Emerging giant symmetric strain response in strongly hole-doped iron-based superconductors*

The symmetry channel and strength of nematic fluctuations, as well as possible nematic order, in the strongly hole-doped iron-based superconductors remain obscure. We address these questions using transport measurements under elastic strain and elucidate the evolution of the elastoresistance induced by hole doping BaFe₂As₂ from optimal to the fully overdoped end-members AFe₂As₂ (A = K, Rb, Cs) [1,2]. Our technical development takes into consideration the extreme thermal expansion of some of these compounds [1]. By decomposing the strain response into the appropriate symmetry channels, we demonstrate the emergence of a giant in-plane symmetric contribution, associated with the growth of strong electronic correlations and their growing strain-sensitivity. We find weakened remnants of the nematic fluctuations that are present at optimal doping, but no change in the symmetry channel of nematic fluctuations with hole doping [2]. Furthermore, we find no indication of a nematic ordered state in the AFe₂As₂ (A = K, Rb, Cs) superconductors above T_c.