Mossbauer Spectroscopy Study Of Nematicity In Ba(Fe0.962cu0.038)(2)As-2 Single 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 BeFe2As2 is 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(Fe0.962Cu0.038)(2)As-2 single crystal by using Mossbauer spectroscopy. The electric field gradient and its in-plane asymmetry on Fe nucleus, which are directly determined by the occupation of individual t (2g ) orbital, are sensitive to the local nematicity of Fe ions. The nematic phase transition happens at T (nem) approximate to 73.8 K in the compound while the band splitting between d ( xz )/d ( yz ) orbitals begins far above T (nem) and 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 T (nem). However, the spin-lattice relaxation signal is only evident below T (nem). These observations show that the role of orbital degrees of freedom is more active in driving nematicity than in Co- or Ni-doped BaFe2As2 compounds, and can be attributed to enhanced electronic localization caused by Cu doping.