Title: Universality Class of a Spinor Bose-Einstein Condensate far from Equilibrium
Abstract: Scale invariance and self-similarity in physics have provided a unified frame- work to classify phases of matter and dynamical properties near equilibrium. This paradigm has been further applied in isolated many-body quantum systems that can exhibit self-similar time evolution during thermalization. This property can appear in a wide range of systems, such as cosmology, quark-gluon matter, cold atoms, and quantum magnets. Recently, the universal dynamics have been experimentally observed in atomic systems, demonstrating that dynamic scaling behaviour are independent of microscopic details. However, an experimental demonstration for categorizing nonequilibrium dynamics in quantum many-body systems has remained a great challenge. Here, we address this question and demonstrate that the universal dynamics in quantum systems can be classified by the symmetry of the order parameters and by the dynamics of topological defects. By using a ferromagnetic atomic condensate in two dimensions, we quench the system to have Z2 symmetry and find self-similar coarsening dynamics driven by the motions of the magnetic domain walls. The universal scaling is characterized by a power law exponent 1/z ≃ 0.58(2), which can be found in inviscid binary fluid or in Model H. In dynamics with isotropic spin symmetry SO(3), on the other hand, the domain coarsening is dif- fusive 1/z ≃ 0.43(2) and proceeds with the annihilation of the spin vortices. Our experiments demonstrate that far-from-equilibrium dynamics in quantum systems can be categorized into a well-defined universality class and can provide a new opportunity to understand the dynamics of strongly correlated quantum systems under extreme conditions, such as the early universe after inflation.