62,223 research outputs found
Failure mechanisms of graphene under tension
Recent experiments established pure graphene as the strongest material known
to mankind, further invigorating the question of how graphene fails. Using
density functional theory, we reveal the mechanisms of mechanical failure of
pure graphene under a generic state of tension. One failure mechanism is a
novel soft-mode phonon instability of the -mode, whereby the graphene
sheet undergoes a phase transition and is driven towards isolated benzene rings
resulting in a reduction of strength. The other is the usual elastic
instability corresponding to a maximum in the stress-strain curve. Our results
indicate that finite wave vector soft modes can be the key factor in limiting
the strength of monolayer materials
Gauss-Bonnet brane gravity with a confining potential
A brane scenario is envisaged in which the -dimensional bulk is endowed
with a Gauss-Bonnet term and localization of matter on the brane is achieved by
means of a confining potential. The resulting Friedmann equations on the brane
are modified by various extra terms that may be interpreted as the X-matter,
providing a possible phenomenological explanation for the accelerated expansion
of the universe. The age of the universe in this scenario is studied and shown
to be consistent with the present observational data.Comment: 14 pages, 4 figures, to appear in PR
Nodal Quasiparticle Lifetimes in Cuprate Superconductors
A new generation of angular-resolved photoemission spectroscopy (ARPES)
measurements on the cuprate superconductors offer the promise of enhanced
momentum and energy resolution. In particular, the energy and temperature
dependence of the on-shell nodal (k_x=k_y) quasiparticle scattering rate can be
studied. In the superconducting state, low temperature transport measurements
suggest that one can describe nodal quasiparticles within the framework of a
BCS d-wave model by including forward elastic scattering and spin-fluctuation
inelastic scattering. Here, using this model, we calculate the temperature and
frequency dependence of the on-shell nodal quasiparticle scattering rate in the
superconducting state which determines the momentum width of the ARPES momentum
distribution curves. For a zero-energy quasiparticle at the nodal momentum k_N,
both the elastic and inelastic scattering rate show a sudden decrease as the
temperature drops below Tc, reflecting the onset of the gap amplitude. At low
temperatures the scattering rate decreases as T^3 and approaches a zero
temperature value determined by the elastic impurity scattering. For T>T_c, we
find a quasilinear dependence on T. At low reduced temperatures, the elastic
scattering rate for the nodal quasiparticles exhibits a quasilinear increase at
low energy which arises from elastic scattering processes. The inelastic
spin-fluctuation scattering leads to a low energy omega^3 dependence which, for
omega>~Delta_0, crosses over to a quasilinear behavior.Comment: 8 pages, 7 figures, minor revision
Bulk photonic metamaterial with hyperbolic dispersion
In this work, we demonstrate a self-standing bulk three-dimensional
metamaterial based on the network of silver nanowires in an alumina membrane.
This constitutes an anisotropic effective medium with hyperbolic dispersion,
which can be used in sub-diffraction imaging or optical cloaks. Highly
anisotropic dielectric constants of the material range from positive to
negative, and the transmitted laser beam shifts both toward the normal to the
surface, as in regular dielectrics, and off the normal, as in anisotropic
dielectrics with the refraction index smaller than one. The designed photonic
metamaterial is the thickest reported in the literature, both in terms of its
physical size 1cm x 1cm x 51 mm, and the number of vacuum wavelengths, N=61 at
l=0.84 mm.Comment: 6 pages, 4 figur
Dynamics of Nucleation in the Ising Model
Reactive pathways to nucleation in a three-dimensional Ising model at 60% of
the critical temperature are studied using transition path sampling of single
spin flip Monte Carlo dynamics. Analysis of the transition state ensemble (TSE)
indicates that the critical nuclei are rough and anisotropic. The TSE,
projected onto the free energy surface characterized by cluster size, N, and
surface area, S, indicates the significance of other variables in addition to
these two traditional reaction coordinates for nucleation. The transmission
coefficient along N is ~ 0.35, and this reduction of the transmission
coefficient from unity is explained in terms of the stochastic nature of the
dynamic model.Comment: In press at the Journal of Physical Chemistry B, 7 pages, 8 figure
Electrohydrodynamic jet printing of PZT thick film micro-scale structures
This paper reports the use of a printing technique, called electrohydrodynamic jet printing, for producing PZT thick film micro-scale structures without additional material removing processes. The PZT powder was ball-milled and the effect of milling time on the particle size was examined. This ball-milling process can significantly reduce the PZT particle size and help to prepare stable composite slurry suitable for the E-Jet printing. The PZT micro-scale structures with different features were produced. The PZT lines with different widths and separations were fabricated through the control of the E-Jet printing parameters. The widths of the PZT lines were varied from 80 μm to 200 μm and the separations were changed from 5 μm to 200 μm. In addition, PZT walled structures were obtained by multi-layer E-Jet printing. The E-Jet printed PZT thick films exhibited a relative permittivity (ɛr) of ∼233 and a piezoelectric constant (d33, f) of ∼66 pC N−1
A Search for the Near-Infrared Counterpart to GCRT J1745-3009
We present an optical/near-infrared search for a counterpart to the
perplexing radio transient GCRT J1745-3009, a source located ~1 degree from the
Galactic Center. Motivated by some similarities to radio bursts from nearby
ultracool dwarfs, and by a distance upper limit of 70 pc for the emission to
not violate the 1e12 K brightness temperature limit for incoherent radiation,
we searched for a nearby star at the position of GCRT J1745-3009. We found only
a single marginal candidate, limiting the presence of any late-type star to >1
kpc (spectral types earlier than M9), >200 pc (spectral types L and T0-T4), and
>100 pc (spectral types T4-T7), thus severely restricting the possible local
counterparts to GCRT J1745-3009. We also exclude any white dwarf within 1 kpc
or a supergiant star out to the distance of the Galactic Center as possible
counterparts. This implies that GCRT J1745-3009 likely requires a coherent
emission process, although whether or not it reflects a new class of sources is
unclear.Comment: 10 pages, 5 figures. Accepted for publication in the Astrophysical
Journa
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