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

The attenuation and velocity shift of sound waves in metallic spin glasses at temperatures well below the freezing temperature have been studied in the context of relaxational dynamics within a metastable state. The damping of the sound wave is critically dependent on the small- lambda form of rho ( lambda ), the density of relaxation modes of frequency lambda . Inclusion of precessional dynamics has only a quantitative effect on the damping at low frequencies. The concentration dependence of the attenuation for RKKY spin glasses has been considered.

Sound attenuation and relaxational dynamics in spin glasses

The surface structures resulting from the deposition of various coverages of Sb on the GaAs(111)B‐(2×2) surface at room temperature, followed by annealing in the 100–375 °C temperature range, have been investigated using scanning tunneling microscopy. At low annealing temperatures Sb islands are observed displaying no ordered atomic structure, between which the As trimer based (2×2) reconstruction of the clean GaAs surface is visible. Annealing above 250 °C causes the formation of Sb trimers which are distinguishable from the remaining As trimers via contrast differences in filled‐ and empty‐state images. Annealing at still higher temperatures leads to the creation of Sb chain pairs oriented along the substrate 〈110〉 directions, coexisting with regions of Sb trimer based reconstruction between the chain pairs. The local periodicity of the patterns of Sb trimers between the chain pairs is dependent on the separation between chain pairs.

Island, trimer, and chain formation on the Sb‐terminated GaAs(111)B surface

Experiments and calculations are used to illustrate the role of nonequilibrium electron transport in determining the performance of InP/In 0.53 Ga 0.47 As heterostructure bipolar transistors. The intrinsic small signal response of devices operated under low voltage bias conditions is subpicosecond. However, with increasing bias the intrinsic speed of devices decreases because of scattering into low velocity subsidiary X- and L-valleys.

Nonequilibrium electron dynamics in bipolar transistors

We have observed a new sub-threshold peak in the I(V) characteristics of double-barrier resonant tunneling structures. By investigating a range of intentionally δ-doped and control layers we can relate this peak to resonant tunneling through the bound state of a shallow donor impurity in the quantum well. We have studied the dependence of the voltage position of this peak on the magnitude of a magnetic field applied either parallel or perpendicular to the plane of the quantum well. In the parallel orientation the donor peak shifts slightly to lower voltage whereas the threshold of the continuum resonance shows the usual large quadratic shift to higher voltage. The amplitude of the donor-related peak is reduced in the presence of a magnetic field. We present a theoretical model to explain this behaviour and show how the results can be used to probe the spatial form of the impurity wavefunction in the quantum well.

High magnetic field studies of resonant tunneling via shallow impurities in δ-doped quantum wells

The assembly of molecular networks into structures such as random tilings and glasses has recently been demonstrated for a number of two-dimensional systems. These structures are dynamically arrested on experimental time scales, so the critical regime in their formation is that of initial growth. Here, we identify a transition from energetic to entropic stabilization in the nucleation and growth of a molecular rhombus tiling. Calculations based on a lattice-gas model show that clustering of topological defects and the formation of faceted boundaries followed by a slow relaxation to equilibrium occur under conditions of energetic stabilization. We also identify an entropically stabilized regime in which the system grows directly into an equilibrium configuration without the need for further relaxation. Our results provide a methodology for identifying equilibrium and nonequilibrium randomness in the growth of molecular tilings, and we demonstrate that equilibrium spatial statistics are compatible with exponentially slow dynamical behavior.

Peter H. Beton

Papers

Sound attenuation and relaxational dynamics in spin glasses

Island, trimer, and chain formation on the Sb‐terminated GaAs(111)B surface

Nonequilibrium electron dynamics in bipolar transistors

High magnetic field studies of resonant tunneling via shallow impurities in δ-doped quantum wells

Entropically stabilized growth of a two-dimensional random tiling.