Lu Ji

Nonlinear Inductance of Intrinsic Josephson Junction Arrays and Its Application to Tunable Filters

Nonlinear inductance of intrinsic Josephson junction arrays in Tl-2212 thin films has been investigated. Misaligned Tl-2212 thin films were fabricated on 20deg vicinal substrates and then patterned into long bridges by standard photolithography and wet chemical etching. Each bridge forms a large array of intrinsic Josephson junctions. We have measured the Josephson inductance of the bridges in the zero-DC-voltage state at 77 K and show that its dependence on the critical current is in a good agreement with theory. Also, we have numerically studied the nonlinearity of the Josephson inductance and present dependence of the distortion factor on the bias and AC signal currents. Based on the obtained results, we have designed a 10-pole narrow-band lumped-element filter with the tunable center frequency in the range from 1900 MHz to 1960 MHz. Simulations show that the filter has good selectivity.

Millimeter-Wave Radiation From Misaligned

We have measured the current-voltage characteristics of misaligned Tl2Ba2CaCu2O8 thin-film intrinsic Josephson junctions (IJJs) embedded in a quasi-optical resonator at 78.5 K. The substrate is regarded as a dielectric resonator to improve high-frequency electromagnetic coupling between the IJJs. The experimental result is fitted well with the electromagnetic simulation. Radiation power larger than 10 pW has been detected at 75.6 GHz. Moreover, the phenomenon of jump voltage is explored, and a possible explanation is discussed.

\hbox{Tl}_{2}\hbox{Ba}_{2}\hbox{CaCu}_{2}\hbox{O}_{8}

High quality Tl2Ba2CaCu2O8 (Tl-2212) superconducting thin films were fabricated on 2?inch double-sided CeO2 buffered r-cut sapphire substrates. The metal cerium target was used as the sputtering source to deposit the CeO2 buffer layer by the RF magnetron reactive sputtering technique, and the inter-diffusion of chemical elements between the CeO2 layer and the substrate was studied by XPS depth profiling. The crack-free Tl-2212 films with dense crystal structure and smooth morphology were homogeneous over the whole area of the film. The XRD measurement confirmed the purity of the Tl-2212 phase and the texture with c axis perpendicular to the surface of the CeO2 buffered sapphire substrate. The electrical and magnetic measurements on the Tl-2212 films showed that the transition temperature Tc was around 106?K, and the critical current densities Jc (77?K, 0?T) for both side films were 2.9 ? 0.1?MA?cm ? 2 and 1.6 ? 0.2?MA?cm ? 2, respectively. The microwave surface resistance Rs on one side film was about 390??? and on the other side 450??? at 10?GHz and 77?K.

Thin-Film Intrinsic Josephson Junctions

Two compact dual-band bandpass filters (BPFs) with closely spaced passbands are presented in this paper. Each of the filters consists of a stub loaded resonator, to which shorted lines are coupled. The ratio of the center frequencies of two passbands can be easily adjusted from 1.2 to 1.1 by changing the gap of the coupled line. In addition, seven transmission zeros (TZs) can be yielded to obtain high passband selectivity and enhance the out of band performances. As an example, two filters are designed, fabricated and measured. Both filters exhibit the merits of high passband selectivity, very low center frequency ratio, and wide stopband suppression.

Fabrication of double-sided Tl2Ba2CaCu2O8 superconducting thin films on large-area sapphire substrates

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.

DESIGN OF DUAL-BAND BANDPASS FILTER WITH CLOSELY SPACED PASSBANDS AND MULTIPLE TRANSMISSION ZEROS

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

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.

The Influence of

We fabricated (Tl0.95Bi0.2)(Sr0.8Ba0.2)2Ca2Cu3Ox (Tl, Bi-1223) superconducting thin films on a LaAlO3(001) substrate by a three-step process including magnetron sputtering and two post-annealing processes. In the post-annealing process we found that the (Tl0.95Bi0.2)(Sr0.8Ba0.2)2Ca1Cu2Ox (Tl, Bi-1212) phase formation played a very important role in obtaining the Tl, Bi-1223 superconducting phase. So, the precursors were annealed at lower temperature to form the Tl, Bi-1212 phase entirely, and then almost pure Tl, Bi-1223 phase superconducting thin films could be obtained by annealing the Tl, Bi-1212 film at higher temperature. The superconducting critical temperature Tc of the Tl, Bi-1223 film annealed in oxygen and in argon was 111 K and 107 K, respectively. The critical current density Jc (77 K, 0 T) of the Tl, Bi-1223 film annealed in oxygen was as high as 3.7 MA cm−2.

\hbox{CeO}_{2}

The preparation and properties of Tl-2212 (Tl2Ba2CaCu2O8) thin films on biaxially textured Ni substrates, buffered with CeO2/YSZ/CeO2 trilayers, have been studied. Tl-2212 thin films were grown using a two-step fabrication process that involved magnetron sputtering and low temperature post-annealing. XRD θ–2θ scans, rocking curves and -scans proved that the Tl-2212 thin films were epitaxially grown on the buffered RABiTS. The critical temperature Tc and critical current Jc (77 K, 0 T) were 103 K and 2.6 MA cm−2, respectively.

Nano-Dots Decoration on Substrates on Flux Pinning Strength in

Tl-2212 superconducting films were fabricated on r-cut sapphire substrates buffered with (00l)-oriented CeO2 films. The buffer layers were deposited by the cerium dioxide sputtering target and the RF magnetron sputtering method. The epitaxial growth of CeO2 films on r-cut sapphire substrates was obtained over a wide range of sputtering parameters, such as temperature, pressure, power and Ar/O2 ratio. The Tl-2212 films grown on these buffer layers subsequently were purely c-axis orientation. The critical transition temperature of the best film was 105.6 K, the critical current density was 2.8 MA/cm2 (77 K, 0 T) and the surface resistance was 435 μΩ (10 GHz, 77 K).