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

Sapkota, A. and Das, P. and Böhmer, A.E. and Ueland, B.G. and Abernathy, D.L. and Bud'Ko, S.L. and Canfield, P.C. and Kreyssig, A. and Goldman, A.I. and McQueeney, R.J.

Volume: 97 Pages:
DOI: 10.1103/PhysRevB.97.174519
Published: 2018

Results of inelastic neutron scattering measurements are reported for two annealed compositions of Ca(Fe1-xCox)2As2,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 CaFe2As2 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 Ba(Fe1-xCox)2As2, wherein the crossover corresponds to microscopically coexisting AFM order and SC at low temperature. The smooth evolution is likely absent in Ca(Fe1-xCox)2As2 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. © 2018 American Physical Society.

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