Crystal-field fluctuations for 4f ions in high-Tc cuprates

Neutron spectroscopy of crystal field (CF) excitations for 4f ions in cuprates like Y1-xRxBa2Cu3O6+y represents one of the powerful tools to probe quasiparticle dynamics over a wide range of time scales. In particular, the CF lifetime measurements provide the information concerning the dynamic susceptibility at the frequency of the CF transition. Conventional approach to the interpretation of linewidth measurements in cuprates is based on the assumption of the predominant contribution of spin-fluctuations through the 4f-3d exchange interaction. This mechanism predicts the ratios of the effective matrix elements for the Stark transitions to be governed by the Lande factors. However, A. Mukherjee et al. when studying the system Y1-xRxBa2Cu3O6+y (R = Er, Ho, Tm) found values clearly disagreeing with those predicted by the exchange mechanism [A. Mukherjee, A.T. Boothroyd, D.McK. Paul et al., Phys. Rev. B 49, 13089 (1994)]. It is argued that the main contribution to the broadening of the linewidth of CF transitions for 4f ions in high-Tc cuprates results from the crystal-field fluctuations. Indeed, a simple assumption of the dominant contribution of the quadrupolar term (k=2) yields for the ratios mentioned above, values given by ratios of Stevens parameters. These values appear to agree with the respective experimental values. Thus, it is concluded that the crystal-field fluctuations induced by the charge and/or orbital fluctuations in copper-oxygen subsystem may be the main origin of the broadening of the linewidth of CF transitions for 4f ions in high-Tc cuprates