C.C. Lam

Thermodynamic fluctuation under high pressure in Hg-1223 superconductors

Excess conductivities of polycrystalline Hg-1223 superconductor have been studied under high-pressure conditions. The critical temperature was chosen as the mean field transition temperature . Above this temperature, the normal state resistivity decreases with increasing pressure. The relative change in resistivity w.r.t. pressure, namely , is about . It is much smaller than the typical value of for most high- superconductors. As a consequence of the change in the normal state resistivity due to the applied pressure, the fluctuation-induced conductivity is also changed. The experimental results were analysed within the framework of Aslamazov-Larkin theory. The logarithmic plots of the excess conductivity versus reduced temperature reveal that there is a cross-over temperature which determines the transition from two- to three-dimensional conductivity of charge carrier transportation. With the help of the cross-over temperature , the values of the coherence length for the Hg-1223 superconductor could be estimated. The change in the fluctuation conductivity is discussed in terms of the coherence length and the thermodynamical critical field .

A new calculating formula of iodometric titration for high Tc superconductors

Abstract In order to determine the effective oxidation state of Cu (p) and oxygen content (y) with the iodometric method, a new formula for calculating p and y of high Tc copper oxides is presented here. The formula of p involves a corrective term of 8CV1/M1, which reduces a systematic calculating error. With this approach, for YBCO, the value of p of YBCO is improved by 5% and y by 0.02, as compared with the conventional method.

Powder XRD investigation of crystal structure modification effects on superconductivity in the Sn-doped YBa2Cu3-xSnxO7-delta systems

The relationships between the crystal structure modification, due to the doping element of Sn, and the critical temperature Tc of Sn-doped YBa2Cu3-x SnxO7- , with x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, have been investigated by means of the peak shift and the peak broadening of the powder x-ray diffraction pattern. The profiles of x-ray diffraction peaks, due to crystal planes (006), (020), (021) and (200), were studied using the multi-peak fitting technique which is based on the Gaussian distribution function. It is interesting to note that the Sn-doped atoms effectively occupy the Cu2 sites in the CuO2 layer. The critical temperature of the Y-123 phase depends more sensitively on the lattice parameters a and b, rather than on the c-axis parameter c. The samples used for this investigation underwent the same treatment to ensure the requirement for maintaining the same oxygen content, particle size and residual stress. In this way, we can make sure that the Sn doping basically causes the peak shift and the peak broadening. Based on our results, we conclude that Sn doping in YBCO leads to an increase of the lattice constants a and b. The greater the expansion in a and b, the lower the critical temperature that results.

Thermal excitation measured by resistivity measurement on the Mg-doped high temperature superconductors

Abstract The thermal excitation that starts to occur from the opening temperature T* in the normal state of Mg-doped YBCO superconductors was studied by the resistivity measurement. There are some characteristic temperatures T* and T F related to the thermal excitation of quasi-particles during the cooling process. The data of T F are close to the onset temperature of the superconductors. The magnitude of the pseudo-gap (Δ PG ) can be measured from the 1n[1/ϱ N (T) − 1/ϱ(T)] vs 1/T curve, it depends on the concentration of the magnesium.

Low-lying energy spectrum of a magnetopolaron bound to a Coulomb impurity in a quantum dot

Abstract By using an exact numerical diagonalization scheme, we studied the low-lying spectrum of a magnetopolaron quantum dot with a confined parabolic potential in the presence of a Coulomb impurity at the center. Continuous ground-state energy transitions induced by the magnetic field are reported. Furthermore, the effect of different electron–LO phonon interaction strengths on the energy spectrum has been predicted.

(Hg, Sb)Ba2Ca2Cu3O8+delta thick films on YSZ substrates

Superconducting thick films of (Hg, Sb)Ba2Ca2Cu3O8+ have been fabricated on polycrystalline yttria-stabilized-zirconia substrates utilizing an Hg-free precursor film reacted with Hg vapour, released from a solid Hg source, in a sealed quartz tube. The resulting films have been studied by x-ray diffraction, scanning electron microscopy, ac susceptibility and resistance measurement techniques. A high quality Hg(Sb)-1223 superconducting thick film on YSZ can be fabricated by using a pre-melted Hg-free precursor film. The zero resistance superconducting transition temperature in the post-growth oxygenated thick film is in excess of 130 K and the transport critical current density for the film is 510 A cm-2 at 77 K.

Flux pinning characteristics and material power law of melt-textured YBa2Cu3O7-

Low-electric-field E-J curves of YBa2Cu3O7- were determined from 0.27 to 1.34 T at various temperatures from 52 to 83 K with the magnetic sweeping method by VSM. Using an empirical material power law E = Ec(J/Jc)n, we obtained the characteristic pinning potential U0(T,H) and real critical current density Jc(T,H) by an appropriate definition of Ec. By further investigation of the changing shape of the pinning potential well, we interpreted the peak effect of U0 with temperature in high temperature superconductors (HTSCs).

Enhancement of critical current density in HgBa2Ca2Cu3O8+δ superconductor doped by Sb

Abstract Antimony-free Hg-1223 and antimony-doped (Hg0.95Sn0.05)-1223 superconductors were synthesized in sealed silica tubes by the solid state reaction method. The precursor can be prepared in air. A series of experimental results obtained from the Sb-doped and Sb-free superconductors by X-ray diffraction, scanning electron microscopy (SEM), thermogravity analysis, resistance measurements and AC susceptibility measurement techniques shows that a small addition of Sb increases the volume fraction of the Hg(Sb)-1223 superconducting phase and enhances the intergrain critical current density Jc, as well as the thermal stability of this superconducting phase. The Tc of the Sb-doped sample decreases by about 4 K as compared with that of the pure Hg-1223 superconductor. SEM observation shows many plate-like superconducting grains which seem to be more dense in their packing for the Sb-doped sample than in the Sb-free sample. The enhancement of current density Jc is due to the Sb doping which improves the coupling status for their intergrains. These grains modify the characteristics and the nature of superconducting grain boundaries. Furthermore, an orthorhombic insulating phase, with a=1.6603 nm, b=1.0618 nm, and c=0.87575 nm, was found to appear in the (Hg0.70Sb0.30)-1223 sample.

Enhancement in flux pinning and irreversibility field by means of a short-time annealing technique for superconductor

We have successfully prepared mercury-based high-temperature superconductors of composition by means of the encapsulation and short-time annealing technique. By measuring the hysteresis loops, the critical current density is determined by using Beans model. Scaling behaviours of the pinning-force density are discussed. By using the short-time annealing technique, the irreversibility line and the flux-pinning properties of the Hg-1223 specimen were enhanced by the introduction of more lattice defects, such as screw dislocations, intergrowth structures and stacking faults, into the Hg-1223 specimen. Furthermore, it is shown that the temperature dependence of the irreversibility line exhibits different features within two temperature regions. In the low-temperature region is proportional to , while in the high-temperature region is proportional to . The observed can be clarified by comparison with the theoretical model.

The Influence of Mobile Holes in Hg1 − x Pb x Ba2Ca2Cu3O8 + δ Superconductors

By means of X-ray diffraction (XRD), neutron diffraction spectra (NDS), electron probe microanalysis (EMPA), and X-ray photo electron spectroscopy (XPS) etc., we studied the superconductivity of doped-Pb Hg-1223 compound. The crystal parameters a and c and the unit cell volume of the Pb-doped sample do not change considerably, but the intercalated O(4) of the sample increases more than that of the pure-phase Hg-1223. As for the valence number of Hg is still 2 and that of Pb is either 2 or 4, the bond length of Cu(2)–O(3) increases, Raman frequency of the AIg mode of the apical O(3) increases, the bond angle of O(2)–Cu(2)–O(2) is smaller and the hole density is approximately uniformly shared between the three CuO2 planes. The above have been affirmed by our experiment data and calculations based on Bond Valence Sums (BVS). Thus, the enhancement in Tc of the Pb doping effect is due to the variation of the electronic state and its distribution.