First-principles density functional theory calculations are performed to examine five postulated diffusion mechanisms for Ni in NiAl: next-nearest-neighbor (NNN) jumps, the triple defect mechanism, and three variants ...
First-principles density functional theory calculations are performed to examine five postulated diffusion mechanisms for Ni in NiAl: next-nearest-neighbor (NNN) jumps, the triple defect mechanism, and three variants of the six-jump cycle. In contrast to most previous theoretical work, which employed empirical interatomic potentials, we provide a more accurate nonempirical description of the mechanisms. For each pathway, we calculate the activation energy and the pre-exponential factor for the diffusion constant. Although our quantum mechanics calculations are performed at 0 K, we show that it is critical to include the effect of temperature on the pre-exponential factor. We predict that the triple defect mechanism and [110] six-jump cycle both are likely contributors to Ni diffusion in NiAl since their activation energies and pre-exponential factors are in very good agreement with experimental data. Although the activation energy and pre-exponential factor of NNN jumps agree well with experiment, experimental evidence suggests that this is not a dominant contributor to Ni diffusion. Lastly, the activation energies of the [100] bent and straight six-jump cycles are 1 eV higher than the experimental value, allowing us to exclude both [100] cycle mechanisms.
In this paper, we focus on compressed sensing and recovery schemes for low-rank matrices, asking under what conditions a low-rank matrix can be sensed and recovered from incomplete, inaccurate, and noisy observations....
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In this paper, we focus on compressed sensing and recovery schemes for low-rank matrices, asking under what conditions a low-rank matrix can be sensed and recovered from incomplete, inaccurate, and noisy observations. We consider three schemes, one based on a certain Restricted Isometry Property and two based on directly sensing the row and column space of the matrix. We study their properties in terms of exact recovery in the ideal case, and robustness issues for approximately low-rank matrices and for noisy measurements.
In this work, multiple radar waveforms are simultaneously transmitted, emitted from different ldquovirtualrdquo antennas. The goal is to process the returns in such a way that the overall ambiguity function is a sum o...
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ISBN:
(纸本)9781424429400
In this work, multiple radar waveforms are simultaneously transmitted, emitted from different ldquovirtualrdquo antennas. The goal is to process the returns in such a way that the overall ambiguity function is a sum of ambiguity functions better approximating the desired thumbtack shape. A 4times4 example involves two spatially separated antennas with each able to transmit and receive simultaneously on two different polarizations. The 4times4 unitary design dictates the scheduling of the waveforms over the four virtual antennas over four PRIs (pulse repetition intervals), and how the matched filtering of the returns over four PRIs is combined in to achieve both perfect separation (of the superimposed returns) and perfect reconstruction. Perfect reconstruction means the sum of the time-autocorrelations associated with each of the four waveforms is a delta function. Conditions for both perfect separation and perfect reconstruction are developed, and a variety of waveform sets satisfying both are presented.
We derive an analytic form of the Wang-Govind-Carter (WGC) [Wang et al., Phys. Rev. B 60, 16350 (1999)] kinetic energy density functional (KEDF) with the density-dependent response kernel. A real-space aperiodic impl...
We derive an analytic form of the Wang-Govind-Carter (WGC) [Wang et al., Phys. Rev. B 60, 16350 (1999)] kinetic energy density functional (KEDF) with the density-dependent response kernel. A real-space aperiodic implementation of the WGC KEDF is then described and used in linear scaling orbital-free density functional theory (OF-DFT) calculations.
Periodic nonuniform sampling is a known method to sample spectrally sparse signals below the Nyquist rate. This strategy relies on the implicit assumption that the individual samplers are exposed to the entire frequen...
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Periodic nonuniform sampling is a known method to sample spectrally sparse signals below the Nyquist rate. This strategy relies on the implicit assumption that the individual samplers are exposed to the entire frequency range. This assumption becomes impractical for wideband sparse signals. The current paper proposes an alternative sampling stage that does not require a full-band front end. Instead, signals are captured with an analog front end that consists of a bank of multipliers and lowpass filters whose cutoff is much lower than the Nyquist rate. The problem of recovering the original signal from the low-rate samples can be studied within the framework of compressive sampling. An appropriate parameter selection ensures that the samples uniquely determine the analog input. Moreover, the analog input can be stably reconstructed with digital algorithms. Numerical experiments support the theoretical analysis.
The paper has the goal of comparing the performance of four different approaches to fuzzy modeling, using parameterized conjunctions, a novel concept named constrained fuzzy sets (CFSs), CFSs with parameterized conjun...
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The paper has the goal of comparing the performance of four different approaches to fuzzy modeling, using parameterized conjunctions, a novel concept named constrained fuzzy sets (CFSs), CFSs with parameterized conjunctions, and unnormalized interval type-2 Takagi Sugeno Kang (IT2 TSK). The theoretical and mathematical backgrounds of the four approaches are briefly described and their performances are compared in approximating a nonlinear function.
We show that edge stresses introduce intrinsic ripples in freestanding graphene sheets even in the absence of any thermal effects. Compressive edge stresses along zigzag and armchair edges of the sheet cause out-of-pl...
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We show that edge stresses introduce intrinsic ripples in freestanding graphene sheets even in the absence of any thermal effects. Compressive edge stresses along zigzag and armchair edges of the sheet cause out-of-plane warping to attain several degenerate mode shapes. Based on elastic plate theory, we identify scaling laws for the amplitude and penetration depth of edge ripples as a function of wavelength. We also demonstrate that edge stresses can lead to twisting and scrolling of nanoribbons as seen in experiments. Our results underscore the importance of accounting for edge stresses in thermal theories and electronic structure calculations for freestanding graphene sheets.
We present a multiscale modeling approach that can simulate multimillion atoms effectively via density-functional theory. The method is based on the framework of the quasicontinuum (QC) approach with orbital-free dens...
We present a multiscale modeling approach that can simulate multimillion atoms effectively via density-functional theory. The method is based on the framework of the quasicontinuum (QC) approach with orbital-free density-functional theory (OFDFT) as its sole energetics formulation. The local QC part is formulated by the Cauchy-Born hypothesis with OFDFT calculations for strain energy and stress. The nonlocal QC part is treated by an OFDFT-based embedding approach, which couples OFDFT nonlocal atoms to local region atoms. The method—QCDFT—is applied to a nanoindentation study of an Al thin film, and the results are compared to a conventional QC approach. The results suggest that QCDFT represents a new direction for the quantum simulation of materials at length scales that are relevant to experiments.
In this paper we examine maximum weight independent sets (MWIS) on branching processes and Erdos-Renyi graphs. Gamarnik, Nowicki, and Swirscsz [1], showed that, if a particular fixed point equation had a unique soluti...
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ISBN:
(纸本)9781424429257
In this paper we examine maximum weight independent sets (MWIS) on branching processes and Erdos-Renyi graphs. Gamarnik, Nowicki, and Swirscsz [1], showed that, if a particular fixed point equation had a unique solution, then the MWIS value converged. This followed by showing that there was no long range dependencies in the MWIS solution. Here we examine what happens when there are multiple solutions to the fixed point equation, and hence there are long range dependencies. Specifically, we show that, under surprisingly weak conditions - which include the possibility of long range dependencies - that the MWIS value converges in probability to a random variable. We give a new method for calculating the MWIS value for Erdos-Renyi graphs. In the case of vertex weights independently drawn from the standard exponential distribution and with multiple solutions to the fixed point equation, we use our method to show improved upper and lower bounds of the MWIS value. Finally, we show how our proof techniques are general enough to be applied to other optimization problems with local constraints such as Maximum Weight Matching and Maximum Weight Color.
We have developed a virtual integration environment (VIE) for the development of neural prosthetic systems. This paper, the second of two companion articles, describes the use of the VIE as a common platform for the i...
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We have developed a virtual integration environment (VIE) for the development of neural prosthetic systems. This paper, the second of two companion articles, describes the use of the VIE as a common platform for the implementation of neural decode algorithms. In this paper, a linear filter decode and a recursive Bayesian algorithm are implemented as separate signal analysis modules of the VIE for the real-time decode of end effector trajectory. The process of implementing each algorithm is described and the real-time behavior as well as computational cost for each algorithm is examined. This is the first report of the real-time implementation of the Mixture of Trajectory Models decode [10]. These real-time algorithms can be easily interfaced with pre-existing modules of the VIE to control simulated and real devices.
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