68 research outputs found
Josephson Effects in a Bose-Einstein Condensate of Magnons
A phenomenological theory is developed, that accounts for the collective
dynamics of a Bose-Einstein condensate of magnons. In terms of such description
we discuss the nature of spontaneous macroscopic interference between magnon
clouds, highlighting the close relation between such effects and the well known
Josephson effects. Using those ideas we present a detailed calculation of the
Josephson oscillations between two magnon clouds, spatially separated in a
magnonic Josephson junction
Many-body theory of spin-current driven instabilities in magnetic insulators
We consider a magnetic insulator in contact with a normal metal. We derive a
self-consistent Keldysh effective action for the magnon gas that contains the
effects of magnon-magnon interactions and contact with the metal to lowest
order. Self-consistent expressions for the dispersion relation, temperature and
chemical potential for magnons are derived. Based on this effective action, we
study instabilities of the magnon gas that arise due to spin-current flowing
across the interface between the normal metal and the magnetic insulator. We
find that the stability phase diagram is modified by an interference between
magnon-magnon interactions and interfacial magnon-electron coupling. These
effects persist at low temperatures and for thin magnetic insulators.Comment: 10 pages and 5 figure
Magnon-polarons in cubic collinear Antiferromagnets
We present a theoretical study of excitations formed by hybridization between
magnons and phonons - magnon-polarons - in antiferromagnets. We first outline a
general approach to determining which magnon and phonon modes can and cannot
hybridize in a system thereby addressing the qualitative questions concerning
magnon-polaron formation. As a specific and experimentally relevant case, we
study Nickel Oxide quantitatively and find perfect agreement with the
qualitative analysis, thereby highlighting the strength of the former. We find
that there are two distinct features of antiferromagnetic magnon-polarons which
differ from the ferromagnetic ones. First, hybridization between magnons and
the longitudinal phonon modes is expected in many cubic antiferromagnetic
structures. Second, we find that the very existence of certain hybridizations
can be controlled via an external magnetic field, an effect which comes in
addition to the ability to move the magnon modes relative to the phonons modes.Comment: arXiv admin note: text overlap with arXiv:1808.0901
Green's function formalism for spin transport in metal-insulator-metal heterostructures
We develop a Green's function formalism for spin transport through
heterostructures that contain metallic leads and insulating ferromagnets. While
this formalism in principle allows for the inclusion of various magnonic
interactions, we focus on Gilbert damping. As an application, we consider
ballistic spin transport by exchange magnons in a metal-insulator-metal
heterostructure with and without disorder. For the former case, we show that
the interplay between disorder and Gilbert damping leads to spin current
fluctuations. For the case without disorder, we obtain the dependence of the
transmitted spin current on the thickness of the ferromagnet. Moreover, we show
that the results of the Green's function formalism agree in the clean and
continuum limit with those obtained from the linearized stochastic
Landau-Lifshitz-Gilbert equation. The developed Green's function formalism is a
natural starting point for numerical studies of magnon transport in
heterostructures that contain normal metals and magnetic insulators.Comment: 13 pages, 8 figure
Time-dependent strain-tuning topological magnon phase transition
Collinear magnets in honeycomb lattices under the action of time-dependent
strains are investigated. Given the limits of high-frequency periodically
varying deformations, we derive an effective Floquet theory for spin system
that results in the emergence of a spin chirality. We find that the coupling
between magnons and spin chirality depends on the details of the strain such as
the spatial dependence and applied direction. Magnonic fluctuations about the
ferromagnetic state are determined, and it is found that spatially homogeneous
strains drive the magnon system into topologically protected phases. In
particular, we show that certain uniform strain fields play the role of an
out-of-plane next-neighbor Dzyaloshinskii-Moriya interaction. Furthermore, we
explore the application of nonuniform strains, which lead to a confinement of
magnon states that for uniaxial strains, propagates along the direction that
preserves translational symmetry. Our work demonstrates a direct and novel way
in which to manipulate the magnon spectrum based on time-dependent strain
engineering that is relevant for exploring topological transitions in quantum
magnonics
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