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Pressure-induced polyamorphism by quantitative structure factor and pair distribution function analysis in two Ce-based metallic glasses
We utilized the pair distribution function method to characterize the pressure-induced polyamorphic transition in Ce60Al20Cu20 and Ce55Al45 metallic glass at room temperature. Using synchrotron high-energy x-ray diffraction we collected scattering information from a large Q-space coverage, which in turn gave a high resolution g(r) that provided accurate local structure information. We observed a sudden change in compressibility and the nearest neighbor distance at 3.50–6.32 GPa for Ce60Al20Cu20 and 2.20–6.89 GPa for Ce55Al45. The origin of the volume collapse seemed to be pressure-induced qualitative changes in bond shortening that corresponded to different coordination spheres. The polyamorphic transitions in these two systems from low-density glass (LDG) to high-density glass (HDG) are associated with local atomic rearrangements.
Data-driven molecular modeling with the generalized Langevin equation
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Parallel streaming between heterogeneous HPC resources for real-time analysis
Performing analysis or generating visualizations concurrently with high performance simulations can yield great benefits compared to post-processing data. Writing and reading large volumes of data can be reduced or eliminated, thereby producing an I/O cost savings. One such method for concurrent simulation and analysis is in transit - streaming data from the resource running the simulation to a separate resource running the analysis. In transit analysis can be beneficial since computational resources may not have certain resources needed for visualization and analysis (e.g. GPUs) and to reduce the impact of performing analysis tasks to the run time of the simulation. When sending and receiving data in transit, data redistribution mechanisms are needed in order to support heterogeneous data layouts that may be required by the simulation and analysis applications. The work described in this paper compares two mechanisms for on-the-fly data redistribution when streaming data in parallel between two distributed memory applications. Finally, our results show that it is often more advantageous to stream data in the same layout as the sender and redistribute data amongst processes on the receiving end than to stream data in the final layout needed by the receiver.
Controlled growth, characterization and thermodynamic behavior of bismuth–tin nanostructures sheathed in carbon nanotubes
We report the controlled synthesis of bismuth–tin (Bi–Sn) nanostructures sheathed in graphitic shells that resemble carbon nanotubes (CNTs). Our approach is based on a simple catalytic chemical vapor deposition over a mixture of Bi{sub 2}O{sub 3} and SnO{sub 2} supplied as starting materials. Shape control of the nanostructures strongly relies on the weight ratio of Bi{sub 2}O{sub 3} and SnO{sub 2}. Sheathed nanoparticles and nanorods are formed at SnO{sub 2} to Bi{sub 2}O{sub 3} weight ratios of less than 4:1. They are composed of two separate crystals: rhombohedral Bi and tetragonal Sn{sub 19}Bi crystals. On the other hand, the sheathed nanowires are formed at SnO{sub 2} to Bi{sub 2}O{sub 3} weight ratios above 4:1. The nanowires have only tetragonal Sn{sub 19}Bi structure with a diameter of approximately 100 nm. Elementary analyses support the core/shell heterostructure of the resulting products. A favorable temperature for the Sn-rich Sn{sub 19}Bi nanowires is in the range of 700–800 °C, more specifically around 750 °C. Thermodynamic analysis reveals that the CNTs play a significant role in the protection of the Bi–Sn nanostructures during phase transition by temperature change. This simple and reproducible method may be extended to the fabrication of similar binary or ternary nanostructures. - Highlights: • Controlled growth of Bi–Sn nanostructures sheathed in CNTs relies on MSnO{sub 2}/MBi{sub 2}O{sub 3}. • Growth mechanism is based on catalytic reaction of C{sub 2}H{sub 2} over an oxide mixture. • Nanoparticles and nanorods consist of rhombohedral Bi and tetragonal Sn{sub 19}Bi. • A favorable temperature for the Sn{sub 19}Bi nanowires is in the range of 700–800 °C. • CNT acts as a protective barrier during phase change of the Bi–Sn nanostructures.
Frequency domain multiplexing of pulse mode radiation detectors
The capability to multiplex scintillation detectors or other pulse mode radiation detectors is necessary in some applications where a large number of detectors is required. Frequency domain multiplexing has been previously implemented for applications in astronomy using amplitude modulation on radiation detectors such as transition-edge sensors. We propose here an alternative method for multiplexing pulse mode radiation detectors in the frequency domain using convolution. We pass the detector signal to a resonator circuit that converts a detector pulse to a damped sinusoid of a specific frequency which gives a unique tag to the detector. We have developed a prototype frequency-domain multiplexed system for four EJ-309 organic scintillator detectors using four resonators of unique frequencies. The resonator outputs are combined using a fan-in circuit which is then connected to a single digitizer input. Using this system, we further demonstrate that the charge collected under the original anode pulse can be estimated from the power spectrum of the damped sinusoid with a relative uncertainty of about 2%. The time-of-arrival of the anode pulse can be estimated using constant fraction discrimination applied to the leading edge of the damped sinusoid with an uncertainty of about 450 ps. We also used a CeBr3 detector to test the performance of our system for spectroscopic applications and found only small degradation in the resolution for a multiplexed detector.
Near-wake dynamics of a vertical-axis turbine
Cross-flow, or vertical-axis, turbines are a promising technology for capturing kinetic energy in wind or flowing water and their inherently unsteady fluid mechanics present unique opportunities for control optimization of individual rotors or arrays. To explore the potential for beneficial interactions between turbines in an array, as well as to characterize important cycle-to-cycle variations, coherent structures in the wake of a single two-bladed cross-flow turbine are examined using planar stereo particle image velocimetry in a water channel experiment. There are three main objectives in the present work. First, the mean wake structure of this high chord-to-radius ratio rotor is described, compared with previous studies, and a simple explanation for observed wake deflection is presented. Second, the unsteady flow is then analysed via the triple decomposition, with the periodic component extracted using a combination of traditional techniques and a novel implementation of the optimized dynamic mode decomposition. The latter method is shown to outperform conditional averaging and Fourier methods, as well as uncover frequencies suggesting a transition to bluff-body shedding in the far wake. Third, vorticity and finite-time Lyapunov exponents are then employed to further analyse the oscillatory wake component. Vortex streets on both sides of the wake are identified, and their formation mechanisms and effects on the mean flow are discussed. Strong axial (vertical) flow is observed in vortical structures shed on the retreating side of the rotor where the blades travel downstream. Time-resolved tracking of these vortices is performed, which demonstrates that vortex trajectories have significant rotation-to-rotation variation within one diameter downstream. This variability suggests it would be challenging to harness or avoid such structures at greater downstream distances.
Magnetocaloric behavior in ternary europium indides EuT 5 In: Probing the design capability of first-principles-based methods on the multifaceted magnetic materials
The most favorable structures and the types of magnetic ordering predicted from first-principles-based methods in a family of closely related transition-metal-rich indides EuT 5 In (T = Cu, Ag, Au) are gauged against relevant experiments. The EuT5In compounds adopt a different structure for each different coinage metal—EuCu 5 In ( hR 42; R $\overline{3}$m, a = 5.0933(7), c = 30.557(6) Å), EuAg 5 In ( oP 28; Pnma , a = 9.121(2), b = 5.645(1), c = 11.437(3) Å), and EuAu 5 In ( tI 14; I 4/ mmm , a = 7.1740(3), c = 5.4425(3) Å)—and crystallize with the Sr 5 Al 9 , CeCu 6 , and YbMo 2 Al 4 structure types, respectively. EuCu 5 In and EuAg 5 In order antiferromagnetically at T N = 12 and 6 K, respectively, whereas EuAu 5 In is ferromagnetic below T C = 13 K. EuCu 5 In exhibits complex magnetism: after the initial drop at T N , the magnetization rises again below 8 K, and a weak metamagnetic-like transition occurs at 2 K in μ 0 H = 1.8 T. The electronic heat capacity of EuCu 5 In, γ = ~400 mJ/(mol K 2 ), points to strong electronic correlations. Spin-polarized densities of states suggest that the magnetic interactions in the three materials studied are supported via mixing 4 f and 5 d states of Eu. As a result, a chemical bonding analysis based on the Crystal Orbital Hamilton populations reveals the tendency to maximize overall bonding as a driving force to adopt a particular type of crystal structure.
Computer simulation of methanol exchange dynamics around cations and anions
In this paper, we present the first computer simulation of methanol exchange dynamics between the first and second solvation shells around different cations and anions. After water, methanol is the most frequently used solvent for ions. Methanol has different structural and dynamical properties than water, so its ion solvation process is different. To this end, we performed molecular dynamics simulations using polarizable potential models to describe methanol-methanol and ion-methanol interactions. In particular, we computed methanol exchange rates by employing the transition state theory, the Impey-Madden-McDonald method, the reactive flux approach, and the Grote-Hynes theory. We observed that methanol exchange occurs at a nanosecond time scale for Na+ and at a picosecond time scale for other ions. We also observed a trend in which, for like charges, the exchange rate is slower for smaller ions because they are more strongly bound to methanol. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. The calculations were carried out using computer resources provided by the Office of Basic Energy Sciences.
Spin coherence in two-dimensional materials
Spin defects in semiconducting solids are promising platforms for the realization of quantum bits. At low temperature and in the presence of a large magnetic field, the central spin decoherence is mainly due to the fluctuating magnetic field induced by nuclear spin flip-flop transitions. Using spin Hamiltonians and a cluster expansion method, we investigate the electron spin coherence of defects in two-dimensional (2D) materials, including delta-doped diamond layers, thin Si films, MoS 2 , and h-BN. We show that isotopic purification is much more effective in 2D than in three-dimensional materials, leading to an exceptionally long spin coherence time of more than 30 ms in an isotopically pure monolayer of MoS 2 .
The addition of methanol to Criegee intermediates
Bimolecular reactions involving stabilized Criegee intermediates (SCI) have been the target of many studies due to the role these molecules play in atmospheric chemistry. Recently, kinetic rates for the addition reaction of the simplest SCI (formaldehyde oxide) and its methylated analogue (acetone oxide) with methanol were reported both experimentally and theoretically. We re-examine the energy profile of these reactions by employing rigorous ab initio methods. Optimized CCSD(T)/ANO1 geometries are reported for the stationary points along the reaction path. Energies are obtained at the CCSD(T)/CBS level of theory. Contributions of full triple and quadruple excitations are computed to assess the convergence of this method. Rate constants are obtained using conventional canonical transition state theory under the rigid rotor harmonic oscillator approximation and with the inclusion of a one-dimensional hindered rotor treatment. These corrections for internal rotations have a significant impact on computed kinetic rate constants. With this approach, we compute rate constants for the addition of methanol to formaldehyde oxide (H 2 COO) and acetone oxide [(CH 3 ) 2 COO] at 298.15 K as (1.2 ± 0.8) × 10 -13 and (2.8 ± 1.3) × 10 -15 cm 3 s -1 , respectively. Additionally, we investigate the temperature dependence of the rate constant, concluding that the transition state barrier height and tunneling contributions shape the qualitative behaviour of these reactions.
Expanding and quantifying the crystal chemistry of the flexible ligand 15aneN5
Tetraazamacrocycles have been very extensively exploited as transition metal ligands for a variety of purposes, including catalysis, medical imaging, pharmaceuticals, etc. However, the pentaazamacrocycles are much less commonly used for similar purposes because of poor availability, difficulties in their synthesis, and less well-known metal coordination properties. 1,4,7,10,13-pentaazacyclopentadane (15aneN5) was initially synthesized by a published synthetic route, which we simplified and shortened with minimal drop in yield. Eight different transition metal complexes were made using typical complexation methods. X-ray crystallography of multiple novel complexes yielded insight into the flexibility in coordination geometry of this interesting macrocycle as well as the first crystal structures of 15aneN5 with Cr 3+ , Mn 3+ , Fe 3+ , Co 3+ , Cu 2+ , and Ru 2+ . A parameter to quantify the coordination geometry adopted by the ligand was devised and applied to all known crystal structures of its metal complexes. Lastly, oxidation of 15aneN5 to a novel diimine macrocycle was observed during complexation with ruthenium.
Near-infrared C-term MCD spectroscopy of octahedral uranium (V) complexes
C-term magnetic circular dichroism (MCD) spectroscopy is a powerful method for probing d–d and f–f transitions in paramagnetic metal complexes. However, this technique remains underdeveloped both experimentally and theoretically for studies of U(V) complexes of Oh symmetry, which have been of longstanding interest for probing electronic structure, bonding, and covalency in 5f systems. In this study, C-term NIR MCD of the Laporte forbidden f–f transitions of [UCl 6 ] - and [UF 6 ] - are reported, demonstrating the significant fine structure resolution possible with this technique including for the low energy Γ 7 → Γ 8 transitions in [UF 6 ] - . The experimental NIR MCD studies were further extended to [U(OC 6 F 5 ) 6 ] - , [U(CH 2 SiMe 3 ) 6 ] - , and [U(NC(tBu)(Ph)) 6 ] - to evaluate the effects of ligand-type on the f–f MCD fine structure features. Theoretical calculations were conducted to determine the Laporte forbidden f–f transitions and their MCD intensity experimentally observed in the NIR spectra of the U(V) hexahalide complexes, via the inclusion of vibronic coupling, to better understand the underlying spectral fine structure features for these complexes. These spectra and simulations provide an important platform for the application of MCD spectroscopy to this widely studied class of U(V) complexes and identify areas for continued theoretical development.
Toward a self-consistent model of the interaction between an ultra-intense, normally incident laser pulse with an overdense plasma
Following a recent work by Sanz et al. [Phys. Rev. E 85, 046411 (2012)], we elaborate upon a one-dimensional model describing the interaction between an ultra-intense, normally incident laser pulse and an overdense plasma. The analytical solutions of the reflected laser field, the electrostatic field, and the plasma surface oscillation are obtained within the cold-fluid approximation. The high-order harmonic spectrum is calculated from the exact solution of the plasma surface oscillations. In agreement with particle-in-cell simulations, two regimes of harmonic generation are predicted: for moderately relativistic laser intensities, or high plasma densities, the harmonic spectrum is determined by the discontinuity in the derivative of the reflected field when the electron plasma boundary oscillates across the fixed ion boundary. For higher intensities, the electron plasma boundary is confined inside the ion region and oscillates at relativistic velocities, giving rise to a train of reflected attosecond pulses. In both cases, the harmonic spectrum obeys an asymptotic ω{sup −4} scaling. The acceleration of electrons and the related laser absorption efficiency are computed by a test particle method. The model self-consistently reproduces the transition between the “anomalous skin effect” and the “J × B” heating predicted by particle-in-cell simulations. Analytical estimates of the different scalings are presented.
Terahertz inline wall thickness monitoring system for plastic pipe extrusion
Conventional and commercially available inline wall thickness monitoring systems for pipe extrusion are usually based on ultrasonic or x-ray technology. Disadvantages of ultrasonic systems are the usual need of water as a coupling media and the high damping in thick walled or foamed pipes. For x-ray systems special safety requirements have to be taken into account because of the ionizing radiation. The terahertz (THz) technology offers a novel approach to solve these problems. THz waves have many properties which are suitable for the non-destructive testing of plastics. The absorption of electrical isolators is typically very low and the radiation is non-ionizing in comparison to x-rays. Through the electromagnetic origin of the THz waves they can be used for contact free measurements. Foams show a much lower absorption in contrast to acoustic waves. The developed system uses THz pulses which are generated by stimulating photoconductive switches with femtosecond laser pulses. The time of flight of THz pulses can be determined with a resolution in the magnitude of several ten femtoseconds. Hence the thickness of an object like plastic pipes can be determined with a high accuracy by measuring the time delay between two reflections on materials interfaces e.g. at the pipe's inner and outer surface, similar to the ultrasonic technique. Knowing the refractive index of the sample the absolute layer thickness from the transit time difference can be calculated easily. This method in principle also allows the measurement of multilayer systems and the characterization of foamed pipes.
An inversion-relaxation approach for sampling stationary points of spin model Hamiltonians
Sampling the stationary points of a complicated potential energy landscape is a challenging problem. Here, we introduce a sampling method based on relaxation from stationary points of the highest index of the Hessian matrix. We illustrate how this approach can find all the stationary points for potentials or Hamiltonians bounded from above, which includes a large class of important spin models, and we show that it is far more efficient than previous methods. For potentials unbounded from above, the relaxation part of the method is still efficient in finding minima and transition states, which are usually the primary focus of attention for atomistic systems.
Influence of solvent on the morphology and photocatalytic properties of ZnS decorated CeO{sub 2} nanoparticles
Herein, we report a theoretical and experimental study on the photocatalytic activity of CeO{sub 2} ZnS, and ZnS decorated CeO{sub 2} nanoparticles prepared by a microwave-assisted solvothermal method. Theoretical models were established to analyze electron transitions primarily at the interface between CeO{sub 2} and ZnS. As observed, the particle morphology strongly influenced the photocatalytic degradation of organic dye Rhodamine B. A model was proposed to rationalize the photocatalytic behavior of the prepared decorated systems taking into account different extrinsic and intrinsic defect distributions, including order-disorder effects at interfacial and intra-facial regions, and vacancy concentration.
Parallel continuous simulated tempering and its applications in large-scale molecular simulations
In this paper, we introduce a parallel continuous simulated tempering (PCST) method for enhanced sampling in studying large complex systems. It mainly inherits the continuous simulated tempering (CST) method in our previous studies [C. Zhang and J. Ma, J. Chem. Phys. 130, 194112 (2009); C. Zhang and J. Ma, J. Chem. Phys. 132, 244101 (2010)], while adopts the spirit of parallel tempering (PT), or replica exchange method, by employing multiple copies with different temperature distributions. Differing from conventional PT methods, despite the large stride of total temperature range, the PCST method requires very few copies of simulations, typically 2–3 copies, yet it is still capable of maintaining a high rate of exchange between neighboring copies. Furthermore, in PCST method, the size of the system does not dramatically affect the number of copy needed because the exchange rate is independent of total potential energy, thus providing an enormous advantage over conventional PT methods in studying very large systems. The sampling efficiency of PCST was tested in two-dimensional Ising model, Lennard-Jones liquid and all-atom folding simulation of a small globular protein trp-cage in explicit solvent. The results demonstrate that the PCST method significantly improves sampling efficiency compared with other methods and it is particularly effective in simulating systems with long relaxation time or correlation time. We expect the PCST method to be a good alternative to parallel tempering methods in simulating large systems such as phase transition and dynamics of macromolecules in explicit solvent.