Quadrupole effects of layered iron pnictide superconductor Ba(Fe _0.9Co_0.1)₂As₂

Elastic constant measurements on iron pnictide Ba(Fe_0.9Co _0.1)₂As₂ with an optimal superconducting transition temperature of TSC = 23K have been performed by the ultrasonic pulse echo method. The shear elastic constant of C₆₆ associated with elastic strain "xy reveals considerable softening of 21% below 300K down to TSC and becomes increasing in the superconducting phase below TSC, while other shear elastic constants of (C11 - C12)=2 and C₄₄ show no sign of softening below 80K down to 4.2 K. The electric quadrupole Ov' existing in the degenerate dy'z and dzx' bands participates in the superconductivity of the present iron pnictide system.