Abstract
Results of inelastic neutron scattering measurements are reported for two annealed compositions of and 0.030, which possess stripe-type antiferromagnetically ordered and superconducting ground states, respectively. In the AFM ground state, well-defined and gapped spin waves are observed for , similar to the parent compound. We conclude that the well-defined spin waves are likely to be present for all corresponding to the AFM state. This behavior is in contrast to the smooth evolution to overdamped spin dynamics observed in , wherein the crossover corresponds to microscopically coexisting AFM order and SC at low temperature. The smooth evolution is likely absent in due to the mutual exclusion of AFM ordered and SC states. Overdamped spin dynamics characterize paramagnetism of the sample and high-temperature sample. A sizable loss of magnetic intensity is observed over a wide energy range upon cooling the sample, at temperatures just above and within the superconducting phase. This phenomenon is unique amongst the iron-based superconductors and is consistent with a temperature-dependent reduction in the fluctuating moment. One possible scenario ascribes this loss of moment to a sensitivity to the -axis lattice parameter in proximity to the nonmagnetic collapsed tetragonal phase and another scenario ascribes the loss to a formation of a pseudogap.
- Received 5 February 2018
- Revised 13 May 2018
DOI:https://doi.org/10.1103/PhysRevB.97.174519
©2018 American Physical Society