Experimental studies of the phase transition in YbIn1-xAgxCu4

We report measurements of the low-temperature specific-heat coefficient gamma = C-p(T)/T, cell volume V(T), Hall coefficient R-H(T), and valence z = 2 + n(f) [where the Yb hole occupation n(f)(T) was determined from Yb-L-3 x-ray absorption] of single crystals of YbIn1-xAgxCu4. Alloying YbInCu4 with Ag increases the temperature T-s(x) of the first-order isomorphic phase transition and causes it to terminate at a critical point at x(c) = 0.195 and T-c = 77 K. The variation of V(T) 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 T-s(x) are of order of the Kondo temperatures T-K(+)(x) of the high-temperature state. The cell volume is found to vary proportionally to 1-n(f)(T). At low temperatures, well away from the transition, the Wilson ratio of the susceptibility chi(0) and specific heat coefficient gamma falls within 20% of the Value predicted for a Kondo impurity, and 1-n(f)(0) and chi(0) are roughly proportional as predicted from the Anderson model. The temperature dependence n(f)(T) for temperatures away from the phase transition also fits the predictions of the Kondo model. The small volume discontinuity Delta V/V-0 observed at T-s suggests that the phase transition is not due to a Kondo volume collapse. The large Hall coefficients R-H(T) observed for x < x(c) and T > T-s(x) suggest instead that a low carrier density in the high-temperature state plays a key role in the phase transition.