bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced positive magnetoresistance effect in GaAs with nanoscale magnetic clusters
Authors: Yuldashev Sh. U. , Shon Y. , Kwon Y. H. , Fu D. J. , Kim D. Y. , Kim H. J. , Kang T. W. , Fan X.
Published: July 2002

The enhanced positive magnetoresistance effect has been observed in GaAs containing nanoscale magnetic clusters. The ferromagnetic metallic clusters were embedded into GaAs by Mn ion implantation and rapid thermal annealing. Positive magnetoresistance in these structures has been observed and attributed to the enhanced geometric magnetoresistance effect in inhomogeneous semiconductors with metallic inclusions. The additional enhancement of positive magnetoresistance under light illumination is due to the higher mobility of photoexcited electrons in comparison with the mobility of holes in p-type GaAs prepared by Mn ion implantation.
Bright pure blue emission from multilayer organic electroluminescent device with purified unidentate organometallic complex
Authors: Im Woo-Bin , Hwang Ha-Keun , Lee Jae-Gyoung , Han Kijong , Kim Youngkyoo
Published: July 2002

Multilayer organic electroluminescent devices (OELDs) were fabricated with highly pure 2-(2-hydroxyphenyl)benzoxazolato lithium (LiPBO), which was obtained through stepwise purification process, as a blue emission layer. The ionization potential of the carefully purified LiPBO was ∼5.82 eV. The multilayer OELD with a hole-blocking layer (HBL) emitted almost pure blue light with the CIE color coordinate of x=0.15 and y=0.08. However, the emission color was redshifted when an electron-transporting layer (ETL) was introduced instead of the HBL. The device with both the HBL and the ETL showed stable and bright blue emission above 14 000 cd/m2 with the color coordinate of x=0.15 and y=0.11, even though the color purity was slightly poorer than that with only the HBL.
Selective growth of InP on focused-ion-beam-modified GaAs surface by hydride vapor phase epitaxy
Authors: Sun Y. T. , Rodrı́guez Messmer E. , Lourdudoss S. , Ahopelto J. , Rennon S. , Reithmaier J. P. , Forchel A.
Published: July 2002

The growth of InP islands on a planar focused-ion-beam (FIB)-modified (001) GaAs substrate was investigated in a hydride vapor phase epitaxy system. InP grew selectively on the FIB-implanted lines, forming continuous stripes, whereas isolated islands were observed outside the implanted area. The impact of the III/V ratio, crystallographic orientation of implanted lines, and implantation dose was explored. The choice of suitable growth conditions makes it possible to obtain continuous InP wires aligned in all possible directions. The results of this work could be used for the fabrication of future optoelectronic integrated circuits, which would include nanoscale structures, e.g., quantum-wire optical devices with GaAs electronic circuits.
Formation of three-dimensional microstructures by electrochemical etching of silicon
Authors: Kleimann P. , Linnros J. , Juhasz R.
Published: July 2002

This letter describes the promising technique of micromachining using the properties of electrochemical etching of (100)-oriented n-type silicon in a hydrofluoric acid electrolyte. The technique is based on electropolishing of a wafer except for areas where vertical structures are needed and does not require a periodic pattern. Predefined steps of a few microns depth prior to the electrochemical etching define the shape and position of the structures. The three-dimensional microstructure width can be adjusted with the etching parameters, also enabling the formation of free-standing structures. The feasibility of this technique is demonstrated by forming high aspect ratio microneedles and tubes.
Modification of Co/Pt multilayers by gallium irradiation—Part 1: The effect on structural and magnetic properties
Authors: Hyndman R. , Warin P. , Gierak J. , Ferré J. , Chapman J. N. , Jamet J. P. , Mathet V. , Chappert C.
Published: July 2002

Atomic force microscopy, transmission electron microscopy, optical, and magneto-optical microscopy have been used to study how structural and magnetic properties are changed when a Co/Pt multilayer is quasihomogeneously irradiated with Ga ions. Under low irradiation fluence, both grain size and texture in the multilayer increase. These effects continue for fluences in excess of 1×1015 Ga ions/cm2, but beyond this dose significant thinning of the multilayer is also observed. Three distinct irradiation-induced magnetic regimes with sharp transitions between each were identified. For Ga fluences less than 5×1012 ions/cm2, the irradiated region retains perpendicular uniaxial anisotropy but with coercivity lower than that of the as-grown film. For fluences between 5×1012 and 1×1013 Ga ions/cm2, a transition from perpendicular to in-plane magnetization was experienced. Very little change of the in-plane magnetic properties of irradiated multilayers is then observed until the sample experiences a ferromagnetic to paramagnetic transition at fluences around 1×1015 Ga ions/cm2. A brief comparison with the effect of irradiating with He ions is given.
Metal electrode–chemisorbate bonding: General influence of surface bond polarization on field-dependent binding energetics and vibrational frequencies
Authors: Wasileski Sally A. , Koper Marc T. M. , Weaver Michael J.
Published: July 2002

An analysis is outlined in which the dependence of the binding energy, Eb, stretching frequency, νM–A, and equilibrium bond length, req, of metal–adsorbate bonds on the external interfacial field (F), and hence surface potential of relevance to electrochemical systems, are described in terms of potential-energy surface and bond-polarization parameters. Density Functional Theory (DFT) calculations for finite metal clusters with monoatomic adsorbates forming polar surface bonds are utilized both to examine metal-, adsorbate-, and field-dependent trends in the required parameters and to examine the applicability of the analytic relations in comparison with numerical calculations. To a very good approximation, the Eb–F dependence is described by the field-dependent static dipole moment, μS, of the metal–adsorbate bond. As expected, the field-induced changes in the potential-energy surface (PES) are determined by the r-dependent μS values, i.e., the dynamic dipole moment, μD. The factors determining the νM–A–F dependence, however, are distinctly more intricate, involving the PES anharmonicity coupled with μD, as well as being influenced significantly by the μD–r dependence in some cases. While both μS and μD are influenced importantly by surface bond polarity, the role of bond polarizability appears to be quite different, so that only a crude correlation between μS and μD is evident. These differences further underscore the disparate nature of the Eb–F and νM–A–F behavior. Nevertheless, an approximate inverse correlation between the νM–A–F and req–F dependence is usually anticipated. The broad-based utility of DFT for assessing as well as predicting the role of surface bond polarization in controlling the field-dependent electrode–adsorbate PES is pointed out.
Significant dielectric enhancement in 0.3BiFeO3–0.7SrBi2Nb2O9
Authors: Gu Haoshuang , Xue Junmin , Wang John
Published: July 2002

Significant dielectric enhancement is observed in 0.3BiFeO3–0.7SrBi2Nb2O9, when a single-phase layered perovskite structure was formed by sintering the mechanically activated oxide composition. The Curie point of xBiFeO3–(1−x)SrBi2Nb2O9 was shifted upward with an increase in the BiFeO3 content. 0.3BiFeO3–0.7SrBi2Nb2O9 exhibits a dielectric constant of 1.84×105 at the Curie point of 750 °C. The lattice dimensions of xBiFeO3–(1−x)SrBi2Nb2O9 decrease slightly with an increase in the content of BiFeO3 over the composition range of x=0–0.2, while 0.3 mol BiFeO3 in SrBi2Nb2O9 led to recovery in the lattice dimensions. The much enhanced dielectric properties observed in 0.3BiFeO3–0.7SrBi2Nb2O9 are therefore due to the enlarged rattling space for both Nb5+ and in particular for smaller Fe3+.
High resolution x-ray spectroscopy using GaAs arrays
Authors: Owens Alan , Bavdaz M. , Peacock A. , Poelaert A. , Andersson H. , Nenonen S. , Sipila H. , Tröger L. , Bertuccio G.
Published: July 2002

We have produced a number of small format gallium arsenide (GaAs) arrays to address the material, electronic, and technological problems that need to be solved in order to develop mega pixel, Fano-limited spectroscopic x-ray imagers. Results will be presented of a series of x-ray measurements carried out on a prototype 5×5 array, fabricated from 40 μm thick epitaxial GaAs. The device has pixel sizes of 200×200 μm2 and pitch 250 μm. As a preliminary investigation of performance, two pixels have been instrumented. Measurements from 5.9 to 98 keV were carried out both in our laboratory and at the Hamburger Synchrotronstrahlungslabor research facility in Hamburg, Germany. Both pixels were found to be remarkably uniform, both in their spectral and spatial response to x-rays. The average nonlinearity in the spectral response is <1% across the energy range 5.9–98 keV. Using a 12 keV, 20×20 μm2 pencil beam, the spatial uniformity was found to be better than 98% over the entire pixel surfaces, consistent with the statistical precision of the measurement. The energy resolution at −40 °C is 400 eV full width at half maximum (FWHM) at 5.9 keV rising to 700 eV FWHM at 98 keV. No difference in energy resolution was found between full area and pencil beam illumination. An analysis of the resolution function has shown that the detector is dominated by electronic noise at low energies and Fano noise at energies above 30 keV. By best-fitting the expected resolution function to the entire data set, we derive a Fano factor of 0.140±0.05, together with a charge transport factor as low as 1.4×10−3. Further improvement in the resolution function has been achieved by replacing the conventional resistive feedback preamplifiers with a new resistorless design, which provides a lower component of electronic noise. In this case, a resolution of 266 eV FWHM at 5.9 keV has been achieved at room temperature (23 °C) and 219 eV FWHM with only modest cooling (−31 °C). The expected Fano noise at this energy is ∼140 eV.
Polymeric nanostructured material for high-density three-dimensional optical memory storage
Authors: Siwick Bradley J. , Kalinina Olga , Kumacheva Eugenia , Miller R. J. Dwayne , Noolandi Jaan
Published: July 2002

The unique properties of a polymer photonic crystal are examined with respect to applications as a medium for high-density three-dimensional optical data storage media. The nanocomposite material was produced from core-shell latex particles, in which the latex cores contained dye-labeled polymer. Nonfluorescent latex shells were attached to the core particles. Upon annealing, the close-packed core-shell particles formed a nanostructured material with the fluorescent particles periodically embedded into the optically inert matrix in a hexagonal close-packed structure. A two-photon laser scanning microscope was used to write bits of information into the material by photobleaching the optically sensitive particles and, under much lower fluence, read out the resulting image. Relative to conventional homogeneous storage media, the nanostructured periodic material is shown to increase the effective optical storage density by at least a factor of 2 by spatially localizing the optically active region and imposing an optically inactive barrier to cross-talk between bits. This polymer photonic crystal has the potential to dramatically improve performance further through the improved capabilities to optimize the photochemical processes and more fully exploiting the periodic nature of the information domains in the image processing.
Biorthogonal approach for explicitly correlated calculations using the transcorrelated Hamiltonian
Authors: Hino Osamu , Tanimura Yoshitaka , Ten-no Seiichiro
Published: July 2002

A biorthogonal formulation is applied to the non-Hermite transcorrelated Hamiltonian, which treats a large amount of the dynamic correlation effects implicitly. We introduce biorthogonal canonical orbitals diagonalizing the non-Hermitian Fock operator. We also formulate many-body perturbation theory for the transcorrelated Hamiltonian. The biorthogonal self-consistent field followed by the second order perturbation theory are applied to some pilot calculations including small atoms and molecules.
Single-crystalline superconducting thin films of electron-doped infinite-layer compounds grown by molecular-beam epitaxy
Authors: Karimoto Shin-ichi , Ueda Kenji , Naito Michio , Imai Tadayuki
Published: July 2002

Single-crystalline Sr1−xLaxCuO2 thin films of electron-doped infinite-layer compounds were grown by molecular-beam epitaxy. Crucial to our success was the use of KTaO3 substrates. The best film showed Tconset=41.5 K and Tczero=39.0 K, which is close to the highest Tconset of 43 K for the bulk value. The resistivity of the optimum-doped film exhibited metallic temperature dependence with a low resistivity of 320 μΩ cm at room temperature and 120 μΩ cm just above Tc.
The isotropic–nematic transition in hard Gaussian overlap fluids
Authors: de Miguel Enrique , Martín del Río Elvira
Published: July 2002

We report a study of the isotropic–nematic (I–N) transition in fluids of molecules interacting through the hard Gaussian overlap (HGO) model. This model is computationally simple and shears some similarities with the widely used hard ellipsoid (HE) fluid. The I–N coexistence properties of HGO fluids have been accurately determined by combining different simulation techniques, including thermodynamic integration and Gibbs–Duhem integration for various molecular elongations, κ, in the range 3⩽κ⩽10. The accuracy of the Gibbs–Duhem integration scheme has been independently assessed by locating the I–N transition using the Gibbs-ensemble simulation technique for the largest elongation considered here (κ=10). The simulation results are analyzed within the context of the simplest version of the decoupling approximation as introduced by Parsons and Lee. The agreement between theoretical predictions and simulation data might be considered satisfactory, particularly for large nonsphericities. A comparison with the simulation results for the I–N properties of HE fluids shows that there exist large quantitative differences between HGO and HE fluids in this region, and this is ascribed to the larger volume excluded by a pair of HGO molecules compared to that of HE. In the light of the results presented here, approximating the distance of closest approach, or the excluded volume in the HE model by the corresponding expressions borrowed from the Gaussian overlap approximation—as is implemented in some theoretical descriptions of the HE fluid—does not seem to be appropriate.
Linear scaling and the 1,2-contracted Schrödinger equation
Authors: Mazziotti David A.
Published: July 2002

A contracted Schrödinger equation (1,2-CSE) is derived for the class of Hamiltonians without explicit interactions including those from Hartree–Fock and density functional theories. With cumulant reconstruction of the two-particle reduced density matrix (2-RDM) from the one-particle-RDM (1-RDM), the 1,2-CSE may be expressed solely in terms of the 1-RDM. We prove that a 1-RDM satisfies the 1,2-CSE if and only if it is an eigenstate of the N-particle Schrödinger equation. The 1,2-CSE is solved through the development and implementation of a reduced, linear-scaling analog of the ordinary power method for finding matrix eigenvalues. The power formula for updating the 1-RDM requires fewer matrix operations than the gradient procedure derived by Li et al. [Phys. Rev. B 47, 10891 (1993)] and Daw [Phys. Rev. B 47, 10895 (1993)]. Convergence of the contracted power method with purification is illustrated with several molecules. While providing a new tool for semiempirical, Hartree–Fock, and density functional calculations, the 1,2-CSE also represents an initial step toward a linear-scaling algorithm for solving higher CSEs which explicitly treat electron correlation.
Be
7
Be
tracer diffusion in a deeply supercooled Zr46.7Ti8.3Cu7.5Ni10Be27.5 melt
Authors: Rehmet A. , Rätzke K. , Faupel F. , Eversheim P. D. , Freitag K. , Geyer U. , Schneider S.
Published: July 2002

Beryllium self-diffusion in the deeply supercooled state of a Zr46.7Ti8.3Cu7.5Ni10Be27.5 alloy has been studied by a radiotracer technique by employing the isotope Be7 in combination with ion-beam sectioning. The temperature dependence of the diffusivity D is Arrhenius like with D(T)=102.6 m2 s−1 exp(−2.6 eV/kBT). The tracer diffusivities consistently extend the size dependence of the diffusion observed in the supercooled liquid state of this alloy to the smallest constituent, but are at variance with previously published results of chemical Be diffusion.
Isolated Fe–Co–Ni nanoparticles in a random arrangement and their magnetic properties
Authors: Yang Choong Jin , Kim Kyung Soo , Wu Jianmin
Published: July 2002

Fe–Co–Ni particles with an average size of 45 and 135 nm are characterized in terms of magnetic phase transformation and magnetic properties at room temperature. The bcc structure of Fe–Co–Ni spherical particles was able to be synthesized from Fe–Co–Ni–Al–Cu precursor films by heating at 600–800 °C for the phase separation of Fe–Co rich Fe–Co–Ni particles, followed by a postheating at 600 °C for 5 h. The average size of nanoparticles was directly determined by the thickness of precursor films. Exchange interactive hysteresis was observed for the nano-composite (Fe–Co–Ni)+(Fe–Ni–Al) films resulting from the short exchange interface between ferromagnetic Fe–Co–Ni particles surrounded by an almost papramagnetic Ni–Al–Fe matrix. Arraying the isolated Fe–Co–Ni nanoparticles in a random arrangement on Al2O3 substrate, the particle assembly showed a behavior of dipole interactive ferromagnetic clusters depending on their volume and interparticle distance.
Excited-state dynamics and carrier capture in InGaAs/GaAs quantum dots
Authors: Zhang L. , Boggess Thomas F. , Gundogdu K. , Flatté Michael E. , Deppe D. G. , Cao C. , Shchekin O. B.
Published: July 2002

Subpicosecond time-resolved photoluminescence upconversion is used to measure the 12 K first-excited-state dynamics in large InGaAs/GaAs self-assembled quantum dots designed for 1.3 μm diode lasers. A comparison with the ground-state dynamics suggests that energy relaxation occurs in a cascade through the multiple discrete levels with an average interlevel relaxation time of ∼250 fs. Excited-state emission is observed from two distinct populations. Due to the ultrafast relaxation from the excited state to the ground state in dots containing only a single exciton, the excited-state emission is dominated by the fraction of dots that capture more than one electron–hole pair. In this case, state filling in the ground state blocks the ultrafast relaxation channel, thereby enhancing the excited-state emission. While state filling and a random capture process dictate the primary features of the excited-state emission, at low excitation levels we find that the rise time of emission from the excited state is influenced by the much denser population of singly occupied dots.
Neutron scattering study of H2 adsorption in single-walled carbon nanotubes
Authors: Ren Y. , Price D. L.
Published: July 2002

H 2 adsorption in single-walled carbon nanotubes (SWCNTs) has been studied with quasielastic and inelastic neutron scattering. At 80 K, under a H2 pressure of 110 atm, H2 molecules gradually condense in the SWCNT sample. After pumping out at 25 K and 20 mTorr, the remaining H2 molecules show a quantum rotational transition at 14.5±0.1 meV, with a peak width that increases linearly with increasing temperature from 4.2 to 35 K. The H2 molecules remain in the sample up to 65 K and then start to desorb with increasing temperature. A broad inelastic scattering spectrum from the adsorbed hydrogen is also observed. The time scale of the dynamics is longer than 15 ps even at 200 K. Our results imply that hydrogen molecules are physisorbed in the interstitial tunnels of the SWCNT bundles.
The thickness dependence of the effect of pressures on magnetic and electronic properties of thin films of La2/3Ca1/3MnO3
Authors: Jacob S. , Roch T. , Razavi F. S. , Gross G. M. , Habermeier H.-U.
Published: July 2002

The discovery of the colossal magnetoresistance and pressure effects introduced new questions concerning the complex interplay among lattice structure, magnetism, and transport in doped perovskite manganites. To observe the interplay between the effect of pressure and strain due to the substrate in the La2/3Ca1/3MnO3 compound, we produced epitaxial films with the thickness varied between 20 and 100 nm on SrTiO3 substrates, using the pulse laser deposition technique and under the same epitaxial growth conditions. We measured magnetic transition temperature TC and resistivity ρ, as a function of pressure up to 8 kbar. Using these results, the dependence of bond compression on pressure, and the lattice distortion induced by the substrate, are discussed. We also report a universal behavior from the plot of d ln TC/dP and dρ/dP versus TC for these films, as well as previously published results on ceramic and thin films of this compound.
Electronic spin detection in molecules using scanning-tunneling- microscopy-assisted electron-spin resonance
Authors: Durkan C. , Welland M. E.
Published: July 2002

By combining the spatial resolution of a scanning-tunneling microscope (STM) with the electronic spin sensitivity of electron-spin resonance, we show that it is possible to detect the presence of localized spins on surfaces. The principle is that a STM is operated in a magnetic field, and the resulting component of the tunnel current at the Larmor (precession) frequency is measured. This component is nonzero whenever there is tunneling into or out of a paramagnetic entity. We have succeeded in obtaining spectra from free radical molecules from which the g factor of a spin entity may be inferred. For the molecules studied here, α,γ-bisdiphenylene-β-phenylallyl, g was found to be 2±0.1.
Cyclotron resonance and magnetotransport measurements in AlxGa1−xN/GaN heterostructures for x=0.15–0.30
Authors: Li Z.-F. , Lu W. , Shen S. C. , Holland S. , Hu C. M. , Heitmann D. , Shen B. , Zheng Y. D. , Someya T. , Arakawa Y.
Published: July 2002

Cyclotron resonance (CR) and magnetotransport experiments have been performed on modulation Si-doped AlxGa1−xN/GaN heterostructures with aluminum fraction x varying from 0.15 to 0.30. A clear CR absorption and Shubnikov–de Haas oscillations have been observed. The CR line shapes are analyzed by calculating the high frequency conductivity of a two-dimensional electron gas. The obtained electron effective mass m* and scattering time τ are found to depend on the aluminum fraction x. For x=0.30 the measured CR frequency shifts significantly upward, which demonstrates the formation of potential fluctuations in AlxGa1−xN/GaN heterostructures with large aluminum fraction x.
Effects of P doping on the thermoelectric properties of β-FeSi2
Authors: Ito Mikio , Nagai Hiroshi , Oda Etsushi , Katsuyama Shigeru , Majima Kazuhiko
Published: July 2002

The effects of P substitution for Si as an n-type dopant on the thermoelectric properties of hot-pressed β-FeSi2 were investigated. The Seebeck coefficient, electrical resistivity, and thermal conductivity of the FeSi2−xPx were measured from room temperature to 1100 K, and then the power factor and figure of merit were evaluated. The Seebeck coefficient of the hot-pressed FeSi2−xPx was negative, indicating that P atoms were definitely substituted for Si atoms as an n-type dopant in the β phase. The samples with x=0.02 and 0.04 had a Seebeck coefficient greater than that of the conventional hot-pressed Fe0.98Co0.02Si2 below 800 K. The electrical resistivity was significantly reduced by P doping, especially in the lower temperature range, and slightly decreased with increasing P content. The log ρ−1/T plots of the P-doped samples exhibited a specific behavior below 480 K, which was not observed in the case of the nondoped sample. The thermal conductivity of the P-doped sample was smaller than that of the nondoped sample in spite of the larger amount of the metallic ε phase. The figure of merit was significantly enhanced by P doping as compared with that of the nondoped β-FeSi2, and a high thermoelectric performance, almost the same as that of the conventional Fe0.98Co0.02Si2, was obtained by P doping into β-FeSi2.
Non-Gaussian transport in strong plasma turbulence
Authors: Annibaldi S. V. , Manfredi G. , Dendy R. O.
Published: July 2002

The transport of test particle ensembles moving in turbulent electrostatic fields governed by the Hasegawa–Mima equation is investigated. It ranges from subdiffusive to ballistic, depending on the size (in terms of thermal ion Larmor radii) of the domain considered, and on the magnitude of the background density gradient. In addition to the electrostatic potential, other fields, notably the vorticity and the Weiss field, prove to be very useful in accounting for particle dynamics and transport. For example, the existence of well defined core-circulation cell vortex structure in the Weiss field gives the most reliable guide to particle trapping, while locales of zero vorticity define regions of filamentary particle flow. Differential transport of guiding center particles and energetic particles gyrating with significant Larmor radius is investigated; the latter is strongly inhibited.
Spectroscopic constants of MH and M2 (M=Tl, E113, Bi, E115): Direct comparisons of four- and two-component approaches in the framework of relativistic density functional theory
Authors: Liu Wenjian , van Wüllen Christoph , Wang Fan , Li Lemin
Published: July 2002

The two-component DFT-ZORA (density functional theory, zeroth order regular approximation) method is implemented into the BDF (Beijing four-component density functional) program package so that systematic and direct comparisons between two- and four-component approaches are made possible for the first time. Different implementations of the ZORA method are also compared in this work. The calculated spectroscopic constants (bond lengths, binding energies, and force constants) for MH and M2 (M=Tl, E113, Bi, E115) by the two- and four-component approaches are very similar. The ionization and excitation energies for the metals obtained by these methods also agree very well with each other. Still, minor higher order relativistic effects beyond ZORA can be identified occasionally, but can be “safely” neglected. Therefore, the applicability of transformed (two-component) Hamiltonians to valence properties is well justified. However, the computational efficiency of four-component DFT compares favorably with that of two-component DFT. The problems of symmetry breaking and different treatments of open-shell systems are discussed by taking the Bi atom as an example.
Magnetic properties and shape memory of Fe-doped Ni52Mn24Ga24 single crystals
Authors: Wu G. H. , Wang W. H. , Chen J. L. , Ao L. , Liu Z. H. , Zhan W. S. , Liang T. , Xu H. B.
Published: July 2002

Single crystals of a pseudoquaternary Heusler alloy, Ni52Mn16Fe8Ga24, have now been synthesized. The substitution of Fe for Mn strengthens the magnetic exchange interactions, increasing the Curie temperature to 381 K. The Fe also increases the Ni magnetic moment to 0.41 μB relative to that in the stoichiometric Heusler composition. The strain accompanying the martensitic transformation is 2.4% in zero field and can be enhanced to 4.2% by a field of 1.2 T. A field-induced strain of 1.15% has been induced along [001] in unstressed crystals with magnetic fields of 1.2 T applied at 291 K.
Transport properties of the advancing interface ohmic contact to AlGaN/GaN heterostructures
Authors: Qiao D. , Yu L. S. , Jia L. , Asbeck P. M. , Lau S. S. , Haynes T. E.
Published: July 2002

The transport properties of the advancing interface ohmic contact to AlGaN/GaN heterostructure field-effect transistors have been investigated. We found that carrier transport across the AlGaN barrier layer is dominated by the tunneling of electrons that originate from the two-dimensional electron gas located at the AlGaN/GaN interface. The observed temperature dependence of the specific contact resistivity is different from that of the contact on highly doped bulk semiconductors, although tunneling current dominates the carrier transport in both cases.
Direct observation of local hot electron transport through Al2O3 tunnel junctions
Authors: Kurnosikov O. , de Jong J. E. A. , Swagten H. J. M. , de Jonge W. J. M.
Published: July 2002

A modified ballistic electron emission microscopy (BEEM) technique using local transport of hot electrons through a buried interface, was successfully applied to study the Al2O3 barrier in the Co/Al2O3/Ru tunnel junction. This technique enabled us to straightforwardly measure an effective barrier height of 1.7 eV and to observe the rise of the barrier height due to continuous current injection into a single point of the junction attributed to charging effects and/or degradation of the barrier structure. Scanning over an area of 510 nm×510 nm showed a spatial inhomogenity of the barrier resulting in different dependencies of the BEEM current on the energy of the injected electrons.
Simple approach to polyampholytes based on chain polarizabilities
Authors: Khan Malek O. , Åkesson Torbjörn , Jönsson Bo
Published: July 2002

The polarizabilities of polyampholytes have been calculated in Monte Carlo simulations. For chains with random charge topologies, the polarizability increases with the chain length and the chain flexibility. For block copolymers, the polarizability initially increases with the block size to a maximum and then decreases. The polarizability is used to characterize the interaction between polyampholytes and external electric fields. As long as the field is not too strong, the polyampholyte response will be linear. This observation is used to describe the chain behavior near charged walls, and for low to moderately charged walls the results agree well with Monte Carlo simulations. At high field strengths the chain properties are dependent on the chain topology and several cases are discussed.
Investigation of nanoscale structure in digital layers of Mn/GaAs and MnGa/GaAs
Authors: Kioseoglou G. , Kim S. , Soo Y. L. , Chen X. , Luo H. , Kao Y. H. , Sasaki Y. , Liu X. , Furdyna J. K.
Published: July 2002

Grazing incidence x-ray scattering (GIXS) and x-ray diffraction (XRD) techniques have been employed to study the microscopic structure of magnetic digital layers of Mn/GaAs and MnGa/GaAs. Samples with various GaAs layer thickness (8 to 16 monolayers) and a half monolayer of either Mn or MnGa were prepared by low-temperature molecular-beam epitaxy. All digital alloys consist of 50 periods of magnetic layers separated by GaAs. High crystalline quality was verified and the periodicity and layer thickness were determined from the GIXS and XRD data. In order to investigate the magnetic properties, we performed magnetization measurements on all samples using superconducting quantum interference device magnetometry (SQUID).
Physical optics treatment of the shadowgraph
Authors: Trainoff Steven P. , Cannell David S.
Published: July 2002

We present an analysis of the shadowgraph method of visualizing convective flows based on physical optics, treating the refractive-index perturbation caused by the flow as a transmission grating. Various patterns in thermal convection of an isotropic fluid as well as normal rolls in electroconvection of a nematic liquid crystal are considered. The results differ significantly from those of geometrical optics, showing that use of the shadowgraph as a quantitative tool for amplitude measurements should not, in general, be based on geometrical optics.
Segment diffusion and nuclear magnetic resonance spin-lattice relaxation of polymer chains confined in tubes: Analytical treatment and Monte Carlo simulation of the crossover from Rouse to reptation dynamics
Authors: Denissov Alexei , Kroutieva Margarita , Fatkullin Nail , Kimmich Rainer
Published: July 2002

The frequency and molecular mass dependences of nuclear magnetic resonance spin-lattice relaxation and the time dependence of the mean-squared segment displacement of Kuhn segment chains confined in static straight and randomly coiled tubes with “soft” and “hard” walls were studied. “Soft” walls were modeled in the form of a cylindrical distribution of a harmonic radial potential. This scenario is analytically solvable in contrast to the situation of “hard” (reflecting) walls corresponding to an infinitely deep square-well radial potential. In the latter case, we have therefore employed Monte Carlo simulations using a modified Stockmayer chain model. In both situations, qualitatively equivalent results were obtained. Depending on the effective tube diameter (or width of the potential well) a crossover from Rouse to reptation behavior occurs which sets on already far beyond the Flory radius of the polymer. In terms of the spin-lattice relaxation dispersion, reptation reveals itself by T1∝M0ω3/4 in the chain mode regime, in good agreement with experimental data for polymers in artificial tubes reported in our previous paper by Kimmich et al. [Chem. Phys. Lett. 307, 147 (1999)].
Design and performance of a THz emission and detection setup based on a semi-insulating GaAs emitter
Authors: Zhao G. , Schouten R. N. , van der Valk N. , Wenckebach W. Th. , Planken P. C. M.
Published: July 2002

We have built a relatively simple, highly efficient, THz emission and detection system centered around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused to a 120 μm diam spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, ±400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1-mm-thick (110) oriented ZnTe crystal or a (110) oriented 0.1-mm-thick GaP crystal. The maximum THz-induced differential signal that we observe is ΔI/I=7×10−3, corresponding to a THz peak amplitude of 95 V/cm. The THz average power was measured to be about 40 μW, to our knowledge, the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques.
Stable field emission with low threshold field from amorphous carbon films due to layer-by-layer hydrogen plasma annealing
Authors: Xu Jun , Huang Xiaohui , Li Wei , Chen Kunji , Xu Jianbin
Published: July 2002

The layer-by-layer hydrogen plasma treatment method, alternatively repeating the process of nanometer thickness film deposition and hydrogen plasma chemical annealing, was applied to fabricate amorphous carbon (a-C) films. It was shown that hydrogen plasma treatment reduced the size of sp2 clusters and resulted in the increase of the optical band gap. Consequently, a stable vacuum electron emission with a low threshold field was achieved from layer-by-layer hydrogen plasma annealed a-C films compared with that from conventionally deposited samples. The threshold electric field was as low as 2 V/μm. The improvement of field emission characteristics could be attributed to the large field enhancement effect due to the inhomogeneous distribution of nanometer scale sp2 clusters.
Oxygen-related dielectric relaxation and leakage characteristics of Pt/(Ba,Sr)TiO3/Pt thin-film capacitors
Authors: Shen Mingrong , Dong Zhenggao , Gan Zhaoqiang , Ge Shuibing , Cao Wenwu
Published: July 2002

The leakage characteristics and dielectric properties of Pt/(Ba,Sr)TiO3/Pt thin-film capacitors were found to be remarkably sensitive to the postannealing temperature in oxygen and nitrogen atmosphere. High leakage currents and low-frequency dielectric relaxation were found in as-deposited capacitors after they had been postannealed in nitrogen at 550 °C and subsequently annealed in oxygen at 350 °C. Such results are related to the mobile oxygen ions and oxygen vacancies accumulated in the (Ba,Sr)TiO3 films. The chemical process of the formation of charged oxygen ions during postannealing is proposed.
Valence excitations in individual single-wall carbon nanotubes
Authors: Stöckli Thomas , Bonard Jean-Marc , Châtelain André , Wang Zhong Lin , Stadelmann Pierre
Published: July 2002

We report on measurements of the plasmon losses of individual single-wall carbon nanotubes by electron energy-loss spectroscopy in a high-resolution transmission electron microscope. The experimental data are compared to simulated excitation probabilities calculated using the hydrodynamic theory of the interaction between a probe electron and a two-dimensional quasifree electron gas confined on a cylindrical shell. Depending on the nanotube geometry, the first- or the second-order oscillation mode dominates the loss spectrum. The resonance energy of the dominant resonance mode is found to depend on the radius of the nanotube.
Designed emitter states in resonant tunneling through quantum dots
Authors: Bryllert T. , Borgstrom M. , Sass T. , Gustafson B. , Landin L. , Wernersson L.-E. , Seifert W. , Samuelson L.
Published: July 2002

Resonant tunneling through a single layer of self-assembled quantum dots (QDs) is compared to tunneling through two layers of vertically aligned (stacked) dots. The difference can be viewed as going from a two-dimensional emitter to a zero-dimensional emitter. The temperature dependence of current peaks originating in tunneling through individual QDs and individual stacks is used to clarify this point. In addition, we show that the statistical size distribution of self-assembled quantum dots causing the inhomogeneous broadening in luminescence experiments can be analyzed in a resonant tunneling experiment.
Negative band gap bowing in epitaxial InAs/GaAs alloys and predicted band offsets of the strained binaries and alloys on various substrates
Authors: Kim Kwiseon , Hart Gus L. W. , Zunger Alex
Published: July 2002

We use pseudopotential theory to provide (1) the band offsets of strained GaAs and InAs on various substrates and (2) the energies Ev(x) and Ec(x) of the valence and conduction bands of InxGa1−xAs alloy, as a function of composition. Results are presented for both the bulk alloy and for the alloy strained on InP or GaAs. We predict that while Ec(x) bows downward for relaxed bulk alloys, it bows upward for strained epitaxial alloys. The calculated alloy offsets are used to discuss electron and hole localization in this system.
Spin valves with interlayer coupling domain biasing
Authors: Lu Z. Q. , Pan G.
Published: July 2002

Spin valves with structure of sub/Ta/NiFe/FeMn/NiFe/Mo/NiFe/Cu/NiFe/CrMnPt/Ta were developed. In this configuration, a longitudinal-biasing field from interlayer coupling stabilized the magnetic domains of the free layer. Planar Hall effect was used to study the magnetization reversal process of the free layer. It was shown that by adjusting the thickness of Mo layer, the interlayer coupling biasing field can provide domain stabilization and was sufficiently strong to constrain the magnetization in coherent rotation. This can prevent Barkhausen noises associated with magnetization reversal. Such a spin valve sensor structure is particularly attractive for high density read heads.
Effect of Pd or Pt addition to Ti/Al ohmic contact materials for n-type AlGaN
Authors: Murai S. , Masuda H. , Koide Y. , Murakami Masanori
Published: July 2002

Effects of Pd or Pt addition to Ti/Al ohmic contact materials used for n-type Al0.2Ga0.8N grown on the GaN substrates were studied where Si with 2×1018 cm−3 were doped in the AlGaN layers. The contact metals were prepared by depositing either by the electron-beam or thermal evaporation technique, and then annealed at high temperatures for 30 s in N2 atmosphere. It was found that the addition of a small amount of Pd or Pt to the Ti/Al ohmic contact materials reduced the contact resistivities of the Ti/Al contacts. The lowest contact resistivity of 4.1×10−6 Ω cm2 was obtained in the Ti(20 nm)/Pd(5 nm)/Al(200 nm) contact after annealing at 600 °C (where a slash “/” indicates the deposition sequence). The reduction of the contact resistivities by the Pd or Pt addition was believed to be due to increase in the donor concentration in the AlGaN layers by forming Pd–Ga or Pt–Ga compounds in the contact metals after annealing at elevated temperatures.
Neutral gas temperature estimates in an inductively coupled CF4 plasma by fitting diatomic emission spectra
Authors: Cruden Brett A. , Rao M. V. V. S. , Sharma Surendra P. , Meyyappan M.
Published: July 2002

In this work we examine the accuracy of plasma neutral temperature estimates by fitting the rotational band envelope of different diatomic species in emission. Experiments are performed in an inductively coupled CF4 plasma generated in a Gaseous Electronics Conference reference cell. Visible and ultraviolet emission spectra are collected at a power of 300 W (∼0.7 W/cm3) and pressure of 30 mTorr. The emission bands of several molecules (CF, CN, C2, CO, and SiF) are fit simultaneously for rotational and vibrational temperatures and compared. Four different rotational temperatures are obtained: 1250 K for CF and CN, 1600 K for CO, 1800 K for C2, and 2300 K for SiF. The vibrational temperatures obtained vary from 1750 to 5950 K, with the higher vibrational temperatures generally corresponding to the lower rotational temperatures. These results suggest that the different species have achieved different degrees of equilibration between rotational and vibrational modes and may not be equilibrated with the translational temperatures. The different temperatures also appear to relate to the expected spacial distribution of the molecules, with species expected to form near the electrode (C2, CO, SiF) having higher temperatures than species formed in the bulk.
Intersubband absorption performed on p-type modulation-doped Si0.2Ge0.8/Si quantum wells grown on Si0.5Ge0.5 pseudosubstrate
Authors: Diehl L. , Sigg H. , Dehlinger G. , Grützmacher D. , Müller E. , Gennser U. , Sagnes I. , Fromherz T. , Campidelli Y. , Kermarrec O. , Bensahel D. , Faist J.
Published: July 2002

We present intersubband absorption measurements performed on p-type quasistrain-compensated modulation-doped Si0.2Ge0.8/Si quantum wells grown on Si0.5Ge0.5 pseudosubstrates. Several intersubband absorption peaks are observed up to room temperature. A strong confinement shift of the resonance occuring between the ground and the first excited heavy hole states has been observed, with the absorption peak shifting from λ=5.3 μm to as short as 3.8 μm. Excellent overall agreement with a 6 band k⋅p calculation is obtained, proving the accuracy of recently predicted values of band offsets.
Light-controllable room temperature negative differential resistance in deep-trench type nitride–oxide tunneling device and its applications
Authors: Chen Fen , Li Baozhen , Feng Kai D
Published: July 2002

This study presents the room-temperature operation of deep-trench type nitride–oxide metal–insulator–semiconductor three-terminal tunneling devices which were fabricated by a standard metal–oxide–semiconductor process. It is instructive to observe a photoinduced N-type negative differential resistance (NDR) with a high peak-to-valley current ratio for device operated under negative polysilicon node bias under tungsten lamp illumination. An explanation was provided for the NDR phenomenon with proper three-terminal biasing. The sudden current drop under light illumination was caused by the sudden reduction of the two-carrier conduction due to Esaki band-to-band tunneling. The NDR amplitude could be modulated by light intensity. The position of the NDR current peak was tunable at different voltages with different p-well biases. The optoelectronic response of nitride–oxide devices we investigate here may open an application window for the nitride–oxide system in silicon-based optoelectronic integrated circuits, wireless communications, and future quantum devices.
Dielectric nonlinearity and stochastic effects in strontium titanate
Authors: Findikoğlu Alp T. , Camassa Roberto , Lythe Grant , Jia Q. X.
Published: July 2002

We report on experiments using compact coplanar waveguide devices, incorporating thin-film superconducting YBa2Cu3O7 electrodes. The substrate is strontium titanate, whose nonlinear dielectric properties are dependent on temperature and applied electric field. Close to zero dc bias, the frequency-doubled microwave power output increases by several orders of magnitude with modest broadband noise input. Experimental results are supported by analytical and numerical calculations based on a nonlinear wave equation.
Mid-infrared (λ≈7.4 μm) quantum cascade laser amplifier for high power single-mode emission and improved beam quality
Authors: Troccoli Mariano , Gmachl Claire , Capasso Federico , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

A quantum cascade laser amplifier has been developed. It was used to obtain high power single-mode emission at λ≈7.4 μm from a quantum cascade distributed feedback laser, together with enhanced beam quality. Laser and amplifier are directly coupled in a master oscillator power amplifier configuration. Peak optical powers of 0.5 W at 80 K have been obtained. Ninety percent of the total power is thereby emitted within a divergence of 20° in the lateral direction. The device showed single mode operation with a side mode suppression ratio of 30 dB in the temperature range from 10 to 280 K. This allowed tuning of the emission wavelength in the range from 7.36 to 7.46 μm. The estimated peak amplifier gain is 6.4 and 4.9 dB at 80 and 300 K, respectively, and the cavity losses are 12.5 and 22 cm−1 at the corresponding temperatures.
High-contrast organic light-emitting diodes on flexible substrates
Authors: Krasnov Alexey N.
Published: July 2002

Present organic light-emitting diode (OLED) displays, including those on flexible substrates, demonstrate relatively poor contrast due to the reflective metal electrode at the rear of the stack. In this letter we report on the use of a contrast-enhancing optical interference filter to improve the contrast of flexible OLED displays. Some other benefits of using the contrast-enhancing technology are also discussed.
Complex dynamics and bifurcations in a Hamiltonian system having a transversal homoclinic orbit to a saddle focus
Authors: Lerman L. M.
Published: July 2002

A Hamiltonian system having a saddle-focus singular point with a transversal homoclinic orbit is considered. The bifurcations that occur upon a variation of the Hamiltonian are studied. Reconstructions of a symbolic system describing the hyperbolic set are investigated. Bifurcations linked with the appearance of Smale horseshoes are pointed out. The results are applied to a system determining traveling-wave solutions of the Landau–Lifshitz equation. Multisolitons with any number of humps can exist.
Complex spiral wave dynamics in a spatially distributed ionic model of cardiac electrical activity
Authors: Courtemanche Marc
Published: July 2002

This study presents computations and analysis of the dynamics of reentrant spiral waves in a realistic model of cardiac electrical activity, incorporating the Beeler–Reuter equations into a two-dimensional cable model. In this medium, spiral waves spontaneously break up, but may be stabilized by shortening the excitation pulse duration through an acceleration of the dynamics of the calcium current. We describe the breakup of reentrant waves based on the presence of slow recovery fronts within the medium. This concept is introduced using examples from pulse circulation around a ring and extended to two-dimensional propagation. We define properties of the restitution and dispersion relations that are associated with slow recovery fronts and promote spiral breakup. The role of slow recovery fronts is illustrated with concrete examples from numerical simulations.