Mössbauer spectroscopy study of nematicity in Ba(Fe_0.962Cu_0.038)₂As₂ 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(Fe_0.962Cu_0.038)₂As₂ single 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 individual t _2g orbital, are sensitive to the local nematicity of Fe ions. The nematic phase transition happens at T _nem ≈ 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. (paper)