Resistivity, magnetization, and specific heat of YbAgCu4 in high magnetic fields

We report measurements of the resistivity ρ, Sommerfeld coefficient of specific heat γ, and magnetization M of polycrystalline YbAgCu₄ in high magnetic fields 0<=B<=18 T [in the case of M(B) to 50 T]. A comparison of the temperature-dependent susceptibility χ(T) as well as field-dependent Sommerfeld coefficient γ and magnetization to Kondo theory for a J=7/2 impurity shows that theory correctly predicts the functional dependence of these quantities on T and B, but the characteristic temperatures determined from the various measurements (120, 98, 77, and 63 K) differ by nearly a factor of 2, which is difficult to understand within the context of Kondo theory even when other possible contributions are considered. In addition the normalized (Wilson) ratio of χ to γ is 1.00 at zero field (compared to 1.14 in Coqblin-Schrieffer theory) and decreases with increasing magnetic field. The magnetoresistance is positive at all temperatures, reaching a value Δρ(B)/ρ(B=0)=0.6 at 25 mK and 18 T. The low-temperature magnetoresistivity Δρ(B) varies as B^1.5. We argue that this is dominated by an ordinary impurity effect. Kohler's rule is clearly violated as the temperature is raised; the scattering rate appears to increase with field below 40 K and decrease with field above 40 K. This behavior is expected for an Anderson lattice when a pseudogap is present. At low temperature the resistivity increases as AT². The coefficient A (corrected for cyclotron-orbit effects) increases with field such that the ratio A(B)/γ(B)² is a constant. Doping with Lu onto the Yb site, or with Ni onto the Cu site, changes the magnitude of the low-temperature resistivity in a manner consistent with the predictions of the theory of ligand-induced disorder in an Anderson lattice.