THE LOWER CRITICAL-FIELD OF HIGH-TEMPERATURE SUPERCONDUCTORS

In view of continuing problems with a reliable assessment of the lower critical field in high-temperature superconductors (HTSs), especially at low temperatures, a new experimental programme on several single c rystalline HTSs has been started. Employing superconducting quantum in terference device magnetometry and novel measuring and evaluation tech niques, precise information on the first penetration field can be obta ined and used to extract the lower critical field H-C1. In most cases, H-C1 is smaller than previously reported, follows a smooth temperatur e dependence down to the lowest temperatures investigated (usually 5 K ) and can be reasonably well described by Bardeen-Cooper-Schrieffer th eory.