bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical order induced by ion irradiation in FePt (001) films
Authors: Ravelosona D. , Chappert C. , Mathet V. , Bernas H.
Published: July 2002

We demonstrate that the long-range order parameter S of sputtered FePt (001) films may be improved by using postgrowth He ion irradiation. This was demonstrated both on disordered (S∼0) and partially ordered (S∼0.4) films in which S was increased up to 0.3 and 0.6, respectively. X-ray diffraction analysis showed that these changes are due to irradiation-induced chemical ordering. The changes in the magnetic hysteresis loops correlate with the expected perpendicular magnetic anisotropy increase. This method may find applications in ultrahigh-density magnetic recording.
Time-resolved four-wave mixing in InAs/InGaAs quantum-dot amplifiers under electrical injection
Authors: Borri P. , Langbein W. , Hvam J. M. , Heinrichsdorff F. , Mao M.-H. , Bimberg D.
Published: July 2002

Time-resolved four-wave mixing in an InAs/InGaAs/GaAs electrically pumped quantum-dot amplifier is measured at room temperature for different applied bias currents going from optical absorption to gain of the device. The four-wave mixing signal from 140 fs pulses shows a transition from a delayed photon-echo response in the absorption regime to a prompt free polarization decay in the gain regime. This corresponds to a pronounced reduction of the dephasing time from 250 fs at zero bias to less than 50 fs at the maximum applied current. The four-wave mixing response at transparency of the device shows a composite structure with both photon echo and free-polarization decay. This is a signature of the digital occupation number in quantum dots, resulting at transparency in a signal from dots occupied with either zero or two excitons corresponding to absorption or gain of the dot ground state.
Simple model for calculating the ratio of the carrier capture and escape times in quantum-well lasers
Authors: Romero B. , Arias J. , Esquivias I. , Cada M.
Published: July 2002

We have developed a simple model for the carrier capture and escape processes in quantum-well (QW) lasers, which yields an analytical expression for the ratio of the carrier capture and escape times. It predicts a decrease in the escape time with injected carrier density due to the state filling effect. It also shows an exponential dependence of the escape time on the effective barrier height and on the inverse of the temperature. A comparison between experimental and calculated values for InGaAs/GaAs QW lasers is presented showing a good agreement.
Mercury depletion as a way of changing the emission spectrum of a fluorescent lamp
Authors: Bakker L. P. , Kroesen G. M. W.
Published: July 2002

We present a promising option for changing the emission spectrum of a fluorescent lamp. In a neon/mercury discharge, neon radiation is produced when the mercury density is sufficiently low. Under certain discharge conditions, radial cathaphoresis causes depletion of mercury atoms in the center of the plasma. This effect is especially well pronounced at high electrical currents and low mercury pressures. We measured the color temperature of a fluorescent lamp containing a neon/mercury discharge at several mercury pressures and currents. The color temperature of this lamp varied from 4000 to 2100 K. We also performed ultraviolet absorption measurements. From these measurements, we obtained the mercury atom density profile in the discharge. A significant decrease of the mercury density in the center of the plasma is indeed observed under certain discharge conditions.
Low temperature deposition of nanocrystalline silicon carbide thin films
Authors: Kerdiles S. , Berthelot A. , Gourbilleau F. , Rizk R.
Published: July 2002

Silicon carbide thin films have been deposited by reactive magnetron sputtering in a pure hydrogen plasma at substrate temperatures, Ts, ranging between 100 and 600 °C. The infrared (IR) absorption spectra and the transmission electron microscopy observations reveal an onset of crystallization at Ts as low as 300 °C. The crystalline fraction increases with Ts and reaches a value of about 60% for Ts=600 °C. Both refractive index n and room temperature dark conductivity σd(RT) show quite consistent behaviors with the structural evolution of the layers. Thus n increases from 1.9 to 2.4 and σd(RT) improves by six orders of magnitude when Ts is raised from 100 to 600 °C.
Microwave-infrared double resonance spectroscopy of an OCS molecule inside a He4 droplet
Authors: Grebenev S. , Havenith M. , Madeja F. , Toennies J. P. , Vilesov A. F.
Published: July 2002

The dynamical behavior of single OCS molecules embedded in large (N4=6000) superfluid 4He droplets has been investigated with high resolution double resonance infrared-microwave spectroscopy. The individual lines for the rotational transitions 2↔1, 3↔2, 4↔3, and 5↔4 are clearly resolved in the difference double resonance spectra and reveal a fine structure. This is especially well resolved for the 3↔2 transition which exhibits five separate lines having widths of about 50 MHz and separated by 200 MHz. The possible origin of the splitting is discussed. © 2000 American Institute of Physics.
Analysis of water–ethanol nucleation rate data with two component nucleation theorems
Authors: Vehkamäki Hanna , Ford Ian J.
Published: July 2002

We generalize the second nucleation theorem to multicomponent systems. Nucleation theorems are used to extract the molecular composition and excess internal energy of the critical cluster from experimental nucleation rates in a water–ethanol mixture. The excess internal energy is found to depend only weakly on temperature and to be almost solely a function of the molecular numbers of water and ethanol in the cluster. We estimate the contribution of the kinetic pre-factor to our analysis, and find that it is small in the case of the first theorem, but significant for the second theorem. We find that capillarity approximation fails to predict the experimental critical size and excess energy in this highly nonideal system.
Simultaneous soft x-ray emissivity profile measurements in poloidally separate locations of the Alcator C-Mod edge plasma
Authors: Sunn Pedersen T. , Granetz R. S.
Published: July 2002

Two high resolution edge x-ray imaging diagnostics have been installed in the Alcator C-Mod tokamak. One array measures the radial soft x-ray emissivity profiles at the top of the plasma with 1.2 mm radial resolution, mapped along flux surfaces to the midplane, whereas the other measures the radial soft x-ray emissivity profiles at the outboard edge with 1.5 mm radial resolution mapped to the midplane. The two diagnostics measure the chord brightness profiles, which are then inverted to get soft x-ray emissivity simultaneously with a 12 μs sampling time. This allows us to determine if the soft x-ray emissivity, and therefore the fluorine density, is constant on a flux surface during steady state high confinement mode conditions, as well as during fast transient edge events, such as edge localized modes or transitions from the high confinement mode to the low confinement mode. Measurements are presented showing that the soft x-ray emissivity is not constant on a flux surface, but instead shows a large poloidal variation, contrary to what is assumed in the inversion routine. The effects of the poloidal variation on the inversion accuracy are estimated numerically. It is found that the emissivity is systematically overestimated at the top, and underestimated at the outboard edge, by less than 15%. The width of the x-ray emissivity pedestal is accurate to within 15%, and the location of the pedestal is accurate to within 1 mm. Measurements showing a poloidal propagation delay for the onset of the transition from high confinement mode (H mode) to low confinement mode (L mode) are also presented.
Accuracy of the energy partitioning data obtained by classical trajectory calculations on potential energy surfaces constructed by interpolation: H2CO→H2 + CO as an example
Authors: Sung Bong June , Kim Myung Soo
Published: July 2002

Capability of the classical trajectory calculation on a potential energy surface (PES) constructed by interpolation in reproducing experimental energy partitioning data has been tested. The title reaction, a prototype polyatomic dissociation which has been heavily investigated over the years, has been taken as the example. The product energy partitioning data obtained by scaling the classical trajectory results on PESs constructed at the moderately high quantum chemical levels of Hartree–Fock (HF), second-order Mo/ller–Plesset (MP2), quadratic configuration interaction single double (QCISD), and B3LYP were nearly level-independent and reproduced the experimental data almost quantitatively. The overall scheme, which is systematic, may become an important quantitative tool for the study of the exit channel dynamics in favorable cases.
A newly developed low drift fast response salinity meter
Authors: Bidokhti A. A.
Published: July 2002

Here a new conductivity probe, which is basically a stainless steel tube with a plastic plug with a hole, plugged in one end, is described. The tube is externally insulated and the other electrode is (usually earthed) a metallic slab fixed in the fluid. The electronic used is of a conventional type and uses a 1 kHz signal whose amplitude is modulated by the conductance across the probes terminals. The signal is amplified, rectified, and filtered. The probe is different from the conventional probes, in that the fluid is sucked through the small hole in the probe tip, and only through the fluid in the tip can an electric current flow. Hence, the output and the dynamic response depend, respectively, on the conductivity of the small volume of the fluid in the tip, and how fast it can be replaced. Tests on this and other types of probes, show that this probe has almost no drift and has a satisfactory dynamic response. Probes with different tip geometries have also been tested and show that the frequency response of the probe (typically 200 Hz) does not change significantly. Possible adaptation of such a probe for oceanographic measurements will be discussed. The author has successfully used such a system for laboratory study of turbulence in stratified fluids.
Simple and inexpensive current-driven magnetic multipole plasma homogenizer
Authors: Machima P. , Bilek M. M. M. , Monteiro O. R. , Brown I. G.
Published: July 2002

Magnetic multipole plasma confinement geometries employing permanent magnets are commonly used to confine laboratory plasmas and improve uniformity. Here we describe the design and operation of a particularly simple and low-cost kind of current-driven multipole system that makes use of a similar magnetic field structure but does not require permanent magnets. The magnetic field is generated by current carrying wires attached to the inside wall of a cylindrical supporting structure to provide a magnetic field resembling that of permanent multipole magnetic homogenizers. We demonstrate the use of the current-driven homogenizer to improve the uniformity of the radial density profile of a vacuum arc generated metal plasma. Our configuration of metal plasma gun, macroparticle filter, and current-driven homogenizer was used for thin film deposition, and was able to produce a relatively homogeneous film thickness profile while maintaining a deposition rate comparable to that at the peak point of the freely expanded beam at the same location.
A simplified six-port model in the radio frequency regime
Authors: Blok F. J. , Verschuur J. W. J.
Published: July 2002

A simplified six-port reflectometer design for the high power radio frequency regime is demonstrated. First a new calibration method for the often-used six-port reflectometer design is presented, where one of the signals is proportional to the forward power only. The presented method is much simpler than other methods given in the literature and, in contrast to these methods, it does not use an overdetermined set of calibration constants. Then a second design restriction is applied, namely that one of the other signals is proportional to the reflected power only. It is shown that this will lead to a nonlinear set of equations with only four calibration constants to be determined. By combining these two results the nonlinear method can be linearized. In doing so the number of calibration constants is increased to six, thereby creating an overdetermined system. It will be shown that, based on noise sensitivity and computational effort, this linearized, overdetermined method is preferred over the nonlinear one.
Multichannel ellipsometer for real time spectroscopy of thin film deposition from 1.5 to 6.5 eV
Authors: Zapien J. A. , Collins R. W. , Messier R.
Published: July 2002

A rotating polarizer multichannel ellipsometer has been optimized for operation well into the ultraviolet (UV) spectral range. With this instrument, 132 spectral points in the ellipsometric parameters (ψ, Δ) over the photon energy range from 1.5 eV (827 nm) to 6.5 eV (191 nm) can be collected in a minimum acquisition time of 24.5 ms, corresponding to one optical cycle of the rotating polarizer. Averages over two and 80 optical cycles (obtained in 49 ms and 1.96 s, respectively) give standard deviations in (ψ, Δ) of less than (0.04°, 0.08°) and (0.007°, 0.015°), respectively, for the energy range from 3.5 to 6.0 eV, as determined from successive measurements of a stable thermally oxidized silicon wafer. Key modifications to previous instrument designs include: (i) a tandem in-line Xe/D2 source configuration for usable spectral output from 1.5 to 6.5 eV; (ii) MgF2 Rochon polarizers for high transmission in the UV without the need for optical activity corrections; (iii) a spectrograph with a grating blazed at 250 nm and two stages of internally mounted order-sorting filters; and (iv) nonuniform grouping of the pixels of the photodiode array detector for a more uniform spectral resolution versus photon energy, with energy spreads per pixel group ranging from 0.02 eV at 1.6 eV to 0.05 eV at 5.1 eV. As an example of the application of this instrument, results of real time spectroscopic ellipsometry studies are reported for the deposition of an amorphous silicon nitride thin film by radio-frequency magnetron sputtering onto a silicon wafer substrate.
Experiments and modeling of an instability of an atmospheric pressure arc
Authors: Karasik Max , Zweben S. J.
Published: July 2002

An instability of a free-burning atmospheric pressure ≈200 A carbon arc is investigated experimentally and modeled analytically. The presence of the instability is found to depend critically on cathode dimensions. In particular, for cylindrical cathodes, the instability occurs only for a narrow range of cathode diameters. Cathode spot motion is proposed as the mechanism of the instability. A simple fluid model combining the effect of the cathode spot motion and the inertia of the cathode jet successfully describes the shape of the arc column during low amplitude instability. The amplitude of cathode spot motion required by the model is in agreement with measurements. The average jet velocity required is approximately equal to that found from applied oscillating transverse magnetic field experiments. The primary reasons for cathode spot motion are most likely cathode vaporization and interaction of arc current with the current distribution in the cathode. Cathode surface temperature distribution is likely to be the reason for cathode geometry dependence.
Self-organized states in sustained toroidal pinches
Authors: Rypdal Kristoffer
Published: July 2002

Toroidal pinches sustained longer than the resistive decay time of the configuration will self-organize into a state characterized by relatively weak magnetic fluctuations compared to those responsible for the initial relaxation. The time-averaged field profiles are not adequately described by Taylor’s theory, because the plasma is in a driven-dissipative state where the global magnetic flux and helicity are not inherently constrained. It is proposed here that the time-averaged fields should attain a state of minimum magnetic energy under the constraint of constant Ohmic energy dissipation rate. This theory relates the field profiles to the resistivity profile η(r). In particular, it predicts that j/B→0 at the boundary if η→∞ there, in agreement with experiment and physical intuition. The theory also predicts the pressure profile.
Dispersive rotation of dipoles in amorphous media
Authors: Binks D. J. , West D. P.
Published: July 2002

The dispersive rotation of dipoles under the influence of an applied electric field is described simply and accurately using a time-dependent diffusion coefficient. There is excellent agreement between theory and measurement. This analysis shows that the dispersive rotational dynamic is described well by a power law in time and hence has no characteristic lifetime, unlike the conventional biexponential description. This has important implications for the characterization of the transient response of any electro-optic process or device based on the rotation of dipoles in an amorphous material such as a polymer or glass.
High-gain Compton free electron laser driven by pre-bunched electrons
Authors: Konoplev I. V. , Phelps A. D. R.
Published: July 2002

A theoretical analysis of high-gain Compton free electron laser dynamics when the radio frequency (rf) field evolves self-consistently with single resonant particles representing a pre-bunched electron beam is presented and the single particle phase space is studied. Using a set of universally scaled equations the optimal parameters of excitation for a free electron laser (FEL) in the single particle approximation are found. Using a new approach the relations between the initial rf field amplitude, the output field amplitude and the particle’s initial detuning from resonance are also found. A set of equations describing the self-consistent evolution of the rf field with e-bunch macro-parameters, such as bunch width and bunch mean phase, is derived from the universally scaled equations under the condition of the uniform initial distribution of electrons in the bunch. The analysis of optical field generation and amplification by compact bunches of electrons is provided. The saturation regime and the physical reason for the saturation in the high-gain Compton FEL driven by pre-bunched electrons are studied and discussed.
Observation of modes at frequencies near the second Alfvén gap in the Tokamak Fusion Test Reactor
Authors: Fredrickson E. , Budny R. V. , Darrow D. , Fu G. Y. , Hosea J. , Phillips C. K. , Wilson J. R. , Van Dam J. W.
Published: July 2002

Modes have been observed near the frequency of the second Alfvén gap during off-axis H-minority heating experiments in the circular cross-section Tokamak Fusion Test Reactor. The observation of these modes is surprising in that the second gap, which is generally opened with ellipticity, is expected to be small, of order (r/R)2. A model is proposed in which the second gap is opened by the fast ion beta, which is shown to be able to introduce mode coupling, much as toroidal effects introduce mode coupling for Toroidal Alfvén Eigenmodes (TAE). With the low inferred energy of the fast ion tail (30–50 keV), the fast ion bounce resonance condition is assumed to drive the modes. The modes are seen with and without accompanying TAE mode activity.
Large amplitude vibrations in the X 2A1 state of C2B
Authors: Léonard C. , Chambaud G. , Rosmus P. , Carter S. , Handy N. C. , Wyss M. , Maier J. P.
Published: July 2002

A three-dimensional potential energy function (PEF) of the A12 electronic ground state of C2B has been generated by electronic structure calculations. The PEF possesses a minimum in an isosceles triangular structure which lies 2204 cm−1 below two equivalent minima having linear equilibrium geometry. The barrier height between the minima relative to the triangular structure has been calculated to the 2383 cm−1. The nuclear motion problem has been solved variationally in Jacobi coordinates for J=0 and 1. Ten vibrational states of A1 and nine of B2 symmetry are calculated to lie below the linear minima. The permutational splitting between the (000)+ and (000)− states in the linear C212 11B has been calculated to be 0.064 cm−1, in C1312C11B this is 0.530 cm−1. Above the energy of the barrier to linearity there are large amplitude vibrations with triangular structure character. In the dense stack of such states vibrational modes of the linear structure are discernible, including their permutational splittings.
Phase equilibria in the polydisperse Zwanzig model of hard rods
Authors: Clarke Nigel , Cuesta José A. , Sear Richard , Sollich Peter , Speranza Alessandro
Published: July 2002

We study the phase behavior of the Zwanzig model of suspensions of hard rods, allowing for polydispersity in the lengths of the rods. In spite of the simplified nature of the model (rods are restricted to lie along one of three orthogonal axes), the results agree qualitatively with experimental observations: the coexistence region broadens significantly as the polydispersity increases, and strong fractionation occurs, with long rods found preferentially in the nematic phase. These conclusions are obtained from an analysis of the exact phase equilibrium equations. In the second part of the paper, we consider the application of the recently developed “moment free energy method” to the polydisperse Zwanzig model. Even though the model contains nonconserved densities due to the orientational degrees of freedom, most of the exactness statements (regarding the onset of phase coexistence, spinodals, and critical points) derived previously for systems with conserved densities remain valid. The accuracy of the results from the moment free energy increases as more and more additional moments are retained in the description. We show how this increase in accuracy can be monitored without relying on knowledge of the exact results, and discuss an adaptive technique for choosing the extra moments optimally.
Strong self-focusing in quasi-stationary laser plasmas
Authors: Malka V. , Galloudec N. Renard-Le , Hüller S. , Pesme D. , Amiranoff F. , Baton S. D. , Modena A. , Mounaix Ph. , Rousseaux C. , Salvati M.
Published: July 2002

Collective Thomson scattering imaging has been used to study the propagation and self-focusing processes taking place during the interaction of a nanosecond laser beam with a preionized gas-jet plasma. The experiments have been carried out with a laser beam power PL exceeding greatly the critical power for ponderomotive self-focusing Pc. It has been found that the position of the ion acoustic waves excited by stimulated Brillouin scattering depends only weakly on the initial focal position of the interaction laser beam. These results, together with theoretical and numerical modeling, demonstrate that in such a regime (PL/Pc≫1) self-focusing is the dominant mechanism governing the localization of the interaction processes.
Structure and dynamics at the aluminum solid–liquid interface: An ab initio simulation
Authors: Jesson Ben J. , Madden Paul A.
Published: July 2002

The interface between the [001] face of crystalline aluminum and the coexisting liquid has been studied in an ab initio molecular dynamics simulation using the orbital-free density functional description of the electronic structure. Direct observation of the equilibrium condition gives a melting temperature in excellent agreement with that obtained from the thermodynamic considerations described in the preceding paper [J. Chem. Phys. 113, 5924 (2000)]. With the resolution which can be achieved, no Friedel-type oscillations in the electron density across the interface can be seen. The atomic density profile shows two or three layers extending into the fluid. The first atomic layer beyond that at which the average atomic density falls to the bulk liquid value shows appreciable in-plane order. Monitoring the instantaneous in-plane “scattering intensity” shows that this layer fluctuates in and out of an ordered state on a time scale of picoseconds. In-plane atomic diffusion is slightly faster than interplane diffusion for these first liquid layers.
On the kinetics of diffusion controlled precipitation under microgravity
Authors: Anestiev L. , Froyen L. , van Vugt L.
Published: July 2002

Metal matrix composites have many desirable characteristics for precision components, e.g., light weight, dimensional stability, and high thermal conductivity. The understanding of the mechanisms and processes taking place at their production is of crucial importance, since these factors are important for the properties of the composite and its future application. A theoretical model of the processes at diffusion controlled formation of TiB2 in liquid Al under microgravity has been proposed. The main features of the developed model are: mechanisms controlling the formation of the TiB2; degree of distribution of the dispersed TiB2 particles; and mean size and size distribution of the dispersed TiB2 particles. The results obtained with the aid of the proposed model were verified on our recent experiments at the STS95 space shuttle mission. The model predicts correctly the TiB2 particles distribution along the specimens and the characteristic split of the reaction zone observed experimentally.
Dependence of Al layer growth mode on Cr underlayer thickness in molecular-beam epitaxy of (001) Al/Cr superlattices
Authors: Kingetsu Toshiki , Kamada Yasuhiro , Yamamoto Masahiko
Published: July 2002

It has been found that (001) Al layers grow on Cr layers in the Stranski–Krastanov (SK) mode [T. Kingetsu, Y. Kamada, and M. Yamamoto, J. Appl. Phys. 87, 159 (2000)]. In the present study, the dependence of the growth behavior of (001) Al/Cr superlattices on the thickness of Cr layers in molecular-beam epitaxy was investigated, using in situ reflection high-energy electron diffraction and ex situ x-ray diffraction analyses. The (001) epitaxial growth proceeds with an orientational relationship of Al(001)[010]‖Cr(001)[110], in the cases where the Cr layer thickness is three monolayers or larger. The critical thickness of the SK growth, the maximum thickness of the two-dimensional growth at the early stage, depends on the thickness of the Cr underlayers. The critical thickness is two monolayers when the Cr underlayer thickness is four monolayers or greater, while the critical thickness increases to three monolayers when the Cr underlayer thickness is reduced to three monolayers. Postdeposition evolution of Al three-dimensional islands also depends on the Cr underlayer thickness, although the thickness where the three-dimensional islands start to occur is about three monolayers in both cases. In the cases where the Cr layer thickness is two monolayers or smaller, epitaxial growth of the superlattices is found to be unstable.
A computer-controlled Gerdien atmospheric ion counter
Authors: Aplin K. L. , Harrison R. G.
Published: July 2002

Accurate measurements of atmospheric ions are desirable in order to investigate atmospheric aerosol processes. A cylindrical capacitor ion counter is described which has a flexible computerized control system, to allow selection of ion mobility by changing the bias voltage across the capacitor. Ion measurements obtained correlate well with the ionization rate measured with an adjacent Geiger counter in clean air. Measurements of air ions using the device in current and voltage decay modes show consistent results. The collecting electrode is exposed directly in the air stream to be sampled, eliminating diffusive losses in intake tubes. The system can measure ion concentrations from 20 ions cm−3 (including typical atmospheric concentrations) and can sample ions rapidly. These characteristics enable comprehensive air conductivity measurements to be made.
Modeling of light-emission spectra measured on silicon nanometer-scale diode antifuses
Authors: Akil N. , Houtsma V. E. , LeMinh P. , Holleman J. , Zieren V. , de Mooij D. , Woerlee P. H. , van den Berg A. , Wallinga H.
Published: July 2002

Electroluminescence (EL) spectra of nanoscale diodes formed after gate-oxide breakdown of n+-polysilicon/oxide/p+-substrate metal–oxide–semiconductor capacitors were measured in reverse and forward bias. The nanoscale diodes, called diode antifuses, are created by the formation of a small link between the n+-poly and the p+-substrate with the properties of a diode. A previously published multimechanism model for avalanche emission from conventional silicon p–n junctions is applied to fit the EL spectra in reverse-biased silicon-diode antifuses. The results show that the light from reverse-biased diode antifuses is caused by the same phenomena as in conventional p–n junctions. Forward-bias spectra of the diode antifuses show different shapes when lightly or highly doped p substrates are used. In the case of a lightly doped p substrate, the EL intensity in the forward mode is increased by about two orders of magnitude in the visible-wavelength range with a maximum intensity in the infrared region. A phonon-assisted electron–hole recombination model is applied to fit the low-energy part of emitted spectra. The visible emission is attributed to the Fowler–Nordheim tunneling current through the SiO2, enabled presumably by electron capture into SiO2 trap levels and intraband transition of hot electrons injected into the Si bulk.
Downscaling of Pb(Zr,Ti)O3 film thickness for low-voltage ferroelectric capacitors: Effect of charge relaxation at the interfaces
Authors: Stolichnov I. , Tagantsev A. , Colla E. , Gentil S. , Hiboux S. , Baborowski J. , Muralt P. , Setter N.
Published: July 2002

In this communication we address two issues essential for low-voltage memory applications of ferroelectric thin films: the size effect on polarization switching, and polarization fatigue. According to the proposed concept, both of these phenomena are controlled by local injection of charge into the interfacial layers of the ferroelectric film. In the experimental part of this work, we show that the entrapped charge relaxation can be enhanced by introducing a thin RuO2 layer into the top interface of the Pt/PZT/Pt ferroelectric capacitor. Capacitors prepared in this way using PZT with a 45/55 Zr/Ti ratio show a substantial improvement of fatigue performance and withstand relatively well the thickness downscaling. As a result, these capacitors exhibited good ferroelectric properties for driving voltage amplitudes as low as 0.6–0.8 V. Our results suggest that control of charge relaxation at the interface is a key issue for development of low-voltage ferroelectric capacitors.
Response to “Comment on ‘Thermal nonequilibrium of the shock-compressed state polymers realized by 1 GPa shock waves’ ” [J. Appl. Phys. 88, 2177 (2000)]
Authors: Nagayama Kunihito , Mori Yasuhito
Published: July 2002

In the previous article, we showed that a very large Grüneisen parameter for poly(methylmethacrylate) is estimated for a very large slope in shock-particle velocity Hugoniot below the kink point at 150 m/s. This large gamma corresponds to the intermolecular vibrational mode, which is supposed to be excited almost exclusively at the shock front. Rosenberg claimed that his shock temperature measurement by gauge method showed no deflection, which could be observed across the Hugoniot kink, if the above condition is fulfilled. Thermal energy transfer between intermolecular to intramolecular vibrational modes does not occur in the time scale realized in experiments. Then, thermalization between the nonequilibrium polymer region and the gauge material is expected to have similar or longer relaxation time than stress relaxation. Rosenberg’s temperature records seem to support this scenario.
Ultrathin zirconium silicate film with good thermal stability for alternative gate dielectric application
Authors: Qi Wen-Jie , Nieh Renee , Dharmarajan Easwar , Lee Byoung Hun , Jeon Yongjoo , Kang Laegu , Onishi Katsunori , Lee Jack C.
Published: July 2002

Zirconium silicate (ZrSixOy) films have been sputtered by comagnetron-reactive sputtering. The composition of ZrSixOy has been controlled by adjusting the sputtering powers of Si and Zr targets to achieve various effective dielectric constants. The sputtered silicate layers showed low equivalent oxide thickness of 14.5 Å with a low leakage of 3.3×10−3 A/cm2 at −1.5 V relative to flat band voltage. The silicate films also exhibited good high-temperature stability and smooth interfacial properties on silicon substrate.
On the Horton–Rogers–Lapwood convective instability with vertical vibration: Onset of convection
Authors: Bardan G. , Mojtabi A.
Published: July 2002

We present a numerical and analytical study of diffusive convection in a rectangular saturated porous cell heated from below and subjected to high frequency vibration. The configuration of the Horton–Rogers–Lapwood problem is adopted. The classical Darcy model is shown to be insufficient to describe the vibrational flow correctly. The relevant system is described by time-averaged Darcy–Boussinesq equations. These equations possess a pure diffusive steady equilibrium solution provided the vibrations are vertical. This solution is linearly stable up to a critical value of the stability parameter depending on the strength of the vibration. The solutions in the neighborhood of the bifurcation point are described analytically as a function of the strength of vibration, and the larger amplitude states are computed numerically using a spectral collocation method. Increasing the vibration amplitude delays the onset of convection and may even create subcritical solutions. The majority of primary bifurcations are of a special type of symmetry-breaking bifurcation even if the system is subjected to vertical vibration.
Giant magneto-impedance in heterogeneous microwires
Authors: Vázquez M. , Garcı́a-Beneytez J. M. , Garcı́a J. M. , Sinnecker J. P. , Zhukov A. P.
Published: July 2002

Recent results concerning the giant magneto-impedance effect, GMI, in magnetic microwires having diameter in the range of a few micrometers and heterogeneous character are introduced in this work. Two types of microwires have been studied, namely, (i) magnetic microwires covered by insulating Pyrex glass coating and (ii) copper wires covered by electrodeposited magnetic microtubes. The magnetic anisotropy distribution in each type of microwire is correlated with giant magneto-impedance. While the main source of anisotropy in glass coated microwires is of magnetoelastic origin created by the stresses induced during quenching and by the coating, in the case of the microtubes the origin is the columnar growth during the electrodeposition. The influence on GMI of various treatments that modify the anisotropy is analyzed. Heterogeneous microwires with different thicknesses of insulating coating and of magnetic tube are analyzed.
Response to “Comment on ‘Numerical study of electrical transport in inhomogeneous Schottky diodes’ ” [J. Appl. Phys. 88, 7366 (2000)]
Authors: Osvald J.
Published: July 2002

In his comment [J. Appl. Phys. 88, 7366 (2000)] Tung brings into question the appropriateness of some of the simulation conditions used in J. Appl. Phys. 85, 1935 (1999) and the conclusion taken from the results. This Response explains that the differences in the conclusions between our work and the work of Sullivan et al. [J. Appl. Phys. 70, 7403 (1991)] are caused by the differences in the parameters of the inhomogeneous structures described. It is also shown that the numerical experiments made by Sullivan et al. [J. Appl. Phys. 70, 7403 (1991)] were done for special diode parameters, and they probably did not support such general conclusions as were made.
The electronic structure and electron affinities of higher chlorine oxide radicals ClOx (x=2–4) from photoelectron spectroscopy of ClOx− anions
Authors: Wang Xue-Bin , Wang Lai-Sheng
Published: July 2002

The electronic structure of ClOx (x=2–4) radicals were experimentally investigated using anion photoelectron spectroscopy of the respective anions at several photon energies. The electron affinities of ClO3 and ClO4 were obtained for the first time and were found to be very high, 4.25±0.10 and 5.25±0.10 eV, respectively. Three low-lying excited states were observed for ClO2 with excitation energies of 2.10 eV (2B2), ∼2.60 eV (A12 and A22). The A12 and A22 states were found to be nearly degenerate. Two low-lying states were observed for ClO3 at ∼1.20 eV (2A2) and ∼2.65 eV (2E), whereas no excited state was observed for ClO4 even at our highest photon energy of 157 nm (7.866 eV). The photoelectron spectra were assigned and compared with available theoretical calculations. The excellent agreement between the experimental and theoretical results confirmed the previous calculations.
CO adsorbed on Cu(001): A comparison between local density approximation and Perdew, Burke, and Ernezerhof generalized gradient approximation
Authors: Favot Fabio , Dal Corso Andrea , Baldereschi Alfonso
Published: July 2002

Adsorption energetics, structural and vibrational properties of the Cu(001)–c(2×2)–CO system are studied with periodic slab calculations within density functional theory in the local density (LDA) and generalized gradient (GGA) approximations by using ultrasoft pseudopotentials. The recent expression of Perdew, Burke, and Ernzerhof (PBE) has been used as the GGA functional [Phys. Rev. Lett. 77, 3865 (1996)]. The fully relaxed geometry of the system is determined for different adsorption sites of the CO molecule; unlike LDA, PBE-GGA predicts correctly the on-top adsorption site of the molecule. C–O and Cu–CO bond lengths, stretching frequencies, Cu-layer displacements, chemisorption energies, and work functions are computed under both approximations and are compared to the experimental values. LDA gives accurate values for the C–O equilibrium distance and the corresponding stretching frequency, while PBE-GGA better describes the adsorption bond and all the remaining properties.
Spectroscopy and dynamics of mixtures of water with acetone, acetonitrile, and methanol
Authors: Venables Dean S. , Schmuttenmaer Charles A.
Published: July 2002

Binary mixtures of water with acetone, acetonitrile, and methanol over their entire range of compositions have been studied spectroscopically and by using molecular dynamics (MD) simulations. We report absorption coefficients and indices of refraction over a frequency range from 3 to 55 cm−1, and from 400 to 1200 cm−1. The far-infrared absorption of the mixtures is substantially less than that for ideal mixtures, and Debye time constants calculated from the spectra are longer for the real than for the ideal mixtures. Significant composition dependence is observed in the high frequency librational spectra of the mixtures, and is reproduced by the MD simulations. Single dipole and angular velocity spectra are also reported, as are detailed changes in the hydrogen bonding environment in the mixtures. There is a loss of tetrahedral water structure on mixing, yet water molecules have a strong tendency to aggregate, especially in the acetone and acetonitrile mixtures. Spatial distribution functions are reported for the acetone/water system.
Exploring the OH+CO reaction coordinate via infrared spectroscopy of the OH–CO reactant complex
Authors: Lester Marsha I. , Pond Bethany V. , Anderson David T. , Harding Lawrence B. , Wagner Albert F.
Published: July 2002

A hydrogen-bonded complex of OH with CO is identified along the reaction coordinate for the OH+CO↔HOCO→H+CO2 reaction. The existence of this linear OH–CO complex is established by infrared action spectroscopy, which accesses vibrational stretching and bending modes of the complex. Complementary electronic structure calculations characterize the OH–CO and OH–OC complexes, the transition state for HOCO formation, and the reaction pathways that connect these complexes directly to the HOCO intermediate.
Morphology of porous media studied by nuclear magnetic resonance line shapes and spin–echo decays
Authors: Allen S. G. , Mallett M. J. D. , Strange J. H.
Published: July 2002

The line shape and spin–echo decays of liquids confined in small pores have been studied as a function of saturation. The samples used were of silica with nominal pore diameters of 60, 200, and 500 Å and had been saturated either with cyclohexane or water. The proton nuclear magnetic resonance linewidth and peak shift were measured as a function of saturation. For water samples the peak shift increased and linewidth decreased up to a critical saturation level above which the peak shift decreased and linewidth increased. This critical saturation point occurs when the surface layer is complete, and is related to the morphology of the pores. It provides evidence to suggest that the internal surface of the 200 Å silica sample was more irregular than the 60 Å sample. For the cyclohexane samples the peak shift increased and the linewidth decreased with saturation. The spin–echo amplitude decays were found to be faster than those seen in previous work. This is attributed to diffusion through a range of field gradients caused by susceptibility variations within the system. The nature of such variations depends on the saturation level. The decays were fitted to a stretched exponential function. Values for the average field gradients and the stretching parameter provide a measure of the range of gradients found and have been deduced for each experiment. Most information on the internal pore morphology is found by studying the systems at low levels of water content. The average field gradient and the stretching exponent may be related to the pore morphology.
Rotation in liquid He4 Lessons from a highly simplified model
Authors: Lehmann Kevin K.
Published: July 2002

This paper presents an analysis of a model problem, consisting of two interacting rigid rings, for the rotation of molecules in liquid He4. Due to Bose symmetry, the excitation of the rotor corresponding to a ring of N helium atoms is restricted to states with integer multiples of N quanta of angular momentum. This minimal model shares many of the same features of the rotational spectra that have been observed for molecules in nanodroplets of ≈103–104 helium atoms. In particular, this model predicts, for the first time, the very large enhancements of the centrifugal distortion constants that have been observed experimentally. It also illustrates the different effects of increasing rotational velocity by increases in the angular momentum quantum number or by increasing the rotational constant of the molecular rotor. It is found that a fixed node, diffusion Monte Carlo and a hydrodynamic model provide upper and lower bounds respectively on the size of the effective rotational constant of the molecular rotor when coupled to the helium.
Spherical SiGe quantum dots prepared by thermal evaporation
Authors: Liao Yu-Cheng , Lin Shih-Yen , Lee Si-Chen , Chia Chih-Ta
Published: July 2002

SiGe alloy quantum dots with spherical shape have been synthesized by the thermal evaporation method. The shape and structure of these dots have been investigated. Transmission electron microscopy images show that they have an average diameter of about 15 nm, and their cores are crystalline. The composition of these dots could be extracted from the Raman peak position of the silicon TO phonon.
Ekman decay of a dipolar vortex in a rotating fluid
Authors: Zavala Sansón L. , van Heijst G. J. F. , Backx N. A.
Published: July 2002

The evolution of quasi-two-dimensional (2D) dipolar vortices over a flat bottom in a rotating fluid system is studied numerically, and the main results are experimentally verified. Our aim is to examine the dipole decay due to bottom friction effects. The numerical simulations are based on the 2D physical model derived by Zavala Sansón and van Heijst [J. Fluid Mech. 412, 75 (2000)], which contains nonlinear Ekman terms, associated with bottom friction, in the vorticity equation. In contrast, the conventional 2D model with bottom friction only retains a linear stretching term in the same equation. It is shown that the dipole trajectory is deflected towards the right (i.e., in the anticyclonic direction) when nonlinear Ekman terms are included. This effect is not observed in simulations based on the conventional model, where the dipole trajectory is a straight line. The basic reason for this behavior is the slower decay of the anticyclonic part of the dipole, with respect to the cyclonic one, due to nonlinear Ekman effects. Another important result is the exchange of fluid between the cyclonic part and the ambient, leaving a tail behind the dipole. By means of laboratory experiments in a rotating tank, these results are qualitatively verified.
Gallium K-edge x-ray absorption study on Mg-doped GaN
Authors: Pan Y. C. , Wang S. F. , Lee W. H. , Lee M. C. , Chen W. K. , Chen W. H. , Jang L. Y. , Lee J. F. , Chiang C. I. , Chang H. , Wu K. T. , Lin D. S.
Published: July 2002

Ga K-edge x-ray absorption measurements were employed to investigate Mg-doping effects in GaN samples. Strong polarization-dependent x-ray absorption near-edge structures become less pronounced with increasing doping concentration, indicating the formation of a mixing-phase structure of cubic and hexagonal phases. Analysis of the extended x-ray absorption region of the spectra revealed doping-related defects such as vacancies, substitutions, and interstitial occupations. They were formed anisotropically in the crystal c axis direction and its perpendiculars. Disorderliness arising from phase mix and defects is believed to have lowered the Debye temperature of the doped GaN films and caused the destructive interference of the absorption fine-structure oscillation functions. These effects were taken into account for the observed large coordination number reductions in our samples.
Solid phase epitaxy of implantation-induced amorphous layer in (11̄00)- and (112̄0)-oriented 6H-SiC
Authors: Satoh Masataka , Nakaike Yuuki , Nakamura Tomonori
Published: July 2002

The epitaxial regrowth of the implantation-induced amorphous layer in (11̄00)- and (112̄0)-oriented 6H-SiC has been investigated at annealing temperatures below 800 °C using Rutherford backscattering spectrometry and transmission electron microscopy. The surface region of sample is amorphized by the Ar+ ion implantation with an energy of 100 keV at a dose of 2×1015/cm2. The implantation-induced amorphous layer epitaxially regrows at annealing temperatures above 700 °C without the inclusion of other polytype crystals such as 3C-SiC and twinned defects. The regrowth rate is estimated to be about 0.4 nm/min at 700 °C and is proportional to the annealing temperature with an activation energy of about 3.4 eV for both (11̄00)- and (112̄0)-oriented 6H-SiC.
Alfvén continuum and high-frequency eigenmodes in optimized stellarators
Authors: Kolesnichenko Ya. I. , Lutsenko V. V. , Wobig H. , Yakovenko Yu. V. , Fesenyuk O. P.
Published: July 2002

An equation of shear Alfvén eigenmodes (AE) in optimized stellarators of Wendelstein line (Helias configurations) is derived. The metric tensor coefficients, which are contained in this equation, are calculated analytically. Two numerical codes are developed: the first one, COBRA (COntinuum BRanches of Alfvén waves), is intended for the investigation of the structure of Alfvén continuum; the second, BOA (Branches Of Alfvén modes), solves the eigenvalue problem. The family of possible gaps in Alfvén continuum of a Helias configuration is obtained. It is predicted that there exist gaps which arise due to or are strongly affected by the variation of the shape of the plasma cross section along the large azimuth of the torus. In such gaps, discrete eigenmodes, namely, helicity-induced eigenmodes (HAE21) and mirror-induced eigenmodes (MAE) are found. It is shown that plasma inhomogeneity may suppress the AEs with a wide region of localization.