L.J. Shen

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 .

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.

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).

Flux Conservation Equations of High‐Tc Superconductors with Different Shapes and Their Application in Electromagnetic Measurements

In consideration of the importance of the specimen shape effects and field orientations in electromagnetic measurements, we extended the conservation equations of one-dimensional (1D) specimens to two-dimensional (2D) ones in parallel fields, the corresponding parameters were calculated and tabulated. Then we discussed the problem of 2D specimen in a perpendicular field. Based on the derivation of general flux conservation equations, we presented their simple applications in measurements by providing the commonly used measurement equations in typical measurement methods.

Pinning characteristics of MgB2 near the melting curve

A less controversial method is used to analyse the imaginary part of the ac susceptibility measurement of MgB2. The pinning potential barrier Uo(T, H) is obtained near the melting curve. It is found that Uo(T, H) depends linearly on both the temperature and the applied field.

Vortex Dynamic Properties of Vortex-Liquid Phase in YBa2Cu3O7–δ Epitaxial Films

The current-voltage characteristics (CVC) of YBa 2 Cu 3 O 7-δ epitaxial films were determined at temperatures and magnetic fields covering the vortex-liquid region. Using functional fitting and scaling method, we derived the temperature and magnetic field dependences of the pinning potential U 0 (T,H) and critical current density J c (T,H), and current, temperature and magnetic field dependences of the effective pinning potential U eff (H, T,J) from the CVC. It is shown that the nonlinear resistance region of the vortex-liquid phase displays a process from vortex plastic motion to free flow with the dynamic exponent changing from -0.5 to 0.

Pressure effect on the spin gap opening temperature for Ca-doped and oxygen-deficient YBCO superconductors

The spin gap opening temperature in the normal state of bulk Ca-doped and oxygen-deficient YBCO superconductors was investigated under various high hydrostatic pressures. Under the ambient pressure, T* can be expressed in terms of Tc as T*(n) = T*(OP) +α(1 - Tc(n)/Tcmax)12. Under various high pressures, this expression is still valid, so, T*(n,P) = T*(OP) +α(P)(1-Tc(n,P)/Tcmax)12. Experiments show that the parameter T*(OP) is pressure-independent, while α(P) decreases linearly with increasing pressure. This indicates that the structural change considerably affects the spin gap behavior.

Characteristic temperatures of pseudo-gap phenomenon in the under-doped high temperature cuprate superconductors

Abstract The anomaly in the normal state of high-T c superconductors is related to the pseudo-gap phenomenon. For the under-doped cuprate superconductors, the relationship between the logarithmic increment in conductivity (w.r.t. the normal conductivity 1/ g 9 N (T)) and the inverse temperature exhibits some specific characters at temperatures T ∗ , T S and T F . The temperatures T ∗ and T S are respectively the opening temperature and the terminating temperature for the cross over in the pseudo-gap phenomenon, while the temperature T F is close to the onset critical temperature of the superconductor. It is shown experimentally that T ∗ , T S and T F depend linearly on the Ca-doping level, while the quantity (2 Δ PS 2 )/(2 Δ T ) − Δ T has a quadratic-polynomial dependence on the Ca-doping level.

Flux dynamic behaviour of a melt-textured YBa2Cu3O7-δ sample

The flux dynamics behaviour of a melt textured YBa2Cu3O7-δ sample was studied by measuring the sweeping rate dependence of magnetization at different temperatures. First, J(T,B) curves were obtained from the extended Bean model in order to discuss the magnetization dependence on the sweeping rate. Then the curves of ln E-ln J and the thermal excited potential U0(T,B) were derived from the magnetic measurements. Thereby the magnetic response of this sample, especially the influence of the field sweeping rate to the second peak was analysed. Thus, the peak effect of this sample and the factors affecting the magnitude and position of the second peak were discussed.

Thermal excitation relevant to the pseudo-gap phenomenon in the normal state of high temperature superconductors

Abstract The thermal excitation spectrum relevant to the pseudo gap phenomenon is established from the conductivity measurement results. The plot of the logarithmic deviation in the conductivity, with respect to the normal state conductivity, against the inverse temperature shows some characteristic temperatures T * and T s , at which the pseudo-gap is opened and the saturation temperature of the created quasi-particles in the pseudo gap level. There is another characteristic temperature T F , which close to the onset critical temperature of the superconductivity. There is a crossover from the strange metallic state to the pseudo-gap state within the temperature interval ΔT = T * − T s . In the temperature interval ΔT = T S − T F , the plot of ln[l/ϱ(T) − l/ϱ N (T)] vs. 1/T exhibits linear relationship. The slope of this linear relationship represents the magnitude of the pseudo-gap Δ SP /k B . The conductivity deviation can be expressed in term of the number of quasi-particles.