Gapless superconductivity and variation of Tc in the heavy-fermion system CeCu₂Si₂

Polycrystalline samples of CeCu2Si2 exhibit bulk superconductivity carried by “heavy fermions”. While usually Tc ≊ (0.55 ± 0.15) K, a few samples showed considerably lower transition temperatures (Tc ⩾ 0.11 K), which could, however, be raised to usual values by proper heat treatment. We found that lower Tc's are accompanied by both slightly reduced values of the spin-fluctuation temperature of the Ce3+ ions and the phenomenon of “gapless superconductivity”. In agreement with recent results by Aliev et al. [5], a CeCu2Si2 single crystal did not become superconducting above 20 mK, but showed a 20% deficiency in Cu occupation