Doping evolution of spin fluctuations and their peculiar suppression at low temperatures in Ca(Fe1−xCox)2As2

Abstract

Results of inelastic neutron scattering measurements are reported for two annealed compositions of ${\mathrm{Ca}(\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2},x=0.026$ 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 $x=0.026$, similar to the parent ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ compound. We conclude that the well-defined spin waves are likely to be present for all $x$ corresponding to the AFM state. This behavior is in contrast to the smooth evolution to overdamped spin dynamics observed in ${\mathrm{Ba}(\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$, wherein the crossover corresponds to microscopically coexisting AFM order and SC at low temperature. The smooth evolution is likely absent in ${\mathrm{Ca}(\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$ due to the mutual exclusion of AFM ordered and SC states. Overdamped spin dynamics characterize paramagnetism of the $x=0.030$ sample and high-temperature $x=0.026$ sample. A sizable loss of magnetic intensity is observed over a wide energy range upon cooling the $x=0.030$ 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 $c$-axis lattice parameter in proximity to the nonmagnetic collapsed tetragonal phase and another scenario ascribes the loss to a formation of a pseudogap.