bluuebunny/crossref_metadata_2025 huggingface dataset:
Computational spectroscopy of ubiquitin: Comparison between theory and experiments
Authors: Choi Jun-Ho , Lee Hochan , Lee Kyung-Koo , Hahn Seungsoo , Cho Minhaeng
Published: January 2007

Using the constrained molecular dynamics simulation method in combination with quantum chemistry calculation, Hessian matrix reconstruction, and fragmentation approximation methods, the authors have established computational schemes for numerical simulations of amide I IR absorption, vibrational circular dichroism (VCD), and two-dimensional (2D) IR photon echo spectra of the protein ubiquitin in water. Vibrational characteristic features of these spectra in the amide I vibration region are discussed. From the semiempirical quantum chemistry calculation results on an isolated ubiquitin, amide I local mode frequencies and vibrational coupling constants were fully determined. It turns out that the amide I local mode frequencies of ubiquitin in both gas phase and aqueous solution are highly heterogeneous and site dependent. To directly test the quantitative validity of thus obtained spectroscopic properties, they compared the experimentally measured amide I IR, 2D IR, and electronic circular dichroism spectra with experiments, and found good agreements between theory and experiments. However, the simulated VCD spectrum is just qualitatively similar to the experimentally measured one. This indicates that, due to delicate cancellations between the positive and negative VCD contributions, the prediction of protein VCD spectrum is critically relied on quantitative accuracy of the theoretical model for predicting amide I local mode frequencies. On the basis of the present comparative investigations, they found that the site dependency of amide I local mode frequency, i.e., diagonal heterogeneity of the vibrational Hamiltonian matrix in the amide I local mode basis, is important. It is believed that the present computational methods for simulating various vibrational and electronic spectra of proteins will be of use in further refining classical force fields and in addressing the structure-spectra relationships of proteins in solution.
How much carbon dioxide can be stored in the structure H clathrate hydrates?: A molecular dynamics study
Authors: Alavi Saman , Woo T. K.
Published: January 2007

The stability of structure H (sH) carbon dioxide clathrate hydrates at three temperature-pressure conditions are determined by molecular dynamics simulations on a 3×3×3 sH unit cell replica. Simulations are performed at 100K at ambient pressure, 273K at 100 bars and also 300K and 5.0kbars. The small and medium cages of the sH unit cell are occupied by a single carbon dioxide guest and large cage guest occupancies of 1–5 are considered. Radial distribution functions are given for guests in the large cages and unit cell volumes and configurational energies are studied as a function of large cage CO2 occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies at three temperature/pressure conditions stated above. At the low temperature, large cage occupancy of 5 is the most stable while at the higher temperature, the occupancy of 3 is the most favored. Calculations are also performed to show that the CO2 sH clathrate is more stable than the methane clathrate analog. Implications on CO2 sequestration by clathrate formation are discussed.
Electron trapping in radio-frequency atmospheric-pressure glow discharges
Authors: Liu D. W. , Shi J. J. , Kong M. G.
Published: January 2007

In this letter, the authors present experimental evidence of electron trapping in radio-frequency (rf) atmospheric-pressure glow discharges. By linking electron density to nanosecond plasma images and optical emission spectroscopy, they show that electron trapping occurs under most discharge conditions. The level of electron trapping increases with increasing discharge current or/and increasing excitation frequency, and manifests itself in the change of the differential conductivity at the point of the gas breakdown. Finally, they demonstrate that electron trapping is largely related to whether the half rf period is shorter than the electron transition time across the electrode gap.
The low field microwave effective linewidth in polycrystalline ferrites
Authors: Mo Nan , Green Jerome J. , Krivosik Pavol , Patton Carl E.
Published: January 2007

High precision measurements on the low and high field effective linewidth ΔHeff at 10GHz have been made on ultradense (UD) and conventionally sintered (CS) polycrystalline yttrium iron garnet (YIG) materials. The high field data confirm previous results on the role of two magnon scattering to low wave number (k) electromagnetic Larmor branch spin waves that lie below the light line. The low field data reveal two important contributions to the effective linewidth. For a field regime from the low k edge of the usual dipole exchange spin wave band down to the point in field (H=HX) where above-the-light-line electromagnetic branch Larmor (EML-HI) spin waves appear, ΔHeff is connected with scattering to relatively high k dipole exchange Larmor (DEL) spin waves. The coupling to these modes comes from grain boundaries in the YIG materials. A grain boundary scattering theory gives reasonable agreement with the data. While the high field effective linewidth due to pseudo in-manifold scattering is larger for the CS samples compared to that for the UD samples, the high k DEL scattering is larger for the UD samples compared to that for the CS samples. This is due to the dominant role of the grain boundaries in the low field ΔHeff. For fields below HX, additional scattering appears for the EML-HI modes. The abrupt appearance of an additional ΔHeff component for H<HX provides direct experimental evidence for the presence of such modes in the spin wave dispersion. Part of the loss contribution for H<HX may be due to subthreshold nonlinear effects.
Influence of trapped electrons on ion-acoustic solitons in plasmas with superthermal electrons
Authors: Abbasi H. , Pajouh H. Hakimi
Published: January 2007

A model for describing the physics of both superthermal and trapped electrons is presented. This is important because most of the space and some of the laboratory plasmas contain a population of superthermal particles. Due to the superthermal electrons, a high-energy tail appears in the electron distribution function that is conveniently modeled by the κ distribution function. The distribution function of trapped particles is modeled based on the simulation result of the Vlasov-Poisson system of equations. An analytical expression for the electron density is obtained. The ion-acoustic solitons are studied in this framework.
Electronic excitations of fluoroethylenes
Authors: Arulmozhiraja Sundaram , Ehara Masahiro , Nakatsuji Hiroshi
Published: January 2007

Several lowest-lying singlet electronic states of vinyl fluoride, trans-, cis-, and 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroethylene were investigated by using symmetry-adapted cluster configuration interaction theory. Basis sets up to Dunning’s aug-cc-pVTZ augmented with appropriate Rydberg functions were utilized for the calculations. Calculated excitation energies show a good agreement with the available experimental values. Even in the troublesome π→π* transitions, the excitation energies obtained in the present study agree well with the experimental values except in one or two fluoroethylenes. Strong mixing between different states was noticed in a few fluoroethylenes; especially the mixing is very strong between π-π* and π-3pπ states in trifluoroethylene. No pure π-σ* excited state was found in almost all the fluoroethylenes. Several assignments and reassignments of features in the experimental spectra were suggested. The present study does not support the existing argument that the interaction between the π-π* and σ-σ* states is the reason behind the blueshift of around 1.25eV in the π-π* excitation energy of tetrafluoroethylene. Possible reasons, including structural changes, for this shift are discussed in detail. Several low-lying triplet excited states were also studied.
Long-time tails of translational and rotational Brownian motion in a suspension of hard spheres
Authors: Hermanns H.-G. , Felderhof B. U.
Published: January 2007

The long-time translational and rotational Brownian motion of a sphere in a suspension of hard spheres is studied on the basis of the linearized Navier-Stokes equations and the fluctuation-dissipation theorem. It is shown that for the rotational long-time coefficient an effective medium conjecture is incorrect. There are short-range velocity correlations that decay at the same rate as the macroscopic flow pattern used in the effective medium conjecture. An estimate of the short-range correction is made on the basis of the pair term in the cluster expansion of the rotational admittance.
Dynamics of supercooled water in various mesopore sizes
Authors: Hwang Dennis W. , Chu Chien-Chih , Sinha Anil K. , Hwang Lian-Pin
Published: January 2007

Double-quantum-filtered NMR and T1 inversion-recovery spectroscopy were employed to exploit the temperature-dependent dynamics of D2O confined in MCM-41. Samples with three pore sizes of 1.58, 2.03, and 2.34nm and two D2O contents were investigated. The reorientation correlation times of confined D2O in variously sized pores exhibit different temperature dependencies. The results reveal that the D2O molecules at fast motion site remain mobile below ∼225K and a liquid-liquid phase transition occurs around this temperature for all samples studied. This temperature is thought to be unreachable for supercooled D2O. Particularly, in 20wt% D2O loaded samples with pore diameters of 1.58 and 2.03nm, the reorientational correlation times of D2O at fast motion site exhibit Arrhenius behavior between 225 and 290K, while other samples show power law dependency. Thus, a liquid phase of the fragile type in bigger pores changes to the strong type in samples with smaller pores.
Probing the O2 (aΔg1) photofragment following ozone dissociation within the long wavelength tail of the Hartley band
Authors: Horrocks S. J. , Ritchie G. A. D. , Sharples T. R.
Published: January 2007

The technique of resonance enhanced multiphoton ionization (REMPI) has been used in conjunction with time-of-flight mass spectrometry (TOFMS), to investigate the dynamics of ozone photolysis in the long wavelength region of the Hartley band (301–311nm). Specifically, both the translational anisotropy and the rotational angular momentum orientation of the O2 (aΔg1; ν=0, J=16–20) fragments have been measured as a function of photolysis wavelength. Within this region, the thermodynamic thresholds for the formation of these products in combination with O (D21) are approached and passed, and consequently these studies have allowed an investigation into the effects on the dynamics of slowing fragment recoil velocities and the increasing importance of vibrationally mediated photolysis. The determined β parameters for all the J states probed follow a similar trend, decreasing from a value typical for the initial B21←A11 excitation responsible for the Hartley band [for example, β=1.40±0.12 for the O2 (aΔg1; J=18) fragment], to a much lower value beyond the thermodynamic threshold for the fragment’s production (for example, β=0.63±0.19 for the J=18 fragment following photolysis at 311nm). This trend, similar to that observed when probing the atomic fragment in a previous set of experiments, [Horrocks et al., J. Chem. Phys. 125, 133313 (2006); Denzer et al., Phys. Chem. Chem. Phys. 16, 1954 (2006)] is consistent with the photodissociation of vibrationally excited ozone molecules beyond the threshold wavelengths and we estimate ∼1∕3 of this to be from excitation in the ν3 asymmetric stretching mode. These observations are substantiated by the values of the β02(2,1) orientation moment measured, which for photolysis at 301nm are negative, indicating that a bond opening mechanism provides the key torque for the departing O2 fragment. The orientation moment becomes positive again for photolysis beyond threshold, however, as the increasing impulsive dissociation again begins to dominate the nature of the rotation of the departing molecular fragment. In addition, a (2+2) REMPI scheme has been utilized to probe the O2 (aΔg1) “low” J fragments, where the majority of the population resides following photolysis within this region. The REMPI-TOFMS technique has been used to confirm the rotational character of a spectral feature through examination of the signal line shapes obtained using different experimental geometries. The dynamical information subsequently obtained, probing the “low” J O2 (aΔg1) fragments on these rotational transitions, has unified previous translational anisotropy results obtained by detecting the O (D21) atomic fragment with data for the O2 (aΔg1; J=16–20) fragments.
Exponential stabilization of chaotic systems with delay by periodically intermittent control
Authors: Li Chuandong , Liao Xiaofeng , Huang Tingwen
Published: January 2007

This paper studies the exponential stabilization problem for a class of chaotic systems with delay by means of periodically intermittent control. A unified exponential stability criterion, together with its simplified versions, is established by using Lyapunov function and differential inequality techniques. A suboptimal intermittent controller is designed with respect to the general cost function under the assumption that the control period is fixed. Numerical simulations on two chaotic oscillators are presented to verify the theoretical results.
Interdiffusion induced changes in the photoluminescence of InXGa1−XAs∕GaAs quantum dots interpreted
Authors: Biswas Dipankar , Kumar Subindu , Das Tapas
Published: January 2007

Interdiffusion in InXGa1−XAs∕GaAs quantum dots (QDs) may occur during high temperature growth and processing, which may create problems in the ultimate device performance. It is simulated through successive high temperature annealing, and the changes at each stage are studied through photoluminescence (PL). Significant changes are observed in the peak energy, linewidth, and intensity of the PL spectra. These have been attributed to relaxation of strain, changes in the composition of InXGa1−XAs, and size distribution of the QDs, which fail to establish proper understanding qualitatively and quantitatively. In this Communication we present appropriate interpretations of the changes in the observed PL through quantum mechanical concepts and computations.
Dust-acoustic Korteweg–de Vries solitons in an adiabatic hot dusty plasma
Authors: Sayed Fatema , Mamun A. A.
Published: January 2007

A rigorous theoretical investigation has been made of dust-acoustic (DA) Korteweg–de Vries (K-dV) solitons by the reductive perturbation method. An unmagnetized dusty plasma consisting of negatively charged adiabatic hot dust fluid and of Boltzmann distributed electrons and ions has been considered. It has been found that the DA K-dV solitons associated with only negative potential can exist in such a dusty plasma. It has been also found that the effects of dust fluid temperature have significantly modified the basic properties (amplitude and width) of the solitary potential structures in such a dusty plasma. The implications of these results to some space and astrophysical plasma situations are briefly discussed.
Effects of anisotropic in-plane strains on the phase diagram of BaxSr1−xTiO3 thin film
Authors: Tao Y. M. , Wu Y. Z.
Published: January 2007

Based on the Landau-Devonshire phenomenological theory, phase diagram of epitaxial BST50∕50 thin films on anisotropic in-plane strains is investigated. Different from BaTiO3 thin films, the paraelectric phase appears under the anisotropic misfit strains on barium strontium titanate (BST)50∕50 thin films at room temperature. The pyroelectric property of the BST films is also calculated. We find that the position of pyroelectric peak greatly depends on anisotropic misfit strains.
Efficient simulation of silicon nanowire field effect transistors and their scaling behavior
Authors: Shin Mincheol
Published: January 2007

We have simulated silicon nanowire field effect transistors in the ballistic transport regime using the effective mass theory and the mode space nonequilibrium Green’s function method. In order to solve the two-dimensional Schrödinger equations on the nanowire cross-sectional planes as a part of the numerical procedure, we have developed an efficient numerical scheme, the product-space method, where the size of the eigenvalue problem is reduced to the number of subband modes that participate in the transport. We have investigated the scaling behavior of the nanowire transistors and found that their device characteristics sensitively depend on the aspect ratio of the channel length and width.
High coercivity L10 FePt films with perpendicular anisotropy deposited on glass substrate at reduced temperature
Authors: Chen J. S. , Lim B. C. , Hu J. F. , Lim Y. K. , Liu B. , Chow G. M.
Published: January 2007

The microstructures and magnetic properties of FePt films grown at 350°C on glass substrates with MgO (200) intermediate layer and CrRu (200) underlayer were investigated. The film with 1nm MgO intermediate layer showed higher degree of chemical ordering than that with 4nm MgO layer due to the compression of the lattice constant of 1nm MgO intermediate layer along the [100] direction. Isolated FePt particles were formed when nominal thickness of FePt was 4nm. The room temperature coercivity of isolated FePt particles with 1nm MgO intermediate layer was as high as 12kOe, significantly larger than that with 4nm MgO intermediate layer (6.3kOe).
Self-formation of quasiperiodic void structure in CaF2 induced by femtosecond laser irradiation
Authors: Hu Xiao , Dai Ye , Yang Luyun , Song Juan , Zhu Congshan , Qiu Jianrong
Published: January 2007

We report the self-formation of quasiperiodic void structure with the length of several hundred micrometers inside the CaF2 crystal. The quasiperiodical voids along the propagation direction of the laser beam were formed spontaneously after the irradiation of a single femtosecond laser beam which was focused at a fixed point inside the crystal sample. The length of the void array varied with the focal depth beneath the sample surface. The possible mechanism of the self-formed void structure was discussed.
Photoreflectance symmetry and amplitude study of quantum dots in microcavity light emitting diode structure: The cavity-ground state exciton resonance
Authors: Rainò G. , De Giorgi M. , Todaro M. T. , De Vittorio M. , Tasco V. , Passaseo A. , Cingolani R.
Published: January 2007

A quantum dot (QD) microcavity emitting around 1.3μm at room temperature is studied by photoreflectance (PR) and photoluminescence. The temperature dependence of the PR spectra line shape and amplitude allows determining the tuning condition of the quantum dot ground state transition with the cavity mode. Our study suggests a way to find the exciton energy when the distinct QD features are hidden by the broadening of the QD dielectric function in combination with the relatively narrow cavity-mode width.
Photoluminescence in crystalline silicon quantum wells
Authors: Cho Eun-Chel , Green Martin A. , Corkish Richard , Reece Peter , Gal Mike , Lee Soo-Hong
Published: January 2007

Crystalline silicon single quantum wells (QWs) with a minimum Si layer thickness of around 1nm were fabricated by high temperature thermal oxidation of separation by implantation of oxygen (SIMOX) and epitaxial layer transfer (ELTRAN®) silicon-on-insulator (SOI) wafers. Not only Si thickness but also defect-free SOI materials may be important factors for observation of quantum-confined transition in the Si QWs. Si QWs fabricated from SIMOX SOI wafers showed strong interface-mediated luminescence, which is independent of Si thickness, at 1.63 and 1.49eV in the Si thickness range from 1to7nm. On the other hand, Si QWs fabricated from ELTRAN SOI wafers showed very strong thickness dependent luminescence in the range of 1.36–1.77eV without interface-mediated luminescence, where the Si thickness range was 3–1nm. The ability to detect quantum-confined luminescence seems to arise from the use of high-quality defect-free ELTRAN SOI wafers, from suppressed interface state luminescence by high temperature oxidation, and, possibly, from interface matching by crystalline silicon oxide. Hydrogen passivation suggests luminescence from a weak SiO state at around 1.59eV, but luminescence due to quantum confinement is more predominant in the ELTRAN Si QWs.
Expansion of the laser ablation vapor plume into a background gas. I. Analysis
Authors: Wen Sy-Bor , Mao Xianglei , Greif Ralph , Russo Richard E.
Published: January 2007

A study of the gas dynamics of the vapor plume generated during laser ablation was conducted including a counterpropagating internal shock wave. The density, pressure, and temperature distributions between the external shock wave front and the sample surface were determined by solving the integrated conservation equations of mass, momentum, and energy. The positions of the shock waves and the contact surface (boundary that separates the compressed ambient gas and the vapor plume) were obtained when the incident laser energy that is transferred to the vapor plume and to the background gas, E, and the vaporized sample mass, M, are specified. The values for E and M were obtained from a comparison of the calculated trajectories of the external shock wave and the contact surface with experimental results for a copper sample under different laser fluences. Thus E and M, which are the two dominant parameters for laser ablation and which cannot be measured directly, can be determined. In addition, the internal shock wave propagation within the vapor plume was determined; the interaction of the internal shock wave with the sample may be one of the mechanisms inducing liquid sample ejection during laser ablation.
On the influence of solute polarizability on the hydrophobic interaction
Authors: Bresme Fernando , Wynveen Aaron
Published: January 2007

The authors have performed molecular dynamics simulations of polarizable solutes in water to investigate how solute polarizability affects solute-solute hydrophophic interactions. A degree of polarization similar to the one expected in biomolecules, corresponding to a dielectric response of ϵ=2–20, results in dramatic changes in the hydrophobic forces. They find that this degree of polarizability is enough to inhibit drying between hydrophobic solutes and to stabilize a reduced water density phase whose density is smaller than the bulk water density. The hydrophobic forces associated with such reduced density states are still very significant with values of the order of several tens of piconewtons. Their results suggest that polarizability plays an important role in determining the hydrophobic force acting between weakly polar surfaces.
Local density augmentation and dynamic properties of hydrogen-and non-hydrogen-bonded supercritical fluids: A molecular dynamics study
Authors: Skarmoutsos Ioannis , Samios Jannis
Published: January 2007

The local density inhomogeneities in neat supercritical fluids were investigated via canonical molecular dynamics simulations. The selected systems under investigation were the polar and hydrogen-bonded fluid methanol as well as the quadrupolar non-hydrogen-bonded carbon dioxide one. Effective local densities, local density augmentation, and enhancement factors were calculated at state points along an isotherm close to the critical temperature of each system (Tr=1.03). The results obtained reveal strong influence of the polarity and hydrogen bonding upon the intensity of the local density augmentation. It is found that this effect is sufficiently larger in the case of the polar and associated methanol in comparison to those predicted for carbon dioxide. For both fluids the local density augmentation values are maximized in the bulk density region near 0.7ρc, a result that is in agreement with experiment. In addition, the local density dynamics of each fluid were investigated in terms of the appropriate time correlation functions. The behavior of these functions reveals that the bulk density dependence of the local density reorganization times is very sensitive to the specific intermolecular interactions and to the size of the local region. Also, the estimated local density reorganization time as a function of bulk density of each fluid was further analyzed and successfully related to two different time-scale relaxation mechanisms. Finally, the results obtained indicate a possible relationship between the single-molecule reorientational dynamics and the local density reorganization ones.
Butterfly C2H2++: New way for the decomposition of the acetylene dication
Authors: Palaudoux J. , Hochlaf M.
Published: January 2007

Using highly correlated ab initio methods, a new transition state is characterized in the lowest singlet potential energy surface of HCCH++. This transition state possesses a dibridged (or “butterfly”) form not observed yet for any acetylic compound. It can be reached either directly or after spin-orbit conversion of triplet HCCH++ ions. In light of these calculations, a reaction pathway for the proton pair formation (i.e., HCCH++→C2+H++H+) is proposed.
Simple systematization of vibrational excitation cross-section calculations for resonant electron-molecule scattering in the boomerang and impulse models
Authors: Sarma Manabendra , Adhikari S. , Mishra Manoj K.
Published: January 2007

Vibrational excitation (νf←νi) cross-sections σνf←νi(E) in resonant e-N2 and e-H2 scattering are calculated from transition matrix elements Tνf,νi(E) obtained using Fourier transform of the cross correlation function ⟨ϕνf(R)∣ψνi(R,t)⟩, where ψνi(R,t)≃e−iHA2−(R)t∕ℏϕνi(R) with time evolution under the influence of the resonance anionic Hamiltonian HA2−(A2−=N2−∕H2−) implemented using Lanczos and fast Fourier transforms. The target (A2) vibrational eigenfunctions ϕνi(R) and ϕνf(R) are calculated using Fourier grid Hamiltonian method applied to potential energy (PE) curves of the neutral target. Application of this simple systematization to calculate vibrational structure in e-N2 and e-H2 scattering cross-sections provides mechanistic insights into features underlying presence/absence of structure in e-N2 and e-H2 scattering cross-sections. The results obtained with approximate PE curves are in reasonable agreement with experimental/calculated cross-section profiles, and cross correlation functions provide a simple demarcation between the boomerang and impulse models.
Ultrathin amorphous Si layer formation by femtosecond laser pulse irradiation
Authors: Izawa Yusaku , Izawa Yasukazu , Setsuhara Yuichi , Hashida Masaki , Fujita Masayuki , Sasaki Ryuichiro , Nagai Hiroyuki , Yoshida Makoto
Published: January 2007

Formation of ultrathin amorphized Si layer by femtosecond laser irradiation is reported in this letter. Below the fluence of ablation threshold, femtosecond laser irradiation induced an amorphization of crystalline Si. The authors confirmed the thickness of amorphous Si layer by transmission electron microscope. The thickness of the amorphized layer was found to be quite uniform and did not depend on the number of irradiated laser pulses and fluence, which was related to the effective light penetration depth.
Optical filter based on two-dimensional photonic crystal surface-mode cavity in amorphous silicon-on-silica structure
Authors: Zhang Ziyang , Dainese Matteo , Wosinski Lech , Xiao Sanshui , Qiu Min , Swillo Marcin , Andersson Ulf
Published: January 2007

An optical filter based on side coupling between silicon wire waveguide and two-dimensional photonic crystal surface-mode cavity is presented. The design is optimized numerically by parallel three-dimensional finite-different time-domain simulations. The device is then fabricated on amorphous silicon-on-silica structure. The drop wavelength is observed around 1580nm. The extinction ratio of the filter is larger than 10dB and the intrinsic quality factor of the surface-mode cavity is approximately 2000.
High-yield synthesis of conductive carbon nanotube tips for multiprobe scanning tunneling microscope
Authors: Konishi H. , Murata Y. , Wongwiriyapan W. , Kishida M. , Tomita K. , Motoyoshi K. , Honda S. , Katayama M. , Yoshimoto S. , Kubo K. , Hobara R. , Matsuda I. , Hasegawa S. , Yoshimura M. , Lee J.-G. , Mori H.
Published: January 2007

We have established a fabrication process for conductive carbon nanotube (CNT) tips for multiprobe scanning tunneling microscope (STM) with high yield. This was achieved, first, by attaching a CNT at the apex of a supporting W tip by a dielectrophoresis method, second, by reinforcing the adhesion between the CNT and the W tip by electron beam deposition of hydrocarbon and subsequent heating, and finally by wholly coating it with a thin metal layer by pulsed laser deposition. More than 90% of the CNT tips survived after long-distance transportation in air, indicating the practical durability of the CNT tips. The shape of the CNT tip did not change even after making contact with another metal tip more than 100 times repeatedly, which evidenced its mechanical robustness. We exploited the CNT tips for the electronic transport measurement by a four-terminal method in a multiprobe STM, in which the PtIr-coated CNT portion of the tip exhibited diffusive transport with a low resistivity of 1.8kΩ∕μm. The contact resistance at the junction between the CNT and the supporting W tip was estimated to be less than 0.7kΩ. We confirmed that the PtIr thin layer remained at the CNT-W junction portion after excess current passed through, although the PtIr layer was peeled off on the CNT to aggregate into particles, which was likely due to electromigration or a thermally activated diffusion process. These results indicate that the CNT tips fabricated by our recipe possess high reliability and reproducibility sufficient for multiprobe STM measurements.
Numerical simulation of the Benjamin-Feir instability and its consequences
Authors: Chalikov Dmitry
Published: January 2007

Full nonlinear equations for one-dimensional potential surface waves were used for investigation of the evolution of an initially homogeneous train of exact Stokes waves with steepness AK=0.01–0.42. The numerical algorithm for the integration of nonstationary equations and the calculation of exact Stokes waves is described. Since the instability of the exact Stokes waves develops slowly, a random small-amplitude noise was introduced in initial conditions. The development of instability occurs in two stages: in the first stage the growth rate of disturbances was close to that established for small steepness by Benjamin and Feir [J. Fluid. Mech. 27, 417 (1967)] and for medium steepness [McLean, J. Fluid Mech. 114, 315 (1982)]. For any steepness, the Stokes waves disintegrate and create random superposition of waves. For AK<0.13, waves do not show a tendency to breaking, which is recognized by approaching a surface to non-single-value shape. Sooner or later, if AK>0.13, one of the waves increases its height, and finally it comes to the breaking point. For large steepness of AK>0.35 the rate of growth is slower than for medium steepness. The data for spectral composition of disturbances and their frequencies are given.
Material deformation in a restricted Euler model for turbulent flows: Analytic solution and numerical tests
Authors: Li Yi , Meneveau Charles
Published: January 2007

The restricted Euler (RE) approximation applied to the equation of the velocity gradient tensor has been shown [Cantwell, Phys. Fluids A 4, 782 (1992)] to reproduce important geometric trends concerning small scale structures in Navier-Stokes turbulence. In this paper, the Lagrangian evolution of material volume elements as they are deformed in a turbulent flow is considered. First, it is shown that the equation for the deformation tensor evolving under RE dynamics admits an analytic solution similar to that of the velocity gradient tensor. The evolution of the eigenvalues and eigenvectors of the Cauchy-Green tensor is obtained from this analytic solution. Comparison with direct numerical simulations (DNS) is performed. Results show that the analytic prediction of shapes and orientations of fluid volume elements agrees well with DNS for an evolution time scale of the order of the Kolmogorov time scale, which is also the time it takes for significant material element deformation. The analysis is repeated in the inertial range, where the deformation due to a filtered velocity field is considered. The analytic solution with filtered velocity gradients as initial conditions is evaluated. Good agreement is found for longer (turn-over) durations, of the order of the eddy turn-over time at the filter scale.
Thermopower profiling of a silicon p-n junction
Authors: Kim Kyeongtae , Park Jisang , Kim Sun Ung , Kwon Ohmyoung , Lee Joon Sik , Park Seungho Ho , Choi Young Ki
Published: January 2007

An ac type thermopower measurement technique was suggested and demonstrated with a simple experimental setup. The thermopower distribution across a silicon p-n junction was measured point by point at every 10nm, so that it was free from the noise due to the built-in potential and photoionization effects, and it was compared with the theoretical result. Although this ac type thermopower measurement technique could not follow the sharp variation of the theoretical thermopower near the p-n junction, it could identify a smooth peak of the thermopower distribution in the depletion layer of the p-n junction.
Size control of catalytic nanoparticles by thermal treatment and its application to diameter control of single-walled carbon nanotubes
Authors: Jeong Goo-Hwan , Suzuki Satoru , Kobayashi Yoshihiro , Yamazaki Akira , Yoshimura Hideyuki , Homma Yoshikazu
Published: January 2007

The authors report size control of catalytic nanoparticles by thermal annealing for diameter-controlled growth of single-walled carbon nanotubes (SWNTs). They found that Co nanoparticle-size gradually decreased through repetitive annealing at 1000°C in Ar ambient. Results of x-ray photoelectron spectroscopy and secondary ion mass spectroscopy show that thermal evaporation is responsible for the decrease. After SWNT growth using this phenomenon, the authors found that thinner SWNTs with a narrower diameter distribution grew as the nanoparticles became smaller. Their results provide a rational and straightforward technique to prepare catalysts having a desirable size and uniformity toward diameter-controlled SWNT growth.
Low dislocation densities and long carrier lifetimes in GaN thin films grown on a SiNx nanonetwork
Authors: Xie J. , Özgür Ü. , Fu Y. , Ni X. , Morkoç H. , Inoki C. K. , Kuan T. S. , Foreman J. V. , Everitt H. O.
Published: January 2007

Significant improvement of structural and optical qualities of GaN thin films on sapphire substrates was achieved by metal organic chemical vapor deposition with in situ SiNx nanonetwork. Transmission electron microscope (TEM) studies revealed that screw- and edge-type dislocations were reduced to 4.4×107 and 1.7×107cm−2, respectively, for a ∼5.5-μm-thick layer. Furthermore, room temperature carrier lifetimes of 2.22 and 2.49ns were measured by time-resolved photoluminescence (TRPL) for samples containing single and double SiNx network layers, respectively, representing a significant improvement over the previous studies. The consistent trends among the TEM, x-ray diffraction, and TRPL measurements suggest that in situ SiNx network reduces line defects effectively as well as the point-defect-related nonradiative centers.
Direct observation of controlled strain-induced second harmonic generation in a Co0.25Pd0.75 thin film on a Pb(ZrTi)O3 substrate
Authors: Jeong Jae-Woo , Shin Sung-Chul , Dadoenkova N. N. , Lyubchanskii I. L. , Valev V. K. , Rasing Th.
Published: January 2007

The authors have observed strain-induced second harmonic generation (SHG) signals from a Co0.25Pd0.75 alloy film deposited on a lead zirconate titanate (PZT) substrate. The strain in the sample was controlled by the inverse piezoelectric effect. The authors demonstrate that it is possible to separate the strain contribution to the SHG signal from the crystallographic contribution and that from the electric polarization in PZT. An estimate of the value of the nonlinear photoelastic tensor components is in very good agreement with previous calculations.
Ni–NiO core-shell nanoclusters with cubic shape by nanocluster beam deposition
Authors: Zhou Y. Z. , Chen J. S. , Tay B. K. , Hu J. F. , Chow G. M. , Liu T. , Yang P.
Published: January 2007

Ni nanoclusters with cubic shape and uniform size below 10nm were fabricated by nanocluster beam deposition. Ni–NiO core-shell structure was achieved when the Ni clusters flew through a chamber with constant oxygen feeding (1–3SCCM, SCCM denotes cubic centimeter per minute at STP). The fraction of oxidized Ni was estimated by means of linear combination fitting to the x-ray absorption near-edge structure data. The hysteresis loop of the oxidized sample (1SCCM O2) showed a shift of ∼181Oe with 100Oe field cooling from 400K and a coercivity of 48Oe at room temperature, while the nonoxidized Ni nanoclusters with size of 4–5nm showed superparamagnetic behavior.
Measurement of piezoelectric coefficient of gallium nitride using metal-insulator-semiconductor capacitors
Authors: Tilak Vinayak , Batoni Paolo , Jiang Jie , Knobloch Aaron
Published: January 2007

Gallium nitride cantilevers with metal-insulator-semiconductor capacitors were fabricated and characterized as a function of strain to determine the effective piezoelectric constant. These cantilevers were tested at room temperature through a combination of cantilever bending experiments, strain gauge measurements, and finite element modeling. By measuring the shift in flatband voltage to applied strain the magnitude of the piezoelectric charge induced is determined. The effective piezoelectric constant e31′ was determined from these measurements to be −0.57±0.03C∕m2.
Wideband backward coupling based on anisotropic left-handed metamaterial
Authors: Zhang Jingjing , Chen Hongsheng , Luo Yu , Yuan Yu , Shen Linfang , Ran Lixin , Kong Jin Au
Published: January 2007

Waveguides using a slab of left-handed metamaterial (LHM) adjacent to a slab of right-handed material (RHM) have shown backward coupling properties over the frequency range in the LHM passband where both permittivity and permeability are negative. In this letter the authors further show that when both slabs are LHMs with different passbands, the resultant backward coupling frequency band is a union of both passbands. The curious fact that the stop band of a metamaterial where permittivity is negative and permeability is positive can support propagating wave is shown theoretically. Backward couplers are demonstrated experimentally to have wideband and dual-band properties by judiciously choosing LHM-LHM and LHM-RHM slabs. This proposed method offers flexibility in producing wideband/multiband coupling and can be used to design and fabricate microwave devices.
Comparative studies of compressibility between nanocrystalline and bulk nickel
Authors: Zhang Jianzhong , Zhao Yusheng , Palosz Bogdan
Published: January 2007

The authors conducted a comparative study of compressibility between nanocrystalline and bulk nickel using synchrotron x-ray diffraction at pressures up to 7.4GPa. Their results present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. It is also observed that the bulk moduli determined using the apparent lattice parameters are 17%–18% lower at the smaller diffraction vectors (Q=2π∕d) than those at the larger Q values. These findings support the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer.
Random surface roughness influence on gas damped nanoresonators
Authors: Palasantzas G.
Published: January 2007

The author investigates quantitatively the influence of random surface roughness on the quality factor Q of nanoresonators due to noise by impinging gas molecules. The roughness is characterized by the amplitude w, the correlation length ξ, and the roughness exponent H that describes fine roughness details at short wavelengths. Surface roughening (decreasing H and increasing ratio w∕ξ) leads to lower Q, which translates to lower sensitivity to external perturbations, and a higher limit to mass sensitivity. The influence of the exponent H is shown to be important as that of w∕ξ, indicating the necessity for precise control of the surface morphology.
Structure and magnetic transport properties of Ge doped La(Co,Si)13 intermetallics
Authors: Guo Yongquan , Li Wei
Published: January 2007

The crystal structure and the magnetic and electronic transport properties of Ge doped LaCo10.5Si2.5−xGex (x=0–0.3) intermetallics have been investigated using powder x-ray diffraction and magnetic and electrical measurements. According to the x-ray diffraction pattern analysis, Ge as a dopant decreases the stability of the cubic LaCo 1:13 phase and stabilizes the Si based La(Co,Si) 1:13 tetragonal phase. From the authors’ structural refinement, the authors found that Ge prefers to occupy the 16l(2) position in the tetragonal phase. LaCo10.5Si2.2Ge0.3 shows ferromagnetic order with a metamagnetic state around 350K. An anomaly in resistivity is observed at around 367K, which is associated with the metamagnetic transition. The resistivity follows ρ(T)=ρo+aTn rule in the temperature range of 4–400K with n=1.17(3) for the half metallic state and n=2 for the normal metallic state, respectively. This type of discontinuity can be interpreted as a crossover from a low temperature metamagnetic half metallic state to the normal ferromagnetic-metallic state.
Multiplexing single electron transistors for application in scalable solid-state quantum computing
Authors: Conrad Vincent I. , Greentree Andrew D. , Hollenberg Lloyd C. L.
Published: January 2007

Single electron transistors (SETs) are nanoscale electrometers of unprecedented sensitivity, and as such have been proposed as readout devices in a number of quantum computer architectures. The authors show that the functionality of a standard SET can be multiplexed so as to operate as both readout device and control gate for solid-state qubit systems based on charge localization and spin-charge transduction. Such multiplexing offers new possibilities for gate density reduction in nanoscale devices, and may therefore play a role in scalable quantum computer architectures.
Efficient coupling of high intensity short laser pulses into snow clusters
Authors: Palchan T. , Pecker S. , Henis Z. , Eisenmann S. , Zigler A.
Published: January 2007

Measurements of energy absorption of high intensity laser pulses in snow clusters are reported. Targets consisting of sapphire coated with snow nanoparticles were found to absorb more than 95% of the incident light compared to 50% absorption in flat sapphire targets.
Domain wall broadening mechanism for domain size effect of enhanced piezoelectricity in crystallographically engineered ferroelectric single crystals
Authors: Rao Wei-Feng , Wang Yu U.
Published: January 2007

Computer modeling and simulation reveal a domain wall broadening mechanism that explains the domain size effect of enhanced piezoelectric properties in domain engineered ferroelectric single crystals. The simulation shows that, under electric field applied along the nonpolar axis of single crystal without domain wall motion, the domain wall broadens and serves as embryo of field-induced new phase, producing large reversible strain free from hysteresis. This mechanism plays a significant role in the vicinity of interferroelectric transition temperature and morphotropic phase boundary, where energy difference between stable and metastable phases is small. Engineered domain configuration fully exploits this domain wall broadening mechanism.
Optimizing levitation force and stability in superconducting levitation with translational symmetry
Authors: Del Valle Nuria , Sanchez Alvaro , Pardo Enric , Chen Du-Xing , Navau Carles
Published: January 2007

From a theoretical analysis based on an energy minimization procedure and the critical-state model, the authors analyze the levitation of an infinitely long superconductor above infinitely long permanent magnets with different arrangements. A discussion on optimum geometries showing large levitation force and good stability is presented from the model results.
Gate capacitance-voltage characteristics of submicron-long-gate diamond field-effect transistors with hydrogen surface termination
Authors: Kasu M. , Ueda K. , Yamauchi Y. , Makimoto T.
Published: January 2007

The radio-frequency characteristics of p-type diamond field-effect transistors with hydrogen surface termination were numerically analyzed using an equivalent-circuit model. From the gate-source capacitance (CGS)-voltage (VGS) results extracted from measured s parameters, the authors found a plateau in CGS within a certain VGS range. This means that a two-dimensional hole gas channel forms parallel to the surface and that the channel is separated by a thin energy-barrier layer with an infinite height from the gate metal. At a high negative VGS, as negative VGS is increased, CGS increases steeply. This results from holes penetrating the energy barrier.
Combined scanning force microscopy and scanning tunneling spectroscopy of an electronic nanocircuit at very low temperature
Authors: Senzier J. , Luo P. S. , Courtois H.
Published: January 2007

The authors demonstrate the combination of scanning force microscopy and scanning tunneling spectroscopy in a local probe microscope operating at very low temperature (60mK). This local probe uses a quartz tuning fork ensuring high tunnel junction stability. They performed the spatially resolved spectroscopic study of a superconducting nanocircuit patterned on an insulating substrate. Significant deviations from the theoretical prediction are observed.
Self-organized nanocrystalline stripe patterns generated during early crystallization of a nonequilibrium metallic glass
Authors: Li Jian-Min
Published: January 2007

The author reports a unique early-stage (the crystallized volume fraction ∼0.20) self-organized nanocrystalline stripe pattern phenomenon that nanoscale phase separation and nucleation simultaneously work together, representing a type that had never been previously observed in a metastable alloy solid. The first observation of mesoscopic self-organized crystallization showed that self-organization is not limited to the case of organic matter, thus potentially offering an insight into the mechanism of the nonequilibrium (nano)crystallization of metallic glasses.
Reproducible hysteresis and resistive switching in metal-CuxO-metal heterostructures
Authors: Dong R. , Lee D. S. , Xiang W. F. , Oh S. J. , Seong D. J. , Heo S. H. , Choi H. J. , Kwon M. J. , Seo S. N. , Pyun M. B. , Hasan M. , Hwang Hyunsang
Published: January 2007

Materials showing reversible resistance switching between high-resistance state and low-resistance state at room temperature are attractive for today’s semiconductor technology. In this letter, the reproducible hysteresis and resistive switching characteristics of metal-CuxO-metal (M-CuxO-M) heterostructures driven by low voltages are demonstrated. The fabrication of the M-CuxO-M heterostructures is fully compatible with the standard complementary metal-oxide semiconductor process. The hysteresis and resistive switching behavior are discussed. The good retention characteristics are exhibited in the M-CuxO-M heterostructures by the accurate controlling of the preparation parameters.