9,023 research outputs found
Electric-field control of magnetic ordering in the tetragonal BiFeO3
We propose a way to use electric-field to control the magnetic ordering of
the tetragonal BiFeO3. Based on systematic first-principles studies of the
epitaxial strain effect on the ferroelectric and magnetic properties of the
tetragonal BiFeO3, we find that there exists a transition from C-type to G-type
antiferromagnetic (AFM) phase at in-plane constant a ~ 3.905 {\AA} when the
ferroelectric polarization is along [001] direction. Such magnetic phase
transition can be explained by the competition between the Heisenberg exchange
constant J1c and J2c under the influence of biaxial strain. Interestingly, when
the in-plane lattice constant enlarges, the preferred ferroelectric
polarization tends to be canted and eventually lies in the plane (along [110]
direction). It is found that the orientation change of ferroelectric
polarization, which can be realized by applying external electric-field, has
significant impact on the Heisenberg exchange parameters and therefore the
magnetic orderings of tetragonal BiFeO3. For example, at a ~ 3.79 {\AA}, an
electric field along [111] direction with magnitude of 2 MV/cm could change the
magnetic ordering from C-AFM to G-AFM. As the magnetic ordering affects many
physical properties of the magnetic material, e.g. magnetoresistance, we expect
such strategy would provide a new avenue to the application of multiferroic
materials.Comment: 4 pages, 4 figure
On two problems in graph Ramsey theory
We study two classical problems in graph Ramsey theory, that of determining
the Ramsey number of bounded-degree graphs and that of estimating the induced
Ramsey number for a graph with a given number of vertices.
The Ramsey number r(H) of a graph H is the least positive integer N such that
every two-coloring of the edges of the complete graph contains a
monochromatic copy of H. A famous result of Chv\'atal, R\"{o}dl, Szemer\'edi
and Trotter states that there exists a constant c(\Delta) such that r(H) \leq
c(\Delta) n for every graph H with n vertices and maximum degree \Delta. The
important open question is to determine the constant c(\Delta). The best
results, both due to Graham, R\"{o}dl and Ruci\'nski, state that there are
constants c and c' such that 2^{c' \Delta} \leq c(\Delta) \leq 2^{c \Delta
\log^2 \Delta}. We improve this upper bound, showing that there is a constant c
for which c(\Delta) \leq 2^{c \Delta \log \Delta}.
The induced Ramsey number r_{ind}(H) of a graph H is the least positive
integer N for which there exists a graph G on N vertices such that every
two-coloring of the edges of G contains an induced monochromatic copy of H.
Erd\H{o}s conjectured the existence of a constant c such that, for any graph H
on n vertices, r_{ind}(H) \leq 2^{c n}. We move a step closer to proving this
conjecture, showing that r_{ind} (H) \leq 2^{c n \log n}. This improves upon an
earlier result of Kohayakawa, Pr\"{o}mel and R\"{o}dl by a factor of \log n in
the exponent.Comment: 18 page
Ultraviolet Diversity of Type Ia Supernovae
Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) probe the
outermost layers of the explosion, and UV spectra of SNe Ia are expected to be
extremely sensitive to differences in progenitor composition and the details of
the explosion. Here we present the first study of a sample of high
signal-to-noise ratio SN Ia spectra that extend blueward of 2900 A. We focus on
spectra taken within 5 days of maximum brightness. Our sample of ten SNe Ia
spans the majority of the parameter space of SN Ia optical diversity. We find
that SNe Ia have significantly more diversity in the UV than in the optical,
with the spectral variance continuing to increase with decreasing wavelengths
until at least 1800 A (the limit of our data). The majority of the UV variance
correlates with optical light-curve shape, while there are no obvious and
unique correlations between spectral shape and either ejecta velocity or
host-galaxy morphology. Using light-curve shape as the primary variable, we
create a UV spectral model for SNe Ia at peak brightness. With the model, we
can examine how individual SNe vary relative to expectations based on only
their light-curve shape. Doing this, we confirm an excess of flux for SN 2011fe
at short wavelengths, consistent with its progenitor having a subsolar
metallicity. While most other SNe Ia do not show large deviations from the
model, ASASSN-14lp has a deficit of flux at short wavelengths, suggesting that
its progenitor was relatively metal rich.Comment: 9 pages, 6 figures, submitted to MNRA
Is \gamma-ray emission from novae affected by interference effects in the 18F(p,\alpha)15O reaction?
The 18F(p,\alpha)15O reaction rate is crucial for constraining model
predictions of the \gamma-ray observable radioisotope 18F produced in novae.
The determination of this rate is challenging due to particular features of the
level scheme of the compound nucleus, 19Ne, which result in interference
effects potentially playing a significant role. The dominant uncertainty in
this rate arises from interference between J\pi=3/2+ states near the proton
threshold (Sp = 6.411 MeV) and a broad J\pi=3/2+ state at 665 keV above
threshold. This unknown interference term results in up to a factor of 40
uncertainty in the astrophysical S-factor at nova temperatures. Here we report
a new measurement of states in this energy region using the 19F(3He,t)19Ne
reaction. In stark contrast with previous assumptions we find at least 3
resonances between the proton threshold and Ecm=50 keV, all with different
angular distributions. None of these are consistent with J\pi= 3/2+ angular
distributions. We find that the main uncertainty now arises from the unknown
proton-width of the 48 keV resonance, not from possible interference effects.
Hydrodynamic nova model calculations performed indicate that this unknown width
affects 18F production by at least a factor of two in the model considered.Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev. Let
Contact Interactions and Resonance-Like Physics at Present and Future Colliders from Unparticles
High scale conformal physics can lead to unusual unparticle stuff at our low
energies. In this paper we discuss how the exchange of unparticles between
Standard Model fields can lead to new contact interaction physics as well as a
pseudoresonance-like structure, an unresonance, that might be observable at the
Tevatron or LHC in, e.g., the Drell-Yan channel. The specific signatures of
this scenario are quite unique and can be used to easily identify this new
physics given sufficient integrated luminosity.Comment: 20 pages, 10 figs; minor text changes, ref added; typos correcte
Demonstrating Additional Law of Relativistic Velocities based on Squeezed Light
Special relativity is foundation of many branches of modern physics, of which
theoretical results are far beyond our daily experience and hard to realized in
kinematic experiments. However, its outcomes could be demonstrated by making
use of convenient substitute, i.e. squeezed light in present paper. Squeezed
light is very important in the field of quantum optics and the corresponding
transformation can be regarded as the coherent state of SU(1; 1). In this
paper, the connection between the squeezed operator and Lorentz boost is built
under certain conditions. Furthermore, the additional law of relativistic
velocities and the angle of Wigner rotation are deduced as well
Multiphase Gas In Galaxy Halos: The OVI Lyman-limit System toward J1009+0713
We have serendipitously detected a strong O VI-bearing Lyman limit system at
z_abs = 0.3558 toward the QSO J1009+0713 (z_em = 0.456) in our survey of
low-redshift galaxy halos with the Hubble Space Telescope's Cosmic Origins
Spectrograph. Its rest-frame equivalent width of W_r = 835 +/- 49 mA is the
highest for an intervening absorber yet detected in any low-redshift QSO
sightline, with absorption spanning 400 km s^-1 in its rest frame. HST/WFC3
images of the galaxy field show that the absorber is associated with two
galaxies lying at 14 and 46 kpc from the QSO line of sight. The bulk of the
absorbing gas traced by H I resides in two strong, blended component groups
that possess a total logN(HI) = 18 - 18.8. The ion ratios and column densities
of C, N, O, Mg, Si, S, and Fe, except the O VI, can be accommodated into a
simple photoionization model in which diffuse, low-metallicity halo gas is
exposed to a photoionizing field from stars in the nearby galaxies that
propagates into the halo at 10% efficiency. We constrain the metallicity firmly
within the range 0.1 - 1 Zsun, and photoionization modeling indirectly
indicates a subsolar metallicity of 0.05 - 0.5 Zsun. The appearance of strong O
VI and nine Mg II components and our review of similar systems in the
literature support the "interface" picture of high-velocity O VI: the total
strength of the O VI shows a positive correlation with the number of detected
components in the low-ionization gas, however the total O VI column densities
still far exceed the values expected from interface models for the number of
detected clouds.Comment: 20 pages, 11 figures, accepted for publication in Ap
Template coexistence in prebiotic vesicle models
The coexistence of distinct templates is a common feature of the diverse
proposals advanced to resolve the information crisis of prebiotic evolution.
However, achieving robust template coexistence turned out to be such a
difficult demand that only a class of models, the so-called package models,
seems to have met it so far. Here we apply Wright's Island formulation of group
selection to study the conditions for the coexistence of two distinct template
types confined in packages (vesicles) of finite capacity. In particular, we
show how selection acting at the level of the vesicles can neutralize the
pressures towards the fixation of any one of the template types (random drift)
and of the type with higher replication rate (deterministic competition). We
give emphasis to the role of the distinct generation times of templates and
vesicles as yet another obstacle to coexistence.Comment: 7 pages, 8 figure
Unparticles-Higgs Interplay
We show that scalar unparticles coupled to the Standard Model Higgs at the
renormalizable level can have a dramatic impact in the breaking of the
electroweak symmetry already at tree level. In particular one can get the
proper electroweak scale without the need of a Higgs mass term in the
Lagrangian. By studying the mixed unparticle-Higgs propagator and spectral
function we also show how unparticles can shift the Higgs mass away from its
Standard Model value, \lambda v^2, and influence other Higgs boson properties.
Conversely, we study in some detail how electroweak symmetry breaking affects
the unparticle sector by breaking its conformal symmetry and generating a mass
gap. We also show that, for Higgs masses above that gap, unparticles can
increase quite significantly the Higgs width.Comment: 14 pages, 7 figures, typos correcte
- …