Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

Spontaneous C4-symmetry breaking phases are ubiquitous in layered quantum materials and compete with other phases such as superconductivity. Phenomena like light induced superconductivity and metallicity are believed to arise from optical excitations selectively melting the C4-symmetry breaking phase, enabling competing phases to emerge. Phenomenological models suggest that light directly modifies the order parameter or its potential. However, clear evidence for such a process is lacking. Here, we measure the order parameter dynamics of the C4 symmetry breaking transition in the prototypical manganite La0.5Sr1.5MnO4. Despite some degrees of freedom appearing to show a coherent response and critical slowing, consistent with previous measurements, we find the order-parameter is overdamped and incoherent. Concomitant changes in the lattice potential of the entire system in under ~25 fs show rapid energy redistribution and lack of selectivity in photoexcitation, suggesting a disorder-driven transition where the order parameter potential plays a minimal role, in contrast to previous models.