bluuebunny/crossref_metadata_2025 huggingface dataset:
Why is Al11B2− not a magic number in TOF-MS?
Authors: Wan Jian , Fournier René
Published: September 2003

Bimetallic anionic and neutral clusters, consisting of group III elements (Aln−1B1, Aln−2B2, Aln−1In1, and Inn−1Al1, n=11–14), have been theoretically investigated by density functional theory at the B3LYP/6-31G* (LanL2DZ for the In element) level. The calculated optimized equilibrium geometries and total energies of neutral and anionic clusters give a satisfactory interpretation of magic number clusters observed in time of flight mass spectra (TOF-MS). Our results show that Al11B2− is the most stable among Aln−2B2− (n=11–14) cluster anions and keeps an icosahedronlike structure, contrary to what had been suggested previously. Whether a magic number turns out in TOF-MS likely depends more on the stability of the neutral clusters than on the stability of the anions. The Al11B2 neutral cluster is less stable than Al12B2, and this is why Al11B2− does not appear as a magic number in TOF-MS. In addition, we found that icosahedral structures do not always hold for the magic cluster anions considered in the present study.
Observation and control of dissociating and autoionizing Rydberg electron wave packets in NO
Authors: Minns R. S. , Verlet J. R. R. , Watkins L. J. , Fielding H. H.
Published: September 2003

The dynamics of predissociating Rydberg electron wave packets are observed using the optical Ramsey method. The time-resolved spectra are hydrogenic and are very well modeled by assuming that only one p Rydberg series contributes to the dynamics. This is in contrast with previous observations of autoionizing Rydberg electron wave packets [Phys. Rev. Lett. 83, 2552 (1999)], which show quite dramatic deviations from hydrogenic behavior above the Born–Oppenheimer limit. The origin of these deviations lies in the interplay between electronic and molecular phase. By exploiting these phases we are able to control the ratio of predissociaton to autoionization.
Control of a collision complex via a conical intersection
Authors: Geppert Dorothee , Hofmann Angelika , de Vivie-Riedle Regina
Published: September 2003

Optimal control theory is applied to prepare excited state wave packets localized in specific regions of a reaction surface which can be used as a sensitive tool to clarify and manipulate characteristic reaction features. In the present example of the Na–H2 collision complex this allows us to study the quantum dynamics in the vicinity of a conical intersection and the population transfer through it in great detail. We could clarify the relaxation mechanism and extract the vibrational distribution of the H2 fragment resulting after the collision. Employing differently shaped laser pulses the ability to control reactions through conical intersections is proven.
Attractive and repulsive interactions in the inelastic scattering of NO by Ar: A comparison between classical trajectory and close-coupling quantum mechanical results
Authors: Aoiz F. J. , Verdasco J. E. , Herrero V. J. , Sáez Rábanos V. , Alexander M. A.
Published: September 2003

State-resolved differential cross sections for the rotationally inelastic scattering of the Ar+NO system have been derived from quasiclassical trajectories and quantum close-coupling calculations on a recent ab initio potential energy surface at the collision energy of a recent high resolution experiment (66 meV). Globally good agreement is obtained between the theoretical predictions and experimental results, although some of the experimental details are not reproduced in the classical calculation. The role of attractive and repulsive interactions in the observed dynamical features is examined.
Corrections to Slater exchange potential in terms of Dirac idempotent density matrix: With an approximate application to Be-like positive atomic ions for large atomic number
Authors: Howard I. A. , March N. H.
Published: September 2003

In earlier studies, we have considered the exchange energy density εx(r) in terms of the Dirac density matrix ρ1(r,r′) for the nonrelativistic limit of large atomic number Z in (i) the Be-like series with configuration (1s)2(2s)2 and (ii) the Ne-like series with closed K+L shells. Subsequently the work of Della Sala and Görling [J. Chem. Phys. 115, 5718 (2001)] has appeared, in which an integral equation for the exchange potential vx(r) is given in terms of the idempotent Dirac density matrix, based on the admittedly drastic approximation that the Hartree–Fock and the Kohn–Sham determinants are equal. Here a formally exact generalization of the integral equation is set up and an approximate solution is presented for the Be series at large Z.
Adsorption induced hydrogen bonding by CH group
Authors: Gao Shiwu , Hahn J. R. , Ho W.
Published: September 2003

We demonstrate a novel mechanism for hydrogen bond formation with CH group in the bimolecular complexes of adsorbed ethylene (C2H4) and oxygen (O2) molecules on a Ag(110) surface. These complexes were created experimentally by manipulation with a scanning tunneling microscope, and characterized theoretically using ab initio density functional theory calculations. We demonstrate unambiguously that hydrogen bonding between a CH group of ethylene and O2, whose interaction is negligible in the gas phase, has been formed in the adsorbed complexes. This unusual hydrogen bond results from adsorption induced electron transfer to the O2, leading to enhanced electrostatic interaction between the hydrogen and oxygen, and has broad implications in biomolecular and chemical systems.
Temperature and solvent dependence of vibrational relaxation of tri-iodide: A simulation study
Authors: Zhang F. S. , Lynden-Bell R. M.
Published: September 2003

Simulations of a flexible model of the tri-iodide ion have been used to study the effects of temperature and solvent on the vibrational energy relaxation rate and the pure dephasing rate of the two stretching modes. These rates were determined in ethanol and 2-methyltetrahydrofuran at 100 K, 200 K, and 300 K, covering the range between glass and mobile liquid states for both solvents. In agreement with experiment the temperature dependence is small. In ethanol the symmetry of the tri-iodide ion is broken at the three temperatures studied, while in methyltetrahydrofuran symmetry is retained except at the lowest temperature. In order to examine different contributions to the relaxation processes, perturbation theory was applied to simulations with rigid ions. Although the large amplitude of the solvent-induced geometry changes suggests that perturbation theory is at best qualitative, reasonable agreement was obtained between the results from the flexible simulation and perturbation theory in 2-methyltetrahydrofuran. Both solvent-induced curvature and solvent force terms were found to contribute to the solvent frequency shifts and the pure dephasing rates. In ethanolic solutions the time correlation functions of the forces decay rapidly, but the curvature term probes a slow dynamical process. At least in our model, the lines would not be completely motionally narrowed in solution in ethanol. Comparisons are made with experiment and previous work.
Transport coefficients of a mesoscopic fluid dynamics model
Authors: Kikuchi N. , Pooley C. M. , Ryder J. F. , Yeomans J. M.
Published: September 2003

We investigate the properties of stochastic rotation dynamics, a mesoscopic model used for simulating fluctuating hydrodynamics. Analytical results are given for the shear viscosity and the friction exerted on a massive solute particle moving within the fluid. We discuss an efficient way of measuring the shear viscosity and viscous friction, and obtain excellent agreement between the theoretical and numerical calculations.
Accelerated molecular dynamics with the bond-boost method
Authors: Miron Radu A. , Fichthorn Kristen A.
Published: September 2003

We present a new method for accelerating molecular-dynamics simulations of infrequent events. The method targets simulation of systems that spend most of the time in local energy minima, with slow transitions in between, as is the case with low-temperature surface diffusion. The potential-energy surface is modified by adding a boost potential in regions close to the local minima, such that all transition rates are increased while relative rates are preserved. The boost potential is an empirical function determined by the deviation of the bond lengths of a specified set of atoms from equilibrium. The method requires no previous knowledge of the processes involved and it can be applied to a wide variety of interaction potentials. Application to the diffusion of Cu atoms on the Cu(100) surface using an embedded-atom potential yields correct rates for adatom hopping, exchange, as well as vacancy and dimer diffusion with speed-ups up to several orders of magnitude.
Phase transitions within the isolated polymer molecule: Coupling of the polymer threading a membrane transition to the helix-random coil, the collapse, the adsorption, and the equilibrium polymerization transitions
Authors: Di Marzio Edmund A. , Kasianowicz John J.
Published: September 2003

The polymer threading a membrane transition (PTM), which is a first-order thermodynamic phase transition for an isolated linear polymer in the limit of infinite molecular weight, is coupled to the other four phase transitions of the isolated polymer molecule. They are (1) the helix–random coil (HR) phase transition which can be diffuse (polypeptides), second-order (DNA) or first-order (collagen) depending on the number of strands, (2) the collapse (C) transition which is usually second-order but can be first-order for polymeric solvents, (3) adsorption onto a surface (SA) which is second-order, (4) a model of equilibrium polymerization (P) which is first-order. In each case an exact expression for the partition function of the coupled pair is given as a one-dimensional summation over products of the individual partition functions corresponding to sides 1 and 2. Using a procedure analogous to evaluation of the grand canonical ensemble the summation can be performed and the character of the transition elucidated in the limit of infinite molecular weight. Given that the solutions on either side are sufficiently diverse there are 15 possible translocation pair couplings. They are PTM–PTM, HR–HR, C–C, SA–SA, P–P, PTM–HR, PTM–C, PTM–SA, PTM–P, HR–C, HR–SA, HR–P, C–SA, C–P, SA–P. The PTM–P coupling is most interesting because one can create polymer in the PTM side even though the P side is in the depolymerization regime. For HR–HR there are eight possible translocation modes. For example, as we raise the temperature we can have H1→H2→R1→R2 in obvious notation. These exact model solutions provide a thermodynamic base for the study of the kinetics of significant technological problems such as the translocation of DNA through pores imbedded in membranes. They also throw light on the nature of polymer–membrane–pore interactions in living cells and viruses.
Multiconfigurational self-consistent field linear response for the polarizable continuum model: Theory and application to ground and excited-state polarizabilities of para-nitroaniline in solution
Authors: Cammi Roberto , Frediani Luca , Mennucci Benedetta , Ruud Kenneth
Published: September 2003

This paper presents the linear response (LR) function for a multiconfigurational self-consistent field (MCSCF) molecular wave function for the integral equation formalism version of the polarizable continuum model (PCM). Both equilibrium and nonequilibrium PCM solvation schemes are described: The nonequilibrium scheme is applied to the calculation of excited state wave functions (Franck–Condon states) and/or of dynamic response properties. An important characteristic of the LR-PCM-MCSCF theory is the explicit inclusion of the effects of solvent dynamics, and this allows us to treat a large variety of time-dependent phenomena. Here, in particular, the theory is applied to the study of the solvent effect on transition energies and on static and dynamic polarizabilities of para-nitroaniline (pNA). The study of the polarizability dispersion of pNA is performed for the ground state and for low-lying electronic excited states including the charge transfer state. We compare our results with available experimental and theoretical data.
Dielectric hole burning: Signature of dielectric and thermal relaxation time heterogeneity
Authors: Jeffrey Kenneth R. , Richert Ranko , Duvvuri Kalyan
Published: September 2003

We have calculated the effects of dielectric hole burning on the basis of the frequency dependent dielectric relaxation ε̂(ω) and specific heat ĉp(ω) of glycerol and assuming heterogeneous dynamics where the dielectric and thermal relaxation times are correlated quantities. The heat generated in the sample is determined from the time dependent electric field and polarization, i.e., without involving steady-state approximations. The calculations mimic the protocol of recent hole-burning experiments on glycerol [K. Duvvuri and R. Richert, J. Chem. Phys. 118, 1356 (2003)], including the high-field burn-process, the phase-cycle employed to eliminate the linear response to the high field, the detection in terms of the electric modulus M(t), and the signal normalization. Without using any fit parameters, this model is capable of quantitatively reproducing the amplitudes, shapes, and peak positions of observed dielectric holes, together with their dependence on the burn frequency and the waiting time between the pump and probe processes. Therefore, frequency selective local heating of the modes of motion associated with the structural relaxation resulting from the dielectric loss fully accounts for the dielectric hole burning effects.
Fractal kinetics in drug release from finite fractal matrices
Authors: Kosmidis Kosmas , Argyrakis Panos , Macheras Panos
Published: September 2003

We have re-examined the random release of particles from fractal polymer matrices using Monte Carlo simulations, a problem originally studied by Bunde et al. [J. Chem. Phys. 83, 5909 (1985)]. A certain population of particles diffuses on a fractal structure, and as particles reach the boundaries of the structure they are removed from the system. We find that the number of particles that escape from the matrix as a function of time can be approximated by a Weibull (stretched exponential) function, similar to the case of release from Euclidean matrices. The earlier result that fractal release rates are described by power laws is correct only at the initial stage of the release, but it has to be modified if one is to describe in one picture the entire process for a finite system. These results pertain to the release of drugs, chemicals, agrochemicals, etc., from delivery systems.
High-resolution photoelectron spectroscopy of HI and DI: Experimental and theoretical analysis of the A 2Σ+ ion system
Authors: Yencha Andrew J. , Baltzer Peter , Cormack Andrew J. , Li Yan , Liebermann Heinz-Peter , Alekseyev Aleksey B. , Buenker Robert J.
Published: September 2003

A combined high-resolution (6 meV) HeI photoelectron (PE) and ab initio theoretical study of the A 2Σ+ ion system of HI and DI has been conducted to elucidate the origin of a peculiar “holelike” feature in the vibrational distribution found in the high-resolution (6 meV) threshold photoelectron (TPE) spectrum of HI. The PE and TPE spectra were found to yield essentially identical results. Ab initio potential energy curves for the low-lying cationic states of HI have been calculated for the first time with and without spin–orbit contributions included. It has been found that the diabatic A 2Σ+ state of HI+ is strongly predissociated due to spin–orbit coupling with the Σ−4, Σ−2, and Π4 repulsive states leading to a complex set of adiabatic curves. It is shown that the adiabatic A 2Σ1/2+ state is only slightly bound (by 260 cm−1 after suitable adjustments of the positions of the various repulsive potentials relative to that of the A state are made based on observed atomic spectral data) and should support at most one vibrational level in both HI+ and DI+. However, using the complex rotational method, it was possible to calculate the energies, predissociation linewidths, and rotational constants of a number of nonstationary vibrational levels (or resonances) of the A 2Σ1/2+ state. Reasonably good agreement has been found between experiment and theory. The observed “hole” in the TPE and PE spectra is attributed to the fact that the stability of the v+=1 and 2 levels is notably less than for v+=0 in the A 2Σ1/2+ state of both HI+ and DI+.
Ground- and excited state proton transfer and tautomerization in 7-hydroxyquinoline⋅(NH3)n clusters: Spectroscopic and time resolved investigations
Authors: Bach Andreas , Tanner Christian , Manca Carine , Frey Hans-Martin , Leutwyler Samuel
Published: September 2003

Mass-selected S1↔S0 two color resonant two photon ionization (2C-R2PI) spectra, fluorescence spectra and fluorescence decay times are measured for supersonically cooled 7-hydroxyquinoline (7HQ)⋅(NH3)n clusters with n=4–10. For n=4, the S1←S0 2C-R2PI spectrum shows a 20 cm−1 broad electronic origin at 27 746 cm−1, followed by an intermolecular vibrational progression with band widths that increase up to ≈45 cm−1. In contrast, the 2C-R2PI spectra of the mixed 7HQ⋅(NH3)3H2O and 7HQ⋅(NH3)2(H2O)2 clusters exhibit narrow bands of 1–2 cm−1 width. The large band widths of 7HQ⋅(NH3)4 are due to a fast (k>1012 s−1) excited state process which is blocked when replacing one or more NH3 molecules by H2O in the cluster. For the n=5–10 clusters, the 2C-R2PI spectra display two broad absorption bands peaking at 25 000 and 27 000 cm−1. The latter is characteristic of the 7-quinolinate (7Q−) anion, implying that ground state proton transfer from 7HQ to the ammonia cluster occurs for n⩾5. Excitation at 27 000 cm−1 leads to fluorescence from the 7Q−* anion clusters at 22 500 cm−1 with τfl=14–27 ns, and also to fluorescence emission from 7KQ* clusters at 18 370 cm−1 with τfl≈7 ns, signaling excited state anion→keto proton transfer. The 25 000 cm−1 absorption band is characteristic of ground state 7KQ, implying that the n⩾5 clusters also undergo ground state enol→keto tautomerization.
Polarization-dependent photoluminescence-excitation spectra of one-dimensional exciton and continuum states in T-shaped quantum wires
Authors: Itoh Hirotake , Hayamizu Yuhei , Yoshita Masahiro , Akiyama Hidefumi , Pfeiffer Loren N. , West Ken W. , Szymanska Marzena H. , Littlewood Peter B.
Published: September 2003

We measured polarization-dependent photoluminescence-excitation spectra of highly uniform T-shaped quantum wires at 5 K. We attribute one peak to the one-dimensional- (1D-) exciton ground state and the continuous absorption band to 1D continuum states. These had similar polarization dependences. We also observed some other peaks above the 1D-exciton ground state and attribute them to exciton states consisting of excited hole subbands. These results show good agreement with a model calculation of a single electron–hole pair in T-shaped geometry with exact diagonalizations of the Coulomb interaction.
Generalized Einstein relation for disordered semiconductors with exponential distributions of tail states and square-root distributions of band states
Authors: Nguyen Thanh H. , O’Leary Stephen K.
Published: September 2003

We present a generalized Einstein relation analysis, applicable for disordered semiconductors with exponential distributions of tail states and square-root distributions of band states. We find that there are substantial deviations from the traditional nondegenerate form of this relation, these deviations arising as a consequence of the degeneracy of the occupation statistics within disordered semiconductors. We use these results to account for some hydrogenated amorphous silicon experimental results.
Positive isotropic resonance field shift of exchange coupled wedged-permalloy/FeMn bilayers
Authors: Zhou S. M. , Yuan S. J. , Wang L. , Lu M. , Du J. , Hu A. , Song J. T.
Published: September 2003

Exchange-coupled wedged-permalloy/uniform-FeMn bilayers are studied by ferromagnetic resonance and magnetometry measurements with applied field parallel to the film plane. An additional weak resonance peak was observed besides an intense resonance, indicating the existence of interfacial diffusion. For all samples, the exchange field measured by the ferromagnetic resonance is close to that by magnetometry measurements. For the wedged-Py/FeMn bilayers, the in-plane isotropic resonance field shift is positive and inversely proportional to the ferromagnetic layer thickness. It is originated from either specific geometry of Py layer thickness or interfacial diffusion.
Stimulated resonance Raman scattering from single crystals of a thiophene/phenylene co-oligomer
Authors: Yanagi Hisao , Yoshiki Atsutoshi , Hotta Shu , Kobayashi Shunsuke
Published: September 2003

We have investigated the stimulated resonance Raman scattering from single crystals of a thiophene/phenylene co-oligomer. The very narrow lines [full width at half maximum (FWHM)=11–20 cm−1] are observed in the same spectral regions where the spectrally narrowed emissions (FWHM≈300 cm−1) occur upon excitation with intense laser pulse. We related these modes to the stimulated resonance Raman scattering. Two lines of Raman shifts of 1466 and 1605 cm−1 are of molecular origin. Another line with an energy shift of ∼1260 cm−1 from excitation energy, on the other hand, implies the presence of the coherence of crystal origin. The associated split lines accompanying this mode are thought to constitute the longitudinal modes in the Fabry–Pérot cavity naturally formed in the crystals.
High-power room-temperature continuous wave operation of 2.7 and 2.8 μm In(Al)GaAsSb/GaSb diode lasers
Authors: Kim J. G. , Shterengas L. , Martinelli R. U. , Belenky G. L.
Published: September 2003

We have fabricated and characterized 2.7 and 2.8 μm wavelength In(Al)GaAsSb/GaSb two-quantum-well diode lasers. All lasers have 2 mm cavity lengths and 100 μm apertures. Continuous wave operation up to 500 mW was recorded at 16 °C from 2.7 μm lasers, while 160 mW was obtained from 2.8 μm lasers. Threshold current densities as low as 350 A/cm2 were recorded from 2.7 μm lasers with external quantum efficiencies of 0.26 photon/electrons. The maximum wall-plug efficiency was 9.2% at a current of 2.4 A. A peak power of 2.5 W was recorded in pulsed-current mode operation at 20 °C at 2.7 μm and 2 W at 2.8 μm. Characteristic temperatures of T0=71 K and T1=86 K were measured from the 2.7 μm devices. T0=59 K and T1=72 K for the 2.8 μm lasers. The devices have differential series resistances of about 0.18 Ω with estimated thermal resistances of about 5 K/W.
Interweaving domain configurations in [001]-poled rhombohedral phase 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 single crystals
Authors: Han Jiaping , Cao Wenwu
Published: September 2003

Domain structures in [001]-poled rhombohedral phase 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 single crystals have been investigated using polarized light microcopy. It was found that the observed domain structures are quite different from those previously assumed four domain states with cross intersecting 71° charged domain walls. The pattern can be interpreted as interweaving of two types of twins in three dimensions. Each local twin pattern contains only two domains with a 109° charged domain wall in either [110] or [11̄0]. The stacking of two 90° rotated twins produces a cross-hatching pattern for transmission optical microscope and an effective 4mm macroscopic symmetry. The pattern becomes simple twins when the imaging is focused at different depth. The domain size is 1–2 μm in all the samples observed and the twin layer thickness is about 50–100 μm.
High-efficiency, high-power, stable 172 nm xenon excimer light source
Authors: Salvermoser M. , Murnick D. E.
Published: September 2003

Stable, continuous-wave light sources at 172 nm, based on the Xe2* excimer molecule, with conversion efficiency of electrical energy to vacuum ultraviolet (VUV) light greater than 50%, are reported. In high-pressure xenon gas, “Saint Elmo’s Fire” corona discharges serve as localized point electron sources with a metal grid at a few kilovolts providing an accelerating electric field. An extended VUV light-emitting region with high-energy conversion efficiency indicates that electron energy loss is predominantly by excitation of Xe atoms rather than by ionization. A room-temperature prototype lamp with variable VUV power to 35 mW/cm2 has been demonstrated.
Surface deformation and ferroelectric domain switching induced by a force microscope tip on a La-modified PbTiO3 thin film
Authors: Labardi M. , Polop C. , Likodimos V. , Pardi L. , Allegrini M. , Vasco E. , Zaldo C.
Published: September 2003

Surface deformation of a ferroelectric (111)-oriented thin film of La-modified PbTiO3 is induced by contact with the tip of a scanning force microscope (SFM). The deformation is accompanied by switching of the out-of-plane polarization of ferroelectric domains revealed by simultaneous piezoresponse force microscopy. The effect shows up in topographic SFM images as strokes in the fast scan direction due to surface deformation occurring below the scanning tip, and is critically dependent on the contact force for which a threshold value is deduced that allows proper SFM characterization of such thin films. At higher force, SFM might be used as a nanoscale tool for investigating fundamental properties like phase transitions under applied stress in such systems.
Electron-beam-induced segregation in InGaN/GaN multiple-quantum wells
Authors: O’Neill J. P. , Ross I. M. , Cullis A. G. , Wang T. , Parbrook P. J.
Published: September 2003

We report a study of the morphology and composition of InxGa1−xN/GaN multiple-quantum-well structures and their sensitivity to electron-beam damage. We have employed high-resolution transmission electron microscopy, energy dispersive x-ray analysis, and scanning transmission electron microscopy. Microstructural analysis was performed to investigate the dynamical effects of electron-beam irradiation on the relative indium distribution within the quantum wells. Exposure to relatively low incident beam illumination, corresponding to current densities at the specimen of ∼100 pA/cm2, was found to induce significant nanoclustering of indium within the multiple-quantum wells. These findings highlight the need for caution when reporting the presence of indium-rich clusters within InGaN/GaN multiple-quantum wells studied in the transmission electron microscope.
X-ray diffraction measurement of doping induced lattice mismatch in n-type 4H-SiC epilayers grown on p-type substrates
Authors: Okojie Robert S. , Holzheu Thomas , Huang XianRong , Dudley Michael
Published: September 2003

High-resolution x-ray diffractometry was used to measure the lattice mismatch and misorientation in n-type 4H-SiC epilayers grown homoepitaxially on p-type 4H-SiC as function of different nitrogen doping levels. The spatially averaged lattice mismatch increased from 1.0×10−5 to 4.0×10−5, 6.3×10−5, 8.8×10−5, and 11.6×10−5 in epilayers doped 4.1×1017 cm−3, 2.6×1018 cm−3, 1.7×1019 cm−3, 2.2×1019, and 4×1019, respectively. The resolved multiple subsidiary peaks in the rocking curve of the epilayers doped 2.2×1019 and 4×1019 cm−3 are likely due to high density of domain boundaries. The increase in mismatch with doping, is attributed to the substitutional nitrogen incorporated preferentially in the host carbon sites of the 4H-SiC epilayer.
Demonstration of a microelectromechanical tunable asymmetric Fabry–Pérot quantum well modulator
Authors: Rabinovich W. S. , Stievater T. H. , Papanicolaou N. A. , Katzer D. S. , Goetz P. G.
Published: September 2003

Asymmetric Fabry–Pérot multiple quantum well modulators can have much higher extinction ratios than are otherwise possible with surface normal devices. This performance comes at the price of very tight tolerances on the epitaxial growth and high sensitivity to temperature and angle of incidence. We demonstrate that a quantum well modulator with an integrated microelectromechanical mirror can remove these sensitivities.
Epitaxial electrodeposition of Cu2O films onto InP(001)
Authors: Liu Run , Bohannan Eric W. , Switzer Jay A. , Oba Fumiyasu , Ernst Frank
Published: September 2003

Cu 2 O (cuprite) films were deposited electrochemically onto InP (001) single-crystal substrates from aqueous solutions at room temperature. X-ray diffraction indicates a unique epitaxial 45°[001] orientation relationship between Cu2O and InP. This reduces the mismatch between corresponding spacings to 2.9%, compared with a mismatch of 27.2% between the lattice parameters of Cu2O and InP. The morphology of the Cu2O film can be influenced via the electrolyte acidity. At a pH of 9.0, Cu2O forms pyramidal islands. At a pH of 12.0, on the other hand, cubelike morphologies of Cu2O are observed. Between a pH of 9.0 and 12.0, the direction of slowest growth changes from 〈111〉 to 〈100〉. In apparent contradiction to the observation of a unique epitaxial orientation relationship, transmission electron microscopy reveals an amorphous oxygen-rich interlayer between the Cu2O and the InP crystal.
Experimental demonstration of a high quality factor photonic crystal microcavity
Authors: Srinivasan Kartik , Barclay Paul E. , Painter Oskar , Chen Jianxin , Cho Alfred Y. , Gmachl Claire
Published: September 2003

Subthreshold measurements of a photonic crystal (PC) microcavity laser operating at 1.3 μm show a linewidth of 0.10 nm, corresponding to a quality factor (Q)∼1.3×104. The PC microcavity mode is a donor-type mode in a graded square lattice of air holes, with a theoretical Q∼105 and mode volume Veff∼0.25 cubic half-wavelengths in air. Devices are fabricated in an InAsP/InGaAsP multi-quantum-well membrane and are optically pumped at 830 nm. External peak pump power laser thresholds as low as 100 μW are also observed.
Spin-polarized scanning tunneling microscope for imaging the in-plane magnetization
Authors: Schlickum U. , Wulfhekel W. , Kirschner J.
Published: September 2003

We present a spin-polarized scanning tunneling microscope (Sp–STM) for imaging the magnetic in-plane component of magnetic surfaces. Magnetic in-plane sensitivity is obtained by using a ferromagnetic ring as a Sp–STM tip. By periodically switching the magnetization of the ring, the spin-dependent tunneling current between the ring and a spin-polarized sample is measured. The topography and the spin polarization can be imaged at the same time. We resolved the 180° domain wall of Fe whiskers and antiferromagnetic coupled Mn layers on Fe(001).
Band offset determination of Zn0.53Cd0.47Se/Zn0.29Cd0.24Mg0.47Se
Authors: Muñoz Martı́n , Lu Hong , Zhou Xuecong , Tamargo Maria C. , Pollak Fred H.
Published: September 2003

The interband transitions of a single quantum well structure of Zn0.53Cd0.47Se/Zn0.27Cd0.23Mg0.50Se (lattice matched to InP) were evaluated using contactless electroreflectance at room temperature. From a comparison of the measured optical transitions with those calculated using the envelope function approximation we determined that the conduction band offset for this system is given by the parameter Qc=ΔEc/ΔE0=0.82±0.02, which yields ΔEc of 590 meV. Such a large conduction band offset may be useful for the design of quantum cascade lasers and other devices based on intersubband transitions.
Resonant absorption quenching and enhancement of optical nonlinearity in Au:BaTiO3 composite films by adding Fe nanoclusters
Authors: Wang W. T. , Chen Z. H. , Yang G. , Guan D. Y. , Yang G. Z. , Zhou Y. L. , Lu H. B.
Published: September 2003

We present an approach to enhance the figure of merit χ(3)/α (with χ(3) being the third-order nonlinear susceptibility and α the optical absorption coefficient) in metal-dielectric composites. Nanocomposite thin films containing metal nanoclusters were obtained by embedding gold, or gold+iron in BaTiO3 matrices using the pulsed laser deposition technique. In the optical absorption spectra, the Au:BaTiO3 films showed a strong absorption peak around 562 nm due to surface plasmon resonance of Au particles. However, when a small amount of Fe was added, the resonant absorption was quenched, and χ(3) was enhanced so that χ(3)/α was significantly increased. Our study suggests a method by which to design optical nonlinear materials with large figure of merits for practical application.
1.55 μm emission from GaInNAs with indium-induced increase of N concentration
Authors: Zhou W. , Uesugi K. , Suemune I.
Published: September 2003

GaInNAs/GaAs multiple quantum well (MQW) structures were grown by metalorganic molecular-beam epitaxy (MOMBE). Increase of the N concentration in GaInNAs with increasing In concentration was observed. This trend of enhanced N incorporation for the higher In concentration made it possible to realize long-wavelength emission of 1.55 μm from a GaInNAs/GaAs MQW grown by MOMBE. This result is compared with the previous reports on the growth of GaInNAs alloys and the main factors which lead to the enhanced N incorporation are discussed.
Single-electron transistors in heterostructure nanowires
Authors: Thelander C. , Mårtensson T. , Björk M. T. , Ohlsson B. J. , Larsson M. W. , Wallenberg L. R. , Samuelson L.
Published: September 2003

Semiconductor-based single-electron transistors have been fabricated using heterostructure nanowire growth, by introducing a double barrier of InP into InAs nanowires. From electrical measurements, we observe a charging energy of 4 meV for the approximately 55 nm diameter and 100 nm long InAs islands between the InP barriers. The Coulomb blockade can be periodically lifted as a function of gate voltage for all devices, which is a typical signature of single-island transistors. Homogeneous InAs nanowires show no such effect for the corresponding voltage ranges.
Self-organized nanostructures in the Ti–Al–N system
Authors: Mayrhofer Paul H. , Hörling Anders , Karlsson Lennart , Sjölén Jacob , Larsson Tommy , Mitterer Christian , Hultman Lars
Published: September 2003

The phenomenon of age hardening could be evidenced in thin film applications. A model system, Ti1−xAlxN was chosen as such coatings are known for their excellent wear resistance enabling improved machining processes like high-speed and dry cutting. Here, we show unambiguously that metastable Ti1−xAlxN coatings initially undergo spinodal decomposition into coherent cubic-phase nanometer-size domains, causing an increase in hardness at elevated temperatures. These intermediate metastable domains transform into their stable phases TiN and AlN during further thermal treatment. Activation energies for the processes indicate defect-assisted segregation of Ti and Al. The findings are corroborated by ab initio calculations. A long-standing discussion on the thermal stability of this important class of ceramics is thus resolved.
Strong anti-Stokes luminescence from H+-irradiated diamond
Authors: Xu Y. , Naramoto H. , Narumi K. , Miyashita K. , Kamiya T. , Sakai T.
Published: September 2003

A strong anti-Stokes luminescence (ASL) peak associated with 3H color centers is observed from H+-irradiated diamond, when the diamond is excited at 514.5 nm of wavelength. The characteristics of 3H emission spectrum are discussed in terms of the vibrational–electronic interaction in diamond. We consider that the ASL peak is due to simultaneous phonon absorption with photon absorption in the process of electronic excitations. According to the mechanism of the ASL, some weak ASL peaks at the high-energy side of the strong ASL peak have been also found.
Effect of annealing profile on defect annihilation, crystallinity and size distribution of germanium nanodots in silicon oxide matrix
Authors: Kan E. W. H. , Choi W. K. , Leoy C. C. , Chim W. K. , Antoniadis D. A. , Fitzgerald E. A.
Published: September 2003

A double-step annealing profile has been used to synthesize germanium nanodots embedded in silicon oxide matrix with low defects, good crystallinity, good size distribution, and shape. A significant reduction in the photoluminescence was observed for samples annealed at temperature higher than 900 °C. The improved crystallinity of the nanodots synthesized via the double-step annealing process was investigated using Raman spectroscopy and transmission electron microscopy diffraction patterns. A mechanism of growth at different annealing temperature profiles is proposed. The optimum annealing profile was 1000 °C for 300 s followed by 700 °C for 60 s. The mean diameter of the dots at such annealing condition was found to be 9.5±1.6 nm with an area density of ∼5×1011 cm−2.
Infrared photonic crystal fiber
Authors: Rave E. , Roodenko K. , Katzir A.
Published: September 2003

An infrared photonic crystal fiber has been made by multiple extrusions of silver halide (AgClxBr1−x) crystalline materials. The core of the fiber consisted of pure AgBr, and the cladding area included fiberoptic elements made of AgCl (with lower refractive index). Fibers of outer diameter 1 mm and length of the order of 1 m were flexible and transparent in the spectral range 3–20 μm. Measurements of the power distribution and the transmission losses in the fibers indicated that they behave like core-clad structures, with an effective core diameter 200 μm. These fibers will be extremely useful for many applications in the middle- and far-infrared.
Time dependence of energy dissipation in resonating silicon cantilevers in ultrahigh vacuum
Authors: Ono Takahito , Wang Dong F. , Esashi Masayoshi
Published: September 2003

The nanomechanical properties of single-crystalline silicon cantilevers as resonating elements are investigated in ultra-high vacuum. Flash heating the single-crystalline silicon cantilever at 1000 °C in ultra-high vacuum dramatically reduced the mechanical energy dissipation of the oscillating cantilever due to the removal of contaminants and natural oxide and reconstruction of the silicon surface. However, the reconstructed silicon surface is not stable and mechanical properties change depending on the surface state. The Q-factor and influence of gas adsorption on ultrathin single crystalline silicon resonators having (100) and (111)-oriented surfaces are presented.
Spatially controlled, nanoparticle-free growth of InP nanowires
Authors: Poole P. J. , Lefebvre J. , Fraser J.
Published: September 2003

A technique for the growth of InP nanowires, which does not rely on the vapor–liquid–solid growth mechanism, is demonstrated using selective-area chemical beam epitaxy. The nanowires are precisely positioned on an InP wafer and are always aligned along the available substrate 〈111〉A directions. They have diameters as small as 40 nm, and typical lengths of 600 nm. They are found to be optically active, with thin embedded InAs layers showing quantum-dot-like behavior with well-defined excited states.
Evaluation of the electrical properties and interfacial reactions for the polycrystalline Si1−xGex(x=0,0.6)/HfO2 gate stack
Authors: Kang Sung Kwan , Nam Suheun , Min Byung Gi , Nam Seok Woo , Ko Dae-Hong , Cho Mann-Ho
Published: September 2003

The effect of interfacial reactions on the electrical properties of a polycrystalline (poly) Si1−xGex/HfO2 gate stack were evaluated in terms of annealing conditions and the results were compared with those of a conventional poly-Si/HfO2 system. In the poly-Si0.4Ge0.6/HfO2 gate stack, silicate formation was the dominant reaction at the poly-Si0.4Ge0.6/HfO2 interface after annealing at 900 °C, resulting in the significant decrease in leakage current. From x-ray photoelectron spectroscopy analysis, the binding states of Hf silicates were clearly observed at a binding energy of about 16.1 eV in Hf 4f spectra and 102.7 eV in Si 2p spectra. However, in the poly-Si/HfO2 gate stack, the accumulation capacitance became undeterminable and the leakage current increased suddenly after annealing at 900 °C due to silicide formation at the poly-Si/HfO2 interface. The differences in reactions between a poly-Si/HfO2 interface and a poly-Si0.4Ge0.6/HfO2 interface are attributed to the accumulation of Ge.
Time evolution of the depth profile of {113} defects during transient enhanced diffusion in silicon
Authors: Colombeau B. , Cowern N. E. B. , Cristiano F. , Calvo P. , Cherkashin N. , Lamrani Y. , Claverie A.
Published: September 2003

The evolution of {113} defects as a function of time and depth within Si implant-generated defect profiles has been investigated by transmission electron microscopy. Two cases are considered: one in which the {113} defects evolve into dislocation loops, and the other, at lower dose and energy, in which the {113} defects grow in size and finally dissolve. The study shows that dissolution occurs preferentially at the near-surface side of the defect band, indicating that the silicon surface is the principal sink for interstitials in this system. The results provide a critical test of the ability of physical models to simulate defect evolution and transient enhanced diffusion.
Physical constraints on magnetic quantum cellular automata
Authors: Parish M. C. B. , Forshaw M.
Published: September 2003

A magnetic cellular computing structure called the bistable magnetic quantum cellular automata (BMQCA) has been investigated using micromagnetic simulations. The time evolution of a number of BMQCA elements and subcircuits was investigated using real-time quantification of the Monte Carlo step. It was found to be essential to include temperature-dependent effects, which were modeled using the Metropolis algorithm. The effects of varying ten different parameters were examined, and it was found that, although individual elements might reliably operate at gigahertz frequencies, the need for adiabatic evolution of the circuit significantly reduces the system speed. This result may have implications for other adiabatic computing systems such as electronic quantum cellular automata and other magnetic logic systems.
Doping of chalcopyrites by hydrogen
Authors: Kılıç Çetin , Zunger Alex
Published: September 2003

First-principles total-energy calculations for hydrogen impurities in CuInSe2 (CIS) and CuGaSe2 (CGS) show that H+ takes up the Cu–Se bond center position, whereas H0 and H− take up tetrahedral interstitial site next to In (in CIS) or Ga (in CGS). Hydrogen creates a negative-U center (i.e., H0 is never stable), with a (+/−) transition level at Ec−0.39 eV in CIS, and Ec−0.57 eV in CGS. However, once combined with the 2VCu−+IIICu2+ complex, hydrogen forms shallower centers with transition levels at Ec−0.15 eV in CIS, and Ec−0.39 eV in CGS. We conclude that hydrogen could convert CIS to n type, but not CGS.
Effects of silicon-on-insulator substrate on the residual stress within 3C-SiC/Si thin films
Authors: Park J.-H. , Kim J. H. , Kim Y. , Lee B.-T. , Jang S.-J. , Moon C.-K. , Song H.-J.
Published: September 2003

Single-crystalline 3C-SiC heteroepitaxial layers were grown on silicon-on-insulator (SOI) and Si wafers, to investigate effects of SOI substrates on the film quality. Residual stress measurement using a laser scan method and the Raman scattering spectroscopy indicated that internal stress within SiC films on SOI were indeed reduced, when compared with that of SiC films on Si.
Large-quantity free-standing ZnO nanowires
Authors: Banerjee D. , Lao J. Y. , Wang D. Z. , Huang J. Y. , Ren Z. F. , Steeves D. , Kimball B. , Sennett M.
Published: September 2003

Large-quantity (grams) one-dimensional ZnO nanowires of different sizes have been synthesized by a simple thermal evaporation of ZnO powder in a tube furnace at a temperature controlled to 1000–1200 °C and pressure to 1–2 Torr air. A mixture of ZnO and graphite powder was used as the source. Fine graphite flakes were used to promote the growth. The graphite flakes are the key for large-quantity yield and were easily removed by oxidation in flowing O2 at about 700 °C for 1–3 h. The scanning- and transmission-electron-microscopic studies show that the diameter and length of the nanowires vary from 20 to 100 nm and 0.5 to 10 μm, respectively. Room temperature photoluminescence studies found that the luminescent intensity depends on the processing conditions. A reduced band edge ultraviolet (380 nm) and deep-band green (520 nm) emission have been observed for these nanowires. Most importantly, the method can be extended to any other oxide nanowires that will be the building block of future nanoscale devices.
Dielectric relaxation behavior in barium stannate titanate ferroelectric ceramics with diffused phase transition
Authors: Xiaoyong Wei , Yujun Feng , Xi Yao
Published: September 2003

The temperature dependence of dielectric permittivity was investigated for two ferroelectric ceramic compositions in a barium stannate titanate solid solution system. The dielectric relaxation behavior was observed in these ferroelectrics with diffused phase transition. In view of the polar nanoregion, the relationship between relaxor ferroelectric and ferroelectrics with a diffused phase transition was explored. The results suggest that, in barium stannate titanate, the relaxor ferroelectric dielectric behavior is universal in the diffused phase transition region, but it only appears when the experimental frequency is close to the relaxation frequency of polar nanoregion at that temperature.
Kinetic control of a particle by voltage sequence for a nonimpact electrostatic micromanipulation
Authors: Saito Shigeki , Himeno Hideo , Takahashi Kunio , Urago Masataka
Published: September 2003

This letter describes a calculation of a voltage sequence to obtain kinetic control of a particle for nonimpact electrostatic micromanipulation. The system consists of conductive objects: A manipulation probe, a spherical particle, and a substrate plate. The particle, initially adhering to the probe tip, is detached by an applied voltage. The electrostatic force acting on the particle during its movement to the substrate is calculated by a numerical boundary element method. We determine the voltage and time sequence for nonimpact deposition of the particle onto the substrate by considering the total work to the particle. The calculation provides the power source requirements for nonimpact particle deposition.
Low-noise microwave magnetrons by azimuthally varying axial magnetic field
Authors: Neculaes V. B. , Gilgenbach R. M. , Lau Y. Y.
Published: September 2003

A technique has been demonstrated to significantly reduce the noise in microwave oven magnetrons. The technique employs permanent magnets to generate an azimuthally varying axial magnetic field. Noise measurements are reported which show dramatic reductions in the noise of kW oven magnetrons operating near 2.45 GHz. The noise reduction near the carrier is some 30 dB. Microwave sidebands are reduced or eliminated. Noise reduction occurs at all anode currents, but is particularly significant at low current near the start-oscillation condition.
Field-effect-induced midinfrared electroluminescence of a quantum-wire-cascade structure by remote δ-doping
Authors: Schmult Stefan , Keck Ingo , Herrle Thomas , Wegscheider Werner , Bichler Max , Schuh Dieter , Abstreiter Gerhard
Published: September 2003

We present a quantum-cascade emitter in the galliumarsenide/aluminum–galliumarsenide (GaAs/AlGaAs) heterosystem whose emission properties are controlled by an additional electric field perpendicular to the transport direction. In our case, the additional field is established by remote δ-silicon doping, which is also responsible for charge carrier supply. The field originating from the δ-doping gives rise to an in-plane confinement creating a quantum-wire cascade. This field-effect quantum-cascade emitter is realized using the cleaved edge overgrowth method. Radiative electronic transitions between discrete energy levels in coupled quantum wires were calculated for such a structure. Without an additional electric field, no significant transport is observed. With a field applied, midinfrared emission is observed at a peak wave number of 1200 cm−1 with a full width at half maximum of 300 cm−1 for a heat-sink temperature of 20 K. The presented sample is an experimental proposal for a unipolar quantum-wire intersubband laser.