An experiment has been performed to investigate the mixed state of type II superconductors by simultaneously measuring the real part of the surface impedance at 1·2 Gc/s and the magnetization. The surface resistance could thus be related to the magnetic condition of the specimen both during the initial magnetization and afterwards. The specimens were in the form of 1·6 mm diameter wires wound into helices and the steady field was applied parallel to the axis of the helix. In the first place tin-indium alloys having 0 to 7 at. % of indium were used, and most of the data were obtained with this alloy system. Lead alloys were used later. The results differ from those of Rosenblum
et al
., who worked at 23 Gc/s, in that the surface resistance was found to be sensitive to the surface condition of the specimen. But consistent results were obtained when the specimens had been chemically or electrolytically polished and a comparison is made between the data at the two frequencies. Expressed as a fraction of its value in the normal state the surface resistance at 1·2 Gc/s is found to obey the same universal function of field and temperature as at the higher frequency when allowance is made for the difference in geometrical arrangement. It is argued that the pinning mechanisms which prevent flux lines moving in response to a small direct current are unlikely to be effective in preventing high frequency oscillations. Microwave power will therefore be absorbed in the specimen owing to the damping of flux line motion by the same viscous force which is held to account for the d. c. resistivity when the current is large enough to overcome the pinning. The microwave surface resistance should therefore be closely related to the d. c. resistivity of an ideal type II superconductor with no pinning centres. In this way most of the features of the microwave data are explained qualitatively but a surface resistance is predicted which in fields much weaker than the upper critical field is larger than the experimental value.
Microwave surface resistance and upper-critical-field anisotropy of MgB2 superconductor
