bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal conductivity, thermal diffusivity, and specific heat of thin samples from transient measurements with hot disk sensors
Authors: Gustavsson Mattias , Karawacki Ernest , Gustafsson Silas E.
Published: July 2002

Transient measurements of thermal conductivity, thermal diffusivity, and specific heat capacity have been performed with hot disk sensors in thin samples of metallic materials. With this new variation of the hot disk method the sample size can be reduced to a volume less than ten cubic centimeters for copper at room temperature. It is also shown that the specific heat capacity can be conveniently measured in transient recordings of slightly longer duration. On comparing with standard values the accuracy turns out to be better than 1% while the precision (standard deviation of the mean from six measurements) on the average is about 0.5% for all values recorded.
Ion implantation doping and isolation of GaN
Authors: Pearton S. J. , Vartuli C. B. , Zolper J. C. , Yuan C. , Stall R. A.
Published: July 2002

N- and p-type regions have been produced in GaN using Si+ and Mg+/P+ implantation, respectively, and subsequent annealing at ∼1100 °C. Carrier activation percentages of 93% for Si and 62% for Mg were obtained for implant doses of 5×1014 cm−2 of each element. Conversely, highly resistive regions (≳5×109 Ω/⧠) can be produced in initially n- or p- type GaN by N+ implantation and subsequent annealing at ∼750 °C. The activation energy of the deep states controlling the resistivity of these implant-isolated materials is in the range 0.8–0.9 eV. These process modules are applicable to the fabrication of a variety of different GaN-based electronic and photonic devices.
Broadband fiber-optic interferometer as ultrasonic transducer suitable for coating adhesion measurements
Authors: Weiss P. , Sigrist M. W.
Published: July 2002

A novel scheme for the measurement and characterization of the adhesion of solid coatings on solid substrates is presented. It represents an appealing alternative to standard methods, which are not satisfactory in many respects. Our technique is based on laser-induced spallation of the coatings under test, combined with an interferometric monitoring of the spallation process which is manifested in the time dependence of the surface displacement of the coating. We induce the spallation of the coating by a shock pulse generated at the backside of the sample by the impact of a Nd:YAG laser pulse. At the coated front surface of the sample, compressive pulses are reflected as tensile pulses. The coating undergoes spallation if the effective tensile stress, which is the superposition of the stresses of the incoming and reflected waves, exceeds the adhesion strength of the coating. In order to measure adhesion strengths, one has to determine the critical tensile stress at the interface corresponding to the threshold of spallation. We present our experimental setup for the laser generation of high-amplitude acoustic waves, which induce the desired spallation of a coating under test. A fiber-optic interferometer has been developed, which features a low noise level of 1 nm and a large bandwidth of 300 MHz necessary for recording the fast transient surface displacements of the sample surface. The performance and versatility of this instrument are analyzed in theory and experiment. An interactive computer program has been developed which allows the correct reconstruction also of large surface displacements from interferometer signals that stretch over many interference fringes.
Three-dimensional analyses for motions of trapped microparticles in the TRISTAN accumulation ring
Authors: Saeki Hiroshi , Momose Takashi , Ishimaru Hajime
Published: July 2002

Signals of high-energy bremsstrahlung different from previous results of periodic pair signals were observed using a gamma-ray detector during microparticle trapping in the TRISTAN accumulation ring. Therefore, more precise three-dimensional analyses for motions of trapped microparticles around the electron beam were carried out. The analyses nearly coincided with the results of observation.
Thermal noise in biomagnetic measurements
Authors: Nenonen Jukka , Montonen Juha , Katila Toiro
Published: July 2002

Studies of weak magnetic fields are generally influenced by magnetic noise emanating from thermal agitation of electric charge (Johnson noise) in electrically conducting materials surrounding the magnetic-field sensor. In this article, the thermal magnetic noise fields generated by slabs with high electric conductivity (copper, aluminum) or high magnetic permeability (mu metal) are studied. The analysis is based both on a previously published phenomenological model and on measurements with an ultrasensitive superconducting magnetometer. Both the spectral densities and spatial correlations of the magnetic field fluctuations are evaluated. The computed correlation coefficients are utilized to develop a practical method for estimating the thermal noise due to thin conducting foils, such as thermal radiation shields in a cryogenic measurement Dewar. Also experiments to reduce the Dewar noise are reported. Finally, estimations are presented for the thermal noise fields arising in the walls of a magnetically shielded room. In practice, thermal magnetic noise, particularly due to the superinsulation in cryogenic Dewars, can be the limiting factor of sensitivity in measurements of weak biomagnetic signals arising in the human heart and brain. The results are useful in the estimation and minimization of the contribution of thermal noise.
A self-calibration technique for a Thomson scattering system
Authors: Smith O. R. P. , Gowers C. , Nielsen P. , Salzmann H.
Published: July 2002

A self-calibration technique, based on the use of two lasers of different wavelengths is described. Such a scheme is essential in the presence of inaccessible optical components, as found on large present day machines. In these devices spectral characteristics may change with time, posing a problem for the calibration of a Thomson scattering diagnostic. From a combination of the scattered spectra generated by the two lasers on the same plasma, the temperature and the unknown, slowly varying spectral transmission of the different channels can be determined. A visible wavelength scattering system on a high-temperature plasma is assumed with laser wavelengths of 800 and 530 nm. Details of the analysis procedure and the results of fitting for the unknown spectral calibrations are presented. A statistical analysis of the error in the fitted calibration curve is included. A practical solution, with reference to ITER, is proposed.
A variable-temperature scanning tunneling microscope capable of single-molecule vibrational spectroscopy
Authors: Stipe B. C. , Rezaei M. A. , Ho W.
Published: July 2002

The design and performance of a variable-temperature scanning tunneling microscope (STM) is presented. The microscope operates from 8 to 350 K in ultrahigh vacuum. The thermally compensated STM is suspended by springs from the cold tip of a continuous flow cryostat and is completely surrounded by two radiation shields. The design allows for in situ dosing and irradiation of the sample as well as for the exchange of samples and STM tips. With the STM feedback loop off, the drift of the tip–sample spacing is approximately 0.001 Å/min at 8 K. It is demonstrated that the STM is well-suited for the study of atomic-scale chemistry over a wide temperature range, for atomic-scale manipulation, and for single-molecule inelastic electron tunneling spectroscopy (IETS).
Ultrafast 1.55 μm all-optical switching using low-temperature-grown multiple quantum wells
Authors: Takahashi R. , Kawamura Y. , Iwamura H.
Published: July 2002

Low-temperature grown surface-reflection all-optical switching has been demonstrated with ultrafast photoresponse (1.5 ps), low switching energy (2 pJ), high-contrast (13 dB), polarization independence, and wide operation wavelength range in the 1.55 μm band using low-temperature-grown Be-doped strained InGaAs/InAlAs multiple quantum wells. The combination of low-temperature growth and Be-doping contributes to the ultrafast photoresponse. Additionally, the introduction of compressive strain and a mirror with 1% reflectivity greatly enhances optical nonlinearities.
Nitrogen containing hydrogenated amorphous carbon for thin-film field emission cathodes
Authors: Amaratunga Gehan A. J. , Silva S. R. P.
Published: July 2002

Field emission measurements using 0.3 μm thick nitrogen containing hydrogenated amorphous carbon films (a-C:H:N) on n++-Si cathodes are reported. Onset emission fields as low as 4 V μm−1 have been obtained using a flat plate anode configuration. Uniform emission is observed over the entire cathode area at current densities below 7×10−2 mA cm−2. At higher current density preferential emission from spots is observed. The spot emission is imaged using the ITO coated plate anode. A model based on the a-C:H:N acting as a space charge interlayer on the n++-Si is proposed to explain the emission at low electric fields.
Electron emission from a single Spindt-type field emitter structure: Correlation of theory and experiment
Authors: Jensen K. L. , Phillips P. M. , Nguyen K. , Malsawma L. , Hor C.
Published: July 2002

The performance characteristics of rf power amplifiers will be significantly enhanced if a cathode source capable of producing current densities greater than 10 A/cm2 under gigahertz modulation can be created. Characterization of single- and multiple-tip arrays is imperative to determine performance characteristics in order to design and implement inductive output amplifiers (IOA): knowledge of the beam spread is paramount in the design of the helix or cavity power extraction region. Nevertheless, a simple analytic model for gated field emitters for understanding the spatial dispersion of the emitted electrons has not emerged. We provide such a model, approximating the tip by a smooth sphere and the gate by a ring of charge (Saturn model), and correlate it with experimental measurements made on a single Spindt-type molybdenum field emitter using a nanofabricated anode whose position from the emitter was determined using laser interferometry. Methods used to correlate theory with experiment are explained, and the dependence of the beam profile on tip sharpness, gate diameter, anode distance, and tip work function are examined. There is good agreement between theory and experiment. Measurement has shown that the rms spread angle is between 15° and 29°.
Synthesis of epitaxial SnxGe1−x alloy films by ion-assisted molecular beam epitaxy
Authors: He Gang , Atwater Harry A.
Published: July 2002

In this letter, we report the synthesis of epitaxial SnxGe1−x/Ge/Si(001) with compositions up to x=0.34 by ion-assisted molecular beam epitaxy with 30–100 eV Ar+ ions produced by an electron cyclotron resonance ionization source with ion to atom flux ratios of the order of unity in the substrate temperature range of 120–200 °C. High flux low energy ion beam irradiation greatly inhibits Sn segregation without interrupting epitaxy.
Organic field-effect transistors with high mobility based on copper phthalocyanine
Authors: Bao Zhenan , Lovinger Andrew J. , Dodabalapur Ananth
Published: July 2002

Organic field-effect transistors that employ copper phthalocyanine (Cu–Pc) as the semiconducting layer can function as p-channel accumulation-mode devices. The charge carrier mobility of such devices is strongly dependent on the morphology of the semiconducting thin film. When the substrate temperature for deposition of Cu–Pc is 125 °C, a mobility of 0.02 cm2/V s and on/off ratio of 4×105 can be obtained. These features along with the highly stable chemical nature of Cu–Pc make it an attractive candidate for device applications.
Effects of electron temperature in high-density Cl2 plasma for precise etching processes
Authors: Samukawa Seiji , Tsukada Tsutomu
Published: July 2002

Silicon etching characteristics are investigated by using radio-frequency (rf) biased ultrahigh frequency (UHF) and other conventional plasmas (electron cycotron resonance plasma, inductive coupled plasma, surface wave plasma) determined by using a Cl2 etchant. The silicon etching rate and its pattern dependence are significantly improved by decreasing the electron temperature when supplying a 600-kHz rf bias. In particular, use of the UHF plasma allows high-rate and microloading-free silicon trench etching. It is suggested that a larger number of negative ions are generated in the UHF plasma because of the extremely low electron temperature. The low-frequency bias accelerates the negative and positive ions alternately to the substrate surface. As a result, the low-frequency biased UHF plasma reduces the charge accumulation on the substrate.
Dielectric properties of strained (Sr,Ca)TiO3/(Ba,Sr)TiO3 artificial lattices
Authors: Tabata Hitoshi , Kawai Tomoji
Published: July 2002

Dielectric superlattices of (Sr,Ca)TiO3/(Ba,Sr)TiO3 have been formed by a pulsed laser ablation technique. The crystal structure is controlled with atomic order accuracy and a large lattice stress of 0.5–1 GPa can be introduced periodically at the interfaces owing to the lattice mismatch of the constituent layers. The (Sr0.3Ba0.7)TiO3/(Sr0.48Ca0.52)TiO3 superlattice shows dramatically large dielectric constant of 900 even at film thickness of 500 Å. The optimum pressure for inducing tetragonality in the (Ba,Sr)TiO3 layers and enhanced dielectric properties occurs at a lattice mismatch of 2.5–3%.
Pure strain effect on reducing the chirp parameter in InGaAsP/InP quantum well electroabsorption modulators
Authors: Yamanaka Takayuki , Wakita Koichi , Yokoyama Kiyoyuki
Published: July 2002

The pure strain dependence of the chirp parameter (linewidth enhancement factor) of electroabsorption in the 1.55 μm window for strain ranging from compression to tension is studied theoretically in InGaAsP strained quantum well (QW) structures. The small-signal chirp parameter for TE polarization is evaluated from calculated electroabsorption (EA) spectra based on k⋅p theory and their Kramers–Krönig transformed refractive index changes. It is found that both compressive and tensile strain in the well layer reduce the chirp parameter. In a tensile-strained QW, almost continuously negative values irrespective of applied electric fields occur at an optimized amount of strain. The compressive-strained QW lowers the chirp parameter to nearly zero or negative values, the amount of the reduction being proportional to strain amount.
Effect of crystallinity on the magnetoresistance in perovskite manganese oxide thin films
Authors: Shreekala R. , Rajeswari M. , Ghosh K. , Goyal A. , Gu J. Y. , Kwon C. , Trajanovic Z. , Boettcher T. , Greene R. L. , Ramesh R. , Venkatesan T.
Published: July 2002

We report our study of the effect of crystallinity on the magnetoresistance in epitaxial and polycrystalline La2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 thin films. Magnetoresistance in epitaxial films exhibits field dependence and temperature dependence similar to bulk single crystals and sintered bulk ceramics. The polycrystalline films exhibit a markedly different behavior. The magnetoresistance in this case shows either a monotonic increase or saturation with decreasing temperature in contrast to that of epitaxial films in which the magnetoresistance peaks close to the ferromagnetic transition temperature. The field dependence in the polycrystalline films is also remarkably different. At low fields, we observe a sharp drop in resistance followed by a more gradual decrease at higher fields. Our data suggest that in addition to the intrinsic magnetoresistance, grain-boundary transport contributes significantly to the magnetoresistance in polycrystalline films.
Temperature-dependent absorption measurements of excitons in GaN epilayers
Authors: Fischer A. J. , Shan W. , Song J. J. , Chang Y. C. , Horning R. , Goldenberg B.
Published: July 2002

Optical absorption measurements were performed on a series of thin GaN epilayers. Sharp spectral features were observed due to the 1s A and B exciton transitions. Using polarization dependent absorption, the C exciton transition was identified. A broad absorption feature was observed at ∼3.6 eV, which is attributed to indirect exciton-phonon absorption. The excitonic structure was found to persist well above room temperature. A fit to the Varshni formula yielded a temperature dependence of E(T)=E(T=0)−11.8×10−4T2(1414+T) eV for the A and B excitons. The exciton absorption linewidth was studied as a function of temperature, indicating that GaN exhibits very large exciton-phonon coupling.
Intermixing and shape changes during the formation of InAs self-assembled quantum dots
Authors: Garcı́a J. M. , Medeiros-Ribeiro G. , Schmidt K. , Ngo T. , Feng J. L. , Lorke A. , Kotthaus J. , Petroff P. M.
Published: July 2002

The initial stages of GaAs overgrowth over self-assembled coherently strained InAs quantum dots (QDs) are studied. For small GaAs coverages (below 5 nm), atomic force microscopy (AFM) images show partially covered island structures with a regular size distribution which are elongated in the [011] direction. Analysis of the AFM profiles show that a large anisotropic redistribution of the island material is taking place during the initial GaAs overgrowth. Short time annealing experiments together with photoluminescence spectroscopy on annealed QDs are consistent with a Ga and In intermixing during the overgrowth. Surface QDs capped with 5 nm or more GaAs show a strong luminescence intensity indicating that surface QDs are remarkably insensitive to surface recombination effects.
Bulk metallic glass matrix composites
Authors: Choi-Yim H. , Johnson W. L.
Published: July 2002

Composites with a bulk metallic glass matrix were synthesized and characterized. This was made possible by the recent development of bulk metallic glasses that exhibit high resistance to crystallization in the undercooled liquid state. In this letter, experimental methods for processing metallic glass composites are introduced. Three different bulk metallic glass forming alloys were used as the matrix materials. Both ceramics and metals were introduced as reinforcement into the metallic glass. The metallic glass matrix remained amorphous after adding up to a 30 vol% fraction of particles or short wires. X-ray diffraction patterns of the composites show only peaks from the second phase particles superimposed on the broad diffuse maxima from the amorphous phase. Optical micrographs reveal uniformly distributed particles in the matrix. The glass transition of the amorphous matrix and the crystallization behavior of the composites were studied by calorimetric methods.
Nonphotolithographic fabrication of organic transistors with micron feature sizes
Authors: Rogers John A. , Bao Zhenan , Raju V. R.
Published: July 2002

This letter describes the use of micromolding in capillaries in combination with screen printing to form organic microstructures for applications in microelectronics. Fabrication of plastic transistors with micron feature sizes demonstrates the approach. The performance of these transistors compares favorably with that of similar devices constructed using conventional methods and inorganic substrates, dielectrics, and conductors.
Mg
x
Zn
1−x
O
as a II–VI widegap semiconductor alloy
Authors: Ohtomo A. , Kawasaki M. , Koida T. , Masubuchi K. , Koinuma H. , Sakurai Y. , Yoshida Y. , Yasuda T. , Segawa Y.
Published: July 2002

We propose a widegap II–VI semiconductor alloy, MgxZn1−xO, for the fabrication of heteroepitaxial ultraviolet light emitting devices based on ZnO. The c-axis oriented MgxZn1−xO films were epitaxially grown by pulsed laser deposition on ZnO epitaxial films and sapphire (0001) substrates using ceramic targets. Solid solution films were prepared with Mg content up to x=0.33, achieving a band gap of 3.99 eV at room temperature. MgO impurity phase segregated at x⩾0.36. Lattice constants of MgxZn1−xO films changed slightly (∼1%), increasing in a axis and decreasing in c-axis direction with increasing x. These films showed ultraviolet photoluminescence at energies from 3.36 (x=0) to 3.87 eV (x=0.33) at 4.2 K.
Optical temperature sensing using upconversion fluorescence emission in Er3+/Yb3+-codoped chalcogenide glass
Authors: dos Santos P. V. , de Araujo M. T. , Gouveia-Neto A. S. , Medeiros Neto J. A. , Sombra A. S. B.
Published: July 2002

Optical temperature sensing using upconversion fluorescence emission in Er3+/Yb3+-codoped Ga2S3:La2O3 chalcogenide glass excited at 1.06 μm is reported. Temperature measurements in the region of 20–225 °C with a resolution of approximately 0.5 °C using excitation powers of a few tens of milliwatts were obtained. The temperature sensing mechanism is independent of variations in the excitation intensity, possible fluctuations of transmission, and utilizes a simple signal detection and processing system. The results also revealed that the glass host material plays an important role in the performance of the sensing system.
Determination of piezoelectric fields in strained GaInN quantum wells using the quantum-confined Stark effect
Authors: Takeuchi Tetsuya , Wetzel Christian , Yamaguchi Shigeo , Sakai Hiromitsu , Amano Hiroshi , Akasaki Isamu , Kaneko Yawara , Nakagawa Shigeru , Yamaoka Yoshifumi , Yamada Norihide
Published: July 2002

We have identified piezoelectric fields in strained GaInN/GaN quantum well p-i-n structures using the quantum-confined Stark effect. The photoluminescence peak of the quantum wells showed a blueshift with increasing applied reverse voltages. This blueshift is due to the cancellation of the piezoelectric field by the reverse bias field. We determined that the piezoelectric field points from the growth surface to the substrate and its magnitude is 1.2 MV/cm for Ga0.84In0.16N/GaN quantum wells on sapphire substrate. In addition, from the direction of the field, the growth orientation of our nitride epilayers can be determined to be (0001), corresponding to the Ga face.
Amorphous silica nanowires: Intensive blue light emitters
Authors: Yu D. P. , Hang Q. L. , Ding Y. , Zhang H. Z. , Bai Z. G. , Wang J. J. , Zou Y. H. , Qian W. , Xiong G. C. , Feng S. Q.
Published: July 2002

We report large-scale synthesis of silica nanowires (SiONWs) using an excimer laser ablation method. Silica was produced in the form of amorphous nanowires at a diameter of ∼15 nm and a length up to hundreds micrometers. The SiONWs emit stable and high brightness blue light at energies of 2.65 and 3.0 eV. The intensity of the emission is two orders of magnitude higher than that of porous silicon. The SiONWs may have potential applications in high-resolution optical heads of scanning near-field optical microscope or nanointerconnections in future integrated optical devices.
Pseudo-Hall effect in spin-valve multilayers
Authors: Li Guohong , Lu Zhengqi , Chai Chunlin , Jiang Hongwei , Lai Wuyan
Published: July 2002

Magnetoresistance and the pseudo-Hall effect in NiFe/Cu/NiFe/FeMn spin-valve multilayers were measured simultaneously in fields rotating in the film plane. Large pseudo-Hall voltages have been observed when the magnetization of the free layer was perpendicular to the sensing current, which was applied along the magnetization of the pinned layer. The pseudo-Hall voltages cannot be explained by treating the anisotropic magnetoresistance of the two permalloy layers independently. Such a cross effect of the free and pinned layers on the anisotropic magnetoresistance is dependent upon the angle between their magnetization.
Origin of nitrogen depth profiles after laser nitriding of iron
Authors: Schaaf P. , Landry F. , Lieb K.-P.
Published: July 2002

In spite of its technological importance, the basic mechanisms of laser nitriding of metals and alloys are hardly understood. The nitrogen depth profiles achieved by laser nitriding of pure iron were measured with high accuracy by resonant nuclear reaction analysis and described by two superimposed diffusion profiles. Using simple estimates, together with the results of marker experiments and laser treatments in N15-isotopically enriched atmospheres, the development of these profiles with the number of pulses can be simulated in excellent agreement with the experimental results.
Ba
(1−x)
Sr
x
TiO
3
thin films by off-axis cosputtering BaTiO3 and SrTiO3
Authors: Cukauskas E. J. , Kirchoefer Steven W. , DeSisto William J. , Pond Jeffrey M.
Published: July 2002

Thin films of Ba(1−x)SrxTiO3 (BST) have been deposited by off-axis cosputtering targets of SrTiO3 (STO) and BaTiO3 (BTO). The films were grown on (100) MgO and LaAlO3 substrates at 550 °C at an argon/oxygen gas pressure of 20 Pa. Reaction of the sputtered species in the plasma resulted in the growth of BST films. The film composition was set by the relative power levels of the STO and BTO targets. X-ray diffractometry indicated the as-deposited films were predominantly (h00) oriented. The calculated lattice parameter was 3.955±0.01 Å, slightly larger than bulk Ba0.5Sr0.5TiO3. Postannealing the films in flowing O2 at 780 °C resulted in greater peak heights and no change in orientation. Capacitance–voltage characteristics measured at 10 GHz yielded Q’s of 65 with nearly 14% tuning for BST on LAO and Q>1000 with a 2% tuning for films on MgO.
Electrochromic mechanism in a-WO3−y thin films
Authors: Lee Se-Hee , Cheong Hyeonsik M. , Zhang Ji-Guang , Mascarenhas Angelo , Benson David K. , Deb Satyen K.
Published: July 2002

The electrochromic mechanism in amorphous tungsten oxide films is studied using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the W6+–O and W6+=O bonds, respectively, and a weaker peak at 220 cm−1 that we attribute to the W4+–O bonds. When lithium or hydrogen ions and electrons are inserted, extra Raman peaks due to W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. Comparison of the Raman spectra of sputtered isotopic a-W16O3−y and a-W18O3−y films confirms these assignments. We conclude that the as-deposited films contain mainly the W4+ and W6+ states, and the W5+ states are generated as a result of reduction of the W6+ states when lithium or hydrogen ions and electrons are inserted. We propose that the optical absorption in the colored films is caused by transitions between the W6+ and W5+, and W5+ and W4+ states.
Room-temperature bonding of lithium niobate and silicon wafers by argon-beam surface activation
Authors: Takagi Hideki , Maeda Ryutaro , Hosoda Naoe , Suga Tadatomo
Published: July 2002

The residual stress originating form the thermal expansion mismatch has been a serious problem in the bonding of piezoelectric crystals onto silicon wafers. The room-temperature bonding method using argon-beam surface activation is applied to the bonding of lithium niobate and silicon. In this method, the surfaces of the specimens are etched by fast argon atom beam and bonded to each other in vacuum. Bonding strength equivalent to that of the bulk material is achieved without any heat treatment. Transmission electron microscope observations show intimate contact at the interface. This method is quite suitable for bonding dissimilar materials with thermal expansion mismatch, because the bonding is performed at room temperature throughout the whole process and no thermal stress is generated.
X-ray imaging and diffraction from surface phonons on GaAs
Authors: Sauer W. , Streibl M. , Metzger T. H. , Haubrich A. G. C. , Manus S. , Wixforth A. , Peisl J. , Mazuelas A. , Härtwig J. , Baruchel J.
Published: July 2002

Surface acoustic waves (SAWs) are excited on the GaAs (001) surface by using interdigital transducers, designed for frequencies of up to 900 MHz. The emitted phonons with wavelengths down to 3.5 μm are visualized and characterized by combined x-ray diffraction techniques. Using stroboscopic topography, the SAW emission of a parallel and a focusing transducer geometry are imaged. High-resolution x-ray diffraction profiles show up to 12 phonon-induced satellite reflections besides the GaAs (004) reflection, with a width of 9 arcsec each. The diffraction pattern is simulated numerically, applying the kinematical scattering theory to a model crystal. From fits to measured diffraction profiles at different excitation voltages, the SAW amplitudes were calculated and found to be in the sub-nm range.
Relationship between sp2 carbon content and E04 optical gap in amorphous carbon-based materials
Authors: Oppedisano C. , Tagliaferro A.
Published: July 2002

A decreasing trend of the optical gap E04 with increasing sp2 carbon content has been noticed in amorphous carbon thin films. This behavior is common to materials grown using different deposition methods and having different composition and local structure. The use of a model of density of states in which the π bands are assumed to be Gaussian shaped allows us to explain such behavior, once the role of network distortion and hydrogen content in determining the width of the bands is taken into account.
Inverse giant magnetoresistance at room temperature in antiparallel biased spin valves and application to bridge sensors
Authors: Marrows C. H. , Stanley F. E. , Hickey B. J.
Published: July 2002

We report a spin-engineering scheme whereby the sign of the giant magnetoresistance in an antiparallel biased spin valve may be selected by varying only layer thicknesses within the structure. The antiferromagnetic coupling in the biased layers leads to either positive exchange bias or positive magnetoresistance in the response of the structure. Hence, spin valves may be fabricated with either positive or negative sensitivities to applied fields in the same sense. When two spin valves of each type are connected in the appropriate Wheatstone bridge configuration, a sensor with four active legs may be constructed. The bias may be set by either the growth field, or by application of uniform field to the entire sensor structure in a single postprocessing anneal step.
Image force effects at contacts in organic light-emitting diodes
Authors: Tutiš E. , Bussac M.-N. , Zuppiroli L.
Published: July 2002

The effect of an image force on the barrier for the injection of the carrier from the electrode into the organic layer is shown to be essential for proper understanding of the metal/organic contact and, in particular, of recent results of internal photoemission experiments for metal/Alq3 interfaces. While the calculation presented does not depend on the details of the carrier transport in the organic layer, the resulting value of the injection barrier is expected to determine the conduction regime of the device.
Electron temperature control by varying size of slits made in a grid
Authors: Kato Kohgi , Shimizu Tetsuji , Iizuka Satoru , Sato Noriyoshi
Published: July 2002

Electron temperature is controlled by varying the length of slits made in a grid immersed in a weakly ionized discharge plasma. The grid, which is kept at floating potential, has six slits in this experiment. With a decrease in the slit length from 6 to 0 cm, the electron temperature decreases from 2.1 to 0.09 eV, being accompanied by an electron-density increase from 0.32×109 to 1.53×109 cm−3 at argon gas pressure of 1.5 mTorr. This method of electron–temperature control is applicable to reactive plasmas in which grids are often covered by insulators.
Observation of an antiparallel magnetic state in Fe3O4/Mn3O4 superlattices
Authors: Chern G. , Horng Lance , Hou T. Y. , Lin M. Z.
Published: July 2002

[ Fe 3 O 4 (20 Å)/Mn 3 O 4 (80 Å)] x20 and [Fe3O4(20 Å)/MgO(80 Å)]x20 superlattices on MgO(001) are fabricated by molecular beam epitaxy in order to compare the magnetic coupling in ferrimagnetic–ferrimagnetic and ferrimagnetic–nonmagnetic systems. The magnetic response is measured as a function of applied-field (−50 to 50 kOe) parallel to the film surface and temperature (5–300 K). A strong reduction of magnetization, from 115 to 45 emu/cm3, is observed only from the Fe3O4/Mn3O4 superlattice at temperature below ∼60 K. This observation indicates that the magnetic moments in two constituents are antiparallel and the Curie temperature (Tc) of Mn3O4 is enhanced for 15 K. In addition, the remanent magnetization shows a compensation point (Tcp) at about 32 K at which the opposing spins are balanced. Detailed magnetic hysteresis loops measured at different temperature further explore magnetic phase transitions as a function of external field and temperature. A possible phase diagram is similar to the previous Gd/Fe multilayered system in that Mn3O4 is parallel and Fe3O4 antiparallel to the applied field below Tcp while Fe3O4 is parallel and Mn3O4 antiparallel to the applied field above Tcp. Moreover, a spin-flop-like phase is observed above a critical external field, H*, ∼10 kOe.
Photon noise and correlations in semiconductor cascade lasers
Authors: Rana Farhan , Ram Rajeev J.
Published: July 2002

A comprehensive model for photon noise and correlations in semiconductor cascade lasers is presented. Photon emission events in different gain sections of cascade lasers are correlated. These correlations are found to be positive and arise because the gain sections are connected electrically. The scaling of photon correlations and intensity noise with the number of cascade sections is discussed. The model presented in this letter is applicable to a variety of cascade laser structures including bipolar interband cascade lasers and unipolar intersubband cascade lasers. For comparison, photon noise and correlations in parallel lasers arrays are also discussed.
Experimental and theoretical scaling laws for transverse diffusive broadening in two-phase laminar flows in microchannels
Authors: Ismagilov Rustem F. , Stroock Abraham D. , Kenis Paul J. A. , Whitesides George , Stone Howard A.
Published: July 2002

This letter quantifies both experimentally and theoretically the diffusion of low-molecular-weight species across the interface between two aqueous solutions in pressure-driven laminar flow in microchannels at high Péclet numbers. Confocal fluorescent microscopy was used to visualize a fluorescent product formed by reaction between chemical species carried separately by the two solutions. At steady state, the width of the reaction–diffusion zone at the interface adjacent to the wall of the channel and transverse to the direction of flow scales as the one-third power of both the axial distance down the channel (from the point where the two streams join) and the average velocity of the flow, instead of the more familiar one-half power scaling which was measured in the middle of the channel. A quantitative description of reaction–diffusion processes near the walls of the channel, such as described in this letter, is required for the rational use of laminar flows for performing spatially resolved surface chemistry and biology inside microchannels and for understanding three-dimensional features of mass transport in shearing flows near surfaces.
Magnetic imaging of NdBa2Cu3Oy thin-film patterns with slots
Authors: Suzuki K. , Li Yijie , Utagawa T. , Tanabe K.
Published: July 2002

We have taken magnetic-field images in a wide temperature range of 4–90 K for square-shaped NdBa2Cu3Oy (Tc=89–92 K) thin-film patterns with an array of slots and a linewidth of 4–40 μm by using a scanning superconducting quantum interference device microscope. The thin-film patterns were cooled below 10 K in a various ambient magnetic field of 0.5–100 μT to investigate the behavior of flux trapping. The number of trapped flux quanta clearly decreases with decreasing the ambient field and the linewidth, and flux quanta are completely excluded from the patterns below the threshold-field strength, which agrees well with the Clem’s criterion [E. Dantsker, S. Tanaka, and J. Clarke, Appl. Phys. Lett. 70, 2037 (1997)]. Moreover, additional flux trapping in the pattern-edge regions is observed by applying a field above 50 μT at temperatures near 77 K.
Spring constant measurement of a bent near-field optical fiber probe
Authors: Chen Sy-Hann , Lin Heh-Nan , Ong Pang-Ming
Published: July 2002

We present a method to determine the spring constant of a bent near-field optical fiber probe based on resonance change. The spring constant of a cylindrical fiber cantilever is first obtained and the fiber is used to measure the mass of a sticky lump around 100 μm in size through resonance change. The known mass is then transferred to the end of a bent fiber probe and its spring constant can be obtained by the same approach. Three different masses have been employed and the spring constant is measured with a total uncertainty of around 10%. The current method has the advantage that particles of uniform sizes and densities are not required.
Strain development and damage accumulation during neon ion implantation into silicon at elevated temperatures
Authors: Cima C. A. , Boudinov H. , de Souza J. P. , Suprun-Belevich Yu. , Fichtner P. F. P.
Published: July 2002

The development of mechanical strain and accumulation of damage in silicon single crystals implanted with Ne ions to doses in the range of 0.1–1.0 ×1017 cm−2 at temperatures from 200 to 600 °C were investigated employing Rutherford backscattering spectrometry, high resolution x-ray diffraction (HRXRD) analysis and cross section transmission electron microscopy (XTEM). Two distinct layers have been found in the implanted material: A near-surface layer (< 0.2 μm thick) where no extended defects are observed and a buried layer (≈0.5 μm thick) containing a dense array of dislocation loops and defect clusters. XTEM analysis revealed a distribution of small spherical cavities presumably filled with Ne, with a diameter <4 nm, extending along the entire depth of the implanted layer. HRXRD studies showed the presence of a positive strain (of expansion), irrespective of the implanted dose and temperature. The findings are discussed in terms of the proposed model which assumes that vacancy-type defects are consumed during the formation of Ne bubbles.
Asynchronous algorithm for integration of reaction–diffusion equations for inhomogeneous excitable media
Authors: Rousseau Guillaume , Kapral Raymond
Published: July 2002

An asynchronous algorithm for the integration of reaction–diffusion equations for inhomogeneous excitable media is described. Since many physical systems are inhomogeneous where either the local kinetics or the diffusion or conduction properties vary significantly in space, integration schemes must be able to account for wide variations in the temporal and spatial scales of the solutions. The asynchronous algorithm utilizes a fixed spatial grid and automatically adjusts the time step locally to achieve an efficient simulation where the errors in the solution are controlled. The scheme does not depend on the specific form of the local kinetics and is easily applied to systems with complex geometries.
Improved external cavity design for cesium D1 (894 nm) diode laser
Authors: Andalkar A. , Lamoreaux S. K. , Warrington R. B.
Published: July 2002

We have developed an external cavity diode laser for use near the cesium D1 transition at 894 nm, producing over 20 mW of single-mode power with a continuous tuning range of up to 25 GHz. Our mechanical design allows simple alignment and optimization of the cavity with very good passive stability, as temperature control of the diode is not coupled with changes in cavity length, and it could easily be used with diodes at other wavelengths. We detail the design and construction, and review the performance of the current system, which has been in operation for several years.
Improved elastic collision modeling in DEGAS 2 for low-temperature plasmas
Authors: Kanzleiter Randall J. , Stotler Daren P. , Karney Charles F. F. , Steiner Don
Published: July 2002

Recent emphasis on low-temperature divertor operations has focused attention on proper treatment of neutral-elastic collisions in low-temperature environments. For like species collisions, as in D++D, quantum mechanical indistinguishability precludes differentiation of small-angle elastic scattering from resonant charge exchange for collision energies <2 eV. The current work improves the low-temperature simulation capabilities of the DEGAS 2 Monte Carlo neutral transport code [D. P. Stotler and C. F. F. Karney, Contrib. Plasma Phys. 34, 392–397 (1994)] by employing a set of quantum mechanical collision cross sections produced by Oak Ridge National Laboratory to provide a comprehensive treatment of ion-neutral elastic collisions. Ion-molecular collisions in the form of D++D2 are included for the first time. An integration technique is utilized that reduces the total collision cross section while keeping the other transport cross sections invariant. The inclusion of ion-molecular elastic collisions results in significant increases in energy exchange between background ions and neutral test species.
Hydrogen bonding and stacking interactions of nucleic acid base pairs: A density-functional-theory based treatment
Authors: Elstner Marcus , Hobza Pavel , Frauenheim Thomas , Suhai Sándor , Kaxiras Efthimios
Published: July 2002

We extend an approximate density functional theory (DFT) method for the description of long-range dispersive interactions which are normally neglected by construction, irrespective of the correlation function applied. An empirical formula, consisting of an R−6 term is introduced, which is appropriately damped for short distances; the corresponding C6 coefficient, which is calculated from experimental atomic polarizabilities, can be consistently added to the total energy expression of the method. We apply this approximate DFT plus dispersion energy method to describe the hydrogen bonding and stacking interactions of nucleic acid base pairs. Comparison to MP2/6-31G*(0.25) results shows that the method is capable of reproducing hydrogen bonding as well as the vertical and twist dependence of the interaction energy very accurately.
Effect of surface roughness on magnetic domain wall thickness, domain size, and coercivity
Authors: Zhao Y.-P. , Gamache R. M. , Wang G.-C. , Lu T.-M. , Palasantzas G. , De Hosson J. Th. M.
Published: July 2002

We study the effect of surface roughness on magnetic domain wall thickness, domain size, and coercivity of thin magnetic films. We show that the roughness increases (decreases) the domain wall thickness and domain size for Bloch walls (Néel walls). The surface roughness affects the domain wall movement and causes the increase of coercivity for Néel walls. The coercivity due to domain rotation for Bloch walls decreases with the increase of roughness. The domain wall thickness, domain size, and coercivity are each related to the demagnetizing factor, which depends on the roughness and type of wall (Bloch wall or Néel wall). The calculated coercivity versus thickness is compared with experimental data of ultrathin Co films, where the thickness dependent roughness parameters are available.
Quantitative compositional analysis of InAs/GaAs quantum dots by scanning transmission electron microscopy
Authors: Zhi D. , Davock H. , Murray R. , Roberts C. , Jones T. S. , Pashley D. W. , Goodhew P. J. , Joyce B. A.
Published: July 2002

We present a quantitative technique for the direct compositional analysis of quantum dots (QDs), in which scanning transmission electron microscopy is applied to a capped InAs/GaAs QD layer in a structure also containing InxGa1−xAs/GaAs quantum well (QW) layers to provide an internal calibration of the In content. By obtaining energy dispersive x-ray analysis line scans through both QWs and QDs, the composition of the QDs can be determined by reference to the known composition of the QWs. In this article the method is described and demonstrated using two InAs/GaAs structures in which the QDs are nominally identical, but with different In composition in the calibration QW layers. We find that the QDs in both structures have an In composition of 65%–67% and the associated wetting layers contain approximately 12% In.
Role of collisions in particle-in-cell modeling of high-density short-pulse laser-plasma interaction
Authors: Weber S. , Bonnaud G. , Gauthier J.-C.
Published: July 2002

One-and-one-half-dimensional particle-in-cell (PIC) modeling with restored short-encounter collisional behavior is used to model the interaction of high-intensity short laser pulses with plasmas. The role of Coulomb collisions in expanding thin plasma targets at solid density is particularly investigated. It is shown that collisions play an important role for plasma expansion and ion acceleration mechanisms, even at high laser intensities.
Relaxation of Bloch oscillations of a magnetic soliton in a nonuniform magnetic field
Authors: Babich I. M. , Kosevich A. M.
Published: July 2002

The influence of damping on the Bloch oscillations arising in the motion of a magnetic soliton in an easy-axis one-dimensional ferromagnet in the presence of a small magnetic field gradient is investigated. The most interesting case, that of solitons with low frequencies and low velocities, is considered. In this case there are two soliton damping regimes. When the magnetic field gradient is greater than a critical value set by the damping, the soliton lifetime is considerably greater than the period of the Bloch oscillations. However, together with these oscillations there arises a translational motion of the soliton due to dissipation. If the magnetic field gradient is smaller than the critical value, then the Bloch oscillations vanish, and the soliton undergoes only translational motion.
A new parametrization of exchange–correlation generalized gradient approximation functionals
Authors: Boese A. Daniel , Handy Nicholas C.
Published: July 2002

A new “HCTH” generalized gradient approximation (GGA) functional is presented. Its 15 parameters have been refined against data from a training set containing 407 atomic and molecular systems. We believe that the much enhanced training set means that the new functional HCTH/407 has a much greater universality than previous GGA functionals. Statistical data is presented for the 407 set for the new functional, as well as other functionals.
Invariant manifold methods for metabolic model reduction
Authors: Roussel Marc R. , Fraser Simon J.
Published: July 2002

After the decay of transients, the behavior of a set of differential equations modeling a chemical or biochemical system generally rests on a low-dimensional surface which is an invariant manifold of the flow. If an equation for such a manifold can be obtained, the model has effectively been reduced to a smaller system of differential equations. Using perturbation methods, we show that the distinction between rapidly decaying and long-lived (slow) modes has a rigorous basis. We show how equations for attracting invariant (slow) manifolds can be constructed by a geometric approach based on functional equations derived directly from the differential equations. We apply these methods to two simple metabolic models.
Characterization of defects in doped InGaAsN grown by molecular-beam epitaxy
Authors: Fleck A. , Robinson B. J. , Thompson D. A.
Published: July 2002

Defects in doped InGaAsN (≈1.5% N) grown by gas source molecular-beam epitaxy are examined through Hall effect measurements. The behavior of the carrier concentration as a function of N content and doping concentration is examined. A Fermi statistics model based upon the experimental results has identified the energy levels and concentrations of three traps in as-grown InGaAsN.
Phase-coherent light scattering spectroscopy. II. Depolarized dynamic light scattering
Authors: Takagi Shinsaku , Tanaka Hajime
Published: July 2002

In our preceding paper, we demonstrate the basic principle of the phase-coherent light scattering (COLS) method, focusing on polarized light scattering. In this paper we describe the principle of depolarized COLS. We explain how to excite coherent orientational motion of molecules in liquids and transverse sound waves in amorphous solids by using the electric-field grating, which is generated by crossing two cw laser beams in the media. We also explain how to observe complex spectra of depolarized light scattering from these anisotropic modes. The advantage of COLS over the conventional method is also discussed.