bluuebunny/crossref_metadata_2025 huggingface dataset:
Centers of mass and rotational kinematics for the relativistic N-body problem in the rest-frame instant form
Authors: Alba David , Lusanna Luca , Pauri Massimo
Published: July 2002

A relativistic kinematics for the N-body problem which solves all the problems raised until now on this topic is constructed by exploting the Wigner-covariant rest-frame instant form of dynamics in the context of parametrized Minkowski theories. The Wigner hyperplanes, orthogonal to the total timelike four-momentum of any N-body configuration, define the intrinsic rest frame and realize the separation of the center-of-mass motion. The point chosen as origin in each Wigner hyperplane can be made to coincide with the covariant noncanonical Fokker–Pryce center of inertia. As is well known, the latter is distinct from the canonical pseudo- four-vector describing the decoupled motion of the center of mass (which possess the same Euclidean covariance as the quantum Newton–Wigner three-position operator) and from the noncanonical pseudo-four-vector known as Møller’s center of energy. Our approach leads to the splitting of the notion of center of mass into an external one, defined in terms of an external Poincaré group realization, and an internal one defined in terms of an internal unfaithful realization of the group inside the Wigner hyperplane. Because of the first class constraints defining the rest frame (vanishing of the internal three-momentum), the latter three internal concepts of center of mass weakly coincide. The resulting unique internal center of mass is thereby a gauge variable which, by a suitable gauge fixing, can be localized at the origin of the Wigner hyperplane. An adapted canonical basis of relative variables is constructed by means of the classical counterpart of the Gartenhaus–Schwartz transformation. The invariant mass of the N-body configuration is the Hamiltonian for the relative motions. Within this general framework, the rotational kinematics can be developed in terms of the same dynamical body frames, orientation-shape variables, spin frame, and canonical spin bases already introduced in the case of the nonrelativistic N-body problem.
Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part II. Interplay between chemistry and structure of layers
Authors: Losurdo Maria , Capezzuto Pio , Bruno Giovanni , Namkoong Gon , Doolittle W. Alan , Brown April S.
Published: July 2002

The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphire substrate/GaN interface, nucleation layer, and of the GaN epilayers grown by rf plasma assisted molecular beam epitaxy is investigated. It is found that a sapphire nitridation temperature as low as 200 °C improves the structural and optical quality of GaN epilayers. This result can be explained by the chemistry of the sapphire nitridation process, which is discussed in the framework of a model considering the competitive formation of AlN and oxynitride (NO). In particular, at 200 °C, NO desorbs from the sapphire surface, yielding an homogeneous 6 Å AlN layer upon N2 plasma nitridation. This low temperature AlN template favors the nucleation of hexagonal GaN nuclei which coalesce completely resulting in a hexagonal GaN buffer layer that homogeneously covers the sapphire substrate. This condition promotes the growth of a high quality GaN epilayer. In contrast, high nitridation temperatures result in a mixed AlN/NO nitrided sapphire surface which induce a perturbed and more defected interface with the occurrence of cubic crystallites in the GaN buffer. A sapphire surface with random GaN islands is found upon annealing of the GaN buffer and this condition results in a low-quality GaN epilayer.
Nonlinear dielectric microwave losses in MgO substrates
Authors: Hein M. A. , Oates D. E. , Hirst P. J. , Humphreys R. G. , Velichko A. V.
Published: July 2002

We have investigated the nonlinear surface impedance and two-tone intermodulation distortion of ten epitaxial YBa2Cu3O7−δ films on MgO substrates, using stripline resonators, at frequencies f=2.3–11.2 GHz and temperatures T=1.7 K−TC. The power dissipation decreased by up to one order of magnitude as the microwave electric field was increased to 100 V/m for T<20 K. The reactance showed only a weak increase. The minimum of the losses correlated with a plateau in the intermodulation signal. The same features were observed for a Nb film on MgO but not for YBa2Cu3O7−δ and Nb on LaAlO3 or sapphire. The anomalous response results from dielectric losses in MgO, most probably due to defect dipole relaxation.
Effect of adsorption on nanoindentation test
Authors: Yang Fuqian
Published: July 2002

The contact stiffness widely used in nanoindentation test is based on the Hertzian contact theory for zero-interfacial friction (slip boundary condition), in which the effect of adhesion energy is ignored. However, in the real situation, the adhesion energy may not be negligible when the contact is adhesive. A solution of the contact stiffness is derived by using adhesive contact theory for both frictionless condition (the famous Johnson–Kendall–Roberts theory) and the sticky condition. The results show that the nominal contact stiffness depends on the adhesion energy and indenter size as well as the load applied to indenter. For a large size of indenter and small load, the adhesion energy controls the contact process; while for a large load, the elastic deformation dominates, in which the nominal contact stiffness approaches the result based on the Hertzian contact theory.
Symmetry and coupling of magnetic and electric order parameters in YMnO3
Authors: Lottermoser Th. , Fiebig M. , Fröhlich D.
Published: July 2002

Second harmonic generation (SHG) is used to probe the coexisting ferroelectric and antiferromagnetic orders in hexagonal YMnO3. SH contributions coupling to the electric and/or magnetic order parameters are identified on the basis of the symmetries of the corresponding order parameters and the spectral dependence. The SH signal from the ferroelectric and antiferromagnetic orders were employed to image the electric and magnetic domain structures and investigate the mutual coupling.
Epitaxial growth of zinc-blende CrAs/GaAs multilayer
Authors: Mizuguchi M. , Akinaga H. , Manago T. , Ono K. , Oshima M. , Shirai M. , Yuri M. , Lin H. J. , Hsieh H. H. , Chen C. T.
Published: July 2002

Zinc-blende CrAs/GaAs multilayers were grown by molecular beam epitaxy. It was certified that each CrAs layer maintains an epitaxial relationship with the zinc-blende GaAs structure judging from the reflection high-energy electron diffraction observation. The film contains thicker zinc- blende CrAs layers in total than the CrAs thin film directly grown on the GaAs substrate which has the critical thickness of 3 nm. It was clarified that the optimum thicknesses of CrAs and GaAs to keep a good epitaxial relationship are 2 ML and 2 ML, respectively. The electronic structure of the multilayer is thought to be close to that of the (Ga, Cr)As thin film which has 50% of Cr content judging from x-ray absorption spectroscopy measurements.
Barocaloric effect: The use of pressure for magnetic cooling in Ce3Pd20Ge6
Authors: Strässle Th. , Furrer A. , Dönni A. , Komatsubara T.
Published: July 2002

The barocaloric effect found in many rare-earth compounds opens the way to novel magnetic refrigeration techniques which are rather based on the application of external pressure than on the application of large external magnetic fields as needed for cooling by the magnetocaloric effect. Here we report on the first observation of this effect in a heavy-fermion system, namely the Kondo lattice compound Ce3Pd20Ge6. Upon isothermal application of uniaxial pressure the compound lowers the magnetic entropy, subsequent adiabatic release of the pressure leads to a cooling by as much as −0.75 K per 0.3 GPa pressure release at 4.4 K. Based on entropy considerations the observed cooling effect can be explained by an increase of the Kondo temperature and by a structurally induced splitting of the Γ8 ground-state quartet upon uniaxial pressure.
Non-Born–Oppenheimer calculations of the polarizability of LiH in a basis of explicitly correlated Gaussian functions
Authors: Cafiero Mauricio , Adamowicz Ludwik
Published: July 2002

We present non-Born–Oppenheimer calculations of the electrical static polarizability of the LiH molecule. This is the first calculation of the non-Born–Oppenheimer polarizability for a system with more than two electrons. The polarizability is calculated by numerical differentiation of the energy obtained at different electric field strengths. The values for the energy are obtained by variational optimization with analytical gradients of the wave function expanded in a basis of explicitly correlated Gaussian functions. We also present a derivation of the integrals and energy gradients needed to perform these calculations. The result for the polarizability is 29.57 a.u.
Resonant light scattering from individual Ag nanoparticles and particle pairs
Authors: Tamaru Hiroharu , Kuwata Hitoshi , Miyazaki Hideki T. , Miyano Kenjiro
Published: July 2002

Light scattering by individual Ag nanoparticles and structures have been studied spectroscopically. Individual particles were selected and manipulated with a micromanipulator installed inside a scanning electron microscope (SEM). With typical particle dimensions of some 100 nm, the plasma resonances of particles and the coupled modes of particle pairs were observed in the visible region. The polarization dependence of the resonance frequencies strongly reflects the shape anisotropy; the effect that would be averaged out for experiments on ensembles. With a simple approximation to take the glass substrate into account, the results are in good agreement with the analytical calculations by Mie scattering, and with numerical calculations by the finite-difference time-domain method, both of which are performed with the morphological parameters obtained from the SEM observation for the corresponding particle or particle pair.
Density-of-states Monte Carlo method for simulation of fluids
Authors: Yan Qiliang , Faller Roland , de Pablo Juan J.
Published: July 2002

A Monte Carlo method based on a density-of-states sampling is proposed for study of arbitrary statistical mechanical ensembles in a continuum. A random walk in the two-dimensional space of particle number and energy is used to estimate the density of states of the system; this density of states is continuously updated as the random walk visits individual states. The validity and usefulness of the method are demonstrated by applying it to the simulation of a Lennard-Jones fluid. Results for its thermodynamic properties, including the vapor–liquid phase coexistence curve, are shown to be in good agreement with high-accuracy literature data.
Schottky diode with Ag on (112̄0) epitaxial ZnO film
Authors: Sheng H. , Muthukumar S. , Emanetoglu N. W. , Lu Y.
Published: July 2002

Silver Schottky contacts were fabricated on (112̄0) n-ZnO epilayers, which were grown on R-plane sapphire substrates by metalorganic chemical-vapor deposition. The flatband barrier height was determined to be 0.89 and 0.92 eV by current–voltage and capacitance–voltage measurements, respectively. The ideality factor was found to be 1.33.
Negative corona in air at atmospheric pressure due to a voltage impulse
Authors: Paillol J. , Espel P. , Reess T. , Gibert A. , Domens P.
Published: July 2002

This article presents a numerical modeling of a negative corona caused by an overvoltage pulse (∼250%), in a point-to-plane electrode system. The field value applied to the cathode is high, thus, the numerical simulation described in this article attempts to show that the field-effect emission can be responsible for the ignition of the corona and for its first stage of development, while the positive-ion bombardment is responsible for sustaining the discharge during the current decay phase. A time-dependent enhancement factor is introduced in the classical Fowler–Nordheim relationship in order to model the switch on of some emissive site at the cathode surface. Preliminary experiments were carried out in order to better understand the ignition and development of the corona. The role of the field-effect emission is underlined and the dimension of the corona is optically determined. The negative corona current pulse and its associated emitted UV light are computed and compared to experimental results.
Finite dimensional Hamiltonian formalism for gauge and quantum field theories
Authors: Hélein Frédéric , Kouneiher Joseph
Published: July 2002

We discuss in this article the canonical structure of classical field theory in finite dimensions within the pataplectic Hamiltonian formulation, where we put forward the role of Legendre correspondence. We define the generalized Poisson 𝔭-brackets which are the analogs of the Poisson bracket on forms. We formulate the equations of motion of forms in terms of 𝔭-brackets. As illustration of our formalism we present three examples: the interacting scalar fields, conformal string theory and the electromagnetic field.
Bi
3.25
La
0.75
Ti
3
O
12
thin films on ultrathin Al2O3 buffered Si for ferroelectric memory application
Authors: Chen San-Yuan , Sun Chia-Liang , Chen Shi-Bai , Chin Albert
Published: July 2002

We have investigated the physical and electrical properties of Bi3.25La0.75Ti3O12 (BLT) thin films on Pt/Ti/SiO2/Si and on Al2O3(6 nm)/Si, which are used for one-transistor-one-capacitor and one-transistor ferroelectric memory, respectively. The BLT thin films on both substrates show good capacitance–voltage characteristics and the same threshold voltage shift of 1.6 V at applied ±10 V bias. However, the leakage current of BLT on Al2O3/Si at −100 kV/cm is two orders of magnitude lower than that on Pt. The comparable memory characteristics and much reduced leakage current of BLT on Al2O3/Si are the strong advantages as compared with BLT on Pt because it is directly related to switching energy and device scaling down.
Microstructure from simulated Brownian suspension flows at large shear rate
Authors: Morris Jeffrey F. , Katyal Bhavana
Published: July 2002

Pair microstructure of concentrated Brownian suspensions in simple-shear flow is studied by sampling of configurations from dynamic simulations by the Stokesian Dynamics technique. Simulated motions are three dimensional with periodic boundary conditions to mimic an infinitely extended suspension. Hydrodynamic interactions through Newtonian fluid and Brownian motion are the only physical influences upon the motion of the monodisperse hard-sphere particles. The dimensionless parameters characterizing the suspension are the particle volume fraction and Péclet number, defined, respectively, as φ=(4π/3)na3 with n the number density and a the sphere radius, and Pe=6πηγ̇a3/kT with η the fluid viscosity, γ̇ the shear rate, and kT the thermal energy. The majority of the results reported are from simulations at Pe=1000; results of simulations at Pe=1, 25, and 100 are also reported for φ=0.3 and φ=0.45. The pair structure is characterized by the pair distribution function, g(r)=P1|1(r)/n, where P1|1(r) is the conditional probability of finding a pair at a separation vector r. The structure under strong shearing exhibits an accumulation of pair probability at contact, and angular distortion (from spherical symmetry at Pe=0), with both effects increasing with Pe. Flow simulations were performed at Pe=1000 for eight volume fractions in the range 0.2⩽φ⩽0.585. For φ=0.2–0.3, the pair structure at contact, g(|r|=2)≡g(2), is found to exhibit a single region of strong correlation, g(2)≫1, at points around the axis of compression, with a particle-deficient wake in the extensional zones. A qualitative change in microstructure is observed between φ=0.3 and φ=0.37. For φ⩾0.37, the maximum g(2) lies at points in the shear plane nearly on the x axis of the bulk simple shear flow Ux=γ̇y, while at smaller φ, the maximum g(2) lies near the compressional axis; long-range string ordering is not observed. For φ=0.3 and φ=0.45, g(2)∼Pe0.7 for 1⩽Pe⩽1000, a slower increase than the g(2)∼Pe predicted theoretically for φ≪1 [Brady and Morris, J. Fluid Mech. 348, 143 (1997)]. Spherical harmonic decomposition of g(r) was performed, and for Pe=1000, expansion convergence is found to be nearly complete when harmonics Ylm to the level l=10 are included.
Absorption spectra and chirality of single-walled 4 Å carbon nanotubes
Authors: Liang WanZhen , Chen GuanHua , Li Zhaoming , Tang Zi-Kang
Published: July 2002

Absorption spectrum of recently discovered single-walled 4 Å carbon nanotubes is measured. The semiempirical PM3 localized-density-matrix method is employed to evaluate the absorption spectra of three possible 4 Å single-walled carbon nanotubes, (3,3), (4,2), and (5,0). Both experimental and calculated results reveal that these nanotubes have finite optical gaps and strong anisotropic optical responses. When the electric field is perpendicular to the nanotubes, they are transparent to visible lights; and this is confirmed and explained by the calculations. Compared to the measured absorption spectrum, calculated absorption spectra are used to determine the chirality of the nanotubes synthesized in the channels of porous zeolites.
Irreversibility in diluted antiferromagnets
Authors: Soukoulis C. M. , Grest Gary S. , Ro Charles , Levin K.
Published: July 2002

We compute the irreversibility phase diagram, and history-dependent magnetizations for diluted Ising antiferromagnets in three dimensions. We use an iterative mean-field technique which has proved quite successful for treating spin glasses. Motion of the domain walls as the magnetic field is varied at low temperature is also discussed. The onset of time-dependent long-range order and magnetization anomalies are predicted for certain regions of the H-T phase diagram from our studies of the free-energy surface. Our results are compared to those of the random field ferromagnet and spin glasses, which have the common feature that on the time scale of a physical measurement the system is trapped in a local minimum of the free-energy surface. Irreversibility then results primarily from the disappearance of a given minimum as the field or temperature are changed and not from tunneling or thermal activation processes.
Variations in the second- and third-order elastic constants in polycrystalline aggregates
Authors: Johnson George C.
Published: July 2002

Expressions for the expected variation in the effective second- and third-order elastic constants of statistically isotropic polycrystalline aggregates with a finite number of crystallites are developed. The analysis is based on the Voigt approach for the effective response of an aggregate and is valid for cubic crystals of the highest symmetry (m3m,432,4̄3m). In this approach, the effective stiffnesses are the averages of the stiffnesses in the constituent crystals. It is shown that the expected variance in the effective elastic stiffness is inversely proportional to the number of crystallites contributing to the average. Numerical evaluations for crystals of various materials show that the variations in the third-order stiffnesses are substantially larger than the variations in the second-order stiffnesses.
Carrier freeze-out effects in semiconductor devices
Authors: Ahmad Nizam
Published: July 2002

The Gunn effect as a function of pressure has been studied in InSb devices. The Gunn threshold field is found to decrease with increasing pressure reaching a minimum at P=9 kbar. It then increases while the oscillations weaken with increasing pressure up to 12 kbar. Low-field Hall and conductivity measurements have also been made in an attempt to obtain a possible explanation for the threshold minimum. It is observed that the impurity level moves away from the conduction band approximately exponentially with increasing pressure. By 9 kbar the donor ionization energy is large enough that considerable freezeout or deionization of donor states occurs. The reduction in carrier number is unfavorable to the Gunn process and hence leads to an increase in the threshold field for pressure above 9 kbar.
Energy losses in magnetically insulated transmission lines due to microparticles
Authors: Gray Eoin W. , Stinnett Regan W.
Published: July 2002

We discuss the effects of high-velocity and hypervelocity microparticles in the magnetically insulated transmission lines of multiterawatt accelerators used for particle beam fusion and radiation effects simulation. These microparticles may be a possible source for plasma production near the anode and cathode in early stages of the voltage pulse, and current carriers during and after the power pulse, resulting in power flow losses. Losses in the current pulse, due to microparticles, are estimated to be approximately 12 mA/cm2 (0.3 kA) as a lower limit, and ∼0.3 A/cm2 (7.2 kA) for microparticle initiated, anode plasma positive ion transport. We have calculated the velocities reached by these microparticles and the effects on them of Van der Waals forces. Field emission from the particles and their effects on cathode and anode plasma formation have been examined. Particle collision with the electrodes is also examined in terms of plasma production, as in the electron deposition in the particles in transit across the anode-cathode gap. Blistering of the electrode surface, thought to be due to H− bombardment was also observed and appears to be consistent with losses due to negative ions previously reported by J. P. VanDevender, R. W. Stinnett, and R. J. Anderson [App. Phys. Lett. 38, 229 (1981)].
Effects of temperature on exchange coupled alloys of Ni80Fe20-FeMn, Ni80Fe20-αFe2O3, and Ni80Fe20-TbCo
Authors: Cain William C. , Meiklejohn William H. , Kryder Mark H.
Published: July 2002

The effect of temperature on the exchange coupling at the interface of three different bilayered materials was studied with the objective of developing temperature-stable single-domain materials for magnetoresistive readback heads. Exchange field (HE) was measured from room temperature to 245 °C for NiFe films coupled to FeMn, αFe2O3, and TbCo. In the permalloy-FeMn system, the exchange field decreases linearly, and reaches zero at about 150 °C, which is close to the Neel temperature of the antiferromagnet. These results agree well with previous work [C. Tsang and Kenneth Lee, J. Appl. Phys. 53, 2605 (1982)]. HE also decreases linearly in the αFe2O3 system, from 6.8 Oe at room temperature to 1.8 Oe at 245 °C. While the αFe2O3 system offers greater temperature stability, the exchange field produced is not very large, and the coercivity is somewhat high. Changes in HE with variations in the thickness of the αFe2O3 layer were also noted. Results for permalloy coupled to ferrimagnetic amorphous TbCo indicate a finite exchange field to at least 250 °C, with samples deposited at 90-V substrate bias having exchange fields of 12 Oe at 250 °C. Room-temperature coercivities of some 400-Å NiFe films coupled to TbCo have been measured to be as low as 1 Oe. These results suggest that films that have a low coercivity and an exchange field large enough to bias a magnetoresistive head can be produced with good temperature stability up to 250 °C.
Novel method for determining the anisotropy constant of MnFe in a NiFe/MnFe sandwich
Authors: Mauri Daniele , Kay Eric , Scholl David , Howard J. Kent
Published: July 2002

We show that an in situ Kerr rotation measurement is a very effective technique for the study of antiferromagnetic (AF) ferromagnetic (F) film couples. Magnetic signals can be obtained even in the case where the (AF) is the top layer up to at least 200 Å of AF thickness. We have used this in situ approach combined with ion milling to study the thickness dependence of the magnetic properties of Mn50Fe50/Ni80Fe20 systems. We observe that the exchange bias field has a surprisingly sharp onset at a critical thickness of AF∼50 Å. We show that this is consistent with a simple model and that the magnetic anisotropy of MnFe can be estimated from the observed critical thickness to be ∼1.35×105 erg/cm3. The exchange field showed the predicted proportionality to the inverse of the F thickness from ∼50 to 400 Å. Auger spectroscopy and spin polarized secondary electron emission have been used to rule out gross artifacts due to ion milling.
Pulsed ruby laser-induced aqueous oxidation of tantalum: X-ray diffraction and x-ray photoelectron spectroscopic study
Authors: Ghaisas S. V. , Malshe A. P. , Patil P. P. , Kanetkar S. M. , Ogale S. B. , Bhide V. G.
Published: July 2002

Tantalum oxide phases have been synthesized by pulsed ruby laser-induced reactive quenching at a tantalum and water interface (Ta:H2O). The structural and chemical states of the oxides have been characterized by x-ray diffraction and x-ray photoelectron spectroscopy, whereas the surface morphology of these laser-treated samples was studied using scanning electron microscopy. The results are significantly different from those resulting from transformations induced when tantalum is laser-treated in air (Ta:air) at comparable energy density.
Dispersion of acoustic surface waves by velocity gradients
Authors: Kwon S. D. , Kim H. C.
Published: July 2002

The perturbation theory of Auld [Acoustic Fields and Waves in Solids (Wiley, New York, 1973), Vol. II, p. 294], which describes the effect of a subsurface gradient on the velocity dispersion of surface waves, has been modified to a simpler form by an approximation using a newly defined velocity gradient for the case of isotropic materials. The modified theory is applied to nitrogen implantation in AISI 4140 steel with a velocity gradient of Gaussian profile, and compared with dispersion data obtained by the ultrasonic right-angle technique in the frequency range from 2.4 to 14.8 MHz. The good agreement between experiments and our theory suggests that the compound layer in the subsurface region plays a dominant role in causing the dispersion of acoustic surface waves.
The nonlinear effects of noise on parametric amplification: An analysis of noise rise in Josephson junctions and other systems
Authors: Bryant Paul , Wiesenfeld Kurt , McNamara Bruce
Published: July 2002

We take a new look at an old problem; namely, the observed ‘‘noise rise’’ in superconducting Josephson junction parametric amplifiers. By exploiting recent insights from dynamical systems theory, we show how the interplay or random noise and (nonchaotic) deterministic dynamics can result in a noise rise like that observed in experiments. Our analysis leads to a universal first-order equation which applies to all similar systems in the high-gain regime. We propose several predictions which can be tested experimentally, including that a similar noise rise should occur in modulated semiconductor injection lasers. We also analyze a previously unknown mode of operation—a ‘‘six-photon’’ mode associated with a symmetry breaking bifurcation—and discuss its potential advantages over the previously studied three- and four-photon modes.
Impedance of an annular-cathode indented-anode electron diode terminating a coaxial magnetically insulated transmission line
Authors: Sanford T. W. L. , Poukey J. W. , Wright T. P. , Bailey J. , Heath C. E. , Mock R. , Spence P. W. , Fockler J. , Kishi H.
Published: July 2002

The impedance of a diode having an annular cathode and indented anode that terminates a coaxial MITL (magnetically insulated transmission line) is measured and compared with a semiempirical model developed from calculations made using the magic code. The measurements were made on the 16-Ω electron accelerator HELIA (high-energy linear induction accelerator) operating at 3 MV. The model agrees with the measurements within the 10% measuring error and shows that the diode operates in either a load- or line-dominated regime depending on AK (anode-cathode) gap spacing. In the load-dominated regime, which corresponds to small AK gaps, the diode impedance is controlled by an effective anode-cathode gap, and the flow is approximately axial. In the line-dominated regime, which corresponds to large AK gaps, the impedance is independent of the AK gap and corresponds to the impedance associated with the minimum current solution of the MITL, with the flow becoming more radial as the AK gap is increased.
Performance aspects of a quantum-well detector
Authors: Goossen K. W. , Lyon S. A.
Published: July 2002

Calculations of the quantum efficiency and detectivity of an infrared detector based on photoemission from a quantum well are presented. The detector is most efficient when a resonant-extended state exists near the top of the well. The quantum efficiency also increases with increasing electron density. However, due to screening, the absorption peaks at a higher energy than the difference of the energies of the resonant-extended and the bound states by an amount which is proportional to the carrier density in the well. This causes the detectivity (D*) to have a maximum with respect to electron density. We have estimated the dark current and found that, for a GaAs quantum-well detector designed for 10-μm operation, the optimal electron density was 2×1011 cm−2 at 77 K. We have also performed calculations for a quantum-well detector for which the light coupling has been enhanced by incorporating a diffraction grating into the detector. For the stated electron density, we find a D* of 2.8×1011 cm Hz1/2/W and a quantum efficiency of 23%. If the electron density is raised to 8×1011 cm−2, the quantum efficiency rises to 61%, but D* falls to 1.9×1010 cm Hz1/2/W.
Ferromagnetism in metastable 304 stainless steel with bcc structure
Authors: Childress J. , Liou S. H. , Chien C. L.
Published: July 2002

We have studied the consequence of structure on the magnetic properties of 304 stainless steel in two distinct crystalline states. Ordinary 304 stainless steel has an fcc structure and is nonmagnetic at room temperature. By using a vapor quenching method, we have fabricated single-phase metastable bcc 304 stainless steel which is strongly ferromagnetic. Films a few μm thick have been made by high-rate sputter deposition onto substrates at room temperature or liquid-nitrogen temperature. Vibrating sample magnetometry and 57Fe Mössbauer spectroscopy reveal that the bcc phase has a magnetization of 130 emu/g, due largely to the Fe moment. The Curie temperature is found to be excess of 550 °C. Upon subsequent annealing above 550 °C, the metastable bcc state transforms back into the usual nonmagnetic fcc phase. The changes in the magnetic properties and the structure of these films during the transformation are examined.
Thickness of p/n junction space-charge layers
Authors: Liou J. J. , Lindholm F. A.
Published: July 2002

A model for finding the space-charge-layer thickness is developed based on the relevant device physics and on a space-charge-layer capacitance model which accounts for the effects of the free-carrier charges in the space-charge layer. Discrepancies of as much as 50% are predicted when the present model is compared with the conventional depletion model. A partitioned charge-based model is used to illustrated the usefulness of the model.
Characterization of GaAs/(GaAs)n(AlAs)m surface-emitting laser structures through reflectivity and high-resolution electron microscopy measurements
Authors: Faist J. , Ganière J.-D. , Buffat Ph. , Sampson S. , Reinhart F.-K.
Published: July 2002

Careful investigation of the reflectivity of two very high finesse integrated Fabry–Perot interferometers is reported. These two structures, made of GaAs active layer (1.7 μm thick) surrounded by two superlattice/AlAs Bragg reflectors, exhibit vertical cw lasing action at and above room temperature when photopumped with thresholds of 16 mW at 300 K and 56 mW at 380 K. Reflectivity measurements together with theoretical calculations show that layer regularity, accurate thickness control, and low interface roughness are key parameters for high-performance structures. Transmission electron microscopy on cleaved wedges and reflection electron microscopy are shown to be unique tools for measuring and characterizing these layers. Electron microscopy, optical reflection, and laser linewidth measurements are correlated and show that the layer flatness is dramatically increased by the introduction of six (2.5 Å) GaAs wells in the AlAs growth of the integrated dielectric reflectors. Reflectives of 97%, Fabry–Perot finesse as high as 100 (61 for direct measurements), and laser linewidths as small as 1.2 Å are reported.
A potential problem for point contacts on a two-dimensional anisotropic medium with an arbitrary resistivity tensor
Authors: Shibata Hisashi , Terakado Ryuiti
Published: July 2002

A two-dimensional anisotropic medium with a resistivity tensor in the general form is considered. In the arbitrarily shaped region with four point contacts on the circumference, the potential problem caused by two current contacts is solved by using affine and conformal mappings. The relations among the resistances that are defined as the potential difference between the remaining contacts per unit current are presented for various combinations of current contacts. The resistance characteristics in this paper include the well-known van der Pauw theorem, which is applicable for electrical characteristic measurements on anisotropic conductors in applied magnetic fields.
Linear and two-photon absorption of rectangular and sawtooth HgTe/CdTe superlattices
Authors: Chang Yia-Chung , Cheung J. , Chiou A. , Khoshnevisan M.
Published: July 2002

Theoretical studies of linear and two-photon absorption spectra of HgTe/CdTe superlattices with rectangular and sawtooth potential profiles are reported. Under optimum condition, the two-photon absorption coefficient can be higher than 1 cm/kW. The possible application of these materials as optical limiters near 10 μm wavelength is discussed.
Remote plasma deposition of aluminum nitride
Authors: Nomura Hideaki , Meikle Scott , Nakanishi Yoichiro , Hatanaka Yoshinori
Published: July 2002

Aluminum nitride thin films have been deposited from the addition of trimethyl aluminum in the afterglow of N2, N2:H2 microwave plasmas over a pressure range of 0.01–0.3 Torr. With N2 as the plasma gas at 0.3 Torr, strongly chemiluminescent reactions of N with CH3 lead to CN incorporation and resulting films are poor insulators. With N2:H2 as the plasma gas or for deposition at low pressures, chemiluminescence is extinguished and films with resistivities ≳1015 Ω cm can be deposited. It is proposed that the improvement results because the fast gas phase reactions between N and CH3 are avoided.
Analysis of arc pressure effect on metal transfer in gas-metal arc welding
Authors: Rhee Sehun , Kannatey-Asibu Elijah
Published: July 2002

Metal transfer has been analyzed using both the static force balance method and the pinch instability theory. The influences of arc pressure as well as electromagnetic pressure in the development of instability in the cylindrical molten metal were considered. Using perturbation theory, the dispersion equation was obtained in terms of the arc current density, and the drop diameters were then calculated. The drop diameters, which were determined by considering the arc effect, were found to be greater than those drop diameters that were obtained without the arc effect. The transition current was obtained on the assumption that necking starts when the arc covers an entire drop, at which point spray transfer occurs as the molten droplet becomes cylindrical in shape. This explains why spray transfer or transition does not appear with carbon dioxide or helium shielding gas since the arc then exists only at the lower half of the droplet.
Ab initio calculation of molecular field interactions in rare-earth transition-metal intermetallics (invited)
Authors: Brooks M. S. S. , Gasche T. , Auluck S. , Nordström L. , Severin L. , Trygg J. , Johansson B.
Published: July 2002

The interaction, KRM, between the rare-earth 4f moment and the transition-metal 3d moments in rare-earth transition-metal intermetallics is shown to depend upon the R-5d moment, which is due to 3d–5d hybridization, and local 4f–5d exchange integrals. Both the R-5d moment and KRM may be calculated ab initio from the local spin-density approximation to density functional theory in self-consistent energy-band calculations with the localized 4f-moments fixed at their Russel–Saunders values. Detailed examples are given for the RFe2 (R=Gd−Yb) series. The exchange integrals are similar to those entering into the density functional version of Stoner theory and their energy dependence must be treated carefully. The calculated local exchange integrals are shown to be related to the molecular fields derived from spin Hamiltonians, hence to the spin-wave spectra. Reasonable agreement with values of the molecular fields extracted from inelastic neutron scattering and high field susceptibility measurements is obtained.
Synthesis and characterization of fine grain diamond films
Authors: Wu Richard L. C. , Rai A. K. , Garscadden Alan , Kee Patrick , Desai Hemant D. , Miyoshi Kazuhisa
Published: July 2002

A fine grain diamond film has been developed by microwave plasma assisted chemical vapor deposition. Various analytical techniques, including Rutherford backscattering, proton recoil analysis, Raman spectroscopy, and X-ray diffraction, were utilized to characterize the diamond films. The grain size of the film was determined from bright and dark field electron micrographs, and found to be 200–1000 Å. The films exhibited good optical transmission between 2.5 and 10 μm, with a calculated absorption coefficient of 490 cm−1. The friction coefficients of this film were found to be 0.035 and 0.030 at dry nitrogen and humid air environments, respectively, and the films had low wear rates.
Doping of Si thin films by low-temperature molecular beam epitaxy
Authors: Gossmann H.-J. , Unterwald F. C. , Luftman H. S.
Published: July 2002

Two-dimensional doping sheets (‘‘δ doping’’) are integral parts of many novel semiconductor device concepts. Deep submicron design rules require junction depths significantly below 100 nm. This level of control is difficult to achieve with ion implantation. We discuss the application of thermal, coevaporative doping with Sb and elemental B during Si molecular beam epitaxy at growth temperatures below ≊300 °C to this problem. We show that it is possible to create structures with very high doping levels, yet with very sharp doping transitions. Delta-doping spikes with a full width at half maximum of <2.7 nm and <4.0 nm have been obtained by secondary-ion mass spectrometry for Sb and B, respectively, with corresponding up-slopes of 2.5 and 0.94 nm/decade. Homogeneously doped films show full activation up to NSb≊6×1020 cm−3 and NB≳1×1021 cm−3. Mobilities agree with bulk values at corresponding concentrations. Mesa-isolated pn junctions exhibit ideality factors of 1.05.
Critical current density and magnetic properties of a melt-processed Y0.8Ho0.2Ba2Cu3Oy superconductor
Authors: Feng Yong , Zhou Lian , Wang Keguang , Zhang Yuheng , Jin Xin , Zhang Yitong , Jin Jirong , Yao Xixian
Published: July 2002

We have prepared a bulk Y0.8Ho0.2Ba2Cu3Oy superconductor by the powder melting process method and investigated its magnetization curves and magnetic relaxation in the temperature range 55–77 K at different fields. The Jc values at a certain temperature and field is estimated by Bean’s model. It is found that Jc decreases with the magnetic field according to a power law, whereas it drops exponentially with temperature. The dependence on temperature of the maximum diamagnetization field Hp and the remanent magnetization Mrem is discussed. The magnetic relaxation indicates that the magnetization M varies logarithmically with time. The relaxation rate dM/d ln t is also studied.
Phase equilibria in the Pt-In-P system
Authors: Lin C. F. , Mohney S. E. , Chang Y. A.
Published: July 2002

X-ray diffraction and electron probe microanalysis were used to determine the phase equilibria in the In-Pt-P system at 725 and 600 °C. The observed binary phases were consistent with the assessed phase diagrams from the literature. Three ternary phases were found. The major difference between the 725 and 600 °C isotherms was the phases in equilibrium with InP. At 600 °C, the binary phases Pt3In7 and PtP2 were in equilibrium with InP. However, a tie-line transition from InP-Pt3In7 to In-PtP2 was found between 600 and 725 °C. The binary phases which were found to be stable in thin film form on InP were consistent with the phase equilibria data. From the tie-line change, the enthalpy of formation of PtP2 at 298 K was estimated to be −350±40 kJ/mol.
Transport properties of epitaxial lift-off films
Authors: Mena R. A. , Schacham S. E. , Young P. G. , Haugland E. J. , Alterovitz S. A.
Published: July 2002

Transport properties of epitaxially lifted-off (ELO) films were characterized using conductivity, Hall, and Shubnikov–de Haas measurements. A 10%–15% increase in the two-dimensional electron gas concentration was observed in these films as compared with adjacent conventional samples. We believe this result to be caused by a backgating effect produced by a charge buildup at the interface of the ELO film and the quartz substrate. This increase results in a significant decrease in the quantum lifetime in the ELO samples, by 17%–30%, but without a degradation in carrier mobility. Under persistent photoconductivity, only one subband was populated in the conventional structure, while in the ELO films the population of the second subband was clearly visible. However, the increase of the second subband concentration with increasing excitation is substantially smaller than anticipated due to screening of the backgating effect.
Interactions of YBa2Cu3O7−x thin films with alkaline earth fluoride substrates
Authors: Peters C. H. , Bernasek S. L. , Venkatesan T. , Pique A. , Harshavardhan K. S. , Wu Y.
Published: July 2002

Film-substrate interactions of YBa2Cu3O7−x (YBCO) thin films grown by pulsed laser deposition on alkaline earth fluoride substrates were investigated. X-ray photoelectron spectroscopy, atomic force microscopy, and x-ray-diffraction measurements showed that the quality of the film and amount of chemical reaction between film and substrate were dependent on the substrate material and deposition temperature. The reaction of YBCO films with CaF2 and MgF2 substrates forms BaF2 and calcium or magnesium oxide species. The reacted film is insulating and has a microscopically rough surface. No reaction was detected in films deposited on BaF2 and SrF2. Physical and thermodynamic properties which may explain the observed order of reactivity are examined.
Characterization of trapping states in polycrystalline-silicon thin film transistors by deep level transient spectroscopy
Authors: Ayres J. R.
Published: July 2002

Polycrystalline-silicon (poly-Si) thin film transistors (TFTs) have been assessed using deep level transient spectroscopy (DLTS) based on the measurement of current transients due to the thermal emission of carriers trapped at deep levels in the poly-Si. Measurements were made on fully hydrogenated TFTs and the DLTS signal was found to vary continuously between 77 K and room temperature, without showing the characteristic peaks normally associated with point defects in single crystal Si. This demonstrates the existence of a continuous distribution of states through the band gap rather than discrete monoenergetic states. Analysis of the DLTS signal suggests the density of states increases rapidly toward the conduction band edge indicating the presence of a relatively high density of tail states. After annealing the TFTs at 450 °C the current DLTS signal was found to increase due to an increase in trap state density, which occurs because Si–H bonds are thermally unstable at this temperature. The increase in signal was over the whole temperature range of the measurement showing that hydrogen passivates states up to at least 0.1 eV below the conduction band as well as states nearer mid-gap. The effects of varying the fill voltage on the measurement and the time dependence of the current transient were also investigated and added weight to arguments that the current transient was due to the thermal emission of carriers trapped at deep levels.
Analysis of the electroluminescence observed during the anodic oxidation of porous layers formed on lightly p-doped silicon
Authors: Ligeon M. , Muller F. , Herino R. , Gaspard F. , Vial J. C. , Romestain R. , Billat S. , Bsiesy A.
Published: July 2002

A detailed analysis of the different characteristics of the electroluminescence that is observed during the anodic oxidation of porous silicon layers formed on lightly p-doped substrates is presented. It is shown that the emission presents characteristics very similar to that of the photoluminescence observed on the same porous layers, and that the same basic mechanisms are involved in the two phenomena. The emission intensity increase with the oxidation level is quantitatively explained by the passivation enhancement provided by the electrochemical oxidation. The spectral shift of the spectra during the oxidation is also discussed: It is shown to result from the decrease in the sizes of the largest emitting crystallites or/and from the significant improvement of the passivation of the smallest ones due to the oxide growth. The effect of the anodizing current density on the emission efficiency is also presented.
Blocking of logarithmic temporal relaxation of magnetic remanence by piezomagnetically induced domains in Fe1−xZnxF2
Authors: Kushauer J. , Binek C. , Kleemann W.
Published: July 2002

Faraday rotation and superconducting quantum interference device magnetometry were used to investigate the relaxation of the weak-field induced remanence, μ, of the random-field Ising model system Fe1−xZnxF2. The stretched logarithmic decay law, as predicted by Nattermann and Vilfan for the case of compact domains with fractal surfaces, was confirmed for freezing fields of 3≤B≤5 T. Virtually constant remanence, μ, was found at low fields, 0.0002≤B≤1.5 T. This ferrimagnetic moment is due to the piezomagnetic effect acting on random-stress induced immobile domains. Chemical etching, which removes surface stress originating from the sample preparation, proves to decrease the remanence by about 50%, part of which is restored after renewed optical polishing.
Method for the determination of photostimulable defect center concentrations, production rates, and effective formation energies
Authors: Thoms M. , von Seggern H.
Published: July 2002

A new method is proposed to determine radiation-induced defect center concentrations in solids which exhibit photostimulable luminescence. The technique relies on the determination of the number of photons released by spontaneous and photostimulated luminescence. A comparison with the x-ray-absorbed energy yields the effective formation energy of spontaneous luminescence photons and photostimulable centers. The method is applied representatively to BaFBr:Eu2+, which is utilized commercially as photostimulable x-ray storage phosphor in image plates for x-ray, γ-ray, electron, and neutron radiography. It is shown that in the utilized material on average 8 photostimulable F(Br−) centers (color centers based on bromine vacancies) and about 5 spontaneously emitted photons are generated by the absorption of 1 keV x-ray energy. Taking recent investigations into account which show that the x-ray generation of spontaneous luminescence and photostimulated luminescence centers is accompanied by the creation of approximately 36 nonphotostimulable defect centers, an average formation energy of about 20 eV per defect center can be calculated.