Pressure-Induced Valence Crossover in Superconducting ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$

Measurement of the Ce valence in the heavy fermion ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ is reported for the first time under pressure and at low temperature ($T=14\mathrm{K}$) in proximity of the superconducting region. ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ 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 $T_{\mathrm{cr}}$ close to zero.

Figure 1

Schematic view of the $P\mathrm{\text{−}}T$ phase diagram of ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ (adapted from Ref. [10]). $T_N$ is the Néel temperature; $P_c$ and $P_v$ are the pressure location of the two superconducting domes. The circles show the locations in the phase diagram where the sample was measured.

Figure 2

X-ray absorption spectra in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ as a function of pressure at 14 K. The spectra were acquired in the partial-fluorescence yield, high-resolution mode PFY-XAS. The arrows show the evolution of main spectral features with increasing pressure.

Figure 3

Computed x-ray absorption spectra in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ as a function of pressure within the Anderson impurity model (solid lines); see text for details. Ticks show the multiplet states for the lowest pressure. An arctan background (dashed line) is added to all the spectra to mimic the edge jump; arrows show the tendency upon pressure increase. Inset: comparison between calculated (lines) and measured (circles) spectra at the two extreme pressures.

Figure 4

$f$-electron number per site ${\overline{n}}_f$ as function of pressure (top scale). The lower scale refers to the dimensionless unit of Ref. 10; we use the approximation $D=2V$. The dashed lines are guides to the eyes. The Ce data were borrowed from Ref. 15.