2,164 research outputs found
Crop area estimation based on remotely-sensed data with an accurate but costly subsample
Research activities conducted under the auspices of National Aeronautics and Space Administration Cooperative Agreement NCC 9-9 are discussed. During this contract period research efforts are concentrated in two primary areas. The first are is an investigation of the use of measurement error models as alternatives to least squares regression estimators of crop production or timber biomass. The secondary primary area of investigation is on the estimation of the mixing proportion of two-component mixture models. This report lists publications, technical reports, submitted manuscripts, and oral presentation generated by these research efforts. Possible areas of future research are mentioned
Flexural phonon scattering induced by electrostatic gating in graphene
Graphene has an extremely high carrier mobility partly due to its planar
mirror symmetry inhibiting scattering by the highly occupied acoustic flexural
phonons. Electrostatic gating of a graphene device can break the planar mirror
symmetry yielding a coupling mechanism to the flexural phonons. We examine the
effect of the gate-induced one-phonon scattering on the mobility for several
gate geometries and dielectric environments using first-principles calculations
based on density functional theory (DFT) and the Boltzmann equation. We
demonstrate that this scattering mechanism can be a mobility-limiting factor,
and show how the carrier density and temperature scaling of the mobility
depends on the electrostatic environment. Our findings may explain the high
deformation potential for in-plane acoustic phonons extracted from experiments
and furthermore suggest a direct relation between device symmetry and resulting
mobility.Comment: Accepted at Physical Review Letter
First-principles method for electron-phonon coupling and electron mobility: Applications to 2D materials
We present density functional theory calculations of the phonon-limited
mobility in n-type monolayer graphene, silicene and MoS. The material
properties, including the electron-phonon interaction, are calculated from
first-principles. We provide a detailed description of the normalized full-band
relaxation time approximation for the linearized Boltzmann transport equation
(BTE) that includes inelastic scattering processes. The bulk electron-phonon
coupling is evaluated by a supercell method. The method employed is fully
numerical and does therefore not require a semi-analytic treatment of part of
the problem and, importantly, it keeps the anisotropy information stored in the
coupling as well as the band structure. In addition, we perform calculations of
the low-field mobility and its dependence on carrier density and temperature to
obtain a better understanding of transport in graphene, silicene and monolayer
MoS. Unlike graphene, the carriers in silicene show strong interaction with
the out-of-plane modes. We find that graphene has more than an order of
magnitude higher mobility compared to silicene. For MoS, we obtain several
orders of magnitude lower mobilities in agreement with other recent theoretical
results. The simulations illustrate the predictive capabilities of the newly
implemented BTE solver applied in simulation tools based on first-principles
and localized basis sets
X-ray-frequency modulation via periodic switching of an external magnetic field
Single x-ray photons can be resonantly scattered and stored with the help of
suitable transitions in the atomic nucleus. Here, we investigate theoretically
means of mechanical-free modulation for the frequency spectra of such x-ray
photons via periodic switching of an external magnetic field. We show that
periodically switching on and off an external magnetic field generating
hyperfine splitting of the nuclear transition leads to the generation of
equidistant narrow sidebands of the resonantly scattered response. This
frequency-comb-like structure depends on the magnitude and orientation of the
applied magnetic field and on the switching period. An analytical approach for
the characterization of the comblike frequency spectrum is presented. The
feasibility of the external control on the frequency modulation of the x-ray
response is discussed in view of possible applications in high-resolution
spectroscopy or quantum technology
Receiver architecture of the thousand-element array (THEA)
As part of the development of a new international radio-telescope SKA (Square Kilometre Array), an outdoor phasedarray prototype, the THousand Element Array (THEA), is being developed at NFRA. THEA is a phased array with 1024 active elements distributed on a regular grid over a surface of approximately 16 m2. The array is organised into 16 units denoted as tiles. THEA operates in the frequency band from 750 to 1500 MHz.\ud
On a tile the signals from 64 antenna elements are converted into two independent RF beams. Two times 16 beams can be made simultaneously with full sensitivity by the real-time digital beam former of the THEA system. At the output of each tile the analog RF signal from a beam is converted into a 2 Ă— 12-bit digital quadrature representation by a receiver system.\ud
A double super-heterodyne architecture is used to mix the signal band of interest to an intermediate frequency of 210 MHz. The IF-signal is shifted to baseband by means of a partly digitally implemented I/Q mixer scheme. After a quadrature mixer stage, the I and Q signals are digitised by means of 12 bit A/D converters at 40 MS/s. Implementing a part of the mixing scheme digitally offers the flexibility to use different I/Q architectures, e.g. Hartley and Weaver mixer setups. This way the effect of RFI in different mixing architectures can be analyzed. After the digital processing, the samples are multiplexed, serialised and transported over fibres to the central adaptive digital beam former unit where the signals from all tiles are combined giving 32 beams.\ud
This paper focuses on the design choices and the final implementation of the THEA system. In particular, the receiver architecture is addressed. A digital solution is presented, which enables switching between a Hartley and a Weaver based mixer scheme
Tailoring laser-generated plasmas for efficient nuclear excitation by electron capture
The optimal parameters for nuclear excitation by electron capture in plasma
environments generated by the interaction of ultra-strong optical lasers with
solid matter are investigated theoretically. As a case study we consider a 4.85
keV nuclear transition starting from the long-lived Mo isomer
that can lead to the release of the stored 2.4 MeV excitation energy. We find
that due to the complex plasma dynamics, the nuclear excitation rate and the
actual number of excited nuclei do not reach their maximum at the same laser
parameters. The nuclear excitation achievable with a high-power optical laser
is up to twelve and up to six orders of magnitude larger than the values
predicted for direct resonant and secondary plasma-mediated excitation at the
x-ray free electron laser, respectively. Our results show that the experimental
observation of the nuclear excitation of Mo and the subsequent
release of stored energy should be possible at laser facilities available
today.Comment: 6 pages, 3 figures, 1 table; minor modifications made; accepted for
publication in Physical Review Letter
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