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

The archetypical heavy-fermion superconductor $\mathrm{Ce}{\mathrm{Cu}}_{2}{\mathrm{Si}}_{2}$ 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 $({T}_{c}=2.3\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. Here we present x-ray absorption measurements under pressure and applied magnetic field at the Ce ${L}_{3}$ edge at 2.7 K, i.e., close to the optimal ${T}_{\mathrm{c}}$. 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.