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David J Wales - One of the best experts on this subject based on the ideXlab platform.
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properties of kinetic transition networks for Atomic Clusters and glassy solids
arXiv: Disordered Systems and Neural Networks, 2017Co-Authors: John W R Morgan, Dhagash Mehta, David J WalesAbstract:A database of minima and transition states corresponds to a network where the minima represent nodes and the transition states correspond to edges between the pairs of minima they connect via steepest-descent paths. Here we construct networks for small Clusters bound by the Morse potential for a selection of physically relevant parameters, in two and three dimensions. The properties of these unweighted and undirected networks are analysed to examine two features: whether they are small-world, where the shortest path between nodes involves only a small number or edges; and whether they are scale-free, having a degree distribution that follows a power law. Small-world character is present, but statistical tests show that a power law is not a good fit, so the networks are not scale-free. These results for Clusters are compared with the corresponding properties for the molecular and Atomic structural glass formers ortho-terphenyl and binary Lennard-Jones. These glassy systems do not show small-world properties, suggesting that such behaviour is linked to the structure-seeking landscapes of the Morse Clusters.
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Atomic Clusters with addressable complexity
Journal of Chemical Physics, 2017Co-Authors: David J WalesAbstract:A general formulation for constructing addressable Atomic Clusters is introduced, based on one or more reference structures. By modifying the well depths in a given interAtomic potential in favour of nearest-neighbour interactions that are defined in the reference(s), the potential energy landscape can be biased to make a particular permutational isomer the global minimum. The magnitude of the bias changes the resulting potential energy landscape systematically, providing a framework to produce Clusters that should self-organise efficiently into the target structure. These features are illustrated for small systems, where all the relevant local minima and transition states can be identified, and for the low-energy regions of the landscape for larger Clusters. For a 55-particle cluster, it is possible to design a target structure from a transition state of the original potential and to retain this structure in a doubly addressable landscape. Disconnectivity graphs based on local minima that have no direct connections to a lower minimum provide a helpful way to visualise the larger databases. These minima correspond to the termini of monotonic sequences, which always proceed downhill in terms of potential energy, and we identify them as a class of biminimum. Multiple copies of the target cluster are treated by adding a repulsive term between particles with the same address to maintain distinguishable targets upon aggregation. By tuning the magnitude of this term, it is possible to create assemblies of the target cluster corresponding to a variety of structures, including rings and chains.
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impurity effects on solid solid transitions in Atomic Clusters
Nanoscale, 2016Co-Authors: D Schebarchov, Be Husic, David J WalesAbstract:We use the harmonic superposition approach to examine how a single atom substitution affects low-temperature anomalies in the vibrational heat capacity (CV) of model nanoClusters. Each anomaly is linked to competing solidlike “phases”, where crossover of the corresponding free energies defines a solid–solid transition temperature (Ts). For selected Lennard-Jones Clusters we show that Ts and the corresponding CV peak can be tuned over a wide range by varying the relative Atomic size and binding strength of the impurity, but excessive atom-size mismatch can destroy a transition and may produce another. In some tunable cases we find up to two additional CV peaks emerging below Ts, signalling one- or two-step delocalisation of the impurity within the ground-state geometry. Results for Ni74X and Au54X Clusters (X = Au, Ag, Al, Cu, Ni, Pd, Pt, Pb), modelled by the many-body Gupta potential, further corroborate the possibility of tuning, engineering, and suppressing finite-system analogues of a solid–solid transition in nanoalloys.
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symmetrisation schemes for global optimisation of Atomic Clusters
Physical Chemistry Chemical Physics, 2013Co-Authors: Mark T Oakley, Roy L Johnston, David J WalesAbstract:Locating the global minima of Atomic and molecular Clusters can be a difficult optimisation problem. Here we report benchmarks for procedures that exploit approximate symmetry. This strategy was implemented in the GMIN program following a theoretical analysis, which explained why high-symmetry structures are more likely to have particularly high or particularly low energy. The analysis, and the corresponding algorithms, allow for approximate point group symmetry, and can be combined with basin-hopping and genetic algorithms. We report results for 38-, 75-, and 98-atom Lennard-Jones Clusters, which are all multiple-funnel systems. Exploiting approximate symmetry reduces the mean time taken to locate the global minimum by up to two orders of magnitude, with smaller improvements in efficiency for LJ55 and LJ74, which correspond to simpler single-funnel energy landscapes.
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from topographies to dynamics on multidimensional potential energy surfaces of Atomic Clusters
Science, 1996Co-Authors: Keith D Ball, Stephen R Berry, Ralph E Kunz, Ana Proykova, David J WalesAbstract:Multidimensional potential energy surfaces for systems larger than about 15 atoms are so complex that interpreting their topographies and the consequent dynamics requires statistical analyses of their minima and saddles. Sequences of minimum-saddle-minimum points provide a characterization of such surfaces. Two examples, Ar19 and (KCI)32, illustrate how topographies govern tendencies to form glasses or “focused” structures, for example, crystals or folded proteins. Master equations relate topographies to dynamics. The balance between glass-forming and structure-seeking characters of a potential energy surface seems governed by sawtooth versus staircase topography and the associated collectivity of the growth process after nucleation.
Peter Wiecha - One of the best experts on this subject based on the ideXlab platform.
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Quantum theory of near-field optical imaging with rare-earth Atomic Clusters
Journal of the Optical Society of America B, 2020Co-Authors: Clément Majorel, Christian Girard, Aurelien Cuche, Arnaud Arbouet, Peter WiechaAbstract:Scanning near-field optical imaging (SNOM) using local active probes provides in general images of the electric part of the photonic local density of states. However, certain Atomic Clusters can supply more information by simultaneously revealing both the magnetic (m-LDOS) and the electric (e-LDOS) local density of states in the optical range. For example, nanoparticles doped with rare-earth elements like europium or terbium provide both electric dipolar (ED) and magnetic dipolar (MD) transitions. In this theoretical article, we develop a quantum description of active systems (rare earth ions) coupled to a photonic nanostructure, by solving the optical Bloch equations together with Maxwell's equations. This allows us to access the population of the emitting energy levels for all atoms excited by the incident light, degenerated at the extremity of the tip of a near-field optical microscope. We show that it is possible to describe the collected light intensity due to ED and MD transitions in a scanning configuration. By carrying out simulations on different experimentally interesting systems, we demonstrate that our formalism can be of great value for the interpretation of experimental configurations including various external parameters (laser intensity, polarization and wavelength, the SNOM probe size, the nature of the sample ...).
Qingqing Cheng - One of the best experts on this subject based on the ideXlab platform.
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carbon defect driven electroless deposition of pt Atomic Clusters for highly efficient hydrogen evolution
Journal of the American Chemical Society, 2020Co-Authors: Qingqing Cheng, Chuangang Hu, Guoliang Wang, Hui YangAbstract:Pt Atomic Clusters (Pt-ACs) display outstanding electrocatalytic performance because of their unique electronic structure with a large number of highly exposed surface atoms. However, the small siz...
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carbon defect driven electroless deposition of pt Atomic Clusters for highly efficient hydrogen evolution
Journal of the American Chemical Society, 2020Co-Authors: Qingqing Cheng, Guoliang Wang, Hui Yang, Zhiqing Zou, Liming DaiAbstract:Pt Atomic Clusters (Pt-ACs) display outstanding electrocatalytic performance because of their unique electronic structure with a large number of highly exposed surface atoms. However, the small size and large specific surface area intrinsically associated with ACs pose challenges in the synthesis and stabilization of Pt-ACs without agglomeration. Herein, we report a novel one-step carbon-defect-driven electroless deposition method to produce ultrasmall but well-defined and stable Pt-ACs supported by defective graphene (Pt-AC/DG) structures. A theoretical simulation clearly revealed that the defective regions with a lower work function and hence a higher reducing capacity compared to those of normal hexagonal sites triggered the reduction of Pt ions preferentially at the defect sites. Moreover, the strong binding energy between Pt and carbon defects effectively restricted the migration of spontaneously reduced Pt atoms to immobilize/stabilize the resultant Pt-ACs. Electrochemical analyses demonstrated the high performance of Pt-ACs in catalyzing the hydrogen evolution reaction, showing a greatly enhanced mass activity, a high Pt utilization efficiency, and excellent stability compared with commercial Pt/C catalysts. The integration of proton exchange membrane water electrolysis with Pt-AC/DG as a cathode exhibited an excellent hydrogen generation activity and extraordinary stability (during 200 h of electrolysis) with a greatly reduced Pt usage compared with commercial Pt/C catalysts.
T Ditmire - One of the best experts on this subject based on the ideXlab platform.
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high intensity femtosecond xuv pulse interactions with Atomic Clusters
High Energy Density Physics, 2010Co-Authors: K Hoffmann, B Murphy, A Helal, N Kandadai, John W Keto, T DitmireAbstract:Abstract High intensity femtosecond extreme-ultraviolet (XUV) pulse interactions with large xenon Clusters have been studied at a wavelength of 38 nm. XUV radiation approaching 1011 W/cm2 is produced by high-order-harmonic conversion from a 35 fs, near-infrared terawatt laser. Resulting ion spectra show high charge states with a hydrodynamic based energy distribution from cluster explosions. It is concluded that continuum lowering in the created cluster nanoplasma makes single photoionization to the high charge states possible. These experiments look toward high intensity cluster interaction experiments in the x-ray range on the Linac Coherent Light Source (LCLS) under development at SLAC.
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explosion of Atomic Clusters irradiated by high intensity laser pulses scaling of ion energies with cluster and laser parameters
Physical Review A, 2000Co-Authors: E Springate, T Ditmire, J W G Tisch, M H R Hutchinson, M B Mason, N Hay, J P MarangosAbstract:Experimental measurements of scaling with cluster and laser parameters of the energies of ions produced in the explosion of Atomic Clusters have been obtained, with a view to experimental optimization of the cluster explosion temperature. The noble-gas Clusters were irradiated with high-intensity, 200-fs laser pulses. Ion energy scalings with cluster size (ranging from ${10}^{2}--{10}^{5}$ atoms per cluster), laser intensity ${(10}^{14}--{10}^{16}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}),$ and laser wavelength (780 and 390 nm) are presented for both Xe and Kr Clusters. Numerical calculations of the interaction, treating the cluster as a spherical nanoplasma, are also presented.
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nuclear fusion from explosions of femtosecond laser heated deuterium Clusters
Nature, 1999Co-Authors: T Ditmire, J Zweiback, V Yanovsky, T E Cowan, G Hays, K B WhartonAbstract:As a form of matter intermediate between molecules and bulk solids, Atomic Clusters have been much studied1. Light-induced processes in Clusters can lead to photo-fragmentation2,3 and Coulombic fission4, producing atom and ion fragments with a few electronvolts (eV) of energy. However, recent studies of thephotoionization of Atomic Clusters with high intensity (>1016 W cm−2) femtosecond laser pulses have shown that these interactions can be far more energetic5,6,7,8,9,10,11,12,13—excitation of large Atomic Clusters can produce a superheated microplasma that ejects ions with kinetic energies up to 1 MeV (ref. 10). This phenomenon suggests that through irradiation of deuterium Clusters, it would be possible to create plasmas with sufficient average ion energy for substantial nuclear fusion. Here we report the observation of nuclear fusion from the explosions of deuterium Clusters heated with a compact, high-repetition-rate table-top laser. We achieve an efficiency of about 105 fusion neutrons per joule of incident laser energy, which approaches the efficiency of large-scale laser-driven fusion experiments. Our results should facilitate a range of fusion experiments using small-scale lasers, and may ultimately lead to the development of a table-top neutron source, which could potentially find wide application in materials studies.
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explosion of Atomic Clusters heated by high intensity femtosecond laser pulses
Physical Review A, 1998Co-Authors: T Ditmire, J W G Tisch, E Springate, M B Mason, N Hay, J P Marangos, Y L Shao, M H R HutchinsonAbstract:Atomic Clusters have long been studied by chemists and physicists because of the unique position that Clusters hold as an intermediate state between molecules and solids [1]. Many studies have traced the properties of materials from their monAtomic characteristics to their bulk state characteristics through an examination of the material as it forms larger and larger Clusters. Recently, there has been much activity in extending these studies to very high intensity, ultrashort laser pulses with peak laser intensities >1015 Wcm −2 and pulse widths of 0.1 to 10 ps [2–11]. There has also been some preliminary theoretical work in this area as well [6,12]. In this parameter regime the physics governing the laser cluster interaction is fundamentally different than in previous studies. At these intensities the laser interaction is non-perturbative and very high order multiphoton ionization and strong electric field tunnel ionization are possible. Consequently, highly charged ions can be produced [2,5,8,10]. Furthermore, the short pulses used are comparable to or shorter than the disassembly times of a cluster in the laser field [6] and, so, the entire laser pulse interacts with an inertially confined body of atoms.
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high intensity laser absorption by gases of Atomic Clusters
Physical Review Letters, 1997Co-Authors: T Ditmire, R A Smith, J W G Tisch, M H R HutchinsonAbstract:We have measured the energy absorption efficiency of high intensity, picosecond laser pulses in low density gases composed of large Atomic Clusters. We find that, though the average density of the resulting plasmas is low, the energy absorption can be very high $(g95%)$, indicating that substantial laser energy is deposited per particle in the plasma. Ion energy measurements confirm that this efficient energy deposition results in plasmas with very high (multi-keV) ion temperatures.
Yan Wang - One of the best experts on this subject based on the ideXlab platform.
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Local bond-electron-energy relaxation of Mo Atomic Clusters and solid skins
RSC Advances, 2015Co-Authors: Wei Zhou, Yan Wang, Yongli Huang, Chang Q. SunAbstract:Mo(100, 110) skins and Atomic Clusters share the common nature of Atomic undercoordination induced local bond strain and the associated quantum entrapment and valence electron polarization.
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coordination resolved local bond contraction and electron binding energy entrapment of si Atomic Clusters and solid skins
Journal of Applied Physics, 2014Co-Authors: Yan Wang, Yongli Huang, Xi Zhang, Ting Zhang, Changqing SunAbstract:Consistency between x-ray photoelectron spectroscopy measurements and density-function theory calculations confirms our bond order-length-strength notation-incorporated tight-binding theory predictions on the quantum entrapment of Si solid skin and Atomic Clusters. It has been revealed that bond-order deficiency shortens and strengthens the Si-Si bond, which results in the local densification and quantum entrapment of the core and valence electrons. Unifying Si Clusters and Si(001) and (111) skins, this mechanism has led to quantification of the 2p binding energy of 96.089 eV for an isolated Si atom, and their bulk shifts of 2.461 eV. Findings evidence the significance of Atomic undercoordination that is of great importance to device performance.