Cu-NQR study of magnetism and superconductivity in Ce_1+xCu_2+ySi₂ under pressure

In order to investigate magnetic properties in the vicinity of the magnetic to superconducting (SC) phase boundary, we have performed Cu-NQR measurements under hydrostatic pressure (P) on two off-stoichiometric Ce_1+xCu_2+ySi₂polycrystals: Ce_0.99Cu_2.02Si₂ (denoted as Ce 0.99) exhibits the anomalous ground state so-called A phase dominated by critical magnetic correlations and Ce_0.975Cu₂Si₂ (Ce 0.975) shows a static magnetic ordering at ambient P. With increasing P which enhances the Kondo temperature, the fraction of the A phase becomes markedly suppressed, and the SC fraction increases correspondingly, although T_c stays nearly constant in the measured pressure range, P ≤ 14.4 kbar. This shows that the P-induced transition from the A to the SC phases is of a first-order type, without passing through a quantum (T → 0) phase transition. However, no superconductivity has been attained in Ce 0.975 up to 19.4 kbar. It is considered that the P-induced superconductivity is sensitive to the sample quality.