S.L. Jia

Effect of Li doping on structure and superconducting transition temperature of Mg1-xLixB2

Abstract We report the preparation of Mg 1− x Li x B 2 compounds. Nearly single phase samples were obtained for x ⩽0.3. The in-plane lattice parameter a decreases with Li doping, while the lattice parameter c does not show obvious change. The superconducting transition temperature of Mg 1− x Li x B 2 decreases with Li doping and loss of superconductivity occurs for x =0.5 sample. The results were discussed in terms of the T c dependence on hole doping and contraction of the lattice parameter a .

Influence of the starting composition Mg1−xB2 on the structural and superconducting properties of the MgB2 phase

We report the preparation of samples with the nominal composition Mg1−xB2 (0⩽x⩽0.5). Nearly single-phase MgB2 was obtained for x=0 with a minor amount of MgO. For 0<x⩽0.5, MgB4 coexists with MgB2 and some minor impurity phases. The amount of MgB4 increases with x. With increasing x, the lattice parameter c of MgB2 increases, the lattice parameter a decreases, and correspondingly Tc decreases. The results are discussed in terms of the presence of Mg vacancies or B interstitials in the MgB2 structure. This work is helpful to understand MgB2 films with different Tcs, as well as the Mg site doping effect for MgB2.

Crystal structures and superconductivity of superconducting phases in the Tl-Ba-Ca-Cu-O system

Abstract We found a series of superconducting phases in the Tl-Ba-Ca-Cu-O system and determined the crystal structures of these superconducting phases by means of the X-ray powder diffraction method. All of these superconducting phases belong to the tetragonal symmetry. They have the same lattice constant a and different lattice constant c . These superconducting phases can be divided into two types. The space group of type I is P4/mmm. The space group of type II is 14/mmm. The chemical formulae of type I and type II can be generalized as TlBa 2 Ca n −1 Cu n O 2 n +2.5 ( n =2, 3, 4) and Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 ( n =1, 2, 3), respectively. T c increases as n increases. On the condition that n is the same, type II has a higher T c than type I. The relations among the crystal structures of the superconducting phases are discussed. We proposed a method which can deduce the crystal structure of the superconducting phase from the d 1 -value of the first diffraction line (00 l ) or lattice parameter c and which is confirmed by experimental data.

The structural change and superconductivity in MgCxNi3 (x=0.5–1.55) and MgxCyNi3 (x=0.75–1.55, y=0.85, 1.0 and 1.45) and the phase diagram of Ni-rich region in Mg–C–Ni ternary system

The structural change and superconductivity in the nominal composition MgCxNi3 (x = 0.5-1.55) and MgxCyNi3 (x = 0.75-1.55, y = 0.85, 1.0 and 1.45) were investigated and the phase diagram of Ni-rich region in Mg-C-Ni ternary system was determined. It was found that in MgCxNi3 system there are two phases: alpha-MgCxNi3 and beta-MgCNi3. alpha-MgCxNi3 phase exists in the MgCxNi3 samples with x 1.0 prepared at the temperature of 900985 degreesC and in the samples with x < 1.0 prepared at the temperature higher than 965 degreesC. It has a large and constant lattice parameter of 0.3810 run. The beta-MgCNi3 phase is superconducting. The investigation results indicate that a homogeneous region with changeable content of Mg does not exist in MgxCyNi3 (x = 0.75-1.55, y = 0.85, 1.0 and 1.45) systems

Superconductivity in the (Sr1-xYx)CuO2 (x=0.00-0.30) system synthesized under high pressure

Abstract Superconductivity has been observed in a new series (Sr 1- x Y x )CuO 2 ( x =0.00−0.30) synthesized under high pressure of 3.5 GPa and 950°C. This new series shows a broad superconducting composition range with x =0.05−0.30. The onset transition temperature of the series is 36–38 K. The preparation condition has a remarkable effect on both the microstructure and superconductivity of the samples.

Structural change and superconductivity in La1.85−2xSr2x+0.15Cu1−xRuxO4 and La1.85Sr0.15Cu1−xRuxO4 systems

The structural changes and superconductivity of La1.85−2xSr2x+0.15Cu1−xRuxO4 (I) and La1.85Sr0.15Cu1−xRuxO4 (II) systems have been investigated. It was observed that the Ru doping strongly suppresses Tc. The critical contents xc for Tc disappearances are respectively 0.0225 and 0.019 for the (I) and (II) systems. It was found that in all La1.85Sr0.15Cu1−xMxO4 systems investigated (M = Ru, Li, V, Sn, Al, Ga, Co, Fe, Mn, Mg, Ni and Zn), the structure parameter c/a ratios decrease with increasing doping content x, which is a common characteristic. The mechanisms for Tc suppression in the La1.85Sr0.15CuO4 system are summarized and discussed briefly.

Structural and superconducting properties of the infinite-layer (Sr, Ca)1−yCuO2 prepared under high pressure

The Rutherford backscattering/channelling technique of 2.1 MeV He ions has been used to study the damage distribution in Si irradiated with 1.0 MeV Ti ions to a dose of 5 x 10(14) ions cn-2 under 7-degrees and 60-degrees incidence. A dechannelling analysis of multiple scattering has been made. The longitudinal damage straggling and lateral damage spread are estimated. The values obtained are compared with transport of ions in matter. The results show that the differences between experimental and calculated values for longitudinal damage straggling and the lateral damage spread are 25% and 28%, respectively. Also, it is observed that the damage distribution shape under 60-degrees irradiation is found in good qualitative agreement with an earlier simulation.

STRUCTURE AND SUPERCONDUCTIVITY IN THE INFINITE-LAYER SR1-XCUO2 SYSTEM PREPARED UNDER HIGH-PRESSURE

Abstract Infinite-layer Sr 1− x CuO 2 ( x = 0.0−0.3) samples have been prepared under high pressure. The average structure of the infinite- layer SrCuO 2 was refined by the Rietveld technique to be tetragonal with a = 3.9269 (2) A and c = 3.4346 (2) A . The infinite- layer Sr 1− x CuO 2 samples could be formed under high pressure for x = 0.0−0.3 in which the Sr deficiency was partially released by forming Cu rich impurity phase (s) such as CuO or Sr 14 Cu 24 O 41 . The microstructure of the Sr 1− x CuO 2 samples was characterized by high-resolution electron microscopy and Sr vacancies were not found to cluster into defect layers along the c -axis. The resistivity-temperature dependence of the Sr 1 − x CuO 2 ( x = 0.0−0.3) samples suggested that the Sr deficient infinite-layer phase might be electron-doped. An indication of superconductivity with T c onset = 60 K was observed in the Sr 1− x CuO 2 samples. When the samples were annealed in an oxidizing atmosphere, the indication of superconductivity remained at 60 K but the superconducting property was not improved remarkably. The origin of the superconductivity was discussed.

High-Tc superconductivity above 110 K in the Tl-Ba-Ca-Cu-O system

High-Tc superconductivity above 110 K has been observed in Tl-Ba-Ca-Cu oxides. The dependence of electrical resistivity and AC magnetic susceptibility on temperature show that the zero-resistance transition temperature is about 114 K and a dramatic magnetic susceptibility drop appears at 117 K. The superconductivity remains stable after several cooling and heating cycles. The observation of superconductivity in this system was carried out at least eight times.

The high-pressure and high-temperature synthesis of HgBa2CaCu2O6+ delta and HgBa2Ca2Cu3O8+ delta superconductors and their characterization

The superconductors HgBa2CaCu2O6+ delta (Hg-1212) and HgBa2Ca2Cu3O8+ delta (Hg-1223) have been synthesized by solid state reaction, carried out at 850 degrees C under 25 kbar, using the nominal composition of HgBa2Ca2Cu3Ox. The results indicate that the optimum sintering time for the Hg-1223 phase is about 0.5 h. With increasing sintering time, the Hg-1223 phase decomposes, and the Hg-1212 phase increases gradually. When the sintering time increased to 3.8 h, we obtained a sample with nearly a single phase of Hg-1212. We synthesized the HgBa2CaCu2O6+ delta superconductor under 27 kbar at various sintering temperatures using the nominal composition of HgBa2CaCu2Ox. The results indicate that the optimum sintering temperature for the Hg-1212 phase is 820-830 degrees C. The effect of annealing at various temperatures in flowing O2 on the superconductivity of HgBa2Ca2Cu3O8+ delta has been investigated.