6,080 research outputs found
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Nonreciprocal Wavefront Engineering with Time-Modulated Gradient Metasurfaces
We propose a paradigm to realize nonreciprocal wavefront engineering using time-modulated gradient metasurfaces. The essential building block of these surfaces is a subwavelength unit cell whose reflection coefficient oscillates at low frequency. We demonstrate theoretically and experimentally that such modulation permits tailoring the phase and amplitude of any desired nonlinear harmonic and determines the behavior of all other emerging fields. By appropriately adjusting the phase delay applied to the modulation of each unit cell, we realize time-modulated gradient metasurfaces that provide efficient conversion between two desired frequencies and enable nonreciprocity by (i) imposing drastically different phase gradients during the up/down conversion processes and (ii) exploiting the interplay between the generation of certain nonlinear surface and propagative waves. To demonstrate the performance and broad reach of the proposed platform, we design and analyze metasurfaces able to implement various functionalities, including beam steering and focusing, while exhibiting strong and angle-insensitive nonreciprocal responses. Our findings open an alternative direction in the field of gradient metasurfaces, in which wavefront control and magnetic-free nonreciprocity are locally merged to manipulate the scattered fields
Lattice Properties of PbX (X = S, Se, Te): Experimental Studies and ab initio Calculations Including Spin-Orbit Effects
During the past five years the low temperature heat capacity of simple
semiconductors and insulators has received renewed attention. Of particular
interest has been its dependence on isotopic masses and the effect of spin-
orbit coupling in ab initio calculations. Here we concentrate on the lead
chalcogenides PbS, PbSe and PbTe. These materials, with rock salt structure,
have different natural isotopes for both cations and anions, a fact that allows
a systematic experimental and theoretical study of isotopic effects e.g. on the
specific heat. Also, the large spin-orbit splitting of the 6p electrons of Pb
and the 5p of Te allows, using a computer code which includes spin-orbit
interaction, an investigation of the effect of this interaction on the phonon
dispersion relations and the temperature dependence of the specific heat and on
the lattice parameter. It is shown that agreement between measurements and
calculations significantly improves when spin-orbit interaction is included.Comment: 25 pages, 12 Figures, 1 table, submitted to PR
Current-induced two-level fluctuations in pseudo spin-valves (Co/Cu/Co) nanostructures
Two-level fluctuations of the magnetization state of pseudo spin-valve
pillars Co(10 nm)/Cu(10 nm)/Co(30 nm) embedded in electrodeposited nanowires
(~40 nm in diameter, 6000 nm in length) are triggered by spin-polarized
currents of 10^7 A/cm^2 at room temperature. The statistical properties of the
residence times in the parallel and antiparallel magnetization states reveal
two effects with qualitatively different dependences on current intensity. The
current appears to have the effect of a field determined as the bias field
required to equalize these times. The bias field changes sign when the current
polarity is reversed. At this field, the effect of a current density of 10^7
A/cm^2 is to lower the mean time for switching down to the microsecond range.
This effect is independent of the sign of the current and is interpreted in
terms of an effective temperature for the magnetization.Comment: 4 pages, 5 figures, revised version, to be published in Phys. Rev.
Let
On the origin of the X-ray emission from a narrow-line radioquasar at z>1
We present new XMM-Newton X-ray observations of the z=1.246 narrow-line
radioquasar RX J1011.2+5545 serendipitously discovered by ROSAT. The flat X-ray
spectrum previously measured by ROSAT and ASCA is shown to be the result of a
steep Gamma~1.8 power law spectrum seen through a moderate intrinsic absorbing
column NH~4E21 cm^-2. The position of the X-ray source is entirely coincident
with the nucleus of the radio source that we have resolved in new sensitive VLA
observations at 3.6 and 6 cm, implying that scattering in the radio lobes is
not responsible for the bulk of X-ray emission. In the EPIC pn image, a faint
patch of X-ray emission is apparent 14'' to the NE of the main X-ray source.
The former is positionally coincident with an apparently extended optical
object with R~21.9, but there is no associated radio emission, thus ruling out
the possibility that this represents a hotspot in a jet emanating from the
primary X-ray source. No reflection features are detected in the X-ray spectrum
of the narrow-line radioquasar, although an Fe line with equivalent width of up
to 600 eV cannot be ruled out.Comment: 7 pages, 6 figures, MNRAS in the pres
The effect of the forget-remember mechanism on spreading
We introduce a new mechanism---the forget-remember mechanism into the
spreading process. Equipped with such a mechanism an individual is prone to
forget the "message" received and remember the one forgotten, namely switching
his state between active (with message) and inactive (without message). The
probability of state switch is governed by linear or exponential
forget-remember functions of history time which is measured by the time elapsed
since the most recent state change. Our extensive simulations reveal that the
forget-remember mechanism has significant effects on the saturation of message
spreading, and may even lead to a termination of spreading under certain
conditions. This finding may shed some light on how to control the spreading of
epidemics. It is found that percolation-like phase transitions can occur. By
investigating the properties of clusters, formed by connected, active
individuals, we may be able to justify the existence of such phase transitions.Comment: 15 pages, 15 figure
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The deep-pocket effect of internal capital markets
We provide evidence that incumbent and entrant firms' access to business group deep pockets affects entry patterns in product markets. Relying on a unique French data set on business groups, our paper shows that entry in manufacturing industries is negatively related to the cash hoarded by incumbent-affiliated groups, and positively related to entrant groups' cash. In line with theoretical predictions, we find that the impact on entry of group cash holdings is more important in environments where financial constraints are pronounced and in more financially dependent sectors. The cash holdings of incumbent and entrant groups also affect the survival rate of entrants in the 3 to 5 year post-entry window. Overall, our findings suggest that internal capital markets operate within corporate groups and affect the product market behavior of affiliated firms by mitigating financial constraints
The Luminosity Function Evolution of Soft X--ray selected AGN in the RIXOS survey
A sample of 198 soft X--ray selected active galactic nuclei (AGN) from the
ROSAT International X--ray Optical Survey (RIXOS), is used to investigate the
X--ray luminosity function and its evolution. RIXOS, with a flux limit of 3E-14
erg s-1 cm-2 (0.5 to 2.0 keV), samples a broad range in redshift over 20 deg^2
of sky, and is almost completely identified; it is used in combination with the
Einstein Extended Medium Sensitivity Survey (EMSS), to give a total sample of
over 600 AGN. We find the evolution of AGN with redshift to be consistent with
pure luminosity evolution (PLE) models in which the rate of evolution slows
markedly or stops at high redshifts z>1.8. We find that this result is not
affected by the inclusion, or exclusion, of narrow emission line galaxies at
low redshift in the RIXOS and EMSS samples, and is insensitive to uncertainties
in the conversion between flux values measured with ROSAT and Einstein. We
confirm, using a model independent Ve/Va test, that our survey is consistent
with no evolution at high redshifts.Comment: 10 pages, LaTeX file, PS figures and mn.sty. Accepted in MNRA
Efecto del procesado físico de diferentes materias primas sobre el rendimiento productivo y composición de la grasa abdominal del broiler
El objetivo de este trabajo fue determinar el efecto de la incorporación en el pienso de diversas materias primas sometidas a procesado físico sobre los rendimientos productivos del broiler. Paralelamente se determinó el efecto de las diferentes dietas experimentales sobre la calidad de la grasa abdominal del pollo como parámetro indicativo de la calidad lipídica de la canal del pollo
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