UPPER CRITICAL FIELD IN HEAVY FERMION SYSTEMS.

For explaining the peculiar temperature dependence of the upper critical field H//c//2 in UBe//1//3 the following mechanism was considered: The H//c//2 of UBe//1//3 is extremely high at low temperatures and its intensity is comparable to the Kondo temperature in this compound. Therefore, the coherent state itself may be affected by the external field. The self-energy and vertex function of a quasielectron become field dependent. This field dependence causes the modification of the g-factor, the spin-orbit interaction and the pairing interaction of quasielectrons. Taking account of the modification and assuming singlet superconductivity, we calculated the H//c//2 of the heavy fermion systems for various values of the parameters. The possibility of the reentrant superconductivity in the high field region is predicted.