Vietnam Academy of Science and Technology: Journals Online
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Mott Transition in the Mass Imbalanced Ionic Hubbard Model at Half Filling
Get PDFThe Mott - Hubbard metal - insulator transition in the half-filled mass imbalanced ionic Hubbard model is investigated using the two-site dynamical mean field theory. We find that for a fixed mass imbalanced parameter r the critical interaction Uc increases when the ionic energy is increased. In the other hand, for a fixed , decreases with increasing the mass imbalance. We also show the existence of BI phase in the system for the case , and calculate the staggered charge density as a function of the interaction for different values of the mass imbalance. Our results in the limiting cases (; or/and ; ) are in good agreement with those obtained from full dynamical mean field theory
Positron Annihilation Spectroscopy in Material studies
No full textPositron annihilation spectroscopy (PAS) is a method dedicated to detection of open-volume type of defects in materials. Nowadays, this technique is of a great interest due to the practical character of obtained results. New devices using monoenergetic positron beams are built. The paper presents the basics of PAS, a description of popular experimental techniques and their application on the basis of results obtained in the frame of collaboration between Joint Institute for Nuclear Research (JINR), Institute of Physics - Vietnamese Academy of Science and Technology (VAST) and Vietnam Atomic Energy Institute (Vinatom).. The positron beam with energy up to 35 keV and other apparatus for PAS studies are under operation in Dubna
Constraints from Spectrum of Scalar Fields in the 3-3-1 Model with CKM Mechanism
Get PDFThe vacuum stability conditions are implemented for the 3-3-1 model of observed fermion masses and mixings. We show that for the Higgs squared mass matrices, the condi- tions for semi-definition of the diagonal elements are most important since other constraints are followed from the first ones
Marine biodiversity in Ha Long bay and Cat Ba archipelago
Get PDFHa Long bay - Cat Ba island located in the Northeast of Vietnam, in the area of Ha Long city (Quang Ninh province) and Cat Hai island district (Hai Phong city) is the largest limestone island in Vietnam. Features of biodiversity include the seven typical ecosystems of tropical limestone islands, namely tropical rainforest ecosystem, cave ecosystem, mangrove ecosystem, tidal ecosystem, coral reef ecosystem, soft-bottom ecosystem. Ha Long - Cat Ba also represents a high level of species diversity with 4,622 species of plants and animals on land and in the sea. In particular, on land there are 2,464 species, accounting for 53.3% and under the sea there are 2158 species, accounting for 46.7%. Up to 16 species and 114 species have been recorded in the Vietnam Red List and in the IUCN List, respectively. Together with hundreds of rare and precious species at national and regional levels, there are species of global conservation value, demonstrating that Ha Long bay - Cat Ba archipelago can possibly be a world heritage site according to the criteria of biodiversity (ix and x criteria)
Investigation of Some Characterizations of Black TiO Nanotubes Via Spectroscopic Methods
Get PDFThe black TiO with substantial Ti and oxygen vacancies exhibit an excellent photoelectrochemical water-splitting performance due to the improved charge transport the extended visible light response. In this study, black TiO nanotube arrays synthesized by the anodization method, and then, they have been investigated some characterizations by spectroscopic methods such as UV-visible reflectance (UV-vis DRS), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and photoluminescence spectrum. The results showed that some highlighted properties of the black TiO2 nanotube arrays and they could apply for water-splitting effect
No Small Hairs in Anisotropic Power-law Gauss-Bonnet Inflation
Get PDFWe will examine whether anisotropic hairs exist in a string-inspired scalar-Gauss-Bonnet gravity model with the absence of potential of scalar field during the inflationary phase. As a result, we are able to obtain the Bianchi type I power-law solution to this model under the assumption that the scalar field acts as the phantom field, whose kinetic is negative definite. However, the obtained anisotropic hair of this model turns out to be large, which is inconsistent with the observational data. We will therefore introduce a nontrivial coupling between scalar and vector fields such as into the scalar-Gauss-Bonnet model with the expectation that the anisotropic hair would be reduced to a small one. Unfortunately, the magnitude of the obtained anisotropic hair is still large. These results indicate that the scalar-Gauss-Bonnet gravity model with the absence of potential of scalar field might not be suitable to generate small anisotropic hairs during the inflationary phase
Lower bound of performance index of anti-sway control of a pendulum using on-off damping radial spring-damper
No full textAn on-off damping radial spring-damper can be attached to a pendulum to suppress its nonlinear sway motion. This paper studies the class of on-off damping controller, which switches the damping level from high to low and back at fixed times every quarter of period. Among this class, this paper shows the solution of the lower bound controller producing the lowest amplitude-frequency curve. The lower bound curve shows some fundamental natures of the system and gives some useful directions for a good on-off damping controller
Mechanical response of outer frames in tuning fork gyroscope model with connecting diamond-shaped frame
Get PDFIn tuning fork micro-gyroscopes, two outer frames are connected by using the linking elements. The driving vibrations of the two outer frames are required to be exactly opposite to generate the opposite sensing modes perpendicular to driving direction. These opposite driving vibrations are provided by a mechanical structure named the diamond-shaped frame. This paper presents mechanical responses of two outer frames in a proposed model of tuning fork gyroscope when an external force with different types is applied to them. The results show that the presence of a diamond-shaped frame guarantees the absolute anti-phase mode for the driving vibrations of outer frames
CONTRIBUTE TO STUDY ON CHEMICAL CONSTITUENTS OF URENA LOBATA (L.) OF VIETNAM
No full textUrena lobata L. is used in Vietnamese traditional medicine for the treatment of several diseases. Tree roots are used to treat rheumatism, dysentery, poor digestion, flu, tonsils, malaria, asthma, goiter. Flowers are used to treat chickenpox, fever, and mental disorders. Branches, leaves or whole trees used to treat injuries bruises, rheumatism, mastitis, bites. Phytochemical investigation of the n-hexan and ethyl acetate extract of Urena lobata L. led to the isolation of β-sitosterol (1), β-sitosterol-3-O-β-D-glucopyranoside (2), a-acetylamino-phenylpropyl a-benzoylamino-phenylpropanoate (3), quercetin (4), and trans-tiliroside (5). Their chemical structures were determined by spectroscopic methods including MS, 1D, 2D NMR and comparing with those reported in previous papers. Two compounds 3, 5 were isolated for the first time from Urena lobata plant
OPTIMIZATION OF OPERATING FRACTURING PARAMETERS FOR IMPROVING OIL PRODUCTION IN LOWER OLIGOCENE E RESERVOIR USING RESPONSE SURFACE METHOD, OFFSHORE VIETNAM: A CASE STUDY
No full textIn the recent days, hydraulic fracturing technique has been widely used to improve oil production with different reservoir characteristics such as low or high formation permeability, low or high formation porosity, formation damage. However, previous research did not mention the optimization for fracturing parameters including the injection rate, injection time, and leak-off coefficient to stimulate the Oligocene E reservoir, which is based on optimum oil production performance at which maximum net present value has been achieved. The problems in the Oligocene reservoir are too low production rate due to high reservoir depth, high closure pressure up to 7,700 psi, low reservoir permeability, low porosity and geological structure with heterogeneous reservoir, high temperature, resulting in low conductivity. To deal with these problems, fracturing technique is the best choice to stimulate this reservoir. The study focuses on optimizing fracturing parameters by applying the CCD and RSM by which economic production performance has been maximized at 119 $ in 10 years. The optimum fracturing parameters have been found as injection rate of 47 bpm, injection time of 119 minutes and leak-off coefficient of 0.003 ft/min0.5 in 50 pounds per thousand gallons of polymer (HPG). The optimal fracture geometry has been obtained with the fracture half-length of 1,449.44 ft and fracture width of 0.567 in. The rest of experimental laboratory is to measure fracture conductivity at 3,400 mD.ft in terms of proppant fracture concentration of 1.78 lb/ft2 and high closure pressure of 7700 psi. The post fractured well shows an increase in oil productivity of 7.5 folds