Nematic-Fluctuation-Mediated Superconductivity Revealed by Anisotropic Strain in Ba ( Fe 1 − x Co x ) 2 As 2

Anisotropic strain is an external field capable of selectively addressing the role of nematic fluctuations in promoting superconductivity. We demonstrate this using polarization-resolved elasto-Raman scattering to probe the evolution of nematic fluctuations under strain in the normal and superconducting states of the paradigmatic iron-based superconductor Ba(Fe1−x1−x_{1-x}Coxx_x)22_2As22_2. In the non-superconducting parent compound BaFe22_2As22_2 we observe a strain-induced suppression of the nematic susceptibility which follows the expected behavior of an Ising order parameter under a symmetry breaking field. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the superconducting critical temperature TcTcT_c. Our results indicate a significant contribution of nematic fluctuations to electron pairing and validate theoretical scenarios of enhanced TcTcT_c near a nematic quantum critical point.