172 research outputs found
Regimes of heating and dynamical response in driven many-body localized systems
We explore the response of many-body localized (MBL) systems to periodic
driving of arbitrary amplitude, focusing on the rate at which they exchange
energy with the drive. To this end, we introduce an infinite-temperature
generalization of the effective "heating rate" in terms of the spread of a
random walk in energy space. We compute this heating rate numerically and
estimate it analytically in various regimes. When the drive amplitude is much
smaller than the frequency, this effective heating rate is given by linear
response theory with a coefficient that is proportional to the optical
conductivity; in the opposite limit, the response is nonlinear and the heating
rate is a nontrivial power-law of time. We discuss the mechanisms underlying
this crossover in the MBL phase, and comment on its implications for the
subdiffusive thermal phase near the MBL transition.Comment: 17 pages, 9 figure
Far-from-equilibrium field theory of many-body quantum spin systems: Prethermalization and relaxation of spin spiral states in three dimensions
We study theoretically the far-from-equilibrium relaxation dynamics of spin
spiral states in the three dimensional isotropic Heisenberg model. The
investigated problem serves as an archetype for understanding quantum dynamics
of isolated many-body systems in the vicinity of a spontaneously broken
continuous symmetry. We present a field-theoretical formalism that
systematically improves on mean-field for describing the real-time quantum
dynamics of generic spin-1/2 systems. This is achieved by mapping spins to
Majorana fermions followed by a 1/N expansion of the resulting two-particle
irreducible (2PI) effective action. Our analysis reveals rich
fluctuation-induced relaxation dynamics in the unitary evolution of spin spiral
states. In particular, we find the sudden appearance of long-lived
prethermalized plateaus with diverging lifetimes as the spiral winding is tuned
toward the thermodynamically stable ferro- or antiferromagnetic phases. The
emerging prethermalized states are characterized by different bosonic modes
being thermally populated at different effective temperatures, and by a
hierarchical relaxation process reminiscent of glassy systems. Spin-spin
correlators found by solving the non-equilibrium Bethe-Salpeter equation
provide further insight into the dynamic formation of correlations, the fate of
unstable collective modes, and the emergence of fluctuation-dissipation
relations. Our predictions can be verified experimentally using recent
realizations of spin spiral states with ultracold atoms in a quantum gas
microscope [S. Hild, et al. Phys. Rev. Lett. 113, 147205 (2014)]
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