Spatially Inhomogeneous Superconducting State near H-c₂ in UPd₂Al₃

Abstract

We have performed ²⁷Al-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 Hc₂. 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 μ₀Hc₂ = 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 Tc(H), and the NMR spectrum is broadened symmetrically in the SC state in the field range of 3 T < μ₀H < μ₀Hc₂. The unusual NMR spectrum near Hc2 suggests that a spatially inhomogeneous SC state such as the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state would be realized.