Non-linear lattice response of SmFeAsO_{1-x}F_{x} superconductor to hydrostatic pressure

Abstract

Hydrostatic pressure Raman measurements have been carried out on the SmFeAsO series of oxypnictides with varying amount of doping (F substitution for O and Co for Fe) and transition temperature (T_{c}), in order to investigate lattice modifications and their connection to doping and superconductivity. Synchrotron XRD data on some of these compounds indicated that at low doping the lattice constants vary smoothly with pressure, but with increasing F concentration there is a deviation from the normal equation of state and these effects are related with modifications in the superconducting FeAs4 tetrahedra. The hydrostatic pressure Raman measurements show that the A1g mode of the rare earth atom for the superconducting compounds deviates from the linear pressure dependence at the same pressures where the XRD results indicate pressure-induced lattice anomalies. A similar anomaly is found for the As phonon of same symmetry. As in cuprates, the effect is diminished in the undoped compounds and it is not related with the F substitution being present in the Sm(Fe_{1-x}Co_{x})AsO as well. The calculated Gr\"uneisen parameter for the Sm phonon ({\gamma \approx}1.5) is very similar to the corresponding values of cuprates and it does not vary with doping. For the Fe mode it has higher value ({\gamma \approx}1.8) than for As ({\gamma \approx}1) indicating a more anharmonic phonon.