Fig. 1.

La_1.65Eu_0.2Sr_0.15CuO₄ structure, equilibrium RXS results, and tr-RXS experiment. (A) Crystal structure of LESCO. The crystal axes are labeled according to the high-temperature tetragonal (HTT) setting, and the CuO₂ planes are highlighted in pale red. (B) Schematic depiction of the pump–probe geometry used to access the $Q=(001)$ peak originating from the layer alternating $\mathit{ab}$-plane anisotropy. (C) Equilibrium temperature dependence of the apical O $K$ RXS intensity at $Q=(001)$ compared with the Cu $L_3$ RXS intensity at $Q=(001)$ and $Q_{\text{CDW}}=(0.2601.5)$. (D) Planar O $K$ tr-RXS intensity at $Q=(001)$ probed along the (0 0 $L$) direction in a $\theta -2\theta$ scan. The equilibrium peak profile is compared with scans taken 0.6 ps and 20 ps after excitation with 1.55 eV pump photons at a fluence of 50 $\mathrm{\mu }$J/cm². (E) tr-RXS intensity at the apical O $K$ resonance for $Q=(001)$ at 20 K as a function of pump–probe delay. The pump–probe response is sensitive only to the anisotropy induced by the LTT structural distortion and becomes unobservable for low fluences.