shaped crystals of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi were separated from the annealed samples and investigated by single-crystal X-ray diffraction (a = 640.46(7), b = 404.07(5), c = 743.66(8) pm, wR2 = 0.0285, 390 F² values, 20 variables for ZrOsSi; a = 629.78(6), b = 388.72(4), c = 727.48(7) pm, wR2 = 0.0350, 397 F² values, 20 variables for NbOsSi, a = 626.80(6), b = 389.36(4), c = 726.22(7) pm, wR2 = 0.0501, 385 F² values, 20 variables for TaOsSi, a = 653.48(8), b = 395.35(4), c = 739.19(8) pm, wR2 = 0.0427, 413 F² values, 20 variables for ZrIrSi and a = 646.34(12), b = 393.57(7), c = 736.8(14) pm, wR2 = 0.0582, 371 F² values, 20 variables for HfIrSi). The striking structural motifs in the new osmium compounds are three-dimensional [OsSi] networks (Os–Si: 240–251 pm) in which the osmium atoms have strongly distorted tetrahedral silicon coordination. High-pressure/high-temperature experiments (9.5 GPa/1520 K) on TaOsSi gave no hint for a structural phase transition. Temperature dependent measurements of the magnetic susceptibility and the electrical conductivity of NbOsSi and TaOsSi showed superconductivity below T_C = 3.5 and 5.5 K, respectively. ²⁹Si solid state MAS NMR investigations of the prepared silicides approved the structural models and showed a correlation between the observed ²⁹Si resonance shifts and the electronegativity of the involved refractory metal.