Abstract

We report measurements of the low-temperature specific-heat coefficient γ=Cp/T, cell volume V, Hall coefficient RH, and valence z=2+nf [where the Yb hole occupation nf was determined from Yb-L3 x-ray absorption] of single crystals of YbIn1-xAgxCu4. Alloying YbInCu4 with Ag increases the temperature Ts of the first-order isomorphic phase transition and causes it to terminate at a critical point at Xc=0.195 and Tc=77K. The variation of V near the critical point is well described by a mean-field equation of state. The phase transition involves a large change in the Kondo temperature, and the transition temperatures Ts are of order of the Kondo temperatures Tk+ of the high-temperature state. The cell volume is found to vary proportionally to 1-nf. At low temperatures, well away from the transition, the Wilson ratio of the susceptibility χ and specific heat coefficient γ falls within 20% of the value predicted for a Kondo impurity, and 1-nf and χ are roughly proportional as predicted from the Anderson model. The temperature dependence nf for temperatures away from the phase transition also fits the predictions of the Kondo model. The small volume discontinuity ΔV/V0 observed at Ts suggests that the phase transition is not due to a Kondo volume collapse. The large Hall coefficients RH observed for x Ts suggest instead that a low carrier density in the high-temperature state plays a key role in the phase transition.