Shaolin Yan

The growth and superconducting properties of epitaxial Tl2Ba2CaCu2O8 thin films

Epitaxial Tl2Ba2CaCu2O8 thin films have been prepared on (001) LaAlO3 substrates by DC magnetron sputtering and a post-annealing process. X-ray diffraction (XRD) theta -2 theta scans, phi scans and rocking curves proved that the thin films are strongly textured with the c axis parallel to the c axis of the substrate and epitaxially grown on the substrate. A critical current density of 7.6*106 A cm-2 at 77 K has been obtained in zero magnetic field, and the films have strong flux pinning force in high magnetic fields. When magnetic fields were applied perpendicular to the Cu-O plane of the film, the critical current density dropped only by a factor of seven in a 4 T magnetic field and a factor of 15 in a 6.5 T magnetic field.

The design of a linear phase superconducting filter with quasi-elliptic response

This paper presents the design of a linear phase superconducting filter with quasi-elliptic response. The coupling structure of the filter contains two trisections and one quadruplet. The two trisections are applied to generate two independent transmission zeros for high selectivity and are realized by meandered open-loop microstrip resonators; the quadruplet is applied for phase equalization and realized by four novel L-shaped microstrip resonators. The filter is designed at 1950 MHz with a bandwidth of 20 MHz. It is fabricated on a LaAlO3 wafer with double-coated Tl2Ba2CaCu2O8 films. The filter shows good selectivity together with flat group delay over 80% of the passband both in the simulation and measurements.

Flux dynamics and vortex phase diagram determined on a ring-shaped Tl2Ba2CaCu2O8 thin film

Abstract By using a sensitive torque magnetometer, the current–voltage (I–V or E–j) characteristics have been determined for a ring-shaped Tl2Ba2CaCu2O8 thin film at temperatures between 4.2 K and 70 K in magnetic fields up to 6 T. In addition to the field-induced vanishing of the vortex glass transition temperature above the crossover field Bcr≈0.7 T as reported earlier [H.-H. Wen, A.Th. Hoekstra, R. Griessen, S.L. Yan, L. Fang, M.S. Si, Phys. Rev. Lett. 79 (1997) 1559], a detailed and systematic analysis performed on these data reveals two new discoveries: (1) An anomalous peak appears on the curve T/Q vs. T below 20 K for all fields below 0.7 T (with Q the normalized dynamical relaxation rate); (2) The E–j curves measured at magnetic fields above 0.7 T can be described by the 2D vortex glass theory with the derived parameters v2D, p and T0 being consistent with those derived from recent transport measurement [H.-H. Wen, H.A. Radovan, F.M. Kamm, P. Ziemann, S.L. Yan, L. Fang, M.S. Si, Phys. Rev. Lett. 80 (1998) 3859]. We attribute the low temperature anomalous peak of T/Q vs. T to the crossover between different collective pinning regimes. The new discoveries further corroborate the field-induced decoupling at Bcr. Finally, we attempt to give a vortex phase diagram based on this analysis.

Critical currents in Tl-2212 and Tl-2223 thin films

Abstract The critical currents of Tl-2212 and Tl-2223 thin films grown by sputtering and post annealing on LaAlO3 have been measured as a function of the angle between the magnetic field and the c-axis at various temperatures and magnetic fields. The results show higher critical current densities in Tl-2223 than in Tl-2212 at low temperatures. Moreover, Jc is strongly field dependent in both films, suggesting a weak pinning structure. Scaling of Jcab(θ) with the c-axis component of the applied field is observed at all temperatures in both films, which confirms the 2D-nature of the flux lines. Furthermore, the thickness dependence of Jc was investigated for Tl-2212. The effect of fast neutron and Au ion irradiation has been investigated on both films. Large enhancements of flux pinning and Jc are observed, in particular at high temperatures, without significant changes of Tc. STM studies of the surface morphology and of the growth mechanism are also presented. Steps of unexpected height are observed on the surface of both films. The surface steps are shown to develop as a consequence of different stacking sequences. A striking feature in Tl-2223 is the occurrence of blocks with non-oriented terraces. These blocks are attributed to the growth of other phases in the films.

A computer-aided method for postproduction tuning of HTS filters

The empirical tuning of high temperature superconducting (HTS) filters is usually time-consuming and expensive. A computer-aided tuning method is presented in this paper. By means of optimization, the method diagnoses detuning of individual resonators quantitatively in the tuning process, and then by using neural network models the method is able to give the operator a decision on how much the corresponding tuning screws should be tuned. So, compared with the empirical tuning method, this method can be used to tune the HTS filters accurately with less time and cost.

Study on chaotic behaviors of RCLSJ model Josephson junctions

Chaotic behaviors of the dc-biased resistively-capacitively-inductively shunted Josephson junctions are studied numerically. The existence of the chaos is proved by the spectrum and strange attractor. We also find out the route to chaos is intermittence. The parameter space in which chaos exits is obtained, and different features of the chaos in different parameter range are also given.

The Influence of CeO2 Nano-Dots Decoration on Substrates on Flux Pinning Strength in Tl2Ba2CaCu2O8 Thin Films

Nano-sized CeO₂ dots were deposited on (001) LaAlO₃ substrate by rf magnetron sputtering. After the processing of the artificial decoration defects on the substrate, 100-nm, 150-nm, 200-nm, and 400-nm-thick Tl₂Ba₂CaCu₂O₈ (Tl-2212) thin films were grown on it, using dc magnetron sputtering and post annealing process. AFM images showed that the CeO₂ nano-dots were distributed well on the substrate surface with 70 nm in diameter and 5 nm in height, and the density of nano-dots was 25±3 dots/μm². SEM images were used to observe the variations on film surface. Transport critical current density J_c was measured both on Tl-2212 film with nano-dots decorated substrate and untreated single crystal LaAlO₃ substrate. From the results, J_c values of Tl-2212 films with nano-dots decorated substrate were lifted significantly, and the pinning strength was improved on Tl-2212 films by nanostructure defects induced from nano-dots.

The Influence of

Nano-sized CeO₂ dots were deposited on (001) LaAlO₃ substrate by rf magnetron sputtering. After the processing of the artificial decoration defects on the substrate, 100-nm, 150-nm, 200-nm, and 400-nm-thick Tl₂Ba₂CaCu₂O₈ (Tl-2212) thin films were grown on it, using dc magnetron sputtering and post annealing process. AFM images showed that the CeO₂ nano-dots were distributed well on the substrate surface with 70 nm in diameter and 5 nm in height, and the density of nano-dots was 25±3 dots/μm². SEM images were used to observe the variations on film surface. Transport critical current density J_c was measured both on Tl-2212 film with nano-dots decorated substrate and untreated single crystal LaAlO₃ substrate. From the results, J_c values of Tl-2212 films with nano-dots decorated substrate were lifted significantly, and the pinning strength was improved on Tl-2212 films by nanostructure defects induced from nano-dots.

\hbox{CeO}_{2}

Tl2Ba2CaCu2Ox (Tl-2212) thin films with enhanced transport critical current densities Jc were fabricated by using conventional dc sputtering and a post-annealing process. In order to employ artificial defects to improve the flux pinning strength, nano-sized CeO2 dots were deposited on the LaAlO3(001) substrates by radiofrequency (rf) magnetron sputtering from a cerium metal target prior to the growth of Tl-2212 films. The critical current density Jc (77 K) of the Tl-2212 thin film on the nanodot-introduced substrate reaches 6 × 106 A cm − 2 at self-field, which is 1.7 times larger than that on the normal substrate. The enhancement of the Jc in Tl-2212 film grown on the substrate with artificial defects is more obvious in magnetic fields. At 0.5 T dc magnetic field, a Jc (77 K) of 3.5 × 104 A cm − 2 is observed, which is about 13 times larger than that on the normal substrate.

Nano-Dots Decoration on Substrates on Flux Pinning Strength in

Epitaxial Ti2Ba2CaCu2Ox thin film dc superconducting quantum interference devices (dc SQUIDs) have been fabricated on bicrystal SrTiO3 (STO) substrates. By using the Tl-2212 single phase film, a flux noise density, S-Phi(1/2), of 2.0 x 10(-5)Phi(0) Hz(-1/2) at liquid nitrogen temperature was obtained in the white noise region, which is more than one order of magnitude lower than previous Tl-based SQUIDs made by multiphase thin films. The Tl-2212 thin film bicrystal grain boundary Josephson junctions have demonstrated resistively shunted junction (RSJ) behaviour. The characteristic voltage, VC, which is the product of critical current I-C and junction resistance R-N, was 45 muV at 95 K. The dependence of the critical current on temperature near T-C ( 105 K) was measured as I-C(T) similar to (1 - T / T-C)(1.5-1.98). The dc SQUIDs can operate at temperatures up to 99 K.