Heavy-Fermion Superconductivity in CeAg${}_2$Si${}_2$ --Interplay of Spin and Valence Fluctuations--

Abstract

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 [$T_{\mathrm{N}}(p=0)=8.6$~K] slowly increases up to $T_{\mathrm{N}}=13.4$~K at 9.4~GPa and then vanishes abruptly at the magnetic critical pressure $p_{\mathrm{c}}\sim 13$~GPa. For the first time, heavy fermion superconductivity is observed in CeAg${}_2$Si${}_2$ . Superconductivity emerges at $\sim 11$~GPa and persists over roughly 10~GPa. Partial- and bulk-transition temperatures are highest at $p=16$~GPa, with a maximal $T_{{\mathrm{c}}^{\mathrm{bulk}}}=1.25$~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 $p_{\mathrm{cr}}\sim 17$~GPa and $T_{\mathrm{cr}}\sim -13K$, respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg${}_2$Si${}_2$.