Pressure-Induced Valence Crossover in Superconducting CeCu2Si2

Measurement of the Ce valence in the heavy fermion CeCu2Si2 is reported for the first time under pressure and at low temperature (T = 14 K) in proximity of the superconducting region. CeCu2Si2 is considered as a strong candidate for a new type of pairing mechanism related to critical valence fluctuations which could set in at high pressure in the vicinity of the second superconducting dome. A quantitative estimate of the valence in this pressure region was achieved from the measurements of the Ce L-3 edge in the high-resolution partial-fluorescence yield mode and subsequent analysis of the spectra within the Anderson impurity model. While a clear increase of the Ce valence is found, the weak electron transfer and the continuous valence change under pressure suggests a crossover regime with the hypothetical valence line terminating at a critical end point T-cr close to zero.