We develop an approach of calculating the many-body path integral based on the linked cluster expansion method. First, we derive a linked cluster expansion and we give the diagrammatic rules for calculating the free-energy and the pair distribution function $g(r)$ as a systematic power series expansion in the particle density. We also generalize the hypernetted-chain (HNC) equation for $g(r)$, known from its application to classical statistical mechanics, to a set of quantum HNC equations (QHNC) for the quantum case. The calculated $g(r)$ for distinguishable particles interacting with a Lennard-Jones potential in various attempted schemes of approximation of the diagrammatic series compares very well with the results of path integral Monte Carlo simulation even for densities as high as the equilibrium density of the strongly correlated liquid $^4$He. Our method is applicable to a wide range of problems of current general interest and may be extended to the case of identical particles and, in particular, to the case of the many-fermion problem.
Tertiary structure formation underlies the folding mechanics of many classes of polymers. A simplified model of helical polymers is a useful system in which to begin studying the formation and properties of compact conformations known from biomolecules. Hyper-phase diagrams and structural transitions are presented for polymers of length 40 and 50 over an array of model parameters.
The static structure factor of amorphous silicon (a-Si) models, containing 400,000 atoms with a density of 2.25 g.cm, has been studied by generating atomistic models using classical molecular-dynamics simulations. The behavior of the structure factor, S(Q), in the limit Q -> 0, is examined to determine the degree of hyperuniformity in a-Si and is compared with the results with those from earlier simulations and small-angle X-ray scattering experiments. The study suggests that the computed value of the relative variance of the number of atoms at large distances, and hence S(Q -> 0), lies in the range from 0.00736 to 0.00758, which is very close to the experimental value of 0.0076 +/- 0.0005, obtained from an extrapolation of transmission X-ray scattering data in the small-angle region. The non-zero value of the structure factor S(0) in a-Si can be attributed to density fluctuations on a very large length scale, which is a characteristic property of the structural and topological ordering of silicon atoms in the amorphous state.
The thermally driven differential mutation algorithm is an evolutionary algorithm dealing with the structural optimization of large amorphous systems represented by empirical potentials. It is a hybrid algorithm that combines a differential mutation evolutionary algorithm with a metropolis selection criterion and a cooling schedule inspired by simulated annealing. In this manuscript, the influence of the cooling rate on the quality of obtained amorphous graphene structures is discussed.
We find a lower bound on the computational complexity of Grover’s quantum search algorithms in low-dimensional networks using the renormalization group (RG). It highlights the competition between Grover’s abstract algorithm, i.e., a rotation in Hilbert space, and quantum transport in an actual geometry. It can be characterized in terms of the quantum walk dimension dwQ and the spatial (fractal) dimension df or, alternatively, the spectral dimension of the network, ds, even when translational invariance is broken.
Molecular dynamics (MD) methods suitable for treating anisotropic fluctuations in condensed matter are discussed. Barostats with two anisotropic factors to control the simulation cell dynamics are introduced in three MD simulation methods; constant pressure, constant surface tension, and canonical hydrostatic. To demonstrate the possibilities of these methods, calculations of two systems, one consisting of anisotropic particles and other consisting of isotropic particles, are presented.
The putative ground-state structures of 13-atom Cu and Ag clusters have been studied using ab initio molecular-dynamics (AIMD) based on density-functional theory (DFT). An ensemble of low-energy configurations, collected along the AIMD trajectory and optimized to nearest local minimum-energy configurations, were studied. An analysis of the results suggests the existence of low-symmetric bilayer structures as strong candidates for the putative ground-state structure of Cu 13 and Ag 13 clusters. These bilayer structures are markedly different from a buckled bi-planar (BBP) configuration and energetically favorable, by about 0.4-0.5 eV, than the latter proposed earlier by others. Our study reveals that the structure of the resulting putative global-minimum configuration is essentially independent of the nature of basis functions (i.e., plane waves vs. pseudoatomic orbitals) employed in the calculations, for a given exchange-correlation functional. The structural configurations obtained from plane-wave-based DFT calculations show a noticeably tighter or dense first-shell of Cu and Ag atoms. A comparison of our results with recent full-potential DFT simulations is presented.
We review recent advances in the analysis of the Wang-Landau algorithm, which is designed for the direct Monte Carlo estimation of the density of states (DOS). In the case of a discrete energy spectrum, we present an approach based on introducing the transition matrix in the energy space (TMES). The TMES fully describes a random walk in the energy space biased with the Wang-Landau probability. Properties of the TMES can explain some features of the Wang-Landau algorithm, for example, the flatness of the histogram. We show that the Wang-Landau probability with the true DOS generates a Markov process in the energy space and the inverse spectral gap of the TMES can estimate the mixing time of this Markov process. We argue that an efficient implementation of theWang-Landau algorithm consists of two simulation stages: the original Wang-Landau procedure for the first stage and a 1/t modification for the second stage. The mixing time determines the characteristic time for convergence to the true DOS in the second simulation stage. The parameter of the convergence of the estimated DOS to the true DOS is the difference of the largest TMES eigenvalue from unity. The characteristic time of the first stage is the tunneling time, i.e., the time needed for the system to visit all energy levels.
We present a computational study of void-induced microstructure in amorphous silicon (a-Si) by generating ultra-large models of a-Si with a void-volume fraction of 0.3%, as observed in small-angle X-ray scattering (SAXS) experiments. The relationship between the morphology of voids and the intensity of scattering in SAXS has been studied by computing the latter from the Fourier transform of the reduced pair-correlation function and the atomic-form factor of amorphous silicon. The effect of low-temperature annealing on scattering intensities and the microstructure of voids has been addressed, with particular emphasis on the shape and size of voids, by studying atomic rearrangements on void surfaces and computing the average radius of gyration of the voids from the spatial distribution of surface atoms and the intensity plots in the Guinier approximation. The study suggests that low-temperature annealing can lead to considerable restructuring of void surfaces, which is clearly visible from the three-dimensional shape of the voids but it may not necessarily reflect in one-dimensional scattering-intensity plots.
Supercooled liquids are kinetically trapped materials in which the transition to a thermodynamically more stable state with long-range order is strongly suppressed. To assess the glass-forming abilities of a liquid empirical rules exist, but a comprehensive microscopic picture of devitrification is still missing. Here we study the crystallization of a popular model glass former, the binary Kob-Andersen mixture, in small systems. We perform trajectory sampling employing the population of the locally favored structure as order parameter. While for large population a dynamical phase transition has been reported, here we show that biasing towards a small population of locally favored structures induces crystallization, and we estimate the free energy difference. This result sheds new light on the competition between local and global structure in glass-forming liquids and its implications for crystallization.
Density functional theory calculations of twin-domain walls in strontium titanate are presented. The two possible domain walls are characterized. The domain wall type is set by the relative phase of the octahedral rotations in the body-centered-tetragonal SrTiO$_3$ on either side of the domain wall. The width of the domain walls is estimated to be approximately 3 nm.
Local minima, also known as inherent structures, are expected to play an essential role in the behavior of spin glasses. Here, we propose techniques to efficiently sample these configurations in Monte Carlo simulations. For the Sherrington-Kirkpatrick and the three-dimensional Edwards-Anderson model their spectra are determined and compared to analytical results.
Laser absorptance (coupling efficiency) changes rapidly during laser metal processing due to temperature and multiple phase changes. We find that there is limited literature data available, but that this property is vital for accurate process simulations, as well as the fundamental understanding of the dynamics of high-power laser-matter interaction. We measure the dynamic, absolute absorbed power with sub-microsecond resolution during a 10 ms laser spot weld in 316L stainless steel from conduction to keyhole welding conditions using an integrating sphere. Our data show several important features including the points of melting and keyhole formation. Conclusions from the optical data are supported by weld cross-section analysis.
Element loss during high-power laser processing of metals can lead to deleterious chemical and mechanical effects that negatively impact the processed material’s quality. Currently, there is no suitable way to measure time-resolved element loss in situ during laser processing. In this work, we show that laser-induced fluorescence (LIF) can be used to temporally resolve individual element vaporization during a laser spot welding of 316L stainless steel. As a proof-of-concept, we measured iron loss during a 500 µs laser spot weld with a time resolution of 5 microseconds. We found that keyhole formation could clearly be identified by a dramatic increase in iron emission from the weld pool. This conclusion was validated by independent, time-resolved measurements of laser absorptance during laser spot welding from our previous work.
A series of tests have been conducted at the Wood Hole Oceanographic Institution's National Ocean Sciences Accelerator Mass Spectrometry facility (NOSAMS) to investigate the effect of sample well geometry and cathode material on C− extraction efficiency and beam currents. Ion current production tests were performed on aluminum cathodes that were prepared by drilling sample wells with various diameters (Ø), ranging from 0.50mm to 1.50mm, and depths ranging from 1.3mm to 4.3mm. Cathodes with sample well diameters of 1mm and 0.75mm had marginally better C− current, while current for the larger sample wells was lower but more consistent. Depth tests showed an obvious difference in ion beam currents, with shallow wells outperforming the deeper wells. Efficiency tests were first conducted on Al cathodes to find an optimum diameter. Cathodes with Ø of 0.50mm, 0.75mm, and 1.00mm were drilled to a depth of 2.3mm, hand pressed with approximately 250μg of Alfa Aesar graphite, and then run to exhaustion. The best performers were cathodes with Ø of 0.75mm, measuring as much as 16.5% efficiency compared to 13% from the 0.50mm and 15% from the 1.00mm cathodes. Cathodes with Zn inserts were then prepared in the same manner, with a 0.75mm diameter, and showed further improvement, increasing the ion source efficiency to as much as 27%.
Conventional radiography based on attenuation of a well-collimated beam remains the mainstay of neutron imaging. The spatial resolution attained with this pinhole-camera method depends on the beam collimation; therefore, achieving the spatial resolution of a few microns is practically difficult, since collimating the neutron beam results in a low flux. The use of focusing devices allows maintaining sufficient spatial resolution without collimating the beam. Therefore, axisymmetric grazing-incidence focusing mirrors (Wolter mirrors) have begun to be introduced to neutron imaging. In this paper, a design of a neutron microscope for NIST Center for Neutron Research (NCNR) is presented. We evaluate the spatial resolution and study field curvature aberrations of Wolter mirrors through ray-tracing simulations. A general formula is found describing the field curvature, and ways to counter these aberrations are discussed.
Lawrence Livermore National Laboratory (LLNL) is developing a high-brightness, quasi-monoenergetic neutron source for fast neutron-based imaging. The intensity of the neutron source is expected to be 1010∼11 n/s/sr with energies set at 7 MeV or 10 MeV with 5% bandwidth at 0-degrees. The neutrons are produced by the D(d,n) reaction using one of two accelerators of consisting of 1 or 2 RFQs (depending on the desired energy) and a DTL. Each accelerator will deliver approximately 100- or 300-uA (depending on the energy) average current deuteron beam onto a pulsed deuterium gas target or a solid target, depending on the application. In this paper, we discuss some of our benchmarking measurements and overall progress in preparation to be online in late 2017. We also discuss our performance expectations and broader set of applications.
The Colorado School of Mines installed a neutron radiography system at the United States Geological Survey TRIGA reactor in 2012. An upgraded beamline could dramatically improve the imaging capabilities of this system. This project performed computational analyses to support the design of a new beamline, with the major goals of minimizing beam divergence and maximizing beam intensity. The new beamline will consist of a square aluminum tube with an 11.43 cm (4.5 in) inner side length and 0.635 cm (0.25 in) thick walls. It is the same length as the original beam tube (8.53 m) and is composed of 1.22 m (4 ft) and 1.52 m (5 ft) flanged sections which bolt together. The bottom 1.22 m of the beamline is a cylindrical aluminum pre-collimator which is 0.635 cm (0.25 in) thick, with an inner diameter of 5.08 cm (2 in). Based on Monte Carlo model results, when a pre-collimator is present, the use of a neutron absorbing liner on the inside surface of the beam tube has almost no effect on the angular distribution of the neutron current at the collimator exit. The use of a pre-collimator may result in a non-uniform flux profile at the image plane; however, as long as the collimator is at least three times longer than the pre-collimator, the flux distortion is acceptably low. Published by Elsevier B.V.
The Los Alamos Neutron Science Center (LANSCE) provides the scientific community with intense sources of neutrons, which can be used to perform experiments supporting civilian and national security research. These measurements include nuclear physics experiments for the defense program, basic science, and the radiation effect programs. This paper focuses on the radiation effects program, which involves mostly accelerated testing of semiconductor parts. When cosmic rays strike the earth's atmosphere, they cause nuclear reactions with elements in the air and produce a wide range of energetic particles. Because neutrons are uncharged, they can reach aircraft altitudes and sea level. These neutrons are thought to be the most important threat to semiconductor devices and integrated circuits. The best way to determine the failure rate due to these neutrons is to measure the failure rate in a neutron source that has the same spectrum as those produced by cosmic rays. Los Alamos has a high-energy and a low-energy neutron source for semiconductor testing. Both are driven by the 800-MeV proton beam from the LANSCE accelerator. The high-energy neutron source at the Weapons Neutron Research (WNR) facility uses a bare target that is designed to produce fast neutrons with energies from 100 keV to almost 800 MeV. The measured neutron energy distribution from WNR is very similar to that of the cosmic-ray-induced neutrons in the atmosphere. However, the flux provided at the WNR facility is typically 5×107 times more intense than the flux of the cosmic-ray-induced neutrons. This intense neutron flux allows testing at greatly accelerated rates. An irradiation test of less than an hour is equivalent to many years of neutron exposure due to cosmic-ray neutrons. The low-energy neutron source is located at the Lujan Neutron Scattering Center. It is based on a moderated source that provides useful neutrons from subthermal energies to ∼100 keV. The characteristics of these sources, and ongoing industry program are described in this paper.
The flux of secondary neutrons generated in collisions of the 660 MeV proton beam with the massive natural uranium spallation target was investigated using a set of monoisotopic threshold activation detectors. Sandwiches made of thin high-purity Al, Co, Au, and Bi metal foils were installed in different positions across the whole spallation target. The gamma-ray activity of products of (n,xn) and other studied reactions was measured offline with germanium semiconductor detectors. Reaction yields of radionuclides with half-life exceeding 100min and with effective neutron energy thresholds between 3.6 MeV and 186 MeV provided us with information about the spectrum of spallation neutrons in this energy region and beyond. The experimental neutron flux was determined using the measured reaction yields and cross-sections calculated with the TALYS 1.8 nuclear reaction program and INCL4-ABLA event generator of MCNP6. Neutron spectra in the region of activation sandwiches were also modeled with the radiation transport code MCNPX 2.7. Neutron flux based on excitation functions from TALYS provides a reasonable description of the neutron spectrum inside the spallation target and is in good agreement with Monte-Carlo predictions. The experimental flux that uses INCL4 cross-sections rather underestimates the modeled spectrum in the whole region of interest, but the agreement within few standard deviations was reached as well. The paper summarizes basic principles of the method for determining the spectrum of high-energy neutrons without employing the spectral adjustment routines and points out to the need for model improvements and precise cross-section measurements.
