Literatura académica sobre el tema "Open system density matrix theory"
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Artículos de revistas sobre el tema "Open system density matrix theory"
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STEFANESCU, E., A. SǍNDULESCU y W. GREINER. "QUANTUM TUNNELING IN OPEN SYSTEMS". International Journal of Modern Physics E 02, n.º 01 (marzo de 1993): 233–58. http://dx.doi.org/10.1142/s0218301393000078.
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We study the barrier penetrability in the frame of the Lindblad theory of open quantum systems. In addition to the diagonal elements of the density matrix, leading to the Gamow’s formula, new terms, describing energy dissipation and spectral line broadening effects are obtained. It is shown that the presence of a dissipative environment increase the barrier penetrability, in accordance with a very simple physical interpretation: for a system initially found in its ground state the dissipation can lead only to transitions to the reaction channels where lower-energy levels exist.
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ISAR, A., A. SANDULESCU, H. SCUTARU, E. STEFANESCU y W. SCHEID. "OPEN QUANTUM SYSTEMS". International Journal of Modern Physics E 03, n.º 02 (junio de 1994): 635–714. http://dx.doi.org/10.1142/s0218301394000164.
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The damping of the harmonic oscillator is studied in the framework of the Lindblad theory for open quantum systems. A generalization of the fundamental constraints on quantum mechanical diffusion coefficients which appear in the master equation for the damped quantum oscillator is presented; the Schrödinger, Heisenberg and Weyl-Wigner-Moyal representations of the Lindblad equation are given explicitly. On the basis of these representations it is shown that various master equations for the damped quantum oscillator used in the literature are particular cases of the Lindblad equation and that not all of these equations are satisfying the constraints on quantum mechanical diffusion coefficients. Analytical expressions for the first two moments of coordinate and momentum are obtained by using the characteristic function of the Lindblad master equation. The master equation is transformed into Fokker-Planck equations for quasiprobability distributions and a comparative study is made for the Glauber P representation, the antinormal ordering Q representation, and the Wigner W representation. The density matrix is represented via a generating function, which is obtained by solving a timedependent linear partial differential equation derived from the master equation. Illustrative examples for specific initial conditions of the density matrix are provided. The solution of the master equation in the Weyl-Wigner-Moyal representation is of Gaussian type if the initial form of the Wigner function is taken to be a Gaussian corresponding (for example) to a coherent wavefunction. The damped harmonic oscillator is applied for the description of the charge equilibration mode observed in deep inelastic reactions. For a system consisting of two harmonic oscillators the time dependence of expectation values, Wigner function and Weyl operator, are obtained and discussed. In addition models for the damping of the angular momentum are studied. Using this theory to the quantum tunneling through the nuclear barrier, besides Gamow’s transitions with energy conservation, additional transitions with energy loss are found. The tunneling spectrum is obtained as a function of the barrier characteristics. When this theory is used to the resonant atom-field interaction, new optical equations describing the coupling through the environment of the atomic observables are obtained. With these equations, some characteristics of the laser radiation absorption spectrum and optical bistability are described.
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Sargsyan, V. V., Z. Kanokov, G. G. Adamian y N. V. Antonenko. "Application of the theory of open quantum systems in nuclear physics". International Journal of Modern Physics: Conference Series 49 (enero de 2019): 1960008. http://dx.doi.org/10.1142/s2010194519600085.
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Projectile-nucleus capture by a target nucleus at bombarding energies in the vicinity of the Coulomb barrier is treated with the reduced-density-matrix formalism. The effects of dissipation and fluctuations on the capture process are taken self-consistently into account within the quantum model suggested. The excitation functions for the capture in the reactions [Formula: see text]O, [Formula: see text]F, [Formula: see text]Mg, [Formula: see text]Si, [Formula: see text]S, [Formula: see text]Ca, [Formula: see text]Ti, [Formula: see text]Cr [Formula: see text] [Formula: see text]Pb are calculated and compared with the experimental data.
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Scheid, Werner, Aurelian Isar y Aurel Sandulescu. "Diffusion and dissipation by linear momentum in spherical environment". International Journal of Modern Physics B 28, n.º 11 (26 de marzo de 2014): 1450077. http://dx.doi.org/10.1142/s0217979214500775.
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An open quantum system is studied consisting of a particle moving in a spherical space with an infinite wall. With the theory of Lindblad the system is described by a density matrix which gets affected by operators with diffusive and dissipative properties depending on the linear momentum and density matrix only. It is shown that an infinite number of basis states leads to an infinite energy because of the infinite unsteadiness of the potential energy at the infinite wall. Therefore only a solution with a finite number of basis states can be performed. A slight approximation is introduced into the equation of motion in order that the trace of the density matrix remains constant in time. The equation of motion is solved by the method of searching eigenvalues. As a side-product two sums over the zeros of spherical Bessel functions are found.
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Zenichowski, Karl, Jadranka Dokić, Tillmann Klamroth y Peter Saalfrank. "Current versus temperature-induced switching of a single molecule: Open-system density matrix theory for 1,5-cyclooctadiene on Si(100)". Journal of Chemical Physics 136, n.º 9 (7 de marzo de 2012): 094705. http://dx.doi.org/10.1063/1.3692229.
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Czerwiński, Artur y Andrzej Jamiołkowski. "Dynamic Quantum Tomography Model for Phase-Damping Channels". Open Systems & Information Dynamics 23, n.º 04 (diciembre de 2016): 1650019. http://dx.doi.org/10.1142/s1230161216500190.
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In this paper we propose a dynamic quantum tomography model for open quantum systems with evolution given by phase-damping channels. Mathematically, these channels correspond to completely positive trace-preserving maps defined by the Hadamard product of the initial density matrix with a time-dependent matrix which carries the knowledge about the evolution. Physically, there is a strong motivation for considering this kind of evolution because such channels appear naturally in the theory of open quantum systems. The main idea behind a dynamic approach to quantum tomography claims that by performing the same kind of measurement at some time instants one can obtain new data for state reconstruction. Thus, this approach leads to a decrease in the number of distinct observables which are required for quantum tomography; however, the exact benefit for employing the dynamic approach depends strictly on how the quantum system evolves in time. Algebraic analysis of phase-damping channels allows one to determine criteria for quantum tomography of systems in question. General theorems and observations presented in the paper are accompanied by a specific example, which shows step by step how the theory works. The results introduced in this paper can potentially be applied in experiments where there is a tendency to look at quantum tomography from the point of view of economy of measurements, because each distinct kind of measurement requires, in general, preparing a separate setup.
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Naicker, V. y F. Petruccione. "A Lie Symmetry Analysis of the Caldeira-Leggett Model". Open Systems & Information Dynamics 17, n.º 04 (diciembre de 2010): 409–17. http://dx.doi.org/10.1142/s1230161210000254.
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An important model in the theory of open quantum systems is the Caldeira-Leggett quantum Brownian motion model for the behaviour of a massive quantum particle weakly interacting with a bath of harmonic oscillators. The model is formulated in terms of a (1 + 2) dimensional partial differential equation (PDE) for the reduced density matrix of the particle. In this paper we analyze this PDE from the perspective of Lie symmetry groups. A Lie point symmetry analysis is performed and a number of canonical reductions are obtained. Of note are new analytic solutions to the model obtained by mapping the model to the linear diffusion equation.
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TOPOL, I. A. y V. I. POLYAKOV. "Method for Molecular Electronic State Multiplet Structure Calculation in the Space of Xα-SW-Orbitals". International Journal of Modern Physics C 02, n.º 01 (marzo de 1991): 510–14. http://dx.doi.org/10.1142/s0129183191000792.
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The SCF-Xα-scattered wave method (Xα-SW) as well as other versions of the density function approach cannot give a proper description of the open-shell many-electron energy levels and thus it is not always possible to reproduce electron spectra adequately by this method. We propose the following way to overcome this drawback of the X α-SW method. First one- and two-particle molecular integrals with Xα-SW molecular orbitals (MO) are calculated numerically. Then these integrals are used to evaluate Hamiltonian matrix elements (both diagonal and off-diagonal) in the basis of configuration state functions. The present scheme allows us to describe molecular electronic spectra in various approximations: a) one-configurational frozen orbitals approach; b) ΔSCF; c) configuration interaction (CI). Our method gives an opportunity to go beyond the muffin-tin (MT) approximation for a potential; inherent in the X α-SW method. In the X α-SW-MO basis it is simple enough to construct the full electron Hamiltonian matrix elements for various open-shell systems.
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Rohr, Daniel R. y Katarzyna Pernal. "Open-shell reduced density matrix functional theory". Journal of Chemical Physics 135, n.º 7 (21 de agosto de 2011): 074104. http://dx.doi.org/10.1063/1.3624609.
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Ying-Hua, Ji y Liu Yong-Mei. "Reduced density matrix identification of open quantum system". Optik 159 (abril de 2018): 162–68. http://dx.doi.org/10.1016/j.ijleo.2018.01.066.
Texto completoTesis sobre el tema "Open system density matrix theory"
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Beyvers, Stephanie. "Selective excitation of adsorbate vibrations on dissipative surfaces". Phd thesis, Universität Potsdam, 2008. http://opus.kobv.de/ubp/volltexte/2008/2551/.
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The selective infrared (IR) excitation of molecular vibrations is a powerful tool to control the photoreactivity prior to electronic excitation in the ultraviolet / visible (UV/Vis) light regime ("vibrationally mediated chemistry").
For adsorbates on surfaces it has been theoretically predicted that IR preexcitation will lead to higher UV/Vis photodesorption yields
and larger cross sections for other photoreactions.
In a recent experiment, IR-mediated desorption of molecular hydrogen from a Si(111) surface on which atomic hydrogen and deuterium were co-adsorbed was achieved, following a vibrational mechanism as indicated by the isotope-selectivity.
In the present work, selective vibrational IR excitation of adsorbate molecules, treated as multi-dimensional oscillators on dissipative surfaces,
has been simulated within the framework of open-system density matrix theory.
Not only potential-mediated, inter-mode coupling poses an obstacle to selective excitation but also the coupling of the adsorbate ("system") modes
to the electronic and phononic degrees of freedom of the surface ("bath") does.
Vibrational relaxation thereby takes place, depending on the availabilty of energetically fitting electron-hole (e/h) pairs and/or phonons (lattice vibrations) in the surface, on time-scales ranging from milliseconds to several hundreds of femtoseconds. On metal surfaces, where the relaxation process of the adsorbate via the e/h pair mechanism dominates, vibrational lifetimes are usually shorter than on insulator or semiconductor surfaces, in the range of picoseconds, being also the timescale of the IR pulses used here.
Further inhibiting factors for selectivity can be the harmonicity of a mode and weak dipole activities ("dark modes") rendering vibrational excitation with moderate field intensities difficult.
In addition to simple analytical pulses, optimal control theory (OCT) has been employed here to generate a suitable electric field to populate the target state/mode maximally.
The complex OCT fields were analyzed by Husimi transformation, resolving the control field in time and energy.
The adsorbate/surface systems investigated were CO/Cu(100), H/Si(100) and 2H/Ru(0001). These systems proved to be suitable models to study the above mentioned effects.
Further, effects of temperature, pure dephasing (elastic scattering processes), pulse duration and dimensionality (up to four degrees of freedom)
were studied. It was possible to selectively excite single vibrational modes, often even state-selective.
Special processes like hot-band excitation, vibrationally mediated desorption and the excitation of "dark modes" were simulated.
Finally, a novel OCT algorithm in density matrix representation has been developed which allows for time-dependent target operators
and thus enables to control the excitation mechanism instead of only the final state.
The algorithm is based on a combination of global (iterative) and local (non-iterative) OCT schemes, such that short, globally controlled time-intervals are coupled locally in time.
Its numerical performance and accuracy were tested and verified and it was successfully applied to stabilize a two-state linear-combination and to enforce a successive "ladder climbing" in a rather harmonic system, where monochromatic, analytical pulses simultaneously excited several states, leading to a population loss in the target state.Die selektive Anregung von Molekülschwingungen mittels Infrarotlicht (IR) ist vorteilhaft, um die Wirkungsquerschnitte nachfolgender photochemischer oder photophysikalischer Prozesse zu steigern, welche durch Elektronenanregung mittels ultraviolettem (UV) bzw. sichtbarem (Vis) Licht ausgelöst werden. Für Adsorbatmoleküle auf Oberflächen wurden theoretische Vorhersagen getroffen, dass eine kombinierte (IR plus UV)-Strategie ("schwingungsvermittelte Chemie") die Ausbeute bei Photodesorption und anderen Photoreaktionen deutlich zu erhöhen vermag. Kürzlich wurde im Experiment gezeigt, dass eine rein IR-vermittelte Desorption möglich ist, welche über einen schwingungsangeregten Mechanismus erfolgt. Hierbei wurde molekularer Wasserstoff von einer Si(111)-Oberfläche desorbiert, an der atomarer Wasserstoff und atomares Deuterium gebunden waren. Eine thermische Anregung, die zum Bindungsbruch führt, konnte hierbei wegen der Isotopenselektivität ausgeschlossen werden. In der vorliegenden Arbeit wurde die selektive IR-Schwingungsanregung von Adsorbaten, die als multidimensionale Oszillatoren auf dissipativen Oberflächen behandelt wurden, mit Hilfe der Dichtematrixtheorie für offene Systeme simuliert. Nicht nur die potentialvermittelte Kopplung zwischen den einzelnen Moden ist ein Hindernis für selektive Anregung, sondern auch die Kopplung der Moden des Adsorbats ("Systems") an elektronische und phononische Freiheitsgrade des Substrats ("Bades"). Die Schwingungsrelaxation verläuft hierbei auf Zeitskalen, die von Millisekunden bis hin zu wenigen hundert Femtosekunden reichen, je nach Verfügbarkeit energetisch geeigneter Elektron-Loch-Paar-Anregungen bzw. Phononen (Gitterschwingungen) in der Oberfläche. Auf Metalloberflächen, bei denen die Schwingungrelaxation des Adsorbats zumeist von einem Elektronen-Loch-Paar-Mechanismus dominiert wird, sind die Schwingungslebensdauern normalerweise kürzer als auf Isolator- oder Halbleiteroberflächen und betragen einige Picosekunden, ebenso wie die Zeitskala der hier gewählten IR-Pulse. Weitere Faktoren, die die selektive Anregung behindern können sind die Harmonizität einer Mode und die geringe Dipolaktivität sogenannter "dunkler Moden", die eine Anregung mit moderat intensiven Feldern erschweren. Zusätzlich zu einfachen analytischen Pulsen wurden Felder mittels Optimaler Kontrolltheorie (OCT) erzeugt, um eine(n) Zielzustand/-mode maximal zu populieren. Komplexe OCT Pulse wurden mit Hilfe der Husimi-Transformation analysiert, welche das Kontrollfeld im Zeit- und Energieraum aufzulösen vermag. Die in dieser Arbeit untersuchten Adsorbat/Oberflächen-Systeme waren CO/Cu(100), H/Si(100) und 2H/Ru(0001), die sich als passende Modelle erwiesen, um oben genannte Faktoren zu untersuchen. Desweiteren wurden die Auswirkungen von Temperatur, reiner Dephasierung (elastische Streuprozesse), Dauer des IR-Pulses und Systemdimensionalit"at (Behandlung von bis zu vier Freiheitsgraden) studiert. Einzelne Schwingungsmoden konnten angeregt werden, in vielen Fällen sogar zustandsselektiv. Spezielle Prozesse wie die Anregung "heißer Banden", Desorption via alleiniger Schwingungsanregung und die Anregung "dunkler Moden" wurden simuliert. Schließlich wurde ein neuer OCT-Algorithmus in Dichtematrixdarstellung entwickelt, der es erlaubt, zeitabhängige Zieloperatoren einzuführen, um nicht nur Kontrolle über den Endzustand einer Anregung, sondern auch über den Anregungsmechanismus zu erlangen. Der Algorithmus basiert auf einer Kombination von globaler (iterativer) und lokaler (nicht-iterativer) optimaler Kontrollschemata in der Art, dass kurze, global kontrollierte Intervalle zeitlich lokal miteinander gekoppelt werden. Nach numerischen Tests wurde der Algorithmus erfolgreich angewandt, um eine Linearkombination aus zwei Zuständen zu stabilisieren, sowie um eine schrittweise "Leiteranregung" in einem harmonischen System zu forcieren, bei dem monochromatische, analytische Pulse mehrere Zustände zugleich anregen und somit einen Populationsverlust im angestrebten Zielzustand zur Folge haben.
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Schirrmeister, Dirk. "Zur Theorie photoinduzierter Dynamik offener Molekularsysteme: Kontrolle von Dissipation durch ultrakurze Laser-Pulse". Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 1998. http://dx.doi.org/10.18452/14380.
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Zusammenfassung in PostScript In dieser Arbeit wird die photoinduzierte Dynamik offener Molekularsysteme unter dem Einfluß intensiver und ultrakurzer Laserpulse untersucht. Die Anregung eines Moleküls durch einen optischen ultrakurzen Laserpuls führt zu Übergängen zwischen verschiedenen elektronischen Zuständen. Dieser Anregungsprozeß wird begleitet von dissipativen Vorgängen wie Energie-- und Phasenrelaxation. Die Beschreibung dieser photoinduzierten Dynamik erfolgt mit Hilfe der Methode der Dichtematrixtheorie. Dabei zeigt die Ableitung der Quanten--Master--Gleichung im Rahmen des Projektionsoperator--Formalismus, daß die wirkenden äußeren Felder einmal direkt im reversiblen Anteil der Bewegungsgleichung auftreten, aber auch einen indirekten Einfluß über den die Dissipation beschreibenden Dissipations--Superoperator ausüben. In dieser Arbeit wird zum ersten Mal die durch ultrakurze Laserpulse induzierte Feldabhängigkeit des Dissipations--Superoperators berücksichtigt. Im Rahmen der Darstellung der Quanten--Master--Gleichung im Floquetbild kann eine anschauliche Deutung dieses feldabhängigen Effektes gegeben werden: die die Dissipation beschreibende frequenzabhängige Spektraldichte der Umgebungsmoden wird feldabhängig bei verschiedenen Frequenzen abgefragt. Analytische Untersuchungen zum Zwei--Niveau--System zeigen, daß die Feldabhängigkeit dann relevant wird, wenn die Pulslänge vergleichbar ist mit der Zeitskala, auf der die Autokorrelationsfunktion der Umgebungsfreiheitsgrade abklingt. Um den Einfluß auf experimentelle Größen zu untersuchen, wird ein zweifarbiges Pump--Test--Experiment zum Laserfarbstoffmolekül IR 125 betrachtet, bei welchem die spektral und zeitlich aufgelöste Transmission auf einer Femtosekunden-- und Pikosekunden--Zeitskala gemessen wurde. Im Rahmen des Modells einer effektiven Schwingungsmode wird eine Anpassungsrechnung an das Experiment vorgenommen. Dabei wird zunächst die Standard-Redfield-Theorie verwendet, um ein Referenzmodell zu gewinnen. Es gelingt, eine gute Übereinstimmung mit dem Experiment zu erreichen. Die exakte Berücksichtigung des Einflusses der internen Konversion zwischen den angeregten elektronischen Zuständen führt zu einem Anstieg der Transmission innnerhalb einer Pikosekunde. Es ist notwendig, die Dichtematrixgleichungen exakt zu lösen, da eine vergleichende Untersuchung mit Hilfe der nichtlinearen Suszeptibilität dritter Ordnung eine deutliche Abweichung zum exakten Resultat zeigt. Ausgehend vom Referenzfall feldunabhängiger Dissipation wird dann die Feldabhängigkeit der Relaxationsraten bestimmt sowie der Einfluß auf Observablen wie der relativen Transmission untersucht. In Übereinstimmung mit den analytischen Ergebnissen zeigt sich, daß der feldabhängige Effekt am größen ausgeprägt ist, wenn die Pulslänge kleiner als die Korrelationszeit der Umgebungsfreiheitsgrade wird und die wirkenden Felder hinreichend intensiv sind.Damit wird eine Kontrolle von Dissipation möglich. Ein Einfluß des feldabhängigen Effektes auf experimentelle Observablen wird vorhergesagt.abstract in PostScript This thesis investigates the influence of intense and ultrashort laser pulses on the photoinduced dynamics of open molecular systems. The excitation of a molecule by an optical ultrashort laser pulse induces transitions between different electronic states. This excitation process is accompanied by the dissipative processes of energy and vibrational relaxation. This excitation process is described within the method of the density matrix theory. Thereby, the derivation of the quantum master equation in the framework of the projection operator formalism demonstrates that the external fields are present in the reversible part of the equation of motion and also exert an indirect influence by acting on the dissipation superoperator which accounts for dissipation. In this thesis the field--dependency of the dissipation superoperator which is induced by the external fields is considered for the first time. By a representation of the quantum master equation in the Floquet picture, an interpretation of this field--dependent effect can be given: the frequency--dependent spectral density of the environmental modes which describe dissipation is determined at different field--dependent frequencies. Analytical investigations for the two level system demonstrate that the field dependence becomes relevant if the pulse length is comparable with the time scale on which the autocorrelation function of the environmental degrees of freedom decays.To investigate the influence on experimental quantities, a two--color pump--probe experiment for the laser dye molecule IR 125 is considered for which the spectrally and temporally resolved transmission on a femtosecond and picosecond time scale has been measured. Within the model of one effective vibrational mode the experimental data is fitted. The standard Redfield theory is used to provide a reference model. A high degree of concurrence between the theory and the results of the experiment is achieved. The exact treatment of internal conversion between the excited electronic states leads to a rise in transmission within one picosecond. It is necessary to solve the density matrix equations exactly because a comparative investigation with the nonlinear susceptibility of third order leads to a clear viation from the exact result. Starting from the reference case of field--independent dissipation, the field--dependency of the relaxation rates is determined and the influence on observables for example the relative transmission is investigated. The analytical results show that the field--dependent effect is strongest if the pulse length becomes smaller than the correlation time of the environmental modes and if the acting fields are sufficiently strong. Thereby, a control of dissipation becomes possible. An influence of the field--dependent effect on experimental observables is predicted.
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Ossipov, Alexandre. "Open Mesoscopic Systems: beyond the Random Matrix Theory". Doctoral thesis, [S.l.] : [s.n.], 2003. http://deposit.ddb.de/cgi-bin/dokserv?idn=969598173.
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Fruchtman, Amir. "Theory and modelling of energy transport in quantum nanostructures". Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:9c00d93c-c839-4342-9dc1-c2917c71a670.
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This thesis is concerned with quantum properties of excitonic energy transport in nanostructures that are embedded in a noisy environment. Of principal interests are ways to exploit this environment to facilitate the transport of energetic excitations. The first research chapter deals with an extension to the 'standard' open quantum system picture, where the Hilbert space is split into three: system, environment, and a wider universe. This division is natural for many biological and artificial nanostructures. A new analytical method, based on a phase space representation of the density matrix, is developed for studying such division. The effects of the wider universe are shown to be captured by a simple correction of the environmental response function. The second research chapter addresses the question: when do second-order perturbative approaches to open quantum systems, which are intuitive and simple to compute, provide adequate accuracy? A simple analytical criterion is developed, and its validity is verified for the case of the much-studied FMO dynamics as well as the canonical spin-boson model. In the third research chapter, an intuitive model of a photocell is studied. The model comprises two light-absorbing molecules coupled to an idealised reaction centre, showing asymmetric dimers are capable of providing a significant enhancement of light-to-current conversion under ambient conditions. This is done by 'parking' the energy of an absorbed photon in a dark state which neither absorbs nor emits light. In the final research chapter, a basic model for what can be thought as a "quantum brachistochrone" problem is investigated. Exotic energy configurations are found to yield considerable enhancement to the exciton's transfer probability, due to similar mechanisms studied in the previous chapter.
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Demir, Nazlı. "Decentralized probabilistic density control of swarm of autonomous agents with conflict avoidance constraints". Thesis, 2014. http://hdl.handle.net/2152/26210.
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This report describes a method to control the density distribution of a large number of autonomous agents. The approach is based on the fact that there are a large number of agents in the system, and hence the time evolution of the probabilistic density distribution of agents can be described as a Markov chain. The main contribution of this paper is the synthesis of a Markov matrix which will guide the multi-agent system density to a desired steady-state density distribution, in a probabilistic sense, while satisfying some motion and safety constraints. Also, an adaptive density control method based on real time density feedback is introduced to synthesize a time-varying Markov ma- trix, which leads to better convergence to the desired density distribution. Finally, a decentralized density computation method is described. This method guarantees that all agents will have a best, and common, density estimate in a finite, with an explicit bound, number of communication updates.text
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Αλατάς, Παναγιώτης. "Application of a non-linear thermodynamic master equation to three-level quantum systems". Thesis, 2014. http://hdl.handle.net/10889/7534.
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In this Master’s thesis, we have focused on the description of three-level quantum systems through master equations for their density matrix, involving a recently proposed non-linear thermodynamic one. The first part is focused on a three-level system interacting with two heat baths, a hot and a cold one. We investigated the rate of heat flow from the hot to the cold bath through the quantum system, and how the steady-state is approached. Additional calculations here refer to the rate of entropy production and the evolution of all elements of the density matrix of the system from an arbitrary initial state to their equilibrium or steady-state value. The results are compared against those of a linear, Lindblad-type master equation designed so that for a quantum system interacting with only one heat bath, the same final Gibbs steady state is attained.
In the second part of this thesis, we focus on the electromagnetically induced transparency (EIT), a phenomenon typically achievable only in atoms with specific energy structures. For a three level system (to which the present study has focused), for example, EIT requires two dipole allowed transitions (the 1-3 and the 2-3) and one forbidden (the 1-2). The phenomenon is observed when a strong laser (termed the control laser) is tuned to the resonant frequency of the upper two levels. Then, as a weak probe laser is scanned in frequency across the other transition, the medium is observed to exhibit both: a) transparency at what was the maximal absorption in the absence of the coupling field, and b) large dispersion effects at the atomic resonance. We discuss the Hamiltonian describing the phenomenon and we present results from two types of master equations: a) an empirically modified Von-Neumann one allowing for decays from each energy state, and b) a typical Lindblad one, with time-dependent operators. In the first case, an analytical solution is possible, which has been confirmed through a direct solution of the full master equation. In the second case, only numerical results can be obtained. We present and compare results from the two master equations for the susceptibility of the system with respect to the probe field, and we discuss them in light also of available experimental data for this very important phenomenon.Η παρούσα εργασία επικεντρώνεται στην περιγραφή των κβαντικών συστημάτων τριών καταστάσεων μέσω εξισώσεων master για την μήτρα πυκνότητας πιθανότητάς τους (density matrix), συμπεριλαμβάνοντας μία πρόσφατα προτεινόμενη μη-γραμμική θερμοδυναμική εξίσωση. Το πρώτο μέρος εστιάζει σε ένα σύστημα τριών καταστάσεων το οποίο βρίσκεται σε αλληλεπίδραση με δύο λουτρά θερμότητας, ένα θερμό και ένα ψυχρό. Εξετάζεται ο ρυθμός ροής θερμότητας από το θερμό προς το ψυχρό λουτρό μέσω του κβαντικού συστήματος, και με ποιον τρόπο επιτυγχάνεται η μόνιμη κατάσταση. Επιπλέον υπολογισμοί αναφέρονται στον ρυθμό παραγωγής της εντροπίας και στην εξέλιξη όλων των στοιχείων της μήτρας πυκνότητας πιθανότητας από μία τυχαία αρχική κατάσταση προς την ισορροπία ή τη μόνιμη κατάσταση. Τα αποτελέσματα παρουσιάζονται συγκριτικά με εκείνα μιας γραμμικής, τύπου Lindblad master εξίσωσης, κατάλληλα σχεδιασμένης ώστε στην ειδική περίπτωση ενός κβαντικού συστήματος σε αλληλεπίδραση με ένα λουτρό θερμότητας επιτυγχάνεται η ίδια τελική μόνιμη κατάσταση Gibbs. Στο δεύτερο μέρος, εστιάζουμε στην ηλεκτρομαγνητικά επαγόμενη διαφάνεια (electromagnetically induced transparency (EIT)), ένα φαινόμενο το οποίο τυπικά είναι εφικτό μόνο σε άτομα με ειδικές ενεργειακές δομές. Για ένα σύστημα τριών καταστάσεων (στο οποίο επικεντρώνεται η παρούσα εργασία), για παράδειγμα, το ΕΙΤ απαιτεί δύο διπολικά επιτρεπτές μεταβάσεις (την 1-3 και την 2-3) και μία απαγορευμένη (την 1-2). Το φαινόμενο παρατηρείται όταν ένα ισχυρό laser (το αποκαλούμενο ως control laser) συντονίζεται στη συχνότητα των δύο άνω ενεργειακών σταθμών. Τότε, καθώς ένα ασθενές probe laser ανιχνεύεται με συχνότητα όμοια με της άλλης επιτρεπόμενης μετάβασης, το μέσο παρατηρείται να εμφανίζει τα εξής: α) διαφάνεια στο σημείο μέγιστης απορρόφησης απουσία του control πεδίου, και β) έντονα φαινόμενα διασποράς στον ατομικό συντονισμό. Θα συζητήσουμε τη Χαμιλτονιανή που περιγράφει το φαινόμενο και θα παρουσιάσουμε αποτελέσματα από δύο εξισώσεις master: α) μία εμπειρική τροποποιημένη Von-Neumann εξίσωση επιτρέποντας τις απώλειες από κάθε ενεργειακή κατάσταση, και β) μία τυπική Lindblad εξίσωση, με χρόνο-εξαρτώμενους τελεστές. Στην πρώτη περίπτωση, είναι πιθανή η εύρεση μιας αναλυτικής λύσης, η οποία έχει επιβεβαιωθεί μέσω μιας άμεσης (direct) λύσης της πλήρους εξίσωσης master. Στη δεύτερη περίπτωση, μπορούν να ληφθούν μόνο αριθμητικά αποτελέσματα. Παρουσιάζονται και συγκρίνονται τα αποτελέσματα που ελήφθησαν από τις δύο master εξισώσεις και αφορούν την επιδεκτικότητα (susceptibility) του συστήματος σε σχέση με το probe πεδίο, και τέλος συζητιούνται σε σχέση με διαθέσιμα πειραματικά δεδομένα γι’ αυτό το πολύ σημαντικό φαινόμενο.
Libros sobre el tema "Open system density matrix theory"
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Akemann, Gernot. Random matrix theory and quantum chromodynamics. Oxford University Press, 2018. http://dx.doi.org/10.1093/oso/9780198797319.003.0005.
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This chapter was originally presented to a mixed audience of physicists and mathematicians with some basic working knowledge of random matrix theory. The first part is devoted to the solution of the chiral Gaussian unitary ensemble in the presence of characteristic polynomials, using orthogonal polynomial techniques. This includes all eigenvalue density correlation functions, smallest eigenvalue distributions, and their microscopic limit at the origin. These quantities are relevant for the description of the Dirac operator spectrum in quantum chromodynamics with three colors in four Euclidean space-time dimensions. In the second part these two theories are related based on symmetries, and the random matrix approximation is explained. In the last part recent developments are covered, including the effect of finite chemical potential and finite space-time lattice spacing, and their corresponding orthogonal polynomials. This chapter also provides some open random matrix problems.
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Schomerus, Henning. Random matrix approaches to open quantum systems. Oxford University Press, 2018. http://dx.doi.org/10.1093/oso/9780198797319.003.0010.
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Over the past decades, a great body of theoretical and mathematical work has been devoted to random-matrix descriptions of open quantum systems. This chapter reviews the physical origins and mathematical structures of the underlying models, and collects key predictions which give insight into the typical system behaviour. In particular, the aim is to give an idea how the different features are interlinked. The chapter mainly focuses on elastic scattering but also includes a short detour to interacting systems, which are motivated by the overarching question of ergodicity. The first sections introduce general notions from random matrix theory, such as the 10 universality classes and ensembles of Hermitian, unitary, positive-definite, and non-Hermitian matrices. The following sections then review microscopic scattering models that form the basis for statistical descriptions, and consider signatures of random scattering in decay, dynamics, and transport. The last section touches on Anderson localization and localization in interacting systems.
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Beenakker, Carlo W. J. Extreme eigenvalues of Wishart matrices: application to entangled bipartite system. Editado por Gernot Akemann, Jinho Baik y Philippe Di Francesco. Oxford University Press, 2018. http://dx.doi.org/10.1093/oxfordhb/9780198744191.013.37.
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This article describes the application of random matrix theory (RMT) to the estimation of the bipartite entanglement of a quantum system, with particular emphasis on the extreme eigenvalues of Wishart matrices. It first provides an overview of some spectral properties of unconstrained Wishart matrices before introducing the problem of the random pure state of an entangled quantum bipartite system consisting of two subsystems whose Hilbert spaces have dimensions M and N respectively with N ≤ M. The focus is on the smallest eigenvalue which serves as an important measure of entanglement between the two subsystems. The minimum eigenvalue distribution for quadratic matrices is also considered. The article shows that the N eigenvalues of the reduced density matrix of the smaller subsystem are distributed exactly as the eigenvalues of a Wishart matrix, except that the eigenvalues satisfy a global constraint: the trace is fixed to be unity.
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Morawetz, Klaus. Nonequilibrium Quantum Hydrodynamics. Oxford University Press, 2018. http://dx.doi.org/10.1093/oso/9780198797241.003.0015.
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The balance equations resulting from the nonlocal kinetic equation are derived. They show besides the Landau-like quasiparticle contributions explicit two-particle correlated parts which can be interpreted as molecular contributions. It looks like as if two particles form a short-living molecule. All observables like density, momentum and energy are found as a conserving system of balance equations where the correlated parts are in agreement with the forms obtained when calculating the reduced density matrix with the extended quasiparticle functional. Therefore the nonlocal kinetic equation for the quasiparticle distribution forms a consistent theory. The entropy is shown to consist also of a quasiparticle part and a correlated part. The explicit entropy gain is proved to complete the H-theorem even for nonlocal collision events. The limit of Landau theory is explored when neglecting the delay time. The rearrangement energy is found to mediate between the spectral quasiparticle energy and the Landau variational quasiparticle energy.
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Horing, Norman J. Morgenstern. Q. M. Pictures; Heisenberg Equation; Linear Response; Superoperators and Non-Markovian Equations. Oxford University Press, 2018. http://dx.doi.org/10.1093/oso/9780198791942.003.0003.
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Three fundamental and equivalent mathematical frameworks (“pictures”) in which quantum theory can be lodged are exhibited and their relations and relative advantages/disadvantages are discussed: (1) The Schrödinger picture considers the dynamical development of the overall system state vector as a function of time relative to a fixed complete set of time-independent basis eigenstates; (2) The Heisenberg picture (convenient for the use of Green’s functions) embeds the dynamical development of the system in a time-dependent counter-rotation of the complete set of basis eigenstates relative to the fixed, time-independent overall system state, so that the relation of the latter fixed system state to the counter-rotating basis eigenstates is identically the same in the Heisenberg picture as it is in the Schrödinger picture; (3) the Interaction Picture addresses the situation in which a Hamiltonian, H=H0+H1, involves a part H0 whose equations are relatively easy to solve and a more complicated part, H1, treated perturbatively. The Heisenberg equation of motion for operators is discussed, and is applied to annihilation and creation operators. The S-matrix, density matrix and von Neumann equation, along with superoperators and non-Markovian kinetic equations are also addressed (e.g. the intracollisional field effect).
Capítulos de libros sobre el tema "Open system density matrix theory"
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Tempel, David G., Joel Yuen-Zhou y Alán Aspuru-Guzik. "Open Quantum Systems: Density Matrix Formalism and Applications". En Fundamentals of Time-Dependent Density Functional Theory, 211–29. Berlin, Heidelberg: Springer Berlin Heidelberg, 2012. http://dx.doi.org/10.1007/978-3-642-23518-4_10.
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Mussardo, Giuseppe. "Particle Spectrum by Semi-classical Methods". En Statistical Field Theory, 882–912. Oxford University Press, 2020. http://dx.doi.org/10.1093/oso/9780198788102.003.0023.
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Chapter 23 discusses the setting of a semi-classical method—based on the Lagrangian density of the model, irrespective of whether or not it describes an integrable system—to address the computation of the particle spectrum of bound states in quantum field theory with a set of degenerate vacua connected by kink excitations. It begins by investigating kinks and anti-kinks and a semi-classical formula for the kink matrix elements, as well as asymptotic behavior. It then goes on to cover such topics as universal mass formula, symmetric wells, asymmetric wells and the double Sine–Gordon model, a benchmark of the semi-classical method.
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Allesina, Stefano y Jacopo Grilli. "Models for large ecological communities—a random matrix approach". En Theoretical Ecology, 74–92. Oxford University Press, 2020. http://dx.doi.org/10.1093/oso/9780198824282.003.0006.
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Lotka and Volterra were among the first to attempt to mathematize the dynamics of interacting populations. While their work had a profound influence on ecology, leading to many of the results that were covered in the preceding chapters, their approach is difficult to generalize to the case of many interacting species. When the number of species in a community is sufficiently large, there is little hope of obtaining analytical results by carefully studying the system of dynamical equations describing their interactions. Here, we introduce an approach based on the theory of random matrices that exploits the very large number of species to derive cogent mathematical results. We review basic concepts in random matrix theory by illustrating their applications to the study of multispecies systems. We introduce tools that can be used to yield new insights into community ecology and conclude with a list of open problems.
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Nitzan, Abraham. "Vibrational Energy Relaxation". En Chemical Dynamics in Condensed Phases. Oxford University Press, 2006. http://dx.doi.org/10.1093/oso/9780198529798.003.0020.
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An impurity molecule located as a solute in a condensed solvent, a solid matrix or a liquid, when put in an excited vibrational state will loose its excess energy due to its interaction with the surrounding solvent molecules. Vibrational energy accumulation is a precursor to all thermal chemical reactions. Its release by vibrational relaxation following a reactive barrier crossing or optically induced reaction defines the formation of a product state. The direct observation of this process by, for example, infrared emission or more often laser induced fluorescence teaches us about its characteristic timescales and their energetic (i.e. couplings and frequencies) origin. These issues are discussed in this chapter. Before turning to our main task, which is constructing and analyzing a model for vibrational relaxation in condensed phases, we make some general observations about this process. In particular we will contrast condensed phase relaxation with its gas phase counterpart and will comment on the different relaxation pathways taken by diatomic and polyatomic molecules. First, vibrational relaxation takes place also in low density gases. Collisions involving the vibrationally excited molecule may result in transfer of the excess vibrational energy to rotational and translational degrees of freedom of the overall system. Analysis based on collision theory, with the intermolecular interaction potential as input, then leads to the cross-section for inelastic collisions in which vibrational and translational/rotational energies are exchanged. If C∗ is the concentration of vibrationally excited molecules and ρ is the overall gas density, the relaxation rate coefficient kgas is defined from the bimolecular rate law When comparing this relaxation to its condensed phase counterpart one should note a technical difference between the ways relaxation rates are defined in the two phases.
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"Performance of Adsorptive Heat Storage Devices for Heat Supply". En Technology Development for Adsorptive Heat Energy Converters, 124–73. IGI Global, 2021. http://dx.doi.org/10.4018/978-1-7998-4432-7.ch005.
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The chapter is focused on modelling of performance of adsorptive heat storage devices and estimation of performance of heat storage devices. Two groups of models of adsorptive heat storage units suggested previous researchers are analyzed. The first one is focused on predicting the heat energy storage density, it being based on Dubinin-Polanyi theory. The second one is devoted to analyzing the kinetic of adsorption processes and performance of the adsorber or adsorptive-desorptive reactor filled with traditional adsorbent or salt which forms crystalline hydrates. The key drawback of both groups of models concerns with considering only one stage of exploitation of adsorptive heat storage device in spite of its operating in two-stage mode, that is, alternating discharge (adsorption) and charge (regeneration). It inhibits estimation of efficiency of adsorptive heat storage device basing on full operating cycle and its involving in heat supply system. Two algorithms for estimation of operating parameters are proposed by authors for closed-type and open-type heat storage devices. The algorithm for calculation of operating parameters of closed type adsorptive heat storage device is proposed: calculation of the mass transfer coefficient, adsorption, useful heat, that is, heat of adsorption, determination of the heat input, it being calculated as heat inputs for heating the adsorbent, device housing, water in the tank, evaporation of water in the tank, heating of the adsorbed water and desorption. Then efficiency factor is calculated. The operating characteristics of a closed-type heat energy storage device were studied when the composite adsorbent ‘silica gel – sodium sulphate' used. The effect of the humid airflow velocity on the efficiency factor is taken into account by introducing a coefficient equal to the value of the adsorption. An increase in the efficiency coefficient was stated when the velocity and relative humidity of the airflow. It is shown that the humid air flow temperature practically does not affect its value. Having been used the suggested algorithm, the optimal operating characteristics of an adsorptive heat storage device of a closed type based on a composite adsorbent ‘silica gel – sodium sulphate' for a private house heating system are revealed to be humid air velocities of 0.6 – 0.8 m/s and relative humidity 40 – 60%. When these operational data applied, the efficiency coefficient is shown to reach the maximum values (about 55%). Algorithm of calculation of operating parameter of open-type heat storage device includes computation of mass transfer coefficient, adsorption, useful heat (heat of adsorption), heat input for heating the adsorbent, device casing, water in the humidifier, evaporation of water, heating the adsorbed water, desorption, and calculating efficiency coefficient. Performance of open-type heat storage device based on the composite adsorbent ‘silica gel – sodium sulphate' is estimated. The optimal operating conditions of the heat accumulating device which allow operating with maximal magnitudes of efficiency coefficients 53 – 57% are stated to be humid airflow speed of 0.6 – 0.8 m/s and relative humidity of 40 – 60%. Correlation between efficiency factors obtained by experiments and calculated with suggested algorithm is confirmed. The possibility of reducing the power consumption when heat storage devices applied in 2,4 – 90 times versus decentralized heating systems on basis of solid fuel boiler, gas boiler and electric boiler is stated when open-type sorptive heat storage device used. Results of the study can be used to develop adsorptive storage devices in decentralized heat supply and ventilation systems and adsorption units for utilization of low-temperature waste heat.
Actas de conferencias sobre el tema "Open system density matrix theory"
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Tseng, Charles C., Ruth L. Sikorski, R. Viskanta y Ming Y. Chen. "Effect of Radiation on Heat Transfer in Open-Cell Foams at High Temperature". En ASME 2011 International Mechanical Engineering Congress and Exposition. ASMEDC, 2011. http://dx.doi.org/10.1115/imece2011-62530.
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There are a variety of foams that can be used in thermal protection and/or thermal insulation systems. At high temperature (> 1000 K) thermal radiation may be important or dominate heat transfer in the foam; however, studies based on more detailed thermal radiation analysis are limited. In this paper foams are considered to be semitransparent, because radiation can penetrate through the pore (or void) space and/or foam skeleton (solid matrix), depending on the materials from which the foams are made. Of particular interest of this study is to understand how the properties such as foam material its density, porosity, etc. affect thermal and radiant energy transfer. Physical and mathematical models are developed to account for conduction and radiation (absorption, emission and scattering) in the porous material. The spectral extinction coefficients of SiC foams are measured experimentally in the laboratory at room temperature, and the radiative transfer equation is solved using the spherical harmonics P1 and the Rosseland diffusion approximations. Parametric calculations have been carried out, and the results are reported in the paper for a range of parameters characterizing heat transfer in SiC foams of different porosities to identify desirable conditions for effectively reducing heat transfer in potential thermal protection concepts.
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Dashti, Jalal, Bader Al-Ajmi, Hawas Farwan, Ahmad Shoeibi, Milton Sanclemente, Alberto Martocchi y Eliana R. Russo. "IDENTIFICATION OF NATURAL OPEN FRACTURES, INDUCED FRACTURES AND MATRIX PERMEABILITY IN CARBONATES WHILE DRILLING". En 2021 SPWLA 62nd Annual Logging Symposium Online. Society of Petrophysicists and Well Log Analysts, 2021. http://dx.doi.org/10.30632/spwla-2021-0084.
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The economic feasibility of a well drilled in tight carbonates is extremely dependent on the level of fracture permeability; hard and dense carbonate formations may not be considered as net pay without the presence of fractures. The evaluation of fractures is a key to reservoir effectiveness characterization for well drilling, completion, development and stimulation of fractured reservoirs. While knowledge of the geological conditions and regional stress is helpful to estimate the characteristics of the natural fracture system in a given reservoir, the true extent of the natural open fracture system in any specific location is typically unknown. Several methods are available to the industry to identify natural fractures near the wellbore, including acoustic and resistivity image logs. In some cases, the poor-quality results of these techniques do not provide reliable information and such data cannot be available in all the wells. When minor downhole losses are accurately detected, it is possible to locate and characterize the natural open fractures intersected by the drill bit while drilling operations. The differential flow (Flow-out minus Flow-in) and the Active Volume System are continually monitored during drilling and integrated with drilling and hydraulic parameters. These readings are processed in a computer-based, data-acquisition system to form a compensated delta-flow signal that identifies the occurrence of downhole fluid losses. The differential flow is measured accurately through a dedicated Coriolis type flow-meter with a Limit Of Detection up to 10 l/min. By accurately detecting and measuring the downhole micro-losses instantaneously at the surface, the responses would be compared to predefined models for fracture characterization; that enables identification of different types of fractures (open natural, induced fractures). The system can detect very fine micro-fractures that might not be visible with wireline images; fracture density plots can then be created to highlight the fracture concentration along the well. Drilling deep wells in Kuwait is challenging due to high pressure, high-temperature formations, with the Bottom Hole Pressure of +15kpsi and Bottom Hole Temperature of +150 Centigrade degrees. In conventional surface systems, the loss detection relies on the Active Volume System and the Paddle type Flow-out sensor; however, these systems usually fail to identify the minor mud losses associated to open fractures. Especially for active pits with a big surface, it is almost impossible to identify few millimetres of mud level decrease and during fluid transfers, mud conditioning will make the job even more difficult to identify minor losses. With flow paddle type of sensors, the flow out information is not displayed as a calibrated value but rather as a percentage of full scale, which can be difficult to interpret. Instead, dedicated Coriolis type flowmeters properly installed, can identify flow rate changes accurately, regardless of any transfer of mud, water or diesel between pits. By applying this technique, it is possible to identify fractures while drilling in different types of wells, such as vertical, highly deviated and horizontal. The data were validated initially through core and image logs and further applied in next drilling campaigns.
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Konstantinov, Mikhail. "POLITICAL IDEOLOGY AS AN EVOLUTIONARY SYSTEM (TO THE THEORY OF COGNITIVE-IDEOLOGICAL MATRICES)". En NORDSCI International Conference. SAIMA Consult Ltd, 2020. http://dx.doi.org/10.32008/nordsci2020/b2/v3/14.
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The aim of the article is to concretize the concept of political ideology in the aspect of its matrix structure and in the context of the cognitive-evolutionary approach. Based on Michael Frieden's morphological approach to the analysis of ideological consciousness, the concept of cognitive-ideological matrices is introduced, which allows us to describe the process of transition from proto-ideological to ideological concepts proper, especially at the level of individual consciousness. The identification of the ideological concept as the main “gene” of conceptual variability and inheritance made it possible to describe the main parameters of the evolution of political ideologies and associate it with changes taking place at the individual consciousness level. The described concept was tested in a series of sociological studies of youth consciousness conducted in 2015-2016 and 2018-2020. As a result of the study, it was possible to first identify the “zero level” of ideology, at which the minds of young respondents are potentially open to the influence of diverse and often mutually exclusive ideological orientations, and second, to pinpoint the changes that have occurred in the cognitive ideological matrices of Rostov-on-Don students over the past five years. This study was conducted by scientists from the southern Federal University.
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Klein, John, George Gilchrist, Jim Karanik, Noe Arcas, Richard Yurman, James Whiteside, Bill Shields y Tony Bartilucci. "Thermal Management of Airbourne Early Warning and Electronic Warfare Systems Using Foam Metal Fins". En ASME 2003 International Electronic Packaging Technical Conference and Exhibition. ASMEDC, 2003. http://dx.doi.org/10.1115/ipack2003-35187.
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This program, addresses the need for thermal management of increasingly powerful and densely packaged electronic devices. Open-celled foams and lattice structures offer the promise of much improved heat transfer between the coolant and the solid structure of the lattice compared to traditional finned heat exchangers. The focus of this program is to evaluate integration of foam and lattice materials as heat exchanger cores and as electronic racks. The potential benefits of this approach include reduction in the volume and weight of the heat exchanger core and/or device junction temperature as well as direct attach cooling of high power electronics. To begin we have selected two major applications, a liquid cooling system heat exchanger, and avionics rack cooling. There is little data on foam metal heat transfer in the regime we anticipate for aircraft applications. Our approach begins with the measurement of heat transfer characteristics of compressed foam metals under conditions suitable for aircraft applications. Basic heat transfer data is being obtained for heat removal from a heated surface by “foam metal fins” with air flowing through the foam. Effective heat transfer coefficient and airflow resistance have been measured. The test method and apparatus are briefly described. Results of heat transfer measurements to date are presented. A theoretical model of “foam metal fins” has been developed and is applied for scaling foam metal fins within our test matrix. Using the model we determine the heat transfer coefficient between the air and foam ligaments. These heat transfer coefficients are compared with cylinders in cross flow. We applied our measured heat transfer characteristics to the design, fabrication and verification test of a highly efficient heat exchanger core. A laboratory scale thermal performance demonstration core was sized based on our test results. Initial tests of a single air / liquid heat exchanger core leg validates our core sizing. Our results can also be applied to cooling of individual electronic components as well as cold plates for electronics.
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Liu, Yanbin, Weilin Zhuge, Yangjun Zhang, Shuyong Zhang, Junyue Zhang y Xuemin Huo. "A Matching Method for Two-Stage Turbocharging System". En ASME Turbo Expo 2014: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/gt2014-26682.
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The turbine system of a two-stage turbocharger composed of high pressure turbine, low pressure turbine and by-pass valve decides distribution and utilization of exhaust gas energy and influence performance of two-stage turbocharger in whole operational conditions. Besides, characteristics of turbine is expressed by envelop line of characteristic lines in different speeds. So turbine can be conveniently selected compared with compressor with similarity theory. Therefore two-stage turbocharger matching begins from turbine system matching in the paper. In two-stage turbocharger, cooler efficiency, cooler loss and by-pass valve open besides turbochargers will influence turbocharging system performance and design of cooler and by-pass valve are important contents of turbocharging system matching. The paper matched inter cooler, by-pass valve open, compressors and turbines jointly. Calculation model for turbocharger matching was built, and turbine performance is get from reference turbine based on similarity theory; influence of compressor ratio distribution, cooler efficiency and pressure drop in cooler imposing on compressor work was analyzed; and influence of turbine flow capacity and by-pass valve imposing on output working in expanding process was studied; the method for matching of two-stage turbocharging system in whole operational condition is studied Matching analysis was made aiming at two-stage turbocharging system of a type of high power density diesel engine, and design for turbocharging system was finished. Matching result using the method is compared to matching result using traditional method. Analysis result proves that using the method matching points in different operational conditions are located in more reasonable zone of compressor MAP.
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Rehman, A., K. S. Ahmed, F. A. Umrani, B. Munir, A. Mehboob, S. M. Ahmad y Z. Kazmi. "Finite Element Modelling of a Generic Rotor-Bearing System and Experimental Validation". En ASME 2015 Dynamic Systems and Control Conference. American Society of Mechanical Engineers, 2015. http://dx.doi.org/10.1115/dscc2015-9901.
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The design and development of the rotating machinery require a precise identification of its dynamic response for efficient operation and failure prevention. Determination of critical speeds and mode shapes is crucial in this regard. In this paper, a finite element model (FEM) based on the Euler beam theory is developed for investigating the dynamic behavior of flexible rotors. In-house code in Scilab environment, an open source platform, is developed to solve the matrix equation of motion of the rotor-bearing system. The finite element model is validated by the impact hammer test and the dynamic testing performed on the rotors supported on a purpose-built experimental setup. Bearing stiffness is approximated by using the Hertzian contact theory. Obtaining the critical speeds and mode shapes further improves the understanding of dynamic response of rotors. This study paves way towards advanced research in rotordynamics in Faculty of Mechanical Engineering, GIK Institute.
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Zaretsky, Erwin V. y Robert E. deLaneuville. "MMC Life System Development (Phase I): A NASA/Pratt & Whitney Cooperative Program". En ASME 1993 Design Technical Conferences. American Society of Mechanical Engineers, 1993. http://dx.doi.org/10.1115/detc1993-0073.
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Abstract Pratt & Whitney Aircraft and NASA Lewis Research Center studied a silicon-carbide-fiber-reinforced, titanium-metal-matrix (MMC) composite designated SiC(SCS-6)/Ti-15V-3Cr-3Al-3Sn. Fracture tests of three nominal 15.24-cm (6-in.) diameter SCS–6/Ti-15-3 rings at 427 °C (800 °F) and room temperature resulted in failure at internal pressures of 0.172 GPa (25 ksi) (average) and 0.235 GPa (34.1 ksi), respectively. Predictions of internal pressure to failure varied from 0.140 to 0.276 GPa (20.3 to 40.1 ksi) at 427 °C (800 °F). Nondestructive evaluation revealed that the rings’ MMC cores had varying density and were not uniformly distributed inside the monolithic casing. Analytical predictions of ring cyclic life at 0.154-GPa (22.4-ksi) internal pressure varied from 1000 to 15 000 cycles. No fatigue life data were obtained with the MMC rings for comparison with theory.
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Raza, Kabeer, Syed Sohail Akhtar, Abul Fazal M. Arif y Abbas Saeed Hakeem. "An Improved Predictive Model for Effective Thermal Conductivity of Polymer Composites With Non-Dilute Filler Concentrations". En ASME 2019 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/imece2019-10960.
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Abstract Most of the predictive models for thermal conductivity of composites are derived based on the assumption that the filler concentration in the matrix is dilute. This assumption leads to inaccurate predictions when filler concentration is essentially non-dilute and hence there is a need to propose a model that could handle a non-dilute filler concentration. In this work an improved and realistic model for effective thermal conductivity of polymer matrix composites with non-dilute filler’s concentrations is derived and validated by experiments. The proposed model can handle fillers with variable size and shapes. The derivation is based on the Bruggeman’s differential effective medium theory where the high volume fractions can be obtained by incrementally adding ‘small volume fractions’ into the ‘existing composite’ at each stage. The proposed model is validated by experimentally produced different series of ceramic particles-polymer composites. Differently sized and shaped alumina (Al2O3) & aluminum nitride (AlN) particulate fillers, and high density polyethylene (HDPE) & polypropylene (PP) matrices were used as the variable ingredients. Using different combinations of filler, matrix and particle size six different series of composites were produced with variable filler concentrations up to 50% by volume. The microstructure of the produced samples was studied by field emission scanning electron microscope to relate the morphology with the predictions. The predictions of proposed model are found in close agreement with the measured thermal conductivities. To understand the detailed effects of different parameters, parametric studies are presented and discussed. It is found that aspect ratio of particulate fillers is the most sensitive parameter to enhance effective thermal conductivity. Overall, the proposed model is proven to be useful in composite material design for heat transfer applications. It is expected that the proposed model will open new doors for the researchers and polymer composite industry to develop new composite designs for achieving ultrahigh thermal conductivities.
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Zhao, J., H. A. DeSmidt y W. Yao. "Nonlinear Breathing Behavior Study of Transverse Crack on a Jeffcott Rotor Under Torsional Excitation". En ASME 2014 Dynamic Systems and Control Conference. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/dscc2014-5824.
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In this paper, Jeffcott rotor model is employed to explore the vibration response of breathing cracked system with unbalance mass. Based on the energy method and Lagrange principle, 6 degree-of-freedom equation of motion is derived in fixed coordinate system. The crack model is established using strain energy release theory of facture mechanics. The stiffness matrix induced by the crack is changing with the variation of crack open area. Zero stress intensity factor (SIF) method is used to determine the crack closure line by computing the SIF for opening mode. By integrating compliant coefficients over newly determined crack open area, the stiffness matrix is updated and vibration response is solved for every time step by Gear’s method. In addition, the breathing behavior of the crack is studied for multiple eccentricity phases and rotation speeds in order to provide effective guidance for damage detection. The paper explores the effect of external torsional loading on the crack breathing behavior. Finally, the coupling of lateral and torsional vibration is investigated to be used as an indicator of damage detection and health monitoring.
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Johnson, Derek, Mahdi Darzi, Chris Ulishney, Mehar Bade y Nima Zamani. "Methods to Improve Combustion Stability, Efficiency, and Power Density of a Small, Port-Injected, Spark-Ignited, Two-Stroke Natural Gas Engine". En ASME 2017 Internal Combustion Engine Division Fall Technical Conference. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/icef2017-3557.
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Two-stroke engines are often used for their low cost, simplicity, and power density. However, these engines suffer efficiency penalties due to fuel short-circuiting. Increasing power density has previously been an area of focus for performance two-stroke engines — such as in dirt bikes. Smaller-displacement engines have also been used to power remote controlled cars, boats, and aircraft. These engines typically rely on gasoline or higher-octane liquid fuels. However, natural gas is an inherently knock-resistant fuel and small natural gas engines and generators could see increased market penetration. Power generators typically operate at a fixed frequency with varied load, which can take advantage of intake and exhaust system tuning. In addition, stationary engines may not be subject to size restrictions of optimal intake and exhaust systems. This paper examines methods to improve combustion stability, efficiency, and power density of a 29cc air-cooled two-stroke engine converted to operate on natural gas. Initial conversion showed significant penalties on delivery ratio, which lowered power density and efficiency. To overcome these issues a tuned intake pipe, two exhaust resonators, and a combustion dome were designed and tested. The engine was operated at 5400 RPM and fueling was adjusted to yield maximum brake-torque (MBT). All tests were conducted under wide-open throttle conditions. The intake and exhaust systems were designed based on Helmholtz resonance theory and empirical data. The engine utilized a two-piece cylinder head with removable combustion dome. The combustion dome was modified for optimal compression ratio while decreasing squish area and volume. With all designs incorporated, power increased from 0.22 kW to 1.07 kW — a factor of 4.86. Efficiency also increased from 7% to 12%. In addition to these performance gains, the coefficient of variation (COV) of indicated mean effective pressure (IMEP) decreased from just above 11% to less than 4%.