THE RESISTIVE UPPER CRITICAL-FIELD OF THE CUPRATE SUPERCONDUCTORS

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