Furen Wang

Fabrication of superconducting magnesium diboride thin films by electron beam annealing

The technique of electron beam annealing for fabricating MgB2 thin films has been explored. MgB2 thin films were prepared by e-beam evaporation of M?B precursor films on 6H?SiC (0001) substrates followed by ex?situ electron beam annealing without any extra Mg vapor or argon gas protection. The very short annealing duration, about 1?s, effectively prevented volatilization and oxidation of Mg and decomposition of MgB2. In comparison with the MgB2 thin films grown by other techniques, our films show medium qualities including a superconducting transition temperature of Tconset ~ 35.3?K, a transition width ?Tc ~ 0.2?K, and a critical current density of Jc(5?K, 0?T) ~ 3.2 ? 106?A?cm ? 2. Such an electron beam annealing technique has potential for the fabrication of large-scale MgB2 thin films as well as MgB2 wires and tapes.

The abnormal temperature-dependent rectification effect in BiFeO3/YBa2Cu3Ox heterostructures

We report the fabrication and properties of heterostructures formed by the BiFeO3 (BFO) ferroelectric and YBa2Cu3Ox (YBCO) superconductor. Four measurement configurations on BFO/YBCO heterojunctions are proposed and showed different I-V characteristics. Moreover, one type of BFO/YBCO heterostructures which had the strongest rectification effect at 15K, showed the abnormal rectification effect dependent on temperature. The rectifying effect enhanced with decreasing temperature below the superconducting transition temperature of YBCO. YBCO electrode played an important role in increasing the rectifying parameter because of the high density of states for quasi-particles in YBCO in the superconducting state.

Dirty limit approach for the proximity effect between a two band superconductor and a ferromagnet

The proximity effect in multilayer structures with dirty two band superconductors and ferromagnets is investigated here. With a homogeneous exchange field and interband scattering in the ferromagnetic layer, the condensate functions that penetrate the ferromagnet are coupled up. This coupling effect is intensively suppressed by the exchange field and modulated by the diffusion coefficients of the two bands. Such coupling also affects the superconductor where the condensate functions of the two bands are also coupled up through the proximity effect. An inhomogeneous exchange field raises the damped oscillation of the condensate functions as the spatial distance from the interface increases. There is a 0−π phase shift when the ferromagnetic layer thickness varies beyond the ferromagnetic coherence length.

Research on high-Tc rf SQUID and its applications

We report on the research work at Peking University on optimizing high-temperature superconductor (HTS) rf superconducting quantum interference device (SQUID) systems and applying them in geophysical survey and magnetocardiography (MCG). Emphasis is placed on the design of comb-shape resonators for HTS rf SQUID systems and the experimental results of two applications: transient electromagnetics and MCG. The magnetic-field sensitivity of HTS magnetometers is now adequate for MCG applications. However, in order to be commercially used, the system still needs some improvements: development of suitable gradiometers and multi-channel systems.

Growth and superconductivity characteristics of MgB2 thin films

We attempt to make MgB2 thin films by using a pulsed-laser-deposition (PLD) and a magnetron sputtering method. We have deposited metal magnesium and boron on various substrates under different vacuum conditions. The PLD method has been employed to fabricate layers of magnesium and boron sandwiches under room temperature and the multi-layer system was then annealed in-situ under different temperatures. We also attempted to co-deposit magnesium and boron under high vacuum (5 × 10−5 Pa) on heated substrates with PLD. We have successfully grown superconducting MgB2 thin films on an STO (100) substrate by magnetron sputtering. The onset transition temperature was 37 K and zero resistance temperature was 34 K.