M. He

Enhanced superconductivity in TlBa2CaCu2Ox thin films by the variation of oxygen content

Enhanced superconducting properties in TlBa2CaCu2Ox (Tl-1212) thin films have been achieved by extra low-temperature annealing in pure argon. The oxygen content could be reduced from the more oxygen-rich film by the latter process. As the oxygen content approached an optimal value, the superconductivity of the Tl-1212 films could be significantly improved. The optimized critical temperature Tc of Tl-1212 thin films was up to 100 K and the critical current density Jc could be enhanced to 1.85×106 A cm-2 at 77 K. Slight changes occurred in the crystal lattice constant and the surface morphology correlated to the improved Tc and Jc.

A method for self-radiation of Josephson junction arrays

We proposed a novel approach to self-radiation of Josephson junction arrays. Bicrystal Josephson junction arrays fabricated on a bicrystal substrate were integrated with a half-wavelength dipole array and embedded in a Fabry–Perot resonator. Considered as a dielectric resonator, the substrate could be stimulated to resonate in an appropriate mode at the same frequency as the Fabry–Perot resonator, which realized a coupling between Josephson junctions as well as a coupling between the junctions and microwave structures by means of a resonant interaction between junctions and resonators. Both simulations and experiments were performed. A YBCO bicrystal Josephson junction array with 166 junctions was measured at liquid nitrogen temperature. The maximal self-radiation power of 10 pW was detected at 75.2 GHz, which was in agreement with the simulation results.

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.

Meandering bicrystal Josephson junction arrays in a hemispherical Fabry?Perot resonator

We have explored the coupling mechanism of a millimeter wave radiation to bicrystal Josephson junction arrays embedded in a Fabry–Perot resonator. In order to optimize the system, three-dimensional simulations and relative experiments were performed. By analyzing the results of computer simulations and experiments, we found that the layout of the array and the distance between the array and the plane mirror should minimize the effects on the electrical field E distribution in the fundamental mode of the Fabry–Perot resonator without the sample. In this case, the uniformity of the E field in the bicrystal arrays was demonstrated together with an improvement in the coupling efficiency of the mm wave irradiation to high-temperature superconductor arrays of Josephson junctions.

Improved coupling of Josephson junction arrays to the open space

The substrates of Josephson junctions have recently been considered as dielectric resonator antennas in order to increase self-emission power from the junctions. Further investigations and interesting phenomena demonstrating this mechanism are presented in this paper. Junction arrays of the same layout were located on various substrates, and Shapiro steps and self-emission were studied in the course of the experiments and simulations. It is demonstrated that suitable substrates can be utilized to improve the coupling of the junctions to the open space for both irradiation and self-emission at the same time.

Anisotropic phase diagram at higher temperature for Tl2Ba2CaCu2O8 thin film

By measuring the temperature dependence of the magnetoresistivity of Tl2Ba2CaCu2O8 thin films, two superconducting transitions were observed for field orientations in the H⊥ab- and H∥ab-planes. The transition at the high temperature (Tch) corresponds to a steep peak in the derivative of the effective activation energy and shifts very moderately with increasing magnetic field. The transition is in good agreement with the value of the upper critical field derived from critical fluctuation theory. Meanwhile, the transition at low temperature (Tcl) was associated with the Josephson coupling and strong intrinsic pinning for the two field orientations, respectively. This is consistent with the different magnetic field dependence of the activation energy calculated from the experimental data between Tch and Tcl.

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}

The dynamics of capacitively coupled intrinsic Josephson junction arrays with a McCumber parameter βc<1 driven by a dc current are investigated by using numerical simulations. For the first time, spatiotemporal chaotic behaviors are found. Analysis of the dynamics of individual junctions shows that the chaos is caused by the strong diffusive coupling between the junctions in the array. The chaotic regions of different parameters are also presented, from which the conditions for chaos are deduced. The properties of the array shunted by an LCR series resonance circuit are also investigated. The results show that the spatiotemporal chaos can be effectively controlled by the shunted LCR circuit. This presents a new spatiotemporal chaos control method.

Thin-Film Intrinsic Josephson Junctions

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.

Spatiotemporal chaos in capacitively coupled intrinsic Josephson junction arrays and control by a shunted LCR series resonance circuit

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.