bluuebunny/crossref_metadata_2025 huggingface dataset:
Orbital optimization in the density matrix renormalization group, with applications to polyenes and β-carotene
Authors: Ghosh Debashree , Hachmann Johannes , Yanai Takeshi , Chan Garnet Kin-Lic
Published: April 2008

In previous work we have shown that the density matrix renormalization group (DMRG) enables near-exact calculations in active spaces much larger than are possible with traditional complete active space algorithms. Here, we implement orbital optimization with the DMRG to further allow the self-consistent improvement of the active orbitals, as is done in the complete active space self-consistent field (CASSCF) method. We use our resulting DMRG-CASSCF method to study the low-lying excited states of the all-trans polyenes up to C24H26 as well as β-carotene, correlating with near-exact accuracy the optimized complete π-valence space with up to 24 active electrons and orbitals, and analyze our results in the light of the recent discovery from resonance Raman experiments of new optically dark states in the spectrum.
Obtaining the two-body density matrix in the density matrix renormalization group method
Authors: Zgid Dominika , Nooijen Marcel
Published: April 2008

We present an approach that allows to produce the two-body density matrix during the density matrix renormalization group (DMRG) run without an additional increase in the current disk and memory requirements. The computational cost of producing the two-body density matrix is proportional to O(M3k2+M2k4). The method is based on the assumption that different elements of the two-body density matrix can be calculated during different steps of a sweep. Hence, it is desirable that the wave function at the convergence does not change during a sweep. We discuss the theoretical structure of the wave function ansatz used in DMRG, concluding that during the one-site DMRG procedure, the energy and the wave function are converging monotonically at every step of the sweep. Thus, the one-site algorithm provides an opportunity to obtain the two-body density matrix free from the N-representability problem. We explain the problem of local minima that may be encountered in the DMRG calculations. We discuss theoretically why and when the one- and two-site DMRG procedures may get stuck in a metastable solution, and we list practical solutions helping the minimization to avoid the local minima.
The density matrix renormalization group self-consistent field method: Orbital optimization with the density matrix renormalization group method in the active space
Authors: Zgid Dominika , Nooijen Marcel
Published: April 2008

We present the density matrix renormalization group self-consistent field (DMRG-SCF) approach that is analogous to the complete active space self-consisted field (CASSCF) method but instead of using for the description of the active space the full configuration interaction (FCI) method, the DMRG-SCF uses the density matrix renormalization group (DMRG) method. The DMRG-SCF approach, similarly to CASSCF, properly describes the multiconfigurational character of the wave function but avoids the exponential scaling of the FCI method and replaces it with a polynomial scaling. Hence, calculations for a larger number of orbitals and electrons in the active space are possible since the DMRG method provides an efficient tool to automatically select from the full Hilbert space the many-body contracted basis states that are the most important for the description of the wave function.
Effects of transverse magnetic field and viscosity on the Richtmyer–Meshkov instability
Authors: Qiu Zhiyong , Wu Zhengwei , Cao Jintao , Li Ding
Published: April 2008

The effects of transverse magnetic field and viscosity on the Richtmyer–Meshkov instability are considered under the framework of two semi-infinite fluids with densities ρ+ and ρ−, magnetic fields B+ and B−, and viscosities μ+ and μ−, respectively. The amplitude of the perturbations is analytically obtained. It is found that the magnetic field provides oscillation and damping, and viscosity provides damping. When both are present, the perturbations of the interface undergo damped oscillation provided that the magnetic field is strong enough; otherwise the perturbations will damp from the beginning.
Vibrational dynamics and structural investigation of 2,2′-dipyridylketone using Raman, IR and UV-visible spectroscopy aided by ab initio and density functional theory calculation
Authors: Sett Pinaky , Mishra Tumpa , Chowdhury Joydeep , Ghosh Manas , Chattopadhyay Subrato , Kumar Sarkar Susil , Kumar Mallick Prabal
Published: April 2008

Detailed investigation on the vibrational and electronic spectra has been carried out in order to study various properties of 2,2′-dipyridylketone molecule in its ground and excited electronic states. To get insight into the structural and symmetry features of the molecule, Raman excitation profiles of several normal modes have been analyzed. The polarized Raman spectra in different environments along with their IR counterpart have been critically surveyed and different normal modes have been assigned. The knowledge in regard to the positions of different excited electronic states has been acquired from the study of electronic absorption spectra. All the experimental observations have been substantiated and corroborated theoretically by the quantum chemical calculation. Possibility of exciton splitting of the La1 band has been explored both from theoretical and experimental points of view.
Hamiltonian replica exchange molecular dynamics using soft-core interactions
Authors: Hritz Jozef , Oostenbrink Chris
Published: April 2008

To overcome the problem of insufficient conformational sampling within biomolecular simulations, we have developed a novel Hamiltonian replica exchange molecular dynamics (H-REMD) scheme that uses soft-core interactions between those parts of the system that contribute most to high energy barriers. The advantage of this approach over other H-REMD schemes is the possibility to use a relatively small number of replicas with locally larger differences between the individual Hamiltonians. Because soft-core potentials are almost the same as regular ones at longer distances, most of the interactions between atoms of perturbed parts will only be slightly changed. Rather, the strong repulsion between atoms that are close in space, which in many cases results in high energy barriers, is weakened within higher replicas of our proposed scheme. In addition to the soft-core interactions, we proposed to include multiple replicas using the same Hamiltonian/level of softness. We have tested the new protocol on the GTP and 8-Br-GTP molecules, which are known to have high energy barriers between the anti and syn conformation of the base with respect to the sugar moiety. During two 25ns MD simulations of both systems the transition from the more stable to the less stable (but still experimentally observed) conformation is not seen at all. Also temperature REMD over 50 replicas for 1ns did not show any transition at room temperature. On the other hand, more than 20 of such transitions are observed in H-REMD using six replicas (at three different Hamiltonians) during 6.8ns per replica for GTP and 12 replicas (at six different Hamiltonians) during 8.7ns per replica for 8-Br-GTP. The large increase in sampling efficiency was obtained from an optimized H-REMD scheme involving soft-core potentials, with multiple simulations using the same level of softness. The optimization of the scheme was performed by fast mimicking [J. Hritz and C. Oostenbrink, J. Chem. Phys. 127, 204104 (2007)].
Thermal history effects and methyl tunneling dynamics in a supramolecular complex of calixarene and para-xylene
Authors: Panesar K. S. , Horsewill A. J. , Cuda F. , Carravetta M. , Mamone S. , Danquigny A. , Grossel M. C. , Levitt M. H.
Published: April 2008

The low-temperature structure and dynamics of guest molecules of p-xylene incorporated in the isopropyl-calix[4] arene(2:1) p-xylene complex have been investigated by solid state nuclear magnetic resonance (NMR). Using one-dimensional H1-decoupled C13 cross-polarization magic-angle-spinning (MAS) NMR and two-dimensional H1–C13 correlation spectroscopy, a full assignment of the C13 and H1 chemical shifts has been made. Using H1 NMR relaxometry, the effects of thermal history on the structure of the system have been investigated. Rapidly cooled samples have H1 spin-lattice relaxation times T1, which at low temperature (T<60K) are typically two orders of magnitude faster than those observed in annealed samples which have been cooled slowly over many hours. In both forms, the low-temperature relaxation is driven by the dynamics of the weakly hindered methyl rotors of the p-xylene guest. The substantial difference in T1 is attributed in the rapidly cooled sample to disorder in the structure of the complex leading to a wide distribution of correlation times and methyl barrier heights. A comparison of the linewidths and splittings in the high resolution C13 MAS spectra of the two forms provides structural insight into the nature of the disorder. Using H1 field-cycling NMR relaxometry, the methyl dynamics of the p-xylene guest in the annealed sample have been fully characterized. The B-field dependence of the H1 T1 maps out the spectral density from which the correlation times are directly measured. The methyl barrier heights are determined from an analysis of the temperature dependence.
Test of the consistency of various linearized semiclassical initial value time correlation functions in application to inelastic neutron scattering from liquid para-hydrogen
Authors: Liu Jian , Miller William H.
Published: April 2008

The linearized approximation to the semiclassical initial value representation (LSC-IVR) is used to calculate time correlation functions relevant to the incoherent dynamic structure factor for inelastic neutron scattering from liquid para-hydrogen at 14 K. Various time correlations functions were used which, if evaluated exactly, would give identical results, but they do not because the LSC-IVR is approximate. Some of the correlation functions involve only linear operators, and others involve nonlinear operators. The consistency of the results obtained with the various time correlation functions thus provides a useful test of the accuracy of the LSC-IVR approximation and its ability to treat correlation functions involving both linear and nonlinear operators in realistic anharmonic systems. The good agreement of the results obtained from different correlation functions, their excellent behavior in the spectral moment tests based on the exact moment constraints, and their semiquantitative agreement with the inelastic neutron scattering experimental data all suggest that the LSC-IVR is indeed a good short-time approximation for quantum mechanical correlation functions.
Characterization of enantiospecific chemisorption on chiral Cu surfaces vicinal to Cu(111) and Cu(100) using density functional theory
Authors: Bhatia Bhawna , Sholl David S.
Published: April 2008

Surfaces of simple fcc metals such as Cu with nonzero and unequal Miller indices are intrinsically chiral. Density functional theory (DFT) calculations are a useful way to study the enantiospecific adsorption of small chiral molecules on these chiral metal surfaces. We report DFT calculations of seven chiral molecules on several structurally distinct chiral Cu surfaces. These surfaces include two surfaces with (111)-oriented terraces and one with (100)-oriented terraces. Calculations are also described on a surface that was modified to mimic the surface structures that typically appear on real metal surfaces following thermally driven fluctuations in step edges. Our results provide initial information on how variation in the surface structure of intrinsically chiral metal surfaces can affect the enantiospecific adsorption of small molecules on these surfaces.
Recoil excitation of vibrational structure in the carbon 1s photoelectron spectrum of CF4
Authors: Thomas T. Darrah , Kukk Edwin , Sankari Rami , Fukuzawa Hironobu , Prümper Georg , Ueda Kiyoshi , Püttner Ralph , Harries James , Tamenori Yusuke , Tanaka Takahiro , Hoshino Masamitsu , Tanaka Hiroshi
Published: April 2008

The carbon 1s photoelectron spectrum of CF4 measured at photon energies from 330to1500eV shows significant contributions from nonsymmetric vibrational modes. These increase linearly as the photon energy increases. The excitation of these modes, which is not predicted in the usual Franck–Condon point of view, arises from the recoil momentum imparted to the carbon atom in the ionization process. A theory is presented for quantitative prediction of the recoil effect; the predictions of this theory are in agreement to the measurements. The experiments also yield the vibrational frequencies of the symmetric and asymmetric stretching modes in core-ionized CF4, the change in CF bond length upon ionization, −0.61pm, and the Lorentzian linewidth of the carbon 1s hole, 67meV.
Dissipative particle dynamics simulation of depletion layer and polymer migration in micro- and nanochannels for dilute polymer solutions
Authors: Fedosov Dmitry A. , Em Karniadakis George , Caswell Bruce
Published: April 2008

The flows of dilute polymer solutions in micro- and nanoscale channels are of both fundamental and practical importance in variety of applications in which the channel gap is of the same order as the size of the suspended particles or macromolecules. In such systems depletion layers are observed near solid-fluid interfaces, even in equilibrium, and the imposition of flow results in further cross-stream migration of the particles. In this work we employ dissipative particle dynamics to study depletion and migration in dilute polymer solutions in channels several times larger than the radius of gyration (Rg) of bead-spring chains. We compare depletion layers for different chain models and levels of chain representation, solvent quality, and relative wall-solvent-polymer interactions. By suitable scaling the simulated depletion layers compare well with the asymptotic lattice theory solution of depletion near a repulsive wall. In Poiseuille flow, polymer migration across the streamlines increases with the Peclet and the Reynolds number until the center-of-mass distribution develops two symmetric off-center peaks which identify the preferred chain positions across the channel. These appear to be governed by the balance of wall-chain repulsive interactions and an off-center driving force of the type known as the Segre–Silberberg effect.
An ab initio study based on a finite nucleus model for isotope fractionation in the U(III)–U(IV) exchange reaction system
Authors: Abe Minori , Suzuki Tatsuya , Fujii Yasuhiko , Hada Masahiko
Published: April 2008

Isotope fractionation in the U(III)–U(IV) reaction system was investigated by a series of atomic relativistic ab initio calculations using the multiconfigurational Dirac–Coulomb Hartree–Fock method. To evaluate the nuclear volume effect on the fractionation, the Fermi statistical distribution function was adopted for nuclear charge density. The isotope fractionation coefficient ε resulting from the nuclear volume difference was evaluated from the total electronic energies of U3+ and U4+, based on the theoretical equation proposed by Bigeleisen [J. Am. Chem. Soc. 118, 3676 (1996)]. The calculated fractionation coefficient ε in the present work for the isotopic pair U235 and U238 at 293K is 0.0031, which is quite close to the experimentally observed value of 0.0027. Discussion is extended to the nuclear volume effects on isotopic fractionations in the Pu(III)–Pu(IV) and Eu(II)–Eu(III) exchange systems.
Efficient chain moves for Monte Carlo simulations of a wormlike DNA model: Excluded volume, supercoils, site juxtapositions, knots, and comparisons with random-flight and lattice models
Authors: Liu Zhirong , Chan Hue Sun
Published: April 2008

We develop two classes of Monte Carlo moves for efficient sampling of wormlike DNA chains that can have significant degrees of supercoiling, a conformational feature that is key to many aspects of biological function including replication, transcription, and recombination. One class of moves entails reversing the coordinates of a segment of the chain along one, two, or three axes of an appropriately chosen local frame of reference. These transformations may be viewed as a generalization, to the continuum, of the Madras–Orlitsky–Shepp algorithm for cubic lattices. Another class of moves, termed T±2, allows for interconversions between chains with different lengths by adding or subtracting two beads (monomer units) to or from the chain. Length-changing moves are generally useful for conformational sampling with a given site juxtaposition, as has been shown in previous lattice studies. Here, the continuum T±2 moves are designed to enhance their acceptance rate in supercoiled conformations. We apply these moves to a wormlike model in which excluded volume is accounted for by a bond-bond repulsion term. The computed autocorrelation functions for the relaxation of bond length, bond angle, writhe, and branch number indicate that the new moves lead to significantly more efficient sampling than conventional bead displacements and crankshaft rotations. A close correspondence is found in the equilibrium ensemble between the map of writhe computed for pair of chain segments and the map of site juxtapositions or self-contacts. To evaluate the more coarse-grained freely jointed chain (random-flight) and cubic lattice models that are commonly used in DNA investigations, twisting (torsional) potentials are introduced into these models. Conformational properties for a given superhelical density σ may then be sampled by computing the writhe and using White’s formula to relate the degree of twisting to writhe and σ. Extensive comparisons of contact patterns and knot probabilities of the more coarse-grained models with the wormlike model show that the behaviors of the random-flight model are similar to that of DNA molecules in a solution environment with high ionic strengths, whereas the behaviors of the cubic lattice model with excluded volume are akin to that of DNA molecules under low ionic strengths.
The permanent electric dipole moment of calcium monodeuteride
Authors: Chen Jinhai , Steimle Timothy C.
Published: April 2008

The sub-Doppler laser induced fluorescence spectra of numerous branch features in the BΣ+2−XΣ+2(0,0) band of calcium monodeuteride were recorded field-free and in the presence of a static electric field of up to 7kV∕cm. The field-free spectra were analyzed to produce an improved set of fine structure parameters for the BΣ+2(v=0) state. The observed electric field induced splittings and shifts were analyzed to produce permanent electric dipole moments of 2.57(3) and 2.51(3)D for BΣ+2(v=0) and XΣ+2(v=0) states, respectively. The permanent electric dipole moment for the XΣ+2(v=0) state of CaH is estimated to be 2.53(3)D.
A GaAs metalorganic vapor phase epitaxy growth process to reduce Ge outdiffusion from the Ge substrate
Authors: Galiana B. , Rey-Stolle I. , Algora C. , Volz K. , Stolz W.
Published: April 2008

A barrier based on GaAs for controlling the Ge out diffusion has been developed by metalorganic vapor phase epitaxy. It is based on a thin GaAs layer (50nm) grown at a low temperature (≈500°C) on top of a predeposition layer, showing that GaAs prevents the Ge diffusing when it is grown at a low temperature. Additionally, two different predeposition monolayers have been compared, concluding that when the Ga is deposited first, the diffusions across the GaAs∕Ge heterointerface decrease.
Neoclassical tearing mode saturation in periodic current sheets
Authors: Militello F. , Ottaviani M. , Porcelli F.
Published: April 2008

The saturation of Neoclassical Tearing Mode islands in a periodic slab configuration is investigated. Several theoretical models, all based on a generalization of Rutherford’s procedure, that aim at reducing the complete system to a single equation of the magnetic island width, are compared against numerical simulations. When the effects of the bootstrap current and of the second derivative of the equilibrium current profile are included, the numerical saturation levels are well matched with the predictions of this equation in a wide region of the stability diagram. However, the numerical results diverge from the standard theory when evaluating the threshold for nonlinear destabilization, since the theoretical value appears to be strongly conservative. In other words, the standard generalization of Rutherford’s equation is not able to capture the minimum value of the linear stability parameter and of the island width such that below them the Neoclassical Tearing Mode is always suppressed. To correct this discrepancy, a new theoretical model in which the transverse propagation of the island affects the bootstrap current term is proposed.
Semiempirical evaluation of post-Hartree–Fock diagonal-Born–Oppenheimer corrections for organic molecules
Authors: Mohallem José R.
Published: April 2008

Recent post-Hartree–Fock calculations of the diagonal-Born-Oppenheimer correction empirically show that it behaves quite similar to atomic nuclear mass corrections. An almost constant contribution per electron is identified, which converges with system size for specific series of organic molecules. This feature permits pocket-calculator evaluation of the corrections within thermochemical accuracy (10−1mhartree or kcal/mol).
Approximating nonequilibrium processes using a collection of surrogate diffusion models
Authors: Calderon Christopher P. , Chelli Riccardo
Published: April 2008

The surrogate process approximation (SPA) is applied to model the nonequilibrium dynamics of a reaction coordinate (RC) associated with the unfolding and refolding processes of a deca-alanine peptide at 300K. The RC dynamics, which correspond to the evolution of the end-to-end distance of the polypeptide, are produced by steered molecular dynamics (SMD) simulations and approximated using overdamped diffusion models. We show that the collection of (estimated) SPA models contain structural information “orthogonal” to the RC monitored in this study. Functional data analysis ideas are used to correlate functions associated with the fitted SPA models with the work done on the system in SMD simulations. It is demonstrated that the shape of the nonequilibrium work distributions for the unfolding and refolding processes of deca-alanine can be predicted with functional data analysis ideas using a relatively small number of simulated SMD paths for calibrating the SPA diffusion models.
An accelerator based fusion-product source for development of inertial confinement fusion nuclear diagnostics
Authors: McDuffee S. C. , Frenje J. A. , Séguin F. H. , Leiter R. , Canavan M. J. , Casey D. T. , Rygg J. R. , Li C. K. , Petrasso R. D.
Published: April 2008

A fusion-product source, utilizing a 150kV Cockraft–Walton linear accelerator, has been refurbished to provide a reliable nuclear diagnostic development tool to the national inertial confinement fusion (ICF) research program. The accelerator is capable of routinely generating DD reaction rates at ∼107∕s when using a 150kV, 150μA deuterium (D) beam onto an erbium (Er) or titanium (Ti) target doped with D, and DHe3 reaction rates at ∼5×105∕s when using a using a 120kV, ∼100μA D beam onto a Er or Ti target doped with He3. The new accelerator is currently being used in a number of projects related to the national ICF program at the OMEGA Laser Fusion Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], which includes the wedge range filter charged-particle spectrometry program [F. H. Seguin et al., Rev. Sci Instrum. 75, 3520 (2004)] and the magnetic recoil neutron spectrometer [J. A. Frenje et al., Rev. Sci. Instrum. 72, 854 (2001)].
Joint excitation probability for two harmonic oscillators in one dimension and the Mott problem
Authors: Dell’Antonio Gianfausto , Figari Rodolfo , Teta Alessandro
Published: April 2008

We analyze a one dimensional quantum system consisting of a test particle interacting with two harmonic oscillators placed at the positions a1 and a2, with a1>0 and ∣a2∣>a1, in the two possible situations: a2>0 and a2<0. At time zero, the harmonic oscillators are in their ground state and the test particle is in a superposition state of two wave packets centered in the origin with opposite mean momentum. Under suitable assumptions on the physical parameters of the model, we consider the time evolution of the wave function and we compute the probability Pn1n2−(t) [Pn1n2+(t)] that both oscillators are in the excited states labeled by n1 and n2>0 at time t>∣a2∣v0−1 when a2<0 (a2>0). We prove that Pn1n2−(t) is negligible with respect to Pn1n2+(t) up to second order in time dependent perturbation theory. The system we consider is a simplified, one dimensional version of the original model of a cloud chamber introduced by Mott [“The wave mechanics of α-ray tracks,” Proc. R. Soc. London, Ser. A 126, 79 (1929)], where the result was argued using euristic arguments in the framework of the time independent perturbation theory for the stationary Schrödinger equation. The method of the proof is entirely elementary and it is essentially based on a stationary phase argument. We also remark that all the computations refer to the Schrödinger equation for the three-particle system, with no reference to the wave packet collapse postulate.
Nanotwins and phases in high-strain Pb(Mg1/3Nb2/3)1−xTixO3 crystal
Authors: Tu C.-S. , Hsieh C.-M. , Chien R. R. , Schmidt V. H. , Wang F.-T. , Chang W. S.
Published: April 2008

This work is a study of the thermal stability of (001)-cut Pb(Mg1/3Nb2/3)1−xTixO3 (x=0.30) single crystals before and after an electric (E)-field poling by means of dielectric permittivity, hysteresis loop, domain structure, polarization current, and x-ray diffraction. An R(RNT)-R(RNT)/T(TNT)-T(TNT)-C transition sequence was observed upon heating in the unpoled sample. R, RNT, T, TNT, and C are the rhombohedral, rhombohedral nanotwin, tetragonal, tetragonal nanotwin, and cubic phases, respectively. R/T indicates coexistence of the R and T phases. R(RNT) and T(TNT) indicate that the RNT and TNT structures mimic monoclinic phases in the R and T matrices, respectively. After a prior E-field poling, an R(RNT)-T(TNT)-C phase sequence takes place upon heating. The dielectric permittivity and current density evidenced an additional polarization at 355 K, which is associated with the vanishing of the dielectric dispersion, which reappears near 410 K and remains up to the Burns temperature TB=510 K. This study suggests that nanotwins (RNT and TNT) can play an important role in high-strain piezoelectric crystals while phase transition takes place. Under E=38 kV/cm, [001] T domains randomly appeared in the matrix, suggesting that the matrix consists of a glassy matrix and ferroelectric nanoclusters.
Improved supermolecular second order Møller–Plesset intermolecular interaction energies using time-dependent density functional response theory
Authors: Heßelmann Andreas
Published: April 2008

The supermolecular second order Møller-Plesset (MP2) intermolecular interaction energy is corrected by employing time-dependent density functional (TDDFT) response theory. This is done by replacing the uncoupled second order dispersion contribution contained in the supermolecular MP2 energy with the coupled dispersion energy obtained from the TDDFT approach. Preliminary results for the rare gas dimers He2, Ne2, and Ar2 and a few structures of the (HF)2 and (H2O)2 dimers show that the conventional MP2 interaction energies are considerably improved by this procedure if compared to coupled cluster singles doubles with perturbative triples [CCSD(T)] interaction energies. However, the quality of the interaction energies obtained in this way strongly depends on the exchange-correlation potential employed in the monomer calculations: It is shown that an exact exchange-only potential surprisingly often performs better than an asymptotically corrected hybrid exchange-correlation potential. Therefore the method proposed in this work is similar to the method by Cybulski and Lytle [J. Chem. Phys., 127, 141102 (2007)] which corrects the supermolecular MP2 energies with a scaled dispersion energy from time-dependent Hartree–Fock. The results in this work are also compared to the combination of density functional theory and intermolecular perturbation theory.
Thermographic measurement of the emittance plot of a single positive ion beamlet
Authors: Kovari M. , Crowley B. , Cox S. J.
Published: April 2008

Measurements have been made of the emittance plot of a partly neutralized positive ion beamlet, using a slit, a polymer target, and an infrared camera. This thermographic approach is intrinsically linear and absolute (since the properties of the target are known and approximately independent of temperature). It is sufficiently sensitive that only one short pulse is required to capture the entire range of the angular coordinate y′. Ions and neutral atoms are both detected with equal sensitivity. The measurement is unaffected by secondary electrons emitted by the target and by electrons traveling with the positive ion beam, as both types of electron carry very little energy. No specialized electronics or beam deflection devices are required. Providing the region downstream of the mask is free of fields, the target can be several meters away from the mask, allowing good resolution in the y′ axis. The target can be as large as required.
Functionalization of 6H-SiC surfaces with organosilanes
Authors: Schoell S. J. , Hoeb M. , Sharp I. D. , Steins W. , Eickhoff M. , Stutzmann M. , Brandt M. S.
Published: April 2008

We demonstrate the successful covalent functionalization of n-type 6H-SiC with organosilanes. In particular, wet-chemically processed self-assembled layers of octadecyltrimethoxysilane and amino-propyldiethoxymethylsilane were investigated. The structural and chemical properties of these layers were studied by contact angle measurements, atomic force microscopy, thermal desorption, and x-ray photoelectron spectroscopy. The organic layers are smooth and wetting angles up to 100° are observed. Desorption temperatures in the range of 830K prove the covalent bonding of the organic molecules to the SiC surface. By utilizing self-assembled layers with functional end groups, attachment of complex molecules was demonstrated by immobilization of proteins on micropatterned organic layers.
Transmission resonances of two-constituent metal/dielectric gratings with subwavelength slits
Authors: Hu Bin , Gu Ben-Yuan , Dong Bi-Zhen , Zhang Yan
Published: April 2008

One of the mechanisms responsible for the extraordinary optical transmission of metallic gratings with narrow slits is attributed to the slit waveguide mode. We show that by constructing two-constituent metal/dielectric gratings with subwavelength slits and changing the material as well as the width of the cladding layer attached to the sidewall of the slit, the transmission resonances can be significantly modified. Since the dielectric cladding layer forms an additional dielectric cavity, the waveguide mode of the dielectric cladding layer channel contributes an extra transmission resonance. Normalized magnetic field intensity distributions confirm this conclusion.
Tailoring the resonances of surface plasmas on fractal-featured metal film by adjusting aperture configuration
Authors: Bao Yong-Jun , Li Hong-Min , Chen Xiao-Chun , Peng Ru-Wen , Wang Mu , Lu Xiang , Shao Jun , Ming Nai-Ben
Published: April 2008

Metal films perforated with two types of Sierpinski carpet patterns (SCP) are fabricated on silicon wafer, where one pattern possesses open square aperture as the building block and the other has square ring-shaped aperture as the building block. Enhanced transmissions are observed in both structures. However, for SCP made of square ring-shaped apertures, resonance of surface plasmon of both surface and localized modes has been identified. This feature does not exist in SCP made of open square apertures. This finding provides an effective way to control the extraordinary transmission through the metal film perforated with fractal-like structures.
Ultracompact waveplates: Approach from metamaterials
Authors: Iwanaga Masanobu
Published: April 2008

Ultracompact waveplates working in subwavelength scale are substantiated in realistic simulation. The optical elements have been designed employing transparent photonic metamaterials of remarkably strong anisotropy. The waveplates are far more efficient at visible wavelengths than usual bulk waveplates and are shown to be compact down to subwavelength dimensions. Similar designs for the telecommunication wavelengths are also presented.
Concentrated-mass cantilever enhances multiple harmonics in tapping-mode atomic force microscopy
Authors: Li Huiling , Chen Yan , Dai Lanhong
Published: April 2008

The natural frequencies of a cantilever probe can be tuned with an attached concentrated mass to coincide with the higher harmonics generated in a tapping-mode atomic force microscopy by the nonlinear tip-sample interaction force. We provide a comprehensive map to guide the choice of the mass and the position of the attached particle in order to significantly enhance the higher harmonic signals containing information on the material properties. The first three eigenmodes can be simultaneously excited with only one carefully positioned particle of specific mass to enhance multiple harmonics. Accessing the interaction force qualitatively based on the high-sensitive harmonic signals combines the real-time material characterization with the imaging capability.
Effect of annealing on metastable shallow acceptors in Mg-doped GaN layers grown on GaN substrates
Authors: Pozina G. , Hemmingsson C. , Paskov P. P. , Bergman J. P. , Monemar B. , Kawashima T. , Amano H. , Akasaki I. , Usui A.
Published: April 2008

Mg-doped GaN layers grown by metal-organic vapor phase epitaxy on GaN substrates produced by the halide vapor phase technique demonstrate metastability of the near-band-gap photoluminescence (PL). The acceptor bound exciton (ABE) line possibly related to the C acceptor vanishes in as-grown samples within a few minutes under UV laser illumination. Annealing activates the more stable Mg acceptors and passivates C acceptors. Consequently, only the ABE line related to Mg is dominant in PL spectra for the annealed samples. The temporal changes in PL are permanent at low temperatures; however, they can be recovered after heating to 100K or higher.
Enhanced near-infrared emission from Ni2+ in Cr3+∕Ni2+ codoped transparent glass ceramics
Authors: Wu Botao , Zhou Shifeng , Ruan Jian , Qiao Yanbo , Chen Danping , Zhu Congshan , Qiu Jianrong
Published: April 2008

Transparent Li2O–Ga2O3–SiO2 glass ceramics containing Cr3+∕Ni2+ codoped LiGa5O8 nanocrystals were synthesized. The steady state emission spectra indicated that the near-infrared emission intensity of Ni2+ at 1300nm in Cr3+∕Ni2+ codoped glass ceramics was enhanced up to about 7.3 times compared with that in Ni2+ single-doped glass ceramics with 532nm excitation. This enhancement in emission intensity was due to efficient energy transfer from Cr3+ to Ni2+, which was confirmed by time-resolved emission spectra. The energy transfer efficiency was estimated to be 85% and the energy transfer mechanism was discussed.
Sexy Beasts and Devoted Mums: Narrating Nature through Dolphin Tourism
Authors: Besio Kathryn , Johnston Lynda , Longhurst Robyn
Published: April 2008

This paper focuses upon a growing activity within New Zealand's ecotourism market: viewing and swimming with dolphins. Drawing on poststructuralist feminist theory we examine some of the ways in which New Zealand dolphin tour operators and others represent dolphins in relation to sex and gender. Three sets of data inform this research: (1) promotional materials, such as postcards, brochures, Internet websites, and advertising; (2) participant-observation on dolphin swim tours; and (3) interviews with tour operators. We argue that dolphins are constructed paradoxically as sexually polyamorous and promiscuous—sexy beasts— and as loving and maternal—devoted mums. These seemingly contradictory narratives about dolphins and nature perform different functions. The discourse of dolphins as sexy beasts can be read as an attempt by tour operators to use sex to enhance the quality of a product. Tourists are offered an opportunity to experience ‘wild nature’ as sexualized ‘other’. The discourse of dolphins as devoted mums can be read as an attempt by tour operators to anthropomorphize dolphins and offers tourists an opportunity to experience ‘domestic nature’. Fostering a connection between humans and dolphins helps to highlight the need for environmental protection to ensure the continuation of the species and ‘tourist dollars’.
Challenging the Contours: Critical Cartography, Local Knowledge, and the Public
Authors: Cidell Julie
Published: April 2008

As with many environmental issues, conflicts over airport noise are often grounded in the different experiences and knowledges of those who measure it and those who suffer its effects. The ways that airport neighbors challenge noise maps reflect the critical cartography literature, which does not take the truth of maps for granted but instead considers the political and other subjectivities behind their construction. At the same time, work in science and technology studies shows that conflicts between local residents and state officials and/or scientists are in part based on their different types of knowledge about a place, with state-centered scientific knowledge generally considered to override local knowledges. This paper brings together these two literatures in an analysis of conflicts over airport noise at Minneapolis–St Paul International Airport to show how members of the public construct their own critical cartographies.
Explaining Spatial Patterns of Innovation: Analytical and Synthetic Modes of Knowledge Creation in the Medicon Valley Life-Science Cluster
Authors: Moodysson Jerker , Coenen Lars , Asheim Bjørn
Published: April 2008

The authors address the dichotomy around ‘proximate’ and ‘distant’ learning processes by looking specifically at the characteristics of the knowledge-creation process. By way of suggesting an alternative conceptualization to the well-known tacit–codified knowledge dichotomy they propose a distinction between ‘analytical’ and ‘synthetic’ modes of knowledge creation. Analytical knowledge creation refers to the understanding and explaining of features of the (natural) world. Synthetic knowledge creation refers to the design or construction of something to attain functional goals. By applying this framework to qualitative empirics from the Medicon Valley life-science cluster, the authors demonstrate the complementarity of globally distributed analytical knowledge creation and locally oriented synthetic knowledge creation.
Choice-Based Estimation of Alonso's Theory of Movement: Methods and Experiments
Authors: Ding Guoxiang , O'Kelly Morton E
Published: April 2008

Alonso's theory of movement (ATM) provides a general framework for modeling spatial interaction. Though conceptually elegant, application of this model is limited due to the difficulty in estimating model parameters. In this paper ATM is applied in retail modeling using choice data in a novel way. The approach permits flexibility with regard to model assumptions about flow constraints; that is, the model is neither attraction constrained nor production constrained. Data aggregated to the census ‘block-group’ scale, with network-based travel distances, have been examined to estimate model parameters by pairwise comparisons and ordinary least squares. The expenditure flow is related to the store properties (attractiveness, relative accessibility) and the spatial separation between origins and destinations. Based on our empirical estimate of a key parameter, and assuming that sales increase at a decreasing rate with respect to store size, we deduce that accessibility is inversely related to store size.
Empty Dwellings: The Use of Council-Tax Records in Identifying and Monitoring Vacant Private Housing in England
Authors: Wyatt Peter
Published: April 2008

There is increasing political concern over the issue of empty dwellings in England. Not only are they perceived as a waste of valuable housing resources—at a time when housing demand greatly exceeds supply—but they are associated with social and environmental problems in the neighbourhoods in which they are situated. This concern has culminated in the enactment of the Housing Act 2004 which allows local authorities to force empty dwellings into use as a way of addressing housing need. But although there are national estimates for the number of empty dwellings in England there is a lack of systematically published data on empty homes at the subdistrict level. This paper describes a method of obtaining detailed information about empty dwellings that makes use of council-tax records. A case study for Leeds was undertaken and the results are reported and compared with residential vacancy statistics that are currently available. The results show that this data source is capable of producing statistical information on vacancy at various geographical scales and revealing the nature of empty dwellings at the local level.
Residential Preferences for Interregional Migration in Sweden: Demographic, Socioeconomic, and Geographical Determinants
Authors: Niedomysl Thomas
Published: April 2008

Policy makers are showing increasing interest in factors that make places attractive for prospective new residents. This paper focuses on residential preferences for interregional migration and aims to explore what place attributes people would value highly if they considered migrating. Special attention is given to differences in preferences between population subgroups in terms of demographic, socioeconomic, and geographical determinants. An ambitious prestudy with 390 respondents was carried out to ascertain relevant place attributes for the main study. The main study was carried out via a survey sent to a national sample of 5000 Swedes, of whom approximately 53% took part. The results give valuable information on how demographic, socioeconomic, and geographical aspects determine residential preferences in Sweden, and make some contributions to methodological issues on researching preferences in a migration context.
Who Wants to Leave the Neighbourhood? The Effect of Being Different from the Neighbourhood Population on Wishes to Move
Authors: van Ham Maarten , Feijten Peteke
Published: April 2008

Little attention has been paid to date to the role of the neighbourhood as a factor influencing residential mobility and the residential choice process. The question addressed here is to what extent neighbourhood characteristics (percentage of rented dwellings, low-income households, and ethnic minorities in the neighbourhood) influence different categories of residents to wish to leave their neighbourhood. The answer to this question can enhance our understanding of residential mobility and of the mechanisms causing segregation by income and ethnic groups. We use data from the 2002 Netherlands Housing Demand Survey, enriched with neighbourhood characteristics. Whether or not people wish to leave their neighbourhood is estimated using a multilevel logistic regression model with cross-level interaction effects between individual and neighbourhood characteristics. The main result shows that, with an increasing percentage of people from an ethnic minority in the neighbourhood, more people have the wish to leave the neighbourhood. However, this is to a lesser extent the case for members of ethnic minorities themselves.
Fiscal Decentralization and Regional Disparities: Evidence from Several European Union Countries
Authors: Ezcurra Roberto , Pascual Pedro
Published: April 2008

This paper investigates for the first time the relationship between fiscal decentralization and regional disparities in a set of European Union countries. Our findings reveal that the devolution of fiscal power to subnational governments is negatively correlated with the level of regional inequality within the sample countries. Therefore, the processes of fiscal decentralization may contribute to a more balanced distribution of resources across space, making up for the central government's loss of redistributive power. In fact, this result is robust to the inclusion of additional explanatory variables in the analysis, and to the choice of the measure used to quantify the degree of dispersion in the spatial distribution of per capita income.
Distinct error-correcting and incidental learning of location relative to landmarks and boundaries
Authors: Doeller Christian F. , Burgess Neil
Published: April 2008

Associative reinforcement provides a powerful explanation of learned behavior. However, an unproven but long-held conjecture holds that spatial learning can occur incidentally rather than by reinforcement. Using a carefully controlled virtual-reality object-location memory task, we formally demonstrate that locations are concurrently learned relative to both local landmarks and local boundaries but that landmark-learning obeys associative reinforcement (showing “overshadowing” and “blocking” or “learned irrelevance”), whereas boundary-learning is incidental, showing neither overshadowing nor blocking nor learned irrelevance. Crucially, both types of learning occur at similar rates and do not reflect differences in levels of performance, cue salience, or instructions. These distinct types of learning likely reflect the distinct neural systems implicated in processing of landmarks and boundaries: the striatum and hippocampus, respectively [Doeller CF, King JA, Burgess N (2008)
Proc Natl Acad Sci USA
105:5915–5920]. In turn, our results suggest the use of fundamentally different learning rules by these two systems, potentially explaining their differential roles in procedural and declarative memory more generally. Our results suggest a privileged role for surface geometry in determining spatial context and support the idea of a “geometric module,” albeit for location rather than orientation. Finally, the demonstration that reinforcement learning applies selectively to formally equivalent aspects of task-performance supports broader consideration of two-system models in analyses of learning and decision making.
Proposals for the collection of multifrequency acoustic data
Authors: Korneliussen Rolf J. , Diner Noel , Ona Egil , Berger Laurent , Fernandes Paul G.
Published: April 2008

Korneliussen, R. J., Diner, N., Ona, E., Berger, L., and Fernandes, P. G. 2008. Proposals for the collection of multifrequency acoustic data. – ICES Journal of Marine Science, 65: 982–994. Acoustic surveys are used to estimate the abundance and distribution of many fish species, and have been based traditionally on data collected at a single acoustic frequency. Although it has been known for some time that the use of additional frequencies can provide information on the nature of the acoustic target, the knowledge and technology required to combine the so-called “multifrequency data” in an appropriate manner has been limited. The use of several transducers of different frequencies is now common on board research vessels and fishing vessels, so multifrequency data are often collected. In order for these data to be combined appropriately, their physical and spatial characteristics from each frequency should be as similar as possible. We detail the requirements deemed necessary to collect multifrequency data in an appropriate manner. They can be stringent and may not always be achievable, so we also consider the consequences of combining acoustic data originating in transducers with varying degrees of spatial separation and with different beam widths.