Absence of magnetic field effect on the cerium valence in CeCu2Si2 at its optimum superconducting critical temperature

The archetypical heavy-fermion superconductor CeCu2Si2 is known to present two distinct superconducting phases under pressure. In the low-pressure region, the superconductivity is mediated by spin fluctuations while the superconducting phase observed in the high-pressure region could be associated with a first-order valence transition (FOVT). However, the critical end point (CEP) of the FOVT was shown to be located at negative temperature and only a continuous valence change (crossover regime) was so far observed at 14 K, i.e., far above the optimal superconducting temperature (Tc = 2.3 K). Here we present x-ray absorption measurements under pressure and applied magnetic field at the Ce L3 edge at 2.7 K, i.e., close to the optimal Tc. It was expected that the applied magnetic field could shift the CEP to positive temperature with the possibility to observe a FOVT. Our data indicate the valence of Ce increases continuously (crossover regime) from 3.11 at ambient pressure up to 3.20 at 8.5 GPa likewise for any applied magnetic field up to 6 T.