Guohui Cao

Positron annihilation study on the stress-field pinning mechanism in (Eu, Y)-123 superconductors

Abstract The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra have been performed on (Y 1− x Eu x )Ba 2 Cu 3 O 7− y ( x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1) superconductors. The mean lifetime and S -parameter, respectively show a characteristic significantly symmetric to Eu-doping concentration. Since lifetime and Doppler broadening spectroscopies are rather sensitive to the variation of the lattice distortion and defects induced by the stress-field of lattice mismatch, such symmetric behaviors of the S -parameter and the mean lifetime suggest a striking characteristic of stress in Y 1− x Eu x -123 superconductor. When the doping concentration x reaches 30%, the density of defects corresponding to stress-field appears the maximum, and there is an optimum distribution of the stress-field so as to increase critical current J c in Eu-doped Y-123 superconductors. Our results of the critical current density obtained by means of measurements of susceptibility χ ′( T , H AC ) and χ ″( T , H AC ) also suggest that the enhancement of the critical current density to reach an optimum when Eu doping x is 30%. We also qualitatively discuss experimental results in the framework of the model of stress-field pinning. The model calculating results also show the stress-field induced by lattice mismatch is an effective pinning center, and further conform that the stress pinning mechanism is dominant in RE-doped Y-123 superconductors.

The low temperature Mössbauer spectra of YBa2Cu2.88Fe0.12O7-x

Abstract The occupied position of Fe in the superconducting phase of Fe-doped YBa2Cu2.8807-x was estimated by point charge model. The Mossbauer spectra were measured in the temperature range from 78 K to 273 K. Lattice softenings were found near 110 and 220 K. The observed variations of hyperfine parameters can be considered tobe related to the lattice softening near 110 K. It is suggested that the lattice softening near 110 K is related to the Peierls instability of the one-dimensional Cu(1)−0 chains.

Superconductivity in Al-substituted Ba8Si46 clathrates

There is a great deal of interest vested in the superconductivity of Si clathrate compounds with sp3 network, in which the structure is dominated by strong covalent bonds among silicon atoms, rather than the metallic bonding that is more typical of traditional superconductors. A joint experimental and theoretical investigation of superconductivity in Al-substituted type-I silicon clathrates is reported. Samples of the general formula Ba8Si46−xAlx, with different values of x were prepared. With an increase in the Al composition, the superconducting transition temperature TC was observed to decrease systematically. The resistivity measurement revealed that Ba8Si42Al4 is superconductive with transition temperature at TC = 5.5 K. The magnetic measurements showed that the bulk superconducting Ba8Si42Al4 is a type II superconductor. For x = 6 sample Ba8Si40Al6, the superconducting transition was observed down to TC = 4.7 K which pointed to a strong suppression of superconductivity with increasing Al content as ...

Study of the pinning behaviour in yttrium-doped Eu-123 superconductors

The magnetic properties of a series of superconductors (Eu1-xYx)Ba2Cu3O7-y (x = 0, 0.2, 0.3, 0.5, 0.7 and 0.8) have been investigated by magnetic measurements from 20 to 90 K. Magnetization hysteresis loops of the Y-doped sample exhibit a strongly developed second peak in the higher temperature range. We discuss the experimental results in the framework of a stress-field pinning model. The stress field induced by the lattice mismatch is also an effective pinning centre, in addition to pinning by 211 phase precipitates and oxygen vacancies. We further suggest that in the (Eu1-xYx)Ba2Cu3O7-y samples the oxygen deficient regions induced by the stress field in Cu-O chains also act as pinning centres, thus causing the second peak in the Jc-B curves of (Eu1-xYx)Ba2Cu3O7-y.

Superconducting and microstructural properties of Fe-doped Tl-1223 cuprates

Abstract The Fe-doping effects on superconductivity, transport and microstructure are investigated by means of Hall coefficient measurements, thermogravimetric analysis, Mossbauer spectroscopy and the calculation of the point-charge-effective-valence model (PCEV model) in Fe-doped Tl 0.5 Pb 0.5 (Sr 0.8 Ba 0.2 ) 2 Ca 2 (Cu 1− x Fe x ) 3 O y (Tl-1223) superconductors. In the doping level of Fe ( x =0–0.05), both zero-resistance temperature T co and carrier concentration n H decrease linearly with Fe dopants increasing. The striking linear behavior of T co and n H suggests the potential effects of charge localization on the transport properties and also shows that Fe dopants occupy different Cu sites in a stable manner with Fe content increasing. The thermogravimetric analysis shows the Fe 3+ ions substituting for Cu 2+ ions can bring excess oxygen atoms into lattice and form some Fe–O defect clusters. Mossbauer measurements are performed to investigate the microstructure of Fe–O defects. In addition, the atomic position of Fe dopant and the electric field gradient (EFG) on Fe sites are investigated under the PCEV model. We found that the signs of the EFG on Fe sites are positive. According to quadrupole splittings deduced from Mossbauer spectroscopies and results of model calculation, it is further confirmed that there is a small shift of Fe ion relative to the original center of the CuO 2 network, and in some different kinds of Fe–O defect clusters, Fe dopants can lead to excess oxygen atoms entering lattice. These results provide a further support for the interpretation about the displacement of Fe dopant and the assignment of Fe site. Our discussion suggests that the microstructural distortion and the excess oxygen defect induced by Fe doping encourage in the localization of cruising carrier on CuO 2 planes, which is one of the reasons to explain the decrease of T c and carrier concentration.

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.

Mössbauer study of quadrupole interaction at the different Cu sites in the Bi-system superconductor

Abstract 57 Fe Mossbauer measurements have been performed on Fe-doped samples of Bi 1.6 Pb 0.4 Sr 2 Ca 3 Cu 4- x Fe x O y ( x =0.06 and 0.08) at room temperature. The Mossbauer spectra show three quadrupole doublets that can be related to different Cu sites. Two doublets (B and C) with larger quadrupole splitting (QS) are assigned with a 57 Fe probe atom on the Cu-O square planar sites of the compound with different local charge environments. Another doublet A with smallest quadrupole splitting is associated with a Fe atom in a planar pyramid site. the doublet with smallest QS possesses relatively the smaller area indicating that the Fe atom preferentially occupies a square planar site rather than a pyramid site. To assure our assignment, we calculated the EFG (electric field gradient) on different Cu sites with the use of the point charge model and of the calculation of an effective ions valence.

Study of phonon anomalies in Tl-system superconductor near Tc

Abstract The phonon softening near T c on iron-doped Tl - 1223 phase samples were observed by 57 Fe Mo¨ssbauer measurements at temperatures ranging from 77K to 250K. Simultaneously, the temperature dependence of positron annihilation parameters on an iron-free sample show the anomaly of the electronic behavior in the temperature range from T c to T c +30K.

Study of lattice softening on Bi-system superconductor by means of 119Sn Mössbauer spectroscopy

Abstract Anomalous behavior of the Mossbauer factor for tin impurity atoms in Bi-system Superconductors is found by Mossbauer spectroscopy. This indicates the occurrence of lattice softening above Tc.

Defect and pinning effect of Sn-doping on (La1−xSrx)2Cu1−xSnxO4 superconductors

Abstract The effect of Sn-doping on (La 1− x Sr x ) 2 Cu 1− x Sn x O 4 ( x =0.075, 0.090 and 0.110) has been studied. Lattice parameters monotonically decrease with Sn dopant increasing. Sn atoms dominantly occupy Cu sites and form Sn–O clusters as point-like defects. Flux pinning properties of ceramic samples of (La 1− x Sr x ) 2 Cu 1− x Sn x O 4 have been investigated by magnetic measurements from 6 to 30 K up to 5 T. The current carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the more richly Sn-doped sample exhibit stronger pinning behavior, especially in the higher temperature range. The intra-granular critical current J c in the rich Sn-doping sample has a significant increase. We discuss the experimental results within a pinning model of point-like defects. From our data we infer that the Sn–O cluster near Cu–O layers provides an effective pinning center.