Very low surface resistance Tl2Ba2CaCu2O8 films of thickness 1.0 μm on (100) LaAlO3 were fabricated and characterized. The films are highly c-axis oriented (x-ray diffraction rocking curve full width half maximum of 0.68°) with good in-plane epitaxy with respect to the substrate. The Tc of the films was 107.6±0.5 K as measured by ac magnetic susceptibility. The surface resistance at 10 GHz was measured with the parallel plate resonator technique and found to be 23±5 μΩ at 4.2 K, 130±20 μΩ at 77 K, and 300±60 μΩ at 95 K. Using a 20 GHz TE011 end-wall replacement copper cavity, the surface resistance was found to be comparable to that of copper, about 18 mΩ, at 104±1 K. The surface resistance of the films is significantly lower than that of all films reported to date at temperatures above 70 K and only slightly higher than the best high-temperature superconductor films reported to date at 4.2 K.
The nonlinearity of microwave surface resistance of high temperature superconductors (HTS) is of considerable interest. We report the first measurements of the Jrd order intercept (TOI) from harmonic generation for HTS. The nonlinearities were measured for a series of coplanar transmission lines composed of ThBa2CaCu20~ or YBa2Cu307. At a fundamental frequency of 1.4 GHz, the TO1 for a 28 p n wide by 5 mm long T I Z B~Z C~C I I Z O~ transmission line was: +52 dBm at 100 K; +68 dBm at 90 K; +72 dBm at 80 K; and s+80 dBm at 50 K. This narrow transmission line handled up to 4 Watts of cw power at 80 K which corresponds to an average rf current density of 2.2 x lo6 A h Z . The 3rd harmonic exhibited the expected slope 3 on a log log scale at both low and high powers and a shift of the TO1 was observed at some intermediate power. The scaling rules for power handling from changes of a T12Ba~CaCu208 coplanar transmission line width, length and temperature were also derived. These data suggest a redistribution of the rf current within the transmission line across its width at high powers. A pronounced TO1 shift was not observed for YBa2Cu307 coplanar transmission lines.
This work develops two techniques for optically probing the spatial profile of microwave frequency superconducting currents. As an application, we measured the effects of high microwave powers on the spatial distribution of current on coplanar superconducting transmission lines fabricated using YBa2Cu3O7−δ and Tl2Ba2CaCu2O8. For both techniques, a focused light-spot served as the spatial probe whose effect was measured through the change in transmitted microwave power. For resonant geometries, the change was due to the kinetic-inductance bolometric effect; for nonresonant geometries, the change was due to the resistive-transition bolometric effect. Kinetic-inductance photoresponse measurements were acquired by setting the microwave frequency on the shoulder of a resonance and measuring the change in the microwave power transmitted through the device that occurred when the absorption of light shifted the frequency of the resonance. These kinetic-inductance measurements were performed as a function of microwave power on a YBa2Cu3O7−δ coplanar device at 74 K and on a Tl2Ba2CaCu2O8 coplanar device at 80 K. Because the photoresponse in this technique is proportional to the square of the local current density underneath the light spot (as opposed to directly proportional), this technique is sensitive to current redistribution on length scales much smaller than the ∼6 μm spatial resolution of our measurements. Extrinsic (defect and grain boundary associated) and intrinsic photoresponses were measured. Both coplanar samples showed no change in the intrinsic spatial distribution of the current as the microwave power was varied. At all temperatures the Tl2Ba2CaCu2O8 sample exhibited substantial extrinsic spatial variations on a distance scale equal to the film’s ∼5 μm grain size; these spatial variations became more dramatic as the temperature T approached the critical temperature Tc. The spatial variations for the finer-grained YBa2Cu3O7 sample were much less pronounced. Behavior consistent with Tc being several degrees lower at the film edges was observed as T approached Tc; the edge-current photoresponse started to narrow, increase faster than linearly with light power, and move away from the film edges toward the center of the strip; lowering the light power lessened these effects. Resistive-bolometric photoresponse measurements performed at temperatures within the resistive transition exhibited the same light power effects near Tc. The kinetic-inductance photoresponse can also be used as a probe of the local quality of unpatterned superconducting films; for this usage an unpatterned film serves as one wall of a resonant cavity and a focused light beam is scanned through the transparent substrate onto the underside of the superconducting film while monitoring the shift in the cavity resonance.
BaTiO 3 (BTO) thin film capacitors were deposited on Ni substrate using rf magnetron sputtering. Microstructural studies by x-ray diffraction and transmission electron microscopy reveal that the as-grown BTO films have the columnar structure and the good interface structure. Postdeposition anneal results in the recrystallization of BTO films. The electrical measurements exhibit that the BTO films have high capacitance densities and low leakage currents.
Epitaxial Tl2Ba2CaCu2O8 films of thickness 0.65±0.05 μm and Tc of 105±1 K were prepared on (100) LaAlO3 through a two-step post-deposition thallination process and patterned by standard photolithographic techniques and ion beam milling. Using the voltage per unit length criteria Ec=1.0 μV/cm, transport critical current density Jc in zero applied field for a 1.8-m-long, 12±1-μm-wide meander line separated by 8±1 μm spaces was measured to be 1.04×107 A/cm2 at 20 K, 1.82×106 A/cm2 at 80 K, and 1.02×105 A/cm2 at 100 K. The uniformity in Jc was measured for eight line segments of about 11.7 cm length, yielding variations in Jc of 1.44–3.02×106 A/cm2 at 80 K. Jc values independent of linewidth were also measured for three 0.7-cm-long lines with widths of 7, 27, and 52 μm. For design of electronic circuits, resistivity may be a more useful design parameter than Jc, and detailed measurements of resistivity ρ as a function of current density J were carried out. At low temperatures (T/Tc<0.2), ρ increased by about two orders of magnitude for a 10% increase in J, and Jc is well defined. At high temperatures (T/Tc≳0.7), ρ is less strongly dependent on J near Jc. At 90 K, where Jc=7.6×105 A/cm2, ρ remained less than 10−10 Ω cm (3000× less than oxygen free high conductivity copper at 90 K) even for J=1.4×106 A/cm2. The results suggest the potential for the use of patterned Tl2Ba2CaCu2O8 films in high Jc electronic applications such as chip-to-chip interconnects operating at temperatures below 90 K.
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