Firstly, I would like to express my sincere gratitude to my co-advisors Prof. Leonid Klushin and Prof. Jihad Touma for their continuous support throughout my Ph.D study. Prof. Klushin with his patience, immense knowledge and realistic approach to problems managed to put me back on track whenever I'm overwhelmed with the difficulties. Prof. Touma, on the other hand, challenged me, taught me how to be curious and passionate about what I do, and showed me an inspiring example of a scientist who's not limited to his science but have something useful to say about life, society, politics, culture, religion and much more. I cannot imagine having a better combination of mentors for my Ph.D study and who ended up as mentors for life.

Experiments with groups of fish inside a circular tank have provided valuable insights into the nature of leadership in social groups. Sophisticated mathematical models were constructed with a view to recovering observed schooling and leadership behavior in such experiments. Here, and with the help of variations on a promising class of such models, we explore a dual set of social concerns, namely the likelihood of permanent evasion from a cohesive group by a controlled individual in confinement. Our minimal model reduces to a leader-follower configuration, with cone-of-vision driven interactions inside a circular domain. We show that the resulting dynamical system sustains a rich supply of non-aligned, straying "follower" states, the dynamics on which displays (chaotic) intermittency between boundary following behavior and infrequent long flights. We map these states in configuration space and explore transitions between them. We demonstrate robustness of observed behavior by considering model variations, as well as alternate leader control trajectory. While it is too early to draw the implications of leader-follower dynamics to collective behavior, we do confirm that a model stray fish relates to a self-organized school bouncing back and forth along the diameter very much like a follower responds to a point leader in our model. We further draw the implications of our results to the study of dynamical systems with discontinuities, robotics, and the study of human behavior in the face of normative control and confinement. Self-organizing behavior of living organisms is ubiquitous. From the smallest to the largest scales, collective structures obtain and have received the attention of biologist, etiologists, sociologists, and increasingly physicists and mathematicians 1-5. Phenomena range from bacteria swimming in formation, to swarming insects, to flocking birds, to schooling fish, to herds of wilder beast, to milling worshipers etc 6-9. Controlled experiments whether on fish in tanks, or pedestrians in crowds are now added to a rich tradition of observational techniques deployed in the subject 10-13. What is common to these works (and associated mathematical models) is an increasing focus on the nature of leadership in groups of animals moving in and out of coherent structures. Here, we identify novel formations of organisms in confinement and suggest a mechanism that gives rise to transitions between them. Our concern is not to mimic reality in its bewildering complexity. Instead, we explore the complexity of behavior sustained by simplified and reasonably grounded models, with a view to evaluating their relevance to one biological (and/or sociological) setting or the other. With this in mind, we adopt a promising recipe for alignment of fish as described in 10 with the addition of interactions via a cone of vision as motivated in 13-15. This model differs from novel recent minimal models which disregard the velocity-velocity alignment effect 16,17 and whose universality is still debatable 15,18,19. We then constrain organisms to move in a circular tank with a reflective boundary, having further split them into a leader bouncing on an invariant trajectory, and a follower which is responding to intermittent forcing by that leader. In pursuit of patterns sustained by our construct, we were lead away from popular concerns with the self-organization of a herd and its leaders, to the seeming inevitability of straying in confinement, and this despite a persistent will to align. We start with a brief description of our model, followed

The stubborn persistence of poverty is perhaps the central puzzle of development economics. Almost all theories of poverty reduction revolve around the idea of convergence: the proposition that, because the relationship between capital stocks and income (i.e., the production function) is convex and monotonically decreasing in slope, the poor should eventually “catch up” to the rich. A great deal of the development economics literature is devoted to exploring why in fact this does not happen in the real world. Classically, explanations have focused on differences in savings rates, population growth, and technological change [7]; conditioning on these variables, convergence should hold. Later theorists, while acknowledging the importance of these forces, emphasized the role of imperfections in credit, insurance, and information markets as well as the influence of institutions—durable political and social norms that shape economic behavior [6].Together, imperfect markets and institutio...

Magnetization measurements were performed on five Pb 1Ϫx Eu x S samples, with 0.0092рxр0.059. Two of the samples were measured at 20 mK in magnetic fields H up to 50 kOe parallel to the ͓111͔ and ͓100͔ directions. The other three samples were measured at 0.6 K in fields up to 180 kOe. The 20 mK data exhibit magnetization steps ͑MST's͒ arising from pairs. These MST's depend on the direction of H, so that anisotropic interactions must be included in the data analysis. The MST's lead to a value J/k B ϭϪ0.228Ϯ0.007 K for the dominant antiferromagnetic exchange constant. Computer simulations identify this J as the nearest-neighbor exchange constant J 1 , and show that the Eu distribution over the cation sites is random or very nearly random. The MST's from pairs exhibit a splitting when H is parallel to ͓100͔. The splitting is tentatively attributed to exchange anisotropy, in which case the above value for J refers to the average exchange constant J. The 20 mK data for H͉͉͓100͔ also show a MST from isolated Eu ions. This MST occurs because such ''singles'' are subjected to a cubic crystal-field anisotropy. ͓S0163-1829͑98͒03813-2͔

The single-ion axial anisotropy parameter D for Co + in CdSe was determined both from the magnetization step (MST) arising from isolated ions and by EPR. The MST, measured at 30 mK, gave 2D =0.95+0.02 cm '. The EPR data, obtained with frequencies in the X band, gave 2D =0.936+0.01 cm '. These values are substantially below the early EPR result of Hoshina, 2D =1.54+0.03 cm '. It is shown that Hoshina's value was based on a misidentification of one of the resonances.

The dinuclear complex bis[(l2-chloro)chloro(1,10-phenanthroline)copper(II)] (1) was synthesized, and
characterized by X-ray, FTIR and thermal analysis. The fitting of magnetic susceptibility and magnetization
curve of (1) indicates the occurrence of weak antiferromagnetic exchange interaction between
copper(II) ions. The electronic structure has been also determined by density functional theory (DFT)
method. Complex (1) displayed potent anticancer activity against B16 (Melanoma), MDA-MB-32
(Breast Adenocarcinoma), A549 (Lung Adenocarcinoma), HT-29 (Colon Adenocarcinoma) and SF
(Astrocytoma) cell lines with an average IC50 value of 0.726 lg/ml compared to 4.88 lg/ml for cisplatin.
Complex (1) has a better therapeutic index and toxicological profile than cisplatin, and has demonstrated
a potential chemotherapeutic property.

In this paper, we report on the growth of manganese oxides thin films by Pulsed Laser Deposition using an MnO target at various oxygen pressures and substrate temperatures ranging from 550 to 800°C. Grazing Incidence X-Ray Diffraction measurements on the grown films revealed that, at low deposition temperature, the dominant phase is Mn 2 O 3 , but as the deposition temperature was raised above 700°C, a phase transformation occurred leading to the formation of Mn 3 O 4 . In a qualitative comparison, in the temperature range of 500-850°C, and at a pressure below 13 Pa, the phase diagram of bulk manganese oxides and our grown films show a fair correlation. The films grown near the transition temperature (T = 700°C) were found to be very thin compared to those grown at lower or higher temperatures, but the surface roughness was found to increase with temperature, as determined by Atomic Force Microscopy.

The growth and magnetic properties of TinSelenide(SnSe) doped with Eu2+ Sn1xEuxSe (x=2.5%) were
investigated. Q-band(34GHz)electron paramagnetic resonance measurements show that the site
symmetry of Eu2+ at 4.2K is orthorhombic and the Lande factor was determined to be g=1.997 (0.01).
The exchange coupling between nearest-neighbor(NN) Eu2+ ions was estimated from magnetization
and magnetic-susceptibility measurements using a model that takes into account the magnetic
contributions of single ions, pairs and triplets.The exchange interaction between Eu2+ nearest
neighbors was found to be antiferromagnetic with an estimated average value of Jp/kB= 0.187 (0.03) K.

Manganese dioxide (MnO 2 ) is considered as one of the most attractive compound among the manganese oxide phases due to its fundamental chemical and physical properties, its use in energy-storage devices, electrochemical applications, and biosensors. There have been limited attempts to grow high quality MnO 2 thin films where high pressure and low substrate temperature are targeted to promote the formation of this phase. In this work, we have exploited the flexibility of Pulsed Laser Deposition (PLD) in order to synthesize thin films of MnO 2 on Si substrates by laser ablation of a MnO target in oxygen gas ambient. Substrate temperature was varied from 25 to 800 °C aiming to grow films of good crystalline quality while investigating the temperature range where MnO 2 phase is expected to be stable. We have also investigated the effect of oxygen pressure which was varied from 10 to 500 mTorr. X-ray diffraction and Fourier Transform Infra-Red analyses have confirmed the formation of the MnO 2 phase for pressures above 250 mTorr, and an optimal deposition temperature of 500 °C, while Mn 2 O 3 is obtained in the range between 550 and 650 o C. Further increase in deposition temperature led to pure Mn 3 O 4 films. Atomic Force Microscopy imaging confirms the nanograined surface structure of the MnO 2 films, with a typical grain size of 30 nm. * Corresponding author: malek.tabbal@aub.edu.lb; phone 961 1 340460-4280; fax 961 1 744 461.

Electron paramagnetic resonance (EPR) studies are carried out on composite samples of SmBa2Cu3O7-δ/(MnFe2O4)x superconductors with 0.00 ≤ x ≤ 0.20 wt% at different temperatures 100K ≤ T ≤ 295K. An EPR signal of Cu 2+ ions in the orthorhombic local symmetry is observed. Moreover, the EPR signal intensity increases as the temperature decreases from room temperature down to 100 K. The temperature dependence of g-factors indicates the pseudogap onset temperature (T*), corresponding to the peak of the observed curves between 127 K and 162 K. The number of spins (N) participating in EPR resonance are calculated as a function of both nanosized MnFe2O4 addition and temperature. In particular, it is shown that any EPR signal for SmBa2Cu3O7-δ/(MnFe2O4)x is probably due to small amount of spurious phases typically present in the samples such as BaCuO2.The presence of BaCuO2 is confirmed by magnetization-field (M-H) hysteresis measurements

The magnetic susceptibility of Pb 1-x Ce x A (A = S, Se and Te) crystals with 0.006 ≤ x ≤ 0.036 were studied in the temperature range from 20 mK up to room temperature. X-band (~9.5 GHz) Electron Paramagnetic Resonance (EPR) showed small shifts in the effective Landé factors that were attributed to crystal-field admixture. The EPR measurements were correlated with the magnetic susceptibility data and resulted in estimating the crystal-field splitting E( 8 ) -E( 7 ) of the lowest 2 F 5/2 manifold for Ce 3+ ions in PbA (A = S, Se and Te) of about 340 K, 440 K and 540 K for Pb 1-x Ce x Te, Pb 1-x Ce x Se, and Pb 1-x Ce x S, respectively. The values for the crystal-field splitting deduced from the magnetic data were found to be in agreement with the calculated ones based on the point charge model. Moreover, the de-Haas van-Alphen magnetic oscillations in the susceptibility measurements of Pb 1-x Ce x Te (x~ 0.05 and 0.07) were observed at ultra-low temperature (20 mK); The oscillations were investigated and the values of the oscillatory period for Pb 1-x Ce x Te (x = 0.0048 and 0.007) are reported.

The effects of the deposition temperature and laser energy on the characteristics of Co 2 MnAl films deposited on GaAs substrates were investigated. The grown films were characterized by AFM for film roughness and surface topography. Film thickness and elemental composition were measured using Rutherford Back Scattering (RBS) technique, while crystalline structure and phase composition were investigated by XRD. The RBS measurements showed that the stochiometry of the films was satisfactory and very close to that of the target Co: 0.5, Mn: 0.25, Al: 0.25. The thickness of the films was found to increase as the laser energy was increased from 200 to 400 mJ, in particular for the films deposited at 400 °C. We also found an increase in the films thicknesses as the deposition temperature was increased for the samples grown at 200 and 300 mJ. The best film quality as deduced from XRD, RBS and AFM results for producing these single layers were those deposited at 600 °C with the laser energy at 300 mJ.

Metal electrodeposition systems display tree-like structures with extensive ramification and a fractal character. Electrolysis is not a necessary route for the growth of such dendritic metal deposits. We can grow beautiful ramification patterns via a simple redox reaction. We present here a study of silver (Ag) deposits from the reduction of Ag þ in (AgNO 3 ) solution by metallic copper. The experiments are carried out in discotic geometry, in a Petri dish hosting a thin AgNO 3 solution film. A variety of deposited structures and patterns is obtained at different Ag þ concentrations, yet with essentially the same fractal dimension averaged at 1.64, typical of diffusion-limited aggregation (DLA). A linear magnetic field of low induction (0.50-1.0 T) applied across the medium causes a notable transformation in the morphology of the deposits. In both the field off and the field on cases, the effect of vertical (hence 3D) heaving seems to be dominant, perhaps explaining the nearly constant fractal dimension. Published by AIP Publishing.

Samih Isber, Young Ju Park, Jagadeesh S. Moodera, and Don Heiman

Spin-oriented electrons were injected into ferromagnetic Co2MnAl by optical absorption in GaAs. Schottky diode heterostructures were fabricated by molecular beam epitaxy growth of the Heusler ferromagnet Co2MnAl epitaxially on GaAs. The GaAs was illuminated near the bandgap at room temperature by circularly polarized light with 1.45 eV. Spin-polarized photocurrent was observed for forward biased conditions. A barrier height of 0.6 eV was found from I-V and photoemission experiments.

X. Gratens, S. Charar, M. Averous, S. Isber, J. Deportes and Z. Golacki


Electron paramagnetic resonance (EPR) experiments were performed on a Pb1-xCexSe crystal at liquid helium temperatures and show very clearly that the doublet Gamma7 is the ground state for cerium ions. The cubic symmetry is shown and the effective Lande´ factor for the Ce3+ is determined to be 1.354 (0.003). An orbital reduction factor is introduced to explain the g experimental value. High-field magnetization results are in good agreement with the EPR results. The nominal Ce composition in PbSe deduced from saturation of the magnetization, x=0.0405 (0.0003), is very closed to the value determined by microprobe analysis (x=0.04). At 1.5 K, an antiferromagnetic interaction between the nearest-neighbor cerium atoms is found, Jex /kB = -0.715 K. The low-field magnetic-susceptibility results show that the magnetic moment of cerium impurities is strongly temperature dependent, explained by the presence of the crystal-field effect and the Kondo effect.

X. Gratens, A. B. Arauzo,* G. Breton, S. Charar, M. Averous and S. Isber

Electron paramagnetic resonance (EPR) experiments are performed at room and liquid helium temperature on layers Pb1-xEuxSe and on the superlattice PbSe/Pb1-xEuxSe, grown by molecular beam epitaxy. EPR measurements show unambiguously that Eu2+ ions replace the Pb2+ ions and that the crystal field is associated with an octahedron of Se2+ ions in the Pb1-xEuxSe epitaxial layer. The data were fitted with the classical cubic spin Hamiltonian. The crystal field coefficients are b4 = 267.4 MHz and b6=23.4 MHz. Low-temperature experiments do not reveal the existence of strain in the epitaxial layers and in the superlattices. The effect of the europium concentration on the EPR spectrum can be characterized by (i) the vanishing of the fine structure, (ii) the appearance of a strong central transition, (iii) the broadening of the transition due to the Eu2+ nearest-neighbor dipolar and exchange interactions.

X. Gratens, S. Isber and S. Charar

Electron paramagnetic resonance (EPR) of Eu2+ ions in Pb1−xEuxSe layers grown by molecular beam epitaxy on Si substrate has been investigated. The measurements were performed on layers with thickness around 1.5 micro-meter and europium concentration x=0.01, 0.06, 0.08, and 0.10. The spin Hamiltonian (SH) parameters of
the single ion anisotropy were obtained from the sample with x=0.01. Room temperature (300 K) EPR measurements showed that the site symmetry of the Eu2+ ions is cubic and the SH parameters were found to be g=1.970±0.001, b4=270.5±0.5 MHz, and b6=−2.0±0.5 MHz. However, at T=4.2 K, it was found that the cubic symmetry is axially distorted along the [111] direction, perpendicular to the layer surface. The change in the EPR spectra at 4.2 K was interpreted by incorporating an axial component in the SH with b2=−41±3 MHz. The observed distortion was attributed to strain induced by thermal expansion caused by a mismatch between the layer and the substrate. For x=0.01, we also observed a partially resolved hyperfine
structure due to the 151Eu and 153Eu isotopes and a structure of lines due to Eu-Eu pair spectra. For samples with higher Eu concentration x=0.06 and x=0.08, the spectrum consisted of the isolated Eu2+ ions EPR lines superimposed on a broad single isotropic line at g~2, which was attributed to signal from Eu clusters. For the
sample with x=0.10, we only observed a single broad line centered on g~2. The number of single Eu2+ ions was determined from the EPR lines intensities at T=300 K. The results show that the distribution of the Eu2+ over the cation sites is not random.