bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomically defined epitaxy and physical properties of strained La0.6Sr0.4MnO3 films
Authors: Izumi M. , Konishi Y. , Nishihara T. , Hayashi S. , Shinohara M. , Kawasaki M. , Tokura Y.
Published: July 2002

La 0.6 Sr 0.4 MnO 3 thin films were fabricated on SrTiO3 (001) substrates using pulsed laser deposition with observing persistent intensity oscillation of reflection high-energy electron diffraction. By atomic force microscopy, the surface of resulting films was confirmed to be extremely flat, showing atomically smooth terraces and 0.4 nm high steps corresponding to a unit cell height of perovskite. The surface terminating atomic layer was unambiguously assigned to the MnO2 layer by coaxial impact collision ion scattering spectroscopy. Crystal symmetry of the films is distorted into a tetragonal one due to the strain to fulfill perfect in-plane matching with the substrate even for films as thick as 100 nm. Even for films as thin as 4 nm (10 unit cells), ferromagnetic transition takes place to induce a metallic state and large negative magnetoresistance is observed as well.
Coulomb blockade and discrete energy levels in Au nanoparticles
Authors: Davidović Dragomir , Tinkham M.
Published: July 2002

We describe a fabrication technique in which a nanoscale Au grain is connected to two Cr electrodes. Electron transport in our samples can be fitted to orthodox theory of single electron tunneling on a single particle. In one sample, we can resolve additional steps in current, which are interpreted as discrete quantum states of electrons added to the grain.
Low loss photoinduced waveguides in rapid thermally annealed films of chalcogenide glasses
Authors: Ramachandran S. , Bishop S. G.
Published: July 2002

Rapid thermal annealing of sputter-deposited chalcogenide glasses is demonstrated to yield superior quality films which are used to fabricate low loss (<0.3 dB/cm) photoinduced channel waveguides in chalcogenide glasses. This process also enhances the photosensitivity and the magnitude of photoinduced index changes possible (Δn∼5%) in these glass systems.
Time-resolved microwave technique for ultrafast charge-carrier recombination time measurements in diamonds and GaAs
Authors: Garnov S. V. , Ritus A. I. , Klimentov S. M. , Pimenov S. M. , Konov V. I. , Gloor S. , Lüthy W. , Weber H. P.
Published: July 2002

Recombination times of laser-excited charge carriers in natural diamond crystals, polycrystalline chemical vapor deposited (CVD) diamond films, and GaAs wafers were measured with 1 ns time resolution by a microwave-radiation technique. A waveguide scheme was applied to record time-dependent reflection and transmission of 140 GHz cw radiation. The measured recombination carrier lifetimes in the bulk of natural and CVD diamond samples were found to be of 1–3 ns. In GaAs, a distinguishing difference between the bulk (15 ns) and surface (3.5 ns) recombination times was observed. To validate the applicability of the developed technique, a computer simulation of the microwave-radiation interaction with excited plane–parallel specimens has been performed applying the Fabry–Perot resonator theory and the classical Drude model.
Nonlinear magneto-optical diffraction from periodic domain structures in magnetic films
Authors: Dadoenkova N. N. , Lyubchanskii I. L. , Lyubchanskii M. I. , Rasing Th.
Published: July 2002

The nonlinear optical diffraction in magnetic films with a laminar domain structure and Bloch-type domain walls is investigated for both s and p polarization of incident light. It is shown that the contribution of magnetic domains and domain walls to the nonlinear diffraction can be separated by a polarization analysis of the scattered light.
Semiconductor photonic dots: Visible wavelength-sized optical resonators
Authors: Suemune I. , Ueta A. , Avramescu A. , Tanaka S. , Kumano H. , Uesugi K.
Published: July 2002

Here we describe a strategy toward constructing semiconductor photonic dots in the ultraviolet to blue region. An array of ZnS dots was grown on a GaAs substrate with a selective growth method. The ZnS dots have a pyramidal structure with the base plane of 800 nm square and the height of 300 nm. The {034} crystallographic planes form the sidewalls of the pyramids. Therefore, the size of the pyramidal dots is uniquely determined by the mask patterning. The optical reflection spectra showed clear resonance peaks which are reasonably assigned by the calculation of the resonance modes. Each resonance showed the Q values on the order of 160–300, a reasonable value to observe the modification of the total spontaneous emission rate in this kind of photonic dots.
Passivation of GaAs using (Ga2O3)1−x(Gd2O3)x, 0⩽x⩽1.0 films
Authors: Kwo J. , Murphy D. W. , Hong M. , Opila R. L. , Mannaerts J. P. , Sergent A. M. , Masaitis R. L.
Published: July 2002

The Ga2O3(Gd2O3) dielectric film was previously discovered to passivate the GaAs surface effectively. We have investigated the systematic dependence of the dielectric properties of (Ga2O3)1−x(Gd2O3)x on the Gd (x) content. Our results show that pure Ga2O3 does not passivate GaAs. Films with x⩾14% are electrically insulating with low leakage current and high electrical breakdown strength. Furthermore, a low interfacial density of states was attained in films with x⩾14%. The results show the important role of Gd2O3 in the (Ga2O3)1−x(Gd2O3)x dielectric films for effective passivation of GaAs.
Fabrication of a substrate-independent aluminum oxide-GaAs distributed Bragg reflector
Authors: Wohlert D. E. , Lin H. C. , Chang K. L. , Pickrell G. W. , Epple J. H. , Hsieh K. C. , Cheng K. Y.
Published: July 2002

We propose a method for forming a top distributed Bragg reflector (DBR) during very-low temperature (VLT) molecular-beam epitaxy (MBE) growth that is independent of the substrate being used. By varying the arsenic overpressure during VLT MBE, it was determined by Auger electron spectroscopy and cross-section transmission electron microscopy that alternating layers of polycrystalline GaAs and amorphous (Al,As) can be deposited. Because these layers are not single crystal, they can be grown on any host lattice. After lateral wet oxidation, the polycrystalline GaAs does not undergo any significant changes; whereas the amorphous (Al,As) becomes an amorphous aluminum oxide. An index step of Δn=1.88 is realized between these two layers which makes it possible to fabricate a high efficiency DBR with very few polycrystal-GaAs/amorphous-Al-oxide pairs on GaAs-, GaP-, or InP-based materials. Using reflectivity measurements, we demonstrate a five pair GaAs/AlAs-based DBR grown on an InP substrate that reflects wavelengths between 1.4 and 2.3 μm up to 95%.
Domain imaging and local piezoelectric properties of the (200)-predominant SrBi2Ta2O9 thin film
Authors: Hu G. D. , Xu J. B. , Wilson I. H.
Published: July 2002

The domain structure of the (200)-predominant SrBi2Ta2O9 (SBT) thin film was detected by an atomic force microscope in the piezoelectric mode. It was found that the content of the grains split by single domain walls is less than 5%. The types of domain walls formed in individual grains were identified by analyzing the dependence of piezoelectric coefficient (d33) on the alternating current driving electric field. Several grains larger than 300 nm were found to be split by non-180° domain walls. To study the switching properties, the (200)-predominant SBT thin film was polarized and imaged over a large area. Unswitchable grains cannot be observed both in the area polarized using +8 V and in the region polarized using −8 V.
Improving the performance of conjugated polymer-based devices by control of interchain interactions and polymer film morphology
Authors: Nguyen Thuc-Quyen , Kwong Raymond C. , Thompson Mark E. , Schwartz Benjamin J.
Published: July 2002

Interchain interactions in conjugated polymer films promote good carrier transport but also reduce the luminescence quantum yield, leading to a fundamental trade-off in optimizing film morphology for device performance. We present two methods to improve the efficiency of light-emitting diodes (LEDs) based on poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) by altering film morphology without changing device architecture. First, “trilayer” LEDs, which use a central MEH-PPV layer with reduced interchain interactions between layers of highly aggregated MEH-PPV near the electrodes, have a higher efficiency than single-layer devices. Second, device efficiency can be improved by annealing MEH-PPV films, so that the reduced emission upon increasing interchain interactions is overcome by more balanced charge injection.
Effect of strain transfer on critical thickness for epitaxial layers grown on compliant substrate
Authors: Huang F. Y.
Published: July 2002

The critical thickness of strained layers grown on a thin-film substrate is analyzed based on an equilibrium theory. Due to the strain transfer from the epitaxial layer to the underneath substrate, the critical thickness increases as the substrate thickness decreases. Ultimately, there exists a threshold thickness for the substrate below which mismatched epitaxial layers of any thickness can be grown without dislocations.
Raman scattering study of Ga1−xMnxN crystals
Authors: Gebicki W. , Strzeszewski J. , Kamler G. , Szyszko T. , Podsiadło S.
Published: July 2002

Raman spectra of Ga1−xMnxN crystals grown by the resublimation method have been investigated. New bands around 300 and 667 cm−1, as well as a broad structure near 600 cm−1, not observed in undoped GaN have been found. The temperature dependence of major Raman bands has been measured. The simple model of GaN lattice dynamics has been presented, and the observed bands have been assigned to disorder-activated phonon modes, in good agreement with the calculated phonon density of states.
Nonuniform bulk second-order optical nonlinearity in PbO/B2O3 glass
Authors: Xu Zhiling , Liu Liying , Fei Yue , Yang Peng , Hou Zhanjia , Xu Lei , Wang Wencheng , Lim Jong Wook , Affatigato Mario , Feller Steve
Published: July 2002

Second-order nonlinearity (SON) of thermally poled PbO/B2O3 glass was investigated. The SON distribution within the glass was obtained by repeatedly grinding the poled glass and measuring its Maker fringes. After grinding away up to one third of the thickness of the poled sample, i.e., about 300 μm, clear fringes with the interference minimum not equal to zero could still be observed, indicating that the area of SON was not confined within thin layers near the two surfaces. Cathode side grinding and anode side grinding had quite different influences on the SON of the poled sample. A model of the anode-charge-layer dependent bulk electric field distribution was proposed and this electric field was regarded as playing a key role in the generation of a nonuniform bulk SON distribution in PbO/B2O3 glass.
Generating-function method for tensor products
Authors: Bégin L. , Cummins C. , Mathieu P.
Published: July 2002

This is the first of two articles devoted to a exposition of the generating-function method for computing fusion rules in affine Lie algebras. The present paper is entirely devoted to the study of the tensor-product (infinite-level) limit of fusions rules. We start by reviewing Sharp’s character method. An alternative approach to the construction of tensor-product generating functions is then presented which overcomes most of the technical difficulties associated with the character method. It is based on the reformulation of the problem of calculating tensor products in terms of the solution of a set of linear and homogeneous Diophantine equations whose elementary solutions represent “elementary couplings.” Grobner bases provide a tool for generating the complete set of relations between elementary couplings and, most importantly, as an algorithm for specifying a complete, compatible set of “forbidden couplings.”
Photoisomerization reaction of CH2BrI following A-band and B-band photoexcitation in the solution phase: Transient resonance Raman observation of the iso-CH2I–Br photoproduct
Authors: Zheng Xuming , Phillips David Lee
Published: July 2002

We present nanosecond transient resonance Raman experiments that investigate the photoproduct species formed following A-band and B-band excitation of bromoiodomethane in room temperature cyclohexane solutions. Density functional theory calculations were also performed for several species that have been proposed as photoproducts for photodissociation of bromoiodomethane in the condensed phase. Comparison of the experimental resonance Raman spectra to density functional theory computational results and results for the closely related iso-CH2I–I and iso-CH2Br–Br species demonstrated that the iso-CH2I–Br species is mainly responsible for a transient absorption spectrum that appears after either A-band or B-band photoexcitation of bromoiodomethane in cyclohexane solution. This is in contrast to previous results for low temperature (12 K) solids where mainly the iso-CH2Br–I species was observed following A-band photoexcitation of bromoiodomethane. Further density functional theory computational results indicate that the iso-CH2I–Br species is noticeably more stable than the iso-CH2Br–I species by about 4.1 kcal/mol. This suggests that although both iso-CH2I–Br and iso-CH2Br–I species may be initially produced following ultraviolet excitation of bromoiodomethane in cyclohexane solution, only the more stable isomer has a sufficiently long lifetime to be observed in our nanosecond time-scale transient resonance Raman experiments. We compare results for the bromoiodomethane ultraviolet photodissociation/photoisomerization reactions in the condensed phase to those of the closely related diiodomethane system and discuss a probable mechanism for the formation of the iso-bromoiodomethane species in the condensed phase.
Magnetic structure of epitaxially grown MnAs on GaAs(001)
Authors: Schippan F. , Behme G. , Däweritz L. , Ploog K. H. , Dennis B. , Neumann K.-U. , Ziebeck K. R. A.
Published: July 2002

We investigate in detail the occurrence of magnetic domains in epitaxially grown MnAs films on GaAs(001) by magnetic force microscopy (MFM). MnAs layers exhibit in their demagnetized state a very complex magnetic domain structure. High resolution MFM images reveal detailed information on the domain wall. Additionally, we imaged magnetic domains in the dependence on the applied magnetic field. This detailed investigation gives new insight into the correlation between film topography and magnetic domain structures. Systematic magnetization measurements in-plane and out-of-plane have shown high anisotropy in our films. The out-of-plane magnetization determined as a function of the applied field reveals that the direction of the magnetic moments in the domain walls are out-of-plane, thus the domain walls are determined as 180° Bloch type.
Apparatus for in situ x-ray absorption fine structure studies on catalytic systems in the energy range 1000 eV<E<3500 eV
Authors: van der Eerden A. M. J. , van Bokhoven J. A. , Smith A. D. , Koningsberger D. C.
Published: July 2002

A new apparatus for in situ x-ray absoprtion fine structure measurements in the medium energy range of 1000–3500 eV has been developed. Measurements can be performed in a gaseous environment (max. pressure 1 bar) at temperatures ranging from 80 to 750 K. Pre-treatments can be performed at 5 bar and 750 K in the same cell, after which XAFS measurements can be done without exposing the sample to ambient air. In a modular set-up several detector systems can be used: fluorescence detection using a gas proportional counter, a photodiode or a microstrip detector. All detectors are highly integrated into the cell, gaining solid angle for detection. Electron yield detection can be used simultaneously using conversion electron yield or total electron yield. The performance of the new apparatus is demonstrated by a study of the K edge of Al in Zeolite Beta. The Al content is as low as 2 wt%. It will be shown that octahedral framework Al is formed while adding gaseous water at room temperature after ammonia removal (at 450 °C) of an NH4-Beta.
Enthalpy–entropy interplay for C36 cages: B3LYP/6-31G* calculations
Authors: Slanina Zdeněk , Uhlı́k Filip , Zhao Xiang , Ōsawa Eiji
Published: July 2002

The recently isolated first lower fullerene C36 is computed primarily at the B3LYP/6-31G* level with some information derived from the semiempirical SAM1 method. Twelve low-energy cages are considered both with fullerene (pentagons and hexagons) and “quasifullerene” (also squares and heptagons) structural pattern. The B3LYP/6-31G* evaluations of the equilibrium isomeric composition at high temperatures agree well with the recently reported solid-state NMR spectra. The fullerene cage with a topological D6h symmetry, that dominates at high temperatures, undergoes a small Jahn–Teller distortion. Some features of C36-related nanotubes are also discussed.
An accurate global ab initio potential energy surface for the X 1A′ electronic state of HOBr
Authors: Peterson Kirk A
Published: July 2002

A global, analytical potential energy surface for the ground electronic state of HOBr has been determined using highly correlated multireference configuration interaction wave functions and explicit basis set extrapolations of large correlation consistent basis sets. The ab initio data have been fit to an analytical functional form that accurately includes both the HOBr and HBrO minima, as well as all dissociation asymptotes. Small adjustments to this surface are made based on the limited experimental data available and by indirectly taking into account the effects of spin–orbit coupling on the OH+Br dissociation channel. Vibrational energy levels are calculated variationally for both HOBr and HBrO up to the OH+Br dissociation limit using a truncation/recoupling method. The HOBr isomer is calculated to contain 708 bound vibrational energy levels, while the HBrO minimum lies above the OH+Br dissociation limit but is calculated to have 74 “quasibound,” localized eigenstates. Infrared intensities for all of these vibrational transitions are also calculated using MRCI dipole moment functions. The assignment of the HOBr states is complicated by strong stretch–bend resonances even at relatively low energies. In contrast to the HOCl case, these state mixings made it particularly difficult to assign the relatively intense OH overtone bands above v1=2. The vibrational density of states of HOBr at the OH+Br dissociation limit is determined to be 0.16 states/cm−1. Comparisons to recent work on HOCl using similar methods are made throughout.
Symmetries of Bianchi I space–times
Authors: Tsamparlis Michael , Apostolopoulos Pantelis S.
Published: July 2002

All diagonal proper Bianchi I space–times are determined which admit certain important symmetries. It is shown that for Homothetic motions, Conformal motions and Kinematic Self-Similarities the resulting space–times are defined explicitly in terms of a set of parameters, whereas Affine Collineations, Ricci Collineations, and Curvature Collineations, if they are admitted, they determine the metric modulo certain algebraic conditions. In all cases the symmetry vectors are explicitly computed. The physical and the geometrical consequences of the results are discussed and a new anisotropic fluid, physically valid solution which admits a proper conformal Killing vector, is given.
Temperature dependence of narrow-band terahertz generation from periodically poled lithium niobate
Authors: Lee Y.-S. , Meade T. , DeCamp M. , Norris T. B. , Galvanauskas A.
Published: July 2002

Femtosecond optical pulses are used to generate narrow-band terahertz wave forms via optical rectification in a periodically poled lithium niobate crystal. By cooling the crystal to reduce losses due to phonon absorption, we are able to obtain bandwidths as narrow as 18 GHz at a carrier frequency of 1.8 THz. Temperature-dependent measurements show insignificant bandwidth broadening between 10 and 120 K, although the terahertz power substantially decreases as the temperature increases. Absolute power measurements indicate a conversion efficiency of at least 10−5.
A simple model for associative desorption and dissociative chemisorption
Authors: Luntz A. C.
Published: July 2002

This paper describes a simple model for state resolved associative desorption, and by assuming detailed balance, of activated dissociative chemisorption. The equations that result from this model are identical to those traditionally used empirically in fits to experimental data. This allows better understanding of parameters used to fit experiments and how to compare these with theory. The role of model parameters in describing activated dissociation when the barrier is almost exclusively along a vibrational coordinate is also examined.
Quantum dynamics study of the Cl+D2 reaction: Time-dependent wave packet calculations
Authors: Yang Ben-Hui , Tang Bi-Yu , Yin Hong-Ming , Han Ke-Li , Zhang John Z. H.
Published: July 2002

The quantum dynamics of the Cl+D2 reaction has been studied by means of time-dependent quantum wave packet calculations on the G3 and BW2 potential energy surfaces. Initial state-specific total reaction probabilities and integral cross sections are calculated, and the thermal rate constant is obtained. On the G3 surface, the effect of the reagent’s rotational excitation on the reactivity is negative, while on the BW2 surface, the rotation of reagent has positive effect on the reactivity. Comparison of the thermal rate constants on the G3 and BW2 surfaces with experimental measurement is shown.
Electronic spectroscopy of B atoms and B2 molecules isolated in para-H2, normal-D2, Ne, Ar, Kr, and Xe matrices
Authors: Tam Simon , Macler Michel , DeRose Michelle E. , Fajardo Mario E.
Published: July 2002

We report spectroscopic observations on B atoms isolated in cryogenic parahydrogen (pH2), normal deuterium (nD2), Ne, Ar, Kr, and Xe matrices, and of B2 molecules in Ne, Ar, Kr, and Xe matrices. The 2s23s(2S)←2s22p(2P) B atom Rydberg absorption suffers large gas-to-matrix blue shifts, increasing from +3000 to +7000 cm−1 in the host sequence: Xe<Kr<Ar≈Ne≈nD2≈pH2. Much smaller shifts are observed for the 2s2p2(2D)←2s22p(2P) B atom core-to-valence transition. We assign pairs of absorption peaks spaced by ≈10 nm in the 315–355 nm region to the B2 (A 3Σu−←X 3Σg−) Douglas–Herzberg transition. We assign a much weaker progression in the 260–300 nm region to the B2 (2)3Πu←X 3Σg− transition. We report a novel progression of strong peaks in the 180–200 nm region which we suspect may be due to B2 molecules, but which remains unassigned. Ultraviolet (UV) absorption spectra of B/pH2 solids show two strong peaks at 216.6 and 208.9 nm, which we assign to the matrix perturbed 2s23s(2S)←2s22p(2P) and 2s2p2(2D)←2s22p(2P) B atom absorptions, respectively. This assignment is supported by quantum path integral simulations of B/pH2 solids reported in the following article in this journal [J. R. Krumrine, S. Jang, G. A. Voth, and M. H. Alexander, J. Chem. Phys. 113, 9079 (2000)]. Laser induced fluorescence emission spectra of B/pH2 solids show a single line at 249.6 nm, coincident with the gas phase wavelength of the 2s23s(2S)→2s22p(2P) B atom emission. The UV laser irradiation results in photobleaching of the B atom emission and absorptions, accompanied by the formation of B2H6.
Enhanced second harmonic generation at surface in transparent nanocrystalline TeO2-based glass ceramics
Authors: Sakai Ryosuke , Benino Yasuhiko , Komatsu Takayuki
Published: July 2002

Transparent TeO2-based glass ceramics consisting of nanocrystalline particles such as 15K2O.15Nb2O5.70TeO2 have been prepared, and the effect of surface layers on the second harmonic (SH) intensity has been examined using the Maker fringe method. An enhanced second harmonic generation (SHG) with fine fringe patterns has been observed at the original surface layers in transparent glass ceramics. The thickness of the surface layers giving a strong SHG is estimated to be around 50 μm, which is much larger than the coherence length of 3.5 μm for the glass ceramics. The glass ceramics without the original surface layers obtained by mechanical polishing show weak SH intensity, and any fringe pattern is not observed. Any evidence for an orientation of crystals at the surface has not been confirmed, and the origin of an enhanced SHG is unclear at this moment.
A comparison of Köhler activation with nucleation for NaCl–H2O
Authors: Mirabel Philippe , Reiss Howard , Bowles Richard K.
Published: July 2002

The Köhler theory of activation is compared with classical nucleation theory for the NaCl–H2O system. We find that for NaCl particles of the size typically found in the atmosphere the two processes are hardly distinguishable. Consequently, the usually preferred choice of simple activation theory over nucleation in the development and application of large scale atmospheric models, is reaffirmed. However, we also find that for small NaCl crystals consisting of 10–50 molecules, nucleation can occur at relative humidities significantly lower than those required for activation.
Measurement of silicon surface recombination velocity using ultrafast pump–probe reflectivity in the near infrared
Authors: Sabbah A. J. , Riffe D. M.
Published: July 2002

We demonstrate that ultrafast pump–probe reflectivity measurements from bulk Si samples using a Ti:sapphire femtosecond oscillator (λ=800 nm) can be used to measure the Si surface recombination velocity. The technique is sensitive to recombination velocities greater than ∼104 cm s−1.
Optimization of the emission characteristics of light emitting diodes by surface plasmons and surface waveguide modes
Authors: Gianordoli S. , Hainberger R. , Köck A. , Finger N. , Gornik E. , Hanke C. , Korte L.
Published: July 2002

In this letter, the influence of metal films and dielectric waveguides on the radiation characteristics of light emitting diodes (LEDs) is investigated. Cross and hexagonal surface grating structures are used to excite surface plasmons and waveguide modes in these media. A beam divergence as small as 17° is achieved for a hexagonal grating coated with a 40 nm Ag film. The maximum quantum efficiency is found for a hexagonal grating with a 10-nm-thick Au film and a 250-nm-thick dielectric layer. The optical power emitted by this LED into the solid angle from −30° to 30° is 10% higher than that of an LED without metal film and dielectric layer.
Direct characterization of terahertz radiation from the dynamics of the semiconductor surface field
Authors: Han P. Y. , Huang X. G. , Zhang X.-C.
Published: July 2002

We report on a time-resolved electro-optic sampling of the photocarrier-induced surface field dynamics by a midband gap probe beam from a femtosecond fiber laser. By measuring the ultrafast surface field variation, we are able to derive the undistorted terahertz wave form radiated from the semiconductor surface that is excited by the femtosecond laser pulses. The derived wave form agrees well with the directly measured terahertz radiation at the far field. The peak frequency of terahertz radiation is found to increase with the carrier density, which can be explained in terms of field dynamics.
Domain nucleation and relaxation kinetics in ferroelectric thin films
Authors: Ganpule C. S. , Nagarajan V. , Ogale S. B. , Roytburd A. L. , Williams E. D. , Ramesh R.
Published: July 2002

The time-dependent relaxation of the remanent polarization in epitaxial lead zirconate titanate (PbZr0.2Ti0.8O3) ferroelectric thin films, containing a uniform two-dimensional grid of 90° domains (c axis in the plane of the film), is examined using piezoresponse microscopy. The 90° domain walls preferentially nucleate the 180° reverse domains during relaxation, with a significant directional anisotropy. Relaxation occurs through the nucleation and growth of reverse domains, which subsequently coalesce and consume the entire region as a function of time.
Group theoretical quantum tomography
Authors: Cassinelli G. , D’Ariano G. M. , De Vito E. , Levrero A.
Published: July 2002

The paper is devoted to the mathematical foundation of quantum tomography using the theory of square-integrable representations of unimodular Lie groups.
Maximal subalgebras of vector fields for equivariant quantizations
Authors: Boniver F. , Mathonet P.
Published: July 2002

The elaboration of new quantization methods has recently developed the interest in the study of subalgebras of the Lie algebra of polynomial vector fields over a Euclidean space. In this framework, these subalgebras define maximal equivariance conditions that one can impose on a linear bijection between observables that are polynomial in the momenta and differential operators. Here, we determine which finite dimensional graded Lie subalgebras are maximal. In order to characterize these, we make use of results of Guillemin, Singer, and Sternberg and Kobayashi and Nagano.
Quantum-classical Liouville description of multidimensional nonadiabatic molecular dynamics
Authors: Santer Mark , Manthe Uwe , Stock Gerhard
Published: July 2002

The quantum-classical Liouville formulation gives a quantum-mechanical density-matrix description of the “quantum” particles of a problem (e.g., the electrons) and a classical phase-space-density description of the “classical” particles (e.g., the nuclei). In order to employ this formulation to describe multidimensional nonadiabatic processes in complex molecular systems, this work is concerned with an efficient Monte Carlo implementation of the quantum-classical Liouville equation. Although an exact stochastic realization of this equation is in principle available, in practice one has to cope with two major complications: (i) The representation of nonlocal phase-space operators in terms of local classical trajectories and (ii) the convergence of the Monte Carlo sampling which is cumbersome due to complex-valued trajectories with rapidly oscillating phases. Several strategies to cope with these problems are discussed, including various approximations to determine the momentum shift associated with a nonadiabatic transition, the on-the-fly generation of new trajectories at curve-crossings, and the localization of trajectories after irreversible electronic transitions. Employing several multidimensional model systems describing ultrafast photoinduced electron transfer and internal conversion, detailed numerical studies are performed which are compared to exact quantum calculations as well as to the “fewest-switches” surface-hopping method. In all cases under consideration, the Liouville calculations are in good agreement with the quantum reference. In particular, the approach is shown to provide a correct quantum-classical description of the electronic coherence.
Conditioning of hydrogenated amorphous carbon thin films for field emission via current stressing
Authors: Carey J. D. , Silva S. R. P.
Published: July 2002

The effects of electrical current stressing on the field emission characteristics of hydrogenated amorphous carbon (a-C:H) thin films are reported. In these a-C:H films an initial conditioning treatment of the film is often required before the onset of stable emission and only after several voltage cycles do the values of the threshold field tend to converge. By stressing of the film by applying a predetermined current through the film, the initial conditioning treatment can be removed and stable and reproducible emission observed. Retesting of the current stressed films shows that the films remain fully conditioned provided a sufficiently high stress current was initially used.
Enhanced coupling to vertical radiation using a two-dimensional photonic crystal in a semiconductor light-emitting diode
Authors: Erchak Alexei A. , Ripin Daniel J. , Fan Shanhui , Rakich Peter , Joannopoulos John D. , Ippen Erich P. , Petrich Gale S. , Kolodziejski Leslie A.
Published: July 2002

Enhanced coupling to vertical radiation is obtained from a light-emitting diode using a two-dimensional photonic crystal that lies entirely inside the upper cladding layer of an asymmetric quantum well structure. A sixfold enhancement in light extraction in the vertical direction is obtained without the photonic crystal penetrating the active material. The photonic crystal is also used to couple pump light at normal incidence into the structure, providing strong optical excitation.
Potential energy surfaces of image potential states
Authors: Jo/rgensen Solvejg , Ratner Mark A. , Mikkelsen Kurt V.
Published: July 2002

Methods for calculating potential energy surfaces of image potential states are presented. The methods cover metal–vacuum interfaces, metal–dielectric medium interfaces, and metal–dielectric medium–vacuum interfaces. The metal is approximated as a perfect conductor or as a Fermi–Thomas metal. Potential energy surfaces and the induced charge densities are presented as functions of the properties of the dielectric medium, the metal and the thickness of the absorbed material.
Extraordinary magnetoresistance in externally shunted van der Pauw plates
Authors: Zhou T. , Hines D. R. , Solin S. A.
Published: July 2002

We show that extraordinary magnetoresistance (EMR) exhibited by a composite van der Pauw (vdP) disk consisting of a semiconductor with an internal shunt can also be obtained from an electrically equivalent, externally shunted structure that is amenable to fabrication in the mesoscopic sizes required for important magnetic sensor applications. As an example, we use bilinear conformal mapping to transform the composite vdP disk into an externally shunted rectangular plate and calculate its EMR by solving Laplace’s equation with appropriate boundary conditions using no adjustable parameters. The calculations are in good agreement with measurements of InSb plates with Au shunts. Room-temperature EMR values as high as 550% at 0.05 T are obtained.
Influence of surface plasmons on the Faraday effect in bismuth-substituted yttrium iron garnet films
Authors: Bremer J. , Vaicikauskas V. , Hansteen F. , Hunderi O.
Published: July 2002

The Kerr rotation and ellipticity of Bi1.8Y1.2Fe5O12 films (Bi:YIG) with the easy axis of magnetization perpendicular to the film plane was investigated in a Kretschmann setup at the wavelength 1.31 μm. The sample was coated by a thin Au film in order to excite plasmons at the free Au surface. An ellipsometric setup, supplemented with a magnetic field oriented perpendicularly to the film plane was used to measure the reflected p and s components of an incoming p polarized wave. The phase difference between the outgoing p and s waves was found by an indirect procedure. A minimum in Kerr rotation and a maximum in Kerr ellipticity could be observed at the surface plasmon resonance angle. Using the Berreman formalism, the experimental curves for the prism/Bi:YIG/Au system could be reproduced by bulk optical constants of the constituents. The asymmetry of the ellipticity curve is demonstrated to arise from a sudden variation in the phase. The effects of the field distribution and the surface roughness are discussed.
Development and characterization of a Z-pinch-driven hohlraum high-yield inertial confinement fusion target concept
Authors: Cuneo Michael E. , Vesey Roger A. , Porter John L. , Chandler Gordon A. , Fehl David L. , Gilliland Terrance L. , Hanson David L. , McGurn John S. , Reynolds Paul G. , Ruggles Laurence E. , Seamen Hans , Spielman Rick B. , Struve Ken W. , Stygar William A. , Simpson Walter W. , Torres Jose A. , Wenger David F. , Hammer James H. , Rambo Peter W. , Peterson Darrell L. , Idzorek George C.
Published: July 2002

Initial experiments to study the Z-pinch-driven hohlraum high-yield inertial confinement fusion (ICF) concept of Hammer, Tabak, and Porter [Hammer et al., Phys. Plasmas 6, 2129 (1999)] are described. The relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling (“hohlraum energetics”) is well understood from zero-dimensional semianalytic, and two-dimensional view factor and radiation magnetohydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pinch-driven secondary hohlraum (26±5 TW), indicating the concept could scale to fusion yields of >200 MJ. A novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry has also been developed. This source will permit investigation of the pinch power balance and hohlraum geometry requirements for ICF relevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [Spielman et al., Phys. Plasmas 5, 2105 (1998)].
Metal foam evolution studied by synchrotron radioscopy
Authors: Banhart John , Stanzick Heiko , Helfen Lukas , Baumbach Tilo
Published: July 2002

High-intensity synchrotron x-ray radioscopy was used to obtain real-time images of foaming metals, thus allowing the formation, growth, and decay of such systems to be studied. Bubble generation, foam coalescence and drainage of an aluminum-based alloy foam were investigated. Although the foaming process appears to be very similar to the formation of aqueous foams, the observed rupture behavior of thin metal films suggests that the processes responsible for metal foam stabilization and destabilization must be quite different.
Analysis of a direct-drive ignition capsule designed for the National Ignition Facility
Authors: McKenty P. W. , Goncharov V. N. , Town R. P. J. , Skupsky S. , Betti R. , McCrory R. L.
Published: July 2002

This paper reviews the current direct-drive ignition capsule designed for the National Ignition Facility (NIF) [M. D. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41, B39 (1999)]. The ignition design consists of a cryogenic deuterium–tritium (DT) shell contained within a very thin CH shell. To maintain shell integrity during the implosion, the target is placed on an isentrope approximately three times that of Fermi-degenerate DT (α=3). One-dimensional studies show that the ignition design is robust. Two-dimensional simulations examine the effects on target performance due to laser imprint, power imbalance, and inner- and outer-target-surface roughness. Results from these studies indicate that the capsule gain can be scaled to the ice/vapor surface deformation at the end of the acceleration stage of the implosion. The physical reason for gain reduction as a function of increasing nonuniformities is examined. Simulations show that direct-drive target gains in excess of 30 can be achieved for an inner-ice-surface roughness of 1 μm rms, an on-target power imbalance of 2% rms, and by using the beam-smoothing technique SSD with 1 THz and two color cycles.
Kinetic theory of dusty plasmas. III. Dust–dust collision integrals
Authors: Tsytovich V. N. , de Angelis U.
Published: July 2002

The dust–dust collision integrals found in the kinetic theory of dusty plasmas [Tsytovich and de Angelis, Phys. Plasmas 6, 1093 (1999); 7, 554 (2000)] are analyzed. Analytic expressions are derived and numerical results are given for the case of thermal distributions to assess their importance and dependence on dusty plasma parameters. Modifications of Debye screening by collective dust effects are obtained. A study of the forces between two test dust particles is presented and the appearance of weakly screened attraction forces at distances larger than the Debye screening length is analyzed. These forces are related to the collective shadow effect in dust charging and operate at distances larger than the previously known noncollective shadow attraction forces. The dependence of the collective attraction forces on dust temperature is analyzed.
Formation and stabilization of pyramidal etch hillocks on silicon {100} in anisotropic etchants: Experiments and Monte Carlo simulation
Authors: Nijdam A. J. , van Veenendaal E. , Cuppen H. M. , van Suchtelen J. , Reed M. L. , Gardeniers J. G. E. , van Enckevort W. J. P. , Vlieg E. , Elwenspoek M.
Published: July 2002

On Si{100} surfaces etched in anisotropic etchants such as aqueous solutions KOH and TMAH, pyramidal etch hillocks are frequently found. Besides these hillocks, we have investigated hillocks that have partially disappeared using scanning electron microscopy (SEM). During re-etching numerous additional pyramidal etch hillocks are formed on the exact spots where SEM pictures were made earlier. These observations suggest that semipermeable particles adhering to the surface are responsible for the development of the pyramidal etch hillocks. In order to investigate the influence of such nanometer scale particles on the etch rate and the surface morphology, Monte Carlo simulations were performed of etching of Si{100} surfaces on which small semimasks are present. The presence of the microscopic semimasks is shown to cause the formation of macroscopic hillocks, which closely resemble experimentally observed hillocks. Removal of the semimask on top of a hillock leads to a vanishing pyramidal etch hillock. In the Monte Carlo model, however, the etch rate as a function of surface orientation has a maximum for {100}, while in reality {100} corresponds to a local minimum. This implies that for typical experimental conditions an etch hillock should not be stable despite a semipermeable particle on top, because of underetching starting from 〈110〉 ridges of the hillock. This paradox can be resolved by assuming that the ridges act as sinks of tiny particles. This gives a reduction in etch rate of the ridges, next to the top, which is necessary for the hillock to remain stable. The exact nature of these masking particles is unknown, but silicate particles are a likely candidate.
Bi
3.25
La
0.75
Ti
3
O
12
thin films prepared on Si (100) by metalorganic decomposition method
Authors: Hou Yun , Xu Xiao-Hong , Wang Hong , Wang Min , Shang Shu-Xia
Published: July 2002

Bi 3.25 La 0.75 Ti 3 O 12 (BLT) thin films have been grown on n-type Si (100) substrates by metalorganic decomposition method. The structural properties of the films were examined by x-ray diffraction. The BLT films exhibit good insulating property with room temperature resistivities in the range of 1012–1013 Ω cm. The current–voltage characteristics show ohmic conductivity in the lower voltage range and space-charge-limited conductivity in the higher voltage range. The capacitance–voltage characteristic hysteresis curves show that the metal-ferroelectric-semiconductor structure has memory effect. The fixed charge density and the surface state density were also calculated. The results obtained indicate that the present BLT films are suitable for making ferroelectric field effect transistor memories.
Hysteresis and limiting cycles in a high frequency ac field
Authors: Klik I. , Yao Y. D.
Published: July 2002

Numerical solutions of Brown’s Fokker-Planck equation are used to analyze the response of a superparamagnetic particle to an ac applied field. The nature of the response is governed by the rate of intrawell relaxation processes: At low driving frequencies they maintain the system close to a quasistationary state, and the response is Markovian. By contrast, probability redistribution effects taking place at high frequencies lead to a non-Markovian response mode which displays memory phenomena. The high frequency limiting cycles are analyzed in detail, and the transition to conventional low frequency hysteresis loops is discussed.
SiGeC/Si superlattice microcoolers
Authors: Fan Xiaofeng , Zeng Gehong , LaBounty Chris , Bowers John E. , Croke Edward , Ahn Channing C. , Huxtable Scott , Majumdar Arun , Shakouri Ali
Published: July 2002

Monolithically integrated active cooling is an attractive way for thermal management and temperature stabilization of microelectronic and optoelectronic devices. SiGeC can be lattice matched to Si and is a promising material for integrated coolers. SiGeC/Si superlattice structures were grown on Si substrates by molecular beam epitaxy. Thermal conductivity was measured by the 3ω method. SiGeC/Si superlattice microcoolers with dimensions as small as 40×40 μm2 were fabricated and characterized. Cooling by as much as 2.8 and 6.9 K was measured at 25 °C and 100 °C, respectively, corresponding to maximum spot cooling power densities on the order of 1000 W/cm2.
Gadolinium silicate gate dielectric films with sub-1.5 nm equivalent oxide thickness
Authors: Gupta J. A. , Landheer D. , McCaffrey J. P. , Sproule G. I.
Published: July 2002

GdSi x O y gate dielectric films were deposited on Si(001) substrates using ultra-high-vacuum electron-beam evaporation from pressed-powder targets. Transmission electron microscopy showed that the films were amorphous as deposited and remained amorphous when annealed to temperatures up to 900 °C. Capacitance–voltage measurements indicate an equivalent oxide thickness (EOT) of 13.4 Å for a film with composition GdSi0.56O2.59 determined by in situ x-ray photoelectron emission spectroscopy. After forming gas annealing at 500 °C the EOT was reduced to 11.0 Å, at a physical thickness of 45 Å. The same film has a low leakage current of approximately 5.7×10−3 A cm−2 at +1 V, a reduction of 8.7×104 compared to current density estimates of SiO2 films with the same specific capacitance.
Polar clusters in relaxor (Pb,La)(Zr,Ti)O3 revealed by second harmonic generation
Authors: Pavel M. , Rychetský I. , Petzelt J.
Published: July 2002

Polarization ordering in Pb1−xLax(Zr0.65Ti0.35)0.98O3 ceramics for x=8%, 9% and 9.5% (PLZT x/65/35) was detected from the temperature dependence of the averaged second harmonic generation (SHG) intensity. In PLZT 8/65/35 it shows two drops near the freezing transition (at about 50 °C) and slightly below the Burns temperature (about 350 °C). The SHG intensity exhibits peaks at 90 and 355 °C detected in both PLZT compositions using slow heating runs, which disappear for faster temperature runs. This indicates an unusual increase in dipole ordering with a very slow relaxation time.
General thermodynamic framework for understanding temperature-entropy diagram of batchwise operating thermodynamic cooling cycles
Authors: Chua H. T. , Ng K. C. , Malek A. , Oo N. M.
Published: July 2002

This article presents a general thermodynamic framework for the temperature-entropy (T-s) diagram of batchwise operating thermodynamic cooling cycles. Our approach is fully consistent with the rigor of classical thermodynamics and captures accurately characteristics of cyclic-steady-state performance of a batchwise operating cooling cycle. The Eulerian viewpoint is adopted to allow for a time invariant description of such a cycle on a T-s plane, and this results in the formation of close loops at all internal energy storage components and their connection(s).
Topographic cross talk in reflection mode near-field optical microscopy on patterned structures
Authors: Rosenberger A. , Münnemann A. , Kiendl F. , Güntherodt G. , Rosenbusch P. , Bland J. A. C. , Eggers G. , Fumagalli P.
Published: July 2002

In scanning near-field optical microscopy the sample topography may have a strong effect on the optical image signal. This cross talk has been investigated in subwavelength-periodically patterned thin-film structures using a reflection-mode near-field optical microscope. A comparison between measured and simulated line scans shows that far-field light waves emitted from the tip aperture play a major role in the imaging process.