33,272 research outputs found
In-flight dissipation as a mechanism to suppress Fermi acceleration
Some dynamical properties of time-dependent driven elliptical-shaped billiard
are studied. It was shown that for the conservative time-dependent dynamics the
model exhibits the Fermi acceleration [Phys. Rev. Lett. 100, 014103 (2008)]. On
the other hand, it was observed that damping coefficients upon collisions
suppress such phenomenon [Phys. Rev. Lett. 104, 224101 (2010)]. Here, we
consider a dissipative model under the presence of in-flight dissipation due to
a drag force which is assumed to be proportional to the square of the
particle's velocity. Our results reinforce that dissipation leads to a phase
transition from unlimited to limited energy growth. The behaviour of the
average velocity is described using scaling arguments.Comment: 4 pages, 5 figure
Newtonian View of General Relativistic Stars
Although general relativistic cosmological solutions, even in the presence of
pressure, can be mimicked by using neo-Newtonian hydrodynamics, it is not clear
whether there exists the same Newtonian correspondence for spherical static
configurations. General relativity solutions for stars are known as the
Tolman-Oppenheimer-Volkoff (TOV) equations. On the other hand, the Newtonian
description does not take into account the total pressure effects and therefore
can not be used in strong field regimes. We discuss how to incorporate pressure
in the stellar equilibrium equations within the neo-Newtonian framework. We
compare the Newtonian, neo-Newtonian and the full relativistic theory by
solving the equilibrium equations for both three approaches and calculating the
mass-radius diagrams for some simple neutron stars equation of state.Comment: 6 pages, 3 figures. v2 matches accepted version (EPJC
Spectroscopy of brown dwarf candidates in IC 348 and the determination of its substellar IMF down to planetary masses
Context. Brown dwarfs represent a sizable fraction of the stellar content of
our Galaxy and populate the transition between the stellar and planetary mass
regime. There is however no agreement on the processes responsible for their
formation. Aims. We have conducted a large survey of the young, nearby cluster
IC 348, to uncover its low-mass brown dwarf population and study the cluster
properties in the substellar regime. Methods. Deep optical and near-IR images
taken with MegaCam and WIRCam at the Canada-France-Hawaii Telescope (CFHT) were
used to select photometric candidate members. A spectroscopic follow-up of a
large fraction of the candidates was conducted to assess their youth and
membership. Results. We confirmed spectroscopically 16 new members of the IC
348 cluster, including 13 brown dwarfs, contributing significantly to the
substellar census of the cluster, where only 30 brown dwarfs were previously
known. Five of the new members have a L0 spectral type, the latest-type objects
found to date in this cluster. At 3 Myr, evolutionary models estimate these
brown dwarfs to have a mass of ~13 Jupiter masses. Combining the new members
with previous census of the cluster, we constructed the IMF complete down to 13
Jupiter masses. Conclusions. The IMF of IC 348 is well fitted by a log-normal
function, and we do not see evidence for variations of the mass function down
to planetary masses when compared to other young clusters.Comment: Accepted to A&A (8 November 2012
Universal behavior of spin-mediated energy transport in S=1/2 chain cuprates: BaCu2Si2O7 as an example
The thermal conductivity of the spin-1/2 chain cuprate BaCu2Si2O7 was
measured along different crystallographic directions in the temperature region
between 0.5 and 300 K. The thermal conductivity along the chain direction
considerably exceeds that along perpendicular directions. Near the
antiferromagnetic transition at T_N = 9.2 K the data indicates enhanced
scattering of phonons by critical fluctuations in the spin system. A comparison
of the data above T_N with available results on similar materials reveals
similarities in the main features of the temperature dependence of the mean
free path of itinerant spin excitations. This universal behavior is most likely
caused by the spin-lattice interaction.Comment: 7 pages, 3 figure
Study of the Fully Frustrated Clock Model using the Wang-Landau Algorithm
Monte Carlo simulations using the newly proposed Wang-Landau algorithm
together with the broad histogram relation are performed to study the
antiferromagnetic six-state clock model on the triangular lattice, which is
fully frustrated. We confirm the existence of the magnetic ordering belonging
to the Kosterlitz-Thouless (KT) type phase transition followed by the chiral
ordering which occurs at slightly higher temperature. We also observe the lower
temperature phase transition of KT type due to the discrete symmetry of the
clock model. By using finite-size scaling analysis, the higher KT temperature
and the chiral critical temperature are respectively estimated as
and . The results are in favor of the double
transition scenario. The lower KT temperature is estimated as .
Two decay exponents of KT transitions corresponding to higher and lower
temperatures are respectively estimated as and
, which suggests that the exponents associated with the KT
transitions are universal even for the frustrated model.Comment: 7 pages including 9 eps figures, RevTeX, to appear in J. Phys.
Characterization of the second- and third-order nonlinear optical susceptibilities of monolayer MoS using multiphoton microscopy
We report second- and third-harmonic generation in monolayer MoS
as a tool for imaging and accurately characterizing the material's nonlinear
optical properties under 1560 nm excitation. Using a surface nonlinear optics
treatment, we derive expressions relating experimental measurements to second-
and third-order nonlinear sheet susceptibility magnitudes, obtaining values of
m V and for the first time for
monolayer MoS, m V.
These sheet susceptibilities correspond to effective bulk nonlinear
susceptibility values of m V and
m V, accounting for the sheet
thickness. Experimental comparisons between MoS and graphene are
also performed, demonstrating 3.4 times stronger third-order sheet
nonlinearity in monolayer MoS, highlighting the material's
potential for nonlinear photonics in the telecommunications C band.Comment: Accepted by 2D Materials, 28th Oct 201
Optimized cross-slot flow geometry for microfluidic extension rheometry
A precision-machined cross-slot flow geometry with a shape that has been optimized by numerical simulation of the fluid kinematics is fabricated and used to measure the extensional viscosity of a dilute polymer solution. Full-field birefringence microscopy is used to monitor the evolution and growth of macromolecular anisotropy along the stagnation point streamline, and we observe the formation of a strong and uniform birefringent strand when the dimensionless flow strength exceeds a critical Weissenberg number Wicrit 0:5. Birefringence and bulk pressure drop measurements provide self consistent estimates of the planar extensional viscosity of the fluid over a wide range of deformation rates (26 s1 "_ 435 s1) and are also in close agreement with numerical simulations performed by using a finitely extensible nonlinear elastic dumbbell model
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