Ultrafast surface melting of orbital order in La_0.5Sr_1.5MnO₄

Understanding how light modifies long-range order in quantum materials is key to improving our ability to control functionality. However, this is challenging if the response is heterogeneous. Here we address the most common form of light-induced heterogeneity-surface melting-and measure the dynamics of orbital order in the layered manganite La_0.5Sr_1.5MnO₄. We isolate the surface dynamics from the bulk by measuring the orbital truncation rod and orbital Bragg peak. After photoexcitation, the orbital Bragg peak shows an unusual narrowing, which suggests an increase in correlation length of the probed volume. By contrast, the correlation length at the surface decreases. These differences can be reconciled if the material is heterogeneous, and light melts a less ordered surface. By isolating the surface response, we determine that the loss of long-range order is an incoherent process, which is probably accompanied by the formation of local polarons.