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Search Results: 1 - 10 of 34555 matches for " Dinh Van Tuan "
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Anomalous Ballistic Transport in Disordered Bilayer Graphene: Dimer Vacancies induced Dirac Semimetal
Dinh Van Tuan,Stephan Roche
Physics , 2015,
Abstract: We report anomalous quantum transport features in bilayer graphene in presence of a random distribution of structural vacancies. By using an efficient real-space Kubo-Greenwood transport methodology, the impact of a varying density of dimer versus non-dimer vacancies is investigated in very large scale disordered models. While non-dimer vacancies are shown to induce localization regimes, dimer vacancies result in an unexpected ballistic regime whose energy window surprisingly enlarges with increasing impurity density. Such counterintuitive phenomenon is explained by the formation of an effective linear dispersion in the bilayer bandstructure, which roots in the symmetry breaking effects driven by dimer vacancies, and provides a novel realization of Dirac semimetals in high dimension.
Full-Filling Subcarrier Power Allocation in OFDMA-Based Cognitive Radio Systems  [PDF]
Tuan Le Van, Hieu Dinh Chi, Kinh Nguyen Viet, Hieu Nguyen Thanh
Wireless Engineering and Technology (WET) , 2014, DOI: 10.4236/wet.2014.51002

When implementing an appropriate windowing, the interference from a Cognitive Radio (CR) system to licensed systems (primary users) will be significantly reduced. Consequently, power allocated to subcarriers can be increased, especially subcarriers having far spectral distance to primary user bands can be allocated full of its maximum possible power. In this paper, we propose a new class of sub-optimal subcarrier power allocation algorithm that significantly reduces complexity of OFDMA-based CR systems. Two sub-optimal proposals, called Pre-set Filling Range (PFR) and Maximum Filling Range (MFR) are studied. Investigations show that this new power allocating algorithm allows CR systems obtain high throughput while retaining low complexity.

Plasmon modes of double-layer graphene at finite temperature
Dinh Van Tuan,Nguyen Quoc Khanh
Physics , 2011,
Abstract: We calculate the dynamical dielectric function of doped double-layer graphene (DLG), made of two parallel graphene monolayers with carrier densities n 1, n2, respectively, and an interlayer separation of d at finite temperature. The results are used to find the dispersion of plasmon modes and loss functions of DLG for several interlayer separations and layer densities. We show that in the case of n 2=0, the temperature plasmon modes are dramatically different from the zero temperature ones.
The Shifts of Band Gap and Binding Energies of Titania/Hydroxyapatite Material
Nguyen Thi Truc Linh,Phan Dinh Tuan,Nguyen Van Dzung
Journal of Composites , 2014, DOI: 10.1155/2014/283034
Abstract: The titania/hydroxyapatite (TiO2/HAp) product was prepared by precipitating hydroxyapatite in the presence of TiO(OH)2 gel in the hydrothermal system. The characteristics of the material were determined by using the measurements such as X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance spectra (DRS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX). The XPS analysis showed that the binding energy values of Ca (2p1/2, 2p3/2), P (2p1/2, 2p3/2), and O 1s levels related to hydroxyapatite phase whereas those of Ti (2p3/2, 2p1/2) levels corresponded with the characterization of titanium (IV) in TiO2. The XRD result revealed that TiO2/HAp sample had hydroxyapatite phase, but anatase or rutile phases were not found out. TEM image of TiO2/HAp product showed that the surface of the plate-shaped HAp particles had a lot of smaller particles which were considered as the compound of Ti. The experimental band gap of TiO2/HAp material calculated by the DRS measurement was 3.6?eV, while that of HAp pure was 5.3?eV and that of TiO2 pure was around 3.2?eV. The shift of the band gap energy of TiO2 in the range of 3.2–3.6?eV may be related to the shifts of Ti signals of XPS spectrum. 1. Introduction TiO2 photocatalysis has gained much attention because of its low cost, nontoxicity, high stability, and easy preparation. However, the slow rates of the photocatalytic chemical transformations, compared with other methods, the low quantum yields, the lack of visible-light utilization, and the low adsorption capacity of TiO2 have hindered it from the practical application. To solve these problems, much effort has been made to enhance the photocatalytic efficiency and visible-light utilization of TiO2 by the additional components doping [1], improving its sensitization and metallization [2], or combining TiO2 and absorbable inorganic materials [3]. Recently, the preparation of titania/hydroxyapatite (TiO2/HAp) materials was attracting considerable attention thanks to the photocatalytic property of TiO2 and great adsorption ability of HAp. Among various features of the TiO2/HAp material which have been studied, the band gap evaluation was an important one because it decided the energy separation between the valence and conduction bands, the quantum effect, and the effect of visible-light utilization of that material [4]. The band gap of HAp was reported in [5] to be 3.95?eV by photoluminescence measurement, meanwhile, in some papers, that of HAp was calculated to be around 4.51–5.4?eV
Assessment of Radiation Dose Caused by Radioactive Gaseous Effluent Released from Nuclear Power Plant Ninh Thuan 1 under Scenario of Normal Working Conditions  [PDF]
Nguyen Tuan Khai, Do Xuan Anh, Le Dinh Cuong, Duong Duc Thang, Vuong Thu Bac, Nguyen Thi Thu Ha, Nguyen Quang Long, Trinh Van Giap, Nguyen Hao Quang
World Journal of Nuclear Science and Technology (WJNST) , 2015, DOI: 10.4236/wjnst.2015.53018
Abstract: Based on guides RG 1.109, RG 1.111 published by United States Nuclear Regulatory Commission (USNRC), our research concentrates in assessing radiation doses caused by radioactive substances released from the nuclear power plant (NPP) Ninh Thuan 1 under the scenario of normal operation using software package NRCDose72 provided by the USNRC. The database including the released radioactive nuclides, meteorology, terrain, population and agricultural production activities have been collected and processed to build the input data for the model calculation. The wind rose distribution obtained from the meteorological data in a five-year period from 2009-2013 showed that the radioactive nuclides released to environment spread in two main wind directions which are North East and South West. The X/Q (s/m3) and D/Q (s/m2) qualities which are, respectively, the ratio of activity concentration to release rate and that of deposition density of radioactive nuclides to release rate were calculated within an area of 80 km radius from the NPP site using XOQDOQ. Population doses were calculated using GASPAR. The XOQDOQ and GASPAR are two specific softwares in NRCDose72 package.
Impact of Vacancies on Diffusive and Pseudodiffusive Electronic Transport in Graphene
Alessandro Cresti,Thibaud Louvet,Frank Ortmann,Dinh Van Tuan,Pawe? Lenarczyk,Georg Huhs,Stephan Roche
Crystals , 2013, DOI: 10.3390/cryst3020289
Abstract: We present a survey of the effect of vacancies on quantum transport in graphene, exploring conduction regimes ranging from tunnelling to intrinsic transport phenomena. Vacancies, with density up to 2%, are distributed at random either in a balanced manner between the two sublattices or in a totally unbalanced configuration where only atoms sitting on a given sublattice are randomly removed. Quantum transmission shows a variety of different behaviours, which depend on the specific system geometry and disorder distribution. The investigation of the scaling laws of the most significant quantities allows a deep physical insight and the accurate prediction of their trend over a large energy region around the Dirac point.
Impact of graphene polycrystallinity on the performance of graphene field-effect transistors
David Jiménez,Aron W. Cummings,Ferney Chaves,Dinh Van Tuan,Jani Kotakoski,Stephan Roche
Physics , 2014, DOI: 10.1063/1.4863842
Abstract: We have used a multi-scale physics-based model to predict how the grain size and different grain boundary morphologies of polycrystalline graphene will impact the performance metrics of graphene field-effect transistors. We show that polycrystallinity has a negative impact on the transconductance, which translates to a severe degradation of the maximum and cutoff frequencies. On the other hand, polycrystallinity has a positive impact on current saturation, and a negligible effect on the intrinsic gain. These results reveal the complex role played by graphene grain boundaries, and can be used to guide the further development and optimization of graphene-based electronic devices.
Insulating Behavior of an Amorphous Graphene Membrane
Dinh Van Tuan,Avishek Kumar,Stephan Roche,Frank Ortmann,M. F. Thorpe,Pablo Ordejon
Physics , 2012, DOI: 10.1103/PhysRevB.86.121408
Abstract: We investigate the charge transport properties of planar amorphous graphene that is fully topologically disordered, in the form of sp2 three-fold coordinated networks consisting of hexagonal rings, but also including many pentagons and heptagons distributed in a random fashion. Using the Kubo transport methodology and the Lanczos method, the density of states, mean free paths and semiclassical conductivities of such amorphous graphene membranes are computed. Despite a large increase in the density of states close to the charge neutrality point, all electronic properties are dramatically degraded, evidencing an Anderson insulating state caused by topological disorder alone. These results are supported by Landauer-Buttiker conductance calculations, which show a localization length as short as 5 nanometers
Multiple Quantum Phases in Graphene with Enhanced Spin-Orbit Coupling: From the Quantum Spin Hall Regime to the Spin Hall Effect and a Robust Metallic State
Alessandro Cresti,Dinh Van Tuan,David Soriano,Aron W. Cummings,Stephan Roche
Physics , 2014, DOI: 10.1103/PhysRevLett.113.246603
Abstract: We report an intriguing transition from the quantum spin Hall phase to the spin Hall effect upon segregation of thallium adatoms adsorbed onto a graphene surface. Landauer-B\"uttiker and Kubo-Greenwood simulations are used to access both edge and bulk transport physics in disordered thallium-functionalized graphene systems of realistic sizes. Our findings not only quantify the detrimental effects of adatom clustering in the formation of the topological state, but also provide evidence for the emergence of spin accumulation at opposite sample edges driven by spin-dependent scattering induced by thallium islands, which eventually results in a minimum bulk conductivity $\sim 4e^{2}/h$, insensitive to localization effects.
Spin Dynamics and Relaxation in Graphene Dictated by Electron-hole Puddles
Dinh Van Tuan,Frank Ortmann,Aron W. Cummings,David Soriano,Stephan Roche
Physics , 2015,
Abstract: The understanding of spin dynamics and relaxation mechanisms in clean graphene and the upper time and length scales on which spin devices can operate are prerequisites to realizing graphene spintronic technologies. Here we theoretically reveal the nature of fundamental spin relaxation mechanisms in clean graphene on different substrates with spin-orbit Rashba fields as low as a few tens of micron eV. Spin lifetimes ranging from 50 picoseconds up to several nanoseconds are found to be dictated by substrate-induced electron-hole characteristics. A crossover in the spin relaxation mechanism from a Dyakonov-Perel type for SiO2 substrates to a broadening-induced dephasing for hBN substrates is described. The energy dependence of spin lifetimes, their ratio for spins pointing out-of-plane and in-plane, and the scaling with disorder provide a global picture about spin dynamics and relaxation in ultraclean graphene in presence of electron-hole puddles.
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