Valence instability of YbCu2Si2through its magnetic quantum critical point.

We provide a complete picture of the valence instabilities of Yb in the heavy-fermion compound YbCu2Si2 by resonant inelastic x-ray scattering measurements (RIXS) at the Yb L3 edge under high pressure (up to 22 GPa) and at low temperatures (down to 7 K) in the vicinity of the magnetic ordered state. The valence shows a significant increase with pressure without reaching the full trivalent state even at the highest pressure. Its pressure dependence is marked by a pronounced change of slope around 10 GPa, a value close to the magnetic critical pressure Pc≈ 8 GPa. Our results suggest that magnetic order in YbCu2Si2 appears for a valence value v < 3. Unlike CeCu2Si2, valence fluctuations do not sustain superconductivity, which is likely destroyed by ferromagnetism. [ABSTRACT FROM AUTHOR]