Elastic properties of iron-based superconductor SrFe2(As1-xPx)2

We have measured the transverse elastic constants C₄₄ and C₆₆ of iron-based superconductor SrFe₂(As_1-xP_x)₂ (Sr122) single crystals as a function of temperature. Under-doped samples show elastic anomalies towards the structural/magnetic transition temperature. Optimal sample shows an upturn at the superconducting transition temperature in both C₄₄ and C₆₆. These behavior is similar to Ba122, while only C₆₆ shows anomaly for Ba122. The elastic anomalies were analyzed by Jahn-Teller formula, and it was found that the Jahn-Teller energy of C₄₄ is much larger than that of C₆₆. This indicates that monoclinic structural fluctuations exist inherently in Sr122 in addition to the known tetragonal fluctuations. Co-existence of these diverse fluctuations and their cooperation are a key to investigate the mechanism and properties of superconductivity in iron based superconductors.