346 research outputs found
Light scattering by magnons in whispering gallery mode cavities
Brillouin light scattering is an established technique to study magnons, the
elementary excitations of a magnet. Its efficiency can be enhanced by cavities
that concentrate the light intensity. Here, we theoretically study inelastic
scattering of photons by a magnetic sphere that supports optical whispering
gallery modes in a plane normal to the magnetization. Magnons with low angular
momenta scatter the light in the forward direction with a pronounced asymmetry
in the Stokes and the anti-Stokes scattering strength, consistent with earlier
studies. Magnons with large angular momenta constitute Damon Eschbach modes are
shown to inelastically reflect light. The reflection spectrum contains either a
Stokes or anti-Stokes peak, depending on the direction of the magnetization, a
selection rule that can be explained by the chirality of the Damon Eshbach
magnons. The controllable energy transfer can be used to manage the
thermodynamics of the magnet by light
Photon-assisted electron transport in graphene
Photon-assisted electron transport in ballistic graphene is analyzed using
scattering theory. We show that the presence of an ac signal (applied to a gate
electrode in a region of the system) has interesting consequences on electron
transport in graphene, where the low energy dynamics is described by the Dirac
equation. In particular, such a setup describes a feasible way to probe energy
dependent transmission in graphene. This is of substantial interest because the
energy dependence of transmission in mesoscopic graphene is the basis of many
peculiar transport phenomena proposed in the recent literature. Furthermore, we
discuss the relevance of our analysis of ac transport in graphene to the
observability of zitterbewegung of electrons that behave as relativistic
particles (but with a lower effective speed of light).Comment: 5 pages, 2 figure
Cascade Boltzmann - Langevin approach to higher-order current correlations in diffusive metal contacts
The Boltzmann - Langevin approach is extended to calculations of third and
fourth cumulants of current in diffusive-metal contacts. These cumulants result
from indirect correlations between current fluctuations, which may be
considered as "noise of noise". The calculated third cumulant coincides exactly
with its quantum-mechanical value. The fourth cumulant tends to its
quantum-mechanical value at high voltages and to a positive value
at V=0 changing its sign at .Comment: 6 pages, 2 eps figures, typo corrected, minor change
Optimal mode matching in cavity optomagnonics
Inelastic scattering of photons is a promising technique to manipulate
magnons but it suffers from weak intrinsic coupling. We theoretically discuss
an idea to increase optomagnonic coupling in optical whispering gallery mode
cavities, by generalizing previous analysis to include the exchange
interaction. We predict that the optomagnonic coupling constant to surface
magnons in yttrium iron garnet (YIG) spheres with radius m
can be up to times larger than that to the macrospin Kittel mode. Whereas
this enhancement falls short of the requirements for magnon manipulation in
YIG, nanostructuring and/or materials with larger magneto-optical constants can
bridge this gap.Comment: Comments welcom
Transport in disordered graphene nanoribbons
We study electronic transport in graphene nanoribbons with rough edges. We
first consider a model of weak disorder that corresponds to an armchair ribbon
whose width randomly changes by a single unit cell size. We find that in this
case, the low-temperature conductivity is governed by an effective
one-dimensional hopping between segments of distinct band structure. We then
provide numerical evidence and qualitative arguments that similar behavior also
occurs in the limit of strong uncorrelated boundary disorder.Comment: 5 pages, 3 figures. version as published in PR
The six-month line in geomagnetic long series
International audienceDaily means of the horizontal components X (north) and Y (east) of the geomagnetic field are available in the form of long series (several tens of years). Nine observatories are used in the present study, whose series are among the longest. The amplitudes of the 6-month and 1-year periodic variations are estimated using a simple but original technique. A remarkably clear result emerges from the complexity of the geomagnetic data: the amplitude of the 6-month line presents, in all observatories, the same large variation (by a factor of 1.7) over the 1920–1990 time span, regular and quasi-sinusoidal. Nothing comparable comes out for the annual line. The 6-month line results from the modulation by an astronomical mechanism of a magnetospheric system of currents. As this latter mechanism is time invariant, the intensity of the system of currents itself must present the large variation observed on the 6-months variation amplitude. This variation presents some similarities with the one displayed by recent curves of reconstructed solar irradiance or the "Earth's temperature". Finally, the same analysis is applied to the aa magnetic index
Topological confinement in bilayer graphene
We study a new type of one-dimensional chiral states that can be created in
bilayer graphene (BLG) by electrostatic lateral confinement. These states
appear on the domain walls separating insulating regions experiencing the
opposite gating polarity. While the states are similar to conventional
solitonic zero-modes, their properties are defined by the unusual chiral BLG
quasiparticles, from which they derive. The number of zero-mode branches is
fixed by the topological vacuum charge of the insulating BLG state. We discuss
how these chiral states can manifest experimentally, and emphasize their
relevance for valleytronics.Comment: 4 pages, 3 figure
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