Discovery of Dynamical Heterogeneity in a Supercooled Magnetic Monopole Fluid

Abstract: Dynamical heterogeneity, in which transitory local fluctuations occur in the conformation and dynamics of constituent particles, is widely hypothesized to be essential to evolution of supercooled liquids into the glass state. Yet its microscopic spatiotemporal phenomenology has remained unobservable in virtually all molecular glass forming liquids. Because recent theoretical advances predict that corresponding dynamical heterogeneity could occur in supercooled magnetic monopole fluids, we searched for such phenomena in Dy2Ti2O7. By measuring its microsecond-resolved spontaneous magnetization fluctuations $M(t,T)$ we discovered a sharp bifurcation in monopole noise characteristics below $T \approx 1500$ mK, with the appearance of powerful spontaneous monopole current bursts. This intense dynamics emerges upon entering the supercooled monopole fluid regime, reaches maximum strength near $T \approx 500$ mK and then terminates along with coincident loss of ergodicity near $T \lesssim 250$ mK. Moreover, the four-point dynamical susceptibility $\chi_4(T, t)$ is observed directly and evolves as predicted when dynamical heterogeneity is present, clearly revealing its diverging length scales $\xi(T)$. This overall phenomenology greatly expands our empirical knowledge of supercooled monopole fluids and, more generally, demonstrates direct detection of the time sequence, magnitude, statistics and correlations of dynamical heterogeneity, access to which may greatly accelerate fundamental vitrification studies.