Fig. 4: Evolution of long range order and phase change on the acoustic time scales.

a, c Parameters extracted from Rietveld refinement of the XRD data. a Temporal evolution of the λ- phase unit cell volume (ΔV_λ, dark orange, filled circles) and microstrain parameter ϕ_λ (light orange, filled squares). c Temporal evolution of the β- phase unit cell volume (ΔV_β,blue) and λ- phase fraction (ΔX_λ, red). Relative changes normalized to the change at 50 ps and errors estimated as described in the Methods section. b, d Simulation with the Thomsen model¹¹ of the parameters shown in (a, c). All details are given in the SI. b λ- crystallites. Calculation of the expected evolution of the volume change (ΔV_{λ, calc,} dark orange) and microstrain (S. D._{λ, calc,} light orange) following ultrafast excitation. Parameters of the model are taken as follows: laser penetration depth of χ_L = 65 nm, sound velocity of v_s = 6.5 × 10³ m s⁻¹; curves normalized to the value at t = 50 ps. d β-crystallites. Calculation of the expected evolution of the volume change (ΔV_β,calc, blue) and λ- phase fraction (ΔX_λ,calc, red) following ultrafast excitation. The corresponding acoustic front propagation time corresponding to z_S = 100 nm, ϕ_PF = z_S/v_s) is shown with solid black lines in (a, b, c, d).