Search

The big events cause substantial deviations in the city traffic, especially railway transportation which happens to be the dominant mode of transportation for most of the major cities. Analysis of big events' impact on railway transportation, therefore, is of great importance to urban planning and transport management, yet is quite challenging because of the lack of readily available data about railway passengers' citywide flow and event participants' choice of transportation mode. Previous works have mainly relied on precise but limited data like sensors, AFC (Automated Fare Collection), or smart-card get-on get-off data to estimate the railway passengers, and did not take a holistic multi-dimensional approach to analyze railway traffic congestion caused by the events. To tackle these challenges, we propose a novel interactive Dashboard system that utilizes millions of smartphone GPS records across Japan. The dashboard can estimate and visualize railway passengers for the stations nearby the event venue as well as other relevant event participants' information. We also introduce a Congestion Index to measure the increase in congestion of stations during events. The dashboard can be highly useful for event organizers, railway administrators, and city planners to assess and compare the impact of big events on railway traffic. [ABSTRACT FROM AUTHOR]

The electron transport properties with the relativistic intrinsic and Rashba spin-orbit (SO) couplings in a graphene nanoribbon are investigated by using the third-nearest-neighbor tight-binding model and the nonequilibrium Green function method. The numerical results display a perfect quantum spin Hall state (QSH) surrounded by a simple insulator state, presented in Rashba SO coupling vs sublattice potential plane. The QSH state is enhanced by the intrinsic SO coupling, indicating a quantum topological property of graphene nanoribbons. [ABSTRACT FROM AUTHOR]

We discover a method of opening the conductive channels of zigzag graphene nanoribbons (ZGNRs) by using hydrogenation chains to separate the nanoribbon into two strips with a ∼0.7 Å distance, although the overall hydrogenation on graphene transforms the highly conductive semimetal sheet into an insulator. Two edge-like states emerge around each hydrogenation chain. The conductance enhancement, made by the hydrogenation chain, is found in nanoribbons with 5 to 7 chains, e.g. 5ZGNRH, 6ZGNRH, 7ZGNRH, and 7ZGNR2H (7ZGNR with two hydrogenation chains). The ZGNRs with hydrogenation chains illustrate their potential in nanoelectronics and carbon electronics as electronic leads and nonlinear devices. [ABSTRACT FROM AUTHOR]

We use density functional theory based nonequilibrium Green's function to self-consistently study the current through the 1,4-benzenedithiol (BDT). The elastic and inelastic tunneling properties through this Au-BDT-Au molecular junction are simulated, respectively. For the elastic tunneling case, it is found that the current through the tilted molecule can be modulated effectively by the external gate field, which is perpendicular to the phenyl ring. The gate voltage amplification comes from the modulation of the interaction between the electrodes and the molecules in the junctions. For the inelastic case, the electron tunneling scattered by the molecular vibrational modes is considered within the self-consistent Born approximation scheme, and the inelastic electron tunneling spectrum is calculated. [ABSTRACT FROM AUTHOR]

Group-IV element clathrates have attracted considerable interest in recent years. Here, we report an ab initio study on the structural stability of carbon clathrates at high pressure and identify fcc-C136 clathrate as the third most stable carbon phase after cubic diamond and hexagonal graphite. A pressure-induced phase transition is predicted to occur around 17 GPa from hexagonal graphite to fcc-C136, which is more stable than other carbon clathrates such as hex-C40 and sc-C46, and the recently predicted metastable M-carbon up to 26 GPa. Phonon dispersion calculations confirm the dynamic stability of fcc-C136 as well as diamond. [ABSTRACT FROM AUTHOR]

With nonequilibrium Green’s function approach combined with density functional theory, we perform an ab initio calculation to investigate transport properties of graphene nanoribbon (GNR) junctions self-consistently. Tight-binding approximation is applied to model the zigzag (ZGNR) electrodes, and its validity is confirmed in comparison to the GAUSSIAN03 periodic boundary condition calculation result of the same system. The origin of abnormal jump points usually appearing in the transmission spectrum is explained with the detailed tight-binding ZGNR band structure. Transport property of an edge-defect ZGNR junction is investigated, and the tunable tunneling current can be sensitively controlled by transverse electric fields. [ABSTRACT FROM AUTHOR]

The structure and vibrational properties of high- and low-density amorphous (HDA and LDA, respectively) ices have been determined using reverse Monte Carlo, molecular dynamics, and lattice dynamics simulations. This combined approach leads to a more accurate and detailed structural description of HDA and LDA ices when compared to experiment than was previously possible. The water molecules in these ices form well connected hydrogen-bond networks that exhibit modes of vibration that extend throughout the solid and can involve up to 70% of all molecules. However, the networks display significant differences in their dynamical behavior. In HDA, the extended low-frequency vibrational modes occur in dense parallel two dimensional layers of water that are approximately 10 nm thick. In contrast, the extended modes in LDA resemble a holey structure that encapsulates many small pockets of nonparticipating water molecules. [ABSTRACT FROM AUTHOR]

We use density functional theory based nonequilibrium Green’s function to calculate the current through the different rodlike molecules at the finite temperatures self-consistently, which was compared to the experimental measurements presented by Reichert et al. [Phys. Rev. Lett. 88, 176804 (2002)] and by Mayor et al. [Angew. Chem. Int. Ed. 42, 5834 (2003)], respectively. Our results agree with the measurements very well, especially for the bias around ±1.0 V. The investigation of the topological effect for the symmetrical molecule reveals the fact that the para position compound provides a considerably larger conductance than the meta one. [ABSTRACT FROM AUTHOR]

Electronic transport properties of alkanedithiols are calculated by a first-principles method based on density functional theory and nonequilibrium Green’s function formalism. At small bias, the I-V characteristics are linear and the resistances conform to the Magoga’s exponential law. The calculated length-dependent decay constant γ which reflects the effect of internal molecular structure is in accordance with most experiments quantitatively. Also, the calculated effective contact resistance R0 is in good agreement with the results of repeatedly measuring molecule-electrode junctions [B. Xu and N. Tao, Science 301, 1221 (2003)]. [ABSTRACT FROM AUTHOR]

The bistable molecular switches have been studied theoretically based on the first-principles calculation. The geometry structures of the switches studied in this paper can be triggered between two symmetrical structures by using an external applied electric field. I-V characteristic curves of the different molecule configurations have been calculated, and distinguishability of these characteristic curves indicates a switching behavior, the performance of which can be improved significantly by some suitable donors and acceptors. [ABSTRACT FROM AUTHOR]

The quantum transport of a gated polythiophene nanodevice is analyzed using density functional theory and nonequilibrium Green’s function approach. For this typical molecular field effect transistor, we prove the existence of two main features of electronic components, i.e., negative differential resistance and good switching. Ab initio based explanations of these features are provided by distinguishing fixed and shifting conducting states, which are shown to arise from the interface and functional molecule, respectively. The results show that proper functional molecules can be used in conjunction with metallic electrodes to achieve basic electronics functionality at molecular length scales. [ABSTRACT FROM AUTHOR]

We use a self-consistent method to study the current of the single molecular transistor modulated by the transverse field in the level of the density functional theory and the nonequilibrium Green function method. The numerical results show that both the polyacene-dithiol molecules and the fused-ring thiophene molecules are the potential high-frequency molecular transistors controlled by the transverse field. The longer molecules of the polyacene-dithiol or the fused-ring thiophene are in favor of realizing the gate-bias controlled molecular transistor. The theoretical results suggest the related experiments. [ABSTRACT FROM AUTHOR]

We study theoretically the vertical electron transport in semiconductor multiple-quantum-well structures, where sequential tunneling between neighboring wells takes place. The nonuniformity of electric field along the growth axis and charge redistribution among the quantum wells, as well as between the inner wells and contacts, are taken into account. A simple and efficient model of charged contact layers is proposed. The calculated I-V curves exhibit regions of conventional N-shaped negative differential conductivity and Z-shaped portions of intrinsic bistability, both arising due to the tunneling resonances. A general explanation of their formation mechanism is given, which is valid for any form of interwell transitions of resonant nature. The conditions of N- and Z-shaped curve observation and controllable transition between them are discussed. [ABSTRACT FROM AUTHOR]

Investigates a body-centered-cubic lattice model with realistic many-body interactions to describe both crystalline and molten states of silicon. Energy of the atoms surrounded by tetrahedral first-neighbors; First-order transition between a solid with diamond structure and a melt; Nucleation and domain growth in certain range of supercooling.

The electronic and magnetic properties of double-impurities-doped TiO2 (rutile) are explored using first-principles calculations within the generalized gradient approximation to examine their potential use as spintronic system. Calculations are performed on all possible sets of double impurities (M1 and M2) from M1=Cr, Mn, Fe, Co, and Ni, and M2=Mo, W, and Re. The results show the overlapping of the highest occupied impurity states with the bottom of the conduction band of the host system and the half-metallic nature in Fe- and W-doped TiO2, making them suitable as spintronic systems. These impurities preferentially substitute adjacent Ti sites and form a stable magnetic complex with oxygen atoms. [ABSTRACT FROM AUTHOR]

The properties of binary H2+ CH4 clathrate hydrates have been estimated using the extended van der Waals and Platteeuw statistical thermodynamic model that takes into account the lattice relaxation, host-guest, and guest-guest interactions as well as the quantum nature of guest behavior in the clathrate cavities. It has been found that at a small methane concentration in the gas phase the stable hydrate phase has cubic structure II (CS-II) and at a methane concentration of 6% stabilizes cubic structure I, which is metastable in the case of the pure hydrogen hydrate. This is in agreement with recent experimental data. The amount of hydrogen storage depends on the methane concentration in the gas phase as well as the thermodynamic conditions of hydrate formation. Hydrogen storage up to 2.6 wt. % can be achieved in the binary H2+ CH4 CS-II hydrate at T = 250K [ABSTRACT FROM AUTHOR]

An original approach accounting for multiple cavity occupancy, host lattice relaxation and the description of the quantum nature of guest behaviour has been used for the estimation of the thermodynamic properties of pure hydrogen and binary C2H6 + H2 hydrates with the possibility of multiple filling of cavities by guest molecules. It has been found that the pure hydrogen cubic structure II (CS-II) hydrate is more thermodynamically stable than the cubic structure I (CS-I) hydrate in a wide range of p–T regions. However, at low pressure, the stabilisation of the CS-I hydrate can be realised for H2–C2H6–H2O systems even with small concentrations of ethane in the gas phase. However, in this case, the amount of stored hydrogen strongly depends on the ethane concentrations in the gas phase. At low concentration of ethane, the amount of hydrogen stored, 2.5 wt%, in CS-I hydrate can be achieved at T = 250 K. We believe that the present approach can be useful for understanding the thermodynamic properties of the binary hydrate and it can support the experimental exploration of novel hydrogen storage materials based on clathrate hydrates. [ABSTRACT FROM PUBLISHER]

Structural, dynamic, and thermodynamic properties of ozone, oxygen, and mixed ozone-oxygen hydrates are investigated. The thermodynamic stability regions of these hydrates are found. Ozone can form hydrates at ambient pressure and temperatures below 230 K. Strong dependence of the binary hydrate formation pressure on the ozone concentration in the gas phase is shown. In the formation of the hydrate, ozone concentrates in the hydrate phase. At an ozone concentration of 5 mol.% in the gas phase, the ozone content in the hydrate reaches 40%. [ABSTRACT FROM AUTHOR]

In the present study, we report the results of a systematic investigation of cage-like water structures using the first-principles calculations. These results show that, in the case of methane hydrate, the following nucleation mechanism can be revealed. The formation of small water cavities filled with methane is the first step of the formation of methane hydrate. It is not necessary to occupy all dodecahedral cages by guest molecules. After that small cavities start to form the H-bonding network with surrounding water molecules and a small number of water molecules is enough for the formation of a stable hydrogen-bonding network. The structural information contained in such nuclei is conserved in the forming crystal. Moreover, the presence of a methane molecule between small cages is also important to prevent the adhesion of cavities. It found that the ozone molecule can also stabilize the small cage since the value of the interaction energy between the ozone guest and the water host framework is very close to that obtained for the methane case. However, ozone affects the structure of large cavities and hence, the second guest is necessary to stabilize the hydrate structure. [ABSTRACT FROM AUTHOR]

Based on first-principles study, tuning of electronic structure of graphene is reported. The emergence of band gap in this semimetal can be accomplished through different mechanisms. In this study, we have reported on the band gap modulations in graphene through chemical functionalization with oxygen, under the application of external stress, and through the creation of vacancies. Our study suggests that all these mechanisms alter either electronic properties or both structural and electronic properties of graphene. As a result, these mechanisms completely destroy the nature of massless Dirac fermions of graphene. Also, we report on the effect of static electric field on the band gap in hydrogenated graphene (graphane). The combined action of structural modifications that involves stretching and compression of C–C bonds in the hexagonal network and charge transfer mechanism are responsible for the gap opening in electronic spectrum of graphene, which is essential for the future application of graphene in electronics. The introduction of strain is a nondestructive method when compared to other methods for band gap engineering in graphene. [ABSTRACT FROM PUBLISHER]

Nature of the interaction potential of different adsorbates on different zigzag single-walled carbon nanotubes is investigated. The intermolecular potentials for H2 absorbed in carbon nanotubes (5, 0), (6, 0), (7, 0), (8, 0), (9, 0), and (10, 0) are computed and sketched. This study is extended to N2 adsorbed on (4, 0) and BH3 adsorbed on (10, 0) tubes. The equilibrium positions of the adsorbates obtained from the potential model serve as an initial guess in designing the CNT + adsorbate complex in the simulation cell and this process considerably reduces the computation time. Further, the hydrogen storage capacity of CNT(10,0) + BH3 complex is calculated. The estimated storage capacity of this system is in the range 6-12 wt.%. [ABSTRACT FROM AUTHOR]

Storage of hydrogen as hydrogen hydrate is a promising alternative technology to liquefied hydrogen at cryogenic temperatures or compressed hydrogen at high pressures. In this paper, computer simulation is performed based on the solid solution theory of clathrates of van der Waals and Platteeuw with some modifications that include in particular the account of multiple cage occupancies and host relaxation. The quasiharmonic lattice dynamics method employed here gives the free energy of clathrate hydrate to first order in the anharmonicity of intermolecular potential and permits to take into account quantum zero-point vibration of host lattice and hydrogen in the cages. It is employed to study the thermodynamic functions of binary (mixed) H2–CH4 hydrates of cubic structure II (sII) and hexagonal structure H (sH). It is shown that at divariant equilibrium "gas phase–gas hydrate" with increasing pressure the filling of large cavities by hydrogen proceeds gradually from single filling to the maximal number of hydrogen molecules in clusters included in large cages (four in sII and five in sH) preserving stability of the hydrogen–methane hydrates sII and sH. The results show that mass fraction of hydrogen in the mixed sH hydrate is significantly lower than in the mixed sII hydrate. Pressure of monovariant equilibrium "IceIh–gas phase–mixed sII hydrate" with increasing methane concentration in the gas phase lowers in comparison with the pressure of pure hydrogen hydrate formation. For the mixed hydrogen + methane sH hydrates, it was demonstrated that thermodynamic stability depends on the filling degree of small cavities by methane molecules and stability area shifts to lower pressure with increasing filling. [ABSTRACT FROM AUTHOR]

This paper presents theoretical predictions for the excess polarizabilities upon excitation from the ground state to the first dipole-allowed excited state (11Bu) of some conjugated oligomers. The excess polarizability was obtained by simulating the Stark shift, which was predicted by the time-dependent density functional theory (TDDFT) with the hybrid Becke-3 Lee–Yang–Parr (B3LYP) potential. The Stark shift in solution was simulated by employing the non-equilibrium integral equation formalism polarizable continuum model (IEFPCM). All the model molecules considered in this study were fully optimized by the Hartree–Fock (HF) method and the density functional theory (DFT) with the B3LYP potential, respectively. For diphenylpolyenes, the excess polarizabilities displayed by the DFT/B3LYP-optimized geometries are more reasonable than those displayed by the HF-optimized geometries when compared with the experimental results. However, this feature is not clearly demonstrated by our results in the cases of oligo(phenylenevinylene)s (OPVs). Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper discusses the excess polarizabilities upon excitation from the ground state to the first dipole-allowed excited state (S1) of diphenylpolyenes by using the time-dependent density functional theory. Two hybrid exchange-correlation (xc) potentials Becke-3 Lee-Yang-Parr (B3LYP) and Perdew-Burke-Ernzerhof (PBE1PBE) were employed. Our calculations indicate that the magnitude of the excess polarizability will decrease while the molecule evolves from the unrelaxed S1 state to the relaxed S1 state. This decreasing trend is found to be independent of substituents, though substituents can change the value of the excess polarizability. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

The geometry and electronic structures of metal 5, 15-di-(4-thiophenyl)-porphyrin (MDTP) have been investigated using first-principles calculations. Several metal atoms, including Cr, Mn, Fe, Co, Ni, Cu and Zn, have been selected. It has been found that the dihedral angle between porphyrin and thiophenyl planes is close to the perpendicular, which means that the π conjugation in whole MDTP molecules is broken. The most stable spin configurations of Cr, Mn, Fe, Co and NiDTPs are S =2, 5/2, 1, 1/2 and 0, respectively. Analysis of metal 3d-orbital splitting in Zn and NiDTP have shown that in the case of NiDTP, the out-of-plane interaction between metal 3d-orbitals and π orbital of porphyrin is larger than that in the case of ZnDTP. The results suggest that the Ni metal will enhance the conductance of DTP because transport properties in molecular systems have strong relations to the molecular π orbital. [ABSTRACT FROM AUTHOR]

The electronic transport through bent single-wall carbon nanotubes is studied. Using a four-orbital per atom tight-binding model, the relaxed configurations of a (10, 0) semiconducting nanotube at different bending angles are first obtained. The optimized structures are then used in calculating conductance and current–voltage characteristics of the systems. These results are used to establish a correspondence between the mechanical deformation and transport properties, with potential applications in, e.g., nanoswitches. The source of the switching behavior is explained in terms of the localized states within the bent region. [ABSTRACT FROM AUTHOR]

Fragmentation of germanium and tin cluster ions (Gex+, Snx+ : x = 4∼20) in the low-energy collisions with a Si surface has been investigated by means of a tandem time-of-flight mass spectrometer. At low incident energies, smaller clusters fragmented by an atom loss process, whereas larger clusters decayed by fission. The favored fragmentation paths for both cluster ions were similar to those for Si cluster ions. The results support the structural similarities among Si, Ge, and Sn clusters in the present size range. For tin cluster ions, low-energy fragmentation patterns were compared with those obtained from theoretical calculations using generalized gradient approximation (GGA) and the B3PW91 exchange-correlation functional. It has been found that the B3PW91 hybrid functional results are consistent with the experimental observations. [ABSTRACT FROM AUTHOR]

Thin films obtained by cluster deposition have attracted strong attention both as a new manufacturing technique to realize high-density magnetic recording media and to create systems with unique magnetic properties. Because the film’s features are influenced by the cluster properties during the flight path, the relevant physical scale to be studied is as large as centimeters. In this paper, a new model of cluster growth processes based on a combination of the Direct Simulation Monte Carlo (DSMC) method and the cluster growth model is introduced to examine the effects of experimental conditions on cluster growth by an adiabatic expansion process. From the macroscopic viewpoint, we simulate the behavior of clusters and inert gas in the flight path under different experimental conditions. The internal energy of the cluster, which consists of rotational and vibrational energies, is limited by the binding energy which depends on the cluster size. These internal and binding energies are used as criteria of the cluster growth. The binding energy is estimated by surface and volume terms. Several types of size distribution of generated clusters under various conditions are obtained by the present model. The results of the present numerical simulations reveal that the size distribution is strongly related to the experimental conditions and can be controlled. [ABSTRACT FROM AUTHOR]

Investigates the convection depth in shear cell under microgravity through an isotope trace method in hydrodynamical simulation. Reliability of the approach; Influence of shear velocities on convection depth; Effect of isotope on field flow.

The geometry, stability, and electronic properties of B@Co12@Bn and Co13@Bn clusters with a wide range n=55 up to 92 are studied by ab initio calculations. We find that B@Co12@B80 and Co13@B80 with closed B80 shell are two stable magic clusters with nearly perfect icosahedral symmetry, and B@Co12@B80 is more stable than Co13@B80 energetically. The strong core-shell bonding yields a very large energy gain of ∼30 eV. This high stability is attributed to the favorable closed-shell atomic and electronic structures. The B@Co12@B80 exhibits a large highest occupied and lowest unoccupied energy gap (0.96 eV) that is close to the value for isolated B80 fullerene. [ABSTRACT FROM AUTHOR]

The authors report ab initio total energy calculations on the first-order structural transition of the ferromagnetic MnCo1-xGe(0.00≤=x≤=0.25) intermetallic compound. They show that increasing Co vacancies induce a transition from an orthorhombic structure at 0≤=x≤=0.08 to a hexagonal structure at x>0.08. A concomitant high-to-low moment magnetic transition and a large magnetovolume effect occur due to the change of the symmetry and the resulting coupling distance between the magnetic atoms. These results provide an excellent account for the experimental results and reveal the crucial role of the Co vacancies in determining the relative structural stability and the magnetic properties of MnCo1-xGe [ABSTRACT FROM AUTHOR]

We have performed a detailed density functional theory study on the structural and electronic properties of NanC60- (n = 1-12) clusters. The calculated vertical detachment energies show good agreement with the experimental data, which confirms the 3p (n = 3p) oscillation rule. The oscillation can be attributed to the combination of the charge depletion distribution induced by removing electrons and the number of the sodium atoms in direct contact with the fullerene. Based on the structural and electronic properties, the Na atoms can be categorized into two groups, one is for the metal atoms directly bonded to the fullerene surface, and the other one is for those without bonding to the fullerene. The Na atoms in group one would donate electrons to both the fullerene and the Na atoms in group two. As the total number of the sodium atoms increases, the number of Na atoms in group one would continue increasing till the size n = 3p - 1 to meet a shoulder from n = 3p - 1 to n = 3p, which accounts for the maximum vertical detachment energy at the size of n = 3p as drawn from the detailed electronic property studies. [ABSTRACT FROM AUTHOR]

Investigates the Magnetic Multi-Valued (MMV) recording process by numerical simulation including thermomagnetic effect. Details on the model and the numerical method; Discussion of the results; Conclusions.

New magnetic configurations in 2D and 3D artificial lattices are studied by solving the non-linear equation based on the classical Heisenberg models for 2D and 3D artificial lattices. The results indicate a strong possibility of use as an ultra-high density magnetic recording media achieved by magnetic multi-valued (MMV) recording. The MMV states are based on the magnetic anisotropic configurations in 2D and 3D artificial lattices. In this paper, the magnetic properties of these artificial lattices and the density of MMV recording are discussed. [ABSTRACT FROM PUBLISHER]

Recently, field of molecular electronics has attracted strong attention as a "post-silicon technology" to enable future nanoscale electronic devices. To realize this molecular device, unimolecular rectifying function is one of the most fundamental requirements using nanotechnology. In the present study, the geometric and electronic structures of alkyl derivative C 37 H 50 N 4 O 4 (PNX) molecule, (donor - spacer - acceptor), a candidate for a molecular rectifying device, has been investigated theoretically using ab initio quantum mechanical calculations. The results suggest that in such donor-acceptor molecular complexes, while the lowest unoccupied orbital concentrates on the acceptor subunit, the highest occupied molecular orbital is localized on the donor subunit. After the optimization of the structure by B3LYP/6-31(d), the approximate potential differences for the optimized PNX molecule have been estimated at the B3LYP/6-311++G(d,p) level of theory, which achieves quite good agreement with experimentally reported results. [ABSTRACT FROM AUTHOR]