We present coplanar resonator measurements of the nonlinear microwave surface impedance of laser ablated and electron-beam coevaporated YBa2Cu3O7−δ thin films at 8 GHz in zero and applied magnetic fields, including the effects of irradiation with heavy ions. Correlations between the changes in surface resistance and reactance as a function of microwave current suggest that there are different contributions to the nonlinear behavior at low and high microwave currents in both zero and applied fields. In zero field, we suggest that microwave-induced flux lines are responsible for the observed nonlinear behavior, resulting in thermal runaway at large, sample dependent, microwave currents. The observed nonlinearity completely masks any possible contribution from the intrinsic pair breaking expected for a d-wave superconductor. In a magnetic field at relatively low currents, the nonlinearity can be described by the nonlinear dynamics of flux lines pinned by defects with a continuous range of pinning strengths. At higher currents, microwave field nucleation of flux lines is again considered important. On irradiation with 3.6 MeV/nucleon Xe-129 ions, the microwave losses and nonlinearity are significantly decreased in both zero and applied fields, which could have important implications for microwave device applications.

Striking features have been found in the nonlinear microwave (8.0 GHz) surface impedance Zs = Rs +j ·Xs of highquality YBaCuO thin films with comparable low power characteristics (Rres ∼35-60 µΩ and λL(15 K) ∼130-260 nm). The surface resistance Rs is found to increase, decrease or remain independent of the microwave field H rf (up to 60 mT) at different temperatures and for different samples. However, the surface reactance Xs always follows the same functional form. Mechanisms which may be responsible for the observed variations in Rs and Xs are briefly discussed.Measurements of the nonlinear microwave surface impedance, Z s , of high-temperature superconductors (HTS) is a powerful tool for studying non-equilibrium processes in these materials. Nonlinear impedance measurements allow one to investigate peculiarities of the rfvortex nucleation, and to study the vortex dynamics at elevated microwave fields. Such measurements may also discriminate between d-wave and s-wave mechanisms of pairing symmetry in HTS, and indicate the presence of magnetic impurities in the materials 1,2 .In the present paper, we report observations of nonmonotonous behavior of R s and the penetration depth, λ (or, equivalently, the surface reactance X s = ωµ 0 λ), of high-quality epitaxial YBaCuO thin films, in microwave fields up to 60 kA/m (∼700 Oe) using the coplanar resonator technique 3 at 8 GHz. For all samples, depending on temperature T , R s demonstrates completely different behavior, whereas λ always preserves the same H rfdependence, irrespective of sample and T . Measurements are presented for very high quality samples over a wide temperature range (12-75 K) which at first time reveal non-monotonous and uncorrelated behavior in R s and X s as a function of H rf . Such a behavior does not agree with any of the existing models for the nonlinear microwave impedance 4-9 . In the following we discuss several mechanisms relevant to these observations. The films are deposited by e-beam co-evaporation onto polished (001)-orientated MgO single crystal substrates 10 × 10 mm 2 . The films are 350 nm thick. The c-axis misalignment of the films are typically less than 1%, and the dc critical current density J c at 77 K is around 2 · 10 6 A/cm 2 . More detailed information on the growth technique can be found in Ref. [ 10]. The values of R s and λ at 15 K are 60, 35, 50 µΩ and 260, 210, 135 nm for samples TF1, TF2 and TF3, respectively. Changes in R s and X s with H rf , ∆R s = R s (H rf ) − R s (0) and ∆X s = X s (H rf ) − X s (0), are plotted in fig. 1 and fig. 2 for all three samples. For sample TF1 for all T and in the whole field range ∆R s ∼ H 2 rf , whereas for samples TF2 and TF3 the behavior of ∆R s (H rf ) changes dramatically with T . For sample TF2 R s changes from decreasing at 15 K to almost H rf -independent behavior at 35 K, and to a rapidly increasing function of H rf between 40-75 K. At 15 K ∆R s diminishes noticeably only at H rf > 10 kA/m showing no features of saturation up to the highest available H rf of ∼40 kA/m. At h...

We have carried out non-linear microwave (8 GHz) surface impedance measurements of three YBaCuO thin films in dc magnetic fields H dc (parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero dc field the three films, deposited by the same method, show a spread of lowpower residual surface resistance, Rres and penetration depth, λ (T = 15 K) within a factor of 1.9. However, they exhibit dramatically different microwave field, H rf dependences of the surface resistance, Rs, but universal Xs(H rf ) dependence. Application of a dc field was found to affect not only absolute values of Rs and Xs, but the functional dependences Rs(H rf ) and Xs(H rf ) as well. For some of the samples the dc field was found to decrease Rs below its zero-field low-power value.

We have performed accurate, reproducible and nondestructive measurements of the surface resistance of high quality YBazCu307, (YBCO) thin films in the temperature range 12 K to T, using a compact rutile dielectric resonator method at 9.6 GHz. The absolute values of the HTS surface resistance have been measured to systematic errors of less than 10 pQ at this frequency using this technique, which employ two resonator configurations with carefully chosen geometries. The resonators have unloaded Q factors of around lo5 at 77 K, increasing to above 5.105 below 15 K We have used three dimensional electromagnetic software ("Superfish") to study the effects of the copper shield on these rutile (TiOz) measurements, and others using lanthanum aluminate (LaA103) as the dielectric material. In the latter case, there are significant uncertainties (as large as 100%) in the measurements of the surface resistance of the YBCO films owing to the smaller field energy filling factor for LaAI03, since it has a lower relative permittivity than rutile.

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