Temperature-enhanced nearly critical magnetic scattering in nearly superconducting FeTe$_{0.87}$S$_{0.13}$

Recent neutron scattering measurements [1] have revealed an unusual temperature-induced enhancement of dynamical magnetism in iron telluride, FeTe, the parent material of the chalcogenide family of iron-based superconductors. Here we report a study of magnetic excitations in the sulfur-doped FeTe$_{0.87}$S$_{0.13}$, where bulk measurements show presence of the filamentary superconductivity [2]. Our neutron measurements probe the bulk of the material, which is nearly critical (filamentary superconductivity emerges in a small volume fraction of the sample, which is supercritical). We observe a peculiar pattern of low-energy magnetic scattering, characteristic of critical magnetic fluctuations, and find that it is also anomalously enhanced with the increasing temperature up to $\sim 100$ K. \\[4pt] [1] I. A. Zaliznyak, \emph{et. al.}, Z. J. Xu, J. M. Tranquada, G. D. Gu, A. M. Tsvelik, M. B. Stone, Phys. Rev. Lett., in press (2011).\\[0pt] [2] Rongwei Hu, E. S. Bozin, J. B. Warren, C. Petrovic, Phys. Rev. B {\bf 80}, 214514 (2009).