Possible signature of broken symmetry state near the quantum critical point in P doped BaFe${}_2$As${}_2$: A Raman spectroscopy study

Abstract

We study the iron-pnictide compound BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ for x $\sim$0.23, with a doping concentration near quantum criticality and enhanced nematic fluctuating state in the doping-temperature phase diagram. Transport measurements confirm the presence of a magneto-structural transition at 60K from the tetragonal to the orthorhombic phase, followed by a superconducting transition below 16K. The temperature and polarisation dependent Raman spectra reveal that there is a phonon mode at 211 cm${}^{-1}$, followed by two broad modes (BM) between 400 and 700 cm${}^{-1}$, having an energy difference of 15 meV, in the temperature range between 300K and 80K. In the non-superconducting state, the phonon mode exhibits expected polarization dependence as well as temperature evolution due to anharmonicity, strong anisotropic and thermally inert behaviour are observed for the BM. Electronic structure calculations for doped and undoped BaFe${}_2$As${}_2$ 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 BM. We believe that reported BM possibly are the signature of electronic Raman scattering involving the crystal field levels of $d$-orbitals of Fe${}^{2+}$ due to local breaking of the C${}_4$ symmetry of the parent compound in the doped system.

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