C. Rillo

Superconducting weak links in YBa2Cu3O7-δ an AC magnetic susceptibility study

AC magnetic susceptibility, X and X, and ac resistivity, ρ, of two different samples of superconducting YBa2Cu3O7-I´ have been measured. The results show a strong non-linear behaviour and are very sensitive to the exciting amplitudes. From the analysis of the data and comparison with appropriate models and other superconducting compounds, evidence for the existence of weak superconducting links is inferred. Finally, the utility of acX measurements for detection of different superconducting phases is made evident.

Solution-based synthesis routes to (Bi 1− x Pb x ) 2 Sr 2 Ca 2 Cu 3 O 10+δ

A comparison of several solution syntheses with a solid-state route to the leadcontaining high Tc phase of the Bi-Sr-Ca-Cu-O system has been performed using DSC, TGA, X-ray diffraction, electron microscopy, magnetic ac susceptibility, and critical-current measurements. A novel polymer solution synthesis route is shown to yield an increased percentage of the 2223 high Tc phase when sintering is performed at the low temperature end of this phases stability range. Under the best preparative conditions given in the literature, however, the properties of samples obtained with the different synthetic methods described in the present study are shown to be similar. Samples derived from solution syntheses have been found to contain higher amounts of carbon precipitates, while samples derived from conventional solid-state synthesis contain CuO precipitates, according to energy dispersive spectroscopy analyses.

Thermogravimetry and neutron thermodiffractometry studies of the H-YBa2Cu3O7 system.

The high Tc superconducting oxide YBa2Cu3O7?x reacts with hydrogen gas. Thermogravimetric, X-ray and neutron scattering experiments allow us to propose a two-step type of hydrogen bonding. Firstly, a few hydrogen atoms fill some oxygen vacancies and may favourably modify the electron state, giving rise to a slight increase in the critical temperature. Secondly, after a prolonged heating period, the collapse of the YBa2Cu3O7?x type framework and of superconductivity were observed, and a new, highly hydrogenated material appeared.

A New Series of Homoleptic, Paramagnetic Organochromium Derivatives: Synthesis, Characterization and Study of Their Magnetic Properties

The synthesis and characterization of the triad of organochromium derivatives [Cr(C(6)Cl(5))(4)](n-) (n=0, 1, 2) are described. By treating [CrCl(3)(thf)(3)] with LiC(6)Cl(5) in 1:5 molar ratio, the salt [Li(thf)(4)][Cr(III)(C(6)Cl(5))(4)] (1) was obtained as a violet solid in 57 % yield. Oxidation of 1 with [N(4-BrC(6)H(4))(3)][SbCl(6)] yielded the neutral complex [Cr(IV)(C(6)Cl(5))(4)] (2) as a brown solid in 71 % yield. The arylation of [CrCl(2)(thf)] with LiC(6)Cl(5) under similar conditions as above gave [[Li(thf)(3)](2)(mu-Cl)](2)[Cr(II)(C(6)Cl(5))(4)] (3) as an extremely air- and water-sensitive red solid in 47 % yield. The crystal and molecular structures of 1 and 3 have been established by X-ray diffraction methods. Complex 3 contains the unusual cation [[Li(thf)(3)](2)(mu-Cl)](+) with an almost linear Li-Cl-Li unit (174.2(6)degrees). All four C(6)Cl(5) groups are sigma-bonded to the Cr(II) center, which is located in a square-planar environment. The local geometry around the Cr(III) center in 1 is, in turn, pseudo-octahedral, since two of the C(6)Cl(5) groups act as standard sigma-bonded monodentate ligands, while the other two act as small-bite didentate ligands coordinated through both the ipso-C and one of the ortho-Cl atoms. Compounds 1-3 are paramagnetic with maximum spin multiplicity each (EPR and magnetization measurements).

Ultimate current resolution of a cryogenic current comparator

The sensitivity and current resolution of an overlapping tube cryogenic current comparator (CCC) are calculated in terms of an equivalent electrical circuit model. The key features for optimum design are given, and theoretical predictions are compared with experiments. We propose that optimum current resolution will be achieved when the overlapping tube is used as single turn sensing coil directly connected to a SQUID, which must have its input coil inductance matched to that of the overlapping tube. The possibility of including a strip wound ferromagnetic core is also analyzed. It is shown that a fractional turn loop sensing coil may enhance the CCC sensitivity above that of the SQUID.

YBa2Cu3O7−δ low field diamagnetic properties: A multiharmonic analysis

Abstract Multiharmonic analysis of the magnetic AC susceptibility of a YBa2Cu3O7−δ powder sintered sample has been performed, measuring nine in-phase, χn′ (T), and out-of-phase, χn″ (T), components. Measurements at zero and low bias DC field, Ho, have been performed. For even n, both χn′ (T) and χ″n (T) are very sensetive to Ho and become zero at real zero field. Odd harmonics (n≥3) strongly depend on Ho and on the weak-link nature. Furthermore, from χn′ and χn″ (T) AC hysteresis loops have been derived and analyzed. The applicability of available theoretical models in the description of the harmonic components has been analyzed.

Calculation of effective inductances of superconducting devices. Application to the cryogenic current comparator

As a consequence of the image effect the inductances of coils and superconducting devices do change. Calculations are performed with the help of commercial software based on the finite element method. The results are applied to the design of an optimum cryogenic current comparator.

Homoleptic organoderivatives of high-valent nickel(III).

Homoleptic perhalophenyl derivatives of divalent nickel complexes with the general formula [NBu(4)](2)[Ni(II)(C(6)X(5))(4)] [X=F (1), Cl (2)] have been prepared by low-temperature treatment of the halo-complex precursor [NBu(4)](2)[NiBr(4)] with the corresponding organolithium reagent LiC(6)X(5). Compounds 1 and 2 are electrochemically related by reversible one-electron exchange processes with the corresponding organometallate(III) compounds [NBu(4)][Ni(III)(C(6)X(5))(4)] [X=F (3), Cl (4)]. The potentials of the [Ni(III)(C(6)X(5))(4)](-)/[Ni(II)(C(6)X(5))(4)](2-) couples are +0.07 and -0.11 V for X=F or Cl, respectively. Compounds 3 and 4 have also been prepared and isolated in good yield by chemical oxidation of 1 or 2 with bromine or the amminium salt [N(C(6)H(4)Br-4)(3)][SbCl(6)]. The [Ni(III)(C(6)X(5))(4)](-) species have SP-4 structures in the salts 3 and 4, as established by single-crystal X-ray diffraction methods. The [Ni(II)(C(6)F(5))(4)](2-) ion in the parent compound 1 has also been found to exhibit a rather similar SP-4 structure. According to their SP-4 geometry, the Ni(III) compounds (d(7)) behave as S=1/2 systems both at microscopic (EPR) and macroscopic levels (ac and dc magnetization measurements). The spin Hamiltonian parameters obtained from the analysis of the magnetic behavior of 3 and 4 within the framework of ligand field theory show that the unpaired electron is centered mainly on the metal atom, with >97 % estimated d(z(2) ) contribution. Thermal decomposition of 3 and 4 proceeds with formation of the corresponding C(6)X(5)--C(6)X(5) coupling compounds.

1:30000 cryogenic current comparator with optimum SQUID readout

We developed a 1:30000 Cryogenic Current Comparator for SET current amplification. A dedicated low-noise, directly-coupled SQUID was used for the readout, which allowed reaching a sensitivity close to ideal. The ratio error was <9/spl times/10/sup -9/. The CCC-SQUID equivalent current input noise was 3.0 fA/Hz/sup 1/2/, measured down to 0.1 Hz.

On the structural and magnetic properties of the new ternary nitride series R2Fe17Nx

Abstract A systematic analysis of the crystal and magnetic properties of the nitrides R 2 Fe 17 N x has been undertaken using X-ray and neutron diffraction, a.c. susceptibility and high-field measurements. The maximum composition corresponds to x max = 3 and to the filling of the distorted 2R–4Fe octahedral site. There is evidence of high anisotropy fields even for the easy-plane systems. Crystal data, as well as magnetic properties, are discussed.