Hidden nematic fluctuation in the triclinic (Ca0.85La0.15)10(Pt3As8)(Fe2As2)5 superconductor revealed by ultrafast optical spectroscopy

We reported the quasiparticle relaxation dynamics of an optimally doped triclinic iron-based superconductor ${({\mathrm{Ca}}_0.85{\mathrm{La}}_0.15)}₁₀({\mathrm{Pt}}₃{\mathrm{As}}₈){({\mathrm{Fe}}₂{\mathrm{As}}₂)}₅$ with bulk ${T}_c=30\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ using polarized ultrafast optical pump-probe spectroscopy. Our results reveal anisotropic transient reflectivity induced by nematic fluctuations develops below ${T}_{\text{nem}}\ensuremath{\approx}120$ K and persists in the superconducting states. Measurements under high pump fluence reveal three distinct, coherent phonon modes at frequencies of 1.6, 3.5, and 4.7 THz, corresponding to ${A}_1g(1), {E}_g$, and ${A}_1g(2)$ modes, respectively. The high-frequency ${A}_1g(2)$ mode corresponds to the $c$-axis polarized vibrations of FeAs planes with a nominal electron-phonon coupling constant ${\ensuremath{\lambda}}_{{A}_1g(2)}=0.139\ifmmode\pm\else\textpm\fi{}0.02$. Our results suggest that, at low temperatures, superconducting and nematic states coexist but compete with each other, and it is possible that ${A}_1g$ phonons are implicated in the formation of Cooper pairs in ${({\mathrm{Ca}}_0.85{\mathrm{La}}_0.15)}₁₀({\mathrm{Pt}}₃{\mathrm{As}}₈){({\mathrm{Fe}}₂{\mathrm{As}}₂)}₅$.