Bai-Ru Zhao

Josephson tunneling in MgB2 break junctions

Josephson effects in break junctions of MgB2 sintered at ambient pressure, have been investigated. The measured I-V characteristics of the junctions and the current step heights of the Shapiro steps can well be fitted with the resistively shunted junction model. Relatively large IcRn was obtained, which ranges from 1.4 to 4.2 mV. The temperature dependence of the Josephson critical current indicated that our break junctions were of the SNS weak link. The apparent deviation in the Fraunhofer pattern implied inhomogeneous current flow in the MgB2 break junctions

A new semi-Heusler ferromagnet NiFeSb: electronic structure, magnetism and transport properties

We report on structural, magnetic, transport, and spin polarization measurements of a new semi-Heusler alloy NiFeSb, which was synthesized successfully by means of electron melt-spinning technique although its ferromagnetic state and crystal structure was predicted to be unstable by the band calculation. The temperature-dependent measurement of magnetizations can both be interpreted well by the spin-wave theory. The electrical resistivity follows a T-5 behavior at low temperatures. Point contact Andreev reflection measurements of the spin polarization yield moderate polarization of 52%

Josephson effects in MgB2/Nb point-contacts

We have systematically investigated the dc and ac Josephson effects in MgB2/Nb point-contacts. The observed I–V characteristics of the point-contacts can be described nicely by the resistively shunted junction (RSJ) models and large characteristic voltages, 2–3 mV, have been obtained. The dependence of the Josephson current step heights on the microwave current amplitude can also be fitted by both dc and ac current-biased RSJ models. The measured temperature dependence of the Josephson critical current agrees well with the Ambegaokar and Baratoff equation at low temperature regions, but there are deviations from it at high temperatures. These results imply that the 39 K MgB2 is more likely a BCS superconductor and has its potential application in superconducting electronics.

Tunneling observation of the coexistence of two distinct gaps in Bi2Sr2CaCu2O8+δ

Abstract Temperature, T , and doping dependent tunneling spectroscopy on Bi 2 Sr 2 CaCu 2 O 8+ δ /natural barrier/Zn thin film (Bi2212/I/Zn) junctions has been investigated systematically. For most of the junctions, the tunneling conductance shows one superconducting gap (SG) at low T and a discontinuous transformation from the SG to the pseudogap (PG) at T c . For some junctions studied, the conductance reveal two distinct gaps: an inner gap which closes when T approaches to T c with BCS-like T -dependence, and an outer gap, coexisting with the inner gap for T below T c , which is almost T -independent below about 120 K. We discuss the possible origins of the coexistence of the two gaps, including the coexistence of the SG and PG and the gaps resulting from electronic or doping inhomogeneity.

PrBa2Cu3O7 thickness dependence of current-voltage behaviors in YBa2Cu3O7/PrBa2Cu3O7 multilayers

Abstract The current-voltage ( I–V ) characteristics were measured for YBa 2 Cu 3 O 7 (24A)/PrBa 2 Cu 3 O 7 (12A, 24Aand 96A) (YBCO/PBCO) multilayers in the perpendicular magnetic fields H ( H | c -axis) up to 10 T. From the analysis of the vortex-glass (VG) phase transition with a transition temperature T g and a dynamical critical exponent z , it is found that the dynamics of vortex system is influenced by the thickness of PrBa 2 Cu 3 O 7 layers. For YBCO(24A)/PBCO(12A) (24/12) multilayers, the I–V behaviors can be almost explained by three-dimensional VG transition. While, for 24/12 multilayers the quasi-two-dimensional VG transition leads a reasonable value of z . In the high fields ( H > 4T) the parameters T g and z of 24/24 are similar to ones of 24/96 rather than ones of 24/12. While, they approach to ones of 24/12 when the values of H are less than 1 T. This behavior is discussed with the current dependence of exponents α in power law ( V ∼ I α ).

Magnetic field and temperature dependence of linear resistivity in YBa2Cu3O7/PrBa2Cu3O7 multilayers

We have studied the temperature dependence of the linear resistivity ρL under several magnetic fields for YBa2Cu3O7 (24Å)/PrBa2Cu3O7 (96Å) multilayers. From a thermally activated flux flow model, theH dependence of an activation energyU seems to show the power-law behaviour above 2 T, while the scaling behaviour of ρL suggests the existence of a quasi-two-dimensional vortex-glass transition. These phenomena may be attributed to the existence of the coupling between YBa2Cu3O7 layers in this multilayers.