The LiHo_xY_{1-x}F_4 magnetic material in a transverse magnetic field B_x perpendicular to the Ising spin direction has long been used to study tunable quantum phase transitions in pure and random disordered systems. We first present analytical and numerical evidences for the validity of an effective spin-1/2 approach to the description of a general dipolar spin glass model with strong uniaxial Ising anisotropy and subject to weak B_x. We relate this toy model to the LiHo_xY_{1-x}F_4 transverse field Ising material. We show that an effective spin-1/2 model is able to capture both the qualitative and quantitative aspects of the physics at small B_x. After confirming the validity of the effective spin-1/2 approach, we show that the field-induced magnetization along the x direction, combined with the local random dilution-induced destruction of crystalline mirror symmetries generates, via the predominant dipolar interactions between Ho^{3+} ions, random fields along the Ising z direction. This identifies LiHo_xY_{1-x}F_4 in B_x as a new random field Ising system. We show that the random fields explain the smearing of the nonlinear susceptibility at the spin glass transition with increasing B_x. In this thesis, we also investigate the phase diagram of non-diluted LiHoF_4 in the presence of B_x, by performing Monte-Carlo simulations. A previous quantum Monte Carlo (QMC) simulation found that even for small B_x where quantum fluctuations are small, close to the classical critical point, there is a discrepancy between experiment and the QMC results. We revisit this problem, focusing on weak B_x close to the classical T_c, using an alternative approach. For small B_x, by applying a so-called cumulant expansion, the quantum fluctuations around the classical T_c are taken into account perturbatively. We derived an effective perturbative classical Hamiltonian, on which MC simulations are performed. With this method we investigate different proposed sources of uncertainty which can affect the numerical results. We fully reproduce the previous QMC results at small B_x. Unfortunately, we find that none of the modifications to the microscopic Hamiltonian that we explore are able to provide a B_x-T phase diagram compatible with the experiments in the small semi-classical B_x regime.

The level set method offers a simple and robust approach to modeling liquid-vapor interfaces that arise in boiling and condensing flows. The current liquid-vapor phase-transition techniques used with the level set method are not able to account for different thermal conductivities and specific heats in each respective phase, nor are they able to accurately account for latent heat absorption and release. This paper presents a new level set based technique for liquid-vapor phase-transition that accounts for different material properties in each respective phase, such as thermal conductivity and specific heat, while maintaining the interface at the saturation temperature. The phase-transition technique is built on the ghost fluid framework coupled with the standard level set method. A new technique is presented for constructing ghost nodes that implicitly captures the immersed boundary conditions and is second order accurate. The method is tested against analytical solutions, and it is used to model film boiling. The new phase-transition technique will greatly assist efforts to accurately capture the physics of boiling and condensing flows. In addition to presenting a new phase transition technique, a coupled level set volume of fluid advection scheme is developed for phase transition flows. The new scheme resolves the mass loss problem associated with the level set method, and the method provides an easy way to accurately calculate the curvature of an interface, which can be difficult with the volume of fluid method. A film boiling simulation is performed to illustrate the superior performance of the coupled level set volume of fluid approach over the level set method and the volume of fluid method.

Theoretical strength of solids is defined as the ultimate strength beyond which plastic deformation, fracture, or decohesion would occur. Understanding the microscopic origin from quantum mechanics and thermoelastic formulation is of great importance to mechanical properties and engineering design of various solids. While quite a few theory models have been made in the past century by several generations of scientists, including Frankel and Born, a general and convincing framework has not been fully established. We study this issue from three respects: (1) Unify various elastic stability criteria for solids that determine an upper bound of theoretical strength; (2) with ab initio method, we test the elastic stability conditions of crystal Au. The phenomenon of bifurcation is observed: under hydrostatic expansion, the rhombohedral modulus reaches zero first of all; while under uniaxial tensile stress, the tetragonal shear modulus first reaches zero; (3) propose a nonlinear theoretical formulation of stability criterion. As an analytic method, this scheme is quite simple, in the mean time, it saves computation resource.

Metal oxides crystallized in perovskite structure are generally modified in two different ways. According to the general structural formula ABO3, the two ways are A-site modification and B-site modification. The primary significance of perovskite metal oxides rests on their importance in electronic devices. A particular class of perovskites, namely Lead Zirconate or modified Lead Zirconate has received a special attention because of their unique antiferroelectricity and various applications in devices. Among the other modifications, A-site modification of PbZrO3 by La is rare and not much explored. Chapter 1 describes various applications of antiferroelectric thin films along with the synthesis and characterization of pure and La modified PbZrO3, which are relevant to the work presented in this thesis. Sol-gel processing and spin coating technique to deposit solid oxide thin films are well known for their low cost of deposition as well as for their ability to achieve better stoichiometric chemical composition. Common crack formation problem of sol-gel grown films can be prevented by ‘drying control chemical adhesive’ like polyvinylpyrrolidone (PVP). Heat treatment of sol-gel derived thin films is generally determined by TGA and DTA. Crystalline phase of deposited solid thin films is determined by XRD whereas effect of modification can be ascertained by XRD peak assignment and relative crystalline peak shifting. Sol-gel grown film thickness is measured by common cross sectional SEM whereas AFM can detail the surface morphology. Chapter 2 summarizes the deposition and characterization of pure and La modified PbZrO3 thin films. Any nonmetal, which is insulator, is dielectric material and show dielectric dispersion in a frequency domain of low field alternative current. Among the most common feature of dielectric dispersion, Maxwell – Wagner type dispersion is well known. Similar kind of dielectric dispersion, named Maxwell – Wagner like dispersion, can be observed while the equivalent circuit consists of parallel G – C along with a series R. Universal power law of ac conductivity is the deciding factor to distinguish the nature of dispersion. Structural phase transition can be determined by dielectric response and it is widely known as dielectric phase transition. Effect of La modification on dielectric phase transition of PbZrO3 thin films depends on stabilization or destabilization of antiferroelectricity. Maximum dielectric constants of pure and modified PbZrO3 thin films depend on the crystallographic orientations of the growth. Chapter 3 presents dielectric properties of pure and La modified PbZrO3 thin films and these properties are correlated to the stabilization or destabilization of antiferroelectricity, relative integrated intensity of (202)O film orientation and trapped electron charge due to oxygen vacancies. Charge storage property of a capacitor is determined by the polarization of the capacitor on application of electric field whereas field dependent integrated area of polarization on withdrawal of electric field determines the recoverable capacitive energy storage. Among the three kinds of capacitors like linear or paraelectric, ferroelectric and antiferroelectric capacitors, antiferroelectric capacitor is known to be best for their ability to store huge amount of recoverable energy. The recoverable energy in antiferroelectrics can be increased by increasing squareness of the P – E hysteresis loop, applicable electric field, polarization or by the all possible combinations of them. Chapter 4 describes the correlation of relative integrated intensity of (202)O [RI(202)O] with critical applied electric field of P – E saturation to provide enhanced squareness of the hysteresis loops. This chapter also describes the variation of charge and recoverable energy storage properties with respect to RI(202)O. Like magnetocaloric effect, electrocaloric effect is capable to alter the temperature of a system by adiabatic polarization or depolarization. From the Maxwell’s relation of thermodynamics, assuming, (∂p ) = (∂s )electrocaloric effect can be calculated from temperature dependent polarization value of a paraelectric, ferroelectric or an antiferroelectric. Chapter 5 presents the electrocaloric effect of pure and La modified PbZrO3 thin films. Summary of present study and discussion have been delineated in Chapter 6 along with the future work which can give more insight into the understanding of antiferroelectric PbZrO3 thin films with respect to Pb and Zr site modification and with respect to different electrodes. (For formulas pl see the pdf file of the thesis)

No description available.

Phase transitions and other intrinsic properties (shape, size, architecture) of molecularly structured aerosol particles are important for understanding their role in planetary atmospheres and for technical applications. By combining bath gas cooling with time resolved mid-infrared spectroscopy and modeling, information is obtained on dynamic processes and intrinsic properties of fluoroform and ethane aerosol particles. The distinct infrared spectral features of fluoroform aerosol particles make it a particularly suitable model system. Homogeneous crystallization rates of the sub-micron sized aerosol particles are determined (JV = 10⁸ - 10¹⁰ cm-³s-¹ or JS = 10³ – 10⁵ cm-²s-¹ at a temperature of T = 78 K), and the controversial question regarding volume versus surface nucleation in freezing aerosols is addressed. It is demonstrated that current state of the art measurements of droplet ensembles cannot distinguish between the two mechanisms due to inherent experimental uncertainties. The evolution of particle shape from spherical supercooled droplets to cube-like crystalline particles and eventually to elongated crystalline particles is recorded and analyzed in detail with the help of vibrational exciton model calculations. Phase behaviour of pure ethane aerosols and ethane aerosols formed in the presence of other ice nuclei under conditions mimicking Titan’s atmosphere provide evidence for the formation of supercooled liquid ethane aerosol droplets, which subsequently crystallize. The observed homogeneous freezing rates (JV = 10⁷ – 10⁹ cm-³s-¹) imply that supercooled ethane could play a similar role in ethane rich regions of Titan’s atmosphere as supercooled water does in the Earth’s atmosphere.

Complex organization is found in many biological systems. For example, biopolymers could possess very hierarchic structure, which provides their functional peculiarity. Understating such, complex organization allows describing biological phenomena and predicting molecule functions. Besides, we can try to characterize the specific phenomenon by some probabilistic quantities (variances, means, etc), assuming the primary biopolymer structure to be randomly formed according to some statistical distribution. Such a formulation is oriented toward evolutionary problems.Artificially constructed biological network is another common object of statistical physics with rich functional properties. A behavior of cells is a consequence of complex interactions between its numerous components, such as DNA, RNA, proteins and small molecules. Cells use signaling pathways and regulatory mechanisms to coordinate multiple processes, allowing them to respond and to adapt to changing environment. Recent theoretical advances allow us to describe cellular network structure using graph concepts to reveal the principal organizational features shared with numerous non-biological networks.The aim of this thesis is to develop bunch of methods for studying statistical and dynamic objects of complex architecture and, in particular, scale-free structures, which have no characteristic spatial and/or time scale. For such systems, the use of standard mathematical methods, relying on the average behavior of the whole system, is often incorrect or useless, while a detailed many-body description is almost hopeless because of the combinatorial complexity of the problem. Here we focus on two problems.The first part addresses to statistical analysis of random biopolymers. Apart from the evolutionary context, our studies cover more general problems of planar topology appeared in description of various systems, ranging from gauge theory to biophysics. We investigate analytically and numerically a phase transition of a generic planar matching problem, from the regime, where almost all the vertices are paired, to the situation, where a finite fraction of them remains unmatched.The second part of this work focus on statistical properties of networks. We demonstrate the possibility to define co-expression gene clusters within a network context from their specific motif distribution signatures. We also show how a method based on the shortest path function (SPF) can be applied to gene interactions sub-networks of co-expression gene clusters, to efficiently predict novel regulatory transcription factors (TFs). The biological significance of this method by applying it on groups of genes with a shared regulatory locus, found by genetic genomics, is presented. Finally, we discuss formation of stable patters of motifs in networks under selective evolution in context of creation of islands of "superfamilies".

Le lien entre le spectre de la matrice de transfert de la formulation de spins du modèle de Potts critique et celui de la matrice de transfert double-ligne de la formulation de boucles est établi. La relation entre la trace des deux opérateurs est obtenue dans deux représentations de l'algèbre de Temperley-Lieb cyclique, dont la matrice de transfert de boucles est un élément. Le résultat est exprimé en termes des traces modifiées, qui correspondent à des traces effectuées dans le sous-espace de l'espace de représentation des N-liens se transformant selon la m ième représentation irréductible du groupe cyclique. Le mémoire comporte trois chapitres. Dans le premier chapitre, les résultats essentiels concernant les formulations de spins et de boucles du modèle de Potts sont rappelés. Dans le second chapitre, les propriétés de l'algèbre de Temperley-Lieb cyclique et de ses représentations sont étudiées. Enfin, le lien entre les deux traces est construit dans le troisième chapitre. Le résultat final s'apparente à celui obtenu par Richard et Jacobsen en 2007, mais une nouvelle représentation n'ayant pas été étudiée est aussi investiguée. / The link between the spectrum of the spin transfer matrix of the critical Potts model and that of the double-row transfer matrix of the loop model is established. The relationship between the two operators is obtained in two different representations of the cyclic Temperley-Lieb algebra, whereof the transfer matrix is an element. The result is given in terms of the modified traces that correspond to tracing out the subspace of the N-link representation space that transforms according to the m th representation of the cyclic group. The thesis consists of three chapters. In the first chapter, basic results about the Potts model in the spin and loop pictures are recalled. In the second chapter, the properties of the cyclic Temperley-Lieb algebra and of some of its representations are studied. Finally, the relationship between the traces of the two operators is constructed in the third chapter. The final result is similar to the one obtained by Jacobsen and Richard in 2007, but a new representation that has not been studied is investigated.

The main topic of this thesis is the study of the ultrafast photo-switching of photo-magnetic molecular materials showing transition between spin states. These molecular crystals are prototypes of molecular bistability between two distinct electronic states, HS and LS. The molecules can be switched between these two states by a light pulse. The emergence of ultrafast techniques allows us to study in real time these photo-switching processes and also the associated out-of-equilibrium dynamics down to the femtosecond scale (10-15s). We have combined probes sensitive to the change of electronic state, on the hand, and to structural rearrangements, on the other hand, in order to observe these photo-switching processes. The measurements of ultrafast transient absorption spectroscopy have been made using the laser plateform at the IPR. Complementary time resolved X-ray diffraction and absorption experiments have been performed on large facilities. The first part of this manuscript is focused on the photo-switching dynamics at molecular scale. It reveals a complicated interaction between electronic and structural degrees of freedom. The generation and damping of coherent optical phonons is identified as a key parameter in the trapping in HS potential. Several experiments on different compounds show the linear and local character of such ultrafast photo-switching. The second part of this thesis presents studies on the complete out of equilibrium dynamics. It reveals a cascading process with activation of elastic and thermal effects at different time scales. Cooperative processes following a light excitation are observed. These complexes dynamics are driven by propagating and diffusive process sensitive to the size of the sample. The study of nanocrystals yields high conversion and faster response to elastics effect than single-crystals. These studies further elucidate the out of equilibrium processes underlying the photo-induced phase transitions on time and length scales, from the molecule to material scale.

科学研究費補助金 研究種目:基盤研究(C) 課題番号:11680482 研究代表者:庄司 多津男 研究期間:1999-2001年度