L.Z. Cao

Alkali metal substitution effects in Mg1-xAxB2 (A = Li and Na)

Abstract We report the alkali metal substitution effects on the structure and superconductivity of Mg 1− x A x B 2 (A=Li and Na) compounds. X-ray diffraction results show that nearly single phase Mg 1− x A x B 2 can be obtained for x ⩽0.15 Li and x ⩽0.2 Na substitution. The lattice parameters a and c decrease with Li substitution. As substituted by Na, both a and c show an increase at low Na concentration, then decrease with substitution at higher Na concentrations. The change of a with Li and Na substitution is explained by two overlapping effects: ion size and an increase of in-plane coupling upon hole doping. The tendency of c -axis contraction in Mg 1− x Li x B 2 is consistent with theoretical prediction. The T c decreases when Li concentration is increased, while the change of T c upon Na substitution is not apparent.

Thermopower and thermal conductivity of superconducting perovskite MgCNi 3

The thermopower and thermal conductivity of superconducting perovskite MgCNi 3 (T c 8 K) have been studied. The thermopower is negative from room temperature to 10 K. Combined with the negative Hall coefficient reported previously, the negative thermopower definitely indicates that the carrier in MgCNi 3 is electron type. The nonlinear temperature dependence of thermopower below 150 K is explained by the electron-phonon interaction renormalization effects. The thermal conductivity is of the order for intermetallics, larger than that of borocarbides, and smaller than MgB 2 . In the normal state, the electronic contribution to the total thermal conductivity is slightly larger than the lattice contribution. The transverse magnetoresistance of MgCNi 3 is also measured. It is found that the classical Kohlers rule is valid above 50 K. An electronic crossover occurs at T * ∼50 K, resulting in the abnormal behavior of resistivity, thermopower, and magnetoresistance below 50 K.

Resistivity and thermoelectric power of Bi2Sr2Ca−xPrxCu2Oy system

Abstract Samples of Bi 2 Sr 2 Ca 1− x Pr x Cu 2 O y have been characterized by resistivity and thermoelectric power measurements. All metallic samples show superconductivity with a maximum T c = 90 K at x = 0.2. The sample of x = 0.6 shows a crossover from hopping conduction at low temperature above T c to metallic conduction at high temperature. For the metallic samples below x = 0.6, the results of thermoelectric power are well fitted by both of a phenomenological band spectrum model and the Nagaosa and Lee model.

Physical Properties of La0.85K0.15MnO3 in Magnetic Field

We have measured the specific heat, resistance and dc susceptibility of the mixed valence manganite La 0.85 K 0.15 MnO 3 as a function of temperature. The sample shows colossal magnetoresistance effects with phase transitions at low temperature. The magnetic specific heat of this sample is proportional to the temperature coefficient of the resistance, following the Fisher-Langer relation. This suggests that the sharp drop in the transition region of the sample is due to the magnetic scattering.

The detailed transport property of the underdoped Bi-2212 system in the pseudogap state

Abstract The detailed character of transport properties in underdoped Bi-2212 systems are analysed in the pseudogap state. In a certain underdoped regime, in-plane resistivity ρab(T) shows a typical S-shaped temperature dependence and a perfect ρ ab (T)=ρ 0 * +β exp (−Δ/T) fit is observed over a wide temperature range from slight above Tc up to T* where the pseudogap begins opening. The underdoped crystal, in contrast to the optimum crystal where no pseudogap is observed, in-plane transverse magnetoresistance shows a slower temperature decreasing rate with temperature increasing in the pseudogap state, and a larger value around T*. About the temperature dependence of thermopower S(T), all underdoped samples completely fall into a scaling curve S/S* vs. T/T* above T*. Whereas, S(T) below T* deviates from this scaling law. The above transport properties should be intimately related with the detailed prospects of Fermi surface destruction in the pseudogap state below T*.

Critical current density of the whiskers

The whiskers with 2212 phase have been grown out by annealing a melt-quenched plate in a stream of gas at for 121 hours. Temperature dependences of the magnetic susceptibility (-T) and the critical current density have been measured. The transition point of the whiskers is 76.2 K, and the value is at 63.4 K, zero magnetic field. near is found, which hints that there may exist some inner grain Josephson junctions in the whisker.

The electronic structure and transport properties of Bi2Sr2−xLaxCaCu2Oy (x=0.0–0.8)

Abstract Experiment on X-ray photoelectron spectroscopy of samples of Bi 2 Sr 2− x La x CaCu 2 O y has been performed to investigate the distribution of La atoms on the cation sites and the change of valence of Bi and Cu atoms. The analysis indicates that La mostly takes the place of Sr at Sr sites. An increase in the average valence of Bi and a decrease in that of Cu have been observed. The resistivity and thermoelectric power (TEP) of the samples have also been measured. The two-band model is applied to analyse the results of TEP.

Transport properties and high Tc superconductivity in single phase Ba2DyCu3O7−δ

Abstract Results of resistivity, ac magnetic susceptibility, Hall effect and superconducting transition temperature measurements for single phase Ba 2 DyCu 3 O 7−δ are reported. The positive value of the Hall coefficient indicates than in the normal state the material conducts via holes with a carrier concentration of 1.0×10 21 cm -3 . A large anomaly in R H near T c is observed.

Transport critical current density of Fe sheath MgB2 tapes sintered at different temperatures

Abstract Fe sheath MgB 2 tapes sintered at 650, 750, 850 and 950 °C, respectively, for 2 h in a high purity argon gas were prepared using the powder-in-tube method. Transport critical current densities of tapes were measured in the fields up to 10 T from 4 to 30 K. Both tapes sintered at 850 and 950 °C completely lost the capacity of carrying superconducting current over the temperature range from 4 to 30 K. Compared to a tape sintered at 650 °C, an improvement in the in-field critical current density J c and irreversibility field μ 0 H * ( T ) was observed in a tape sintered at 750 °C. At 20 K, the J c value was ∼1.32×10 5 A/cm 2 in self-field and the μ 0 H * value 4.2 T for the tape sintered at 750 °C, whereas the J c in self-field and μ 0 H * values were ∼5.9×10 4 A/cm 2 and 2.8 T for that sintered at 650 °C.

Crystallinity and transport properties in Bi2Sr2Ca1-xPrxCu2Oy single crystals

Bi2Sr2Ca1-xPrxCu2Oy (x ranging from 0 to 0.5) single crystals have been grown by a self-flux method from Bi-rich melts. The crystals obtained manifest slim and plate-like shapes. The crystallinity is studied by x-ray diffraction. The x-ray θ-scanning rocking curve shows that the slim crystal has perfect crystal structure while the plate-like crystal is mosaic along the c-axis. The in-plane resistivity ρab(T) has been measured for crystals with different Pr doping levels. Tc is suppressed gradually upon doping while the residual resistivity increases strongly. A superconductor-insulator transition is observed which is attributed to the tetragonal-orthorhombic phase transition when the Pr content reaches about 0.5. The effect of oxygen doping on in-plane resistivity ρab(T) is also studied for x = 0.22 and 0.40 crystals.