Quadrupole Effects of Layered Iron Pnictide Superconductor Ba(Fe0.9Co0.1)2As2

Elastic constant measurements on iron pnictide Ba(Fe0.9Co0.1)(2)As-2 with an optimal superconducting transition temperature of T-SC = 23 K have been performed by the ultrasonic pulse echo method. The shear elastic constant of C-66 associated with elastic strain epsilon(xy) reveals considerable softening of 21% below 300 K down to T-SC and becomes increasing in the superconducting phase below T-SC, while other shear elastic constants of (C-11 -C-12)/2 and C-44 show no sign of softening below 80 K down to 4.2 K. The electric quadrupole O-v' existing in the degenerate d(y'z) and d(zx') bands participates in the superconductivity of the present iron pnictide system.