Time delay in thin slabs with self-focusing Kerr-type nonlinearity

Time delays for an intense transverse electric (TE) wave propagating through a Kerr-type nonlinear slab are investigated. The relation between the bidirectional group delay and the dwell time is derived and it is shown that the difference between them can be separated into three terms. The first one is the familiar self interference time, due to the dispersion of the medium surrounding the slab. The other two terms are caused by the nonlinearity and oblique incidence of the TE wave. It is shown that the electric field distribution along the slab may be expressed in terms of Jacobi elliptic functions while the phase difference introduced by the slab is given in terms of incomplete elliptic integrals. The expressions for the field intensity dependent complex reflection and transmission coefficients are derived and the multivalued oscillatory behavior of the delay times for the case of a thin slab is demonstrated

From alcohol consumption to cultural conceptualization of drinking : anthropological research based on youth population in Belgrade

U disertaciji se, iz antropološke perspektive, istražuje povezanost konzumacije alkoholnih pića i populacije mladih kao ciljne grupe. Konzumaciji alkohola pristupilo se kao društvenoj praksi i činu koji se odigrava u specifičnim socijalnim kontekstima. Pod društvenim i kulturnim konceptualizacijama pijenja se podrazumevaju svi oni oblici ponašanja i praksi koji se formiraju oko konzumacije alkohola, bilo da su u pitanju svakodnevni rituali, poput pića nakon napornog dana, ili neki rekurentni događaj, ritual životnog ciklusa (poput proslavljanja punoletstva); alkoholna pića igraju uloge markera i graničnika između rada i dokolice. S tim u vezi, cilj je bio istražiti deljene prakse pijenja među mladima, kao vid participativnih aktivnosti. Kroz ove aktivnosti, osmišljavaju se lični i kolektivni identiteti, podložni manipulisanju. Pijenje predstavlja integrativni deo svakodnevice mladih, a na osnovu subjektivnih iskustava, koja su u sadejstvu sa aproprijacijama vladajućih normi u društvu, mladi konceptualizuju pijenje i opijanje pre svega kao vid užitka i provođenja slobodnog vremena, ali i kao socijalni problem ili, pak, kao „alatku“ za osporavanje vladajućih normi. Osnovna hipoteza u radu jeste da je alkoholno piće, iako je nastalo na hemijskim osnovama i u našim telima izaziva metaboličke promene, simbolički prazno dok se značenja u njega ne „upišu“. Ova značenja koreliraju sa istorijskim, političkim, ekonomskim i društvenim parametrima. Mladima se pristupilo ne kao homogenoj grupi, koja deli jedan zajednički identitet, već kao polimorfnoj. Mladi putem praksi pijenja pre svega osmišljavaju koncepte zabave i provođenja slobodnog vremena (dokolice). Metodološki postupak zasnovan je na tri nivoa: pisanim upitnicima, intervjuima i posmatranju sa učestvovanjem. Ovako sprovedena kombinacija kvantitativnog i kvalitativnih metoda omogućila je uvid u etnoeksplikacije o piću i pijenju, mestima i nemestima pijenja, stilovima pijenja (pre svega binge-drinking) i kafanama kao etnografskim arenama u kojima se odigravaju visoko ritualizovane forme ponašanja. Parkovi, kućne žurke, klubovi i kafane dobijaju na taj način oznaku mesta konvivijalnosti. Dobijeni rezultati pokazuju da se identiteti, putem praksi pijenja kao kroz posebnu prizmu transgresije sopstva, artikulišu, preinačuju i transformišu, čime se potvrđuje i druga premisa u disertacijiThe dissertation tackles the relation between alcohol consumption and youth as focus group, from anthropological perspective. In these terms, alcohol consumption is understood as a cultural practice, which takes place in specific social context. Cultural and social conceptualizations of drinking entail all those behavioral patterns and practices that shape and stem from alcohol consumption as such, whether the drinking occasion is a part of daily rituals (such as having a drink after a long day), or a recurrent life event (such as coming of age); alcohol plays as border marker between work and leisure. Bearing that in mind, the goal of the disseration was to inquire shared drinking practices among youth population, as a form of participative activity. Through these activities, individual and collective identities are being (re)imagined and manipulated. Drinking is an integrative part of the „ordinary“ and daily, and based on personal experiences, which interact with appropriation of norms in the society, young people conceptualize drinking as an aspect of enjoyment and hedonism, but also as a social problem or a means to challenge the proclaimed norms. The first premise is that an alcoholic drink, although it is a chemical „compound“ and triggers metabolic processes in our bodies, is symbolically empty, until the meanings are inscribed. These meanings correlate with historical, political, economic and social parameters in a given society. The youth is perceived not as a homogenous group that shares a single unique identity, but as a polymorph one. Through drinking practices, young people construct the notions of fun and leisure. Three levels of methodological conduct were combined: questionnaires, in-depth informal interviews and participant observation. Through such mixed methods, the insight into following is gained: ethno-explications on drinks and drinking; places and spaces of drinking and drinking styles (binge-drinking being the prominent style) and kafana as an ethnographic arena in which highly ritualized forms of practice take place. Public parks, house parties, clubs and kafana are marked as places of conviviality. The results show that indeed, identities are transformed through drinking practices, and transgression of self is articulated and shifted, thus confirming the second premise

Synthesis and characterization of complex and metal hydrides for hydrogen energy applications

Cilj ove doktorske disertacije jeste ispitivanje desorpcionih osobina kompozitnih materijala na bazi magnezijum hidrida (MgH2) kao pogodnih materijala za skladištenje vodonika. Ispitvani su kompoziti MgH2-neoksidna keramika (titan-diborid (TiB2) i silicijum-karbid (SiC)) i MgH2- kompleksni hidridi (litijum-alanat (LiAlH4) i natrijum-amid (NaNH2)). Praćeno je ponašanje materiljala u zavisnosti od koncetracije aditiva i vremena mlevenja. Pomoću rendgenostrukturne analize (RSA), skenirajuće elektronske mikroskopije (SEM) i laserske metode za određivanje raspodele veličine čestica, ispitivan je uticaj mikrostrukture i morfologije na desorpcione osobine koje su praćene pomoću temperaturski programirane desorpcije (TPD) i diferencijalne skenirajuće kalorimetrije. Ustanovljeno je da se u kompozitinim sistemima MgH2 - TiB2 - SiC javljaju tri desorpciona maksimuma od kojih je visokotemperaturni maksimum blago pomeren ka većim temperaturama u odnosu na komercijalni MgH2. Postojanje više desorpcionih maksimuma ukazuje na činjenicu da se vodonik u ovom sistemima nalazi u nekoliko oblika (vezan u vodi, hidroksidu ili kao molekul vodonika). Pokazano je da TiB2 dovodi do značajnog smanjenja energije aktivacije za reakciju desorpcije vodonika (Ea=173 kJ/mol), dok SiC utice na sniženje temperature početka desorpcije kod srednjetemperaturskog pika, tj. potpomaže međufaznu reakciju između magnezijumovog hidrida i hidroksida. U slučaju kompozita MgH2-LiAlH4 ustanovljeno je da najnižu temperaturu desorpcije i najmanju energiju aktivacije za reakciju desorpcije vodonika (Ea=94 kJ/mol) poseduje kompozit mleven 15 minuta. Ovakvo ponašanje je posledica kratkog vremena mlevenja usled čega dobar deo litijum-alanata ostaje neizmenjen, dok kod većih vremena mlevenja dolazi do dekompozicije ovog jedinjenja i formiranja produkata koji imaju negativan uticaj na reakciju desorpcije. Slično ponašanje je primećeno i kod NaNH2-MgH2 kompozita. I kod ovog kompozita je pokazano da se energija aktivacije drastično smanjuje (Ea=168 kJ/mol) u odnosu na čist hidrid ako se koristi kratko vremena mlevenja (15 minuta). Povećanje vremena mlevenja rezultuje sporijom kinetikom, što se može pripisati direktnom uticaju dekompozicije NaNH2, dok veće količine ovog jedinjenja dovode do sprezanja ovog efekta sa povećanjem količine Mg(OH)2. Utvrđeno je da smanjenje veličine kristalita i čestica kod svih kompozita utiču kako na temperaturu tako i na kinetiku desorpcije. Takođe je pokazano da promena morfologije čestica sa ljuspasog oblika (čisti nemleveni MgH2) na sunđerasti oblik, koji je karakterističan za kompozite, utiče na desorpciju vodonika. Pokazano je da se proces desorpcije kod svih sintetisanih nanokompozita, kao i kod čistog MgH2, najbolje opisuje kinetičkim mehanizmom Avrami-Erofejeva (Avrami-Erofeev) koji se zasniva na principu dekompozicije MgH2 mehanizmima nukleacije i rasta. Utvrđeno je da se katalizovani uzorci ponašaju po spomenutom kinetičkom modelu sa vrednošću parametra n=4, dok je vrednost ovog parametra n=3 za čist MgH2.The goal of this doctoral dissertation was investigation of desorption characteristics of composties based on magnesium hydride (MgH2). This type of hydrides are capable for hydrogen storage. Two kinds of composites are investigated: MgH2-non-oxide ceramics (titanium-diboride (TiB2) and silicon-carbide (SiC)) and MgH2-complex hydrides (lithium-alanate (LiAlH4) and sodiumamide (NaNH2)). It was studied behavior of mentioned nano-materials in dependance of additive concentration and milling time. Influence of microstructure and morphology on desorption properties was investigated by X-ray Diffraction Analysis (XRD), Scanning Electron Microscopy (SEM) and Laser diffraction method for particle size distribution (LD-PSD). Desorption features are analysed by temperature programmed desorption device (TPD) and differential scanning calorimetry (DSC). It is found that MgH2-TiB2-SiC composite system has three desorption maxima. Hightemperature maximum is slightly shifted toward higher temperatures in comparison to commercial MgH2. Existence of several desorption maxima in composites indicate presence of hydrogen in several forms (bonded in water, hydroxide and in molecular form of hydrogen). In composite with TiB2, activation energy for hydrogen desorption reaction is significantly decreased (Ea=173 kJ/mol), while SiC affect on lowering of desorption temperature onset (intermediate-temperature peak), i.e. helps interface reaction between magnesium hydride and magnesium hydroxide. In the case of MgH2-LiAlH4 composite it is concluded that the lowest desorption temperature and the lowest activation energy (Ea=94 kJ/mol) for hydrogen desorption reaction possess composite milled for 15 minutes. Such behavior is a consequence of short milling time, which makes that large quantities of lithium-alanate remain unchanged. Longer milling time cause decomposition of this compound and generate products with negative influence on hydrogen desorption process. Similar behavior was noticed for NaNH2-MgH2 composites. It has been shown that activation energy for hydrogen desorption drastically decrease (Ea=168 kJ/mol) in comparison to pure magnesium hydride, if milling time is very short (15 minutes). Longer milling time cause slower kinetics, which is a consequence of NaNH2 decomposition. Higher quantities of this compound leads to the combination of this effect and cause enhancement of Mg(OH)2 quantity. It is concluded that decrease of grain size and particle size affects on desorption temperature and kinetics in all composites. Also, the change of particle’s morphology from flakelike particles (pure unmilled MgH2) to particles with sponge-like shape (characteristic for all catalysed composites), affect hydrogen desorption. It is shown for all synthesized composites (and for pure MgH2) that Avrami-Erofeev kinetic mechanism describes whole desorption process very well. This desorption mechanism is based on two processes: nucleation and growth. Obtained values for Avrami-Erofeev kinetic parameter, n, are: 4 for all catalysed composites and 3 for pure MgH2

Towards automated design of quantum cascade lasers

We present an advanced technique for the design and optimization of GaAs/AlGaAs quantum cascade laser structures. It is based on the implementation of the simulated annealing algorithm with the purpose of determining a set of design parameters that satisfy predefined conditions, leading to an enhancement of the device output characteristics. Two important design aspects have been addressed: improved thermal behavior, achieved by the use of higher conduction band offset materials, and a more efficient extraction mechanism, realized via a ladder of three lower laser states, with subsequent pairs separated by the optical phonon energy. A detailed analysis of performance of the obtained structures is carried out within a full self-consistent rate equations model of the carrier dynamics. The latter uses wave functions calculated by the transfer matrix method, and evaluates all relevant carrier–phonon and carrier–carrier scattering rates from each quantized state to all others within the same and neighboring periods of the cascade. These values are then used to form a set of rate equations for the carrier density in each state, enabling further calculation of the current density and gain as a function of the applied field and temperature. This paper addresses the application of the described procedure to the design of lambda~9 µm GaAs-based mid-infrared quantum cascade lasers and presents the output characteristics of some of the designed optimized structures. © 2005 American Institute of Physic

Dynamics and nonequilibrium states in the Hamiltonian mean-field model: A closer look

We critically revisit the evidence for the existence of quasistationary states in the globally coupled XY (or Hamiltonian mean-field) model. A slow-relaxation regime at long times is clearly revealed by numerical realizations of the model, but no traces of quasistationarity are found during the earlier stages of the evolution. We point out the nonergodic properties of this system in the short-time range, which makes a standard statistical description unsuitable. New aspects of the evolution during the nonergodic regime, and of the energy distribution function in the final approach to equilibrium, are disclosed

The influence of uncertainties and parameter structural dependencies in distribution system state estimation

This paper evaluates a number of uncertain parameters that affect the accuracy of distribution system state estimation, and ranks their importance using an efficient sensitivity analysis technique, Morris screening method. The influence of the uncertain parameters on state estimation performance is analysed globally and zonally. Furthermore the dependence structure between the critical variable and state estimation accuracy is analysed using copula to establish their relationship at different section of the bivariate space. The sensitivity of the critical parameter at different ranges is also studied and ranked using Morris screening methods to present the variation of state estimation performance when the critical variable is allocated at different sections within the feasible range. Accurate assessment of the importance of various uncertain parameters and the analysis of the dependence structure can inform power system operators which parameters will require the greatest levels of mitigation or increased monitoring accuracy in order to have satisfactory performance of distribution system state estimation

Feasibility study of applicability of recurrence quantification analysis for clustering of power system dynamic responses

A methodology based on Recurrence Quantification Analysis (RQA) for the clustering of generator dynamic behavior is presented. RQA is a nonlinear data analysis method, which is used in this paper to extract features from measured generator rotor angle responses that can be used to cluster generators in groups with similar oscillatory behavior. The possibility of extracting features relevant to damping and frequency of oscillations present in power systems is studied. The k-Means clustering algorithm is further used to cluster the generator responses in groups exhibiting well or poorly damped oscillations, based on the extracted features from RQA. The effectiveness of RQA is investigated using simulated responses from a modified version of the IEEE 68 bus network, including renewable energy resources

Kompleksna modulacija sila tokom preciznog hvata šake primenom ponavljane transkranijalne magnetne stimulacije pražnjenjima u teta frekvenciji iznad dorzalnog premotornog korteksa

Background/Aim. Adaptive control and fingertip force synchronization of precise grasp stability during unimanual manipulation of small objects represents an illustrative example of highly fractionated movements that are foundation of fine motor control. It is assumed that this process is controlled by several motor areas of the frontal lobe, particularly applicable to the primary motor (M-1) and dorsal premotor cortex (PMd). Aiming to examine the role of PMd during fine coordination of fingertip forces we applied theta burst repetitive magnetic stimulation (TBS) to disrupt neural processing in that cortical area. Methods. Using a single-blind, randomized, crossover design, 10 healthy subjects (29 ± 3.9 years) received single sessions of continuous TBS (cTBS600), intermittent TBS (iTBS600), or sham stimulation, separate from one another at least one week, over the PMd region of dominant hemisphere. Precision grasp and lift were assessed by instrumented device, recording grip (G) and load (L) forces, during three manipulation tasks (ramp-and-hold, oscillation force producing and simple lifting tasks), with each hand separately, before and after interventions. Results. We observed the improvement of task performance related to constant error (CE) in oscillation task with the dominant hand (DH) after the iTBS (p = 0.009). On the contrary, the cTBS reduced variable error (VE) for non-dominant hand (NH), p = 0.005. Considering force coordination we found that iTBS worsened variables for NH (G/L ratio, p = 0.017; cross-correlation of the G and L, p = 0.047; Gain, p = 0.047). Conclusion. These results demonstrate the ability of TBS to modulate fingertip forces during precision grasping and lifting, when applied over PMd. These findings support the role of PMd in human motor control and forces generation required to hold small objects stable in our hands.Uvod/Cilj. Adaptivna kontrola i sinhronizacija sila prstiju šake tokom preciznog hvata pri manipulisanju malim predmetima jednom rukom predstavlja ilustrativni primer visoko frakcionisanih pokreta koji predstavljaju temelj motorne kontrole preciznih pokreta. Pretpostavlja se da ovim procesom upravlja nekoliko motornih oblasti frontalnog režnja, i to prvenstveno primarni motorni (M-1) i dorzalni premotorni korteks (PMd). Cilj istraživanja bio je ispitivanje uloge PMd-a tokom vršenja pokreta koji zahtevaju finu koordinaciju sila prstiju šake. U istraživanju smo primenili ponavljanu magnetnu stimulaciju pražnjenjima u teta frekvenciji, kako bi ometali neuralno procesiranje u toj oblasti moždane kore. Metode. Primenom jednostrano slepe studije, uz nasumičnu raspodelu i ukršteni dizajn, 10 zdravih ispitanika (29 ± 3,9 godina) bilo je izloženo pojedinačnim sesijama kontinuirane magnetne stimulacije (cTBS600), ili intermitentne ponavljane magnetne stimulacije (iTBS600), pražnjenjima u teta frekvenciji kao i prividnoj stimulaciji iznad PMd regiona dominantne hemisfere, odvojenih međusobno, najkraće nedelju dana. Precizanost hvata šake i podizanja procenjivani su uređajem koji je registrovao silu stiska (G) i silu podizanja (L) prilikom izvođenja tri zadatka (zadatak sa zadatim profilom L, zadatak sa oscilatornim variranjem nivoa L i zadatak sa podizanjem), koji su izvođeni sa obe ruke odvojeno, i to pre i nakon svake intervencije. Rezultati. Nakon primene iTBS protokola zabeleženo je poboljšanje izvođenja iskazano konstantnom greškom (CE) u zadatku sa oscilatornim variranjem nivoa L, kada je izvođen dominantnom rukom (DH), p = 0.009. Suprotno tome, primena cTBS protokola dovela je do smanjenja promenjive greške (VE) za nedominantnu ruku (NH), p = 0.005. Sa aspekta koordinacije sila utvrđeno je da je iTBS protokol doveo do pogoršanja rezultata praćenih pokazatelja za nedominantnu ruku (G/L odnos, p = 0.017; korelacija G i L, p = 0.047; prirast sile p = 0.047). Zaključak. Rezultati našeg istraživanja ukazuju na mogućnost modulacije sila prstiju šake tokom preciznog hvata i podizanja, ukoliko se TBS primeni iznad PMd-a. Dobijeni nalazi podržavaju ulogu PMd u motornoj kontroli i generisanju sila neophodnih za stabilno držanje malih predmeta kod ljudi