Debasmita Swain1,Soumyadeep Ghosh2,Kousik Bera3,Sven Friedemann4,Haranath Ghosh5,Anushree Roy1,Sitikantha Das1
Indian Institute of Technology, Kharagpur1,Lawrence Berkeley National Laboratory2,Indian Institute of Technology Kharagpur3,University of Bristol4,RRCAT, Indore5
Debasmita Swain1,Soumyadeep Ghosh2,Kousik Bera3,Sven Friedemann4,Haranath Ghosh5,Anushree Roy1,Sitikantha Das1
Indian Institute of Technology, Kharagpur1,Lawrence Berkeley National Laboratory2,Indian Institute of Technology Kharagpur3,University of Bristol4,RRCAT, Indore5
The concept of nematicity is still not well settled in case of Fe based Superconductors. Signatures of nematic ordering exist well above the structural transition temperature i.e. the presence of orthorhombic distortion in the tetragonal symmetry broken phase. Tetragonal symmetry dictates that both Fe d<sub>xz</sub> and d<sub>yz</sub> orbitals should be degenerate, whereas studies like ARPES, X-ray pair distribution function etc. show the unequal occupation of Fe d<sub>xz</sub> and d<sub>yz</sub> orbitals in the symmetry broken orthorhombic phase. By using Raman spectroscopy, we have studied P doped BaFe<sub>2</sub>As<sub>2</sub> for x~0.23, whose doping composition lies near quantum criticality and enhanced nematic fluctuation in the doping temperature phase diagram. From transport measurement it is observed that P doped BaFe<sub>2</sub>As<sub>2</sub> exhibits a tetragonal to orthorhombic phase transition around 60K followed by a superconducting transition below ~16K. A room temperature micro-Raman spectrum of the studied compound shows a phonon mode around ~211cm<sup>-1</sup> with two broad high frequency modes around 515 cm<sup>-1</sup> and 635cm<sup>-1</sup>. The separation between these two modes remains around 15 mev at room temperature. Temperature dependent Raman measurement shows that the B<sub>1g</sub> mode (phonon mode around ~211cm<sup>-1</sup>) follows standard anharmonicity behavior within a temperature range from 80 to 300K, whereas with varying temperature these broad modes show a drop in intensity which is typically observed for electronic Raman modes. In the non-superconducting state, strong anisotropic behavior is observed for these BMs in polarization dependent Raman scattering. Electronic structure calculations for P doped and undoped BaFe<sub>2</sub>As<sub>2</sub> show that while Fe d<sub>xz</sub> and d<sub>yz</sub> 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. Hence, we believe that these reported broad modes bear signatures of crystal field splitting of Fe d orbitals due to local breaking of tetragonal C<sub>4</sub> symmetry in the doped system.