  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.
