Investigation on the nature of the concentration-induced non-fermi-liquid behavior in YbCu3.5Al1.5

YbCu5-xAlx exhibits long range antiferromagnetic order for x > 1.5 with T-N rising up to 2 K for the composition YbCu3Al2. This development of anti-fe rromagnetic order is due to a valence shift from the non-magnetic Yb 4f(14) ground state YbCu5 to the magnetic 4f(13) ground state in Yb 4f(14). At th e critical concentration x = 1.5 the system is in a state of magnetic insta bility, where typical non-Fermi-liquid (nFl)-properties are observed, e.g., a - log T contribution in C/T and a power law in p = p(0) + cT(alpha)with alpha < 2. We have measured the specific and electrical resistance of YbCu3 .5Al1.5 in variuos magnetic fields in order to clarify the nature of the no n-Fermi-liquid behavior in this series. The specific heat divided by temper ature in zero field can be described by a Self Consistent Renormalization ( SCR)-model for a 3D antiferromagnet down to the lowest measured temperature and over more than a decade of temperature. The resistance data clearly de viate from the Fermi-liquid (FI)-T-2-behavior in zero magnetic field, where as with increasing field, the range of the Fi-behavior enlarges. The deviat ed SCR parameters indicate that the system is not exactly at the quantum cr itical point (QCP) and the application of external magnetic field pushes th e system further away from the QCP. A hyperscaling analysis of the specifis heat data taken in various magnetic fields in the region where C/T is prop ortional to - log T points to collective effects responsible for the nFl-be havior in YbCu3.5Al1.5. The scaling disappears for T < 240 mK which would p oint to a crossover from weakly interacting to non Gaussian strongly intera cting spin fluctuations as the cause of the nFl-behavior.