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 crystalline HTSs has been started. Employing superconducting quantum interference device magnetometry and novel measuring and evaluation techniques, precise information on the first penetration field can be obtained and used to extract the lower critical field Hcl. In most cases, Hcl is smaller than previously reported, follows a smooth temperature dependence down to the lowest temperatures investigated (usually 5 K) and can be reasonably well described by Bardeen-Cooper-Schrieffer theory. (author)