J. M. Wu

Microwave effects on YBa2Cu3Oy Josephson junctions with step-edge, bi-epitaxial, and sandwich geometries

Fabrication and characterization of YBa2Cu3Oy Josephson junctions with step‐edge, bi‐epitaxial, and sandwich geometries are performed in this study to investigate the effects of microwave irradiation on the I–V characteristics. I–V curves under microwave irradiation reveal Shapiro steps. The width of the constant voltage steps were modulated by the microwave power. The period of the magnetic induction ΔB detected from the V–Φ curves for superconducting quantum interference devices is temperature dependent. Additionally, Tc and Ic of the proximity sandwich junctions are enhanced by the microwave irradiation.

Effects of magnetic fields and microwave irradiation on step-edge and bi-epitaxial YBa/sub 2/Cu/sub 3/O/sub y/ SQUIDs

Current-voltage (I-V) curves of step-edge YBa/sub 2/Cu/sub 3/O/sub y/ junctions, SQUIDs, and Junctions in parallel in a magnetic field and under microwave irradiation were measured. I/sub c/-/spl Phi/ curves for a single Josephson junctions show Fraunhofer diffraction pattern. The voltage of SQUIDs and Josephson junctions in parallel at a constant bias current is modulated by external magnetic fields. In additional to integer Shapiro steps, non-integer constant-voltage steps were observed under microwave irradiation in bi-epitaxial SQUIDs. The non-integer voltage steps can be modulated by the microwave power and the applied magnetic field.

Influence of magnetic field on the flux motion in superconducting YBCO/PYBCO superlattice

Angular dependent critical current densities J/sub c/(H,/spl theta/) for the YBa/sub 2/Cu/sub 3/O/sub 7-y//Pr/sub 0.5/Y/sub 0.5/Ba/sub 2/Cu/sub 3/O/sub 7-y/ (120 /spl Aring//24 /spl Aring/)/spl times/8 superlattice under various magnetic fields were measured to investigate the influence of the applied magnetic fields on the dimensionality of the flux pinning and flux dynamics. J/sub c/(H,/spl theta/) was obtained by applying current through x- or y-direction and analyzed by Kes model. It is found that the dissipation of the flux line system in the mixed state is mainly due to the pancake motion along ab-plane as H<1 T, while the flux lines begin to move along c-axis when the magnetic field reached 1 T. The causes will be discussed.

Temperature dependent noise in YBa2Cu3Oy thin film

Abstract We measured the noise power spectral density at a constant current (3 mA in this report) and various temperatures to study the origin of the noise in the high-T c superconducting YBa 2 Cu 3 O y thin film. The range of the measuring temperature T was from 11 to 90 K. It is found that the noise power spectrum curves all overlap and decrease smoothly as the frequency f increases for T 85.6 K. Furthermore, the temperature dependent noise power density at f = 0.2 Hz can be fitted to the thermal fluctuation model. This result suggests that the noise may come from the resistance fluctuation which is caused by the thermal fluctuation. The causes will be also discussed in terms of the flux motion of the high-T c superconductors.

Saturated flux pinning in the YBa2Cu3Oy/PrBa2Cu3Oy superlattice

The temperature dependent zero-field-cooled, field-cooled and remanent moment were measured at various applied magnetic field (from 10 Oe to 4000 Oe) for the YBa2Cu3Oy/PrBa2Cu3Oy (96Å/48Å) superlattice to investigate the saturated flux pinning. The results showed that MREM=MFC−MZFC as H≤195 Oe while MREM>MFC−MZFC as H >203 Oe. This critical magnetic field (203 Oe in this report) is defined as Hs. Hs will be studied together with the penetration field H* which is obtained from both the magnetic relaxation for various applied magnetic field and the hysteresis curve. It is found that the Hs is closely related to H*. The result are discussed by using the bean’s critical state model.

Synthesis, characterization and flux pinning of YBa2Cu3Oy/Y1−xPrxBa2Cu3Oy superlattices

Growth, surface morphology and flux pinning of YBa2Cu3Oy/Y1−xPrxBa2Cu3Oy (x=1 and 0.5) superlattices were systematically investigated. The superlattices were in situ prepared using an off-axis radio frequency magnetron sputtering system. The x-ray diffraction shows satellite peaks around the main diffraction peaks. The resistivity in the superconducting mixed state shows thermal activated behavior.

Angular Dependence of the Flux Pinning For YBa2Cu3Oy/PrBa2Cu3Oy Superlattice

The angular dependence of the magnetic relaxation for YBa2Cu3Oy/PrBa2Cu3Oy (YBCO/PBCO) superlattice was measured under magnetic field to investigate the flux pinning. The applied magnetic field was 0.1 Tesla. The direction of the applied magnetic field makes an angle of 10o,20°,30o,45°... with the c-axis of YBCO/PBCO superlattice. Based on the Anderson-Kim model we derive the pinning energy of this film. The pinning energy is angular independent. The results are discussed.

Synthesis and Characterization of YBa2Cu3OY and LaSrGaOy Thin Films

The X-ray diffraction, Scanning Tunneling Microscope and Scanning Force Microscope were ultilized to characterize the surface morphology and the crystal structure of YBa2Cu3OY(YBCO) and LaSrGaOy (LSGO) thin films on MgO(OOl) and SrTiO3(001) substrates. YBCO and LSGO films were prepared by the rf magnetron sputtering and the pulse laser ablation. The X-ray diffraction pattern shows that YBCO and LSGO films on SrTiO3(001) and MgO(001) were epitaxial, with the crystal c-axis perpendicular to the plane of the substrate. Spiral dislocation and layer-cake-like morphology were observed in YBCO films. Tc of the as grown YBCO films was about 90 K with the critical current density greater than 106 A/cm2 at 77 K.