Gauge/gravity duality

In this dissertation we use gauge/gravity duality to investigate various phenomena of strongly coupled field theories. Of special interest are quantum phase transitions, quantum critical points, transport phenomena of charges and the thermalization process of strongly coupled medium. The systems studied in this thesis might be used as models for describing condensed matter physics in a superfluid phase near the quantum critical point and the physics of quark-gluon plasma (QGP), a deconfinement phase of QCD, which has been recently created at the Relativistic Heavy Ion Collider (RHIC). Moreover, we follow the line of considering different gravity setups whose dual field descriptions show interesting phenomena of systems in thermal equilibrium, slightly out-of-equilibrium and far-from-equilibrium. We first focus on systems in equilibrium and construct holographic superfluids at finite baryon and isospin charge densities. For that we use two different approaches, the bottom-up with an U(2) Einstein-Yang-Mills theory with back-reaction and the top-down approach with a D3=D7 brane setup with two coincident D7-brane probes. In both cases we observe phase transitions from a normal to a superfluid phase at finite and also at zero temperature. In our setup, the gravity duals of superfluids are Anti-de Sitter black holes which develop vector-hair. Studying the order of phase transitions at zero temperature, in the D3=D7 brane setup we always find a second order phase transition, while in the Einstein-Yang-Mills theory, depending on the strength of the back-reaction, we obtain a continuous or first order transition. We then move to systems which are slightly out-of-equilibrium. Using the D3/D7 brane setup with Nc coincident D3-branes and Nf coincident D7-brane probes, we compute transport coefficients associated with massive N = 2 supersymmetric hypermultiplet fields propagating through an N = 4 SU(Nc) super Yang-Mills plasma inthe limit of Nf << Nc. Introducing a baryon number density and arbitrary constant electric and magnetic fields, we compute all components of the conductivity tensor associated with transport of baryon number charge. Determining the contribution that the flavor degrees of freedom make to the stress-energy tensor, we are able to identify the rates of energy and momentum loss of the flavor field to the plasma. Going towards systems far-from-equilibrium, we investigate a time-dependent geometry consisting of a mirror moving in the bulk of the Anti-de Sitter space. This geometry can be seen as a toy model which is relevant to the formulation of holographic thermalization in strongly coupled field theory. For this configuration, we establish a procedure for calculating time-dependent two-point functions of scalar fluctuations, based on a WKB approximation. We test our method on two sample trajectories for the mirror, and find that the singularity structure of the two-point functions is in agreement with the geometric optics

Gas-Absorption in Emulsionen

In the present work, absorption of pure oxygen (O2) and pure carbon dioxide (CO2) into aqueous emulsions of three different n-alkanes (n-heptane, n-dodecane and n-hexadecane) has been studied in a stirred tank at the stirring speed of 1000 min-1. The interfacial tensions have been measured with a tensiometer using the ring method. Positive spreading coefficients are calculated for all oils but the value for n-heptane is noticeably higher than the values for n-dodecane and n-hexadecane. The full range of oil volume fraction (0-100%) has been covered. Phase inversion (O/W to W/O) occurs at 60-65 % n-dodecane and n-hexadecane as identified with an electrical conductivity sensor, whereas observation of de-emulsification suggests the inversion region to be 50%-60% n-heptane. The volumetric mass transfer coefficient, kLa, has been evaluated from the pressure decrease under isochoric and isothermal (298.2 K) conditions with a pseudo-homogeneous mass transfer model. As expected, the solubility of both gases increases linearly with oil volume fraction; only for CO2-water-n-heptane system the values are low as compared to linear interpolation of the measured pure phase solubilities. In addition, a fluorescence technique with an oxygen-sensitive fluorophore, soluble only in the water phase, has been applied to measure the oxygen-water volumetric mass transfer coefficient, (kLa)GW, in emulsions of n-dodecane and n-hexadecane in order to gain more insight into the mass transfer mechanism. For O/W emulsions, both O2 and CO2 have similar trends in the variation of kLa with increasing dispersed oil volume fraction. A kLa maximum at low oil volume fractions (1-2%) is found for both n-dodecane and n-hexadecane; the “shuttle” mechanism might be responsible for this maximum. Differently, O/W emulsions of n-heptane always show higher kLa than pure water; this can be explained by a bubble covering mechanism enabled by its high positive spreading coefficient. The oxygen mass transfer rate is strongly enhanced by up to the factor of 38 at 50% n-heptane. Such strong effects have never reported before. For O2 absorption into O/W emulsions of n-dodecane and n-hexadecane, both (kLa)GW and kLa have the same trend with increasing oil volume fraction. The low values of spreading coefficient for n-dodecane and n-hexadecane and the strong decrease of both (kLa)GW and kLa towards the phase inversion region suggest the serial transport mechanism: gas → water → oil. For W/O emulsions, both O2 and CO2 have similar trends in the variation of kLa with increasing dispersed water volume fraction. In W/O emulsions of n-hexadecane, kLa decreases monotonously as expected due to increasing viscosity. For W/O emulsions of both n-dodecane and n-heptane, the kLa values do not decrease but increase substantially. The reason for this surprising trend is not clear.In dieser Arbeit wurde die Absorption von reinem Sauerstoff (O2) und von reinem Kohlendioxid (CO2) in wässrigen Emulsionen dreier n-Alkane (n-Heptan, n-Dodecan und n-Hexadecan) in einem Rührkessel bei der Rührerdrehzahl von 1000 min-1 untersucht. Die Grenz¬flächen-spannungen wurden mit einem Tensiometer nach der Ringmethode bestimmt. Aus den Messwerten errechnen sich für alle Öle positive Spreitungskoeffizienten, allerdings ist der Wert für n-Heptan deutlich größer als die Werte für die beiden anderen Öle. Es wurde der gesamte Bereich des Öl-Volumenanteils (0-100%) betrachtet. Die Phasenumkehr (O/W zu W/O) wurde durch Messen der elektrischen Leitfähigkeit untersucht; für n-Dodecan und n-Hexadecan erfolgt der Umschlag bei Ölanteilen von 60-65%. Beobachten des Entmischungsvorgangs deutet für n-Heptan den Umschlag im Bereich von 50-60% Ölanteil an. Die volumenbezogene Stoffübergangszahl kLa wurde aus dem Druckabfall unter isochoren und isothermen (298,2 K) Bedingungen mit einem pseudo-homogenen Modell ermittelt. Erwartungsgemäß variieren die Löslichkeiten beider Gase linear mit dem Ölanteil; einzig für CO2 in n-Heptan-Emulsionen sind die Messwerte kleiner als durch lineare Interpolation berechnet. Zusätzlich wurde eine optische Methode mit einem einzig in der wässrigen Phase löslichen Fluorophor verwendet, um über die daraus ermittelte Stoffübergangszahl (kLa)GW Hinweise zum Mechanismus des Stofftransports zu gewinnen. In O/W-Emulsionen zeigen die kLa-Werte beider Gase ähnliche Einflüsse des Ölanteils. Bei kleinen Anteilen (1-2%) von n-Dodecan und n-Hexadecan gibt es ein kLa-Maximum, das mit dem “Shuttle“-Mechanismus erklärt werden könnte. In O/W-Emulsionen von n-Heptan sind die kLa-Werte beider Gase durchweg höher als in reinem Wasser, was zusammen mit dem hohen Wert des Spreitungskoeffizienten den Transport über einen auf den Blasen spreitenden Ölfilm nahelegt. Bei 50% n-Heptan ist der Sauerstofftransport um den Faktor 38 erhöht; so hohe Werte wurden niemals zuvor berichtet. Für die Sauerstoffabsorption in n-Dodecan und n-Hexadecan ähneln sich die Verläufe von kLa und (kLa)GW. Die niedrigen Spreitungskoeffizienten und die starke Abnahme zum Inversionspunkt hin, legen den Transportweg Gas → Wasser → Öl nahe. In W/O-Emulsionen ist mit steigendem Volumenanteil der dispersen Wasserphase für beide Gase der Verlauf der kLa-Werte ähnlich. In W/O-Emulsionen von n-Hexadecan nimmt der kLa-Wert ab, wie es für steigende Emulsionsviskosität zu erwarten ist. In W/O-Emulsionen von n-Dodecan und n-Heptan dagegen steigen die kLa-Werte sogar deutlich an. Die Ursache für den unerwarteten Verlauf konnte im Rahmen dieser Arbeit nicht geklärt werden

The roles of student engagement motivations in learning and managing.

The purpose of this study is to explore how the combined influence of intrinsic motivation and extrinsic one on the engagement about behavioral, emotional, and cognitive of university students to bring academic achievements. To do this, an experimental questionnaire is designed to collect data from Vietnamese students in Danang and employed Structural Equation Modeling (SEM) to estimate the relationships among proposed variables. The results from 393 samples indicated that intrinsic motivation is the crucial element stimulating their efforts and engagement to obtain learning success. In contrast, extrinsic motivation is found to not bring a positive influence on the cognitive engagement of students. Therefore, this is an important research result that contributes to the theoretical foundation of student engagement as well as makes sense for the management of higher educatio

Oxygen Absorption into Stirred Emulsions of n-Alkanes

Absorption of pure oxygen into aqueous emulsions of n-heptane, n-dodecane, and n-hexadecane, respectively, has been studied at 0 to 100% oil volume fraction in a stirred tank at the stirring speed of 1000 min−1. The volumetric mass transfer coefficient, , was evaluated from the pressure decrease under isochoric and isothermal (298.2 K) conditions. The O/W emulsions of both n-dodecane and n-hexadecane show a maximum at 1-2% oil fraction as reported in several previous studies. Much stronger effects never reported before were observed at high oil fractions. Particularly, all n-heptane emulsions showed higher mass-transfer coefficients than both of the pure phases. The increase is by upto a factor of 38 as compared to pure water at 50% n-heptane. The effect is tentatively interpreted by oil spreading on the bubble surface enabled by a high spreading coefficient. In W/O emulsions of n-heptane and n-dodecane increases with the dispersed water volume fraction; the reason for this surprising trend is not clear

Jamming Signal Cancellation by Channel Inversion Power Control for Preserving Covert Communications, Journal of Telecommunications and Information Technology, 2023, nr 2

Uninformed jammers are used to facilitate covert communications between a transmitter and an intended receiver under the surveillance of a warden. In reality, the signals the uniformed jammer emits to make the warden’s decision uncertain have inadvertently interfered with the detection of the intended receiver. In this paper, we apply truncated channel inversion power control (TCIPC) to both the transmitter and the uninformed jammer. The TCIPC scheme used on the uninformed jammer may help the intended receiver remove jamming signals using the successive interference cancellation (SIC) technique. Under the assumption that the warden knows the channel coefficient between two intended transceivers and achieves the optimal detection power threshold, we form the optimization problem to maximize the effective transmission rate (ETR) under covertness and decoding constraints. With the aim of enhancing covertness-related performance, we achieve the optimal power control parameters and determine system parameter-related constraints required for the existence of these solutions. According to the simulations, the use of the TCIPC scheme on the uninformed jammer significantly improves covertness-related performance in comparison to that of random power control (RPC) and constant power control (CPC) schemes. In addition, simulation results show that, for the TCIPC scheme: 1) the maximum ETR tends to converge as the transmitter’s or the uninformed jammer’s maximum transmit power increases, and 2) there exists an optimal value of the transmitter’s predetermined transmission rate to achieve the optimal performance

An Approximate Evaluation of BER Performance for Downlink GSVD-NOMA with Joint Maximum-likelihood Detector, Journal of Telecommunications and Information Technology, 2022, nr 3

Generalized Singular Value Decomposition (GSVD) is the enabling linear precoding scheme for multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) systems. In this paper, we extend research concerning downlink MIMO-NOMA systems with GSVD to cover bit terror rate (BER) performance and to derive an approximate evaluation of the average BER performance. Specifically, we deploy, AT the base station, the well-known technique of joint-modulation to generate NOMAsymbols and joint maximum-likelihood (ML) to recover the transmitted data at end user locations. Consequently, the joint ML detector offers almost the same performance, In terms of average BER as ideal successive interference cancellation. Next, we also investigate BER performance of other precoding schemes, such as zero-forcing, block diagonalization, and simultaneous triangularization, comparing them with GSVD. Furthermore, BER performance is verified in different configurations in relation to the number of antennas. In cases where the number of transmit antennas is greater than twice the number of receive antennas, average BER performance is superior

Bilateral Heterogeneity in an Upwelling Mantle via Double Subduction of Oceanic Lithosphere

Vietnam is a major field of Cenozoic volcanism in Southeast (SE) Asia. Two contrasting models have been proposed to explain the mantle upwelling and volcanism in this region; collision of the Indian and Eurasian continents or subduction of the Pacific or Indo-Australian oceanic lithosphere. To place constraints on the origin of the intraplate volcanism in SE Asia, new geochronological and geochemical data for Cenozoic basalts in Vietnam are presented. Based largely on Sr-Nd-Pb isotope systematics, it was found that the sources of basalts from Central and Southern Vietnam are chemically distinct forming a sharp boundary at 13.5°N. The basalts north of the boundary show isotopic features similar to Enriched Mantle type 2 (EM2) ocean island basalts. Whereas the basalts south of the boundary show isotopic features similar to Enriched Mantle type 1 (EM1) ocean island basalts. The EM1 and EM2 basalts display positive Sr anomalies and elevated Pb/Ce and Th/La ratios, respectively. Such features suggest the origins of the sources through the recycling of deeply-subducted crustal lithologies. Furthermore, subduction of dense oceanic lithosphere can induce a convecting cell in the upper mantle. Therefore, we suggest that the chemically different basalts from Central and Southern Vietnam represent the surface expression of melting in two different convecting cells, one is driven by subduction of the Pacific plate and the other by subduction of the Indo-Australian plate