Оценка влияния икосаэдрических "магических" чисел на термическую стабильность малых нанокластеров серебра

A comparative analysis of thermally induced structural transitions in silver nanoclusters, the number of atoms of which corresponded to the “magic” numbers of the icosahedral (Ih) structure with variation of their initial morphology, was carried out by the molecular dynamics method using the modified tight-binding potential TB-SMA. It is shown that, in the case of the initial fcc phase, the formation of the Ih modification, depending on the particle size, occurred either at the stage of preliminary thermal relaxation or during further heating. At the initial amorphous morphology, the nature of the structural transitions underwent significant changes. Thus, even in the case of Ag55 clusters, the icosahedral structure was formed only in 50-60% of the experiments performed. Based on the data obtained, it was concluded that to create a stable Ih structure, it is necessary to use the thermal cycling procedure.

STRUCTURAL STABILITY OF Ag AND Ag NANOCLUSTERS WITH A CHANGE IN THE INITIAL MORPHOLOGY

В статье методом молекулярной динамики с использованием модифицированного потенциала сильной связи TB-SMA (second moment approximation of tight-binding) проводится сравнительный анализ характера термически индуцированных структурных переходов в нанокластерах серебра, число атомов в которых соответствует «магическим» числам икосаэдрической структуры, при вариации их начальной морфологии. Показано, что в случае начальной ГЦК конфигурации формирование Ih модификации происходит либо на этапе предварительной термической релаксации, либо в ходе дальнейшего нагрева. При начальной аморфной морфологии характер структурных переходов претерпевает значительные изменения. Так, например, формирующаяся Ih модификация обладает большей стабильностью в области высоких температур и точка плавления нанокластеров смещается на величину более 100 К. Такой эффект обусловлен более плавным изменением удельной потенциальной энергии нанокластера в сравнении со случаем, когда устойчивая Ih конфигурация формируется при низких температурах. Полученные данные могут быть использованы при процессах создания нанокластеров серебра с требуемым внутренним строением. This article provides a comparative analysis of thermally induced structural transitions in silver nanoclusters with a change in their initial morphology. The study was executed by the molecular dynamics method using the modified TB-SMA (second moment approximation of tight-binding) tight binding potential. The number of atoms in nanoclusters corresponds to the icosahedral structure «magic» numbers. It is shown that for nanoclusters with the initial FCC configuration, the Ih modification is formed either at the stage of preliminary thermal relaxation or during further heating. For nanoclusters with an initial amorphous morphology, the nature of structural transitions undergoes significant changes. For example, the formed Ih modification is more stable at high temperatures and the melting point of nanoclusters shifts by more than 100 K. This effect is due to a smoother change in the specific potential energy of the nanocluster in comparison with the case when a stable Ih configuration is formed at low temperatures. The data obtained can be used in processes to create silver nanoclusters with the required internal structure.

ROLE OF «MAGIC» HCP NUMBERS IN STABILITY OF THE INTERNAL STRUCTURE OF Ag AND Ag NANOCLUSTERS

В настоящее время серебро активно применяется в микроэлектронике, в основном благодаря своей высокой электро- и теплопроводности. Учет процессов взаимодействия между металлом и световой волной (плазмонные эффекты) дает совершенно новые технические приложения серебра. Эти приложения становятся возможными благодаря сильному взаимодействию между падающим светом и свободными электронами в наноструктурах. К настоящему времени уже стало понятно, что размер, форма и структура наночастиц определяют их плазмонные свойства, в том числе резонансные частоты. Следовательно, подгонкой размера, внешнего вида металлической наноструктуры и ее внутреннего строения, можно управлять светом с очень большой степенью точности. В данной работе методом молекулярной динамики с использованием модифицированного потенциала сильной связи TB-SMA (second moment approximation of tight-binding) были изучены границы термической стабильности различной исходной структурной фазы в малых кластерах серебра с числом атомов, соответствующим «магическим» числам ГПУ структуры. Было показано, что характер термически индуцированных структурных переходов в исследуемых группах нанокластеров резко отличается. Данный факт может позволить создать малые кластеры серебра с требуемым внутренним строением. Currently, silver is actively used in microelectronics, mainly due to its high electrical and thermal conductivity. Taking into account the processes of interaction between a metal and a light wave (plasmon effects) gives completely new technical applications of silver. These applications are made possible by the strong interaction between incident light and free electrons in nanostructures. By now, it has already become clear that the size, shape, and structure of nanoparticles determine their plasmon properties, including resonance frequencies. Therefore, by adjusting the size, appearance of the metal nanostructure and its internal structure, it is possible to control light with a very high degree of accuracy. In this work, the boundaries of thermal stability of various initial structural phases in small silver clusters with the number of atoms corresponding to the «magic» numbers of the hcp structure were studied by the molecular dynamics method using the modified tight-binding potential TB-SMA (second moment approximation of tight-binding). It was shown that the nature of thermally induced structural transitions in the groups of nanoclusters under study differs sharply. This fact can make it possible to create small silver clusters with the required internal structure.

Beta-decay half-lives of the extremely neutron-rich nuclei in the closed-shell N = 50, 82, 126 groups

The β--decay half-lives of extremely neutron-rich nuclei are important for understanding nucleosynthesis in the r-process. However, most of their half-lives are unknown or very uncertain, leading to the need for reliable calculations. In this study, we updated the coefficients in recent semi-empirical formulae using the newly updated mass (AME2020) and half-life (NUBASE2020) databases to improve the accuracy of the half-life prediction. In particular, we developed a new empirical model for better calculations of the β--decay half-lives of isotopes ranging in Z = 10 – 80 and N = 15-130. We examined the β--decay half-lives of the extremely neutron-rich isotopes at and around the neutron magic numbers of N = 50, 82, and 126 using either five different semi-empirical models or finite-range droplet model and quasi-particle random phase approximation (FRDM+QRPA) method. The β--decay rates derived from the estimated half-lives were used in calculations to evaluate the impact of the half-life uncertainties of the investigated nuclei on the abundance of the r-process. The results show that the half-lives mostly range in 0.001 < T1/2 < 100 s for the nuclei with a ratio of N/Z < 1.9; however, they differ significantly for those with the ratio of N/Z > 1.9. The half-life differences among the models were found to range from a few factors (for N/Z < 1.9 nuclei) to four orders of magnitude (for N/Z > 1.9). These discrepancies lead to a large uncertainty, which is up to four orders of magnitude, in the r-process abundance of isotopes. We also found that the multiple-reflection time-of-flight (MR-TOF) technique is preferable for precise mass measurements because its measuring timescale applies to the half-lives of the investigated nuclei. Finally, the results of this study are useful for studies on the β-decay of unstable isotopes and astrophysical simulations.

Thermodynamic analysis of Al clusters formation over aluminum melt

The formation of the aluminum nanoparticles with the size of up to 60 atoms in a gas phase is theoretically studied. Thermodynamic modeling has been applied to investigate the effect of the synthesis conditions on the distribution of the nanoparticles. The magic numbers of the particles have been estimated and found to be consistent with the available data. Furthermore, the simulations showed that higher amounts of larger nanoparticles are obtained during condensation from the supercooled aluminum vapor. In contrast, lower amounts of smaller clusters may be formed in a gas phase over the aluminum melt. Varying the temperature and concentration of supercooled aluminum vapor in a broad range results in no significant change in cluster size distribution. This effect is governed by the equilibrium shift.

Isotopic shift in magic nuclei within relativistic mean-field formalism

The ground-state properties such as binding energy, root-mean-square radius, pairing energy, nucleons density distribution, symmetry energy, and single-particle energies are calculated for the isotopic chain of Ca, Sn, Pb, and Z = 120 nuclei. The recently developed G3 and IOPB-I forces along with the DD-ME1 and DD-ME2 sets are used in the analysis employing the relativistic mean-field approximation. To locate the magic numbers in the superheavy region and to explain the observed kink at neutron number N=82 for Sn isotopes, a three-point formula is used to see the shift of the observable and other nuclear properties in the isotopic chain. Unlike the electronic configuration, due to strong spin-orbit interaction, the higher spin orbitals are occupied earlier than the lower spin, causing the possible kink at the neutron magic numbers. We find peaks at the known neutron magic number with the confirmation of sub-shell, shell closure respectively at N=40, 184 for Ca and 304120.

Magic Numbers in Boson 4He Clusters: The Auger Evaporation Mechanism

The absence of magic numbers in bosonic 4He clusters predicted by all theories since 1984 has been challenged by high-resolution matter-wave diffraction experiments. The observed magic numbers were explained in terms of enhanced growth rates of specific cluster sizes for which an additional excitation level calculated by diffusion Monte Carlo is stabilized. The present theoretical study provides an alternative explanation based on a simple independent particle model of the He clusters. Collisions between cluster atoms in excited states within the cluster lead to selective evaporation via an Auger process. The calculated magic numbers as well as the shape of the number distributions are in quite reasonable agreement with the experiments.