Magnetic fluctuations at the field-tuned vs. concentration-tuned quantum phase transition in [formula omitted

Abstract

CeCu 6 - x Au x 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., χ - 1 ( q , E , T ) = c - 1 [ θ α ( q ) + ( T - i E ) α ] , where θ ( q ) is a generalized Curie–Weiss temperature. This scaling suggests local criticality, with an anomalous exponent α ≈ 0.75 . Here we report on an INS study at the field-induced magnetic QPT of CeCu 5.8 Au 0.2 where the Néel temperature is driven to zero at a critical field of B ≈ 0.4 T , supplemented by specific-heat data. The INS data can be described better by the spin-density-wave scenario, i.e., χ - 1 ( Q AF , E , T ) = a - 1 ( T 3 / 2 - i bE ) 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.