Nonlinear microwave surface impedance of YBCO films: latest results and present understanding

Disorder Effects on the Intrinsic Nonlinear Current Density in ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$

“…In agreement with (1) and (7), it is through the expansion of the superfluid density in terms of that is defined by the relation . Here, can be expressed as (8) with (9) and (10) In order to model the varying crystal quality of the samples used to obtain the results shown in Fig. 1 we vary the impu- rity parameters , and use (7)-(10) to extract the penetration depth (at zero current) and the nonlinear current density scale .…”

“…While it is known that impurity substitution (particularly Zn and Ni) can strongly affect the nonlinear microwave properties of YBCO films [9], the effect of disorder on the nonlinear behavior in as-grown films has not been studied systematically and compared quantitatively with theory. This issue is important since the nonlinear response and its unwanted intermodulation products can severely limit the applications of superconducting devices used for e.g.…”

Fundamental microwave-power-limiting mechanism of epitaxial high-temperature superconducting thin-film devices

“…6,7 However, by comparing measurements on devices with different-magnetic field configurations, it is argued that a field-driven mechanism cannot consistently describe the onset of nonlinear effects for all field configurations. 8 Current-driven mechanisms are usually based on the onset of nonlinear effects if the rf transport current density exceeds a certain limiting value which is characteristic of the nonlinear mechanism. If several currentdriven mechanisms exist, the mechanism with the lowest limiting value then naturally defines the onset of nonlinear effects and, thus, the power-handling capability.…”

“…A strong support for the idea of a current-driven limitation comes from power-handling capability measurements of the same thin superconducting films performed in two separate devices of highly different field configurations which give similar results as functions of the rf current density. 8 From a dc perspective, there are three main currentlimiting mechanisms: ͑i͒ grain-boundary Josephson junction behavior, ͑ii͒ pair breaking, and ͑ii͒ vortex depinning and motion.…”

Heating and high frequency nonlinearities in the surface impedance of high Tcsuperconductors

“…Microwave losses are characterized by a decrease of the quality factor Q and a downward shift of the resonant frequency. The surface impedance of HTSC has been studied by many groups [1][2], however the physical origin of the observed nonlinearities is still under debate and the subject of present-day experimental investigation [3][4][5]. It has been proposed that a simple way to differentiate among the mechanisms leading to a nonlinear surface impedance is the examination of the r parameter [6].…”