THE NONOHMIC BEHAVIOR OF BULK HIGH-TEMPERATURE SUPERCONDUCTORS

The current voltage characteristics of granular high temperature superconductors become non-ohmic in a way which depends on both the temperature and the magnetic field. We have studied this behaviour in the lead stabilized 2:2:2:3 phase of bismuth strontium calcium copper oxide and have found that there are some samples where a single family of V(I) characteristics may be swept through either by changing the temperature or by changing the magnetic field. The magnetic field and temperature dependence of these materials can thus be described by a single magnetic field and temperature dependent parameter. This result spans both the low magnetic field regime (fields less than one flux quantum per grain boundary) and high magnetic fields and both the region of the characteristics where log(V) varies linearly with log(I) and the region of non-linear behaviour. The dissipation is almost independent of the angle between the magnetic field and the measurement current. These results impose a number of severe constraints upon possible theories for the dissipation. © 1990.