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