3,807 research outputs found
Determination of the lowest energy structure of Ag from first-principles calculations
The ground-state electronic and structural properties, and the electronic
excitations of the lowest energy isomers of the Ag cluster are calculated
using density functional theory (DFT) and time-dependent DFT (TDDFT) in real
time and real space scheme, respectively. The optical spectra provided by TDDFT
predict that the D dodecahedron isomer is the structural minimum of
Ag cluster. Indeed, it is borne out by the experimental findings.Comment: 4 pages, 2 figures. Accepted in Physical Review A as a brief repor
Unexpected Magnetism of Small Silver Clusters
The ground-state electronic, structural, and magnetic properties of small
silver clusters, Ag (2n22), have been studied using a linear
combination of atomic Gaussian-type orbitals within the density functional
theory. The results show that the silver atoms, which are diamagnetic in bulk
environment, can be magnetic when they are grouped together in clusters. The
Ag cluster with icosahedral symmetry has the highest magnetic moment per
atom among the studied silver clusters. The cluster symmetry and the reduced
coordination number specific of small clusters reveal as a fundamental factor
for the onset of the magnetism.Comment: 4 pages, 4 figure
Low-frequency electrostatic waves in the ionospheric E-region: a comparison of rocket observations and numerical simulations
International audienceLow frequency electrostatic waves in the lower parts of the ionosphere are studied by a comparison of observations by instrumented rockets and of results from numerical simulations. Particular attention is given to the spectral properties of the waves. On the basis of a good agreement between the observations and the simulations, it can be argued that the most important nonlinear dynamics can be accounted for in a 2-D numerical model, referring to a plane perpendicular to a locally homogeneous magnetic field. It does not seem necessary to take into account turbulent fluctuations or motions in the neutral gas component. The numerical simulations explain the observed strongly intermittent nature of the fluctuations: secondary instabilities develop on the large scale gradients of the largest amplitude waves, and the small scale dynamics is strongly influenced by these secondary instabilities. We compare potential variations obtained at a single position in the numerical simulations with two point potential-difference signals, where the latter is the adequate representation for the data obtained by instrumented rockets. We can demonstrate a significant reduction in the amount of information concerning the plasma turbulence when the latter signal is used for analysis. In particular we show that the bicoherence estimate is strongly affected. The conclusions have implications for studies of low frequency ionospheric fluctuations in the E and F regions by instrumented rockets, and also for other methods relying on difference measurements, using two probes with large separation. The analysis also resolves a long standing controversy concerning the supersonic phase velocities of these cross-field instabilities being observed in laboratory experiments
Two distinct superconducting phases in LiFeAs
A non-trivial temperature evolution of superconductivity including a temperature-induced phase transition between two superconducting phases or even a time-reversal symmetry breaking order parameter is in principle expected in multiband superconductors such as iron-pnictides. Here we present scanning tunnelling spectroscopy data of LiFeAs which reveal two distinct superconducting phases: at = 18 K a partial superconducting gap opens, evidenced by subtle, yet clear features in the tunnelling spectra, i.e. particle-hole symmetric coherence peak and dip-hump structures. At Tc = 16 K, these features substantiate dramatically and become characteristic of full superconductivity. Remarkably, the distance between the dip-hump structures and the coherence peaks remains practically constant in the whole temperature regimeT ≤ . This rules out the connection of the dip-hump structures to an antiferromagnetic spin resonance
The overdensities of galaxy environments as a function of luminosity and color
We study the mean environments of galaxies in the Sloan Digital Sky Survey as
a function of rest-frame luminosity and color. Overdensities in galaxy number
are estimated in and spheres
centered on galaxies taken from the SDSS spectroscopic sample. We
find that, at constant color, overdensity is independent of luminosity for
galaxies with the blue colors of spirals. This suggests that, at fixed
star-formation history, spiral-galaxy mass is a very weak function of
environment. Overdensity does depend on luminosity for galaxies with the red
colors of early types; both low-luminosity and high-luminosity red galaxies are
found to be in highly overdense regions.Comment: submitted to ApJ
Chandra Multiwavelength Project: Normal Galaxies at Intermediate Redshift
(abridged) We have investigated 136 Chandra extragalactic sources without
broad optical emission lines, including 93 galaxies with narrow emission lines
(NELG) and 43 with only absorption lines (ALG). Based on fx/fo, Lx, X-ray
spectral hardness and optical emission line diagnostics, we have conservatively
classified 36 normal galaxies (20 spirals and 16 ellipticals) and 71 AGNs. We
found no statistically significant evolution in Lx/LB, within the limited z
range. We have built log(N)-log(S), after correcting for completeness based on
a series of simulations. The best-fit slope is -1.5 for both S and B energy
bands, which is considerably steeper than that of the AGN-dominated cosmic
background sources, but slightly flatter than the previous estimate, indicating
normal galaxies will not exceed the AGN population until fx ~ 2 x 10-18 erg s-1
cm-2 (a factor of ~5 lower than the previous estimate). A group of NELGs appear
to be heavily obscured in X-rays, i.e., a typical type 2 AGN. After correcting
for intrinsic absorption, their X-ray luminosities could be Lx > 10^44 erg s-1,
making them type 2 quasar candidates. While most X-ray luminous ALGs (XBONG -
X-ray bright, optically normal galaxy candidates) do not appear to be
significantly absorbed, we found two heavily obscured objects, which could be
as luminous as an unobscured broad-line quasar. Among 43 ALGs, we found two E+A
galaxy candidates with strong Balmer absorption lines, but no [OII] line. The
X-ray spectra of both galaxies are soft and one of them has a nearby close
companion galaxy, supporting the merger/interaction scenario rather than the
dusty starburst hypothesis.Comment: 31 pages, 9 figures, accepted for publication in ApJ (20 June 2006,
v644), replaced with minor correction
The polar Catalysmic Variable 1RXS J173006.4+033813
We report the discovery of 1RXS J173006.4+033813, a polar cataclysmic
variable with a period of 120.21 min. The white dwarf primary has a magnetic
field of B = 42+6-5 MG, and the secondary is a M3 dwarf. The system shows
highly symmetric double peaked photometric modulation in the active state as
well as in quiescence. These arise from a combination of cyclotron beaming and
ellipsoidal modulation. The projected orbital velocity of the secondary is K2 =
390+-4 km/s. We place an upper limit of 830+-65 pc on the distance.Comment: ApJ Accepted. 12 Pages, 13 Figures, 6 table
- …