THE RESISTIVE UPPER CRITICAL-FIELD OF THE CUPRATE SUPERCONDUCTORS

At certain doping levels, the resistive transitions of cuprate superconductors show a parallel shift to lower temperatures under the application of magnetic fields. This allows a resistive upper critical field to be defined more easily than for materials such as YBa2Cu3O7, in which the field broadens the transition. The temperature dependence of this field is very unusual, with a steep rise at low temperatures. In this paper we review the experimental situation, and discuss the implications of the results far the (H,T) diagram of the cuprates.