bluuebunny/crossref_metadata_2025 huggingface dataset:
Dielectric properties of material with random off-center defects: Monte Carlo simulation of relaxor ferroelectrics
Authors: Su C.-C. , Vugmeister B. , Khachaturyan A. G.
Published: July 2002

A Ginzburg–Landau type theory of interaction of randomly distributed local dipoles in a paraelectric crystal is developed. The interaction is caused by the polarization of the host lattice generated by these dipoles. The obtained effective Hamiltonian of the dipole–dipole interaction is employed for the Monte Carlo simulation of ferroelectric properties of a system with off-center dopant ions producing local dipoles. The computer simulation shows that at low dopant ion concentration the paraelectric state transforms into a macroscopically paraelectric state consisting of randomly oriented polar clusters. These clusters amplify the effective dipole moment and dramatically increase the dielectric constant. The interaction between the clusters results in a spectrum of relaxation time and transition to the relaxor state. The real and imaginary parts of the susceptibility of this state are calculated. At intermediate dopant concentration, the material undergoes a diffuse phase transition into a ferroelectric state smeared within a temperature range. A further increase in the dopant concentration makes the transition sharper and closer to the conventional ferroelectric transition. The results obtained are compared with the behavior of the K1−xLixTaO3 relaxor ferroelectric.
Transverse dynamics of dispersive Alfvén waves. II. Driving of a reduced magnetohydrodynamic flow
Authors: Laveder D. , Passot T. , Sulem P. L.
Published: July 2002

The nonlinear dynamics resulting from transverse and quasi-transverse instabilities of a finite-amplitude dispersive Alfvén wave propagating along an ambient magnetic field is studied by direct numerical simulations of the three-dimensional Hall-magnetohydrodynamic (Hall-MHD) equations. When the pump wave has a moderate amplitude and a long enough wavelength, one observes the generation of nonlinear structures in the form of helical filaments for the transverse magnetic field intensity and the density fluctuations. An interesting feature is the development of a quasi-incompressible turbulent flow, with a longitudinal characteristic scale large compared to the Alfvén wavelength, that remains spectrally well separated from the wave throughout the evolution. The coexistence of this “reduced MHD” flow with nonlinear Alfvén waves was predicted on the basis of an asymptotic analysis [A. Gazol, T. Passot, and P. L. Sulem, Phys. Plasmas 6, 3114 (1999)] carried out in the long-wavelength limit. Whereas in this regime the generation of the reduced MHD flow is negligible, it becomes significant on a time scale of a few wave periods when dispersion is increased. Increasing the dispersion also leads to a faster destabilization of the wave and to a more rapid dissipation, a remarkable effect due to enhanced instability growth rates. In the case of a larger amplitude pump, or of an Alfvén wavelength close to the ion-inertial length, the helical structures get fragmented and the spectral gap observed at early times between the large-scale flow and the waves rapidly disappears, leading to a fully three-dimensional MHD turbulent flow.
The properties of fast and slow oblique solitons in a magnetized plasma
Authors: McKenzie J. F. , Doyle T. B.
Published: July 2002

This work builds on a recent treatment by McKenzie and Doyle [Phys. Plasmas 8, 4367 (2001)], on oblique solitons in a cold magnetized plasma, to include the effects of plasma thermal pressure. Conservation of total momentum in the direction of wave propagation immediately shows that if the flow is supersonic, compressive (rarefactive) changes in the magnetic pressure induce decelerations (accelerations) in the flow speed, whereas if the flow is subsonic, compressive (rarefactive) changes in the magnetic pressure induce accelerations (decelerations) in the flow speed. Such behavior is characteristic of a Bernoulli-type plasma momentum flux which exhibits a minimum at the plasma sonic point. The plasma energy flux (kinetic plus enthalpy) also shows similar Bernoulli-type behavior. This transonic effect is manifest in the spatial structure equation for the flow speed (in the direction of propagation) which shows that soliton structures may exist if the wave speed lies either (i) in the range between the fast and Alfven speeds or (ii) between the sound and slow mode speed. These conditions follow from the requirement that a defined, characteristic “soliton parameter” m exceeds unity. It is in this latter slow soliton regime that the effects of plasma pressure are most keenly felt. The equilibrium points of the structure equation define the center of the wave. The structure of both fast and slow solitons is elucidated through the properties of the energy integral function of the structure equation. In particular, the slow soliton, which owes its existence to plasma pressure, may have either a compressive or rarefactive nature, and exhibits a rich structure, which is revealed through the spatial structure of the longitudinal speed and its corresponding transverse velocity hodograph.
Stratified shear flows in a model of turbulence-shear flow interaction
Authors: del-Castillo-Negrete D. , Carreras B. A.
Published: July 2002

In magnetically confined plasmas there is evidence of localized regions of improved confinement. These regions are usually associated with shear flows with radial structure, and an important problem is to understand how such flows emerge. To address this problem a reaction–diffusion type model of turbulence-shear flow interaction that incorporates the mechanism of turbulence suppression by shear, and parameterizes turbulent transport as a nonlinear diffusivity is considered. The fixed points of the model correspond to the L (low confinement) and H (high confinement) modes of the system, and it is shown that for a range of parameter values the H-mode fixed point has a finite-k instability. Numerical results show that this instability leads, in the nonlinear regime, to the formation of stratified shear layers and jets in which bands of intense shear and suppressed turbulence alternate with bands of low shear and enhanced turbulence. Approximate analytical solutions of the model corresponding to high-confinement modes with radial structure are presented.
Generation of harmonic Langmuir mode by beam-plasma instability
Authors: Gaelzer Rudi , Ziebell Luiz F. , Yoon Peter H.
Published: July 2002

In this article, numerical solutions of the generalized weak turbulence equation [P. H. Yoon, Phys. Plasmas 7, 4858 (2000)] are carried out. In the generalized weak turbulence theory, the generation of the 2ωpe-harmonic Langmuir mode is treated as a fundamental process in turbulent beam-plasma interaction process, in addition to, and concomitant to, the well-known nonlinear processes such as Langmuir and ion-sound mode coupling and wave-particle interactions. The present numerical analysis shows that the harmonic mode, which is a solution to a nonlinear dispersion equation, hence a “nonlinear” eigenmode, grows primarily due to an induced emission process, which is a “linear” wave-particle interaction process. The harmonic Langmuir mode generation has been observed since the late 1960s in laboratory experiments, simulations, and in space. However, adequate and quantitative theoretical explanation has not been forthcoming. The present work represents a step toward an understanding of such a phenomenon.
Formation and steady-state maintenance of field reversed configuration using rotating magnetic field current drive
Authors: Guo H. Y. , Hoffman A. L. , Brooks R. D. , Peter A. M. , Pietrzyk Z. A. , Tobin S. J. , Votroubek G. R.
Published: July 2002

Rotating magnetic fields (RMF) have been used to both form and maintain field reversed configurations (FRC) in quasisteady state. These experiments differ from steady-state rotamaks in that the FRCs are similar to those formed in theta-pinch devices, that is elongated and confined inside a flux conserver. The RMF creates an FRC by driving an azimuthal current which reverses an initial positive bias field. The FRC then expands radially, compressing the initial axial bias flux and raising the plasma density, until a balance is reached between the RMF drive force and the electron–ion friction. This generally results in a very high ratio of separatrix to flux conserver radius. The achievable final conditions are compared with simple analytic models to estimate the effective plasma resistivity. The RMF torque on the electrons is quickly transferred to the ions, but ion spin-up is limited in these low density experiments, presumably by ion-neutral friction, and does not influence the basic current drive process. However, the ion rotation can result in a rotating n=2 distortion if the separatrix radius is too far removed from the plasma tube wall.
Molecular-beam epitaxial growth of insulating AlN on surface-controlled 6H–SiC substrate by HCl gas etching
Authors: Onojima Norio , Suda Jun , Matsunami Hiroyuki
Published: July 2002

Insulating AlN layers were grown on surface-controlled 6H–SiC subtrates by molecular-beam epitaxy (MBE) using elemental Al and rf plasma-excited nitrogen (N*). HCl gas etching was introduced as an effective pretreatment method of substrate for MBE growth of AlN. 6H–SiC substrates pretreated by HCl gas etching had no surface polishing scratches and an atomically flat surface. In addition, evident (∛×∛)R30° surface reconstruction was observed even before thermal cleaning. AlN layers grown on this substrate had no defects related to surface polishing scratches and excellent insulating characteristics.
Measurement of true spontaneous emission spectra from the facet of diode laser structures
Authors: Lewis G. M. , Smowton P. M. , Thomson J. D. , Summers H. D. , Blood P.
Published: July 2002

Measurement of the spontaneous emission and gain spectra provides a complete characterization of a semiconductor gain medium, however, this requires the observation of emission in two directions to avoid amplification of the spontaneous emission spectrum. We show that both the gain spectrum and the true spontaneous emission spectrum can be obtained from amplified spontaneous emission (ASE) spectra measured from the end of a segmented-contact device. The spontaneous emission spectra agree with spectra measured through a top contact window. If the carrier populations are fully inverted at low photon energy, it is possible to convert the ASE-derived spontaneous emission into real units.
Magnetic-field generation and electron acceleration in relativistic laser channel
Authors: Kostyukov I. Yu. , Shvets G. , Fisch N. J. , Rax J. M.
Published: July 2002

The interaction between energetic electrons and a circularly polarized laser pulse inside an ion channel is studied. Laser radiation can be resonantly absorbed by electrons executing betatron oscillations in the ion channel and absorbing angular momentum from the laser. The absorbed angular momentum manifests itself as a strong axial magnetic field (inverse Faraday effect). The magnitude of this magnetic field is calculated and related to the amount of the absorbed energy. Absorbed energy and generated magnetic field are estimated for the small and large energy gain regimes. Qualitative comparisons with recent experiments are also made.
Large N limit of SO(N) scalar gauge theory
Authors: Toprak Erdal , Turgut O. Teoman
Published: July 2002

In this paper we study the large Nc limit of SO(Nc) gauge theory coupled to a real scalar field following ideas of Rajeev [Int. J. Mod. Phys. A 9, 5583 (1994)]. We will see that the phase space of this resulting classical theory is Sp1(H)/U(H+) which is the analog of the Siegel disk in infinite dimensions. The linearized equations of motion give us a version of the well-known ’t Hooft equation of two dimensional quantum chromodynamics.
Low-temperature fabrication of L10 ordered FePt alloy by alternate monatomic layer deposition
Authors: Shima T. , Moriguchi T. , Mitani S. , Takanashi K.
Published: July 2002

L 1 0 ordered FePt alloy films with large magnetic anisotropy have been successfully prepared by alternating Fe(001) and Pt(001) monatomic layers on MgO (001) substrates at low temperatures below 230 °C. In addition to the fundamental (002) peak, (001) and (003) superlattice peaks have clearly been observed in the x-ray diffraction patterns for all the samples, indicating the formation of L10 ordered structure. The magnetization measurements show that all the samples are perpendicularly magnetized. Large uniaxial magnetic anisotropy (Ku=3.0×107 erg/cc) and high chemical ordering (long-range order parameter S=0.7±0.1) have been obtained even at the substrate temperature Ts=200 °C. The magnetization curves show good magnetic squareness (Mr/Ms∼0.9) for the samples grown at Ts⩾160 °C.
Braided oscillators
Authors: Yildiz A.
Published: July 2002

A generalized oscillator algebra is proposed and the braided Hopf algebra structure for this generalized oscillator is investigated. Using the solutions for the braided Hopf algebra structure, two types of braided Fibonacci oscillators are introduced. This leads to two types of braided Biedenharn–Macfarlane oscillators as special cases of the Fibonacci oscillators. We also find the braided Hopf algebra solutions for the three dimensional braided space. One of these, as a special case, gives the Hopf algebra given in the literature.
The multiple sum formulas for 12j coefficients of SU(2) and uq(2)
Authors: Ališauskas Sigitas
Published: July 2002

The expressions for 12j coefficients of the both kinds (without and with braiding) of the SU(2) group and the quantum algebra uq(2) are considered. Using Dougall’s summation formula of the very well-poised hypergeometric F45(1) series and its q-generalization, several fourfold sum formulas [with each sum related to the balanced F45(1) or φ45 series] for the q-12j coefficients of the second kind (without braiding) are derived. Applying q-generalizations of rearrangement formulas of the very well-poised hypergeometric F56(−1) series [which correspond to a new expression for the Clebsch–Gordan coefficients of SU(2) and uq(2)], the new expressions with five sums [of the F34(1) and F23(1) or φ45 and φ23 type] are derived for the q-12j coefficients of the first kind (with braiding) instead of the usual expansions in terms of q-6j coefficients. Stretched and doubly stretched q-12j coefficients [as triple, double, or single sums, related to composed or separate hypergeometric F34(1) and F45(1) or φ33 and φ45 series and, particularly, to q-9j or q-6j coefficients] are considered.
Group invariant classification of separable Hamiltonian systems in the Euclidean plane and the O(4)-symmetric Yang–Mills theories of Yatsun
Authors: McLenaghan Raymond G. , Smirnov Roman G. , The Dennis
Published: July 2002

We present a new and effective method of determining separable coordinate systems for natural Hamiltonians with two degrees of freedom in flat Riemannian space. The method is based on intrinsic properties of the associated Killing tensors and their invariants under the group of rigid motions E(2). Applications to the Hamiltonian systems derived by the late V. A. Yatsun from O(4)-symmetric Yang–Mills theories are presented. In addition, an equivalence between separability of two-dimensional Hamiltonian systems and the existence of Pfaffian quasi-bi-Hamiltonian representations is specified.
Ohmic contacts to p-type GaN mediated by polarization fields in thin InxGa1−xN capping layers
Authors: Gessmann T. , Li Y.-L. , Waldron E. L. , Graff J. W. , Schubert E. F. , Sheu J. K.
Published: July 2002

Low-resistance ohmic contacts are demonstrated using thin p-type InGaN layers on p-type GaN. It is shown that the tunneling barrier width is drastically reduced by polarization-induced electric fields in the strained InGaN capping layers resulting in an increase of the hole tunneling probability through the barrier and a significant decrease of the specific contact resistance. The specific contact resistance of Ni (10 nm)/Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2×10−2 Ω cm2 and 6×10−3 Ω cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.
Requirements for x-ray magnetic circular dichroism on paramagnetic biological systems and model compounds
Authors: Funk Tobias , Friedrich Stephan , Young Anthony , Arenholz Elke , Cramer Stephen P.
Published: July 2002

We have built an end station for x-ray magnetic circular dichroism (XMCD) measurements on proteins and paramagnetic compounds. Our current setup reaches a base temperature of 2.6 K and magnetic fields up to 6 T and is operated at beamline 4.02 of the Advanced Light Source. In this article we discuss magnetic field and low temperature requirements needed to perform XMCD experiments on magnetically saturated samples. For a typical 3d transition metal paramagnetic system we find that fields above 4 T at a temperature of 2.6 K saturate the magnetization of the sample to more than 80%. We discuss principal considerations for a setup operated at low temperatures on a synchrotron and show that infrared heat shielding is unavoidable to obtain the base temperature at the sample. We show first experimental results from the vanadium (IV) compound VOSO4X[H2O].
Noncollinear spin transfer in Co/Cu/Co multilayers (invited)
Authors: Stiles M. D. , Zangwill A.
Published: July 2002

This article has two parts. The first part uses a single point of view to discuss the reflection and averaging mechanisms of spin transfer between current-carrying electrons and the ferromagnetic layers of magnetic/nonmagnetic heterostructures. The second part incorporates both effects into a matrix Boltzmann equation and reports numerical results for current polarization, spin accumulation, magnetoresistance, and spin-transfer torques for Co/Cu/Co multilayers. When possible, the results are compared quantitatively with relevant experiments.
The pump power dependence of the femtosecond relaxation of CdSe nanoparticles observed in the spectral range from visible to infrared
Authors: Burda Clemens , Link Stephan , Mohamed Mona B. , El-Sayed Mostafa
Published: July 2002

The pump power dependence of the relaxation dynamics of CdSe nanoparticles (NPs) was studied with femtosecond pump probe spectroscopy at observation wavelengths of the first exciton transition at 560 nm, the near infrared (NIR) absorption at 2 μm, and the transient mid-infrared (IR) absorption at 4.5 μm. Excitation with less than one photon per particle leads to bleaching of the excitonic transitions, and the bleach intensity is initially linear to the pump power. At higher pump power the bleach intensity levels off, when complete saturation of the excitonic transition is reached. At the same time, increasing pump power causes an acceleration of the bleach decay, which is due to additional Auger processes when multiple excitons are formed in the NPs. In addition, the pump power effect was investigated for the NIR and IR regions, at 2 and 4.5 μm wavelength, respectively. Whereas the IR transients are very similar to the ones observed for the bleach, the NIR transients behave completely different. No pump power dependence was found for the transients at 2 μm when pumped in a power range from 0.5 to 5 μJ per pulse. The results show that the fs transients in the visible (bleach) and IR (absorption) regions are due to electron relaxation in the conduction band and the NIR transients are due to the relaxation of the hole. Furthermore, it suggests that in the investigated CdSe NPs, Auger processes act much more efficiently on the electrons than for the holes.
Mechanically milled nanostructured (Sm,Pr)12.5Co85.5Zr2 magnets with TbCu7 structure
Authors: Tang Hong , Zhou Jian , Sellmyer David J.
Published: July 2002

Nanostructured (Sm,Pr)(Co,Zr) magnets with the TbCu7-type structure have been synthesized by mechanically milling (Sm1−xPrx)12.5Co85.5Zr2 alloys (0⩽x⩽0.8) followed by appropriate annealing. Magnetic properties, structure, and microstructure have been investigated. It is found that single-phase (Sm,Pr)(Co,Zr)7 magnets with the TbCu7 structure and with nanoscale grain size (14–19 nm) form in the whole composition range. Intrinsic coercivity Hci decreases from 20.7 to 5.6 kOe with increasing Pr content from 0 to 0.8, while energy products (BH)max shows an optimum value of 12.6 MGOe (Hci of 17.9 kOe) at x=0.2 due to increase in remanent magnetization. Remanence ratio is observed in the range of 0.64–0.69, with an optimum value of ∼0.69 at x=0.3. The high coercivity is ascribed to the formation of nanocrystalline (Sm,Pr)(Co,Zr)7 phase of the TbCu7-type structure with large anisotropy, and the remanence enhancement may result from the exchange-coupling interactions between nanosized (Sm,Pr)(Co,Zr)7 grains. The magnetic properties can be understood in term of the anisotropy of the magnetically hard phase and the interactions between the nanosized grains.
Effect of boron on structure and magnetic properties of magnetostrictive compound Dy0.7Pr0.3Fe2
Authors: Ren W. J. , Zhang Z. D. , Liu J. P. , Zhao X. G. , Liu W. , Geng D. Y. , Jin X. M.
Published: July 2002

Crystal structure, magnetic properties and magnetostriction of Dy0.7Pr0.3(Fe1−xBx)2 (0⩽x⩽0.20) have been investigated by means of x-ray diffraction, ac initial susceptibility, a vibrating sample magnetometer, and a standard strain technique. The matrix of Dy0.7Pr0.3Fe2 alloy consists of (Dy,Pr)Fe2 phase with a cubic MgCu2-type structure and some amount of (Dy,Pr)Fe3 phase with a rhombohedral PuNi3-type structure. The introduction of boron effectively restrains the emergence of the iron-rich phase and thus decreases the amount of (Dy,Pr)Fe3 phase. Dy0.7Pr0.3(Fe1−xBx)2 alloy with x=0.05 contains small amount of (Dy,Pr)(Fe,B)3 phase, and those with x=0.10 and x=0.15 are essentially of a single (Dy,Pr)(Fe,B)2 phase. An unidentified minor phase appears when x=0.20. The lattice parameter of (Dy,Pr)(Fe,B)2 phase decreases monotonically with the boron substitution up to x=0.20, indicating that the boron atoms occupy the substitutional sites. The Curie temperature for (Dy,Pr)(Fe,B)2 phase obviously increases compared with the boron free one. The saturation magnetization at room temperature increases with increasing boron content for the alloys studied, suggesting that the partial boron substitution is beneficial to the increment of the exchange interactions in the Dy0.7Pr0.3Fe2 system. The increment of the magnetization originates from the decrement of iron content, because the Fe moment that aligns antiparallel with the Dy moment almost keeps constant in this system. Boron substitution for iron increases the lattice distortion and anisotropy, thus causes the decrease of the linear magnetostriction λa=λ∥−λ⊥ at the room temperature.
Unusual photon tunneling in the presence of a layer with a negative refractive index
Authors: Zhang Z. M. , Fu C. J.
Published: July 2002

Recent experiments give evidence of a negative refractive index at microwave frequencies in a microstructured material. This discovery may allow some unique features associated with negative- refraction materials to be observed and applied. This letter describes the calculated results for photon tunneling via evanescent fields in the presence of a layer of negative-refraction material, also known as a left-handed material (LHM) in contrast to the conventional right-handed materials (RHMs). We show that photons may tunnel through a much greater distance when a LHM that has the same magnitudes of refractive index, relative permeability, and thickness as those of the RHM (which could also be air or vacuum) is included between two semi-infinite media.
Coercivity and energy product of thin Sm–Co layers
Authors: Andreescu R. , O’Shea M. J.
Published: July 2002

We have prepared homogeneous SmCox (x=5–26) sputtered thin films of thickness 500 nm with excess Co compared to SmCo5. The purpose is to determine how any spring exchange incorporated by the excess Co improves the energy product. Films are subjected to a 30 s or 20 min anneal to crystallize the Sm–Co. In the case of both anneal methods x-ray diffraction indicated no preferential orientation of the Sm–Co film after annealing. The best value of the maximum energy product, BHmax, is 19 MG Oe (151 kJ/m3) and is found in a sample with x of 12 after a 30 s anneal at 700 °C. We also compare these results with our earlier work or Sm–Co thin films prepared initially as SmCo5/Co multilayers and find that homogeneous films tend to have a larger BHmax.
Fabrication and characterization of Co1−xFex alloy nanowires
Authors: Fodor P. S. , Tsoi G. M. , Wenger L. E.
Published: July 2002

Co 1−x Fe x alloy nanowires with 40 nm diam and x=0–1.0 were fabricated by electrodeposition in nanopores of alumina templates. The crystalline structure of the nanowires is concentration dependent and shows a transition from the cobalt hexagonal-closed-packed structure (hcp) to a face-centered-cubic structure (fcc) in the concentration range 0<x<0.1. For an iron content greater than 15 at % the structure becomes body-centered-cubic (bcc). The nanowires with an aspect ratio of about 8 show a highly anisotropic magnetization with the easy axis along the length of the wire. The squareness of the hysteresis loops is between 72% and 93% for magnetic fields parallel to the wires while the squareness is less than 13% for fields perpendicular to the wires. The coercivity along the easy axis reaches a maximum of 2150 Oe for x=0.55 and is almost concentration independent for 0.5<x<0.6.
Finite element modeling of a magnetic test to determine whether a weld is annealed
Authors: Sablik M. J. , Weidner C. , Rubin S. W.
Published: July 2002

Using finite element modeling and building in the effect of grain size and dislocation density on magnetic properties, it is shown how a magnetic test can be used to determine if a weld is annealed or not. In a weld, the grain size increases as one goes through the heat-affected zone from the base metal to the fusion zone. In an unannealed weld, dislocation density increases very sharply from the base metal to the fusion zone, so much so that its effect dominates the magnetic properties, causing permeability to decrease from the base metal to the fusion zone. In the annealed weld, this very sharp increase in dislocation density from base metal to fusion zone is not present, and the grain size effect dominates, which causes the permeability to slightly or moderately increase (instead of decrease) from the base metal to the fusion zone. This change in behavior of the permeability from the base metal to the fusion zone determines whether the weld is annealed or not.
Investigation on corrosion resistance of CoCrPt–C granular thin film media
Authors: Xu Y. J. , Zhang J. , Shi J. R. , Ji R. , Wang J. P.
Published: July 2002

The multilayer granular structure has been developed and has proved to be a promising candidate for extremely high-density magnetic recording. Recently, the properties of friction and wear of the structure have been studied and the in-situ nanoscale protection concept has been proposed. In this paper, two series of CoCrPt–C multilayer samples with various interlayer carbon concentration and with/without carbon overcoat (top layer) are compared, having been prepared by using facing target sputtering (FTS) and normal dc sputtering (DCS), respectively. The corrosion resistance of the sample is investigated using environmental and electrochemical methods. Optical microscopy and time-of-flight secondary ion mass spectrometry are used to characterize the corrosion test samples. For the samples prepared by FTS, both top and interlayer carbon can provide protection for the media. In terms of the samples prepared by DCS, the carbon layer may increase the porous density during the corrosion process. It is concluded that the sp3/sp2 ratio of the carbon is the most important consideration in nanoscale protection for the granular thin film media. Raman spectroscopy results further confirm the conclusion.
The methods of gluing manifolds in general relativity
Authors: Nozari Kourosh , Mansouri Reza
Published: July 2002

Some areas of modern theoretical physics such as modern cosmology contain different manifolds which must be glued together along a common boundary. These boundaries can be spacelike, timelike, or lightlike hypersurfaces. This paper shows how this gluing for different hypersurfaces is possible. Two different approaches are considered and the extent to which these approaches are equivalent are discussed. In particular, we will construct a distributional approach for dynamics of lightlike hypersurfaces in general relativity. Since Einstein’s equations are nonlinear PDEs, for discontinuous metrics such as signature changing metrics, product of distributions are unavoidable. To glue two different manifolds which admit signature change, we consider this problem in the context of Colombeau’s new theory of generalized functions. Some examples are given for clarification.
Density matrix variational theory: Application to the potential energy surfaces and strongly correlated systems
Authors: Nakata Maho , Ehara Masahiro , Nakatsuji Hiroshi
Published: July 2002

The density matrix variational theory (DMVT) algorithm developed previously [J. Chem. Phys. 114, 8282 (2001)] was utilized for calculations of the potential energy surfaces of molecules, H4, H2O, NH3, BH3, CO, N2, C2, and Be2. The DMVT(PQG), using the P, Q, and G conditions as subsidiary condition, reproduced the full-CI curves very accurately even up to the dissociation limit. The method described well the quasidegenerate states and the strongly correlated systems. On the other hand, the DMVT(PQ) was not satisfactory especially in the dissociation limit and its potential curves were always repulsive. The size consistency of the method was discussed and the G condition was found to be essential for the correct behavior of the potential curve. Further, we also examined the Weinhold–Wilson inequalities for the resultant 2-RDM of DMVT(PQG) calculations. Two linear inequalities were violated when the results were less accurate, suggesting that this inequality may provide a useful N-representability condition for the DMVT.
Magnetization reversal of elliptical Co/Cu/Co pseudo-spin valve dots
Authors: Dao N. , Whittenburg S. L. , Hao Y. , Ross C. A. , Malkinski L. M. , Wang J. Q.
Published: July 2002

We present our recent simulated results on Cr (5 nm)/ Cu (5 nm)/ Co (5 nm)/ Cu (3 nm)/ Co (2 nm) pseudo-spin valve dots. The simulated results agree qualitatively with the experimental results. Three different sizes of elliptical dots, 150 nm×105 nm, 175 nm×70 nm, and 200 nm×60 nm, were simulated. Our simulations show that in these types of dots magnetization reversal occurs by the formation of domain walls: 90° for 175 nm×70 nm and 360° for 200 nm×60 nm. No domain wall was observed in the reversal of the 150 nm×105 nm dots. For such dots, the simulated loops show a small two-step reversal pattern with the thin upper layer partially reversing followed by complete reversal of both layers at higher fields. In the larger dots, a two-step reversal is clearly observed both in the simulation and experiment.
Diffusion equation on curved surfaces. I. Theory and application to biological membranes
Authors: Faraudo Jordi
Published: July 2002

We analyze how the transport by diffusion of a substance over a curved surface is affected by the local curvature. We show that these effects are completely characterized by the geodesic curvature κg(r) or equivalently by the Gaussian curvature K(r). The mean square geodesic distance traveled by a diffusing molecule on the surface and the probability distribution are obtained perturbately as a function of the surface curvature. The obtained corrections in the curvature are shown to be relevant in the understanding of lateral diffusion on biomembranes.
Density functional theory and Monte Carlo simulations for hard sphere fluids in square and rectangular channels
Authors: Jagannathan Kamakshi , Yethiraj Arun
Published: July 2002

The density distribution of hard spheres in square and rectangular channels is studied using density functional theory and grand canonical ensemble Monte Carlo simulations. The theory uses the weighted density approximation for the excess Helmholtz free energy functional with the Curtin–Ashcroft approximation for the weighting function. The theoretical predictions are in good quantitative agreement with the simulations except for very high densities. The theory predicts pronounced layering in the channel as the density is increased, with high values of density at the surfaces and even higher densities at the corners where any two surfaces meet. Interesting interference effects are observed in the density profiles when compared to the case of a hard sphere fluid in slitlike pores.
Design optimization of electric machines based on the estimation of permeability distribution
Authors: Wakao S. , Yamashita A. , Fujishima Y. , Yamamura T.
Published: July 2002

We propose an optimization approach based on the estimation of permeability distribution, which enables us to make an effective objective function of optimization methods for the efficient design of magnetic devices. By the proposed approach, we can improve magnetic device characteristics by evaluating the deviation in the permeability distribution of the device as an objective function. In this approach, we separate the iron areas and calculate the permeability of each block to consider the permeability distribution. To verify the proposed approach, some numerical results, i.e., the shape optimization of ventilation holes in an induction motor, which demonstrate the validity of the proposed approach, are also presented. The optimization concept based on the estimation of permeability distribution is effective and can be readily applied to the practical design optimization of various magnetic devices.
Isotropic vector hysteresis modeling with feed-forward neural networks
Authors: Makaveev Dimitre , Dupré Luc , De Wulf Marc , Melkebeek Jan
Published: July 2002

A new vector hysteresis model is presented, based on the function approximation capabilities of feed-forward neural networks. Two-dimensional circular and elliptical magnetization of laminated SiFe steel sheets can be successfully handled by the model. A feed-forward neural network with four inputs, derived at each time step from the time-dependent magnetic induction vector, yields an accurate prediction of the magnetic field strength vector. Measurement results for a steel sheet sample are used to train and test the neural network. The model accuracy is good and can be easily adapted to the requirements of the application by extending or reducing the network training set and thus the required amount of measurement data. Besides, the presented technique is fast, requires no large data set, and applies standard neural network algorithms. Future extension of the model to other magnetization patterns is possible.
Efficient three-dimensional numerical magnetic shield model
Authors: Oti John
Published: July 2002

We describe an accurate, flexible, and efficient three-dimensional magnetic shield model that is suitable for use within generic micromagnetic models. The model accounts for both shield saturation effects (to first order) and fringing field edge effects due to the finite size of the shields. The model is described for shields with shapes in the form of rectangular segments; however, it is equally applicable to other shield shapes. In this method, each shield object is treated as a field source owing to “magnetic charge” distributions that are induced on its bounding surfaces. The charge distributions are obtained from image boundary conditions imposed on the total local fields acting at numerical mesh points defined for the shield surfaces. A numerical relaxation procedure is applied to ensure self-consistent charge distributions. We compare this approach with traditional magnetic image models, discuss its properties and demonstrate its application in calculating the distortion of the interaction field between a soft recording head and magnetic transitions in recording media.
Low-temperature properties of small heavy fermion particles
Authors: Schlottmann P.
Published: July 2002

Heavy fermions arise from the hybridization of extended conduction states with strongly correlated localized f states. Depending on the position of the Fermi level the system can be metallic or semiconducting. In nanosized particles the extended states have discrete energies and their energy spacing competes with TK. We consider a small Kondo insulator particle described by the symmetric Anderson model within the mean-field slave-boson approximation at low T. The formation of a ferromagnetic ground state is prevented for small system sizes. The susceptibility, entropy, and specific heat are thermally activated at low T with a gap of the order of TK for the insulator, but only of the order of spTK/D in the metallic case, where sp is the mean energy level spacing.
A study of spin dynamics in the a-Fe90Sc10 spin glass
Authors: van Lierop J. , Ryan D. H. , Cadogan J. M.
Published: July 2002

Spin dynamics in a-Fe90Sc10 have been examined with zero-field muon spin relaxation (ZF-μSR) and selective excitation double Mössbauer (SEDM) spectroscopy. Analysis of the μSR data yields Tsg∼105 K in agreement with both χac and conventional Mössbauer data. SEDM shows fluctuation rates in good agreement with μSR, but reveals an abrupt change in relaxation behavior from spin-flip to diffusive above 90 K (0.9Tsg).
All-optical Mach–Zehnder modulator using a photochromic dye-doped polymer
Authors: Kang Jae-Wook , Kim Jang-Joo , Kim Eunkyoung
Published: July 2002

An all-optical Mach–Zehnder modulator is demonstrated which is composed of a polymer waveguide doped with a photochromic dye in the core and a thick light blocking metal layer on it. The metal layer was opened on one arm of the Mach–Zehnder modulator, so that only one arm of the modulator could be irradiated by modulation light, thus allowing a differential phase shift. The optical modulator exhibited an extinction ratio of about −12 dB at a wavelength of 1.55 μm. A simple kinetic model developed to delineate the refractive index change in the dye-doped polymer film was applied to predict the evolution of the modulation characteristics.
Ab initio study of the isomerization of retinal chromophore and its derivatives
Authors: Lee Han Myoung , Kim Jongseob , Kim Cheol-Ju , Kim Kwang S.
Published: July 2002

The structures, vibrational frequencies, electronic properties, and cis-trans photoisomerization process of retinal chromophore and its derivatives (i.e., Schiff base and protonated Schiff base) are studied using the density-functional theory with Becke’s three-parameter exchange functional together with the correlation functionals of Lee–Yang–Parr (B3LYP) and the second order Møller–Plesset perturbation theory (MP2). The optical transition energies for photoisomerization are reported at the configuration interaction (CI) level with single excitations (CIS) as well as at the B3LYP/6-31G* level using the random-phase-approximation (RPA). For slightly simplified model systems of retinal chromophore and its derivatives, the RPA values are very close to those of the complete active-space self-consistent-field (CASSCF) method and the multireference CI (MRCI) method, and are also in reasonable agreement with the experiments. We have also tried to investigate the solvent effect of the vertical transition energies in the presence of one or two water molecules. The present study deals with the mechanism of the cis-trans (or trans-cis) photoisomerization based on the molecular orbital (MO) analysis.
Comparison of all atom, continuum, and linear fitting empirical models for charge screening effect of aqueous medium surrounding a protein molecule
Authors: Takahashi Takuya , Sugiura Junnnosuke , Nagayama Kuniaki
Published: July 2002

To investigate the role hydration plays in the electrostatic interactions of proteins, the time-averaged electrostatic potential of the B1 domain of protein G in an aqueous solution was calculated with full atomic molecular dynamics simulations that explicitly considers every atom (i.e., an all atom model). This all atom calculated potential was compared with the potential obtained from an electrostatic continuum model calculation. In both cases, the charge-screening effect was fairly well formulated with an effective relative dielectric constant which increased linearly with increasing charge–charge distance. This simulated linear dependence agrees with the experimentally determined linear relation proposed by Pickersgill. Cut-off approximations for Coulomb interactions failed to reproduce this linear relation. Correlation between the all atom model and the continuum models was found to be better than the respective correlation calculated for linear fitting to the two models. This confirms that the continuum model is better at treating the complicated shapes of protein conformations than the simple linear fitting empirical model. We have tried a sigmoid fitting empirical model in addition to the linear one. When weights of all data were treated equally, the sigmoid model, which requires two fitting parameters, fits results of both the all atom and the continuum models less accurately than the linear model which requires only one fitting parameter. When potential values are chosen as weighting factors, the fitting error of the sigmoid model became smaller, and the slope of both linear fitting curves became smaller. This suggests the screening effect of an aqueous medium within a short range, where potential values are relatively large, is smaller than that expected from the linear fitting curve whose slope is almost 4. To investigate the linear increase of the effective relative dielectric constant, the Poisson equation of a low-dielectric sphere in a high-dielectric medium was solved and charges distributed near the molecular surface were indicated as leading to the apparent linearity.
Dielectric relaxation of electrolyte solutions using terahertz transmission spectroscopy
Authors: Asaki M. L. T. , Redondo A. , Zawodzinski T. A. , Taylor A. J.
Published: July 2002

We use terahertz (THz) transmission spectroscopy to obtain the frequency dependent complex dielectric constants of water, methanol, and propylene carbonate, and solutions of lithium salts in these solvents. The behavior of the pure solvents is modeled with either two (water) or three (methanol and propylene carbonate) Debye relaxations. We discuss the effects of ionic solvation on the relaxation behavior of the solvents in terms of modifications to the values of the Debye parameters of the pure solvents. In this way we obtain estimates for numbers of irrotationally bound solvent molecules, the numbers of bonds broken or formed, and the effects of ions on the higher-frequency relaxations.
Colloid aggregation induced by oppositely charged polyions
Authors: Harnau Ludger , Hansen Jean-Pierre
Published: July 2002

The “polymer reference interaction site model” integral equation formalism is used to determine the pair structure of binary colloidal dispersions involving large and small polyions of opposite charge. Two examples of such bidisperse suspensions are considered in detail, namely mixtures of charged spherical colloids and oppositely charged polyelectrolyte chains, and binary mixtures of oppositely charged large and small clay platelets. In both cases clear evidence is found for aggregation of the larger particles induced by the polyionic counterions, signalled by a strong enhancement of long wavelength concentration fluctuations.
Dilution and cluster contributions to hopping transport in a bias field
Authors: Sin J. M. , Soos Z. G.
Published: July 2002

Hole transport in molecularly doped polymers (MDPs) is modeled as random walks on fixed donors (Ds) embedded in a polymer matrix. Dilution p<1 corresponds to placing individual Ds, dimers D2, or tetramers D4 randomly on a fraction p of sites in a face-centered-cubic lattice. Monte Carlo simulations of the drift velocity vD(E) in a bias field E have maxima in dilute (p=8%) systems of D2 or D4 that are related to the formation and polarization of clusters of nearest-neighbor donors. Marcus or small-polaron hopping rates with fixed parameters account for the concentration, field, and temperature dependencies of the mobility, μ(E,T)=vD(E,T)/E, of D=TTA (tritolylamine) in polystyrene and of related systems with D2 or D4 in PS. The compensation temperature is lower for D2 than for D at p=20%, consistent with stronger positional disorder for dimers. The anomalous broadening of photocurrents in D4 with increasing E is due to cluster polarization. The parameter σ=700 K for energetic disorder is used throughout and is ∼25% smaller than in the Gaussian disorder model. Spatially correlated energies yield the characteristic field dependence of μ(E,T). Although not quantitative, the comprehensive treatment of dilution in TTA:PS and related MDPs clearly supports Marcus hopping rates and stronger geometrical than energetic disorder.
Plasmon beam instability and plasmon Landau damping of ion acoustic waves
Authors: Mendonça J. T. , Bingham R.
Published: July 2002

A new view is proposed on wave propagation in a plasma, where the resonant interaction of ion acoustic waves with quasiparticles (plasmons) is considered. The distribution of quasiparticles is used to describe broadband electron plasma turbulence. Resonant interactions occur when the phase velocity of the ion acoustic wave is nearly equal to the group velocity of the electron plasma waves (or plasmons) associated with the turbulent spectrum. It is shown that plasmon Landau damping can take place, and beam type instabilities are discussed.
Optical constants of InN thin films on (111) GaAs grown by reactive magnetron sputtering
Authors: Yang H. F. , Shen W. Z. , Qian Z. G. , Pang Q. J. , Ogawa H. , Guo Q. X.
Published: July 2002

The optical constants of InN thin films on (111) GaAs substrates grown by rf reactive magnetron sputtering under different growth conditions have been investigated both theoretically and experimentally in the range from the free-carrier absorption region to the above band gap region (0.2–3.0 eV). A model of dielectric function for InN is developed, which includes four different mechanisms corresponding to interband transition contribution, excitonic contribution, free-carrier contribution, and background dielectric contribution. The contribution of free-carrier absorption to dielectric function has been introduced for the study of optical constants in InN thin films. By employing the model, we obtain the refractive index, extinction coefficient, and absorption coefficient as a function of photon energy, as well as the free-carrier concentration. The refractive index (n) and the extinction coefficient (k) of InN thin films near the band gap are found to be almost independent of the growth conditions. In contrast, in the free-carrier absorption region, the values of n and k are strongly related to the free-carrier concentration for different samples. The relationship between the free-carrier concentration and the film quality has also been studied.
Magnetic properties of n-GaMnN thin films
Authors: Thaler G. T. , Overberg M. E. , Gila B. , Frazier R. , Abernathy C. R. , Pearton S. J. , Lee J. S. , Lee S. Y. , Park Y. D. , Khim Z. G. , Kim J. , Ren F.
Published: July 2002

GaMnN thin films were synthesized using gas-source molecular-beam epitaxy. Mn concentrations between 3 and 12 at. % were investigated. No evidence of second-phase formation was observed by powder x-ray diffraction or high-resolution cross section transmission electron microscopy in films with 9% or less Mn. The films were n type as determined by capacitance–voltage or Hall analysis. Magnetic characterization performed using a squid magnetometer showed evidence of ferromagnetic ordering at room temperature for all samples. In agreement with theoretical predictions, material with 3% Mn showed the highest degree of ordering per Mn atom. At 320 K, the samples show a nonzero magnetization indicating a TC above room temperature.