Nuclear magnetic resonance study of magnetic correlation in the heavy-fermion superconductor CeCu₂Si₂

Magnetic correlation in the heavy-fermion superconductor CeCu₂Si₂ has been investigated by Cu nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR). For CeCu_2.02Si₂ with T_c=0.72 K, a magnetic transition has been found at around 0.8 K in applied fields up to 3.5 T, which presumably also exists in zero magnetic field. Below the transition temperature, Cu NMR loses intensity suddenly without any broadening and shift of the spectrum, and the spin-echo decay rate, 1/T₂, at H=13.3 kOe, increases with decreasing temperature, which is different from the behaviour expected in a static magnetically ordered state. The magnetic transition just above T_c is quite unusual in the sense that the ordered state is not in a completely static regime, but possesses some dynamic aspect. The unusual magnetic state is considered to coexist with superconductivity below T_c. The authors present a magnetic phase diagram in the field-temperature plane for CeCu_2.02Si₂.