Fulde-Ferrell state in heavy-fermion superconductors

Within the weak-coupling model we study the upper critical field of S-wave and D-wave superconductors in the clean limit close to the Chandrasekhar-Clogston limit. We find for parameters derived from dH_c2(T)/dT_T=Tc of a single crystal of the recently discovered heavy-fermion superconductor UPd₂Al₃ that the Fulde-Ferrell (FF) state is possible for the S-wave and one of the D-wave superconductors of D_6h symmetry but not for the hybrid state. Indeed, choosing a g factor slightly less than 2, we can describe the temperature dependence of H_c2(T) for both H∥Rc and H∥a extremely well. On the other hand the D-wave model predicts the transition into the FF state at T/T_c=0.29 in sharp contrast to the experimental observation T/T_c0=0.8.