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Register a new userIntermediate valence in single crystalline Yb$_2$Si$_2$Al
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Yb$_2$Si$_2$Al may be a prototype for exploring different aspects of the Shastry-Sutherland lattice, formed by planes of orthogonally coupled Yb ions. Measurements of the magnetic susceptibility find incoherently fluctuating Yb$^{3+}$ moments coexisting with a weakly correlated metallic state that is confirmed by measurements of the electrical resistivity. Increasing signs of Kondo coherence are found with decreasing temperature, including an enhanced Sommerfeld coefficient and Kadowaki-Woods ratio that signal that the metallic state found at the lowest temperatures is a Fermi liquid where correlations have become significantly stronger. A pronounced peak in the electronic and magnetic specific heat indicates that the coupling of the Yb moments to the conduction electrons leads to an effective Kondo temperature that is approximately 30 K. The valence of Yb$_2$Si$_2$Al has been investigated with electron spectroscopy methods. Yb$_2$Si$_2$Al is found to be strongly intermediate valent ($v_F=2.68(2)$ at 80 K). Taken together, these experimental data are consistent with a scenario where a coherent Kondo lattice forms in Yb$_2$Si$_2$Al from an incoherently fluctuating ensemble of Yb moments with incomplete Kondo compensation, and strong intermediate valence character.
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We present the crystal growth as well as the structural, chemical and physical chracterization of SmRh$_2$Si$_2$ single crystals. Their ground state is antiferromagnetic, as indicated by the behaviour of the magnetic susceptibility and the specific heat at the second order phase transition observed at T$_{rm N}$ = 64 K. The Sommerfeld coefficient is small and similar to that of LuRh$_2$Si$_2$ with $gamma_0approx$ J/(molK$^2$). Susceptibility measurements show no Curie-Weiss behaviour at high temperatures which is a consequence of the large Van-Vleck contribution of the excited multiplets of Sm$^{3+}$. Previous angle-resolved photoemission studies showed that at 10 K, the valence of the Sm ions is smaller than three at the surface as well as in the bulk, suggesting a possible Kondo screening of the Sm$^{3+}$ ions. This could not be observed in our thermodynamic and transport measurements.
We present thermopower measurements on Yb(Rh$_{1-x}$Co$_x$)$_2$Si$_2$. Upon Co substitution the Kondo temperature is decreasing and the single large thermopower minimum observed for YbRh$_2$Si$_2$ splits into two minima. Simultaneously, the absolute thermopower values are strongly reduced due to a weaker exchange coupling between the $4f$ and the conduction electron states with increasing $x$. Pure YbCo$_2$Si$_2$ is considered a stable, trivalent system. Nevertheless, we still observe two minima in the thermopower indicative of weak residual Kondo scattering. This is in line with results from photo emission spectroscopy revealing a tiny contribution from Yb$^{2+}$. The value at the high-$T$ minimum in $S(T)$ is found to be proportional to the Sommerfeld coefficient for the whole series. This unexpected finding is discussed in relation to recent measurements of the valence and Fermi surface evolution with temperature.
We present experimental results of electrical resistivity, Hall coefficient, magnetic susceptibility, and specific heat for single crystals of Ce-based intervalent compound CeNiSi$_2$. The results show similar behaviors observed in Yb-based intervalent compounds and support recent thoery of the Anderson lattice, in which the Fermi-liquid coherence is gloval over the whole lattice. There is a low-temperature scale $T_{coh} sim$ 50 K for the onset of Fermi-liquid coherence, in addition to a high-temperature scale $T_K^* sim$ 150 K for the Kondo-lattice condensation. Therefore, we conclude that two energy scales are generic in intermediate valence compounds based on Ce where the orbital degeneracy is smaller and where the size of the $4f$ orbital is larger than those based on Yb.
We report Resonant inelastic x-ray scattering measurements (RIXS) in YbCu$_2$Si$_2$ at the Yb L$_{3}$ edge under high pressure (up to 22 GPa) and at low temperatures (down to 7 K) with emphasis on the vicinity of the transition to a magnetic ordered state. We find a continuous valence change towards the trivalent state with increasing pressure but with a pronounced change of slope close to the critical pressure. Even at 22 GPa the Yb$^{+3}$ state is not fully achieved. The pressure where this feature is observed decreases as the temperature is reduced to 9 GPa at 7K, a value close to the critical pressure (itshape{p ormalfont{$_c$}} ormalfont $approx$ 7.5 GPa) where magnetic order occurs. The decrease in the valence with decreasing temperature previously reported at ambient pressure is confirmed and is found to be enhanced at higher pressures. We also compare the f electron occupancy between YbCu$_2$Si$_2$ and its Ce-counterpart, CeCu$_2$Si$_2$.
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