Possible Raman signature of broken symmetry states near the quantum critical point in P doped BaFe2As2: Experiment and theory

Single crystals of the iron-pnictide compound BaFe₂(As1_-xP_x)₂ for x ∼0.23, with a doping concentration near quantum criticality and enhanced nematic fluctuating state in the doping-temperature phase diagram, have been studied. Transport measurements confirm the presence of a magneto-structural transition at 60 K from the tetragonal to the orthorhombic phase, followed by a superconducting transition below 16 K. Micro-Raman spectra of the compound exhibit a phonon mode at 211 cm^-1 and two broad modes (BMs) at 515 cm^-1 and 635 cm^-1, which corresponds to an energy difference of 15 meV. The temperature dependent Raman measurements reveal that the intensity of these BMs drop over the temperature range between 80 K and 300 K. In the non-superconducting state, strong anisotropic behaviour is observed for these BMs in polarization dependent Raman scattering. Electronic structure calculations for doped and undoped BaFe₂As₂ show that while Fe d_xz and d_yz orbitals do not split in the tetragonal phase, the splitting energy is 13.5 meV in the orthorhombic phase of the doped system, which is reasonably close to the experimentally observed value of the energy separation of the BMs. We believe that the reported BMs are consistent with the signature of electronic Raman scattering involving the crystal field levels of d-orbitals of Fe^2＋ due to local breaking of the C₄ symmetry in the doped system.