Shunzhou Li

Magnetic relaxation and critical current density of the new superconductor MgB2

Magnetic relaxation rate, critical current density and transport properties have been investigated on MgB2 bulks from 1.6 K to Tc at magnetic fields up to 8 T. A vortex phase diagram is depicted based on these measurements. A large separation between the bulk irreversibility field Hirr(T) and the upper critical field Hc2(T) has been found. It is thus proposed that there is a quantum vortex liquid due to strong quantum fluctuation of vortices at 0 K. It is also found that the magnetic relaxation rate is weakly dependent on temperature but strongly dependent on field indicating a trivial influence of thermal fluctuation on the vortex depinning process. Therefore, the phase line Hirr(T) is attributed to quantum vortex melting in the rather clean system at a finite temperature.

Hole doping dependence of the coherence length in La2 − xSrxCuO4 thin films

By measuring the field and temperature dependence of magnetization on systematically doped La2 − xSrxCuO4 thin films, the critical current density jc(0) and the collective pinning energy Up(0) are determined in single-vortex creep regime. Together with the published data of superfluid density, condensation energy and anisotropy, for the first time we derive the doping dependence of the coherence length or vortex core size in wide-doping regime directly from the low-temperature data. It is found that the coherence length drops in the underdoped region and increases in the overdoped side with the increase of hole concentration. The result in the underdoped region clearly prevents from taking the pseudogap energy scale as the upper critical field.

Effects of Al doping on the superconducting and structural properties of MgB2

We have investigated the structural and superconducting properties of a series of Al doped MgB2 samples. The results show that the superconductivity of MgB2 is destroyed when Al is alloyed into MgB2 as reported previously. A structural instability was observed from the data of X-ray diffraction. Further study by TEM shows that the Al substitution leads to an ordering occupation on Mg sites. This corresponds to a superstructure with a repeat period of 2c along the c-axis. The superstructure was identified to be a form of MgAlB4 structure which intergrows with MgB2. Measurements on the low doping level samples showed that the upper critical field B-c2, irreversible field B-irr and the critical current density J(c) of these samples were reduced as the All content was increased. Analysis of the upper critical field data suggests that the density of states near the Fermi surface N(E-F) is reduced by Al doping. This is consistent with the decrease in the superconducting transition temperature T-c. The field dependence of pinning force is also discussed for these samples

A 12-Pole Narrowband Highly Selective High-Temperature Superconducting Filter for the Application in the Third-Generation Wireless Communications

An ultra-selective high-temperature superconducting bandpass filter was designed and fabricated to satisfy the demands of the third-generation wireless communications. This filter was designed to have 12-pole quasi-elliptic response with 4.68-MHz bandwidth in the third-generation communications band (a fractional bandwidth of 0.0023). Full-wave simulations were conducted by using Sonnet software. A novel, compact, and low radiation resonator with a high-quality (Q) factor value was developed to reduce the parasitical coupling. In order to satisfy the demand of high band-edge steepness, three pairs of transmission zeros at finite frequencies were introduced in the cascaded quadruplet coupling structure. The filter was fabricated on a 2-in-diameter 0.43-mm-thick sapphire wafer with double-sided YBCO films. The measurements showed that the passband center frequency (f0) of the filter is 2.038 GHz at 67 K with a midband insertion loss of 0.67 dB and the return loss better than 15 dB. The measured response of the filter also exhibited ultra-high band-edge steepness of 140-220 dB/MHz. Better than 60 dB of out-of-band rejections for frequencies very close to the band edge (|f-f0|>2.7 MHz), better than 90 dB at frequencies 7.5 MHz away from the center frequency and up to 100 dB of wideband rejections were achieved

A 24-pole high Tc superconducting filter for mobile communication applications

This paper presents a highly selective 24-pole Chebyshev HTS bandpass filter. The filter was designed at 1748 MHz centre frequency with 75 MHz bandwidth, which was suitable for the GSM base station. The filter was fabricated using double-sided YBCO films on a 3 in diameter, 0.43 mm thick sapphire substrate. The simulation of the filter performance was carried out using Sonnet software. A novel resonator structure was proposed. The resonator exhibited a very high Q value of about 30 000 at 77 K. The filter showed an excellent selectivity. At the band edge of the filter, the slope was about 17 dB MHz(-1). Out of the band, the rejection level is better than -90 dB.

Superconducting filter with a linear phase for third-generation mobile communications

A linear phase filter using two cross- coupled quadruplet structures to achieve self- equalization was designed at 2012.5 MHz with 5 MHz bandwidth for a third- generation mobile communications system. This filter was fabricated using double- sided YBCO films on a 2 inch diameter, 0.5 mm thick MgO substrate. In the measurement, it showed good matching in the passband, with reflection better than - 15 dB. Moreover, the group delay variation is less than 50 ns over 89% of the filter bandwidth.

Absence of a true vortex-glass phase above the Bragg glass line in Bi2Sr2CaCu2O8+δ

Abstract In magnetic measurements on Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) single crystals, a general peak with a dynamical feature on both S – H and S – T curves was found with S the magnetic relaxation rate. At higher fields, the characteristic exponent μ , becomes negative, together with the positive curvature of log E vs. log j and the scaling based on the 2D vortex glass (VG) theory or plastic creep theory, we conclude that the vortex motion above the second peak is plastic when j →0 and there is no VG phase at finite temperatures in Bi-2212. The peak of S is then explained as the crossover between different meta-stable vortex states.

Intrinsic percolative superconductivity in heavily overdoped high-temperature superconductors

Magnetic measurements on heavily overdoped La 2 − x Sr x CuO 4, Tl2Ba2CuO6, Bi2Sr2CuO6, Y 1 − x (Ca x)Ba2Cu3O7 − δ and Bi2Sr2CaCu2O8 single crystals reveal a new type of magnetization hysteresis loops characterized by the vanishing of the usual central peak near zero field. Since this effect has been observed in various systems with very different structural details, it probably reflects a generic behavior for all high-temperature superconductors. This easy penetration of magnetic flux can be understood in the picture of percolative superconductivity due to the inhomogeneous electronic state in heavily overdoped regime.

Dimensional crossover of vortex dynamics induced by Gd substitution on Bi2212 single crystals

The vortex dynamics of Bi2Si2Ca1-xGdxCu2O8+delta single crystals is investigated by magnetic relaxation and hysteresis measurements. By substituting Ca with Gd, it is found that the interlayer Josephson coupling is weakened and the anisotropy is increased, which leads to the change of vortex dynamics from 3D elastic to 2D plastic vortex creep. Moreover, the second magnetization peak, which can be observed in samples near the optimal doping, is absent in the strongly underdoped (with 2D vortex) region

Study on miniature pulse tube cryocooler for space application

Abstract Pulse tube refrigerator (PTR) is a new type of mechanical cryocooler with the potential of long-term operation in space. Theoretical and experimental studies are currently on the way in the Cryogenic Laboratory of Chinese Academy of Sciences (CL/CAS) in order to develop a 85 K/250 mW class pulse tube cryocooler to be used to cool space-borne infrared devices. A theoretical model is established based on the analyses of the thermodynamic behavior of gas parcels in oscillating flow regenerators. It helps us to understand the cooler and can be used to study the influence of DC flow on the refrigeration performance. The flow resistance of the regenerator is an important factor for the cooler performance. A test bench, including a hot-wire anemometer has been set up to investigate the flow resistance characteristics of regenerators with oscillating flow. The results of measurement are correlated and served for practical design. The Oxford type linear pressure wave generator with flexure bearings is also under development in CL/CAS. The prototype miniature pulse tube cryocooler, driven by a linear pressure wave generator of 1.06 cm 3 maximum swept volume, provides at present 200 mW net cooling power at 81 K with 28.4 W input power. Improvements are being made to further increase the cooling power and reduce the input power.