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

We present the pressure-temperature phase diagram of the antiferromagnet CeAg 2 Si 2 established via resistivity and calorimetry measurements under quasi-hydrostatic conditions up to 22.5 GPa. With increasing pressure, the Néel temperature [ T N ( p = 0 ) = 8.6 K] slowly increases up to T N = 13.4 K at 9.4 GPa and then vanishes abruptly at the magnetic critical pressure p c ∼ 13 GPa. For the first time, heavy fermion superconductivity is observed in CeAg 2 Si 2 . 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 T c bulk = 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 p cr ∼ 17 GPa and T cr ∼ − 13 K , respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg 2 Si 2 . [ABSTRACT FROM AUTHOR]