Ho Chi Minh City Institute of Physics

Ho Chi Minh City, Vietnam

Ho Chi Minh City Institute of Physics

Ho Chi Minh City, Vietnam
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Lan T.N.,University of Michigan | Lan T.N.,Ho Chi Minh City Institute of Physics | Zgid D.,University of Michigan
Journal of Physical Chemistry Letters | Year: 2017

Ab initio quantum chemistry calculations for systems with large active spaces are notoriously difficult and cannot be successfully tackled by standard methods. We generalize a Green's function QM/QM embedding method called self-energy embedding theory (SEET) that has the potential to be successfully employed to treat large active spaces. In generalized SEET, active orbitals are grouped into intersecting groups of a few orbitals, allowing us to perform multiple parallel calculations yielding results comparable to the full active-space treatment. We examine generalized SEET on a series of examples and discuss a hierarchy of systematically improvable approximations. © 2017 American Chemical Society.

Lan T.N.,Graduate University for Advanced Studies | Lan T.N.,Ho Chi Minh City Institute of Physics | Kurashige Y.,Graduate University for Advanced Studies | Kurashige Y.,Japan Institute for Molecular Science | And 3 more authors.
Journal of Chemical Theory and Computation | Year: 2015

We have developed a new computational scheme for high-accuracy prediction of the isotropic hyperfine coupling constant (HFCC) of heavy molecules, accounting for the high-level electron correlation effects, as well as the scalarrelativistic effects. For electron correlation, we employed the ab initio density matrix renormalization group (DMRG) method in conjunction with a complete active space model. The orbital-optimization procedure was employed to obtain the optimized orbitals required for accurately determining the isotropic HFCC. For the scalar-relativistic e ffects, we initially derived and implemented the Douglas-Kroll- Hess (DKH) hyperfine coupling operators up to the third order (DKH3) by using the direct transformation scheme. A set of 4d transition-metal radicals consisting of Ag atom, PdH, and RhH2 were chosen as test cases. Good agreement between the isotropic HFCC values obtained from DMRG/DKH3 and experiment was archived. Because there are no available gas-phase values for PdH and RhH2 radicals in the literature, the results from the present high-level theory may serve as benchmark data. (Graph Presented). © 2014 American Chemical Society.

Tsunoda R.T.,SRI International | Thampi S.V.,Physical Research Laboratory | Nguyen T.T.,Ho Chi Minh City Institute of Physics | Yamamoto M.,Kyoto University
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2013

The development of plasma structure in the nighttime equatorial F layer, known as equatorial spread F (ESF), appears to be influenced (if not controlled) by the presence of large-scale wave structure (LSWS). To understand this process, knowledge of the properties of LSWS is crucial. A major obstacle, in this regard, is the virtual lack of LSWS data; most instruments in use today are unable to detect spatial structure in the absence of horizontal drift. There appears to be some information about LSWS contained in two ionogram signatures, multi-reflected echoes (MREs) and satellite traces (STs). How useful they can be depends, however, on knowledge of their relationship to LSWS. Results from a case study are presented herein, which show that MREs are likely produced by a tilted, or shallowly modulated, reflecting surface in the bottomside F layer, whereas STs are produced by more steeply sloped surfaces. The accumulated results are shown to lead to a working hypothesis for LSWS development followed by ESF. Differences between LSWS and a traveling ionospheric disturbance are also pointed out. © 2012 Elsevier Ltd.

Tsunoda R.T.,SRI International | Nguyen T.T.,Ho Chi Minh City Institute of Physics | Le M.H.,Institute of Geophysics
Journal of Geophysical Research A: Space Physics | Year: 2015

Plasma structure in nighttime equatorial F layer, referred to as equatorial spread F (ESF), displays climatology whose seasonal variation depends on longitude. At longitudes where ESF favors equinoxes, times when maxima occur can be predicted in terms of the day of year, when E region sunset is simultaneous in conjugate hemispheres (i.e., "sunset nodes"). Aside from occurrences around equinoxes, there are only three longitudes where ESF also occurs during a solstice; one is the central Pacific region. Here ESF activity is strong during the June solstice, when solar activity is high. To understand this puzzling behavior, ESF climatology over the Kwajalein Atoll was compared with properties of the postsunset rise (PSSR) of the F layer and seeding activity in the troposphere. The key findings are as follows: (1) Maxima in PSSR velocity (VPSSR) are better aligned with equinoxes than with sunset nodes; hence, seasonal pattern of VPSSR, not only sunset nodes, should be included in interpretation of ESF climatology. (2) The source of VPSSR during solstice appears to differ from that during equinoxes. (3) Equinoctial maxima in VPSSR could be related to a semiannual variation in equatorial electrojet strength and its contribution to polarization of the F region dynamo current. (4) Enhanced VPSSR during the June solstice is interpreted in terms of tidal forcing with a wave number of 2. (5) Displacements of maxima in ESF climatology from maxima in VPSSR are shown to be consistent with deep convective activity. ©2014. American Geophysical Union. All Rights Reserved.

Lan T.N.,Ho Chi Minh City Institute of Physics | Hai T.H.,Ho Chi Minh City Institute of Physics
Computational Materials Science | Year: 2010

We use the Monte Carlo method to study the influence of the low field on the blocking temperature of magnetic nanoparticle systems at different sample conditions, namely the poly-dispersity and the concentration. We find the increase of the blocking temperature at the low field in the dilute sample, and the slow decrease in the concentrated sample. We extract the barrier distribution to explain and compare the magnetic properties of two above samples. We also provide short discussions about the origin of the increase of the peak temperature at low field and the influence of the interaction on the blocking state of particles in the high field. © 2010 Elsevier B.V. All rights reserved.

Haug H.,Goethe University Frankfurt | Cao H.T.,Ho Chi Minh City Institute of Physics | Thoai D.B.T.,Ho Chi Minh City Institute of Physics
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We formulate a Langevin-Gross-Pitaevski equation for spatially homogeneous systems together with the semiclassical Boltzmann equations for the excited states of polaritons in a microcavity. The gain of the coherent amplitude is due to the polariton-polariton scattering from the excited states to the ground state and has been obtained by an adiabatic elimination the corresponding three-point polarization. The Langevin-Gross-Pitaevski equation contains in addition to the gain the cavity losses as well as the fluctuations from the cavity losses and from the eliminated polarization. In analogy to the semiconductor laser theory the homogeneously broadened linewidth of the condensate amplitude can be evaluated analytically above threshold using the dissipation-fluctuation theorem. A linewidth enhancement is found because of the changes in the dispersive part of the gain function with the number of excited states and of the blueshift of the ground state. The latter mechanism causes well above threshold the homogeneously broadened linewidth to increase again after a remarkably narrow linewidth is reached. This decoherence mechanisms is inherent to all nonequilibrium condensates due to the Gross-Pitaevski nonlinearity and the fluctuations of the condensate population © 2010 The American Physical Society.

Le T.D.,Ho Chi Minh City Institute of Physics
Progress of Theoretical Physics | Year: 2011

Observed spectra of quasars are affected by the variance of the fine-structure constant (α = e2 /4πε0hc ). We propose a new method of using QSO spectra to investigate the possible temporal or spatial dependence of the fine-structure constant. Making use of the technique to reanalyze the optical spectra of the fine-structure transitions in [Ne III], [Ne V], [O III], [O I] and [S II] multiplets from 14 Seyfert 1.5 galaxies in the low-z range 0.035 〈 z 〈 0.281, we obtain a conservative value of α2(t)/α2(0) = 0.9999099 ± 0.0000553. This result represents a factor of ∼ 33.4 improvements on the constraint on α2(t)/α2(0) compared with the result using the same data published in the literature.

Haug H.,Goethe University Frankfurt | Doan T.D.,Ho Chi Minh City Institute of Physics | Tran Thoai D.B.,Ho Chi Minh City Institute of Physics
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

The space and time dependent nonequilibrium Keldysh-Green functions are employed to derive the scattering rates between the condensed microcavity polaritons described by a Gross-Pitaevskii equation and an uncondensed higher lying exciton reservoir. Slowly varying center coordinates and rapidly varying relative coordinates are assumed. For particle-particle and particle-phonon interactions the scattering rates which provide gain to the condensate are calculated explicitly. These processes result in scattering rates which are quadratic and linear in the density of reservoir excitons, respectively. The resulting quantum Boltzmann equation for the reservoir is simplified by assuming local thermal equilibrium to rate equations for the exciton density and their temperature. Using the microscopically calculated (not phenomenologically chosen) transition amplitudes for CdTe microcavity polaritons we demonstrate that our model is able to describe the spontaneous pattern formation for a ring-shaped nonresonant excitation as seen in recent experiments © 2014 American Physical Society.

Haug H.,Goethe University Frankfurt | Doan T.D.,Ho Chi Minh City Institute of Physics | Thoai D.B.T.,Ho Chi Minh City Institute of Physics
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We use the nonequilibrium Gross-Pitaevskii equation in the form recently derived [H. Haug, Phys. Rev. B 89, 155302 (2014)PRBMDO1098-012110.1103/PhysRevB.89.155302] to study spontaneous pattern formation and the connected superfluid current patterns. Using a nonresonant excitation beam with ring structure we get depending on the details of the structure and of the pump power, different spatial patterns of the condensate density. The corresponding phase profiles allow to identify, e.g., vortex-antivortex pairs, but beyond that, yield an image of the superfluid flow patterns linked with the structured condensate density. The fast superfluid flow driven by the spatially changing phase with velocities of the order of several mμ/ps is found to be often supersonic. In order to test dynamically the stability of the spontaneously formed flow patterns under external perturbations, we apply an additional resonant Laguerre-Gauss beam with angular momentum. This beam causes complex response of the phase patterns. This response is shown to be basically an oscillation or rotation of the vortex-antivortex pair depending on the strength of the extra beam. The rotation is induced via a ring of vortices induced by the Laguerre-Gaus beam. The main result of these studies is the extraordinary stability of the vortex-antivortex pair even under strongly perturbing external fields. © 2015 American Physical Society.

Haug H.,Goethe University Frankfurt | Doan T.D.,Ho Chi Minh City Institute of Physics | Cao H.T.,Ho Chi Minh City Institute of Physics | Thoai D.B.T.,Ho Chi Minh City Institute of Physics
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We analyze the decay of the first- and second-order correlations, g (1 )(t) and g (2 )(t), using a Langevin equation of the order parameter for polaritons in a microcavity, pumped nonresonantly with a noise-free laser diode. We consider a coupling of the condensate to the excited states by two types of scattering mechanism both derived from the polariton-polariton interaction: (a) one particle is scattered from an excited state to the ground state, while another excited-state particle is scattered to a higher-energy state, and (b) two ground-state particles are scattered into low-lying excited states with opposite momenta. This nonresonant scattering rate increases with the condensate population. We calculate from the Langevin equation for the order parameter the temporal decay of g (1 )(t) and the linewidth κ analytically. Our results make close contacts with the linewidth enhancement factor well known from semiconductor lasers. A semiclassical evaluation for the polariton kinetics yields a first-order correlation function, which is in good agreement with experimental results and with the results of Schwendimann and Quattropani, although our formulation is more complete because it contains also the dispersive effects of both types of scattering processes and uses no fitting parameters. Our analysis also provides an understanding of the rather different pump dependencies of the linewidth reported in the literature. The calculated second-order correlation function g (2 )(0) is shown to stay for larger pump values as observed on a plateau above the coherent limit again due to the nonresonant scattering processes. © 2012 American Physical Society.

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