bluuebunny/crossref_metadata_2025 huggingface dataset:
Harmonic heat flow in isotropic layered systems and its use for thin film thermal conductivity measurements
Authors: Reichling M. , Grönbeck H.
Published: July 2002

A theoretical model is presented describing harmonic heat flow in a two layer system heated by a modulated Gaussian laser beam. Amplitude and phase of the modulated temperature rise in the layers, as well as in the backing substrate and adjacent atmosphere, are calculated by solving the three-dimensional heat conduction equation with a source term including exponential absorption of the laser light in one or two layers. Heat conduction is assumed to be isotropic throughout the system, however, a thermal contact resistance between the two layers can be taken into account. Results are presented for single and double layer systems of gold and various dielectric thin film materials on glass substrates. Amplitude and phase of the harmonic temperature variation are calculated either as a function of position in the sample system or at the surface as a function of the laser beam modulation frequency. It is found that both amplitude and phase of the calculated temperature rise exhibit typical thin film features in their frequency dependence, however, the phase is more sensitive to thin film phenomena than the amplitude. The phase shows typical extrema in that frequency region, where the thermal diffusion length in the film is equal to the film thickness. Based on these findings, we demonstrate how these calculations can be utilized for the interpretation of thin film thermal parameter measurements. The influence of thermal wave interference is demonstrated, and it is discussed how the main thermal parameters like conductivity, effusivity, and thermal contact resistance of the thin film system can be extracted from measurements by a fit of theoretical curves to experimental data. Applying a simple one-dimensional thermal expansion model, surface displacements for thin film systems are calculated and the applicability of photothermal surface displacement for thin film conductivity measurements is discussed.
Frictional behavior of highly oriented pyrolytic graphite
Authors: Ruan Ju-Ai , Bhushan Bharat
Published: July 2002

Friction of a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface and a roughened graphite surface against a silicon nitride tip is measured using a friction force microscope. The cleaved graphite surface is an atomically smooth surface of (0001) plane with a small fraction of line-shaped regions. It is observed that the coefficient of friction is extremely small (<0.006) for a freshly cleaved HOPG surface of (0001) plane. However, the coefficient of friction is more than an order of magnitude larger in some line-shaped regions on the surface compared with that of smooth regions of (0001) planes. Transmission electron microscopy analysis indicates that the line-shaped regions consist of graphite planes of different orientations [other than (0001)] as well as amorphous carbon. This result suggests that (0001) graphite planes have the lowest coefficient of friction in comparison with other surface orientations or amorphous carbon. The coefficient of friction of a roughened graphite surface is found to be much larger than that of cleaved graphite. These observations may explain the large differences between the atomic-scale coefficient of friction of graphite (usually cleaved) (about 0.006 or even less) and its macroscopic counterpart (about 0.1).
Magnetic transitions at high fields in (Fe,Mn)3Si alloys
Authors: Al-Kanani H. J. , Booth J. G. , Cable J. W. , Fernandez-Baca J. A.
Published: July 2002

In the series of ordered DO3 solid solutions Fe3−xMnxSi, field-induced transitions have previously been found to occur for antiferromagnetic alloys with values of x near 1.70. The present work has determined a region of composition in which antiferromagnetism is the low-field, low-temperature state and gives the variation of the critical field as a function of temperature and composition. The critical field decreases approximately quadratically with temperature and rises to beyond 6 T in the range of composition from 1.6≤x≤1.8. Neutron diffraction indicates that the antiferromagnetic axis is shifted from the [111] direction and that in a 5 T field the antiferromagnetic component of the scattering is reduced considerably from that in zero field. However, no corresponding increase in ferromagnetic component is observed within experimental error and Arrott plots suggest the high-field state is still largely antiferromagnetic.
High-temperature ohmic contact to n-type 6H-SiC using nickel
Authors: Crofton J. , McMullin P. G. , Williams J. R. , Bozack M. J.
Published: July 2002

Specific contact resistances measured at elevated temperatures for Ni ohmic contacts to 6H-SiC were reported. The specific contact resistances were measured with the linear transmission line method at both room temperature and at 500 °C and yielded values <5×10−6 Ω cm2 at both temperatures. The trend shows a decreasing contact resistance at higher temperatures. The annealed metal film is a nickel silicide with substantial mixing of C throughout the silicide layer.
Analysis of two-step-growth conditions for GaN on an AlN buffer layer
Authors: Sasaki T. , Matsuoka T.
Published: July 2002

Gallium nitride is grown by metalorganic vapor phase epitaxy with and without a low-temperature-grown AlN buffer layer. Variations in the surface morphology and the layer properties are compared between two-step growth and direct growth to study the effects of various growth conditions. It is found that (i) conditions that stabilize the GaN(0001) surface serve as guidelines for obtaining mirrored surfaces, and (ii) raising GaN growth temperature improves crystallographic, electrical, and luminescence properties of GaN. The observed improvement in the layer properties with increase in GaN growth temperature suggests that increasing N2 dissociation pressure does not affect GaN properties. GaN growth conditions are analyzed thermodynamically to show that NH3 in the growth ambients has the potential to suppress thermal dissociation of GaN.
Modeling the ion energy dependence of the sp3/sp2 bonding ratio in amorphous diamondlike films produced with a mass-separated ion beam
Authors: Koponen I. , Hakovirta M. , Lappalainen R.
Published: July 2002

The formation of amorphous diamondlike films by deposition of energetic carbon atoms is modeled. Formation of diamondlike bonding is attributed to transient high pressure created by the ion impact. The relaxation to energetically favored graphitic state is assumed to proceed through succession of metastable states, taking place until the glass transition point is reached. The model illustrates from the new point of view the processes involved in the formation of diamondlike films. It predicts qualitatively similar dependence of film properties on the energy of the incoming carbon atoms as observed in experiments.
Interaction of an edge dislocation with a wedge crack
Authors: Zhang Tong-Yi , Tong Pin , Ouyang Hao , Lee Sanboh
Published: July 2002

The elastic interaction of an edge dislocation with a wedge crack is investigated using Muskhelishvili’s complex potential treatment of elasticity. The stress intensity factors induced by the dislocation, the strain energy, and the image force acting on the dislocation are formulated, calculated, and plotted. Especially, the influence of wedge angle on the shielding effects and on image forces is investigated. The results show that the radial component of the image force is inversely proportional to the distance from the crack tip to the dislocation and the tangential image force increases with increasing wedge angle as the dislocation position is kept unchanged. When the slip plane passes the crack tip, whether the edge dislocation shields or antishields the crack tip depends on its Burgers vector. In this case, the slip plane of the edge dislocation of Burgers vector b1+ib2=beiθmax inclined at an angle θmax with respect to the positive real axis corresponding to the maximum mode I crack shielding increases slightly to a maximum and then decreases with increasing wedge angle. Regardless of wedge angle, the maximum mode II shielding (or anti-shielding) occurs when the dislocation is located at the positive real axis.
Green’s functions in thermal-wave physics: Cartesian coordinate representations
Authors: Mandelis Andreas
Published: July 2002

A self-consistent integral formulation of Green’s functions in thermal wave-physics is presented in one, two, and three dimensions and for infinite, semi-infinite, and spatially bounded geometries. Furthermore, several applications are explicitly worked out based on either Dirichlet or Neumann boundary conditions and resulting in well-known or novel integral expressions for propagating thermal-wave fields in thermally homogeneous media and in experimentally useful geometries. It is hoped that the Green’s functions methodologies will enhance their use by investigators who wish to take advantage of their elegance, mathematical simplicity, and computational power.
Atomic force microscopy growth modeling of SiC buffer layers on Si(100) and quality optimization
Authors: Ferro G. , Monteil Y. , Vincent H. , Thevenot V. , Tran Min Duc , Cauwet F. , Bouix J.
Published: July 2002

The effect of various growth conditions has been studied in order to modelize and optimize the SiC buffer layers obtained by reactive chemical-vapor deposition (RCVD) on Si(100) substrates. First, thermodynamic calculations have been carried out on the Si–H2–CxHyClz system to simulate the RCVD process and to foresee the nature and the evolution of the deposit with varying parameters. The experiments have confirmed some of the thermodynamic results, such as the carbon deposition in specific conditions, and brought out complementary information on the kinetics point of view. Due to our low heating up rate, no SiC island formation was characterized at the early stage of growth. The obtained SiC layers are ultrathin with a very particular morphology. We propose a new model of growth based on atomic force microscopy observations to explain the resulting morphology. The optimal conditions have been deduced to elaborate ultrathin, smooth, and monocrystalline β-SiC buffer layers.
Anneal-tunable Curie temperature and transport of La0.67Ca0.33MnO3
Authors: Worledge D. C. , Snyder G. Jeffrey , Beasley M. R. , Geballe T. H. , Hiskes Ron , DiCarolis Steve
Published: July 2002

Resistivity measurements on a La0.67Ca0.33MnO3 film are reported for a series of argon anneals at successively higher temperatures. Tc, the ferromagnetic ordering temperature, increases uniformly with increasing annealing temperature and annealing time. Hence, Tc can be tuned by appropriate annealing. In order to fully anneal these samples, i.e., achieve bulk properties, it proves sufficient to anneal them in argon. Further annealing in oxygen produces only minor changes in the resistivity. Data from Tc up to 1200 K show activated conduction with ρ=BTeEa/kT, the temperature dependence predicted by the Emin–Holstein theory of adiabatic polaron hopping. Their model fits both data from the partially annealed and fully annealed samples better than the variable range hopping or semiconductor models which have been used by previous workers. The activation energy Ea and resistivity coefficient B decrease with increasing maximum anneal temperature. These changes, together with the increase in Tc, are consistent with an anneal induced relaxation of the Mn–O–Mn bond angle. The time dependent resistivity during annealing at a fixed temperature follows the equation ρ=ρ0{1−D ln[1+(t−t0)/τ]}, making it possible to acquire data in a reversible regime, and also to obtain an estimate of the annealing activation energy.
Measurements of stress during vapor deposition of copper and silver thin films and multilayers
Authors: Shull Alison L. , Spaepen Frans
Published: July 2002

Copper and silver single layer and multilayered thin films were thermal vapor deposited onto cantilevered substrates [Si(100) with native oxide] near room temperature in ultrahigh vacuum. The total force per unit width (F/w) in the film during and after deposition was determined from the change in substrate curvature measured in situ by a laser scanning technique. The intrinsic component of F/w was obtained by subtraction of the thermal component, which was obtained by measuring the product of the biaxial modulus of the film (Yf) and the difference in coefficients of thermal expansion of the substrate and the film (Δαs−f) while each sample was still in the ultra-high vacuum deposition chamber. For all samples, the measured value of YfΔαs−f was substantially lower than the calculated value based on the {111} biaxial modulus and the coefficients of thermal expansion of the bulk materials, even though x-ray diffraction indicated strong {111} film texture. During deposition, a general trend in F/w was found regardless of the initial surface material as long as a heterogeneous interface was formed: following an initial compressive transient, F/w goes through a broad tensile maximum, then decreases and eventually becomes net compressive. This work conclusively demonstrated the presence of a compressive component of F/w. The evolution of the general features is unaffected by stopping and then resuming deposition of the same material. When a Ag/Cu multilayer is deposited continuously or intermittently (i.e., with periods of time without deposition between deposition of each layer), the evolution of the features of F/w is different during deposition of Cu layers from that during deposition of Ag layers, but is the same for each Cu layer and each Ag layer. After deposition ends, F/w changes with time in the tensile direction regardless of whether the final F/w during deposition is tensile or compressive. This change is reversed upon further deposition of the same material and therefore is a surface change from a dynamic to a static configuration (not a bulk change such as grain growth). The magnitude of the change in F/w after deposition increases linearly with the thickness of the top layer (the layer that was most recently deposited). The (111) Ag/Cu interface stress was determined for the first time, to the best of our knowledge, based on measured discontinuities in F/w associated with interface formation during deposition of multilayers. These discontinuities were different for Cu onto Ag and Ag onto Cu, and were repeatably reproduced for each interface during continuous deposition of multilayered films. A value of −0.21±0.10 N/m was found for the (111) Ag/Cu interface stress. The sign (compressive) is significant; it is the same as that of earlier experimental determinations, but opposite to that of theoretical calculations. The absolute value of the interface stress is low due to the incomplete formation of each interface at the early stage of layer formation.
A generalized solidification model and microstructural verification for the Nd–Fe–B–Ti–C system processed by rapid solidification
Authors: Kramer M. J. , Li C. P. , Dennis K. W. , McCallum R. W. , Sellers C. H. , Branagan D. J. , Shield J. E.
Published: July 2002

For the Nd2Fe14B (2-14-1) compound, the optimal grain size should be smaller than the size of the single domain size of 150 nm. Transition metal carbides (TMC) also reduce the quench rate necessary to achieve the optimal or overquenched condition. This allows inert gas atomization (IGA) to produce viable magnetic materials. In this article we will demonstrate that optimal microstructure for the 2-14-1 can be produced by IGA with the addition of TiC. Moreover, a solidification model will be presented to show (1) how recalescence is a critical feature to the evolution of the microstructure in rapidly solidified materials and (2) the role TMC and other solute phases have on inhibiting grain growth so that lower quench rates can be employed.
Structural and magnetic properties of Mn epitaxially grown on Cu(111) (abstract)
Authors: Grigorov I. L. , Walker J. C.
Published: July 2002

Structural and magnetic properties of the epitaxial Mn films vary widely with the structure and magnetic ordering of the substrate material. On the surfaces with hexagonal symmetry and appropriate lattice spacing, Mn undergoes a peculiar reconstruction to a phase with the surface unit cell area three times that of fcc Mn.12 In order to investigate magnetic properties of this phase Mn/Cu multilayers were grown on Cu(111) single crystals. The quality of the film surface and in-plane lattice spacings were monitored by in situ reflection high-energy electron diffraction. The initial growth of Mn on Cu(111) was characterized as nonpseudomorphic fcc with the successive reconstruction to the expanded phase for the thicknesses above 2 Å. The reconstruction was observed in all repeats despite gradual deterioration of the surface quality. Magnetic properties of the films were investigated by SQUID magnetometry in the temperature range between 2 and 150 K. In magnetic fields up to 5 T magnetic moment of the films showed virtually linear field dependence even at 2 K. By fitting our data to the solution of the mean-field Hamiltonian we determined that above 20 K the dc susceptibility χ(T) followed Curie–Weiss law with ΘN=30 K and C=0.15 emu/cm3. These observations give us a reason to argue that the expanded structure of Mn is antiferromagnetic with a transition temperature around or below 2 K and spin density on the order of 1022 spins/cm3.
Shock waves from a water-confined laser-generated plasma
Authors: Berthe L. , Fabbro R. , Peyre P. , Tollier L. , Bartnicki E.
Published: July 2002

Generation of a high amplitude shock wave by laser plasma in a water confinement regime has been investigated for an incident 25–30 ns/40 J/λ=1.064 μm pulsed laser beam. Experimental measurements of temporal and spatial profiles of induced shock waves for this regime of laser shock processing of materials were performed using a velocimetry interferometer system for any reflector system. Above a 10 GW/cm2 laser intensity threshold, a saturation of the peak pressure is shown to occur while the pressure pulse duration is reduced by parasitic plasma occurring in the confining water. The observation of the interaction zone with a fast camera system shows that this breakdown plasma, which mainly occurs at the very surface of the water rather than within the water volume, limits the efficiency of the process. This plasma absorbs the incident laser energy, and the power density reaching the target gradually decreases with increasing power densities while the shock-wave duration is correspondingly reduced. Both pressure measurements and plasma observations allow explaining the current limit of high amplitude shock-waves generation by laser plasma in the water-confinement mode and open new research areas for the understanding of breakdown plasma effects at the surface of the confining water.
Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: Application to Tm3+ and Ho3+ ions in LiYF4
Authors: Walsh Brian M. , Barnes Norman P. , Di Bartolo Baldassare
Published: July 2002

The measurement of branching ratios, cross sections and radiative lifetimes for rare earth ions in solids is considered. The methods are applied to Tm and Ho in YLF as a test case. De-activation rates for electric dipole and magnetic dipole emission are calculated for many of the lower lying manifolds in Tm:YLF and Ho:YLF in the context of the Judd-Ofelt theory to determine radiative lifetimes. Measured values for the branching ratios as well as the absorption and emission cross sections are also presented for many of the excited state manifolds. From these measurements, a methodology is developed to extract measured values for the radiative lifetimes. These results are compared with the Judd-Ofelt theory as a guide for consistency and for determining the accuracy of the Judd-Ofelt theory in predicting branching ratios and radiative lifetimes. The parameters generated by the methods covered here have potential applications for more accurate modeling of Tm:Ho laser systems.
Preparation of soft magnetic films of nanocrystalline Fe–Cu–Nb–Si–B alloy by facing targets sputtering
Authors: Naoe Masahiko , Matsumiya Hiroaki , Ichihara Takayuki , Nakagawa Shigeki
Published: July 2002

Soft magnetic thin films of nanocrystalline Fe–Cu–Nb–Si–B alloy were deposited using the facing targets sputtering (FTS) apparatus. It was found that the Fe–Cu–Nb–Si–B single layers thinner than 100 nm revealed good soft magnetic properties, of which the saturation magnetization 4πMs and the relative permeability μr were 11.3 kG and 500, respectively. However, when these films were thicker than 100 nm, their soft magnetic properties degraded due to the perpendicular magnetic anisotropy. On the other hand, the soft magnetic properties of the post-annealed films were improved owing to the release of stress in the films. Especially, μr of the post-annealed films with thickness of 120 nm increased drastically up to around 6200. Furthermore, Fe–Cu–Nb–Si–B/Al multilayers revealed superior soft magnetic properties due to the magnetostatic coupling between the two ferromagnetic layers. These multilayers post-annealed at 300 °C revealed softer magnetic properties than single layers. They exhibited very low coercivity Hc of 0.63 Oe, large 4πMs of 13.2 kG and high μr of 4600.
Elastic and high pressure properties of ZnO
Authors: Ahuja R. , Fast Lars , Eriksson O. , Wills J. M. , Johansson B.
Published: July 2002

We have studied the elastic and structural properties of ZnO by means of accurate first-principles total energy calculations using the full potential linear muffin tin orbital method. The calculations are based on the density functional theory and we have used the local density Hedin–Lundqvist parametrization and the generalized gradient approximation of Perdew and Wang for the exchange and correlation potential. The calculated values for the equilibrium volume, bulk modulus, and elastic constants are generally in very good agreement with experiments. At elevated pressures ZnO undergoes a structural phase transition from the relatively open wurtzite structure into the more dense NaCl atomic arrangement. The calculated transition pressure is in good agreement with experiment.
Possible solution to the problem of high built-up stresses in diamond-like carbon films
Authors: Kumar Sushil , Dixit P. N. , Sarangi D. , Bhattacharyya R.
Published: July 2002

The various issues relating to the nature of high built-up stresses in diamond like carbon (DLC) films are presented and analyzed and the utility of pulse plasma technique in growing low residual stress DLC films is emphasized. Subsequently, sufficiently thick (2.2 μm) and hard (2000 kg/mm2) DLC films of significantly low stress (≈0.1 GPa) were deposited by the pulse plasma enhanced chemical vapor deposition (PECVD) technique. Stress values were found to be less than 0.5 GPa even with wide variation in pulse parameters (power density 0.4–2.0 W/cm2, dwell time 10–150 ms and duty cycle 10%–70%). A possible growth mechanism operating during pulse plasma discharge of such low residual stress and hard DLC films appears to involve the three phenomena: (i) relaxation of adions/adatoms, (ii) control of the substrate temperature, and (iii) creation of a hard/soft multilayer structure. To examine the role of substrate heating during the pulse plasma discharge, films were also deposited on deliberately heated substrates, using pulse plasma discharge, by using methane, acetylene, and benzene as hydrocarbon sources. An observation of direct correlation of the residual stresses and the degree of order of the film network has been made. Nitrogen dilution of the feedstock was also investigated, and further stress reduction has been observed, but not to the extent that occurs in continuous wave (cw) discharge grown films. This may be because constituent atoms in the film already approach close to a critical coordination number set by the fully constrained network (FCN) model. Other film properties like optical band gap (Eg), refractive index, and room temperature electrical conductivity (σRT) have also been estimated.
Optical and structural studies of the effect of spacers in modulation-doped ZnSe/Zn1−xCdxSe quantum wells
Authors: Na M. H. , Chang H. C. , Zhao J. , Haetty J. , Gardella J. A. , Luo H. , Petrou A.
Published: July 2002

The effect of spacers in modulation-doped Zn1−xCdxSe/ZnSe:Cl multiple quantum wells (MD-MQWs) was investigated by photoluminescence (PL) and time-of-flight secondary-ion-mass spectrometry (TOF-SIMS). A comparison was made between structures with and without spacers as a function of annealing temperature. The diffusion of Cl and Cd was monitored by TOF-SIMS depth profiling and photoluminescence. Although TOF-SIMS does not show any significant diffusion of Cl and Cd in both structures at temperatures up to 385 °C, the PL results indicate the modification of optical properties in the Zn1−xCdxSe/ZnSe:Cl MD-MQWs due to annealing. Up to an annealing temperature of 385 °C, the MD-MQWs with spacers show superior optical quality in the quantum well regions, while quenching of the quantum well band-edge PL and strong enhancement of deep-level emission were observed from the MD-MQWs without spacers. This phenomenon suggests that the radiative deep-level emission may provide more efficient channel for electron–hole recombination with increasing annealing temperature.
Standing surface waves in a dust-contaminated large-area planar plasma source
Authors: Ostrikov K. N. , Yu M. Y. , Sugai H.
Published: July 2002

The effect of charged particulates or dusts on surface wave produced microwave discharges is studied. The frequencies of the standing electromagnetic eigenmodes of large-area flat plasmas are calculated. The dusts absorb a significant amount of the plasma electrons and can lead to a modification of the electromagnetic field structure in the discharge by shifting the originally excited operating mode out of resonance. For certain given proportions of dusts, mode conversion is found to be possible. The power loss in the discharge is also increased because of dust-specific dissipations, leading to a decrease of the operating mode quality factor.
Raman line profile in polycrystalline silicon
Authors: Pivac B. , Furić K. , Desnica D. , Borghesi A. , Sassella A.
Published: July 2002

Raman spectroscopy is applied to the study of the structure of polycrystalline silicon films. The analysis of the transversal optical phonon Raman line shows its complex structure consisting of two dominant contributions, centered at about 519 and 517 cm−1 and attributed to two dominant groups of grains with different size. The profile of this Raman line is demonstrated to give deeper information about the film morphology, directly influenced by the deposition temperature in terms of the ratio of amorphous to crystalline phases, as well as the grain-size distribution, and the film stress.
Hafnium and zirconium silicates for advanced gate dielectrics
Authors: Wilk G. D. , Wallace R. M. , Anthony J. M.
Published: July 2002

Hafnium and zirconium silicate (HfSixOy and ZrSixOy, respectively) gate dielectric films with metal contents ranging from ∼3 to 30 at. % Hf, or 2 to 27 at. % Zr (±1 at. % for Hf and Zr, respectively, within a given film), have been investigated, and films with ∼2–8 at. % Hf or Zr exhibit excellent electrical properties and high thermal stability in direct contact with Si. Capacitance–voltage measurements show an equivalent oxide thickness tox of about 18 Å (21 Å) for a 50 Å HfSixOy (50 Å ZrSixOy) film deposited directly on a Si substrate. Current–voltage measurements show for the same films a leakage current of less than 2×10−6 A/cm2 at 1.0 V bias. Hysteresis in these films is measured to be less than 10 mV, the breakdown field is measured to be EBD∼10 MV/cm, and the midgap interface state density is estimated to be Dit∼1–5×1011 cm−2 eV−1. Au electrodes produce excellent electrical properties, while Al electrodes produce very good electrical results, but also react with the silicates, creating a lower ε layer at the metal interface. Transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy indicate that the dielectric films are amorphous silicates, rather than crystalline or phase-separated silicide and oxide structures. TEM shows that these films remain amorphous and stable up to at least 1050 °C in direct contact with Si substrates.
First-principles investigation of the stability of 3d monolayer/Fe(001) against bilayer formation
Authors: Asada T. , Blügel S. , Bihlmayer G. , Handschuh S. , Abt R.
Published: July 2002

Based on the full-potential linearized augmented plane-wave method combined with the generalized gradient approximation, we determine the ground-state spin configurations and the total energies of 3d transition- metal monolayer and bilayer films on Fe(001) within the c(2×2) unit cell. We find by energy analysis that V, Cr, and Mn layers prefer the layered antiferromagnetic coupling, and Fe, Co, and Ni layers favor the ferromagnetic coupling to Fe(001). One exception is the Mn monolayer, which favors the c(2×2) ferrimagnetic superstructure. We discuss the stability of the 3d transition-metal monolayer films on Fe(001) against the bilayer formation and find that, with the exception of Cr, all 3d monolayers on Fe(001) are stable against bilayer formation. We have confirmed that the interlayer relaxations do not change the overall features of the present results.
Hall effect of epitaxial double-perovskite Sr2FeMoO6 thin films
Authors: Westerburg W. , Martin F. , Jakob G.
Published: July 2002

We prepared high epitaxial thin films of the compound Sr2FeMoO6 with narrow rocking curves by pulsed laser deposition. The diagonal and nondiagonal elements of the resistivity tensor were investigated at temperatures from 4 K up to room temperature in magnetic fields up to 8 T. An electronlike ordinary Hall effect and a holelike anomalous Hall contribution are observed. Both coefficients have reversed sign compared to the colossal magnetoresistive manganites. We found at 300 K an ordinary Hall coefficent of −1.87×10−10 m3/A s, corresponding to a nominal charge carrier density of four electrons per formula unit. At low temperature only a small negative magnetoresistance is observed which vanishes at higher temperatures. The temperature coefficient of the resistivity is negative over the whole temperature range. A Kondo like behavior is observed below 30 K while above 100 K variable range hopping like transport occurs.
Inelastic neutron scattering analysis of low frequency motion in proteins: A normal mode study of the bovine pancreatic trypsin inhibitor
Authors: Smith Jeremy , Cusack Stephen , Pezzeca Ulrik , Brooks Bernard , Karplus Martin
Published: July 2002

An improved understanding of the function of macromolecules of biological interest requires the characterization of their low-frequency internal fluctuations (ℏw≪kBT where kBT∼200 cm−1 at room temperature), which dominate the atomic displacements. To explore the potential or inelastic neutron scattering for supplying information about such motions, we employ the formalism of Zemach and Glauber [A. C. Zemach and R. J. Glauber, Phys. Rev. 101, 118 (1956)] to calculate the one-phonon vibrational incoherent scattering function, Svibinc (q,w), of a small protein, BPTI. An in vacuo normal mode analysis of BPTI [B. Brooks and M. Karplus, Proc. Natl. Acad. Sci. U.S.A. 80, 6571 (1983)], is used for the calculation of Svibinc (q,w). The dependence of Svibinc (q,w) on energy transfer and momentum transfer is investigated and the results are placed in the context of present instrumental capabilities. Although the overall energy dependence of Svibinc (q,w) has a simple form, the variations in the

hydrogen atom displacements in the different modes have a significant effect on the scattering. Mode anisotropy is strongly manifested in oriented spectra calculated without instrumental resolution broadening. Contributions from individual atoms and residues to the unbroadened whole molecule scattering are examined in detail. They are found to depend significantly on the atom or residue examined. This suggests that inelastic neutron scattering has considerable potential for the investigation of local dynamic variations in proteins by use of a deuterated protein with specifically protonated residues.

Orientational averaging and instrumental resolution broadening of Svibinc (q,w) indicate that the one-phonon scattering detectable on the most suitable current instrument would show strong features arising principally from the frequency distribution of the vibrational modes. Multiphonon scattering is small at low momentum transfers in the low frequency range of most interest (<30 cm−1) but increases rapidly with increasing energy and momentum transfers. It is shown to originate from the lowest frequency modes. The momentum transfer dependence of the one-phonon Svibinc (q,w) is investigated. An expression is found which can be used to extrapolate the calculated one-phonon scattering to zero momentum transfer using experimentally obtainable ranges of momentum transfer. This permits the extraction of an amplitude-weighted frequency distribution function. At current resolutions this function is similar in form to the unweighted frequency distribution function, knowledge of which would be very useful for analysis of protein thermodynamics and dynamics.
High-resolution measurement of the infrared rotation–vibration spectrum of the negative molecular ion 15NH−: Isotopic shifts and equilibrium constants
Authors: Miller Harold C. , Farley John W.
Published: July 2002

A coaxial ion beam/laser beam spectrometer has been used to measure 52 transitions in the fundamental rotation–vibration band of the X 2Π electronic state of the molecular anion 15NH−. Infrared P, Q, and R-branch transitions (2950–3320 cm−1) are excited by a color center laser beam to autodetaching states, and the fast neutral NH is detected. The uncertainty (1 SD) is 0.01 cm−1. The spectroscopic constants returned by a least-squares fit to an effective Hamiltonian have been combined with our previous results on 14NH− to yield equilibrium vibrational constants ωe and ωexe for both isotopes (cm−1): ωe (14NH−)=3191.5±2.1, ωexe (14NH−)=85.6±1.9, ωe (15NH−)=3184.4±2.1, and ωexe (15NH−)=85.2±1.9. There is good agreement with the theoretical calculation for 14NH− by Manz, Zilch, Rosmus, and Werner: ωe=3179, ωexe=89. The electron affinity has been measured: EA(15NH)=0.3761±0.0025 eV.
Deuterium NMR study of methyl group dynamics in L-alanine
Authors: Beshah Kebede , Olejniczak Edward T. , Griffin Robert G.
Published: July 2002

Deuterium quadrupole echo spectroscopy is used to study the dynamics of the CD3 group in polycrystalline L-alanine-d3. Temperature-dependent quadrupole echo line shapes, their spectral intensities and τ dependence, and the anisotropy of the 2H spin-lattice relaxation were employed to determine the rate and mechanism of the –CD3 group motion. The rigid lattice Pake pattern observed at low temperature (T<−120 °C) transforms to a triplet spectrum characteristic of threefold jumps in the intermediate exchange regime (−120 to −70 °C) and this in turn to a Pake pattern of reduced breadth at higher temperatures (T>−70 °C). The quadrupole echo line shapes and their τ dependence, which are especially sensitive to the rate and mechanism of the motion, can be simulated quantitatively with the threefold jump model. We find Ea=20.0 kJ/mol for this process which is higher than is observed for most methyl groups, probably because of steric crowding in the L-Ala molecule. Finally, we observe a line shape due to the presence of multiple crystallographic forms which suggests that this technique can be extended to studies of the dynamics of more complex systems.
The microwave spectra of CO in the electronically excited states (a 3Πr and a′ 3Σ+)
Authors: Yamamoto Satoshi , Saito Shuji
Published: July 2002

The rotational transitions of CO in the electronically excited states (a 3Πr and a′ 3Σ+) are observed in the millimeter and submillimeter wave regions. The vibrational satellites are detected up to v=6 for the a 3Πr state and up to v=2 for the a′ 3Σ+ state. The observed spectra are found to be perturbed by the interaction between the a 3Πr and a′ 3Σ+ states, and they are not fitted to conventional formulations for the 3Πr and 3Σ+ states within the experimental uncertainties. A simultaneous deperturbation analysis for the a 3Πr (v=0–7) states and the a′ 3Σ+(v=0–3) states is carried out by using the observed transition frequencies and the reported data of the Λ-type doubling rf transitions in the a 3Πr state. The detailed molecular constants are obtained for the a 3Πr and a′ 3Σ+ states. These constants show smooth change on the vibrational quantum number. This indicates that they are almost free from the interaction between the a 3Πr and a′ 3Σ+ states.
Brownian dynamics simulation of a lipid chain in a membrane bilayer
Authors: Pastor Richard W. , Venable Richard M. , Karplus Martin
Published: July 2002

A Brownian dynamics simulation of a lipid chain is used to model the motional properties of a dipalmitoyl phosphatidylcholine bilayer. The effects of the bilayer environment on the chain are represented by a mean field derived from an extension of the Marcelja model. The simulation was run 44 million steps, the equivalent of approximately 0.66 μs for a viscosity of 2.2 cp. The results are compared with those of a 30 million step simulation of the chain in the absence of the mean field. Deuterium order parameters for the methylene groups along the chain and the average chain length calculated from the mean field trajectory are shown to converge to the experimentally determined values for DPPC with an appropriate choice of parameters. An analysis of the torsional dynamics of the chain, including transition rates and kink probabilities, is carried out. It is demonstrated that kink formation is sometimes, though not always, concerted. A comparison of the membrane and free chain simulations implies that the internal dynamics of a hydrocarbon chain in a bilayer is very similar to that of a neat alkane. Thus, the significant limits on the orientational freedom of the chain, as expected by the nonzero order parameters, are induced by a potential that has only a small effect on the local motions.
The unrestricted natural orbital–complete active space (UNO–CAS) method: An inexpensive alternative to the complete active space–self-consistent-field (CAS–SCF) method
Authors: Bofill Josep M. , Pulay Peter
Published: July 2002

Full configuration interaction in the space of fractionally occupied unrestricted Hartree–Fock natural orbitals is proposed as an inexpensive alternative to the complete active space–self-consistent-field method for ground states. In most cases, this method requires only a fraction of the computational effort of the latter and gives comparable results, as demonstrated on several examples. Gradient evaluation, while more complex in formulation, is computationally not significantly more expensive than unrestricted Hartree–Fock gradients.
Nonequilibrium solvation effects on reaction rates for model SN2 reactions in water
Authors: Gertner Bradley J. , Wilson Kent R. , Hynes James T.
Published: July 2002

Molecular dynamics (MD) simulations of the model SN2 reaction Cl−+CH3Cl→ClCH3+Cl− in water, and variants thereof, are presented. The resulting transmission coefficients κ, that measure the deviations of the rates from the transition state theory (TST) rate predictions due to solvent-induced recrossings, are used to assess the validity of the generalized Langevin equation (GLE)-based Grote–Hynes (GH) theory. The GH predictions are found to agree with the MD results to within the error bars of the calculations for each of the 12 cases examined. This agreement extends from the nonadiabatic regime, where solvent molecule motions are unimportant and κ is determined by static solvent configurations at the transition state, into the polarization caging regime, where solvent motion is critical in determining κ. In contrast, the Kramers theory predictions for κ fall well below the simulation results. The friction kernel in the GLE used to evaluate the GH κ values is determined, from MD simulation, by a fixed-particle time correlation function of the force at the transition state. When this is expressed as a (Fourier) friction spectrum in frequency, marked similarities to the pure solvent spectrum are observed, and are used to identify the water solvent motions that determine the transmission coefficient κ. The deviations of κ from unity, the TST value, are dominated by solvent motions (translational and reorientational) which on the time scale of the recrossings are essentially static configurations. The deviations from the frozen solvent, nonadiabatic limit values κNA are dominated by the hinderd rotations (librations). Finally, the underlying assumptions of the GLE and the GH theory are discussed within the context of the simulation results.
Microscopic theory of reversible pressure broadening in hole-burning spectra of impurities in glasses
Authors: Laird Brian B. , Skinner J. L.
Published: July 2002

Recently, Sesselmann et al. [Phys. Rev. B 36, 7601 (1987)] have examined the effect of pressure changes on hole-burning spectra of dye molecules in polymer glasses, finding that the hole shift and broadening are linear in the pressure change and its magnitude, respectively. We develop a statistical, microscopic theory of this effect, and of the inhomogeneous line shape itself. In the limit that the density of solvent perturbers becomes large, the general theory predicts that both the inhomogeneous line shape and the hole shape after a pressure change will be Gaussian, in qualitative agreement with experiment. By considering a specific model for the solute–solvent interaction, we then provide a quantitative analysis of the experiments that shows that the pressure broadening is due to changes in the local environment of each chromophore. As a further test of the theory, we make a prediction as to the frequency dependence of the pressure-dependent hole shift, which can be easily tested by experiment.
Theoretical studies of the first- and second-row transition-metal methyls and their positive ions
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry , Barnes Leslie A.
Published: July 2002

The metal–carbon bond dissociation energies (D0) and geometries for the first- and second-row transition-metal methyl neutrals and positive ions are determined. The computed D0 values for the positive ions compare favorably with experiment, except for RuCH+3, RhCH+3, and PdCH+3 where the experimental values are 10–15 kcal/mol larger. The computed D0 values for the hydride and methyl positive ions are similar for all metals in both transition rows except for Cu and Ag. However, for the neutral systems the D0 values for the methyls are smaller, especially on the right-hand side of both transition rows where the differences approach 15 kcal/mol. In general, the dissociation energies do not follow simple trends, as the individual D0 values are significantly affected by the relative spacings between the atomic states of the metal. The study of all of the methyl neutral and ions of both transition rows presented here provides a consistent set of data for the dissociation energies, thereby allowing a critical assessment of the experimental data for these molecular species, and an enhanced understanding of the different bonding mechanisms.
The ice/water interface: A molecular dynamics simulation using the simple point charge model
Authors: Karim Omar A. , Kay Paul A. , Haymet A. D. J.
Published: July 2002

The ice/water interface is investigated using the simple point charge (SPC) model for water. Molecular dynamics simulations at constant NVE demonstrate that the model interface is stable on the time scale of at least 300 ps. The density profile and mean square displacement of molecules are measured, and the results are nearly identical to those obtained earlier using the TIP4P model for water. This is additional evidence that the properties measured in the simulation are intrinsic to the ice/water interface and do not depend on the particular model potentials employed.
Predissociation linewidths of the (1,0)–(12,0) Schumann–Runge absorption bands of O2 in the wavelength region 179–202 nm
Authors: Cheung A. S.-C. , Yoshino K. , Esmond J. R. , Chiu S. S.-L. , Freeman D. E. , Parkinson W. H.
Published: July 2002

A nonlinear least-squares method of retrieving predissociation linewidths from the experimental absolute absorption cross sections of Yoshino et al. [Planet. Space Sci. 31, 339 (1983)] has been applied to the (1,0)–(12,0) Schumann–Runge bands of oxygen. Predissociation linewidths deduced for the Schumann–Runge bands are larger than the theoretical predictions of Julienne [J. Mol. Spectrosc. 63, 60 (1976)] and the latest measurements of Lewis et al. [J. Quant. Spectrosc. Radiat. Transfer 36, 187 (1986)]. The larger linewidths found will have an impact on calculations of solar flux penetration into the Earth’s atmosphere and of the photodissociation rates of trace species in the upper atmosphere. Systematic variation of predissociation linewidths with rotational quantum number is observed in the bands (v′,0) with v′=6, 8, 9, 11, and 12.
Singlet–triplet energy gaps in fluorine-substituted methylenes and silylenes
Authors: Shin Seung Koo , Goddard William A. , Beauchamp J. L.
Published: July 2002

We report singlet and triplet state splittings (ΔEST) for fluorine-substituted methylenes and silylenes using dissociation-consistent configuration interaction (CI) (based on generalized valence bond wave functions). These relatively simple CI calculations emphasize correlation consistency between the singlet and triplet states. Values of ΔEST for CH2, CF2, SiH2, and SiF2 are in excellent agreement with available experimental results, and we expect the predictions for the other cases CHF (14.5) and SiHF (41.3) to be equally accurate. This result strongly suggests that the correct choice among the experimental values for ΔEST of CHF is 14.7±0.2 kcal/mol.
Effective core potentials and accurate energy curves for Cs2 and other alkali diatomics
Authors: Krauss M. , Stevens W. J.
Published: July 2002

Energy curves of Cs2 that correlate to the ground (SS) and first excited asymptote (SP) are calculated using compact effective potentials (CEP) and core polarization potentials (CPP) which reduce the alkali atom to a single valence electron. Dissociation energies and equilibrium internuclear separations are in good agreement with experimental values. The long-range properties of the energy curves are analyzed to determine the region where the chemical interactions begin. Analogous energy curves and spectroscopic constants are obtained for the Rb2 molecule. The ground state singlet and triplet energy curves are also determined for K2. For completeness, the ground state spectroscopic constants are also reported for the Li and Na neutral and cation homonuclear diatomic molecules to illustrate the accuracy of the CEP and CPP for all the alkali atoms. Both doublet and quartet energy curves of the homonuclear anions also were examined. The dissociation energies and electron detachment energies of the doublet ground state for Rb−2 and Cs−2 are in good agreement with experiment. An assignment of the photoelectron spectra of Cs−2 is possible from the electronic structure of the ground state and the excitation energies of the neutral states. Quartet excited states of Cs−2 are calculated to be bound relative to the 3Σ+u neutral state but are metastable with respect to the ground 1Σ+g state. The accuracy of the ionic energy curves shows that the CEP and CPP are transferrable to the ionic systems.
Bifurcation structure of the classical Morse oscillator
Authors: Knop W. , Lauterborn W.
Published: July 2002

The classical damped and driven Morse oscillator is investigated with the new tools of nonlinear dynamics. Bifurcation diagrams, fixed-point diagrams, and phase diagrams are given for two damping constants. The fixed-point diagrams include the unstable branches and reveal the connection between bifurcations as well as their recurrent structure.