220 research outputs found
Single-layer -MoS under electron irradiation from molecular dynamics
Irradiation with high-energy particles has recently emerged as an effective
tool for tailoring the properties of two-dimensional transition metal
dichalcogenides. In order to carry out an atomically-precise manipulation of
the lattice, a detailed understanding of the beam-induced events occurring at
the atomic scale is necessary. Here, we investigate the response of
-MoS to the electron irradiation by molecular dynamics
means. Our simulations suggest that an electron beam with energy smaller than
75 keV does not result in any knock-on damage. The displacement threshold
energies are different for the two nonequivalent sulfur atoms in -MoS
and strongly depend on whether the top or bottom chalcogen layer is considered.
As a result, a careful tuning of the beam energy can promote the formation of
ordered defects in the sample. We further discuss the effect of the electron
irradiation in the neighborhood of a defective site, the mobility of the sulfur
vacancies created and their tendency to aggregate. Overall, our work provides
useful guidelines for the imaging and the defect engineering of -MoS
using electron microscopy.Comment: 8 pages, 5 figure
The role of mass, equation of state and superfluid reservoir in large pulsar glitches
Observations of pulsar glitches may provide insights on the internal physics
of neutron stars and recent studies show how it is in principle possible to
constrain pulsar masses with timing observations. The reliability of these
estimates depend on the current uncertainties about the structure of neutron
stars and on our ability to model the dynamics of the superfluid neutrons in
the internal layers. We assume a simplified model for the rotational dynamics
of a neutron star and estimate an upper bound to the mass of 25 pulsars from
their largest glitch and average activity: the aim is to understand to which
extent the mass constraints are sensitive to the choice of the unknown
structural properties of neutron stars, like the extension of the superfluid
region and the equation of state. Reasonable values, within the range measured
for neutron star masses, are obtained only if the superfluid domain extends for
at least a small region inside the outer core, which is compatible with
calculations of the neutron S-wave pairing gap. Moreover, the mass constraints
stabilise when the superfluid domain extends to densities over nuclear
saturation, irrespective of the equation of state tested.Comment: 11 pages, 6 figure
A universal formula for the relativistic correction to the mutual friction coupling time-scale in neutron stars
Vortex-mediated mutual friction governs the coupling between the superfluid
and normal components in neutron star interiors. By, for example, comparing
precise timing observations of pulsar glitches with theoretical predictions it
is possible to constrain the physics in the interior of the star, but to do so
an accurate model of the mutual friction coupling in general relativity is
needed. We derive such a model directly from Carter's multifluid formalism, and
study the vortex structure and coupling time-scale between the components in a
relativistic star. We calculate how general relativity modifies the shape and
the density of the quantized vortices and show that, in the quasi-Schwarzschild
coordinates, they can be approximated as straight lines for realistic neutron
star configurations. Finally, we present a simple universal formula (given as a
function of the stellar compactness alone) for the relativistic correction to
the glitch rise-time, which is valid under the assumption that the superfluid
reservoir is in a thin shell in the crust or in the outer core. This universal
relation can be easily employed to correct, a posteriori, any Newtonian
estimate for the coupling time-scale, without any additional computational
expense.Comment: 20 pages, 7 figure
Mesoscopic pinning forces in neutron star crusts
The crust of a neutron star is thought to be comprised of a lattice of nuclei
immersed in a sea of free electrons and neutrons. As the neutrons are
superfluid their angular momentum is carried by an array of quantized vortices.
These vortices can pin to the nuclear lattice and prevent the neutron
superfluid from spinning down, allowing it to store angular momentum which can
then be released catastrophically, giving rise to a pulsar glitch. A crucial
ingredient for this model is the maximum pinning force that the lattice can
exert on the vortices, as this allows us to estimate the angular momentum that
can be exchanged during a glitch. In this paper we perform, for the first time,
a detailed and quantitative calculation of the pinning force \emph{per unit
length} acting on a vortex immersed in the crust and resulting from the
mesoscopic vortex-lattice interaction. We consider realistic vortex tensions,
allow for displacement of the nuclei and average over all possible orientation
of the crystal with respect to the vortex. We find that, as expected, the
mesoscopic pinning force becomes weaker for longer vortices and is generally
much smaller than previous estimates, based on vortices aligned with the
crystal. Nevertheless the forces we obtain still have maximum values of order
dyn/cm, which would still allow for enough
angular momentum to be stored in the crust to explain large Vela glitches, if
part of the star is decoupled during the event.Comment: 17 pages, 16 figures, 5 table
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