There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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’

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

Scientific Reports 6: Article number: 34474; published online: 29 September 2016; updated: 08 May 2017. As the authors of references 25 and 26 also employed plasma-assisted molecular beam epitaxy for the synthesis of hexagonal boron nitride films, the authors would like to make the following changesto the Introduction section of their Article:

/pdf/corrigendum-hexagonal-boron-nitride-tunnel-barriers-grown-on-953toj2xfg.pdf

Corrigendum: Hexagonal Boron Nitride Tunnel Barriers Grown on Graphite by High Temperature Molecular Beam Epitaxy.

Polymer thin films that emit and absorb circularly polarised light have been demonstrated with the promise of achieving important technological advances; from efficient, high-performance displays, to 3D imaging and all-organic spintronic devices. However, the origin of the large chiroptical effects in such films has, until now, remained elusive. We investigate the emergence of such phenomena in achiral polymers blended with a chiral small-molecule additive (1-aza[6]helicene) and intrinsically chiral-sidechain polymers using a combination of spectroscopic methods and structural probes. We show that – under conditions relevant for device fabrication – the large chiroptical effects are caused by coupling of electric and magnetic transition dipole moments (natural optical activity), not structural chirality as previously assumed, and may occur because of local order in a cylinder blue phase-type organisation. This disruptive mechanistic insight into chiral polymer thin films will offer new approaches towards chiroptical materials development after almost three decades of research in this area.

/pdf/a-unified-model-to-explain-the-large-chiroptical-effects-in-4cbx0okoff.pdf

A Unified Model to Explain the Large Chiroptical Effects in Polymer Systems Through Natural Optical Activity

Gunn effect oscillator

(AlGa)As–GaAs–(AlGa)As double barrier resonant tunneling diodes with different δ‐doping levels in the GaAs quantum well (QW) together with undoped control samples have been investigated. A new subthreshold peak in the I(V) characteristics is observed and assigned to resonant tunneling through the bound state of a shallow donor impurity in the QW. By comparing the I(V) characteristics for wafers grown at different temperatures between 480 and 630 °C, the effects of Si segregation from contact layers into the well can be identified. This constitutes a new technique of assessment of donors which is sensitive to areal doping densities as low as 107 cm−2. The effect of low growth temperature (480 °C) and δ doping on peak/valley ratios of the resonances on the I(V) characteristic is also assessed.

Effect of Si δ doping and growth temperature on the I(V) characteristics of molecular‐beam epitaxially grown GaAs/(AlGa)As resonant tunneling devices

A novel fabrication process, based on selective wet etching and photolithography, has been developed to produce quantum wire resonant tunnelling diodes contacted via a free-standing GaAs bridge Current-voltage characteristics of quantum wire resonant tunnelling diodes have been studied at 300 mK in the presence of a magnetic field oriented either parallel to or perpendicular to the current For the smallest device, in the presence of a magnetic field oriented perpendicular to the current and parallel to the wire, we are able to deduce the probability density of the lowest three bound states from the magnetic field dependence of the current and show that the one-dimensional confining potential is close to parabolic In the presence of a magnetic field oriented parallel to the current a continuous transition from electrostatic (at low field) to magnetic (at high field) confinement is observed

Peter H. Beton

Papers

Corrigendum: Hexagonal Boron Nitride Tunnel Barriers Grown on Graphite by High Temperature Molecular Beam Epitaxy.

A Unified Model to Explain the Large Chiroptical Effects in Polymer Systems Through Natural Optical Activity

Gunn effect oscillator

Effect of Si δ doping and growth temperature on the I(V) characteristics of molecular‐beam epitaxially grown GaAs/(AlGa)As resonant tunneling devices

A novel approach in fabrication and study of laterally quantum-confined resonant tunnelling diodes