12,936 research outputs found
Compositional imprints in density-distance-time: a rocky composition for close-in low-mass exoplanets from the location of the valley of evaporation
We use an end-to-end model of planet formation, thermodynamic evolution, and
atmospheric escape to investigate how the statistical imprints of evaporation
depend on the bulk composition of planetary cores (rocky vs. icy). We find that
the population-wide imprints like the location of the "evaporation valley" in
the distance-radius plane and the corresponding bimodal radius distribution
clearly differ depending on the bulk composition of the cores. Comparison with
the observed position of the valley (Fulton et al. 2017) suggests that close-in
low-mass Kepler planets have a predominately Earth-like rocky composition.
Combined with the excess of period ratios outside of MMR, this suggests that
low-mass Kepler planets formed inside of the water iceline, but still
undergoing orbital migration. The core radius becomes visible for planets
losing all primordial H/He. For planets in this "triangle of evaporation" in
the distance-radius plane, the degeneracy in compositions is reduced. In the
observed diagram, we identify a trend to more volatile-rich compositions with
increasing radius (R/R_Earth3: H/He).
The mass-density diagram contains important information about formation and
evolution. Its characteristic broken V-shape reveals the transitions from solid
planets to low-mass core-dominated planets with H/He and finally to
gas-dominated giants. Evaporation causes density and orbital distance to be
anti-correlated for low-mass planets, in contrast to giants, where closer-in
planets are less dense, likely due to inflation. The temporal evolution of the
statistical properties reported here will be of interest for the PLATO 2.0
mission which will observe the temporal dimension.Comment: 24 pages, 12 figures. Accepted in ApJ. Minor changes relative to v
Top-squark mixing effects in the supersymmetric electroweak corrections to top quark production at the Tevatron
Taking into account the mixing effects between left- and right-handed
top-squarks, we calculate the genuine supersymmetric eletroweak correction to
top quark production at the Tevatron in the minimal supersymmetric model. The
analytic expressions of the corrections to both the parton level cross section
and the total hadronic cross section are presented. Some numerical examples are
also given to show the size of the corrections.Comment: 11 pages, 3 figures, available at reques
Mechanism of phonon localized edge modes
The phonon localized edge modes are systematically studied, and two
conditions are proposed for the existence of the localized edge modes: (I)
coupling between different directions (, or ) in the interaction;
(II) different boundary conditions in three directions. The generality of these
two conditions is illustrated by different lattice structures: one-dimensional
(1D) chain, 2D square lattice, 2D graphene, 3D simple cubic lattice, 3D diamond
structure, etc; and with different potentials: valence force field model,
Brenner potential, etc.Comment: 5 pages, 8 fig
Graphene-based tortional resonator from molecular dynamics simulation
Molecular dynamics simulations are performed to study graphene-based
torsional mechanical resonators. The quality factor is calculated by
, where the frequency and life time are
obtained from the correlation function of the normal mode coordinate. Our
simulations reveal the radius-dependence of the quality factor as
, which yields a maximum value at some proper
radius . This maximum point is due to the strong boundary effect in the
torsional resonator, as disclosed by the temperature distribution in the
resonator. Resulting from the same boundary effect, the quality factor shows a
power law temperature-dependence with power factors bellow 1.0. The theoretical
results supply some valuable information for the manipulation of the quality
factor in future experimental devices based on the torsional mechanical
resonator.Comment: (accepted by EPL). New email address for Jin-Wu Jiang after
22/Nov/2011: [email protected]
Why edge effects are important on the intrinsic loss mechanisms of graphene nanoresonators?
Molecular dynamics simulations are performed to investigate edge effects on
the quality factor of graphene nanoresonators with different edge
configurations and of various sizes. If the periodic boundary condition is
applied, very high quality factors () are obtained for all kinds
of graphene nanoresonators. However, if the free boundary condition is applied,
quality factors will be greatly reduced by two effects resulting from free
edges: the imaginary edge vibration effect and the artificial effect. Imaginary
edge vibrations will flip between a pair of doubly degenerate warping states
during the mechanical oscillation of nanoresonators. The flipping process
breaks the coherence of the mechanical oscillation of the nanoresonator, which
is the dominant mechanism for extremely low quality factors. There is an
artificial effect if the mechanical oscillation of the graphene nanoresonator
is actuated according to an artificial vibration (non-natural vibration of the
system), which slightly reduce the quality factor. The artificial effect can be
eliminated by actuating the mechanical oscillation according to a natural
vibration of the nanoresonator. Our simulations provide an explanation for the
recent experiment, where the measured quality factor is low and varies between
identical samples with free edges.Comment: accepted by J. Appl. Phy
A theoretical study of thermal conductivity in single-walled boron nitride nanotubes
We perform a theoretical investigation on the thermal conductivity of
single-walled boron nitride nanotubes (SWBNT) using the kinetic theory. By
fitting to the phonon spectrum of boron nitride sheet, we develop an efficient
and stable Tersoff-derived interatomic potential which is suitable for the
study of heat transport in sp2 structures. We work out the selection rules for
the three-phonon process with the help of the helical quantum numbers attributed to the symmetry group (line group) of the SWBNT. Our calculation
shows that the thermal conductivity diverges with length as
with exponentially decaying , which results from the competition between boundary scattering
and three-phonon scattering for flexure modes. We find that the two flexure
modes of the SWBNT make dominant contribution to the thermal conductivity,
because their zero frequency locates at where is
the rotational angle of the screw symmetry in SWBNT.Comment: accepted by PR
Self-repairing in single-walled carbon nanotubes by heat treatment
Structure transformation by heat treatment in single-walled carbon nanotubes
(SWCNT) is investigated using molecular dynamics simulation. The critical
temperature for the collapse of pure SWCNT is as high as 4655 K due to strong
covalent carbon-carbon bonding. Above 2000 K, the cross section of SWCNT
changes from circle to ellipse. The self-repairing capability is then
investigated and two efficient processes are observed for the SWCNT to repair
themselves. (1) In the first mechanism, vacancy defects aggregate to form a
bigger hole, and a bottleneck junction is constructed nearby. (2) In the second
mechanism, a local curvature is generated around the isolate vacancy to smooth
the SWCNT. Benefit from the powerful self-repairing capability, defective SWCNT
can seek a stable configuration at high temperatures; thus the critical
temperature for collapse is insensitive to the vacancy defect density.Comment: accepted by Journal of Applied Physic
Elastic and non-linear stiffness of graphene: a simple approach
The recent experiment [Science \textbf{321}, 385 (2008)] on the Young's
modulus and third-order elastic stiffness of graphene are well explained in a
very simple approach, where the graphene is described by a simplified system
and the force constant for the non-linear interaction is estimated from the
Tersoff-Brenner potential.Comment: 4 pages, 4 figure
- …