bluuebunny/crossref_metadata_2025 huggingface dataset:
Vacancy-induced enhancement of magnetic interactions in (Ca, Na)-doped lanthanum manganites
Authors: Tovstolytkin A. I. , Pogorily A. M. , Podyalovskii D. I. , Kalita V. M. , Lozenko A. F. , Trotsenko P. O. , Ryabchenko S. M. , Belous A. G. , V’yunov O. I. , Yanchevskii O. Z.
Published: September 2007

Structural, electric, and magnetic properties of the bulk polycrystalline samples with nominal composition La0.7Ca0.3−xNaxMnO3 (x=0−0.10) have been investigated in the work. As follows from the results of chemical and x-ray structural analyses, the increase in x brings about the rise in the number of structural vacancies in both lanthanum and oxygen sublattices. The influence of the variation of the vacancy number on peculiar features of the transition from ferromagnetic metallic state to paramagnetic state with activated conductivity has been studied in detail. The conclusion is made that the rise in the number of structural vacancies gives rise not only to the enhancement of magnetic inhomogeneity and broadening of the magnetic transition, but also to the increase in the temperature of magnetic transition. It is demonstrated that the changes in magnetic parameters also result in the increase of the temperatures of resistivity and magnetoresistance peaks as well as in the change of the character of the temperature dependence of electric resistance.
Noise characterization of light-emitting devices based on conjugated copolymers of fluorene and thiophene moieties
Authors: Ke Lin , Tang Weihua , Song Yang , Chen Zhi Kuan , Chua Soo Jin
Published: September 2007

Degradation induced changes in the structural and optical properties of polyfluorene-based light-emitting diodes are examined by using electroluminescence and low frequency noise (LFN) spectroscopic techniques. The materials studied are poly[2,7-(9,9′-dihexylfluorene)-alt-bithiophene] (P1) and poly[2,7-(9,9′-dihexylfluorene)-alt-thieno[3,2-b]thiophene] (P2). Improved emission spectra for a light-emitting device based on polymer P2 in terms of current efficiency, spectra stability, and lifetime are observed. A polymer P2-based device also presents long lifetime predicted by the smaller slope in the initial LFN spectra. Correlation of device LFN spectra with polymer structure change and lifetime is established. The increase in noise level predicts the undergoing degradation in bulk material and the increase in the noise slope predicts the fluctuation of carrier number and change in polymer structure. The redshift in emission spectrum for P2 after long-time driving is also picked up by the LFN spectrum.
Far-infrared, Raman spectroscopy, and microwave dielectric properties of La(Mg0.5Ti(0.5−x)Snx)O3 ceramics
Authors: Babu G. Santosh , Subramanian V. , Murthy V. R. K. , Lin I-Nan , Chia Chia-Ta , Liu Hsiang-Lin
Published: September 2007

La ( Mg 0.5 Ti ( 0.5 − x ) Sn x ) O 3 perovskite ceramics with composition (x=0.0−0.5) are prepared by the solid state reaction method. The ceramics are characterized by x-ray diffraction, far-infrared reflectance, Raman spectroscopy, and microwave dielectric properties. The symmetry of ceramics is monoclinic with P21/n space group. Intrinsic dielectric constant and loss are estimated by fitting reflectance to the four-parameter semiquantum model. Transverse optic phonon mode strengths and average phonon damping are calculated. The modes corresponding to B-site ordering are identified in Raman spectra and the A1g mode of La(MgTi)0.5O3 is analyzed by assuming two merging modes. The variation of long-range order is correlated with full width half maximum of the A1g mode. Microwave measurements are carried out in the frequency range of 8–10 GHz. The dielectric constant (ε′) is found to gradually decrease from 28.4 to 19.7 with an increase in tin concentration, whereas the temperature coefficient of resonant frequency (τf) decreases from −68 to −84 ppm/°C. The product of the quality factor and resonant frequency (Q×f) obtained for La(MgTi)0.5O3 is 55,000 GHz, that decreases to 46,000 for x=0.25 composition and then increases to 63,000 GHz for La(MgSn)0.5O3.
Anisotropic optical constants of electroluminescent conjugated polymer thin films determined by variable-angle spectroscopic ellipsometry
Authors: Winfield Jessica M. , Donley Carrie L. , Kim Ji-Seon
Published: September 2007

This article reports on in-plane and out-of-plane refractive index (n) and extinction coefficient (k) values measured using variable-angle spectroscopic ellipsometry for poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) thin films of different molecular weights (Mn=9−255 kg∕mol), both in the pristine and annealed states. The in-plane n and k values are generally larger than the out-of-plane values for all pristine films leading to a measurable optical anisotropy which becomes much stronger as F8BT molecular weight increases. This indicates that polymer chains lie preferentially in the plane of the substrate and this configuration is more energetically favorable for longer polymer chains. Upon annealing, a larger reduction in kout-of-plane than in kin-plane is measured leading to a further increase in optical anisotropy. A redistribution of oscillator strengths and a broadening toward lower energies in absorption spectra are also observed indicating significant restructuring of F8BT chains upon annealing.
Spectroscopic investigation of light-emitting porous silicon photoetched in aqueous HF∕I2 solution
Authors: Adachi Sadao , Oi Mitsuru
Published: September 2007

The optical properties of porous silicon (PSi) photoetched in aqueous HF∕I2 solution are investigated using spectroellipsomety (SE), electroreflectance (ER), photovoltage (PV), photoconductivity (PC), photoluminescence (PL), and Fourier transform infrared (FTIR) spectroscopy. The PSi layers were formed in a HF∕I2 solution on n-Si substrates under Xe lamp illumination. The SE ε(E) and related data show an interference oscillation in the region below E∼3 eV, where the PSi material is nearly transparent. The PV and PC spectra reveal three individual peaks A, B, and C at ∼1.2, ∼1.7, and ∼2.5 eV, respectively, arising from the PSi layer itself. Peak C is also observed in the ER spectrum, together with a broadened E1 peak at ∼3.4 eV. Change in the fundamental-absorption-edge nature (EgX) from the indirect gap in crystalline silicon to the quasidirect gap in PSi is found in the PV and PC spectra. The PL spectrum shows a broad peak at ∼2.0 eV(B). Peaks A, B, and C observed in the PSi layer may originate from the nondirect optical transitions at and above the lowest absorption edges EgX (A and B) and EgL(C). The quantum-mechanical size effect, i.e., a relaxation of the momentum conservation, makes possible the nondirect or quasidirect transitions at and above EgX and EgL in porous materials. The FTIR data support that the PL emission is due to the surface-sensitive quantum confinement effect.
Effects of oxygen partial pressure on packing density and laser damage threshold of TiO2 thin films
Authors: Yao Jianke , Fan Zhengxiu , Jin Yunxia , Zhao Yuanan , He Hongbo , Shao Jianda
Published: September 2007

TiO 2 films are deposited by electron beam evaporation as a function of oxygen partial pressure. The packing density, refractive index, and extinction coefficient all decrease with the increase of pressure, which also induces the change of the film’s microstructure, such as the increase of voids and H2O concentration in the film. The laser-induced damage threshold (LIDT) of the film increases monotonically with the rise of pressure in this experiment. The porous structure and low nonstoichiometric defects absorption contribute to the film’s high LIDT. The films prepared at the lowest and the highest pressure show nonstoichiometric and surface-defects-induced damage features, respectively.
Self-fabricated single mode waveguide in fluoride glass excited by self-channeled plasma filaments
Authors: Cho Sung-Hak , Chang Won-Seok , Kim Jae-Goo , Whang Kyoung-Hyun
Published: September 2007

Self-fabricated permanent structure of single mode waveguide in optical fluoride glasses was demonstrated using the self-channeled plasma filament excited by a femtosecond (130fs) Ti:sapphire laser (λp=790nm). The photoinduced refractive index modification in ZrF4–BaF2–LaF3–AlF3–NaF glasses reached a length of approximately 10–15mm from the input surface of the optical glass with the diameters ranging from 5to8μm at the input intensities of more than 1.0×1012W∕cm2. The graded refractive index profiles were fabricated to be a symmetric form from the center of optical fluoride glass, and a maximum value of refractive index change (Δn) was measured to be 1.3×10−2. The beam profile of the output beam transmitted through the modified region showed that the photoinduced refractive index modification produced a permanent structure of single mode waveguide.
Universal frequency-dependent ac conductivity of conducting polymer networks
Authors: Papathanassiou A. N. , Sakellis I. , Grammatikakis J.
Published: September 2007

A model based on the aspect of the distribution of the length of conduction paths accessible for electric charge flow reproduces the universal power-law dispersive ac conductivity observed in polymer networks and, generally, in disordered matter. Power exponents larger than unity observed in some cases are physically acceptable within this model. A saturation high frequency region is also predicted, in agreement with experimental results. There exists not a “universal fractional power law” (and it is useless searching for a unique common critical exponent) but a qualitative universal behavior of the ac conductivity in disordered media.
Active tuning of photonic device characteristics during operation by ferroelectric domain switching
Authors: Dayal Kaushik , Bhattacharya Kaushik
Published: September 2007

Ferroelectrics have many unusual properties. Two properties that are often exploited are first, their complex, nonlinear optical response and second, their strong nonlinear coupling between electromagnetic and mechanical fields through the domain patterns or microstructure. The former has led to the use of ferroelectrics in optical devices and the latter is used in ferroelectric sensors and actuators. We show the feasibility of using these properties together in nanoscale photonic devices. The electromechanical coupling allows us to change the domain patterns or microstructure. This in turn changes the optical characteristics. Together, these could provide photonic devices with tunable properties. We present calculations for two model devices. First, in a photonic crystal consisting of a triangular lattice of air holes in barium titanate, we find the change in the band structure when the domains are switched. The change is significant compared to the frequency spread of currently available high-quality light sources and may provide a strategy for optical switching. Second, we show that periodically poled 90° domain patterns, despite their complex geometry, do not cause dispersion or have band gaps. Hence, they may provide an alternative to the antiparallel domains that are usually used in quasiphase matching and allow for tunable higher-harmonic generation.
Polarized spectral characterization and laser demonstration of Nd3+:Li2Gd4(MoO4)7 crystal
Authors: Zhu Haomiao , Lin Yanfu , Chen Yujin , Gong Xinghong , Tan Qiguang , Luo Zundu , Huang Yidong
Published: September 2007

Detailed polarized spectral properties of Nd3+:Li2Gd4(MoO4)7 crystal grown by the Czochralski method have been investigated. The Judd-Ofelt intense parameters Ω2,4,6, the absorption and emission cross sections, and the branching ratios were calculated for each polarization of the main transitions. The large absorption cross section (16.6×10−20cm2 of π polarization) and broad absorption band (7nm of π polarization) around 804nm indicate that this crystal can be pumped efficiently by diode lasers. The broad emission bands from the F3∕24 multiplet show that the crystal is a promising medium for tunable and short pulse lasers. Pumped by a Ti:sapphire laser, the maximum 514mW quasi-cw laser output around 1060nm has been obtained with a 1.5% output couple, the threshold is 35mW, and the slope efficiency is 32%.
Measurement of energy transfer at an isolated substrate in a pulsed dc magnetron discharge
Authors: Čada M. , Bradley J. W. , Clarke G. C. B. , Kelly P. J.
Published: September 2007

The power density delivered by particles to an electrically isolated substrate in an asymmetric bipolar pulsed dc unbalanced magnetron has been quantified. The plasma source was operated in argon with a titanium target, and measurements were made using both a calorimeter probe and time-resolved Langmuir probe incorporated into a specially made substrate holder. The main results from the calorimeter probe show clearly that with increased pulse frequency (from dc to 350kHz) and reduced duty cycle (90%–50%), the particle power density (from ions, electrons, sputtered Ti, and backscattered Ar) at the substrate increases significantly. For instance, at 350kHz and 60% duty cycle, the total power density is 83mW∕cm2, about 60% higher than in dc mode for the same time-average discharge power. However, from an inventory of the individual particle contributions to the total power density derived from time-resolved Langmuir measurements and a simple model of the substrate sheath and plasma internal processes, we predict values of power density much lower than those measured. The measured and calculated values are in close agreement for the results obtained in dc mode but diverge at high frequencies. It is believed that this is due to the Langmuir probe measurements being unable to observe the presence of high-energy ions, created during the transient peaks in the electron temperature at the transitions from on off and off on [J. W. Bradley et al., Plasma Sources Sci. Technol. 11, 165 (2002)] which subsequently bombard the substrate. This paper shows conclusively the benefit of pulsing the magnetron over and above dc operation for enhancing the ion power per depositing neutral in the ion assisted deposition process.
Sulfur surface chemistry on the platinum gate of a silicon carbide based hydrogen sensor
Authors: Kahng Yung Ho , Tobin R. G. , Loloee Reza , Ghosh Ruby N.
Published: September 2007

We have investigated the effects of sulfur contamination on a Pt-gate silicon carbide based field-effect gas sensor, under ultrahigh vacuum conditions, at a temperature of 527°C. Exposure to hydrogen sulfide, even in the presence of hydrogen or oxygen at partial pressures of 20–600 times greater than the H2S level, rapidly coated the gate with a monolayer of sulfur. Sulfur contamination reduced the magnitude of the sensor’s response to alternating hydrogen and oxygen pulses by about 70%, as compared to the uncontaminated gate. There was no evidence of irreversible changes in device behavior due to sulfur deposition and removal. The adsorbed sulfur could not be removed by exposure to hydrogen at the pressures accessible. Oxygen was effective at removing the sulfur. The rate of sulfur oxidation was suppressed at high sulfur coverages, but not as strongly as on low-index single-crystal surfaces. These results are discussed in the context of prior experiments on Pt crystals and films.
Annealing effects on electrical and optical properties of ZnO thin-film samples deposited by radio frequency-magnetron sputtering on GaAs (001) substrates
Authors: Liu H. F. , Chua S. J. , Hu G. X. , Gong H. , Xiang N.
Published: September 2007

The effects of thermal annealing on Hall-effect measurement and photoluminescence (PL) from undoped n-type ZnO/GaAs thin-film samples have been studied. The evolutions of carrier concentration, electrical resistivity, and PL spectrum at various annealing conditions reveal that the dominant mechanism that affects the electrical and PL properties is dependent on the amount of thermal energy and the ambient pressure applied during the annealing process. At low annealing temperatures, annihilation of native defects is dominant in reducing the carrier concentration and weakening the low-energy tail of the main PL peak, while the GaAs substrate plays only a minor role in carrier compensations. For the higher temperatures, diffusion of Ga atoms from the GaAs substrate into ZnO film leads to a more n-type conduction of the sample. As a result, the PL exhibits a high-energy tail due to the high-level doping.
A growth diagram for plasma-assisted molecular beam epitaxy of In-face InN
Authors: Gallinat C. S. , Koblmüller G. , Brown J. S. , Speck J. S.
Published: September 2007

We investigated the role of temperature and In∕N flux ratios to determine suitable growth windows for the plasma-assisted molecular beam epitaxy of In-face (0001) InN. Under vacuum, InN starts decomposing at 435°C as defined by the release of N2 from the InN crystal and a buildup of an In adlayer and liquid In droplets on the sample surface. At temperatures greater than 470°C, InN decomposition was characterized by a release of both In vapor and N2 in the absence of a significant accumulation of an In adlayer. No growth was observed at substrate temperatures above 500°C or at temperatures in which the decomposition rates were higher than the growth rates. A growth diagram was then constructed consisting of two growth regimes: the “In-droplet regime” characterized by step-flow growth and relatively flat surfaces and the “N-rich regime” characterized by rough, three-dimensional surfaces. The growth diagram can then be used to predict the surface structure of films grown at varying substrate temperatures and In fluxes. A 2.5 monolayer In adlayer was observed during In-droplet growth, suggesting that an In wetting layer was necessary for step-flow growth.
Fabrication and characterization of MgO-based magnetic tunnel junctions for spin momentum transfer switching
Authors: Assefa Solomon , Nowak J. , Sun J. Z. , O’Sullivan E. , Kanakasabapathy S. , Gallagher W. J. , Nagamine Y. , Tsunekawa K. , Djayaprawira D. D. , Watanabe N.
Published: September 2007

Current-induced spin-torque switching was demonstrated on sub-100 nm magnetic tunnel junction devices fabricated on 200 mm substrates utilizing 180 nm complimentary metal–oxide–semiconductor back-end-of-the-line (BEOL) technology. Low resistance-area (RA) product and high tunneling magnetoresistance (TMR) were achieved by using substrates containing a CoFeB free layer and a thin MgO barrier. To obtain the desired sub-100 nm features, photoresist trimming was applied on patterns created by a 248 nm lithography tool. Furthermore, the magnetic stack was defined using an ion beam etch that stopped on the thin MgO barrier. Field-sweep measurements on elliptical devices that are 80 nm wide and 160 nm long indicated RA∼4 Ω μm2 and TMR∼90%. Upon injecting current into the devices while applying an external offset field of 28 Oe, current-induced switching occurred from parallel (P) to antiparallel (AP) state at +1.3 mA, and from AP to P state at −1.25 mA. BEOL process integration on 200 mm substrates enabled statistical analysis of device properties, such as the observation of two breakdown mechanisms in the devices.
Experimental and computational study of the mean energy of electrons backscattered from surface films
Authors: Dapor Maurizio , Rau Eduard I. , Sennov Ruslan A.
Published: September 2007

Experimental measurements and Monte Carlo simulation of the mean energy of backscattered electrons by Au/Si targets irradiated by electron beams have been made as a function of the Au layer thickness for primary electron energies in the range 8–20 keV. The simulated trends are compared to experimental data. The mean backscattered energy as a function of the layer thickness presents a maximum whose position depends on the primary energy. The simulated and experimental behaviors are in satisfactory agreement. Similarities and differences between experimental and Monte Carlo results are illustrated and briefly discussed. A somewhat unexpected effect is observed: the maximum of the mean backscattered energy of layered targets is higher than that of the elements constituting the system. This effect is discussed on the basis of semi-empirical considerations.
Direct approach for the electron transport through molecules
Authors: Ortiz Diego O. , Seminario Jorge M.
Published: September 2007

A very simple, straightforward, and easy to implement fully ab initio procedure for the determination of current-voltage characteristics in molecular junctions is presented. Application of this procedure predicts reasonably well the experimental findings for low bias voltages of a break junction experiment and can help us to characterize its geometry.
Raman scattering from turbostratic graphitelike BC4 under pressure
Authors: Solozhenko Vladimir L. , Kurakevych Oleksandr O. , Kuznetsov Alexei Yu.
Published: September 2007

Raman spectra of turbostratic graphitelike BC4(t-BC4) were measured at room temperature and pressures up to 45 GPa on both compression and decompression. At ambient conditions, these Raman spectra contain lines that were not previously observed in Raman spectra of other graphitelike phases. Raman bands of the starting BC4 were assigned on the basis of the lattice dynamics of the ideal graphite structure and relaxation of the q-vector selection rule. The frequency of the E2g(2) in-plane mode at 1561 cm−1 exhibits nonlinear behavior under pressure with the initial pressure coefficient of 7.3 cm−1/GPa. The general behavior of the E2g(2) band at high pressures is consistent with the model of pressure-induced buckling of layers.
Analytical formulas and scaling laws for peak interaction forces in dynamic atomic force microscopy
Authors: Hu Shuiqing , Raman Arvind
Published: September 2007

Determining the peak interaction force between an oscillating nanoscale tip and a sample surface has been a fundamental yet elusive goal in amplitude-modulated atomic force microscopy. Closed form analytical expressions are derived using nonlinear asymptotic theory for the peak attractive and repulsive forces that approximate with a high degree of accuracy the numerically simulated peak forces under ambient or vacuum conditions. Scaling laws involving van der Waals, chemical forces, nanoscale elasticity, and oscillator parameters are identified to demonstrate approximate similitude for the peak interaction forces under practical operating conditions.
Vibrational resonance enhanced broadband multiphoton absorption in a triphenylamine derivative
Authors: Lu Changgui , Cui Yiping , Huang Wei , Yun Binfeng , Wang Zhuyuan , Hu Guohua , Cui Jing , Lu Zhifeng , Qian Ying
Published: September 2007

Multiphoton absorption of 2,5-bis[4-(2-N,N-diphenylaminostyryl)phenyl]-1,3,4-oxadiazole was experimentally studied by using femtosecond laser pulses. This material demonstrates a very broad multiphoton absorption band of around 300nm width with two peaks of 1250 and 1475nm. The first peak results from the three-photon absorption process while the second is attributed to the vibrational resonance enhanced four-photon absorption process. Combination of these two processes provides a much broader multiphoton absorption band. In this letter, the analytical solution to nonlinear transmission of a three-photon absorption process is also given when the incident beam has a Gaussian transverse spatial profile.
Electronic memory device based on a single-layer fluorene-containing organic thin film
Authors: Pearson Christopher , Ahn Jin H. , Mabrook Mohammed F. , Zeze Dagou A. , Petty Michael C. , Kamtekar Kiran T. , Wang Changsheng , Bryce Martin R. , Dimitrakis Panagiotis , Tsoukalas Dimitris
Published: September 2007

The authors report on a simple bistable switching device based on a thin film of an electroactive polymer containing a fluorene group sandwiched between aluminum electrodes. No additional materials (e.g., metallic or semiconductive nanoparticles) are required for the devices to operate. This improves considerably the scalability of the memory elements at nanometer dimensions. The device operation is thought to depend on charge trapping at fluorenone defects within the organic film.
Enhanced dielectric and ferroelectric properties of Pb(Zr0.8Ti0.2)O3∕Pb(Zr0.2Ti0.8)O3 multilayer films
Authors: Liu Hong , Gong Xiaogang , Liang Jin-e , Li Xuedong , Xiao Dingquan , Zhu Jianguo , Pu Zhaohui
Published: September 2007

A series of Pb(Zr1−xTix)O3 multilayer films consisted of Pb(Zr0.8Ti0.2)O3 and Pb(Zr0.2Ti0.8)O3 were deposited on Pt∕Ti∕SiO2∕Si substrates by using radio frequency magnetron sputtering. All the films comprise six periodicities of Pb(Zr0.8Ti0.2)O3∕Pb(Zr0.2Ti0.8)O3 with periodicity thickness of 133nm, but the layer thicknesses of rhombohedral phase and tetragonal phase in one periodicity are varied. The films with two layer thickness ratio of 1:3 possess enhanced dielectric and ferroelectric properties: dielectric constant εr=328 at 10kHz, dielectric loss tgδ=0.0098, and sharply enhanced remanent polarization Pr=32.6μC∕cm2. The layer structure and interlayer stress of Pb(Zr1−xTix)O3 multilayer films play important roles in the electric enhancement.
Effect of overlayers on critical-point parameters in the analysis of ellipsometric spectra
Authors: Jung Y. W. , Ghong T. H. , Kim Y. D. , Aspnes D. E.
Published: September 2007

As the complete removal of overlayers may not be possible in general, the authors investigate the effect of incomplete removal on critical-point parameters in the analysis of ellipsometric data. Using an approximate analytic expression, they show that energies and broadening parameters are much less affected by overlayers than amplitudes and phases. These conclusions are confirmed by false-data calculations for GaAs and overlayer-removal data for CdTe.
Dielectric tunable properties of low dielectric constant Ba0.5Sr0.5TiO3–Mg2TiO4 microwave composite ceramics
Authors: Chou Xiujian , Zhai Jiwei , Yao Xi
Published: September 2007

Ba 0.5 Sr 0.5 Ti O 3 – Mg 2 Ti O 4 composite ceramics are fabricated via the conventional solid-state reaction method. The microstructures, dielectric tunability, and microwave properties of composite ceramics are investigated. The dielectric constant is tailored from 335 to 35 by manipulating the addition of Mg2TiO4 from 50% to 80% weight ratio and the tunability is 10.8% measured at 10kHz for the 80% Mg2TiO4 addition. The composite ceramics with high Q value (>200) at L band are useful for potential tunable microwave device applications in the wireless communication system.
How do the grains slide in fine-grained zirconia polycrystals at high temperature?
Authors: Daraktchiev Maren , Schaller Robert , Gremillard Laurent , Epicier Thierry , Chevalier Jerome , Fantozzi Gilbert
Published: September 2007

Degradation of mechanical properties of zirconia polycrystals is hardly discussed in terms of solution-precipitation grain-boundary sliding due to experimental controversies over imaging of intergranular amorphous phases at high and room temperatures. Here, the authors applied the techniques of mechanical spectroscopy and transmission electron microscopy (TEM) to shed light on the amorphization of grain interfaces at high temperature where the interface reaction determines the behavior of fine-grained zirconia polycrystals. They present mechanical spectroscopy results, which yield evidences of an intergranular amorphous phase in silica doped and in high-purity zirconia. Quenching of zirconia polycrystals reveals an intergranular amorphous phase on TEM images at room temperature.
Gate-tunable graphene spin valve
Authors: Cho Sungjae , Chen Yung-Fu , Fuhrer Michael S.
Published: September 2007

The authors perform nonlocal four-probe spin-valve experiments on graphene contacted by ferromagnetic Permalloy electrodes. They observe sharp switching and often sign reversal of the nonlocal resistance at the coercive field of the electrodes, indicating the presence of a spin current between injector and detector. The nonlocal spin-valve signal changes magnitude and sign with back-gate voltage, and is observed up to T=300K. The gate voltage variation of the spin-valve signal may result from quantum-coherent transport, as evidenced by Fabry-Pérot-like oscillations of the current.
Self-sustained pulsation in the oxide-confined vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots
Authors: Kuzmenkov A. G. , Ustinov V. M. , Sokolovskii G. S. , Maleev N. A. , Blokhin S. A. , Deryagin A. G. , Chumak S. V. , Shulenkov A. S. , Mikhrin S. S. , Kovsh A. R. , McRobbie A. D. , Sibbett W. , Cataluna M. A. , Rafailov E. U.
Published: September 2007

The authors report the observation of strong self-pulsations in molecular-beam epitaxy–grown oxide-confined vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots. At continuous-wave operation, self-pulsations with pulse durations of 100–300ps and repetition rates of 0.2–0.6GHz were measured. The average optical power of the pulsations was 0.5–1.0mW at the laser continuous-wave current values of 1.5–2.5mA.
Electromechanical coupling coefficient of 0.70Pb(Mg1∕3Nb2∕3)O3–0.30PbTiO3 single crystal resonator with arbitrary aspect ratio
Authors: Huang Naixing , Zhang Rui , Cao Wenwu
Published: September 2007

Based on mode coupling theory and the fundamental energy ratio definition of the electromechanical coupling coefficient, a unified formula has been derived to calculate the effective electromechanical coupling coefficient keff of 0.70Pb(Mg1∕3Nb2∕3)O3–0.30PbTiO3 (PMN-30%PT) ferroelectric single crystal cylindrical resonator with arbitrary aspect ratio. The keff dependence on resonator aspect ratio of PMN-30%PT crystal shows similar characteristics as that of the conventional piezoelectric ceramics Pb(ZrxTi1−x)O3 (PZT). However, compared to PZT-5 ceramics, PMN-30%PT single crystal has a smaller aspect ratio where the keff changes dramatically and there is also a stronger coupling between thickness and radial modes causing much larger mode splitting.
Magnetoelectric laminate composite based tachometer for harsh environment applications
Authors: Myers Robert , Islam Rashed Adnan , Karmarkar Makarand , Priya Shashank
Published: September 2007

This study reports the design, fabrication, and characterization of a tachometer utilizing magnetoelectric (ME) laminate composites with sandwich structure consisting of Pb(Zr,Ti)O3 (PZT) and Galfenol. High temperature characterization of Galfenol shows that it can sustain the magnetic property over 500°C. The Curie temperature of PZT compositions was in the range of 325–340°C. The magnitude of the ME coefficient was found to scale with the dimensionless ratio (d g/S), where d is the piezoelectric strain constant, g is the piezoelectric voltage constant, and S is the elastic compliance. The tachometer design is based on the principle that when ME composite is exposed to oscillating magnetic field, it generates voltage with the same frequency.
Phase dependence of single crystalline transistors of tetrathiafulvalene
Authors: Jiang Hui , Yang Xianjin , Cui Zhenduo , Liu Yongchang , Li Hongxiang , Hu Wenping , Liu Yunqi , Zhu Daoben
Published: September 2007

The aim of this study is to establish the structure-properties relationship of tetrathiafulvalene (TTF). After this study (1) TTF crystals with α and β phases were controllably obtained by the cast method with careful selection of the solvents, and (2) organic field-effect transistors (OFETs) of α and β phases both exhibited field-effect performance, and the performance of α phase was much better than that of β phase. The highest mobility of α phase reached 1.2cm2V−1s−1, while β phase showed the maximum mobility only about 0.23cm2V−1s−1, indicating the strong performance dependence of OFETs performance on the crystal phases.
Dirac equation in the background of the Nutku helicoid metric
Authors: Birkandan T. , Hortaçsu M.
Published: September 2007

We study the solutions of the Dirac equation in the background of the Nutku helicoid metric. This metric has curvature singularities, which necessitates imposing a boundary to exclude this point. We use the Atiyah-Patodi-Singer [Math. Proc. Cambridge Philos. Soc. 77, 43 (1975)] nonlocal spectral boundary conditions for both the four and the five dimensional manifolds.
Dielectric relaxations and formation mechanism of giant dielectric constant step in CaCu3Ti4O12 ceramics
Authors: Ni Lei , Chen Xiang Ming
Published: September 2007

The dielectric relaxation behaviors of CaCu3Ti4O12 ceramics were evaluated together with the mixed-valence structure, and the formation mechanism of a giant dielectric constant step was discussed. The giant dielectric constant step was bounded by two dielectric relaxations in low and high temperature ranges. The low-temperature dielectric relaxation was intrinsic, and it was very similar to the electronic ferroelectricity, while the high temperature relaxorlike dielectric peak was assigned to be the result of defect ordering since it could be suppressed by O2 annealing. Both the low and high temperature dielectric relaxations were the thermal activated relaxation process following the Arrhenius law. Moreover, it was supposed that the giant dielectric constant step resulted from the competing balance between the low and high temperature dielectric relaxations.
Anomalous ferromagnetism in spray pyrolysis deposited multiferroic BiFeO3 films
Authors: Siwach P. K. , Singh H. K. , Singh Jai , Srivastava O. N.
Published: September 2007

Oriented polycrystalline BiFeO3 (BFO) films are deposited on single crystal LaAlO3 (001) substrates by nebulized spray pyrolysis technique at low temperature of ∼300°C. Annealing of the as deposited films in air and oxygen at ∼550°C for 1h is found to produce crystallization and BFO phase formation. X-ray diffraction results show rhombohedral disordered perovskite structure with space group R3m. As revealed by atomic force microscopy, grain size increases with oxygen annealing. Magnetic measurements show well-defined hysteretic loops at room temperature for both the films. However, oxygenation leads to dramatic increase in room temperature ferromagnetism as evidenced by large saturation magnetization of ∼250emu∕cm3. This enhanced magnetization is attributed to the presence of iron oxide rich nanoclusters due to oxygen annealing.
Aging-induced double ferroelectric hysteresis loops in BiFeO3 multiferroic ceramic
Authors: Yuan G. L. , Yang Y. , Or Siu Wing
Published: September 2007

Double ferroelectric hysteresis (P-E) loops are observed in aged BiFeO3 multiferroic ceramic at room temperature and are explained according to an aging effect driven by charged defects with high activation energy. The aged BiFeO3 shows R3c defect symmetry and allows a nonzero charged defect-induced polarization (PD) below the ferroelectric Curie temperature. This PD is strong enough to recover the switched 180° ferroelectric domains upon the removal of E, but it is rather weak to effectively switch back the 71°/109° ferroelectric domains. As a result, the traits of the double P-E loops weaken with increasing the maximum E from 121to167kV∕cm.
Enhancement of the light output performance for GaN-based light-emitting diodes by bottom pillar structure
Authors: Wang Chien-Chun , Ku Han , Liu Chien-Chih , Chong Kwok-Keung , Hung Chen-I , Wang Yeong-Her , Houng Mau-Phon
Published: September 2007

A three-dimensional model with finite difference and time domain was established to investigate the enhancement of the light output intensity of GaN light-emitting diodes (LEDs) with bottom pillar (BP) structure. Through comparing the normalized light extraction intensity of GaN LEDs with or without BP in different dimensions, the theoretical results show that the light output intensity in the LED with BP structure involved could be enhanced by about 30%. The influence of BP structure on the light output intensity of a LED could be explained by the physical model of light interaction. In addition, the experimental results also show the same trend to the theoretical calculations.
Multicarrier recombination in colloidal quantum dots
Authors: Pandey Anshu , Guyot-Sionnest Philippe
Published: September 2007

The rates of multicarrier recombination are measured in colloidal quantum dots with and without hole surface trapping and with hole extraction in type II core/shell systems. We report that hole trapping or the physical separation of electronic and hole wavefunctions into different semiconductor domains of a type II system have an insignificant effect on the multicarrier recombination dynamics. These observations are inconsistent with the accepted Auger transition mechanism.
Reliability properties of metal-oxide-semiconductor capacitors using HfO2 high-κ dielectric
Authors: Chen Chun-Heng , Chang Ingram Yin-Ku , Lee Joseph Ya-Min , Chiu Fu-Chien , Chiouand Yan-Kai , Wu Tai-Bor
Published: September 2007

Metal-oxide-semiconductor capacitors with atomic layer deposited HfO2 dielectric were fabricated. The charge-to-breakdown (QBD) characteristics of the metal-oxide-semiconductor structure were investigated. The Weibull slopes β with thicknesses of 9.12, 8.2, and 6.51nm are 3.42, 2.90, and 1.83, respectively. Using the cell-based analytic model, the defect sizes a0 were extracted to be about 0.98 and 1.64nm for exponent σ values of 0.6 and 1, respectively. A comparison with conventional SiO2-gated capacitors was made.
Low-voltage-driven pentacene thin-film transistor with an organic-inorganic nanohybrid dielectric
Authors: Lee Kwang H. , Choi Jeong-M. , Im Seongil , Lee Byoung H. , Im Kyo K. , Sung Myung M. , Lee Seungjun
Published: September 2007

The authors report on the fabrication of pentacene-based thin-film transistors (TFTs) with a 13nm-thick nanohybrid superlattice-type dielectric composed of ten units of molecular aluminum oxide (AlOx)-self-assembled multilayer (SAMu) lattice on indium-tin-oxide (ITO) glass or on n+-Si substrate. The AlOx-SAMu nanohybrid layers showed high dielectric capacitances of 187 and 233nF∕cm2 on ITO glass and on n+-Si substrate, respectively, along with a high dielectric strength of 4MV∕cm in both cases. Our pentacene-TFTs showed a maximum field effect mobility of 0.92cm2∕Vs, operating at −3V with an on/off current ratio of ∼103. Load-resistance inverter using our pentacene-TFT demonstrated a decent voltage gain of ∼5.
Optically stimulated luminescence in NaMgF3:Eu2+
Authors: Dotzler C. , Williams G. V. M. , Rieser U. , Edgar A.
Published: September 2007

Optically stimulated luminescence (OSL) and thermoluminescence (TL) measurements were performed on polycrystalline NaMgF3:Eu2+ as well as sintered and quenched NaMgF3:Eu2+ after exposure to ionizing radiation. The authors find a range of TL traps and the sintering and quenching process reduces the concentration of shallow traps. The resultant time integrated OSL intensity is linear from microgray dose levels to approximately 100Gy, and hence this material is suitable for a personal and environmental dosimetry, where low dose levels are encountered and high sensitivity is required.
Growth of plasmonic gold nanostructures by electron beam induced deposition
Authors: Graells S. , Alcubilla R. , Badenes G. , Quidant R.
Published: September 2007

The authors report on the growth of organometallic dots for optical applications using electron beam induced gold deposition on a transparent substrate. The effects of the substrate material and the deposition parameters, such as beam current and water vapor pressure, on both the deposition rate and gold purity are investigated. Ex situ annealing of the sample is used as a purity improvement method. Scattering optical measurements on the purified dots reveal that they support localized surface plasmon resonances. This technique opens new perspectives in the fabrication of substantial height-base aspect ratio plasmonic nanostructures and may become particularly relevant for nonflat substrates.
Direct measurements of neutral density depletion by two-photon absorption laser-induced fluorescence spectroscopy
Authors: Aanesland A. , Liard L. , Leray G. , Jolly J. , Chabert P.
Published: September 2007

The ground state density of xenon atoms has been measured by spatially resolved laser-induced fluorescence spectroscopy with two-photon excitation in the diffusion chamber of a magnetized Helicon plasma. This technique allows the authors to directly measure the relative variations of the xenon atom density without any assumptions. A significant neutral gas density depletion was measured in the core of the magnetized plasma, in agreement with previous theoretical and experimental works. It was also found that the neutral gas density was depleted near the radial walls.
Interplay of built-in potential and piezoelectric field on carrier recombination in green light emitting InGaN quantum wells
Authors: Schwarz Ulrich T. , Braun H. , Kojima K. , Kawakami Y. , Nagahama S. , Mukai T.
Published: September 2007

The authors measure the interplay of piezoelectric field and built-in potential on carrier recombination and quantum confined Stark effect in green light emitting InGaN quantum wells by modulating the external bias voltage of the pn junction. Time-resolved electroluminescence shows a temporal separation of carrier injection into the active region and radiative recombination within the InGaN quantum wells. During the time when the bias voltage is off, the piezoelectric field is partially compensated by the built-in potential of the pn junction, resulting in an increased radiative recombination rate.
ZnO-based light-emitting metal-insulator-semiconductor diodes
Authors: Hwang Dae-Kue , Oh Min-Suk , Lim Jae-Hong , Choi Yong-Seok , Park Seong-Ju
Published: September 2007

The ZnO-based metal-insulator-semiconductor (MIS) diode was fabricated by using an insulator ZnO layer and an n-ZnO layer grown by radio frequency magnetron sputtering. The current-voltage of the ZnO MIS diodes showed a good diode characteristic with a threshold voltage of 8.9V and a band-edge emission at 380nm at room temperature. The electroluminescence emission of ZnO MIS was attributed to the generation of holes in the insulating ZnO layer at the high threshold voltage of 8.9V via an impact ionization process.
Concerning the 506cm−1 band in the Raman spectrum of silicon nanowires
Authors: Prades J. D. , Arbiol J. , Cirera A. , Morante J. R. , Fontcuberta i Morral A.
Published: September 2007

Silicon nanowires synthesized on an amorphous substrate by the vapor-liquid-solid method were studied using Raman spectroscopy and ab initio calculations. The authors show that the Raman spectrum of Si nanowires is compatible with the existence of two distinct phases. The Raman spectra revealed a peak centered around 517cm−1 as expected for the nanosized diamond cubic phase (Si-I). However, two new contributions, centered around 506 and 516cm−1, arose which they attributed to the diamond hexagonal phase (Si-IV). The identification of these Raman peaks clarifies the controversy surrounding the vibration modes of Si-IV previously reported in the literature.
Large enhancement in polarization response and energy density of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) by interface effect in nanocomposites
Authors: Chu Baojin , Lin Minren , Neese Breet , Zhou Xin , Chen Qin , Zhang Q. M.
Published: September 2007

The authors report the finding of large enhancement in the electric energy density and electric displacement level in nanocomposites of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor ferroelectric polymer/ZrO2 nanoparticles. Through the interface effect, the presence of the nanoparticles (with only 1.6vol% of ZrO2 nanoparticles in the composite) raises the maximum electric displacement D from 0.085C∕m2 under 400MV∕m in the neat terpolymer to more than 0.11C∕m2 under 300MV∕m in the nanocomposites. Consequently, a high energy density of 10.5J∕cm3 can be achieved under a lower field (300MV∕m) in the nanocomposites compared with the neat terpolymer.
Enhancement of optical absorption and photocurrent of 6H-SiC by laser surface nanostructuring
Authors: Zhao Q. Z. , Ciobanu F. , Malzer S. , Wang L. J.
Published: September 2007

Nanoscale ripple structures with spatial periods of up to 155nm are generated by a single beam ultrashort pulsed laser structuring of 6H-SiC surface. The period of nanoripples does not depend on the laser fluence and the number of pulses. Optical absorption and photocurrent measurements on SiC samples are investigated before and after laser nanostructuring. Nearly 40% enhancement of optical absorption and a factor of 3 increase of photocurrent are achieved after laser nanostructuring.
Hyper-Raman scattering enhanced by anisotropic dimer plasmons on artificial nanostructures
Authors: Ikeda Katsuyoshi , Takase Mai , Sawai Yoshitaka , Nabika Hideki , Murakoshi Kei , Uosaki Kohei
Published: September 2007

Silver nanodimers with a small gap of a few nanometers aligned on glass substrates were used to enhance hyper-Raman scattering of crystal violet dye molecules. When localized surface plasmon of the dimer array was resonantly excited along the interparticle axis, hyper-Raman intensity was significantly enhanced. Moreover, the spectral appearance was slightly different between the two excitation polarizations, suggesting a possibility of two resonance contributions at one-photon and two-photon energies. Since the plasmonic property of dimer arrays can be controlled by the dimer geometry, the dimer arrays are expected to be well-defined substrates for surface-enhanced hyper-Raman spectroscopy.
Optical properties of highly luminescent zinc oxide tetrapod powders
Authors: Fischer A. M. , Srinivasan S. , Garcia R. , Ponce F. A. , Guaño S. E. , Di Lello B. C. , Moura F. J. , Solórzano I. G.
Published: September 2007

Zinc oxide tetrapod powders with high luminescent characteristics have been grown by simple evaporation and direct oxidation of zinc. Their structural and optical properties have been studied by transmission electron microscopy and by cathodoluminescence. The legs of the tetrapods have a pure hexagonal structure and form an arrowhead joint at the center. The tetrapod legs exhibit strong band edge and weak green luminescence. Monochromatic cathodoluminescence images show that the band-edge emission originates from the bulk of the legs, while the green emission emanates from the surface, indicative of it being related to oxygen vacancies. Low temperature cathodoluminescence shows a direct correlation of near-band-edge emission with specific sites within the structure.
High efficiency tandem organic light-emitting devices with Al∕WO3∕Au interconnecting layer
Authors: Zhang Hongmei , Dai Yanfeng , Ma Dongge , Zhang Hongmei
Published: September 2007

An interconnecting layer of Al (2nm)∕WO3 (3nm)∕Au (16nm) was studied for application in tandem organic light-emitting devices. It can be seen that the Al∕WO3∕Au structure plays the role of an excellent interconnecting layer. The introduction of WO3 in the connection unit significantly improves the device efficiency as compared to the case of Al∕Au. Thus, the current efficiency of the two-unit tandem devices is enhanced by two factors with respect to the one-unit devices. The green two-unit tandem device of indium tin oxide/MoO3∕4,4′-N,N′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl(NPB)/tris(8-hydroxylquinoline) aluminum (Alq3):10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano[6,7,8-ij]quinolizin-11-one (C545T)∕Alq3∕LiF∕Al∕WO3∕Au∕MoO3∕NPB∕Alq3:C545T∕Alq3∕LiF∕Al showed a maximum current efficiency of 33.9cd∕A and a power efficiency of 12.0lm∕W.
Measurement of silicon dioxide surface phonon-polariton propagation length by attenuated total reflection
Authors: Chen Dye-Zone A. , Chen Gang
Published: September 2007

The authors present an experimental measurement of the propagation length of surface phonon-polaritons on amorphous silicon dioxide using the method of attenuated total reflection. The measurements yield a propagation length of 10.8±1.3μm for the silicon dioxide surface phonon-polariton resonance at 9μm, which is in good agreement with the calculated value.