391 research outputs found
A Fully Differential Digital CMOS Pulse UWB Generator
A new fully-digital CMOS pulse generator for impulse-radio Ultra-Wide-Band (UWB) systems is presented. First, the shape of the pulse which best fits the FCC regulation in the 3.1-5 GHz sub-band of the entire 3.1-10.6 GHz UWB bandwidth is derived and approximated using rectangular digital pulses. In particular, the number and width of pulses that approximate an ideal template is found through an ad-hoc optimization methodology. Then a fully differential digital CMOS circuit that synthesizes the pulse sequence is conceived and its functionality demonstrated through post-layout simulations. The results show a very good agreement with the FCC requirements and a low power consumptio
RoboTAP: Target priorities for robotic microlensing observations
Context. The ability to automatically select scientifically-important transient events from an alert stream of many such events, and to conduct follow-up observations in response, will become increasingly important in astronomy. With wide-angle time domain surveys pushing to fainter limiting magnitudes, the capability to follow-up on transient alerts far exceeds our follow-up telescope resources, and effective target prioritization becomes essential. The RoboNet-II microlensing program is a pathfinder project, which has developed an automated target selection process (RoboTAP) for gravitational microlensing events, which are observed in real time using the Las Cumbres Observatory telescope network.
Aims. Follow-up telescopes typically have a much smaller field of view compared to surveys, therefore the most promising microlensing events must be automatically selected at any given time from an annual sample exceeding 2000 events. The main challenge is to select between events with a high planet detection sensitivity, with the aim of detecting many planets and characterizing planetary anomalies.
Methods. Our target selection algorithm is a hybrid system based on estimates of the planet detection zones around a microlens. It follows automatic anomaly alerts and respects the expected survey coverage of specific events.
Results. We introduce the RoboTAP algorithm, whose purpose is to select and prioritize microlensing events with high sensitivity to planetary companions. In this work, we determine the planet sensitivity of the RoboNet follow-up program and provide a working example of how a broker can be designed for a real-life transient science program conducting follow-up observations in response to alerts; we explore the issues that will confront similar programs being developed for the Large Synoptic Survey Telescope (LSST) and other time domain surveys
Correlation energy and spin polarization in the 2D electron gas
The ground state energy of the two--dimensional uniform electron gas has been
calculated with fixed--node diffusion Monte Carlo, including backflow
correlations, for a wide range of electron densities as a function of spin
polarization. We give a simple analytic representation of the correlation
energy which fits the density and polarization dependence of the simulation
data and includes several known high- and low-density limits. This
parametrization provides a reliable local spin density energy functional for
two-dimensional systems and an estimate for the spin susceptibility. Within the
proposed model for the correlation energy, a weakly first--order polarization
transition occurs shortly before Wigner crystallization as the density is
lowered.Comment: Minor typos corrected, see erratum: Phys. Rev. Lett. 91, 109902(E)
(2003
A fourfold coordinated point defect in silicon
Due to their technological importance, point defects in silicon are among the
best studied physical systems. The experimental examination of point defects
buried in bulk is difficult and evidence for the various defects usually
indirect. Simulations of defects in silicon have been performed at various
levels of sophistication ranging from fast force fields to accurate density
functional calculations. The generally accepted viewpoint from all these
studies is that vacancies and self interstitials are the basic point defects in
silicon. We challenge this point of view by presenting density functional
calculations that show that there is a new fourfold coordinated point defect in
silicon that is lower in energy
Small Fermi energy and phonon anharmonicity in MgB_2 and related compounds
The remarkable anharmonicity of the E_{2g} phonon in MgB_2 has been suggested
in literature to play a primary role in its superconducting pairing. We
investigate, by means of LDA calculations, the microscopic origin of such an
anharmonicity in MgB_2, AlB_2, and in hole-doped graphite. We find that the
anharmonic character of the E_{2g} phonon is essentially driven by the small
Fermi energy of the sigma holes. We present a simple analytic model which
allows us to understand in microscopic terms the role of the small Fermi energy
and of the electronic structure. The relation between anharmonicity and
nonadiabaticity is pointed out and discussed in relation to various materials.Comment: 5 pages, 2 figures replaced with final version, accepted on Physical
Review
First-principle study of excitonic self-trapping in diamond
We present a first-principles study of excitonic self-trapping in diamond.
Our calculation provides evidence for self-trapping of the 1s core exciton and
gives a coherent interpretation of recent experimental X-ray absorption and
emission data. Self-trapping does not occur in the case of a single valence
exciton. We predict, however, that self-trapping should occur in the case of a
valence biexciton. This process is accompanied by a large local relaxation of
the lattice which could be observed experimentally.Comment: 12 pages, RevTex file, 3 Postscript figure
Oscillator strengths with pseudopotentials
The time-dependent local-density approximation (TDLDA) is shown to remain
accurate in describing the atomic response of IB elements under the additional
approximation of using pseudopotentials to treat the effects of core electrons.
This extends the work of Zangwill and Soven who showed the utility of the
all-electron TDLDA in the atomic response problem.Comment: 13 pages including 3 Postscript figure
Phonon-induced spin relaxation of conduction electrons in aluminum
Spin-flip Eliashberg function and temperature-dependent spin
relaxation time are calculated for aluminum using realistic
pseudopotentials. The spin-flip electron-phonon coupling constant
is found to be . The calculations agree with experiments
validating the Elliott-Yafet theory and the spin-hot-spot picture of spin
relaxation for polyvalent metals.Comment: 4 pages; submitted to PR
Phonon spectrum and soft-mode behavior of MgCNi_3
Temperature dependent inelastic neutron-scattering measurements of the
generalized phonon density-of-states for superconducting MgCNi_3, T_c=8 K, give
evidence for a soft-mode behavior of low-frequency Ni phonon modes. Results are
compared with ab initio density functional calculations which suggest an
incipient lattice instability of the stoichiometric compound with respect to Ni
vibrations orthogonal to the Ni-C bond direction.Comment: 4 pages, 5 figure
Ab initio Calculations of Multilayer Relaxations of Stepped Cu Surfaces
We present trends in the multilayer relaxations of several vicinals of
Cu(100) and Cu(111) of varying terrace widths and geometry. The electronic
structure calculations are based on density functional theory in the local
density approximation with norm-conserving, non-local pseudopotentials in the
mixed basis representation. While relaxations continue for several layers, the
major effect concentrates near the step and corner atoms. On all surfaces the
step atoms contract inwards, in agreement with experimental findings.
Additionally, the corner atoms move outwards and the atoms in the adjacent
chain undergo large inward relaxation. Correspondingly, the largest contraction
(4%) is in the bond length between the step atom and its bulk nearest neighbor
(BNN), while that between the corner atom and BNN is somewhat enlarged. The
surface atoms also display changes in registry of upto 1.5%. Our results are in
general in good agreement with LEED data including the controversial case of
Cu(511). Subtle differences are found with results obtained from semi-empirical
potentials.Comment: 21 pages and 3 figure
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