CHANGE IN THE STRUCTURE OF TITANIUM NANOCLUSTERS UNDER THERMAL EXPOSURE: MOLECULAR DYNAMIC SIMULATION

Исследование структуры нанокластеров при различных температурах является актуальной задачей современного материаловедения. Данный факт обусловлен перспективой их применения при создании материалов с уникальными физическими, механическими, химическими и эксплуатационными свойствами. Компьютерное моделирование проводилось методом классической молекулярной динамики в программном комплексе LAMMPS. Для описания межатомного взаимодействия в кластере использовалась модификация многочастичного потенциала Финниса-Синклера. Проведено изучение структуры нанокластеров титана различного размера. Они получены при различных скоростях охлаждения из жидкого состояния. Увеличение скорости охлаждения приводит к формированию субблочной структуры и росту числа атомов с неупорядоченным окружением. Они обусловлены тем, что большие скорости охлаждения препятствуют равновесному протеканию процессов перестройки атомной структуры с формированием дальнего порядка. Областей с икосаэдрической структурой не обнаружено. Показано, что температура кристаллизации и энергия связи уменьшаются при убывании размера нанокластера. Рост скорости охлаждения увеличивает разницу температур точек начала и конца кристаллизации, соответственно. Результаты моделирования свидетельствуют о менее выраженной размерной зависимости температуры кристаллизации - её оценочное значение для макроскопической системы (810 К) гораздо ниже значения для массивного титана (1940 К). Investigation of the structure of nanoclusters at different temperatures is an urgent task of modern materials science. This fact is due to the prospect of their application in the creation of materials with unique physical, mechanical, chemical and operational properties. Computer simulation was carried out by the method of classical molecular dynamics in the LAMMPS software package. To describe the interatomic interaction in the cluster, a modification of the Finnis-Sinclair many-body potential was used. The structure of titanium nanoclusters of various sizes has been studied. They are obtained at various cooling rates from the liquid state. An increase in the cooling rate leads to the formation of a subblock structure and an increase in the number of atoms with a disordered environment. They are due to the fact that high cooling rates impede the equilibrium process of rearrangement of the atomic structure with the formation of long-range order. No regions with an icosahedral structure were found. It is shown that the crystallization temperature and binding energy decrease with decreasing nanocluster size. An increase in the cooling rate increases the temperature difference between the start and end points of crystallization, respectively. The simulation results indicate a less pronounced dimensional dependence of the crystallization temperature - its estimated value for a macroscopic system (810 K) is much lower than the value for bulk titanium (1940 K). Keywords: nanocluster, binding energy, crystallization temperature, cooling rate, structure, molecular dynamics method.

A microscopic model of wave-function decoherence in the double-slit experiment

The act of measurement on a quantum state is supposed to “decohere” and “collapse” the state into one of several eigenstates of the operator corresponding to the observable being measured. This measurement process is sometimes described as outside standard quantum-mechanical evolution and not calculable from Schr¨odinger’s equation [2]. Progress has, however, been made in studying this problem with two main calculation tools - one uses a time-independent Hamiltonian [18], while a rather more general approach proving that decoherence occurs under some generic conditions [21]. The two general approaches to the study of wave-function collapse are as follows. The first approach, called the “consistent” or “decoherent”’ histories approach [11], studies microscopic histories that diverge probabilistically and explains collapse as an event in our particular history. The other, referred to as the “environmental decoherence” approach[8, 21] studies the effect of the environment upon the quantum system, to explain wave-function decoherence. Then collapse is produced by irreversible effects of various sorts. In the “environmental decoherence” approach, one writes down a Markovian-approximated Master equation to study the time-evolution of the reduced density matrix and obtains the long-time dependence of the off-diagonal elements of this matrix. The calculation in this paper studies the evolution of a quantum system under the “environmental” approach, with a rather important analytic difference. We start from the Schr¨odinger equation for the state of the system, with a time-dependent Hamiltonian that reflects the actual microscopic interactions that are occurring. Then we systematically solve for the time-evolved state, without invoking a Markovian approximation when writing out the effective time-evolution equation, i.e., keeping the evolution unitary until the end. This approach is useful and it allows the system wave-function to explicitly “un-collapse” if the measurement apparatus is sufficiently small. However, in the limit of a macroscopic system, collapse is a temporary state that will simply take extremely long (of the order of multiple universe lifetimes) to reverse. While this has been attempted previously [12], we study a particularly simple and calculable example. We make some connections to the work by Linden et al [21] while doing so. The calculation in this paper has interesting implications for the interpretation of the Wigner’s friend experiment, as well as the Mott experiment, which is explored in Sections V and VI (especially the enumerated points in Section VI). The upshot is that as long as Wigner’s friend is macroscopically large (or uses a macroscopically large measuring instrument), no one needs to worry that Wigner would see something different from his friend. Indeed, Wigner’s friend does not even need to be conscious during the measurement that she conducts. In particular, as a result of the mathematical analysis, the short-time behavior of a collapsing system, at least the one considered in this paper, is not exponential. Instead, it is the usual Fermigolden rule result. The long-term behavior is, of course, still exponential. This is a second novel feature of the paper - we connect the short-term Fermi-golden rule (quadratic-in-time behavior) transition probability to the exponential long-time behavior of a collapsing wave-function in one continuous mathematical formulation.

Multiscale modeling of glioma pseudopalisades: contributions from the tumor microenvironment

Gliomas are primary brain tumors with a high invasive potential and infiltrative spread. Among them, glioblastoma multiforme (GBM) exhibits microvascular hyperplasia and pronounced necrosis triggered by hypoxia. Histological samples showing garland-like hypercellular structures (so-called pseudopalisades) centered around the occlusion site of a capillary are typical for GBM and hint on poor prognosis of patient survival. We propose a multiscale modeling approach in the kinetic theory of active particles framework and deduce by an upscaling process a reaction-diffusion model with repellent pH-taxis. We prove existence of a unique global bounded classical solution for a version of the obtained macroscopic system and investigate the asymptotic behavior of the solution. Moreover, we study two different types of scaling and compare the behavior of the obtained macroscopic PDEs by way of simulations. These show that patterns (not necessarily of Turing type), including pseudopalisades, can be formed for some parameter ranges, in accordance with the tumor grade. This is true when the PDEs are obtained via parabolic scaling (undirected tissue), while no such patterns are observed for the PDEs arising by a hyperbolic limit (directed tissue). This suggests that brain tissue might be undirected - at least as far as glioma migration is concerned. We also investigate two different ways of including cell level descriptions of response to hypoxia and the way they are related .

Microscopic Insights into Charge Formation and Energetics in N-Doped Organic Semiconductors

<p>In the molecular doping of organic semiconductors</p><p>(OSC), achieving efficient charge generation</p><p>and managing the energetic cost for charge</p><p>release from local molecular charge transfer</p><p>complexes (CTCs) to the host matrix is of</p><p>central importance. Experimentally tremendous</p><p>progress has been made in this direction.</p><p>However, the relation between OSC film</p><p>structure on a nanoscopic level including different</p><p>inter-molecular geometrical arrangements</p><p>and the macroscopic properties of doped OSC</p><p>films is usually only established quite indirectly.</p><p>Explicit microscopic insights into the underlying</p><p>doping mechanisms and resulting electronic</p><p>structure are still scarce and mostly limited</p><p>to the study of the individual molecular constituents</p><p>or isolated bi-molecular dopant:host</p><p>complexes. In the present study we investigate</p><p>n-type doping of the frequently investigated</p><p>OSC materials ZnPC and F8ZnPc and</p><p>their mixtures which are n-doped with 2-Cyc-</p><p>DMBI. We report significant electronic differences</p><p>for complexes with nominally the same</p><p>material composition but different geometrical</p><p>structures. One specific important finding in</p><p>this context is that complexes containing two</p><p>adjacent dopant molecules show much reduced</p><p>ionization energy values, leading to substantially</p><p>reduced energy cost for charge release. Furthermore our results demonstrate that important</p><p>trends towards macroscopic system behavior</p><p>can already be obtained with increasing</p><p>size and varying composition of the relatively</p><p>small molecular dopant-host complexes considered,</p><p>including systematic shifts in the Fermi</p><p>level energies in the doped OSC.</p>

Microscopic Insights into Charge Formation and Energetics in N-Doped Organic Semiconductors

<p>In the molecular doping of organic semiconductors</p><p>(OSC), achieving efficient charge generation</p><p>and managing the energetic cost for charge</p><p>release from local molecular charge transfer</p><p>complexes (CTCs) to the host matrix is of</p><p>central importance. Experimentally tremendous</p><p>progress has been made in this direction.</p><p>However, the relation between OSC film</p><p>structure on a nanoscopic level including different</p><p>inter-molecular geometrical arrangements</p><p>and the macroscopic properties of doped OSC</p><p>films is usually only established quite indirectly.</p><p>Explicit microscopic insights into the underlying</p><p>doping mechanisms and resulting electronic</p><p>structure are still scarce and mostly limited</p><p>to the study of the individual molecular constituents</p><p>or isolated bi-molecular dopant:host</p><p>complexes. In the present study we investigate</p><p>n-type doping of the frequently investigated</p><p>OSC materials ZnPC and F8ZnPc and</p><p>their mixtures which are n-doped with 2-Cyc-</p><p>DMBI. We report significant electronic differences</p><p>for complexes with nominally the same</p><p>material composition but different geometrical</p><p>structures. One specific important finding in</p><p>this context is that complexes containing two</p><p>adjacent dopant molecules show much reduced</p><p>ionization energy values, leading to substantially</p><p>reduced energy cost for charge release. Furthermore our results demonstrate that important</p><p>trends towards macroscopic system behavior</p><p>can already be obtained with increasing</p><p>size and varying composition of the relatively</p><p>small molecular dopant-host complexes considered,</p><p>including systematic shifts in the Fermi</p><p>level energies in the doped OSC.</p>

Higher order interactions in complex networks of phase oscillators promote abrupt synchronization switching

Synchronization processes play critical roles in the functionality of a wide range of both natural and man-made systems. Recent work in physics and neuroscience highlights the importance of higher-order interactions between dynamical units, i.e., three- and four-way interactions in addition to pairwise interactions, and their role in shaping collective behavior. Here we show that higher-order interactions between coupled phase oscillators, encoded microscopically in a simplicial complex, give rise to added nonlinearity in the macroscopic system dynamics that induces abrupt synchronization transitions via hysteresis and bistability of synchronized and incoherent states. Moreover, these higher-order interactions can stabilize strongly synchronized states even when the pairwise coupling is repulsive. These findings reveal a self-organized phenomenon that may be responsible for the rapid switching to synchronization in many biological and other systems that exhibit synchronization without the need of particular correlation mechanisms between the oscillators and the topological structure.