Mössbauer spectroscopy study of nematicity in Ba(FeCu)Assingle crystal: enhanced orbital effect.

The origin of the nematic order remains unclear due to the strong coupling between orbital, spin and lattice degrees of freedom in iron-based superconductors. Although the driving force of hole-doped BeFeAsis still controversial, the nematic fluctuation of electron-doped compounds is generally believed to be spin fluctuation driven. Here, we present a comprehensive study of the nematic phase transition in Ba(FeCu)Assingle crystal by using Mössbauer spectroscopy. The electric field gradient and its in-plane asymmetry on Fe nucleus, which are directly determined by the occupation of individualorbital, are sensitive to the local nematicity of Fe ions. The nematic phase transition happens at≈ 73.8 K in the compound while the band splitting between/orbitals begins far aboveand reaches 18.8 meV at 30 K. The temperature evolution of the hyperfine parameters proves the existence of electron-phonon interaction and non-Fermi-liquid behaviour near. However, the spin-lattice relaxation signal is only evident below. These observations show that the role of orbital degrees of freedom is more active in driving nematicity than in Co- or Ni-doped BaFeAscompounds, and can be attributed to enhanced electronic localization caused by Cu doping.