Recent developments in the characterization of superconducting films by microwaves

Abstract

We describe and analyze selected surface impedance data recently obtained by different groups on cuprate, ruthenate and diboride superconducting films on metallic and dielectric substrates for fundamental studies and microwave applications. The discussion includes a first review of microwave data on MgB 2 , the weak-link behaviour of RABiTS-type YBa 2 Cu 3 O 7-δ tapes, and the observation of a strong anomalous powerdependence of the microwave losses in MgO at low temperatures. We demonstrate how microwave measurements can be used to investigate electronic, magnetic, and dielectric dissipation and relaxation in the films and substrates. The impact of such studies reaches from the extraction of microscopic information to the engineering of materials and further on to applications in power systems and communication technology.Keywords: Surface impedance, superconducting films, microwave dissipation, relaxation.Address: Dr. Matthias Hein, Dept. of Physics, Univ. of Wuppertal, D-42097 Wuppertal, Germany. Tel. +49(202) 439-2747, Fax +49(202) Email: mhein@venus.physik.uni-wuppertal.de M. Hein et al., EUCAS 2001, invited paper B3-01, page 2 of 19 Microwave measurements in condensed matter physics Theoretical backgroundThis paper illustrates the relevance of microwave measurements for the investigation of electronic and magnetic transport and ordering in condensed matter. While most work has been done on solids, the described techniques could also be adapted to soft matter or liquids.The presentation is based on a selection of examples and does not attempt to provide a complete description of activities in this field.The complex-valued surface impedance Z s =R s +iX s at a microwave frequency ω=2πf is related to the permeability and conductivity of a material, µ and σ, in the local limit by ) (where R s and X s are the surface resistance and reactance [1]. The material parameters µ and σ are complex-valued and frequency dependent, e.g., via electronic, magnetic, or dielectric relaxation. The generalized conductivity σ characterizes electrical conduction through transport and displacement currents.The electrodynamic response of metallic materials provides information on the electronic properties within a surface layer of the order of the London penetration depth ~200 nm, and on a variable time scale ∼1/ω. Such information is complementary to thermodynamic bulk properties and low-frequency transport data. The high frequencies also help to derive Fermi surface properties, which would require strong magnetic fields at very low temperatures otherwise [2]. We will focus in the following on superconductors, for which Z s bears information on the density of states and quasiparticle excitations above and below the transition temperature T c [3]. It is also related to the phase purity, grain connectivity and interface effects of technical materials, which cannot readily be studied at similar resolution by At present at least 50 renowned academic and industrial groups study superconductors at microwave frequencies, to ...

Microwave properties of MgB/sub 2/ thin films

“…The inset shows values below 20 K indicating at 4.2 K. Frequency scaling according to an assumed square law results in at 10 GHz, which is comparable to that of high-quality YBCO thin film. This value is also in accordance with data obtained by the other group [30].…”

Microwave surface impedance of MgB2thin film

“…The inset shows R s values below 20 K indicating R s = 19.4 µ at 4.2 K. Frequency scaling according to an assumed square law results in R s = 37.6 µ at 10 GHz, which is comparable to that of high-quality YBCO thin film. This value is also in accordance with data obtained by the other group [37].…”

Effects of the two-gap nature on the microwave conductivity of polycrystallineMgB2films with a critical temperature of39KLee1, Lee2, Han3 et al. 2005Phys. Rev. B135150View full textAdd to dashboardCite

“…It is seen that the R S eff values of both films are lower than ~ 10 µΩ at 7 K, a value comparable to the surface resistance of high-quality MgB 2 films including c-axis oriented films [19][20][21] . This shows that the surface resistance of our film is affected little by their polycrystalline nature at low temperatures.…”