SciPost Phys. 11, 009 (2021) ·
published 13 July 2021

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In chiral magnets a magnetic helix forms where the magnetization winds around a propagation vector $\boldsymbol{q}$. We show theoretically that a magnetic field $\boldsymbol{B}_\perp(t) \perp \boldsymbol{q}$, which is spatially homogeneous but oscillating in time, induces a net rotation of the texture around $\boldsymbol{q}$. This rotation is reminiscent of the motion of an Archimedean screw and is equivalent to a translation with velocity $v_{screw}$ parallel to $\boldsymbol{q}$. Due to the coupling to a Goldstone mode, this nonlinear effect arises for arbitrarily weak $\boldsymbol{B}_\perp(t) $ with $v_{screw} \propto {\boldsymbol{B}_\perp}^2$ as long as pinning by disorder is absent. The effect is resonantly enhanced when internal modes of the helix are excited and the sign of $v_{screw}$ can be controlled either by changing the frequency or the polarization of $\boldsymbol{B}_\perp(t)$. The Archimedean screw can be used to transport spin and charge and thus the screwing motion is predicted to induce a voltage parallel to $\boldsymbol{q}$. Using a combination of numerics and Floquet spin wave theory, we show that the helix becomes unstable upon increasing $\boldsymbol{B}_\perp$ forming a `time quasicrystal' which oscillates in space and time for moderately strong drive.
SciPost Phys. 9, 057 (2020) ·
published 21 October 2020

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Weakly pumped systems with approximate conservation laws can be efficiently described by a generalized Gibbs ensemble if the steady state of the system is unique. However, such a description can fail if there are multiple steady state solutions, for example, a bistability. In this case domains and domain walls may form. In onedimensional (1D) systems any type of noise (thermal or nonthermal) will in general lead to a proliferation of such domains. We study this physics in a 1D spin chain with two approximate conservation laws, energy and the $z$component of the total magnetization. A bistability in the magnetization is induced by the coupling to suitably chosen Lindblad operators. We analyze the theory for a weak coupling strength $\epsilon$ to the nonequilibrium bath. In this limit, we argue that one can use hydrodynamic approximations which describe the system locally in terms of space and timedependent Lagrange parameters. Here noise terms enforce the creation of domains, where the typical width of a domain wall goes as $\sim 1/\sqrt{\epsilon}$ while the density of domain walls is exponentially small in $1/\sqrt{\epsilon}$. This is shown by numerical simulations of a simplified hydrodynamic equation in the presence of noise.
Jan Gelhausen, Michael Buchhold, Achim Rosch, Philipp Strack
SciPost Phys. 1, 004 (2016) ·
published 23 October 2016

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The fields of quantum simulation with cold atoms [1] and quantum optics [2] are currently being merged. In a set of recent pathbreaking experiments with atoms in optical cavities [3,4] lattice quantum manybody systems with both, a shortrange interaction and a strong interaction potential of infinite range mediated by a quantized optical light field were realized. A theoretical modelling of these systems faces considerable complexity at the interface of: (i) spontaneous symmetrybreaking and emergent phases of interacting manybody systems with a large number of atoms $N\rightarrow\infty$, (ii) quantum optics and the dynamics of fluctuating light fields, and (iii) nonequilibrium physics of driven, open quantum systems. Here we propose what is possibly the simplest, quantumoptical magnet with competing short and longrange interactions, in which all three elements can be analyzed comprehensively: a Rydbergdressed spin lattice [5] coherently coupled to a single photon mode. Solving a set of coupled evenodd sublattice Master equations for atomic spin and photon meanfield amplitudes, we find three key results. (R1): Superradiance and a coherent photon field can coexist with spontaneously broken magnetic translation symmetry. The latter is induced by the shortrange nearestneighbor interaction from weakly admixed Rydberg levels. (R2): This broken evenodd sublattice symmetry leaves its imprint in the light via a novel peak in the cavity spectrum beyond the conventional polariton modes. (R3): The combined effect of atomic spontaneous emission, drive, and interactions can lead to phases with anomalous photon number oscillations. Extensions of our work include nanophotonic crystals coupled to interacting atoms and multimode photon dynamics in Rydberg systems.
Prof. Rosch: "We thank the referee for the p..."
in Submissions  report on Bistabilities and domain walls in weakly open quantum systems