bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical properties of singly charged conjugated oligomers: A coupled-cluster equation of motion study
Authors: Ye A. , Shuai Z. , Kwon O. , Brédas J. L. , Beljonne D.
Published: September 2004

We have implemented a coupled-cluster equation of motion approach combined with the intermediate neglect of differential overlap parametrization and applied it to study the excited states and optical absorptions in positively and negatively charged conjugated oligomers. The method is found to be both reliable and efficient. The theoretical results are in very good agreement with experiments and confirm that there appear two subgap absorption peaks upon polaron formation. Interestingly, the relative intensities of the polaron-induced subgap absorptions can be related to the extent of the lattice geometry relaxations.
Numerical characterization of hydrothermal waves in a laterally heated shallow layer
Authors: Bucchignani Edoardo
Published: September 2004

Instabilities of a buoyancy-thermocapillary flow in a three-dimensional differentially heated layer have been examined by means of direct numerical simulations, in order to have a more detailed description of the flow field with respect to that obtained experimentally. The main aim of this work is the numerical modelization of some experiments performed by Riley and Neitzel [J. Fluid Mech. 359, 143 (1998)], considering a cavity 30 mm×1 mm×50 mm filled with a silicone oil (Prandtl number equal to 13.9). Hydrothermal waves have been observed and examined in detail. Further simulations have been performed in order to describe also a basic unicellular flow and a steady multicellular flow. The numerical code is based on the vorticity–velocity formulation of the Navier–Stokes equations for an incompressible fluid, solved in time by means of a fully implicit approach.
Double-resonance versus pulsed Fourier transform two-dimensional infrared spectroscopy: An experimental and theoretical comparison
Authors: Cervetto Valentina , Helbing Jan , Bredenbeck Jens , Hamm Peter
Published: September 2004

In this study we focus on the differences and analogies of two experimental implementations of two-dimensional infrared (2D-IR) spectroscopy: double-resonance or dynamic hole burning 2D-IR spectroscopy and pulsed Fourier transform or heterodyne detected photon echo spectroscopy. A comparison is done theoretically as well as experimentally by contrasting data obtained from both methods. As an example we have studied the strongly coupled asymmetric and symmetric carbonyl stretching vibrations of dicarbonylacetylacetonato rhodium dissolved in hexane. Both methods yield the same peaks in a 2D-IR spectrum. Within certain approximations we derive an analytic expression which shows that the 2D-IR spectra are broadened in one frequency dimension in the double-resonance experiment by convolution with the pump pulse spectral width, while the spectral resolution in the other frequency direction is the same in both cases.
Coupled-cluster connected quadruples and quintuples corrections to the harmonic vibrational frequencies and equilibrium bond distances of HF, N2, F2, and CO
Authors: Ruden Torgeir A. , Helgaker Trygve , Jørgensen Poul , Olsen Jeppe
Published: September 2004

Using the coupled-cluster method, we have examined the contributions from the connected quadruple and quintuple virtual excitations to the harmonic vibrational frequencies and equilibrium bond distances of HF, N2, F2, and CO. Whereas the largest quadruples contributions are −18.8 cm−1 to the harmonic frequency of N2 and 0.43 pm to the bond distance of F2, the largest quintuples contributions are −3.9 cm−1 to the harmonic frequency of N2 and 0.03 pm to the bond distances of N2 and F2. As we improve the description by going from the coupled-cluster singles-and-doubles model with a perturbative triples correction [CCSD(T)] to the coupled-cluster singles-doubles-triples-and-quadruples model, the mean and maximum absolute errors in the calculated frequencies relative to experiment are reduced from 11.3 and 15.8 cm−1, respectively, to 3.2 and 4.7 cm−1; for the bond distances, the mean and maximum absolute errors are reduced from 0.16 and 0.47 pm, respectively, to 0.04 and 0.13 pm. The calculations presented here confirm previous observations that, to some extent, the relatively small errors in the CCSD(T) equilibrium bond distances and harmonic frequencies arise from a cancellation of errors in the approximate (perturbative) treatment of the connected triples and the neglect of higher-order connected excitations. Further inclusion of quintuples contributions, relativistic corrections, and adiabatic corrections reduces the mean and maximum absolute errors to 1.1 and 2.3 cm−1, respectively, for the harmonic frequencies and to 0.02 and 0.05 pm, respectively, for the bond distances.
High Schmidt number scalars in turbulence: Structure functions and Lagrangian theory
Authors: Borgas Michael S. , Sawford Brian L. , Xu Shuyi , Donzis Diego A. , Yeung P. K.
Published: September 2004

We demonstrate the existence of Batchelor’s viscous-convective subrange using direct numerical simulation (DNS) results to confirm the logarithmic dependence of the scalar structure function on the separation for the scalar field generated by stationary isotropic turbulence acting on a uniform mean scalar gradient. From these data we estimate the Batchelor constant B̃θ≈5. By integrating a piecewise continuous representation of the scalar variance spectrum we calculate the steady-state scalar variance as a function of Reynolds number and Schmidt number. Comparison with DNS results confirms the Reλ−1 behavior predicted from the spectral integration, but with a coefficient about 60% too small. In the large Reynolds number limit the data give a value of 2.5 for the mechanical-to-scalar time scale ratio. The dependence of the data for the scalar variance on Schmidt number agrees very well with the spectral integration using the values of the Batchelor constant estimated from the structure function. We also carry out an exact Lagrangian analysis of the scalar variance and structure function, explicitly relating the Batchelor constant to the Lyapunov exponent for the separation of pairs of fluid particles within the turbulence dissipation subrange. Our results, particularly for the scalar variance, illustrate explicitly the singular nature of the zero diffusivity limit. For finite values of the Schmidt number and Reynolds number the viscous-convective subrange contribution to the variance can be significant even at moderate values of the Reynolds number.
A multiparameter data acquisition system based on universal serial bus interface for electron momentum spectrometer
Authors: Ning C. G. , Deng J. K. , Su G. L. , Zhou H. , Ren X. G.
Published: September 2004

A versatile multiparameter data acquisition system based on universal serial bus (USB) interface was designed and has been used on the electron momentum spectromenter. Digitized data were first buffered in a FIFO memory in an event-by-event mode with a check bit, and then transferred to computer through the USB interface. USB interface combined with a microcontroller unit provides much flexibility for data acquisition and experimental controls. The operation performance of the system is demonstrated in the measurement of electron momentum spectra of CH2F2 molecules.
Structure of the NaxClx+1− (x=1–4) clusters via ab initio genetic algorithm and photoelectron spectroscopy
Authors: Alexandrova Anastassia N. , Boldyrev Alexander I. , Fu You-Jun , Yang Xin , Wang Xue-Bin , Wang Lai-Sheng
Published: September 2004

The application of the ab initio genetic algorithm with an embedded gradient has been carried out for the elucidation of global minimum structures of a series of anionic sodium chloride clusters, NaxClx+1− (x=1–4), produced in the gas phase using electrospray ionization and studied by photoelectron spectroscopy. These are all superhalogen species with extremely high electron binding energies. The vertical electron detachment energies for NaxClx+1− were measured to be 5.6, 6.46, 6.3, and 7.0 eV, for x=1–4, respectively. Our ab initio gradient embedded genetic algorithm program detected the linear global minima for NaCl2− and Na2Cl3− and three-dimensional structures for the larger species. Na3Cl4− was found to have C3v symmetry, which can be viewed as a Na4Cl4 cube missing a corner Na+ cation, whereas Na4Cl5− was found to have C4v symmetry, close to a 3×3 planar structure. Excellent agreement between the theoretically calculated and the experimental spectra was observed, confirming the obtained structures and demonstrating the power of the developed genetic algorithm technique.
Quantum effects in light and heavy liquid water: A rigid-body centroid molecular dynamics study
Authors: Hernández de la Peña L. , Kusalik P. G.
Published: September 2004

The centroid molecular dynamics (CMD) method is applied to the study of liquid water in the context of the rigid-body approximation. This rigid-body CMD technique, which is significantly more efficient than the standard CMD method, is implemented on the TIP4P model for water and used to examine isotopic effects in the equilibrium and dynamical properties of liquid H2O and D2O. The results obtained with this approach compare remarkably well with those determined previously with path integrals simulations as well as those obtained from the standard CMD method employing flexible models. In addition, an examination of the impact of quantization on the rotational and librational motion of the water molecule is also reported.
Application of the Zeeman patterns in Ov and Hα spectra to the local plasma diagnostics of the TRIAM-1M tokamak
Authors: Shikama T. , Kado S. , Zushi H. , Iwamae A. , Tanaka S.
Published: September 2004

The positions of emission of berylliumlike oxygen ions in the core region as well as the hydrogen atoms in the boundary region of the limiter shadow have been measured by means of the difference of the Zeeman patterns in the spectral shape in the poloidal section of the TRIAM-1M super conducting tokamak [H. Zushi et al., Nucl. Fusion 43, 1600 (2003)]. For determining magnetic field strength, the σ components of the Zeeman spectra are resolved by a linear polarizer. In addition to the emission region, the local hydrogen neutral temperature and the recycling flow velocity are measured in the boundary region, and the bulk ion temperature is measured in the core region.
Evidence for partial quenching of orbital angular momentum upon complex formation in the infrared spectrum of OH-acetylene
Authors: Marshall Mark D. , Davey James B. , Greenslade Margaret E. , Lester Marsha I.
Published: September 2004

The entrance channel leading to the addition reaction between the hydroxyl radical and acetylene has been examined by spectroscopic characterization of the asymmetric CH stretching band of the π-hydrogen bonded OH-acetylene reactant complex. The infrared action spectrum observed at 3278.6 cm−1 (origin) consists of seven peaks of various intensities and widths, and is very different from those previously reported for closed-shell HF/HCl-acetylene complexes. The unusual spectrum arises from a partial quenching of the OH orbital angular momentum in the complex, which in turn is caused by a significant splitting of the OH monomer orbital degeneracy into A′2 and A″2 electronic states. The magnitude of the A′2−2A″ splitting as well as the A rotational constant for the OH-acetylene complex are determined from the analysis of this b-type infrared band. The most populated OH product rotational state, jOH=9/2, is consistent with intramolecular vibrational energy transfer to the ν2 C≡C stretching mode of the departing acetylene fragment. The lifting of the OH orbital degeneracy and partial quenching of its electronic orbital angular momentum indicate that the electronic changes accompanying the evolution of reactants into products have begun to occur in the reactant complex.
Pulsed electron beam generation in a simple discharge device
Authors: Rhee M. J. , Strikovski M.
Published: September 2004

We report a phenomenon of pulsed electron beam generation in a low-pressure gas discharge device. The cathode of the device is of simple geometry without a hollow cavity that is employed in the transient hollow cathode discharge (THCD) devices. The discharge and beam characteristics are measured and found to be similar to that of THCDs such as pseudospark and channelspark.
Phase boundary detection for dilution refrigerators
Authors: ter Haar E. , Martin R. V.
Published: September 2004

We describe a device to conveniently measure the positions of the phase boundaries in a dilution refrigerator. We show how a simple modification of a standard capacitive level gauge (segmentation of one of the electrodes) permits a direct calibration of the capacitance versus phase boundary position. We compare this direct calibration with the indirect procedure that must be adopted for a conventional capacitive level gauge. The device facilitates the correct adjustment of the 3He/4He fraction in the dilution refrigerator.
Modeling of configurations and third-order nonlinear optical properties of C36 and C34X2 (X=B,N)
Authors: Li X.-D. , Cheng W.-D. , Wu D.-S. , Lan Y.-Z. , Zhang H. , Gong Y.-J. , Li F.-F. , Shen J.
Published: September 2004

Using the ab initio method, the geometrical structures of C36 and the X (B,N)-doped isomers C34X2 have been optimized. On the basis of the optimized structures, then, the third-order nonlinear optical polarizabilities γ in the different optical processes of the third-harmonic generation, electric-field induced second-harmonic generation and degenerate four-wave mixing, and two-photon absorption (TPA) cross sections δ are calculated by using TDB3LYP method coupled with the sum-over-states method. The calculated results show that the one-photon allowed excitation process dominate the two-photon excitation process for C36-D6h, whereas the two-photon allowed excitation process dominate the one-photon excitation process for C36-D2d and C34X2 (B,N). It is found that the largest resonant TPA peaks of dopant fullerenes have a blueshift and the TPA cross sections have an enhancement compared with those of the parent fullerenes of isomers C36-D6h and C36-D2d.
Relaxor and nonlinear behaviors of SrTiO3∕BaTiO3 multilayers derived by a sol–gel process
Authors: Zhai Jiwei , Hung T. F. , Chen Haydn
Published: September 2004

Multilayered SrTiO3∕BaTiO3 thin films were fabricated on LaNiO3-buffered Pt∕Ti∕SiO2∕Si substrates by a sol–gel deposition method. X-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy were employed to examine the microstructure and crystalline phase of grown films. The dielectric properties were investigated as a function of multiplayer periodicity. Results showed that dielectric responses of these films were extremely similar to those of relaxor ferroelectric ceramics. The hysteresis loop decreased as the individual layer thickness decreased for a fixed total thickness of about 360nm. The dielectric constants, however, increased first then decreased as the individual layer thickness increased.
Theoretical investigation of excited states of C3
Authors: Terentyev A. , Scholz R. , Schreiber M. , Seifert G.
Published: September 2004

In this work, we present ab initio calculations for the potential energy surfaces of C3 in different electronic configurations, including the singlet ground state [X̃ 1Σg+,(1A1)], the triplet ground state [ã 3Πu,(3B1, 3A1)], and some higher excited states. The geometries studied include triangular shapes with two identical bond lengths, but different bond angles between them. For the singlet and triplet ground states in the linear geometry, the total energies resulting from the mixed density functional—Hartree-Fock and quadratic configuration interaction methods reproduce the experimental values, i.e., the triplet occurs 2.1 eV above the singlet. In the geometry of an equilateral triangle, we find a low-lying triplet state with an energy of only 0.8 eV above the energy of the singlet in the linear configuration, so that the triangular geometry yields the lowest excited state of C3. For the higher excited states up to about 8 eV above the ground state, we apply time-dependent density functional theory. Even though the systematic error produced by this approach is of the order of 0.4 eV, the results give different prospective to insight into the potential energy landscape for higher excitation energies.
Interrupted escape and the emergence of exponential relaxation
Authors: Lubchenko Vassiliy , Silbey Robert J.
Published: September 2004

A simple statistical theory of irreversible processes in a subsystem coupled to (or “interrupted” by) a stochastic bath is formulated. The theory does not explicitly invoke time scale separation that underlies the standard description of nonequilibrium phenomena and is intrinsic to the concept of quasiequilibrium in the canonical ensemble. Arbitrary statistics and speed of bath fluctuations are straightforwardly treated by the theory. Except in the case of an extremely slow, nonequilibrium bath, the ultimate statistics of interrupted escape are shown to be Poisson, which is solely a consequence of the stationary nature of interactions in a sufficiently dense system. In the limit of a fast bath, the corresponding relaxation rate is shown to equal the initial rate of decay, thus validating a wide class of Golden Rate type expressions at long times. This true exponentiality thus appears when the time scale separation takes place. The theory also applies to a number of specific phenomena including transport in a fluctuating or disordered medium, gated reactions, the line shape theory, and the quantum Zeno effect. The general nature of motional narrowing phenomena is demonstrated and related to the bath mediated slowing down of a decay process with a nearly deterministic uninterrupted escape probability. The corresponding survival probability is shown also to exhibit discernible oscillations around the exponential background. Mathematical tools necessary for using the theory in specific applications are exposed in some detail.
High-pressure nuclear magnetic resonance probe designed for a narrow bore magnet system
Authors: Zahl Achim , Igel Peter , Weller Manfred , van Eldik Rudi
Published: September 2004

The development of a high-pressure nuclear magnetic resonance (NMR) probe designed for a 400 MHz narrow bore NMR spectrometer is reported. As for the earlier constructed system for wide bore magnets, the electrical leads of the radio frequency (rf) circuit, the tubes for the thermostating and high-pressure transmitting fluids, as well as the connections for the temperature control unit are all fitted through the bottom of the high-pressure vessel. The sample can easily be removed and rapidly exchanged through a top plug in the vessel. There is no need to modify the magnet in order to perform the high-pressure experiments. In comparison to the already published wide bore design, the spectral resolution could be improved to be better than 6×10−9 by selecting a different titanium alloy and a more symmetrical design. The high-pressure vessel and the rf circuit are placed in an aluminum jacket which has the same outer dimensions as a commercial narrow bore probe and can be fitted into the magnet in the same way.
Nanoscale measurements of the absolute temperature from the tunneling of the free electron gas
Authors: Pavlov A.
Published: September 2004

The direct measurement of the absolute temperature using tunneling of the free electron gas is reported. The tunneling nanojunction is established between the copper oxide and a platinum tip in a scanning tunneling microscope. The method is based on linear temperature dependence of the relative Fermi-level shift. The energy difference shifted linearly with temperature to a value of the order of kBT in the range from room temperature up to 1250K. The energy spectrum of the free electrons is measured. A microelectromechanical systems device is fabricated to be a prototype of the microthermometer. The tunneling current spectrum is measured at resonant frequency.
Anomalous change in electron density at nuclear sites in nanosize zinc ferrite
Authors: Upadhyay C. , Verma H. C.
Published: September 2004

Nanosize samples of zinc ferrite with different particle sizes synthesized by citrate precursor route have been studied by x-ray diffraction, transmission electron microscopy and Mössbauer spectroscopy. The observations suggest that the electronic processes depend sensitively on size, and a big variation occurs in the pattern of temperature dependence of isomer shift on changing the size from 5nm to 6nm and from 6nm to 7nm. The electron density at the nuclear sites first increases, then decreases and again increases as the temperature of the nanosize samples is increased from 12K to 400K. The electric-field gradient varies considerably as the size is changed, but remains almost independent of temperature for a particular size. It is concluded that different aspects of hyperfine interactions in nanophase become significant at different sizes.
Time-resolved electroluminescence studies of III-nitride ultraviolet photonic-crystal light-emitting diodes
Authors: Shakya J. , Lin J. Y. , Jiang H. X.
Published: September 2004

Transient responses of III-nitride photonic-crystal (PC) ultraviolet (UV) light-emitting diodes (LEDs) were measured by picosecond time-resolved electroluminescence (EL) spectroscopy. Triangular arrays of PCs with different diameters/periodicities were fabricated on 333nmUV LEDs for enhancing light extraction efficiency using electron-beam lithography and inductively coupled-plasma dry etching. With the incorporation of PCs on LEDs, the EL decay time constant τ decreases systematically with the increase of the etched sidewall area indicating the strong effect of the surface recombination. The surface recombination velocities on the p-type epitaxial surface and on the sidewall of etched holes on LEDs were determined to be 1.73×104cm∕s and 1.48×105cm∕s, respectively. The angular distribution of light emission from LEDs with PCs shows slight narrowing in far-field pattern. Because of the increased transient response along with enhanced light extraction, the incorporation of PCs in UV LEDs provide an effective method to control the modulation speed of UV LEDs, which could be very useful for many applications.
Structural and optical properties of Ba(Cox,Ti1−x)O3 thin films fabricatedby sol-gel process
Authors: Li Y. W. , Sun J. L. , Meng X. J. , Chu J. H. , Zhang W. F.
Published: September 2004

Ba ( Co x , Ti 1 − x ) O 3 thin films were prepared on fused quartz substrate by a sol-gel method. The results of x-ray diffraction showed that the films are perovskite phase and the change of lattice constant caused by different Co concentration is undetectable. Optical transmittance measurement indicated that Co doping has effect on the energy band structure. The energy gap of Ba(Cox,Ti1−x)O3 decreased linearly with the increase of Co concentration. It is inferred that the energy level of conduction bands decline with the Co content increasing. This result is similar to the phenomena found in binary semiconductor where the band gap decreases with the increasing of average atomic number.
Assessment of thermodynamic models for dense gas dynamics
Authors: Guardone A. , Vigevano L. , Argrow B. M.
Published: September 2004

The accuracy of thermodynamic models in the computation of nonclassical gasdynamic phenomena is discussed, to investigate their suitability in connection with the design of experimental apparatuses aimed at the observation of nonclassical wavefields. The Soave–Redlich–Kwong and the Peng–Robinson are preliminarily suggested as alternative thermodynamic models to the Martin–Hou usually considered in nonclassical gasdynamics of fluorinated substances. The validity of these models is assessed by comparisons to reference experimental data for fluorinated R13, R125, C318, FC-72, FC-75, and SF6 and to the Martin–Hou model itself. The three models are found to exhibit a comparable accuracy for reduced volumes ranging from 1.4 to 2.5 and near the critical isotherm, i.e., in the thermodynamic region where heavier fluorinated substances such as PP10 are expected to exhibit nonclassical gasdynamic phenomena. The Soave–Redlich–Kwong and the Peng–Robinson models are then used to supplement previous numerical results for a three-discontinuity nonclassical shock-tube experiment with fluid PP10, which was designed under the Martin–Hou model. Under the initial conditions chosen for the experiment, the three models agree in predicting the formation of a supersonic nonclassical rarefaction wave, with wave Mach number in the range 1.01–1.02, thus providing further confidence on the possibility of experimentally observing nonclassical gasdynamic behavior in fluid PP10.
Comparison of thermal and piezoresistive sensing approaches for atomic force microscopy topography measurements
Authors: King William P. , Kenny Thomas W. , Goodson Kenneth E.
Published: September 2004

Atomic force microscope cantilevers with integrated piezoresistive displacement sensors are widely used for nanometer-scale topographic measurement and force sensing. Heated cantilevers used in thermomechanical data storage are a promising alternative for topographic measurement. For both cantilever types, this letter models and predicts cantilever displacement sensitivity and noise-limited displacement resolution. The predictions for the thermal cantilever sensitivity compare well with data. Comparing the thermal cantilever with a similarly sized piezoresistive cantilever, the thermal cantilever provides more than one order of magnitude improved performance in both sensitivity and resolution over the piezoresistive cantilever.
Ferroelectric hysteresis loops of (Pb,Ca)TiO3 thin films under spherical indentation
Authors: Algueró M. , Calzada M. L. , Bushby A. J. , Reece M. J.
Published: September 2004

Ferroelectric switching under indentation forces has been studied for a set of (Pb,Ca)TiO3 ferroelectric thin films. A radius of contact of 10.2μm and an average pressure of 1.5GPa have been estimated by Hertzian theory. The indentation stress field prevents the ferroelectric hysteresis loops from becoming saturated. Ferroelectric switching is hindered and apparent coercive fields as high as 400kVcm−1 were observed. As a consequence, small remnant polarizations were obtained. The effect is proposed to be caused by a combination of a decrease of the electric polarization that can be switched by 180° domain wall movements, as a consequence of stress induced 90° domain wall movements under the indentation force, and the clamping of the 90° domain walls by the indentation stress field.
Arrays of microlenses of complex shapes prepared by reaction-diffusion in thin films of ionically doped gels
Authors: Campbell Christopher J. , Baker Eric , Fialkowski Marcin , Grzybowski Bartosz A.
Published: September 2004

This letter describes a wet-stamping technique for the fabrication of highly regular arrays of microlenses through reaction-diffusion-swelling processes occurring in a thin film of ionically doped gelatin. Geometrical parameters of the lenses depend on and can be controlled by the geometries of the stamped features and the concentrations of chemicals diffusing and reacting in the gelatin matrix. Surface topographies of the lenses are studied experimentally, and are reproduced by a lattice gas reaction-diffusion model.
Electrochemical growth of Co nanodots on patterned Si substrates
Authors: Rastei M. V. , Meckenstock R. , Bucher J. P. , Devaux E. , Ebbesen Th.
Published: September 2004

A silicon substrate, prestructured by a focused ion beam, is used as a nano-electrode template to perform a selective electrodeposition of cobalt nanodots. It is found that the shape of the magnetic nanostructures determines to a large extent their magnetic properties. Using magnetic force microscopy, a transition from in-plane to out-of-plane single domain magnetization is observed when the diameter of the 400nm height dots is reduced from 400 to 200nm. Additional ferromagnetic resonance measurements confirm that the magnetic anisotropy is imposed mainly by the shape of the cylindrical dots.
Microwave liquid-crystal variable phase grating
Authors: Yang Fuzi , Sambles J. R.
Published: September 2004

A voltage-controlled variable phase grating, at microwave frequencies, is described and its response characterized. It comprises a stack of 71 aluminium strips of 1mm thickness separated by 75μm spaces, filled with aligned nematic liquid crystal. For microwaves polarized normal to the grating strips there are a set of resonant transmitted frequencies. By varying the voltages applied across the liquid crystal layers and their distribution, a variable phase microwave grating is realized. This allows low-voltage control of output beam profile and intensity.
Size- and shape-controlled GaN nanocrystals grown on Si(111) substrate by reactive epitaxy
Authors: Wu Chung-Lin , Chou Li-Jen , Gwo Shangjr
Published: September 2004

We report the investigation of coherent GaN nanocrystals spontaneously formed by nitrogen-plasma-assisted reactive epitaxy with Ga droplets supported on the single crystalline Si3N4(0001)∕Si(111) surface. The distribution of grown GaN nanocrystals, as revealed by scanning electron microscopy and cross-sectional transmission electron microscopy, is very uniform in size (∼16nm) and shape and the distribution width is significantly narrower than that of Ga droplets deposited in the Volmer–Weber mode. By using high-resolution electron microscopy, the shape and crystalline structure of the self-assembled GaN nanocrystals can be determined to be truncated triangular pyramids formed by the facets of the GaN wurtzite lattice.
Anharmonic decay of phonons in strain-free wurtzite AlN
Authors: Tischler J. G. , Freitas J. A.
Published: September 2004

We present Raman scattering measurements on high-quality freestanding AlN single crystals. Polarization studies provide clear identification of all allowed phonons. We report the smallest phonon linewidths observed in AlN, which provide a direct measurement of the anharmonic decay of phonons in this material. Also from the Raman mode frequencies and reported index of refraction values we estimated the ordinary and extraordinary dielectric constant values. The calculated values are considerably different from previously reported values of the dielectric constant of AlN.
Effects of Ti–W codoping on the optical and electrical switching of vanadium dioxide thin films grown by a reactive pulsed laser deposition
Authors: Soltani M. , Chaker M. , Haddad E. , Kruzelecky R. V. , Margot J.
Published: September 2004

Thin films of thermochromic VO2, V1−xWxO2 and V1−x−yWxTiyO2 (x=0.014, and y=0.12) were synthesized onto quartz substrates using a reactive pulsed laser deposition technique. The films were then characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The W and Ti dopant effects on the semiconductor-to-metal phase transition of VO2 were investigated by measuring the temperature dependence of their electrical resistivity and their infrared transmittance. Remarkably strong effects of Ti–W codoping were observed on both the optical and electrical properties of V1−x−yWxTiyO2 films. The IR transmittance was improved, while the transition temperature could be varied from 36°C for W-doped VO2 film to 60°C for Ti–W codoped VO2 film. In addition, at room temperature, a higher temperature coefficient of resistance of 5.12%∕°C is achieved. Finally, both optical and electrical hysteresis are completely suppressed by Ti–W codoping the VO2 films.
Part-per-million gas detection from long-baseline THz spectroscopy
Authors: Harmon S. A. , Cheville R. A.
Published: September 2004

We report a long-baseline THz time domain spectrometer based on a White cell design capable of detecting gas species in the low part-per-million range in near real time. Coherent transients from methyl chloride vapor are observed directly in the time domain using a 5.0m path length at pressures down to 1Pa. Both phase sensitive (lock-in) detection and direct signal averaging using a rapid-scanning delay line are used for data acquisition.
Carrier transport in transparent oxide semiconductor with intrinsic structural randomness probed using single-crystalline InGaO3(ZnO)5 films
Authors: Nomura Kenji , Kamiya Toshio , Ohta Hiromichi , Ueda Kazushige , Hirano Masahiro , Hosono Hideo
Published: September 2004

We have investigated carrier transport in a crystalline oxide semiconductor InGaO3(ZnO)5 using single-crystalline thin films. When carrier concentration is less than 2×1018cm−3, logarithm of electrical conductivity decreases in proportion to T−1∕4 and room-temperature Hall mobility was as low as ∼1cm2(Vs)−1. When carrier concentration was increased to 4×1018cm−3, the conduction mechanism changed to degenerate conduction and room-temperature Hall mobility was steeply increased to >10cm2(Vs)−1, showing metal–insulator transition behavior. These results are explained by percolation conduction over distribution of potential barriers formed around conduction band edge. The potential distribution is a consequence of potential modulation originating from random distribution of Ga3+ and Zn2+ ions in the crystal structure of InGaO3(ZnO)5.
Aluminum-induced crystallization of amorphous silicon–germanium thin films
Authors: Gjukic M. , Buschbeck M. , Lechner R. , Stutzmann M.
Published: September 2004

Aluminum-induced layer exchange (ALILE) was used to crystallize amorphous silicon–germanium (a-Si1−xGex) alloys. A bilayer structure of aluminum (Al) and a-Si1−xGex was deposited on quartz substrates and annealed below the eutectic temperature of the binary Al–Ge alloy (420°C). The annealing process results in an almost complete exchange of the two layers and leads to the crystallization of the initially amorphous Si1−xGex thin films. Elastic recoil detection and Raman spectroscopy were used for structural characterization. The polycrystalline Si1−xGex (poly-Si1−xGex) samples show good structural properties over the entire composition range. In particular, no significant phase segregation was observed. Thus, ALILE has a high potential for the fabrication of polycrystalline Si1−xGex layers.
High external efficiency in a monomode full-photonic-crystal laserunder continuous wave electrical injection
Authors: Talneau A. , LeGratiet L. , Gentner J. L. , Berrier A. , Mulot M. , Anand S. , Olivier S.
Published: September 2004

We demonstrate continuous wave with an external efficiency over 0.15W∕A and monomode operation of a full photonic crystal (PhC) laser. The optical confinement and a wavelength selectivity better than 25dB are all ensured by a unique PhC. We take advantage of the feedback on the fundamental mode which exists within the photonic gap for some specific designs of the PhC guide. These designs are calculated using the plane wave expansion method.
Superconducting transition edge sensor using dilute AlMn alloys
Authors: Deiker S. W. , Doriese W. , Hilton G. C. , Irwin K. D. , Rippard W. H. , Ullom J. N. , Vale L. R. , Ruggiero S. T. , Williams A. , Young B. A.
Published: September 2004

We have fabricated a bolometer using a transition-edge sensor (TES) made of Al doped with Mn to suppress the superconducting critical temperature (Tc) of Al from ∼1Kto∼100mK. The resulting detector exhibits low-frequency noise consistent with theory, with a noise-equivalent power of 7.5×10−18W∕√Hz. The addition of Mn impurities did not significantly increase the heat capacity of the TES. In addition, the detector is surprisingly insensitive to applied magnetic fields. The use of AlMn alloy films in arrays of TES detectors has advantages in simplicity of fabrication when compared to traditional bilayer fabrication techniques.
Porous nanotubes of Co3O4: Synthesis, characterization, and magnetic properties
Authors: Wang R. M. , Liu C. M. , Zhang H. Z. , Chen C. P. , Guo L. , Xu H. B. , Yang S. H.
Published: September 2004

Stoichiometric Co3O4 porous nanotubes have been synthesized through a simple modified microemulsion method. The structural and the chemical information of the as-grown nanotubes have been investigated by means of x-ray diffraction, electron microscopy, electron energy loss spectroscopy, and dynamic force microscopy. The results reveal that the as-grown materials are formed by concentric stacking of Co3O4 (111) planes or weaved porous nanotubes with diameters ranging from tens to ∼200nm and sidewall thickness ranging from 2to∼20nm. Magnetic property of the sample demonstrates a magnetic transition temperature at 8.4K, indicating macroscopic quantum confinement effects from the sidewall thickness of the porous nanotube.
Enhancement of organic light-emitting device performances with Hf-doped indium tin oxide anodes
Authors: Chen T.-H. , Liou Y. , Wu T. J. , Chen J. Y.
Published: September 2004

We have enhanced the luminance and the power efficiency of organic light-emitting devices with Hf-doped indium tin oxide (ITO) anodes instead of a CuPc layer. The Hf-doped ITO layer with a thickness of 15nm was deposited on top of the ITO anode. Less than 10mol.% of Hf was doped in ITO films by adjusting the sputtering rates of both sources. The highest work function of the Hf-doped ITO layers was 5.4eV at the Hf concentrations about 10mol.%. The driving voltages of the device have been reduced by 1V. A luminance of 1000cd∕m2 at 7mA∕cm2, a current efficiency of 14cd∕A, and a power efficiency of 6lm∕W at 6mA∕cm2 have been achieved in the device with a 4mol.% Hf-doped ITO layer (work function=5.2eV). In general, the performance was about 50% better than the device with a CuPc buffer layer.
Heteroepitaxy of high-quality Ge on Si by nanoscale Ge seeds grown through a thin layer of SiO2
Authors: Li Qiming , Jiang Ying-Bing , Xu Huifang , Hersee Stephen , Han Sang M.
Published: September 2004

We demonstrate that high-quality Ge can be grown on Si covered with a thin layer of chemical SiO2. When the oxidized Si substrate is exposed to Ge molecular beam, 7-nm-wide seed pads form in the oxide layer and “touchdown” on the underlying Si. Upon continued exposure, Ge selectively grows on the seed pads rather than on SiO2, and the seeds coalesce to form an epitaxial lateral overgrowth (ELO) layer. The Ge ELO layer is characterized by transmission electron microscopy and etch-pit density (EPD). The Ge ELO layer is free of dislocation network, but stacking faults exist near the Ge-SiO2 interface. A fraction of these stacking faults propagate to the surface, resulting in EPD less than 2×106cm−2. The high quality Ge ELO layer is attributed to a high density of nanoscale Ge seed pads interspaced by 2–12-nm-wide SiO2 patches.
Magnetic properties and band structures of half-metal-type Co2CrGa Heusler alloy
Authors: Umetsu R. Y. , Kobayashi K. , Kainuma R. , Fujita A. , Fukamichi K. , Ishida K. , Sakuma A.
Published: September 2004

The saturation magnetic moment Ms and the Curie temperature TC of the half-metal-type Co2CrGa Heusler alloy have been investigated. The value of Ms at 4.2K for the L21-type Co2CrGa alloy is 3.01μB∕f.u., in agreement with the generalized Slater-Pauling line and the theoretical calculation. The Curie temperature TC and the phase-transformation temperature Tt from the L21 to B2-type structure are 495 and 1050K, respectively. The band calculations disclose that the spin polarization is also relatively high in the B2-type structure, although its estimated TC is about 100K lower than that of the L21-type structure.
Probing ion transport at the nanoscale: Time-domain electrostatic force spectroscopy on glassy electrolytes
Authors: Schirmeisen A. , Taskiran A. , Fuchs H. , Roling B. , Murugavel S. , Bracht H. , Natrup F.
Published: September 2004

We have carried out time-domain electrostatic force spectroscopy on two different ion-conducting glasses using an atomic force microscope. We compare the electrostatic force spectroscopic data obtained at different temperatures with macroscopic electrical data of the glasses. The overall consistency of the data shows that electrostatic force spectroscopy is capable of probing the ion dynamics and transport in nanoscopic subvolumes of the samples.
Strain-sensitive size modulations in ZnSe∕ZnS quantum dots grownon GaAs substrates
Authors: Kim Y. G. , Joh Y. S. , Song J. H. , Sim E. D. , Baek K. S. , Chang S. K. , Lee J. I.
Published: September 2004

Strain effects on sizes and emission energies of ZnSe∕ZnS quantum dots (QDs) have been investigated. The initial strain in the ZnSe QD layer was altered by adjusting the thickness of the ZnS buffer. Consistent blueshifts of the ground-state emission from 4 monolayer ZnSe QDs were observed with increasing the thickness of the ZnS buffer from 20 to 40nm. Atomic-force microscopy revealed that the blueshifts are due to a continuous QD size reduction. We estimated the emission energy as a function of the initial strain in the ZnSe QD layer, which shows that the band-gap engineering is possible through the strain modification.
A new experimental design for noncontact giant magnetoresistance measurements using the magnetorefractive effect
Authors: Vopsaroiu M. , Matthew J. A. D. , Thompson S. M.
Published: September 2004

Electrical magnetoresistance measurements of spin dependent materials or devices are very difficult without surface damaging or sample contamination, especially for thin-film giant magnetoresistance (GMR) multilayers. Moreover, the in situ determination of the GMR profile is almost impossible using electrical measurements. We propose a novel experimental design that allows the convenient measurement of magnetoresistance profiles using a noncontact method based on the magnetorefractive effect. This technique is applicable to metallic samples or devices showing magnetotransport properties and is also suitable for in situ measurements. The experiment involves infrared (IR) reflectivity measurements as a function of the applied magnetic field. By introducing IR optical fibers, the experimental setup has been substantially simplified while the need for costly optical components and time consuming alignments has been eliminated. Theoretical simulations of this experiment are also presented prior to the introduction of the proposed design.
Characterization of CsI photocathodes at grazing incidence for use in a unit quantum efficiency x-ray streak camera
Authors: Lowney D. P. , Heimann P. A. , Padmore H. A. , Gullikson E. M. , MacPhee A. G. , Falcone R. W.
Published: September 2004

The performance of CsI photocathodes has been characterized for use with grazing incidence soft x rays. The total electron yield and pulsed quantum efficiency of a CsI photocathode has been measured in a reflection geometry as a function of photon energy (100 eV to 1 keV), angle of incidence, and the electric field between the anode and photocathode. The total electron yield and pulsed quantum efficiency increase as the x-ray penetration depth approaches the secondary electron escape depth. Unit quantum efficiency in a grazing incidence geometry is demonstrated. A weak electric-field dependence is observed for the total yield measurements; while no significant dependence is found for the pulsed quantum efficiency. The effect of the pulse height distribution on the detective quantum efficiency is discussed. Theoretical predictions agree accurately with experiment.
An atom trap system for practical 81Kr dating
Authors: Du X. , Bailey K. , Lu Z.-T. , Mueller P. , O’Connor T. P. , Young L.
Published: September 2004

81 Kr  (t1/2=2.3×105 yr, 81Kr/Kr∼6×10−13) is a long-lived cosmogenic isotope, which is ideal for dating old groundwater and ice in the age range of 50,000 years to 1 million years. Here, we describe the apparatus and performance of an atom-counting system for practical 81Kr dating. This system is based upon the atom trap trace analysis method that was first demonstrated in 1999. Since then, significant improvements have been made to increase the system efficiency and to reduce the required krypton sample size. For a modern krypton gas sample of 100 μl STP, which contains 1.2×106 81Kr atoms, the system can accumulate approximately 240 81Kr counts in 20 h, thereby reaching a counting efficiency of 2×10−4. Detailed studies have been conducted to characterize the performance of this system. This system has been calibrated with a low-level counting method and has been used for 81Kr dating of ancient groundwater from the Nubian Aquifer (Egypt). It can also be used to measure the isotopic abundance of a fission-produced isotope 85Kr (t1/2=10.76 year,85Kr/Kr∼2×10−11).
Comparison of different methods of electron cyclotron emission-correlation radiometry for the measurement of temperature fluctuations in the plasma core
Authors: Watts Christopher , Hartfuss H. J. , Häse M.
Published: September 2004

ECE correlation radiometry is a technique for measuring high frequency electron temperature fluctuations in the plasma core of fusion relevant devices. Two correlation methods, based on different physical principles, have been developed at W7-AS and TEXT-U. Depending on the required application, the two techniques can offer complementary advantages. At W7-AS a comparison of both techniques has been carried out. In all measurements both techniques yield identical results to within measurement error. The advantages of each method are discussed.
Cryogenic amplifier for ∼1 MHz with a high input impedance using a commercial pseudomorphic high electron mobility transistor
Authors: Robinson A. M. , Talyanskii V. I.
Published: September 2004

A cryogenic amplifier for ∼100 kHz to a few MHz is presented which uses a commercially available pseudomorphic GaAs/InGaAs high electron mobility transistor and dissipates less than 0.5 mW in the cryogenic stage. The input-referred voltage noise and current noise of the amplifier at approximately 2 MHz are measured to be approximately 0.7 nV Hz−1/2 and 25 fA Hz−1/2, respectively. A superconducting resonant circuit can be used to provide a high input impedance over a narrow bandwidth, or a low input impedance can be used to yield a large bandwidth. Although suitable for many applications, the amplifier has been developed for measurements of current noise from a high-impedance source, and its long-term stability is such that by averaging over 30 min, it could be used to detect approximately 0.6 fA Hz−1/2.
Evidence of linear lattice expansion and covalency enhancement in rutile TiO2 nanocrystals
Authors: Li Guangshe , Boerio-Goates Juliana , Woodfield Brian F. , Li Liping
Published: September 2004

Lattice variations and bonding characteristics in rutile TiO2 nanocrystals were examined by x-ray diffraction and x-ray photoelectron spectroscopy. With a reduction in the physical dimensions, rutile TiO2 nanocrystals show a linear lattice expansion and an anomalous covalency enhancement in apparent contradiction to the ionicity increase in BaTiO3 and CuO nanocrystals as reported recently by S. Tsunekawa et al. [Phys. Rev. Lett. 2000, 85, 3440] and V. R. Palkar et al. [Phys. Rev. B 1996, 53, 2167]. A surface defect dipole model is proposed to explain these physical phenomena in terms of the strong interactions among the surface dipoles that produce an increased negative pressure. The covalency enhancement is interpreted according to the critical properties of the increased TiO bond lengths in the expanded lattice.
Mechanical tuning of holographic polymer-dispersed liquid crystal reflection gratings
Authors: Holmstrom Scott A. , Natarajan Lalgudi V. , Tondiglia Vincent P. , Sutherland Richard L. , Bunning Timothy J.
Published: September 2004

Holographic polymer-dispersed liquid crystal reflective structures have been formed using a highly elastic photopolymer. The elasticity allows for mechanical tuning of the reflection notch by over 120nm in the visible spectral range using compressive stress. The shift in the central position of the reflection notch is related to the strain in the system and an effective elastic modulus for the structure is obtained.