X. J. Zhao

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.

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

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.

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

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

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).

An improved technique for the growth of (Tl0.95Bi0.2)(Sr0.8Ba0.2)2Ca2Cu3Ox superconducting thin films in oxygen and?argon

High-quality Tl-2212 superconducting films were prepared on CeO₂ coated r-cut sapphire substrates. An in situ two-temperature deposition process was introduced in the growth of CeO₂ buffer layers. XPS measurements showed that the deposition process can sharply decrease the mutual diffusion at the interface. The critical temperature T_c of Tl-2212 film grown on the buffer layer was 108.2 K, the critical current density J_c (77K, 0T) was above 6 MA/cm², and the microwave surface resistance R_s(77K, 10 GHz) was below 200 μΩ.

The Growth of (00l)-Oriented CeO2 Buffer Layers on R-Cut Sapphire Substrates for the Epitaxy of Tl-2212 Superconducting Films

We report off-chip self-emission at 76.9 GHz from intrinsic Josephson junction arrays in misaligned Tl2Ba2CaCu2O8 + δ thin films at 78 K. The substrate was considered as a dielectric resonator antenna to improve the coupling between arrays and electromagnetic waves. Three-dimensional numerical simulations were performed to guide and explain the experiments. Sharp radiation peaks with a maximum radiation power of 17.7 pW were observed. Furthermore, we investigated the angular dependence of the radiation power in terms of experiment combined with numerical simulations.

Fabrication of CeO2 buffer layers for epitaxial growth of Tl-2212 films by the in situ two-temperature process

Tl2Ba2CaCu2O8 (Tl-2212) films were prepared on r-cut sapphire substrates buffered with CeO2 by dc magnetron sputtering and post-annealing method. The in situ two-temperature process was used to grow CeO2 buffer layers. XRD and AFM measurements showed that CeO2 films deposited at temperature of 370-470°C were excellent c-axis orientation and had smooth surface. SEM observations demonstrated that the Tl-2212 films’ surface morphology was changed from condensing crystal structure to plate-like structure when the CeO2 deposition temperature was increased. The best Tl-2212 film’s critical temperature Tc can reach to 108.3 K, and critical current density Jc obtained at 5.33 MA/cm2 (77 K, 0 T).

Self-emission from intrinsic Josephson junction arrays in misaligned Tl2Ba2CaCu2O8 + δ thin films at 78 K

The preparation and superconductivity of Tl-2212 films grown on CeO2-buffered MgO substrates were investigated. AFM showed that the annealing of MgO substrates remarkably improved their crystal quality of surface layers, and led to the surface morphology of substrates evolving into unattached outgrowth structure with ultra-smooth surface. XRD measurements showed that CeO2 buffer layers grown on MgO annealed at the suitable conditions were pure (00l) phase. The 500 nm thick Tl-2212 films grown on these CeO2 films subsequently possessed excellent c-axis orientation and superconductivity. The best Tl-2212 film reached a low surface resistance of about 324 μΩ (10 GHz, 77 K), a critical current density as high as 3.5 MA/cm2 (77 K, 0 T) and a critical transition temperature of about 108.4 K.