MAGNETISM AND SUPERCONDUCTIVITY BY NMR STUDY

Abstract

Results of NMR measurement on high Tc oxide superconductor are reviewed together with heavy electron superconductor. In high Tc compounds, superconductivity and antiferromagnetism compete with each other, while in heavy electron system both coexist or compete. The property of Cooper pair is discussed in high T , superconductor together with the heavy electron superconductor. In the several systems where superconductivity and magnetism interact with each other, heavy electron superconductor and high T, oxide superconductor are of particular interest. There are several similar aspects in both systems. In these systems antiferromagnetic ordering and superconductivity appear in coexistence or in competition [I]. The property of superconductivity is unconventional in heavy electron system [I] which is also seen in high Tc superconductor in NMR measurement. There is a possibility of non-phonon mechanism, an attractive interaction through a magnetic fluctuations, being responsible for the occurrence of superconductivity, although any direct evidence for this is not proved. In this paper a recent review on NMR measurements of high Tc superconductor will be presented together with a brief review on heavy electron systems [2]. 1. Heavy electron system 1.1 MAGNETISM. The neutron diffraction measurements have observed the antiferromagnetic correlation, sometimes static antiferromagnetic ordering, appearing in the ground state of heavy electron system, which seems to be a general property of this system [I]. The static antiferromagnetic ordering is reflected in the line broadening associated with the internal field and in the anomaly in the nuclear spin-lattice relaxation rate T ~ ~ in NMR. In CeA12 NMR line broadening due to antiferromagnetic ordering has been reported [3]. A rapid decrease of T;' due to a gap opening associated with SDW ordering is observed in URunSi2 [4]. Recent NMR measurements have revealed that the systems CeCuzSiz 153, CeA13 161 and CeInCuz [7] are ordered at low temperature. 1.2 SUPERCONDUCTING PROPERTY. TI in CeCu2Si2 [8], UBel3 [9], URuzSiz [4] and UPt3 [lo] in superconducting state all show an unconventional behavior. T ; ~ below T, decreases monotonously and in proportion to T? at low temperature. These behaviors are explained in terms of an anisotropic superconductivity, the gap disappearing on lines at the Fermi surface. 2. High Tc oxide superconductor 2.1.1 (LaxBal-,), Cu04. LaaCu04 is an antiferromagnet with TN N 240 K . With replacing La by Ba, TN decreases rapidly [ll] and then superconductivity appears for x ;C 2.5 % [12]. Figure 1 shows a NQR spectrum of La at 4.2 K 1131. The complicated structure is well explained by a large electric quadrupole interaction together with a small Zeeman energy due I L?CuO4 Zero Field T = 1 3 K I