39 research outputs found
Bogomolny equations for the BPS Skyrme models with impurity
We show that the BPS Skyrme model, as well as its (2+1) dimensional baby
version (restricted), can be coupled with an impurity in the BPS preserving
manner. The corresponding Bogomolny equations are derived.Comment: arXiv admin note: substantial text overlap with arXiv:1512.0123
Giant crystal-electric-field effect and complex magnetic behavior in single-crystalline CeRh3Si2
Single-crystalline CeRh3Si2 was investigated by means of x-ray diffraction,
magnetic susceptibility, magnetization, electrical resistivity, and specific
heat measurements carried out in wide temperature and magnetic field ranges.
Moreover, the electronic structure of the compound was studied at room
temperature by cerium core-level x-ray photoemission spectroscopy (XPS). The
physical properties were analyzed in terms of crystalline electric field and
compared with results of ab-initio band structure calculations performed within
the density functional theory approach. The compound was found to crystallize
in the orthorhombic unit cell of the ErRh3Si2 type (space group Imma -- No.74,
Pearson symbol: oI24) with the lattice parameters: a = 7.1330(14) A, b =
9.7340(19) A, and c = 5.6040(11) A. Analysis of the magnetic and XPS data
revealed the presence of well localized magnetic moments of trivalent cerium
ions. All physical properties were found to be highly anisotropic over the
whole temperature range studied, and influenced by exceptionally strong
crystalline electric field with the overall splitting of the 4f1 ground
multiplet exceeding 5700 K. Antiferromagnetic order of the cerium magnetic
moments at TN = 4.70(1)K and their subsequent spin rearrangement at Tt =
4.48(1) K manifest themselves as distinct anomalies in the temperature
characteristics of all investigated physical properties and exhibit complex
evolution in an external magnetic field. A tentative magnetic B-T phase
diagram, constructed for B parallel to the b-axis being the easy magnetization
direction, shows very complex magnetic behavior of CeRh3Si2, similar to that
recently reported for an isostructural compound CeIr3Si2. The electronic band
structure calculations corroborated the antiferromagnetic ordering of the
cerium magnetic moments and well reproduced the experimental XPS valence band
spectrum.Comment: 32 pages, 12 figures, to appear in Physical Review
Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Vir
The spectral energy distribution (SED) in chemically peculiar stars may be
significantly affected by their abundance anomalies. The observed SED
variations are usually assumed to be a result of inhomogeneous surface
distribution of chemical elements, flux redistribution and stellar rotation.
However, the direct evidence for this is still only scarce. We aim to identify
the processes that determine the SED and its variability in the UV and visual
spectral domains of the helium-weak star CU Vir. We used the model atmospheres
to obtain the emergent flux and predict the rotationally modulated flux
variability of the star. We show that most of the light variations in the vby
filters of the Stromgren photometric system are a result of the uneven surface
distribution of silicon, chromium, and iron. Our models are only able to
explain a part of the variability in the u filter, however. The observed UV
flux distribution is very well reproduced, and the models are able to explain
most of the observed features in the UV light curve. The variability observed
in the visible is merely a faint gleam of that in the UV. While the amplitude
of the light curves reaches only several hundredths of magnitude in the visual
domain, it reaches about 1 mag in the UV. The visual and UV light variability
of CU Vir is caused by the flux redistribution from the far UV to near UV and
visible regions, inhomogeneous distribution of the elements and stellar
rotation. Bound-free transitions of silicon and bound-bound transitions of iron
and chromium contribute the most to the flux redistribution. This mechanism can
explain most of the rotationally modulated light variations in the filters
centred on the Paschen continuum and on the UV continuum of the star CU Vir.
However, another mechanism(s) has to be invoked to fully explain the observed
light variations in the u filter and in the region 2000-2500 A.Comment: 14 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Surprising variations in the rotation of the chemically peculiar stars CU Virginis and V901 Orionis
CU Vir and V901 Ori belong among these few magnetic chemically peculiar stars
whose rotation periods vary on timescales of decades. We aim to study the
stability of the periods in CU Vir and V901 Ori using all accessible
observational data containing phase information. We collected all available
relevant archived observations supplemented with our new measurements of these
stars and analysed the period variations of the stars using a novel method that
allows for the combination of data of diverse sorts. We found that the shapes
of their phase curves were constant, while the periods were changing. Both
stars exhibit alternating intervals of rotational braking and acceleration. The
rotation period of CU Vir was gradually shortening until the year 1968, when it
reached its local minimum of 0.52067198 d. The period then started increasing,
reaching its local maximum of 0.5207163 d in the year 2005. Since that time the
rotation has begun to accelerate again. We also found much smaller period
changes in CU Vir on a timescale of several years. The rotation period of V901
Ori was increasing for the past quarter-century, reaching a maximum of 1.538771
d in the year 2003, when the rotation period began to decrease. A theoretically
unexpected alternating variability of rotation periods in these stars would
remove the spin-down time paradox and brings a new insight into structure and
evolution of magnetic upper-main-sequence stars.Comment: 5 pages, 3 figure
Study of the Sigma-nucleus potential by the (pi^-,K^+) reaction on medium-to-heavy nuclear targets
In order to study the Sigma-nucleus optical potential, we measured inclusive
(pi^-,K^+) spectra on medium-to-heavy nuclear targets: CH_2, Si, Ni, In and Bi.
The CH_2 target was used to calibrate the excitation energy scale by using the
elementary process p + pi^- -> K^+ + Sigma^-, where the C spectrum was also
extracted. The calibration was done with +-0.1 MeV precision. The angular
distribution of the elementary cross section was measured, and agreed well with
the previous bubble chamber data, but with better statistics, and the
magnitudes of the cross sections of the measured inclusive (pi^-,K^+) spectra
were also well calibrated. All of the inclusive spectra were found to be
similar in shape at a region near to the Sigma^- binding energy threshold,
showing a weak mass-number dependence on the magnitude of the cross section.
The measured spectra were compared with a theoretical calculation performed
within the framework of the Distorted Wave Impulse Approximation (DWIA). It has
been demonstrated that a strongly repulsive \sig-nucleus potential with a
non-zero size of the imaginary part is required to reproduce the shape of the
measured spectra.Comment: 21 pages, 24 figures, submitted to PR
Feasibility study of the positronium imaging with the J-PET tomograph
A detection system of the conventional PET tomograph is set-up to record data
from e+ e- annihilation into two photons with energy of 511 keV, and it gives
information on the density distribution of a radiopharmaceutical in the body of
the object. In this paper we explore the possibility of performing the three
gamma photons imaging based on ortho-positronium annihilation, as well as the
possibility of positronium mean lifetime imaging with the J-PET tomograph
constructed from plastic scintillators. For this purposes simulations of the
ortho-positronium formation and its annihilation into three photons were
performed taking into account distributions of photons' momenta as predicted by
the theory of quantum electrodynamics and the response of the J-PET tomograph.
In order to test the proposed ortho-positronium lifetime image reconstruction
method, we concentrate on the decay of the ortho-positronium into three photons
and applications of radiopharmaceuticals labeled with isotopes emitting a
prompt gamma quantum. The proposed method of imaging is based on the
determination of hit-times and hit-positions of registered photons which
enables the reconstruction of the time and position of the annihilation point
as well as the lifetime of the ortho-positronium on an event-by-event basis. We
have simulated the production of the positronium in a cylindrical phantom
composed of a set of different materials in which the ortho-positronium
lifetime varied from 2.0 ns to 3.0 ns, as expected for ortho-positronium
created in the human body. The presented reconstruction method for total-body
J-PET like detector allows to achieve a mean lifetime resolution of about 40
ps. Recent Positron Annihilation Lifetime Spectroscopy measurements of
cancerous and healthy uterine tissues show that this sensitivity may allow to
study the morphological changes in cell structures.Comment: accepted in PMB
(http://iopscience.iop.org/article/10.1088/1361-6560/aafe20
Nonlinearity and Topology
The interplay of nonlinearity and topology results in many novel and emergent
properties across a number of physical systems such as chiral magnets, nematic
liquid crystals, Bose-Einstein condensates, photonics, high energy physics,
etc. It also results in a wide variety of topological defects such as solitons,
vortices, skyrmions, merons, hopfions, monopoles to name just a few.
Interaction among and collision of these nontrivial defects itself is a topic
of great interest. Curvature and underlying geometry also affect the shape,
interaction and behavior of these defects. Such properties can be studied using
techniques such as, e.g. the Bogomolnyi decomposition. Some applications of
this interplay, e.g. in nonreciprocal photonics as well as topological
materials such as Dirac and Weyl semimetals, are also elucidated