Ruijuan Nie

The supercurrent spatial distribution of grain boundary junctions

Abstract We carried out a series of measurement on several different sizes of YBCO grain boundary junctions by using an improved method, in which we used the resistively shunted junction (RSJ) model to get a more accurate I c ( B ) relation regardless of the magnitude of the critical current. From the I c ( B ) dependence of the YBCO GBJs the supercurrent distribution are derived. For the latter a spatial resolution of about 0.4 μm is obtained. The measured correlation function shows that the supercurrent density across the grain boundary is spatially inhomogeneous on even μm scale.

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.

Improving on the yield and performance of high Tc rf SQUIDs

Abstract A method to control and determined the critical currents I c of step-edge junctions has been developed. Using this method, the critical currents of step-edge junctions are easily adjusted to within a range useful for obtaining high performance rf SQUID magnetometers or gradiometers.

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.

Optimizing the slew rate of SQUID system

Abstract We study optimization of the slew rate of SQUID (Superconducting QUantum Interference Device) systems both theoretically and experimentally. Considering the total equivalent noise flux input in the SQUID, both for SQUID electronics with one pole and for electronics with two poles and one zero, we get new formulas for slew rate and find the optimum bandwidth for the maximum slew rate. For SQUID electronics with one pole, the optimum bandwidth and corresponding maximum slew rate are both proportional to S Φ −1 . While for systems with two poles and one zero, the optimum bandwidth and corresponding maximum slew rate are proportional to S Φ −1 and S Φ −2 respectively. The SQUID sensors and a multi flux quanta electronic simulator are used in the experiment and the results are in good agreement with analysis.

Fabrication of low noise High-Tc bicrystal junction rf SQUID

Abstract We fabricate rf superconducting quantum interference devices (SQUIDs) using a YBCO bicrystal junction, which is made on a chip consisting of a 10×12mm2 YSZ bicrystal substrate with a 24° angle of in-plane misorientation. The magnetic field sensitivity is 84 fT/Hz 1 2 . The tank frequency at which the rf SQUID operated is near 800MHz. According to our result, the boundary across the superconducting loop does not show significant effects on the 1/f noise level.

The effect of excess current on high-c superconducting step-edge Josephson junctions

Abstract In this paper, we discuss the effect of excess current on SQUID performance, and point out that excess current may be the reason leading to a lower yield rate of rf SQUIDs.

Size effect of 1/f noise in normal state of YBa2Cu3O7☆

Abstract The low-frequency 1/f noises of bridge-shaped YBa 2 Cu 3 O 7 (YBCO) film samples are measured in temperature range of 100–300 K. The noise level of three large bridges is in the order of magnitude typical for high quality YBCO film. For three small bridges, the noise level is size-dependent. It seems that there is a characteristic length in the order of a few tens μm, below which the noise level decreases rapidly.