N. Balchev

Structure, superconducting and magnetotransport properties of Ru1−xSnxSr2Gd1.4Ce0.6Cu2Oy (0≤x≤0.1)

Samples with nominal compositions Ru1−xSnxSr2Gd1.4Ce0.6Cu2Oy (0≤x≤0.1) were synthesized and their structure, superconducting and magnetotransport properties were investigated. It was shown that the Sn-doping enhances the crystal growth in Ru-1222. A maximum in the dependences of the lattice parameters and Tc on the dopant content x was observed. Small doping levels significantly increase the Tc of the Ru-1222 samples, prepared at the same conditions—from 20 K for the undoped sample to 35 K for the x = 0.02 and 0.03 ones. It was found that the Sn-doping enhances the upper critical field Hc2(0), extrapolated to T = 0, and the weak link behavior at the expense of the intragranular superconductivity in Ru-1222. The observed phenomena are discussed.

Superconductivity and critical fields in undoped and Sn-doped MoSr2YCu2O8−δ

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB_2 superconductor was investigated in magnetic fields up to 16 T. The upper critical field of the major fraction of the investigated sample was determined from ac susceptibility and resistance data and was found to increase up to Hc2(0) = 13 T at T = 0 corresponding to a coherence length of 5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc and at intermediate temperatures. This positive curvature of Hc2(T) is similar to that found for the borocarbides YNi_2B_2C and LuNi_2B_2C indicating that MgB_2 is in the clean limit.

Possibilities for formation of the 110 K phase 2223 in Sb or V doped Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O materials

Superconducting oxide materials with nominal composition Bi2−xSbxSr2Ca2Cu3Oy (x=0.05−0.3) and Bi1.6Pb0.4−xVxSr2Ca2Cu3Oy (x=0.1 0.4) were synthesized. It was found that after prolonged synthesis, formation of the 2223 phase in the Bi-Sb-Sr-Ca-Cu-O system is possible. However, the critical temperatures of the samples are around 90 K and are lower than those of the superconducting materials from the Bi-Pb-Sr-Ca-Cu-O and Bi-Pb(Sb)-Sr-Ca-Cu-O systems. It was also found that V inhibits the formation of the phase 2223, raises the resistance of the samples and has a negative effect on theTc and the phase composition of the Bi-Pb(V)-Sr-Ca-Cu-O materials.

Superconductivity at 103 K in CdBa2(Ca0.7Y0.3)Cu2Oy

A cadmium analogue of the mercury system with nominal composition CdBa2(Ca1−xYx)Cu2Oy has been synthesized. Thex=0 samples contain about 12 vol.% of the 1212 phase but are not superconducting. Thex=0.3 samples are superconducting atTon = 103 K. The EDX analysis of 18 microcrystals shows a broad cationic distribution of the different components. The observed broad superconducting transition is attributed to the variousTc of the different microcrystals.A cadmium analogue of the mercury system with nominal composition CdBa{sub 2}(Ca{sub 1{minus}x}Y{sub x})Cu{sub 2}O{sub y} has been synthesized. The x=0 samples contain about 12 vol.% of the 1212 phase but are not superconducting. The x=0.3 samples are superconducting at T{sub on}= 103K. The EDX analysis of 18 microcrystals shows a broad cationic distribution of the different components. The observed broad superconducting transition is attributed to the various T{sub c} of the different microcrystals.

Superconductivity in nearly single-phase Bi-Pb-Sr-Ca-Cu-O samples with different nominal compositions

The preparation conditions, phase composition, and superconducting properties of Bi-Pb-Sr-Ca-Cu oxide materials from different nominal compositions have been investigated. Nearly single-phase samples from Bi2Pb0.4Sr2Ca3Cu4Oy, as well as from the proposed compositions Bi1.8Pb0.4Sr2Ca3Cu4Oy and Bi1.8Pb0.4Sr2Ca2.5Cu3.5Oy were obtained by a solid-state reaction in air. Samples with the third nominal composition showed the best superconducting properties (Ton=111 K and zero resistance atT0=103 K). A possible mechanism for the 2223 phase formation in the three investigated compositions has been discussed.

Synthesis and Phase Composition of FeSr2YCu2O z

The phase composition of the new superconductor FeSr2YCu2Oz prepared by a solid state reaction at ambient pressure was investigated. The presence of a tetragonal 123, orthorhombic 123, or an intergrowth of 123/1212 phases was established in dependence of the synthesis temperature and atmosphere. The temperature dependence of the susceptibility has a paramagnetic behavior. Diamagnetic effects were observed at 40 and 9 K in the samples prepared without a reducing annealing process. A magnetoresistive effect of about 6% was observed at 5 K.

Structure and Diamagnetic Properties of (Pb0.6SnyCu0.4 − y)Sr2(Y1 − xCax)Cu2Oz

The effect of Sn doping in (Pb0.6SnyCu0.4 − y)Sr2(Y1 − xCax)Cu2Oz with 0 ≤ y ≤ 0.3 and 0 ≤ x ≤ 0.7 was investigated. It was established that a nearly pure 1212 phase can be obtained at 0 ≤ y ≤ 0.1 and 0 ≤ x ≤ 0.3. The obtained XRD patterns as well as the results of the EDX and ICP-AES analyses showed that Sn substitution is possible in the (Pb,Cu)-1212 phase. Superconductivity was observed at 0.4 ≤ x ≤ 0.7. The onset of the diamagnetic transitions varied from 10 to 30 K. The influence of the strong Pb deficiency on the superconducting properties of the samples was discussed.

Structure and Superconducting Properties of Cd0.8Ba2(Y0.7Ca0.4)Cu3.5Oy

A new Cd-containing superconductor with nominal composition ofCd0.8Ba2(Y0.7Ca0.4)Cu3.5Oy was synthesized and investigated. The obtained Cd and Ca-doped 123 phase exhibits an orthorhombic (Tc=80 K) or tetragonal (Tc=65 K) modification depending on the reaction atmosphere. It was shown that the combined Cd and Ca substitution facilitates the 123 phase formation. The results of the EDX analysis, as well as the comparison of the obtained lattice parameters with those of undoped, Cd-doped, and Ca-doped 123 have shown that both Cd and Ca enter the 123 phase and form a new Cd–Ba–Y–Ca–Cu–O superconducting compound.