Elastic softening of the shear modulus in Fe-based superconductors*

A strong softening of the elastic shear modulus C$_{\mathrm{66}}$ has been observed as one approaches the spin-density-wave (SDW) transition in Ba122 from high temperature [1,2]. A smaller softening is still observed for superconducting Co-doped Ba122 crystals, followed by distinct hardening below T$_{\mathrm{c\thinspace }}$[1,2]. This elastic response has been taken as evidence either for electronic-magnetic nematic fluctuations [1], or as evidence for a structural quantum critical point near optimal doping [2]. Here we study the elastic response of various Fe-based superconductors by a recently developed technique based upon a three-point bending experiment in a high-resolution capacitance dilatometer. We measure the temperature dependence of the Young's modulus, which for thin slabs can be shown to be closely related to C$_{\mathrm{66}}$ for a given orientation. This is confirmed by measurements on Co-doped Ba122, for which we find very similar results as previously reported [1,2]. We will report on new measurements of the Young's modulus on other Fe-based Ba122 systems in order to study the universality of the elastic response at the SDW and superconducting transitions. [1] R. M. Fernandes, et al., Phys. Rev. Lett. 105, 157003 (2010). [2] M. Yoshizawa, et al., J. Phys. Soc. Jpn. 81, 024604 (2012).