bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical transitions and the mobility edge in amorphous semiconductors: A joint density of states analysis
Authors: Orapunt Farida , O’Leary Stephen K.
Published: October 2008

We determine the contributions to the joint density of states function attributable to the various types of optical transitions that occur within an amorphous semiconductor. We consider four types of optical transitions in this analysis: (1) those from the valence band extended electronic states to the conduction band extended electronic states, (2) those from the valence band extended electronic states to the conduction band localized electronic states, (3) those from the valence band localized electronic states to the conduction band extended electronic states, and (4) those from the valence band localized electronic states to the conduction band localized electronic states. We perform this analysis within the framework of a general empirical density of states model, with square-root functional dependencies in the band regions and exponential functional dependencies in the tail regions. In order to keep the analysis as general as possible, our joint density of states results are determined as functions of the valence band and conduction band mobility edge locations, these edge locations demarcating the extended electronic states from their localized counterparts. The dependence of these joint density of states results on a number of key density of states modeling parameters is then determined. Finally, we apply this joint density of states formalism to the specific case of hydrogenated amorphous silicon, and draw conclusions about the optical response of this particular material.
Homogeneous nucleation under shear in a two-dimensional Ising model: Cluster growth, coalescence, and breakup
Authors: Allen Rosalind J. , Valeriani Chantal , Tănase-Nicola Sorin , ten Wolde Pieter Rein , Frenkel Daan
Published: October 2008

We compute rates and pathways for nucleation in a sheared two-dimensional Ising model with Metropolis spin flip dynamics using forward flux sampling (FFS). We find a peak in the nucleation rate at intermediate shear rate. We analyze the origin of this peak using modified shear algorithms and committor analysis. We find that the peak arises from an interplay between three shear-mediated effects: Shear-enhanced cluster growth, cluster coalescence, and cluster breakup. Our results show that complex nucleation behavior can be found even in a simple driven model system. This work also demonstrates the use of FFS for simulating rare events, including nucleation, in nonequilibrium systems.
Raman scattering on quadrupolar vibrational modes of spherical nanoparticles
Authors: Ristić D. , Ivanda M. , Furić K. , Desnica U. V. , Buljan M. , Montagna M. , Ferrari M. , Chiasera A. , Jestin Y.
Published: October 2008

The Raman light to vibration coupling coefficients C(ν) of quadrupolar and symmetrical vibrational modes of spherical nanoparticles embedded in a matrix are calculated. In contrast to the symmetrical mode, the C(ν) of the quadrupolar modes consists of the longitudinal and transversal sound velocity contributions. It is shown that depending on the ratio of longitudinal and transverse sound velocities, these two contributions can interfere constructively or destructively resulting in enhancing or vanishing of some radial modes. The calculated results were compared to the polarized Raman spectra of CdSxSe1−x and Ge nanoparticles. By using the known size distribution of the CdSxSe1−x particles found by high-resolution transmission electron microscopy (HRTEM), the calculated low frequency (LF) polarized Raman spectra agree with the experiment. In the second case, the size distribution of Ge particles determined from the LF Raman spectra showed good agreement with the HRTEM results. The inverse problem of the determination of sound velocities of nanoparticles from the polarized low frequency Raman spectra by using the calculated C(ν) and known size distribution of the nanoparticles was demonstrated for the hafnium oxide nanoparticles in a silica matrix.
Tunable transport properties of n-type ZnO nanowires by Ti plasma immersion ion implantation
Authors: Liao L. , Zhang Z. , Yang Y. , Yan B. , Cao H. T. , Chen L. L. , Li G. P. , Wu T. , Shen Z. X. , Tay B. K. , Yu T. , Sun X. W.
Published: October 2008

Single-crystalline, transparent conducting ZnO nanowires were obtained simply by Ti plasma immersion ion implantation (PIII). Electrical transport characterizations demonstrate that the n-type conduction of ZnO nanowire could be tuned by appropriate Ti-PIII. When the energy of PIII is increased, the resistivity of ZnO decreases from 4×102 to 3.3×10−3 Ω cm, indicating a semiconductor-metal transition. The failure-current densities of the metallic ZnO could be up to 2.75×107 A/cm2. Therefore, this facile method may provide an inexpensive alternative to tin doped indium oxide as transparent conducting oxide materials.
High-throughput screening of shape memory alloy thin-film spreads using nanoindentation
Authors: Dwivedi Arpit , Wyrobek Thomas J. , Warren Oden L. , Hattrick-Simpers Jason , Famodu Olubenga O. , Takeuchi Ichiro
Published: October 2008

We have demonstrated the utility of nanoindentation as a rapid characterization tool for mapping shape memory alloy compositions in combinatorial thin-film libraries. Nanoindentation was performed on Ni–Mn–Al ternary composition spreads. The indentation hardness and the reduced elastic modulus were mapped across a large fraction of the ternary phase diagram. The large shape memory alloy composition region, located around the Heusler composition (Ni2MnAl), was found to display significant departure in these mechanical properties from the rest of the composition spread. In particular, the modulus and the hardness values are lower for the martensite region than those of the rest of the phase diagram.
Computing conformational free energy by deactivated morphing
Authors: Park Sanghyun , Lau Albert Y. , Roux Benoît
Published: October 2008

Despite the significant advances in free-energy computations for biomolecules, there exists no general method to evaluate the free-energy difference between two conformations of a macromolecule that differ significantly from each other. A crucial ingredient of such a method is the ability to find a path between different conformations that allows an efficient computation of the free energy. In this paper, we introduce a method called “deactivated morphing,” in which one conformation is morphed into another after the internal interactions are completely turned off. An important feature of this method is the (shameless) use of nonphysical paths, which makes the method robustly applicable to conformational changes of arbitrary complexity.
The classical point electron in Colombeau’s theory of nonlinear generalized functions
Authors: Gsponer Andre
Published: October 2008

The electric and magnetic fields of a pole-dipole singularity attributed to a point-electron singularity in the Maxwell field are expressed in a Colombeau algebra of generalized functions. This enables one to calculate dynamical quantities quadratic in the fields which are otherwise mathematically ill-defined: the self-energy (i.e., “mass”), the self-angular momentum (i.e., “spin”), the self-momentum (i.e., “hidden momentum”), and the self-force. While the total self-force and self-momentum are zero, therefore ensuring that the electron singularity is stable, the mass and spin are diverging integrals of δ2-functions. Yet, after renormalization according to standard prescriptions, the expressions for mass and spin are consistent with quantum theory, including the requirement of a gyromagnetic ratio greater than 1. The most striking result, however, is that the electric and magnetic fields differ from the classical monopolar and dipolar fields by δ-function terms which are usually considered as insignificant, while in a Colombeau algebra these terms are precisely the sources of the mechanical mass and spin of the electron singularity.
Feedback control of thermal lensing in a high optical power cavity
Authors: Fan Y. , Zhao C. , Degallaix J. , Ju L. , Blair D. G. , Slagmolen B. J. J. , Hosken D. J. , Brooks A. F. , Veitch P. J. , Munch J.
Published: October 2008

This paper reports automatic compensation of strong thermal lensing in a suspended 80 m optical cavity with sapphire test mass mirrors. Variation of the transmitted beam spot size is used to obtain an error signal to control the heating power applied to the cylindrical surface of an intracavity compensation plate. The negative thermal lens created in the compensation plate compensates the positive thermal lens in the sapphire test mass, which was caused by the absorption of the high intracavity optical power. The results show that feedback control is feasible to compensate the strong thermal lensing expected to occur in advanced laser interferometric gravitational wave detectors. Compensation allows the cavity resonance to be maintained at the fundamental mode, but the long thermal time constant for thermal lensing control in fused silica could cause difficulties with the control of parametric instabilities.
Electrical and structural characterization of a single GaSb∕InAs∕GaSb quantum well grown on GaAs using interface misfit dislocations
Authors: Eyink Kurt G. , Tomich David H. , Mitchel William C. , Grazulis Lawrence , Carlin John A. , Mahalingam Krishnamurthy , Jallipalli Anitha , Balakrishnan Ganesh , Huffaker Diana , Elhamri Said
Published: October 2008

Interface misfit formation has been used for the growth of high mobility GaSb∕InAs single quantum wells (SQW) formed on GaAs substrates. The SQW structure was topped with 800Å GaSb, followed by 100Å GaSb:Si (5×108cm−3), 10nm GaSb, 10nm InAs, and finally 250nm GaSb on a GaAs substrate. The structural quality was examined using high resolution x-ray diffraction and transmission electron microscopy. Reciprocal space mapping indicated that the GaSb was completely relaxed. A high resolution x-ray rocking curve showed good agreement between the proposed structure and the simulation, assuming that all layers were relaxed to the GaSb lattice, and clearly showed interference fringing from individual layers. Atomic force microscopy showed the film appeared textured, and that the final growth occurred by step flow growth. The observed peak-to-peak roughness was 7nm over a 100×100μm2 square area. Plane view transmission electron microscopy analysis showed a nearly regular array of Lomer dislocations responsible for the relaxation of the strain in the two films. The mobility of the SQW was measured as a function of temperature. The room temperature mobility was 13900cm2∕Vs and a peak mobility of 25200cm2∕Vs was observed at ∼60K, and remained nearly constant at lower temperatures. The sheet concentration was 2.3×1012∕cm2 at room temperature, and dropped to 7.35×1011∕cm2 at 60K. Magnetoresistance measurements revealed the presence of Shubnikov–de Haas oscillations at 1.2K; indicating the presence of a good quality two-dimensional electron gas (2DEG). Further support for the high quality of the 2DEG is provided by the observation of sharply defined quantum Hall plateaus.
Additional data on the (4)Σ1+ state of RbCs and potential determination
Authors: Nogueira Andreia F. , Fellows C. E. , Bergeman T.
Published: October 2008

We report measurements of nine additional term values of the (4)Σ1+ state of RbCs. These data plus previously published data on band origins and on term values from higher rovibrational levels yield a more accurate determination of the potential of this state, which confirms the form obtained in ab initio relativistic calculations.
Multireference Fock-space coupled-cluster and equation-of-motion coupled-cluster theories: The detailed interconnections
Authors: Musial Monika , Bartlett Rodney J.
Published: October 2008

The multireference Fock-space coupled-cluster (FS-CC) equations constructed via the effective Hamiltonian approach are reduced to those obtained through a partitioning technique for the matrix diagonalization. This allows finding a better route to solve the FS-CC equations for the (1,1) sector by the direct diagonalization of the properly constructed matrix, which is identical to that obtained with the intermediate Hamiltonian approach. The detailed connections with the equation-of-motion CC (EOM-CC) approach also provides alternative routes for studying the higher-sectors of Fock space, and for the treatment of properties other than the energy. The approach studied in this work is formulated at the CC singles, doubles (CCSD) and CC singles, doubles, triples (CCSDT) level. The performance of the method is illustrated by numerical examples for the C2, C2H4, and H2CO molecules in comparison with the EOM-CC method and available experimental data.
Calculating interaction energies in transition metal complexes with local electron correlation methods
Authors: Hill J. Grant , Platts James A.
Published: October 2008

The results of density fitting and local approximations applied to the calculation of transition metal–ligand binding energies using second order Møller–Plesset perturbation theory are reported. This procedure accurately reproduces counterpoise corrected binding energies from the canonical method for a range of test complexes. While counterpoise corrections for basis set superposition error are generally small, this procedure can be time consuming, and in some cases gives rise to unphysical dissociation of complexes. In circumventing this correction, a local treatment of electron correlation offers major efficiency savings with little loss of accuracy. The use of density fitting for the underlying Hartree–Fock calculations is also tested for sample Ru complexes, leading to further efficiency gains but essentially no loss in accuracy.
Active-space two-electron reduced-density-matrix method: Complete active-space calculations without diagonalization of the N-electron Hamiltonian
Authors: Gidofalvi Gergely , Mazziotti David A.
Published: October 2008

Molecular systems in chemistry often have wave functions with substantial contributions from two-or-more electronic configurations. Because traditional complete-active-space self-consistent-field (CASSCF) methods scale exponentially with the number N of active electrons, their applicability is limited to small active spaces. In this paper we develop an active-space variational two-electron reduced-density-matrix (2-RDM) method in which the expensive diagonalization is replaced by a variational 2-RDM calculation where the 2-RDM is constrained by approximate N-representability conditions. Optimization of the constrained 2-RDM is accomplished by large-scale semidefinite programming [Mazziotti, Phys. Rev. Lett. 93, 213001 (2004)]. Because the computational cost of the active-space 2-RDM method scales polynomially as ra6 where ra is the number of active orbitals, the method can be applied to treat active spaces that are too large for conventional CASSCF. The active-space 2-RDM method performs two steps: (i) variational calculation of the 2-RDM in the active space and (ii) optimization of the active orbitals by Jacobi rotations. For large basis sets this two-step 2-RDM method is more efficient than the one-step, low-rank variational 2-RDM method [Gidofalvi and Mazziotti, J. Chem. Phys. 127, 244105 (2007)]. Applications are made to HF, H2O, and N2 as well as n-acene chains for n=2–8. When n>4, the acenes cannot be treated by conventional CASSCF methods; for example, when n=8, CASSCF requires optimization over approximately 1.47×1017 configuration state functions. The natural occupation numbers of the n-acenes show the emergence of bi- and polyradical character with increasing chain length.
Interaction between two nearby diffusion-controlled reactive sites in a plane
Authors: Strieder William
Published: October 2008

The complete series solution for two diffusion-controlled chemically reactive sites, each of radius a, located on an inert plane an arbitrary center-to-center distance d apart, is presented. Simple analytical forms are developed to calculate the site reaction rates in terms of d∕a. The impact of these results on a number of important problems in chemical physics is discussed.
Dressing of driven colloidal particles in a subcritical liquid suspension
Authors: Chakrabarti J. , Löwen H.
Published: October 2008

At equilibrium, colloidal particles in a subcritical liquid suspension are surrounded by a drying layer if the colloid has solvophobic interaction. Using Brownian dynamics computer simulations, we investigate the nonequilibrium response of this layer to a strong external driving force. We find that the driven colloidal particle dresses itself with more particles than in the equilibrium drying layer. The effective interaction between two such dressed particles exhibits a deep drive-induced attraction due to a stretched joint gas bubble.
Effects of Y doping on the structural stability and defect properties of cubic HfO2
Authors: Chen G. H. , Hou Z. F. , Gong X. G. , Li Quan
Published: October 2008

First-principles calculations have been performed to study the structural and electronic properties of pure and Y-doped cubic HfO2. It is found that Y doping in HfO2 would increase the stability of the cubic phase relative to the monoclinic phase by reducing the energy difference and the phase transition pressure. This result is consistent with the observed stabilization of the cubic phase of HfO2 by the addition of Y. The calculated formation energy of the VO–YHf complex defect in different charged states indicates that the single positively charged state (VO–YHf)+ is more stable than the neutral state (VO–YHf)0 and the double positively charged state (VO–YHf)++ in Y-doped cubic HfO2. Because the number of d-electrons of Y is less than that of Hf by one and substitutional Y for Hf introduces holes in the oxygen p-band, Y doping would make the highest occupied defect level induced by (VO–YHf)+ fall into the valence band rather than the energy gap, which explains the experimental observation that gap states related to oxygen vacancy defects become nondetectable in Y-doped HfO2 films.
Spin-dependent injection model for Monte Carlo device simulation
Authors: López H. , Oriols X. , Suñé J. , Cartoixà X.
Published: October 2008

We develop a spin-dependent injection model that can be used in time-dependent simulators of spintronic semiconductor devices. The physical basis of the method and the algorithm implementation are described. We test the validity of our model by simulating a two terminal ballistic semiconductor with the Monte Carlo technique and comparing the outcome to a simple low bias equivalent circuit, with excellent agreement. The results obtained demonstrate that the methodology proposed is suitable for use in time-dependent simulators of spintronic devices in order to make qualitative predictions.
Effect of intermixing on bulk and interface Raman modes in GaAs:AlAs superlattice waveguide structures
Authors: Scrutton P. , Fung B. , Helmy A. S.
Published: October 2008

Spatially resolved Raman spectroscopy at room temperature is used to study quantum well intermixing in GaAs:AlAs superlattice structures. Phonon modes are probed from the side facet along the [110] direction. The intermixing leads to the appearance of interfacial alloy modes and degraded the intensity of the superlattice interface (IF) modes, which can be used as a sensitive indicator of superlattice quality. These changes in the Raman spectra, along with spatially resolved photoluminescence, are used to distinguish the degree of intermixing in samples intermixed by impurity free vacancy diffusion at 850–950 °C, and to investigate the bandgap modulation in a periodically intermixed bandgap grating fabricated using ion implantation induced disordering. The shift of the GaAs-transverse optic mode to the GaAs-like transverse optic alloy mode and the degradation of the GaAs-IF mode are shown to provide the best indication of the extent of intermixing. Due to the large contrast in the GaAs-IF mode intensity between as-grown and intermixed superlattice, using IF modes is found to be a promising route for characterizing the resolution of the superlattice bandgap grating.
The enhanced negative magnetoresistance of Fe/Tb multilayer at multiextreme conditions
Authors: Ohashi Masashi , Oomi Gendo , Ohmichi Eiji , Osada Toshihito , Takano Katsuyoshi , Sakurai Hiroshi , Itoh Fumitake
Published: October 2008

Magnetoresistance (MR) in Fe/Tb magnetic multilayer is studied under multiextreme conditions, i.e., high magnetic field, low temperature, and high pressure. The negative MR is observed to be 24.6% in [Fe(12 nm)/Tb(15 nm)]25 at 4.2 K, and MR is not saturated completely even up to 30 T. With increasing pressure, the magnitude of MR tends to be suppressed, indicating that magnetic order in the Tb layers is suppressed by applying pressure.
A stabler non contact atomic force microscopy imaging using a tuning fork for air and liquid environments: The zero phase mode atomic force microscopy
Authors: Van L. Pham , Kyrylyuk V. , Thoyer F. , Cousty J.
Published: October 2008

We describe a method to perform noncontact atomic force microscopy (NC-AFM) in liquid and in air using a quartz crystal tuning fork. The latter, thanks to its increased stability originating from its higher stiffness, compared to cantilevers, enables very low oscillation amplitudes and thus, should yield a very high sensitivity to the force interactions. However, applying conventional NC-AFM technique fails, due to the phase lock loop demodulator becoming unstable as the input signal to noise ratio decreases when lowering the oscillation amplitude. Therefore we have developed so-called zero phase mode atomic force microscopy based on phase modulation. Comparisons with conventional frequency modulation mode using the same mechanical setup demonstrate gains in sensitivity and in scanning rates. High resolution topographic images, as well as simultaneous dissipation images can be recorded using this very convenient and effective technique.
Influence of band non-parabolicity on the quantized gate capacitance in δ-doped MODFED of III–V and related materials
Authors: Bhattacharya Sitangshu , Dan Surya Shankar , Mahapatra Santanu
Published: October 2008

We have investigated analytically the influence of band non-parabolicity on the quantized gate capacitance in n-channel inversion layers of AlxGa1−xAs∣GaAs, In1−xAsxSb∣InSb, and In1−xAlxAs∣In1−xGaxAsyP1−x δ-doped modulation field effect devices, whose channel electrons obey the three, two, and the parabolic energy band models of Kane. The quantized gate capacitance has been investigated by including the effects of electric subbands under quantum mechanical treatment on GaAs, InSb, and In1−xGaxAsyP1−y lattices matched to InP as channel materials. The oscillatory dependence of the quantized gate capacitance as a function of surface electric field and gate bias signatures directly the two-dimensional quantum confinement of the carriers. The influence of the band non-parabolicity of the confined carriers significantly influences the value of the gate capacitance. The result of the gate capacitances for the parabolic energy band model forms a special case of our generalized theoretical formalism.
Interfacial effects on the thermal conductivity of a-Ge thin films grown on Si substrates
Authors: Alvarez-Quintana J. , Rodríguez-Viejo J.
Published: October 2008

We use the 3ω method to measure the effective thermal conductivity of thin films of a-Ge with thicknesses of 20–150 nm in the temperature range of 30–300 K. By using a moving shadow mask, the films are grown on the same Si (001) substrate in a single deposition run to minimize changes in the microstructure. We observe a reduction in the effective conductivity of the films with the decreasing layer thickness. From the measured data we estimate values for both the film thermal conductivity and the thermal boundary resistance (TBR) between SiO2/a-Ge/Si at the different temperatures. An experimental value of the interface resistance of 2×10−8 m2 K/W is obtained at 300 K. The temperature dependence of the TBR differs appreciably from calculations based on the diffusive mismatch model. The values derived for the intrinsic thermal conductivity of the films, kfilm(300 K)=0.64 W/mK, agree with predictions from the minimum thermal conductivity model and with values measured by Cahill and Pohl [Phys. Rev. B 37, 8773 (1988)] for thicker films.
On linear stability of Rayleigh–Bénard Poiseuille flow of viscoplastic fluids
Authors: Métivier Christel , Nouar Chérif
Published: October 2008

The present paper deals with the onset of the two-dimensional Rayleigh–Bénard convection for a plane channel flow of viscoplastic fluid. The influence of the yield stress on the instability and stability conditions characterized by the Rayleigh numbers denoted, respectively, RaL and RaE is investigated in the framework of linear analysis using modal and energetic approaches. The results show that the yield stress, represented by the Bingham number B, delays the onset of convection. For low values of the Reynolds number Re, the critical conditions RaL and RaE tend to be equal and the difference RaL−RaE increases with increasing Re, highlighting the non-normality of the linear operator. For Re<1 and large B (B≥O(10)), it is shown that the critical Rayleigh number increases as B2 and the critical wave number evolves according to B1/4.
Elastic phase response of silica nanoparticles buried in soft matter
Authors: Tetard Laurene , Passian Ali , Lynch Rachel M. , Voy Brynn H. , Shekhawat Gajendra , Dravid Vinayak , Thundat Thomas
Published: October 2008

Tracking the uptake of nanomaterials by living cells is an important component in assessing both potential toxicity and in designing future materials for use in vivo. We show that the difference in the local elasticity at the site of silica (SiO2) nanoparticles confined within a macrophage enables functional ultrasonic interactions. By elastically exciting the cell, a phase perturbation caused by the buried SiO2 nanoparticles was detected and used to map the subsurface populations of nanoparticles. Localization and mapping of stiff chemically synthesized silica nanoparticles within the cellular structures of a macrophage are important in basic as well as applied studies.
Systematic investigation of the thermoelectric properties of TlMTe2 (M=Ga, In, or Tl)
Authors: Matsumoto Hideaki , Kurosaki Ken , Muta Hiroaki , Yamanaka Shinsuke
Published: October 2008

We performed a systematic investigation of the thermoelectric properties of TlMTe2, where M was Ga, In, or Tl, to elucidate the role of Tl in determining the properties, especially thermal conductivity (κ). TlGaTe2 and TlInTe2 exhibited predictable characteristics, i.e., semiconductorlike behaviors in the electrical properties, very low κ (around 0.5 Wm−1 K−1), and relatively high thermoelectric figures of merit ZT (around 0.4). However, TlTe exhibited unexpected characteristics, i.e., an exceptionally high κ of around 4 Wm−1 K−1. In addition, it was revealed that this high κ of TlTe was due, not only to its large electronic contribution, but also to its high lattice thermal conductivity. The present study implies that the extremely low κ observed in many thallium tellurides cannot be explained solely by the presence of Tl.
Reversible change in electrical and optical properties in epitaxially grown Al-doped ZnO thin films
Authors: Noh Jun Hong , Jung Hyun Suk , Lee Jung-Kun , Kim Jin Young , Cho Chin Moo , An Jae-sul , Hong Kug Sun
Published: October 2008

Aluminum-doped ZnO (AZO) films were epitaxially grown on sapphire (0001) substrates using pulsed laser deposition. As-deposited AZO films had a low resistivity of 8.01×10−4 Ω cm. However, after annealing at 450 °C in air, the electrical resistivity of the AZO films increased to 1.97×10−1 Ω cm because of a decrease in the carrier concentration. Subsequent annealing of the air-annealed AZO films in H2 recovered the electrical conductivity of the AZO films. In addition, the conductivity change was reversible upon repeated air and H2 annealing. A photoluminescence study showed that oxygen interstitial (Oi′) is a critical material parameter allowing for the reversible control of the electrical conducting properties of AZO films.
Local structure analysis of Ga1−xAlxN epitaxial layer
Authors: Robouch B. V. , Kisiel A. , Robouch P. , Kutcherenko I. , Vodopyanov L. K. , Ingrosso L. , Marcelli A.
Published: October 2008

We used the strained-tetrahedron model to interpret published extended x-ray absorption fine structure data on the wurtzite structure of epitaxial GaAlN thin films grown on a sapphire substrate. Site occupation preference coefficients were determined and the nearest neighbor and next-nearest neighbor interion distances were estimated. The three observed ion-pair preference coefficients and the related configuration population coefficients strongly deviate from the random distribution, which indicates that the occurrence of tetrahedra with A1l+G3a is highly improbable. Moreover, instead of the eight lines expected, the GaAlN phonon spectra display only four strong lines. Finally, the analysis suggests that beyond a concentration of 50% of Al, these thin films grow less homogeneously whatever the method of preparation.
Microwave-assisted synthesis of sensitive silver substrate for surface-enhanced Raman scattering spectroscopy
Authors: Xia Lixin , Wang Haibo , Wang Jian , Gong Ke , Jia Yi , Zhang Huili , Sun Mengtao
Published: October 2008

A sensitive silver substrate for surface-enhanced Raman scattering (SERS) spectroscopy is synthesized under multimode microwave irradiation. The microwave-assisted synthesis of the SERS-active substrate was carried out in a modified domestic microwave oven of 2450MHz, and the reductive reaction was conducted in a polypropylene container under microwave irradiation with a power of 100W for 5min. Formaldehyde was employed as both the reductant and microwave absorber in the reductive process. The effects of different heating methods (microwave dielectric and conventional) on the properties of the SERS-active substrates were investigated. Samples obtained with 5min of microwave irradiation at a power of 100W have more well-defined edges, corners, and sharper surface features, while the samples synthesized with 1h of conventional heating at 40°C consist primarily of spheroidal nanoparticles. The SERS peak intensity of the ∼1593cm−1 band of 4-mercaptobenzoic acid adsorbed on silver nanoparticles synthesized with 5min of microwave irradiation at a power of 100W is about 30 times greater than when it is adsorbed on samples synthesized with 1h of conventional heating at 40°C. The results of quantum chemical calculations are in good agreement with our experimental data. This method is expected to be utilized for the synthesis of other metal nanostructural materials.
Synthesis of CuInS2 fluorescent nanocrystals and enhancement of fluorescence by controlling crystal defect
Authors: Uehara Masato , Watanabe Kosuke , Tajiri Yasuyuki , Nakamura Hiroyuki , Maeda Hideaki
Published: October 2008

For this study, Cu–In–S nanocrystals were developed as a low toxic fluorescent. The stoichiometric CuInS2 nanocrystals were synthesized facilely by heating a solution of metal complexes and dodecanethiol. The fluorescence would be originated from the crystal defect. We intentionally introduced the crystal defect in nanocrystal with the prospect that the fluorescence intensity would be increased. The defect structure of products was analyzed using Raman spectroscopy and other techniques. The nanocrystals have many defects without phase separation as observed in the bulk material. Consequently, the fluorescence quantum yield achieved ∼5%. Moreover, the fluorescence quantum yield was increased up to 15% by the ZnS coating.
Magnetic properties of very-high-spin organic π-conjugated polymers based on Green’s function theory
Authors: Fu Hua-Hua , Yao Kai-Lun , Liu Zu-Li
Published: October 2008

Magnetic properties of two very-high-spin organic π-conjugated polymers have been investigated theoretically by means of the many-body Green’s function method with random phase approximation. The polymers are designed with a large density of cross-links and alternating connectivity of radical modules with unequal spin quantum numbers (S), macrocyclic S=2 or 3, and cross-linking S=1∕2 modules, which permit large net S values for either antiferromagnetic or ferromagnetic exchange coupling between the modules. The numerical results reveal that, ascribing to the zero-temperature spin fluctuations, the sublattice magnetizations of the two polymers are both smaller than their classical spin values and the ground-state magnetizations of them are also smaller than their predicted values in the antiferromagnetic exchange coupling case. However, these magnetic behaviors do not occur in the ferromagnetic exchange coupling case. On the basis of our synthesis of the temperature dependence of the magnetic susceptibility multiplied by temperature, and through comparing the theoretical results with the experimental measurements, it is concluded that the magnetic exchange couplings between the modules within the two high-spin polymers should be ferromagnetic exchange couplings, which are consistent with other theoretical results drawn from the investigations into the ground-state properties of the two organic polymers.
Injected carrier distribution in a pentacene field effect transistor probed using optical second harmonic generation
Authors: Yamada Daisuke , Manaka Takaaki , Lim Eunju , Tamura Ryousuke , Weis Martin , Iwamoto Mitsumasa
Published: October 2008

The injected carrier distribution in a pentacene field effect transistor (FET) was examined using optical second harmonic generation (SHG) measurements. Results showed that the SHG signal representing the profile of carriers injected from the source electrode distributed along the channel depends on the biasing conditions. The enhanced SHG around the drain electrode under the biasing condition u[=(Vgs−Vth)/Vds]<1 was discussed based on the formation of an exhausted region of the distributed injected carriers. Organic FET (OFET) analysis, as a system of the Maxwell–Wagner (MW) effect element, suggests that injected carriers that are distributed along the pentacene-SiO2 interface in the region between x=0 (source electrode) and x=uL (L: channel length) decrease the SHG intensity in this region. The SHG experiments with MW analysis described herein are helpful in understanding the performance of the OFET channel region.
Verified finite element simulation of multiferroic structures: Solutions for conducting and insulating systems
Authors: Blackburn John F. , Vopsaroiu Marian , Cain Markys G.
Published: October 2008

Composite multiferroics are an exciting class of engineered materials with a wide range of existing and potential applications. This paper describes an original finite element (FE) simulation for composite multiferroic devices. Detailed analysis is given on the flow of electric/magnetic fields between the composite regions with attention given to the conductivity of the magnetostrictive layers (piezoelectric assumed to be insulating). The simulation is verified against an existing FE code and against a theoretical analysis, also presented here. The issue of boundary and continuity conditions is discussed in detail and approximations made in both FE and analytical treatments are specified. We derive the I-V curves of the composite device when acting as a magnetic field detector, in both conducting and insulating cases. The magnetic field detector acts as a voltage source: we calculate its open-circuit voltage and internal impedance in each case.
Switchable Ni–Mn–Ga Heusler nanocrystals
Authors: Zayak A. T. , Beckman S. P. , Tiago Murilo L. , Entel P. , Chelikowsky James R.
Published: October 2008

We examined bulklike Heusler nanocrystals using real-space pseudopotentials constructed within density functional theory. The nanocrystals were made of various compositions of Ni–Mn–Ga in the size range from 15 up to 169 atoms. Among these compositions, the closest to the stoichiometric Ni2MnGa were found to be the most stable. The Ni-based nanocrystals retained a tendency for tetragonal distortion, which is inherited from the bulk properties. Surface effects suppress the tetragonal structure in the smaller Ni-based nanocrystals, while bigger nanocrystals develop a bulklike tetragonal distortion. We suggest the possibility of switchable Ni–Mn–Ga nanocrystals, which could be utilized for magnetic nanoshape-memory applications.
Characterization of GaAs grown on SiGe/Si graded substrates using p-n junction diodes
Authors: Chen K. P. , Yoon S. F. , Ng T. K. , Tanoto H. , Lew K. L. , Dohrman C. L. , Fitzgerald E. A.
Published: October 2008

The effect of GaAs buffer thickness on the electrical characteristics of a p+-n− junction diode is presented. The GaAs diodes are grown on a Ge/graded SixGe1−x/Si virtual substrate. Electrical characterization and visual observation provided vital insights into the origin of improvement in the electrical characteristics of the diode as the GaAs buffer thickness is increased. A high breakdown voltage of −9.2 V was achieved as the buffer thickness was increased from 50 to 600 nm, as the ideality factor reduces from 2.2 to 1.7, indicating reduction in the recombination current in the depletion region. This improvement is found to be related to the suppression mechanism of the wedge-shaped dislocation formation in the growth direction. This evidence provides important insights for the integration of III-V semiconductor devices, such as heterojunction bipolar transistor, with Si-based substrates.
A fast sonochemical approach for the synthesis of solution processable ZnO rods
Authors: Palumbo Marco , Henley S. J. , Lutz T. , Stolojan V. , Silva S. R. P.
Published: October 2008

A solution based sonochemical synthesis method for ZnO rods is presented with a resulting growth rate in excess of 15 times faster than previously reported. Such material is solution processable and could be exploited in the fabrication of transparent conductors and/or large area electronics via inkjet printing methods or solution based self-assembly techniques. To understand the crystal structure and defects chemistry, the as-synthesized wurtzite crystal structures were compared and contrasted with rods grown by the more traditional and well characterized hydrothermal growth method. Fluorescence spectra were recorded and the emission characteristics correlated with the structural and conductive properties of the ZnO rods. In particular, the sonochemical crystals appear to have a higher degree of order with fewer defects. This study represents a first step toward the tailoring of the electronic properties of ZnO rods. In particular, we will concentrate on the influence that native defects have on electrical conduction and on photoluminescence. Furthermore, we show how the intensity of the ultrasonic power exploited in this synthesis has a direct influence on the crystal quality as revealed by a comparative study. An optimum value between 30% and 35% of the maximum amplitude of a 20 kHz ultrasonic probe was found to give the best conditions for the growth of crystals with fewer defects density, while at ca. 25% of the maximum amplitude we observed the higher intensities for the fluorescence spectra both in the ultraviolet and in the visible range.
Electronic spectroscopy of the jet-cooled arsenic dicarbide (C2As) free radical
Authors: Wei Jie , Grimminger Robert A. , Sunahori Fumie X. , Clouthier Dennis J.
Published: October 2008

The Δr2-X̃Πr2 band system of the jet-cooled arsenic dicarbide (C2As) free radical has been recorded by laser-induced fluorescence (LIF) techniques in the 685–588nm region. The radical was produced in a pulsed electric discharge jet using a precursor mixture of AsCl3 vapor and methane in high pressure argon. A series of weak bands involving all three excited state vibrations was observed for both C212As and C213As. High-resolution spectra of the Π1∕22 component of the 000 bands of both isotopomers were rotationally analyzed, leading to the conclusion that the upper state is Δ2 with a small spin-orbit splitting (A=2.78cm−1). Ground and excited state molecular structures of r0″(CC,abinitio)=1.2933Å, r0″(CAs)=1.734(4)Å and r0′(CC,abinitio)=1.2276Å, r0′(CAs)=1.830(3)Å were derived from the B values and our density functional predictions of the CC bond lengths. Single vibronic level emission spectra were recorded for many of the LIF bands and these were used to obtain the ground state vibrational frequencies and spin-orbit splittings. These data were satisfactorily fitted to a Renner–Teller model which gave C212As parameters of ε=0.695(8), ω1=1704.8(20)cm−1, ω2=161.6(8)cm−1, ω3=663.6(12)cm−1, and a spin-orbit constant A=857.7(11)cm−1.
R12-calibrated H2O–H2 interaction: Full dimensional and vibrationally averaged potential energy surfaces
Authors: Valiron Pierre , Wernli Michael , Faure Alexandre , Wiesenfeld Laurent , Rist Claire , Kedžuch Stanislav , Noga Jozef
Published: October 2008

The potential energy surface of H2O–H2 is of great importance for quantum chemistry as a test case for H2O-molecule interactions. It is also required for a detailed understanding of important astrophysical processes, namely, the collisional excitation of water, including the pumping of water masers and the formation of molecular hydrogen on icy interstellar dust grains. We have calculated the interaction for H2O–H2 by performing both rigid-rotor (five-dimensional) and non-rigid-rotor (nine-dimensional) calculations using the coupled-cluster theory at the level of singles and doubles with perturbative corrections for triple excitations [CCSD(T)] with moderately large but thoroughly selected basis set. The resulting surface was further calibrated using high precision explicitly correlated CCSD(T)-R12 calculations on a subset of the rigid-rotor intermolecular geometries. The vibrationally averaged potential is presented in some details and is compared with the most recent rigid-rotor calculations. We explain, in particular, as to why vibrationally averaged rigid-rotor geometries are a better choice than equilibrium geometries. Our fit of the vibrationally averaged surface provides for the first time an accuracy of ∼3 cm−1 in the van der Waals minimum region of the interaction. The overall accuracy of the nine-dimensional surface and fit is lower but remains of the order of 3%–4% of the anisotropy in the domain spanned by the vibrational functions.
Comparison of sampling efficiency between simulated tempering and replica exchange
Authors: Zhang Cheng , Ma Jianpeng
Published: October 2008

We compared the sampling efficiency of simulated tempering and replica exchange. Our results indicate that simulated tempering is superior to replica exchange if the parameters for temperature transition in simulated tempering are adjusted to be proportional to the partition function. It is shown that, in simulated tempering, the rate of traversing energy space of different temperatures is much higher than that in replica exchange, especially in the case of low tempering frequency and/or larger temperature separations.
Phase-field simulations of stress-strain behavior in ferromagnetic shape memory alloy Ni2MnGa
Authors: Wu P. P. , Ma X. Q. , Zhang J. X. , Chen L. Q.
Published: October 2008

The evolution of strain, magnetic domain structure, and martensite microstructure during compressive stress loading and unloading of Ni2MnGa was studied using a phase-field model at several selected magnetic fields. We observed a typical quasiplastic behavior at zero field and a pseudoelastic behavior at 300 kA/m. At an intermediate field, 150 kA/m, the stress-strain relation is partially pseudoelastic. It was demonstrated that the magnetic domain structure has little influence on the recovered strain while the demagnetization factor impacts the strain reversal.
Electron acceleration in a warm magnetized plasma-filled cylindrical waveguide
Authors: Kumar S. , Yoon M.
Published: October 2008

The effect of plasma-electron collision and their thermal motion is investigated on the externally injected electron in a warm magnetized plasma-filled cylindrical waveguide of cross section of 13.68 cm2. The numerical results are presented for the external electron-energy gain and its trajectory in a nonrelativistic γe=1 and stationary v0=0 warm magnetized plasma-filled waveguide. Results shows that for an electron-cyclotron frequency ωc greater than the electron-plasma frequency ωp, a 100 keV electron acquires a 1.74 MeV energy in a 2.5 cm distance for plasma density n0=1.08×1017/m3, magnetic field B0=0.193 T, microwave frequency f=7.64 GHz, plasma-electron thermal velocity vth=0.2c, and plasma-electron collision frequency ν=4 GHz, which is lower than the 7 MeV electron energy in a cold magnetized plasma-waveguide case. Space-charge effects and other nonlinear effects are assumed to be negligible in this model.
Structural, electrical, and optical properties of InAsxSb1−x epitaxial films grown by liquid-phase epitaxy
Authors: Gao Fubao , Chen NuoFu , Zhang X. W. , Wang Yu , Liu Lei , Yin Zhigang , Wu Jinliang
Published: October 2008

The InAsxSb1−x films were grown on (100) GaSb substrates by liquid-phase epitaxy, and their structural, electrical, and optical properties were investigated. The high-resolution x-ray diffraction results reveal that the single crystalline InAsxSb1−x films with a midrange composition are epitaxially grown on the GaSb substrates. Temperature dependence of the Hall mobility was theoretically modeled by considering several predominant scattering mechanisms. The results indicate that ionized impurity and dislocation scatterings dominate at low temperatures, while polar optical phonon scattering is important at room temperature (RT). Furthermore, the InAsxSb1−x films with the higher As composition exhibit the better crystalline quality and the higher mobility. The InAs0.35Sb0.65 film exhibits a Hall mobility of 4.62×104 cm2 V−1 s−1. The cutoff wavelength of photoresponse is extended to about 12 μm with a maximum responsivity of 0.21 V/W at RT, showing great potential for RT long-wavelength infrared detection.
Enhanced dielectric responses in Mg-doped CaCu3Ti4O12
Authors: Li M. , Cai Gemei , Zhang D. F. , Wang W. Y. , Wang W. J. , Chen X. L.
Published: October 2008

We report the effects of the Mg doping on the dielectric properties of CaCu3Ti4O12 in the frequency range of 200 Hz–200 kHz and in the temperature range of 58–473 K. It is found that the incorporations of Mg2+ result in increases in the dielectric constant by 12%–20% and decreases in the dielectric loss by 41%–64% (with the minimum of 0.042 for CaCu2.7Mg0.3Ti4O12) at room temperature and at 1 kHz. The x-ray photoemission spectroscopy measurements reveal that the content of Cu3+ increases with the increasing concentration of Mg2+. The enhanced dielectric response may be related to the influence of Cu3+ and/or Mg2+. In other words, Mg2+ is an effective ion to optimize the dielectric properties of CaCu3Ti4O12.
Performances of compact integrated superconducting magnetometers for biomagnetic imaging
Authors: Granata C. , Vettoliere A. , Rombetto S. , Nappi C. , Russo M.
Published: October 2008

In the present paper, performances of compact fully integrated superconducting quantum interference device (SQUID) magnetometers, recently developed, have been investigated in view of their employment in large multichannel systems for biomagnetic imaging. The analysis has been focused on SQUID sensors having a pickup loop side length of 3 and 4 mm based on a design aimed to maximize the magnetic flux transferred from the detection coil to the SQUID in comparison with a magnetometer with 9 mm side length having a suitable sensitivity for biomagnetic applications. The performance study has been consisted in the computation of the magnetic responses to a current dipole which is the most fundamental approach used in biomagnetism. The results have shown that the dipole current sensitivity of 4 mm long side compact magnetometers is suitable for application in multichannel systems for magnetoencephalography and magnetocardiography.