8,821 research outputs found
Engineering magnetic domain-wall structure in permalloy nanowires
Using Lorentz transmission electron microscopy we investigate the behavior of
domain walls pinned at non-topographic defects in Cr(3 nm)/Permalloy(10
nm)/Cr(5 nm) nanowires of width 500 nm. The pinning sites consist of linear
defects where magnetic properties are modified by a Ga ion probe with diameter
~ 10 nm using a focused ion beam microscope. We study the detailed change of
the modified region (which is on the scale of the focused ion spot) using
electron energy loss spectroscopy and differential phase contrast imaging on an
aberration (Cs) corrected scanning transmission electron microscope. The signal
variation observed indicates that the region modified by the irradiation
corresponds to ~ 40-50 nm despite the ion probe size of only 10 nm. Employing
the Fresnel mode of Lorentz transmission electron microscopy, we show that it
is possible to control the domain wall structure and its depinning strength not
only via the irradiation dose but also the line orientation.Comment: Accepted for publication in Physical Review Applie
CAFE: Calar Alto Fiber-fed Echelle spectrograph
We present here CAFE, the Calar Alto Fiber-fed Echelle spectrograph, a new
instrument built at the Centro Astronomico Hispano Alem\'an (CAHA). CAFE is a
single fiber, high-resolution (70000) spectrograph, covering the
wavelength range between 3650-9800\AA. It was built on the basis of the common
design for Echelle spectrographs. Its main aim is to measure radial velocities
of stellar objects up to 13-14 mag with a precision as good as a few
tens of . To achieve this goal the design was simplified at maximum,
removing all possible movable components, the central wavelength is fixed, so
the wavelentgth coverage; no filter wheel, one slit and so on, with a
particular care taken in the thermal and mechanical stability. The instrument
is fully operational and publically accessible at the 2.2m telescope of the
Calar Alto Observatory.
In this article we describe (i) the design, summarizing its manufacturing
phase; (ii) characterize the main properties of the instrument; (iii) describe
the reduction pipeline; and (iv) show the results from the first light and
commissioning runs. The preliminar results indicate that the instrument fulfill
the specifications and it can achieve the foreseen goals. In particular, they
show that the instrument is more efficient than anticipated, reaching a
20 for a stellar object as faint as 14.5 mag in 2700s
integration time. The instrument is a wonderful machine for exoplanetary
research (by studying large samples of possible systems cotaining massive
planets), galactic dynamics (high precise radial velocities in moving groups or
stellar associations) or astrochemistry.Comment: 12 pages, 23 figures; Acepted for publishing in A&A, 201
Inhomogenized sudden future singularities
We find that sudden future singularities may also appear in spatially
inhomogeneous Stephani models of the universe. They are temporal pressure
singularities and may appear independently of the spatial finite density
singularities already known to exist in these models. It is shown that the main
advantage of the homogeneous sudden future singularities which is the
fulfillment of the strong and weak energy conditions may not be the case for
inhomogeneous models.Comment: REVTEX 4, 5 pages, no figures, a discussion of the most general case
include
On the 2-point function of the O(N) model
The self-energy of the critical 3-dimensional O(N) model is calculated. The
analysis is performed in the context of the Non-Perturbative Renormalization
Group, by exploiting an approximation which takes into account contributions of
an infinite number of vertices. A very simple calculation yields the 2-point
function in the whole range of momenta, from the UV Gaussian regime to the
scaling one. Results are in good agreement with best estimates in the
literature for any value of N in all momenta regimes. This encourages the use
of this simple approximation procedure to calculate correlation functions at
finite momenta in other physical situations
Performance analysis of energy detection over hyper-Rayleigh fading channels
This study investigates the performance of energy detection (ED)-based spectrum sensing over two-wave with diffused power (TWDP) fading channels, which have been found to provide accurate characterisation for a variety of fading conditions. A closed-form expression for the average detection probability of ED-based spectrum sensing over TWDP fading channels is derived. This expression is then used to describe the behaviour of ED-based spectrum sensing for a variety of channels that include Rayleigh, Rician and hyper-Rayleigh fading models. Such fading scenarios present a reliable behavioural model of machine-to-machine wireless nodes operating in confined structures such as in-vehicular environments
- âŠ