Concept of the Critical Nucleus in Nucleation

Reconsideration on the concept of critical nucleus for single component systems leads to the result that the size n^h_K of a kinetic critical nucleus for which the probabilities of its decay and growth balance is not equal to the size n^ of the thermodynamic one for which the reversible work of nucleus formation takes the maximum value. n^h_K is in general smaller than n^, and there exist two values for n^h_K, the larger is kinetically unstable but the smaller is stable. The difference between n^ and the larger n^h_K increases but the difference between the two values of n^h_K decreases with the degree of supersaturation or supercooling, and in the critical state two values of n^h_K coincide and it diminishes to 8/27 of n^ for three dimensional homogeneous nucleation and to 1/4 of n^ for two dimensional disc nucleation on a substrate. Beyond this critical state n^h_K does not exist and for a nucleus with any size the probabilty of growth is higher than that of decay

Investigations of the role of disorder in heavy fermion compounds

The purpose of this work was to investigate the role of disorder in strongly correlated electron systems. From our study of Fe2VAl and the two heavy fermion compounds UPd2-xSn and URh2Ge2-xSnx we conclude that in these materials the disorder can control the physical properties. For Fe2VAl, we have disproved a Kondo-insulating behavior as was previously proposed. The Kondo-insulating behavior does not play a role in the anomalous resistivity of Fe2VAl, instead the 'semiconducting-like' behavior is due to the ability of Al-vacancies to localize the electrons. For the heavy fermion compounds UPd2-xSn we have observed that disorder drives the anomalous physical behavior. Surprisingly, the levels of disorder for cubic and orthorhombic UPd2-xSn are similar, but the 'efficiency' of disorder appears to be higher in the first case. A description of the resistivity, magnetoresistivity and Hall effect of cubic UPd2-xSn in terms of conventional heavy fermion behavior does not consistently account for our observations. Alternatively, we attributed it to disorder induced diffusive electronic transport. Further, for cubic UPd2-xSn we have determined the magnetic structure and ground state, which is characterized by small antiferromagnetic domains with correlation lengths of about ~100Å. Finally, we have studied in detail the influence of crystallographic disorder on the class of compounds URh2Ge2-xSnx. Regarding the electronic transport properties, we find for all samples/ground states a non-metallic behavior. Our Hall effect measurements disproves a (pseudo)gap scenario of the anomalous electronic transport. We conclude that the 'semiconducting-like' resistivity arises from disorder induced localization. Finally, we observe a close resemblance of the Hall-constants of UPd1.85Sn and URh2Ge2-xSnx , RH-R0 ~ χ, which probably indicates that this phenomenon is of more general relevance.Ziel der vorliegenden Arbeit war die Klärung des Einflusses von Unordnung auf die Eigenschaften stark korrelierter Elektronensysteme. Für Fe2VAl wurde gezeigt, dass dieses Material nicht – wie behauptet - als Kondo-Isolator zu betrachten ist. Stattdessen können die Materialeigenschaften von Fe2VAl auf Unordnungseinflüsse zurückgeführt werden. Hierbei wurden Art und Grad der Unordnung bestimmt. Weiterhin wurde für das Schwer-Fermion-System UPd2-xSn nachgewiesen, dass das hier beobachtete anomale physikalische Verhalten durch kristallographische Unordnung getriggert wird. Entsprechend wird das Verhalten von kubischen UPd2-xSn komplett durch die kristallographische Unordnung kontrolliert. So zeigt sich, dass eine Beschreibung des Widerstandes, Magnetowiderstandes sowie Hall-Effektes nicht im Rahmen konventioneller Schwer-Fermionen-Terminologie möglich ist. Weiterhin haben wir den magnetischen Grundzustand bestimmt, welcher antiferromagnetisch ist, allerdings mit einer aufgrund der kristallographischen Unordnung kleinen Kohärenzlänge von ~100Å. In der Ausgangsverbindung URh2Ge2 kann der Grad der Unordnung auf verschiedene Weisen variiert werden – durch unterschiedliche Abkühlgeschwindigkeiten während der Probenherstellung, Wärmebehandlung oder isoelektronische Substitution. Hinsichtlich der elektronischen Transporteigenschaften finden wir für alle Proben/Grundzustände ein nicht-metallisches Verhalten. Aufgrund von Hall-Effektmessungen kann eine (Pseudo)Energielücke als Quelle dieses Verhaltens ausgeschlossen werden. Daher folgern wir, dass dieses Verhalten aus unordnungsinduzierter elektronischer Lokalisierung entspringt. Spinabhängige Streuung scheint von sekundärer Bedeutung für den elektronischen Transport zu sein, trägt allerdings zum Magnetowiderstand bei. Schliesslich beobachten wir ein sehr ähnliches Verhalten der Hall-Konstanten für UPd1.85Sn und URh2Ge2-xSnx , RH-R0 ~ χ, was möglicherweise ein Hinweis auf ein allgemeineres Phänomen darstellen könnte

Theoretical and numerical studies of nucleation kinetics

この論文は国立情報学研究所の電子図書館事業により電子化されました。Reconsideration on the concept of critical nucleus for single component systems leads to the result that the size n_k of a kinetic critical nucleus for which the probabilities of its decay and growth balance is not equal to the size n^* of the thermodynamic one for which the reversible work of nucleus formation takes the maximum value. n_k is in general smaller than n^*, and there exist two values for n_k, the larger is kinetically unstable but the smaller is stable. The difference between n^* and the larger n_k increases but the difference between the two values of n_k decreases with supersaturation and or temperature, and at the critical state two values of n_k coincide and it diminishes to 8/27 of n^* for three dimensional homogeneous nucleation and to 1/4 of n^* for two dimensional disc nucleation on a substrate. Beyond this critical state n_k does not exist and for a nucleus with any size the probabilty of growth is higher than that of decay. The height of the nucleation barrier, i.e., the reversible work of critical nucleus formation, is found to be the main parameter quantitatively controlling the distinction between n^* and n_k. It is shown that when the distinction between the two kinds of the critical nuclei is significant, the attachment and the detachment rates of monomers do not differ appreciably

Mathematical Simulation of Convective Heat Transfer in the Low-Temperature Storage of Liquefied Natural Gas

The article shows the results of mathematical modeling of convective heat transfer in the low-temperature storage of liquefied natural gas. Regime of natural convection in an enclosure with different intensity of the heat flux at the external borders are investigated. Was examined two-dimensional nonstationary problem within the model of Navier-Stokes in dimensionless variables "vorticity - stream function - temperature". Distributions of hydrodynamic parameters and temperatures that characterize the basic regularities of the processes are obtained. Circulating flows are determined and carried out the analysis of vortices formation mechanism and the temperature distribution in solution at conditions of natural convection when the Grashof number (Gr=10{6}). A significant influence of heat transfer rate on solutions boundary on flow structure and temperature field in LNG storage tanks

Genuine Saddle Point and Nucleation Potential for Binary Systems

A generalized nucleation potential is constructed for binary systems. The potential consists of the reversible work of cluster formation plus additional terms arising from various kinetic effects. We show that the major nucleation flux passes through the saddle point (termed the genuine saddle point) of this generalized nucleation potential. The generalized nucleation potential reduces to the kinetic potential of a unary system when one component vanishes. The genuine saddle point concept provides a convenient way to identify systems and conditions for which the ridge crossing phenomenon occurs. Our theory agrees approximately with exact numerical results

Numerical Simulation of the Kinetic Critical Nucleus

Our main interest is to see whether the number density indicates a peak at the kinetically stable critical nucleus due to its kinetical stability. We have numerically calculated the time evolution of the number densities of clusters in the case of water vapor nucleation. We employ the condition in which the difference between the size of the thermodynamic crtitical nucleus and that of the kinetic one is appreciable. The results show that the peak does not appear in the number densities of clusters. The reason is thought to be that the kinetical stability is not strong enough in our condition

Field-Dependent Critical Current in Type-II Superconducting Strips: Combined Effect of Bulk Pinning and Geometrical Edge Barrier

Recent theoretical and experimental research on low-bulk-pinning superconducting strips has revealed striking dome-like magnetic-field distributions due to geometrical edge barriers. The observed magnetic-flux profiles differ strongly from those in strips in which bulk pinning is dominant. In this paper we theoretically describe the current and field distributions of a superconducting strip under the combined influence of both a geometrical edge barrier and bulk pinning at the strip's critical current Ic, where a longitudinal voltage first appears. We calculate Ic and find its dependence upon a perpendicular applied magnetic field Ha. The behavior is governed by a parameter p, defined as the ratio of the bulk-pinning critical current Ip to the geometrical-barrier critical current Is0. We find that when p > 2/pi and Ip is field-independent, Ic vs Ha exhibits a plateau for small Ha, followed by the dependence Ic-Ip ~ 1/Ha in higher magnetic fields.Comment: 4 pages, 2 figures, Fig. 1 revised, submitted to Phys. Rev.

Analysis of influence of heat insulation on the thermal regime of storage tanks with liquefied natural gas

Is numerically investigated the process of convective heat transfer in the reservoirs of liquefied natural gas (LNG). The regimes of natural convection in a closed rectangular region with different intensity of heat exchange at the external borders are investigated. Is solved the time-dependent system of energy and Navier-Stokes equations in the dimensionless variables “vorticity – the stream function”. Are obtained distributions of the hydrodynamic parameters and temperatures, that characterize basic regularities of the processes. The special features of the formation of circulation flows are isolated and the analysis of the temperature distribution in the solution region is carried out. Is shown the influence of geometric characteristics and intensity of heat exchange on the outer boundaries of reservoir on the temperature field in the LNG storage

Paucity and preferential suppression of transgenes in late replication domains of the D. melanogaster genome

<p>Abstract</p> <p>Background</p> <p>Eukaryotic genomes are organized in extended domains with distinct features intimately linking genome structure, replication pattern and chromatin state. Recently we identified a set of long late replicating euchromatic regions that are underreplicated in salivary gland polytene chromosomes of <it>D. melanogaster</it>.</p> <p>Results</p> <p>Here we demonstrate that these underreplicated regions (URs) have a low density of <it>P</it>-<it>element </it>and <it>piggyBac </it>insertions compared to the genome average or neighboring regions. In contrast, <it>Minos</it>-based transposons show no paucity in URs but have a strong bias to testis-specific genes. We estimated the suppression level in 2,852 stocks carrying a single <it>P</it>-<it>element </it>by analysis of eye color determined by the mini-<it>white </it>marker gene and demonstrate that the proportion of suppressed transgenes in URs is more than three times higher than in the flanking regions or the genomic average. The suppressed transgenes reside in intergenic, genic or promoter regions of the annotated genes. We speculate that the low insertion frequency of <it>P-elemen</it>ts and <it>piggyBac</it>s in URs partially results from suppression of transgenes that potentially could prevent identification of transgenes due to complete suppression of the marker gene. In a similar manner, the proportion of suppressed transgenes is higher in loci replicating late or very late in Kc cells and these loci have a lower density of <it>P-elements </it>and <it>piggyBac </it>insertions. In transgenes with two marker genes suppression of mini-<it>white </it>gene in eye coincides with suppression of <it>yellow </it>gene in bristles.</p> <p>Conclusions</p> <p>Our results suggest that the late replication domains have a high inactivation potential apparently linked to the silenced or closed chromatin state in these regions, and that such inactivation potential is largely maintained in different tissues.</p