Ji Lu

Coupling of a Tl2Ba2CaCu2O8 Thin Film Intrinsic Josephson Junction and a Fabry—Perot Resonator

We study the electromagnetic coupling mechanism of a Tl2Ba2CaCu2O8 (Tl-2212) thin film intrinsic Josephson junction to a hemispherical Fabry—Perot resonator. An effective model to analyze coupling mechanism is put forward. The dielectric substrate is used as a dielectric resonator antenna and the Josephson junction, and a superconducting film is used as the feed line to excite a resonance mode inside the dielectric resonator antenna. To confirm this method, two Josephson junction samples with different dimensions of substrate and shapes of superconducting film are fabricated and tested under microwave irradiations. At the same time, numerical simulations of the antenna characteristics and the field distribution of these samples are performed by numerical simulation. The different coupling intensities of the two samples with the Fabry—Perot resonator fit well with the numerical simulation results. The proposed model is important for Josephson junctions used in the microwave field.

Formation of epitaxial Tl2Ba2Ca2Cu3O10 superconducting films by dc-magnetron sputtering and triple post-annealing method

For obtaining pure phase Tl2Ba2Ca2Cu3O10 (Tl-2223) films with good superconducting properties, the growth technique is improved by dc magnetron sputtering and a triple post-annealing process. The triple post-annealing process comprises annealing twice in argon and once in oxygen at different temperatures. In the first low-temperature annealing phase in argon, Tl2Ba2CaCu2O8 (Tl-2212) is obtained to effectively minimize evaporation in the next step. With the increase of temperature in the second annealing stage in argon, the previously prepared Tl-2212 inter-phase is converted into Tl-2223 phase. An additional annealing in oxygen is also adopted to improve the properties of Tl-2223 films, each containing an optimal oxygen content value. The results of X-ray diffraction (XRD) θ–2θ scans, ω scans and rotational scans show that each of the Tl-2223 films has a high phase purity and an epitaxial structure. Smooth films are observed by scanning electron microscopy (SEM). The critical temperatures Tc of the films are measured to be about 120 K and the critical current densities Jc can reach 4.0 MA/cm2 at 77 K at self field.

Energy gap suppression and excess current in Tl2Ba2CaCu2O8 intrinsic Josephson junctions

Intrinsic Josephson junctions in misaligned Tl2Ba2CaCu2O8 thin film were fabricated on LaAlO3 substrate. The temperature dependence of the critical current is investigated around liquid nitrogen temperature. In the current voltage characteristic, large voltage jump and lack of resistive branch are observed, which shows good consistency with the intrinsic Josephson junctions. By analyzing the large gap voltage in the curve, great suppression of the energy gap is found. Through discussing the temperature dependence of the gap voltage in liquid nitrogen temperature, it is shown that this phenomenon can be caused by the non-equilibrium quasiparticle injection. The temperature influence on the excess current also confirms the non-equilibrium effect.

Characteristics of Off-Chip Millimeter-Wave Radiation from Serial Josephson Junction Arrays

We investigate the self-emissions from serial high-temperature superconductor bicrystal Josephson junction arrays embedded in a quasi-optical resonator. A bicrystal substrate is used as a dielectric resonator antenna, which increases the coupling strength between the junction array and the electromagnetic (EM) wave. Both three-dimension (3D) electromagnetic simulations and experiments are performed. Strong off-chip radiations are measured from the junction array at 78 GHz and 78 K. The proposed method and the experimental results are important for millimeter wave applications in junction arrays.

Fabrication and Properties of Double-Side Tl2Ba2CaCu2O8 Thin Film on CeO2 Buffered Sapphire Substrate

The double-side Tl2 Ba2 CaCu2O8 (Tl-2212) superconducting thin films were fabricated on CeO2 buffered sapphire substrates. The reactive magnetron sputtering technique was used to grow CeO2 buffer thin films on sapphire substrates. Making use of the metal cerium as a sputtering source, the depositing rate is much higher compared with the CeO2 target. The Tl-2212 thin films on CeO2 buffered sapphire substrates were fabricated by a dc magnetron sputtering and post-annealing process. The x-ray diffraction indicates that the thin film is pure Tl-2212 phase with the c-axis perpendicular to the substrate surfaces, and epitaxially grown on the CeO2 buffered sapphire. The critical transition temperature Tc is around 106K, the critical current density Jc is around 3.5MA/cm2 at 77K, and the microwave surface resistance Rs at 77K and 10 GHz of the film is as low as 390 μΩ.

Inductance of Long Intrinsic Josephson Junction Arrays Composed of Misaligned Tl2Ba2CaCu2O8 Thin Films

We observe and measure the inductance of long intrinsic Josephson junction arrays composed of misaligned Tl2Ba2CaCu2O8 thin films grown on LaAlO3 substrates. The array consists of about 9.1×103 intrinsic Josephson junctions, where 90° phase shift between ac voltage across the array and ac current flowing through has been measured. Furthermore, the voltage is proportional to the frequency of the current. The measured inductance values of the intrinsic Josephson junction arrays are basically consistent with the theoretically calculated results, confirming that the inductance is mainly due to the Josephson effect. The dependence of the array inductance on its critical current is also discussed.