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Constructing specific structures from the bottom up with artificial units is an important interdisciplinary topic involving physics, chemistry, materials, and so on. In this work, we theoretically demonstrated the feasibility of using superatoms as building blocks to assemble a complex at atomic-level precision. By using a series of actinide-based endohedral metallofullerene (EMF) superatoms that can form one, two, three and four chemical bonds, a planar complex with intra- and inter-molecular interactions was assembled on the Au(111) surface. This complex is composed of two parts, containing ten and eight superatoms, respectively. The electronic structure analysis shows that the electron density inside each part is connected and the closed-shell electronic arrangement system is designed. There is also an obvious van der Waals boundary by physical adsorption between the two parts, and a stable complex is formed. Since this complex is realized by the first-principles calculations of quantum mechanics, our results help not only achieve atomic-level precision construction with artificial superatomic units but also maintain atomic-level functional properties. [ABSTRACT FROM AUTHOR]

The mysterious properties of individual U atoms on transition metal surfaces play indispensable parts in supplementing our understanding of uranium-transition metal systems, which are important subjects for both nuclear energy applications and fundamental scientific studies. By using scanning tunneling microscopy and density functional theory calculations, the adsorptions, reactions and electronic properties of individual U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces were comparatively studied for the first time in this work. Upon the deposition of a small amount of U onto Cu(111) or Ag(111) at 8 K, individual U atoms show relatively high activity and can either be adsorbed on intact substrate surfaces or induce various surface vacancies surrounded by clusters of substrate atoms. By contrast, the majority of U atoms tend to dispersedly adsorb on intact surfaces of Au(111) and Ru(0001) rather than producing surface vacancies at the same temperature. In all cases, Kondo resonance manifested as asymmetric dip feature around Fermi energy is only observed in the differential tunneling conductance spectra of single U adatoms on Ag(111). [ABSTRACT FROM AUTHOR]

Designing the novel cluster units is a major concern in the cluster assembled materials. With the aid of particle swarm optimization algorithm and density functional theory calculations, we have identified the geometric and electronic structures of 13-atom MgnLi12−nSc (n = 1–8) nanoclusters. The icosahedral structures are discovered as their respective global minima except Mg8Li4Sc, of which the cage structure is destroyed due to a magnesium atom outside the external metal cage. As the magnesium atoms increases, there is a sharp energy drop thereafter Mg5Li7Sc, and a large gain energy in MgnLi12−nSc (n = 3–5) is indicative of their especial stability. Molecular orbital analysis shows that the high stable clusters have the characteristics of magnetic superatoms. Then, the second stable magnetic cluster of Mg4Li8Sc is chosen as a possible building unit and the five-membered face-sharing motif is adopted. Mg4Li13Sc2 with the supermolecular properties are successfully constructed. Though the outer cage doped, this could to be a useful way to form the magnetic superatoms. Our findings further enrich building units that may aid in developing cluster-materials and tailoring their properties. [ABSTRACT FROM AUTHOR]

We present the first measurements of photoelectron spectra of atomic clusters embedded in superfluid helium (He) nanodroplets. Owing to the large absorption cross section of xenon (Xe) around 100 eV photon energy (4d inner-shell ionization), direct dopant photoionization exceeds charge transfer ionization via the ionized He droplets. Despite the predominant creation of Xe2+ and Xe3+ by subsequent Auger decay of free Xe atoms, for Xe embedded in He droplets only singly charged , k = 1, 2, 3 fragments are observed. Broad Xe+ ion kinetic-energy distributions indicate Coulomb explosion of the ions due to electron transfer to the primary Auger ions from surrounding neutral atoms. The electron spectra correlated with Xe ions emitted from the He nanodroplets contain a low-energy feature and nearly unshifted Xe photolines. These results pave the way to extreme ultraviolet and x-ray photoelectron spectroscopy of clusters and molecular complexes embedded in He nanodroplets. [ABSTRACT FROM AUTHOR]

As the first step of cluster-assembled materials, it is crucial to design stable clusters. In this paper, a type of D3h symmetry 15-coordinate core-shell superatoms TM@Li15 is proposed by encapsulating a transition-metal atom (TM = Sc, Ti, Y, Zr, Nb, Hf, and Ta) into the Li15 cage, of which the center is located in the assigned transition-metal atom. The unique structures are identified as the global minima through CALYSPO structure prediction method and frequency calculations. Energy calculations also confirm that they are very stable. Molecular orbital analysis reveals that all the D3h symmetry isomers TM@Li15 (TM = Sc, Ti, Y, Zr, Nb, Hf, and Ta) are with the superatomic properties, where their stability can be well understood based on the superatom and magnetic superatom models. Our findings enrich the superatom family and may offer building units for cluster-assembling materials. [ABSTRACT FROM AUTHOR]

As the first step of cluster-assembled materials, it is crucial to design stable clusters. In this paper, a type of D3h symmetry 15-coordinate core-shell superatoms TM@Li15 is proposed by encapsulating a transition-metal atom (TM = Sc, Ti, Y, Zr, Nb, Hf, and Ta) into the Li15 cage, of which the center is located in the assigned transition-metal atom. The unique structures are identified as the global minima through CALYSPO structure prediction method and frequency calculations. Energy calculations also confirm that they are very stable. Molecular orbital analysis reveals that all the D3h symmetry isomers TM@Li15 (TM = Sc, Ti, Y, Zr, Nb, Hf, and Ta) are with the superatomic properties, where their stability can be well understood based on the superatom and magnetic superatom models. Our findings enrich the superatom family and may offer building units for cluster-assembling materials. [ABSTRACT FROM AUTHOR]

A three-dimensional molecular dynamic approach is employed to investigate the ionization dynamics of small Xe400 clusters irradiated by intense lasers (I = 1016Wcm-2) in the near infrared wavelength region (ƛ = 800 nm). The pulse duration of the incident laser is varied from a few cycles (τ = 2T0 with T0 as one laser time cycle) to many cycles (τ = 8T0). In the case of pulse durations of a few cycles, the carrier-envelope (CE) phase of the incident laser electric field is found to be an important parameter that affects the ionization dynamics of Xe clusters. The fractions of various ionized Xe species are observed to be different for the two values of the CE phases (ɸ = 0 and ɸ = π/2) in the case of the shorter laser pulse duration of τ = 2T0. The nature of the instantaneous electric field (the rising or falling edge of the field) at the time of birth of the electron due to ionization decides the extent of ionization. The difference in time-evolution of the electric field for the two values of the CE phase leads to an observable disparity in the yield of various ionic species. For the case of a pulse duration of many cycles (τ = 8T0), these differences average out and we do not observe any change in the yield of various ionic species for the two values of the CE phase. [ABSTRACT FROM AUTHOR]

The new mechanism of x-ray generation by clusters at their irradiation by femtosecond laser pulses has been suggested. We develop photo-recombination theory for electrons which first leave atomic clusters due to the outer ionization and then go into the ground quantum state of the neighbouring charged cluster. The charged cluster field is considered as a quantum potential well. The conclusion has been made that non-dipole x-ray photons at the photo-recombination into charged clusters have an energy which is larger than that for the well-known photo-recombination on separate atomic ions inside the cluster. [ABSTRACT FROM AUTHOR]

Developing accurate self-assembly is the key for constructing functional materials from a bottom-up approach. At present, it is mainly hindered by building blocks and driving modes. We design a new self-assembly method based on the magnetic coupling between spin-polarized electrons. First-principles calculations show that spin-polarized electrons from different endohedral metallofullerene (EMF) superatoms can pair each other to ensure a one-dimensional extending morphology. Furthermore, without ligand passivation, the EMF superatoms maintain their electronic structures robustly in self-assembly owing to the core-shell structure and the atomic-like electron arrangement rule. Therefore, it should noted that the magnetic coupling of monomeric electron spin polarization can be an important driving mechanism for high-precision self-assembly. These results represent a new paradigm for self-assembly and offer fresh opportunities for functional material construction at the atomic level. [ABSTRACT FROM AUTHOR]

In this work we use magnetic deflection of V, Nb, and Ta atomic clusters to measure their magnetic moments. While only a few of the clusters show weak magnetism, all odd-numbered clusters deflect due to the presence of a single unpaired electron. Surprisingly, for the majority of V and Nb clusters an atomic-like behavior is found, which is a direct indication of the absence of spin–lattice interaction. This is in agreement with Kramers degeneracy theorem for systems with a half-integer spin. This purely quantum phenomenon is surprisingly observed for large systems of more than 20 atoms, and also indicates various quantum relaxation processes, via Raman two-phonon and Orbach high-spin mechanisms. In heavier, Ta clusters, the relaxation is always present, probably due to larger masses and thus lower phonon energies, as well as increased spin–orbit coupling. [ABSTRACT FROM AUTHOR]

The clustering behavior of helium atoms in thorium dioxide has been investigated by first-principles calculations. The results show that He atoms tend to form a cluster around an octahedral interstitial site (OIS). As the concentration of He atoms in ThO2 increases, the strain induced by the He atoms increases and the octahedral interstitial site is not large enough to accommodate a large cluster, such as a He hexamer. We considered three different Schottky defect (SD) configurations (SD1, SD2, and SD3. When He atoms are located in the SD sites, the strain induced by the He atoms is released and the incorporation and binding energies decrease. The He trimer is the most stable cluster in SD1. Large He clusters, such as a He hexamer, are also stable in the SDs. [ABSTRACT FROM AUTHOR]

A multilayer deposition method has been developed with the potential to capture and process atomic clusters generated by a high flux cluster beam source. In this deposition mode a series of sandwich structures each consisting of three layers—a carbon support layer, cluster layer and polymer release layer—is sequentially deposited to form a stack of isolated cluster layers, as confirmed by through-focal aberration-corrected HAADF STEM analysis. The stack can then be diced into small pieces by a mechanical saw. The diced pieces are immersed in solvent to dissolve the polymer release layer and form small platelets of supported clusters. [ABSTRACT FROM AUTHOR]

The temperature-dependent structural and optical properties of Ge60Te40 during transformation from amorphous to crystalline phases are studied by spectroscopic ellipsometry, grazing incidence x-ray diffraction (GIXRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) as well as ab initio molecular dynamics (MD) simulations. Our findings, derived from GIXRD results and dielectric functions, indicate that the crystallization process of Ge60Te40 occurs at ~260 °C and the crystalline phase adopts the rhombohedral GeTe phase along with the cubic Ge phase. Our calculations further indicate that the Ge atoms also prefer to clump together in amorphous Ge60Te40. Such a segregation of Ge leads to a high-temperature phase-change in Ge60Te40 samples. According to the XPS results, the local atomic environment between amorphous and crystalline states gets a little bit of change. And the result of the atomic cluster alignment suggests that both the amorphous and crystalline phases have the rhombohedral local structure. [ABSTRACT FROM AUTHOR]

Intense pulses from a short wavelength free-electron laser turn xenon nanoparticles into a high energy density nanoplasma within femtoseconds. Recently, the generation of multiply charged xenon ions during the initial phase of plasma evolution has been studied by energy-resolved XUV fluorescence detection as a function of cluster size and cluster composition [1]. In the present contribution we give a detailed analysis of the corresponding radiative transitions after resonant excitation of the 4d electron shell at intensities of 2 × 1012 − 2.45 × 1015 W cm−2. The evaluation of charge-state specific fluorescence yields as a function of FEL power density demonstrates that plasma effects such as ionization potential lowering, electron impact excitation, ionization, and energy redistribution govern the laser-induced non-equilibrium dynamics in xenon clusters. [ABSTRACT FROM AUTHOR]

We investigate the fragmentation dynamics of atomic clusters ionized by intense extreme-ultraviolet pulses from a high-order harmonic generation source. It is demonstrated that the transient modification of the trimer fragment yield induced by a weak near-infrared (NIR) probe pulse provides valuable information on the cluster disintegration process. For medium-sized Ar clusters and delays of around 5 ps, we observe a transition from a collective response, where efficient absorption of the NIR laser energy takes place, to a state where the interaction of the probe laser pulse with individual fragments dominates. The cluster disintegration is faster for smaller clusters, in agreement with a simple physical picture taking into account the measured expansion speeds of excited and ionic fragments. For Xe clusters, we find a significantly increased fragmentation time of about 40 ps, attributed to the larger atomic mass and to more efficient electron–ion recombination processes. [ABSTRACT FROM AUTHOR]

Anomalously enhanced nonlinear electromagnetic coupling can arise from ordered driven collective motions in many electron systems. The augmented strength of the interaction can be expressed as an effective increase in the fine structure constant α in which α → Z2α, where Z specifies the number of electrons involved in the ordered response to the external field. The present work illustrates this phenomenon in the x-ray range with the observation of the 5-photon nonlinear excitation of Xe(L)* hollow atom states that are generated by intense (∼7 × 1015 W cm−2) Xe(M) radiation at ∼1 keV. The nonlinear cross section experimentally determined for the + Xe → [Xeq+(L)]* + qe− amplitude is ∼ 2 × 10−21 cm2. The matching theoretical cross section corresponds to Z = 18, an outcome indicating the participation of the full Xe(4d105s25p6) supershell, a dynamic feature of Xe that also plays a significant role in the linear photoionization of neutral Xe atoms in the kilovolt region. For the high-intensity 5γ nonlinear coupling, the outcome for the Xe(L)* hollow atom excitation is an enhancement of the strength of the interaction by a factor of ∼1012 and, with Z2α > 1, a fundamentally new region of strong coupling is entered. The experimental value of is likewise shown to be in very good accord with an earlier analysis that estimated the upper bound of cross sections for high-order multi-photon cross sections in the combined high-Z and high-intensity limit. These results forecast the general presence of comparably enhanced coupling strengths in the interaction of sufficiently intense (I ≥ 7 × 1015 W cm−2) x-rays with high-Z atoms and molecules. [ABSTRACT FROM AUTHOR]

The behaviours of hydrogen and helium in tungsten are vitally important in fusion research because they can result in the degradation of the material. In the present work, we carry out density-functional theory calculations to investigate the clustering of hydrogen and helium atoms at interstitial sites, vacancy and small vacancy clusters (Vacm, m = 2, 3), and the influence of hydrogen and helium on vacancy evolution in tungsten. We find that hydrogen atoms are extremely difficult to aggregate at interstitial sites to form a stable cluster in tungsten. However, helium atoms are energetically favourable to cluster together in a close-packed arrangement between (1 1 0) planes forming helium monolayer structure, where the helium atoms are not perfectly in one plane. Both hydrogen and helium prefer to aggregate stably in vacancy and small vacancy cluster forming VacmXn (X = H, He). The concentrations of VacmHn (m = 1) clusters relative to temperature are evaluated through the law of mass action. The present calculations also show that the emission of a 〈1 1 1〉 dumbbell self-interstitial atom (SIA) from Hen to form VacHen and from VacHen to form Vac2Hen may take place for n > 5 and n > 9, respectively. According to the present results, we predict that a helium monolayer structure could nucleate for He atom platelet lying on (1 1 0) plane in tungsten, and the helium platelet formation on (1 1 0) plane in molybdenum observed by the experiment may be due to the initial monolayer arrangement of He atoms at interstitial sites. Meanwhile, our results contribute to the understanding for nucleation and the development of the voids and blisters in tungsten that are observed in the experiments. [ABSTRACT FROM AUTHOR]

The achievement of controlled saturation with high spectral resolution (∼2–3 eV) provides a refined diagnostic for the quantitative study of the propagation of intense beams of x-rays. This work demonstrates the characteristics of saturation spectroscopy with an analysis of Kr(L) autoionizing transitions at ħω = 1652 eV. The data include single-pulse x-ray spatial morphologies, corresponding Thomson images of the electron density, and spatially resolved transversely observed x-ray spectra that are recorded longitudinally along the direction of propagation of a Kr26+ (λ26 = 7.504 Å) beam in a Krn cluster medium. The results reveal the complex interactions associated with the propagation of the 7.504 Å x-ray beam and, based on sets of quantitative criteria, attribute the observed behavior to the saturation of 2p → 3d transitions to autoionizing states of Kr15+ and Kr16+ ionic species. [ABSTRACT FROM AUTHOR]

We report on the aggregation behaviors of Cr atoms deposited on flat and curved silicone oil substrates by the sputtering technique. It is found that the Cr atoms are prone to form quasi-circular clusters first and then these clusters connect with each other to form ramified or network-shaped aggregates. In the case of a flat liquid surface, large quasi-circular clusters are frequently observed and their sizes increase with the deposition time. In the case of a curved surface with a larger curvature, the Cr atoms and clusters tend to move toward to the oil drop edge, and ramified aggregates with small widths are shaped. In the case of a curved surface with smaller curvature, large quasi-circular clusters and ramified aggregations can coexist. Based on the motion features of the Cr atoms and clusters on different liquid surfaces, various distinct film morphologies are discussed in detail. [ABSTRACT FROM AUTHOR]

Clusters excited by intense laser pulses are a unique source of warm dense matter, that has been the subject of intensive experimental studies. The majority of these investigations concern atomic clusters, whereas the evolution of molecular clusters excited by intense laser pulses is less explored. In this work we trace the dynamics of clusters triggered by a few-cycle driving pulse through the detection of XUV fluorescence induced by a delayed 800 nm ignition pulse. Striking differences among fluorescence dynamics from different ionic species are observed. [ABSTRACT FROM AUTHOR]