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 non-linear 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 one-dimensional (1D) systems any type of noise (thermal or non-thermal) 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 non-equilibrium bath. In this limit, we argue that one can use hydrodynamic approximations which describe the system locally in terms of space- and time-dependent 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 many-body systems with both, a short-range 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 symmetry-breaking and emergent phases of interacting many-body systems with a large number of atoms N\rightarrow\infty, (ii) quantum optics and the dynamics of fluctuating light fields, and (iii) non-equilibrium physics of driven, open quantum systems. Here we propose what is possibly the simplest, quantum-optical magnet with competing short- and long-range interactions, in which all three elements can be analyzed comprehensively: a Rydberg-dressed spin lattice [5] coherently coupled to a single photon mode. Solving a set of coupled even-odd sublattice Master equations for atomic spin and photon mean-field 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 short-range nearest-neighbor interaction from weakly admixed Rydberg levels. (R2): This broken even-odd 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 nano-photonic crystals coupled to interacting atoms and multi-mode 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