NMR study of strongly correlated electron systems

Various types of ground states in strongly correlated electron systems have been systematically investigated by means of NMR/NQR at low temperatures under high magnetic field and pressure. We focus on two well-known heavy-electron families, CeCu₂X₂ (X = Si and Ge) (Ce(122)) and UM₂Al₃ (M = Ni and Pd) (U(123)). The Cu NQR experiments on CeCu₂X₂ under high pressure indicate that the physical property of CeCu₂Ge₂ at high pressure, i.e. above the transition at 7.6 GPa from antiferromagnetic (AF) to superconductivity, are clearly related to tha CeCu₂Si₂ at ambient pressure. In addition to the H-T phase diagram established below 7 T, NMR and specific heat experiments on polycrystal CeCu_2.05Si₂ have revealed the presence of a new phase above 7 T. In a high-quality polycrystal of UPd₂Al₃ with a record high-T_c of 2 K at ambient pressure and the narrowest Al NQR line width, the nuclear-spin lattice relaxation rate, ²⁷(1/T₁) measured in zero field has been found to obey the T³ law down to 0.13 K, giving strong evidence that the energy gap vanishes along lines on the Fermi surface. Thus it seems that all heavy-electron superconductors exhibit lines of zero gap, regardless of their different magnetic properties.