Y. Kitaoka et al., NMR-STUDY IN NOVEL SUPERCONDUCTORS - HEAVY-FERMION SYSTEM AND HIGH-T-C CUPRATE, Journal of physics and chemistry of solids, 56(12), 1995, pp. 1931-1935
From the NMR and NQR studies we show the unconventional superconductin
g properties in the antiferromagnetic (AF) heavy-fermion (HF), UPd2Al3
and high-T-c cuprate systems. In UPd2Al3 with T-N=14.5 K and T-c=2 K,
the superconductivity is shown to be of d-wave pairing type with line
s of vanishing gap on the Fermi surface. The potential scattering asso
ciated with some defects at the Al site is the cause for the T-c suppr
ession and the appearance of the density of states (DOS) at the Fermi
level. This unusual impurity effect is consistent with a d-wave scenar
io, because the potential scattering leads to a gap opening for an ani
sotropic s-wave scenario. In high-T-c cuprate which are argued to be e
ither a d-wave or an anisotropic s-wave, the potential scatterer like
Zn in the plane induces the gapless superconductivity as well, which c
an not be interpreted in terms of an anisotropic s-wave scenario if th
e order parameter does not change sign largely. It is shown from the N
MR experiment at the Al site for Al-doped La1.85Sr0.15CuO4 that local
moments at near neighbor Cu sites induced around Al impurity do not pl
ay a role to suppress the T-c and to induce the DOS at all. The NMR st
udies in novel superconductors have provided important clues to identi
fy their symmetry of order parameter to be d-wave.