Optical investigation of BaFe2(As0.77P0.23)2 : Spin-fluctuation-mediated superconductivity under pressure

Temperature-dependent reflectivity measurements in the frequency range 75--8000 ${\mathrm{cm}}^{\ensuremath{-}1}$ were performed on ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{0.77}{\mathrm{P}}_{0.23})}_{2}$ single crystals under pressure up to $\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The obtained optical conductivity spectra have been analyzed to extract the electron-boson spectral density ${\ensuremath{\alpha}}^{2}F(\mathrm{\ensuremath{\Omega}})$. A sharp resonance peak was observed in ${\ensuremath{\alpha}}^{2}F(\mathrm{\ensuremath{\Omega}})$ upon the superconducting transition, persisting throughout the applied pressure range. The energy and temperature dependences of this peak are consistent with the superconducting gap opening. Furthermore, several similarities with other experimental probes such as inelastic neutron scattering (INS) [D. S. Inosov et al., Nat. Lett. 6, 178 (2010)] give evidence for the coupling to a bosonic mode, possibly due to spin fluctuations. Moreover, electronic correlations have been calculated via spectral weight analysis, which revealed that the system stays in the strongly correlated regime throughout the applied pressure range. However, a comparison to the parent compound showed that the electronic correlations are slightly decreased with P doping. The investigation of the phase diagram obtained by our optical study under pressure also revealed the coexistence of the spin-density wave and the superconducting regions, where the coexistence region shifts to the lower pressure range with increasing P content. Moreover, the optimum pressure range, where the highest superconducting transition temperature has been obtained, shows a nonlinear decrease with increasing P content.