bluuebunny/crossref_metadata_2025 huggingface dataset:
Stimulated Brillouin backscatter in the presence of transverse plasma flow
Authors: Hinkel D. E. , Berger R. L. , Williams E. A. , Langdon A. B. , Still C. H. , Lasinski B. F.
Published: July 2002

Three-dimensional simulations show that stimulated Brillouin backscattered (SBS) light can be deflected in a direction opposite to transverse plasma flow. When the backscatter gain occurs predominantly in the region beyond where the incident light is deflected by transverse flow, and when the backscatter gain from the deflected incident light region is detuned from the undeflected incident light region by axial flow gradients, the SBS deflection correlates well with the steering of the incident beam. The level of Brillouin backscatter gain in the presence of transverse flow is less than that in the absence of transverse flow because of convective damping, where ion acoustic waves are swept out of the high intensity regions(s) of a beam.
Role of electron beam temperature in a high gain Compton free-electron laser
Authors: Seo Y. , Choi E. K.
Published: July 2002

Investigated are the effects of longitudinal electron temperature on the eigenmodes of a high gain Compton free-electron laser. It is noted that the electron beam temperature reduces the growth rate by destroying the refractive guiding property of the electron beam.
Effects of perpendicular velocity profile on sheared slab ηi instability at the tokamak internal transport barrier
Authors: Li Jiquan , Qu Wenxiao , Long Yongxing , Huang Lin , Dong Jiaqi
Published: July 2002

Stability of the sheared slab ηi mode at the tokamak internal transport barrier (ITB) off and near the minimum-q magnetic surface is studied under the assumption that the profile of the perpendicular rotation velocity has a Gaussian distribution. The results show that the effects of the perpendicular velocity profile with shear and curvature depend on the regimes with finite or weak magnetic shear where the ITB is located; on the rotation direction along the ion or electron diamagnetic drift; and on the mode structure of the eigen perturbation. The stabilizing or destabilizing roles of the perpendicular velocity shear and curvature are numerically analyzed in detail.
Local approximation for the phase velocity transform
Authors: Mattor Nathan
Published: July 2002

A recent paper [N. Mattor, Phys. Plasmas 5, 1822 (1998)] introduced the phase velocity transform. This was used to derive closed fluid moment equations from a collisionless kinetic equation. Here, a local approximation for this transform is introduced, which allows much simpler calculation. A local approximation to the Hilbert transform is also given.
Feedback stabilization of resistive shell modes in a reversed field pinch
Authors: Fitzpatrick R. , Yu E. P.
Published: July 2002

A reactor relevant reversed field pinch (RFP) must be capable of operating successfully when surrounded by a close-fitting resistive shell whose L/R time is much shorter than the pulse length. Resonant modes are largely unaffected by the shell resistivity, provided that the plasma rotation is maintained against the breaking effect of nonaxisymmetric eddy currents induced in the shell. This may require an auxiliary momentum source, such as a neutral beam injector. Nonresonant modes are largely unaffected by plasma rotation, and are expected to manifest themselves as nonrotating resistive shell modes growing on the L/R time of the shell. A general RFP equilibrium is subject to many simultaneously unstable resistive shell modes; the only viable control mechanism for such modes in a RFP reactor is active feedback. It is demonstrated than an N-fold toroidally symmetric arrangement of feedback coils, combined with a strictly linear feedback algorithm, is capable of simultaneously stabilizing all intrinsically unstable resistive shell modes over a wide range of different RFP equilibria. The number of coils in the toroidal direction N, at any given poloidal angle, must be greater than, or equal to, the range of toroidal mode numbers of the unstable resistive shell modes. However, this range is largely determined by the aspect-ratio of the device. The optimum coil configuration corresponds to one in which each feedback coil slightly overlaps its immediate neighbors in the toroidal direction. The critical current which must be driven around each feedback coils is, at most, a few percent of the equilibrium toroidal plasma current. The feedback scheme is robust to small deviations from pure N-fold toroidal symmetry or a pure linear response of the feedback circuits.
Effect of a resistive vacuum vessel on dynamo mode rotation in reversed field pinches
Authors: Fitzpatrick R. , Guo S. C. , Den Hartog D. J. , Hegna C. C.
Published: July 2002

Locked (i.e., nonrotating) dynamo modes give rise to a serious edge loading problem during the operation of high current reversed field pinches. Rotating dynamo modes generally have a far more benign effect. A simple analytic model is developed in order to investigate the slowing down effect of electromagnetic torques due to eddy currents excited in the vacuum vessel on the rotation of dynamo modes in both the Madison Symmetric Torus (MST) [Fusion Technol. 19, 131 (1991)] and the Reversed Field Experiment (RFX) [Fusion Eng. Des. 25, 335 (1995)]. This model strongly suggests that vacuum vessel eddy currents are the primary cause of the observed lack of mode rotation in RFX. The eddy currents in MST are found to be too weak to cause a similar problem. The crucial difference between RFX and MST is the presence of a thin, highly resistive vacuum vessel in the former device. The MST vacuum vessel is thick and highly conducting. Various locked mode alleviation methods are discussed.
Collective phenomena in the inner magnetosphere
Authors: Baker Daniel N. , Kanekal S. G. , Klimas A. J. , Vassiliadis D. , Pulkkinen T. I.
Published: July 2002

The Earth’s magnetosphere exhibits substantial complexity in many of its physical properties. Particle populations wax and wane and magnetic fields fluctuate on virtually all observed time scales, from less than 1 minute to many days. Much of the variability of the magnetosphere and its interaction with the ionosphere can be ascribed to the phenomena termed “substorms” and “storms.” Ample evidence is found that these geospace disturbances, though exhibiting event-to-event differences, are remarkably repetitive and have basic underlying similarities. The ring current development, radiation belt particle changes, and basic substorm patterns suggest a high degree of coherence in the phenomena. Observations, modeling, and basic physical properties are discussed here that point to a relatively ordered, low-dimensional underlying dynamics in the magnetosphere. These results suggest that nonlinear processes and couplings introduce much of the observed complexity in magnetospheric particle and field changes.
Pellet driven disruptions in tokamaks
Authors: Strauss H. R. , Park W.
Published: July 2002

Pellet injection can trigger ballooning like instability in tokamaks, driven by the large pressure perturbation of the pellet cloud. The instability is easier to excite when pellets are injected on the low magnetic field side of the tokamak, which is the usual case. This is because the pellet pressure perturbation tends to add to the background pressure drive. When pellets are injected on the high field side, the instability is harder to excite, because the pellet pressure perturbation opposes the background pressure drive. Nonlinearly, the instability causes fragmentation of the pellet cloud, and can give anomalous pellet penetration. This effect might explain recent experimental observations.
Anisotropies in magnetic field evolution and local Lyapunov exponents
Authors: Tang X. Z. , Boozer A. H.
Published: July 2002

The natural occurrence of small scale structures and the extreme anisotropy in the evolution of a magnetic field embedded in a conducting flow is interpreted in terms of the properties of the local Lyapunov exponents along the various local characteristic (un)stable directions for the Lagrangian flow trajectories. The local Lyapunov exponents and the characteristic directions are functions of Lagrangian coordinates and time, which are completely determined once the flow field is specified. The characteristic directions that are associated with the spatial anisotropy of the problem, are prescribed in both Lagrangian and Eulerian frames. Coordinate transformation techniques are employed to relate the spatial distributions of the magnetic field, the induced current density, and the Lorentz force, which are usually followed in Eulerian frame, to those of the local Lyapunov exponents, which are naturally defined in Lagrangian coordinates.
Axial length and separatrix radius behavior of field-reversed configuration plasma in dynamic compression of mirror distance
Authors: Kitano K. , Yamanaka K. , Okada S. , Goto S.
Published: July 2002

The axial magnetic compression experiment of the field-reversed configuration (FRC) plasma is reported. The FRC produced in the theta-pinch system is translated into the confinement region. The separatrix length of the translated FRC is decided by the mirror distance. The compression is done in a manner as shortening the distance in time. The compression coil is installed inside the chamber to raise the strength of the confinement field at the neighborhood of the mirror. The mirror distance is compressed to be 70% of the original one. The increment of the separatrix radius is observed to be 14%. This is nearly consistent with the adiabatic calculation. The decay rate of the radius has a constant value. From the line integrated density signal measured by the interferometer, no n=2 rotational instability is observed even in the case of the compression.
Inductive classification of low-mode and high-mode edge parameters
Authors: Deranian R. D. , Groebner R. J. , Pham D. T.
Published: July 2002

A pattern-recognition algorithm has been used to search for edge operational space diagrams which discriminate well between the low-mode (L-mode) and high-mode (H-mode) states in the DIII-D [J. L. Luxon, R. Anderson, F. Batty et al., in Proceedings of the 11th Conference on Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] tokamak. The study uses a large database of electron temperatures Te, electron densities ne, electron pressures Pe, and their gradients, obtained for a fixed magnetic configuration. The data are evaluated at the edge of the plasma in the region where the H-mode transport barrier forms. An inductive classification algorithm shows that operational space diagrams which include either ∇Pe or ∇Te provide significantly better discrimination between the L-mode and H-mode states than diagrams based on Te and ne alone. These conclusions are based on standard metrics of pattern recognition. The successful application of a classification algorithm to this simple system provides confidence that classification techniques can be applied to larger and more complex databases of edge parameters. Moreover, these results suggest that edge pressure or temperature gradients should be considered on an equal footing with edge temperature as potential control parameters for the H-mode transition.
Particle acceleration by a fast ordinary mode in an electron–positron plasma
Authors: Machabeli G. Z. , Vladimirov S. V. , Melrose D. B. , Luo Q.
Published: July 2002

The possibility of nonresonant particle acceleration in an electron–positron plasma of a pulsar magnetosphere is investigated. A mechanism is proposed in which modulations of a fast superluminal (with phase velocity exceeding the speed of light) longitudinal ordinary mode (caused by a beat wave of two transverse electromagnetic waves propagating along the magnetic field) stimulate nonresonant quasilinear diffusion leading to a redistribution of plasma particles in pitch angle. The resulting perpendicular momenta of the particles lead to synchrotron radiation which is in the γ-ray range.
Direct-drive high-convergence-ratio implosion studies on the OMEGA laser system
Authors: Marshall F. J. , Delettrez J. A. , Epstein R. , Glebov V. Yu. , Harding D. R. , McKenty P. W. , Meyerhofer D. D. , Radha P. B. , Seka W. , Skupsky S. , Smalyuk V. A. , Soures J. M. , Stoeckl C. , Town R. P. J. , Yaakobi B. , Li C. K. , Séguin F. H. , Hicks D. G. , Petrasso R. D.
Published: July 2002

A series of direct-drive implosion experiments, using room-temperature, gas-filled CH targets, are performed on the University of Rochester’s OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The target performance at stagnation and its dependence on beam smoothing and pulse shaping is investigated. Compressed core conditions are diagnosed using x-ray and neutron spectroscopy, and x-ray imaging. The individual beams of OMEGA are smoothed by spectral dispersion in two dimensions (2D SSD) with laser bandwidths up to ∼0.3 THz, with 1 ns square to 2.5 ns shaped pulses. A clear dependence of target performance on pulse shape and beam smoothing is seen, with the target performance (yield, areal density, and shell integrity) improving as SSD bandwidth is applied.
Valence-band discontinuities at AlAs-based heterojunction interfaces
Authors: Kelly M. K. , Niles D. W. , Colavita E. , Margaritondo G. , Henzler M.
Published: July 2002

We extended our empirical method for estimating heterojunction band discontinuities to systems containing AlAs. The necessary data were provided by a synchrotron-radiation photoemission study of the AlAs-Ge interface formation. Our results suggest that the ‘‘15%–85%’’ rule deduced for Ga1−xAlxAs-GaAs cannot be extrapolated to AlAs-GaAs.
Transient enhanced diffusion during rapid thermal annealing of boron implanted silicon
Authors: Cho K. , Numan M. , Finstad T. G. , Chu W. K. , Liu J. , Wortman J. J.
Published: July 2002

Enhanced diffusion of ion implanted boron in silicon during the first few seconds (transient) of rapid thermal annealing (RTA) has previously tentatively been attributed to either interstitial boron diffusion or damage enhanced diffusion. We have performed various anneal sequences of 11B implanted Si combined with post-implantation of 10B and measured the boron concentration profiles with secondary ion mass spectroscopy. The data show that the enhanced diffusion found in the tail of the profiles after RTA is not caused by fast diffusion of boron ending up at interstitial sites after the slowing down during implantation. Rather the enhanced diffusion in the tail is associated with the implantation damage annealing.
Observations on intensity oscillations in reflection high-energy electron diffraction during epitaxial growth of Si(001) and Ge(001)
Authors: Aarts J. , Gerits W. M. , Larsen P. K.
Published: July 2002

Intensity oscillations have been found in the specular beam of reflection high-energy electron diffraction patterns during growth of Si(001) and Ge(001) by molecular beam epitaxy. The reported results demonstrate the dependence of the amplitude and damping of the oscillations on different parameters such as substrate temperature, electron beam angle of incidence, and azimuth.
Photoluminescence studies of ZnTe-CdTe strained-layer superlattices
Authors: Miles R. H. , Wu G. Y. , Johnson M. B. , McGill T. C. , Faurie J. P. , Sivananthan S.
Published: July 2002

Photoluminescence from ZnTe-CdTe strained-layer superlattices has been observed for the first time. Superlattices with CdTe and ZnTe layer thicknesses between 20 and 51 Å have been compared with CdxZn1−xTe alloys. The superlattices display intense visible photoluminescence which is observed at lower energies than for corresponding alloys. Both the intensities and energies of the luminescence indicate good superlattice structure. Second order k⋅p calculations have been performed and are found to be in agreement with experiment. Substrates greatly lattice mismatched to the superlattice are shown not to play a role in determining the band gap as the lattice constant jumps to that of the free-standing superlattice.
Temperature dependence of electrical properties of non-doped and nitrogen-doped beta-SiC single crystals grown by chemical vapor deposition
Authors: Suzuki Akira , Uemoto Atsuko , Shigeta Mitsuhiro , Furukawa Katsuki , Nakajima Shigeo
Published: July 2002

Electrical properties of non-doped and nitrogen-doped n-type β-SiC films grown on Si substrates have been investigated at 70–1000 K. Those of non-doped films remarkably depend on the Si/C ratio of the source gases. The highest mobilities of non-doped films are 510 and 1330 cm2 V−1 s−1 at 296 and 71 K, respectively. The carrier concentrations are as low as 6×1016 cm−3 even at 1000 K. Ionization energies of 34 –37 meV for nitrogen donors and 19–25 meV for unknown donors of non-doped films are obtained. Mobilities of both non-doped and nitrogen-doped films are dominated by lattice scattering at high temperatures and by impurity scattering at low temperatures. Nitrogen donors strongly affect the impurity scattering.
Time-of-flight measurements of minority-carrier transport in p-silicon
Authors: Tang D. D. , Fang F. F. , Scheuermann M. , Chen T. C.
Published: July 2002

The electric field dependence of electron mobilities in p-type silicon with doping density of 4.5×1016 cm−3 at room temperature was measured using a time-of-flight technique. It was found that the electron mobility at zero field is very close to that in n-type silicon of equivalent doping density; however, it decreases rapidly with electric field in a range from 0 to 0.2 kV/cm and more gradually at higher fields. The effect is attributed to the electron-hole scattering.
Limitations to the open circuit voltage of amorphous silicon solar cells
Authors: Hack M. , Shur M.
Published: July 2002

In this paper, we investigate the open circuit voltage of amorphous silicon alloy p-i-n and n-i-p solar cells and show that the low open circuit voltage of some devices can be caused by a built-in potential smaller than normal arising from a low band-gap p+ layer. For these solar cells, in good agreement with experimental data, the introduction of a boron profile in 200–500 Å of the intrinsic region near the p+-i interface can enhance the open circuit voltage by about 150–200 mV to a value close to the bulk recombination limit.
Boron heavy doping for Si molecular beam epitaxy using a HBO2 source
Authors: Tatsumi Toru , Hirayama Hiroyuki , Aizaki Naoaki
Published: July 2002

Boron doping with a high carrier concentration has been realized in Si molecular beam epitaxy (MBE) using a HBO2 source with the usual Knudsen cell. Maximum carrier concentration has reached 6×1020 cm−3 at crucible temperatures of 900 °C. From a comparison between activation energy for vapor pressure and carrier concentration dependence on crucible temperature, it was concluded that boron evaporated in the form of HBO2 (g) from the HBO2 source. The boron profile achieved by Knudsen cell shutter opening and closing was sufficiently steep. The oxygen concentration in the epitaxial film depended on the growth temperature. Above 750 °C, the oxygen concentration decreased to under the detection limit of secondary ion mass spectroscopy, 1018 cm−3. These results make it possible to use boron as the p-type dopant in silicon MBE without using ion imbedding or a very high temperature crucible.
Defects in neutron irradiated SiC
Authors: Nagesh V. , Farmer J. W. , Davis R. F. , Kong H. S.
Published: July 2002

Deep level transient spectroscopy and resistivity measurements have been used to characterize defects in as-grown and neutron irradiated epitaxially grown 3C-SiC on Si(100) substrates. The thick epilayers were free of defects; neutron irradiation induced an electron trap with an activation energy of 0.49 eV. The SiC-Si interface has a large density of defects and dislocations. Most of the irradiation defects are confined to the lower two-thirds of the band gap. Ninety percent of these defects can be removed by annealing at 350 °C.
Model of rf discharges at frequencies greater than the ionic plasma frequency
Authors: Pointu A. M.
Published: July 2002

The extension of a previous model using an asymmetrical double-probe theory is set out to describe rf discharges in the opposite limit of mainly capacitive electrode sheaths. It reasonably agrees with available experimental results for an excitation frequency equal to or greater than 13.56 MHz.
Resonant two-phonon Raman scattering in GaAs: A sensitive probe for implantation damage and annealing
Authors: Wagner J. , Hoffman Ch.
Published: July 2002

We have used resonant Raman scattering by two longitudinal optical (LO) phonons in GaAs to probe ion implantation induced damage and its annealing. It was found that the strength of the two-LO phonon peak is very sensitive to ion bombardment induced damage as demonstrated by the change in the Raman spectrum after sputtering with low-energy Ar ions. For the study of ion implanted and thermally annealed material the two-LO phonon scattering is more sensitive to the lattice perfection than the first-order Raman spectrum especially in the region where electrical activation of the dopant sets in. Spatially resolved Raman spectroscopy revealed considerable variations of the crystalline perfection across the annealed sample. The latter study was particularly facilitated by using optical multichannel detection.
Efficient XeCl/H2 Raman shifting to a blue-green region
Authors: Cheng Xusan , Lou Qihong , Wang Runwen , Wang Zhijiang
Published: July 2002

90% photon conversion of 18 MW (1 J, 55 ns) power XeCl laser radiation to Raman Stokes beams is reported with 39% photon efficiency and ∼60% peak power conversion efficiency to the third Stokes order S3 (499 nm). Relaxation oscillations of higher order Si(i≥3) pulses have been observed.
Growth of CdZnTe on Si by low-pressure chemical vapor deposition
Authors: Goela Jitendra S. , Taylor Raymond L.
Published: July 2002

Epitaxial layers of Cd1−xZnxTe (0.04≤x≤0.62) have been grown on (100) and (111) silicon substrates by a low-pressure chemical vapor deposition (LPCVD) process. Elemental metals were used as sources of cadmium and zinc and dimethyltelluride was used as a source of tellurium. The characterization of layers by x-ray diffraction, scanning electron microscope, and scanning electron microscope x-ray dispersive analysis shows that hot wall LPCVD is a viable technique to grow large-area epitaxial layers of CdZnTe on different substrates.
High performance silver ohmic contacts to YBa2Cu3O6+x superconductors
Authors: Tzeng Y. , Holt A. , Ely R.
Published: July 2002

Silver-YBa2Cu3O6+x superconductor ohmic contacts with specific contact resistance on the order of 10−8 Ω cm2 have been demonstrated. Alloying the contacts in O2 at 500 °C enhances the contact conductivity by two orders of magnitude.
Very shallow p+-n junction formation by low-energy BF+2 ion implantation into crystalline and germanium preamorphized silicon
Authors: Ozturk M. C. , Wortman J. J. , Fair R. B.
Published: July 2002

Very low energy (6 keV) BF+2 ion implantation has been used to form very shallow (≤1000 Å) junctions in crystalline and Ge+ preamorphized Si. Low-temperature furnace annealing was used to regrow the crystal, and rapid thermal annealing was used for dopant activation and radiation damage removal. In preamorphized samples, Ge+ implantation parameters were found to have an influence on B diffusion. Our results show that for temperatures higher than 950 °C, B diffusion, rather than B channeling, becomes the dominant mechanism in determining the junction depth. Computer simulations of the profiles show regions of retarded and enhanced B diffusion, which depend on surface and end-of-range damage, respectively.
Transmission electron microscopy study of the formation of epitaxial CoSi2/Si (111) by a room-temperature codeposition technique
Authors: d’Anterroches Cecile , Yakupoglu H. Nejat , Lin T. L. , Fathauer R. W. , Grunthaner P. J.
Published: July 2002

Co and Si have been codeposited on Si (111) substrates near room temperature in a stoichiometric 1:2 ratio in a molecular beam epitaxy system. Annealing of these deposits yields high-quality single-crystal CoSi2 layers. Transmission electron microscopy has been used to examine as-deposited layers and layers annealed at 300, 500, and 600 °C. Single-crystal epitaxial grains of CoSi2 embedded in a matrix of amorphous Co/Si are observed in as-deposited samples, while the layer is predominantly single-crystal, inhomogeneously strained CoSi2 at 300 °C. At 600 °C, a homogeneously strained single-crystal layer with a high density of pinholes is observed. In contrast to other solid phase epitaxy techniques used to grow CoSi2 on Si (111), no intermediate silicide phases are observed prior to the formation of CoSi2.
Time-dependent approach to double-barrier quantum well oscillators
Authors: Liu H. C.
Published: July 2002

Microwave and millimeter-wave oscillation frequencies and efficiencies of heterojunction double-barrier resonant-tunneling structures are investigated theoretically from a time-dependent point of view. Calculations for two specific devices show that the oscillation characteristics depend strongly on the device parameters, i.e., barrier heights and widths and the well width. Results also show that oscillations in frequencies above limits predicted by a transit tunneling time consideration are less efficient, but not vanishing.
Sputtering of silicon dioxide near threshold
Authors: Todorov S. S. , Fossum E. R.
Published: July 2002

The sputtering rate for silicon dioxide by argon ion bombardment at energies appropriate for ion beam deposition (<100 eV) has been measured. It has been found that the energy dependence of the oxide sputtering rate at these low energies is easily predicted by assuming the yield is limited by the metallic component of the binary target. This assumption is shown to predict also the sputtering rate of other metallic oxides.
Reduced optical waveguide losses of a partially disordered GaAs/AlGaAs single quantum well laser structure for photonic integrated circuits
Authors: Werner J. , Kapon E. , Von Lehmen A. C. , Bhat R. , Colas E. , Stoffel N. G. , Schwarz S. A.
Published: July 2002

We present TE and TM waveguide loss measurements of partially disordered GaAs/AlGaAs quantum well separate confinement laser structures. Disordering is accomplished by silicon ion implanation and subsequent annealing. Propagation losses as low as 9 cm−1 are observed at the lasing wavelength of the corresponding untreated laser wafer. The waveguides are shown to be compatible with fabrication and dimensional requirements of high quality semiconductor lasers; ridge waveguide lasers fabricated in unimplanted portions of the same wafer exhibit threshold currents of only 8 mA. The results show that impurity-induced quantum well disordering is suitable for monolithic integration of low-threshold quantum well lasers and transparent optical waveguides.
Molecular beam epitaxial growth of ultrathin buried metal layers: (Al,Ga)As/NiAl/(Al,Ga)As heterostructures
Authors: Harbison J. P. , Sands T. , Tabatabaie N. , Chan W. K. , Florez L. T. , Keramidas V. G.
Published: July 2002

We report the first growth of epitaxial NiAl metallic layers buried within monocrystalline GaAs/AlAs/NiAl/AlAs/GaAs heterostructures deposited entirely within a molecular beam epitaxy growth chamber. The layer growth sequence is monitored by reflection high-energy electron diffraction. Cross-sectional transmission electron microscopy shows that the metal layers and the III-V overgrowth are monocrystalline and of high quality. Thin, buried NiAl layers over the entire thickness range investigated (3–100 nm) are electrically continuous (69 μΩ cm at 3 nm). The heterostructures formed by this process can be used for the fabrication of thin-metal buried-layer devices utilizing ballistic transport or quantum mechanical tunneling across thin metal bases.
Electrical characteristics of Schottky diodes fabricated using plasma assisted chemical vapor deposited diamond films
Authors: Gildenblat G. Sh. , Grot S. A. , Wronski C. R. , Badzian A. R. , Badzian T. , Messier R.
Published: July 2002

Schottky diodes were fabricated using gold and aluminum contacts to thin diamond films obtained by a microwave plasma assisted chemical vapor deposition process. The current-voltage and capacitance-voltage-frequency characteristics of these devices are similar to those fabricated on a crystalline diamond base formed by traditional ultrahigh pressure process.
Epitaxial growth of ZrN on Si(100)
Authors: Barnett S. A. , Hultman L. , Sundgren J.-E. , Ronin F. , Rohde S.
Published: July 2002

Epitaxial ZrNx (x=1.0±0.05) films have been grown on Si(100) substrates using reactive magnetron sputtering. The films were deposited from a Zr sputtering target in a pure N2 discharge with a growth rate of 700 nm/h and substrate temperatures Ts=750 and 900 °C. X-ray diffractometer results showed complete (100) preferred orientation for both temperatures. Transmission electron microscopy (TEM) observations for samples grown at Ts=750 °C indicated that the (100) grains were almost randomly oriented about the surface normal, but that a small fraction of epitaxial grains were present. Electron channeling and TEM showed that ZrN films were epitaxial for Ts=900 °C. The epitaxial relationships were found to be ZrN(100)//Si(100) and ZrN[011]//Si[011]. The resistivity of the films was 25–35 μΩ cm.
Differential long-term and multilineage engraftment potential from subfractions of human CD34+cord blood cells transplanted into NOD/SCID mice
Authors: Hogan Christopher J. , Shpall Elizabeth J. , Keller Gordon
Published: July 2002

Over the past decade xenotransplantation systems have been used with increasing success to gain a better understanding of human cells that are able to initiate and maintain the hematopoietic systemin vivo. The nonobese diabetic/severe combined immunodeficiency (SCID) mouse has been a particularly useful model. Human cells capable of hematopoietic repopulation in this mouse, termed SCID-repopulating cells, have been assumed to represent the most primitive elements of the hematopoietic system, responsible for long-term maintenance of hematopoiesis. However, we demonstrate that SCID-repopulating cells present in the CD34+cell fraction of cord blood can be segregated into subpopulations with distinct repopulation characteristics. CD34+/CD38+progenitors can repopulate recipients rapidly, but can only maintain the graft for 12 weeks or less and have no secondary repopulation potential. Conversely, the more primitive CD34+/CD38−subpopulation repopulates recipients more gradually, can maintain the graft for at least 20 weeks, and contains cells with serial repopulation potential throughout the engraftment period. Additionally, a much higher frequency of T cell precursors are found among SCID-repopulating cells in the CD34+/CD38−subpopulation. These findings demonstrate that cells with variable repopulation potential comprise the human CD34+population and that short- and long-term potential of human precursors can be evaluated in the mouse model.
Functional MRI detection of pharmacologically induced memory impairment
Authors: Sperling Reisa , Greve Douglas , Dale Anders , Killiany Ronald , Holmes Jennifer , Rosas H. Diana , Cocchiarella Andrew , Firth Paul , Rosen Bruce , Lake Stephen , Lange Nicholas , Routledge Carol , Albert Marilyn
Published: July 2002

To examine alterations in brain activation associated with
pharmacologically induced memory impairment, we used functional MRI
(fMRI) to study the effects of lorazepam and scopolamine on a
face–name associative encoding paradigm. Ten healthy young subjects
were scanned on four occasions, 2 weeks apart; they were administered
i.v. saline during two placebo-scanning sessions and then alternately
administered i.v. lorazepam (1 mg) or scopolamine (0.4 mg) in a
double-blind, randomized, cross-over design. Both the extent and
magnitude of activation within anatomic regions of interest (ROIs) were
examined to determine the reproducibility of activation in the placebo
sessions and the regional specificity of the pharmacologic effects.
Activation within all ROIs was consistent across the two placebo scans
during the encoding of novel face–name pairs (compared with visual
fixation). With the administration of either lorazepam or scopolamine,
significant decreases were observed in both the extent and magnitude of
activation within the hippocampal, fusiform, and inferior prefrontal
ROIs, but no significant alterations in activation in the striate
cortex were found. Both medications impaired performance on postscan
memory measures, and significant correlations between memory
performance and extent of activation were found in hippocampal and
fusiform ROIs. These findings suggest that pharmacologic effects can be
detected with fMRI by using a reproducible experimental paradigm and
that medications that impair memory also diminish activation in
specific brain regions thought to subserve complex memory processes.
Cytosolic malate dehydrogenase confers selectivity of the nucleic acid-conducting channel
Authors: Hanss Basil , Leal-Pinto Edgar , Teixeira Avelino , Christian Robert E. , Shabanowitz Jeffery , Hunt Donald F. , Klotman Paul E.
Published: July 2002

We have described previously a cell surface channel that is highly selective for nucleic acids. Nucleic acid conductance is 10 pS and the channel is at least 10,000-fold more selective for oligodeoxynucleotides than any anion tested (1). Herein we provide evidence that the nucleic acid-conducting channel (NACh) is a heteromultimeric complex of at least two proteins; a 45-kDa pore-forming subunit (p45) and a 36-kDa regulatory subunit (p36). Reconstitution of p45 in planar lipid bilayers resulted in formation of a channel which gated in the absence of nucleic acid and which was more selective for anions (including oligonucleotide) than cations. This channel exhibited transitions from one level of current to another (or to the closed state); however the incidence of transitions was rare. Channel activity was not observed when p36 was reconstituted alone. Reconstitution of p36 with p45 restored nucleic acid dependence and selectivity to the channel. Protein sequence analysis identified p36 as cytosolic malate dehydrogenase (cMDH). Experiments were performed to prove that cMDH is a regulatory subunit of NACh. Selective activity was observed when p45 was reconstituted with pig heart cMDH but not with mitochondrial MDH. Both the enzyme substrate
l
-malate and antiserum raised against cMDH block NACh activity. These data demonstrate that a nucleic acid conducting channel is a complex of at least two proteins, p45 and cMDH. Furthermore, these data establish that cMDH confers nucleic acid selectivity of the channel.
B lymphocyte-restricted expression of prion protein does not enable prion replication in prion protein knockout mice
Authors: Montrasio Fabio , Cozzio Antonio , Flechsig Eckhard , Rossi Daniela , Klein Michael A. , Rülicke Thomas , Raeber Alex J. , Vosshenrich Christian A. J. , Proft Juliane , Aguzzi Adriano , Weissmann Charles
Published: July 2002

Prion replication in spleen and neuroinvasion after i.p. inoculation of mice is impaired in forms of immunodeficiency where mature B lymphocytes are lacking. In spleens of wild-type mice, infectivity is associated with B and T lymphocytes and stroma but not with circulating lymphocytes. We generated transgenic prion protein knockout mice overexpressing prion protein in B lymphocytes and found that they failed to accumulate prions in spleen after i.p. inoculation. We conclude that splenic B lymphocytes are not prion-replication competent and that they acquire prions from other cells, most likely follicular dendritic cells with which they closely associate and whose maturation depends on them.
Lack of a role for transforming growth factor-β in cytotoxic T lymphocyte antigen-4-mediated inhibition of T cell activation
Authors: Sullivan Timothy J. , Letterio John J. , van Elsas Andreas , Mamura Mizuko , van Amelsfort Jocea , Sharpe Shellyann , Metzler Barbara , Chambers Cynthia A. , Allison James P.
Published: July 2002

Similarities in the phenotypes of mice deficient for cytotoxic T lymphocyte antigen-4 (CTLA-4) or transforming growth factor-β1 (TGF-β1) and other observations have led to speculation that CTLA-4 mediates its inhibitory effect on T cell activation via costimulation of TGF-β production. Here, we examine the role of TGF-β in CTLA-4-mediated inhibition of T cell activation and of CTLA-4 in the regulation of TGF-β production. Activation of AND TCR transgenic mouse T cells with costimulatory receptor-specific antigen presenting cells results in efficient costimulation of proliferation by CD28 ligation and inhibition by CTLA-4 ligation. Neutralizing antibody to TGF-β does not reverse CTLA-4-mediated inhibition. Also, CTLA-4 ligation equally inhibits proliferation of wild-type, TGF-β1−/−, and Smad3−/−T cells. Further, CTLA-4 engagement does not result in the increased production of either latent or active TGF-β by CD4+T cells. These results indicate that CTLA-4 ligation does not regulate TGF-β production and that CTLA-4-mediated inhibition can occur independently of TGF-β. Collectively, these data demonstrate that CTLA-4 and TGF-β represent distinct mechanisms for regulation of T cell responses.
Anion-templated self-assembly of tetrahedral cage complexes of cobalt(II) with bridging ligands containing two bidentate pyrazolyl-pyridine binding sites
Authors: Paul Rowena L. , Bell Zöe R. , Jeffery John C. , McCleverty Jon A. , Ward Michael D.
Published: July 2002

The bridging ligands L
1
and L
2
contain two
N
,
N-
bidentate pyrazolyl-pyridine units linked to a central aromatic spacer unit (1,2-phenyl or 2,3-naphthyl, respectively). Reaction with Ni(II) salts and treatment with the anions tetrafluoroborate or perchlorate result in formation of dinuclear complexes having a 2:3 metal:ligand ratio, with one bridging and two terminal tetradentate ligands. In contrast, reaction of L
1
and L
2
with Co(II) salts, followed by treatment with tetrafluoroborate or perchlorate, results in assembly of cage complexes having a 4:6 metal:ligand ratio; these complexes have a metal ion at each corner of an approximate tetrahedron, and a bis-bidentate bridging ligand spanning each edge. The central cavity is occupied by a tetrahedral counterion that forms multiple hydrogen-bonding interactions with the methylene protons of the bridging ligands. The anionic guest fits tightly into the central cavity of the cage to which it is ideally complementary in terms of shape, size, and charge. Solution NMR experiments show that the central anion acts as a template for cage formation, with a mixture of Co(II) and the appropriate bridging ligand alone giving no assembly into a cage until the tetrahedral anion is added, at which point cage assembly is fast and quantitative. The difference between the structures of the complexes with Ni(II) and Co(II) illustrate how the uncoordinated anions can exert a profound influence on the course of the assembly process.
Two-photon excitation microscopy of tryptophan-containing proteins
Authors: Lippitz M. , Erker W. , Decker H. , van Holde K. E. , Basché T.
Published: July 2002

We have examined the feasibility of observing single protein molecules by means of their intrinsic tryptophan emission after two-photon excitation. A respiratory protein from spiders, the 24-meric hemocyanin, containing 148 tryptophans, was studied in its native state under almost
in vivo
conditions. In this specific case, the intensity of the tryptophan emission signals the oxygen load, allowing one to investigate molecular cooperativity. As a system with even higher tryptophan content, we also investigated latex spheres covered with the protein avidin, resulting in 340 tryptophans per sphere. The ratio of the fluorescence quantum efficiency to the bleaching efficiency was found to vary between 2 and 180 after two-photon excitation for tryptophan free in buffer solution, in hemocyanin, and in avidin-coated spheres. In the case of hemocyanin, this ratio leads to about four photons detected before photobleaching. Although this number is quite small, the diffusion of individual protein molecules could be detected by fluorescence correlation spectroscopy. In avidin-coated spheres, the tryptophans exhibit a higher photostability, so that even imaging of single spheres becomes possible. As an unexpected result of the measurements, it was discovered that the population of the oxygenated state of hemocyanin can be changed by means of a one-photon process with the same laser source that monitors this population in a two-photon process.
Cortical cell death induced by IL-1 is mediated via actions in the hypothalamus of the rat
Authors: Allan Stuart M. , Parker Lisa C. , Collins Barry , Davies Ralph , Luheshi Giamal N. , Rothwell Nancy J.
Published: July 2002

The cytokine IL-1 mediates diverse forms of neurodegeneration, but its mechanism of action is unknown. We have demonstrated previously that exogenous and endogenous IL-1 acts specifically in the rat striatum to dramatically enhance ischemic and excitotoxic brain damage and cause extensive cortical injury. Here we tested the hypothesis that this distant effect of IL-1 is mediated through polysynaptic striatal outputs to the cortex via the hypothalamus. We show that IL-1β injected into the rat striatum with the excitotoxin α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (S-AMPA) caused increased expression of IL-1β (mRNA and protein) mainly in the cortex where maximum injury occurs. Marked increases in IL-1β mRNA and protein were also observed in the hypothalamus. S-AMPA, injected alone into the striatum, caused only localized damage, but administration of IL-1β into either the striatum or the lateral hypothalamus immediately after striatal S-AMPA resulted in widespread cell loss throughout the ipsilateral cortex. Finally we showed that the cortical cell death produced by striatal coinjection of S-AMPA and IL-1β was significantly reduced by administration of the IL-1 receptor antagonist into the lateral hypothalamus. These data suggest that IL-1β can act in the hypothalamus to modify cell viability in the cortex. We conclude that IL-1-dependent pathways project from the striatum to the cortex via the hypothalamus and lead to cortical injury, and that these may contribute to a number of human neurological conditions including stroke and head trauma.
The copper transporter CTR1 provides an essential function in mammalian embryonic development
Authors: Kuo Yien-Ming , Zhou Bing , Cosco Dominique , Gitschier Jane
Published: July 2002

Copper serves as an essential cofactor for a variety of proteins in all living organisms. Previously, we described a human gene (CTR1;SLC31A1)that encodes a high-affinity copper-uptake protein and hypothesized that this protein is required for copper delivery to mammalian cells. Here, we test this hypothesis by inactivating theCtr1gene in mice by targeted mutagenesis. We observe early embryonic lethality in homozygous mutant embryos and a deficiency in copper uptake in the brains of heterozygous animals.Ctr1−/−embryos can be recovered at E8.5 but are severely developmentally retarded and morphologically abnormal. Histological analysis reveals discontinuities and variable thickness in the basement membrane of the embryonic region and an imperfect Reichert's membrane, features that are likely due to lack of activity in the collagen cross-linking cupro-enzyme lysyl oxidase. A collapsed embryonic cavity, the absence of an allantois, retarded mesodermal migration, and increased cell death are also apparent. In the brains of heterozygous adult mice, which at 16 months are phenotypically normal, copper is reduced to approximately half compared with control littermates, implicating CTR1 as the required port for copper entry into at least this organ. A study of the spatial and temporal expression pattern ofCtr1during mouse development and adulthood further shows that CTR1 is ubiquitously transcribed with highest expression observed in the specialized epithelia of the choroid plexus and renal tubules and in connective tissues of the eye, ovary, and testes. We conclude that CTR1 is the primary avenue for copper uptake in mammalian cells.
Prospective measurements of dehydroepiandrosterone sulfate in a cohort of elderly subjects: Relationship to gender, subjective health, smoking habits, and 10-year mortality
Authors: Mazat Lise , Lafont Sylviane , Berr Claudine , Debuire Brigitte , Tessier Jean-François , Dartigues Jean-François , Baulieu Etienne-Emile
Published: July 2002

The decrease with age of the adrenal-secreted dehydroepiandrosterone sulfate (DHEAS) in serum has suggested that it may be causally related to longevity. For the PAQUID [People (Personnes) Aged (Agées) About What (Quid, in Latin)] cohort of elderly subjects, we have previously reported higher DHEAS in men than in women, a decrease with age and, among men, a negative correlation between the DHEAS level and mortality at 2 and 4 years. Here, with an 8-year followup in 290 subjects, we show a global decrease of 2.3% per year for men and 3.9% per year for women. However, in approximately 30% of cases, there was an increase of DHEAS. We observed no relationship between the evolution of DHEAS level and functional, psychological, and mental status, possibly because of selection by death. In women, no association was found between mortality and DHEAS level. In men, the relative risk (RR) of death was higher for the lowest levels of DHEAS (RR = 1.9,P= 0.007), with RR = 6.5,P= 0.003 for those under 70 years old, a result indicating heterogeneity of the population. There was an effect of subjective health on mortality that disappeared after adjustment of DHEAS levels, suggesting its relation with these DHEAS levels. Death RR was much higher in smokers with a low DHEAS level than in nonsmokers with high DHEAS (RR = 6.7,P= 0.001). We submit that the involvement of DHEAS is possibly different according to gender, that association between low DHEAS level and mortality only for men under 70 years old possibly reflects heterogeneity of the population, and that DHEAS level is a reliable predictor of death in male smokers.
Functional and physiological consequences of genetic variation at phosphoglucose isomerase: Heat shock protein expression is related to enzyme genotype in a montane beetle
Authors: Dahlhoff Elizabeth P. , Rank Nathan E.
Published: July 2002

Allele frequency variation at the phosphoglucose isomerase (PGI) locus in Californian populations of the beetleChrysomela aeneicollissuggests that PGI may be undergoing natural selection. We quantified (i) apparent Michaelis-Menten constant (Km) of fructose 6-phosphate at different temperatures and (ii) thermal stability for three common PGI genotypes (1–1, 1–4, and 4–4). We also measured air temperature (Ta) and beetle body temperature (Tb) in three montane drainages in the Sierra Nevada, California. Finally, we measured 70-kDa heat shock protein (Hsp70) expression in field-collected and laboratory-acclimated beetles. We found that PGI allele 1 predominated in the northernmost drainage, Rock Creek (RC), which was also significantly cooler than the southernmost drainage, Big Pine Creek (BPC), where PGI allele 4 predominated. Allele frequencies and air temperatures were intermediate in the middle drainage, Bishop Creek (BC). Differences among genotypes inKm(1–1 > 1–4 > 4–4) and thermal stability (4–4 > 1–4 > 1–1) followed a pattern consistent with temperature adaptation. In nature,Tbwas closely related toTa. Hsp70 expression in adult beetles decreased with elevation and differed among drainages (BPC > BC > RC). After laboratory acclimation (8 days, 20°C day, 4°C night) and heat shock (4 h, 28–36°C), Hsp70 expression was greater for RC than BPC beetles. In RC, field-collected beetles homozygous for PGI 1–1 had higher Hsp70 levels than heterozygotes or a 4–4 homozygote. These results reveal functional and physiological differences among PGI genotypes, which suggest that montane populations of this beetle are locally adapted to temperature.