Observation of ergodicity breaking dynamics near the spin-stripe transition in amorphous FeGe thin-films

Fluctuations originating from a magnetic order can lead to a phase transition which intricately depends on the topology of the magnetic energy landscape. The dynamical response of such a system close to the phase transition is usually represented by a normal distribution and thus the dynamical parameters are obtained from the mean and the variance value. Here by using coherent resonant soft X-ray scattering we obtain the higher order moments of the fluctuation distributions near a spin-spiral stripe melting transition in an amorphous FeGe thin film. The higher order moments i.e skew and excess kurtosis directly estimate the degree of asymmetry in the distributions thereby providing valuable insights into the nature of the dynamics governing the phase transition. We find that the system is ergodic very close to the transition, as reflected by a normal fluctuation distribution. At slightly lower temperature the distributions are significantly skewed, implying non-ergodicity. Ergodicity breaking is at the heart of the well-known phenomena of Many Body Localization (MBL) where local functions retains memory of the initial state by reaching some stationary values which are non-thermal. As a result, systems with strong ergodicity breaking can sustain much longer quantum coherence.