1,945 research outputs found
Cosmic Duality in Quintom Universe
In this paper we study the duality in two-field Quintom models of Dark Energy. We find that an expanding universe dominated by Quintom-A field is dual to a contracting universe with Quintom-B field
Low Temperature Precursor Route for Highly Efficient Spherically Shaped LED-Phosphors M2Si5N8:Eu2+ (M = Eu, Sr, Ba)
The highly efficient nitridosilicate phosphors M2Si5N8 (M = Sr, Ba, Eu) for phosphor-converted pc-LEDs were synthesized at low temperatures using a novel precursor route involving metal amides M(NH2)2. These precursors have been synthesized by dissolution of the respective metals in supercritical ammonia at 150°C and 300 bar. The thermal behavior and decomposition process of the amides were investigated with temperature programmed powder X-ray diffractometry and thermoanalytical measurements (DTA/TG). These investigations rendered the amides as suitable intermediates for reaction with silicon diimide (Si(NH)2). Thus, the desired nitridosilicate phosphors were obtained at relatively low temperatures around 1150−1400°C which is approximately 300°C lower compared to common synthetic approaches starting from metals or oxides. The influence of the thermal treatment on the phosphor morphology has been studied extensively. The accessibility of spherical phosphor particles represents another striking feature of this route since it improves light extraction from the crystallites due to decreasing light guiding and decreasing re-absorption inside the phosphor particle. The synthesized luminescent materials M2Si5N8:Eu2+ (M = Sr, Ba) exhibit quantum efficiencies and emission band widths (FWHM 70−90 nm) comparable to standard phosphor powders. Employment of Eu(NH2)2 as dopant reagent for synthesis of Ba2Si5N8:Eu2+ proved favorable for the formation of spherical crystallites compared to doping with Eu metal, halides, or oxide
Inflationary universe in loop quantum cosmology
Loop quantum cosmology provides a nice solution of avoiding the big bang
singularity through a big bounce mechanism in the high energy region. In loop
quantum cosmology an inflationary universe is emergent after the big bounce, no
matter what matter component is filled in the universe. A super-inflation phase
without phantom matter will appear in a certain way in the initial stage after
the bounce; then the universe will undergo a normal inflation stage. We discuss
the condition of inflation in detail in this framework. Also, for slow-roll
inflation, we expect the imprint from the effects of the loop quantum cosmology
should be left in the primordial perturbation power spectrum. However, we show
that this imprint is too weak to be observed.Comment: 21 pages, 4 figures; accepted for publication in JCA
A Single Scalar Field Model of Dark Energy with Equation of State Crossing -1
In this paper we study the possibility of building models of dark energy with
equation of state across -1 and propose explicitly a model with a single scalar
field which gives rise to an equation of state larger than -1 in the past and
less than -1 at the present time, consistent with the current observations.Comment: 4 pages, 1 figure, the version accepted by JCAP, presentation
improved and references adde
Development of an Integrated DBH Estimation Model Based on Stand and Climatic Conditions
Using Korean National Forest Inventory (NFI) data, our study developed a model to estimate stand mean diameter at breast height (DBH) reflecting the influence of site and climate factors on forest growth for the major tree species in South Korea. A DBH estimation model was developed using stand-level variables (stand age, site index and number of trees per hectare) as independent factors. The spatial autocorrelation of residuals of the model was identified using semi-variogram analysis for each tree species. Further, a residual model, in which residuals were estimated by climatic factors (mean temperature, sum temperature in the growing season and precipitation), was developed assuming that the spatial autocorrelation of residuals reflects the differences in regional climatic conditions. Linear regression analysis showed that residuals of all tree species were significantly correlated with temperature and precipitation. The DBH and residual models were integrated to estimate the current DBH under different climatic factors (temperature and precipitation) and stand-level variables. This model had high reliability (R2 = 0.74–0.79), and no obvious dependencies or patterns in residuals were noted. Our results indicated that temperature increases caused by climate change would negatively affect the DBH estimate of coniferous trees, but not of oak species
Reconstruction of a Nonminimal Coupling Theory with Scale-invariant Power Spectrum
A nonminimal coupling single scalar field theory, when transformed from
Jordan frame to Einstein frame, can act like a minimal coupling one. Making use
of this property, we investigate how a nonminimal coupling theory with
scale-invariant power spectrum could be reconstructed from its minimal coupling
counterpart, which can be applied in the early universe. Thanks to the coupling
to gravity, the equation of state of our universe for a scale-invariant power
spectrum can be relaxed, and the relation between the parameters in the action
can be obtained. This approach also provides a means to address the Big-Bang
puzzles and anisotropy problem in the nonminimal coupling model within Jordan
frame. Due to the equivalence between the two frames, one may be able to find
models that are free of the horizon, flatness, singularity as well as
anisotropy problems.Comment: 31 pages, 4 figure
Janus monolayers of transition metal dichalcogenides.
Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements
Natural Phantom Dark Energy, Wiggling Hubble Parameter and Direct Data
Recent direct data indicate that the parameter may wiggle with
respect to . On the other hand the luminosity distance data of supernovae
flatten the wiggles of because of integration effect. It is expected
that the fitting results can be very different in a model permitting a wiggling
because the data of supernovae is highly degenerated to such a model. As
an example the natural phantom dark energy is investigated in this paper. The
dynamical property of this model is studied. The model is fitted by the direct
data set and the SNLS data set, respectively. And the results are quite
different, as expected. The quantum stability of this model is also shortly
discussed. We find it is a viable model if we treat it as an effective theory
truncated by an upperbound.Comment: 14 pages, 2 figures, discussions on the stability added, conclusions
not change
Hessence: A New View of Quintom Dark Energy
Recently a lot of attention has been drawn to build dark energy model in
which the equation-of-state parameter can cross the phantom divide .
One of models to realize crossing the phantom divide is called quintom model,
in which two real scalar fields appears, one is a normal scalar field and the
other is a phantom-type scalar field. In this paper we propose a non-canonical
complex scalar field as the dark energy, which we dub ``hessence'', to
implement crossing the phantom divide, in a similar sense as the quintom dark
energy model. In the hessence model, the dark energy is described by a single
field with an internal degree of freedom rather than two independent real
scalar fields. However, the hessence is different from an ordinary complex
scalar field, we show that the hessence can avoid the difficulty of the Q-balls
formation which gives trouble to the spintessence model (An ordinary complex
scalar field acts as the dark energy). Furthermore, we find that, by choosing a
proper potential, the hessence could correspond to a Chaplygin gas at late
times.Comment: Latex2e, 12 pages, no figure; v2: discussions and references added,
14 pages, 3 eps figures; v3: published versio
Strong coupling in Horava gravity
By studying perturbations about the vacuum, we show that Horava gravity
suffers from two different strong coupling problems, extending all the way into
the deep infra-red. The first of these is associated with the principle of
detailed balance and explains why solutions to General Relativity are typically
not recovered in models that preserve this structure. The second of these
occurs even without detailed balance and is associated with the breaking of
diffeomorphism invariance, required for anisotropic scaling in the UV. Since
there is a reduced symmetry group there are additional degrees of freedom,
which need not decouple in the infra-red. Indeed, we use the Stuckelberg trick
to show that one of these extra modes become strongly coupled as the parameters
approach their desired infra-red fixed point. Whilst we can evade the first
strong coupling problem by breaking detailed balance, we cannot avoid the
second, whatever the form of the potential. Therefore the original Horava
model, and its "phenomenologically viable" extensions do not have a
perturbative General Relativity limit at any scale. Experiments which confirm
the perturbative gravitational wave prediction of General Relativity, such as
the cumulative shift of the periastron time of binary pulsars, will presumably
rule out the theory.Comment: 11 page
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