Manh Tuong Nguyen

Automatic discovery of community structures in complex networks is a fundamental task in many disciplines, including physics, biology, and the social sciences. The most used criterion for characterizing the existence of a community structure in a network is modularity, a quantitative measure proposed by Newman and Girvan (2004). The discovery community can be formulated as the so-called modularity maximization problem that consists of finding a partition of nodes of a network with the highest modularity. In this letter, we propose a fast and scalable algorithm called DCAM, based on DC (difference of convex function) programming and DCA (DC algorithms), an innovative approach in nonconvex programming framework for solving the modularity maximization problem. The special structure of the problem considered here has been well exploited to get an inexpensive DCA scheme that requires only a matrix-vector product at each iteration. Starting with a very large number of communities, DCAM furnishes, as output results, an optimal partition together with the optimal number of communities [Formula: see text]; that is, the number of communities is discovered automatically during DCAM's iterations. Numerical experiments are performed on a variety of real-world network data sets with up to 4,194,304 nodes and 30,359,198 edges. The comparative results with height reference algorithms show that the proposed approach outperforms them not only on quality and rapidity but also on scalability. Moreover, it realizes a very good trade-off between the quality of solutions and the run time.

... Unit: DCMP Chair: Valentino Cooper, Oak Ridge National Laboratory. Abstract: T23.00015 : Atomic Structures and Magnetic Properties of Fe-rich Fe$_{1-x}$Co$_x$ Alloys: A Genetic Algorithm Search. 5:18 PM–5:30 PM. Preview Abstract. Authors: Manh Cuong Nguyen (Ames ...

Cu2Te is one of the most commonly used conductive back-contacting materials for high-efficiency CdTe-based solar cells. However, the detailed crystal structure of Cu2Te is still undetermined blocking property investigations of the Cu2Te-based solar cell systems. Some models have been proposed but all of them have positive formation energy. We have performed adaptive genetic algorithm crystal structure search to find low energy crystal structures of Cu2Te. We found a new layered-structure edged by Te atoms with negative formation energy from first-principles calculations within local density approximation. This layered structure consists of tilted Cu2Te ribbon arrays. Structural and electronic properties of the newly found Cu2Te structure will be discussed in detail. Supported in part US DOE, NSF of China, and US NSF.

Using a motif-network search scheme, we studied the tetrahedral structures of the dilithium/disodium transition metal orthosilicates A2MSiO4 with A = Li or Na and M = Mn, Fe or Co. In addition to finding all previously reported structures, we discovered many other different tetrahedral-network-based crystal structures which are highly degenerate in energy. These structures can be classified into structures with 1D, 2D and 3D M-Si-O frameworks. A clear trend of the structural preference in different systems was revealed and possible indicators that affect the structure stabilities were introduced. For the case of Na systems which have been much less investigated in the literature relative to the Li systems, we predicted their ground state structures and found evidence for the existence of new structural motifs.

ABSTRACT The compound known as Zr2Co11 is a ferromagnet with high uniaxial anisotropy. Although a lot of experimental work has been done on this compound, its crystal structure is still unsolved. We performed adaptive Genetic Algorithm (GA) search on its atomic structure, in order to have a better understanding of this compound. The validity of our method was verified by locating all the stable phases in Zr-Co alloy system. The search for Zr2Co11 was performed with up to 117 atoms per unit cell and a narrow composition window near 15.38% Zirconium was explored. We found that Zr2Co11 compound has a structure derived from CaCu5 prototype and complex mixed phases can be formed. Simulated XRD and TEM patterns of our models are in agreement with the experimental results. Calculated magnetic properties provide explanations of the high uniaxial anisotropy in this system.

We consider a developing country with three sectors in economy: consumption goods, new technology, and education. Productivity of the consumption goods sector depends on new technology and skilled labor used for production of the new technology. We show that there might be three stages of economic growth. In the first stage the country concentrates on production of consumption goods; in the second stage it requires the country to import both physical capital to produce consumption goods and new technology capital to produce new technology; and finally the last stage is one where the country needs to import new technology capital and invest in the training and education of high skilled labor in the same time.

In a developing country with three sectors: consumption goods, new tech- nology, and education, the productivity of the consumption goods depends on a new technology and skilled labor used to produce this new technol- ogy. There can be three stages of economic growth. In the rst stage the country concentrates on the production of consumption goods; in the second the country must import both physical capital and new technology capital to produce consumption goods and new technology; in the third the country must import capital and invest in the training and education of high skilled labor.

ABSTRACT Application layer multicast (ALM) algorithms are either similar or conceptually based on network layer multicast’s cost functions. In this research work, a new application-network cross layer multi-variable cost function is proposed. It optimizes the variable requirements and available resources from both the application and the network layers. It can dynamically update the available resources required for reaching a particular node on the ALM’s media distribution tree. Mathematical derivation and theoretical analysis have been provided for the newly proposed cost function so that it can be applied in more general cases of different contexts. An evaluation platform of an overlay network built over a convergent underlay network comprised of a simulated Internet topology and a real 4G mobile WiMAX IEEE802.16e wireless network is constructed. The performance of the newly proposed cross-layer multi-variable cost function is evaluated and compared with conventional cost functions in scalable video coding-based services such as video multicast and video conferencing services using the evaluation platform. Intensive evaluation results have shown that the newly proposed cross-layer multi-variable cost function outperforms the conventional cost function and improves the performance of multimedia services on dynamic convergent network environments.

We attempt to describe the stress distributions of granular packings using lattice-based layer-by-layer stochastic models that satisfy the constraints of force and torque balance and nontensile forces at each site. The inherent asymmetry in the layer-by-layer approach appears to lead to an asymmetric force distribution, in disagreement with both experiments and general symmetry considerations. The vertical force component probability distribution is robust and in agreement with predictions of the scalar q model of Liu et al. [Science 269, 513 (1995)] and Coppersmith et al. [Phys. Rev. E 53, 4673 (1996)] while the distribution of horizontal force components is qualitatively different and depends on the details of implementation.

Density functional calculations are performed to study the energetic, structural, and electronic properties of graphene and silicene functionalized with hydrogen. Our calculations predict that H atoms bind much more strongly to silicene than to graphene. The adsorbed H atoms tend to cooperatively stabilize each other leading to a two-dimensional nucleation and growth mechanism. The different structural and electronic modifications induced by H in fully functionalized graphene and silicene (known as graphane and silicane) are also explained. Finally, the electronic properties of defective graphane with multiple hydrogen vacancies are investigated. Engineering the vacancies in graphane offers a way to modify the electronic properties of this material.

ABSTRACT The concerns about the supply and resource of rare earth (RE) metals have generated a lot of interests in searching for high performance RE-free permanent magnets. Alnico alloys are traditional non-RE permanent magnets and have received much attention recently due their good performance at high temperature. In this paper, we develop an accurate and efficient cluster expansion energy model for alnico 5–7. Monte Carlo simulations using the cluster expansion method are performed to investigate the structure of alnico 5–7 at atomistic and nano scales. The alnico 5–7 master alloy is found to decompose into FeCo-rich and NiAl-rich phases at low temperature. The boundary between these two phases is quite sharp (∼2 nm) for a wide range of temperature. The compositions of the main constituents in these two phases become higher when the temperature gets lower. Both FeCo-rich and NiAl-rich phases are in B2 ordering with Fe and Al on α-site and Ni and Co on β-site. The degree of order of the NiAl-rich phase is much higher than that of the FeCo-rich phase. A small magnetic moment is also observed in NiAl-rich phase but the moment reduces as the temperature is lowered, implying that the magnetic properties of alnico 5–7 could be improved by lowering annealing temperature to diminish the magnetism in NiAl-rich phase. The results from our Monte Carlo simulations are consistent with available experimental results.

ABSTRACT We demonstrate bandwidth- and wavelength-tunable comb filters with sharp-cutoff and low-dispersion characteristics using a 16-tap PLC-based optical transversal filter. Such filters have no limitation of the wavelength-channel number owing to the cascaded Mach-Zehnder interferometer structure.

ABSTRACT We consider the problem of constructing public-key encryption schemes that are resilient to chosen-ciphertext and key-leakage attach. In this paper, we present a generic construction of public-key encryption that is resilient to this attack from any 1-universal hash proof system and any 4-wise independent hash function. As shown in the paper [6] they can be instantiated with efficient schemes based on standard intractability assumptions such as Decisional Diffie-Hellman, Quadratic Residuosity, and Pailliers Decisional Composite Residuosity.

A large amount of parameters related to both operating conditions and material design affects the electrical ageing of the low voltage rotating machine insulation. Accelerated ageing tests are usually undertaken in order to develop a theory to describe the electrical ageing process and to determine a lifetime model of these materials. However, to the best of our knowledge, there is no complete model allowing the prediction of an insulation lifetime from accelerated ageing tests, since there are many possible failure mechanisms and various synergetic effects between them. Another problem with accelerated ageing tests is that results of the tests tend to have a great deal of scatter. In the present work, we propose the use of the design of experiments (DoE) method, which is a useful statistical approach that would lead to a reliable and significant interpretation of the different ordering parameters of the insulation ageing process. Using the DoE method, the analysis of accelerated ageing test results allows identifying the factors that most influence the results, and those that do not, as well as details such as the existence of interactions and synergies between these factors. In the following, results from accelerated ageing tests on PEI varnishes, largely used in rotating machines insulation, are presented and analyzed with the DoE method.

Advanced materials characterization techniques with ever-growing data acquisition speed and storage capabilities represent a challenge in modern materials science, and new procedures to quickly assess and analyze the data are needed. Machine learning approaches are effective in reducing the complexity of data and rapidly homing in on the underlying trend in multi-dimensional data. Here, we show that by employing an algorithm called the mean shift theory to a large amount of diffraction data in high-throughput experimentation, one can streamline the process of delineating the structural evolution across compositional variations mapped on combinatorial libraries with minimal computational cost. Data collected at a synchrotron beamline are analyzed on the fly, and by integrating experimental data with the inorganic crystal structure database (ICSD), we can substantially enhance the accuracy in classifying the structural phases across ternary phase spaces. We have used this approach to ...

As a distinct class of ultra-hard materials, transition metal borides are found to have superior mechanical properties that challenge the traditional materials. In this work, we explored new stable structures for rhenium borides with different stoichiometries using genetic algorithm in combination with first-principles calculations. Based on theoretical calculations, ReB in a P-3m1 structure is found to be stable against decomposition reactions below 10 GPa and ReB3 in a P-6m2 structure is stable above 22 GPa. Two new phases of Re(2)B are predicted to be thermodynamically stable at pressures higher than 55 GPa and 80 GPa respectively. We also show that a C2/m structure discovered for ReB(4) has energy lower than that of the R-3m structure reported earlier (Wang et al 2013 J. Alloys Compd. 573 20). Elastic and vibrational properties from first-principles calculations indicate that the low-energy structures obtained in our search are mechanically and dynamically stable and are promisi...

By using density functional theory calculations at the PBE+U level, we investigated the properties of hematite (0001) surfaces decorated with adatoms/vacancies/substituents. For the most stable surface termination over a large range of oxygen chemical potentials (muO), the vacancy formation and adsorption energies were determined as a function of muO. Under oxygen-rich conditions, all defects are metastable with respect to the ideal surface. Under oxygen-poor conditions, O vacancies and Fe adatoms become stable. Under ambient conditions, all defects are metastable; in the bulk, O vacancies form more easily than Fe vacancies, whereas at the surface the opposite is true. All defects, that is, O and Fe vacancies, Fe and Al adatoms, and Al substituents, induce important modifications to the geometry of the surface in their vicinity. Dissociative adsorption of molecular oxygen is likely to be exothermic on surfaces with Fe/Al adatoms or O vacancies.

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