He Yu-Sheng

A study on the microwave responses of YBCO and TBCCO thin films by coplanar resonator technique

YBa2Cu3O7(YBCO) thin films have been prepared by thermal coevaporation on LaAlO3(LAO) substrates, and Tl2Ba2CaCu2O8(TBCCO) thin films are synthesized by magnetron sputtering method on LAO substrates. The transition temperature Tc is90 K for YBCO/LAO and 104 K for TBCCO/LAO. Microwave responses of the films are studied systematically by coplanar resonator technique. Energy gaps of the films obtained are Δ0 = 1.04kBTc for YBCO films and Δ0 = 0.84kBTc for TBCCO films by analysing the temperature dependence of resonant frequencies of coplanar resonator. Penetration depth at 0 K λ0 = 198 nm for YBCO films and λ0 = 200 nm for TBCCO films could also be obtained by using the weak coupling theory and two fluid theory. Results of penetration depth and energy gap confirm the weak coupling properties of the films. In addition, microwave surface resistances Rs of YBCO/LAO and TBCCO/LAO are also investigated by analysing the quality factor and insert loss of the coplanar resonator. Surface resistance of TBCCO/LAO is less than that of YBCO/LAO, so that TBCCO/LAO films may have more potential applications.

The microwave response of MgB2 /Al2O3 superconducting thin films

Microwave characteristics of MgB2/Al2O3 superconducting thin films were investigated by coplanar resonator technique. The thin films studied have different grain sizes resulting from different growth techniques. The experimental results can be described very well by a grain-size model which combines coplanar resonator theory and Josephson junction network model. It was found that the penetration depth and surface resistance of thin films with smaller grain sizes are larger than those of thin films with larger grain sizes.

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.

A scanning superconducting quantum interference device microscope for room temperature samples

We have constructed a scanning low-Tc superconducting quantum interference device (SQUID) microscope, in which the SQUID is mounted on the lower end of a copper rod and cooled to liquid helium temperature. There is a 65µm thick sapphire window under the SQUID. The sample at room temperature underneath the window can be scanned to produce magnetic images. The microscope has a spatial resolution of 100-150µm and a magnetic field sensitivity of 3-60pT/√ Hz in a magnetically unshielded environment. We have used this scanning SQUID microscope to measure various room temperature samples.

The structure-induced abnormal critical current characteristics of vortex line in Bi2212 whisker

The current-voltage characteristics in different magnetic fields are measured on a whisker (2 mm x 0.037 mm x 0.007 mm) of highly anisotropic material Bi2212 with field H parallel to c-axis and H parallel to b-axis, respectively. When H parallel to c-axis, the j(c) versus T relation remains normal for all fields. However, when the field H is along the b-axis, we obtained an abnormal j(c) versus T relation with a peak value appearing at T = 25 K in H = 4 T. The possible explanations are discussed

The high temperature superconducting filters and its application progress

Microwave filters are essential elements in many areas of RF/microwave engineering. The crowded frequency spectrum has put forward higher requirements on channel filters. Not only very high close-to-band rejections are required to prevent interference to or from closely neighboring channels, but also very low insertion loss is required to reduce system noise figure. Some RF/microwave system require in-band group-delay and amplitude flatness to minimize signal degradation even further. Surface resistance of high temperature superconducting (HTS) material is about 1000 times less than the best conventional metals such as copper and silver at microwave frequency. So a significant effort has been put into the research and realization of high performance microwave HTS filters since the discovery of HTS materials. HTS microwave filters with remarkably high performance have been constructed in the past 30 years. For example, by selecting appropriate resonators with high quality factors and employing well-designed coupling structures with week coupling coefficients, some HTS filters can have fractional band width (FBW) blow 0.1% with very high selectivity. The insertion loss of some HTS filters can be as low as 0.05 dB. With above excellent performance, HTS filters can not only eliminate unwanted interference but also minimize noise figure of the microwave system. HTS filters and their front-end subsystems have also been successfully applied to many fields and brought multiple economic returns and social benefit. It is reported that HTS receiver front-end subsystem, which integrate with narrow-band highly selective HTS filters and low noise amplifiers (LNA) in a cryocooler, has been employed in more than 10000 base stations in America. According to the promotion materials, the ultra-low-loss and high sensitivity of the HTS receiver can translate into 10%–15% or more increased base station coverage area and data throughput. In some cases carriers could add 8% or more usable resource blocks and capacity due to the high selectivity of the HTS receiver, valued at hundreds of millions of dollars. Design technology of HTS filters has been developed rapidly at an incredible speed. In recent years, many efforts have concentrated on novel HTS filter design technologies, including those in high power handling capability filters, multiplexer or multiband filters and frequency tunable filters. Some research groups are trying to develop all-HTS integrated microwave front-end receivers, which integrate HTS filter, oscillators, mixers and antenna into a whole chip. The all-HTS integrated front-end receiver could make the system compact, efficient, and be able to exploit the full potential of the superconductive components. In this paper, the research status, trend of development and application progress about HTS filters are reviewed. The best performances of HTS filters ever reached in specifications such as insertion loss, FBW, out of band rejection, skirt slope and in band group delay are firstly reviewed. Then the remarkable progress in the most recent years which stand for the research trends of HTS filter technology is presented in detail. At last the application of HTS filters and their front-end subsystems to mobile communications, space or satellite communication technology, radar detective system, deep space detection and radio astronomy is also reviewed.

The effect of current along c direction on the ab plane superconducting properties of Bi2212 crystal

Abstract We found a normal-state reentrant behavior in zero field near the critical temperature of a Bi2212 crystal when we applied current along c-axis direction and measured the ab plane voltage. A similar phenomenon is observed in a Zn substituted Bi2212 crystal. The new phenomenon may be due to destroying of the Cooper pairs in the ab plane by the injected normal electrons.