Discovery of Dynamical Heterogeneity in a Supercooled Magnetic Monopole Fluid

Dynamical heterogeneity in which transitory local fluctuations occur in the conformation and dynamics of constituent particles, is essential for evolution of supercooled liquids into the glass state. Yet its microscopic spatiotemporal phenomenology has remained unobservable in virtually all supercooled glass forming liquids. Recent theoretical advances predict that corresponding dynamical heterogeneity could also occur in supercooled magnetic monopole fluids. Motivated thus, we searched for dynamical heterogeneity when entering the supercooled monopole fluid of Dy2Ti2O7. By measuring microsecond-resolved spontaneous magnetization noise M(t,T) at temperatures between 15 mK<T<2500 mK we discover a sharp bifurcation in monopole noise characteristics beginning below T≈ 1500 mK, with the appearance of powerful monopole current bursts. This unique new form of dynamical heterogeneity first emerges upon entering the supercooled monopole fluid regime, reaches maximum intensity near T≈ 500 mK and then terminates along with coincident loss of ergodicity near T250 mK. Surprisingly, however, low intensity monopole noise representing activity at approximately 2% of the Dy sites persists below T250 mK, implying a population of dynamical monopoles trapped within the spin-ice ground state as T→ 0. This overall phenomenology significantly expands our knowledge of both supercooled monopole fluids and of the ground state of spin-ice. More generally, we demonstrate how comprehensive direct detection of the time sequence, magnitude and statistics of dynamical heterogeneity can greatly accelerate fundamental vitrification studies.