Photoinduced Excited-State Dynamics in Co-Doped BaFe₂As₂ Superconducting Films

We report on nonlinear optical dynamics and structural reconstruction of Co-doped BaFe₂As₂single-crystal superconducting films within a broad range of temperatures. Films were grown by pulsed laser deposition on (La,Sr)(Al,Ta)O3 [LSAT] and LaAlO₃ [LAO] crystalline substrates with epitaxial SrTiO₃ [STO] template buffer layer used for higher epitaxial bonding of BaFe₂As₂. Samples show superconducting transition with T_c~15.5-20 K. Temperature-dependent optical transmittance measurements in the range from 290 K down to 10 K were conducted using a supercontinuum laser source, revealing a distinct increase of the transmittance within the wavelengths range from λ=400 nm to λ=1.6 µm, as the sample temperature drops below T_c. Reflectivity measurements at λ=800 nm also show an abrupt decrease of the reflectance below T_c. Drude-Lorentz model was applied to fit the optical transmittance spectra in order to demonstrate the contribution of a free-carriers component in the optical signal. The noticeable change of both, carrier scattering rate and plasma frequency near ~150 K is associated with the spin density wave (SDW) transition in BaFe₂As₂. The angle-resolved hemispherical elastic light scattering surface metrology measurements performed with a light scatterometer at cryogenic temperatures indicate that this transition is accompanied by surface reconstruction. High-resolution angle-resolved hemispherical scattering imaging of multi-scale nonequilibrium processes in BaFe₂As₂reveals that the small change in the elastic energy upon SDW formation results in the splitting of the film into domains due to the same or different structural phases since this process is accompanied by tetragonal-to-orthorhombic phase transition. Several competitive transition processes at ~150 K alter the correlation length of the surface and optical properties. In the region of superconducting transition, BaFe₂As₂films demonstrate abrupt changes in surface morphology (surface autocorrelation length, optical isotropy, and root-mean-square roughness). A strong correlation between the observed temperature dependence of the surface statistics and quasiparticle dynamics near T_cwas found. Time-resolved measurements of transient reflectivity performed in a relatively high-fluence regime, of several mJ/cm², reveal a strong dependence of the excited-states-dynamics on sample temperature, morphology, and laser excitation energy. Femtosecond photoexcitation demonstrates complex reflectance traces attributed to two distinct ultrafast processes: (i) instantaneous formation of the nonequilibrium state of quasiparticles with its subsequent thermalization within ~3 ps with characteristic time τ₁, and (ii) slower few-picoseconds phonon-phonon scattering. Temperature-dependence of τ₁ shows its noticeable drop as the temperature rises above Tc. Observed nonlinear optical dynamics correlate with the temperature dependence of the surface morphology near T_c and provide a new understanding of light-induced processes in the superconducting state of BaFe₂As₂.