NMR study in novel superconductors: Heavy-fermion system and high-T c cuprate

Abstract

From the NMR and NQR studies, we show the unconventional superconducting properties in the antiferromagnetic (AF) heavy-fermion (HF), UPd 2Al 3 and high- T c cuprate systems. In UPd 2Al 3 with T N = 14.5 K and T c = 2 K, the superconductivity is shown to be of d-wave pairing type with lines of vanishing gap on the Fermi surface. The potential scattering associated with some defects at the Al site is the cause for the T c suppression and the appearance of the density of states (DOS) at the Fermi level. This unusual impurity effect is consistent with a d-wave scenario, because the potential scattering leads to a gap opening for an anisotropic s-wave scenario. In high- T c cuprate which are argued to be either a d-wave or an anisotropic s-wave, the potential scatterer like Zn in the plane induces the gapless superconductivity as well, which can not be interpreted in terms of an anisotropic s-wave scenario if the order parameter does not change sign largely. It is shown from the NMR experiment at the Al site for Al-doped La 1.85Sr 0.15CuO 4 that local moments at near neighbor Cu sites induced around Al impurity do not play a role to suppress the T c and to induce the DOS at all. The NMR studies in novel superconductors have provided important clues to identify their symmetry of order parameter to be d-wave.