Acoustic wave propagation in bulk GaN Semiconductor in the presence of a slowly changing a.c electric field and a constant electric field has been studied. Analytical expression for the attenuation (amplification) coefficient has been obtained. It is shown that the a.c electric field is acting as a modulator and that the acoustoelectric gain increases to a maximum value and then fall off as the acoustic wave frequency is increased. Also for a a.c electric field of 4 x 104 V/CM the acoustoelectric gain remains zero until the acoustic wave frequency is about 5.8x1012Hz then a resonant amplification peak appears. If the a.c electric field is increased a second resonant peak accurs at 8.7x1012Hz. It is therefore suggested that the sample can be used as a maser.
https://arxiv.org/abs/1005.2574
We consider wireless communication networks where network users are subject to critical events such as emergencies and crises. If a critical event occurs to a user, the user needs to send critical traffic as early as possible. However, most existing medium access control (MAC) protocols are not adequate to meet the urgent need for data transmission by users with critical traffic. In this paper, we devise a class of distributed MAC protocols that achieve coordination using the finite-length memory of users containing their own observations and traffic types. We formulate a protocol design problem and find optimal protocols that solve the problem. We show that the proposed protocols enable a user with critical traffic to transmit its critical traffic without interruption from other users after a short delay while allowing users to share the channel efficiently when there is no critical traffic. Moreover, the proposed protocols require short memory and can be implemented without explicit message passing.
https://arxiv.org/abs/1005.0693
This paper focuses on data structures for multi-core reachability, which is a key component in model checking algorithms and other verification methods. A cornerstone of an efficient solution is the storage of visited states. In related work, static partitioning of the state space was combined with thread-local storage and resulted in reasonable speedups, but left open whether improvements are possible. In this paper, we present a scaling solution for shared state storage which is based on a lockless hash table implementation. The solution is specifically designed for the cache architecture of modern CPUs. Because model checking algorithms impose loose requirements on the hash table operations, their design can be streamlined substantially compared to related work on lockless hash tables. Still, an implementation of the hash table presented here has dozens of sensitive performance parameters (bucket size, cache line size, data layout, probing sequence, etc.). We analyzed their impact and compared the resulting speedups with related tools. Our implementation outperforms two state-of-the-art multi-core model checkers (SPIN and DiVinE) by a substantial margin, while placing fewer constraints on the load balancing and search algorithms.
https://arxiv.org/abs/1004.2772
Based on the full band electronic structure calculations, first we consider the effect of n-type doping on the optical absorption and the refractive index in wurtzite InN and GaN. We identify quite different dielectric response in either case; while InN shows a significant shift in the absorption edge due to n-type doping, this is masked for GaN due to efficient cancellation of the Burstein-Moss effect by the band gap renormalization. For high doping levels the intraband absorption becomes significant in InN. Furthermore, we observe that the free-carrier plasma contribution to refractive index change becomes more important than both band filling and the band gap renormalization for electron densities above 10$^{19}$~cm$^{-3}$ in GaN, and 10$^{20}$~cm$^{-3}$ in InN. As a result of the two different characteristics mentioned above, the overall change in the refractive index due to n-type doping is much higher in InN compared to GaN, which in the former exceeds 4\% for a doping of 10$^{19}$~cm$^{-3}$ at 1.55~$\mu$m wavelength. Finally, we consider intrinsic InN under strong photoexcitation which introduces equal density of electron and holes thermalized to their respective band edges. The change in the refractive index at 1.55~$\mu$m is observed to be similar to the n-doped case up to a carrier density of 10$^{20}$~cm$^{-3}$. However, in the photoexcited case this is now accompanied by a strong absorption in this wavelength region due to $\Gamma^v_5 \to \Gamma^v_6$ intravalence band transition. Our findings suggest that the alloy composition of In$x$Ga${1-x}$N can be optimized in the indium-rich region so as to benefit from high carrier-induced refractive index change while operating in the transparency region to minimize the losses. These can have direct implications for InN-containing optical phase modulators and lasers.
https://arxiv.org/abs/0911.2312
A software platform for global optimisation, called PaGMO, has been developed within the Advanced Concepts Team (ACT) at the European Space Agency, and was recently released as an open-source project. PaGMO is built to tackle high-dimensional global optimisation problems, and it has been successfully used to find solutions to real-life engineering problems among which the preliminary design of interplanetary spacecraft trajectories - both chemical (including multiple flybys and deep-space maneuvers) and low-thrust (limited, at the moment, to single phase trajectories), the inverse design of nano-structured radiators and the design of non-reactive controllers for planetary rovers. Featuring an arsenal of global and local optimisation algorithms (including genetic algorithms, differential evolution, simulated annealing, particle swarm optimisation, compass search, improved harmony search, and various interfaces to libraries for local optimisation such as SNOPT, IPOPT, GSL and NLopt), PaGMO is at its core a C++ library which employs an object-oriented architecture providing a clean and easily-extensible optimisation framework. Adoption of multi-threaded programming ensures the efficient exploitation of modern multi-core architectures and allows for a straightforward implementation of the island model paradigm, in which multiple populations of candidate solutions asynchronously exchange information in order to speed-up and improve the optimisation process. In addition to the C++ interface, PaGMO’s capabilities are exposed to the high-level language Python, so that it is possible to easily use PaGMO in an interactive session and take advantage of the numerous scientific Python libraries available.
https://arxiv.org/abs/1004.3824
This paper presents an application of evolutionary search procedures to artificial neural networks. Here, we can distinguish among three kinds of evolution in artificial neural networks, i.e. the evolution of connection weights, of architectures, and of learning rules. We review each kind of evolution in detail and analyse critical issues related to different evolutions. This article concentrates on finding the suitable way of using evolutionary algorithms for optimizing the artificial neural network parameters.
https://arxiv.org/abs/1004.3557
We address quantum invisibility in the context of electronics in nanoscale quantum structures. We make use of the freedom of design that quantum corrals provide and show that quantum mechanical objects can be hidden inside the corral, with respect to inelastic electron scattering spectroscopy in combination with scanning tunneling microscopy, and we propose a design strategy. A simple illustration of the invisibility is given in terms of an elliptic quantum corral containing a molecule, with a local vibrational mode, at one of the foci. Our work has implications to quantum information technology and presents new tools for nonlocal quantum detection and distinguishing between different molecules.
https://arxiv.org/abs/0811.1782
Most of the web user’s requirements are search or navigation time and getting correctly matched result. These constrains can be satisfied with some additional modules attached to the existing search engines and web servers. This paper proposes that powerful architecture for search engines with the title of Probabilistic Semantic Web Mining named from the methods used. With the increase of larger and larger collection of various data resources on the World Wide Web (WWW), Web Mining has become one of the most important requirements for the web users. Web servers will store various formats of data including text, image, audio, video etc., but servers can not identify the contents of the data. These search techniques can be improved by adding some special techniques including semantic web mining and probabilistic analysis to get more accurate results. Semantic web mining technique can provide meaningful search of data resources by eliminating useless information with mining process. In this technique web servers will maintain Meta information of each and every data resources available in that particular web server. This will help the search engine to retrieve information that is relevant to user given input string. This paper proposing the idea of combing these two techniques Semantic web mining and Probabilistic analysis for efficient and accurate search results of web mining. SPF can be calculated by considering both semantic accuracy and syntactic accuracy of data with the input string. This will be the deciding factor for producing results.
https://arxiv.org/abs/1004.1794
This is a scientific strategy for the detection and characterization of extrasolar planets; that is, planets orbiting other stars. As such, it maps out over a 15-year horizon the techniques and capabilities required to detect and measure the properties of planets as small as Earth around stars as large as our own Sun. It shows how the technology pieces and their development fit together to achieve the primary goal of the strategy: if planets like Earth exist around stars within some tens of light years of our own Solar System, those planets will be found and their basic properties characterized. Essential to this strategy is not only the search for and examination of individual planets, but also a knowledge of the arrangement, or architecture, of planetary systems around as large a number of stars as possible; this is the second goal of the strategy. The final goal of the strategy is the study of disks around stars, important both to understand the implications of the variety of exoplanet systems for planet formation, and to determine how many nearby stars have environments around them clean enough of debris that planets may be sought and, if found, characterized.
https://arxiv.org/abs/0808.2754
As the amount of personal information stored at remote service providers increases, so does the danger of data theft. When connections to remote services are made in the clear and authenticated sessions are kept using HTTP cookies, data theft becomes extremely easy to achieve. In this paper, we study the architecture of the world’s largest service provider, i.e., Google. First, with the exception of a few services that can only be accessed over HTTPS (e.g., Gmail), we find that many Google services are still vulnerable to simple session hijacking. Next, we present the Historiographer, a novel attack that reconstructs the web search history of Google users, i.e., Google’s Web History, even though such a service is supposedly protected from session hijacking by a stricter access control policy. The Historiographer uses a reconstruction technique inferring search history from the personalized suggestions fed by the Google search engine. We validate our technique through experiments conducted over real network traffic and discuss possible countermeasures. Our attacks are general and not only specific to Google, and highlight privacy concerns of mixed architectures using both secure and insecure connections.
https://arxiv.org/abs/1003.3242
We present an ab-initio and analytical study of the Jahn-Teller effect in two diluted magnetic semiconductors (DMS) with d4 impurities, namely Mn-doped GaN and Cr-doped ZnS. We show that only the combined treatment of Jahn-Teller distortion and strong electron correlation in the 3d shell may lead to the correct insulating electronic structure. Using the LSDA+U approach we obtain the Jahn-Teller energy gain in reasonable agreement with the available experimental data. The ab-initio results are completed by a more phenomenological ligand field theory.
https://arxiv.org/abs/1003.0365
We investigate acceptor and donor states in GaN nanocrystals doped with a single substitutional impurity. Quantum dots (QD’s) of zinc-blende structure and spherical shape are considered with the radius ranging from 4.5 to 67.7 A. The size-dependent energy spectra are calculated within the sp3d5s* tight-binding model, which yields a good agreement with the confinement-induced blue shifts observed in undoped QD’s. The computed binding energy is strongly enhanced with respect to the experimental bulk value when the dopant is placed at the center of the smallest QD’s. It decreases with increasing QD size following a scaling law that extrapolates to the bulk limit. In order to estimate the degree of localization of the bound carriers we analyze their wave functions and average radii. The resulting physical picture points to a highly localized acceptor hole, mostly distributed over the nearest-neighbor anion shell, and a much more extended donor electron. We also study off-center impurities in intermediate-size QD’s. The acceptor binding energy is approximately independent of the dopant position unless it is placed within a surface shell of thickness of the order of the bulk Bohr radius, where the ionization energy abruptly drops. On the contrary, the donor binding energy gradually decreases as the impurity is moved away from the center toward the QD surface.
https://arxiv.org/abs/1003.0298
This note compares the performance of two multidimensional search and optimization algorithms: Group Search Optimizer and Central Force Optimization. GSO is a new state-of-the-art algorithm that has gained some notoriety, consequently providing an excellent yardstick for measuring the performance of other algorithms. CFO is a novel deterministic metaheuristic that has performed well against GSO in previous tests. The CFO implementation reported here includes architectural improvements in errant probe retrieval and decision space adaptation that result in even better performance. Detailed results are provided for the twenty-three function benchmark suite used to evaluate GSO. CFO performs better than or essentially as well as GSO on twenty functions and nearly as well on one of the remaining three. Includes update 24 February 2010.
https://arxiv.org/abs/1002.2798
Since their conception in 1975, Genetic Algorithms have been an extremely popular approach to find exact or approximate solutions to optimization and search problems. Over the last years there has been an enhanced interest in the field with related techniques, such as grammatical evolution, being developed. Unfortunately, work on developing genetic optimizations for low-end embedded architectures hasn’t embraced the same enthusiasm. This short paper tackles that situation by demonstrating how genetic algorithms can be implemented in Arduino Duemilanove, a 16 MHz open-source micro-controller, with limited computation power and storage resources. As part of this short paper, the libraries used in this implementation are released into the public domain under a GPL license.
https://arxiv.org/abs/1002.2012
We report on photoluminescence (PL) measurements at 85 K for GaN/AlGaN surface quantum wells (SQW’s) with a width in the range of 1.51-2.9 nm. The PL spectra show a redshift with decreasing SQW width, in contrast to the blueshift normally observed for conventional GaN QW’s of the same width. The effect is attributed to a strong coupling of SQW confined exciton states with surface acceptors. The PL hence originates from the recombination of surface-acceptor-bound excitons. Two types of acceptors were identified.
https://arxiv.org/abs/0905.1972
We present initial results from a population synthesis model aimed at determining the star formation rate of the Milky-Way. We find that a total star formation rate of 0.68 to 1.45 Msun/yr is able to reproduce the observed number of young stellar objects in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the 3D Galactic distributions of YSOs and interstellar dust, and using model SEDs to predict the brightness and color of the synthetic YSOs at different wavelengths. This is the first Galaxy-wide measurement derived from pre-main-sequence objects themselves, rather than global observables such as the total radio continuum, Halpha, or FIR flux. The value obtained is slightly lower than, but generally consistent with previously determined values. We will extend this method in the future to fit the brightness, color, and angular distribution of YSOs, and simultaneously make use of multiple surveys, to place constraints on the input assumptions, and reduce uncertainties in the star formation rate estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic star formation rate, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.
https://arxiv.org/abs/1001.3672
When someone mentions the name of a known person we immediately recall her face and possibly many other traits. This is because we possess the so-called associative memory, that is the ability to correlate different memories to the same fact or event. Associative memory is such a fundamental and encompassing human ability (and not just human) that the network of neurons in our brain must perform it quite easily. The question is then whether electronic neural networks (electronic schemes that act somewhat similarly to human brains) can be built to perform this type of function. Although the field of neural networks has developed for many years, a key element, namely the synapses between adjacent neurons, has been lacking a satisfactory electronic representation. The reason for this is that a passive circuit element able to reproduce the synapse behaviour needs to remember its past dynamical history, store a continuous set of states, and be “plastic” according to the pre-synaptic and post-synaptic neuronal activity. Here we show that all this can be accomplished by a memory-resistor (memristor for short). In particular, by using simple and inexpensive off-the-shelf components we have built a memristor emulator which realizes all required synaptic properties. Most importantly, we have demonstrated experimentally the formation of associative memory in a simple neural network consisting of three electronic neurons connected by two memristor-emulator synapses. This experimental demonstration opens up new possibilities in the understanding of neural processes using memory devices, an important step forward to reproduce complex learning, adaptive and spontaneous behaviour with electronic neural networks.
https://arxiv.org/abs/0905.2935
Coalescence overgrowth of pattern-grown GaN nanocolumns (NC) on c-plane sapphire substrate with metal organic chemical vapour deposition is demonstrated. The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry. We present further growth optimization and innovative characterization of MOCVD layers, overgrown on the columnar structure with varying diameter of colums. Nanoimprint lithography was applied to open circular holes of 250, 300, 450, 600 nm in diameter on the SiO2 layer, deposited on the GaN layer on c-plane sapphire template.
https://arxiv.org/abs/1001.1792
Trapping of hot electron behavior by trap centers located in buffer layer of a wurtzite phase GaN MESFET has been simulated using an ensemble Monte Carlo simulation. The results of the simulation show that the trap centers are responsible for current collapse in GaN MESFET at low temperatures. These electrical traps degrade the performance of the device at low temperature. On the opposite, a light-induced increase in the trap-limited drain current, results from the photoionization of trapped carriers and their return to the channel under the influence of the built in electric field associated with the trapped charge distribution. The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible, and the predicted drain current and other electrical characteristics for the simulated device including trapping center effects show close agreement with the available experimental data.
https://arxiv.org/abs/0912.4630
The large scale content distribution systems were improved broadly using the replication techniques. The demanded contents can be brought closer to the clients by multiplying the source of information geographically, which in turn reduce both the access latency and the network traffic. The system scalability can be improved by distributing the load across multiple servers which is proposed by replication. If a copy of the requested object (e.g., a web page or an image) is located in its closer proximity then the clients would feel low access latency. Depending on the position of the replicas, the effectiveness of replication tends to a large extent. A QoS based overlay network architecture involving an intelligent replica placement algorithm is proposed in this paper. Its main goal is to improve the network utilization and fault tolerance of the P2P system. In addition to the replica placement, it also has a caching technique, to reduce the search latency. We are able to show that our proposed architecture attains less latency and better throughput with reduced bandwidth usage, through the simulation results.
https://arxiv.org/abs/0912.2296
The size distribution and total mass of objects in the Oort Cloud have important implications to the theory of planets formation, including the properties of, and the processes taking place in the early solar system. We discuss the potential of space missions like Kepler and CoRoT, designed to discover transiting exo-planets, to detect Oort Cloud, Kuiper Belt and main belt objects by occultations of background stars. Relying on published dynamical estimates of the content of the Oort Cloud, we find that Kepler’s main program is expected to detect between 0 and ~100 occultation events by deca-kilometer-sized Oort Cloud objects. The occultations rate depends on the mass of the Oort cloud, the distance to its “inner edge”, and the size distribution of its objects. In contrast, Kepler is unlikely to find occultations by Kuiper Belt or main belt asteroids, mainly due to the fact that it is observing a high ecliptic latitude field. Occultations by Solar System objects will appear as a photometric deviation in a single measurement, implying that the information regarding the time scale and light-curve shape of each event is lost. We present statistical methods that have the potential to verify the authenticity of occultation events by Solar System objects, to estimate the distance to the occulting population, and to constrain their size distribution. Our results are useful for planning of future space-based exo-planet searches in a way that will maximize the probability of detecting solar system objects, without hampering the main science goals.
https://arxiv.org/abs/0912.0948
This paper reports the results of an experiment on the use of Kak’s B-Matrix approach to spreading activity in a Hebbian neural network. Specifically, it concentrates on the memory retrieval from single neurons and compares the performance of the B-Matrix approach to that of the traditional approach.
https://arxiv.org/abs/0908.3184
Results of the first ab initio simulations of InN/GaN multiquantum well (MQW) system are presented. The DFT results confirm the presence of the polarization charge at InN/GaN interfaces, i.e. at polar InN/GaN heterostructures. These results show the potential jumps which is related to the presence of dipole layer at these interfaces. An electrostatic polarization analysis shows that the energy minimum condition can be used to obtain the field in InN/GaN system, employing standard polarization parameters. DFT results are in good agreement with polarization data confirming the existence of electric field leading to separation of electron and holes in QWs and emergence of Quantum Confined Stark Effect (QCSE).
https://arxiv.org/abs/0911.3036
The next generation of telescopes will acquire terabytes of image data on a nightly basis. Collectively, these large images will contain billions of interesting objects, which astronomers call sources. The astronomers’ task is to construct a catalog detailing the coordinates and other properties of the sources. The source catalog is the primary data product for most telescopes and is an important input for testing new astrophysical theories, but to construct the catalog one must first detect the sources. Existing algorithms for catalog creation are effective at detecting sources, but do not have rigorous statistical error control. At the same time, there are several multiple testing procedures that provide rigorous error control, but they are not designed to detect sources that are aggregated over several pixels. In this paper, we propose a technique that does both, by providing rigorous statistical error control on the aggregate objects themselves rather than the pixels. We demonstrate the effectiveness of this approach on data from the Chandra X-ray Observatory Satellite. Our technique effectively controls the rate of false sources, yet still detects almost all of the sources detected by procedures that do not have such rigorous error control and have the advantage of additional data in the form of follow up observations, which will not be available for upcoming large telescopes. In fact, we even detect a new source that was missed by previous studies. The statistical methods developed in this paper can be extended to problems beyond Astronomy, as we will illustrate with an example from Neuroimaging.
https://arxiv.org/abs/0910.5449
A new style of temporal debugging is proposed. The new URDB debugger can employ such techniques as temporal search for finding an underlying fault that is causing a bug. This improves on the standard iterative debugging style, which iteratively re-executes a program under debugger control in the search for the underlying fault. URDB acts as a meta-debugger, with current support for four widely used debuggers: gdb, MATLAB, python, and perl. Support for a new debugger can be added in a few hours. Among its points of novelty are: (i) the first reversible debuggers for MATLAB, python, and perl; (ii) support for today’s multi-core architectures; (iii) reversible debugging of multi-process and distributed computations; and (iv) temporal search on changes in program expressions. URDB gains its reversibility and temporal abilities through the fast checkpoint-restart capability of DMTCP (Distributed MultiThreaded CheckPointing). The recently enhanced DMTCP also adds ptrace support, enabling one to freeze, migrate, and replicate debugging sessions.
https://arxiv.org/abs/0910.5046
About one-fourth of the universe is thought to consist of dark matter. Yet there is no clear understanding about the nature of these particles. Commonly discussed dark matter candidates includes the so called WIMPs or weakly interacting massive particles with masses from about 10GeV to 1TeV. These particles can gravitate to form a new class of objects in dark matter halos or around the galactic centre. We study in some detail many properties of these objects; which are dark matter dominated and bound by their self gravity; their formation and possibilities of their detection. Implications of the presence of such objects for star formation are also discussed. These objects could provide the possibility of forming primordial black holes distinct from the usual Hawking black holes and they could also provide a scenario for short duration gamma ray bursts, avoiding the baryon load problem.
https://arxiv.org/abs/0910.2306
We explore the use of the Cell Broadband Engine (Cell/BE for short) for combinatorial optimization applications: we present a parallel version of a constraint-based local search algorithm that has been implemented on a multiprocessor BladeCenter machine with twin Cell/BE processors (total of 16 SPUs per blade). This algorithm was chosen because it fits very well the Cell/BE architecture and requires neither shared memory nor communication between processors, while retaining a compact memory footprint. We study the performance on several large optimization benchmarks and show that this achieves mostly linear time speedups, even sometimes super-linear. This is possible because the parallel implementation might explore simultaneously different parts of the search space and therefore converge faster towards the best sub-space and thus towards a solution. Besides getting speedups, the resulting times exhibit a much smaller variance, which benefits applications where a timely reply is critical.
https://arxiv.org/abs/0910.1264
A single source network is said to be memory-free if all of the internal nodes (those except the source and the sinks) do not employ memory but merely send linear combinations of the symbols received at their incoming edges on their outgoing edges. In this work, we introduce network-error correction for single source, acyclic, unit-delay, memory-free networks with coherent network coding for multicast. A convolutional code is designed at the source based on the network code in order to correct network-errors that correspond to any of a given set of error patterns, as long as consecutive errors are separated by a certain interval which depends on the convolutional code selected. Bounds on this interval and the field size required for constructing the convolutional code with the required free distance are also obtained. We illustrate the performance of convolutional network error correcting codes (CNECCs) designed for the unit-delay networks using simulations of CNECCs on an example network under a probabilistic error model.
https://arxiv.org/abs/0903.1967
We study the linear and nonlinear optical properties of wurtzite GaN under c-axial stress field, using density functional theory calculations. The fully structural optimization at each c-axial strain was performed. The calculated dielectric functions show that tensile c-axial strain effectively improves the linear optical efficiency, especially for the band-edge transitions, and significantly increase the mobility of electrons in the conduction band. Second-order nonlinear optical susceptibilities show that the tensile c-axial strain will enhance the zero- and low-frequency nonlinear responses of GaN. The enhancement of the nonlinear optical property is explained by the reduction of the polarization of wurtzite GaN under tensile c-axial strains. Based on these findings, we propose a method for improving the electrical and optical properties of the crystal through imposing appropriate stress on the high symmetry crystalline directions.
https://arxiv.org/abs/0909.1350
Fractional Brownian motion (fBm) emerged as a useful model for self-similar and long-range dependent Internet traffic. Approximate performance measures are known from large deviations theory for single queuing systems with fBm through traffic. In this paper we derive end-to-end performance bounds for a through flow in a network of tandem queues under fBm cross traffic. To this end, we prove a rigorous sample path envelope for fBm that complements previous approximate results. We find that both approaches agree in their outcome that overflow probabilities for fBm traffic have a Weibullian tail. We employ the sample path envelope and the concept of leftover service curves to model the remaining service after scheduling fBm cross traffic at a system. Using composition results for tandem systems from the stochastic network calculus we derive end-to-end statistical performance bounds for individual flows in networks under fBm cross traffic. We discover that these bounds grow in O(n (log n)^(1/(2-2H))) for n systems in series where H is the Hurst parameter of the fBm cross traffic. We show numerical results on the impact of the variability and the correlation of fBm traffic on network performance.
https://arxiv.org/abs/0909.0633
We present detailed simulations of the Pan-STARRS-1 (PS1) multi-epoch, multi-band 3-pi Survey in order to assess its potential yield of transiting planets and eclipsing binaries. This survey differs from dedicated transit surveys in that it will cover the entire Northern sky but provide only sparsely sampled light curves. Since most eclipses would be detected at only a single epoch, the 3-pi Survey will be most sensitive to deep eclipses (> 0.10 mag) caused by Jupiters transiting M dwarfs and eclipsing stellar/substellar binaries. The survey will also provide parallaxes for the ~400,000 stars within 100 pc which will enable a volume-limited eclipse search, reducing the number of astrophysical false positives compared to previous magnitude-limited searches. Using the best available empirical data, we constructed a model of the extended solar neighborhood that includes stars, brown dwarfs, and a realistic binary population. We computed the yield of deeply eclipsing systems using both a semi-analytic and a full Monte Carlo approach. We examined statistical tests for detecting single-epoch eclipses in sparsely sampled data and assessed their vulnerability to false positives due to stellar variability. Assuming a short-period planet frequency of 0.5% for M dwarfs, our simulations predict that about a dozen transiting Jupiters around low-mass stars (M < 0.3 Msun) within 100 pc are potentially detectable in the PS1 3-pi Survey, along with ~300 low-mass eclipsing binaries (both component masses < 0.5 Msun), including ~10 eclipsing field brown dwarfs. Extensive follow-up observations would be required to characterize these candidate eclipsing systems, thereby enabling comprehensive tests of structural models and novel insights into the planetary architecture of low-mass stars.
https://arxiv.org/abs/0909.0006
Observations of the HI 21cm transition line promises to be an important probe into the cosmic dark ages and epoch of reionization. One of the challenges for the detection of this signal is the accuracy of the foreground source removal. This paper investigates the extragalactic point source contamination and how accurately the bright sources ($\gtrsim 1$ ~Jy) should be removed in order to reach the desired RMS noise and be able to detect the 21cm transition line. Here, we consider position and flux errors in the global sky-model for these bright sources as well as the frequency independent residual calibration errors. The synthesized beam is the only frequency dependent term included here. This work determines the level of accuracy for the calibration and source removal schemes and puts forward constraints for the design of the cosmic reionization data reduction scheme for the upcoming low frequency arrays like MWA,PAPER, etc. We show that in order to detect the reionization signal the bright sources need to be removed from the data-sets with a positional accuracy of $\sim 0.1$ arc-second. Our results also demonstrate that the efficient foreground source removal strategies can only tolerate a frequency independent antenna based mean residual calibration error of $\lesssim 0.2 %$ in amplitude or $\lesssim 0.2$ degree in phase, if they are constant over each days of observations (6 hours). In future papers we will extend this analysis to the power spectral domain and also include the frequency dependent calibration errors and direction dependent errors (ionosphere, primary beam, etc).
https://arxiv.org/abs/0908.2639
The functions standardized as part of ISO C 1999 and their addendums improved very little the security options from the previously available library. The largest flaw remained that no function asked for the buffer size of destination buffers for any function copying data into a user-supplied buffer. According to earlier research we performed, we know that error condition handling was the first solution to security vulnerabilities, followed by precondition validation. The standard C functions typically perform little precondition validation and error handling, allowing for a wide range of security issues to be introduced in their use. ISO/IEC TR 24731, titled as “TR 24731: Safer C library functions”, defines 41 new library functions for memory copying, string handling (both for normal and wide character strings), time printing, sorting, searching etc. Another innovation it brings is a constraint handling architecture, forcing error handling when certain security-related preconditions are violated when the functions are called. It also specifies the null-termination of all strings manipulated through its function and introduces a new unsigned integer type that helps preventing integer overflows and underflows. It is currently implemented by Microsoft as part of their Visual Studio 2005 and above. We examine the architecture of our implementation of ISO/IEC TR 24731. We first introduce our architectural philosophy before informing the reader about the Siemens Four View Model, an architectural methodology for the conception of large-scale software systems. Afterwards, we examine each of the view, as architected for our library. Finally, we conclude with other software engineering matters that were of high importance in the development of our implementation.
https://arxiv.org/abs/0906.2512
We present the radio detection of 18 BL Lac objects from our survey of over 575 square degrees of sky. These 18 objects are located within 20 arcsec of the X-ray position, of which 11 have a measured red-shift. All candidates are radio emitters above ~1 mJy and fall within the range of existing samples on the two colour, alpha_ro vs alpha_ox, diagram with a transitional population of three (3) evident. Two unusual sources have been identified, a candidate radio quiet BL Lac, RX J0140.9-4130, and an extreme HBL, RX J0109.9-4020, with Log(nu_peak)~19.2. The BL Lac Log(N)-Log(S) relation is consistent with other samples and indicates the ROSAT All Sky Survey (RASS) could contain (2000+-400) BL Lac objects.
https://arxiv.org/abs/0907.4214
We present first-principles calculations of AlGaN/GaN superlattice, clarifying the microscopic origin of the light emission and revealing the effect of local polarization within the quantum well. Profile of energy band and distributions of electrons and holes demonstrate the existence of a main active site in the well responsible for the main band-edge light emission. This site appears at the position where the local polarization becomes zero. With charge injection, the calculated optical spectra show that the broadening of the band gap at the active site leads to the blueshift of emission wavelength.
https://arxiv.org/abs/0907.3878
Music genre classification is an essential tool for music information retrieval systems and it has been finding critical applications in various media platforms. Two important problems of the automatic music genre classification are feature extraction and classifier design. This paper investigates inter-genre similarity modelling (IGS) to improve the performance of automatic music genre classification. Inter-genre similarity information is extracted over the mis-classified feature population. Once the inter-genre similarity is modelled, elimination of the inter-genre similarity reduces the inter-genre confusion and improves the identification rates. Inter-genre similarity modelling is further improved with iterative IGS modelling(IIGS) and score modelling for IGS elimination(SMIGS). Experimental results with promising classification improvements are provided.
http://arxiv.org/abs/0907.3220
We continue our presentation of VINE. We begin with a description of relevant architectural properties of the serial and shared memory parallel computers on which VINE is intended to run, and describe their influences on the design of the code itself. We continue with a detailed description of a number of optimizations made to the layout of the particle data in memory and to our implementation of a binary tree used to access that data for use in gravitational force calculations and searches for SPH neighbor particles. We describe modifications to the code necessary to obtain forces efficiently from special purpose `GRAPE’ hardware. We conclude with an extensive series of performance tests, which demonstrate that the code can be run efficiently and without modification in serial on small workstations or in parallel using OpenMP compiler directives on large scale, shared memory parallel machines. We analyze the effects of the code optimizations and estimate that they improve its overall performance by more than an order of magnitude over that obtained by many other tree codes. Scaled parallel performance of the gravity and SPH calculations, together the most costly components of most simulations, is nearly linear up to maximum machine sizes available to us (120 processors on an Origin~3000). At similar accuracy, performance of VINE, used in GRAPE-tree mode, is approximately a factor two slower than that of VINE, used in host-only mode. Optimizations of the GRAPE/host communications could improve the speed by as much as a factor of three, but have not yet been implemented in VINE.
https://arxiv.org/abs/0802.4253
This paper extends the treatment of single-neuron memories obtained by the B-matrix approach. The spreading of the activity within the network is determined by the network’s proximity matrix which represents the separations amongst the neurons through the neural pathways.
https://arxiv.org/abs/0906.0798
We present an explanation of recently observed giant magnetic moment and room-temperature ferromagnetism in the dielectric GaN doped with Gd. Our approach uses the polarization mechanism of exchange interaction, which occurs if the d-level of Gd appears in the bandgap close to the valence band edge. Calculated ferromagnetic critical temperature and the value of the magnetic moment well correspond to experimental findings.
https://arxiv.org/abs/0905.0500
By harnessing the superposition and entanglement of physical states, quantum computers could outperform their classical counterparts in solving problems of technological impact, such as factoring large numbers and searching databases. A quantum processor executes algorithms by applying a programmable sequence of gates to an initialized register of qubits, which coherently evolves into a final state containing the result of the computation. Simultaneously meeting the conflicting requirements of long coherence, state preparation, universal gate operations, and qubit readout makes building quantum processors challenging. Few-qubit processors have already been shown in nuclear magnetic resonance, cold ion trap and optical systems, but a solid-state realization has remained an outstanding challenge. Here we demonstrate a two-qubit superconducting processor and the implementation of the Grover search and Deutsch-Jozsa quantum algorithms. We employ a novel two-qubit interaction, tunable in strength by two orders of magnitude on nanosecond time scales, which is mediated by a cavity bus in a circuit quantum electrodynamics (cQED) architecture. This interaction allows generation of highly-entangled states with concurrence up to 94%. Although this processor constitutes an important step in quantum computing with integrated circuits, continuing efforts to increase qubit coherence times, gate performance and register size will be required to fulfill the promise of a scalable technology.
https://arxiv.org/abs/0903.2030
The mechanism of recrystallization in epitaxial (1000) GaN film, introduced by indentation technique, is probed by lattice dynamic studies using Raman spectroscopy. The recrystallized region is identified by Micro-Raman area mapping. Pop-in bursts in loading lines indicate nucleation of dislocations and climb of dislocations. These processes set in plastic motion of lattice atoms under stress field at the center of indentation for the initiation of recrystallization process. A planar defect migration mechanism is evolved. A pivotal role of vacancy migration is pointed out, for the first time, as the rate limiting factor for the dislocation dynamics initiating the recrystallization process in GaN.
https://arxiv.org/abs/0807.0841
The optical properties of focused ion beam-engraved perfectly aligned and spatially controlled multi-quantum-well InGaN-GaN nanopillars were investigated. Crystalline MQW nanopillars with a diameter of 30 to 95 nm and high aspect ratios showed a maximum of three-fold enhancement in emission intensity per unit active area. A light emitting contour map of Taiwan is drawn with a nanopillar structure to demonstrate the site control of the technique adopted in the present study. Raman scattering studies were used to characterize the newly created surfaces. Unknown peaks in GaN and InGaN nanostructures are identified for surface optical (SO) phonon modes with proper assignments of wave vectors using multiple excitations, and the SO mode for the ternary phase is reported for the first time.
https://arxiv.org/abs/0905.0189
We present an ab-initio study of the Mn substitution for Ga in GaN using the Heyd-Scuseria-Ernzerhof hybrid functional (HSE). Contrary to semi-local functionals, the majority Mn t$_{2}$ manifold splits into an occupied doublet and an unoccupied singlet well above the Fermi-level resulting in an insulating groundstate, which is further stabilized by a sizeable Jahn-Teller distortion. The predictions are confirmed using $GW$ calculations and are in agreement with experiment. A transition from a localized to a delocalized Mn hole state is predicted from GaN to GaAs.
https://arxiv.org/abs/0904.2140
The spin relaxation time $T_{1}$ in zinc blende GaN quantum dot is investigated for different magnetic field, well width and quantum dot diameter. The spin relaxation caused by the two most important spin relaxation mechanisms in zinc blende semiconductor quantum dots, {i.e.} the electron-phonon scattering in conjunction with the Dresselhaus spin-orbit coupling and the second-order process of the hyperfine interaction combined with the electron-phonon scattering, are systematically studied. The relative importance of the two mechanisms are compared in detail under different conditions. It is found that due to the small spin orbit coupling in GaN, the spin relaxation caused by the second-order process of the hyperfine interaction combined with the electron-phonon scattering plays much more important role than it does in the quantum dot with narrower band gap and larger spin-orbit coupling, such as GaAs and InAs.
https://arxiv.org/abs/0809.4103
In this paper, we present an envelope function analysis in order to design the emission spectra of a white quantum well light emitting diode. The nanometric heterostructure that we are dealing with is a multiple quantum well, consisting periods of three single quantum wells with various well thicknesses. With the aid of 6x6 Luttinger Hamiltonian, we employ the combination of two methods, k.p perturbation and transfer matrix method, to acquire electron and hole wavefunctions analytically. The envelope function approximation was considered to obtain these wavefunctions for a special basis set. While adjacent valence subbands have been studied exactly, the conduction bands are approximated as parabolic. The effect of Stokes shift has been also taken into account. The dipole moment matrix elements for interband atomic transitions are evaluated via correlation between electron and hole envelope functions, for both orthogonal polarizations. This has simplified the calculation of photoluminescence intensity. Spatial variations in hole/electron wavefunctions have been examined with the introduction of piezoelectric and spontaneous polarizations internal field. We theoretically establish the possibility of a highly efficient InGaN red emitter, resulting in a uniform luminescence in red, green and blue emissions from the while light emitting diode, through adjusting material composition, potential slope, and thickness.
https://arxiv.org/abs/0903.2956
We present 7mm Very Large Array continuum images of the starburst galaxy M82. On arcsecond scales, two-thirds of the 7mm continuum consists of free-free emission from HII regions. In the subarcsecond resolution map, we identify 14 compact sources, including 9 bright HII regions with N_Lyc > 10^51 sec^-1. Four of the HII regions have rising spectra, implying emission measures > 10^8 cm^-6 pc. Except for one compact source with peculiar features, all other compact radio sources are found in dust lanes and do not have optical or near-infrared continuum counterparts. Four regions of extended, high brightness (EM > 10^7 cm-6 pc) radio emission are found in our high resolution map, including some as large as ~2”, or 30 pc, representing either associations of small HII regions, or sheetlike structures of denser gas. The good correlation between 7 mm emission and Spitzer IRAC 8 micron continuum-removed PAH feature suggests that PAH emission may track the recently formed OB stars. We find an excellent correlation between molecular gas and star formation, particularly dense gas traced by HCN, down to the ~ 45 pc scale in M82. We also find star formation efficiencies (SFEs) of 1-10% on the same scale, based on CO maps. The highest SFE are found in regions with the highest dense gas fractions.
https://arxiv.org/abs/0903.1858
Extrasolar multiple-planet systems provide valuable opportunities for testing theories of planet formation and evolution. The architectures of the known multiple-planet systems demonstrate a fascinating level of diversity, which motivates the search for additional examples of such systems in order to better constrain their formation and dynamical histories. Here we describe a comprehensive investigation of 22 planetary systems in an effort to answer three questions: 1) Are there additional planets? 2) Where could additional planets reside in stable orbits? and 3) What limits can these observations place on such objects? We find no evidence for additional bodies in any of these systems; indeed, these new data do not support three previously announced planets (HD 20367b: Udry et al. 2003, HD 74156d: Bean et al. 2008, and 47 UMa c: Fischer et al. 2002). The dynamical simulations show that nearly all of the 22 systems have large regions in which additional planets could exist in stable orbits. The detection-limit computations indicate that this study is sensitive to close-in Neptune-mass planets for most of the systems targeted. We conclude with a discussion on the implications of these non-detections.
https://arxiv.org/abs/0903.0652
Throughput optimal scheduling policies in general require the solution of a complex and often NP-hard optimization problem. Related literature has shown that in the context of time-varying channels, randomized scheduling policies can be employed to reduce the complexity of the optimization problem but at the expense of a memory requirement that is exponential in the number of data flows. In this paper, we consider a Linear-Memory Randomized Scheduling Policy (LM-RSP) that is based on a pick-and-compare principle in a time-varying network with $N$ one-hop data flows. For general ergodic channel processes, we study the performance of LM-RSP in terms of its stability region and average delay. Specifically, we show that LM-RSP can stabilize a fraction of the capacity region. Our analysis characterizes this fraction as well as the average delay as a function of channel variations and the efficiency of LM-RSP in choosing an appropriate schedule vector. Applying these results to a class of Markovian channels, we provide explicit results on the stability region and delay performance of LM-RSP.
https://arxiv.org/abs/0903.0604
The effect of edge-type dislocation wall strain field on the Hall mobility in n-type epitaxial GaN was theoretically investigated through deformation potential within the relaxation time approach. It was found that this channel of scattering can play a considerable role in the low-temperature transport at the certain set of the model parameters. The low temperature experimental data were fitted by including this mechanism of scattering along with ionized impurities and charge dislocation ones.
https://arxiv.org/abs/0902.4774
Great strides have been made in the last two decades in determining how galaxies evolve from their initial dark matter seeds to the complex structures we observe at z=0. The role of mergers has been documented through both observations and simulations, numerous satellites that may represent these initial dark matter seeds have been discovered in the Local Group, high redshift galaxies have been revealed with monstrous star formation rates, and the gaseous cosmic web has been mapped through absorption line experiments. Despite these efforts, the dark matter simulations that include baryons are still unable to accurately reproduce galaxies. One of the major problems is our incomplete understanding of how a galaxy accretes its baryons and subsequently forms stars. Galaxy formation simulations have been unable to accurately represent the required gas physics on cosmological timescales, and observations have only just begun to detect the star formation fuel over a range of redshifts and environments. How galaxies obtain gas and subsequently form stars is a major unsolved, yet tractable problem in contemporary extragalactic astrophysics. In this paper we outline how progress can be made in this area in the next decade.
https://arxiv.org/abs/0902.4717