Among the many non-equilibrium processes encountered in nature we deal with two different but related aspects. One is the non-equilibrium relaxation process that is at the origin of \'aging phenomena••, and the other one is a non-equilibrium phase transition, called ••dynamic phase transition••. One of the main purposes of our research is to explore more realistic situations than studied previously. Indeed, in the study of aging phenomena certain kinds of disorder effects are considered, and we introduce the ••surface•• as a spatial boundary to the system undergoing the dynamic phase transition. In order to observe these processes as clearly as possible, we study in both cases simple spin systems. Using Monte Carlo simulations we first investigate aging in three-dimensional Ising spin glasses as well as in two-dimensional Ising models with disorder quenched to low temperatures. The time-dependent dynamical correlation length L(t) is determined numerically and the scaling behavior of various two-time quantities as a function of L(t)/L(s) is discussed where t and s are two different times. For disordered Ising models deviations of L(t) from algebraic growth law show up. The generalized scaling forms as a function of L(t)/L(s) reveal a generic simple aging scenario for Ising spin glasses as well as for disordered Ising ferromagnets. We also study the local critical phenomena at a dynamic phase transition by means of numerical simulations of kinetic Ising models with surfaces subjected to a periodic oscillating field. We examine layer-dependent quantities, such as the period-averaged magnetization per layer Q(z) and the layer susceptibility ¥ö(z), and determine local critical exponents through finite size scaling. Both for two and three dimensions, we find that the values of the surface exponents differ from those of the equilibrium critical surface. It is revealed that the surface phase diagram of the non-equilibrium system is not identical to that of the equilibrium system in three dimensions. / Ph. D.

権利表示を行うこと, 出典表示を行うこと, 出版社版へのリンクを表示すること / 京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19488号 / 理博第4148号 / 新制||理||1596(附属図書館) / 32524 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 大西 明, 准教授 菅沼 秀夫, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21104号 / 工博第4468号 / 新制||工||1694(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 杉村 博之, 教授 田中 功, 准教授 野瀬 嘉太郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20915号 / 理博第4367号 / 新制||理||1627(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 菅沼 秀夫, 教授 田中 貴浩, 教授 川合 光 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

The metamagnetic transition from an antiferromagnetic (AF) to the ferromagnetic (FM) state in FeRh alloys and the accompanying magnetocaloric effect (MCE) have been investigated with a particular attention to the sample preparation routes. Direct measurements of the adiabatic temperature change show that the MCE in FeRh remains partly reversible despite the hysteresis and exceeds the effect in the benchmark material Gd by 15 %. The AF−FM transition is strongly affected by the microstructure that is formed depending on the heat treatment parameters. This can explain the discrepancy in the reported data over 80 years of research. The effect on the magnetic properties is found to originate from the interaction of the major α'-phase with the secondary γ-phase that has been typically ignored for its negligible magnetic contribution. The nominal composition of the magnetic α'-phase is found to differ from the actual one for binary and substituted FeRh alloys. The elements can be redistributed within the two phases in such a way, that the actual amount of the doping element in the α'-phase that experiences the AF−FM transition is greatly reduced. This demonstrates the significance of microstructural studies, especially when comparing experimental results with theoretical calculations and developing routes to tune and optimize the magnetocaloric properties of materials.

Spherical agglomerates of benzoic acid crystals have been successfully prepared by drowning-out crystallization in three solvent partial miscible mixtures. Benzoic acid is dissolved in ethanol, bridging liquid is added and this mixture is fed to the agitated crystallizer containing water. Fine crystals are produced by crystallization of the substance, and the crystals are agglomerated by introduction of an immiscible liquid called the bridging liquid. The concentration of solute, agitation rate, feeding rate, amount of bridging liquid and temperature are found to have a significant influence on the physico-mechanical properties of the product. The product particle characterization includes the particle size distribution, morphology and mechanical strength. Many of the solvents such as chloroform, toluene, pentane, heptane, cyclo hexane, diethyl ether and ethyl acetate were used as bridging liquids. Among the selected solvents ethyl acetate and di ethyl ether could not form the spherical agglomerates. Characteristics of the particles are changing with the bridging liquid used. Range of the operation for spherical agglomeration is very narrow and was shown that only at certain conditions the spherical agglomerates are produced. Increased amount of bridging liquid, decrease in feeding rate and temperature causes the particle size to increase. Particle morphology depends on the bridging liquid used, amount of bridging liquid and the temperature. Particles look completely spherical from the experiments where toluene is used as bridging liquid. The mechanical strength of single agglomerates has been determined by compression in a materials testing machine, using a 10N load cell. For single particle compression an approximate estimation of the true stress is presented. Compression characteristics for single agglomerates are compared with data on particle bed compression. Low elastic recovery and high compressibility of the single particles and of bed of particles reveals that the spherical agglomerates prepared in this work have a plastic behavior which is expected to be favorable for direct tabletting. Some of the stress–strain curves are J-shaped with no clear fracturing of the particles, and are well correlated by an exponential–polynomial equation. / QC 20101119

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Topological materials have discovered ultrahigh magnetoresistance, chiral anomalies, the inherent anomalous Hall effect, and unique Fermi arc surface states. Topological materials now include insulators, metals, and semimetals. Weyl semimetals (WSM) are topological materials that show linear dispersion with crossings in their band structure which creates the pair of Weyl nodes of opposite chirality. WSMs have topological Fermi arc surface states connecting opposing chirality Weyl nodes. Spin-orbit coupling can result in the band opening in Dirac nodal rings, and creating the pair of Weyl nodes either by breaking the time-reversal or spatial inversion symmetry (but not both) 1-3. The chirality of a Weyl node is set by the Berry flux through a closed surface in reciprocal space around it. The purpose of this thesis was to characterize and investigate the thermodynamic stability of WSM. To accomplish these goals, quantum mechanical modeling at the level of density functional theory (DFT) was used. WloopPHI, a Python module, integrates the characterization of WSMs into WIEN2k, a full-potential all-electron density functional theory package. It calculates the chirality of the Weyl node (monopole charge) with an enhanced Wilson loop method and Berry phase approach. First, TaAs, a well-characterized Weyl semimetal, validates the code theoretically. We then used the approach to characterize the newly discovered WSM YRh6Ge4, and we found a set of Weyl points into it. Further, we study the stability of the kagome-based materials A3Sn2S2, where A is Co, Rh, or Ru, in the context of the ternary phase diagrams and competing binary compounds using DFT. We demonstrated that Co3Sn2S2 and Rh3Sn2S2 are stable compounds by examining the convex hull and ternary phase diagrams. It is feasible to synthesize Co3Sn2S2 by a chemical reaction between SnS, CoSn and Co9S8. Moreover, Rh3Sn2S2 can be produced by SnS, RhSn and Rh3S4. On the other hand, we found that Ru3Sn2S2 is a thermodynamically unstable material with respect to RuS2, Ru3S7 and Ru. Our work provides some insights for confirming materials using the DFT approach. 1. S. M. Young et al. Dirac Semimetal in Three Dimensions. Physical Review Letters108(14) (2012), 140405. 2. J. Liu and D. Vanderbilt. Weyl semimetals from noncentrosymmetric topological insulators. Physical Review B 90(15) (2014), 155316. 3. H. Weng et al. Weyl Semimetal Phase in Noncentrosymmetric Transition-Metal Monophosphides. Physical Review X 5(1) (2015), 011029. / Thesis / Master of Applied Science (MASc)

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