bluuebunny/crossref_metadata_2025 huggingface dataset:
NMR measurements of power-law behavior in the spin-wave and critical regions of ferromagnetic EuO
Authors: Bykovetz N. , Birang B. , Klein J. , Lin C. L.
Published: May 2010

Precision continuous-wave NMR measurements have been carried out over the entire magnetization curve of EuO and are presented in tabular form. Two very closely spaced resonances are observed and are attributed to domain and domain-wall signals. Both of the signals are useful for analysis in the spin-wave region. Only the domain signal is measurable above ∼50 K. The latter is used for fitting Tc and the critical exponent β. The critical-region fits agree with previous measurements within experimental error. The low-temperature data exhibit a clear-cut T2 behavior, at variance with the expectations of conventional spin-wave theory. This result is discussed in relation to two semiempirical spin-wave schemes, one formulated by Bykovetz, and one by Koebler. The NMR signal at 4.2 K gives no indication of a quadrupole splitting in contradiction to the interpretation of several previous spin-echo NMR spectra observed in EuO. This issue remains unresolved.
Magneto-optical and magnetoresistive properties of CoFe–MgO nanocomposite films
Authors: Kravets A. F. , Kravets V. G. , Korenivski V.
Published: May 2010

Nanocomposite CoFe–MgO films, where magnetic CoFe grains are embedded into an insulating MgO matrix, show a pronounced correlation between the tunneling magnetoresistance (TMR) and the magnetorefractive effect properties. The effect is due to the contribution of the spin-dependent conductivity to the optical conductivity of the films. Furthermore, an enhanced magneto-optical Kerr rotation is observed and explained as due to the changes in the band structures at the surface of the magnetic grains and the respective changes in the off-diagonal optical conductivity. The observed effects may prove useful for noncontact characterization of TMR materials.
Thickness dependence of magnetic and structural properties in Fe80Ga20 thin films
Authors: Javed A. , Morley N. A. , Gibbs M. R. J.
Published: May 2010

This work focuses on the effect of film thickness on the microstructural and magnetic properties of polycrystalline Fe80Ga20 films of different thickness (20 nm≤t≤200 nm) fabricated on Si⟨100⟩ substrates. From x-ray diffraction, a ⟨110⟩ texture was normal to the film plane in all films. For all film thicknesses, the lattice constant perpendicular to the film plane was less than the bulk, and first decreased and then increased toward the bulk value with increasing t. From atomic force microscopy data, both surface roughness and grain size were found to increase with film thickness. Using the Villari effect, the effective magnetostriction constant λeff was measured. Using the Szymczak model, it was found that λbulk=91±5 ppm, close to the bulk isotropic polycrystalline value (λiso=98 ppm). The surface magnetostriction λs contribution to λeff was found to be −1065±271 ppm nm. It was found that the 20 nm thick film had weak uniaxial anisotropy, while all the other films were magnetically isotropic. An extension of Néel’s model to saturation field showed that the experimental data obeyed this model and the surface term affected the effective saturation field (Hseff) significantly.
Magnetic property of polycrystalline magnetic garnet for voltage driven type magneto-optic spatial light phase modulator
Authors: Mito S. , Kim J. , Chung K. H. , Yamada K. , Kato T. , Takagi H. , Lim P. B. , Inoue M.
Published: May 2010

In this study, we investigated the magnetization response of the polycrystalline magnetic garnet film for stress. The calculation suggests that the balance of the magnetic anisotropic energy under magnetic field is an essential factor to modulate the magnetization by stress. We fabricated the magnetophotonic crystal based voltage driven type magneto-optic spatial light modulator, and measured the magneto-optic response for stress. The experimental results had a good agreement with our calculation. In our results, the magnitude and linearity of the modulation by stress was depending on static bias field, stress, and magnetostriction constant. The obtained results implied that the calculation could estimate the device performance by deciding material composition and prepare stress inside garnet.
Effect of copper addition on crystallization and properties of hafnium containing HITPERM alloys (invited)
Authors: Turgut Z. , Christy L. , Huang M. , Horwath J. C.
Published: May 2010

This paper presents properties and nanocrystallization characteristics of HITPERM based Fe68.8Co17.2Hf7Cu1B6 and Fe69.6Co17.4Hf7B6 alloys. Both alloys differing only in copper content were subjected to series of characterizations in order to study the effect of copper. Evolution of magnetic properties as a function time has been investigated by isothermal annealing experiments and optimum annealing conditions of amorphous precursors have been reported. Resulting magnetic properties (ac and dc) measured at room temperature are presented. Nanocrystallization kinetics studied by using time dependant magnetization measurements was discussed in the framework of Johnson–Mehl–Avrami model. Results of thermal stability measurements conducted in air at 500 K for up to 1000 h are reported.
Incompressibility of osmium metal at ultrahigh pressures and temperatures
Authors: Armentrout Matt M. , Kavner Abby
Published: May 2010

Osmium is one of the most incompressible elemental metals, and is used as a matrix material for synthesis of ultrahard materials. To examine the behavior of osmium metal under extreme conditions of high pressure and temperature, we measured the thermal equation of state of osmium metal at pressures up to 50 GPa and temperatures up to 3000 K. X-ray diffraction measurements were conducted in the laser heated diamond anvil cell at GeoSoilEnviroCARS and the High Pressure at the Advanced Photon Source and beamline 12.2.2 at the advanced light source. Ambient temperature data give a zero pressure bulk modulus of 421 (3) GPa with a first pressure derivative fixed at 4. Fitting to a high temperature Birch–Murnaghan equation of state gives a room pressure thermal expansion of 1.51(0.06)×10−5 K−1 with a first temperature derivative of 4.9(0.7)×10−9 K−2 and the first temperature derivative of bulk modulus of be dK0/dT=−0.055 (0.004). Fitting to a Mie–Grüneisen–Debye equation of state gives a Grüneisen parameter of 2.32 (0.08) with a q of 7.2 (1.4). A comparison of the high pressure, temperature behavior among Re, Pt, Os, shows that Os has the highest bulk modulus and lowest thermal expansion of the three, suggesting that Os-based ultrahard materials may be especially mechanically stable under extreme conditions.
H-atom interaction with amorphous hydrocarbon films: Effect of surface temperature, H flux and exposure time
Authors: Erradi A. , Clergereaux R. , Gaboriau F.
Published: May 2010

In the present paper, we study the interaction between atomic hydrogen generated in a microwave afterglow with amorphous hydrogenated carbon films. A simple surface model is described and compared with the experimental results. Erosion rate is time dependent and exhibits a transient regime before reaching a constant value. Estimate of the modified film thickness by ellipsometry shows that thickness increases with time and becomes constant and equal to 1.4 nm when reaching the permanent regime. In addition, this limit is independent on the conditions, e.g., on hydrogen flux and temperature. Erosion rate depends linearly on hydrogen flux arriving at the surface and shows an exponential increase with surface temperature. A simple model proposed in the paper is in good agreement with the experimental data and allows giving an estimate of the erosion activation energy Ea=0.2 eV. This value is in agreement with the energy involved in the reaction between hydrogen atom and carbon atom in sp3 hybridization.
ac losses and field and current density distribution during a full cycle of a stack of superconducting tapes
Authors: Yuan Weijia , Campbell A. M. , Coombs T. A.
Published: May 2010

Starting from an existing model by Clem et al., this paper has analyzed how the current density and magnetic field distribution of a stack of superconducting tapes with ac transport currents or applied fields will change in a full cycle. This paper assumes when the ac current or field starts to change in the other direction, a new penetrated region will begin to penetrate from the superconductor surface. If we assume Jc is constant in the critical region, this paper demonstrates that the Claassen formula (7) can be used to calculate the exact ac losses. If Jc depends on local Bz, we can use Eq. (9) to quickly predict the ac losses. This approach does not need to calculate a complete ac cycle. This saves considerably computation time while gives a result which is in close agreement with that calculated from a complete ac cycle. The calculation method can be applied for calculating a superconducting pancake coil if the coil radius is much larger than the tape width.
The role of misorientation and coincident site lattice boundaries in Goss-textured Galfenol rolled sheet
Authors: Chun Hyunsuk , Na Suok-Min , Mudivarthi Chaitanya , Flatau Alison B.
Published: May 2010

In this paper, the role of misorientation angle and coincident site lattice (CSL) boundaries in promoting Goss-textured abnormal grain growth (AGG) in polycrystalline (Fe81Ga19)+1.0 mol % NbC rolled sheet is investigated through electron backscattering diffraction (EBSD) patterns captured by orientation imaging microscopy. Data on the magnetostrictive variation as a function of annealing time under an argon atmosphere are presented along with the area fraction of grains, average grain size, and grain boundary character distributions (GBCD). The GBCD results are used to show changes in the CSL boundary distribution are too small to explain the observed Goss-textured AGG. Finally, GBCD results are presented that support the high energy grain boundary model as suitable for explaining Goss-textured AGG in polycrystalline (Fe81Ga19)+1.0 mol % NbC rolled sheet.
Micron-focused ion beamlets
Authors: Chowdhury Abhishek , Bhattacharjee Sudeep
Published: May 2010

A multiple beam electrode system (MBES) is used to provide focused ion beamlets of elements from a compact microwave plasma. In this study, a honeycomb patterned plasma electrode with micron size apertures for extracting ion beamlets is investigated. The performance of the MBES is evaluated with the help of two widely adopted and commercially available beam simulation tools, AXCEL-INP and SIMION, where the input parameters are obtained from our experiments. A simple theoretical model based upon electrostatic ray optics is employed to compare the results of the simulations. It is found that the results for the beam focal length agree reasonably well. Different geometries are used to optimize the beam spot size and a beam spot ∼5–10 μm is obtained. The multiple ion beamlets will be used to produce microfunctional surfaces on soft matter like polymers. Additionally, the experimental set-up and plans are presented in the light of above applications.
Microwave circuit model of the three-port transistor laser
Authors: Then H. W. , Feng M. , Holonyak N.
Published: May 2010

Based on an earlier charge control analysis, we have constructed a microwave circuit model of a three-port quantum-well (QW) transistor laser (TL) by extending Kirchhoff’s law to include electron-photon interaction, to yield an electrical-optical form of Kirchhoff’s law. The TL circuit model includes both intrinsic device elements and extrinsic parasitic elements, and fits accurately measured microwave S-parameters upto 20 GHz and matches also measured eye-diagram data up to 13 Gb/s (equipment-limited). The TL model yields both electrical and optical device parameters as well as physical quantities such as QW charge density, nQW∼1016 cm−3, which is useful in the analysis of the device physics of TL operation. The low density indicates that the base QW charge level is not as important as the current driving the QW and supplying electron-hole recombination, and implies that the quasi-Fermi level is discontinuous in the TL base. The model is used to simulate a directly modulated TL up to 40 Gb/s, for example, a TL employed in an optical communication link.
Space-charge limited conduction in doped polypyrrole devices
Authors: Anjaneyulu P. , Sangeeth C. S. Suchand , Menon Reghu
Published: May 2010

Doping dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements were carried out on polypyrrole devices in metal-polymer-metal sandwich structure. Temperature dependent I-V measurements infer that space-charge limited conduction (SCLC) with exponential trap distribution is appropriate for the moderately doped samples, whereas trap-free SCLC is observed in lightly doped samples. Trap densities and energies are estimated, the effective mobility is calculated using the Poole–Frenkel model, and the mobility exhibits thermally activated behavior. Frequency dependent capacitance-voltage characteristics show a peak near zero bias voltage, which implies that these devices are symmetric with a negligible barrier height at the metal-polymer interface. Low frequency capacitance measurements have revealed a negative capacitance at higher voltages due to the processes associated with the injection and redistribution of space-charges.
On the construction of coherent states of position dependent mass Schrödinger equation endowed with effective potential
Authors: Chithiika Ruby V. , Senthilvelan M.
Published: May 2010

In this paper, we propose an algorithm to construct coherent states for an exactly solvable position dependent mass Schrödinger equation. We use point canonical transformation method and obtain ground state eigenfunction of the position dependent mass Schrödinger equation. We fix the ladder operators in the deformed form and obtain explicit expression of the deformed superpotential in terms of mass distribution and its derivative. We also prove that these deformed operators lead to minimum uncertainty relations. Further, we illustrate our algorithm with two examples, in which the coherent states given for the second example are new.
Electron spin resonance in CuCrS2 chrome-copper disulphides synthesized by different methods
Authors: Abramova Galina , Pankrats Anatolii , Petrakovskii German , Rasch Julia C. E. , Boehm Martin , Vorotynov Aleksandr , Tugarinov Vasilii , Szumszak Rita , Bovina Asya , Vasil’ev Viktor
Published: May 2010

The electron spin resonance (ESR) in CuCrS2 disulphides is found to be strongly dependent on a synthesis method used. At a temperature of 300 K, a polycrystalline CuCrS2 sample is paramagnetic with a g-value of 1.95 at 40 K, it undergoes the magnetic transition. In the temperature range 4.2–290 K, a single-crystal sample prepared by a chemical vapor transport method exhibits the ESR features typical of a ferromagnet. It is shown that these features are related to the presence of a small amount of the single-crystal CuCr2S4 impurity in the CuCrS2 single crystal.
Simplified theory of carrier back-scattering in semiconducting carbon nanotubes: A Kane’s model approach
Authors: Bhattacharya Sitangshu , Mahapatra Santanu
Published: May 2010

We present a simplified yet analytical formulation of the carrier backscattering coefficient for zig-zag semiconducting single walled carbon nanotubes under diffusive regime. The electron-phonon scattering rate for longitudinal acoustic, optical, and zone-boundary phonon emissions for both inter- and intrasubband transition rates have been derived using Kane’s nonparabolic energy subband model. The expressions for the mean free path and diffusive resistance have been formulated incorporating the aforementioned phonon scattering. Appropriate overlap function in Fermi’s golden rule has been incorporated for a more general approach. The effect of energy subbands on low and high bias zones for the onset of longitudinal acoustic, optical, and zone-boundary phonon emissions and absorption have been analytically addressed. 90% transmission of the carriers from the source to the drain at 400 K for a 5 μm long nanotube at 105 V m−1 has been exhibited. The analytical results are in good agreement with the available experimental data.
Optical metrology of Ni and NiSi thin films used in the self-aligned silicidation process
Authors: Kamineni V. K. , Raymond M. , Bersch E. J. , Doris B. B. , Diebold A. C.
Published: May 2010

The thickness-dependent optical properties of nickel metal and nickel monosilicide (NiSi) thin films, used for self-aligned silicidation process, were characterized using spectroscopic ellipsometry. The thickness-dependent complex dielectric function of nickel metal films is shown to be correlated with the change in Drude free electron relaxation time. The change in relaxation time can be traced to the change in grain boundary (GB) reflection coefficient and grain size. A resistivity based model was used as the complementary method to the thickness-dependent optical model to trace the change in GB reflection coefficient and grain size. After silicidation, the complex dielectric function of NiSi films exhibit non-Drude behavior due to superimposition of interband absorptions arising at lower frequencies. The Optical models of the complete film stack were refined using x-ray photoelectron spectroscopy, Rutherford backscattered spectroscopy, and x-ray reflectivity (XRR).
Enhancement of stiffness of vertically standing Si nanosprings by energetic ions
Authors: Nagar Rupali , Mehta B. R. , Singh J. P. , Patzig C. , Rauschenbach B. , Kanjilal D.
Published: May 2010

In this study, the compressive strength of individual Si nanosprings grown by glancing angle deposition was determined by atomic force microscope based force-distance spectroscopy. Different irradiation conditions were employed to separately investigate the role of inelastic electronic excitations and elastic nuclear stopping on the stiffness of Si nanosprings. This was achieved by using different ion beams at various energies such that the energy deposited per unit volume (ε) ranged from less than 1 eV nm−3 to ∼MeV nm−3. For moderate values of energy densities, a logarithmic dependence of the stiffness change on ε was observed. Interestingly, stiffness of the nanosprings increased up to 32% without any visible deformation and is explained in terms of densification of structures caused via electronic excitations induced by energetic ions. However, for very high energy densities (∼MeV nm−3), an anisotropic deformation of nanosprings initiated by nuclear energy losses occurred predominantly and the stiffness of the nanosprings was observed to increase up to 170%. The present study demonstrates the role of electronic excitations and nuclear stopping in enhancing stiffness of Si nanosprings, and indicates that higher surface-to-volume ratio and shape anisotropy of nanostructures aid in delaying the process of anisotropic deformation during irradiation.
Electronic properties of BN/C nanotube heterostructures
Authors: Fan Yingcai , Zhao Mingwen , He Tao , Wang Zhenhai , Zhang Xuejuan , Xi Zexiao , Zhang Hongyu , Hou Keyu , Liu Xiangdong , Xia Yueyuan
Published: May 2010

We perform first-principles calculations to investigate the geometric and electronic properties of (10,0) and (5,5) BN/C nanotube heterostructures. We show that both of them have smooth interfaces which are free from bond mismatch and vacancy defect. Interface states appear in the band gaps, due to the discontinuity of π-π bonding of carbon nanotube segments, and exhibit asymmetric distribution in the two segments. The charge redistribution in the region near the interfaces gives rise to a build-in electric field and modulates the static electric potential profiles in the heterostructures. The band scheme diagrams of these heterostructures are also presented.
Magnetoelectric effect in lead-free BNKLBT ceramic/terfenol-D continue fiber composite laminates
Authors: Lo C. Y. , Choy S. H. , Or S. W. , Chan H. L. W.
Published: May 2010

A magnetostrictive-piezoelectric laminated composite has been developed by sandwiching a lead-free BNKLBT ceramic plate polarized in the thickness direction between two terfenol-D continuous fiber composite plates. This lead-free magnetoelectric (ME) laminated composite has a large ME voltage sensitivity of 2.5 V/Oe at the resonance frequency of 130.9 kHz under a low magnetic bias field (HBias) of 0.6 kOe. This work shows the potential of BNKLBT lead-free ceramics in ME sensing application.
Interlayer exchange coupling and current induced magnetization switching in magnetic tunnel junctions with MgO wedge barrier
Authors: Skowroński Witold , Stobiecki Tomasz , Wrona Jerzy , Rott Karsten , Thomas Andy , Reiss Günter , van Dijken Sebastiaan
Published: May 2010

Current induced magnetization switching and interlayer exchange coupling (IEC) in sputtered CoFeB/MgO/CoFeB exchange-biased magnetic tunnel junctions with an extremely thin (0.96–0.62 nm) MgO wedge barrier is investigated. The IEC is found to be ferromagnetic for all samples and the associated energy increases exponentially down to a barrier thickness of 0.7 nm. Nanopillars with resistance area product ranging from 1.8 to 10 Ω μm2 and sizes of 0.13 μm2 down to 0.03 μm2 and tunneling magnetoresistance values of up to 170% were prepared. We found, that the critical current density increases with decreasing MgO barrier thickness. The experimental data and theoretical estimations show that the barrier thickness dependence of the spin transfer torque can largely be explained by a reduction in the tunnel current polarization at very small barrier thickness.
Effects of nanoscale porosity on thermoelectric properties of SiGe
Authors: Lee Hohyun , Vashaee Daryoosh , Wang D. Z. , Dresselhaus Mildred S. , Ren Z. F. , Chen Gang
Published: May 2010

The recent achievement of the high thermoelectric figure of merit in nanograined materials is attributed to the successful optimization of the consolidation process. Despite a thermal conductivity reduction, it has been experimentally observed that the porous nanograined materials have lower thermoelectric figure of merit than their bulk counterpart due to significant reduction in the electrical conductivity. In this paper, nanoscale porosity effects on electron and phonon transport are modeled to predict and explain thermoelectric properties in porous nanograined materials. Electron scattering at the pores is treated quantum mechanically while phonon transport is treated using a classical picture. The modeling results show that the charge carriers are scattered more severely in nanograined materials than the macroscale porous materials, due to a higher number density of scattering sites. Porous nanograined materials have enhanced Seebeck coefficient due to energy filtering effect and low thermal conductivity, which are favorable for thermoelectric applications. However, the benefit is not large enough to overcome the deficit in the electrical conductivity, so that a high sample density is necessary for nanograined SiGe.
Anomalous electrical transport characteristics of orthorhombic HoMnO3/Nb-doped SrTiO3 epitaxial heterojunctions
Authors: Wang Weitian , Zhang Lixin , Yuan Dehua , Sun Yuming
Published: May 2010

Orthorhombic HoMnO3 films with c-axis and a-axis orientations were prepared epitaxially on Nb-1.0 wt %-doped SrTiO3 single crystal substrates by using pulsed laser deposition technique to fabricate all-oxide heterojunctions. The temperature dependent current-voltage measurements display diodelike rectifying behaviors, and the forward current was perfectly fitted using the thermionic emission model. An anomalous increase in the reverse-bias current with decreasing temperature was observed in c-axis-oriented HoMnO3/Nb-doped SrTiO3 junctions, which was not found in a-axis-oriented ones. The tunneling through the junction, which depends on the effective barrier height at the interface, was used to explain the results.
The Ho2MnRuO7 pyrochlore oxide: Magnetic structure versus magnetic frustration
Authors: Retuerto M. , Martínez-Lope M. J. , de la Calle C. , Martínez-Coronado R. , García-Hernández M. , Alonso J. A. , Fernández-Díaz M. T.
Published: May 2010

A pyrochlore-like phase of composition Ho2MnRuO7 has been synthesized by a soft-chemistry procedure followed by thermal treatments at moderate temperatures up to 900 °C for 12 h in air. It has been characterized by x-ray diffraction and neutron powder diffraction (NPD), as well as dc and ac susceptibilities. As in the parent Ho2Mn2O7 oxide, the magnetic Mn and Ru ions statistically occupy the 16c sites in a cubic unit cell with space group Fd3¯m, which define a potentially frustrated three-dimensional array of corner sharing (Mn,Ru)4 tetrahedra. The dc and ac magnetic susceptibilities of Ho2MnRuO7 display a sharp increase near 60 K. In addition, the field-cooled and zero-field-cooled curves diverge below 30 K. The ac data present frequency variability below 60 K and broad frequency dependent maxima at lower temperatures (∼30 K), suggesting a spin-glass like behavior similar to Ho2Mn2O7. However, a low-temperature NPD study of the magnetic structure unveils an antiferromagnetic coupling of two subsets of Mn4+/Ru4+ spins, indicating that the magnetic frustration is partially relieved by the random distribution of Mn and Ru over the 16c sites; at lower temperatures there is a polarization of the Ho3+ magnetic moments, which also participate in the magnetic structure. Under an external magnetic field the Ho moments become totally polarized, giving a saturation magnetization at 2 K of 10.8 μB/f.u. at 5 T.
Multilevel resistance switching in Cu/TaOx/Pt structures induced by a coupled mechanism
Authors: Yang Y. C. , Chen C. , Zeng F. , Pan F.
Published: May 2010

We report on multilevel bipolar resistance switching in Cu/amorphous-TaOx/Pt structures controlled by a coupled mechanism. The devices could be reproducibly programmed into three nonvolatile resistance states, and the on/off ratios between all neighboring states are >20. It is speculated that the switching between the high resistance state and the intermediate resistance state originates from a phase transformation between Ta2O5 and TaO2, while the low resistance state is induced by the formation of Cu filaments. This structure might be promising for developing multilevel logic and memory devices with high reliability. It may also serve as a model system for investigating the coupling effect between different switching mechanisms.
High power microwave generation from coaxial virtual cathode oscillator using graphite and velvet cathodes
Authors: Menon Rakhee , Roy Amitava , Singh S. K. , Mitra S. , Sharma Vishnu , Kumar Senthil , Sharma Archana , Nagesh K. V. , Mittal K. C. , Chakravarthy D. P.
Published: May 2010

High power microwave (HPM) generation studies were carried out in KALI-5000 pulse power system. The intense relativistic electron beam was utilized to generate HPMs using a coaxial virtual cathode oscillator. The typical electron beam parameters were 350 kV, 25 kA, and 100 ns, with a few hundreds of ampere per centimeter square current density. Microwaves were generated with graphite and polymer velvet cathode at various diode voltage, current, and accelerating gaps. A horn antenna setup with diode detector and attenuators was used to measure the microwave power. It was observed that the microwave power increases with the diode voltage and current and reduces with the accelerating gap. It was found that both the peak power and width of the microwave pulse is larger for the velvet cathode compared to the graphite cathode. In a coaxial vircator, velvet cathode is superior to the graphite cathode due to its shorter turn on time and better electron beam uniformity.
Optical spectroscopy investigation on distribution of Eu3+ in nanostructured glass ceramics
Authors: Yu Yunlong , Weng Fangyi , Chen Daqin , Huang Ping , Wang Yuansheng
Published: May 2010

The time-resolved luminescence spectra of the Eu3+ doped SiO2–Al2O3–NaF–YF3–EuF3 precursor glass reveal that Eu3+ ions locate in both the oxygen-coordination environment and the fluorine-coordination one. After crystallization induced by heat treatment, the orthorhombic YF3 nanocrystals with mean size of 22 nm embedded homogeneously in the glassy matrix. The Stark splitting emission, the low electric dipole D50→F72 transition, the disappearance of the O2−–Eu3+ charge transfer band and the reduction in Ω2 value indicate the partition of Eu3+ into the YF3 lattice. Moreover, the nearly single-exponential luminescence decay curves of the Eu3+:D50 and D51 levels for the 0.1% Eu3+ doped glass ceramic evidence that Eu3+ ions mainly occupy the Y3+ sites.
Relationship between carbon nanotube density and hysteresis characteristics of carbon nanotube random network-channel field effect transistors
Authors: Hongo Hiroo , Nihey Fumiyuki , Yorozu Shinichi
Published: May 2010

We have investigated the hysteresis in the transfer characteristics of carbon nanotube (CNT) field effect transistors (FETs). The relationship between the hysteresis width and channel CNT density was studied for FETs of spun-on random network CNTs. The hysteresis width in the transfer characteristics decreases with an increase in the channel CNT density. To explain this relationship, we carried out simulations combining two models such as: the hysteresis model and the gate electric field model. Considering the finite channel thickness and trap-assisted tunneling current, the simulation described the experiments well. The simulation implies the possibility of reducing hysteresis by relaxing the strong gate electric field at the CNT surface. We also discuss other possibilities of hysteresis reduction on FETs whose channels are of nanoscale cylindrical material.
Structural, electronic, and optical properties of α, β, and γ-TeO2
Authors: Li Yanlu , Fan Weiliu , Sun Honggang , Cheng Xiufeng , Li Pan , Zhao Xian
Published: May 2010

First-principles calculations of the structural, electronic, and optical properties of TeO2 polymorphs were performed with the density functional theory plane-wave pseudopotential method. The results reveal that all the three crystalline TeO2 phases are wide-gap semiconductors and the lone electron pairs have contributions near the Fermi energy level. The layer structure of β-TeO2 leads to the obvious anisotropy of the complex dielectric function. Considering the lattice contribution of dielectric constants, we predict the static dielectric constants of TeO2 polymorphs. For α-TeO2, the calculated values of 19.0 for ε1⊥ and 25.3 for ε1∥ agree well with the experimental value, and the β- and γ-phases also belong to the high dielectric constant materials. Besides, a special collective plasma resonance for γ-TeO2 has been found in lower energy. It corresponds to the small peak in the imaginary part of dielectric function, and reflects the abrupt reduction in the reflectivity spectrum.
Cluster spin glass behavior in Bi(Fe0.95Co0.05)O3
Authors: Xu Qingyu , Zhou Shengqiang , Wu D. , Uhlarz Marc , Tang Y. K. , Potzger Kay , Xu M. X. , Schmidt Heidemarie
Published: May 2010

The static and time-dependent magnetic properties of Bi(Fe0.95Co0.05)O3 have been investigated by dc and ac magnetization measurements. Cluster spin glass has been confirmed to be the ground state due to the frustration of the interaction between the ferromagnetic clusters by canted antiferromagnetically arranged Fe spins (Tc>350 K) and by ferrimagnetically arranged Co and Fe spins (Tc∼250 K). Two freezing temperatures Tf of about 260 and 100 K have been clearly identified from the abrupt change in magnetization relaxation. The higher Tf is related to the canted antiferromagnetically arranged Fe spins, and the lower Tf is related to the ferrimagnetically arranged Co and Fe spins. The cluster spin glass behavior with two freezing temperatures has been confirmed by ac susceptibility measurements.
Enhancement of Faraday rotation effect in heterostructures with magneto-optical metals
Authors: Dong Lijuan , Jiang Haitao , Chen Hong , Shi Yunlong
Published: May 2010

We theoretically study the transmission and Faraday rotation effect in the heterostructure composed of an all-dielectric photonic crystal and a magnetic metal. Under the tunneling mechanism, the electromagnetic fields can totally enter the structure and localize at the interface between photonic crystal and magnetic metal. Because of the localized electromagnetic fields, the transmission and Faraday rotation in the heterostructure are simultaneously boosted.
Charge sensitive infrared phototransistor for 45 μm wavelength
Authors: Wang Zhihai , Komiyama Susumu , Ueda Takeji , Patrashin Mikhail , Hosako Iwao
Published: May 2010

The detection wavelength of charge-sensitive infrared phototransistors (CSIPs), originally developed for 15 μm wavelength radiation, is expanded to longer wavelengths of ∼45 μm. The CSIPs are fabricated on GaAs/AlGaAs crystals with bilayer two-dimensional electron gas. Photoresponse at targeted wavelengths is confirmed. Low quantum efficiency of photoresponse, on the order of 10−4, has been ascribed to electron traps (Al–O complex) contained in an AlGaAs barrier layer. Several possible approaches for improving the detector performance are suggested.
Giant electrorheological fluid comprising nanoparticles: Carbon nanotube composite
Authors: Li Jiaxing , Gong Xiuqing , Chen Shuyu , Wen Weijia , Sheng Ping
Published: May 2010

We have fabricated suspensions exhibiting the giant electrorheological (GER) effect comprising nanoparticles—multiwall carbon nanotubes (MCNTs) composite particles dispersed in silicone oil. This type of GER fluids display dramatically enhanced antisedimentation characteristic without sacrificing the yield stress. The nanoparticles-nanotubes composites were fabricated by modifying the coprecipitation method with MCNTs and urea-coated barium titanyl-oxylate (BTRU) nanoparticles as the components. The composite solid particles are denoted MCNT-BTRU. In the best cases, stabilized suspensions with MCNT-BTRU particles dispersed in silicone oil have been maintained for several months without any appreciable sedimentation being observed. Both the sedimentary and rheological properties of the MCNT-BTRU suspension were systematically studied and compared with their BTRU counterparts. Yield stress as high as 194 kPa was obtained in the MCNT-BTRU suspensions. The MCNT-BTRU based GER fluids, with their antisedimentation characteristic, may have broad engineering applications.
Extraction of the tunnel magnetocapacitance with two-terminal measurements
Authors: Chang Yin-Ming , Li Kai-Shin , Huang Hongbo , Tung Mean-Jue , Tong Shi-Yuan , Lin Minn-Tsong
Published: May 2010

The tunnel magnetocapacitance (TMC) of the magnetic tunnel junction has been investigated with a series of complex impedance spectra measured at varying magnetic field. To avoid the circuit complication in the four-terminal measurement with high frequency operation, two-terminal approach was developed by elimination of spin independent contribution apart from the junction area. A subsequent fitting process based on the difference spectra analysis gave the TMC ratio of −0.43% with the opposite dependence on the field as compared to the tunnel magnetoresistance (TMR) of 30.67%. This technique would be applied in the further development and integration of spintronics devices.
Two-dimensional subwavelength-focused imaging using a near-field probe at a λ/4 working distance
Authors: Markley Loïc , Eleftheriades George V.
Published: May 2010

Two-dimensional (2D) subwavelength imaging using a near-field antenna array probe is demonstrated experimentally at a probe-to-object separation distance of λ/4. Field perturbations caused by the presence of small objects are detected by monitoring the input reflection coefficient as the probe is scanned. The probe is designed to produce a subwavelength focal spot with a 0.217λ full-width half-maximum beam width. Propagating components are suppressed through destructive interference, enhancing the evanescent field enough to resolve objects spaced by 0.259λ. The poor resolving capability of a conventional single-element probe at this working distance is also addressed; it is shown that even objects spaced over a wavelength apart are inaccurately detected. A full 2D imaging experiment is carried out using eight scattered objects. The single element probe is unable to resolve any of the objects, while all but the two most closely spaced pairs are resolved by the array probe.
Indentation deformation mechanism in glass: Densification versus shear flow
Authors: Rouxel T. , Ji H. , Guin J. P. , Augereau F. , Rufflé B.
Published: May 2010

Although the characteristic time constant for viscous relaxation of glass is so large at room temperature that viscous flow would be hardly detectable, a permanent deformation can be easily achieved at ambient temperature by applying a sharp contact loading—a Vickers indenter for instance—for few seconds only. We provide direct evidence for densification and volume conservative shear flow by means of atomic force microscopy topological analysis of the indentation profile and volume on as-quenched and densified specimens (pressure up to 25 GPa). We show that both possible mechanisms contribute to different extents depending on the glass composition. A major finding is that densification predominates in glasses with relatively low atomic packing density but that shear flow relays on once densification is achieved.
Raman scattering, electronic, and ferroelectric properties of Nd modified Bi4Ti3O12 nanotube arrays
Authors: Zhou Di , Gu Haoshuang , Hu Yongming , Qian Zheli , Hu Zhenglong , Yang Kun , Zou Yanan , Wang Zhao , Wang Yu , Guan Jianguo , Chen Wanping
Published: May 2010

Neodymium-modified bismuth titanate, Bi4−xNdxTi3O12 (BNdT), nanotube arrays were fabricated by sol-gel method utilizing experimentally prepared porous anodic aluminum oxide (AAO) templates with pore diameters of about 200 nm and 100 nm, respectively. The as-prepared nanotube arrays exhibit orthorhombic perovskite polycrystalline structure of BNdT, which have outer diameters of about 200 and 100 nm, corresponding to the pores diameters of the AAO templates employed, and with wall thicknesses of about 9.7 nm and 12 nm, respectively. The phonon vibration modes corresponding to the Bi atoms in the Bi2O2 layers weaken and broaden with increasing Nd content. The changes of Raman internal modes originated from the vibrations of atoms inside the TiO6 octahedral indicate the increase in octahedron tilting and structural distortion. The leakage current and polarization-electric field response curves of BNdT nanotube arrays were measured, and the hysteresis loop illustrates a good ferroelectric property of as-prepared BNdT nanotube array at room temperature. The dielectric constant and dissipation factor were measured in the frequency region from 1 kHz to 1 MHz indicating polarization relaxation phenomenon.
Effects of Li+ on structure and spectroscopic properties of Er3+/Li+ codoped Sb2O3–Na2O–SiO2 glasses
Authors: Liu Lu , Wang Yuxiao , Bai Yunfeng , Zhang Xueru , Yang Kun , Yang Le , Song Yinglin
Published: May 2010

Judd–Ofelt (JO) analysis of Er3+ ions spectral properties in Er3+ doped and Er3+/Li+ codoped Sb2O3–Na2O–SiO2 glasses are performed based on the optical absorption spectra at room temperature. Three phenomenological parameters Ω2, Ω4, and Ω6 are determined. The effects of Li+ on the JO parameters are discussed in terms of the variations in Er3+ bonding nature and ligand fields around Er3+ sites.
Spectroscopic investigation of 2.02 μm emission in Ho3+/Tm3+ codoped transparent glass ceramic containing CaF2 nanocrystals
Authors: Chen Q. J. , Zhang W. J. , Qian Q. , Yang Z. M. , Zhang Q. Y.
Published: May 2010

Tm 3 + and Ho3+ codoped transparent glass ceramic (GC) containing CaF2 nanocrystals were fabricated by melt-quenching and subsequent thermal treatment. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of CaF2 nanocrystals among the glass matrix. Energy-dispersive x-ray spectroscopy results evidenced the incorporation of Tm3+ and Ho3+ into the CaF2 nanocrystals. Judd–Ofelt parameters were calculated based on the absorption spectra, the smaller Ω2 and larger Ω6 imply that Tm3+ and Ho3+ ions has entered into a symmetrical ionic crystal field. 2.02 μm emission spectral of the GC samples were recorded at room temperature with an excitation of 808 nm laser diode. The enhancement of the emission at 2.02 μm in the GC samples could be attributed to more efficient cross relaxation process of Tm3+:H34+Tm3+:H36→2Tm3+:H34, and energy transfer from Tm3+ to Ho3+ benefited from the incorporation of rare earth ions into CaF2 nanocrystals.
The spin entropy suppression induced by Fe3+ in NaCo2O4
Authors: Tang G. D. , Xu X. N. , Tang C. P. , Qiu L. , Lv L. Y. , Wang Z. H. , Du Y. W.
Published: May 2010

Through the measurements of magnetothermopower on NaCo2−xFexO4, we find that the spin entropy is suppressed by doping with high-spin Fe3+ (S=5/2) which is revealed by Mössbauer spectra. The Mössbauer spectra and magnetic properties indicate that the Co4+ concentration is enhanced by Fe doping. A suitable theoretical model is adopted to explain commendably the spin entropy suppression.
Probing triplet-triplet annihilation zone and determining triplet exciton diffusion length by using delayed electroluminescence
Authors: Luo Yichun , Aziz Hany
Published: May 2010

The literature shows that triplet-triplet annihilation (TTA) can provide a substantial contribution to the electroluminescence (EL) of fluorescent organic light-emitting devices (OLEDs). In this study, we utilized delayed EL measurements to probe the TTA emission zone of archetypical 8-hydroxyquinoline aluminum (Alq3) based OLEDs. The results demonstrate that the TTA emission zone of these devices is much larger than the prompt emission zone of singlet states that are formed in the electron-hole recombination. The larger TTA emission zone is attributed to the longer diffusion length of the Alq3 triplet states (60 nm) than that of Alq3 singlet states (20 nm).
Kinetic Monte Carlo simulations of temperature programed desorption of O/Rh(111)
Authors: Franz T. , Mittendorfer F.
Published: May 2010

We present a kinetic Monte Carlo simulation based on ab initio calculations for the thermal desorption of oxygen from a Rh(111) surface. Several models have been used for the parametrization of the interaction between the adsorbed atoms. We find that models based on a parametrization with only pairwise interactions have a relatively large error in the predicted adsorption energies. This error can be significantly reduced by including three- and four-body interactions. In addition, we find that a significant amount of atoms adsorb in a second adsorption site — the hcp-hollow site — at an elevated temperature. Consequently, only a many-body multisite model of the oxygen interactions yields appropriate desorption spectra for the full coverage range, while more simple models only capture the correct shape in the low-coverage case. Our parametrization allows us to predict the adsorption energies of an arbitrary configuration of adsorbates with a mean average error of less than 6 meV/atom.
Enhancing magnetoelectric effect via the curvature of composite cylinder
Authors: Wang H. M. , Pan E. , Chen W. Q.
Published: May 2010

We solved analytically the magnetoelectric (ME) effect in a bilayered piezoelectric/piezomagnetic cylinder under harmonic excitation. We revealed that at a fixed thickness ratio of the layers, the static or low-frequency ME effect can be substantially enhanced by increasing the curvature of the cylinder. In the megahertz frequency domain, on the other hand, we observed that the peak ME effect can be considerably increased by decreasing the curvature. We further showed that at a fixed curvature, the ME effect can be tuned to be around the resonant frequency for giant output by varying the boundary condition and thickness ratio.
Effects of the sintering temperature on the diffused phase transition and the spin-glassy behavior in Pb0.95La0.05(Fe2/3W1/3)0.65Ti0.35O3 ceramics
Authors: Hong Cheng-Shong , Chu Sheng-Yuan , Hsu Chi-Cheng
Published: May 2010

In this paper, the effect of the sintering temperature on the low-field dielectric behavior of nonstoichiometric Pb0.95La0.05(Fe2/3W1/3)0.65Ti0.35O3 relaxor ferroelectrics is investigated. The x-ray patterns and the scanning electron microscope images are used to detect the pyrochlore phase and the perovskite structure. The electric properties of the resistivity, the space charge polarization, the temperature-dependent dielectric constant and dielectric loss are discussed. The diffused phase transition and the ordering state are fitted and discussed by using the empirical law and two ordering models. Furthermore, the glassy behavior is determined by using the Curie–Weiss law and the spin-glass model. According to the experimental data and fitting results, the dielectric picture is changed from the short range order relaxorlike behavior to the long range order normal ferroelectric state as increasing the sintering temperature and the glassy behavior is weakened at the lowest and highest sintering temperature at which the pyrochlore phase PWO4 is induced. Therefore, it is suggested that the 1:1 ordered domain is enhanced by increasing the sintering temperature and the glassy behavior is related to not only the ordering degree also the polar defect pairs. For more ordering degree and polar defect pairs, the glassy is weakened and the correlation of neighboring polar microregions is enhanced.
Raman spectroscopic study of the electrical properties of 6H–SiC crystals grown by hydrogen-assisted physical vapor transport method
Authors: Peng Yan , Xu Xiangang , Hu Xiaobo , Jiang Kai , Song Sheng , Gao Yuqiang , Xu Huayong
Published: May 2010

Raman spectroscopy has been used to determine the carrier concentration of 6H–SiC crystal so as to understand the effects of hydrogen on the electrical property of 6H–SiC crystal grown by hydrogen-assisted physical vapor transport method. The spatial distribution of the carrier concentration is determined in a longitudinally-cut sample grown by an on-off hydrogen supply based on the empirical relationship between the frequency of the LO-phonon-plasmon-coupled modes (LOPC modes) and carrier concentration. It is found that the carrier concentration dramatically drops when the supply of hydrogen turns on and decreases more quickly in the hydrogen-assisted region than that in the undoped region. It is proposed that the vapor phase shifts toward more C-rich condition in case of hydrogen-assisted growth. As a consequence, the N incorporation in 6H–SiC crystal is depressed and the inactive electrically center Vc+H complexes are formed during hydrogen-assisted physical vapor transport SiC crystal growth.
Structural, electronic, and magnetic properties of Co doped SnO2 nanoparticles
Authors: Sharma Aditya , Singh Abhinav Pratap , Thakur P. , Brookes N. B. , Kumar Shalendra , Lee Chan Gyu , Choudhary R. J. , Verma K. D. , Kumar Ravi
Published: May 2010

We present a detailed study on the structural, electronic, and magnetic properties of chemically synthesized Sn1−xCoxO2 (x=0.00 to 0.05) nanoparticles. X-ray diffraction and transmission electron microscope measurements were performed to analyze the crystal structure and morphology of Sn1−xCoxO2 nanoparticles. The energy dispersive x-ray analysis measurements were performed to check the possible presence of any impurity elements in the nanocrystals. The near edge x-ray absorption fine structure (NEXAFS) experiments at Sn M5,4-edge and Co L3,2-edge were performed to probe the local environment of Sn and Co ions in the SnO2 matrix. The NEXAFS at Co L3,2-edge, along with multiplet calculations, indicate that the Co is substituted at the Sn site in SnO2 matrix with +2 charge state and do not form metallic clusters and other oxide phases. The ferromagnetic nature of these materials was confirmed by x-ray magnetic circular dichroism and room temperature magnetization hysteresis loop measurements.