J.K.F. Yau

Effect of Pb substitution in bulk superconducting MgB2

Abstract The samples of Mg 1− x Pb x B 2 ( x =0, 0.01, 0.03, 0.05, and 0.06) were successfully synthesized. The temperature dependence of resistance was measured at variant external magnetic field ranged from 0 to 6.15 T. The effects of Pb-doping on the critical temperatures in zero magnetic field, the upper critical fields and the coherence lengths of the samples are discussed. The experiment results show that the critical temperature of Mg 1− x Pb x B 2 decreases with increasing the content of Pb. Contrarily, the absolute value of derivative of H c2 , namely |d H c2 /d T |, increases as the content of Pb is increasing. The results imply that the impurities and defects due to Pb-doping play an important role in the system of Mg 1− x Pb x B 2 . Finally, according to the relationship between H c2 and T , the value of coherence lengths at zero temperature of the samples was estimated.

Study of synthesis technology and superconductivity of MgB2

Abstract For fabricating a dense and superconducting bulk MgB 2 samples, a synthesis technique was employed. Our experimental results show that there is a suitable sintering temperature range, say from 800 to 850 °C. In this temperature range, the critical temperature of the samples is 38 K above, and the superconducting transition is sharper than that of those sintered out of the range. The temperature dependence of resistance of MgB 2 in high temperature region was also measured. The temperature dependence of the resistance, at temperature lower than 700 °C, obeys a form of a + bT n , where n ∼2.9. However, the resistance of the sample exhibits a negative temperature coefficient, as T is higher than 700 °C. This indicates that MgB 2 has semiconducting behavior.

Can the effective pinning potential of MgB2 behave logarithmically in the high temperature range

Abstract The pinning potential barrier U o ( T , H ) is the main parameter describing the flux pinning of MgB 2 . Comparing the experimental results published recently by Thompson et al. and Bygoslavsky et al., one can prove theoretically that in the high temperature range, U o ( T , H ) is of logarithmic form similar to that suggested for high T c superconductors by Zeldov et al.. Meanwhile, the major pinning centers of MgB 2 employ the planar pinning mechanism. Furthermore, it is found that U o ( T , H ) depends on the temperature linearly and also on the magnetic field with the form H −1∼−2 at high temperature range.

AC response studying on MgB2 superconductor by Maley’s method

Abstract We have measured the imaginary susceptibility of MgB 2 at different frequencies and at various dc applied magnetic fields up to 4 T. The method proposed firstly by Maley et al. on fixed applied magnetic field relaxation measurement was used to analyze the data. The information of the flux activation energy so extracted is found to be a nonlinear function of the current density, indicating a logarithmic relation U ( H , J )= U 0 ( H )ln( J 0 / J ) of the current density in this material. The dependence of the activation energy on temperature is determined by a power law U ( T )∝(1−( T / T irr ) 2 ). The functional dependence U 0 ( H )∝ H −0.8 on the applied magnetic field is determined to be in the sample.

Thermally assisted flux flow character of MgB2

Abstract As a new superconductor, MgB 2 has many different characteristic features in contrast to the traditional HTSC. The investigation of these features is important both for theoretical study and application of this material. Several groups reported that from the vortex phase diagram of MgB 2 , 3D vortex glass phase and vortex liquid phase are found. But the flux dynamic mechanism held by this new superconductor could not be considered as a merely a copy of those of the HTSC. While the former is a s-wave superconductor, the latter is d-wave superconductor. In the present article, we present an extensive investigation on the flux mechanism in MgB 2 by transport measurement from 5 K to T c of the sample at magnetic fields up to 7 T, and deal with the experimental data with the linear simulation method within framework of the thermally assisted flux flow theory, which was first proposed by Kes et al. Finally, we obtained an exact expression of the pinning barrier U 0 in the liquid phase of vortex matter of MgB 2 .

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-line shape and second peak effect in high-Tc superconductors

The origin of the second peak effect in melt-textured YBa2Cu3O7-δ was investigated using the sweeping rate dependence of magnetic hysteresis loops. An anomalous change of the vortex-line shape was found around the second peak position. This phenomenon was ascribed to the competition between elastic energy Eel and pinning energy Epin of the flux system. By comparing it with the results of crystal samples such as YBCO, NCCO and Pb-doped BSCCO, we concluded that this characteristic is uniform for these high-Tc superconductors and should be responsible for the widely observed second peak effect in magnetization measurements.

Accurate temperature control of microwave heat treatment for ceramic superconductors

A heat treatment method has been developed for the fabrication of ceramic superconductors using continuous microwave irradiation. By adjusting the sizes of the samples, isothermal heat treatment can be achieved as a result of thermal equilibrium between energy absorbed from microwaves and heat dissipated to the environment. Superconducting Bi2Sr2CaCu2O8+x (Bi2212) was made in a few hours. The mechanism of the method is explained by numerical calculation based on finite element analysis.

Pinning effects and current density broadening of resistance on MgB2

Abstract We prepared the indium doped MgB 2 bulk sample by conventional solid state reaction. Systematic measurements of resistance versus temperature were performed on the sample with different transport current and different applied magnetic field. The curves of resistance in opposition to temperature are broadened with different transport current in transmitting temperature range. We present a method for the first time to analyze the experimental data and explore the pinning mechanisms of MgB 2 for such kind of experiments. By analyzing on the experimental data, we found that the effective potential U eff ( T , H , J ) is of logarithmic form in our experimental temperature and applied magnetic field regime. The curve U 0 ( T , J ) is linear dependent on temperature but the slope coefficient is changing lower near onset critical temperature.

Investigating E–J characteristics of melt-textured YBa2Cu3O7−δ

Abstract We studied the E – J characteristics of a melt-textured YBa 2 Cu 3 O 7− δ sample by measuring its sweeping rate dependence of magnetization with a vibrating sample magnetometer. Firstly, we derived the relationship between the electric field and sweeping rates from flux conservation equation, and furthermore developed a method of obtaining ln E –ln J curves and thermal excitation potential U 0 ( T , B ) from magnetization measurement. Then, curves of ln E –ln J and U 0 ( T , B ) were deduced from the magnetic measurements on a melt-textured YBa 2 Cu 3 O 7− δ sample prepared by MTG method, and thereby different dynamic behaviors were studied in accordance with our experimental data.