Ho Chi Minh City Institute of Physics

We present numerical studies of the exciton polariton condensation kinetics in microcavities for the combined action of polariton-polariton and polariton-acoustic phonon scattering both for quasi-stationary and picosecond pulse excitation, respectively. For excitations of nearly resonant polaritons mainly the polariton-polariton scattering mechanism results in a condensation on the minimum of the lower polariton branch at relatively low areal polariton densities of the order of ${10}^{9}--{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The simultaneously acting polariton-phonon scattering increases the number of condensed particles considerably by providing a heat dissipation from the polariton gas to the lattice. For nonresonant excitations at large $k$ values the considered scattering mechanisms cannot support a polariton condensation. Above (below) laser threshold there exists a (no) polariton condensate and the polariton distribution of the noncondensed particles can be fitted by a Bose-Einstein (Maxwell-Boltzmann) distribution. Our results for $3\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ pulse excitation are in very good agreement with recent experimental observations of Yamamoto et al.

The intense modifications in the ionosphere‐thermosphere system in the equatorial and low‐latitude regions associated with the dynamic and electrodynamic coupling from high to low latitudes and chemical changes during geomagnetic storms are important space weather issues. In the second half of October 2003, the intense solar activity resulted in one intense and two major geomagnetic storms on 29 and 30 October. In this paper we present and discuss the ionospheric sounding observations carried out from Palmas and São José dos Campos, Brazil (the Brazilian sector), and Ho Chi Minh City, Vietnam, and Okinawa, Japan (the East Asian sector), during these storms. The two sectors are separated by about 12 hours in local time (so while one sector is in daytime, the other one is in nighttime) and provide valuable information related to the storm‐time longitudinal differences. Copious storm‐time changes were observed in both sectors. It should be pointed out that the two longitudinal sectors investigated in the present study clearly show the global nature of the storm‐time effects. However, the responses to the storm‐time effects are also associated with the local time in the two sectors. The present investigations show that there are both similarities and differences in the storm‐time response in the two sectors. During the storm main phases, with sharp decreases of the Dst index, both sectors showed (dusk or dawn periods) fast uplifting of the F layer associated with magnetospheric electric field penetration. Although in the East Asian sector, Ho Chi Minh City and Okinawa are located fairly close in longitude, with only 2 hour difference in local lime, on occasions the storm‐time responses have been very different. Some differences in the latitudinal response of the F region were also observed in the two sectors. Both positive and negative storm phases have been observed at all the four stations. A comparison of the ionospheric parameters obtained from the TIMEGCM model runs and the observed ionospheric parameters at the four stations shows a reasonable agreement during the quiet periods. During the geomagnetic disturbance period, when there were sharp decreases in Dst , some of the observed rapid uplifts of the F region peak heights are not reproduced by the model results. Also, sometimes the model foF2 results differ considerably from the observed foF2 variations. The period investigated represents an extreme storm situation for validation of the model and points to ways in which the model might be improved in the future.

Abstract According to the so-called upwelling paradigm, development of equatorial plasma bubbles (EPBs) involves (1) appearance of an upwelling (i.e., local uplift with a zonal width of ~ 400 km) in the bottomside of the equatorial F layer, (2) its growth via the F-region interchange instability during the post-sunset rise (PSSR) of the F layer, and (3) launching of EPBs, which starts near the end of PSSR, from within the confines of the upwelling. In this description, the PSSR is presumed to be the primary driver of the paradigm, with upwelling growth dependent on PSSR strength. As constructed, the paradigm describes EPB development when PSSR is strong (i.e., high solar activity), but not when it is weak. We, show, for the first time, that when PSSR is weak (e.g., low solar activity), upwelling growth can still be comparable in strength to what would be considered a strong PSSR, and that this growth drives EPB development. Given that EPBs do not develop outside of upwellings, regardless of solar activity, we are led to conclude, against mainstream thinking, that the controlling driver for EPB development is upwelling growth, not PSSR. For continued progress toward understanding EPB development, a crucial next step is to identify the source mechanism for upwelling growth, especially when PSSR is weak, and to better understand the complexities of the underlying physics.

The first-order spatial and the second-order temporal coherences of condensed microcavity polaritons are calculated in terms of Boltzmann equations for the excited states and the ground state supplemented by a Master equation for the probability to find a given number of particles in the condensate. The resulting first-order spatial coherence agrees both in its pump power dependence and in its variation with distance with the results of a recent double-slit experiment. Inserting the calculated rates between the excited states and the condensate with various saturation and depletion models, we solve for stationary situations the Master equation for the ground-state population. The resulting second-order correlations for the various models are compared with recent measurements.

The kinetics of the boson condensation for micro-cavity (MC) polaritons interacting with thermal acoustic phonons is studied within rate equations, taking into account the finite MC cross-section. For a smoothly switched on cw excitation and finite polariton lifetimes we find a build-up of a large population in the lowest state of the polariton spectrum for sufficiently large deformation potential coupling at sufficiently high densities, which are still below the saturation density. The critical influence of the value of the heavy-hole deformation potential is illustrated, as well as the favorable influence of positive values of the detuning. This spontaneous polariton condensation would result in a perpendicular emission out of the micro-cavity.

The Sun was very active in the early part of November 2004. During the period of 8–10 November 2004, intense geomagnetic disturbances with two superstorms were observed. In a companion paper (hereinafter referred to as paper 1), the effects observed in the F region during the intense geomagnetic disturbances in the early part of November 2004 in the Latin American sector were presented. In the present paper, we investigate the effects observed in the F region during the intense geomagnetic disturbances in the early part of November 2004 in the east Asian sector. We have used the ionospheric sounding observations at Ho Chi Minh City (Vietnam) and Okinawa, Yamagawa, Kokubunji, and Wakkanai (Japan) in the present investigations. Also, GPS observations in the east Asian sector (several longitude zones) have been used to study the effect in the F region during the intense geomagnetic disturbances. The ion density versus latitudinal variations obtained by the DMSP F15 satellite orbiting at about 800 km altitude in the east Asian sector and the magnetic field data obtained at several stations in the Japanese meridian are also presented. Several important features from these observations in both the sectors during this extended period of intense geomagnetic disturbances are presented. The east Asian sector showed very pronounced effects during the second superstorm, which was preceded by two storm enhancements. It should be mentioned that around the beginning of the night on 10 November, ionospheric irregularities propagating from higher midlatitude region to low‐latitude region were observed in the Japanese sector. The most intense geomagnetic field H component in that sector was observed on 10 November at L = 2.8, indicating that the auroral oval and the heating got further to low latitudes and the ionospheric irregularities observed in the Japanese sector on this night are midlatitude ionospheric disturbances associated with the second superstorm. The absence of ionospheric irregularities in the Japanese sector during the 8 November superstorm suggests that the magnetosphere‐ionosphere system was possibly preconditioned (primed) when the second interplanetary structure impacted the magnetosphere.

Abstract 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 ( V PSSR ) are better aligned with equinoxes than with sunset nodes; hence, seasonal pattern of V PSSR , not only sunset nodes, should be included in interpretation of ESF climatology. (2) The source of V PSSR during solstice appears to differ from that during equinoxes. (3) Equinoctial maxima in V PSSR could be related to a semiannual variation in equatorial electrojet strength and its contribution to polarization of the F region dynamo current. (4) Enhanced V PSSR 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 V PSSR are shown to be consistent with deep convective activity.

Abstract We investigated a postmidnight field‐aligned irregularity (FAI) event observed with the Equatorial Atmosphere Radar at Kototabang (0.2°S, 100.3°E, dip latitude 10.4°S) in Indonesia on the night of 9 July 2010 using a comprehensive data set of both neutral and plasma parameters. We examined the rate of total electron content change index (ROTI) obtained from GPS receivers in Southeast Asia, airglow images detected by an all‐sky imager, and thermospheric neutral winds and temperatures obtained by a Fabry‐Perot interferometer at Kototabang. Altitudes of the F layer ( h ′ F ) observed by ionosondes at Kototabang, Chiang Mai, and Chumphon were also surveyed. We found that the postmidnight FAIs occurred within plasma bubbles and coincided with kilometer‐scale plasma density irregularities. We also observed an enhancement of the magnetically equatorward thermospheric neutral wind at the same time as the increase of h ′ F at low‐latitude stations, but h ′ F at a station near the magnetic equator remained invariant. Simultaneously, a magnetically equatorward gradient of thermospheric temperature was identified at Kototabang. The convergence of equatorward neutral winds from the Northern and Southern Hemispheres could be associated with a midnight temperature maximum occurring around the magnetic equator. Equatorward neutral winds can uplift the F layer at low latitudes and increase the growth rate of Rayleigh‐Taylor instabilities, causing more rapid extension of plasma bubbles. The equatorward winds in both hemispheres also intensify the eastward Pedersen current, so a large polarization electric field generated in the plasma bubble might play an important role in the generation of postmidnight FAIs.

We investigate with a Boltzmann approach the relaxation kinetics of II-VI compound microcavity polaritons after a nonresonant pump pulse for the example of CdTe. In these polar materials large vacuum Rabi microcavity splittings guarantee robust polariton modes in the strong-coupling regime with an extremely small effective mass at the bottom of the lower branch, but with very short cavity lifetimes. Our kinetic treatment for stationary and picosecond-pulsed pumping takes the polariton--acoustic-phonon as well as the polariton-polariton scattering into account. For cavities with a vacuum Rabi splitting around $\ensuremath{\Omega}=10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, we find for nonresonant picosecond-pulse excitation a relaxation scenario which leads to a transient bosonic condensation in the lowest polariton state and coherent laser action at densities smaller than the exciton saturation density. For microcavities with a large vacuum Rabi splitting of $\ensuremath{\Omega}=26\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ we find due to the steep increase of radiative losses and the simultaneous steep decrease of the scattering rates with decreasing energy a nonequilibrium phase transition with a macroscopically occupied state in the bottleneck region at a finite transverse momentum and a connected coherent laser emission under an angle of 17\ifmmode^\circ\else\textdegree\fi{}. All of our results for medium and large Rabi vacuum splitting are in good qualitative agreement with corresponding observations of Dang and co-workers

The coherent ultrafast optical injection and the temporal evolution of charge and spin currents in semiconductors is analyzed using a microscopic many-body theory. The approach is based on the semiconductor Bloch equations and includes light-field-induced intraband and interband excitations, excitonic effects, and carrier--LO-phonon and carrier-carrier scattering processes. The relaxation effects are treated both in the second Born-Markov approximation and on the level of quantum kinetic theory including memory effects. The dynamics of the charge and spin currents is evaluated numerically for a one-dimensional model system. The dependence of the currents and their decay on the temperature, the excitation intensities, and the frequencies of the incident light fields is discussed. Whereas the overall decay dynamics is described well within the Markov approximation, the quantum kinetic theory predicts additional oscillatory signatures in the current transients.

We present a new theoretical approach, unrestricted self-energy embedding theory (USEET), that is a Green's function embedding theory used to study problems in which an open, embedded system exchanges electrons with the environment. USEET has a high potential to be used in studies of strongly correlated systems with an odd number of electrons and open shell systems such as transition metal complexes important in inorganic chemistry. In this paper, we show that USEET results agree very well with common quantum chemistry methods while avoiding typical bottlenecks present in these methods.

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for HAl co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

We investigate with a Boltzmann approach the spin relaxation kinetics of microcavity polaritons after an excitation pulse with near-resonant polarized light and calculate the polarization of the emitted light around and above the threshold for stimulated emission. Considering only the optically active excitons with an angular momentum $m=\ifmmode\pm\else\textpm\fi{}1$, we calculate the corresponding $2\ifmmode\times\else\texttimes\fi{}2$ single-particle density matrix. Our kinetic treatment takes the polariton-acoustic phonon as well as the polariton-polariton scattering as the dominant relaxation processes into account. Both processes are spin conserving. Particularly for excitation with circular light, we find in isotropic crystals above threshold a nearly complete circular polarization degree which lasts (typically $40--60\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$) much longer than the exciting $3\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ pulses due to the dominance of the stimulated spin-conserving scattering processes over the spontaneous spin-flip processes. These and other results also for linearly polarized pump light are in very good agreement with corresponding experiments on GaAs microcavities. In addition, we present time- and wave-number-dependent results which too are in qualitative agreement with the available angle- and polarization-resolved luminescence measurements.

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 $\ensuremath{\kappa}$ 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.

The semiclassical approach consisting of Boltzmann equations for the excited states and the ground state supplemented by a Master equation for the probability distribution of the condensate population is solved for picosecond pulsed microcavity polaritons. With the simple birth- and death-type Master equation for the condensate population that disregards the condensate phase, one gets results, which, compared to quasistationary solutions, are in better agreement with the experiments. First, the time-dependent solutions show the influence of the change to the coherent state already at pump powers close above the condensate threshold. Second, and even more important for the interpretation of corresponding pulsed experiments, the time-dependent solution give results for the second-order correlation function with seemingly larger correlations above threshold, although no polariton-polariton interaction in the ground state has been included.

While mutlifunctional catalytic activities of metal-free carbon-based materials have been extensively studied, there has been no similar report for metal-free BN-based materials. In this paper, we have performed, for the first time, an assessment of the bifunctional catalytic activity of C-doped BN nanoflakes for oxygen reduction and hydrogen evolution reactions (ORR and HER) using quantum chemical calculations. A comparison between single and double C-doped systems, which in turn have one and two unpaired electrons, has been carefully carried out. While single and double C-doped systems are both catalytically active for ORR, the latter with multiple active sites is more efficient for ORR than the former with only one active site. However, the former is more efficient for HER than the latter on which hydrogen too strongly adsorbs. To explore the catalytic performance of our systems, we discuss the results in comparison with many functionalized materials reported recently. We believe that our study reveals new possibilities for the development of ORR/HER bifunctional catalysts using metal-free C-doped BN-based materials.

Research and application of GIS and remote sensing technology combined with field survey in coastal areas of Ly Son island, Quang Ngai province had been carried out to establish the distribution map of submarine habitats. Depth-invariant index was used to correct water column’s affects on spectral reflectance of each habitat. The results of satellite - image classification showed that coral reef ecosystems living with coverage of over 25% were quite low in the south and southeast, and a small portion was in the north of the Lon island. In addition, seagrass ecosystems developed quite well with high coverage in the south and the north of Lon island. Overall, mainly common types of sea bed in Ly Son area were dead coral and seagrass. The accuracy assessment after classification showed that the overall accuracy of the satellite - image interpretation process was 94% and the kappa coefficient was 0.93.

We successfully prepared Au nanoparticles using the modified polyol methods and design of experiments. The desirable crystal structure and particle size of Au nanoparticles with various kinds of polyhedral and spherical shapes as well as various kinds of their morphologies or complete and rough spherical crystal surfaces were experimentally predicted in TEM and XRD measurements according to the theoretical calculation and data. The fine crystal formation of Au nanostructures by modified polyol methods with EG, PVP, and NaBH4 is of great importance to their practical applications. Our research shows that the critical nucleation, growth, and formation of sizes, shapes, and morphologies of Au nanoparticles were experimentally discussed in modified polyol methods and design of experiments. In this context, our particle size calculation can hold the greatest attraction for researchers in the field of nanoparticles.

Based on survey data in the period of 2013 - 2015 and historical data between 1996 - 1998, the paper showed the variation of water quality parameters by time. The concentration of total suspended sediment, chlorophyll-a, dissolved inorganic nitrogen, dissolved inorganic phosphorus and ratio of N:P changed significantly by season. The results of assessing water quality by UNTRIX (integrated concentration of Chlorophyll-a, dissolved inorganic nitrogen, dissolved inorganic phosphorus and level of saturated oxygen) indicated the quality levels were increased from meso-eutrophication in 1996 (average UNTRIX: 4.54 ± 0.76) to oligo-eutrophication in the period of 2013 - 2015 (average UNTRIX from 2.62 ± 0.66 to 3.81 ± 0.45).

A method for detecting glypican 3 (GPC3) liver cancer cells by coupling of anti-glypican 3 antibody (anti-GPC3) and magnetite nanoparticles (NPs) was investigated to detect GPC3 by enzyme-linked immunosorbent assay (ELISA) in this study. Magnetite NPs with the average size of 11 nm were synthesized by using co-precipitation method of Fe2+ and Fe3+ in NH3·H2O solution. First, silica was coated on the magnetite NPs using Stöber method to obtain Fe3O4/SiO2 core-shell structures and then 3-aminopropyltriethoxysilane (APTES) was treated on the Fe3O4/SiO2 by silanization reaction to achieve Fe3O4/SiO2/APTES nanostructures. After modified by APTES, the nanostructures were activated by glutaraldehyde (GA) to obtain functional groups on the nanostructures surface to bind with anti-GPC3 by covalent immobilization. The UV–vis spectroscopy was carried out to investigate the binding of anti-GPC3 to the NPs and binding efficiency (88.35%) was estimated by the Bradford method. The NPs bound anti-GPC3 (NPs/anti-GPC3) can detect GPC3 by using ELISA at low concentration (0.16 ng/ml).