Facultatea de Fizică şi Inginerie / Faculty of Physics and Engineering
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Item Şcoala doctorală – element-cheie în organizarea ştiinţei moderne [Articol](Academia de Ştiinţe a Moldovei, 2011) Paladi, FlorentinItem Vibronic Interaction in Tetrameric Clusters with Two-Electron Transfer: Magnetic Properties [Articol](American Scientific Publishers, 2009) Boldyrev, S. I.; Paladi, FlorentinThe influence of vibronic interactions on the magnetic properties of the tetrameric square-planar mixed-valence d0-d0-d1-d1 cluster with two-electron transfer is investigated. The dynamic vibronic problem has been solved taking into account the Heisenberg and double exchange. It was shown that the double exchange and vibronic coupling leads to two types of pseudo-Jahn-Teller problems: (1An1+1Ad1+1B1+1B2+1E) ⊗ (b2+e) and (3B2+3A2+3En+3Ed) ⊗ (b2+e). The dependence of magnetic properties on vibronic interaction was studied. All peculiarities of magnetic moments take place in the case of intermediate vibronic coupling. The results show that the vibronic interaction essentially affects the magnetic properties of the system, such as for the tetrameric tetrahedral cluster with one extra electron.Item Contribuţii la teoria relaxării stărilor metastabile în lichide subrăcite şi sticle [Articol](CEP USM, 2009) Paladi, FlorentinThe theory of phase transitions in supercooled liquids and glasses based on the cluster concept, and taken into accountthe recent discovery of the generation and extinction of crystal nuclei in supercooled liquids at very low temperatures,has been developed. In particular, the generation and extinction of nuclei was found at low temperatures in supercooled liquids of o-benzylphenol, salol and 2,2’-dihydroxybenzophenone, and was brought a new concept of irreversible structural relaxation in supercooled liquids and glasses. The practical value of this research is determined by the importance of understanding the connection between physical properties, microscopic structure of the substance and macroscopicconditions of materials processing, which is vital to produce new materials with advanced technological properties. However, the results allow to formulate the essence of a new research development in the study of complex cluster systems bymethods of statistical physics, numerical calculation and computer modeling.Item Measuring heterogeneity in stochastic models [Articol](CEP USM, 2007) Paladi, Florentin; Doţenco, Denis; Gamurari, VisarionUn model teoretic care descrie interacţiunea într-o populaţie eterogenă de agenţi a fost dezvoltat şi analizat în cadrul discuţiilor recente despre decalajul dintre modelele computaţionale ABM şi modelele analitice stocastice [1]. A fost demonstrat că acest model probabilistic descrie exact rezultatul previzibil al simulărilor computaţionale care trebuie săfie suficient de lungi pentru a echilibra starea sistemului. Acest rezultat depinde de distribuţia stărilor în sistem şi este definit atât cu ajutorul valorii medii, cât şi al coeficientului de variaţie pentru payoff. Ultima mărime este independentă de scara valorilor şi denotă cât de inegale sunt distribuţiile în sistem, iar, prin urmare, şi nivelul de eterogenitate al modelului nostru stocastic. Rezultatele obţinute sunt raportate pentru valoarea medie şi coeficientul de variaţie pentru payoff, calculate pentru modelul prezentat în [1]. În lucrare nu este specificată o formă funcţională particulară pentru a evita alegeri arbitrare, în cazul unui sistem format din patruzeci agenţi cu 1221759 distribuţii posibile în şase clustere după trei stări diferite.Item Thermal and dielectric studies of 2,2’-dihydroxybenzophenone [Articol](Akadémiai Kiadó, 2005) Tomitaka, S.; Mizukami, M.; Paladi, Florentin; Oguni, M.Thermal and dielectric properties of 2,2’-dihydroxybenzophenone were studied in relation with the potential progress of crystal nucleation and growth below the ordinary glass transition temperature, Tgα. Differential scanning calorimetry was carried out in a range 100-350 K. The α glass transition was found to occur at Tgα=239 K. Crystallization and fusion were observed to take place when the sample was cooled down to 103 K, but not observed when cooled to 203 K. Crystal nucleation was interpreted as having happened during annealing for a short time at 103 K which is much below the Tgα. Heat capacities were measured in a range 7-350 K by an intermittent heating method with an adiabatic calorimeter. The temperature, enthalpy and entropy of fusion were determined to be 334.46 K, 20.07 kJ mol-1 and 60.01 J K-1mol-1, respectively. Crystal growth was found to proceed even at 220 K below the Tgα, but no glass transition was detected below 220 K. Dielectric losses were measured in a temperature range of 100-250 K and a frequency range of 30Hz-10 kHz. β-Relaxation process was found dielectrically with the activation energy of 22.6 kJ mol-1, and the corresponding glass transition was expected to occur at 76.9 K. It is discussed, based on the “structurally ordered clusters aggregation” model for supercooled liquids and glasses, that the β process is potentially attributed to the crystal nucleation progressing at 103 K.Item Modele computaţionale ABM de cercetare a sistemelor complexe [Articol](2009) Paladi, FlorentinScopul principal al acestui articol este cercetarea aspectelor microscopice ale modelelor stocastice, care pot fi comparate cu modelele computaţionale bazate pe agenţi (ABM), precum şi stabilirea conexiunii dintre modelele ABM şi procesele stocastice. O aplicare a abordării propuse se referă la procesul de nucleere, fenomen întâlnit pe larg în natură tehnologie, care poate fi considerat ca un reprezentant al fenomenelor de agregare în sistemele complexe. În special, noi propunem un model generic stocastic pentru formarea centrelor de cristalizare. În general, este cunoscut faptul că la baza mecanism ului tranziţiilor de fază de gradul întâi este apariţia acestor centre de cristalizare şi atât nucleul definit ca un cluster de molecule sau atomi, cât şi lucrul de nucleere, o barieră de energie pentru tranziţia de fază, sunt principalele mărimi termodina mice în teoria dată. Totodată, din punct de vedere statistic, formarea cristalitului de dimensiune critică este un fenomen aleator, cu o probabilitate determinată considerabil de valoarea lucrului de nucleere.Item The kinetics of phase transitions: I. The role of heterogeneity [Articol](2010) Paladi, Florentin; Gamurari, VisarionA kinetic model involving one order parameter was developed to study the impact of het- erogeneity on the nucleation. The role of asymmetry on phase transitions in the presence of an intermediate metastable state is described in comparison with the generic model. In particular, the discovery of anomalous generation and extinction of crystal nuclei in nonequilibrium supercooled liquids reflects experimentally the effect of the real heterogeneous structure and its irreversible relaxation on the stability of nuclei. It was found that the cubic term in the order parameter of the Landau-type potential associated with the asymmetry of the system serves to increase the stability of crystalline state for the suitably varied control parameter. The cases which allow analytical solution of the problem and the realization of different transition scenarios are also discussed.Item The kinetics of phrase transitions II. The role of interface between [Articol](2010) Paladi, FlorentinThe present developments in the physics of complex systems, in particular, the structural relaxation of supercooled liquids and glasses, are discussed by using a stochastic cluster based model. We are able to depict the impact of the interface between the nucleus considered as a cluster of a certain number of molecules and the liquid phase for the enhancement of the overall nucleation process. It is also shown that even a relatively simple stochastic model, which appears phenomenological if it is not agent-based, can describe precisely the outcomes from multiple agent-based simulation runs where probabilistic insight is lacking and which should be long enough to equilibrate the states of large systems.Item Computational study of non-stationary cluster size distribution and rate of nucleation in case of pre-existing clusters [Articol](2006) Gamurari, Visarion; Paladi, Florentin; Eremeev, Vitalie; Al-Sabayleh, Moayad Abidalraheem Obidhe dynamics of a cluster-type system formed by constant total number of molecules, M, is studied when clusters change their sizes by nearest-size transitions (the Szilard model) with time-dependent boundary conditions and non-zero initial cluster size distribution. We compare these results to those reported previously [5], when non-stationary cluster size distribution was considered under the condition that only monomers are present in the system at the initial moment t=0, and describe the effect of arbitrary pre-existing clusters on the time evolution of the interacting clusters with different sizes, n. The average group size problem is also solved, and the process of relaxation in the system is studied.