Yang Qian-Sheng

Electron tunneling spectroscopy of Bi2Ca1Sr2Cu2Oy superconductor

Abstract We investigated the electron tunneling spectroscopy of single crystal Bi2Ca1Sr2Cu2Oy superconductor of Tc=84 K, using a point contact system. A well-formed gap structure has been observed at liquid helium temperature. In the analysis of our data, we found that an excellent fit to the experimental dI dV curve could be achieved by assuming the density of states of the form N(E)=Re [ E E 2 −Δ 2 ], where Δ is complex and independent of energy. A parallel conductance due to the shunt resistance other than NS tunneling mechanism has been considered and the least square calculation resulted in a reduced energy gap of 2Δ = 5.0 . Discussions are presented.

Superconducting film injected by ions from pulsed energetic dense plasma source

The oxygen and silicon ions have been obtained respectively from pulsed energetic dense oxygen plasma and silane plasma generated by electrodeless discharge. The oxygen ions have been injected into superconducting Nb films and the Si ions into superconducting YBCO films in order to investigate the variation of their superconductivity with the ions injected into them. Auger profile data show that the injection depths range from 20 to 40nm in the films, depending on the injection condition and film material. The resistance-temperature relations (R-T curves) indicate that the superconductivity remains unchanged in the photoresist-masked part of the film, but is significantly changed in the exposed part. The evenness of the film surface remains unchanged after injection. This technique may serve as an alternative to the planar inhibiting fabrication technique in the fabrication of the multi-layer structure of superconducting films and also possibly to the conventional plasma source ion implantation technique in material surface processing.

A novel filter scheme of data processing for SQUID-based magnetocardiogram

We present a new filter scheme for magnetocardiogram (MCG) signal processing based on the quasi-periodic characteristic of the signals. The key points of this scheme are to determine the exact numbers of data points in each cardiac cycle by using electrocardiogram (ECG) data acquired simultaneously with the MCG signal and to normalize the MCG data sequence in each cycle into an identical length. Compared with conventional filters, the scheme has the advantage of more powerful noise suppression with less signal distortion. The desire for having high quality output signals from raw MCG data acquired in a simple shielded room or even in unshielded environment may be realized with the scheme.

Femtosecond Optical Detection of Quasiparticle Dynamics in Single-Crystal Bi2Sr2CaCu2O8+δ

Quasiparticle dynamics of an optimally doped Bi2Sr2CaCu2O8+δ single crystal is investigated by the femtosecond pump-probe technique. Temperature dependences of amplitude of the photoinduced differential reflectivity and the relaxation time show the evidence of strong phonon bottleneck. The experimental results are analysed by the Rothwarf–Taylor model.

An economical magnetocardiogram system based on high-T-c SQUIDs

An economical magnetocardiogram (MCG) system is built in our laboratory. It mainly consists of a MCG data acquisition stage equipped with two high-Tc superconducting quantum interference device (SQUID) magnetometers, a data processing stage with digital filtering and a one-layer μ-metal magnetically shielded room in conjunction with a high-Tc SQUID based active compensation. Experimental results show that a noise level of pico-tesla in MCG profiles, which is necessary for clinical applications, may be achieved with the system. Moreover, stable and convenient operations of the system are demonstrated with simulating MCG measurements.

Fabrication of Intrinsic Josephson Junctions on Bi2Sr2CaCu2O8+? Single Crystals with Reproducible Surface Junctions

Intrinsic Josephson junctions on Bi2Sr2CaCu2O8+δ single crystals are successfully fabricated using photolithography and Ar ion milling. Here we discuss the properties of the surface CuO2/metal bilayers and surface junctions prepared with different crystal cleavage conditions and metal film deposition techniques. We show that, by cleaving the crystal at liquid nitrogen temperature in vacuum, the contents of the (interstitial) oxygen in the Bi-O layers become stable upon cleavage, leading to a CuO2/metal bilayer and to surface junctions with reproducible properties. These results can be useful for practical device fabrications as well as for the studies of the contact properties between high Tc superconductors and normal metals.

Analysis of Step Etching on SrTiO3 Substrates for the Step-Edge YBCO Josephson Junctions

A way to determine some important etching parameters in the step fabrication for high Tc step-edge Josephson junctions is described based on an analysis of the dynamics of the etching process. The optimum thickness of the etch mask is defined with negligible recession of the mask edge during the etch. Under this condition, the equilibrium angle of the steps etched on the SrTiO3 (STO) substrate with an Nb mask has been calculated to be about 76°. With optimized mask thickness, its sharp sidewall and straight edge, high-quality steps on STO substrates with step height from 200-300 nm and step angle above 70° are made. Josephson junctions and dc-SQUIDs with high reproducibility and less parameter scatter are obtained on the step substrates. PACS: 74. 50. +r, 74. 76. Bz, 85. 25. Dq

Research on high-T-c SQUID based non-destructive evaluation

A non-destructive evaluation system based on high-Tc dc-SQUID (superconducting quantum interference device) incorporating a gradient field excitation has been built. By using this system a 1mm-diameter hole at a depth of 2mm inside an aluminium plate at room temperature can be easily detected and imaged in an unshielded environment. The relation between the spatial resolution, or the smallest detectable flaw size and experimental parameters is briefly analysed in terms of a simple metal ring model. The result shows that the spatial resolution depends strongly on the sensor-sample separation as well as on some other parameters, such as signal-to-noise ratio of excitation, excitation frequency and material conductivity.A non-destructive evaluation system based on high-Tc dc-SQUID (superconducting quantum interference device) incorporating a gradient field excitation has been built. By using this system a 1mm-diameter hole at a depth of 2mm inside an aluminium plate at room temperature can be easily detected and imaged in an uushielded environment. The relation between the spatial resolution, or the smallest detectable flaw size, and experimental parameters is briefly analysed in terms of a simple metal ring model. The result shows that the spatial resolution depends strongly on the sensor-sample separation as well as on some other parameters, such as signal-to-noise ratio of excitation, excitation frequency and material conductivity.

Magnetic field induced phase branches of the superconducting transition in two-dimensional square π-loop arrays

Based on the results of explicit forms of free energy density for each possible arrangement of magnetization fluxes in large-scale two-dimensional (2D) square π-loop arrays given by Li et al [2007 Chin. Phys. 16 1450], the field-cooled superconducting phase transition is further investigated by analysing the free energy of the arrays with a simplified symmetrical model. Our analytical result is exactly the same as that obtained in Lis paper by means of numerical calculations. It is shown that the phase transition splits into two branches with either ferromagnetic or anti-ferromagnetic flux ordering, which depends periodically on the strength of external magnetic flux e through each loop and monotonically on the screen parameter β of the loops in the arrays. In principle, the diagram of the phase branches is similar to that of its one-dimensional counterpart. The influence of thermal fluctuation on the flux ordering during the transition from normal to superconducting states of the π-loop arrays is also discussed.

Microwave Response of Bi2Sr2CaCu2O8+δ Surface Intrinsic Josephson Junctions with Bending Pancake Vortices

We study the microwave response of surface intrinsic Josephson junctions on Bi2Sr2CaCu2O8+δ, in which bending pancake vortex lines are introduced in a controllable way. It is found that the bending vortices can greatly influence the response. In some cases, typical Shapiro steps that lie far above the quasiparticle branch are observed, with the step interval satisfying the Josephson relation and their amplitude versus the square root of microwave power following the Bessel function behaviour. In the other cases, current steps that lie on the quasiparticle branch are observed, but only one or two steps appear at the same time under the variation of the microwave power.