Ca-doped Samples Research Articles

Electrical transport and superconductivity in the (Y 0.8Ca 0.2)Ba 2Cu 3O y system with variable oxygen content

The effect of oxygen content and thermal treatment has been studied in the (Y 0.8Ca 0.2)Ba 2Cu 3O y system. The introduction of Ca altered the normal state and superconducting properties substantially. Particularly intriguing is that Ca-doped samples with more oxygen-vacancy disorder have higher superconducting transition temperatures ( T c) and lower resistivities, contrary to the non-Ca-doped system. The correlation between T c and carrier concentration cannot be described by the empirical relation observed in many high- T c oxides.

Preparation and properties of Ca-doped “Yb-124” superconductors

We successfully synthesized Ca-doped Yb-124 samples, (Yb 1− y Ca y ) (Ba 1− x Sr x ) 2Cu 4O 8 0.1≦x≦0.3, y=0.1), using an O 2-hot-isostatic-pressing (O 2-HIP) technique. The samples were characterized with respect to the crystal structure and superconducting properties. The Meissner onset temperature was found to increase up to 87 K by Ca-doping. For the Ca-doped samples, the transition temperature decreased with increasing Sr content. The site for doped Ca ions was discussed.

Specific heat and penetration depths of YBa 2Cu 4O 8 and YBa 1.8Ca 0.2Cu 4O 8 near Tc

Specific-heat data around the critical temperature T c ≈90 K of polycrystalline YBa 1.8Ca 0.2Cu 4O 8 show a discontinuity ΔC p / T c ≈22 mJ/ mol K 2. In undoped YBa 2Cu 4O 8 and in YBa 2Cu 3O 7, the corresponding anomalies are 15 mJ/mol K 2 and 55 mJ/mol K 2. The in-plane London penetration depth of the Ca-doped sample, as deduced from isothermal magnetization data, is λ ab(0)≈1700 Å, compared to λ ab(0)≈2000 Å in undoped YBa 2Cu 4O 8, and 1400 Å in YBa 2Cu 3O 7.

Flux Pinning and Weak Link Structure in Ca2Cu1O3 Doped LnBa2Cu3O7-y (Ln=La and Y)

AbstractFlux pinning and weak link structure were studied on the effect of Ca doping in LnBa2Cu3O7-y system (Ln=La and Y). The magnitude and the magnetic field dependence of the critical current density were improved by Ca doping. A small amount of impurity phase of such as Ca2Cu1O3 may work as a desirable flux pinning center. Moreover, it was found that the current-temperature characteristics for Ca-doped samples showed the evidence of two kinds of superconducting phases which have different transition temperature Tc and Tc'. The experimental result agrees well with the Ambegaokar-Baratoff theory for asymmetric Josephson junctions (S-I-S') in the temperature range of T < Tc' and with the proximity junction theory (S-N-S) in the range of Tc'<T<Tc.

Thermoluminescence and recovery processes in pure and doped NaCl after 20 K irradiation

The thermoluminescence (TL) spectra after X-ray irradiation at 20 K have been investigated for pure as well as divalent cation doped NaCl. The F-centre decay has also been determined in pure and Ca and Mg doped NaCl for comparison purposes. A clear decrease in F-centre concentration appears to correlate with glow peaks at 44 and 50 K for pure and Ca-doped samples. Main glow peak appearing at 69 K is not associated to any appreciable F-centre decay step. Below liquid nitrogen temperature (LNT) all peaks show both σ and π exciton emission bands. Above LNT, the glow peaks for doped samples show the σ emission together with another band at 410 nm, whereas pure samples still present the intrinsic emission bands.