Heavy-fermion superconductivity in CeAg2Si2 - Interplay of spin and valence fluctuations

We present the pressure-temperature phase diagram of the antiferromagnet CeAg2Si2 established via resistivity and calorimetry measurements under quasi-hydrostatic conditions up to 22.5 GPa. With increasing pressure, the Néel temperature [TN(p=0)=8.6 K] slowly increases up to TN=13.4 K at 9.4 GPa and then vanishes abruptly at the magnetic critical pressure pc∼13 GPa. For the first time, heavy fermion superconductivity is observed in CeAg2Si2. Superconductivity emerges at ∼11 GPa and persists over roughly 10 GPa. Partial- and bulk-transition temperatures are highest at p=16 GPa, with a maximal Tcbulk=1.25 K. In the pressure region of superconductivity, Kondo and crystal-field splitting energies become comparable and resistivity exhibits clear signatures of a Ce-ion valence crossover. The crossover line is located at a rapid collapse in resistivity as function of pressure and extrapolates to a valence transition critical endpoint at critical pressure and temperature of pcr∼17 GPa and Tcr∼−13K, respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg2Si2.