Pressure-induced valence crossover in superconducting CeCu2Si2

Measurement of the Ce valence in the heavy fermion CeCu(2)Si(2) is reported for the first time under pressure and at low temperature (T=14 K) in proximity of the superconducting region. CeCu(2)Si(2) 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.