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.

/pdf/large-area-synthesis-of-high-quality-and-uniform-graphene-1mcd5d9hvr.pdf

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

We study a model of pinned bilayer Wigner crystals (WC) and focus on the effects of interlayer coherence (IC) on pinning. We consider both a pseudospin ferromagnetic WC (FMWC) with IC and a pseudospin antiferromagnetic WC (AFMWC) without IC. Our central finding is that a FMWC can be pinned more strongly due to the presence of IC. One specific mechanism is through the disorder induced interlayer tunneling, which effectively manifests as an extra pinning in a FMWC. We also construct a general ``effective disorder'' model and effective pinning Hamiltonian for the case of FMWC and AFMWC respectively. Under this framework, pinning in the presence of IC involves interlayer spatial correlation of disorder in addition to intralayer correlation, leading to enhanced pinning in the FMWC. The pinning mode frequency $({\ensuremath{\omega}}_{\mathrm{pk}})$ of a FMWC is found to decrease with the effective layer separation, whereas for an AFMWC the opposite behavior is expected. An abrupt drop of ${\ensuremath{\omega}}_{\mathrm{pk}}$ is predicted at a transition from a FMWC to AFMWC. Possible effects of in-plane magnetic fields and finite temperatures are addressed. Finally we discuss some other possible ramifications of the FMWC as an electronic supersolid-like phase.

/pdf/pinned-bilayer-wigner-crystals-with-pseudospin-magnetism-35w0375ncd.pdf

Pinned Bilayer Wigner Crystals with Pseudospin Magnetism

In this chapter, we present recent progress in electrical and phase coherent superconducting transport in topological insulators (TIs) with an emphasis on TI nanoribbons (TINRs). We first overview signatures of topological transport in the surface states by analysing the temperature and axial magnetic field dependence of the electrical conductance in the TINRs. In particular, the Aharonov–Bohm effect is studied in more detail as it may provide insight into the exact quantisation of surface modes in one dimension. Finally, we discuss how topological surface states affect superconducting transport across a Josephson junction. We present the temperature and phase dependence of the critical current as powerful probes to investigate topological superconductivity and study the quantum interference patterns of the supercurrent in the axial magnetic field.

Electrical and superconducting transport in topological insulator nanoribbons

Two-dimensional hexagonal boron nitride (hBN) that hosts bright room-temperature single-photon emitters (SPEs) is a promising material platform for quantum information applications. An important step towards the practical application of hBN is the on-demand, position-controlled generation of SPEs. Several strategies have been reported to achieve the deterministic creation of hBN SPEs. However, they either rely on a substrate nanopatterning procedure that is not compatible with integrated photonic devices or utilize a radiation source that might cause unpredictable damage to hBN and underlying substrates. Here, we report a radiation- and lithography-free route to deterministically activate hBN SPEs by nanoindentation with an atomic force microscope (AFM) tip. The method is applied to thin hBN flakes (less than 25 nm in thickness) on flat silicon-dioxide-silicon substrates that can be readily integrated into on-chip photonic devices. The achieved SPEs yields are above 30% by utilizing multiple indent sizes, and a maximum yield of 36% is demonstrated for the indent size of around 400 nm. Our results mark an important step towards the deterministic creation and integration of hBN SPEs with photonic and plasmonic on-chip devices.

Creating Quantum Emitters in Hexagonal Boron Nitride Deterministically on Chip-Compatible Substrates.

Nanoelectromechanical systems (NEMS) based on nanomaterials, especially NEMS resonators made of suspended nanowires, nanotubes or nanosheets have emerged as promising devices for many sensing applications. However, operation of such NEMS resonators in liquids is usually difficult due to strong viscous damping. Here we present our progress in developing NEMS devices based on suspended nanomaterials that can operate in liquids. We present our measurements performed on suspended metallic nanowires driven by AC currents in a magnetic field in liquids.

/pdf/towards-nems-fluid-sensors-based-on-suspended-nanomaterials-2jbyqvl4ha.pdf

Towards NEMS Fluid Sensors Based on Suspended Nanomaterials

Bi2Te2Se (BTS221) bulk crystals were recently discovered as an intrinsic 3D topological insulator. We have synthesized this material, and studied the transport properties of BTS221 from the thermoelectrics perspective. Temperature (T) dependent resistivity measurement indicates surface dominant transports in our sample at low T. We also report Seebeck measurement between 50K to room T.

/pdf/thermoelectric-transport-in-topological-insulator-bi2te2se-iizfizmqr1.pdf

Yong P. Chen

Papers

Pinned Bilayer Wigner Crystals with Pseudospin Magnetism

Electrical and superconducting transport in topological insulator nanoribbons

Creating Quantum Emitters in Hexagonal Boron Nitride Deterministically on Chip-Compatible Substrates.

Towards NEMS Fluid Sensors Based on Suspended Nanomaterials

Thermoelectric transport in topological insulator Bi2Te2Se bulk crystals