KIP-Veröffentlichungen

Quantum many-body systems driven far from equilibrium can show universal self-similar scaling dynamics associated with an approach to a non-thermal fixed point. The characterisation of non-equilibrium universality classes remains an open problem.

In this thesis, we investigate the far-from-equilibrium dynamics of a one-dimensional spin-1 Bose gas. First, we study its microscopic dynamics, identifying rogue waves in the velocity fields, which form real-time instantons in the transverse spin. Their statistics lead to an additional scaling exponent governing the time evolution of a characteristic timescale.

We derive a low-energy effective field theory of the spin-1 gas, taking the form of a double sine-Gordon model for the spinor phase.

This model accounts for the subdiffusive and diffusion-type scaling observed in the full microscopic theory.

It demonstrates that scaling dynamics at non-thermal fixed points go beyond simple domain-size growth processes and connects the type of scaling behaviour to the occupation of the minima of the sinusoidal potential by the double sine-Gordon field. Numerical as well as experimental results support the validity of the effective model.

Furthermore, we investigate the symmetry content of the spin-1 gas using symmetry witnesses derived from Ward identities.

We show that the dynamical restoration of symmetry from an explicitly symmetry-broken state occurs long before thermalisation.

We further demonstrate that different quenches can lead to two distinct non-thermal fixed points, each associated with a different emergent symmetry.

Finally, three-dimensional simulations of a quasi-one-dimensional condensate reconcile the long-standing discrepancies between experimentally and numerically observed scaling exponents.

These findings contribute to a deeper understanding of non-equilibrium physics, establishing a double sine-Gordon non-equilibrium universality class to which the spin-1 Bose gas belongs to, and underline the role of emergent symmetries in determining the non-thermal fixed point the system approaches.