Robert B. Hammond

Epitaxial Tl2CaBa2Cu2O8 thin films with low 9.6 GHz surface resistance at high power and above 77 K

We report measurements of the dc and microwave properties of epitaxial thin films of Tl2CaBa2Cu2O8 on LaAlO3. ac magnetic susceptibility measurements yield critical temperatures at the transition midpoint Tc∼100 K and 10–90% transition widths of ∼1.0 K. We observed a critical current density of 1.06×106 A/cm2 at 77 K measured by dc transport. We measured a low‐power surface resistance of Rs=0.2 mΩ at 9.55 GHz and 77 K. This is ∼50 times lower than oxygen‐free high‐conductivity (OFHC) Cu at the same frequency and temperature. At 90 K, RS rises to 0.4 mΩ, and at 95 K, RS=0.7 mΩ. We measured the microwave power dependence of RS at 77, 90, and 95 K. At high microwave field (≳30 G, 77 K) Rs=0.8 mΩ, more than ten times lower than OFHC Cu.

Measured performance at 77 K of superconducting microstrip resonators and filters

Results on three types of passive microwave devices fabricated and tested using epitaxial thin films of Tl/sub 2/CaBa/sub 2/Cu/sub 2/O/sub 8/ grown on LaAlO/sub 3/ are reported. A microstrip ring resonator with unloaded Q of 2740 at 77 K and 33 GHz is described. A superconducting 4.6 GHz band-reject filter with unloaded Q greater than 15000 when operated at 77 K is reported. In addition, results on a multiple microstrip bandpass filter are presented. >

Clear correlations observed between YBa2Cu3O7−δ thin‐film properties and GHz microwave resonator performance

We have fabricated and measured 5 GHz microstrip resonators from a series of YBa2Cu3O7−δ thin films grown on LaAlO3(001) substrates by in situ laser ablation. We have studied the correlations between unloaded quality factor and various film properties, such as transition temperature, width of transition, critical current density, narrowness of x‐ray rocking curve, sharpness of electron channeling pattern, and most important substrate temperature during growth. We found that in general, higher transition temperature, higher critical current density, sharper transition, sharper channeling pattern, and narrower x‐ray rocking curve correlate positively with good microwave performance. The best quality factor exists for a narrow growth temperature window (around 800 °C). We also report the dependence of quality factor on device power for each film.

Preparation of superconducting Tl‐Ca‐Ba‐Cu thin films by chemical deposition

A chemical deposition process for the preparation of superconducting thin films of Tl2CaBa2Cu2O8 (2122) and Tl2Ca2Ba2Cu3O10 (2223) has been developed. Oriented, superconducting 2122 and 2223 thin films have been successfully fabricated on single‐crystal yttria‐stabilized zirconia and magnesium oxide substrates (〈100〉 orientation). Epitaxial films have been prepared on magnesium oxide. Chemical analysis of the film composition by energy dispersive x‐ray analysis is in agreement (2122 vs 2223) with the phases indicated by powder x‐ray diffraction. The thin films of these compounds exhibited superconducting transition temperatures above 105 K as determined by variable‐temperature resistivity and ac magnetic susceptibility measurements. In addition, microwave surface resistance measurements at 150 GHz show that these films have very low losses at 77 K (1 mΩ).

Microwave surface resistance in Tl‐based superconducting thin films

We report measurements of microwave surface resistance in Tl‐based superconductor thin films made by laser ablation followed by a post‐deposition thermal process. The films were measured by using cavity methods. The data at 9.5 and 148 GHz indicate that the residual resistance scales as f2. At 77 K, the 9.5 GHz surface resistance is ten times smaller than oxygen‐free high‐conductance copper at the same temperature and frequency. The 9.5 GHz measurement also indicates that the film‐substrate interface does not cause more microwave loss than the film surface.

Microwave near-field imaging of electric fields in a superconducting microstrip resonator

We describe the use of a cryogenic near-field scanning microwave microscope to image microwave electric fields from superconducting and normal-metal microstrip resonators. The microscope employs an open-ended coaxial probe and operates from 77 to 300 K in the 0.01–20 GHz frequency range with a spatial resolution of about 200 μm. We describe the operation of the system and present microwave images of Cu and Tl2Ba2CaCu2O8 microstrip resonators, showing standing wave patterns at the fundamental and second harmonic frequencies.

Frequency following imaging of electric fields from resonant superconducting devices using a scanning near-field microwave microscope

We have developed a scanning near-field microwave microscope that operates at cryogenic temperatures. Our system uses an open-ended coaxial probe with a 200 /spl mu/m inner conductor diameter and operates from 77 to 300 K in the 0.01-20 GHz frequency range. In this paper, we present microwave images of the electric field distribution above a Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ microstrip resonator at 77 K, measured at several heights. In addition, we describe the use of a frequency-following circuit to study the influence of the probe on the resonant frequency of the device.

Epitaxial Tl2Ba2CaCu2O8 thin films on LaAlO3 and their microwave device properties

Microwave data on epitaxial Tl2Ba2CaCu2O8 thin films grown on LaAlO3 substrates are reported. The authors fabricated and tested microstrip resonators using these films and performed measurements of the loss tangent of lanthanum aluminate substrates at 5 GHz and 77 K. Loss tangent values of 2 to 4X10-5 were measured. The authors extracted surface resistance for the superconducting thin films from unloaded Q measurements on microstrip resonators. They infer scaled surface resistance values of approximately 300 (mu) ohms at 10 GHz and 77 K from unloaded Q measurements at 5.6, 11.1, and 16.3 GHz. The measured surface resistances scale very closely with the square of the frequency.

High-temperature superconducting Josephson mixers from deliberate grain boundaries in Tl2CaBa2Cu2O8

Deliberate grain boundaries have been fabricated in epitaxial films of Tl2CaBa2Cu2O8 by growing the films on substrates with localized surface damage and on substrates with surface steps produced with an anisotropic etching process. Bowtie antennas were fabricated to couple millimeter wave radiation into the weak link junctions, which were used as the nonlinear elements. Heterodyne mixing of signals at 55 GHz and 94 GHz was observed in the integrated antenna/detector structures at temperatures below 101 K in the Tl-based films. In addition, observations were made at 55 GHz, using these structures as subharmonic mixers. We have designed an integrated microwave package to measure the noise and conversion efficiency of the grain-boundary junctions at frequencies between 9 GHz and 20 GHz.

Tl-Ca-Ba-Cu-O Thin Film Passive Microwave Devices

We report results on several passive microwave devices fabricated and tested using epitaxial thin films of Tl2CaBa2Cu2O8 grown on LaAlO3. We report microstrip resonators with unloaded Q’s of 15,000 at 80K and 4.83 GHz, and 1440 at 80K and 33 GHz. We report a superconducting 4.6 GHz bandreject filter with unloaded Q greater than 15,000 when operated at 77K.