bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-dimensional magnetic properties of soft magnetic composite material at different frequencies
Authors: Li Yongjian , Lin Zhi Wei , Liu Hongxun , Wang Yi , Guo Youguang , Zhu Jianguo , Yang Qingxin
Published: March 2011

Due to their intrinsic isotropic magnetic properties, soft magnetic composite (SMC) materials are applied in electrical machines in which the magnetic energy can be transported in three dimensions (3-D). However, in real applications, complicated magnetic properties such as anisotropy and nonlinearity, are found, in particular, at ahigh frequency range. This paper studies the 3-D magnetic properties of SMC materials under complicated magnetizations, such as circular, elliptical, and spherical excitations. The magnetic flux density vector B loci, magnetic field strength vector H loci, and core losses at magnetization frequencies ranging from 50–1000 Hz were measured and discussed using an improved 3-D testing system. Experimental results show that rotational core losses are greater than alternating losses at the same magnitude of flux density. In addition, rotational loss increases sharply around the saturation point, but is not observed in alternating loss.
Spin transition in Gd3N@C80, detected by low-temperature on-chip SQUID technique
Authors: Chen L. , Carpenter E. E. , Hellberg C. S. , Dorn H. C. , Shultz M. , Wernsdorfer W. , Chiorescu I.
Published: March 2011

We present a magnetic study of the Gd3N@C80 molecule, consisting of a Gd-trimer via a nitrogen atom, encapsulated in a C80 cage. This molecular system can be an efficient contrast agent for magnetic resonance imaging (MRI) applications. We used a low-temperature technique able to detect small magnetic signals by placing the sample in the vicinity of an on-chip SQUID. The technique implemented at the National High Magnetic Field Laboratory has the particularity of being able to operate in high magnetic fields of up to 7 T. The Gd3N@C80 shows a paramagnetic behavior and we find a spin transition of the Gd3N structure at 1.2 K. We perform quantum mechanical simulations, which indicate that one of the Gd ions changes from a 8S7/2 state (L = 0, S = 7/2) to a 7F6 state (L = S = 3, J = 6), likely due to a charge transfer between the C80 cage and the ion.
Demagnetizing factors for two parallel ferromagnetic plates and their applications to magnetoelectric laminated sensors
Authors: Liverts E. , Grosz A. , Zadov B. , Bichurin M. I. , Pukinskiy Y. J. , Priya S. , Viehland D. , Paperno E.
Published: March 2011

An analytical expression is derived to approximate the magnetometric demagnetizing factors for two parallel ferromagnetic plates having the shape of rectangular prisms. The magnetometric demagnetizing factors relate the average magnetic fields within the plates’ volumes to an external magnetic field. Knowing this relationship is essential for describing the response of magnetoelectric sensors comprising two parallel magnetostrictive plates. It is shown that two separate ferromagnetic layers provide better field sensitivity than a single layer with a doubled thickness. The obtained results are in a good agreement with numerical calculations and experimental data.
Time evolution of spin accumulation induced from electric field in ferromagnet
Authors: Hosono Kazuhiro , Nozaki Yukio , Yamaguchi Akinobu , Tatara Gen
Published: March 2011

The spin transport induced from an electric field in the ferromagnet with a uniform magnetization is studied microscopically. Taking into account the time dependence of the electric field and the spin relaxation due to a spin-orbit interaction at impurity, we derive the spin diffusion equation including the time evolution. We also discuss a relationship between a frequency dependence of the spin accumulation and the spin diffusion length.
Dependence of magnetoresistance on dopant density in phosphorous doped silicon
Authors: Porter N. A. , Marrows C. H.
Published: March 2011

The transverse magnetoresistance at 280 K is reported as a function of phosphorous dopant density in silicon ranging from 1011 to 4×1018 cm−3. A reduction in magnetoresistance from 55% to 1.8% in an 8 T magnetic field occurred for dopant densities greater than 1016 cm−3. This reduction is related to the decrease in mobility due to increased scattering from ionized donors. A quadratic relationship of magnetoresistance with mobility is proposed in agreement with classical theory.
Magnetic behaviors of classical spin model on the Shastry–Sutherland lattice: Monte Carlo simulation
Authors: Qin M. H. , Zhang G. Q. , Wang K. F. , Gao X. S. , Liu J.-M.
Published: March 2011

We study the magnetic phase diagram of a classical Heisenberg spin model on the Shastry–Sutherland lattice using the Monte Carlo method. The simulated results indicate that the particular collinear phase region can be enlarged due to the implementation of easy-axis anisotropy, leading to the broadening of the magnetization plateau in the classical Heisenberg model. Also, the random exchange term is taken into account to study the effect of inhomogeneity on the steplike magnetization feature. It is indicated that the multi-step magnetic behaviors in the low temperature range remain observable when the inhomogeneity is in the appropriate range.
High temperature magnetic properties of magnesium ferrite nanoparticles
Authors: Franco A. , Silva M. S.
Published: March 2011

Magnetic properties such as Curie temperature (TC), saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) of nanoparticles of magnesium ferrites (MgFe2O4) were studied in a broad range of temperatures varying from room temperature to 800 K. The magnetization decreases with increasing temperature, approaching 0 at ∼ 750 K. The Curie temperature, determined by means of the inverse susceptibility versus temperature, was ∼738 K. The saturation magnetization, coercivity, and remanence decreased with increasing temperature, being close to 0 at temperatures near TC. However, for temperatures 100 K above room temperature, these magnetic properties were still the same as those at room temperature. The coercivity temperature dependence could be expressed in terms of T3/4, indicating that MgFe2O4 nanoparticles may form a system of random and noninteracting identical particles. The results are discussed in terms of interparticle interactions induced by the thermal fluctuations, cation distribution, and other imperfections that exert fields on Mg2+ ions that could increase with temperature.
Spatial profile of spin excitations in multilayered rectangular nanodots studied by microfocused Brillouin light scattering
Authors: Madami M. , Carlotti G. , Gubbiotti G. , Scarponi F. , Tacchi S. , Ono T.
Published: March 2011

An array of multilayered rectangular NiFe/Cu/NiFe dots (lateral dimensions 720 × 430 nm, thicknesses 10/10/10 nm) has been fabricated by electron-beam lithography. The magnetization reversal process has been studied by the magneto-optical Kerr effect, showing that the dipolar coupling between the two magnetic layers plays an important role and competes with shape anisotropy, determining the characteristic profile of the hysteresis curve. Moreover, both conventional and microfocused Brillouin light scattering have been exploited to study the eigenmodes spectrum in the gigahertz range, achieving a direct visualization of the spatial profile of the different modes. The results are in good agreement with both the modes frequency and the spatial profiles obtained by micromagnetic simulations.
Doped electron localization: Electron paramagnetic resonance probing of La0.3Ca0.7MnO3 compound
Authors: Auslender M. , Shames A. I. , Rozenberg E. , Felner I. , Mogilyansky D. , Pestun A. , Mukovskii Ya.M. , Gorodetsky G.
Published: March 2011

The X-band electron paramagnetic resonance (EPR) was studied in the electron-doped ceramic La0.3Ca0.7MnO3. The model fittings of measured temperature dependences of the EPR linewidth and intensity were employed to probe the paramagnetic spin correlations and spin dynamics. These fittings revealed notable ferromagnetic short-range correlations and pure ion–ion spin relaxation in the paramagnetic state of La0.3Ca0.7MnO3. The latter fact supports the local nature of ferromagnetic double exchange coupling in this very case. It is suggested that such character of ferromagnetic exchange interactions results in the localization of carriers and establishing of charge ordered antiferromagnetic insulating ground state. This finding may be considered a strong prerequisite for the electron–hole doping asymmetry observed in the phase diagram of La–Ca manganites.
Effect of Co and Cu substitution on the magnetic entropy change in Ni46Mn43Sn11 alloy
Authors: Das Rahul , Sarma S. , Perumal A. , Srinivasan A.
Published: March 2011

We report the observation of magnetic entropy change due to martensitic phase transformations of bulk off-stoichiometric Heusler alloys with compositions Ni46Mn43Sn11, Ni44Mn43Co2Sn11, and Ni44Mn43Cu2Sn11, prepared by arc melting method. The martensitic transition of the parent ternary alloy (Ni46Mn43Sn11) shifts to lower temperatures upon Co and Cu substitution. Inverse magnetocaloric effect was observed in these alloys near the martensitic transformation temperature when subjected to an applied magnetic field. Ni46Mn43Sn11, Ni44Mn43Co2Sn11, and Ni44Mn43Cu2Sn11 alloys exhibited maximum positive magnetic entropy change of 7.9, 11.3, and 18.8 J kg−1 K−1, respectively, under an applied magnetic field of 1.8 T.
Large magnetocaloric effect at low magnetic field in Ni50−xCoxMn35In15 ribbons
Authors: Guan W. , Liu Q. R. , Gao B. , Yang S. , Wang Y. , Xu M. W. , Sun Z. B. , Song X. P.
Published: March 2011

Recently, the magnetocaloric effect (MCE) of Ni-Mn-based Heusler alloys has attracted more attention due to its metamagnetic shape-memory effect. However, most MCE investigations focused on the bulk alloys developed at high magnetic field (>2 T). In this paper, by employing the melt-spinning technique, a large MCE at low magnetic field was found in a ribbon of Ni50−xCoxMn35In15. The magnetic entropy change ΔS reaches the maximum value of 5.35 J/kg K at 1 T field in the Ni49CoMn35In15 ribbon near room temperature, and its corresponding temperature span arrives 24 K. The refrigerant capacity is enhanced with the increase of the Co substitution. Such a large MCE at low magnetic field in the Ni50−xCoxMn35In15 ribbon with a wide temperature range provides a potential candidate for room temperature magnetic refrigeration.
A tuning fork based wide range mechanical characterization tool with nanorobotic manipulators inside a scanning electron microscope
Authors: Acosta Juan Camilo , Hwang Gilgueng , Polesel-Maris Jérôme , Régnier Stéphane
Published: March 2011

This study proposes a tuning fork probe based nanomanipulation robotic system for mechanical characterization of ultraflexible nanostructures under scanning electron microscope. The force gradient is measured via the frequency modulation of a quartz tuning fork and two nanomanipulators are used for manipulation of the nanostructures. Two techniques are proposed for attaching the nanostructure to the tip of the tuning fork probe. The first technique involves gluing the nanostructure for full range characterization whereas the second technique uses van der Waals and electrostatic forces in order to avoid destroying the nanostructure. Helical nanobelts (HNB) are proposed for the demonstration of the setup. The nonlinear stiffness behavior of HNBs during their full range tensile studies is clearly revealed for the first time. Using the first technique, this was between 0.009 N/m for rest position and 0.297 N/m before breaking of the HNB with a resolution of 0.0031 N/m. For the second experiment, this was between 0.014 N/m for rest position and 0.378 N/m before detaching of the HNB with a resolution of 0.0006 N/m. This shows the wide range sensing of the system for potential applications in mechanical property characterization of ultraflexible nanostructures.
Dielectric relaxation of Sr1–1.5xBixTiO3 sol-gel thin films
Authors: Okhay Olena , Wu Aiying , Vilarinho Paula M. , Tkach Alexander
Published: March 2011

The dielectric response of Sr1–1.5xBixTiO3 films (0.002 ≤ x ≤ 0.167), prepared by sol-gel and deposited on Si/SiO2/TiO2/Pt substrates, is analyzed as a function of frequency and temperature. The hysteretic behavior of the polarization versus the electric field is studied as well. Between 100 Hz and 1 MHz, the real part of the dielectric permittivity ɛ′ exhibit a relaxation between ∼60 and 260 K, shifting to high temperatures with increasing the Bi content. In the imaginary part of the dielectric permittivity ɛ′′ of these films two relaxations are induced by Bi doping below the temperature of the ɛ′ relaxation. The first relaxation observed in films with 0.002 ≤ x ≤ 0.10 follows the Arrhenius law with an activation energy of U = 64–80 meV and a preexponential term τ0 = (0.3–10.8) × 10−14 s almost independent on the Bi content and is ascribed to the individual hopping of dipoles created by the off-center Bi ions. The second relaxation observed in the films with 0.04 ≤ x ≤ 0.167 is described by the Vögel–Fulcher relation with U = 2–38 meV, τ0 = 5 × 10−10–5 × 10−6 s and a freezing temperature Tf = 50–102 K, increasing with Bi content, and is attributed to the presence of polar clusters of interacting Bi ions. Slim P(E) hysteresis loops are observed at low temperatures, confirming the appearance of a polar state. The effect of Bi incorporation in ST films is qualitatively the same as the relaxorlike behavior observed in Bi-doped ST ceramics and the dissimilarities are explained based on the influence of the substrate and by a higher homogeneity of the dopant distribution in the sol-gel derived films.
Analysis of electronic structures of quantum dots using meshless Fourier transform k·p method
Authors: Zhao Qiuji , Mei Ting
Published: March 2011

We develop a complete Fourier transform k·p method and present its application for a theoretical investigation on electronic structures of quantum dots with consideration of the built-in strain effects. The Fourier transform technique is applied to the periodic position-dependent Hamiltonian, and a simple and neat expression of the Hamiltonian matrix in the Fourier domain is formulated due to the orthogonality of exponential functions. Spurious solutions can be avoided due to the truncation of high Fourier frequencies. A kinetic Hamiltonian matrix in momentum domain is formulated by entering the analytical Fourier transform of the quantum-dot shape function into the neat Hamiltonian matrix easily, which allows meshless numerical implementation. The formulation of strain Hamiltonian matrix is done by convolution of Fourier series of strain components and Fourier series of the quantum-dot shape functions. Therefore, an original Fourier transform-based k·p approach is developed by combining the kinetic Hamiltonian matrix and the strain Hamiltonian. This approach is adopted to study the dimension effect and strain effect on the ground states of electrons and holes of pyramidal quantum dots that are truncated to different heights. The ground-state energy variation shows that the electron state is the most sensitive to these effects and the strain effect on E1, LH1, and HH1 is more prominent for sharperquantum dots. This investigation shows that band mixing between the conduction band and valence band, and band mixing between heavy-hole and light-hole bands are reduced due to the strain effect, whereas this effect is more prominent for nontruncated pyramidal quantum dots due to the stress concentration. Among the three ground states, light-hole states are more weakly confined in the nonpyramidal quantum dot and shift to the tip of the pyramid due to the strain.
Bulk phase two dimensional chiral growth of 6, 13 Pentacenequinone on SiO2
Authors: De Marco P. , Fioriti F. , Bisti F. , Parisse P. , Santucci S. , Ottaviano L.
Published: March 2011

6,13 Pentacenequinone (PQ) ultrathin films (3 and 5 nm nominal thickness) have been grown by means of ultrahigh vacuum deposition onto 100 nm thick SiO2/Si(100). The structure and morphology of the thin films have been studied with field emission-scanning electron microscopy, tapping mode atomic force microscopy, and x-ray diffraction. The growth begins with PQ molecules standing almost upright (with respect to the substrate) and aggregating into two-dimensional 3.62 nm thick pseudodendritic islands. The islands are characterized by two preferential growth directions (at 117°), a clear evidence of a chiral growth. The thickness of the islands and the angle formed by the two directions of preferential growth, allow a straightforward assignment to a PQ initial growth in the “bulk” phase. Above the critical thickness of 3.62 nm the PQ growth proceeds in a Stranski−Krastanov mode, with the formation of “bulk” and “thin-film” phase crystallites.
Analytical approach of Lamb waves coupling in rough isotropic plates
Authors: Valier-Brasier Tony , Potel Catherine , Bruneau Michel , Gatignol Philippe
Published: March 2011

The present work aims at contributing to the investigation of an analytical method to describe Lamb waves which undergoes Lamb waves coupling when propagating along the rough surfaces of finite extent of isotropic, solid, and infinite plates. The motions considered are assumed to be independent of one of the coordinates for which the component of the displacement is equal to zero (two-dimensional problem). In some respect, the analytical approach is an extension of an analytical model describing the coupling of SH waves due to ridges (on the surface of a plate) parallel to the polarization of the waves, but it is treated in a somewhat different manner because the acoustic field involves here both the longitudinal and the transversal displacements of the Lamb waves. The formalism relies on an integral formulation, using Green’s functions which permit to express Lamb waves perturbations, to describe the coupling process between the longitudinal and the transversal components of the forward and the backward propagating Lamb waves, due to the roughness (including the effects of both its depth and its slope). A methodology is given to solve the problem, and results on specific examples (periodic and random rough surfaces) are discussed in order to support the method.
Electrical, magnetic, and optical properties in multiferroic Bi5Ti3FeO15 thin films prepared by a chemical solution deposition route
Authors: Bai W. , Gao Y. Q. , Zhu J. Y. , Meng X. J. , Lin T. , Yang J. , Zhu Z. Q. , Chu J. H.
Published: March 2011

Perovskite Aurivillius structure Bi5Ti3FeO15 (BTF) polycrystalline thin films were prepared by a chemical solution deposition route. The microstructure, electrical, and magnetic properties were investigated and in particular the optical properties of the BTF films were addressed. The coexistence of the ferroelectric and weak ferromagnetic properties at room temperature demonstrates the multiferroic behavior of the BTF films. The optical dielectric functions in the photon energy range of 1.13–4.13 eV have been extracted by fitting the experimental data in terms of the Tauc-Lorentz and Cauchy models. A direct interband transition is allowed and the energy bandgap is determined to be ∼3.3 eV for the BTF films. The weak absorption below the bandgap edge was observed and described by the Urbach band tail rule. Finally, the optical dispersion behaviors of the BTF films in the transparent region were explained using a single oscillator model.
Structural templating and growth behavior of copper phthalocyanine thin films deposited on a polycrystalline perylenetetracarboxylic dianhydride layer
Authors: Hong Dajung , Do Young Rag , Kwak Hyon Tae , Yim Sanggyu
Published: March 2011

Structural templating and the growth behavior of copper phthalocyanine (CuPc) thin films deposited on a polycrystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) layer were examined using x-ray diffraction (XRD) and atomic force microscopy. The templated CuPc films were found to grow on both the (110) and (102) plane of the α-PTCDA layer, indicating the intermolecular π-π interaction is strong enough to induce templating even on the underlying layer which is tilted at an angle of 25° from the substrate surface as well as on the surface-parallel layer. In contrast to the large growth exponent (β) values for other single layer molecular thin films, a significantly small β value of 0.17 ± 0.06 was obtained on the PTCDA/CuPc heterolayer thin film system. The XRD and scaling behavior studies suggest that this relatively slow surface roughening can be rationalized by the lack of surface parallel crystalline ordering at the initial stage of CuPc film growth on the PTCDA layer.
Transport in graphene tunnel junctions
Authors: Malec C. E. , Davidović D.
Published: March 2011

We present a technique to fabricate tunnel junctions between graphene and Al and Cu, with a Si backgate, as well as a simple theory of tunneling between a metal and graphene. We map the differential conductance of our junctions versus probe and backgate voltage, and observe fluctuations in the conductance that are directly related to the graphene density of states. The conventional strong-suppression of the conductance at the graphene Dirac point cannot be clearly demonstrated, but a more robust signature of the Dirac point is found: the inflection in the conductance map caused by the electrostatic gating of graphene by the tunnel probe. We present numerical simulations of our conductance maps, confirming the measurement results. In addition, Al causes strong n doping of graphene, Cu causes a moderate p doping, and in high resistance junctions, phonon resonances are observed, as in STM studies.
Si passivation effects on atomic bonding and electronic properties at HfO2/GaAs interface: A first-principles study
Authors: Wang Weichao , Xiong Ka , Gong Cheng , Wallace Robert M. , Cho Kyeongjae
Published: March 2011

A theoretical study on atomic structures and electronic properties of the interface between GaAs and HfO2 is reported. The intrinsic gap states are mainly originated from Ga dangling bonds, partial Ga-oxidation, and As−As dimers in the reconstructed interface structures. Si passivation interlayer can introduce two types of Si local bonding configuration of Si interstitial or substitutional defects (SiHf). SiHf–passivated interfaces are found to be energetically stable and can suppress the interfacial flat bandgap state stemming from partial Ga-oxidation into the valence band of bulk GaAs. Furthermore, gap states near the conduction bandedge are partially reduced. With the increase of Si concentration at the interface, the charge state of interfacial Ga decreases from +1.26 to between +0.73 and +0.80, and this change shows a Ga oxidation state transformation from Ga2O3 (+1.7) to Ga2O (+0.52) states. The metastable Si interstitials also eliminate Ga2O3-oxidation state and creates Ga2O-like Ga charge state at the interface. However, the gap states near the conduction bandedge cannot be passivated by substitutional (SiHf) nor by interstitial (Sii) silicon. The detailed nature of the gap states examined in this modeling study would facilitate further development of interface passivation and the optimization of Si-passivation layers.
The influence of spontaneous and field-induced spin reorientation transitions on the magnetocaloric properties of HoZn and ErZn
Authors: de Sousa V. S.R. , von Ranke P. J. , Gandra F. C.G.
Published: March 2011

We report a theoretical investigation on the magnetocaloric properties of the cubic CsCl-type HoZn and ErZn compounds. Several anomalies in the magnetocaloric quantities, ΔST and ΔTS, are observed due to spontaneous and/or field-induced spin reorientation transitions in these compounds. In HoZn, a discontinuity in the isothermal entropy change and in the adiabatic temperature change around T1 = 23 K is ascribed to the spontaneous reorientation transition. Under a magnetic field variation from 0 up to 2 T in the 〈110〉 and 〈100〉 directions, an almost table-like behavior in ΔST is predicted between T1 and TSR1. The peak around the ferromagnetic–paramagnetic transition temperature in the magnetocaloric quantities shows a dependence on the direction of the applied field. For μ0ΔH = 2 T, it reaches 11.9 J/kg K (magnetic field along the 〈111〉 direction) and 7.9 J/kg  K (magnetic field in the 〈100〉 direction). In ErZn there is also a dependence of ΔST and ΔTS on field direction. From the analysis of the spin reorientations in both compounds we have built spin reorientation diagrams that summarize their temperature and field dependence. Our theoretical approach is based on a model Hamiltonian that includes exchange, crystal field, and quadrupolar interactions.
Bandgap expansion and dielectric suppression of self-assembled Ge nanocrystals
Authors: Goh Eunice S. M. , Chen T. P. , Huang S. F. , Liu Y. C. , Sun C. Q.
Published: March 2011

The bandgap and optical properties (dielectric functions and optical constants) of dome-shaped Ge nanocrystals (nc-Ge) with average sizes of ∼6 nm in height and ∼13 nm in diameter have been investigated using spectroscopic ellipsometry based on the Forouhi-Bloomer optical dispersion model. As compared to bulk crystalline Ge, the nc-Ge exhibited a bandgap expansion of ∼0.2 eV and a significant reduction in the dielectric function. The bandgap expansion and dielectric suppression are discussed in terms of the quantum confinement effect as well as the bond contraction model.
Impact of interfacial magnetism on magnetocaloric properties of thin film heterostructures
Authors: Kirby B. J. , Lau J. W. , Williams D. V. , Bauer C. A. , Miller Casey W.
Published: March 2011

Polarized neutron reflectometry was used to determine the depth profile of the magnetic moment per Gd atom, mGd, in a Gd(30 nm)/W(5 nm) multilayer. Despite sharp interfaces observed by transmission electron microscopy, mGd is systematically suppressed near the Gd-W interfaces. Because the peak magnetic entropy change is proportional to mGd2/3, this results in a reduction of the maximum achievable magnetocaloric effect in Gd-W heterostructures. By extension, our results suggest that creating materials with Gd-ferromagnet interfaces may increase the mGd relative to the bulk, leading to enhanced magnetocaloric properties.
Surface nanostructuring of laser ablated copper in ambient gas atmosphere and a magnetic field
Authors: Pandey Pramod K. , Thareja R. K.
Published: March 2011

A comprehensive study on deposition of copper nanoclusters back on to the laser ablated target in combined ambient of gas and a magnetic field is reported. The characteristics and mechanism of the inverse plume deposition are investigated by characterizing the laser ablated plasma plume using time resolved imaging and optical emission of the plume. The morphology of the deposited particles in presence of magnetic field depends on the nature of the ambient gas used; the elliptical shape of reverse deposited particles changes to spherical with the increase in mass of the ambient gas. The structural, morphological, and optical properties of the deposited nanoclusters were studied by using x-ray diffraction, scanning electron microscope and photoluminescence spectra respectively. The possibility of enhanced deposition rates in combined ambient gas and a magnetic field has potentials of industrial applications of the process.
The influence of the ionized impurity delta-layer potential in the quantum well on impurity binding energy
Authors: Tulupenko V. , Abramov A. , Belichenko Ya. , Akimov V. , Bogdanova T. , Poroshin V. , Fomina O.
Published: March 2011

The influence of thermal ionization of an impurity delta-doped layer situated either in the center or on the edge of a quantum well (QW) on impurity binding energy is investigated theoretically for the case of Si0.8Ge0.2/Si QW. It is shown that the Hartree potential created by free electrons and by ionized impurities at high temperatures superimposes on the original (at low temperature) QW energy profile. Resulting new QWs have their own impurity binding energies. It is of interest that energies are nearly the same for center- and edge-doped QWs, contrary to those at low temperatures. The obtained results are explained on the basis of Coulomb’s law when decreasing the mean distance between free electron and impurity atom with temperature involves an increase in the impurity binding energy.
Surface states and photo-induced charge transfer on oxygen-terminated chemical vapor deposition diamond
Authors: Deferme W. , Mackova A. , Haenen K. , Nesládek M.
Published: March 2011

Time-of-Flight (ToF) experiments were carried out to study the electrical transport and charge trapping properties of oxidized freestanding single crystalline (100) CVD diamond crystals. It is shown that the diamond surface and properties of the electrical contacts are of extreme importance, influencing measured charge carrier mobility values. While sputtered Al contacts yield textbook ToF spectra and electron mobilities μe exceeding 1860 cm2/Vs, thermally deposited contacts suffer from enhanced trapping. We propose a model in which the observed polarization effects can be explained by charge transfer to oxygen-related surface states, which interact with surface adsorbates, according to the well-known transfer-doping model.
CF4 plasma treatment on nanostructure band engineered Gd2O3-nanocrystal nonvolatile memory
Authors: Wang Jer-Chyi , Lin Chih-Ting
Published: March 2011

The effects of CF4 plasma treatment on Gd2O3 nanocrystal (NC) memory were investigated. For material analysis, secondary ion mass spectrometry and x-ray photoelectron spectroscopy analyses were performed to characterize the fluorine depth profile of the Gd2O3-NC film. In addition, an UV–visible spectrophotometer was used to obtain the Gd2O3 bandgap and analyzed to suggest the modified structure of the energy band. Moreover, the electrical properties, including the memory window, program/erase speed, charge retention, and endurance characteristics were significantly improved depending on the CF4 plasma treatment conditions. This can be explained by the physical model based on the built-in electric field in the Gd2O3 nanostructure. However, it was observed that too much CF4 plasma caused large surface roughness induced by the plasma damage, leading to characteristics degradation. It was concluded that with suitable CF4 plasma treatment, this Gd2O3-NC memory can be applied to future nonvolatile memory applications.
Analysis of thermal diffusivity of Ti thin film by thermoreflectance and periodic heating technique
Authors: Matsui Genzou , Kato Hideyuki
Published: March 2011

Thermal diffusivity of Ti thin film with several hundred nanometers thickness has been measured by means of thermoreflectance (TR) technique and periodic heating using front heating and front detection configuration. Ti thin films were prepared on Si substrates by dc sputtering method. Then thin Mo layers as reflection layers were coated on Ti thin films. Surface of the Mo layer is irradiated by sinusoidally intensity modulated heating laser. Temperature response at the heated area is measured by a probe laser beam with constant intensity, as a TR signal. Phase lag between the phase of TR signal and that of heating laser beam was obtained from 100 kHz to 2.6 MHz. To analyze thermal diffusivity of Ti thin films using the phase lag data, we developed a three-layer analytical model such as Mo coating (100 nm)/thin film/semi-infinite substrate. The calculated phase lag using analytical model is in good agreement with the experimental data for the whole frequency range. The thermal diffusivity of two Ti thin films is determined to be 5 × 10−6 m2/s, which is 53% of the bulk one.
Fourier transform infrared spectroscopy of 2′-deoxycytidine aggregates in CDCl3 solutions
Authors: Biemann Lars , Häber Thomas , Maydt Daniela , Schaper Klaus , Kleinermanns Karl
Published: March 2011

We investigated the self-aggregation of 2′-deoxy-3′,5′-bis(tert-butyldimethylsilyl)-cytidine dC(TBDMS)2 in CDCl3 solutions by Fourier transform infrared (FT-IR) spectroscopy and report the formation of larger aggregates than dimers in this solvent for the first time. The hydrogen bonding patterns in these complexes, which occur with increasing concentration may serve as a model for DNA super-structures such as triplexes. From the IR spectra, wavelength dependent absolute extinction coefficients of the monomer, dimer as well as a contribution from larger clusters which are supposedly trimers are deduced on the basis of a simple deconvolution method. Our results are supported by RI-B3LYP/TZVP calculations within the conductorlike screening model framework, to account for solvent effects in the ab initio calculations.
Performance analysis of the continuous trace gas preconcentrator
Authors: Muntz E. P. , Han Y.-L.
Published: March 2011

In gas molecule detection systems, certain trace gas components can go undetected. This is due to ultralow yet dangerous concentrations combined with limitations of the detection methods. To remedy this problem, a preconcentrator can be included in a system to increase the trace gas concentrations, before the gas samples enter the detection unit. The widely used adsorption/desorption preconcentrators enable detection by interrupting the sampled gas flow for significant periods, in order to accumulate detectable periodic concentrations of trace gas molecules. The recently patented continuous trace gas preconcentrator (CTGP) provides a unique approach for enhancing the trace gas concentration, without stopping the flow. In this study, a performance model is developed for the CTGP, by application of the Poiseuille flow coefficients for long tubes. Based on the Cercignani–Lampis scattering kernel, Sharipov calculated the Poiseuille flow coefficients for various geometries and numerous operating Knudsen numbers. The concentrations of sampled molecules were analyzed in this study using Sharipov’s flow coefficients. The results presented here reinforce the potential benefits of the CTGP.
Bulk diffusion induced structural modifications of carbon-transition metal nanocomposite films
Authors: Berndt M. , Abrasonis G. , Kovács Gy. J. , Krause M. , Munnik F. , Heller R. , Kolitsch A. , Möller W.
Published: March 2011

The influence of transition metal (TM = V,Co,Cu) type on the bulk diffusion induced structural changes in carbon:TM nanocomposite films is investigated. The TMs have been incorporated into the carbon matrix via ion beam co-sputtering, and subsequently the films have been vacuum annealed in the temperature range of 300 – 700 °C. The structure of both the dispersed metal rich and the carbon matrix phases has been determined by a combination of elastic recoil detection analysis, x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The as-grown films consist of carbidic (V and Co) and metallic (Cu) nanoparticles dispersed in the carbon matrix. Thermal annealing induces surface segregation of Co and Cu starting at ≥ 500 °C, preceded by the carbide-metal transformation of Co-carbide nanoparticles at ∼ 300 °C. No considerable morphological changes occur in C:V films. In contrast to the surface diffusion dominated regime where all the metals enhance the six-fold ring clustering of C, in the bulk diffusion controlled regime only Co acts as a catalyst for the carbon graphitization. These results are consistent with the metal-induced crystallization mechanism in the C:Co films. The results are discussed on the basis of the metal-carbide phase stability, carbon solubility in metals or their carbides, and interface species.
Methodology for turbulence code validation: Quantification of simulation-experiment agreement and application to the TORPEX experiment
Authors: Ricci Paolo , Theiler C. , Fasoli A. , Furno I. , Gustafson K. , Iraji D. , Loizu J.
Published: March 2011

A methodology for plasma turbulence code validation is discussed, focusing on quantitative assessment of the agreement between experiments and simulations. The present work extends the analysis carried out in a previous paper [P. Ricci et al., Phys. Plasmas 16, 055703 (2009)] where the validation observables were introduced. Here, it is discussed how to quantify the agreement between experiments and simulations with respect to each observable, how to define a metric to evaluate this agreement globally, and—finally—how to assess the quality of a validation procedure. The methodology is then applied to the simulation of the basic plasma physics experiment TORPEX [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)], considering both two-dimensional and three-dimensional simulation models.
Thermodynamic properties in the molecular dynamics ensemble applied to the Gaussian core model fluid
Authors: Mausbach Peter , Sadus Richard J.
Published: March 2011

The thermodynamic properties of pressure, energy, isothermal pressure coefficient, thermal expansion coefficient, isothermal and adiabatic compressibilities, isobaric and isochoric heat capacities, Joule–Thomson coefficient, and speed of sound are considered in a classical molecular dynamics ensemble. These properties were obtained using the treatment of Lustig [J. Chem. Phys. 100, 3048 (1994)] and Meier and Kabelac [J. Chem. Phys. 124, 064104 (2006)], whereby thermodynamic state variables are expressible in terms of phase-space functions determined directly from molecular dynamics simulations. The complete thermodynamic information about an equilibrium system can be obtained from this general formalism. We apply this method to the Gaussian core model fluid because the complex phase behavior of this simple model provides a severe test for this treatment. Waterlike and other anomalies are observed for some of the thermodynamic properties of the Gaussian core model fluid.
Weak field magnetoresistance of narrow-gap semiconductor InSb
Authors: Yang R. , Gao K. H. , Zhang Y. H. , Chen P. P. , Yu G. , Wei L. M. , Lin T. , Dai N. , Chu J. H.
Published: March 2011

The weak antilocalization effect of InSb film in perpendicular as well as tilted magnetic field is investigated. It is found that the InSb film has quasi-two-dimensional feature and the Nyquist mechanism dominates decoherence. The two dimensionality is also verified further and the influence of roughness effect and Zeeman effect on weak antilocalization effect is studied by systematically investigating the anisotropy of weak field magnetoresistance with respect to magnetic field. It is also found that the existence of in-plane field can effectively suppress the weak antilocalization effect of InSb film and the roughness effect plays an important role in the anisotropy.
Silicide formation process of Pt added Ni at low temperature: Control of NiSi2 formation
Authors: Ikarashi Nobuyuki , Masuzaki Koji
Published: March 2011

Transmission electron microscopy (TEM) and ab initio calculations revealed that the Ni-Si reaction around 300 °C is significantly changed by adding Pt to Ni. TEM analysis clarified that NiSi2 was formed in a reaction between Ni thin film (∼1 nm) and Si substrate, while NiSi was formed when Pt was added to the Ni film. We also found that the Ni-adamantane structure, which acts as a precursor for NiSi2 formation around the reaction temperature, was formed in the former reaction but was significantly suppressed in the latter reaction. Theoretical calculations indicated that Pt addition increased stress at the Ni-adamantane structure/Si-substrate interface. The increase in interface stress caused by Pt addition should raise the interface energy to suppress the Ni-adamantane structure formation, leading to NiSi2 formation being suppressed.
Size dependent optical properties of Si quantum dots in Si-rich nitride/Si3N4 superlattice synthesized by magnetron sputtering
Authors: So Yong-Heng , Gentle Angus , Huang Shujuan , Conibeer Gavin , Green Martin A.
Published: March 2011

A spectroscopic ellipsometry compatible approach is reported for the optical study of Si quantum dots (QDs) in Si-rich nitride/silicon nitride (SRN/Si3N4) superlattice, which based on Tauc-Lorentz model and Bruggeman effective medium approximation. It is shown that the optical constants and dielectric functions of Si QDs are strongly size dependent. The suppressed imaginary dielectric function of Si QDs exhibits a single broad peak analogous to amorphous Si, which centered between the transition energies E1 and E2 of bulk crystalline Si and blue shifted toward E2 as the QD size reduced. A bandgap expansion observed by the TL model when the size of Si QD reduced is in good agreement with the PL measurement. The bandgap expansion with the reduction of Si QD size is well supported by the first-principles calculations based on quantum confinement.
Multiferroic control of magneto-current through a resonant tunneling diode
Authors: Kim Heesang , Kim Nammee
Published: March 2011

We study the resonant tunneling magneto-resistance (TMR) of a multiferroic resonant tunneling diode, which is hybridized of ferroelectric double barriers and a ferromagnetic quantum well. Using the nonequilibrium Green’s function method we show that magneto-currents through the resonant tunneling diode and its current spin polarization can be manipulated by changing the relative direction of an applied magnetic field to the spontaneous magnetic field in the diluted magnetic semiconductor quantum well as well as by reversing dipole polarization directions in ferroelectric barriers. With these results, we propose a multiferroic spin device to control the amount of current and spin polarization of current through it.
Rarefied gas dynamics on a planetary scale
Authors: Goldstein D. B. , Varghese P. L.
Published: March 2011

Large scale rarefied gas dynamic effects are found in the tenuous atmospheres of several planets and satellites in our Solar System. These phenomena range from thickened shock structures in supersonic flows, to strong thermal nonequilibrium in a stratified atmosphere, to thermal and velocity slip at the body surface. Sublimated atmospheric components in particular are dominated by the detailed nature of the gas-surface interaction. Some direct simulation Monte Carlo (DSMC) results for global scale flows on the Jovian moon Io are presented to illustrate key phenomena in both the sublimated and the volcanic plume components of the atmosphere. The simulations stretch the limits of applicability of DSMC; computational approaches beyond those limits are described.
Four-level polarization discriminator based on a surface plasmon polaritonic crystal
Authors: Benetou M. I. , Thomsen B. C. , Bayvel P. , Dickson W. , Zayats A. V.
Published: March 2011

A compact, four-level polarization discriminator based on a surface plasmon polaritonic crystal (SPPC) has been experimentally demonstrated. It is able to uniquely resolve and spatially separate four signals that have been linearly polarized at azimuth angles 0°, 45°, 90°, and 135°. It exploits the excitation of multiple surface plasmon polariton eigenmodes in nondegenerate directions when the SPPC is illuminated with monochromatic light. The device is planar and of micrometer scale, which makes it suitable for on-chip integration and miniaturization of photonic circuits.
Chemical potential perturbation: A method to predict chemical potentials in periodic molecular simulations
Authors: Moore Stan G. , Wheeler Dean R.
Published: March 2011

A new method, called chemical potential perturbation (CPP), has been developed to predict the chemical potential as a function of density in periodic molecular simulations. The CPP method applies a spatially varying external force field to the simulation, causing the density to depend upon position in the simulation cell. Following equilibration the homogeneous (uniform or bulk) chemical potential as a function of density can be determined relative to some reference state after correcting for the effects of the inhomogeneity of the system. We compare three different methods of approximating this correction. The first method uses the van der Waals density gradient theory to approximate the inhomogeneous Helmholtz free energy density. The second method uses the local pressure tensor to approximate the homogeneous pressure. The third method uses the Triezenberg–Zwanzig definition of surface tension to approximate the inhomogeneous free energy density. If desired, the homogeneous pressure and Helmholtz free energy can also be predicted by the new method, as well as binodal and spinodal densities of a two-phase fluid region. The CPP method is tested using a Lennard-Jones (LJ) fluid at vapor, liquid, two-phase, and supercritical conditions. Satisfactory agreement is found between the CPP method and an LJ equation of state. The efficiency of the CPP method is compared to that for Widom's method under the tested conditions. In particular, the new method works well for dense fluids where Widom's method starts to fail.
The effect of rotational and translational energy exchange on tracer diffusion in rough hard sphere fluids
Authors: Kravchenko Olga , Thachuk Mark
Published: March 2011

A study is presented of tracer diffusion in a rough hard sphere fluid. Unlike smooth hard spheres, collisions between rough hard spheres can exchange rotational and translational energy and momentum. It is expected that as tracer particles become larger, their diffusion constants will tend toward the Stokes–Einstein hydrodynamic result. It has already been shown that in this limit, smooth hard spheres adopt “slip” boundary conditions. The current results show that rough hard spheres adopt boundary conditions proportional to the degree of translational–rotational energy exchange. Spheres for which this exchange is the largest adopt “stick” boundary conditions while those with more intermediate exchange adopt values between the “slip” and “stick” limits. This dependence is found to be almost linear. As well, changes in the diffusion constants as a function of this exchange are examined and it is found that the dependence is stronger than that suggested by the low-density, Boltzmann result. Compared with smooth hard spheres, real molecules undergo inelastic collisions and have attractive wells. Rough hard spheres model the effect of inelasticity and show that even without the presence of attractive forces, the boundary conditions for large particles can deviate from “slip” and approach “stick.”
On the equivalence of two commonly used forms of semiclassical instanton theory
Authors: Althorpe Stuart C.
Published: March 2011

Semiclassical instanton theory gives an approximate description of deep tunneling by means of periodic orbits on the inverted potential energy surface. There are two versions of the theory, one derived by taking a semiclassical limit of the exact flux-side time-correlation function and the other by starting from the “Im F” premise, in which the partition function is analytically continued into the complex plane. Here, we provide a derivation showing that the two versions of the theory are exactly equivalent. Unlike a previous derivation (which was restricted to a system-bath model), our derivation is completely general, and thus establishes that the “Im F” premise, which is behind such methods as quantum transition-state theory and ring polymer molecular dynamics rate-theory, is correct in the steepest-descent limit.