bluuebunny/crossref_metadata_2025 huggingface dataset:
Noncollinear spin states and competing interactions in half-metals and magnetic perovskites
Authors: Skomski R. , Zhou J. , Dowben P. A. , Sellmyer D. J.
Published: May 2005

The high-field spin structure of magnetic perovskites and related magnetoresistive materials is investigated by model calculations. Competing exchange as well as real-structure-dependent random field, random anisotropy, and Dzyaloshinskii–Moriya interactions yield a noncollinear magnetic structure that may be called a spin colloid. The noncollinear structure, which contributes to the zero- and finite-temperature spin mixing and reduces the magnetoresistance, is strongly field dependent.
Magnetic field and external-pressure effect on ferroelectricity in manganites: Comparison between GdMnO3 and TbMnO3
Authors: Noda K. , Nakamura S. , Nagayama J. , Kuwahara H.
Published: May 2005

We have investigated dielectric properties in Mott insulators GdMnO3 and TbMnO3 under magnetic fields and external quasihydrostatic pressures. In the case of GdMnO3, thermal hysteresis for dielectric constant ε and discontinuous lattice distortion were observed at ferroelectric transition temperature (TC), and ferroelectric spontaneous polarization was suppressed by the application of external pressure. These results indicate that the ferroelectric transition in GdMnO3 is a first-order displacive-type one. On the other hand, the thermal hysteresis and discontinuous lattice striction were not observed at TC in TbMnO3. The peak of ε corresponding to the ferroelectric transition was shifted toward higher temperatures by the application of external pressure in TbMnO3. The ferroelectric transition of TbMnO3 was thought to be a second-order order–disorder-type one.
Nanoconstricted structure for current-confined path in current-perpendicular-to-plane spin valves with high magnetoresistance
Authors: Fukuzawa H. , Yuasa H. , Koi K. , Iwasaki H. , Tanaka Y. , Takahashi Y. K. , Hono K.
Published: May 2005

We have successfully observed a nanoconstricted structure for current-confined-path (CCP) effect in current-perpendicular-to-plane-giant-magnetoresistance (CPP-GMR) spin valves. By inserting an AlCu nano-oxide layer (NOL) formed by ion-assisted oxidation (IAO) between a pinned layer and a free layer, the MR ratio was increased while maintaining a small area resistance product (RA). The cross-sectional high-resolution transmission electron microscopy image of the sample with RA=380mΩμm2, ΔRA=16mΩμm2, and MR ratio=4.3% showed that an amorphous oxide layer is a main part of the NOL that blocks the electron conduction perpendicular to plane. Some parts of the NOL are punched through crystalline, metallic channels having a diameter of a few nanometers, which are thought to work as nanoconstricted electron conduction paths between the pinned layer and the free layer. Nano-energy-dispersive-x-ray-spectrum analysis also showed that Cu is enriched in the metallic channels, whereas Al is enriched in the amorphous oxide region, indicating that the metallic channel is made of Cu and the oxide is made of Al2O3. The nanoconstricted structure with good segregation between the metallic channel and the oxide layer enables us to realize a large MR ratio in CCP-CPP spin valves.
Microscopic study of magnetostatic spin waves
Authors: Rivkin K. , DeLong L. E. , Ketterson J. B.
Published: May 2005

A relatively new method is developed to numerically calculate the spin-wave-related properties of a magnetic body of arbitrary shape. Starting with a discrete dipole approximation and the linearized Landau–Lifshitz equation, the resonant frequencies and the associated amplitudes of the individual moments are obtained for all modes; from this information we are able to calculate the energy absorbed by the various modes excited by a position- and time-dependent external magnetic field. The method has been demonstrated for a number of cases including thin disks and rings and for equilibrium configurations ranging from the saturated high-field limit to the vortex states at low fields.
A spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) with a ferromagnetic semiconductor for the channel
Authors: Sugahara Satoshi , Tanaka Masaaki
Published: May 2005

We propose and theoretically analyze a metal-oxide-semiconductor field-effect-transistor type of spin transistor (spin MOSFET) employing a ferromagnetic semiconductor (FS) for the channel. A ferromagnetic Schottky junction between the FS channel and a ferromagnetic metal (FM) is used for the source and drain. The output characteristics of the spin MOSFET depend on the relative magnetization configuration of the FS channel and FM source/drain. A large magnetocurrent ratio can be obtained and it is insensitive to the drain-source bias conditions, owing to the spin-filter effect of the FS/FM Schottky junction. Furthermore, excellent transistor performance, such as high transconductance and small subthreshold swing, is predicted. A new nonvolatile memory architecture using a single spin MOSFET cell is also presented, in which the programming current can be drastically reduced using the electrical manipulation of magnetization reversal of the FS channel.
Three-dimensional (3D) eddy current time-domain integral equation with Coulomb gauge condition
Authors: Kwon O-Mun , Chari M. V. K. , Salon Sheppard J.
Published: May 2005

A method is proposed to find eddy currents in massive bodies with relative motion. An integral equation solution in the time domain is proposed in this paper. This method can also be applied to magnetic-resonance imaging magnets to study transient effects occurring due to the sudden application of current. These effects may include perturbations of the homogeneity of the main field or the far field.
Mn-doped Cu2O thin films grown by rf magnetron sputtering
Authors: Pan Liqing , Zhu Hao , Fan Chongfei , Wang Weigang , Zhang Yue , Xiao John Q.
Published: May 2005

Cu 2 O has excellent optical properties and could be a good host semiconductor for dilute magnetic semiconductors because doped Cu2O is a p type and direct wide-band-gap semiconductor. In this work, Mn-doped Cu2O thin films were deposited on thermally oxidized silicon substrates by radio-frequency magnetron sputtering. X-ray diffraction results show that highly (200) oriented (Mn0.06Cu0.94)2O films can be achieved at elevated substrate temperature. X-ray photoelectronic spectroscopy confirms that the valence of copper in the films is only +1, and no other valence states. The Mn-doped Cu2O films show primary paramagnetic behaviors above 25K. Very weak ferromagnetic property interspersed with paramagnetic phase appears near 5K. The high magnetic moment of 5.3μB per Mn ion and high resistivity suggest the valence state of manganese is mainly +1.
Effect of grain size on cluster size
Authors: Endo Hiraku , Uesaka Yasutaro , Nakatani Yoshinobu , Hayashi Nobuo , Fukushima Hiroshi
Published: May 2005

A cluster size of a perpendicular medium was calculated from the correlation between grains in M-H loops. The cluster size was affected by physical properties as follows. When grain size is small and exchange stiffness constant A is not zero, the cluster size increases with decreasing grain-size. The increment of the cluster size decreases with increasing grain size distribution. The cluster size divided by the average grain size (normalized cluster size) increases with an increasing intergranular exchange coupling constant he. The normalized cluster size can be estimated from the grain-size distribution and the intergranular exchange coupling constant.
Epitaxial PtMn∕NiFe exchange-biased bilayers containing directly deposited ordered PtMn
Authors: Choi Young-suk , Petford-Long Amanda K. , Ward Roger C. C.
Published: May 2005

Pt Mn ∕ Ni Fe exchange-biased bilayers have been grown epitaxially on Si (001) using molecular-beam epitaxy. Spontaneous formation of the chemically ordered face-centered-tetragonal phase of PtMn layer was confirmed without postgrowth magnetic-field annealing, whose Néel axis is perpendicular to the PtMn∕NiFe interface. The exchange anisotropy field stabilizes above a PtMn thickness of 15nm which is much lower than that for polycrystalline PtMn-based exchange-biased systems. For comparison, PtMn∕NiFe exchange-biased bilayers have been prepared epitaxially on MgO (001) substrate. Spontaneous formation of the chemically ordered PtMn layer was also confirmed with Néel axis parallel to the PtMn∕NiFe interface. The exchange anisotropy field of the bilayer on MgO stabilizes beyond a PtMn thickness of 15nm as well.
The effect of thermal fluctuation on tilted perpendicular media
Authors: Cheng X. Z. , Jalil M. B. A.
Published: May 2005

We investigated via micromagnetic simulation the effect of thermal fluctuation on the stability and transition width of a tilted media with easy axis oriented at 45° and 135° with respect to the vertical along the down-track direction. A dynamic model of the recording process is presented which incorporates a Westmijze head field rotated by 90°. Thermal fluctuations are modeled via a stochastic Landau–Lifshitz–Gilbert equation with a temperature-dependent white-noise field. The dependence of the transition parameter a between adjacent bits and signal-to-noise ratio on temperature is investigated, and comparison between these values of the perpendicular medium and those of the two types of tilted media is performed.
Transport properties in chromium-doped Ti2O3 thin films
Authors: Wang Zhenjun , Tang Jinke , Spinu Leonard
Published: May 2005

In this paper, we report the transport properties of Cr-doped Ti2O3 thin films. The thin films were grown on α-Al2O3 (012) substrates by pulsed-laser deposition. X-ray diffraction and transmission electron microscopy results show that the films are single corundum phase. All of (CrxTi1−x)2O3 show semiconducting behavior. Without doping, pure Ti2O3 thin films show positive magnetoresistance (MR) of 23% at 2K. The MR behavior changed dramatically after doping with Cr. Sample (Cr0.1Ti0.9)2O3 shows MR=−360% at 2K. All of the Cr-doped films are ferromagnetic up to room temperature.
Magnetic properties of a spin system in a longitudinal magnetic field
Authors: Jiang Wei , Bai Bao-Dong , Lo Veng-Cheong , Liu Wei
Published: May 2005

The hysteresis loops and susceptibility of spin system with the crystal field have been studied within the framework of the effective-field theory with the differential technique. The effects of the external longitudinal magnetic field on the magnetization and susceptibility are discussed in detail. Numerical results are performed and analyzed for the cases of the honeycomb and square lattice. A number of interesting phenomena have been found due to the applied longitudinal magnetic field, such as, the shape of hysteresis loops and susceptibility are dependent on the longitudinal magnetic field and crystal field. The peak of susceptibility of the honeycomb lattice is bigger than that of the square lattice when the parameters are the same in the two systems.
Observation of charge ordering and the ferromagnetic phase transition in single crystal LSMO using rf transverse susceptibility
Authors: Woods G. T. , Poddar P. , Srikanth H. , Mukovskii Ya. M.
Published: May 2005

We performed temperature and field-dependent reversible transverse susceptibility (TS) measurements on a La0.85Sr0.15MnO3 single crystal at radio frequency (10MHz) using a resonant tunnel-diode oscillator technique. These dynamic experiments probe the coupled electronic, magnetic and structural transition in LSMO giving rise to the ferromagnetic transition and the charge-ordered phase. The effective magnetic anisotropy fields are directly probed across the entire range from the Curie temperature to lower temperatures well into the charge-ordered state. Overall, the TS experiments are demonstrated to be useful for studying magnetic anisotropy in the presence of structural and electronic transitions in these strongly coupled systems.
Electrically controlled exchange bias for spintronic applications
Authors: Binek Ch. , Hochstrat A. , Chen X. , Borisov P. , Kleemann W. , Doudin B.
Published: May 2005

Exchange coupling between a magnetoelectric (111)-oriented Cr2O3 single crystal and a CoPt multilayer with perpendicular anisotropy exhibits an exchange bias field proportional to the applied axial electric field. Extrapolation from bulk to thin film magnetoelectric pinning system suggests promising spintronic applications due to coupling between the electric field-controlled magnetization and the magnetization of a neighbor ferromagnetic layer. Pure voltage control of magnetic configurations of tunneling magnetoresistance spin valves is an attractive alternative to current-induced magnetization switching.
Magnetic-flux induced persistent currents in quasiperiodic mesoscopic rings
Authors: Hu X. F. , Peng R. W. , Cao L. S. , Huang X. Q. , Wang Mu , Hu A. , Jiang S. S.
Published: May 2005

Under the tight-binding approximation, we study the persistent current (PC) in the magnetic-flux-threaded mesoscopic ring which is constructed according to the Fibonacci-class model. It is shown that the energy spectra form band structures and the subbands present a self-similarity. The PC is determined by the magnetic flux, the site energy, and the Fermi level. It is found that the large PC can be observed if the Fermi level reaches a specific value. Our investigation provides detailed information on the structural influence on the PC and contributes a mechanism to understand the large PC observed in the experiments.
Magnetoresistance in amorphous TbFeCo films with perpendicular magnetic anisotropy
Authors: Rahman M. Tofizur , Liu Xiaoxi , Matsumoto Mitsunori , Morisako Akimitsu , Meguro Sakae , Akahane Koichi
Published: May 2005

The magnetoresistance of TbFeCo amorphous films with perpendicular magnetic anisotropy have been studied at room temperature. TbFeCo films within a thickness range of 50–300nm were deposited by a dc-magnetron sputtering. Sharp change in magnetoresistance (MR) has been found at the magnetization reversal region. The MR ratio is sensitively dependent on the film thickness and the magnetic domain pattern. Films with smaller domain pattern show higher MR ratio. The MR ratio is decreased with the decrease in film thickness. It may be considered that the observed MR is contributed from the domain wall. The maximum MR ratio is about 3.7% for the film with thickness of 300nm at room temperature. No MR has been found when the magnetic field is applied parallel to the film plane.
Modeling of giant magnetoresistance isolator for high speed digital data transmission utilizing spin valves
Authors: Park S. , Kim J. , Jo S.
Published: May 2005

Giant magnetoresistance isolator was modeled for effective transmission of high speed digital data, and the output voltage characteristics depending on pulse width of input signal, inductor coil turn, and existence of external circuit of the planar inductor were investigated in time domain. The planar coil was modeled as a lumped element R,L,C circuit and the coil current was calculated using SPICE. The corresponding magnetic field was substituted to the measured magnetoresistance-H curve to obtain the resistance values of the spin valves, which were used to obtain the output voltage wave form of the isolator. Effective maximum transmission speed of 250Mbit∕s was estimated. Even though the coil current wave form was much deformed with damping, the transmitted output voltage wave form was very close to the rectangular wave form of the input signal due to hysteretic characteristics of the spin valves.
Superparamagnetic behavior of cobalt nanodots on Al2O3(0001)
Authors: Espinosa J. , Shi H. , Lederman D.
Published: May 2005

Submonolayer ultrathin films of Co were grown on Al2O3(0001) via molecular beam epitaxy at room temperature. In situ noncontact atomic force microscopy showed that the as-grown sample was relatively smooth. Images obtained after annealing the film in situ at 400°C revealed the formation of small dots with an average diameter of ≈17.6nm and a height of 0.39nm. The magnetization as a function of temperature for zero-field cooling with the magnetic field parallel to the surface of the sample provided the evidence of superparamagnetic behavior of the nanoparticles with a blocking temperature of ≈130K. The average magnetic moment per atom of the particles was estimated by adjusting the Langevin function for the magnetic hysteresis loop measured at room temperature with the field in the plane of the sample, and the result is in good agreement with the bulk cobalt moment.
Effect of Ni100−xCrx seedlayer on transport and magnetic properties in PtMn-based spin valves
Authors: Tsunekawa K. , Nagai M. , Djayaprawira D. D. , Watanabe N.
Published: May 2005

The effect of cosputtered Ni100−xCrx seedlayer on transport and magnetic properties in PtMn-based synthetic spin valves has been studied. We found that magnetoresistance (MR) ratio increases with increasing Cr content, and reached over 15% when the Cr content is in the range of 38–41at.%. The MR ratio drastically decreases to less than 2% when the Cr content is higher than 41at.%. We found that the dependence of transport and magnetic properties on the Cr content of the NiCr seed layer is strongly correlated with the crystallographic orientation of the PtMn and CoFe∕Ru∕CoFe∕Cu∕CoFe∕NiFe∕Cu layers grown on the seedlayer.
Influence of antiphase boundary density on the conduction noise properties of epitaxial magnetite thin films
Authors: Arora S. K. , Sofin R. G. S. , Shvets I. V. , Kumar Ravi , Khan M. Wasi , Srivastava J. P.
Published: May 2005

Low frequency conduction noise (1∕f noise) properties of epitaxial magnetite (Fe3O4) thin films having a varying density of antiphase boundaries (APBs) were investigated as a function of temperature and frequency. Temperature dependence of noise exhibits a similar behavior to that of resistivity for all the films. The magnitude of normalized noise (Sv∕V2) decreases with the increasing film thickness, which correlates well with the density of APBs. The quantitative feature of noise, i.e., Hooge parameter has a strong thickness dependence at low temperatures which implies that the APBs play an important role in determining the transport mechanism in epitaxial Fe3O4 films.
Room-temperature growth of epitaxial (111) Fe3O4 films with conductive Cu underlayer
Authors: Huang Po-Hsiang , Lai Chih-Huang , Huang R. T.
Published: May 2005

Epitaxial (111) Fe3O4 film on the Cu underlayer was obtained by using reactive ion-beam sputtering with high incident energy at room temperature. X-ray φ scans and transmission electron microscope diffraction pattern revealed unusual 12-fold symmetry of the (111) Fe3O4 films on Cu (001) underlayers due to the presence of two kinds of epitaxial (111) grains in Fe3O4 films. The saturation magnetization of the 70nm thick (111) Fe3O4 films is 291emu∕cm3. A clear Verwey transition of Fe3O4 films was observed around 116K. The root-mean-square roughness of the Fe3O4 surface is only 3.5Å.
Conductance quantization by the application of magnetic fields in ballistic Ni nanocontacts
Authors: Sekiguchi K. , Saitoh E. , Miyajima H.
Published: May 2005

We investigated the magnetoresistance effect in ballistic Ni nanocontacts in the regime of conductance quantization. In magnetoresistance curves, we found that the conductance is quantized by the application of magnetic fields. The quantization is direct evidence showing the contribution of magnetostriction. The magnetostriction has the potential to control the quantum transport and giant magnetoresistance effect.
Magnetism in the low-doping regime (x<0.50) of Bi1−xSrxMnO3 perovskites
Authors: Frontera C. , García-Muñoz J. L. , Aranda M. A. G. , Hervieu M. , Ritter C. , Calleja A. , Capdevila X. G. , Respaud M.
Published: May 2005

The magnetic properties of low-doped Bi1−xSrxMnO3 manganites (x<0.50) are investigated. Although a progressive dilution by Sr doping of the ferromagnetic ground state of pure BiMnO3 was previously proposed, it is found that compounds with x=0.25, 0.33, and 0.5 present an antiferromagnetic ground state. However, it is confirmed that a strong ferromagnetic component is easily induced under field in the compounds with x=0.25 and 0.33, both presenting the same type of charge and polaron ordering.
Ferromagnetic and electrical properties in ferromagnetic semiconductor Ti1−xCoxO2−δ:Sb thin films grown by pulsed laser deposition
Authors: Seong Nak-Jin , Lee Woo-Sung , Jung Myung-Hwa , Yoon Soon-Gil
Published: May 2005

Rutile Ti1−xCoxO2−δ:Sb thin films were grown on R-Al2O3 substrates by pulsed laser deposition and investigated for the magnetic and electrical properties with deposition temperature. The Sb doping effect in rutile Ti1−xCoxO2−δ thin films was also investigated with deposition temperature. The Ti0.97Co0.03O2−δ:Sb films exhibited a ferromagnetic property and an anomalous Hall effect at room temperature. In Ti0.97Co0.03O2−δ:Sb films deposited at 700 °C, the segregation of Co and Sb elements occurred at near interface between the films and substrate. The Sb doping effect in Ti0.97Co0.03O2−δ films deposited at 500 °C increases the charge carrier density, film resistivity, and decreases the saturation magnetization. An anomalous Hall resistivity of the films increases from 0.1 to 0.14μΩcm at 8 kOe by codoping of Sb in Ti0.97Co0.03O2−δ films.
Programmable spintronics logic device based on a magnetic tunnel junction element
Authors: Wang Jianguo , Meng Hao , Wang Jian-Ping
Published: May 2005

A programmable spintronics logic device was designed and fabricated based on a single pinned magnetic tunnel junction (MTJ) element. In this work, a current input line C passing through the MTJ element itself was introduced. Two separated input current lines (A and B) could switch the magnetization of the pinned layer under the heat assistance from line C. Full logic functions (AND, OR, NAND, NOR, XOR, and XNOR) can be realized based on a normal pinned and a synthetic pinned MTJ element. A Wheatstone bridge was engineered to read this single MTJ element logic device. MTJ elements with 1μm2 and normal pinned structure: (Ta30Å∕NiFe40Å∕MnIr35Å∕CoFe30Å∕(Al7Å)+oxidation∕CoFe30Å∕NiFe40Å∕Ta200Å), have low resistance of 6.3Ω and high resistance of 7.2Ω, which gives the magnetoresistive (MR) ratio of 14%. Approximately a 3-mV output difference is obtained between logical 1 and 0.
Evaluation of the mechanical deformation in incompressible linear and nonlinear magnetic materials using various electromagnetic force density methods
Authors: Lee Se-Hee , He Xiaowei , Kim Do-Kyung , Elborai Shihab , Choi Hong-Soon , Park Il-Han , Zahn Markus
Published: May 2005

Mechanical deformation in incompressible linear and nonlinear magnetic materials was evaluated using various conventional electromagnetic volume and surface force density methods. These conventional force density methods are the Maxwell stress tensor method, Korteweg-Helmholtz force density method (KH), magnetic charge method, magnetizing current method, and Kelvin force density method (KV). The total force values obtained using these different force density methods were found to be the same and equal to the total force using the principle of virtual work, but the distribution of force density values calculated using the given force density methods was found to be different from each other. Using the given five force density methods, the mechanical deformations were evaluated and compared to one another. The KH and KV in incompressible material were shown to give the same mechanical deformation by employing the finite element method (FEM), verifying the theoretical equivalence. To implement the KV, the derivative of magnetic field intensity with respect to the geometrical position was calculated using a linear shape function of FEM along with the nodal field values in each element. A magnetic systems was tested to compare the mechanical deformation in linear and nonlinear magnetic materials.
Coupled electrical and magnetic properties in (La,Sr)FeO3−δ
Authors: Zhou X.-D. , Cai Q. , Yang J. , Kim M. , Yelon W. B. , James W. J. , Shin Y.-W. , Scarfino B. J. , Anderson H. U.
Published: May 2005

This article is aimed at studying the temperature dependence oxygen nonstoichiometry, magnetic moments, Fe3+ fraction, Néel temperature and conductivity of (La,Sr)FeO3−δ. It is found that the magnetic properties in La0.60Sr0.40FeO3−δ is determined by Fe3+ and its concentration, and the conductance is resulted from Fe4+ ions, which act as electron holes. Both of magnetic and electrical properties were directly governed by oxygen nonstoichiometry (δ). When δ=0.2, the compound has maximum Fe3+, the saturation magnetic moments and Néel temperature are 3.8 μB and 410 °C, both at the highest level, whereas the total carrier concentration is at the minimum.
Finite element method-based calculation of the theoretical limit of sensitivity for detecting weak magnetic fields in the human brain using magnetic-resonance imaging
Authors: Hatada Tomohisa , Sekino Masaki , Ueno Shoogo
Published: May 2005

Detection of weak magnetic fields induced by neuronal electrical activities using magnetic-resonance imaging is a potentially effective method for functional imaging of the brain. In this study, we performed a numerical analysis of the theoretical limit of sensitivity for detecting weak magnetic fields induced in the human brain. The limit of sensitivity was estimated from the intensities of signal and noise in the magnetic-resonance images. The signal intensity was calculated with parameters which are commonly used in measurements of the human brain. The noise due to the head was calculated using the finite element method. The theoretical limit of sensitivity was approximately 10−8T.
First-principles studies on the reactions of O2 with silicon clusters
Authors: Li S. F. , Gong X. G.
Published: May 2005

The reactions of an O2 molecule with the neutral and positively charged Sin(n=3–16) clusters are studied with first-principles calculations. Neutral Sin(n=4,5,6,7,10,14) and charged Sin+(n=4,5,6,7,13,15) clusters show higher inertness to O2 molecule adsorption, which is in good agreement with experimental results. Both charge transfer and hybridizations between Si and O play an important role in the dissociative adsorption of O2 molecule. We find that the spin triplet-single conversion of O2 molecule is always accompanied with O2 dissociatively chemisorbed on the Sin clusters.
Following the aggregation of amyloid-forming peptides by computer simulations
Authors: Melquiond Adrien , Boucher Geneviève , Mousseau Normand , Derreumaux Philippe
Published: May 2005

There is experimental evidence suggesting that the toxicity of neurodegenerative diseases such as Alzheimer’s disease may result from the soluble intermediate oligomers. It is therefore important to characterize extensively the early steps of oligomer formation at atomic level. As these structures are metastable and short lived, experimental data are difficult to obtain and they must be complemented with numerical simulations. In this work, we use the activation-relaxation technique coupled with a coarse-grained energy model to study in detail the mechanisms of aggregation of four lys–phe–phe–glu (KFFE) peptides. This is the shortest peptide known to form amyloid fibrils in vitro. Our simulations indicate that four KFFE peptides adopt a variety of oligomeric states (tetramers, trimers, and dimers) with various orientations of the chains in rapid equilibrium. This conformational distribution is consistent with all-atom molecular-dynamics simulations in explicit solvent and is sequence dependent; as seen experimentally, the lys–pro–gly–glu (KPGE) peptides adopt disordered structures in solution. Our unbiased simulations also indicate that the assembly process is much more complex than previously thought and point to intermediate structures which likely are kinetic traps for longer chains.
Adsorption and diffusion of Mg, O, and O2 on the MgO(001) flat surface
Authors: Geneste Grégory , Morillo Joseph , Finocchi Fabio
Published: May 2005

A thorough investigation of the adsorption and diffusion of Mg, O, and O2 on MgO(001) terraces is performed by first-principles calculations. The single Mg adatom weakly binds to surface oxygens, diffuses, and evaporates easily at room temperatures. Atomic O strongly binds to surface oxygens, forming peroxide groups. The diffusion of the O adatom is strongly influenced by the spin polarization, since energy barriers are significantly different for the singlet and triplet states. The crossing of the two Born–Oppenheimer surfaces corresponding to the distinct spin states is also analyzed. Although the O2 molecule does not stick to the perfect surface, it chemisorbs on surface nonstoichiometric point defects such as O vacancies or Mg adatoms, forming in the latter case new chemical species on the surface. We show that the oxidation rate limiting factor in an O2 atmosphere is the concentration of point defects (O vacancies and Mg adatoms) in the growing surface. The simulated O core-level shifts for the various adsorption configurations enable a meaningful comparison with the measured values, suggesting the presence of peroxide ions on growing surfaces. Finally, the computed energy barriers are used to estimate the Mg and O surface lifetimes and diffusion lengths, and some implications for the homoepitaxial growth of MgO are discussed.
On the computational calculation of surface free energies for the disordered semihexagonal reconstructed Au(100) surface
Authors: Grochola Gregory , Snook Ian K. , Russo Salvy P.
Published: May 2005

Previously we developed a general method for calculating the free energy of any surface constrained to a distinct surface excess number/density. In this paper we show how to combine a range of such surfaces, whose free energies have been calculated, to produce an ad hoc semigrand canonical ensemble of surfaces from which ensemble surface properties can be calculated, including the ensemble surface free energy. We construct such an ensemble for the disordered Au(100) semihexagonal reconstructed surface using a Glue model potential at 1000 K and calculate the ensemble surface free energy to be 0.08818eV∕Å2. The ensemble average surface lateral density was found to be 1.375 (with respect to the bulk), which is in agreement with previous grand canonical Monte Carlo studies.
NMR paramagnetic relaxation due to the S=5∕2 complex, Fe(III)-(
tetra-p
-sulfonatophenyl)porphyrin: Central role of the tetragonal fourth-order zero-field splitting interaction
Authors: Schaefle Nathaniel , Sharp Robert
Published: May 2005

The metalloporphyrins, Me-TSPP [Me=Cr(III), Mn(III), Mn(II), Fe(III), and TSPP=meso-(tetra-p-sulfonatophenyl)porphyrin], which possess electron spins S=3∕2, 2, 5∕2, and 5∕2, respectively, comprise an important series of model systems for mechanistic studies of NMR paramagnetic relaxation enhancement (NMR-PRE). For these S>1∕2 spin systems, the NMR-PRE depends critically on the detailed form of the zero-field splitting (zfs) tensor. We report the results of experimental and theoretical studies of the NMR relaxation mechanism associated with Fe(III)-TSPP, a spin 5∕2 complex for which the overall zfs is relatively large (D≈10cm−1). A comparison of experimental data with spin dynamics simulations shows that the primary determinant of the shape of the magnetic relaxation dispersion profile of the water proton R1 is the tetragonal fourth-order component of the zfs tensor. The relaxation mechanism, which has not previously been described, is a consequence of zfs-induced mixing of the spin eigenfunctions of adjacent Kramers doublets. We have also investigated the magnetic-field dependence of electron-spin relaxation for S=5∕2 in the presence of a large zfs, such as occurs in Fe(III)-TSPP. Calculations show that field dependence of this kind is suppressed in the vicinity of the zfs limit, in agreement with observation.
Substrate concentration dependence of the diffusion-controlled steady-state rate constant
Authors: Dzubiella J. , McCammon J. A.
Published: May 2005

The Smoluchowski approach to diffusion-controlled reactions is generalized to interacting substrate particles by including the osmotic pressure and hydrodynamic interactions of the nonideal particles in the Smoluchoswki equation within a local-density approximation. By solving the strictly linearized equation for the time-independent case with absorbing boundary conditions, we present an analytic expression for the diffusion-limited steady-state rate constant for small substrate concentrations in terms of an effective second virial coefficient B2*. Comparisons to Brownian dynamics simulations excluding hydrodynamic interactions show excellent agreement up to bulk number densities of B2*ρ0≲0.4 for hard sphere and repulsive Yukawa-like interactions between the substrates. Our study provides an alternative way to determine the second virial coefficient of interacting macromolecules experimentally by measuring their steady-state rate constant in diffusion-controlled reactions at low densities.
Structural and magnetic properties of epitaxial L21-structured Co2(Cr,Fe)Al films grown on GaAs(001) substrates
Authors: Hirohata A. , Kurebayashi H. , Okamura S. , Kikuchi M. , Masaki T. , Nozaki T. , Tezuka N. , Inomata K.
Published: May 2005

We have successfully grown both L21 polycrystalline Co2CrAl and epitaxial L21-structured Co2FeAl films onto GaAs(001) substrates under an optimized condition. Both structural and magnetic analyses reveal the detailed growth mechanism of the alloys, and suggest that the Co2CrAl film contains atomically disordered phases, which decreases the magnetic moment per f.u., while the Co2FeAl film satisfies the generalized Slater–Pauling behavior. By using these films, magnetic tunnel junctions (MTJs) have been fabricated, showing 2% tunnel magnetoresistance (TMR) for the Co2CrAl MTJ at 5K and 9% for the Co2FeAl MTJ at room temperature (RT). Even though the TMR ratio still needs to be improved for future device applications, these results explicitly include that the Co2(Cr,Fe)Al full Heusler alloy is a promising compound to achieve half-metallicity at RT by controlling both disorder and surface structures in the atomic level by manipulating the Fe concentration.
Super- and sub-Poissonian photon statistics for single molecule spectroscopy
Authors: He Yong , Barkai Eli
Published: May 2005

We investigate the distribution of the number of photons emitted by a single molecule undergoing a spectral diffusion process and interacting with a continuous wave laser field. The spectral diffusion is modeled based on a stochastic approach, in the spirit of the Anderson–Kubo line shape theory. Using a generating function formalism we solve the generalized optical Bloch equations and obtain an exact analytical formula for the line shape and Mandel’s Q parameter. The line shape exhibits well-known behaviors, including motional narrowing when the stochastic modulation is fast and power broadening. The Mandel parameter, describing the line shape fluctuations, exhibits a transition from a quantum sub-Poissonian behavior in the fast modulation limit to a classical super-Poissonian behavior found in the slow modulation limit. Our result is applicable for weak and strong laser fields, namely, for arbitrary Rabi frequency. We show how to choose the Rabi frequency in such a way so that the quantum sub-Poissonian nature of the emission process becomes strongest. A lower bound on Q is found and simple limiting behaviors are investigated. A nontrivial behavior is obtained in the intermediate modulation limit, when the time scales for spectral diffusion and the lifetime of the excited state become similar. A comparison is made between our results and previous ones derived, based on the semiclassical generalized Wiener–Khintchine formula.
Monte Carlo simulation study of droplet nucleation
Authors: Neimark Alexander V. , Vishnyakov Aleksey
Published: May 2005

A new rigorous Monte Carlo simulation approach is employed to study nucleation barriers for droplets in Lennard-Jones fluid. Using the gauge cell method we generate the excess isotherm of critical clusters in the size range from two to six molecular diameters. The ghost field method is employed to compute the cluster free energy and the nucleation barrier with desired precision of (1–2)kT. Based on quantitative results obtained by Monte Carlo simulations, we access the limits of applicability of the capillarity approximation of the classical nucleation theory and the Tolman equation. We show that the capillarity approximation corrected for vapor nonideality and liquid compressibility provides a reasonable assessment for the size of critical clusters in Lennard-Jones fluid; however, its accuracy is not sufficient to predict the nucleation barriers for making practical estimates of the rate of nucleation. The established dependence of the droplet surface tension on the droplet size cannot be approximated by the Tolman equation for small droplets of radius less than four molecular diameters. We confirm the conclusion of ten Wolde and Frenkel [J. Chem. Phys. 109, 9901 (1998)] that integration of the normal component of the Irving–Kirkwood pressure tensor severely underestimates the nucleation barriers for small clusters.
Density functional theory of adsorption of mixtures of charged chain particles and spherical counterions
Authors: Bryk P. , Pizio O. , Sokołowski S.
Published: May 2005

We propose a microscopic density functional theory to describe nonuniform ionic fluids composed of chain molecules with charged “heads” and spherical counterions. The chain molecules are modeled as freely jointed chains of hard spheres, the counterions are oppositely charged spheres of the same diameter as all segments of chain molecules. The theory is based on the approach of Yu and Wu [J. Chem. Phys. 117, 2368 (2002)] of adsorption of chain molecules and on theory of adsorption of electrolytes [O. Pizio, A. Patrykiejew, and S. Sokołowski, J. Chem. Phys. 121, 11957 (2004)]. As an application of the proposed formalism we investigate the structure and adsorption of fluids containing segments of different length in a slitlike pore.
Characterizing the nature of virtual amorphous silicon
Authors: Choudhary Devashish , Clancy Paulette
Published: May 2005

Virtual samples of approximations to real amorphous silicon, a-Si, have been prepared using several different processing routes. These include a fast quench from the melt followed by a long slow annealing period using molecular dynamics, a Reverse Monte Carlo approach, and an ab initio minimization. The characterization of these virtual a-Si samples includes a consideration of structural data (the radial distribution function, angular order, etc.), electronic properties (through the density of states), and thermodynamic information (chiefly the nature of the phase transformation from a-Si to liquid). The properties of a-Si are compared to network models, via the continuous random network model, and to experiment. We investigated the stability of virtual a-Si and consider its implications for use in future simulation studies. We have demonstrated the necessity for the accuracy provided by ab initio-based models to describe the interatomic potentials. Throughout this study, we have monitored the role of order in determining physical properties, as characterized by traditional routes (such as angular correlations) and more novel ones (the signature cell method).
On the acceleration of the convergence of singular operators in Gaussian basis sets
Authors: Pachucki Krzysztof , Cencek Wojciech , Komasa Jacek
Published: May 2005

Gaussian type wave functions do not reproduce the interparticle cusps which result in a slow convergence of the expectation values of the operators involved in calculations of the relativistic and QED energy corrections. Methods correcting this deficiency are the main topic discussed in this paper. Benchmark expectation values of the singular operators for several few-electron systems are presented.
Angle-averaged efficiencies for linear mode conversion between Langmuir waves and radiation in an unmagnetized plasma
Authors: Cairns Iver H. , Willes A. J.
Published: May 2005

Linear mode conversion of Langmuir waves propagating in density irregularities leads to radio waves near the electron plasma frequency. Most previous work on this mechanism is for a single Langmuir wave interacting with a single planar density structure. This paper presents analytic and numerical estimates of the average conversion efficiency resulting from averaging over the distributions of the incoming Langmuir wave vectors and the orientations and length scales of density irregularities in an unmagnetized plasma. Significant mode conversion requires Langmuir wave vectors very closely aligned with the density gradient, within an angular width of order Ve∕c radians (Ve is the electron thermal speed and c the speed of light), and this width plays a major role in determining the average conversion efficiencies. Specifically, the two-dimensional (2D) averaged efficiency for planar irregularities with a uniform distribution of orientations is a factor of order Ve∕(2πc) smaller than the peak values ∼50% in the mode conversion window. Moreover, the analogous 3D average efficiency is estimated to be a factor Ve2∕(2π2c2) smaller than the peak values. For coronal and interplanetary values of Ve, the 2D and 3D average efficiencies are then 10−2–10−3 and 10−5–10−6, respectively. Appropriate averaging is thus vital when predicting the efficiency of linear mode conversion and inferring what mechanisms produce radio phenomena in space and the laboratory.
Statistical mechanics based model for negative bias temperature instability induced degradation
Authors: Zafar Sufi
Published: May 2005

A physics based model for negative bias temperature instability (NBTI) induced degradation is proposed. Like previous models, this model attributes NBTI to depassivation of Si–H bonds at the SiO2∕Si interface. The two distinguishing features of the proposed model are: (i) statistical mechanics is applied to calculate the decrease in interfacial Si–H density as a function of stressing conditions, and (ii) hydrogen diffusion in the oxide is assumed to be dispersive and the diffusing species is identified with the positively charged hydrogen ion (Hi+). The model assumes that as Hi+ diffuses away from the interface into the oxide, interfacial and bulk traps are created. Based on these model assumptions, an equation for the threshold voltage shift (ΔVt) is derived as a function of stressing time, oxide field, temperature, oxide thickness, and initial Si–H density at the interface. The model predicts that ΔVt would increase with a power law dependence at earlier stressing times and would saturate at longer times. The power law increase at earlier stress times is attributed to dispersive hydrogen diffusion and ΔVt saturation at longer stress times is ascribed to occur when all bonded hydrogen (Si–H) has been removed from the interface. These and other model predictions are verified using published NBTI data from various research groups for p-channel field effect transistors (pFETs). In addition, the model is shown to be compatible with NBTI data for HfO2 pFETs.
Efficient coupling of thermal electron Bernstein waves to the ordinary electromagnetic mode on the National Spherical Torus Experiment
Authors: Taylor G. , Efthimion P. C. , LeBlanc B. P. , Carter M. D. , Caughman J. B. , Wilgen J. B. , Preinhaelter J. , Harvey R. W. , Sabbagh S. A.
Published: May 2005

Efficient coupling of thermal electron Bernstein waves (EBW) to ordinary-mode (O-mode) electromagnetic radiation has been measured in plasmas heated by energetic neutral beams and high harmonic fast waves in the National Spherical Torus Experiment (NSTX) [M. Ono, S. Kaye, M. Peng et al., Proceedings of the 17th IAEA Fusion Energy Conference, edited by S. Spak (IAEA, Vienna, Austria, 1999), Vol. 3, p. 1135]. The EBW to electromagnetic mode coupling efficiency was measured to be 0.8±0.2, compared to a numerical EBW modeling prediction of 0.65. The observation of efficient EBW coupling to O mode, in relatively good agreement with numerical modeling, is a necessary prerequisite for implementing a proposed high power EBW current drive system on NSTX.
Wave packets in a bifurcating region of an energy landscape: Diels-Alder dimerization of cyclopentadiene
Authors: Lasorne B. , Dive G. , Desouter-Lecomte M.
Published: May 2005

Quantum dynamics in a valley ridge inflection (VRI) point region is analyzed in the case of the Diels-Alder endo-dimerization of cyclopentadiene pointed out recently by [Caramella et al., J. Am. Chem. Soc. 124, 1130 (2002)]. The VRI point is located along the reaction path connecting the bispericyclic symmetrical transition structure put in evidence by Caramella et al. and the transition state of the Cope rearrangement. Dynamics is carried out by using constrained Hamiltonian methodology. The active coordinates are the first formed C–C bond length and the difference between the two other C–C bond lengths which achieve the dimerization as 4+2 or 2+4 adducts. A two-dimensional (2D) minimum-energy surface have been computed at the Becke 3 Lee–Yong–Parr∕6-31G* level. The energy landscape can be classified as an uphill ridge-pitchfork VRI bifurcation according to a recent classification of bifurcation events [W. Quapp, J. Mol. Struct. 695–696, 95 (2004)]. Dynamics does not describe the thermal reaction but concerns wave packets which could be prepared by pulse reagents, i.e., by coherent control. We analyze how the shape and initial location on the ground potential-energy surface are linked to the synchronous or asynchronous mechanism of the final step after the first transition state. We use a one-dimensional model of optimum control theory to check the feasibility of such a coherent preparation. The wave-packet evolution in the VRI domain is well explained by semiclassical predictions even with the negative curvature of the unstable ridge. Finally, a crude model of dissipation has been introduced to test the stability of the 2D predictions.
Quantum diffusion in liquid para-hydrogen from ring-polymer molecular dynamics
Authors: Miller Thomas F. , Manolopoulos David E.
Published: May 2005

We have used the ring-polymer molecular dynamics method to calculate approximate Kubo-transformed velocity autocorrelation functions and self-diffusion coefficients for low-pressure liquid para-hydrogen at temperatures of 25 and 14 K. The resulting diffusion coefficients are shown to be consistent with experimental shear viscosities and the established finite-size relation D(L)≃D(∞)−2.837kBT∕6πηL, where kB is the Boltzmann constant, T the absolute temperature, η the shear viscosity, and L the length of the (cubic) simulation cell. The diffusion coefficients D(L) obtained in simulations with finite system sizes are therefore too small. However, the extrapolation to infinite system size corrects this deficiency and leads to excellent agreement with experimental results. This both demonstrates the influence of system-size effects on quantum mechanical diffusion coefficients and provides further evidence that ring-polymer molecular dynamics is an accurate as well as practical way of including quantum effects in condensed phase molecular dynamics.