Microwave surface resistance of MgB2

Abstract

The microwave power and frequency dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by the breakdown in the ω2 law for poor-quality samples at all temperatures. The performance of MgB2 at 10 GHz and 21 K was compared directly with that of high-quality YBCO films. The surface resistance of MgB2 was found to be approximately three times higher at low microwave power and showed an onset of nonlinearity at microwave surface fields ten times lower than the YBCO film. It is clear that MgB2 films are not yet optimized for microwave applications.

Microwave properties of MgB/sub 2/ thin films

“…The s-wave nature of the order parameter of MgB suggests an exponential temperature dependence of in the low temperature region (below about 20 K) [15], [17], [18], if the impurity and defect related residual surface resistance is low enough. In contrast to oxide superconductors, grain boundaries do not exhibit weak link behavior, i.e., one important potential source of extrinsic residual losses is expected not to be present.…”

Frequency dependence of the microwave surface resistance of MgB2 by coaxial cavity resonator

“…In granular SC, deviations from this law have been highlighted [10] and have been ascribed to normal-material inclusions at the grain boundaries. The frequency dependence of the mw surface resistance of MgB 2 has not been comprehensively investigated; in the literature, there are only few papers concerning results obtained mainly in films [4,5,15] and these have been obtained measuring R s at two or three different frequencies. R s ðf Þ of bulk samples has been investigated in the range of frequencies 10-100 MHz by Dmitriev et al [7].…”

“…Although MgB 2 SC has been indicated by several authors as suitable material for mw application [1][2][3], the frequency dependence of R s in this material has not been comprehensively investigated. Up to now, in the literature there are only few papers discussing results of frequency dependence of R s , obtained mainly in MgB 2 films [4,5]. Nevertheless, several authors in order to compare R s values obtained in different samples use the f 2 scaling to compare results at different frequencies [6].…”

STUDY OF THE ELECTRODYNAMIC RESPONSE OF MgB2 SINTERED PELLETS AND THIN FILMS

“…Very recently, however, a more careful analysis of µSR data showed [27] results consistent with a twogap model, while the peculiar exponential behavior expected for a s-wave superconductor was indeed observed in polycrystalline samples [28], in wires [29], and recently in single crystals [30]. Measurements carried out on films coming from different sources show also a fully-gapped superconductor behavior, even if quite different values of the strong coupling ratio are reported [31,32]. In the following the temperature dependence of the magnetic penetration depth, λ(T), at low frequency (4 MHz) is studied, using a single coil-mutual inductance technique [33].…”