We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistance R[sub s] of 1 x 1 cm[sup 2] superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE[sub 01[delta]] oscillation mode the highly reproducible quality factor of about 10[sup 5] results in a sensitivity of [+-]50 [mu][Omega] for R[sub s] measurements. The temperature dependence of R[sub s] can be measured up to values as high as 1 [Omega]. We have investigated several YBa[sub 2]Cu[sub 3]O[sub 7] thin films prepared by high oxygen pressure d.c. sputtering on LaAlO[sub 3] and NdGaO[sub 3]. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivity [rho](T) with [rho](300 K)/[rho](100 K) values of about 3.7. Those films show, besides the initial fall-off just below T[sub c], a further strong decrease of R[sub s] at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. For T [much lt] T[sub c]/2 these films exhibit an exp(-[alpha] T[sub c]/T) dependence of R[sub s]more » with [alpha]-values around 0.4. These observations may be explained by a superconducting energy gap with 2[Delta]/kT[sub c][approx]0.8 for charge carriers localized in the CuO chains for YBa[sub 2]Cu[sub 3]O[sub 7]. 25 refs., 4 figs.« less

Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique

A simple technique has been developed for the high-frequency characterization of superconducting thin films. A microwave resonator is formed by sandwiching a high-purity c-axis-oriented sapphire rod between a pair of superconducting thin films. For the TE/sub 011/ resonant mode, the theoretical treatment allows for the surface resistance and the RF current density to be calculated from the unloaded Q-value and the dissipated power. This technique is especially useful since it has: no sample preparation; no calibration; great sensitivity; great accuracy; great repeatability; great dynamic range; high internal power levels with only moderate input power levels; and broad temperature coverage (4.2 to over 120 K). A round robin experiment using HTS thin films was performed to deconvolute the films' surface resistance and also to provide a statistical analysis of the method's reproducibility. The standard error for a single measurement is better than 2%. A Nb thin film was measured to verify the technique. >

A sapphire resonator for microwave characterization of superconducting thin films

We present the development, realization and setup of dielectric resonators, for the purpose of measuring the surface impedance at microwave frequencies of superconducting thin films. We focus o n resonators designed to operate in dc magnetic fields, optimized for the measurements of the variation of the surface impedance with the applied field. Two resonators, operating at 8 and 48 GHz, are presented. We discuss different approaches to the measurement of the resonator parameters, with particular attention to the nonidealities of real setups in a cryogenic environment. Finally, we present some sample measurement of high-Tc and low-Tc superconducting films.

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Dielectric Resonators for the Measurements of the Surface Impedance of Superconducting Films

To accurately determine the surface resistance of high-temperature superconducting films, multifrequency measurements of S/sub 21/, S/sub 11/, and S/sub 22/ and sophisticated data processing are required. As a result, surface resistance measurements and calculations for varying temperatures are very time consuming. In this paper, we introduce a simplified method for calculations of the unloaded Q (Q/sub 0/) factor, which require measurements of S/sub 11/ and S/sub 22/ at the lowest temperature only. For all other temperatures, only S/sub 21/ measurements are needed. The method has been shown to give sufficiently accurate Q/sub 0/ values and, hence, the surface resistance of superconducting samples, as compared to results obtained from S/sub 21/, S/sub 11/, and S/sub 22/ measurements using the transmission-mode Q factor technique. The presented method has been tested under different coupling coefficients and frequencies.

Simplified method for measurements and calculations of coupling coefficients and Q/sub 0/ factor of high-temperature superconducting dielectric resonators

Challenging microwave resonant measurement techniques for conducting material characterization

Perturbation formulas for TE/sub 011/-mode dielectric rod resonator and for a TE/sub 011/-mode circular cavity resonator are derived to determine the surface impedance Z/sub s/ of superconductors from measured values of resonant frequencies and unloaded Q. The relation between the maximum surface current density of a superconductor, J/sub s/, and output power from a signal generator is derived. On the basis of these analytical results, a measurement technique is proposed to evaluate the temperature and J/sub s/ dependencies of Z/sub s/ for superconductors. The measured results of the temperature dependence of Z/sub s/ for YBCO and copper plates are presented. From these results, it is verified that the dielectric resonator is suitable for measuring the surface reactance for YBCO. From these Z/sub s/ values the temperature dependences of the skin depth and the penetration depth and those of the complex conductivity are obtained on the basis of the two-fluid model. These measured values agree well with the theoretical curves. >

T. Imai

Papers

Microwave measurement of temperature and current dependences of surface impedance for high-T/sub c/ superconductors