bluuebunny/crossref_metadata_2025 huggingface dataset:
Inter-peptide interaction and delocalization of amide I vibrational excitons in myoglobin and flavodoxin
Authors: Choi Jun-Ho , Ham Sihyun , Cho Minhaeng
Published: October 2002

The amide I vibrational normal-mode analyses of two globular proteins, myoglobin and flavodoxin, were carried out and the spatial delocalization and phase correlation of each normal mode was studied. The Hessian matrix in the restricted space constructed by the N peptides was obtained by using both ab initio quantum chemistry calculation results and empirical transition dipole coupling model. For both of the two proteins, the amide I normal modes are fairly delocalized and on average about 15% of the polypeptides in myoglobin and flavodoxin are participated in the formation of each amide I normal mode. The analysis of vibrational phase correlation of myoglobin reveals that the high-frequency amide I normal modes are mostly (symmetric) A-mode like, whereas the low-frequency ones are (antisymmetric) E-mode like. For flavodoxin, among the three distinct bands at 1653, 1636, and 1628 cm−1, the high-frequency modes are found to be fairly localized in the four α helices, the low-frequency modes around 1628 cm−1 are strictly localized in the five β sheets, and the midfrequency modes around 1636 cm−1 are delocalized in both the α helices and β sheets. Effects of the diagonal disorder on the amide I band of both proteins were studied. We found that the sharp peak at 1628 cm−1 in the resolution enhanced amide I infrared spectrum of flavodoxin is a direct signature that the through-space interactions between two different β sheets exist.
Unification of the Jaynes–Cummings model and Planck’s radiation law
Authors: van Wonderen A. J. , Lendi K.
Published: September 2002

By combining iterative methods with Laplace transformation, we construct the solution of a dissipative Jaynes–Cummings model. The dissipative part of the model is based on the standard Markovian master equation for a harmonic oscillator that is coupled to a heat bath of nonzero temperature. Besides photon loss, we also take into account frequency detuning between atom and field. Before commencing the iteration, we subject the matrix elements of the density operator to a transformation that depends on temperature. As a result, the pole structure of all Laplace transformed matrix elements is improved. It becomes manifest which poles do not contribute to the asymptotic behavior of the density operator. In proving that our iterative process yields convergent results, we assume upper bounds on: the matrix elements of the density operator, the matrix elements of the initial density operator, the damping parameter of the heat bath, and the temperature of the heat bath. All of these bounds are physically acceptable. The photon field may start from a coherent state or a number state. For experiments in a microwave cavity, temperatures of the order of 0.1 [K] are allowed. As an application, the evolution of the atomic density matrix is studied. We propose a limit for which this matrix converges to the state of maximum von Neumann entropy. The time, the cubed initial energy density, and the inverse of the damping parameter must tend to infinity equally fast. The photon field is assumed to be in a number state at time zero, whereas the initial state of the atom can be chosen freely.
Dust grain acceleration during plasma expansion
Authors: Djebli M. , Bahamida S. , Annou R.
Published: September 2002

A dusty plasma adiabatic expansion model, where an exact ion charging current as well as ion-dust charge exchange reaction produced neutrals are taken into account, is proposed and numerically solved. Particle cooling, dust fronts associated with excitation of dust sound waves along with acceleration of bursts of grains, are demonstrated.
Wigner functions with boundaries
Authors: Dias Nuno Costa , Prata João Nuno
Published: September 2002

We consider the general Wigner function for a particle confined to a finite interval and subject to Dirichlet boundary conditions. We derive the boundary corrections to the “stargenvalue” equation and to the time evolution equation. These corrections can be cast in the form of a boundary potential contributing to the total Hamiltonian which together with a subsidiary boundary condition is responsible for the discretization of the energy levels. We show that a completely analogous formulation (in terms of boundary potentials) is also possible in standard operator quantum mechanics and that the Wigner and the operator formulations are also in one-to-one correspondence in the confined case. In particular, we extend Baker’s converse construction to bounded systems. Finally, we elaborate on the applications of the formalism to the subject of Wigner trajectories, namely in the context of collision processes and quantum systems displaying chaotic behavior in the classical limit.
A new parametrization and all integral realizations of the Lorentz group
Authors: Louck James D.
Published: September 2002

A simple linear transformation of the biquaternionic parameters α and β of the group SL(2, C), which is two-to-one homomorphic to the restricted Lorentz group ℒ, is used to express each element of SL(2, C) and ℒ in terms of the first column of an element of ℒ and the quaternionic parameters α. This parametrization is shown to bring the determination of the integral realizations of the restricted Lorentz group under the purview of classical Diophantine analysis, involving the expression of a given pair of integers, respectively, as a sum of three squares and a sum of four squares. These solutions are further constrained by the condition that a set of four integers, which is linear in the solution of the three squares problem and in the solution of the four squares problem, have a common factor. These are all classical problems addressed and solved by Euclid, Euler, Fermat, Gauss, Jacobi, and others. The corresponding realizations of SL(2, C) fall into three distinct classes: those in which the elements of the matrix are Gaussian integers, Gaussian integers divided by √, and Gaussian half-odd integers. These results apply also to the principal subgroups of SL(2, C).
Dielectric response of plasma in reversed field pinches near the ion cyclotron frequency
Authors: Svidzinski V. A.
Published: September 2002

The dielectric tensor of reversed field pinch (RFP) plasma is calculated near the magnetic axis for cylindrical azimuthally symmetric equilibria in the limit k⊥ρL→0. Particle orbits are calculated to first order in the drift approximation near the axis such that geometrical corrections to gyrophase are included. It is demonstrated that these corrections are important for evaluation of the plasma heating rate near ion cyclotron resonance in RFPs while they can be neglected in tokamak modeling. Geometrical effects of this kind become important in RFPs because the ratio of poloidal and toroidal magnetic field components is substantially larger in RFPs relative to that in tokamaks.
The dynamical elliptic quantum Gaudin models and their solutions
Authors: Hou Boyu , Shi Kangjie , Yue Ruihong , Zhao Shaoyou
Published: September 2002

In this paper, we construct the Hamiltonians of both periodic and open elliptic quantum Gaudin models and show their relations with the elliptic quantum group, and the boundary elliptic quantum group, respectively. We define the eigenstates of these two models to be the Bethe vectors with η=0 of the elliptic quantum group and the boundary elliptic quantum group, respectively. Then, the Hamiltonians are exactly diagonalized by using the algebraic Bethe ansatz method.
Evidence for thermal anisotropy effects on shear modified ion acoustic instabilities
Authors: Scime E. E. , Keesee A. M. , Spangler R. S. , Koepke M. E. , Teodorescu C. , Reynolds E. W.
Published: September 2002

Inclusion of thermal anisotropy effects is shown to be required to describe recently reported experimental measurements as shear-modified, ion acoustic instabilities. For the reported experimental conditions, isotropic theory yields no instability growth that depends on the magnitude of the shear in the parallel flow.
Wetting morphologies on substrates with striped surface domains
Authors: Brinkmann Martin , Lipowsky Reinhard
Published: October 2002

The wetting and dewetting of chemically structured substrates with striped surface domains is studied theoretically. The lyophilic stripes and the lyophobic substrate are characterized by different contact angles θγ and θδ, respectively. We determine the complete bifurcation diagram for the wetting morphologies (i) on a single lyophilic stripe and (ii) on two neighboring stripes separated by a lyophobic one. We find that long channels can only be formed on the lyophilic stripes if the contact angle θγ is smaller than a certain threshold value θch(V) which depends only weakly on the volume V and attains the finite value θch(∞) in the limit of large V. This asymptotic value is equal to θch(∞)=arccos(π/4)≃38° for all lyophobic substrates with θδ⩾π/2. For a given value of θγ<θch(∞), the extended channels spread onto the lyophilic stripes with essentially constant cross section.
A topological method for geodesic connectedness of space–times: Outer Kerr space–time
Authors: Luis Flores José , Sánchez Miguel
Published: September 2002

The geodesic connectedness of outer Kerr space–time is proven by using a topological method. The proof is based on arguments about Brouwer’s degree for the solutions of functional equations. The applicability of these topological arguments for dealing with geodesics in space–times is stressed.
Reduction of edge emission in electron beam diodes
Authors: Hegeler F. , Friedman M. , Myers M. C. , Sethian J. D. , Swanekamp S. B.
Published: September 2002

This paper presents measurements of the enhanced current density along the edges of a large area electron beam as well as successful techniques that eliminated this edge effect/beam halo. The beam current is measured with a Faraday cup array at the anode, and the spatial, time-integrated current density is obtained with radiachromic film. Particle-in-cell simulations support the experimental results. Experiments and simulations show that recessing the cathode reduces the electric field at the edge and eliminates the edge effect. However, the cathode recess structure itself emits under long-term repetitive operation. In contrast, using a floating, metallic, electric field shaper that is electrically insulated from the cathode eliminates the beam halo and mitigates electron emission from its surface during repetitive operation.
Nonlinear frequency shifts of plasma eigenmodes
Authors: Yoon Peter H. , Gaelzer Rudi
Published: September 2002

In the present article, the classic problem of nonlinear frequency shifts of electrostatic plasma eigenmodes in an unmagnetized plasma (i.e., the Langmuir and ion-acoustic waves) is revisited. In the standard literature, only the frequency shift of Langmuir waves by the finite-amplitude Langmuir waves themselves is usually treated. In the present approach, the discussion is generalized to include the ion-sound waves. The significance of the present article is that the analytical approach employed in the present discussion can be utilized to resolve certain apparently singular terms in the induced scattering coefficients of the wave kinetic equations. The detailed discussion of such a problem will be reported in a forthcoming article.
Power dissipation mode transition by a magnetic field
Authors: You S. J. , Chung C. W. , Bai K. H. , Chang H. Y.
Published: September 2002

We measured electrical characteristics of transversely magnetized capacitively coupled plasma at low pressure (10 mTorr). From these measurements, we found that the power characteristics of the magnetized discharge were different from those of the unmagnetized discharge. As the magnetic field increases, a square dependence of power characteristic at high current changes to a linear dependence. This can be understood as a power dissipation mode transition by a magnetic field. A calculation from a simple sheath model agrees well with the experimental data.
Enhanced second-order nonlinear optical response of LiNbO3 films upon Er doping
Authors: Gonzalo J. , Chaos J. A. , Suárez-Garcı́a A. , Afonso C. N. , Pruneri V.
Published: September 2002

Erbium-doped LiNbO3 films have been produced in a single-step process by alternate pulsed laser deposition. The dopant is incorporated in submonolayers whose nominal indepth separation is varied in the range 1.7–4.0 nm to lead respectively to Er concentrations in the range 3.4–0.6×1020 atoms/cm−3. All the films exhibit the characteristic Er3+ photoluminescence at 1.54 μm with lifetime values as high as 3 ms. The d33 nonlinear coefficients determined from second-harmonic generation experiments are in the range 22–28 pm/V, the films having nominal Er submonolayer indepth separation of 3–4 nm exhibiting d33 values slightly above that of the bulk material. The comparison of the d33 values obtained in the Er-doped films to those reported earlier for similar undoped films shows clearly that both the structural quality and the second-harmonic performance of the films can be enhanced by Er doping.
Reduction of the Hilbert space in strongly correlated systems
Authors: Chung Myung-Hoon , Kwon Jae-Hoon
Published: September 2002

Defining commutation relations between symmetry operators and fundamental operators, we set up the symmetry group for a many-particle Hamiltonian. Using the irreducible representations of the symmetry group, we decompose the Hilbert space. We discuss the advantage of this approach to find the dimensions of reduced Hilbert spaces in numerical exact diagonalization.
Atomic and molecular adsorption on Rh(111)
Authors: Mavrikakis M. , Rempel J. , Greeley J. , Hansen L. B. , Nørskov J. K.
Published: October 2002

A systematic study of the chemisorption of both atomic (H, O, N, S, C), molecular (N2, CO, NO), and radical (CH3, OH) species on Rh(111) has been performed. Self-consistent, periodic, density functional theory (DFT-GGA) calculations, using both PW91 and RPBE functionals, have been employed to determine preferred binding sites, detailed chemisorption structures, binding energies, and the effects of surface relaxation for each one of the considered species at a surface coverage of 0.25 ML. The thermochemical results indicate the following order in the binding energies from the least to the most strongly bound: N2<CH3<CO<NO<H<OH<O<N<S<C. A preference for threefold sites for the atomic adsorbates is observed. Molecular adsorbates, in contrast, favor top sites with the exceptions of NO (hcp) and OH (fcc or bridge tilted). Surface relaxation leads to insignificant changes in binding energies but to considerable changes in the spacing between surface rhodium atoms, particularly for on-top adsorption where the rhodium atom directly below the adsorbate is lifted above the plane of the surface. RPBE binding energies are found to be in remarkable agreement with the available experimental values. All atomic adsorbates, except for H, have a significant diffusion barrier [between 0.4 and 0.6 eV (RPBE)] on Rh(111). Atomic H and molecular/radical adsorbates appear to be much more mobile on Rh(111), with an estimated diffusion barrier between 0.1 and 0.2 eV (RPBE). Finally, the thermochemistry for dissociation of CO, NO, and N2 on Rh(111) has been examined. In all three cases, decomposition is found to be thermodynamically preferable to desorption.
Phase dependence of Thomson scattering in an ultraintense laser field
Authors: He Fei , Lau Y. Y. , Umstadter Donald P. , Strickler Trevor
Published: September 2002

The Thomson scattering spectra of an electron by an ultraintense laser field are computed. It is found that the electron orbit, and therefore its nonlinear Thomson scattering spectra, depend critically on the amplitude of the ultraintense laser field and on the phase at which the electron sees the laser electric field. Contrary to some customary notions, the Thomson scattering spectra, in general, do not occur at integer multiples of the laser frequency and the maximum frequency is proportional to the first instead of the third power of the electric field strength for the case of an ultraintense laser. The implications of these findings are discussed.
Electrical properties, texture, and microstructure of vicinal YBa2Cu3O7−δ thin films
Authors: Pedarnig J. D. , Rössler R. , Delamare M. P. , Lang W. , Bäuerle D. , Köhler A. , Zandbergen H. W.
Published: September 2002

Vicinal YBa2Cu3O7−δ (YBCO) thin films of thickness h=20–480 nm are grown by pulsed-laser deposition on 10° miscut (001) SrTiO3 substrates. The anisotropic resistivities, c-axis texture, and critical temperature drastically depend on the thickness of vicinal films. High-resolution electron microscopy reveals a defect microstructure with strong bending of the YBCO lattice near the SrTiO3 interface and improved film microstructure at larger distances to the substrate. The required layer thickness for microstructure relaxation and increase of electrical conductivity are significantly larger than the critical thickness of c-axis oriented YBCO films.
Controlled group V intermixing in InGaAsP quantum well structures and its application to the fabrication of two section tunable lasers
Authors: Teng J. H. , Dong J. R. , Chua S. J. , Lai M. Y. , Foo B. C. , Thompson D. A. , Robinson B. J. , Lee A. S. W. , Hazell John , Sproule Irwin
Published: October 2002

We report the technique of controlled group V quantum well intermixing (QWI) in a compressively strained In0.76Ga0.24As0.85P0.15/In0.76Ga0.24As0.52P0.48 multiquantum well laser structure and its application to the fabrication of two-section tunable lasers. The blueshift of the band-gap energy was enhanced by capping the samples with films of SiO2 or low-temperature grown InP, while suppressed by a SixNy film with a refractive index of about 2.1. Spatially selective band-gap tuning was achieved by patterning the dielectric film into dot and strip arrays with different surface coverage. Time-of-flight secondary ion mass spectra showed that the enhanced blueshift was caused by the interdiffusion of group V atoms between the quantum wells and barriers. A group V interstitial interdiffusion mechanism is proposed for the sample capped with SiO2 and this is supported by the even more efficient intermixing induced by low-temperature InP, which contains a high concentration of excess phosphorus. A two-section tunable laser operating around 1.55 μm was fabricated using this QWI technology. A tuning range of about 10 nm was demonstrated by simply changing the current injected into the phase tuning section.
Field and temperature-dependent attempt frequency for dynamic-thermal reversal of interacting magnetic grains
Authors: Wang Xiaobin , Bertram H. Neal
Published: October 2002

The field and temperature dependence of the attempt frequency is important for the analysis of dynamic-thermal reversal of magnetic grains, especially at relatively short times. Here we present a general approach by which the attempt frequency can be easily evaluated for interacting magnetic grains or subsystems. For the case of large energy barriers relative to thermal energy, a general asymptotic solution to the Fokker–Planck equation is developed. The stochastic equations are based upon a dynamic-thermal reversal model. Dynamic coercivity versus pulse time for the whole time range, from purely dynamic reversal to purely thermal reversal, is calculated. Our approach is illustrated by specific examples of an independent grain, two ferromagnetic coupled grains, and two antiferromagnetic coupled grains. The effect of the exchange interaction is emphasized.
Atomic force microscope anodic oxidation studied by spectroscopic microscopy
Authors: Lazzarino M. , Heun S. , Ressel B. , Prince K. C. , Pingue P. , Ascoli C.
Published: October 2002

Atomic force microscope (AFM) induced local oxidation is a versatile and promising nanofabrication process used successfully to produce quantum devices. Nevertheless, little information is available on the chemical and structural properties of the grown oxide. We address this open issue by a spectromicroscopic study of nanoscopic oxide patterns grown by AFM anodic oxidation on n-type silicon substrate. We show that AFM oxidation produces chemically uniform, stoichiometric SiO2, and that its chemical and structural properties do not depend on the applied voltage. The observed electrostatic shift of the oxide binding energies allows a simple estimation of the electrical properties of the AFM induced oxide.
Growth of high-density Si nanoparticles on Si3N4 and SiO2 thin films by hot-wire chemical vapor deposition
Authors: Zhu Jianhong , Leach W. Thomas , Stanley Scott K. , Ekerdt John G. , Yan Xiaoming
Published: October 2002

High-density (>1×1012 cm−2) Si nanoparticles have been successfully grown on Si3N4 and SiO2 thin films by hot-wire chemical vapor deposition (HW-CVD) using disilane, in which Si atoms are generated on a heated tungsten filament and, after desorbing, impinge on the substrate. The highest density, 1.1×1012 cm−2 as measured by scanning electron microscopy (SEM) and 2.1×1012 cm−2 by scanning tunneling microscopy, is achieved by depositing 1.8 monolayer Si on Si3N4 at 600 °C and a disilane pressure of 1.2×10−6 Torr. The corresponding Si nanoparticles have an average size of about 5 nm. Different densities are reported because scanning tunneling microscopy imaged Si nanoparticles of ∼4 nm, beyond the resolution of SEM. At temperatures above 600 °C, parallel thermal chemical vapor deposition (CVD) during HW-CVD becomes important. Parallel thermal CVD broadens the size distribution of Si nanoparticles and causes coalescence of neighboring nanoparticles, leading to a decrease of nanoparticle density. High densities can be realized above 600 °C if a two-step procedure is used to reduce parallel thermal CVD by starting the growth at a lower temperature.
How nanocavities in amorphous Si shrink under ion beam irradiation: An in situ study
Authors: Ruault M.-O. , Fortuna F. , Bernas H. , Ridgway M. C. , Williams J. S.
Published: September 2002

Nanocavities were formed in Si substrates by conventional H implantation and thermal annealing, after which the samples were amorphized by Si ion irradiation. The size evolution of the nanocavities was monitored in situ during further ion irradiation with Si or As at temperatures of 300 or 600 K. The decrease in nanocavity diameter during ion irradiation depended linearly on the ion fluence. The rate of shrinkage differed according to the ion beam-induced atomic displacement rate and had little or no temperature dependence. These in situ results shed new light on possible ion-beam-induced nanocavity shrinkage mechanisms.
Application of low-temperature area-selective regrowth for ultrashallow sidewall GaAs tunnel junctions
Authors: Oyama Yutaka , Ohno Takeo , Tezuka Kenji , Suto Ken , Nishizawa Jun-ichi
Published: September 2002

Low-temperature (290 °C) area-selective regrowth by the intermittent injection of precursors in an ultrahigh vacuum was applied for the fabrication of ultrashallow sidewall GaAs tunnel junctions with the junction area in the order of 10−8 cm2. The tunnel junctions on the normal mesa orientation have shown the record peak current density up to 31 000 A/cm2 and negative differential conductance of −1.4×10−5 S at 100 μm long strip structure. The peak current density of tunnel junctions has shown strong sidewall orientation dependences with the order of {111}A>{110}>{111}B.
Formation of a quantum dot in a single-walled carbon nanotube using the Al top-gates
Authors: Park J. W. , Choi J. B. , Yoo K.-H.
Published: September 2002

We have fabricated gate-controlled carbon-nanotube single-electron devices by utilizing the line-shaped Al top-gates. A quantum dot is formed in the single-walled carbon nanotube between two Al top-gates fabricated using the electron-beam lithography technique. The deposited top-gates flatten the single-walled carbon nanotube locally and the deformed regions play the role of tunneling barrier, whose potential is controlled by the top-gates. We have also investigated the temperature dependence of the conductance G for the devices with the Al top-gates. The power-law dependence, G∝Tα, is observed at high temperatures. However, the exponent α increases as the barrier potential is enhanced.
Sensitive detection of voids in solid materials by refraction-enhanced synchrotron radiation imaging
Authors: Imamura Keiko , Ehara Norishige , Umetani Keiji , Miyamoto Keiko , Kanemaki Yoshihide , Uesugi Kentarou , Inada Yoichi , Ogata Haruki , Nakajima Yasuo , Fukuda Mamoru
Published: September 2002

Voids in opaque materials (minute air bubbles) were imaged with synchrotron radiation in a refraction enhancement mode. The voids are imaged by an enhanced x-ray intensity inside the bubble, surrounded by a border region with decreased x-ray intensity, thus allowing sensitive detection of air bubbles in plastic materials. As those “impurities” could not be depicted with conventional radiography, and optical inspection is not useful if the matrix is opaque, this in-situ imaging technique offers the potential to obtain information of air inclusions, voids, and cracking that appear inadvertently in opaque plastics and possibly in metals as well.
Near-band-edge photoluminescence of wurtzite-type AlN
Authors: Kuokstis E. , Zhang J. , Fareed Q. , Yang J. W. , Simin G. , Khan M. Asif , Gaska R. , Shur M. , Rojo C. , Schowalter L.
Published: October 2002

Temperature-dependent photoluminescence (PL) measurements were performed for A-plane and C-plane bulk AlN single crystals and epitaxial layers on sapphire. A strong near-band-edge (NBE) emission and deep-level luminescence were observed. At low excitations, the emission spectra are dominated by free and bound excitonic transitions and their LO-phonon replicas. At high excitations, the broadening and redshift of the NBE band is attributed to dense electron–hole plasma formation. The PL spectra differences of bulk single crystals and epilayers is explained by the electron–hole plasma expansion peculiarities.
Wavelength tunable ultrashort pulse generation from a passively mode-locked asymmetric-quantum-well semiconductor laser
Authors: Brennan M. J. , Milgram J. N. , Mascher P. , Haugen H. K.
Published: September 2002

Optical pulses are generated from a passively mode-locked dual asymmetric-quantum-well semiconductor laser mounted in a linear external cavity. The devices are fabricated with two sections, one of which contains a bend in the waveguide for external cavity coupling. Pulses 2.0–3.9 ps in duration have been generated, which are tunable over 61 nm, from 954 to 1015 nm. Pulse compression yields durations as short as 470 fs with peak powers as high as 1.5 W.
Characterization and control of the HfO2/Si(001) interfaces
Authors: Hoshino Y. , Kido Y. , Yamamoto K. , Hayashi S. , Niwa M.
Published: September 2002

The HfO2/Si(001) interfaces formed by reactive dc sputter deposition of Hf buffer layer followed by HfO2 stacking were analyzed by high-resolution transmission electron microscopy, medium energy ion scattering (MEIS), and photoelectron spectroscopy using synchrotron-radiation lights. The present MEIS analysis determined the elemental depth profiles and revealed that no Hf buffer layer resulted in growth of SiO2 at the interface, and that the presence of the Hf layer led to the formation of Si-rich silicate-like interlayers. The binding energy shifts of Si-2p3/2 identified the chemical bonds of the interfacial layers and confirmed the formation of SiO2 (no buffer layer) and silicate layers (presence of the buffer layers) at the interfaces. The Hf-buffer layer suppresses the O diffusion toward the interface and thus the thicker the buffer layer, the thinner the Hf-silicate interlayer. The deposition condition of HfO2 (1.3 nm)/Hf (1.3 nm) has achieved the highest permittivity of 28 for HfO2 (3.6 nm) and 8 for the silicate layer (1.7 nm).
Demonstration and optical characteristics of electro-optic Bragg modulators in periodically poled lithium niobate in the near-infrared
Authors: Abernethy J. A. , Gawith C. B. E. , Eason R. W. , Smith P. G. R.
Published: September 2002

We report the infrared operation of a bulk optical Bragg modulator based on electro-optically induced refractive index gratings in z-cut periodically poled lithium niobate. Efficiencies in the first order of 45% for 1064 nm e-polarized light and 30% for o-polarized light were achieved, with maximum on/off ratios of 15:1 and 9:1, respectively. Field-induced light scattering effects due to poling are observed at higher drive voltages and compromised device performance due to these scattering effects is predicted to limit long-wavelength operation of these devices.
All-optical cw laser polarization conversion at 1.55 μm by two-beam coupling in nematic liquid crystal films
Authors: Khoo I. C. , Ding J.
Published: September 2002

We have demonstrated efficient all-optical polarization conversion of cw 1.55 μm laser in nematic liquid crystal films. The process is mediated by coherent two-beam coupling effect between an incident polarized laser and its orthogonally polarized noise component scattered by the director axis fluctuations. We report here a quantitative theory that accounts for severe pump depletion and high-efficiency polarization conversion and demonstrate good agreement with experimental observations.
Method to measure shear moduli of nonequilibrium phases pseudomorphically stabilized in superlattices
Authors: Pankov V. , Evstigneev M. , Prince R. H.
Published: September 2002

A method to measure shear moduli of nonequilibrium phases pseudomorphically stabilized in superlattices is proposed. The method is based on the use of a stress-free coherent superlattice as a modeling structure whereby the lattice misfit strain is distributed between the individual layers according to their shear moduli and Poisson ratios. The method is applied to estimate the shear modulus of the nonequilibrium rocksalt-type (B1) AlN phase pseudomorphically stabilized in AlN/TiN superlattices. The value of shear modulus of B1–AlN in these superlattices was found to increase from 288±50 GPa to 457±80 GPa with decreasing bilayer thickness from 1 to 0.6 nm.
Random lasing following two-photon excitation of highly scattering gain media
Authors: Zacharakis Giannis , Papadogiannis Nektarios A. , Papazoglou Theodore G.
Published: September 2002

We present experimental evidence of laser emission following two-photon excitation of dye-infiltrated random gain media with optical properties similar to biological tissue. The excitation was performed with femtosecond laser pulses at 800 nm and the emission (at 480 nm) was recorded with a spectrograph streak camera system. The coherent properties of the random lasing emission were also investigated by performing single photon counting. The applications of coherent laser light that can be emitted deep inside a random medium are far reaching, particularly for imaging and therapeutic purposes.
Self-sustained photoelectron discharge
Authors: Bokhan P. A. , Zakrevsky D. E.
Published: September 2002

This letter reports a discharge structure with a self-sustained photoelectron discharge. Specific to this discharge is its ability to generate electron beams with an energy of several keV and virtually 100% efficiency. A mechanism underlying the high efficiency of electron beam generation observed is proposed; the conditions required for pulse and continuous photoelectron discharge are determined.
Fabrication of stable nanopatterns on metals
Authors: Sekiba Daiichiro , Bertero Stefania , Buzio Renato , Buatier de Mongeot Francesco , Boragno Corrado , Valbusa Ugo
Published: September 2002

Nanopatterns on metal surfaces can be easily created by ion sputtering. However, due to the fast diffusion processes characterizing these materials, the nanostructures are often unstable at room temperature and above. This effect prevents the use of such patterned substrates in nanotechnology applications. In this letter, we present a simple oxidation process able to stabilize these features durably. The method has been tested on Cu, but its generality suggests that it can be applied to many other metals.
Large ferroelectric response in Bi4−xNdxTi3O12 films prepared by sol–gel process
Authors: Melgarejo R. E. , Tomar M. S. , Bhaskar S. , Dobal P. S. , Katiyar R. S.
Published: September 2002

Neodymium-substituted Bi4Ti3O12 (i.e., Bi4−xNdxTi3O12) were synthesized by sol–gel process for different compositions. Thin films were deposited on Pt (i.e., Pt/TiO2/SiO2/Si) substrate by spin coating. Materials were characterized by x-ray diffraction and Raman spectroscopy. This study indicates that the material makes a solid solution for the compositions: x=0.00, 0.26, 0.46, 0.75, 0.85, 1.00, and 2.00, where an Nd ion replaces the Bi site. The prominent effect of Nd substitution is observed in low-frequency Raman modes. Sol–gel derived thin films of Bi3.54Nd0.46Ti3O12 on a Pt substrate and postannealed at 700 ° C were tested for ferroelectric response which showed high remnant polarization (Pr=25 μC/cm2).
Purely antiferromagnetic spin waves (antimagnons) in tetragonal magnets and ways of exciting them
Authors: Turov E. A. , Mirsaev I. F.
Published: October 2002

Purely antiferromagnetic oscillations (antimagnons) in tetragonal magnets with the crystal-chemical structure of the trirutiles (Fe2TeO6, etc.) are investigated theoretically. These oscillations were considered by Ukrainian physicists back in 1988, but not for tetragonal crystals and only in the case of an antiferromagnetic (AF) structure. In the present study, besides the AF phases the ferromagnetic phase in the same system of four magnetic sublattices is also considered. Ways of exciting the antimagnons, which are a fourth form of spin waves (in addition to the ferromagnons, quasiferromagnons, and quasiantiferromagnons), are also discussed. Independently of the means of excitation (electric or magnetic field, hypersound), antimagnons are defined as spin waves for which the total local ferromagnetic vector M(r) does not take part in the oscillations, but only the AF vector (or vectors) L(r). The eigenfrequencies and the corresponding susceptibilities are calculated. The presentation of this material is prefaced by a review of the status of the problem on the whole. It is noted that in the majority of cases the effects under consideration arise as a dynamic manifestation of the magnetoelectric and antiferroelectric interactions, even in magnets for which the static magnetoelectric effect is absent.
Influence of structural inhomogeneity on the luminescence properties of silicon nanocrystallites
Authors: Blonskiı̆ I. V. , Brodyn M. S. , Vakhnin A. Yu. , Zhugayevych A. Ya. , Kadan V. M. , Kadashchuk A. K.
Published: October 2002

The features of the photoluminescence and the manifestation of thermally stimulated and tunneling luminescence due to the separation of nonequilibrium charge carriers between the photoexcited silicon core of a nanocrystallite and its peripheral layers of SiOx and SiO2 are investigated for different forms of nanostructured silicon. A model is proposed wherein a “two-stroke charge piston” acts in turn on the electron and hole components by means of an Auger process which occurs under restricted volume conditions and brings about a spatial separation of the charge carriers.
Permanent grayscales in bistable twisted nematic liquid-crystal displays
Authors: Xie Z. L. , Meng Z. G. , Wong M. , Kwok H. S.
Published: September 2002

Permanent grayscales in a π bistable twisted nematic-liquid-crystal display have been demonstrated. This display can be switched between the −22.5° and 157.5° twist states by a combination of a strong in-plane electric field and a vertical electric field for creating both planar anchoring breaking and an electrohydrodynamic backflow. The switching of the twist states during the Frederiks transition is sufficiently inhomogeneous such that domains of −22.5° and 157.5° twisted states can coexist permanently. The ratio of these domains can be controlled by the vertical electric field.
Photoluminescence upconversion in 4H–SiC
Authors: Wagner Mt. , Ivanov I. G. , Storasta L. , Bergman J. P. , Magnusson B. , Chen W. M. , Janzén E.
Published: September 2002

Efficient photoluminescence upconversion is observed in 4H–SiC samples containing both the UD-3 defect and the titanium impurity. In this process, the titanium photoluminescence emission with no-phonon (NP) lines at 2.848 eV (A0) and 2.789 eV (B0) can be excited by tuning the laser to the NP line of UD-3 at 1.356 eV. In samples containing either only UD-3 or only titanium, a different photoluminescence upconversion process can be observed, which occurs at photon energies higher than ∼1.5 eV without exhibiting sharp features. At least one of the two processes generates both free electrons and free holes and can, therefore, be a candidate for an important recombination channel.
Improved transmission characteristics of moderate refractive index contrast photonic crystal slabs
Authors: Böttger G. , Liguda C. , Schmidt M. , Eich M.
Published: September 2002

Finite two-dimensional (2D)-photonic crystal slab waveguide structures from moderate refractive index materials have been investigated theoretically and results were compared with experimental data. 3D-finite difference time domain (3D-FDTD) simulations reveal a strong dependence of the transmission on the etching depth. For structures with etching depths less than the waveguide core thickness, both a substantial leakage of optical power to the substrate and a spatial mismatch of the transmitted field to the waveguide mode were found, leading to unsatisfactory transmissions. These losses occur mainly on the airband side of the spectrum where the optical field is concentrated in the holes. However, hole depths extending into the substrate by only 0.5 μm lead to an almost perfect mode match at transmission values exceeding 90%.
Enhancing the electroluminescent properties of organic light-emitting devices using a thin NaCl layer
Authors: Kang S. J. , Park D. S. , Kim S. Y. , Whang C. N. , Jeong K. , Im S.
Published: September 2002

We report on the fabrication of organic light-emitting devices (OLEDs) using a thin NaCl interlayer as an electron-injection medium. The results show that the device containing the NaCl layer has a higher brightness and electroluminescent efficiency than the device without this layer. We also fabricated similar-structured comparable devices, which were prepared with a LiF layer as a different electron-injection medium. The maximum electroluminescent efficiency of the NaCl (1 nm)/Al cathode device was 2.85 cd/A, which is higher than the 2.25 cd/A of the LiF (1 nm)/Al cathode device. The ultrathin NaCl layer modified the carrier injection properties. In conclusion, the NaCl layer between a cathode and an emitting layer of OLEDs can be used as the carrier injection layer to improve the EL properties.
Identity Construction in Latvia's “Singing Revolution”: Why Inter-Ethnic Conflict Failed to Occur
Authors: Ginkel John
Published: September 2002

Introduction

Among the nationalist revolutions that spread across Eastern Europe in the late 1980s and early 1990s, Latvia seceded from the Soviet Union through a relatively benign process marked by the virtual absence of violent conflict. Several issues were conflated in this movement. Latvia's independence was about democracy, historical redress, and national autonomy. Yet, each of these areas included an ethnic component. Would a Latvian democracy make room for Russians? Would Latvians seek revenge against Russians for the Soviet annexation of the Baltic republics? In the event of conflict between Moscow and Riga, whom would Russophone residents of Latvia support? The demographic situation of Latvia—featuring a near balance of Latvians and mostly Russophone non-Latvians at the time of independence—suggested the potential for ethnic conflict. The forecast of conflict, though, oversimplified the roles that ethnicity and national identity play in affecting political actions. The potential for conflict was predicated on the assumption that individuals naturally identify primarily with others within their ethnic group and act competitively against members of the ethnic “other.” The fact, then, that Latvians and Russians did not clash violently during Latvia's “Singing Revolution” begs the question why inter-ethnic conflict did not occur in this case. This article explains this lack of conflict by focusing on the formation of Latvian identity in the period immediately preceding independence. I argue that individuals in nationalizing states intentionally act with reference to their national identity, and this sense of national identity is not some fixed, exogenous variable. Instead, it is socially constructed. We cannot hope to explain ethnic conflict processes without first understanding the factors that drive ethnic and national identity during chaotic times of change.
An AscI Boundary Library for the Studies of Genetic and Epigenetic Alterations in CpG Islands
Authors: Dai Zunyan , Weichenhan Dieter , Wu Yue-Zhong , Hall Julia L , Rush Laura J. , Smith Laura T. , Raval Aparna , Yu Li , Kroll Daniela , Muehlisch Joerg , Frühwald Michael C. , de Jong Pieter , Catanese Joe , Davuluri Ramana V. , Smiraglia Dominic J. , Plass Christoph
Published: October 2002

Knudson's two-hit hypothesis postulates that genetic alterations in both alleles are required for the inactivation of tumor-suppressor genes. Genetic alterations include small or large deletions and mutations. Over the past years, it has become clear that epigenetic alterations such as DNA methylation are additional mechanisms for gene silencing. Restriction Landmark Genomic Scanning (RLGS) is a two-dimensional gel electrophoresis that assesses the methylation status of thousands of CpG islands. RLGS has been applied successfully to scan cancer genomes for aberrant DNA methylation patterns. So far, the majority of this work was done using NotI as the restriction landmark site. Here, we describe the development of RLGS using AscI as the restriction landmark site for genome-wide scans of cancer genomes. The availability of AscI as a restriction landmark for RLGS allows for scanning almost twice as many CpG islands in the human genome compared with using NotI only. We describe the development of an AscI–EcoRV boundary library that supports the cloning of novel methylated genes. Feasibility of this system is shown in three tumor types, medulloblastomas, lung cancers, and head and neck cancers. We report the cloning of 178 AscI RLGS fragments via two methods by use of this library.

[Supplemental material is available online at http://www.genome.org.]
Expression Profile Analysis of the Low-Oxygen Response in Arabidopsis Root Cultures[W]
Authors: Klok Erik Jan , Wilson Iain W. , Wilson Dale , Chapman Scott C. , Ewing Rob M. , Somerville Shauna C. , Peacock W. James , Dolferus Rudy , Dennis Elizabeth S.
Published: October 2002

We used DNA microarray technology to identify genes involved in the low-oxygen response of Arabidopsis root cultures. A microarray containing 3500 cDNA clones was screened with cDNA samples taken at various times (0.5, 2, 4, and 20 h) after transfer to low-oxygen conditions. A package of statistical tools identified 210 differentially expressed genes over the four time points. Principal component analysis showed the 0.5-h response to contain a substantially different set of genes from those regulated differentially at the other three time points. The differentially expressed genes included the known anaerobic proteins as well as transcription factors, signal transduction components, and genes that encode enzymes of pathways not known previously to be involved in low-oxygen metabolism. We found that the regulatory regions of genes with a similar expression profile contained similar sequence motifs, suggesting the coordinated transcriptional control of groups of genes by common sets of regulatory factors.
Structural study of the low-temperature phase of TlH2PO4 at 180 K
Authors: Álvarez-Otero E. , Madariaga G. , Peral I. , Folcia C. L. , Ríos S.
Published: September 2002

The low-temperature phase of TlH2PO4 has been studied by X-ray diffraction. A structural model is proposed with space group P\bar 1. This model is compared with the structure of the antiferroelectric phase of TlD2PO4 at room temperature to analyze the expected isomorphism at low temperature. Given the structural distortion of TlH2PO4, such isomorphism present in the common high-temperature phase is not recovered in this phase. Through the analysis of the displacements of the PO4 groups there is some evidence that the ordering of the H atoms responsible for the appearance of antiferroelectricity seems to be incomplete. An exhaustive study of the detected ferroelastic domains is also performed.
Analyzing experimental electron density with the localized-orbital locator
Authors: Tsirelson Vladimir , Stash Adam
Published: September 2002

The localized-orbital locator, which describes the features of bonding in terms of the local kinetic energy, is approximately expressed as a function of electron density and its first and second derivatives. Calculations based on accurate electron densities derived from X-ray diffraction data are carried out for crystals with different types of chemical bonds. It is demonstrated that the localized-orbital locator reveals the features of atomic interactions in a solid state and allows the covalent, ionic and van der Waals bonds to be distinguished.
Aminoguanidinium(1+) pentafluorozirconate: multiple redetermination and comparisons
Authors: Ross II C. R. , Bauer M. R. , Nielson R. M. , Abrahams S. C.
Published: September 2002

The structure of CN4H7ZrF5 reported by Bukvetskii et al. [Koord. Khim. (1992). 18, 576–579] has been independently redetermined on the basis of measurements on three different crystals. Assuming all four resulting structures are drawn from a normal distribution, normal probability analysis of the atomic coordinates taken in pairs reveals joint standard uncertainties that are underestimated by factors as large as 16.5 for the x(Zr) coordinate. Unit-cell parameters in the four crystals similarly have joint uncertainties, under the same assumption, that are underestimated by factors as large as 83.0 for the b axis. The variations in axial lengths from crystal to crystal and the declines in standard reflection intensities by 13–15% in at least two of the crystals measured are consistent with the inference that the distribution is not normal. Rather, the differences observed may be assumed to be caused by small but highly significant radiation-induced structural changes. The large underestimations hence reflect physical differences among the four irradiated crystals. The determinations show that the CN4H7
+1 cation is exactly planar except for the two H atoms bonded to the terminal N atom; the plane of this NH2 group is normal to that of the cation. The average length of the three independent C—N bonds is 1.318 (11) Å; the N—N bond length is 1.397 (3) Å. Distorted ZrF7 pentagonal bipyramids share edges, forming chains linked by N—H...F bonds to the CN4H7
+1 ions.
Crystallization of biological macromolecules using agarose gel
Authors: Biertümpfel Christian , Basquin Jérôme , Suck Dietrich , Sauter Claude
Published: September 2002

Gellified media prevent convection and crystal sedimentation, and provide an attractive growth environment for optimising biological crystals. Agarose gels are particularly easy to use and they are compatible with most of the common crystallization methods. They also offer new possibilities like counter-diffusion techniques. This paper gives a brief overview of their general properties and presents an application of a counter-diffusion setup combining agarose gel and capillaries to the crystallization of proteins and protein / nucleic acid complexes.