98 research outputs found
Microscopic origin of level attraction for a coupled magnon-photon system in a microwave cavity
We discuss various microscopic mechanisms for level attraction in a
hybridized magnon-photon system of a ferromagnet in a microwave cavity. The
discussion is based upon the electromagnetic theory of continuous media where
the effects of the internal magnetization dynamics of the ferromagnet are
described using dynamical response functions. This approach is in agreement
with quantized multi-oscillator models of coupled photon-magnon dynamics. We
demonstrate that to provide the attractive interaction between the modes, the
effective response functions should be diamagnetic. Magneto-optical coupling is
found to be one mechanism for the effective diamagnetic response, which is
proportional to photon number. A dual mechanism based on the Aharonov-Casher
effect is also highlighted, which is instead dependent on magnon number.Comment: New Journal of Physics, Focus on Cavity Optomagnonics Issu
Tunable focusing in natural hyperbolic magnetic media
While optical effects such as negative refraction and imaging obtained from slab lenses with plane parallel sides are widely studied using metamaterials, it is less well known that these effects can occur naturally in certain materials. We discuss a class of natural hyperbolic materials that not only display similar effects but also allow one to modify the focal length of a slab lens with an externally applied magnetic field. This is possible because antiferromagnets are gyrotropic and support magnetic polaritons whose frequencies are sensitive to magnetic fields. In addition, a rich caustic structure emerges at low temperatures, when damping should be small. These materials also produce slab focusing at higher temperatures, although the caustic structure disappears
Parallel axis theorem for free-space electron wavefunctions
We consider the orbital angular momentum of a free electron vortex moving in
a uniform magnetic field. We identify three contributions to this angular
momentum: the canonical orbital angular momentum associated with the vortex,
the angular momentum of the cyclotron orbit of the wavefunction, and a
diamagnetic angular momentum. The cyclotron and diamagnetic angular momenta are
found to be separable according to the parallel axis theorem. This means that
rotations can occur with respect to two or more axes simultaneously, which can
be observed with superpositions of vortex states
Spin-wave chirality and its manifestations in antiferromagnets
As first demonstrated by Tang and Cohen in chiral optics, the asymmetry in
the rate of electromagnetic energy absorption between left and right
enantiomers is determined by an optical chirality density [1]. Here, we
demonstrate that this effect can exist in magnetic spin systems. By
constructing a formal analogy with electrodynamics, we show that in
antiferromagnets with broken chiral symmetry the asymmetry in local spin-wave
energy absorption is proportional to a spin-wave chirality density, which is a
direct counterpart of optical zilch. We propose that injection of a pure spin
current into an antiferromagnet may serve as a chiral symmetry breaking
mechanism, since its effect in the spin-wave approximation can be expressed in
terms of additional Lifshitz invariants. We use linear response theory to show
that the spin current induces a nonequilibrium spin-wave chirality density.Comment: 6 pages (plus Supplemental Material, 6 pages), 1 figure, published
versio
Is the angular momentum of an electron conserved in a uniform magnetic field?
We show that an electron moving in a uniform magnetic field possesses a time-varying ``diamagnetic'' angular momentum. Surprisingly this means that the kinetic angular momentum of the electron may vary with time, despite the rotational symmetry of the system. This apparent violation of angular momentum conservation is resolved by including the angular momentum of the surrounding fields
Cavity Optomechanics of Topological Spin Textures in Magnetic Insulators
Collective dynamics of topological magnetic textures can be thought of as a
massive particle moving in a magnetic pinning potential. We demonstrate that
inside a cavity resonator this effective mechanical system can feel the
electromagnetic radiation pressure from cavity photons through the
magneto-optical inverse Faraday and Cotton-Mouton effects. We estimate values
for the effective parameters of the optomechanical coupling for two spin
textures -- a Bloch domain wall and a chiral magnetic soliton lattice. The
soliton lattice has magnetic chirality, so that in circularly polarized light
it behaves like a chiral particle with the sign of the optomechanical coupling
determined by the helicity of the light and chirality of the lattice. Most
interestingly, we find a level attraction regime for the soliton lattice, which
is tunable through an applied magnetic field.Comment: 7 pages, 3 figures, published versio
Excitation of magnon spin photocurrents in antiferromagnetic insulators
In the circular photogalvanic effect, circularly polarized light can produce
a direct electron photocurrent in metals and the direction of the current
depends on the polarization. We suggest that an analogous nonlinear effect
exists for antiferromagnetic insulators wherein the total spin of light and
spin waves is conserved. In consequence, a spin angular momentum is expected to
be transfered from photons to magnons so that a circularly polarized
electromagnetic field will generate a direct magnon spin current. The direction
of the current is determined by the helicity of the light. We show that this
resonant effect appears as a second order light-matter interaction. We find
also a geometric contribution to the spin photocurrent, which appears for
materials with complex lattice structures and Dzyaloshinskii-Moriya
interactions.Comment: 10 pages, 2 figures, published versio
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