Laser-excited N and R fluorescence lines of heavily doped ruby have been studied up to 26 GPa at low temperatures. While the intensity of the R lines at ambient pressure significantly decreases with decreasing temperature, the intensity of N lines originating from exchange-coupled Cr ion pairs is enhanced at low temperatures. The pressure induced wavelength shift of the N lines at 19 K is well fitted with an empirical formula similar to the equation for the R 1 line, showing that the intense N line could be used as an alternative pressure scale at low temperatures. We also observe continuous increase in non-hydrostaticity with increasing pressure at low temperatures when silicone oil and 4:1 mixture of methanol and ethanol are used as pressure media. V
Pressure dependence of the Yb valence in YbNiGe 3 has been investigated up to 15.6 GPa at 300 K and up to 7.7 GPa at 17 K by means of x-ray absorption spectroscopy in Lα 1 partial fluorescence yield mode and resonant x-ray emission spectroscopy around the Yb L 3 absorption edge. The Yb valence in YbNiGe 3 at ambient pressure strongly fluctuates with the mean valence of v = 2.52 ± 0.01 at 300 K. The Yb valence rapidly changes toward a trivalent state with pressure up to ∼5 GPa, slowly increases up to ∼10 GPa, and then reaches a saturated value of v ∼ 2.87 at 15.6 GPa. The Yb valence at 17 K slightly decreases compared to that at 300 K; v ∼ 2.45 at ambient pressure and ∼2.72 at 7.7 GPa. We found that the pressure-induced change in the intensity of a quadrupole component in the x-ray absorption spectra shows the same trend as the Yb valence in YbNiGe 3. In contrast to YbNiGe 3 , the Yb valence in YbNiSi 3 is nearly 3 at ambient pressure with almost no temperature dependence.
Pressure dependence of the Ce valence in CeCu(2)Ge(2) has been measured up to 24 GPa at 300 K and to 17 GPa at 18-20 K using x-ray absorption spectroscopy in the partial fluorescence yield. A smooth increase of the Ce valence with pressure is observed across the two superconducting (SC) regions without any noticeable irregularity. The chemical pressure dependence of the Ce valence was also measured in Ce(Cu(1-x)Ni(x))(2)Si(2) at 20 K. A very weak, monotonic increase of the valence with x was observed, without any significant change in the two SC regions. Within experimental uncertainties, our results show no evidence for the valence transition with an abrupt change in the valence state near the SC II region, challenging the valence-fluctuation mediated superconductivity model in these compounds at high pressure and low temperature.
The correlation between the 4f -conduction electron (c-f ) hybridization and the transition temperature T 0 into the unconventional antiferromagnetically (AFM) ordered state in Ce(Ru 1−x Fe x ) 2 Al 10 has been investigated. The impact of pressure on the crystal structure and the electronic structure has been measured using x-ray diffraction and resonant x-ray emission spectroscopy at the Ce L 3 absorption edge in a diamond anvil cell. Our results show that the lattice spacings imply some correlation with the disappearance of T 0 . Analyses of the measured valence states, however, show only a weak correlation with the corresponding critical disappearance of the AFM order within experimental errors. Our results may be explained by a scenario based on the Kondo-Heisenberg model beyond the conventional Doniach phase diagram.
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