1,038 research outputs found
Polarization of Fast Radio Bursts: radiation mechanisms and propagation effects
Fast radio bursts (FRBs) are observed to be highly polarized. Most have high
linear polarization but a small fraction show significant circular
polarization. We systematically investigate a variety of polarization
mechanisms of FRBs within the magnetar theoretical framework considering two
emission sites inside and outside the magnetosphere. For each site, we discuss
both intrinsic radiation mechanisms and propagation effects. Inside the
magnetosphere, we investigate the polarization properties of both coherent
curvature radiation and inverse Compton scattering by charged bunches and
conclude that both mechanisms produce 100\% linear polarization at an on-axis
geometry but can produce circular polarization if the viewing angle is off
axis. The lack of circular polarization for the majority of bursts requires
that the bunches have a large transverse dimension size. Resonant cyclotron
absorption within magnetosphere may produce high circular polarization if
electrons and positrons have an asymmetric Lorentz factor distribution. Outside
the magnetosphere, the synchrotron maser emission mechanism in general produces
highly linearly polarized emission. Circular polarization would appear at
off-beam angles but the flux is greatly degraded and such bursts are not
detectable at cosmological distances. Synchrotron absorption in a nebula with
ordered magnetic field may reduce the circular polarization degree. Cyclotron
absorption in a strongly magnetized medium may generate significant circular
polarization. Faraday conversion in a medium with field reversal can convert
one polarization mode to another. The two absorption processes require
stringent physical conditions. Significant Faraday conversion may be realized
in a magnetized dense environment involving binary systems or supernova
remnants.Comment: 30 pages, 15 figures. Accepted for publication in MNRA
Robust Intrinsic Ferromagnetism and Half Semiconductivity in Stable Two-Dimensional Single-Layer Chromium Trihalides
Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are crucial
to develop low-dimensional spintronic devices. Using density functional theory,
we show that single-layer chromium trihalides (SLCTs) (CrX,X=F, Cl, Br and
I) constitute a series of stable 2D intrinsic FM semiconductors. A
free-standing SLCT can be easily exfoliated from the bulk crystal, due to a low
cleavage energy and a high in-plane stiffness. Electronic structure
calculations using the HSE06 functional indicate that both bulk and
single-layer CrX are half semiconductors with indirect gaps and their
valence bands and conduction bands are fully spin-polarized in the same spin
direction. The energy gaps and absorption edges of CrBr and CrI are
found to be in the visible frequency range, which implies possible
opt-electronic applications. Furthermore, SLCTs are found to possess a large
magnetic moment of 3 per formula unit and a sizable magnetic anisotropy
energy. The magnetic exchange constants of SLCTs are then extracted using the
Heisenberg spin Hamiltonian and the microscopic origins of the various exchange
interactions are analyzed. A competition between a near 90 FM
superexchange and a direct antiferromagnetic (AFM) exchange results in a FM
nearest-neighbour exchange interaction. The next and third nearest-neighbour
exchange interactions are found to be FM and AFM respectively and this can be
understood by the angle-dependent extended Cr-X-X-Cr superexchange interaction.
Moreover, the Curie temperatures of SLCTs are also predicted using Monte Carlo
simulations and the values can further increase by applying a biaxial tensile
strain. The unique combination of robust intrinsic ferromagnetism, half
semiconductivity and large magnetic anisotropy energies renders the SLCTs as
promising candidates for next-generation semiconductor spintronic applications.Comment: 12 pages, 14 figures. published in J. Mater. Chem.
The Directional Transport of Self-Propelled Ellipsoidal Particles Confined in 2D Channel
Transport phenomenon of self-propelled ellipsoidal particles confined in a
smooth corrugated channel with a two-dimensional asymmetric potential and
Gaussian colored noise is investigated. Effects of the channel, potential and
coloured noise are discussed. The moving direction changes from along x axis to
opposite x axis with increasing load f. Proper size of pore is good at the
directional transport, but too large or too small pore size will inhibit the
transport speed. Large x axis noise intensity will inhibit the directional
transport phenomena. Proper y axis noise intensity will help to the directional
transport. Transport reverse phenomenon appears with increasing self-propelled
speed v0. Perfect sphere particle is more easier for directional transport than
needlelike ellipsoid particle
The plasma suppression effect can be ignored in realistic FRB models invoking bunched coherent radio emission
One widely discussed mechanism to produce highly coherent radio emission of
fast radio bursts (FRBs) is coherent emission by bunches, either via curvature
radiation or inverse Compton scattering (ICS). It has been suggested that the
plasma oscillation effect can significantly suppress coherent emission power by
bunches. We examine this criticism in this paper. The suppression factor
formalism was derived within the context of radio pulsars in which radio waves
are in the low-amplitude, linear regime and cannot directly be applied to the
large-amplitude, non-linear regime relevant for FRBs. Even if one applies this
linear treatment, plasma suppression is not important for two physical reasons.
First, for an efficient radiation mechanism such as ICS, the required plasma
density is not high so that a high-density plasma may not exist. Second, both
bunched coherent mechanisms demand that a large global parallel electric field
() must exist in the emission region in order to continuously
inject energy to the bunches to power an FRB. In order to produce typical FRB
duration via coherent curvature or ICS radiation, a parallel electric field
must be present to balance the acceleration and radiation back-reaction. The
plasma suppression factor should be modified with the existence of
. We show that the correction factor for curvature radiation,
, increases with and becomes 1 when
reaches the radiation-reaction-limited regime. We conclude that the plasma
suppression effect can be ignored for realistic FRB emission models invoking
bunched coherent radio emission.Comment: 9 pages, 2 figures. Accepted for publication in MNRA
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