bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron transport in multiprobe quantum wires anomalous magnetoresistance effects
Authors: Blaikie R. J. , Cumming D. R. S. , Cleaver J. R. A. , Ahmed H. , Nakazato K.
Published: July 2002

The magnetoresistance anomalies that are observed in multiprobe quantum wires (such as quenching of the Hall effect and negative bend resistance) have been investigated using a semiclassical billiard-ball model that includes the effects of diffuse boundary scattering. This modeling predicts that two peaks are expected in the magnetoresistance of a quantum wire in which there is a significant amount of diffuse boundary scattering. One peak is due to diffuse boundary scattering in the wire and the other due to specular boundary scattering in the junctions at either end of the wire. The modeling also predicts that the well-known quenching of the Hall effect and negative bend resistance anomalies are both expected to be enhanced by diffuse boundary scattering. This is explained in terms of the way in which diffuse boundary scattering affects the angular distribution of the electrons entering the junctions in the multiprobe wires. ‘‘Diffuse collimation’’ of the electron distribution occurs, increasing the probability for direct transmission of the electrons across the junctions. Experiments performed on wires fabricated in GaAs/AlxGa1−xAs high-mobility heterostructure material, using implanted p-type gates to provide the lateral confinement, have confirmed the twin-peak structure in the magnetoresistance. Although the diffuse boundary scattering magnetoresistance peak has been observed often before, this is the first unambiguous observation of the junction scattering peak. Other device geometries are investigated using the semiclassical model, and a prediction is made for negative longitudinal resistance in a multiprobe wire in which the voltage probes are shadowed from either the current source or the drain. This phenomenon was experimentally verified with devices fabricated in GaAs/AlxGa1−xAs high-mobility heterostructure material using surface Schottky gates or wet etching to provide the lateral confinement. Thus, the trio of negative resistance effects in multiprobe quantum wires has been completed; in addition to the negative Hall resistance and the negative bend resistance a negative longitudinal resistance has now been measured.
Photoluminescence of CdTe thin films containing a mixed crystal orientation
Authors: Brink D. J. , Kunert H. W.
Published: July 2002

Photoluminescence spectroscopy was employed to study the effect of mixed crystal orientation on defect centers in CdTe thin films grown by metallo-organic chemical vapor deposition on a GaAs substrate. We show that some novel spectral features at 1.36 and 1.32 eV are observed when a substantial degree of both (211) and (331) CdTe are present and that these features disappear gradually if the sample is subjected to a series of low-temperature annealing steps under an inert atmosphere. Optimum annealing conditions, as indicated by the enhancement of the excitonic region, are established. Activation energies are estimated for the 1.59, 1.47, 1.36, and 1.32 eV bands and quantitative measurements are provided for the shift in transition energy due to temperature and annealing procedure.
Study of effects of interelectrode spacing and preheating of source gases on hydrogenated amorphous silicon films prepared at high growth rates
Authors: Das Debabrata , Chattopadhyay S. , Barua A. K. , Banerjee Ratnabali
Published: July 2002

The effect of interelectrode spacing on the properties of hydrogenated amorphous silicon (a-Si:H) films grown at high radio-frequency (rf) power density by rf plasma enhanced chemical vapor deposition method, with control of dusty plasma conditions by heating both the electrodes, was investigated. The formation of precursors responsible for gas phase polymerization itself was sought to be controlled by preheating of the source gas mixture. Optimization of the interelectrode spacing for film characteristics was carried out for this novel deposition technique combining cathode heating and preheating of the source gases. The films were characterized by infrared vibrational spectroscopy, absorption and reflection measurements in the visible and near infrared regions, measurements of dark and photo-conductivity (with light induced degradation), and electron spin resonance spectroscopy.
Femtosecond photoinduced dichroism in polydiacetylene 4bcmu film
Authors: Zheng L. X. , Feng Z. G. , Knopf F. Carl
Published: July 2002

Femtosecond pulses were used to study the photoexcitation dynamics and transient photoinduced dichroism in polydiacetylene 4BCMU film. Ultrafast photoinduced dichroism and optical Kerr gate response were observed. These measurements allowed estimation of the diffusion constant (D∼0.1 cm2/s), nonlinear index of refraction (n2∼10−8 esu), and third-order susceptibility (Δχ(3)∼6 × 10−10 esu), in the film.
Electronic consequences of random layer-thickness fluctuations in AlAs/GaAs superlattices
Authors: Mäder Kurt A. , Wang Lin-Wang , Zunger Alex
Published: July 2002

We study the effects of a few types of atomic disorder on the electronic and optical properties of AlAs/GaAs (001) and (111) superlattices: (i) atomic intermixing across the interfaces; (ii) replacing a single monolayer in a superlattice by one containing the opposite atomic type (isoelectronic δ doping); and (iii) random layer-thickness fluctuations in superlattices (SL). Type (i) is an example of lateral disorder, while types (ii) and (iii) are examples of vertical disorder. Using three-dimensional empirical pseudopotential theory and a plane-wave basis, we calculate the band gaps, electronic wave functions, and optical matrix elements for systems containing up to 2000 atoms in the computational unit cell. Spin-orbit interactions are omitted. Computationally much less costly effective-mass calculations are used to evaluate the density of states and eigenstates away from the band edges in vertically disordered SLs. Our main findings are: (i) Chemical intermixing across the interface can significantly shift the SL energy levels and even change the identity (e.g., symmetry) of the conduction-band minimum in AlAs/GaAs SLs; (ii) any amount of thickness fluctuations in SLs leads to band-edge wave-function localization; (iii) these fluctuation-induced bound states will emit photons at energies below the ‘‘intrinsic’’ absorption edge (red shift of photoluminescence); (iv) monolayer fluctuations in thick superlattices create a gap level whose energy is pinned at the value produced by a single δ layer with ‘‘wrong’’ thickness; (v) (001) AlAs/GaAs SLs with monolayer thickness fluctuations have a direct band gap, while the ideal (001) superlattices are indirect for n<4; (vi) there is no mobility edge for vertical transport in a disordered superlattice, because all the states are localized; however, the density of states retains some of the features of the ordered-superlattice counterpart. We find quantitative agreement with experiments on intentionally disordered SLs [A. Sasaki, J. Cryst. Growth 115, 490 (1991)], explaining the strong intensity and large red shift of the photoluminescence in the latter system. We provide predictions for the case of unintentional disorder.
Influence of Cr3+-substitution on the electrical and magnetic properties of Ni1.05Sn0.05Fe1.9O4 ferrites
Authors: Babbar V. K. , Chandel J. S.
Published: July 2002

The effect of Cr3+-substitution on dc resistivity, dielectric constant, initial permeability, saturation magnetization, and Curie temperature have been studied for Ni1.05Sn0.05CrxFe1.9−xO4 ferrites, wherein x varies from 0.0 to 0.5 in steps of 0.1. The dc resistivity increases slowly for x≤0.3 followed by a rapid increase for x≳0.3 and has been explained on the basis of Verwey hopping mechanism and dilution effect of Cr3+ ions. The behavior of dielectric constant for x≤0.4 is attributed to interfacial polarization and follows the Koops model, whereas for x≳0.4 a complex dielectric model seems to be applicable. The saturation magnetization and the Curie temperature decrease continuously with increased substitution of Cr3+ ions and have been explained by using the concepts of sublattice magnetizations and exchange interactions. The decrease in initial permeability is attributed mainly to the decrease in saturation magnetization. The dispersions in initial permeability and magnetic loss tangent have been discussed on the basis of domain wall movements.
Temperature dependence of the photostimulated luminescence of BaFCl:Eu2+ after ultraviolet irradiation
Authors: Zhang Xueqiang , Wang Yongsheng , Liu Jian , Zhang Guangyin , Xiong Guangnan , Xu Xurong
Published: July 2002

The variation of intensity of the photostimulated luminescence (PSL) of BaFCl:Eu2+ after ultraviolet irradiation with temperature has been investigated. The results show that the PSL intensity is independent of temperature in the readout process, but the storage ability is temperature dependent. The measurements support the mechanism of PSL involving the F-center–interstitial-halogen pairs.
Structure damage in reactive-ion and laser etched InP/GalnAs microstructures
Authors: Dubowski J. J. , Rosenquist B. E. , Lockwood D. J. , Labbé H. J. , Roth A. P. , Lacelle C. , Davies M. , Barber R. , Mason B. , Sproule G. I.
Published: July 2002

Etching of a chemical-beam-epitaxy-grown InP/InGaAs multilayer structure with reactive ion etching (RIE) and laser-assisted dry etching ablation (LADEA) is carried out in order to evaluate the extent of the damage induced by these two etching methods. Micro-Raman spectroscopy indicates a systematic broadening of the phonon lines as a function of depth of a RIE fabricated crater. In contrast, LADEA which is based on the application of an excimer laser for the removal of the products of chemical reaction, shows no measurable changes in the phonon line widths when compared to as-grown material. The results suggest that LADEA has potential for the photoresistless fabrication of damage free microstructures.
Approximations for the resonant-tunneling diode current: Implications for triple-barrier devices
Authors: Boykin Timothy B.
Published: July 2002

Resonant-tunneling diodes designed to have features in their current-density-voltage (J-V) characteristics in addition to the main peak often incorporate triple-barrier structures. In designing such structures, much attention is paid to the alignment of the quasibound levels in the two wells in order to achieve additional peaks or kinks in the J-V curve. Unfortunately, many such devices fail to display these additional features. It is commonly thought that this failure is solely due to the limitations of coherent tunneling models, but this is not always the case. Here we demonstrate that the simplest and most commonly employed approximation for the tunneling current density (the one-dimensional approximation) is often incorrect for triple-barrier devices and that when a more accurate approximation (the two-dimensional approximation) is used the J-V characteristics can be markedly different.
Compton backscattering of intracavity storage ring free-electron laser radiation
Authors: Dattoli G. , Giannessi L. , Torre A. , Altobelli G. , Gallardo J.
Published: July 2002

We discuss the γ-ray production by Compton backscattering of intracavity storage ring free-electron laser radiation. We use a semi-analytical model which provides the buildup of the signal combined with the storage ring damping mechanism and derive simple relations yielding the connection between backscattered photons, brightness and the intercavity laser equilibrium intensity.
Sequential phase formation by ion-induced epitaxy in Fe-implanted Si(001). Study of their properties and thermal behavior
Authors: Behar M. , Bernas H. , Desimoni J. , Lin X. W. , Maltez R. L.
Published: July 2002

The epitaxial growth of FeSi2 silicides was studied by using ion-beam epitaxial crystallization (IBIEC) of Fe-implanted Si(001) samples. By employing Rutherford backscattering/channeling spectrometry and transmission electron microscopy it was possible to determine that the IBIEC process produces a γ-, α-, and β-FeSi2 phase sequence, with increasing Fe concentration along the implantation profile. The critical concentrations for γ→α and α→β phase transitions are 11 and 21 at. %, respectively. A study of the thermal behavior of these phases shows that the γ- and α-FeSi2 are metastable with respect to the β-FeSi2 phase. The γ to β-FeSi2 transition starts at 700 °C via an Ostwald ripening process. In addition a 800 °C, 1 h anneal of high Fe concentration samples produces a complete α and γ to β-FeSi2 transformation. Finally, it is demonstrated that a regular or a rapid thermal annealing on Fe-implanted Si samples induces only the formation of a β-FeSi2 phase.
Molecular-beam-epitaxial growth and characterization of Zn1−xCaxSySe1−y alloys for blue-green laser applications
Authors: Jobst B. , Lunz U. , Zerlauth S. , Cerva H. , Hommel D. , Landwehr G.
Published: July 2002

The quaternary alloy Zn1−xMgxSySe1−y has been successfully used as a cladding layer in ZnSe-based laser diodes. Another group-IIA element that can replace magnesium in this alloy is calcium. A comparative study of the physical properties of the sulfides and selenides of calcium and magnesium is our starting point. Furthermore, Zn1−xCaxSySe1−y layers have been grown by molecular-beam epitaxy over a wide composition range on (100) GaAs substrates. Experimental data from various characterization methods indicate poor structural and optical properties which seem to be caused by the high ionicity of the compound as well as by a strong affinity between calcium and sulfur. The formation of microtwins and indications of phase separation have been observed.
Plasma-charging damage to gate SiO2 and SiO2/Si interfaces in submicron n-channel transistors: Latent defects and passivation/depassivation of defects by hydrogen
Authors: Awadelkarim O. O. , Fonash S. J. , Mikulan P. I. , Chan Y. D.
Published: July 2002

New experimental results are presented which provide evidence for hydrogen passivation and depassivation of plasma-charging-induced defects in gate oxides and at oxide/silicon interfaces. The devices used in this study were 0.5 μm n-channel metal–oxide–semiconductor field-effect transistors fabricated on 200 mm boron-doped silicon substrates. The processing included Cl2/HBr-based chemistries for the polycrystalline silicon gate definition etch, and CHF3/CF4-based chemistries for the contact etch. Plasma-charging defects resulting from the processing are shown to have the following properties: (i) plasma-induced charging defects are latent (electrically inactive) directly after our processing and before postmetallization annealing (PMA); (ii) these defects continue to be latent after N2 and Ar anneals done at temperatures T in the range 200 °C≤T≤400 °C; (iii) these defects are also latent after our standard PMA done in forming gas at 400 °C; (iv) these defects are electrically activated by room-temperature Fowler–Nordheim stress, and (v) equivalently these defects are electrically activated by annealing below 400 °C in hydrogen-rich ambients. We show hydrogen passivation/depassivation is responsible for this behavior. This passivation/depassivation has been previously suggested to occur for defects at SiO2/Si interface; here it is also proposed to describe defect–hydrogen interactions in the bulk gate oxide for defects caused by plasma-charging damage.
Microstructural study of yttria stabilized zirconia buffered sapphire for YBa2Cu3O7−δ thin films
Authors: Rao M. S. R. , D’Souza C. P. , Apte P. R. , Pinto R. , Gupta L. C. , Srinivas S. , Bhatnagar A. K.
Published: July 2002

Yttria stabilized zirconia (YSZ) buffer layers were grown by rf-magnetron sputtering on the r (11̄02) plane of sapphire for YBa2Cu3O7−δ (YBCO) thin film deposition. Microstructural changes of YSZ buffer layers grown using different sputtering conditions (5, 10, and 20 mTorr; Ar/O2 gas mix ratio of 9:1 and 1:1) were monitored by atomic force microscopy (AFM). Films grown using a lower oxygen partial pressure (5 mTorr) and a higher Ar/O2 ratio (9:1) showed smooth surface morphology and the average surface roughness increased with an increase in oxygen partial pressure. YBCO films in situ grown by pulsed laser deposition on sapphire with a YSZ buffer layer deposited using optimized sputtering parameters (5 mTorr gas pressure, 9:1 Ar/O2 ratio) yielded the highest critical density, Jc≊4.5×106 A cm−2 at 77 K. An excellent correlation between microstructure and Jc has been found and AFM has proved to be important for the study of the microstructure of films.
The magnetic entropy change properties of La1−zRz(Fe1−x−yCoxAly)13 compounds
Authors: Wu Jianmin , Li Feng , Cui Liya , Tai Lichi
Published: July 2002

The structures, magnetic properties, and magnetic-entropy-changes of La1−zRz(Fe1−x−yCoxAly)13 (R=Ce, Pr, Nd, Sm, Gd, and Dy) were studied. It was found that the crystal structures of La(Fe0.73−xCoxAl0.27)13 maintain the LaCo13-type cubic structure. The relationship between saturation magnetic moments per 3d atom and the number of 3d electrons in the compounds of La(Fe0.73−xCoxAl0.27)13 system can be explained by the rigid-band model at Co-rich side and disagrees with the model at Fe-rich side. When 0.02≤x≤1.0, the compounds have a nearby room temperature Curie temperature, a high saturation magnetization and a high magnetic-entropy-change. When x=0.02 and the external magnetic field is 1.11 MA/m, the maximum magnetic-entropy-change of the La(Fe0.71Co0.02Al0.27)13 compound is 13 kJ/m3⋅K which is about half of that of metal Gd at the same external magnetic field. The saturation magnetization of the compound increases and the maximum magnetic-entropy-change decreases when La is substituted by Ce. The compounds in which La is substituted by other rare-earth elements R (R=Pr, Nd, Sm, Gd, and Dy) decomposed into 1:13-type, 2:17-type, and α-Fe phase. The second rare-earth elements added exist mainly in the 2:17-type phase.
Observation of time-varying photoconductivity and persistent photoconductivity in porous silicon
Authors: Frello T. , Veje E. , Leistiko O.
Published: July 2002

We have observed time-varying photoconductivity and persistent photoconductivity in porous silicon, both with time-evolution scales of the order of several minutes or hours. The time evolutions depend on the wavelength and the intensity of the illuminating light. The data indicate the presence of at least two competing mechanisms, one is tentatively related to photoinduced creation of charge carriers in the silicon substrate followed by diffusion into the porous silicon layer, and the other is tentatively related to desorption of hydrogen from the porous silicon.
Pulsed laser sputtering of the (100)GaAlAs surface
Authors: Vivet L. , Dubreuil B. , Gibert-Legrand T. , Barthe M. F.
Published: July 2002

We have studied the pulsed laser sputtering of (100)Ga1−xAlxAs (x=0.545) surface with 337 nm photons, starting from the threshold for particle emission (a few tens of mJ/cm2) up to about 300 mJ/cm2. Atoms and molecules sputtered from the irradiated surface are detected, their relative number measured, and their time of flight determined using laser resonant ionization mass spectrometry. After laser irradiation the surface is examined by scanning electron microscopy and electron microprobe analysis. From the shot number and the fluence dependencies of the sputtering yield, it is shown that two sputtering regimes exist. For low fluence (<150 mJ/cm2), the sputtering results mainly from the absorption and excitation of defect sites. At higher fluences, the process is quite similar to thermal evaporation. One observes preferential emission of As in the form of As2 molecules and the correlated Ga and Al enrichment of the surface with formation of GaAl microsized structures. However, As preferential laser sputtering is rapidly counterbalanced by excess surface Ga and Al atoms leading to a stationary sputtering regime after a few thousand laser shots. A simple analytical model was developed which permits to define the conditions for such equilibrium achievement.
Current amplification with asymmetric direct current superconducting quantum interference devices at 77 K
Authors: Kleiner R. , Koelle D. , Ludwig F. , Dantsker E. , Miklich A. H. , Clarke John
Published: July 2002

One can use a direct current (dc) superconducting quantum interference device (SQUID) as a current amplifier by injecting a signal current Is into part of the superconducting loop and detecting the resultant change in critical current Ic. The current gain max ‖dIc/dIs‖ can be increased by making the inductances of the two arms of the SQUID asymmetric, thereby skewing the transfer function Ic vs Is. Detailed simulations of the device performance include an analysis of the impact of inductance and junction asymmetry on the transfer function. At 77 K a maximum current gain of 5 should be achievable albeit for frequencies below 1 GHz. An increase in the operating speed to 2 GHz can be achieved by reducing the gain to 2. We have fabricated asymmetric YBa2Cu3O7−x dc SQUIDs on SrTiO3 bicrystal substrates and operated them at 77 K. Using a second, readout SQUID to monitor changes in the critical current, we have achieved a low-frequency current gain up to 2.3. By coupling two SQUIDs together, we have increased the gain to 8.5.
Magnetomechanical damping in plasma sprayed iron–chromium based coatings
Authors: Karimi A. , Giauque P. H. , Martin J. L.
Published: July 2002

The damping capacity (Q−1) of thermally sprayed Fe–Cr–X coatings has been investigated in the range of frequencies between f=10 Hz and 10 kHz, and deformations between ε=10−4 and 10−3, using a cantilever method. The magnetomechanical hysteresis loss was determined based on the modal analysis technique of flat beam and was found to be very sensitive to internal stress of the samples. Heat treatments usually enhanced the loss capability, but only high-temperature annealing restored the whole damping capacity of the plasma sprayed coatings to be comparable to that of cast alloys of the same chemical composition. The variation of Q−1 versus vibration amplitude first increased rapidly, passed through a maximum, and then declined relatively slowly to its initial values. The position of the maximum damping (Q−1Max) was shifted towards lower strains with annealing time and temperature. As expected in ferromagnetic materials and in contrast to strain amplitude effect, the vibration frequency did not significantly influence damping behavior. The structure of magnetic domains were observed using the magneto-optical Kerr effect and their modification following heat treatments was associated with different values of the damping capacity. Accordingly, the beneficial effect of annealing on damping capacity was found to arise on the one hand from improved mobility of unpinned domain walls and on the other hand from growth of 90° domains leading to equiaxed domain configurations. The irreversible movement of domain walls upon application of an external stress in general occurred suddenly and abruptly between two pinned positions.
A graph theoretic approach to quantum cellular design and analysis
Authors: Lusth John , Jackson David Jeff
Published: July 2002

In this article, graph theory is investigated as a tool for designing and analyzing quantum cellular automata (QCA). A method is presented for constructing graphs from an arbitrary automata. The constructed graphs are used to model both the static and dynamic states of QCA. Several fundamental theorems concerning the constructed graphs are presented relating QCA structure and behavior to graph colorings. Using these theorems, some simple QCA devices are analyzed and improvements, if applicable, are suggested.
Temperature-dependent exciton behavior in quaternary GaInAsSb/AlGaAsSb strained single quantum wells
Authors: Shen W. Z. , Chang Y. , Shen S. C. , Tang W. G. , Zhao Y. , Li A. Z.
Published: July 2002

We report the temperature-dependent exciton behavior in a quaternary Ga0.67In0.33As0.01Sb0.99/Al0.25Ga0.75As 0.2Sb0.98 strained single-quantum-well (SQW) structure by photoluminescence spectroscopy. Strong exciton resonances are observed and have been attributed to localized excitons below 80 K and to free excitons at high temperatures. Nevertheless, we show that the experimental results of stronger exciton–phonon coupling in the quaternary SQW structure would lead to partial ionization of free excitons at temperatures above 125 K, in good agreement with the line-shape analysis of the luminescence spectra which clearly shows the presence of band-to-band recombination.
Investigation of current channel migration in a conducting plasma between planar electrodes
Authors: Spanjers G. G. , Yadlowsky E. J. , Hazelton R. C. , Moschella J. J.
Published: July 2002

Magnetic-field penetration is characterized in a current conducting plasma between planar electrodes used as a plasma opening switch [G. G. Spanjers, E. J. Yadlowsky, R. C. Hazelton, and J. J. Moschella, J. Appl. Phys. 77, 3657 (1995)]. The experiment is performed in a regime where Hall effects [Amnon Fruchtman, Phys. Fluids B 3, 1908 (1991)] are predicted to describe the current channel migration. Measurements of the magnetic-field penetration in two cases with opposite electrical polarity indicate that the Hall effects are not the dominate process. A one-dimensional resistive magnetohydrodynamic model is used to show that current channels in agreement with those measured can be predicted through a coupling of a plasma snowplow motion with resistive diffusion.
Spatially resolved photoluminescence study on T-shaped quantum wires fabricated by cleaved edge overgrowth method
Authors: Someya Takao , Akiyama Hidefumi , Sakaki Hiroyuki
Published: July 2002

We have fabricated GaAs T-shaped edge quantum wire (T-QWR) structures by the cleaved edge overgrowth method. The geometries of the samples were designed in such a way that we can separately detect photoluminescence (PL) from T-QWRs as well as two adjacent quantum wells (QWs) from spatially resolved PL measurements. Each PL peak is unambiguously assigned and analyzed to precisely determine the quantized energy of excitons in T-QWRs and the adjacent QWs. Special care is made in the cleavage and the growth procedures to realize sharp and efficient PL from these structures.
A new field-effect transistor based on the metal–insulator transition
Authors: Katayama Kozo , Hisamoto Digh , Nakamura Yoshitaka , Kobayashi Nobuyoshi , Nagai Ryo
Published: July 2002

We propose a field-effect tunnel transistor based on the metal–insulator transition. The principle of the switching is the metal–insulator transition, which occurs at the sheet resistance RQ (∼h/e2=25.8 kΩ). The modulation of the sheet resistance around RQ by the control gates can be magnified by the phase transition. As a result, high transconductance and high current drivability more than 10 times greater than the ultimate silicon metal-oxide-semiconductor transistors are obtained. The device is a thin-film silicon-on-insulator structure with dual gates, one on each side of the channel. A very thin granular metal film is deposited on the Si layer. Each metal island forms a Schottky contact with the Si layer, which is completely depleted. The electrons in the metal tunnel between the islands through the Si. The metal film can have a higher Coulomb gap and current drivability than is obtained with a single tunnel junction. A temperature of less than 1/20 of the Coulomb gap energy is required to reduce the leakage current by three orders of magnitude with the Coulomb blockade mechanism. Using the Wentzel–Kramers–Brillouin approximation, we calculated the tunneling probability between the islands and evaluated the sheet resistance of the metal film. Changing the gate voltage can modulate the sheet resistance in spite of the very narrow spacing between the islands. In the high resistance regime, the Coulomb blockade can operate and the resistance is three orders of magnitude higher than the bare tunnel resistance. In the ‘‘on’’ state, on the other hand, a very low sheet resistance of less than 1 kΩ per square is obtained.
Enhancement of superconducting transition temperature in (Bi2Sr2Ca3Cu4O12+δ)1(Bi2Sr2CaCu2O8+δ)1 superlattice films by charge transfer
Authors: Hatano Takeshi , Ishii Akira , Nakamura Keikichi
Published: July 2002

(Bi2Sr2Ca3Cu4O12+δ)1(Bi2Sr2CaCu2O8+δ)1 superlattice films synthesized by sequential radio frequency magnetron sputtering on MgO substrates were studied by x-ray diffraction method, Rutherford backscattering technique, magnetization measurements and Hall effect experiments. Superconducting transition temperature observed in the (Bi2Sr2Ca3Cu4O12+δ)1(Bi2Sr2CaCu2O8+δ)1 superlattice films (55–58 K) was higher than those of Bi2Sr2Ca3Cu4O12+δ (40–47 K) and Bi2Sr2CaCu2O8+δ (50–52 K) single phase films. The results suggest that optimum carrier concentration is achieved in CuO2 layers in the superlattice films by charge transfer from the overdoped Bi2Sr2CaCu2O8+δ units to the underdoped Bi2Sr2Ca3Cu4O12+δ units.
The suppression of phonon induced noise in niobium superconducting tunnel junction x-ray detectors
Authors: Poelaert A. , Peacock A. , Rando N. , Verhoeve P. , Videler P.
Published: July 2002

An investigation into the phonon contamination of x-ray sensitive superconducting tunnel junctions arising from the x-ray photoabsorption in various substrates has been conducted. Results are presented on the design of a superconducting tunnel junction (STJ) which substantially reduces or even eliminates phonon induced noise from the substrate. Such noise is the predominant feature in x-ray spectra from junctions due to the bulk of the photons being absorbed in the substrate rather than in the thin superconducting film. The design involves the choice of a suitable buffer sandwich between the substrate and the STJ. Such a buffer would appear not only to attenuate the phonons created in the x-ray photoabsorption in the substrate but also to scatter the phonons inelastically, introducing a frequency down-conversion. Such a process ensures that few phonons of energy sufficient to break Cooper pairs in the superconducting film of the STJ enter the junction.
‘‘Invisibility’’ in certain intersecting particles and arrays of such particles in a solid host
Authors: Radchik A. V. , Paley A. V. , Smith G. B.
Published: July 2002

Theoretical results recently obtained for polarizability of intersecting spherical particles have been compared with those for polarizability of intersecting cardioidal particles resembling the spherical shape. Calculations based on a modified Parseval Rule have been performed for a number of metals in a dielectric matrix in a quasistatic approximation. A large difference in the polarizabilities has been found in spite of a striking similarity in their shapes. An unexpected small value for both the real and the imaginary part of the polarizability of cardioidal shaped metal particles occurs at wavelengths where simple metal spheres absorb strongly. A Maxwell-Garnett model has been used to compare the optical properties of composite materials containing the cardioidal and intersecting spherical particles of interest. It has been found that a composite containing cardioid shaped particles in a dielectric matrix can have an unusual response. Its absorption is very close to zero, while the refractive index is close to that of the fully dense host.
Influence of rapid thermal annealing and internal gettering on Czochralski-grown silicon. II. Light beam induced current study of recombination centers
Authors: Ehret E. , Maddalon-Vinante C.
Published: July 2002

Results concerning the generation of recombination centers induced by rapid thermal annealing and by the combination of this heat pulse with an internal gettering process are reported. The influence of both the duration and the ambient of the rapid thermal annealing is studied by measurements of the minority-carrier diffusion length. This work shows that (i) a short heat pulse at high temperature is sufficient to induce a strong degradation of this diffusion length. The activation of the pre-existing impurities and a modification of the intrinsic defect population during the annealing may occur, (ii) the rapid thermal annealing has still an influence on the generation of recombination centers after thermal treatments at high temperature and for long durations. Indeed, this memory effect is observed even after the internal process, as observed in a previous work for the oxygen precipitation. However, we conclude that the recombination centers generated cannot be totally correlated with the oxygen precipitation. Introducing hydrogen during the RTA shows that the nature of defects responsible for the modification of the minority carrier diffusion length and oxygen precipitation must be different.
Two-photon ionization of trimethylamine using KrF laser radiation
Authors: Werner P. W. , Schamiloglu E.
Published: July 2002

The multiphoton ionization of a low-pressure (3×10−5 Torr) fill of trimethylamine using an electron-beam pumped KrF laser has been studied. A microwave resonator probe was used to measure the resultant free-electron density. The scaling of electron density with the square of the incident laser intensity was verified in the low flux limit. The electron production decreased significantly from the I2 scaling at laser fluxes exceeding 4 MW/cm2. These results are consistent with a simple two-photon model describing the interaction in the low flux limit and a saturation of the electron production in the high-flux limit.
Diagnostics of high-power ion beam interaction with composite targets
Authors: Utkin A. V. , Kanel G. I. , Razorenov S. V. , Baumung K. , Rusch D.
Published: July 2002

The interaction of a high-power proton beam with 10- to 75-μm-thick double layered aluminum targets has been investigated. The experiments were carried out at the Karlsruhe Light Ion Facility using laser Doppler velocimetry with subnanosecond temporal resolution. The results allow us to determine with high accuracy the range of 1.53 MeV protons in the hot aluminum ablation plasma and to reveal the contribution of the thermal conductivity to the state of the target. An analytical model based on an acoustic approximation was established to interpret the ablative acceleration of the target foils during the whole rise time of the accelerator voltage until the maximum ion range is reached. From this model follows that a shock wave is induced into the adjacent material only when the expansion velocity of the ablation zone exactly equals the sound velocity in the target.
Dielectric permittivity of some smectic phases in ferroelectric liquid crystals derived from (2s)-2-(6-hydroxy-2-naphthyl) propionic acid
Authors: Chern G. , Lin H. C. , Wu S. L. , Hsieh W. J.
Published: July 2002

A study of the dielectric response as a function of frequency (20–106 Hz) and temperature in smectic ferroelectric liquid crystals HmPBNP, m=6,8–12,16, has been carried out. For m=10–12, it reveals a transition sequence of five phases: crystal–SmC*γ–SmC*–SmC*α–SmA. Three Goldstone modes have been identified in different temperature intervals associated with SmC*γ, SmC*, and SmC*α phases, respectively. The ferroelectric phases almost totally disappear for m=6. An unexpected antiferroelectric (AF)-like phase accompanied by the ferroelectric phases forms for m=8. The formation of the AF-like phase might be explained by the fast increase of the relaxation frequencies of the Goldstone modes due to the reduction of the alkyl chain in H8PBNP.
Modeling of electron scattering in thin manganese films on silicon by Monte Carlo methods
Authors: Tökési K. , Némethy A. , Kövér L. , Varga D. , Mukoyama T.
Published: July 2002

The electron energy distribution of the backscattered electrons from manganese and manganese films deposited on a silicon substrate was studied. The Monte Carlo technique was used to simulate the backscattered electron energy distributions and these were compared with the measured reflected electron spectra. A good agreement was found in general between our calculations and the experimental results. In addition, the applicability of Tougaard’s method for the determination of the energy loss function from reflected electron energy loss spectroscopy and simulated backscattered electron spectra of manganese films deposited on a silicon substrate were investigated.
Optical absorption of free-standing porous silicon films
Authors: Chan M. H. , So S. K. , Cheah K. W.
Published: July 2002

The optical absorptions of anodically etched p+ and n+ porous silicon (PS) films were investigated by photothermal deflection spectroscopy. Si–H stretching overtones and combination bands of Si–F and Si–H were observed. The defect model in hydrogenated amorphous silicon was used to explain the Urbach edge and the subgap absorptions of PS. The dangling bond defect densities in PS were estimated.
Optical absorption of p-type GexSi1−x quantum wells
Authors: Fu Y. , Willander M. , Xu Wenlan
Published: July 2002

We have applied the 6×6 k⋅p Hamiltonian to calculate the valence subband structure of p-type GexSi1−x quantum wells. When the sheet density of carriers is 5×1012 cm−2 or less, normal incident radiation absorption in the p-type GexSi1−x quantum well is due to the inter-valence band mixings. Subbands become more independent when their energy separations are high, so that high-energy optical absorption is weak. The transition from ground subband to the first excited subband contributes mostly to the normal incident radiation absorption, as demonstrated in many measurements. The work can be extended to design and optimize p-type GexSi1−x quantum well infrared detector.
Magnetic hysteresis in stressed 15-nm-thick Ni films
Authors: Nowak J. , Puppin Ezio , Callegaro Luca
Published: July 2002

The magnetic hysteresis loops of mechanically stressed 15-nm-thick Ni film, measured by the magneto-optical technique, were analyzed using the Stoner–Wohlfarth model. The influence of external stress on the shape of the hysteresis loop is accurately described by stress-induced uniaxial anisotropy. The energy of the domain structure during magnetization reversal is equivalent to an additional uniaxial anisotropy. The processes of domain appearance and disappearance correspond to two different magnitudes of this anisotropy, while the moment of the change of anisotropy magnitude corresponds to the transformation of the Néel walls.
Neutron scattering and magnetization clouds in dilute Pd based alloys
Authors: Parra R. E. , González A. C.
Published: July 2002

Neutron scattering data for dilute PdMn, PdCo, and PdFe alloys show forward peaks at small values of K, which indicate an inhomogeneous distribution of the magnetization in these alloys. These inhomogeneities indicate the existence of magnetization clouds that produce the ferromagnetism in these materials; the shape of the peaks at small K also suggest a quasielastic contribution to the neutron diffuse scattering. We, therefore, calculated the neutron scattering cross sections for these alloys using a local moment model; we also developed an analytic expression for the magnetization clouds that can be compared with experimental results. The calculations were made using Monte Carlo methods in the dilute alloy region of impurity concentration.
Polarization analysis of the electron spin resonance lines in the S=1 one-dimensional antiferromagnet Ni(C3H10N2)2NO2ClO4
Authors: Hagiwara M. , Katsumata K.
Published: July 2002

The results of electron spin resonance (ESR) measurements on the S=1 one-dimensional Heisenberg antiferromagnet Ni(C3H10N2)2NO2ClO4 (NINO) are reported. We have analyzed the polarization dependence of the ESR transition between the singlet ground state and the first excited triplet which is forbidden for an ordinary Hamiltonian. Our results are consistent with the theory by Sakai and Shiba [J. Phys. Soc. Jpn. 63, 867 (1994)] proposed to explain the forbidden transition.
Magnetization behavior of (NBu4)2Mn2[Cu(opba)]3 and related solvated ferromagnets
Authors: Chavan S. A. , Ganguly R. , Jain V. K. , Yakhmi J. V.
Published: July 2002

Direct current magnetization studies are reported for the organic ferromagnet (NBu4)2Mn2[Cu(opba)]3 (TC=22 K), where opba stands for o-phenylenebis (oxamato), and related compounds obtained by solvating it with dimethylsulphoxide, methanol, and acetonitrile, which have TC values of 15, 12, and 14.5 K, respectively. M(H) plots for the first two compounds show very little width and criss-cross at low applied field values (<50 Oe). Zero field-cooled magnetization and field-cooled magnetization curves for (NBu4)2Mn2[Cu(opba)]3 nearly overlap and its remanence plot changes sign at 11.5 K, exhibiting a compensation behavior. Negative remanent magnetization has also been observed for the other three compounds. The results are discussed in terms of the ferrimagnetic nature of the building blocks.
Two-dimensional solitons in the classical Heisenberg antiferromagnet with nonmagnetic impurities
Authors: Subbaraman Kala , Zaspel Craig E. , Drumheller John E.
Published: July 2002

Experimental evidence is presented for the existence of nonlinear localized spin excitations (solitons) in the classical two-dimensional Heisenberg magnet doped with nonmagnetic impurities. These solitons are shown to provide the dominant mechanism for spin relaxation in electron paramagnetic resonance linewidths immediately above the ordering temperature for layered Mn compounds with a small impurity concentration. The new gapless excitations are pinned by the nonmagnetic impurities thereby becoming topologically unstable and lower in energy than the nonlinear excitations (skyrmions) that may exist in the pure material. Concentration-dependent results preclude a spin-wave mechanism for this effect.
Construction of novel magnets for generating astatic fields in x-ray topography
Authors: Nakajima Tetsuo , Yoshizawa Masami
Published: July 2002

Superconducting (S-type) and normal-conducting (N-type) magnets, which can freely change the direction and strength of the fields, were constructed due to an incidental requirement concerning x-ray diffraction. Both magnets comprise three pairs of solenoids, the axes of which intersect orthogonally at the central point. Each pair of solenoids is independently excited by bipolar electric power sources. Alternating and direct current magnetic fields applied to an arbitrary direction [h1k1l1] in a single crystal can be varied to another direction [h2k2l2], parallel to an arbitrary curved surface. The components (Hx, Hy, and Hz) from each solenoid were simultaneously measured using three Hall elements. The prescribed magnetic field (H) was accurately synthesized by computer control of the electric power sources through the direction cosines, Hi/H (i=x, y, and z). The magnet was used to take synchrotron radiation x-ray topographs of α-Fe-3% Si in magnetic fields swept in the (110) plane from [002] via [22̄2], [11̄0], [222], and [002̄] to [002]. In the S type, an inner metal Dewar having a transmittable part of an x-ray beam made of a confocal beryllium cylinder was installed in order to generate temperatures higher than 4.2 K.
Potential model for the subband structures in the inversion layer of narrow-gap semiconductor heterostructures
Authors: Liu Kun , Chu J. H. , Wu L. J. , Ou H. J. , Tang D. Y.
Published: July 2002

Based on the capacitance–voltage (CV) characteristics of semiconductor metal-insulator- semiconductor (MIS) heterostructures and the nonparabolic characteristics of narrow-gap semiconductors (NGSs), a surface potential model has been developed to derive subband structures of two-dimensional electron gas systems in an inversion layer of NGS MIS heterostructures. CV measurement was also performed on a p-type InSb MIS heterostructure, and two onsets of electron filling more than one subband were observed. Subband structures were also obtained using the presented potential model.
Modal properties of second-harmonic generation in doped-silica planar waveguides
Authors: Kester J. J. , Ranon P. M. , Dajani I. , Pribyl S. , White W. R.
Published: July 2002

Planar waveguides of germania-doped silica have been optically modified to allow second-harmonic generation in various waveguiding modes. After optical modification, the propagation of only the fundamental wave produced film-generated second-harmonic light in the same waveguide mode with which it had been prepared. The relative efficiencies of several mode combinations were all measured to be within an order of magnitude of one another and in reasonable agreement with theoretical predictions. Furthermore, we discuss the discrepancy between the theoretical calculations and experimental observations of the modal properties of second-harmonic generation in fibers.
Theoretical study of tunneling current in the access region of various heterojunction field-effect transistor structures
Authors: Jansen Philippe , Mizuta Hiroshi , Yamaguchi Ken , Wagner Mathias
Published: July 2002

The source resistance of a heterojunction field-effect transistor (HFET), whose reduction is mandatory for high-performance devices, consists of an ohmic contact resistance and an access resistance. The access region is located between the geometrical source and the geometrical source side of the gate contact. By means of a quantum-mechanical modeling program, the effect of changes in layer structure in the access region of a HFET is studied. A new heterojunction structure using a Si planar doped layer is designed to improve the linearity and reduce the access resistance by more than ten times for a specific transistor layout. Thanks to the higher sensitivity of the modeling program to structural information, the contribution of the tunneling current and the change of equilibrium as a function of temperature is investigated.
X-ray detection by superconducting tunnel junctions via phonon propagation in the substrate
Authors: Poelaert A. , Erd C. , Peacock A. , Rando N. , Verhoeve P. , Kozorezov A. G. , Wigmore J. K.
Published: July 2002

The x-ray performance of Nb-Al-AlOx-Al-Nb superconducting tunnel junctions deposited on sapphire has been studied for phonon mediated detection of x rays absorbed in the substrate in the energy range 750 to 6000 eV. Two separate channels of phonon propagation can be identified. One produces a discrete signal peak, due to high frequency phonons originating from the x-ray absorption sites in a shallow layer below the junction. The other contributes to a monotonic signal tail, due to low frequency phonons, reaching the junction after diffusive or multiple scattering at surfaces.
Effects of carrier-storage layer of InAs/GaAs short period superlattice on photocurrent in GaAs/GaAlAs multi-quantum well structure
Authors: Matsui Y. , Kusumi Y.
Published: July 2002

Negative differential characteristics in the relations between photoinduced currents and forward bias voltages are investigated considering accumulations of photogenerated carriers in a storage layer on a GaAs/GaAlAs multi-quantum well structure. The nonlinear characteristics become obvious when the storage layer is an InAs/GaAs short period superlattice and can be observed at 300 K under weak photoexcitations (e.g., 2.5 mW/cm2). On the other hand, the phenomenon is extremely reduced in the case of a storage layer of an InGaAs alloy or InAs, and disappears without the storage layer. These results correspond with the differences of carrier lifetime in the storage layer. The enhanced negative differential characteristics can be due to the smaller trap densities and the larger electron mobility in the storage layer of the InAs/GaAs short period superlattice. Finally, the applications of the characteristics to the selectivity functions of visual neurons are discussed.