62,685 research outputs found
On simultaneous arithmetic progressions on elliptic curves
In this paper we study elliptic curves which have a number of points whose
coordinates are in arithmetic progression. We first motivate this diophantine
problem, prove some results, provide a number of interesting examples and,
finally point out open questions which focus on the most interesting aspects of
the problem for us.Comment: 22 page
Theoretical study of loss compensation in long-range dielectric loaded surface plasmon polariton waveguides
In this paper, a theoretical study of loss compensation in long-range dielectric loaded surface plasmon waveguides (LR-DLSPPs) is presented. Although extendable to other gain materials, rare-earth doped double tungstates are used as gain material in this work. Two different structures are studied and the effect of the different waveguide geometrical parameters on the material gain required to fully compensate the propagation losses are reported. The simulations were performed at 1.55 micrometer wavelength. A material gain as low as 12.5 dB/cm was determined as sufficient to obtain complete loss compensation in one of the proposed waveguide structures supporting sub-micron lateral mode dimension
OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A
Our aim is to find and classify OB stars in Sextans A, to later determine
accurate stellar parameters of these blue massive stars in this low metallicity
region .
Using UBV photometry, the reddening-free index Q and GALEX imaging, we built
a list of blue massive star candidates in Sextans A. We obtained low resolution
(R 1000) GTC-OSIRIS spectra for a fraction of them and carried out
spectral classification. For the confirmed O-stars we derive preliminary
stellar parameters.
The target selection criteria and observations were successful and have
produced the first spectroscopic atlas of OB-type stars in Sextans A. From the
whole sample of 18 observed stars, 12 were classified as early OB-types,
including 5 O-stars. The radial velocities of all target stars are in agreement
with their Sextans A membership, although three of them show significant
deviations. We determined the stellar parameters of the O-type stars using the
stellar atmosphere code FASTWIND, and revisited the sub-SMC temperature scale.
Two of the O-stars are consistent with relatively strong winds and enhanced
helium abundances, although results are not conclusive. We discuss the position
of the OB stars in the HRD. Initial stellar masses run from slightly below 20
up to 40 solar masses.
The target selection method worked well for Sextans A, confirming the
procedure developed in Garcia \& Herrero (2013). The stellar temperatures are
consistent with findings in other galaxies. Some of the targets deserve
follow-up spectroscopy because of indications of a runaway nature, an enhanced
helium abundance or a relatively strong wind. We observe a correlation between
HI and OB associations similar to the irregular galaxy IC1613, confirming the
previous result that the most recent star formation of Sextans A is currently
on-going near the rim of the H\,{\sc I} cavity
Unbounded violations of bipartite Bell Inequalities via Operator Space theory
In this work we show that bipartite quantum states with local Hilbert space
dimension n can violate a Bell inequality by a factor of order (up
to a logarithmic factor) when observables with n possible outcomes are used. A
central tool in the analysis is a close relation between this problem and
operator space theory and, in particular, the very recent noncommutative
embedding theory. As a consequence of this result, we obtain better Hilbert
space dimension witnesses and quantum violations of Bell inequalities with
better resistance to noise
Matrix Product State Representations
This work gives a detailed investigation of matrix product state (MPS)
representations for pure multipartite quantum states. We determine the freedom
in representations with and without translation symmetry, derive respective
canonical forms and provide efficient methods for obtaining them. Results on
frustration free Hamiltonians and the generation of MPS are extended, and the
use of the MPS-representation for classical simulations of quantum systems is
discussed.Comment: Minor changes. To appear in QI
Rheological effects in the linear response and spontaneous fluctuations of a sheared granular gas
The decay of a small homogeneous perturbation of the temperature of a dilute
granular gas in the steady uniform shear flow state is investigated. Using
kinetic theory based on the inelastic Boltzmann equation, a closed equation for
the decay of the perturbation is derived. The equation involves the generalized
shear viscosity of the gas in the time-dependent shear flow state, and
therefore it predicts relevant rheological effects beyond the quasi-elastic
limit. A good agreement is found when comparing the theory with molecular
dynamics simulation results. Moreover, the Onsager postulate on the regression
of fluctuations is fulfilled
Dependence of the drag over super hydrophobic and liquid infused surfaces on the textured surface and Weber number
Direct Numerical Simulations of a turbulent channel flow have been performed. The lower wall of the channel is made of staggered cubes with a second fluid locked in the cavities. Two viscosity ratios have been considered, m=μ1/μ2=0.02 and 0.4 (the subscript 1 indicates the fluid in the cavities and 2 the overlying fluid) mimicking the viscosity ratio in super–hydrophobic surfaces (SHS) and liquid infused surfaces (LIS) respectively. A first set of simulations with a slippery interface has been performed and results agree well with those in literature for perfect slip conditions and Stokes approximations. To assess how the dynamics of the interface affects the drag, a second set of DNS has been carried out at We=40 and 400 corresponding to We+≃10−3 and 10−2. The deformation of the interface is fully coupled to the Navier-Stokes equation and tracked in time using a Level Set Method. Two gas fractions, GF=0.5 and 0.875, have been considered to assess how the spacing between the cubes affects the deformation of the interface and therefore the drag. For the dimensions of the substrate here considered, under the ideal assumption of flat interface, staggered cubes with GF=0.875 provide about 20% drag reduction for We=0. However, a rapid degradation of the performances is observed when the dynamics of the interface is considered, and the same geometry increases the drag of about 40% with respect to a smooth wall. On the other hand, the detrimental effect of the dynamics of the interface is much weaker for GF=0.5 because of the reduced pitch between the cubes
Internal energy fluctuations of a granular gas under steady uniform shear flow
The stochastic properties of the total internal energy of a dilute granular
gas in the steady uniform shear flow state are investigated. A recent theory
formulated for fluctuations about the homogeneous cooling state is extended by
analogy with molecular systems. The theoretical predictions are compared with
molecular dynamics simulation results. Good agreement is found in the limit of
weak inelasticity, while systematic and relevant discrepancies are observed
when the inelasticity increases. The origin of this behavior is discussed
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