Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Statistical method for testing the neutral mutation hypothesis by DNA polymorphism.

BACKGROUND Materials by design is an appealing idea that is very hard to realize in practice. Combining the best of different ingredients in one ultimate material is a task for which we currently have no general solution. However, we do have some successful examples to draw upon: Composite materials and III-V heterostructures have revolutionized many aspects of our lives. Still, we need a general strategy to solve the problem of mixing and matching crystals with different properties, creating combinations with predetermined attributes and functionalities. ADVANCES Two-dimensional (2D) materials offer a platform that allows creation of heterostructures with a variety of properties. One-atom-thick crystals now comprise a large family of these materials, collectively covering a very broad range of properties. The first material to be included was graphene, a zero-overlap semimetal. The family of 2D crystals has grown to includes metals (e.g., NbSe 2 ), semiconductors (e.g., MoS 2 ), and insulators [e.g., hexagonal boron nitride (hBN)]. Many of these materials are stable at ambient conditions, and we have come up with strategies for handling those that are not. Surprisingly, the properties of such 2D materials are often very different from those of their 3D counterparts. Furthermore, even the study of familiar phenomena (like superconductivity or ferromagnetism) in the 2D case, where there is no long-range order, raises many thought-provoking questions. A plethora of opportunities appear when we start to combine several 2D crystals in one vertical stack. Held together by van der Waals forces (the same forces that hold layered materials together), such heterostructures allow a far greater number of combinations than any traditional growth method. As the family of 2D crystals is expanding day by day, so too is the complexity of the heterostructures that could be created with atomic precision. When stacking different crystals together, the synergetic effects become very important. In the first-order approximation, charge redistribution might occur between the neighboring (and even more distant) crystals in the stack. Neighboring crystals can also induce structural changes in each other. Furthermore, such changes can be controlled by adjusting the relative orientation between the individual elements. Such heterostructures have already led to the observation of numerous exciting physical phenomena. Thus, spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system. The possibility of positioning crystals in very close (but controlled) proximity to one another allows for the study of tunneling and drag effects. The use of semiconducting monolayers leads to the creation of optically active heterostructures. The extended range of functionalities of such heterostructures yields a range of possible applications. Now the highest-mobility graphene transistors are achieved by encapsulating graphene with hBN. Photovoltaic and light-emitting devices have been demonstrated by combining optically active semiconducting layers and graphene as transparent electrodes. OUTLOOK Currently, most 2D heterostructures are composed by direct stacking of individual monolayer flakes of different materials. Although this method allows ultimate flexibility, it is slow and cumbersome. Thus, techniques involving transfer of large-area crystals grown by chemical vapor deposition (CVD), direct growth of heterostructures by CVD or physical epitaxy, or one-step growth in solution are being developed. Currently, we are at the same level as we were with graphene 10 years ago: plenty of interesting science and unclear prospects for mass production. Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of the heterostructures, making the science and applications more achievable.

2D materials and van der Waals heterostructures

Due to the presence of dissipationless edge states, the quantum anomalous Hall (QAH) insulator has garnered significant attention for both fundamental research and practical application. However, the majority of QAH insulators suffer from a low Chern number (C = 1), and the Chern number is basically unadjustable, which constrains their potential application in spintronic devices. Here, based on a tight-binding model and first-principles calculations, we propose that two-dimensional (2D) ferromagnetic monolayer NdN2 exhibits a high-Chern-number QAH effect with C = ±3, accompanied by a nontrivial band gap of 97.4 meV. More importantly, by manipulating the magnetization orientation in the xz plane, the Chern number of 2D NdN2 can be further tuned between C = ±3 and C = ±1. When the magnetization vector is confined to the xy plane, the monolayer NdN2 would exhibit either a Dirac half-semimetal or in-plane QAH phase. Moreover, the QAH effect with a higher Chern number C = 9 can be achieved by constructing a multilayer van der Waals heterostructure composed of monolayers NdN2 and BN with alternative stacking order. These findings provide a reliable platform for exploring the novel QAH effect and developing high-performance topological devices.

Quantum anomalous Hall effect with a high and tunable Chern number in monolayer NdN2.

ARCHER [1] is a parallel Monte Carlo radiation transport code being developed for various hardware platforms including multi-core CPUs, the emerging Nvidia graphics processing units (GPUs) and Intel Xeon Phi coprocessors. Previous studies have demonstrated the accuracy and speed for CT imaging and radiotherapy dosimetry applications. Radiation shielding applications, however, require ARCHER to be more flexible in defining and handling geometries, in addition to the voxel phantom geometries that are commonly used in medical physics calculations. In this paper, we present the development of a general geometry module used for radiation protection and shielding problems in ARCHER. The methods of geometry design and particle transport for the GPUs are introduced. Preliminary results comparing against the MCNP code for cases involving Cs-137 0.662MeV gammas are reported.

http://neams.rpi.edu/jiw2/papers/RPSD2014%20ARCHER%20Shielding.pdf

Development of CSG-based radiation shielding module for ARCHER: preliminary results for photons

Liquid air energy storage system based on fluidized bed heat transfer

The characteristics of InGaAs quantum dot laser diodes have been investigated by inserting InGaAs wells of different thicknesses in the active zone. The simulation results indicated that the maximum output power of the InGaAs quantum dot laser diode with a 0 nm-thick InGaAs well is 0.72 W at an injection current of 1.1 A. However, the quantum dot laser diode with a 12 nm-thick InGaAs well has a maximum electro-optical conversion efficiency of 40.6%. The properties of carrier transport in the active zone were analyzed via the potential difference of the band offset among potential barrier, potential well, and waveguide layer. It is observed that electron leakage increases owing to the inserted InGaAs well. Auger recombination not only increases the carrier injection efficiency but also is acts as the key factor of carrier loss in the active zone. Moreover, the inserted InGaAs well can improve carrier transport in the active zone and increase the carrier utilization efficiency. The power-loss mechanism of the quantum dot laser diode was elucidated from the optical and carrier losses. It is found that optical loss is the key factor that affects the output power.

Influence of potential well thickness on the carrier transport characteristics of InGaAs quantum dot laser diodes.

The latest release of MCNP6 contains the capability to represent geometry in unstructured meshes. The unstructured mesh features, however, have been tested with only limited examples to date. The aim of this paper is to examine the use of the new unstructured mesh features for space radiation flux calculations involving a space habitat during a solar particle event. High energy proton transport, alongside its secondary particles, a modeling capability integrated from MCNPX, was tested with MCNP6’s unstructured mesh feature to gain insight into the potential uses and limitations of MCNP6’s development. Abaqus was used to generate an unstructured tetrahedral mesh of a space habitat structure, which was then used with MCNP6.1.1 Beta to simulate a Solar Particle Event (SPE) consisting of a high flux of protons of energies up to 500 MeV. Trial simulations were performed using 1 st and 2 nd order tetrahedral meshes, however it is concluded

http://neams.rpi.edu/jiw2/papers/M&C2015%20Zieb.pdf

Wei Ji

Papers

Quantum anomalous Hall effect with a high and tunable Chern number in monolayer NdN2.

Development of CSG-based radiation shielding module for ARCHER: preliminary results for photons

Liquid air energy storage system based on fluidized bed heat transfer

Influence of potential well thickness on the carrier transport characteristics of InGaAs quantum dot laser diodes.

Preliminary investigation of mcnp6 unstructured mesh geometry for radiation flux calculations involving space environment