Valence transitions in the heavy-fermion compound YbCuAl as a function of temperature and pressure

Abstract

We report on direct measurements of the Yb valence in the heavy Fermion compound YbCuAl as a function of temperature and pressure using resonant x-ray emission spectroscopy. The increase of the Yb${}^{2+}$ component at $T<100$ K and ambient pressure, well described by the single impurity Anderson model, is found to compensate for the thermal contraction of the unit cell volume. Under pressure, the Yb valence increases continuously up to 25 GPa, albeit a marked leveling off close to the critical pressure, at $P\ensuremath{\ge}13$ GPa. This finding is reminiscent of a recent report on YbCu${}_{2}$Si${}_{2}$ and further confirms the interplay between electronic and magnetic fluctuations near the magnetic instability point of rare-earth intermediate-valence materials.