bluuebunny/crossref_metadata_2025 huggingface dataset:
Onset of the nonlinear regime and finite-resistivity effects for the resistive kink instability
Authors: Firpo Marie-Christine
Published: February 2004

The development of the resistive kink instability into the nonlinear regime is analyzed dynamically. Two small parameters are involved: the fixed dimensionless resistivity η and the time-dependent amplitude A of the m=1 mode, that measures the amount of nonlinearities in the system. While, for A small, the outer domain is predicted to be ruled by linear ideal magnetohydrodynamic equations, one tracks self-consistently the location of the critical layer and derives the dominant dynamical equations inside it. As A becomes of order η1/2, a nonlinear exponential growth due to a balance between convective and mode coupling terms is predicted for moderately low values of η and the early nonlinear regime below this threshold is quantitatively computed. The nonlinear saturation of the instability should take place for A at most of the order η1/3. This analysis provides a general pattern to model the onset of the nonlinear regime of internal modes.
National Ignition Facility targets driven at high radiation temperature: Ignition, hydrodynamic stability, and laser–plasma interactions
Authors: Hinkel D. E. , Haan S. W. , Langdon A. B. , Dittrich T. R. , Still C. H. , Marinak M. M.
Published: February 2004

A target design driven indirectly to ignition at a radiation temperature of 350 eV for the National Ignition Facility (NIF) is reported in integrated radiation-hydrodynamic simulations which detail the necessary specifications to achieve ignition and burn. The target is further analyzed to determine its hydrodynamic stability as well as its vulnerability to laser–plasma interactions. This target shows enhanced hydrodynamic stability over targets previously designed at lower radiation temperatures [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 2480 (1995); W. J. Krauser, N. M. Hoffman, D. C. Wilson et al., ibid.3, 2084 (1996); D. C. Wilson, P. A. Bradley, N. M. Hoffman et al., ibid.5, 1953 (1998); P. A. Bradley and D. C. Wilson, ibid.6, 4293 (1999)]. To control laser–plasma instabilities, both polarization and temporal smoothing of the spatially smoothed NIF laser beams is necessary. Analyses of laser scatter in target blow-off at peak power demonstrate saturation in both the 300 and 350 eV designs by nonlinear processes such as rescatter of the scattered laser light and the Langmuir decay instability.
Pullback transformations in gyrokinetic theory
Authors: Qin H. , Tang W. M.
Published: February 2004

The pullback transformation of the distribution function is a key component of gyrokinetic theory. In this paper, a systematic treatment of this subject is presented, and results from applications of the uniform framework developed are reviewed. The focus is on providing a clear exposition of the basic formalism which arises from the existence of three distinct coordinate systems in gyrokinetic theory. The familiar gyrocenter coordinate system, where the gyromotion is decoupled from the rest of particle’s dynamics, is noncanonical and nonfibered. For the phase space (cotangent bundle T*M) associated with a configuration space M, a nonfibered coordinate system (X, V) is a coordinate system where X is not necessarily the coordinates for the configuration space M, and V is not necessarily the coordinates for the cotangent fiber Tx*M at each x. On the other hand, Maxwell’s equations, which are needed to complete a kinetic system, are initially only defined in the fibered laboratory phase space coordinate system. The pullback transformations provide a rigorous connection between the distribution functions in gyrocenter coordinates and Maxwell’s equations in laboratory phase space coordinates. This involves the generalization of the usual moment integrals originally defined on the cotangent fiber of the phase space to the moment integrals on a general six-dimensional symplectic manifold. The resultant systematic treatment of the moment integrals enabled by the pullback transformation is shown to be an important step in the proper formulation of gyrokinetic theory. Without this vital element, a number of prominent physics features, such as the presence of the compressional Alfvén wave and a proper description of the gyrokinetic equilibrium, cannot be readily recovered.
Picosecond pulse generation by internal gain switching in laser diodes
Authors: Tarakanov Yu. A. , Ilyushenkov D. S. , Chistyakov V. M. , Odnoblyudov M. A. , Gurevich S. A.
Published: February 2004

A laser diode structure realizing internal gain switching is suggested for high-power picosecond optical pulse generation. A thin wideband barrier layer is incorporated in the active region of this structure, in order to provide additional control of the carrier distribution and the gain dynamics. The dynamic laser output response to the nanosecond pumping current pulses is modeled and analyzed. The pulse generation mechanism is studied and the possibility to generate optical pulses with power density of 0.416 W/μm2 is demonstrated. This result compares well with that provided by the best modern picosecond laser diodes operating in Q-switching mode.
The bounce-kinetic model for driven nonlinear Langmuir waves
Authors: Barnes D. C.
Published: February 2004

The bounce-kinetic model is developed and applied to driven nonlinear Langmuir waves. The waves are described in terms of an eikonal with slow envelope variation. It is assumed that the bounce frequency of trapped electrons is large. A kinetic equation involving only slowly varying quantities is derived and it is shown that the characteristic equations form a Hamiltonian system. Conservation of particles, momentum, and energy are shown to depend on first-order corrections to this kinetic equation. The low order correction moments are derived exactly from these conservation laws, eliminating the need for a complicated boundary layer treatment of the separatrix. Previous results for nonlinear Langmuir waves are reproduced by a simplified version of this theory which neglects variations of the amplitude envelope and phase velocity. A particle-in-cell method is proposed for solution of the nonlinear kinetic problem. Extensions of this method required to correctly describe small amplitude waves are suggested. Such an extended model may be useful for the modeling of laser-plasma interaction in the trapping regime.
Real-space imaging of current distributions at the submicron scale using magnetic force microscopy: Inversion methodology
Authors: Rous P. J. , Yongsunthon R. , Stanishevsky A. , Williams E. D.
Published: February 2004

We report a method, based upon magnetic force microscopy (MFM), that permits the direct imaging of current distributions with submicron resolution. Magnetic force microscopy is used to measure the curvature of the magnetic field generated by a current-carrying structure. Maximum entropy deconvolution of the MFM phase image, followed by the application of a numerical inversion procedure derived from the Biot–Savart law, yields the current distribution in the sample. Careful theoretical analysis of the spatial resolution of this method shows that the lateral resolution is noise limited to approximately one quarter of the tip height. Since tip elevations of 100 nm are typical, we anticipate that this method has a spatial resolution of tens of nanometers. The method was used to determine the current distribution in the vicinity of a (1×9) μm2 slit-like defect embedded in a 11.5-μm-wide current-carrying metallic line. Current crowding and constriction are observed in the images and are resolved at the submicron level. The observed current distributions are found to be in good agreement with finite-element calculations of the current density for equivalent lines, confirming both the fidelity and the resolution of the imaging method.
Polarization relaxation anisotropy in Pb(Zn1/3Nb2/3)O3-PbTiO3 single-crystal ferroelectrics as a function of fatigue history
Authors: Ozgul Metin , Furman Eugene , Trolier-McKinstry Susan , Randall Clive A.
Published: February 2004

Polarization relaxation was studied in Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) single crystals that show fatigue anisotropy. To excite prepoled crystals, a modest dc voltage (<1/2 of the coercive field) was applied along the poling direction. Upon removal of the voltage, the polarization decay in the time domain was measured. Experimental data were modeled with a stretched exponential function. Stretching exponent (β〈hkl〉) and characteristic time (τ〈hkl〉) constants for polarization relaxation were determined from data over four decades in the time domain at different stages of bipolar cycling. β〈hkl〉 values after 101 cycles were 0.146±0.002 and 0.247±0.0004 in the 〈001〉 and 〈111〉 orientations, respectively. The β〈111〉 constant increased up to 0.453±0.104 after 105 cycles in 〈111〉 oriented crystals that show fatigue. However, much less change is observed in β〈001〉 as a function of cycling for 〈001〉 crystals. Characteristic time constants for relaxation (τ〈hkl〉) were calculated for 〈001〉 and 〈111〉 orientations as 0.401±0.048 s and 57.46±0.10 s, respectively. These results suggest a faster polarization relaxation in 〈001〉 than in the 〈111〉 orientation of rhombohedral PZN-PT ferroelectric crystals.
Evolution of relativistic ions incessantly accelerated by an oblique shock wave
Authors: Usami Shunsuke , Ohsawa Yukiharu
Published: February 2004

The evolution of a large number of nonthermal, energetic particles that encounter with an oblique shock wave is studied with theory and simulations. First, an attempt is made to analytically discuss conditions under which particles can cross the shock front multiple times. Also, the change in the parallel momentum of a relativistic particle at the shock front is examined in detail. Then, the time variation of an energy distribution function of 5000 energetic particles is investigated with simulations; where field profiles are obtained from particle simulations of shock waves, and test particle orbits are calculated by use of these fields. The shock speed vsh and propagation angle θ0 are taken to be vsh∼c cos θ0, for which incessant acceleration is expected. The development of these particles in the momentum space is also shown.
Modeling and experimental investigation of cantilever dynamics in force detected single electron tunneling
Authors: Klein Levente J. , Williams Clayton C.
Published: February 2004

The dynamic response of a voltage biased oscillating cantilever probe is investigated through experimental and theoretical analysis as it approaches a dielectric surface. When the tip reaches the appropriate gap single electron tunneling events are detected between the metallic tip and the surface. The tunneling events cause a decrease of the electrostatic force and force gradient acting between tip and sample. The change in the electrostatic force is detected as an abrupt decrease of the cantilever oscillation amplitude. Additionally, due to the nonlinear interaction between tip and sample, the cantilever oscillation amplitude in very close proximity of the sample can have multiple values. Typically, as the tip-sample gap is reduced, a transition between two stable cantilever oscillation modes is detected as an abrupt increase in the oscillation amplitude. If this transition occurs at a gap larger than the tunneling gap, no tunneling event is detected. A theoretical model that includes both the electrostatic and mechanical effects has been developed to investigate the cantilever response in close proximity of the sample. The model, which includes the effects of the single electron tunneling events, is in good agreement with the measurements.
Stable trapping of electrons and holes in deposited insulating oxides: Al2O3, ZrO2, and HfO2
Authors: Afanas’ev V. V. , Stesmans A.
Published: February 2004

Charge trapping in high-permittivity metal oxides (Al2O3, ZrO2, and HfO2) grown on (100)Si using various types of chemical vapor deposition (CVD) was studied using generation of electron-hole pairs in the oxide by 10 eV photons. For most of the CVD methods, thin (≈5 nm) oxide films exhibit positive charging suggesting hole trapping as most efficient charge trapping process. Negative charge is observed only in as-deposited nitrogen-containing films grown from Hf(NO3)4. The trapped positive charge depends only weakly on the HfO2 thickness indicating that holes are trapped in a silicon oxide interlayer grown between the Si and HfO2 during deposition, which is further affirmed by enhanced positive charging after additional oxidation of the samples at high temperatures. The work function of the metal electrode material has a large influence on hole trapping in thin oxides, indicating electron exchange between the metal and defect states in the oxide. In addition, trapping of positive charge correlates with liberation of atomic hydrogen during electron-hole pair generation pointing to a considerable contribution of protonic species to the charge. Electron trapping increases with thickness of oxide layers and appears strongly sensitive to both the method of oxide deposition and post-deposition processing. This suggests the electron traps to be related to defects and/or impurities incorporated into the oxide film during deposition.
Effectiveness of BaZrO3 buffer layer in SmBa2Cu3Oy epitaxial growth on MgO substrate: A first-principles study
Authors: Oba Fumiyasu , Sugawara Yoshihiro , Hasegawa Katsuya , Izumi Teruo , Shiohara Yuh , Hirayama Tsukasa , Yamamoto Takahisa , Ikuhara Yuichi
Published: February 2004

The atomic structure and energies of SmBa2Cu3O6(Sm123)/BaZrO3(BZO) and Sm123/MgO interfaces have been investigated using first-principles calculations. The interfacial energies were evaluated for various atomic configurations under relevant conditions of the chemical potentials. For the Sm123/BZO, an interface composed of a BaO layer is found to be energetically favorable, irrespective of the chemical potentials. This is much lower in energy than the Sm123/MgO interfaces where the preferable configuration even varies with the chemical potential conditions. The stability of the Sm123/BZO interface is attributed to the local atomic arrangement and chemical composition common to Sm123 and BZO, and such an atomic structure is confirmed by high-resolution transmission electron microscopy. The results suggest that the insertion of a BZO buffer layer facilitates the epitaxial growth of Sm123 films on MgO substrates because of the energetically favorable film/buffer layer interface.
Particle acceleration during the counterstreaming instability in magnetized pair plasmas
Authors: Saito Shinji , Sakai Jun-Ichi
Published: February 2004

The electromagnetic counterstreaming instability in unmagnetized pair (electron-positron) plasmas, which is influenced by the external magnetic field parallel to the streaming direction, is investigated both analytically and numerically. It is shown from the linear theory of relativistic cold four fluids model that the generation of a quasi-static magnetic field whose wave vector is perpendicular to the streaming direction is restrained by the increasing external magnetic field. The linear theory is also confirmed by particle-in-cell simulation. The theory and simulation show that the counterstreaming instability changes its character from magnetic to electrostatic nature when the external magnetic field increases as ωce>1.5ωpe. The electrostatic waves growing due to the electrostatic counterstreaming instability play an important role for producing fast electrons and positrons with energy of MeV. The process of high-energy particle production in relativistic shocks in magnetized pair plasmas may be applied to gamma-ray burst events.
Scaling cross sections for ion-atom impact ionization
Authors: Kaganovich Igor D. , Startsev Edward A. , Davidson Ronald C.
Published: February 2004

Knowledge of ion-atom ionization cross sections is of great importance for many applications. When experimental data and theoretical calculations are not available, approximate formulas are frequently used. Based on experimental data and theoretical predictions, a new fit for ionization cross sections by fully stripped ions is proposed.
Microstructure and growth mechanism of laser grown carbon microrods as a function of experimental parameters
Authors: Fauteux C. , Longtin R. , Pegna J. , Boman M.
Published: February 2004

Carbon microrods were grown from ethylene using laser-assisted chemical vapor deposition. The precursor pressure and laser power were varied. The internal structure of the carbon rods was characterized by Raman spectroscopy at 514.5 nm using an Ar+ laser. The rods were cast in epoxy resin and polished in order to allow a radial analysis of the cross section. Each spectrum has been analyzed by noting the peak positions and width changes. Intensities of the G and D peaks were compared between spectra and allowed the determination of the crystal size as a function of radius. An analysis of the spectra gave insight in the bonding conditions as well as the growth mechanism of the rods as a function of the experimental parameters.
Structural characterization of porous low-k thin films prepared by different techniques using x-ray porosimetry
Authors: Lee Hae-Jeong , Soles Christopher L. , Liu Da-Wei , Bauer Barry J. , Lin Eric K. , Wu Wen-li , Grill Alfred
Published: February 2004

Three different types of porous low-k dielectric films, with similar dielectric constants, are characterized using x-ray porosimetry (XRP). XRP is used to extract critical structural information, such as the average density, wall density, porosity, and pore size distribution. The materials include a plasma-enhanced-chemical-vapor-deposited carbon-doped oxide film composed of Si, C, O, and H (SiCOH) and two spin cast silsesquioxane type films—methylsilsesquioxane with a polymeric porogen (porous MSQ) and hydrogensilsesquioxane with a high boiling point solvent (porous HSQ). The porous SiCOH film displays the smallest pore sizes, while porous HSQ film has both the highest density wall material and porosity. The porous MSQ film exhibits a broad range of pores with the largest average pore size. We demonstrate that the average pore size obtained by the well-established method of neutron scattering and x-ray reflectivity is in good agreement with the XRP results.
Martensitic transformation in Ni2MnGa films: A ferromagnetic resonance study
Authors: Dubowik J. , Kudryavtsev Y. V. , Lee Y. P.
Published: February 2004

Off-stoichiometric Ni2MnGa polycrystalline films, deposited by the flash-evaporation technique on the heated mica substrates, show a martensitic phase transformation at 310 K. At room temperature, the films have a tetragonal structure (a=b=0.598 nm, c=0.576 nm) close to the bulk Ni2MnGa with c/a=0.96. The austenite to martensite transformation brings about an anomalous minimum in the effective magnetization and a strong increase in the ferromagnetic resonance linewidth in the martensitic phase just below the transformation temperature. The results are discussed in terms of a qualitative model that combines the ferromagnetic resonance response with a specific microstructure of the polycrystalline Ni2MnGa film.
Crystallization of metallic glasses under the influence of high density dc currents
Authors: Holland Troy B. , Löffler Jörg F. , Munir Zuhair A.
Published: February 2004

The effect of a dc current on the crystallization of Vit1A (Zr42.6Ti12.4Cu11.25Ni10Be23.75) and PCNP (Pd40Cu30Ni10P20) metallic glasses was investigated. Samples were isothermally annealed with and without the current, at 623 and 577 K for the two glasses, respectively. Small-angle neutron scattering analyses showed that in the absence of a current, the annealed Vit1A samples were amorphous, but the imposition of a current enhanced the crystallization process, increasing both the size and volume fraction of the crystallites. Similar general observations were seen for the PCNP glass. Differential scanning calorimetry patterns of Vit1A samples indicate a lower thermal stability of samples annealed with a current.
Elastic properties of β-SiC films by Brillouin light scattering
Authors: Djemia Philippe , Roussigné Yves , Dirras Guy F. , Jackson Kamili M.
Published: February 2004

Brillouin light scattering has been used to investigate elastic properties of a monocrystalline and of 〈111〉 textured polycrystalline 3C polytype silicon carbide films that have been deposited on silicon substrate by chemical vapor deposition. Taking advantage from the detection of different acoustic modes, a complete elastic characterization of the films has been achieved. The three unknown elastic constants of the monocrystalline 3C-SiC, namely, c11=395 GPa, (c11−c12)/2=136 GPa, and c44=236 GPa have been selectively determined, respectively, from the frequency of the longitudinal and of the shear horizontal bulk modes traveling parallelly to the film surface. These determinations are in agreement with the frequency of the observed Rayleigh surface mode, of the pseudosurface mode, and of the bulk waves propagating at different angles from the normal of the single crystal film plane and consistent with existing theoretical calculations of β-SiC elastic constants. Finally, the calculated Voigt average values of the effective elastic constants for the 〈111〉 textured 3C-SiC polycrystalline film using the single crystal constants provides a good agreement with our experimental results (C11=500 GPa, C33=535 GPa, C44=165 GPa, C66=210 GPa, and C13=50 GPa) and compare fairly well with the α-SiC published one. These results confirm that the elastic constants of silicon carbide are slightly influenced by the polytypism.
Melting and crystallization behavior of low-pressure chemical-vapor-deposition amorphous Si films during excimer-laser annealing
Authors: Voogt F. C. , Ishihara R. , Tichelaar F. D.
Published: February 2004

Results are presented of cross-sectional transmission-electron microscopy and time-resolved optical reflectivity investigations into the excimer-laser annealing of low-pressure chemical-vapor-deposition amorphous Si films. It is found that, in the initial stages of the laser pulse, a thin surface layer melts. This is directly followed by explosive crystallization of the film into to small, columnar, and defect-rich grains. As more laser energy is consumed, the Si film melts in from the surface for a second time, eventually leading to complete melting of the film. It is argued that melting along grain boundaries and defects is a crucial step in obtaining large, single-crystalline grains in the super-lateral growth regime.
Noninvasive lateral detection of Coulomb blockade in a quantum dot fabricated using atomic force microscopy
Authors: Nemutudi R. , Kataoka M. , Ford C. J. B. , Appleyard N. J. , Pepper M. , Ritchie D. A. , Jones G. A. C.
Published: February 2004

Atomic force microscope (AFM) lithography is used to fabricate a charge detector. Experimental results are presented showing the lateral detection of Coulomb blockade events using a narrow constriction located in close proximity to a quantum dot. Both the dot and the constriction are patterned by writing oxide lines on a semiconductor surface with a conducting tip of an AFM. On a shallow two-dimensional electron gas (2DEG), the written oxide lifts the bottom of the conduction band above the Fermi level, leaving the 2DEG depleted beneath it. With its inherent ability to draw fine lines, the AFM presents a suitable fabrication technique for noninvasive experiments in which a phenomenon, such as Coulomb blockade, can be detected without necessarily injecting current across the dot.
Laser induced electron acceleration in vacuum
Authors: Singh K. P.
Published: February 2004

Electron acceleration by a plane polarized laser wave has been studied in vacuum. Relativistic equations of motion have been solved exactly for electron trajectory and energy as a function of laser intensity, phase θ of the laser wave and initial electron energy. The electric field of the laser wave is taken as E=x̂A0 cos(ωt−kz+θ). Electron energy is maximum when θ=π/2 and (ωt−kz)=(2n−1)π, where n=1,2,3,… . The peak electron energy increases with laser intensity and initial electron energy. If a propagating laser pulse is abruptly stopped by a thin foil, the highly energetic electrons will continue to move forward inertially and escape from the pulse, as well as the thin foil, without much loss in the energy, if their stopping distance is much larger than the laser skin depth and the thin foil thickness, respectively.
O-X mode conversion in an axisymmetric plasma at electron cyclotron frequencies
Authors: Weitzner Harold
Published: February 2004

The cold plasma dielectric function is employed to represent the plasma response of an axisymmetric steady state to incident waves at frequencies on the order of the electron cyclotron frequency. A boundary layer analysis of Maxwell’s equation valid near the mode conversion region is carried out to obtain an approximate system of wave equations. The independent variables of the wave equations are the flux surface label and poloidal angle. The case is analyzed in which the O mode cut-off surface and X mode cut-off surface intersect. An integral representation of solutions is given. It is shown that for a wide class of incident waves all such waves are transmitted with total mode conversion and no mode reflection. This work cannot assure that solutions with mode reflection do not occur, although a broad search for solutions, possibly with mode reflection, was unsuccessful.
Optical, wave measurements, and modeling of helicon plasmas for a wide range of magnetic fields
Authors: Tysk Shane M. , Denning C. Mark , Scharer John E. , Akhtar Kamran
Published: February 2004

Helicon waves are excited in a plasma wave facility by a half-turn double-helix antenna operating at 13.56 MHz for static magnetic fields ranging from 200 to 1000 G. A non-perturbing optical probe located outside the Pyrex™ plasma chamber is used to observe 443 nm Ar II emission that is spatially and temporally correlated with the helicon wave. The Ar II emission is measured along with wave magnetic and Langmuir probe density measurements at various axial and radial positions. 105 GHz interferometry is used to verify the bulk temperature corrected Langmuir probe measurements. The measured peak Ar II emission phase velocity is compared to the measured wave magnetic field phase velocity and code predicted wave phase velocity for the transition and blue mode regimes. Very different properties of the optical emission peak phase and wave characteristics for the transition and helicon modes of operation are observed. Comparison of the experimental results with the ANTENAII code [Y. Mouzouris and J. E. Scharer, IEEE Trans. Plasma Sci. 24, 152 (1996)] is carried out for the wave field measurements for the two regimes of operation.
An extended multi-component model for the change of threshold current of semiconductor lasers as a function of time under the influence of defect annealing
Authors: Lam S. K. K. , Mallard R. E. , Cassidy Daniel T.
Published: February 2004

We model the change of threshold current of a semiconductor laser diode as a function of time under the influence of defect annealing. Our approach describes an analytical multi-component model (MCM) based on a logistic equation which accounts for finite resources of fuel for the growth of nonradiative recombination defect complexes. We attribute the observable effect of the annealing on the threshold current to a reduction of the internal loss in the laser, αi, as opposed to a reduction in the number of nonradiative recombination centers. In this study, we use the extended MCM to examine the intricate state when degradation and annealing co-exist in a semiconductor laser.
Enhancement and anisotropy of the Landau g factor in modulation-doped Al0.22Ga0.78N/GaN heterostructures
Authors: Zheng Z. W. , Shen B. , Gui Y. S. , Qiu Z. J. , Jiang C. P. , Tang N. , Liu J. , Chen D. J. , Zhou H. M. , Zhang R. , Shi Y. , Zheng Y. D. , Guo S. L. , Chu J. H. , Hoshino K. , Arakawa Y.
Published: February 2004

Spin splitting of the two-dimentional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN heterostructures has been studied by means of magnetotransport measurements at low temperatures and high magnetic fields. The spin splitting is observed in Shubnikov–de Haas oscillations at a magnetic field higher than 5.4 T and a temperature of 1.4 K. The effective g factor g* is enhanced due to the exchange interaction of the 2DEG at high densities. The ratio of the transverse effective g factor g⊥* and the longitudinal effective g factor g∥* is g⊥*/g∥*=2.6 indicating the large difference between g∥* and g⊥*. It is demonstrated that the anisotropy of the g* is due to the strong polarization-induced electric field at the heterointerface.
Plasma measurements in a 100 W cylindrical Hall thruster
Authors: Smirnov A. , Raitses Y. , Fisch N. J.
Published: February 2004

Conventional annular Hall thrusters become inefficient when scaled to low power. Their lifetime decreases significantly due to the channel wall erosion. Cylindrical Hall thrusters, which have lower surface-to-volume ratio and, thus, seem to be more promising for scaling down, exhibit performance comparable with conventional annular Hall thrusters of the similar size. Plasma potential, ion density, and electron temperature profiles were measured inside the 2.6 cm cylindrical Hall thruster with the use of stationary and slow movable emissive and biased Langmuir probes. Potential drop in the 2.6 cm cylindrical Hall thruster is localized mainly in the cylindrical part of the channel and in the plume, which suggests that the thruster should suffer lower erosion of the channel walls due to fast ion bombardment. Plasma density has a maximum of about (2.6–3.8)×1012 cm−3 at the thruster axis. At the discharge voltage of 300 V, the maximum electron temperature is about 21 eV, which is not enough to produce multiple ionization in the accelerated flux of Xe+ ions.
Amplification of coherent anti-Stokes Raman scattering by a metallic nanostructure for a high resolution vibration microscopy
Authors: Hayazawa Norihiko , Ichimura Taro , Hashimoto Mamoru , Inouye Yasushi , Kawata Satoshi
Published: February 2004

On the basis of the mechanism of surface enhanced Raman scattering, it is shown that coherent anti-Stokes Raman scattering (CARS) of molecules attached to isolated gold nanoparticles are strongly enhanced and the signal from each particle is well localized. In addition to well-known advantages of CARS, the surface enhanced CARS combined with a scanning system of metallic nanoprobe tip can realize high spatial resolution CARS microscopy beyond the diffraction limit of light by locally enhancing the weak signals from the small sample volume. This concept is realized by tip-enhanced coherent anti-Stokes Raman spectroscopy using a metallic nanoprobe of near-field scanning optical microscope.
Initiation and prevention of the transit time instability in large area diodes
Authors: Friedman M. , Myers M. , Swanekamp S. B. , Hegeler F. , Sethian J. D.
Published: February 2004

Optimum pumping of krypton fluoride (KrF) lasers requires electron beams that are stable against the transit time instability and spatially uniform. Eliminating the enhanced current density (beam halo) along the edges of a large area homogenous electron beam has been achieved by surrounding the flat diode emitting area with a protruding metallic frame. However, modification to eliminate the beam halo can trigger the transit time instability in an otherwise stable diode. Feeding the instability are growing transverse electromagnetic (TEM) waves that propagate along the anode–cathode (AK) gap reflecting back and forth from the protruding frame. Suppressing these TEM waves was achieved by inserting a slab of honeycomb ceramic in front of the emitting cathode. Electron emission was improved and the presence of the dielectric prevented TEM waves from being generated in the cathode–anode gap. Only surface waves can exist and propagate along the AK gap when the ceramic slab is inserted in the diode. These waves do not couple to the electron beam and cannot support the instability. In such a diode no beam halo and no instability were observed.
Characterization of carbon nanotubes produced by arc discharge: Effect of the background pressure
Authors: Waldorff Erik I. , Waas Anthony M. , Friedmann Peretz P. , Keidar Michael
Published: February 2004

Single walled carbon nanotubes (SWNT) produced by the anodic arc discharge over a range of constant background pressures of helium (100–1000 Torr) were examined under a high-resolution transmission electron microscope, and a Raman spectrometer. It was found that the average SWNT diameter is about 2 nm and fairly independent of the background pressure. Analysis of the relative purity of SWNTs samples suggests that highest SWNT relative concentration can be obtained at background pressure of about 200–300 Torr. Measured anode ablation rate increases linearly with background pressure. The model of the anodic arc discharge was developed. It was found that the predicted anode ablation rate agrees well with experiment suggesting that electron temperature in the anodic arc is about 0.5 eV.
Ultimate limits to inertial mass sensing based upon nanoelectromechanical systems
Authors: Ekinci K. L. , Yang Y. T. , Roukes M. L.
Published: February 2004

Nanomechanical resonators can now be realized that achieve fundamental resonance frequencies exceeding 1 GHz, with quality factors (Q) in the range 103⩽Q⩽105. The minuscule active masses of these devices, in conjunction with their high Qs, translate into unprecedented inertial mass sensitivities. This makes them natural candidates for a variety of mass sensing applications. Here we evaluate the ultimate mass sensitivity limits for nanomechanical resonators operating in vacuo that are imposed by a number of fundamental physical noise processes. Our analyses indicate that nanomechanical resonators offer immense potential for mass sensing—ultimately with resolution at the level of individual molecules.
Thermal stability of metastable silicon phases produced by nanoindentation
Authors: Ge Daibin , Domnich Vladislav , Gogotsi Yury
Published: February 2004

Raman spectroscopy and transmission electron microscopy are used to investigate the temperature effects on the stability of metastable silicon phases (Si–III and Si–XII) produced by nanoindentation. It is found that the thickness of the specimen beneath the residual imprint plays an important role in the phase transformation sequence during heating up to 200 °C. Amorphization is preferred in nanoindents located in thin and loosely constrained areas; formation of Si–IV from Si–III/Si–XII is observed in the residual imprints located in the areas with an intermediate thickness; and the formation of an unidentified “Si–XIII” structure, which precedes the formation of Si–IV, is observed in nanoindents constrained by the bulk wafer. The phase transformation sequence in the indented samples under annealing is established as follows: Si–XII→Si–III→Si–XIII (thick sample only)→a-Si or Si–IV→nanocrystalline Si–I→Si–I.
Vortex motion in YBa2Cu3O7−δ twinned single crystals and epitaxial films
Authors: Bernstein P. , Hamet J. F.
Published: February 2004

The transport properties of superconductive YBa2Cu3O7−δ (YBCO) twinned single crystals and epitaxial thin films are similar in the flux creep regime. In particular, the activation energy of the individual vortices is proportionnal to the temperature in the low temperature range. As a matter of fact, data from the literature show that this energy takes the form Ea=4πνkBT, where ν is an integer. To investigate this behavior, the current–voltage characteristics of epitaxial YBCO thin film microbridges were measured. The activation energy of these samples has proved to take the form given above. This common behavior of single crystals and thin films is interpreted in the framework of a critical state model according to which the vortices move along the sample twin boundaries that behave as arrays of weak links.
Inductively coupled Ar/CH4/H2 plasmas for low-temperature deposition of ordered carbon nanostructures
Authors: Denysenko I. B. , Xu S. , Long J. D. , Rutkevych P. P. , Azarenkov N. A. , Ostrikov K.
Published: February 2004

The results of numerical simulations, optical emission spectroscopy (OES), and quadrupole mass spectrometry (QMS) of inductively coupled Ar/CH4/H2 plasmas in the plasma enhanced chemical vapor deposition (PECVD) of self-assembled vertically aligned carbon nanostructures (CNs) are presented. A spatially averaged (global) discharge model is developed to study the densities and fluxes of the radical neutrals and charged species, the effective electron temperature, methane conversion factor under various growth conditions. The numerical results show a remarkable agreement with the OES and QMS data. It is found that the deposited cation fluxes in the PECVD of CNs generally exceed those of the radical neutrals.
Effects of particles with large gyroradii on resistive magnetohydrodynamic stability
Authors: Svidzinski V. A. , Prager S. C.
Published: February 2004

Fast ions in tokamaks are known to have a significant influence on global plasma instabilities. In normal mode analyses for tokamaks, the perturbed electric and magnetic fields have been evaluated at the position of the particle’s guiding center. The effect of spatial variation of the perturbed fields within the gyroradius for resistive internal magnetohydrodynamic (MHD) modes is considered in the present paper. The resulting tearing mode stability for the reversed field pinch (RFP) is investigated. Such effects are important for neutral beam injected particles in current RFP experiments and for fusion-generated alpha particles. The fast particle dielectric response is evaluated from the linearized Vlasov equation, and inserted into a cylindrical MHD model for the bulk plasma. The response is found for the simplified particles distribution function assuming that equilibrium magnetic field is uniform within the gyro-orbit. The effect of large gyroradii is strong, and can be either stabilizing or destabilizing (depending on the radial distribution of the fast particle density). The effect is maximal when the fast particles reside near the tearing-resonant surface.
Dust quasiatom in a plasma
Authors: Mendonça J. T. , Shukla P. K.
Published: February 2004

A quasiatomic structure of the high-frequency oscillations of the electron density inside the plasma Debye sphere is proposed. The paper studies the internal oscillations of the plasma shielding, which screens the electric charge of a dust particle immersed in a plasma. It is shown that high-frequency oscillations can be established inside the Debye sphere. An equation similar to a time-independent Schrödinger equation is derived. The ground state and the first excited state of the oscillating Debye sphere are studied in detail. The relevance of the investigation to enhanced scattering cross sections in dusty plasmas is discussed.
Size dependence of ordering in FePt nanoparticles
Authors: Takahashi Y. K. , Koyama T. , Ohnuma M. , Ohkubo T. , Hono K.
Published: February 2004

We have investigated the size effect of A1→L10 ordering of FePt nanoparticles in FePt–Al2O3 granular and FePt/SiO2 particulate films by transmission electron microscopy (TEM). The TEM results have shown convincingly that ordering does not progress when the particle size has a diameter of less than 4 nm. Calculation of the order parameter profile from the surface to the volume of the FePt nanoparticles based on diffuse-interface theory justified the experimentally observed size dependence of the ordering. The transition length from disorder to order depends on the interfacial energy, hence the critical particle size of ordering should vary depending on the type of matrix and substrate.
Influence of postannealing on polycrystalline pentacene thin film transistor
Authors: Kang S. J. , Noh M. , Park D. S. , Kim H. J. , Whang C. N. , Chang C.-H.
Published: February 2004

We studied systematically the influence of in situ postannealing treatment of ultrahigh vacuum grown polycrystalline pentacene thin film transistor. The gradual grain growth with the elimination of defects and misoriented crystallites is confirmed in x-ray diffraction (XRD) data and the atomic force microscopy image as the annealing temperature increases. The XRD data reveal that the pentacene molecules are packed parallel to each other in an upright position with a tilting angle of 15.5°. The postannealing results in the enhanced field effect mobility of pentacene organic thin film transistors increases from 0.19±0.04 to 0.49±0.05 cm2/V s after annealing at 90 °C. We suggest that the abnormally small on/off current ratio (∼103) due to the large leakage current is attributed to the conduction via impurity levels originated from the structural isomers of pentacene.
Electronic structure of a silole derivative-magnesium thin film interface
Authors: Mäkinen A. J. , Uchida M. , Kafafi Z. H.
Published: February 2004

Photoemission spectroscopy of an interface between Mg and a silole derivative, 2,5-bis[6′-(2′,2″-bipyridyl)]-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PyPySPyPy) reveals the formation of two gap states both at metal-on-organic (Mg/PyPySPyPy) and organic-on-metal (PyPySPyPy/Mg) interfaces. The appearance of gap states is interpreted as the result of electron transfer from Mg to PyPySPyPy, leading to the formation of a charge transfer complex. A strong correlation found between the metal and organic core level shifts, and the changes in the Fermi level position at the Mg/PyPySPyPy interface further supports the model of charge transfer between the metal and the organic. Despite the similar chemical character of the two interfaces, the binding energy (BE) of the gap states at the PyPySPyPy/Mg interface is 0.2–0.3 eV higher than the gap state BE measured at the reversed interface.
Coexistence of low threshold lasing and strong coupling in microcavities
Authors: Lagoudakis Pavlos G. , Martin M. D. , Baumberg Jeremy J. , Malpuech Guillaume , Kavokin Alexey
Published: February 2004

We report the coexistence of low threshold lasing and strong coupling in a high-quality semiconductor microcavity under near-resonant optical pumping. A sharp laser mode splits from the lower-polariton branch and approaches the bare cavity mode frequency as the pump power increases. The lasing is produced by low density localized exciton states, which are weakly coupled to the cavity mode. The appearance of this lasing mode distinguishes quantum-well excitons into those which are strongly or weakly coupled with the cavity mode.
Optical-strain characteristics of anisotropic polymer films fabricated from a liquid crystal diacrylate
Authors: Escuti Michael J. , Cairns Darran R. , Crawford Gregory P.
Published: February 2004

The optomechanical characteristics of oriented polymer films made from a photopolymerizable liquid crystal diacrylate (BASF LC242) were examined, with general implications for oriented films of similar materials being used and considered for display-component applications. The birefringence of these uniaxial films before and after polymerization was determined by measuring the retardation between crossed polarizers, (resulting in Δn=0.142±0.002 at 633 nm for the cured polymer films). Optical-strain characteristics were also determined by measuring the transmittance of the films between crossed polarizers at two wavelengths (612 and 633 nm) during the application of a monotonically increasing tensile strain. Under the conservative assumption that Poisson’s ratio is constant for the relatively small strains in our experiment, we develop a strained-waveplate model to detect changes in birefringence directly from the modulation in transmittance with increasing strain. We show that strain applied along the axis of the polymer chains did not substantially affect the birefringence, and strain applied perpendicularly caused only a slight decrease (∼1% decrease for 10% strain). These results highlight the robustness of fully polymerized reactive mesogen optical films to withstand the moderate strains anticipated during manufacturing processes and in-service deformation caused by bending or impact.
Electron spin resonance signal of nanocomposite of conducting polypyrrole with inorganic clay
Authors: Kim B. H. , Hong S. H. , Joo J. , Park I.-W. , Epstein A. J. , Kim J. W. , Choi Hyoung J.
Published: February 2004

The sharp and broad electron spin resonance (ESR) signals from nanocomposites of conducting polypyrrole (PPy) with inorganic Na+-montmorillonite (MMT) clay were simultaneously observed. The decomposed linewidths (ΔHP-P) of the sharp and broad ESR peaks of the nanocomposites at room temperature were 0.44 G and 21 G, respectively. However, we observed one broad ΔHP-P(∼12 G) for the conducting PPy without the Na+–MMT clay. From the analyses of the observed ESR signals, we propose that the narrow ΔHP-P originates from the weak spin–orbit interaction of the intercalated nanolayers of the nanocomposites, while the broad ΔHP-P originates from the bulky conducting PPy on the outside of the clay particles. The thermogravimetric and differential scanning calorimeter results showed that intercalation in the nanocomposites induced the thermal stability. Coulomb interactions between the positively charged polymers of the intercalated nanolayers and the negatively charged surface of the clay play an important role in the sharp ESR signal and the enhanced thermal stability.
Shape memory and ferromagnetic shape memory effects in single-crystal Ni2MnGa thin films
Authors: Dong J. W. , Xie J. Q. , Lu J. , Adelmann C. , Palmstrøm C. J. , Cui J. , Pan Q. , Shield T. W. , James R. D. , McKernan S.
Published: February 2004

Epitaxial Ni2MnGa and Ni2Mn1.2Ga0.8 thin films have been grown by molecular beam epitaxy on GaAs (001) substrates with Sc0.3Er0.7As interlayers. Structural characterization of as-grown films confirms the epitaxially stabilized single crystal structure of the films and indicates that the films grow pseudomorphically on GaAs (001) substrates in a tetragonal structure (a=b=5.65 Å, c=6.18 Å). The films are ferromagnetic at room temperature with coercivity of ∼50 Oe, saturation magnetization of ∼250 emu/cm3, and weak in-plane magnetic anisotropy. The Curie temperature of the films is found to be ∼340 K. However, while the films were attached to the substrate martensitic phase transformations were not observed. In order to observe martensitic phase transformations, free-standing Ni2MnGa bridges and cantilevers were fabricated using front and back side photolithography together with a combination of dry and wet etching. After removal of the substrate, the free-standing bridges and cantilevers showed a unique temperature dependent shape. Observation using a polarized-light optical microscope during repeated thermocycling showed large movement of the cantilevers, confirming a two-way shape memory effect in the free-standing films. Using 100 μm long free-standing bridges, field induced strain or the ferromagnetic shape memory effect was observed in a stoichiometric Ni2MnGa sample at 135 K with the magnetic fields perpendicular to the sample surface.