Spatially Inhomogeneous Superconducting State near Hc2 in UPd2Al3

We have performed Al-27-NMR measurements on single-crystalline UPd2Al3 with the field parallel to the c-axis to investigate the superconducting (SC) properties near the upper critical field of superconductivity H-c2. The broadening of the NMR linewidth below 14 K indicates the appearance of the internal field at the Al site, which originates from the antiferromagnetically ordered moments of U 5f electrons. In the SC state well below mu H-0(c2) = 3.4 T, the broadening of the NMR linewidth due to the SC diamagnetism and a decrease in the Knight shift are observed, which are well-understood by the framework of spin-singlet superconductivity. In contrast, the Knight shift does not change below T-c(H), and the NMR spectrum is broadened symmetrically in the SC state in the field range of 3 T < mu H-0 < mu H-0(c2). The unusual NMR spectrum near H-c2 suggests that a spatially inhomogeneous SC state such as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state would be realized.