bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic properties of ultrathin Co/Ge(111) and Co/Ge(100) films
Authors: Cheng W. C. , Tsay J. S. , Yao Y. D. , Lin K. C. , Yang C. S. , Lee S. F. , Tseng T. K. , Neih H. Y.
Published: July 2002

The orientation of the magnetization and the occurrence of interfacial ferromagnetic inactive layers for ultrathin Co films grown on Ge(111) and Ge(100) surfaces have been studied using the in situ surface magneto-optic Kerr effect. On a Ge(111) substrate, cobalt films (⩽28 monolayers) with in-plane easy axis of magnetization have been observed; however, on a Ge(100) substrate, ultrathin Co films (14–16 monolayers) with canted out-of-plane easy axis of magnetization were measured. The ferromagnetic inactive layers were formed due to the intermixing of Co and Ge and lowering the Curie temperature by reducing Co film thickness. The Co–Ge compound inactive layers were 3.8 monolayers thick for Co films grown on Ge(111) and 6.2 monolayers thick for Co films deposited on Ge(100). This is attributed to the difference of the density of surface atoms on Ge(111) and Ge(100).
Effects of Ta on the magnetic structure of permalloy
Authors: Moghadam Nassrin Y. , Stocks G. M. , Kowalewski M. , Butler W. H.
Published: July 2002

In permalloy (Py) based heterostructures Ta has deleterious effects on the magnetic properties resulting in magnetic dead layers. Using the Korringa–Kohn–Rostoker coherent potential approximation method, it is shown that the average moment of Py (Ni0.8Fe0.2) is rapidly quenched by Ta substitutional additions. Locally self-consistent multiple-scattering method calculations for large supercell models of Py0.9Ta0.1 show that Ta additions also result in substantial fluctuations in the size of the local moments. The configuration dependent reductions in the local moments are larger for Ni than for Fe sites and are the largest for Ni sites closest to Ta.
Band-structure calculations of specular reflection in spin valves
Authors: Chen Jian , Fernandez-de-Castro Juan
Published: July 2002

Band structure calculations are carried out on three types of spin-valve structures: (1) simple spin valves, (2) dual spin valves, and (3) spin valves with a synthetic antiferromagnet. The effect of specular reflection is studied by comparing the transport properties of spin valves with resistive metallic and insulating layers at the outer boundaries. In the spin valve with a synthetic antiferromagnet, an insulating layer needs to be inserted inside the reference layer to achieve similar enhancement to the giant magnetoresistance as the other two types of spin valves with insulating outer boundaries. Results are analyzed in terms of s–d scattering in the different spin channels with different boundary conditions.
Magnetic microstructure of the spin reorientation transition
Authors: Vedmedenko E. Y. , Oepen H. P. , Kirschner J.
Published: July 2002

The scenario of the magnetization reorientation in second-order perpendicular anisotropy approximation is theoretically studied by means of Monte–Carlo simulations. The microstructure is investigated as a function of the difference between first-order anisotropy and demagnetizing energy Keff=K1−ED and the second-order anisotropy K2. An influence of the second-order perpendicular anisotropy on the spin reorientation transition is found when Keff vanishes. The broadening and coalescing of domain walls found earlier for K2=0 is prevented by positive K2. The domain wall width and energy are determined by K2. For K2>0 the transition via a canted vortex-like structure is found which yields the smooth, continuous connection between the vertical domain structure and the vortex structure with in-plane magnetization.
Cooperative freezing in spin glasses and magnetic nanostructures
Authors: Skomski Ralph , Leslie-Pelecky D.
Published: July 2002

Relaxation processes in disordered nanostructures and spin glasses are investigated by examining the role of the magnetic anisotropy. To abstract from freezing processes associated with phase transitions, the consideration is restricted to a one-dimensional bond-disordered Ising spin glass. Using a Mattis-type gauge transformation, the spin glass is mapped onto a disordered ferromagnet. Based on the behavior of the spin chain we argue that spin-glass freezing is not necessarily related to a phase transition. The magnetic anisotropy, which is largely ignored in the Glauber approach to Ising dynamics, gives rise to cooperative deviations from the Arrhenius behavior and mimics Vogel–Fulcher relaxation.
Frequency and temperature dependence of ferromagnetic linewidth in exchange biased Permalloy
Authors: Lubitz P. , Rubinstein M. , Krebs J. J. , Cheng S.-F.
Published: July 2002

Ferromagnetic resonance linewidths were measured at 9.5 and 35 GHz and in the temperature range from 77 to 350 K for thin Permalloy (Py) films exchange biased by adjacent layers of NiO, CoO, or IrMn. Compared to unoxidized Py alone, for which the linewidth is nearly temperature independent in this range and scales linearly with frequency, exchange biased Py has broader lines with distinctive temperature dependences for each bias material at 9.5 GHz. Different temperature dependences were observed at 35 GHz. Our results are consistent with relaxation related to thermally driven reversal of the antiferromagnetic grains. Intrinsic damping and inhomogeneities also add to the widths. The qualitative features of the temperature and frequency dependences of the linewidths can be described with the usual expression for the slow relaxation linewidth mechanism. The temperature dependence of the relaxation time is taken from Néel’s expression for the reversal time using appropriate rate prefactors and activation energies.
Interaction effects in disordered multilayers
Authors: Antel W. J. , Laidler H. , Marrows C. H.
Published: July 2002

Interaction effects in a series of disordered Co/Cu multilayers have been studied at low applied fields. The samples had Cu thickness tCu chosen to give antiferromagnetic (AF) and ferromagnetic (FM) coupling. From measured remanence curves, both switching field distributions and ΔM curves have been determined. AF coupled films exhibit globally demagnetizing interactions. However, the interactions in the FM coupled films exhibit magnetizing interactions for thinner Cu layers and competing magnetizing and demagnetizing interactions for thicker Cu spacer layers.
Spin-filter spin-valve films with an ultrathin CoFe free layer
Authors: Fukuzawa H. , Iwasaki H. , Kamiguchi Y. , Koi K. , Sahashi M.
Published: July 2002

The concept of spin-filter spin-valve (SFSV) films is reviewed. The dependence of the free layer structure on the spin-filter effect was investigated by using model films and two types of free layers were compared, a single CoFe layer and a conventional NiFe/Co free layer. At the same magnetic thickness of the free layer, the SFSV films with a CoFe free layer showed a larger magnetoresistance (MR) ratio than those with a NiFe/Co free layer. This is partly attributed to the thinner CoFe free layer thickness, which is due to the fact that the CoFe free layer has higher Bs than the NiFe/Co free layer. Moreover, SFSV films with a CoFe free layer still showed a larger MR ratio when the free layer thickness was the same. It suggests that other factors contribute to the high MR performance, such as the quality of the interface between a free layer and a high conductance layer. Film performance of MR 9% to 10%, ΔRs 1.5–2.0 Ω, Hcea∼3 Oe, and λs⩽±0.5 ppm was obtained with a single CoFe free layer and synthetic antiferromagnetic pinned structure.
Thermal stability of PtMn based synthetic spin valves using thin oxide layer
Authors: Sugita Yasunari , Kawawake Yasuhiro , Satomi Mitsuo , Sakakima Hiroshi
Published: July 2002

Thermal stability of PtMn based synthetic spin valves with a thin oxide layers (OL) in pinned and/or free layers has been studied. Temperature dependence of the magnetoresistance(MR) curves and thermal treatment in the magnetic field show that the OL do not deteriorate thermal stability of the spin valves. PtMn based synthetic spin valves with Ta/NiFeCr seedlayer exhibit a stronger (111) orientation and better MR properties than those with a Ta seedlayer. Furthermore, PtMn based synthetic spin valves on a Ta/NiFeCr seedlayer with and without thin OL in the pinned layer show good thermal stability. An aniferromagnetic coupling of CoFe/Ru/CoFe in these spin valves with a Ta/NiFeCr seedlayer, even though the Ru interlayer was oxidized, is more thermally stable than that in the spin valves with a Ta seedlayer at more than 400 °C.
Atomic engineering of spin valves using Ag as a surfactant
Authors: Yang David X. , Shashishekar B. , Chopra Harsh Deep , Chen P. J. , Egelhoff W. F.
Published: July 2002

In this study, dc magnetron sputtered NiO (50 nm)/Co (2.5 nm)/Cu(1.5 nm)/Co (3.0 nm) bottom spin valves were studied with and without Ag as a surfactant. At Cu spacer thickness of 1.5 nm, a strong positive coupling >13.92 kA/m (>175 Oe) between NiO-pinned and “free” Co layers leads to a negligible giant magnetoresistance (GMR) effect (<0.7%) in Ag-free samples. In contrast, spin valves deposited in the presence of ≈1 monolayer of surfactant Ag have sufficiently reduced coupling, 5.65 kA/m (71 Oe), which results in an order of magnitude increase in GMR (8.5%). Using transmission electron microscopy (TEM), the large positive coupling in Ag-free samples could directly be attributed to the presence of numerous pinholes. In situ x-ray photoelectron spectroscopy shows that, in Ag-containing samples, the large mobile Ag atoms float out to the surface during successive growth of Co and Cu layers. Detailed TEM studies show that surfactant Ag leaves behind smoother interfaces less prone to pinholes. The use of surfactants also illustrates their efficacy in favorably altering the magnetic characteristics of GMR spin valves, and their potential use in other magnetoelectronics devices and multilayer systems.
Electronic structure and magnetic interactions in Mn doped semiconductors
Authors: Schulthess T. C. , Butler W. H.
Published: July 2002

The electronic structures of several magnetic semiconductors were calculated using first-principles based electronic structure techniques. Calculations were performed both for periodic supercells and for disordered systems which were treated within the coherent potential approximation. Our results differ from the conventional atomic model for the Mn impurity. We find strong hybridization between the majority Mn d states and the majority valence band. We find that substitutional Mn impurities add states and electrons to the majority valence band but not to the minority. We find relatively strong carrier induced magnetic interactions between impurities. The sign of these interactions may however, change sign as a function of the distance between the impurities.
Brillouin scattering and diffracted magneto-optical Kerr effect from arrays of dots and antidots (invited)
Authors: Grimsditch M. , Guedes I. , Vavassori P. , Metlushko V. , Ilic B. , Neuzil P. , Kumar R.
Published: July 2002

The magnetic properties of nanoarrays have been investigated using Brillouin scattering, magneto-optical Kerr effect (MOKE) and diffracted-MOKE (DMOKE) techniques. The anisotropies in negative arrays are found to be due to the shape of the holes and not due to the array itself. The DMOKE results allow us to extract the domain pattern at remanence.
High frequency spin dynamics in magnetic heterostructures (invited)
Authors: Stamps R. L.
Published: July 2002

Fast reversal processes in magnetic particles and arrays involve fundamental magnetic dynamic and relaxation processes. Exchange and dipolar interactions determine equilibrium ground states and strongly influence linear and nonlinear dynamics. Calculations are used to show how high frequency resonances in arrays of densely packed magnetic particles can affect reversal times, possibly leading to dramatic decreases in switching rates. High frequency excitations and dynamic processes in interface exchange coupled magnets are also discussed, with emphasis on exchange biased materials. The exchange bias effect is closely related to interface magnetic structure and magnetization processes in systems of ferromagnets exchange coupled to antiferromagnets. It is shown how magnetization processes in the antiferromagnet can be studied through observation of dynamic effects in the ferromagnetic component.
Electronic structure of epitaxial NixPd1−x alloy films on Cu3Au(100)
Authors: Muschiol U. , Seider M. , Schneider C. M. , Lin M.-T.
Published: July 2002

We studied epitaxially grown single-crystalline chemically disordered ultrathin NixPd1−x alloy films on Cu3Au(100) by means of photoemission using magnetic circular dichroism in the angular distribution. We clearly observe a well-defined three-dimensional band dispersion in 15 monolayer films. On the basis of relativistic selection rules, this dispersion can be related to specific electronic states. Since the spin–orbit coupling (ΔEso) is smaller for Ni than for Pd, the alloy will assume an average value. We find that this average spin–orbit coupling clearly shows up in the electronic structure and correlates directly to the composition of the NiPd alloy film. The results are discussed in terms of a simple qualitative model.
Magneto-optical Kerr effect in laser-patterned La2/3Sr1/3MnO3 epitaxial thin films
Authors: Bibes M. , Hrabovsky D. , Martı́nez B. , Fert A. R. , Trtı́k V. , Varela M. , Fontcuberta J.
Published: July 2002

In this study, we have performed magneto-optical Kerr effect (MOKE) measurement on epitaxial La2/3Sr1/3MnO3 thin films containing artificial interfaces created by laser-patterning the SrTiO3 substrate. The observed increase of the resistivity and of the high-field magnetoresistance when measuring the films across the interface arrays are related to the reduction of the magnetization of the interfaces with respect to the rest of the film. As observed by the local MOKE probe, the structural disorder in the manganite film induced by the underlying patterned substrate leads to a large spin disorder responsible for a strong high-field susceptibility of the resistance.
Thermomagnetic hysteresis effects in NiMn and NiMnPd thin films
Authors: Öner Y. , Lue C. S. , Ross Joseph H. , Rathnayaka K. D. D. , Naugle D. G.
Published: July 2002

dc magnetization measurements, for zero-field cooled (MZFC) and field-cooled (MFC) cases, have been carried out for flash-evaporated Pd-doped NiMn thin films. These included reentrant phases (Ni76−xPdx)Mn24, for 0⩽x⩽5, and Ni75Mn23Pd2, a pure spinglass phase. The studies were performed over the temperature range 3–300 K. Low-field magnetization measurements show the irreversibility effect (MZFC and MFC diverge) at temperatures below the Curie temperature Tc. In Ni75Mn23Pd2, MZFC falls below MFC, as usually observed. However, in reentrant compositions, MZFC crosses MFC upon warming into the ferromagnetic regime, where it stays above MFC at temperatures below Tc. This unusual behavior is attributed to a model of Imry and Ma in which, in a ferromagnet with antiferromagnetic impurities, the impurities can couple to the host ferromagnetic alignment and force the system to break into domains antiferromagnetically coupled to each other. Field-cooled hysteresis measurements indicate the uniaxial anisotropy in these samples to be small, in contrast with the rigid uniaxial anisotropy reported for the corresponding polycrystalline bulk samples. Since the lattice-orbit coupling is weak in the amorphous phase, this clearly demonstrates that the physical origin of the unidirectional anisotropy is associated with the spin-orbit coupling.
Effects of weak periodic pressure waves on time to ignition of fuel mixtures
Authors: Vainshtein P. , Gutfinger C.
Published: July 2002

The accumulation of small-amplitude gas dynamic perturbations accelerates the process of self-ignition of a homogeneous explosive mixture. In the present paper the cumulative effect of forced acoustic oscillations is studied when timescales of piston motions in a closed cylinder are comparable with the acoustic timescale. It is shown that the most significant shortening of the time to ignition takes place in a resonant system, where periodic shock waves travel back and forth in the cylinder.
Nuclear magnetic resonance radiation damping in inhomogeneous radio frequency fields: The toroid cavity detector
Authors: Momot Konstantin I. , Johnson Charles S.
Published: July 2002

A theory is presented for radiation damping (RD) in the toroid cavity nuclear magnetic resonance detector, a cylindrically symmetric inhomogeneous-rf field detector in which the magnitude of B1 is inversely proportional to the distance from the cylindrical symmetry axis. The equations of motion of the magnetization components are obtained and discussed. Numerical simulations of conventional- and composite-pulse experiments are presented, along with a discussion of the effects of RD on the evolution of magnetization. Preliminary simulations of RD in the presence of inhomogeneous line broadening are also presented. The signature effect of radiation damping in the TCD is the winding or unwinding of magnetization gratings that has recently been observed by other researchers. The observed magnitude of the effect is linked to the effective filling factor, which currently appears to be limited by the stray inductance of the detection circuit. The results are of interest in connection with recent findings regarding the interaction of RD with the dipolar demagnetizing field.
Ruijsenaars’ commuting difference operators and invariant subspace spanned by theta functions
Authors: Komori Yasushi
Published: July 2002

We study a family of mutually commutative difference operators introduced by Ruijsenaars. The conjugations of these operators with an appropriate function give the Hamiltonians of some relativistic quantum systems. These operators can be regarded as elliptic analogs of the Macdonald operators and their coefficients consist of the Jacobi theta functions. We show that these operators act on the space of meromorphic functions on the Cartan subalgebra of affine Lie algebras and that the space spanned by characters of a fixed positive level is invariant under the action of these operators.
Photodissociation of 1,2-C2H2Br2 at 248 nm: Competition between three-body formation Br+Br+C2H2 and molecular Br2 elimination
Authors: Lee Y. R. , Chou C. C. , Lee Y. J. , Wang L. D. , Lin S. M.
Published: July 2002

The photodissociation of 1,2-C2H2Br2 at 248 nm has been studied by product translational spectroscopy. The results show that the molecule dissociates exclusively into the products (1) Br2+C2H2 and (2) Br (fast)+Br (slow)+C2H2 with a branching ratio ∼0.2:0.8. While the cleavages of the C–Br bonds are not symmetric, producing the Br atoms at unequal velocities, the anisotropy of the products indicates that both reactions occur in a fraction of a rotational period. Following an asynchronous concerted reaction, the triple products were simulated with the P(Et) distributions coupled by asymmetric angular distributions. A mechanism consistent with the measured results is proposed that the Br2 elimination is a result of a fast intersystem crossing from the ππ* pumped state while the triple products occur via a simultaneous asymmetric scission of the C–Br bonds along the nσ* state.
Are lithium hydride clusters purely ionic? Study using model potentials and density-functional theory
Authors: Bertolus Marjorie , Brenner Valérie , Millié Philippe
Published: July 2002

A study of the LinHn and Lin+1Hn+ clusters with n up to 7 and Li14H13+ using both density-functional theory (DFT) and a model potential is presented. The combined use of these two methods has shown great efficiency. It has enabled us to perform exhaustive explorations of the potential energy surfaces and to study a large number of isomers. Our results show that the structures of the Li–H clusters are very close to typical ionic structures and that the Li–H bond is almost totally ionic. Moreover, the stability of the small clusters is not directly related to the number of ionic bonds formed, and families of structures exist throughout the sizes studied. Our study has also shown that a very simple ionic model potential yields good starting structures and not unreasonable energies. This simple model, however, exhibits two problems: The number of minima in the potential is too large compared to DFT, and its accuracy is by far insufficient to enable one to interpret experimental results. Finally, a good agreement is observed between B3LYP results and the available experimental data.
Linear and nonlinear response functions of the Morse oscillator: Classical divergence and the uncertainty principle
Authors: Wu Jianlan , Cao Jianshu
Published: July 2002

The algebraic structure of the quantum Morse oscillator is explored to formulate the coherent state, the phase-space representations of the annihilation and creation operators, and their classical limits. The formulation allows us to calculate the linear and nonlinear quantum response functions for microcanonical Morse systems and to demonstrate the linear divergence in the corresponding classical response function. On the basis of the uncertainty principle, the classical divergence is removed by phase-space averaging around the microcanonical energy surface. For the Morse oscillator, the classical response function averaged over quantized phase space agrees exactly with the quantum response function for a given eigenstate. Thus, phase-space averaging and quantization provide a useful way to establish the classical-quantum correspondence of anharmonic systems.
Electron–vibration coupling in time-dependent density-functional theory: Application to benzene
Authors: Bertsch G. F. , Schnell A. , Yabana K.
Published: July 2002

As a test of the time-dependent density-functional theory (TDDFT) for electron–vibration coupling, we apply it to the optical properties of the π–π* transitions in benzene. Quantities calculated are the envelopes of the Franck–Condon factors of the electronic transitions and the oscillator strengths of symmetry-forbidden transitions. The strengths of the π–π* transitions span three orders of magnitude and are reproduced to better than 35% by the theory. Comparable agreement is found for the Franck–Condon widths. We conclude that rather detailed information about the effects of the electron–vibrational coupling can be obtained with the TDDFT.
Nonlinear resonance reflection from and transmission through a dense glassy system built up of oriented linear Frenkel chains: Two-level model
Authors: Jarque E. Conejero , Malyshev V. A.
Published: July 2002

A theoretical study of the resonance optical response of assemblies of oriented short (as compared to an optical wavelength) linear Frenkel chains is carried out. Despite the fact that the energy spectrum of a single chain is composed of the bands of Frenkel exciton states, a two-level model is used to describe the optical response of a single linear chain. We account for only the (on-resonance) optical transition between the ground state and the state of the one-exciton band bottom as having the dominating oscillator strength as compared to the other states of the one-exciton manifold. The (off-resonance) process of creation of two excitons per chain is neglected because it requires a higher excitation frequency due to the quasi-fermionic nature of one-dimensional Frenkel excitons. A distribution of linear chains over length resulting in fluctuations of all exciton optical parameters, such as the transition frequency and dipole moment as well as the radiative rate, are taken explicitly into account. We show that both transmittivity and reflectivity of the film may behave in a bistable fashion, originating from saturation of the nonlinear refraction index, and analyze how the effects found depend on the film thickness and on the inhomogeneous width of the exciton optical transition. Estimates of the driving parameters show that films of oriented J-aggregates of polymethine dyes at low temperatures seem to be suitable species for the experimental verification of the behavior found.
Wafer-fused p-i-n InGaAs/Si photodiode with photogain
Authors: Kang Y. , Mages P. , Clawson A. R. , Lau S. S. , Lo Y. H. , Yu P. K. L. , Pauchard A. , Zhu Z. , Zhou Y.
Published: July 2002

Using high temperature fusion of InGaAs/InP on Si, we demonstrated an InGaAs/Si p-i-n photodiode with a photomultiplication gain of 11 at relatively low bias of −15 V. The photodiode exhibited a dark current of 30 nA at −5 V, and a responsivity of 0.3 A/W at 1.32 μm wavelength without antireflection coating. The measured device behavior were consistent with that predicted by a model of charged interface states resulting from the fusion.
Binary complexes of HCN with H2, HD, and D2 formed in helium nanodroplets
Authors: Moore D. T. , Ishiguro M. , Miller R. E.
Published: July 2002

Rotationally resolved infrared spectra have been obtained for HCN–oH2, pH2–HCN, HCN–pD2, HD–HCN, and oD2–HCN binary complexes formed in liquid helium nanodroplets. The isolation provided by the droplets enables us to observe both ortho and para complexes, while in the previous gas phase infrared study only the more strongly bound J=1 complexes were observed. Of particular interest is the observation that the complexes correlating with J=0 and J=1 hydrogen bind to opposite ends of the HCN molecule. In an effort to understand this behavior we have carried out ab initio calculations at the CCSD(T) level, exploring the potential energy surface near the corresponding minima. The dramatic difference between the binding of ortho and para hydrogen to HCN can be at least partially understood by averaging the calculated surface over the appropriate rotational wave functions.
Quantum relaxation dynamics using Bohmian trajectories
Authors: Maddox Jeremy B. , Bittner Eric R.
Published: July 2002

We present a new Bohmian trajectory based treatment of quantum dynamics suitable for dissipative systems. Writing the density matrix in complex-polar form, we derive and define quantum equations of motion for Liouville-space trajectories for a generalized system coupled to a dissipative environment. Our theory includes a vector potential which mixes forward and backwards propagating components and pulls coherence amplitude away from the diagonal region of the density matrix. Quantum effects enter via a double quantum potential, Q(x,y), which is a measure of the local curvature of the density amplitude. We discuss how decoherence can be thought of as a balancing between localization brought on by contact with a thermal environment which increases the local curvature of the density matrix and delocalization due to the internal pressure of the quantum force which seeks to minimize the local curvature. The quantum trajectories are then used to propagate an adaptive Lagrangian grid which carries the density matrix, ρ(x,y), and the action, A(x,y), thereby providing a complete hydrodynamiclike description of the dynamics.
A geometric model for cold water and liquid–liquid transitions
Authors: Guisoni Nara , Henriques Vera Bohomoletz
Published: July 2002

Water is an associated liquid in which the main intermolecular interaction is the hydrogen bond (HB) which is limited to four per atom, independently of the number of neighbors. We have considered a hydrogen bond net superposed on Bernal’s geometric model for liquids, which allows for different local environments for the liquid particles. In this study, a mean-field treatment of the two-dimensional version of the model is discussed. Under pressure the model exhibits three phases of different densities and a coexistence line ending in a critical point between low and high density phases. Entropy of the HB network plays an essential role in defining the slope of the coexistence line. The model behavior might be of interest in describing supercooled water and liquid–liquid transitions of other substances.
Production of relativistic electrons by irradiation of 43-fs-laser pulses on copper film
Authors: Oishi Yuji , Nayuki Takuya , Nemoto Koshichi , Okano Yasuaki , Hironaka Yoichiro , Nakamura Kazutaka G. , Kondo Ken-ichi
Published: July 2002

The energy spectra of fast electrons produced by ultrashort, high-intensity laser pulses were directly measured using a magnetic spectrometer with an imaging plate. The typical temperature was 350 keV for irradiation on 30 μm thick copper film by pulses of width 43 fs, intensity 2.7×1018 W/cm2, repetition rate 10 Hz without artificial prepulses and was found to be close to the ponderomotive potential. In addition, the energy spectra of high-energy photons, which are expected to be produced from the electrons, were calculated.
Growth and holographic characterization of nonstoichiometric sillenite-type crystals
Authors: Vogt H. , Buse K. , Hesse H. , Krätzig E. , Garcı́a R. R.
Published: July 2002

We have grown Bi12GeO20 and Bi12SiO20 single crystals from melts with different GeO2 and SiO2 contents, respectively, to investigate the influence of an intrinsic defect, the antisite defect (Bi on Ge or Si site), on the light-induced charge transport. The optical absorption and the effective trap density of the crystals increase with decreasing GeO2 and SiO2 content in the melt. Furthermore, a variation of the photoconductivity is observed. Our results can be described by a one-center model with the antisite defect Bi3+/4+ on Ge or Si site as dominant photorefractive center.
Generation–recombination noise of DX centers in AlN:Si
Authors: Goennenwein S. T. B. , Zeisel R. , Ambacher O. , Brandt M. S. , Stutzmann M. , Baldovino S.
Published: July 2002

Generation–recombination noise is observed in Si-doped aluminum nitride (AlN:Si). Both the magnitude and the characteristic frequency of the generation–recombination noise power density are found to be thermally activated. Using a model based on charge carrier number fluctuations in a two-level system, transition energies and potential barriers of the DX center formed by Si donors in AlN are quantitatively determined.
Energy bounds for the spinless Salpeter equation
Authors: Hall Richard L. , Lucha Wolfgang , Schöberl Franz F.
Published: July 2002

We study the spectrum of the Salpeter Hamiltonian H=βm2+p2+V(r), where V(r) is an attractive central potential in three dimensions. If V(r) is a convex transformation of the Coulomb potential −1/r and a concave transformation of the harmonic-oscillator potential r2, then both upper and lower bounds on the discrete eigenvalues of H can be constructed, which may all be expressed in the form E=minr>0[βm2+P2/r2+V(r)] for suitable values of P here provided. At the critical point r=r̂ the relative growth to the Coulomb potential h(r)=−1/r must be bounded by dV/dh<2β/π.
An ansatz-based variational path integral centroid approach to vibrational energy relaxation in simple liquids
Authors: Poulsen Jens Aage , Rossky Peter J.
Published: July 2002

Within the framework of Feynman–Kleinert variational theory, simple equations for correlation functions of arbitrary nonlinear operators are derived. These correlation functions, meaningful only for short times, are extended to longer times by a well established ansatz. The approach is tested against exact results for the quantum force correlation function in a two-dimensional Helium cluster problem and is found to perform very well. The theory is also applied to calculate quantum force correlation functions for a diatomic in a larger Neon bath. The results indicate that the method is effective for vibrational energy relaxation processes in simple liquids when quantum effects are significant.
Spatially resolved SiF and SiF2 densities in inductively driven discharges containing C2F6 and C4F8
Authors: Hebner G. A.
Published: July 2002

Laser-induced fluorescence was used to measure the spatially resolved SiF and SiF2 radical densities in inductively driven discharges containing fluorocarbon gases. Measurements of the spatially resolved SiF and SiF2 densities were performed in C2F6 and C4F8 discharges as functions of the induction coil power, pressure, and bias power above a silicon surface. The SiF density had a maximum at a radial distance of 2–3 cm from the center of the plasma, and then monotonically decreased towards the edge of the plasma region. While the density was a function of the plasma parameters, the general shape of the SiF spatial distribution was generally independent of the plasma conditions and gas. The SiF2 density had a maximum at a radial distance of approximately 7 cm from the center of the plasma. At this location, the SiF2 plasma-induced optical emission was approximately 10% of the value in the center of the discharge. The SiF2 spatial distribution was dependent on the induction coil power. The location of the peak in the SiF2 density did not depend on the plasma confinement structures or material.
Coherency of magnetic switching in single and multilayered ferromagnetic films with strong out-of-plane anisotropy
Authors: Vavassori P. , Bottoni G.
Published: July 2002

The magnetization reversal in two metal-evaporated samples for magnetic recording, which display a strong out-of-plane anisotropy, has been studied by means of magneto-optic Kerr vector magnetometry, a technique that allows for the determination (in three dimensions) of the magnetization vector direction as a function of the external field. One sample is a single CoNiO thin film and the other is composed by three CoNiO layers. The results of the vector magnetometry analysis indicate that the magnetization of the two CoNiO samples rotates coherently out-of-plane, without domains formation, until the external field reverses and reaches a critical value Hcr. The comparison and the detailed analysis of the results allowed us to evidence the different degree of coherency of the magnetic switching in the two samples investigated and to relate it to their different structure.
Description of a high-density homogeneous electron gas with the Yasuda density matrix functional
Authors: Cioslowski Jerzy , Ziesche Paul , Pernal Katarzyna
Published: July 2002

Description of a spin-unpolarized homogeneous electron gas with the Yasuda functional applied to the exact 1-matrix gives rise to a formalism that correctly accounts for the logarithmic divergence of the correlation energy per volume at the high-density limit. The linear dependence of the on-top pair distribution function for antiparallel spins on the rS parameter is also correctly reproduced. A comparison of the relevant proportionality constant with its exact counterpart fixes the value of the adjustable parameter that enters the Yasuda functional.
Comparison of polarizable and nonpolarizable models of hydrogen fluoride in liquid and supercritical states: A Monte Carlo simulation study
Authors: Jedlovszky Pál , Mezei Mihaly , Vallauri Renzo
Published: July 2002

Structural and thermodynamic properties of a polarizable and two pairwise additive effective interaction potential models of hydrogen fluoride are analyzed and compared with experimental data in the liquid and supercritical phase as well as along the vapor–liquid coexistence line. Pair correlation functions and thermodynamic data are obtained from Monte Carlo simulations at two liquid and four supercritical thermodynamic state points. Vapor–liquid equilibrium properties have been calculated from a set of Gibbs ensemble Monte Carlo simulations. It is found that the polarizable model is clearly superior over the two nonpolarizable ones in describing the temperature and density variation of several thermodynamic and structural properties. Thus the experimentally observed elongation of the hydrogen bonds with decreasing density is only reproduced by the polarizable model. Similarly, among the three models only the polarizable one can correctly describe the dependence of the density on the pressure and temperature in the entire range of the liquid state, although the density of this model is always somewhat lower than that of real HF. Consistently, the vapor–liquid coexistence curve is also much better reproduced by the polarizable than by the other two models. All three models underestimate the critical temperature, although the polarizable model is again in a considerably better agreement with the experimental data than the other two. All three models reproduce the experimental fact that the energy of evaporation of HF goes through a maximum as a function of the temperature.
The reorganization of the lamellar structure of a single polyethylene chain during heating: Molecular dynamics simulation
Authors: Zhang Xiu-bin , Li Ze-sheng , Lu Zhong-yuan , Sun Chia-Chung
Published: July 2002

Molecular dynamics simulation starting from a lamellar crystal model of a single polyethylene chain is performed to investigate the lamellar reorganization during heating at the molecular level. It is shown that three stages are involved in the process of the reorganization: at temperatures 300 K<T⩽370 K the lamella thickens, in the region of 370 K500 K the lamella starts to melt and becomes a random coil in the end. Particularly, the thickening process is investigated in our simulations. It is found that the lamellar thickening occurs in discrete steps and is driven mainly by the van der Waal attraction between the chain segments. There are two mechanisms for the lamellar thickening. At lower temperature the thickening occurs by the sliding diffusion of adjacent chain segments, while at higher temperature the recrystallization after the partial melting of thinner stems leads to the thickening.
Effects of temperature and pressure on phase transitions in a ternary microemulsion system
Authors: Nagao Michihiro , Seto Hideki , Takeda Takayoshi , Kawabata Youhei
Published: July 2002

Temperature variation experiments of small angle neutron scattering (SANS) and neutron spin echo (NSE) were carried out in order to compare effects of temperature and pressure on a structural formation in a ternary microemulsion system composed of AOT (Aerosol-OT; dioctyl sulfosuccinate sodium salt), D2O, and n-decane. From SANS measurements, a phase transition from one-phase dense water-in-oil droplet to two-phase coexistence with a lamellar and a disordered structure was observed with increasing temperature, similar to the case of pressure variation. Another phase transition was observed at a higher temperature above the lamellar phase, although such a subsequent phase transition has not been observed at higher pressure. The characteristic features of structural phase transitions by temperature and by pressure were compared by introducing a reduced temperature and pressure. The dynamical property observed from the NSE measurement was different between the high-temperature phase and the high-pressure phase. These results indicate that the mechanism of the phase transition induced by temperature is different from that by pressure.
Application of the z-transform to composite materials
Authors: Gao Yi Qin , Marcus R. A.
Published: July 2002

Applications of the z-transform were made earlier to interfacial electron transfer involving semi-infinite solids, e.g., semiconductor/liquid and metal/liquid interfaces and scanning tunneling microscopy. It is shown how the method is readily adapted to treat composite materials, such as solid/solid interfaces or “molecular wire”/solid interfaces.
Reentrant spin glass behavior in layered manganite La1.2Sr1.8Mn2O7 single crystals
Authors: Chun S. H. , Lyanda-Geller Y. , Salamon M. B. , Suryanarayanan R. , Dhalenne G. , Revcolevschi A.
Published: July 2002

We report a detailed study of ac/dc magnetization and longitudinal/transverse transport properties of La1.2Sr1.8Mn2O7 single crystals below Tc=118 K. We find that the resistivity minimum at 40 K is related to the reentrant spin glass phase at the same temperature, accompanied by additional anomalous Hall effects. The carrier concentration from the ordinary Hall effects remains constant during the transition and is close to the nominal doping level (0.4 holes/Mn). The spin glass behavior comes from the competition between ferromagnetic double exchange and antiferromagnetic superexchange interactions, which leads to phase separation, i.e., a mixture of ferromagnetic and antiferromagnetic clusters, representing the canted antiferromagnetic state.
Defects in N/Ge coimplanted GaN studied by positron annihilation
Authors: Nakano Yoshitaka , Kachi Tetsu
Published: July 2002

We have applied positron annihilation spectroscopy to study the depth distributions and species of defects in N-, Ge-, and N/Ge-implanted GaN at dosages of 1×1015 cm−2. For all the implanted samples, Ga vacancies introduced by ion-implantation are found to diffuse into much deeper regions of the GaN layers during the implantation and to change into some other vacancy-type defects by the annealing at 1300 °C. In particular, markedly different defects turn out to be newly created in the electrically activated regions for both the Ge- and N/Ge-implanted samples after annealing, indicating that these new defects are probably associated with the presence of the implanted Ge dopant atoms.
Efficient 230–280 nm emission from high-Al-content AlGaN-based multiquantum wells
Authors: Hirayama Hideki , Enomoto Yasushi , Kinoshita Atsuhiro , Hirata Akira , Aoyagi Yoshinobu
Published: July 2002

We demonstrated efficient ultraviolet (UV) photoluminescence (PL) with the wavelength ranging from 230 to 280 nm from AlxGa1−xN(AlN)/AlyGa1−yN multiquantum wells (MQWs) grown on SiC by metalorganic vapor phase epitaxy. We systematically investigated the PL intensity of the AlGaN-based MQWs with wide-band-gap AlGaN barriers as functions of QW thickness and Al content of barriers. Single-peak, efficient PL emission was obtained at 282–234 nm at 77 K from AlxGa1−xN(AlN)/AlyGa1−yN 5-layer MQWs with approximately 1.5-nm-thick active layers by changing the Al content of the AlxGa1−xN barriers from 53% to 100%. The efficiency of the deep-UV emission from AlGaN-based QWs was as high as that of blue emission from InGaN-based QWs at 77 K.
Field-induced director dynamics in a nematic liquid crystal: A molecular site dependence?
Authors: Luckhurst G. R. , Miyamoto T. , Sugimura A. , Timimi B. A.
Published: July 2002

Studies of the field-induced alignment of the nematic director for low molar mass materials using vibrational spectroscopy have revealed some fascinatingly unexpected results. Several studies have found that the relaxation time for the director alignment is dependent on the group in the molecule used to monitor the director orientation. Seemingly this undermines the basic concept of the Leslie–Ericksen hydrodynamic theory of nematics. Here we report complementary studies using deuterium NMR spectroscopy of perdeuteriated 4-pentyl-4′-cyanobiphenyl where the director is aligned by a magnetic or an electric field. This particular technique was chosen because the spectral peaks associated with each rigid group in the molecule are clearly resolved and of comparable intensity. We have investigated the director alignment using different temperatures, electric field strengths and angles between the magnetic and electric fields, each of which influences the director relaxation time. For all of the experiments we find that the relaxation times are independent of the group used to determine the director orientation during the alignment process.
Polarizability model of emission from ceramic cathodes
Authors: Boscolo I. , Cialdi S.
Published: July 2002

We show that properly electroded ceramic disks are strong and robust electron emitters when excited with short voltage pulses. They can operate in two different regimes: (I) a regime at low exciting voltage, ∼0.7 kV per millimeter thickness and (II) a regime at high exciting voltage, ∼2 kV/mm. In the first regime the excitation pulse must be bipolar, positive–negative, the output current results in several tens of milliamperes per cm2. The first positive semiwave charges the cathode surface and the negative semiwave expels the previously accumulated electrons. This behavior of ceramic electroded disks, before as sink and next as source of electrons, is due to the succession of an attractive and then repulsive electron field on the unelectroded zones of the front surface. The two opposite oriented fields are generated by the voltage applied at the rear electrode. In the second regime, the excitation pulse can be either monopolar or bipolar, the emission process is governed by the building up of a plasma sheet as a consequence of the excitation pulse. The high-voltage exciting pulse initiates a discharge, and so the plasma, at the triple metal–insulator–vacuum point because of the strong longitudinal component of the electric field. The plasma cloud expands over the front surface becoming a dynamical electrode. Screening electrons accumulate in front of the sample, either in the plasma sheet or on the front surface of the material. The component of the electric field perpendicular to the surface (due to either the negative semiwave of a bipolar pulse or to the space charge of the huge amount of electrons no longer attracted by bound positive charge) pushes screening electrons out from the cathode.
Deep hole traps in n-GaN films grown by hydride vapor phase epitaxy
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Fang Z.-Q. , Look D. C. , Molnar R. J. , Osinsky A. V.
Published: July 2002

Concentrations of deep hole traps were measured in a set of hydride vapor phase epitaxy grown samples with thicknesses varying from 2.6 to 68 μm. Results were obtained from low temperature capacitance–voltage measurements before and after illumination and from deep level transient spectroscopy measurements with optical injection (ODLTS). The former revealed the presence of high densities (∼1015 to 1016 cm−3) of hole traps whose concentration decreased with sample thickness in a manner similar to that found for the dislocation density. Capacitance versus temperature measurements in the dark and after illumination suggested that these traps form a band of states rather than a single level in the GaN band gap. It is suggested that such states could be associated with dislocations. The main hole traps observed by ODLTS were deep hole traps, of energy near Ev+0.9 eV. Their density was also observed to substantially decrease with sample thickness.
The influence of finite bandwidth excitation sources in degenerate four-wave mixing spectroscopy
Authors: Wasserman Thierry A. W. , Vaccaro Patrick H. , Johnson Bruce R.
Published: July 2002

The effects of finite-bandwidth excitation are incorporated into the theoretical treatment of degenerate four-wave mixing (DFWM) spectroscopy by employing a general description of applied electromagnetic fields, thereby permitting the transient nature of typical laser sources to be taken explicitly into account. A computationally efficient perturbative (weak-field) formalism is developed by exploiting exponentially-decaying wave forms as a flexible basis for expanding the temporal envelope functions of incident quasimonochromatic pulses. This approach has the distinct advantage of yielding time-domain response integrals that can be evaluated analytically since they are almost as simple in form as their monochromatic counterparts. The resulting frequency-domain expressions for the induced DFWM signal polarization reflect the pronounced influence of finite-bandwidth excitation without needing to cobble such behavior into phenomenological rates for population decay and coherence dissipation. Spectral line shapes are examined for both stationary and nonstationary ensembles of isolated (gas-phase) target molecules, with the latter analyses demonstrating the systematic degradation of spatial discrimination introduced by nonmonochromatic implementations of the sub-Doppler (phase-conjugate) four-wave mixing scheme. In keeping with recent experimental findings, the precipitous drop in signal intensity accompanying the action of collisional dephasing processes is predicted to be tempered greatly by the use of short-duration excitation pulses.