Surface Impedance and Granularity in Superconducting Cuprates
- PDF / 247,940 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 58 Downloads / 183 Views
SURFACE IMPEDANCE AND GRANULARITY IN SUPERCONDUCTING CUPRATES
J. HALBRITTER Appl. Phys. Stanford University, STANFORD, CA 94305 and Kernforschungszentrum, Postfach 3640, 7500 KARLSRUHE, W.- Germany
ABSTRACT The rf residual surface resistance Rres is an important figure of merit for superconducting microwave devices and for establishing the homogeneity of superconducting surfaces. In granular superconductors not only are high residual rf surface resistance values Rres observed, but also strong field dependencies of Rres(T, (o, Hrf) and the penetraton depth Xres(T, co, Hrf) are encountered even at very low field levels (of order 1 Oe). These observations can be explained in terms of granularity consisting of weak links and of insulating interfaces, which provide additional degrees of freedom due to fluxoid motion and electromagnetic strip line modes. These degrees of freedom cause additional interaction mechanisms with applied rf fields.
These mechanisms allow a coherent description of the observed T, (o, Hrf and Hdc dependence of Rres and X.res for the granular cuprates and for similar effects in Pb, Nb, Nb 3 Sn and NbN. In addition, on the basis of such rf data, it is possible to quantify the density of weak links and their critical fields and currents.
Mat. Res. Soc. Symp. Proc. Vol. 195. 01990 Materials Research Society
392
SUMMARY Results from surface impedance measurements on the superconducting cuprates in the GHz range exhibit high rf residual losses, Rres, and a strong field dependence of the rf response, even at low fields. A typical example is shown in Fig. 1. Such dependencies are found for the surface resistance and the penetration depth Xres, as function of both dc and rf fields (see Ref. 1 - 4 for examples and references therein). The observed behavior can be classified into three regimes according to the field and frequency depencencies indicated in Fig. 1: I: II: III:
2 2 Co.Lo 7Xres or Rres c- (0 H cOgtokres or Rres *- o)2 H cO0oTres or Rres - 4dc(Hsat - H)
As discussed in detail elsewhere 5 we attribute these field dependencies to locally weak superconductivity, caused by planar defects. We note that similar results 5 have been found for NbN, Nb3Sn, Pb and for Nb. 6 "Defects define materials." As we have stressed in an earlier paper, this statement is especially true for the superconducting cuprates, particularly for their electrical conduction. It follows from the fact that as a function of doping they are near a metal-insulator transition. Thus defects can render the material insulating or weakly superconducting as discussed in Refs. 6 and 7. Planar defects in these materials can be classified phenomenologically by their ability to interrupt or reduce the normal or supercurrent crossing the defect. The effect on normal conduction is described by a grain boundary resistance Rb, and on the supercurrent by a critical current Jc- This Jc governs the Josephson penetration depth Xj *- 114jc and the critical field Hcl -J c, of the junction at which flux first penetrates. For YBa2Cu3Ox this clas
Data Loading...
