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’

Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.

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Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils

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Observation of long-lived persistent spin polarization in a topological insulator.

The search of novel topological states, such as the quantum anomalous Hall insulator and chiral Majorana fermions, has motivated different schemes to introduce magnetism into topological insulators. A promising scheme is using the magnetic proximity effect (MPE), where a ferromagnetic insulator magnetizes the topological insulator. Most of these heterostructures are synthesized by growth techniques which prevent mixing many of the available ferromagnetic and topological insulators due to difference in growth conditions. Here, we demonstrate that MPE can be obtained in heterostructures stacked via the dry transfer of flakes of van der Waals ferromagnetic and topological insulators (Cr2Ge2Te6/BiSbTeSe2), as evidenced in the observation of an anomalous Hall effect (AHE). Furthermore, devices made from these heterostructures allow modulation of the AHE when controlling the carrier density via electrostatic gating. These results show that simple mechanical transfer of magnetic van der Waals materials provides another possible avenue to magnetize topological insulators by MPE.

Gate-Tunable Anomalous Hall Effect in Stacked van der Waals Ferromagnetic Insulator-Topological Insulator Heterostructures.

Transmission electron microscopy is applied to investigate the effect of postannealing on misfit dislocations in an In{sup 0.2}Ga{sup 0.8}As/GaAs(001) heterostructure. An orthogonal array of 60{degree} dislocations along [110] and [110] directions was observed in the interfaces of the samples grown by MBE at 520C. When the as-grown samples were annealed at temperatures ranging from 600 to 800C, the 60{degree} dislocations were gradually reoriented by dislocation reactions occurring at the 90{degree} intersections followed by nonconservative motion driven by dislocation line tension and the residual elastic misfit strain. The final result of this process was a dislocation array lying along [100] and {center_dot} [010] directions. The reoriented u = dislocation has a Burgers vector b = a/2 , which is the same as that of 60{degree} dislocation, but the edge component of its Burgers vector in the (001) interfacial plane is larger than that of 60{degree} dislocation by a factor of {radical}2, resulting in a greater contribution to elastic strain relaxation. This nonconservative reorientation of 60{degree} dislocations to form the u= dislocations represents a new strain relaxation mechanism in diamond or zinc blende semiconductor heterostructures.

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Reorientation of Misfit Dislocations During Annealing in InGaAs/GaAs(001) Interfaces

We report wafer-scale graphene synthesized by chemical vapor deposition (CVD) on copper foils at ambient pressure. Graphene films up to 4 inches in size are synthesized and transferred to SiO2/Si. Spectroscopic Raman mapping demonstrates that the synthesized films consist primarily of monolayer graphene (with as high as ~90% area coverage). Low temperature transport measurements are performed on devices made from such CVD graphene. We observe ambipolar field effect (with on/off ratio ~5 and carrier mobilities up to ~3000 cm/Vs) and the hall-mark “half-integer” quantum Hall effect. We also observe weak localization of carriers and extract phase coherence length up to 0.3 μm.

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Wafer-scale Graphene Synthesized by Chemical Vapor Deposition at Ambient Pressure

We report our spectroscopic studies of the $d \ ^3\Pi$ state of ultra-cold $^7$Li$^{85}$Rb using resonantly-enhanced multi-photon ionization and depletion spectroscopy with bound-to-bound transitions originating from the metastable $a \ ^3\Sigma^+$ state. We evaluate the potential of this state for use as the intermediate state in a STIRAP transfer scheme from triplet Feshbach LiRb molecules to the $X \ ^1\Sigma^+$ ground state, and find that the lowest several vibrational levels possess the requisite overlap with initial and final states, as well as convenient energies. Using depletion measurements, we measured the well depth and spin-orbit splitting. We suggest possible pathways for short-range photoassociation using deeply-bound vibrational levels of this electronic state.

Yong P. Chen

Papers

Observation of long-lived persistent spin polarization in a topological insulator.

Gate-Tunable Anomalous Hall Effect in Stacked van der Waals Ferromagnetic Insulator-Topological Insulator Heterostructures.

Reorientation of Misfit Dislocations During Annealing in InGaAs/GaAs(001) Interfaces

Wafer-scale Graphene Synthesized by Chemical Vapor Deposition at Ambient Pressure

The $d \: ^3 \Pi$ state of LiRb