bluuebunny/crossref_metadata_2025 huggingface dataset:
Na
0.5
K
0.5
NbO
3
/SiO
2
/Si
thin film varactor
Authors: Cho Choong-Rae , Koh Jung-Hyuk , Grishin Alex , Abadei Saeed , Gevorgian Spartak
Published: July 2002

Perfectly c-axis oriented micrometer thick Na0.5K0.5NbO3(NKN) films have been prepared on a thermally grown ultrathin SiO2 template layer onto a Si(001) wafer by the pulsed laser deposition technique. A x-ray diffraction θ–2θ scan reveals multiple-cell structuring of single phase NKN film along the polar axis, while films grown onto amorphous ceramic (Corning) glass show a mixture of slightly c-axis oriented NKN and pyrochlore phases. This implies a small amount of SiO2 crystallites distributed in an amorphous matrix inherit Si(001) orientation and promotes highly oriented NKN film growth. NKN film dielectric permittivity ε′ was found to vary from 114.0 to 107.2 in the frequency range 1 kHz–1 MHz, while the resistivity was on the order of 2.6×1010 Ω cm @ 20 kV/cm. The planar interdigital variable reactance device (varactor) based on the NKN/SiO2/Si thin film structure possesses a dissipation factor of 0.8% at 1 MHz and zero bias, electrical tunability of 3.1%, and nA order leakage current at 20 V bias at room temperature.
Transient and steady-state space-charge-limited currents in polyfluorene copolymer diode structures with ohmic hole injecting contacts
Authors: Campbell Alasdair J. , Bradley Donal D. C. , Antoniadis Homer , Inbasekaran Mike , Wu Weishi W. , Woo Ed P.
Published: July 2002

We report detailed measurements on diode structures containing the electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-bis-N,N′-(4-methoxyphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine). Ohmic injection of holes is achieved with an oxygen plasma cleaned indium tin oxide (ITO) electrode, untreated ITO coated with a film of poly(ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT/PSS) and plasma cleaned ITO with PEDOT/PSS. Transient dark injection and time-of-flight mobility measurements and steady state current density versus voltage measurements are then entirely consistent with a positive carrier, trap-free, space-charge-limited current. Injection limited behavior is observed, however, for untreated ITO without PEDOT/PSS and for evaporated Au contacts.
High rate epitaxial lift-off of InGaP films from GaAs substrates
Authors: Schermer J. J. , Bauhuis G. J. , Mulder P. , Meulemeesters W. J. , Haverkamp E. , Voncken M. M. A. J. , Larsen P. K.
Published: July 2002

Centimeter sized, crack-free single crystal InGaP films of 1 μm thickness were released from GaAs substrates by a weight-induced epitaxial lift-off process. At room temperature, the lateral etch rate of the process as a function of the applied Al0.85Ga0.15As release layer thickness was found to have a maximum of 3 mm/h at 3 nm. Using 5-nm-thick AlAs release layers, the etch rate increased exponentially with temperature up to 11.2 mm/h at 80 °C. Correlation of the experimental data with the established theoretical description of the process indicate that the model is qualitatively correct but fails to predict the etch rates quantitatively by orders of magnitude.
Low resistance and high thermal stability of spin-dependent tunnel junctions with synthetic antiferromagnetic CoFe/Ru/CoFe pinned layers
Authors: Sun J. J. , Shimazawa K. , Kasahara N. , Sato K. , Saruki S. , Kagami T. , Redon O. , Araki S. , Morita H. , Matsuzaki M.
Published: July 2002

In this work, submicron-size (down to 0.273 μm2) spin–dependent tunnel junctions with resistance as low as ∼30 Ω μm2 have been fabricated, where the tunneling barrier of AlOx was formed by in situ natural oxidation. These junctions annealed at 250 °C for 5 h showed tunneling magnetoresistance (TMR) of 14.3% and 25.8% for the pinned layers of CoFe/RuRhMn and CoFe/PtMn, respectively, while the TMR is further increased to 31.6% for a synthetic antiferromagnetic pinned layer of CoFe/Ru/CoFe/PtMn due to less interdiffusion at CoFe/Ru interface. The investigation has indicated that the growth of ultrathin Al layer is very sensitive to the surface roughness of bottom ferromagnetic electrode, and large surface roughness leads to small junction resistance.
Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes
Authors: Peumans P. , Bulović V. , Forrest S. R.
Published: July 2002

We demonstrate a method for efficient photon harvesting in organic thin films, thereby increasing the efficiency of organic photovoltaic cells. By incorporating an exciton-blocking layer (EBL) inserted between the photoactive organic layers and the metal cathode, we achieved an external power conversion efficiency of 2.4%±0.3% in vacuum-deposited ultrathin organic bilayer photovoltaic (PV) cells employed in a simple light trapping geometry. Ultrathin (∼100 Å) cells incorporating the transparent, conductive EBL have an internal quantum efficiency as high as 33%±4% over a spectral region matched to the solar spectrum. The very thin organic layers have a low series resistance, allowing for efficient power conversion in organic PV cells under intense (>15 suns) AM1.5 illumination. This device structure demonstrates that control of exciton diffusion in solid-state organic devices leads to a significant increase in the photon-to-carrier conversion efficiency.
Model for decomposition and nanocrystallization of deeply undercooled Zr41.2Ti13.8Cu12.5Ni10Be22.5
Authors: Löffler Jörg F. , Johnson William L.
Published: July 2002

From in situ small-angle neutron scattering performed at temperatures in the undercooled liquid regime, we derive a model for the crystallization pathway of Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit1). Vit1 first decomposes on the nanometer scale, increasing drastically the nucleation probability. In the later stages nanocrystallization occurs in one of the decomposed amorphous phases. The growth kinetics of the nanocrystals corresponds to a chemical relaxation process in which they equilibrate with the remaining amorphous matrix. Based on our model, a chemical diffusion constant is derived whose temperature dependence follows an Arrhenius law and is comparable with the expected self-diffusion constant of Ti in Vit1, as determined in independent studies of diffusion.
Thermodynamic Properties of Air and Mixtures of Nitrogen, Argon, and Oxygen From 60 to 2000 K at Pressures to 2000 MPa
Authors: Lemmon Eric W. , Jacobsen Richard T , Penoncello Steven G. , Friend Daniel G.
Published: July 2002

A thermodynamic property formulation for standard dry air based upon available experimental p–ρ–T, heat capacity, speed of sound, and vapor–liquid equilibrium data is presented. This formulation is valid for liquid, vapor, and supercritical air at temperatures from the solidification point on the bubble-point curve (59.75 K) to 2000 K at pressures up to 2000 MPa. In the absence of reliable experimental data for air above 873 K and 70 MPa, air properties were predicted from nitrogen data in this region. These values were included in the determination of the formulation to extend the range of validity. Experimental shock tube measurements on air give an indication of the extrapolation behavior of the equation of state up to temperatures and pressures of 5000 K and 28 GPa. The available measurements of thermodynamic properties of air are summarized and analyzed. Separate ancillary equations for the calculation of dew and bubble-point pressures and densities of air are presented. In the range from the solidification point to 873 K at pressures to 70 MPa, the estimated uncertainty of density values calculated with the equation of state is 0.1%. The estimated uncertainty of calculated speed of sound values is 0.2% and that for calculated heat capacities is 1%. At temperatures above 873 K and 70 MPa, the estimated uncertainty of calculated density values is 0.5% increasing to 1.0% at 2000 K and 2000 MPa. In addition to the equation of state for standard air, a mixture model explicit in Helmholtz energy has been developed which is capable of calculating the thermodynamic properties of mixtures containing nitrogen, argon, and oxygen. This model is valid for temperatures from the solidification point on the bubble-point curve to 1000 K at pressures up to 100 MPa over all compositions. The Helmholtz energy of the mixture is the sum of the ideal gas contribution, the real gas contribution, and the contribution from mixing. The contribution from mixing is given by a single generalized equation which is applied to all mixtures used in this work. The independent variables are the reduced density and reduced temperature. The model may be used to calculate the thermodynamic properties of mixtures at various compositions including dew and bubble-point properties and critical points. It incorporates the most accurate published equation of state for each pure fluid. The mixture model may be used to calculate the properties of mixtures generally within the experimental accuracies of the available measured properties. The estimated uncertainty of calculated properties is 0.1% in density, 0.2% in the speed of sound, and 1% in heat capacities. Calculated dew and bubble-point pressures are generally accurate to within 1%.
Optimization of planar Hall resistance using biaxial currents in a NiO/NiFe bilayer: Enhancement of magnetic field sensitivity
Authors: Kim D. Y. , Park B. S. , Kim C. G.
Published: July 2002

We present the optimized planar Hall resistance (PHR) obtained by using biaxial currents in a NiO (30 nm)/NiFe (30 nm) bilayers. The measured PHR, Rxy, had a drift resistance due to the intrinsic and extrinsic characteristics caused by magnetization and sample geometry, respectively. The drift voltage due to drift resistance restricted the PHR ratio and could be compensated for by using the auxiliary current Ix for the sensing current Iy to enhance PHR ratio. A huge PHR ratio over 3000% (±1500%) with the linearity and small hysteresis for the magnetic field experimentally obtained using biaxial currents and could be explained by the anisotropic characteristic of the magnetoresistance, which is influenced by the exchange coupling field (Hex) induced by the antiferromagnetic NiO layer.
Reduction of threading defects in GaN grown on vicinal SiC(0001) by molecular-beam epitaxy
Authors: Xie M. H. , Zheng L. X. , Cheung S. H. , Ng Y. F. , Wu Huasheng , Tong S. Y. , Ohtani N.
Published: July 2002

We observe a significant reduction of threading dislocations in GaN grown on vicinal substrates of SiC(0001). Using scanning tunneling microscopy, we find films grown on vicinal substrates maintain the surface misorientation of the substrate and display terraces with straight edges. On top of the terraces there is no spiral mound, which is the main feature found for films grown on singular substrates. Transmission electron microscopy studies confirm that threading screw dislocations are reduced by two orders of magnitude while edge dislocations are reduced by one order.
Small-core multiconfiguration-Dirac–Hartree–Fock-adjusted pseudopotentials for post-
d
main group elements: Application to PbH and PbO
Authors: Metz Bernhard , Stoll Hermann , Dolg Michael
Published: July 2002

Relativistic pseudopotentials (PPs) of the energy-consistent variety have been generated for the post-d group 13–15 elements, by adjustment to multiconfiguration Dirac–Hartree–Fock data based on the Dirac–Coulomb–Breit Hamiltonian. The outer-core (n−1)spd shells are explicitly treated together with the nsp valence shell, with these PPs, and the implications of the small-core choice are discussed by comparison to a corresponding large-core PP, in the case of Pb. Results from valence ab initio one- and two-component calculations using both PPs are presented for the fine-structure splitting of the ns2np2 ground-state configuration of the Pb atom, and for spectroscopic constants of PbH (X 2Π1/2, 2Π3/2) and PbO (X 1Σ+). In addition, a combination of small-core and large-core PPs has been explored in spin-free-state shifted calculations for the above molecules.
Determination of the band offset and the characteristic interdiffusion length in quantum-well lasers using a capacitance–voltage technique
Authors: Arias J. , Esquivias I. , Larkins E. C. , Bürkner S. , Weisser S. , Rosenzweig J.
Published: July 2002

In this work, a capacitance–voltage (C–V) technique, based on a combination of measured and simulated C–V characteristics, was applied to characterize In0.35Ga0.65As/GaAs multiquantum-well laser structures at room temperature. A theoretical model, including the self-consistent solution of Poisson and Schrödinger equations, was developed to simulate the C–V characteristics and the carrier concentration profiles. Measured C–V carrier concentration profiles were used to obtain the average impurity concentration in active regions. The comparison between experimental and simulated results was used to determine the conduction band offset, yielding ΔEc/ΔEg≈0.8. In the case of samples with postgrowth quantum-well intermixing, this technique was applied to extract the characteristic interdiffusion length.
From molecules to solids with the DMol3 approach
Authors: Delley B.
Published: July 2002

Recent extensions of the DMol3 local orbital density functional method for band structure calculations of insulating and metallic solids are described. Furthermore the method for calculating semilocal pseudopotential matrix elements and basis functions are detailed together with other unpublished parts of the methodology pertaining to gradient functionals and local orbital basis sets. The method is applied to calculations of the enthalpy of formation of a set of molecules and solids. We find that the present numerical localized basis sets yield improved results as compared to previous results for the same functionals. Enthalpies for the formation of H, N, O, F, Cl, and C, Si, S atoms from the thermodynamic reference states are calculated at the same level of theory. It is found that the performance in predicting molecular enthalpies of formation is markedly improved for the Perdew–Burke–Ernzerhof [Phys. Rev. Lett. 77, 3865 (1996)] functional.
Diagnostics of inductively coupled chlorine plasmas: Measurements of the neutral gas temperature
Authors: Donnelly V. M. , Malyshev M. V.
Published: July 2002

We report measurements of the bulk, neutral gas temperature in a chlorine transformer-coupled plasma. A trace amount (2%–5%) of N2 was added to the discharge and the rotational temperature of the C3Πu state was determined from the C3Πu→B3Πg emission in the ultraviolet. This temperature has been shown by others to be equal to the rotational temperature of ground-state N2, which is the thermally equilibrated (translational and rotational) gas temperature (Tg). The gas temperature 3 cm above the wafer is equal to, or only slightly above the wall temperature (300 K) throughout the low-power, capacitively coupled regime (<60 W, 0.024 W/cm3). Between the lowest (130 W, 0.053 W/cm3) and highest (900 W, 0.36 W/cm3) inductively coupled mode powers investigated, Tg increases sublinearly with power (and electron density). The high-power (900 W) Tg increases with increasing pressure (650, 750, 900, and 1250 K at 2, 5, 10, and 20 mTorr, respectively). Mechanisms of neutral gas heating are discussed. The energy released in dissociation of Cl2 appears to be the dominant heating mechanism.
Combined effects of screening and band gap renormalization on the energy of optical transitions in ZnO and GaN
Authors: Reynolds D. C. , Look D. C. , Jogai B.
Published: July 2002

The energy positions of the optical transitions in both GaN and ZnO were investigated when the samples were excited simultaneously with a HeCd laser and an Ar+ ion laser. The increased number of free electrons excited by the Ar+ ion laser will effectively screen both the free exciton and bound exciton transitions, resulting in a blueshift. The increased number of free electrons also produces many-body effects, which lead to a reduction of the band gap energy and thus a redshift. The resultant of screening and renormalization results in a redshift of the optical transitions in ZnO but a nearly vanishing shift in GaN.
Hydrogen bond thermodynamic properties of water from dielectric constant data
Authors: Suresh S. J. , Naik V. M.
Published: July 2002

We apply statistical mechanical principles to derive simple expressions relating the hydrogen bond thermodynamic properties to the static dielectric constant of water. The approach followed by us was to develop an expression for the Kirkwood’s structure factor (g) of water, taking into account the dipolar correlations between a central molecule and H-bonded neighbors present in infinite number of shells surrounding the central molecule. The number of H-bonded neighbors in a specific shell was related to the probability P for the various donor/acceptor sites of any given water molecule to be associated. Neglecting cooperativity effects, we evaluated P by focusing only on the correct counting of H-bonds formed between various association sites rather than on the oligomer distribution. The theory yielded an extremely simple expression for the structure factor (g) of the fluid at any given temperature in terms of the enthalpy (H) and entropy (S) changes associated with bond formation. The proposed theory was then combined with the Kirkwood–Frohlich theory for evaluating the dielectric constant (ε0). We have demonstrated that the theory correctly predicts the dielectric constant of ice-I without the use of any adjustable parameters. We have then deduced estimates for H-bond thermodynamic properties (H=−5.58 kcal/mole of H-bonds; S=−8.89 cal/deg⋅mole of H-bonds) by fitting the theoretical results for ε0 of liquid water to available experimental data over temperatures ranging from 0 °C to the critical point of water. The error in the theoretical values was found to be within 1% of the corresponding experimental values over the entire range of temperatures studied. To further test the theory, we have demonstrated that the temperature variation of the average number of H-bonds per water molecule, calculated using the proposed theory with the above mentioned values for H and S, compares quite well with those estimated from various available spectroscopic and molecular simulation studies.
Variational principle for nonlinear gyrokinetic Vlasov–Maxwell equations
Authors: Brizard Alain J.
Published: July 2002

A new variational principle for the nonlinear gyrokinetic Vlasov–Maxwell equations is presented. This Eulerian variational principle uses constrained variations for the gyrocenter Vlasov distribution in eight-dimensional extended phase space and turns out to be simpler than the Lagrangian variational principle recently presented by H. Sugama [Phys. Plasmas 7, 466 (2000)]. A local energy conservation law is then derived explicitly by the Noether method. In future work, this new variational principle will be used to derive self-consistent, nonlinear, low-frequency Vlasov–Maxwell bounce-gyrokinetic equations, in which the fast gyromotion and bounce-motion time scales have been eliminated.
Superconducting multiturn flux transformers for radio frequency superconducting quantum interference devices
Authors: Yi H. R. , Zhang Y. , Schubert J. , Zander W. , Zeng X. H. , Klein N.
Published: July 2002

This article describes three planar layouts of superconducting multiturn flux transformers integrated with a coplanar resonator for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. The best magnetic field noise values of 22 and 11.5 fT/Hz1/2 in the white noise regime were obtained for the layout with two input coils and the layout with the labyrinth resonator, respectively. Excess low-frequency noise (about 200 fT/Hz1/2 at 10 Hz) was present. Computer simulation showed that the loss in this trilayer system was dominated by the high loss tangent of the dielectric film used for the separation of the upper and lower superconducting films. The rf coupling coefficient krf between the resonator and the flip-chip-coupled SQUID was also estimated. The values krf2≈14×10−3 obtained for the layout with two input coils, and krf2≈45×10−3 for the layout with the labyrinth resonator were considerably higher than the typical value of krf2≈7×10−3 for the single-layer coplanar resonator. These high coupling coefficients have compensated the somewhat degraded unloaded quality factor of the resonator, thus securing the optimum operation of the rf SQUID.
Quantitative nanoscale metrology study of Cu/SiO2 interconnect technology using transmission x-ray microscopy
Authors: Su X. , Stagarescu C. , Xu G. , Eastman D. E. , McNulty I. , Frigo S. P. , Wang Yuxin , Retsch Cornelia C. , Noyan I. C. , Hu C.-K.
Published: July 2002

This letter describes quantitative nondestructive measurements of multilayer submicron Cu/SiO2 interconnect structures such as Cu lines, vias, and W lines with lateral dimensions down to 300 nm and electromigration defect structures using scanning transmission x-ray microscopy employing a 0.2 μm x-ray beam. Typical measurement accuracies are ⩽60 nm for widths and lengths and ⩽10% in height. The high-resolution and nondestructive nature of this technique provide a very powerful probe of physical properties of nanoscale and submicron materials and structures.
Three-dimensional centrifugal-flow instabilities in the lid-driven-cavity problem
Authors: Albensoeder S. , Kuhlmann H. C. , Rath H. J.
Published: July 2002

The classical rectangular lid-driven-cavity problem is considered in which the motion of an incompressible fluid is induced by a single lid moving tangentially to itself with constant velocity. In a system infinitely extended in the spanwise direction the flow is two-dimensional for small Reynolds numbers. By a linear stability analysis it is shown that this basic flow becomes unstable at higher Reynolds numbers to four different three-dimensional modes depending on the aspect ratio of the cavity’s cross section. For shallow cavities the most dangerous modes are a pair of three-dimensional short waves propagating spanwise in the direction perpendicular to the basic flow. The mode is localized on the strong basic-state eddy that is created at the downstream end of the moving lid when the Reynolds number is increased. In the limit of a vanishing layer depth the critical Reynolds number approaches a finite asymptotic value. When the depth of the cavity is comparable to its width, two different centrifugal-instability modes can appear depending on the exact value of the aspect ratio. One of these modes is stationary, the other one is oscillatory. For unit aspect ratio (square cavity), the critical mode is stationary and has a very short wavelength. Experiments for the square cavity with a large span confirm this instability. It is argued that this three-dimensional mode has not been observed in all previous experiments, because the instability is suppressed by side-wall effects in small-span cavities. For large aspect ratios, i.e., for deep cavities, the critical three-dimensional mode is stationary with a long wavelength. The critical Reynolds number approaches a finite asymptotic value in the limit of an infinitely deep cavity.
Reflection-mode scattering-type scanning near-field optical microscope using a laser trapped gold colloidal particle as a scattering probe
Authors: Konada Takeshi , Sasaki Hiroko , Yamaguchi Mitsushiro , Suzuki Yoshimasa , Sasaki Yasuo
Published: July 2002

We have developed a reflection-mode scattering-type scanning near-field optical microscope using a laser trapped gold colloidal particle as a scattering probe and succeeded in observing the reflectance change of an opaque semiconductor sample with the alternating layers of GaAs and Al0.55Ga0.45As. The spatial resolution became as high as 200 nm when using a 200 nm gold colloidal particle. The results indicated that the resolution obtained in the experiment is in good agreement with the trapped particle size and overcame the diffraction limit (420 nm) of the lens system.
CoPtCr:C nanocomposite films for high density recording
Authors: Zeng H. , Yan M. L. , Liu Y. , Sellmyer D. J.
Published: July 2002

We report results on the magnetic properties of nanocomposite films of the form (Co86Cr14)xPt1−x:C, which were prepared by magnetron sputtering and subsequent annealing. X-ray diffraction shows that these films consist of CoPtCr grains with the face-centered-tetragonal structure embedded in an amorphous C matrix. Magnetic properties such as coercivity (Hc), remanence ratio (S), and coercive squareness (S*) are strongly dependent on the Pt and C composition and annealing temperatures. Compared with previously reported CoPt:C films with a equiatomic Co and Pt ratio, lower exchange coupling and higher S and S* at the same C concentration were obtained. Hc values ranging from 2 to 10 kOe and S close to 1 were achieved for film thicknesses down to 10 nm. These features make CoPtCr:C nanocomposite films very attractive for next-generation high-density recording.
Dielectric relaxation in SrTiO3–SrMg1/3Nb2/3O3 and SrTiO3–SrSc1/2Ta1/2O3 solid solutions
Authors: Lemanov V. V. , Smirnova E. P. , Sotnikov A. V. , Weihnacht M.
Published: July 2002

Dielectric properties of ceramic samples of (1−x)SrTiO3–xSrMg1/3Nb2/3O3 and (1−x)SrTiO3–xSrSc1/2Ta1/2O3 solid solutions were studied at x⩽15% and x⩽10%, respectively. The real and imaginary parts of the dielectric constant were measured at frequencies between 10 Hz and 1 MHz and in a temperature range of 4.2–300 K. Remarkable relaxation was found between 80 and 300 K. The relaxation was suggested to be due to local charge compensation processes. The activation energy for these processes was determined to be (0.25±0.05) eV with τ0=(10−12…10−13) s.
Rotational giant magnetoimpedance in soft magnetic wires: Modelization through Fourier harmonic contribution
Authors: Gómez-Polo C. , Vázquez M. , Knobel M.
Published: July 2002

A method to investigate the giant magnetoimpedance effect based on Fourier analysis is introduced. The study is carried out on a FeCoSiB amorphous wire with vanishing magnetostriction subjected to joule heating (current annealing) treatment that induces an enhancement of circumferential magnetic anisotropy and modifies the magnetoimpedance response of the samples. Experimental results are interpreted within the framework of the classical electrodynamical model, where the circumferential permeability plays the dominant role in the field dependence of the complex impedance of the sample. A rotational magnetization model is employed to determine the circular magnetization process, and a mean value of the circumferential permeability is obtained through the harmonic components obtained through Fourier analysis of the time derivative of the circular magnetization. This simple model is able to reproduce the observed experimental behavior, i.e., evolution of the field dependence of the complex impedance with annealing and the asymmetrical field dependence under a dc biased electrical current.
Negative coercivity in epitaxially grown (110) DyFe2/YFe2 superlattices
Authors: Beaujour J-M. L. , Gordeev S. N. , Bowden G. J. , de Groot P. A. J. , Rainford B. D. , Ward R. C. C. , Wells M. R.
Published: July 2002

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2/YFe2 superlattice samples with a (110) growth direction. Detailed magnetization curves have been obtained for YFe2 dominated multilayer samples [wDyFe2/4wYFe2]×16 with w=45, 50, and 55 Å. In particular, it is shown that the formation of magnetic exchange springs in the magnetically soft YFe2 layers, can be used to engineer multilayer samples with a negative coercivity. Further, by using asymmetric field cycling procedures, we have investigated the irreversible parts of the M–B loop, associated with the switching of the DyFe2 multilayers.
High power continuous wave microwave system at 3.7 GHz
Authors: Bora D. , Dani S. , Gangopadhyay S. , Jadav B. , Jha M. , Kadia B. R. , Khilar P. L. , Kulkarni S. V. , Kushwah M. , Patel A. P. , Parmar K. G. , Parmar K. M. , Parmar P. , Rajnish K. , Raghuraj S. , Rao S. L. , Samanta K. K. , Sathyanarayana K. , Shah P. , Sharma P. K. , Srinivas Y. S. S. , Trivedi R. G. , Verghese G. ,   RF Group
Published: July 2002

The lower hybrid current drive (LHCD) system is an important system in superconducting steady state tokamak (SST-1). It is used to drive and maintain the plasma current for 1000 s with a duty cycle of 17%. The LHCD system is being designed to launch 1 MW of radio frequency (rf) power at 3.7 GHz. The rf source is comprised of two high power klystron amplifiers, each capable of delivering 500 kW rf power. In this article, the results obtained during installation and commissioning of these klystrons are presented. Two klystrons (model TH2103D) have been successfully installed and commissioned on dummy loads, delivering ∼200 kW power for more than 1000 s. The maximum output power that could be obtained is limited due to the available direct current (dc) power supply. The test system is comprised of a TH2103D klystron, a low power rf (3.7 GHz/25 W) source, two high power four port circulators, two high power dual directional couplers, two arc detector systems, and two dummy water loads. To avoid rf breakdown in the rf components of the transmission line, the system has been pressurized with dry air to 3 bar. To energize and operate the klystron, a high voltage dc power supply, a magnet power supply, an ion pump power supply, a −65 kV floating anode modulator power supply, and a filament power supply are used. An arc detector unit has been installed to detect and initiate action within a few microseconds to protect the klystron, waveguides, and other rf passive components during arcing. To protect the klystron in the event of an arc, a fast responding (<10 μs), rail gap based pressurized crowbar unit has been used. The entire system is water cooled to avoid excess temperature rise during high power continuous wave operation of the klystron and other rf components. The tube requires initial conditioning. Thereafter, the output rf power is studied as a function of beam parameters such as cathode voltage and beam current.
An approximate deconvolution model for large-eddy simulation with application to incompressible wall-bounded flows
Authors: Stolz S. , Adams N. A. , Kleiser L.
Published: July 2002

The approximate deconvolution model (ADM) for the large-eddy simulation of incompressible flows is detailed and applied to turbulent channel flow. With this approach an approximation of the unfiltered solution is obtained by repeated filtering. Given a good approximation of the unfiltered solution, the nonlinear terms of the filtered Navier–Stokes equations can be computed directly. The effect of nonrepresented scales is modeled by a relaxation regularization involving a secondary filter operation. Large-eddy simulations are performed for incompressible channel flow at Reynolds numbers based on the friction velocity and the channel half-width of Reτ=180 and Reτ=590. Both simulations compare well with direct numerical simulation (DNS) data and show a significant improvement over results obtained with classical subgrid scale models such as the standard or the dynamic Smagorinsky model. The computational cost of ADM is lower than that of dynamic models or the velocity estimation model.
Fabrication of ferroelectric SrBi2Ta2O9 capacitor films using plasma-assisted metalorganic chemical vapor deposition and their electrical properties
Authors: Moon B. K. , Hironaka K. , Isobe C. , Hishikawa S.
Published: July 2002

The fabrication of SrBi2Ta2O9 (SBT) films using plasma-assisted metalorganic chemical vapor deposition (P-MOCVD) has been investigated. Optimizing the process conditions under plasma environment, amorphous SBT films were successfully deposited at a substrate temperature below 300 °C, suggesting that the P-MOCVD process effectively utilizes plasma energy to promote the reaction and decomposition of metal organic source molecules. The amorphous SBT films were crystallized to the bilayered perovskite SBT films by a postannealing at 725 °C. Thin SBT capacitors fabricated using P-MOCVD showed a good step coverage and the excellent ferroelectric properties including endurance. Low voltage operation below 1.5 V was successfully achieved using a 75 nm SBT capacitor, in which the signal level derived from the hysteresis curve suggests the feasibility of application to a 64 Mbit ferroelectric random access memories.
On the density matrix based approach to time-dependent density functional response theory
Authors: Furche Filipp
Published: July 2002

The formulation of time-dependent Kohn–Sham (TDKS) response theory based on the noninteracting one-particle density matrix is reanalyzed in detail. A transparent derivation starting from a von-Neumann-type equation of motion for the TDKS one-particle density matrix is presented. The resulting scheme has a simple structure and leads to compact expressions for frequency-dependent response properties. A systematic treatment of excited states is inferred from a pole analysis of the frequency-dependent density matrix response. A variational principle for excitation energies is established. Excited state properties are straightforward by analytical derivative techniques. The theory provides a particularly suitable starting point for linear scaling implementations. Magneto-optic properties such as rotatory strengths and the rotatory dispersion are accessible from the TDKS current-density response. The formalism is gauge-invariant. Various new sum rules within the adiabatic approximation (AA) are derived. It is shown that there is no “assignment problem” for excited states in the density matrix based formulation; the common density based approach is included as a special case. Merits and limitations of the AA are discussed.
Micromagnetic evaluation of magnetostatic interactions distribution in structured particulate media
Authors: Stancu Alexandru , Stoleriu Laurentiu , Cerchez Mihai
Published: July 2002

A complex analysis of magnetostatic interactions in particulate media is performed by means of a micromagnetic model. The properties of the interaction field distribution are presented for the case of structured two-dimensional media. The vector properties of the interaction field distribution are also analyzed. The hypothesis concerning interactions in scalar Preisach-type models are validated while the same hypothesis used in vector Preisach models are not entirely in agreement with the physical reality in particulate media.
Assessment of the quality of orbital energies in resolution-of-the-identity Hartree–Fock calculations using deMon auxiliary basis sets
Authors: Hamel Sébastien , Casida Mark E. , Salahub Dennis R.
Published: July 2002

The Roothaan–Hartree–Fock (HF) method has been implemented in deMon–DynaRho within the resolution-of-the-identity (RI) auxiliary-function approximation. While previous studies have focused primarily upon the effect of the RI approximation on total energies, very little information has been available regarding the effect of the RI approximation on orbital energies, even though orbital energies play a central role in many theories of ionization and excitation. We fill this gap by testing the accuracy of the RI approximation against non-RI-HF calculations using the same basis sets, for the occupied orbital energies and an equal number of unoccupied orbital energies of five small molecules, namely CO, N2, CH2O, C2H4, and pyridine (in total 102 orbitals). These molecules have well-characterized excited states and so are commonly used to test and validate molecular excitation spectra computations. Of the deMon auxiliary basis sets tested, the best results are obtained with the (44) auxiliary basis sets, yielding orbital energies to within 0.05 eV, which is adequate for analyzing typical low resolution polyatomic molecule ionization and excitation spectra. Interestingly, we find that the error in orbital energies due to the RI approximation does not seem to increase with the number of electrons. The absolute RI error in the orbital energies is also roughly related to their absolute magnitude, being larger for the core orbitals where the magnitude of orbital energy is large and smallest where the molecular orbital energy is smallest. Two further approximations were also considered, namely uniterated (“zero-order”) and single-iteration (“first-order”) calculations of orbital energies beginning with a local density approximation initial guess. We find that zero- and first-order orbital energies are very similar for occupied but not for unoccupied orbitals, and that the first-order orbital energies are fairly close to the corresponding fully converged values. Typical root mean square errors for first-order calculations of orbital energies are about 0.5 eV for occupied and 0.05 eV for unoccupied orbitals. Also reported are a few tests of the effect of the RI approximation on total energies using deMon basis sets, although this was not the primary objective of the present work.
Central-cell corrections and shallow donor states in strong magnetic fields
Authors: Jayam Sr. Gerardin , Navaneethakrishnan K.
Published: July 2002

Ionization energies and the central-cell corrections have been calculated for a few shallow donors in Si, GaP, and GaAs. We have assumed a short range potential with two parameters for the strength and the range for each donor, representing the central-cell effects. These parameters are fixed using the experimentally available ionization energies for each donor in a semiconductor. In the presence of a magnetic field the donor ionization energies are estimated using a variational procedure. Our results show that the ionization energies and the central-cell corrections increase with magnetic field. Our results are compared for GaAs with the recent work by Heron et al. [R. J. Heron, R. A. Lewis, P. E. Simmonds, R. P. Starret, A. V. Skougarevsky, R. G. Clark, and C. R. Stanley, J. Appl. Phys. 85, 893 (1999)].
Magnetic properties of heteroepitaxial La0.7Sr0.3MnO3/SrTiO3 superlattices
Authors: Sahana M. , Walter T. , Dörr K. , Müller K.-H. , Eckert D. , Brand K.
Published: July 2002

We have investigated the magnetic properties of La0.7Sr0.3MnO3(LSMO)/SrTiO3(STO) superlattices on a number of samples with the thickness dLSMO⩾dSTO in order to understand the interface magnetism and the possible interlayer magnetic coupling. A series of multilayers with several thickness combinations of dLSMO and dSTO, varying from 1.9 to 10 nm was fabricated on SrTiO3(100) substrates by pulsed laser deposition. A ferromagnetic transition was observed for all studied films and the transition temperature decreased with dLSMO, without showing a clear dependence on dSTO. The spontaneous magnetization was found to decrease with dLSMO though the decrease is not as strong as expected for a magnetically disordered surface of the LSMO layers.
Magnetization reversal and coercivity of a single-domain Co/Pt dot measured with a calibrated magnetic force microscope tip
Authors: Lohau J. , Carl A. , Kirsch S. , Wassermann E. F.
Published: July 2002

The magnetization reversal and the coercivity of a nanofabricated single-domain 230 nm diam Co/Pt multilayer dot with perpendicular magnetic anisotropy are measured quantitatively with magnetic force microscopy (MFM). During MFM imaging, a variable external magnetic field ranging between −1 kOe<H<1 kOe is applied in the z direction in order to simultaneously reverse the magnetizations of both the single-domain Co/Pt dot and the MFM tip. The hysteresis loop of the Co/Pt dot can be extracted unambiguously from the resulting MFM image contrast, since we have used a calibrated MFM tip, for which the magnetization reversal has been determined independently also by MFM using a nanofabricated current carrying ring.
Introduction of n-electron valence states for multireference perturbation theory
Authors: Angeli C. , Cimiraglia R. , Evangelisti S. , Leininger T. , Malrieu J.-P.
Published: July 2002

The present work presents three second-order perturbative developments from a complete active space (CAS) zero-order wave function, which are strictly additive with respect to molecular dissociation and intruder state free. They differ by the degree of contraction of the outer-space perturbers. Two types of zero-order Hamiltonians are proposed, both are bielectronic, incorporating the interactions between electrons in the active orbitals, therefore introducing a rational balance between the zero-order wave function and the outer-space. The use of Dyall’s Hamiltonian, which puts the active electrons in a fixed core field, and of a partially contracted formalism seems a promising compromise. The formalism is generalizable to multireference spaces which are parts of a CAS. A few test applications of the simplest variant developed in this paper illustrate its potentialities.
Evidence for shallow acceptors in GaN
Authors: Reynolds D. C. , Look D. C. , Jogai B. , Molnar R. J.
Published: July 2002

Two low-temperature photoluminescence lines in GaN, in the region of energies commonly interpreted as longitudinal optical-phonon replicas of free excitons, donor-bound excitons, or acceptor-bound excitons, are reinterpreted as acceptor-bound excitons (A0X’s) collapsing to n=2 and n=3 excited states, respectively, of the acceptors involved. Application of this model to two sets of A0X-related lines in hydride-vapor-phase-grown GaN gives acceptor energies of 85±1, and 115±1 meV, respectively. The existence of such shallow acceptor states, if confirmed, is of great technological importance.
Highly conductive epitaxial CdO thin films prepared by pulsed laser deposition
Authors: Yan M. , Lane M. , Kannewurf C. R. , Chang R. P. H.
Published: July 2002

Epitaxial growth of both pure and doped CdO thin films has been achieved on MgO (111) substrates using pulsed laser deposition. A maximum conductivity of 42 000 S/cm with mobility of 609 cm2/V s is achieved when the CdO epitaxial film is doped with 2.5% Sn. The pure CdO epitaxial film has a band gap of 2.4 eV. The band gap increases with doping and reaches a maximum of 2.87 eV when the doping level is 6.2%. Both grain boundary scattering and ionized impurity scattering are found to contribute to the mobility of CdO films.
Semianalytical ion current model for radio-frequency driven collisionless sheaths
Authors: Bose Deepak , Govindan T. R. , Meyyappan M.
Published: July 2002

We propose a semianalytical ion dynamics model for a collisionless radio-frequency biased sheath. The model uses bulk plasma and electrode boundary conditions to predict the ion impact energy distribution and electrical properties of the sheath. The proposed model accounts for ion inertia and ion current modulation at bias frequencies that are of the same order of magnitude as the ion plasma frequency. A relaxation equation for ion current oscillations is derived, which is coupled with a damped potential equation in order to model ion inertia effects. We find that inclusion of ion current modulation in the sheath model shows marked improvements in the predictions of the sheath electrical properties and ion energy distribution function.
Minimization of energy input to fluids for rock-fracturing experiments
Authors: Doiphode P. , Chaturvedi S.
Published: July 2002

Rock fracturing using electrically produced shocks in water is emerging as an environment-friendly substitute for fracturing by explosives. This involves producing underwater pressure waves or shocks of the desired intensity in a water-filled cavity drilled in the rock. We have numerically studied different options in an attempt to minimize the electrical energy consumption in this process, given a desired final pressure in the cavity. The first option is to follow different thermodynamic paths, e.g., isentropic and single shock, from the initial to the final pressure of water. It is found that isentropic compression allows a reduction of 2–3 times in energy input as compared to compression by a single shock. The second option is to replace water by other fluids. It has been found that the use of aqueous solutions at high electrolyte concentrations can reduce the energy consumption by over 30%.
Theory of space-charge field with a moving fringe in photorefractive polymers
Authors: Yuan Baohong , Sun Xiudong , Zhou Zhongxiang , Li Yan , Jiang Yongyuan , Hou Chunfeng
Published: July 2002

Resonance of space-charge field can occur in photorefractive polymers through a moving fringe and can be described by a forced oscillation equation with damping. The resonance frequency and the magnitudes of both the amplitude and the imaginary part of the space-charge field at the resonance frequency are functions of three experimental parameters: total incident intensity, applied electric field, and grating wave vector. Thus, the diffraction efficiency of grating and the gain coefficient of two-wave mixing are dependent on the three experimental parameters. Our theory can also be applied to inorganic photorefractive crystals.
Carrier dynamics modeling in a precharged Si/CaF2 heterostructure
Authors: Liniger M. , Kholod Alexander N. , Ménard S. , Borisenko V. E. , Bassani F. , Guirleo G. , Arnaud d’Avitaya F.
Published: July 2002

We examine theoretically charge carrier transport across a Si/CaF2 layered heterostructure by using an equivalent lumped-element circuit. We consider a structure has been precharged by a negative pulse. The resulting charge accumulation produces a nonmonotonic current–voltage (I–V) characteristic during a subsequent positive voltage sweep. We examine the dependence of the I–V characteristic on the magnitude and duration of the precharging bias pulse.
Surface evolution of polycrystalline Al films deposited on amorphous substrates at elevated temperatures
Authors: Eisenmenger-Sittner C.
Published: July 2002

A combined experimental and theoretical study of the surface evolution of sputtered aluminum coatings deposited on amorphous glass substrates (roughness <2 nm) at two different substrate temperatures (160 °C and 280 °C) is presented. The substrate temperature is a key parameter in thin film deposition controlling the surface morphology as well as the internal structure of the coating. In the intermediate substrate temperature regime (0.3<T/TM<0.6) the morphological evolution of the growth surface determines the overall film structure to a high degree. Several mechanisms compete in the development of surface features: island formation in the precoalescence phase, kinetic roughening by fluctuations in the incoming particle beam, surface diffusion, and two- or three-dimensional grain growth. Their respective dominance may shift in dependence on the deposition time and therefore on the film thickness. The morphology of the film surface is quantitatively characterized by atomic force microscopy. The dependence of the root-mean-square roughness on the film thickness shows a power-law behavior for thicknesses below 100 nm for both deposition temperatures. Above this thickness only a slight increase in roughness is observed. The presence of lateral correlations is studied by the one-dimensional power spectral densities (1DPSDs) of the film surfaces. The 1DPSDs are evaluated in the framework of two point autocorrelation functions, of Mullins’ and Herring’s model of surface diffusion, and by the q-state Potts model describing grain growth. This allows the identification of the dominant mechanisms for feature formation during different phases of film growth. The range of applicability of the growth models as well as their connection to the experimental observations are discussed.