bluuebunny/crossref_metadata_2025 huggingface dataset:
Three types of stable structures of hydrogenated silicon clusters
Authors: Watanabe M. O. , Murakami H. , Miyazaki T. , Kanayama T.
Published: July 2002

We have studied stable structures of hydrogenated Si clusters grown from silane gas in an ion trap developed for cluster growth. The grown clusters were extracted from the ion trap through two different pathways and were analyzed by mass spectrometers. The Si6Hx+ cations were stably grown, among them clusters with x=1, 7, and 13 were always observed, regardless of the growth conditions or the extraction pathways. The stable structures of these clusters were theoretically investigated. Clusters of x=1, 7, and 13 corresponded to the compact structure, the intermediate structure which has both compact and bulklike configurations, and the bulklike sp3 structure, respectively.
A universal characterization of nonlinear self-oscillation and chaos in various particle-wave-wall interactions
Authors: Lee Hae June , Lee Jae Koo , Hur Min Sup , Yang Yi
Published: July 2002

The comprehensive parameter space of self-oscillation and its period-doubling route to chaos are shown for bounded beam-plasma systems. In this parametrization, it is helpful to use a potentially universal parameter in close analogy with free-electron-laser chaos. A common parameter, which is related to the velocity slippage and the ratio of bounce to oscillation frequencies, is shown to have similar significance for different physical systems. This single parameter replaces the dependences on many input parameters, thus suitable for a simplifying and diagnostic measure of nonlinear dynamical and chaotic phenomena for various systems of particle-wave interactions. The results of independent kinetic simulations verify those of nonlinear fluid simulations.
Single-electron transistors fabricated from a doped-Si film in a silicon-on-insulator substrate
Authors: Sakamoto T. , Kawaura H. , Baba T.
Published: July 2002

We propose doped-thin-Si-film single-electron transistors (DS-SETs), which are fabricated from a highly doped Si film in a silicon-on-insulator substate by electron-beam lithography with a high-resolution resist (calixarene) and dry etching with CF4 gas. Because the structure can be well controlled, the DS-SET with a 45-nm-diam island shows nearly ideal characteristics of SETs with a charging energy of 1.4 meV. The results demonstrate that single-electron tunneling occurs through a single island without any isolated islands formed in potential fluctuations. We also discuss the discreteness of energy levels in a Si island.
A design of planar multi-turn flux transformers for radio frequency SQUID magnetometers
Authors: Zhang Y. , Yi H. R. , Schubert J. , Zander W. , Banzet M. , Braginski A. I.
Published: July 2002

We propose a design for planar multi-turn flux transformer for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. This transformer is integrated with a coplanar resonator, e.g., on a 1 cm2 LaAlO3 substrate. Its pickup loop is connected with two input coils, separate for dc (low-frequency) and rf currents. A double-hole washer SQUID is coupled to these coils in a flip-chip configuration to form a magnetometer. The separation of rf and dc current paths in the transformer made it possible to demonstrate the rf SQUID magnetometer operation.
Characterization of substrate/thin-film interfaces with x-ray microdiffraction
Authors: Noyan I. C. , Jordan-Sweet J. , Liniger E. G. , Kaldor S. K.
Published: July 2002

The (buried) interface between a polycrystalline Al thin-film feature and its substrate (single crystal Si) was characterized with x-ray microdiffraction. Using a focused x-ray beam (effective spot size on the specimen ∼2×12 μm) with the Si 004 reflection, topographic images of the Si around and under the metallization feature were constructed. Comparison with shear-lag model calculations indicate that the interface is not fully coupled despite the absence of surface cracks.
Mono-sized single-wall carbon nanotubes formed in channels of AlPO4-5 single crystal
Authors: Tang Z. K. , Sun H. D. , Wang J. , Chen J. , Li G.
Published: July 2002

An alternative approach to the synthesis of mono-sized and parallel-aligned single-wall carbon nanotubes (SWCNs) is reported. The SWCNs are formed in 0.73 nm sized channels of microporous aluminophosphate crystallites by pyrolysis of tripropylamine molecules in the channels. They are characterized through transmission electron microscopy, polarized Raman scattering, and electrical transport measurements. Our results would open a door to further detailed studies on the intrinsic properties of carbon nanotubes now in progress.
Size dependence of transverse mode structure in oxide-confined vertical-cavity laser diodes
Authors: Hegarty S. P. , Huyet G. , McInerney J. G. , Choquette K. D. , Geib K. M. , Hou H. Q.
Published: July 2002

We analyze the transverse mode structure of selectively oxidized vertical-cavity surface-emitting semiconductor lasers as a function of the aperture size and injection current. Measurements of the spectral splittings, the near and far field sizes and the response to pulsed operation demonstrate that a parasitic thermal lens can be a dominant factor in determining the transverse resonator properties of small (<3 μm) lasers.
Reentrant metal–insulator-type transition induced by high fluence chromium ion implantation of La0.7Ca0.3MnO3 thin films
Authors: Cohen L. F. , de Silva P. S. I. P. N. , Malde N. , Akther Hossain A. K. M. , Thomas K. A. , Chater R. , MacManus-Driscoll J. D. , Tate T. , Mathur N. D. , Blamire M. G. , Evetts J. E.
Published: July 2002

Thin films of colossal magnetoresistance (MR) material La0.7Ca0.3MnO3 were implanted with 200 keV Cr ions over a range of fluence from 1×1013 to 5×1015 ions/cm2. Resistance measurements were made in zero and applied magnetic fields of up to 8 T. At fluences of 1×1014 and 5×1014 Cr+/cm2, the resistance was much greater than in the unimplanted material and the metal–insulator transition temperature was suppressed to values below 20 K. For the highest fluence (5×1015 ions/cm2), a reentrant metal–insulator-type transition was observed and the resistance dropped significantly. Furthermore, improvement in the low-field MR was observed between the virgin and high fluence implanted films for fields less than 500 mT. These results are interpreted in terms of changes in magnetic properties with depth, defect creation, and the influence of oxygen deficiency.
Properties of Ba(Mg1/3Ta2/3)O3 thin films prepared by metalorganic solution deposition technique for microwave applications
Authors: Joshi P. C. , Desu S. B.
Published: July 2002

We report on the properties of Ba(Mg1/3Ta2/3)O3 thin films prepared by the metalorganic solution deposition technique. Bulk Ba(Mg1/3Ta2/3)O3 ceramics have shown excellent dielectric properties at microwave frequencies; however, the high sintering temperature of bulk material is the major obstacle in their use as dielectric resonators to miniaturize microwave circuits. It was possible to obtain an ordered-perovskite phase of 0.3-μm-thick Ba(Mg1/3Ta2/3)O3 films with trigonal symmetry at an annealing temperature of 700 °C, which is much lower than the bulk sintering temperatures. The electrical measurements were conducted on Pt/Ba(Mg1/3Ta2/3)O3/Pt capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 22.2 and 0.009, respectively. The dielectric constant of thin films was comparable to the typical values (εr∼23.5–25) reported for bulk ceramics. The temperature coefficient of capacitance was −145 ppm/°C in the measured temperature range of 25–125 °C. The leakage current density was lower than 10−7 A/cm2 at an applied electric field of 0.5 MV/cm. The high dielectric constant, which is comparable to bulk, low dielectric loss, and good temperature and bias stability suggest the suitability of Ba(Mg1/3Ta2/3)O3 thin films for microwave communications and integrated capacitor applications.
Highly efficient photorefractive polymer-dispersed liquid crystals
Authors: Golemme A. , Kippelen B. , Peyghambarian N.
Published: July 2002

We report on high-resolution photorefractive polymer-dispersed liquid crystals that lead to high diffraction efficiency at lower applied fields compared with photorefractive polymers. The diffraction efficiency reaches 100% internally for 105-μm-thick samples at a field of 8 V/μm. Net optical gain and diffraction efficiencies of 56% are demonstrated in 53-μm-thick devices. Subsecond response times have also been obtained using polymeric matrices with good transport properties.
Tunable wavelength filters with Bragg gratings in polymer waveguides
Authors: Oh Min-Cheol , Lee Hyung-Jong , Lee Myung-Hyun , Ahn Joo-Heon , Han Seon Gyu , Kim Hae-Guen
Published: July 2002

Tunable wavelength filters are demonstrated based on the thermo-optic refractive index change of the polymer waveguide with Bragg reflection grating. For the low-loss waveguide operating around 1.55 μm, fluorinated polymers are incorporated. Bragg reflection gratings are fabricated using a phase mask and a high-index polymer. The Bragg reflector exhibits a narrow bandwidth of less than 1.0 nm, a crosstalk of −20 dB, an insertion loss of 3.2 dB, and a flat-top passband. The peak wavelength of Bragg reflection is shifted over 11 nm with a slight insertion loss change. The thermo-optic tuning efficiency is 22 nm/W and the peak shift is linearly proportional to the heating power.
Interdiffusion and magnetism of the Co/Rh interface studied by magnetization induced second harmonic generation
Authors: Manders F. , Veenstra K. J. , Kirilyuk A. , Rasing Th. , van den Berg H. A. M. , Persat N.
Published: July 2002

The nonlinear magneto-optical response from a Rh/Co/Cu multilayer shows a strong temperature dependence which can be directly related to changes at the Co/Rh interface. The activation energy for these changes can be extracted by in situ magnetization induced second harmonic generation measurements during annealing.
Temperature-dependent recombination in polymer composite light-emitting diodes
Authors: Bozano L. , Tuttle S. E. , Carter S. A. , Brock P. J.
Published: July 2002

We study the temperature dependence of the current–voltage and radiance–voltage curves in double-carrier injected polymer light-emitting devices comprised of poly(2-methoxy,5-(2′-ethyl-hexoxy)–p-phenylene vinylene) (MEH–PPV) and MEH–PPV/SiO2 as the active layer. The quantum efficiency increases significantly as the temperature is decreased in agreement with an increase in the recombination efficiency with decreasing temperature. Moreover, the bimolecular recombination efficiency saturates at low temperatures and high currents to a very high value for both the composite and plain MEH–PPV devices with the nanoparticles serving as charge traps only at moderately low current densities. Finally, we find that the order of magnitude improvement in radiance observed in some polymer/nanoparticle composites is due to an increase in the effective electric field across the device.
Real-time measurement of the evolution of carrier mobility in thin-film semiconductors during growth
Authors: Brenot R. , Vanderhaghen R. , Drévillon B. , Roca i Cabarrocas P.
Published: July 2002

A method which allows real-time microwave mobility measurements in thin-film semiconductors is described. Carriers mobility is determined during growth by combining two diagnostics: time-resolved microwave conductivity (TRMC) and spectroscopic ellipsometry (SE). TRMC provides the product of the number of free carriers, generated by a laser pulse, by their microwave mobility. The number of photogenerated carriers is calculated from real-time SE measurements. Therefore, the mobility of excess carriers in the growing layer can be deduced from TRMC measurements. In order to illustrate this technique, a TRMC setup has been implemented in situ together with real-time SE to analyze the growth of microcrystalline silicon (μc-Si) by radio frequency glow discharge. An increase of the average carrier mobility as a function of the film thickness is observed, which is compared with the increase of the crystalline fraction evidenced by SE.
Growth mode and strain relaxation of InAs on InP (111)A grown by molecular beam epitaxy
Authors: Li Hanxuan , Daniels-Race Theda , Wang Zhanguo
Published: July 2002

Growth mode and strain relaxation of molecular-beam-epitaxy grown InAs/InAlAs/InP (111)A system have been investigated using reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy, and photoluminescence measurements. In direct contrast to the well-studied InAs/GaAs system, our experimental results show that the InAs grown on InAlAs/InP (111)A follows the Stranski–Krastanov mode. Both self-organized InAs quantum dots and relaxed InAs islands are formed depending on the InAs coverage. Intense luminescence signals from both the InAs quantum dots and wetting layer are observed. The luminescence efficiency of (111)A samples is comparable to that of (001) samples, suggesting the feasibility of fabricating quantum dot optoelectronic devices on InP (111)A surfaces.
Observation of intermixing at the buried CdS/Cu(In, Ga)Se2 thin film solar cell heterojunction
Authors: Heske C. , Eich D. , Fink R. , Umbach E. , van Buuren T. , Bostedt C. , Terminello L. J. , Kakar S. , Grush M. M. , Callcott T. A. , Himpsel F. J. , Ederer D. L. , Perera R. C. C. , Riedl W. , Karg F.
Published: July 2002

A combination of x-ray emission spectroscopy and x-ray photoelectron spectroscopy using high brightness synchrotron radiation has been employed to investigate the electronic and chemical structure of the buried CdS/Cu(In, Ga)Se2 interface, which is the active interface in highly efficient thin film solar cells. In contrast to the conventional model of an abrupt interface, intermixing processes involving the elements S, Se, and In have been identified. The results shed light on the electronic structure and interface formation processes of semiconductor heterojunctions and demonstrate a powerful tool for investigating buried interfaces in general.
Effects of surface reconstruction on III–V semiconductor interface formation: The role of III/V composition
Authors: Nosho B. Z. , Weinberg W. H. , Barvosa-Carter W. , Bennett B. R. , Shanabrook B. V. , Whitman L. J.
Published: July 2002

Using molecular-beam epitaxy and in situ scanning tunneling microscopy, we demonstrate how different reconstructions associated with different III–V growth surfaces can create interfacial roughness, and that an understanding of this phenomenon can be used to control the roughness on the atomic scale. Specifically, the different compositions of a clean InAs(001)-(2×4) surface (V/III=0.5 ML/0.75 ML) and an Sb-terminated one (∼1.7 ML/1 ML) cause the InSb-like interfacial surface to have a bilevel morphology. This surface roughness can be eliminated by depositing additional In to exactly compensate for the difference. It is likely that similar types of roughness occur in all heterostructures where the growth surface reconstruction changes at the interfaces, and that a similar procedure will be equally effective at reducing that roughness.
Structural, defect, and device behavior of hydrogenated amorphous Si near and above the onset of microcrystallinity
Authors: Guha S. , Yang J. , Williamson D. L. , Lubianiker Y. , Cohen J. D. , Mahan A. H.
Published: July 2002

High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.
Linear arrays of CaF2 nanostructures on Si
Authors: Viernow J. , Petrovykh D. Y. , Men F. K. , Kirakosian A. , Lin J.-L. , Himpsel F. J.
Published: July 2002

Linear arrays of CaF2 stripes and dots, about 7 nm wide, are fabricated by self-assembly on stepped Si(111). Stripes are grown on a CaF1 passivation layer, dots directly on Si. The stripes have a precision of ±1 nm, are continuous, do not touch each other, and are attached to the top of the step edges. The stripe repulsion and their counter-intuitive attachment are explained via a reversal of the stacking at the CaF2/Si(111) interface. The dot density is 3×1011 cm−2=2 Teradots/in.2. These arrays may serve as masks in nanolithography.
Spin-dependent tunneling junctions with Fe55Ni45 electrodes and in situ resistive measurement of oxide growth
Authors: Wee Andrew T. A. , Sin Kyusik , Wang Shan X.
Published: July 2002

We have fabricated Si/Ta/Fe55Ni45/FeMn/Fe55Ni45/Al2O3/Fe55Ni45/Ta spin-dependent tunneling junctions, in which the tunneling barrier is formed by plasma oxidation. The as-deposited Fe55Ni45/FeMn bilayer has a surface exchange energy of 0.13 erg/cm2. A maximum tunneling magnetoresistive ratio of 16.5% is obtained in the junctions. We also developed an in situ resistance measurement technique to monitor tunneling barrier thickness during plasma oxidation. The plasma oxidation data are fitted to an oxide growth model based on cation hopping current and sputtering.
Temperature evolution of multiple tunnel junction devices made with disordered two-dimensional arrays of metallic islands
Authors: Pépin A. , Vieu C. , Mejias M. , Jin Y. , Carcenac F. , Gierak J. , David C. , Couraud L. , Launois H. , Cordan A. S. , Leroy Y. , Goltzené A.
Published: July 2002

The temperature behavior of multiple tunnel junction (MTJ) devices made with sub-5-nm gold islands is investigated. A smooth decrease of the Coulomb gap with increasing temperatures is observed. The critical temperature beyond which the Coulomb blockade effect is suppressed is found to change as a function both of the average size of the islands and of the size of the two-dimensional (2D) array of islands forming the MTJ. This latter property is attributed to the role of disorder in the 2D array. Results are compared with Monte Carlo simulations of current transport through highly disordered 2D arrays which reproduce the experimental evolution of the Coulomb gap with temperature.
Imaging of microwave intermodulation fields in a superconducting microstrip resonator
Authors: Hu Wensheng , Thanawalla A. S. , Feenstra B. J. , Wellstood F. C. , Anlage Steven M.
Published: July 2002

Nonlinearities give rise to intermodulation distortion in superconducting microwave devices and currently limit their use to low power applications. We have developed a cryogenic imaging technique to spatially resolve intermodulation distortion and used it to image an 8.2 GHz high temperature superconducting Tl2Ba2CaCu2O8 microwave resonator. The images reveal that the fundamental and intermodulation electric fields obey a fixed relation throughout the device. We note that further refinements of intermodulation theory in resonant devices may be required to fully describe the data.
n-type conduction in Ge-doped CuGaSe2
Authors: Schön J. H. , Oestreich J. , Schenker O. , Riazi-Nejad H. , Klenk M. , Fabre N. , Arushanov E. , Bucher E.
Published: July 2002

In order to prepare n-type CuGaSe2 as-grown, p-type CuGaSe2 single crystals were at first doped by Ge implantation. Thermal healing of the implantation damage in vacuum resulted in strong electrical compensation of the material, but not in n-type conduction. This limitation was overcome by annealing of implanted samples in Zn atmosphere, resulting in n-type conduction of CuGaSe2 with a carrier concentration at room temperature of up to 1016 cm−3. The samples were analyzed by photoluminescence, resistivity, and Hall effect measurements. It was found that the Zn–Ge codoping minimizes the formation of Cu vacancies, which act as acceptor levels and lead to self-compensation, by the formation of ZnCu defects. Furthermore, the number of electrically active Ge dopants is increased by a rise of the GeGa concentration compared to the GeCu defect density. The possibility of n-type conduction in Ga-rich CuIn1−xGaxSe2 compounds opens the possibility of the preparation of homojunction photovoltaic devices and might lead to improved solar cell performance of large band-gap chalcopyrites.
Comparison of the k⋅p and direct diagonalization approaches to the electronic structure of InAs/GaAs quantum dots
Authors: Wang L. W. , Williamson A. J. , Zunger Alex , Jiang H. , Singh J.
Published: July 2002

We present a comparison of the 8-band k⋅p and empirical pseudopotential approaches to describing the electronic structure of pyramidal InAs/GaAs self-assembled quantum dots. We find a generally good agreement between the two methods. The most significant differences found in the k⋅p calculation are (i) a reduced splitting of the electron p states (3 vs 24 meV), (ii) an incorrect in-plane polarization ratio for electron-hole dipole transitions (0.97 vs 1.24), and (iii) an over confinement of both electron (48 meV) and hole states (52 meV), resulting in a band gap error of 100 meV. We introduce a “linear combination of bulk bands” technique which produces results similar to a full direct diagonalization pseudopotential calculation, at a cost similar to the k⋅p method.
An apertureless near-field microscope for fluorescence imaging
Authors: Yang T. J. , Lessard Guillaume A. , Quake Stephen R.
Published: July 2002

We describe an apertureless near field microscope for imaging fluorescent samples. Optical contrast is generated by exploiting fluorescent quenching near a metallized atomic force microscope tip. This microscope has been used to image fluorescent latex beads with subdiffraction limit resolution. The use of fluorescence allows us to prove that the contrast mechanism is indeed spectroscopic in origin.
The dependence of arsenic transient enhanced diffusion on the silicon substrate temperature during ultralow energy implantation
Authors: Whelan S. , Van den Berg J. A. , Zhang S. , Armour D. G. , Goldberg R. D.
Published: July 2002

The redistribution of As during high-temperature annealing has been investigated as a function of the Si(100) substrate temperature (−120 °C, +25 °C, and +300 °C) during 2.5 keV implantation (to 1.5×1015atoms/cm2). Each implant produced a damaged near-surface region, the extent of which varied with implant temperature. Samples implanted at each temperature were annealed in a nitrogen ambient with a few percent oxygen for 10 s at 550, 925, and 975 °C. The changes in implant damage and dopant distributions both prior to and following annealing were investigated using medium energy ion scattering and secondary ion mass spectrometry. Transient enhanced diffusion (TED) of the dopant was observed for all implant temperatures after 925 °C annealing with the 25 °C implant showing the deepest diffusion. Between 925 and 975 °C annealing, the As diffusion rate in the 300 °C exceeded that of the 25 °C implant. Significantly, the −120 °C implant displayed less TED of As compared to the higher temperature implants following annealing at 975 °C. The results indicate that the diffusion is affected by the nature of the post-implant damage and the high arsenic concentrations.
Enhancement of soft x-ray emission from a cryogenically cooled Ar gas jet irradiated by 25 fs laser pulse
Authors: Mocek T. , Kim C. M. , Shin H. J. , Lee D. G. , Cha Y. H. , Hong K. H. , Nam C. H.
Published: July 2002

Soft x-ray spectra (40–180 Å) produced by the interaction of 25 fs laser pulses at an intensity of ∼7×1016 W/cm2 with a cryogenically cooled Ar gas jet have been measured. New spectral lines from Ar8+ and Ar9+ charge states appeared with decreasing gas temperature. Nonlinear increase of x-ray line emission from Ar7+, Ar8+, and Ar9+ was observed with cooling, which saturated below certain temperature. The drastic change in the spectrum is attributed to efficient collisional heating and collisional ionization of growing (102–103 atoms) Ar clusters from the cooled jet.
Energy modulation of nonrelativistic electrons with a CO2 laser using a metal microslit
Authors: Bae Jongsuck , Ishikawa Ryo , Okuyama Sumio , Miyajima Takashi , Akizuki Taiji , Okamoto Tatsuya , Mizuno Koji
Published: July 2002

A metal microslit has been used as an interaction circuit between a CO2 laser beam and nonrelativistic free electrons. Evanescent waves which are induced on the slit by illumination of the laser light modulate the energy of electrons passing close to the surface of the slit. The electron-energy change of more than ±5 eV for the 80 keV electron beam has been observed using the 7 kW laser beam at the wavelength of 10.6 μm.
Evidence of interdot electronic tunneling in vertically coupled In0.4Ga0.6As self-organized quantum dots
Authors: Urayama J. , Norris T. B. , Kochman B. , Singh J. , Bhattacharya P.
Published: July 2002

Ultrafast differential transmission spectroscopy with a resonant pump reveals evidence of electronic tunneling among the excited levels of vertically aligned In0.4Ga0.6As self-organized quantum dots. This evidence of tunneling is observed as a rapid spectral redistribution of electrons within a few hundred femtoseconds of optical excitation. Measurements show that this spectral spread is independent of carrier density and, therefore, indicate that carrier–carrier scattering is not the main mechanism for carrier redistribution. Instead, electronic tunneling is responsible for the interdot coupling; tunneling rate calculations agree reasonably with the experiment, supporting this conclusion.
Low-frequency noise reduction in YBa2Cu3O7−δ superconducting quantum interference devices by antidots
Authors: Selders P. , Wördenweber R.
Published: July 2002

It is demonstrated that the low-frequency noise due to vortex motion in high-temperature superconducting quantum interference devices (SQUIDs) in ambient magnetic fields can strongly be reduced by a simple arrangement of antidots patterned into the SQUID. Sputter-deposited YBa2Cu3O7−δ radio-frequency SQUIDs (rf-SQUIDs) with step edge junctions are characterized before and after patterning of antidots in the vicinity of the Josephson junction. No deterioration of the rf-SQUIDs due to the introduction of the antidots can be detected. In contrary, the onset of the increase of the low-frequency noise in an applied magnetic field is shifted from 10 μT for the bare SQUID to 40 (field cooled) and 18 μT (zero-field cooled) for the rf-SQUIDs with antidots. The reduction of low-frequency noise in ambient field is explained by trapping of vortices by the antidots. The comparison of zero-field and field-cooled experiments demonstrates that flux penetrating the washer does not affect the low-frequency noise as long as the vortices are homogeneously distributed and the flux is properly pinned in the vicinity of the junction.
Moment invariants for nonlinear Hamiltonian systems
Authors: Radak Branislav
Published: July 2002

In this paper we study the behavior of the moments of a particle distribution as it is transported through a Hamiltonian system. Functions of moments that remain invariant for an arbitrary nonlinear Hamiltonian system are constructed perturbatively. The perturbation expansion is carried out according to the degree of nonlinearity of the symplectic mapping associated with the Hamiltonian. The existence of these moment invariants is entirely a consequence of the symplectic nature of the flow generated by Hamiltonian systems. These invariants may be of use in accelerator design, in the general area of charged particle beam transport, and in other areas of Hamiltonian dynamics.
Measurement of friction drag force on discontinuous glass and polymer fibers
Authors: Tresso Riccardo , Munoz David R.
Published: July 2002

This article describes the development of a piezoelectric force transducer specially designed to measure the fluctuating drag force experienced by a discontinuous micro fiber immersed in a turbulent airflow. The newly designed measurement tool possesses a fundamental frequency of approximately 940 Hz with a resolution of 10−4 N. A correlation for the friction drag coefficient as a function of the Reynolds number was found. This equation is of similar form but with differing slope and intercept to that previously published for continuous fibers. Although the driving frequency of the flow system was underestimated, resulting in limited utility of the instrument to the fine scale forces imposed on micro fibers, the sensor provided a necessary measurement of average drag forces. It may also be a useful tool for measuring small forces with lower frequency excitation than observed among the micro fibers exposed to a turbulent air stream. This study represents a first attempt at measuring dynamic drag forces on discontinuous micro fibers.
Nonrandom behavior of amphiphilic dimers in solution
Authors: Wu D.-W. , Aranovich G. L. , Donohue M. D.
Published: July 2002

A simple lattice theory is developed for amphiphilic dimers. An analytical solution is derived by taking into account the most important configurations of nearest neighbors. Numerical calculations and Monte Carlo simulations were performed to compare with analytical solutions.
Continuous causal representation of space–time via sphere orderings
Authors: Low Robert J.
Published: July 2002

A space–time can be regarded as a partially ordered set, or poset, where the partial ordering is causal precedence. The question of when this poset can be represented by spheres in Euclidean space is investigated, and it is shown that spaces which can be so represented are rare, if the position and radius of the spheres are required to be continuous functions on the space–time.
Simultaneous measurements of plasma parameters in the Saha Institute of Nuclear Physics Tokamak using a new low cost optoisolator
Authors: Ghosh J. , Chattopadhyay P. K. , Pal R. , Bal B. , Bhattacharya P. S.
Published: July 2002

In the Saha Institute of Nuclear Physics Tokamak (SINP-TOKAMAK), simultaneous measurement of signals from some diagnostics, like, electrostatic probes, etc., is not possible because pickup noises due to the formation of ground and other loops affect them quite severely. To avoid these loops and also to isolate some diagnostics from the ground, we developed a new type of optical isolator using a very low cost optical isolator chip (MC2TE made by Motorola) in a novel configuration. We continuously kept the light emitting diode on and used a feedback arrangement at the receiving side of the photoemitted signal for achieving good isolation for very low signals with reasonably high frequency response and good linearity. The method is highly cost effective in low budget tokamak experiments.
Novel device for the generation of controlled prepulses in a Ti:sapphire laser amplifier chain
Authors: Ziener Ch. , Stobrawa G. , Schwoerer H. , Uschmann I. , Sauerbrey R.
Published: July 2002

For the generation of a well defined prepulse in high intensity laser plasma interactions, we implemented a newly developed prepulse device into a multiterawatt titanium:sapphire amplifier system. The temporal delay between the prepulse and main pulse is variable between −350 ps and + 100 ps. If the laser pulse is focused on a target, the spatial overlap of the two pulses in the focal point is corrected automatically over the entire range. It is possible to attenuate the main pulse as well as the prepulse independently of each other by more than one order of magnitude. We investigated the dependence of the Kα emission of the laser produced plasma of a silicon target on the delay between the prepulse and main pulse. An increase of the yield of the Kα emission by a factor of 2 for a temporal delay of several tens of picoseconds between the prepulse and main pulse was measured.
On a class of invariant coframe operators with application to gravity
Authors: Itin Yakov , Kaniel Shmuel
Published: July 2002

Let a differential 4D-manifold with a smooth coframe field be given. Consider the operators on it that are linear in the second order derivatives and quadratic in the first order derivatives of the coframe, both with coefficients that depend on the coframe variables. The article exhibits the class of operators that are invariant under a general change of coordinates, and, also, invariant under the global SO(1,3)-transformation of the coframe. A general class of field equations is constructed. We display two subclasses in it. The subclass of field equations that are derivable from action principles by free variations and the subclass of field equations for which spherical-symmetric solutions, Minkowskian at infinity, exist. Then, for the spherical-symmetric solutions, the resulting metric is computed. Invoking the geodesic postulate, we find all the equations that are experimentally (by the three classical tests) indistinguishable from Einstein field equations. This family also includes, of course, Einstein equations. Moreover, it is shown, explicitly, how to exhibit it. The basic tool employed in the article is an invariant formulation reminiscent of Cartan’s structural equations. The article sheds light on the possibilities and limitations of the coframe gravity. It may also serve as a general procedure to derive covariant field equations.
Highly sensitive reflection high-energy electron diffraction measurement by use of micro-channel imaging plate
Authors: Saiki Koichiro , Kono Takaomi , Ueno Keiji , Koma Atsushi
Published: July 2002

Micro-channel plate (MCP) was used to intensify the image on the fluorescent screen in reflection high-energy electron diffraction. The primary electron current could be reduced by four orders of magnitude to get the image of the same intensity as in the measurement without MCP. The reduction of electron beam enabled observation of ionic crystal surfaces without causing electrolysis and charging effect.
Reformulation of the standard model in a generalized differential geometry on the discrete space M4×X2
Authors: Okumura Yoshitaka , Mori Kohei
Published: July 2002

The standard model is reconstructed in a generalized differential geometry (GDG) on the product space M4×X2 by reformulating the work of Coquereaux et al. that dealt with the same theme based on the noncommutative geometry (NCG). A GDG on M4×X2 is constructed by adding the basis χn(n=1,2) of a differential form on the discrete space X2 to the ordinary basis dxμ on Minkowski space M4, and so it is a direct generalization of the differential geometry on the continuous manifold. A GDG is a version of NCG. The Yang–Mills–Higgs Lagrangian and the Dirac Lagrangian are reconstructed by using the fermion representation similar to that in a SO(10) grand unified model.
Signs of the cusps in binary lenses
Authors: Bozza V.
Published: July 2002

The cusps of the caustics of any gravitational lens model can be classified into positive and negative ones. This distinction lies on the parity of the images involved in the creation/destruction of pairs occurring when a source crosses a caustic in a cusp. In this paper, we generalize the former definition of the sign of the cusps. Then we apply it to the binary lens. We demonstrate that the cusps on the axis joining the two lenses are positive while the others are negative. To achieve our objective, we combine catastrophe theory, usually employed in the derivation of the properties of caustics, with perturbative methods, in order to simplify calculations and get readable results. Extensions to multiple lenses are also considered.
Generalized smooth and weak-discontinuous unsteady waves
Authors: Sano Yukio , Miyamoto Isamu
Published: July 2002

A theorem of equivalence regarding the weak discontinuity of the solutions (ũ(1),ũ(2),…,ũ(n),ũ) of an underdetermined system of n quasi-linear partial differential equations in one spatial dimension is proven. The theorem demonstrates the theoretical existence of a smooth wave and a weak-discontinuous wave in a generalized unsteady wave, which consists of a shock wave, new (n+1) elementary waves, and a rarefaction wave. A ũ(j)−ũ(i) path has anomalous characteristics such as a peak, an inflection, and a discontinuity in slope.
Effect of the chromophore donor group and ferrocene doping on the dynamic range, gain, and phase shift in photorefractive polymers
Authors: Hendrickx Eric , Van Steenwinckel David , Persoons André , Samyn Celest , Beljonne David , Brédas Jean-Luc
Published: July 2002

We have studied the photorefractive performance of poly(N-vinylcarbazole)-based composites doped with various concentrations of two structurally related dipolar chromophores, at 780 nm. The two chromophores had different electron donor groups, N,N-diethylamine and julolidine, respectively. Complete internal diffraction and gain coefficients >130 cm−1 were obtained for polymers doped with these chromophores. The polymers prepared with the chromophore having the strongest electron donor group, the julolidine group, had the largest dynamic range, but proved to be slower and had a smaller photorefractive phase shift.
Structure sensitivity in the CO oxidation on rhodium: Effect of adsorbate coverages on oxidation kinetics on Rh(100) and Rh(111)
Authors: Hopstaken M. J. P. , Niemantsverdriet J. W.
Published: July 2002

Temperature-programmed reaction spectroscopy has been used to study the surface reaction between CO and O-atoms on Rh(100) and Rh(111) at a range of different adsorbate coverages. Comparison of the reaction on both surfaces in the low coverage regime, where the kinetics can be described by a straightforward Langmuir–Hinshelwood mechanism reveals that the CO oxidation is structure sensitive, with the rate constant being an order of magnitude higher on the Rh(100) than on the Rh(111) surface. As a consequence, the selectivity of the CO+O reaction to CO2 is about 100% on Rh(100), whereas on Rh(111) the oxidation reaction competes with CO desorption. At low CO coverage, CO oxidation is an elementary step on Rh(100) for a broad range of oxygen coverages. We report kinetic parameters Ea=103±5 kJ/mol and ν=1012.7±0.7 for θO=θCO→0 on Rh(100). The activation energy for CO oxidation on Rh(100) decreases continuously with increasing O-coverage. At low coverage (θO<0.25 ML) we attribute this to destabilization of CO, leading to an increase in the CO2 formation rate. At higher coverage (θO>0.25 ML) O-atoms become destabilized as well, as lateral interactions between O-atoms come into play at these coverages. The interactions result in a greatly enhanced rate of reaction at higher coverages.
Structural and thermodynamic description of supercritical argon with ab initio potentials
Authors: Bomont Jean-Marc , Bretonnet Jean-Louis , Pfleiderer Till , Bertagnolli Helmut
Published: July 2002

The self-consistent integral equation method is applied to calculate the structure and the thermodynamic properties of supercritical argon considered in a recent neutron-scattering experiment by Pfleiderer et al. [J. Chem. Phys. 111, 2641 (1999)]. Two kinds of potentials, different in nature, are used for the calculations. One is an empirical standard potential and the other is founded upon ab initio quantum chemical calculations. The small discrepancies between the two approaches are discussed, and the results are compared to the recent measurements for structure and experimental thermodynamic properties as well.
0°–360° bistable nematic liquid crystal display with large dΔn and high contrast
Authors: Xie Z. L. , Zheng C. Y. , Xu S. Y. , Gao H. J. , Kwok H. S.
Published: July 2002

A new mode of 0°–360° bistable twisted nematic (BTN) liquid crystal display is developed by using a parameter space method. This new mode possesses a large dΔn value that provides a possibility of using a 4.8–7 μm cell gap and a 6.5 V operating voltage for the new 0°–360° BTN cells. The high-contrast ratios above 80 within −80°–80° viewing-angle ranges are obtained experimentally in a horizontal direction, and the highest contrast ratio achieved is up to 250. In the contrast ratio, the experiment’s results concur with the simulation results.
Improved efficiency with variational Monte Carlo using two level sampling
Authors: Dewing M.
Published: July 2002

A two level sampling method is applied to variational Monte Carlo (VMC) that samples the one- and two-body parts of the wave function separately. The method is demonstrated on a single Li2 molecule in free space and 32 H2 molecules in a periodic box. This simple modification increases the efficiency of a VMC simulation by up to 72%.
Optical spectra of CuO2 and matrix effect upon its structure
Authors: Caspary N. , Savchenko E. V. , Thoma A. , Lammers A. , Bondybey V. E.
Published: July 2002

Optical absorption and laser-induced fluorescence (LIF) spectra of CuO2 in solid matrices are investigated, and previous visible studies are extended into the infrared. Several new electronic states are observed and their vibrational frequencies determined. The matrix data are compared with the results of recent gas phase CuO2− photodetachment studies, and with ab initio calculations. We also discuss the unusually large matrix effects and medium-induced electronic and vibrational frequency shifts observed for CuO2.
Improvement of the thermal stability of NiSi films by using a thin Pt interlayer
Authors: Liu J. F. , Chen H. B. , Feng J. Y. , Zhu J.
Published: July 2002

The effect of a thin interlayer of Pt on the stability of NiSi films on Si(111) substrates has been investigated. Both x-ray diffraction (XRD) data and sheet resistance measurements show a remarkable improvement in the thermal stability of NiSi due to the Pt interlayer. Detailed study on the XRD data shows PtSi and NiSi form a solid solution following a Vegard’s law. It was found in Ni/Pt/Si samples that a transition in NiSi texture from (200)NiSi‖(111)Si to (002)NiSi‖(111)Si took place before the nucleation of NiSi2, which may contribute to the enhanced stability of NiSi films.
Cross sections for anion formation within solids by electron capture: CF4 embedded in solid Kr
Authors: Nagesha K. , Sanche L.
Published: July 2002

We discuss in detail the considerations necessary to determine the absolute stable anion formation (SAF) cross section for a molecule embedded within the bulk of condensed matter, using the low energy electron transmission (LEET) method. We show that the LEET method is applicable at any energy below the ionization potential of the charge trapping molecule and the first exciton energy of the matrix. As an example, charge trapping due to SAF from CF4 molecules is investigated. In order to generalize the LEET method, we solve in the Appendix a classical electrostatic boundary value problem using the method of images for a charge embedded in a dielectric medium surrounded by two other media of different dielectrics.
Positive definiteness of entropy production in the nonlinear Robertson formalism
Authors: Nettleton R. E.
Published: July 2002

The Robertson formalism, which derives from the Liouville equation via a projection operator a kinetic equation obeyed exactly by the information-theoretic phase space distribution, is used to calculate Ṡ, the time-derivative of the Jaynesian model of entropy. The Robertson kinetic equation involves an operator T̂ for which Robertson gives an expression whose terms must be regrouped to consider times t≫τ, with τ the relaxation time of a variable η(t) having properties typical of fast variables of extended thermodynamics. When driving forces are applied, causing the system to approach a steady state with constant η, one can show that Ṡ≧0 as t→∞ for experimentally-accessible states. In practice, this holds so long as t≫τ, i.e., for the time scale of most measurements in simple fluids. It is also found that Ṡ=S̈(0)t>0 as t→0 for an arbitrary nonequilibrium initial state.
Generalized weak turbulence theory
Authors: Yoon Peter H.
Published: July 2002

In this article we present the derivation of a generalized weak turbulence kinetic equation for unmagnetized collisionless plasmas in a uniform medium. For the sake of simplicity the present formulation assumes longitudinal electrostatic interaction only, and the effects of spontaneous thermal fluctuations are ignored. In spite of these simplifications, the present formalism represents a generalization of the existing weak turbulence theory in that a nonlinear eigenmode excited in a turbulent plasma with frequency close to twice the plasma frequency is incorporated into the discussion. Traditional weak turbulence theory emphasizes various linear and nonlinear interactions among wave modes in quiescent plasmas (i.e., Langmuir and ion-sound waves). In contrast, the present formalism describes linear and nonlinear interactions among Langmuir, ion-sound, and the new nonlinear eigenmode. Nonlinear wave kinetic equations for these modes are systematically derived, and the particle kinetic equation which generalizes the well known quasilinear diffusion equation, is also derived.
Comment on “Quantum confinement effects on the optical and dielectric properties for mesocrystals of BaTiO3 and SrBi2Ta2O9” [J. Appl. Phys. 87, 474 (2000)]
Authors: Scott J. F.
Published: July 2002

It is shown that the effects interpreted as being due to quantum confinement in SrBi2Ta2O9 are unrelated to confinement and that the energy shift from bulk is zero.
Geometric phases for corotating elliptical vortex patches
Authors: Shashikanth B. N. , Newton P. K.
Published: July 2002

We describe a geometric phase that arises when two elliptical vortex patches corotate. Using the Hamiltonian moment model of Melander, Zabusky, and Styczek [J. Fluid Mech. 167, 95–115 (1986)] we consider two corotating uniform elliptical patches evolving according to the second order truncated equations of the model. The phase is computed in the adiabatic setting of a slowly varying Hamiltonian as in the work of Hannay [J. Phys. A 18, 221–230 (1985)] and Berry [Proc. R. Soc. London, Ser. A 392, 45–57 (1984)]. We also discuss the geometry of the symplectic phase space of the model in the context of nonadiabatic phases. The adiabatic phase appears in the orientation angle of each patch—it is similiar in form and is calculated using a multiscale perturbation procedure as in the point vortex configuration of Newton [Physica D 79, 416–423 (1994)] and Shashikanth and Newton [J. Nonlinear Sci. 8, 183–214 (1998)], however, an extra factor due to the internal stucture of the patch is present. The final result depends on the initial orientation of the patches unlike the phases in the works of Hannay and Berry [J. Phys. A 18, 221–230 (1985)]; [Proc. R. Soc. London, Ser. A 392, 45–57 (1984)]. We then show that the adiabatic phase can be interpreted as the holonomy of a connection on the trivial principal fiber bundle π:T2×S1→S1, where T2 is identified with the product of the momentum level sets of two Kirchhoff vortex patches and S1 is diffeomorphic to the momentum level set of two point vortex motion. This two point vortex motion is the motion that the patch centroids approach in the adiabatic limit.
Relationship between the lateral length and thickness of the platelets in naturally occurring strained layer superlattice structures
Authors: Ferguson Ian T. , Norman Andrew G. , Seong Tae-Yeon
Published: July 2002

The molecular beam epitaxial growth of InAs0.5Sb0.5 onto (001) surfaces below 430 °C results in the formation of a “natural” strained layer superlattice (n-SLS). Transmission electron micrographs of 〈110〉 cross sections showed the existence of two different alloy compositions that formed a tetragonally distorted interleaved platelet structure in which the interfaces were highly regular and ran approximately parallel to the growing surface. It is found that the structure of the n-SLS can be changed systematically by carefully controlling the InAs0.5Sb0.5 growth conditions. A simple relationship is derived relating the lateral size of the platelet to its thickness that is found to hold experimentally for n-SLS structures. It is suggested that the n-SLS structure occurs because it corresponds to the minimum free energy configuration of the growing crystal. A similar formalization has been applied to periodic modulations in other material systems.