SciPost Phys. 7, 029 (2019) ·
published 9 September 2019
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Coarsening dynamics, the canonical theory of phase ordering following a
quench across a symmetry breaking phase transition, is thought to be driven by
the annihilation of topological defects. Here we show that this understanding
is incomplete. We simulate the dynamics of an isolated spin-1 condensate
quenched into the easy-plane ferromagnetic phase and find that the mutual
annihilation of spin vortices does not take the system to the equilibrium
state. A nonequilibrium background of long wavelength spin waves remain at the
Berezinskii-Kosterlitz-Thouless temperature, an order of magnitude hotter than
the equilibrium temperature. The coarsening continues through a second much
slower scale invariant process with a length scale that grows with time as
$t^{1/3}$. This second regime of coarsening is associated with spin wave energy
transport from low to high wavevectors, bringing about the the eventual
equilibrium state. Because the relevant spin waves are noninteracting, the
transport occurs through a dynamic coupling to other degrees of freedom of the
system. The transport displays features of a spin wave energy cascade,
providing a potential profitable connection with the emerging field of spin
wave turbulence. Strongly coupling the system to a reservoir destroys the
second regime of coarsening, allowing the system to thermalise following the
annihilation of vortices.
