Showing papers by "David K. Christen published in 1987"
TL;DR: The independence of the superconductive and magnetic behavior indicates that superconductivity in this system is highly anisotropic, supported by magnetization measurements below T/sub c/, which also yield an antiferromagnetic, Curie-Weiss-like temperature FTHETA/sub M/ of +2.3 K.
Abstract: The magnetic susceptibility and magnetization of the superconductive compound Gd/sub 1/Ba/sub 2/Cu/sub 3/O/sub z/ has been studied in the temperature range from 15 to 270 K The magnetic susceptibility chi of the material, which has a superconductive transition temperature T/sub c/ near 95 K, follows a Curie-Weiss law both above and below T/sub c/ with the full Gd magnetic moment, p/sub eff/ = 784mu/sub B/, being observed The independence of the superconductive and magnetic behavior indicates that superconductivity in this system is highly anisotropic This conclusion is supported by magnetization measurements below T/sub c/, which also yield an antiferromagnetic, Curie-Weiss-like temperature FTHETA/sub M/ of +23 K
30 citations
TL;DR: These results give clear experimental evidence for a nearly complete decoupling of the magnetic and superconductive layers, demonstrating that the superconductivity is highly anisotropic.
Abstract: The magnetic properties of the superconductive materials HoBa/sub 2/Cu/sub 3/O/sub z/ and YBa/sub 2/Cu/sub 3/O/sub z/ have been measured and compared. Both had superconductive transition temperatures T/sub c/ in low magnetic fields near 90 K and exhibited nearly complete magnetic-flux exclusion. The susceptibility of the Ho-based materials followed a Curie-Weiss law both above and below T/sub c/. These results give clear experimental evidence for a nearly complete decoupling of the magnetic and superconductive layers, demonstrating that the superconductivity is highly anisotropic.
24 citations
TL;DR: The effect of fast neutron damage on the superconducting transition temperature Tc and the critical current density jc of the compound La1.85Sr0.15CuO4 has been investigated in this paper.
Abstract: The effect of fast neutron damage on the superconducting transition temperature Tc and the critical current density jc of the compound La1.85Sr0.15CuO4 has been investigated. Irradiation to a fluence of 1.3×1018 n/cm2 (E > 0.1 MeV) resulted in a decrease of Tc of about 3 K while jc at 4.2 K and H = 2 T increased by a factor of two to 1.2×104 A/cm2.
21 citations
TL;DR: In this paper, the effects of lattice disorder on the superconducting and electronic properties were investigated by irradiation with nitrogen ions of energy 45 and 340 keV, resulting in a nearly uniform damage profile without the introduction of new chemical species.
Abstract: Thin films (100-120 nm) of hexagonal structure MoN have been fabricated by reaction of Mo films in an NH 3 atmosphere. The as-formed films possessed superconducting transition temperatures T c ≃ 13 K, with resistance ratios r = R(296K)/R(T c ) in the range 5 to 10, low-temperature normal state resistivities ρ o = 4 to 10 μ Ω-cm, and extrapolated upper critical fields H c2 (0) = 4.0 to 5.0 T. Thin film X-ray diffraction patterns revealed no visible second phase, with measured lattice parameters close to literature values. The effects of lattice disorder on the superconducting and electronic properties were investigated by irradiation with nitrogen ions of energy 45 and 340 keV, resulting in a nearly uniform damage profile without the introduction of any new chemical species. The results indicate that ordered hexagonal MoN shows some of the unusual properties characteristic of moderate-to-high T c transition metal compounds, but is relatively insensitive to degradation of the superconducting properties by lattice disorder. For ion fluences Φ up to 2 × 1016N-ions/cm2, T c is found to decrease monotonically and saturate at 9.5 K, almost 3/4 the initial value, while H c2 (0) undergoes a gradual increase to 11T.
17 citations
TL;DR: In this article, the largest critical current densities are associated with the presence of coherent second-phase precipitates, and are significatly higher than for materials containing only grain boundaries or dislocations as flux pinning sites.
Abstract: For various thermal processing conditions, measurements of the electrical transport and superconducting magnetic properties have been correlated with the microstructure as determined by X-ray diffraction and transmission electron microscopy. We find the largest critical current densities Jc are associated with the presence of coherent second-phase precipitates, and are significatly higher than for materials containing only grain boundaries or dislocations as flux pinning sites. Low-temperature annealing in oxygen results in nearly perfect, defect-free grains, separated by clean, planar grain boundaries. We conclude that the temperature-dependent diamagnetic response below Tc cannot be explained by the observed gross microstructure.
10 citations
TL;DR: The superconductive state magnetization of the high-Tc compound Y1Ba2Cu3Oz has been investigated in this article, where single-phase polycrystalline samples of this archetype were prepared by solidstate reaction.
Abstract: The superconductive state magnetization of the high-T
c compound Y1Ba2Cu3Oz has been investigated. Single-phase, polycrystalline samples of this archetype were prepared by solidstate reaction. The onset of the superconductive transition, measured magnetically, was 90.7 K. In addition to low-field dc magnetization results, measurements of the magnetic critical current density J
c (H,T) are presented as a function of magnetic field and temperature. The low values of J
c, especially at high temperature, are discussed in terms of intergranular supercurrents that flow within individual grains in superconductive layers of reduced dimensionality.
9 citations
01 Jan 1987-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, a defect production and annealing model was proposed to fit WHH theory at all ion doses, with a maximum H c2 (0) at 4 × 10 14 N-ions/cm 2.
Abstract: Thin films of VN have been formed by reacting films of vanadium metal, which were previously prepared by e-beam evaporation, with N 2 gas. The superconducting transition temperature T 2 , the electrical resistivity ϱ, the X-ray diffraction spectra, and the temperature dependent upper critical field H c2 of these films were studied as a function of nitrogen ion irradiation. T c is found to decrease exponentially with incident ion fluence to a total change of 42%, while the residual resistivity increases, both saturating at about the same fluence. This exponential behavior is derived from a simple defect production and annealing model. H c2 is found to fit WHH theory at all ion doses, with a maximum H c2 (0) at 4 × 10 14 N-ions/cm 2 . The electronic heat capacity coefficient γ * , calculated from the slope of H c2 at T c , is observed to be correlated with T c ; therefore, γ * also decreases with fluence.
5 citations
TL;DR: In this article, thin superconducting films were obtained by ablating material from high-Tc bulk superconductor ceramics onto a nearby substrate using focused excimer laser radiation.
Abstract: Thin superconducting films of YBa2Cu3O7 and HoBa2Cu3O7 have been obtained by ablating material from high-Tc bulk superconductor ceramics onto a nearby substrate using focused excimer laser radiation. Subsequent annealing in oxygen at elevated temperatures was necessary to achieve the superconducting phase. A range of deposition and annealing conditions was investigated to yield films of the desired stoi chiometry and phase.
4 citations
TL;DR: In this article, the effects of varying oxygen content on the superconducting transition temperature Tc, resistivity, and crystal structure of HoBa2Cu3O7−x (0 < x < 1) have been investigated.
Abstract: The effects of varying oxygen content on the superconducting transition temperature Tc, resistivity, and crystal structure of HoBa2Cu3O7−x (0 < x < 1) have been investigated. The variation of Tc wltn 8 is very similar to that previously reported by several authors for YBa2Cu3O7−x [1,2]. In particular a plateau in the Tc vs x curve is observed for values of x between 0.3 and 0.4 witfi the value of Tc,(10 % onset measured magnetically) remaining relatively constant at about 55 K. The width of the superconducting transition, as measured both resistively and magnetically, is substantially narrower for x= 0.35 + 0.05 than for slightly larger or smaller values of x. High resolution powder x-ray diffraction analysis was used to measure the lattice constants of both the orthorhombic and tetragonal phases as x was varied. With increasing x the temperature coefficient of the resistivity changed from positive (metallic) for x ≤0.2 to negative (semiconducting) for x ≥, 0.3. Since the general shape of the Tc vs x curve was unaffected by the complete substitution of magnetic Ho for Y, these results suggest an ordering of the oxygen vacancies in the a-b plane and the existence of an ordered compound with a Tc of 55 K.
1 citations
TL;DR: In this paper, the superconductive and magnetic properties of rare-earth based polycrystalline samples of high Tc super-conductive compounds with magnetic onset temperatures between 90 and 95 K were investigated.
Abstract: Magnetic properties have been investigated for several rare-earth based, polycrystalline samples of high Tc superconductive “123” compounds with magnetic onset temperatures between 90 and 95 K. Analysis of the susceptibility × of samples based on Gd, Ho, and Dy indicates that the superconductive and magnetic properties are independent of one another to a substantial degree. Studies of the critical current density Jc are reported.