Evidence for fully-gapped superconductivity in heavy-fermion CeCu$_2$Si$_2$

The discovery of superconductivity in heavy-fermion CeCu$_2$Si$_2$ in 1979 has opened a new playground for unconventional superconductivity in strongly-correlated systems. However, even in this archetypal heavy-fermion superconductor, the symmetry and the structure of the superconducting gap, which are intimately related to the paring mechanism, are still elusive. Here, to investigate the low-energy quasiparticle excitations in the superconducting state of CeCu$_2$Si$_2$ ($T_c=0.6$~K), we performed specific heat, thermal conductivity, and penetration depth measurements down to 60~mK. We found that specific heat and penetration depth exhibit exponential $T$-dependence at low $T$. Moreover, thermal conductivity has no residual $T$-linear term and shows little $H$-dependence. These behavior are in marked contrast to nodal superconductors. From the data taken with different experimental configurations, the detailed superconducting gap structure will be discussed.