EXPERIMENTAL STUDIES OF THE PHASE-TRANSITION IN YBIN1-XAGXCU4

We report measurements of the low-temperature specific-heat coefficien t gamma = C-p(T)/T, cell volume V(T), Hall coefficient R-H(T), and val ence 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. Al loying YbInCu4 with Ag increases the temperature T-s(x) of the first-o rder isomorphic phase transition and causes it to terminate at a criti cal point at x(c) = 0.195 and T-c = 77 K. The variation of V(T) near t he 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 tem peratures 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 a way from the transition, the Wilson ratio of the susceptibility chi(0) and specific heat coefficient gamma falls within 20% of the Value pre dicted for a Kondo impurity, and 1-n(f)(0) and chi(0) are roughly prop ortional as predicted from the Anderson model. The temperature depende nce n(f)(T) for temperatures away from the phase transition also fits the predictions of the Kondo model. The small volume discontinuity Del ta 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) observ ed for x < x(c) and T > T-s(x) suggest instead that a low carrier dens ity in the high-temperature state plays a key role in the phase transi tion.