133,735 research outputs found
Role of Interlayer Coupling on the Evolution of Band Edges in Few-Layer Phosphorene
Using first-principles calculations, we have investigated the evolution of
band-edges in few-layer phosphorene as a function of the number of P layers.
Our results predict that monolayer phosphorene is an indirect band gap
semiconductor and its valence band edge is extremely sensitive to strain. Its
band gap could undergo an indirect-to-direct transition under a lattice
expansion as small as 1% along zigzag direction. A semi-empirical interlayer
coupling model is proposed, which can well reproduce the evolution of valence
band-edges obtained by first-principles calculations. We conclude that the
interlayer coupling plays a dominated role in the evolution of the band-edges
via decreasing both band gap and carrier effective masses with the increase of
phosphorene thickness. A scrutiny of the orbital-decomposed band structure
provides a better understanding of the upward shift of valence band maximum
surpassing that of conduction band minimum.Comment: 25 pages, 9 figure
Collapse of Vacuum Bubbles in a Vacuum
Motivated by the discovery of a plenitude of metastable vacua in a string
landscape and the possibility of rapid tunneling between these vacua, we
revisit the dynamics of a false vacuum bubble in a background de Sitter
spacetime. We find that there exists a large parameter space that allows the
bubble to collapse into a black hole or to form a wormhole. This may have
interesting implications to inflationary physics.Comment: 8 pages including 6 figures, LaTex; references adde
A Search for Single Radio Pulses and Bursts from Southern AXPs
We observed four southern AXPs in 1999 near 1400 MHz with the Parkes 64-m
radio telescope to search for periodic radio emission. No Fourier candidates
were discovered in the initial analysis, but the recent radio activity observed
for the AXP XTE J1810-197 has prompted us to revisit these data to search for
single radio pulses and bursts. The data were searched for both persistent and
bursting radio emission at a wide range of dispersion measures, but no
detections of either kind were made. These results further weaken the proposed
link between rotating radio transient sources and magnetars. However, continued
radio searches of these and other AXPs at different epochs are warranted given
the transient nature of the radio emission seen from XTE J1810-197, which until
very recently was the only known radio-emitting AXP.Comment: 3 pages, including 1 table. To appear in the proceedings of "40 Years
of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007,
McGill University, Montreal, Canad
Low Frequency Observations of Millisecond Pulsars with the WSRT
With LOFAR beginning operation in 2008 there is huge potential for studying
pulsars with high signal to noise at low frequencies. We present results of
observations made with the Westerbork Synthesis Radio Telescope to revisit,
with modern technology, this frequency range. Coherently dedispersed profiles
of millisecond pulsars obtained simultaneously between 115-175 MHz are
presented. We consider the detections and non-detections of 14 MSPs in light of
previous observations and the fluxes, dispersion measures and spectral indices
of these pulsars. The excellent prospects for LOFAR finding new MSPs and
studying the existing systems are then discussed in light of these results.Comment: 5 pages, 5 figures, To appear in the proceedings of "40 Years of
Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007,
McGill University, Montreal, Canad
On spurious numerics in solving reactive equations
1. Motivation and objectives
Consider 3D reactive Euler equations of the form
Ut + F(U)x + G(U)y + H(U)z = S(U), (1.1)
where U, F(U), G(U), H(U) and S(U) are vectors. Here, the source term S(U) is restricted to be homogeneous in U; that is, (x, y, z) and t do not appear explicitly in S(U). If physical viscosities are present, viscous flux derivative should be added. If the time scale of the ordinary differential equation (ODE) Ut = S(U) for the source term is orders of magnitude smaller than the time scale of the homogeneous conservation law Ut +F(U)x +G(U)y +H(U)z = 0, then the problem is said to be stiff due to the source terms. In combustion or high speed chemical reacting flows the source term represents the chemical reactions which may be much faster than the gas flow, leading to problems of numerical stiffness. Insufficient spatial/temporal resolution may cause an incorrect propagation speed of discontinuities and nonphysical states for standard numerical methods that were developed for non-reacting flows. See Wang et al. (2012) for a comprehensive overview of the last two decades of development. Schemes designed to improve the prediction of propagation speed of discontinuities for systems of stiff reacting flows remain a challenge for algorithm development (Wang et al. 2012). Wang et al. also proposed a new high order finite difference method with subcell resolution for advection equations with stiff source terms for a single reaction for (1.1) to overcome this difficulty. Research for multi-species (or more species and multi-reactions) is forthcoming.
The objective of this study is to gain a deeper understanding of the behavior of high order shock-capturing schemes for problems with stiff source terms and discontinuities and on corresponding numerical prediction strategies. The studies by Yee et al. (2012) and Wang et al. (2012) focus only on solving the reactive system by the fractional step method using the Strang splitting (Strang 1968). It is a common practice by developers in computational physics and engineering simulations to include a cut off safeguard if densities are outside the permissible range. Here we compare the spurious behavior of the same schemes by solving the fully coupled reactive system without the Strang splitting vs. using the Strang splitting. Comparison between the two procedures and the effects of a cut off safeguard is the focus the present study. The comparison of the performance of these schemes is largely based on the degree to which each method captures the correct location of the reaction front for coarse grids. Here “coarse grids” means standard mesh density requirement for accurate simulation of typical non-reacting flows of similar problem setup. It is remarked that, in order to resolve the sharp reaction front, local refinement beyond standard mesh density is still needed.
For reacting flows there are different ways in formulating (1.1). The present study considers the following two commonly used formulations. These are using all the species variables vs. using the total density and Ns − 1 number of species variables (Ns is the total number of species)
Non-Gaussianity of a single scalar field in general covariant Ho\v{r}ava-Lifshitz gravity
In this paper, we study non-Gaussianity generated by a single scalar field in
slow-roll inflation in the framework of the non-relativistic general covariant
Ho\v{r}ava-Lifshitz theory of gravity with the projectability condition and an
arbitrary coupling constant , where characterizes the
deviation of the theory from general relativity (GR) in the infrared. We find
that the leading effect of self-interaction, in contrary to the case of minimal
scenario of GR, is in general of the order ,
where is a slow-roll parameter, and
are the dimensionless coupling coefficients of the six-order operators of the
Lifshitz scalar, and have no contributions to power spectra and indices of both
scalar and tensor. The bispectrum, comparing with the standard one given in GR,
is enhanced, and gives rise to a large value of the nonlinearity parameter
.We study how the modified dispersion relation with high order
moment terms affects the evaluation of the mode function and in turn the
bispectrum, and show explicitly that the mode function takes various asymptotic
forms during different periods of its evolution. In particular, we find that it
is in general of superpositions of oscillatory functions, instead of plane
waves like in the minimal scenario of GR. This results in a large enhancement
of the folded shape in the bispectrum.Comment: Added new references and corrected some typos. 5 figures, revtex4.
Phys. Rev. D86, 103523 (2012
Bounds on general entropy measures
We show how to determine the maximum and minimum possible values of one
measure of entropy for a given value of another measure of entropy. These
maximum and minimum values are obtained for two standard forms of probability
distribution (or quantum state) independent of the entropy measures, provided
the entropy measures satisfy a concavity/convexity relation. These results may
be applied to entropies for classical probability distributions, entropies of
mixed quantum states and measures of entanglement for pure states.Comment: 13 pages, 3 figures, published versio
Using dark modes for high-fidelity optomechanical quantum state transfer
In a recent publication [Y.D. Wang and A.A. Clerk, Phys. Rev. Lett. 108,
153603 (2012)], we demonstrated that one can use interference to significantly
increase the fidelity of state transfer between two electromagnetic cavities
coupled to a common mechanical resonator over a naive sequential-transfer
scheme based on two swap operations. This involved making use of a delocalized
electromagnetic mode which is decoupled from the mechanical resonator, a
so-called "mechanically-dark" mode. Here, we demonstrate the existence of a new
"hybrid" state transfer scheme which incorporates the best elements of the
dark-mode scheme (protection against mechanical dissipation) and the
double-swap scheme (fast operation time). Importantly, this new scheme also
does not require the mechanical resonator to be prepared initially in its
ground state. We also provide additional details on the previously-described
interference-enhanced transfer schemes, and provide an enhanced discussion of
how the interference physics here is intimately related to the optomechanical
analogue of electromagnetically-induced transparency (EIT). We also compare the
various transfer schemes over a wide range of relevant experimental parameters,
producing a "phase diagram" showing the the optimal transfer scheme for
different points in parameter space.Comment: 39 pages, 11 figures NJP 14 (Focus issue on Optomechanics
A nonlinear quantum model of the Friedmann universe
A discussion is given of the quantisation of a physical system with finite
degrees of freedom subject to a Hamiltonian constraint by treating time as a
constrained classical variable interacting with an unconstrained quantum state.
This leads to a quantisation scheme that yields a Schrodinger-type equation
which is in general nonlinear in evolution. Nevertheless it is compatible with
a probabilistic interpretation of quantum mechanics and in particular the
construction of a Hilbert space with a Euclidean norm is possible. The new
scheme is applied to the quantisation of a Friedmann Universe with a massive
scalar field whose dynamical behaviour is investigated numerically.Comment: 11 pages of text + 4 pages for 8 figure
Dynamic polarizabilities and related properties of clock states of ytterbium atom
We carry out relativistic many-body calculations of the static and dynamic
dipole polarizabilities of the ground 6s^2 ^1S_0 and the first excited states of Yb. With these polarizabilities, we compute several
properties of Yb relevant to optical lattice clocks operating on the 6s^2
^1S_0 - 6s6p ^3P^o_0 transition. We determine (i) the first four {\em magic}
wavelengths of the laser field for which the frequency of the clock transition
is insensitive to the laser intensity. While the first magic wavelength is
known, we predict the second, the third and the forth magic wavelengths to be
551 nm, 465 nm, and 413 nm. (ii) We reevaluate the effect of black-body
radiation on the frequency of the clock transition, the resulting clock shift
at being Hz. (iii) We compute long-range
interatomic van der Waals coefficients (in a.u.) C_6(6s^2 ^1S_0 +6s^2 ^1S_0) =
1909(160), C_6(6s^2 ^1S_0 + 6s6p ^3P_0) =2709(338) , and . Finally, we determine the atom-wall interaction
coefficients (in a.u.), C_3 (6s^2 ^1S_0) =3.34 and .
We also address and resolve a disagreement between previous calculations of the
static polarizability of the ground state.Comment: 11 pages, 1 figur
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