3,316 research outputs found
Crossover of skyrmion and helical modulations in noncentrosymmetric ferromagnets
The coupling between angular (twisting) and longitudinal modulations arising
near the ordering temperature of noncentrosymmetric ferromagnets strongly
influences the structure of skyrmion states and their evolution in an applied
magnetic field. In the precursor states of cubic helimagnets, a continuous
transformation of skyrmion lattices into the saturated state is replaced by the
first-order processes accompanied by the formation of multidomain states.
Recently the effects imposed by dominant longitudinal modulations have been
reported in bulk MnSi and FeGe. Similar phenomena can be observed in the
precursor regions of cubic helimagnet epilayers and in easy-plane chiral
ferromagnets (e.g. in the hexagonal helimagnet CrNb3S6)
Probability of radiation of twisted photons by classical currents
The general formula for the probability of radiation of a twisted photon by a
classical current is derived. The general theory of generation of twisted
photons by undulators is developed. It is proved that the probability to record
a twisted photon produced by a classical current is equal to the average number
of twisted photons in a given state. The general formula for the projection of
the total angular momentum of twisted photons with given the energy, the
longitudinal projection of momentum, and the helicity is obtained. The symmetry
property of the average number of twisted photons produced by a charged
particle moving along a planar trajectory is found. The explicit formulas for
the average number of twisted photons generated by undulators both in the
dipole and wiggler regimes are obtained. It is established that, for the
forward radiation of an ideal right-handed helical undulator, the harmonic
number of the twisted photon coincides with its projection of the total
angular momentum . As for the ideal left-handed helical undulator, we obtain
that . It is found that the forward radiation of twisted photons by a
planar undulator obeys the selection rule that is an even number. It
turns out that the average number of twisted photons produced by the undulator
and detected off the undulator axis is a periodic function of in a certain
spectral band of the quantum numbers .Comment: 36 pp; some misprints corrected; formulas (48)-(51) change
Properties of an ultrarelativistic charged particle radiation in a constant homogeneous crossed electromagnetic field
The properties of radiation created by a classical ultrarelativistic scalar
charged particle in a constant homogeneous crossed electromagnetic field are
described both analytically and numerically with radiation reaction taken into
account in the form of the Landau-Lifshitz equation. The total radiation
naturally falls into two parts: the radiation formed at the entrance point of a
particle into the crossed field (the synchrotron entrance radiation), and the
radiation coming from the late-time asymptotics of a particle motion (the
de-excited radiation). The synchrotron entrance radiation resembles, although
does not coincide with, the ultrarelativistic limit of the synchrotron
radiation: its distribution over energies and angles possesses almost the same
properties. The de-excited radiation is soft, not concentrated in the plane of
motion of a charged particle, and almost completely circularly polarized. The
photon energy delivering the maximum to its spectral angular distribution
decreases with increasing the initial energy of a charged particle, while the
maximum value of this distribution remains the same at the fixed observation
angle. The ultraviolet and infrared asymptotics of the total radiation are also
described.Comment: 14 pp, 3 fig
Spintronics via non-axisymmetric chiral skyrmions
Micromagnetic calculations demonstrate a peculiar evolution of
non-axisymmetric skyrmions driven by an applied magnetic field in confined
helimagnets with longitudinal modulations. We argue that these specific
solitonic states can be employed in nanoelectronic devices as an effective
alternative to the common axisymmetric skyrmions which occur in magnetically
saturated states
A Chandra X-ray Observatory Study of PSR J1740--5340 and Candidate Millisecond Pulsars in the Globular Cluster NGC 6397
We present a deep Chandra X-ray Observatory study of the peculiar binary
radio millisecond pulsar PSR J1740--5340 and candidate millisecond pulsars
(MSPs) in the globular cluster NGC 6397. The X-rays from PSR J1740--5340 appear
to be non-thermal and exhibit variability at the binary period. These
properties suggest the presence of a relativistic intrabinary shock formed due
to interaction of a relativistic rotation-powered pulsar wind and outflow from
the unusual "red-straggler/sub-subgiant" companion. We find the X-ray source
U18 to show similar X-ray and optical properties to those of PSR J1740--5340,
making it a strong MSP candidate. It exhibits variability on timescales from
hours to years, also consistent with an intrabinary shock origin of its X-ray
emission. The unprecedented depth of the X-ray data allows us to conduct a
complete census of MSPs in NGC 6397. Based on the properties of the present
sample of X-ray--detected MSPs in the Galaxy we find that NGC 6397 probably
hosts no more than 6 MSPs.Comment: 10 pages, 6 figures, 3 tables; accepted for publication in The
Astrophysical Journa
Chandra X-ray Observations of 12 Millisecond Pulsars in the Globular Cluster M28
We present a Chandra X-ray Observatory investigation of the millisecond
pulsars (MSPs) in the globular cluster M28 (NGC 6626). In what is one of the
deepest X-ray observations of a globular cluster, we firmly detect seven and
possibly detect two of the twelve known M28 pulsars. With the exception of PSRs
B1821-24 and J1824-2452H, the detected pulsars have relatively soft spectra,
with X-ray luminosities 10^30-31 ergs s^-1 (0.3-8 keV),similar to most
"recycled" pulsars in 47 Tucanae and the field of the Galaxy, implying thermal
emission from the pulsar magnetic polar caps. We present the most detailed
X-ray spectrum to date of the energetic PSR B1821-24. It is well described by a
purely non-thermal spectrum with spectral photon index 1.23 and luminosity
1.4x10^33Theta(D/5.5 kpc)^2 ergs s^-1 (0.3-8 keV), where Theta is the fraction
of the sky covered by the X-ray emission beam(s). We find no evidence for the
previously reported line emission feature around 3.3 keV, most likely as a
consequence of improvements in instrument calibration. The X-ray spectrum and
pulse profile of PSR B1821--24 suggest that the bulk of unpulsed emission from
this pulsar is not of thermal origin, and is likely due to low-level
non-thermal magnetospheric radiation, an unresolved pulsar wind nebula, and/or
small-angle scattering of the pulsed X-rays by interstellar dust grains. The
peculiar binary PSR J1824-2452H shows a relatively hard X-ray spectrum and
possible variability at the binary period, indicative of an intrabinary shock
formed by interaction between the relativistic pulsar wind and matter from its
non-degenerate companion star.Comment: 9 pages, 6 figures. Accepted for publication in the Astophysical
Journa
The Light Curve and Internal Magnetic Field of the Mode-Switching Pulsar PSR B0943+10
A number of radio pulsars exhibit intriguing mode-switching behavior. Recent
observations of PSR B0943+10 revealed correlated radio and X-ray mode switches,
providing a new avenue for understanding this class of objects. The large X-ray
pulse fraction observed during the radio quiet phase (Q mode) was previously
interpreted as a result of changing obscuration of X-rays by dense
magnetosphere plasma. We show that the large X-ray pulse fraction can be
explained by including the beaming effect of a magnetic atmosphere, while
remaining consistent with the dipole field geometry constrained by radio
observations. We also explore a more extreme magnetic field configuration,
where a magnetic dipole displaced from the center of the star produces two
magnetic polar caps of different sizes and magnetic field strengths. These
models are currently consistent with data in radio and X-rays and can be tested
or constrained by future X-ray observations.Comment: 5 pages, 5 figures, submitted to ApJ
On the nature of the solar-wind-Mars interaction
Plasma measurements near Mars on the U.S.S.R. Mars-2, -3, and -5 spacecraft are considered. The data are compared with simultaneous magnetic measurements. Strong evidence is obtained in favor of a direct interaction and mass exchange between the solar wind plasma and the gaseous envelope of Mars
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