Magnetic fluctuations at the field-tuned vs. concentration-tuned quantum phase transition in CeCu6-xAux

CeCu6-xAux has become a model system to study magnetic quantum phase transitions (QPT). In this system, the QPT can be tuned by Au concentration x, pressure, or magnetic field. Previous inelastic neutron-scattering (INS) experiments have demonstrated an anomalous scaling of the dynamical susceptibility at the concentration-tuned (zero-field) QPT for x 0.1, i.e., chi(-1)(q; E, T) = c(-1) [theta(alpha)(q) + (T - iE)(alpha)], where theta(q) is a generalized Curie-Weiss temperature. This scaling suggests local criticality, with an anomalous exponent alpha approximate to 0.75. Here we report on an INS study at the field-induced magnetic QPT of CeCu5.8Au0.2 where the Neel temperature is driven to zero at a critical field of B approximate to 0.4T, supplemented by specific-heat data. The INS data can be described better by the spin-density-wave scenario, i.e., chi(-1) (Q(AF); E, T) = a(-1) (T-3/2 - ibE) where Q(AF) is the antiferromagnetic ordering wave vector, than by a local quantum critical point. This constitutes direct microscopic evidence for a difference in quantum fluctuation spectra at a magnetic QPT driven by different tuning parameters. (c) 2006 Published by Elsevier B.V.