Mössbauer spectroscopy study of nematicity in Ba(Fe 0.962 Cu 0.038) 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 BeFe2As2is 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)2As2single 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 t2gorbital, are sensitive to the local nematicity of Fe ions. The nematic phase transition happens at Tnem≈ 73.8K in the compound while the band splitting between dxz/dyzorbitals begins far above Tnemand reaches 18.8meV at 30K. The temperature evolution of the hyperfine parameters proves the existence of electron-phonon interaction and non-Fermi-liquid behaviour near Tnem. However, the spin-lattice relaxation signal is only evident below Tnem. These observations show that the role of orbital degrees of freedom is more active in driving nematicity than in Co- or Ni-doped BaFe2As2compounds, and can be attributed to enhanced electronic localization caused by Cu doping.