bluuebunny/crossref_metadata_2025 huggingface dataset:
New experimental evidence about secondary processes in phenylphthalein-dimethylether and 1,1′-bis(p-methoxyphenyl)cyclohexane
Authors: Prevosto D. , Sharifi S. , Capaccioli S. , Rolla P. A. , Hensel-Bielowka S. , Paluch M.
Published: September 2007

The slow secondary (β) process of 1,1′-bis (4-methoxyphenyl) cyclohexane and phenolphthalein dimethylether has been investigated by dielectric spectroscopy. New experimental results about the pressure dependence of the two processes are reported, as well as new data about the dependence of the characteristic relaxation frequency on the cooling rate used to vitrify the system in isobaric conditions. Previous investigations on these systems suggested that the first one is not a true Johari-Goldstein relaxation and both processes should originate from the flip flop motion of the phenyl ring. The results herein reported evidence that the characteristic frequency of the β process of phenolphthalein dimethylether is more sensitive to pressure variation and to the vitrification procedure than that of 1,1′-bis (4-methoxyphenyl) cyclohexane. Such results suggest an intermolecular origin for the secondary process in phenolphthalein dimethylether and an intramolecular origin for the other one, which do not completely agree with the previous interpretation. We evidence that the microscopic mechanism at the basis of these two processes is still an open question, which should be debated on the basis of new experimental investigations.
Improved two-equation model for thin layer fluid flowing down an inclined plane problem
Authors: Abderrahmane H. Ait , Vatistas G. H.
Published: September 2007

We are presenting a substantial improvement to the two-equation model proposed by Usha and Uma [Phys. Fluids 16, 7 (2004)] for the thin film of fluid flowing down an inclined plane. Incorporating the shear stress in the parabolic velocity profile, we satisfy exactly a tangential dynamical condition at the free surface. This correction has significant implications on the occurrence conditions for both heteroclinic and Hopf bifurcations of long stationary waves.
Electronic properties of the interface between p-CuI and anatase-phase n-TiO2 single crystal and nanoparticulate surfaces: A photoemission study
Authors: Kumarasinghe A. R. , Flavell W. R. , Thomas A. G. , Mallick A. K. , Tsoutsou D. , Chatwin C. , Rayner S. , Kirkham P. , Warren S. , Patel S. , Christian P. , O’Brien P. , Grätzel M. , Hengerer R.
Published: September 2007

We present a study of the growth of the p-type inorganic semiconductor CuI on n-type TiO2 anatase single crystal (101) surfaces and on nanoparticulate anatase surfaces using synchrotron radiation photoemission spectroscopy. Core level photoemission data obtained using synchrotron radiation reveal that both the substrate (TiO2) and the overlayer (CuI) core levels shift to a lower binding energy to different degrees following the growth of CuI on TiO2. Valence band photoemission data show that the valence band maximum of the clean substrate differs from that of the dosed surface which may be interpreted qualitatively as due to the introduction of a new density of states within the band gap of TiO2 as a result of the growth of CuI. The valence band offset for the heterojunction n-TiO2∕p-CuI has been measured using photoemission for both nanoparticulate and single crystal TiO2 surfaces, and the band energy alignment for these heterojunction interfaces is presented. With the information obtained here, it is suggested that the interface between p-CuI and single crystal anatase-phase n-TiO2 is a type-II heterojunction interface, with significant band bending. The measured total band bending matches the work function change at the interface, i.e., there is no interface dipole. In the case of the nanoparticulate interface, an interface dipole is found, but band bending within the anatase nanoparticles remains quite significant. We show that the corresponding depletion layer may be accommodated within the dimension of the nanoparticles. The results are discussed in the context of the functional properties of dye-sensitized solid state solar cells.
Ab initio statistical mechanics of surface adsorption and desorption. I. H2O on MgO (001) at low coverage
Authors: Alfè D. , Gillan M. J.
Published: September 2007

We present a general computational scheme based on molecular dynamics (MD) simulation for calculating the chemical potential of adsorbed molecules in thermal equilibrium on the surface of a material. The scheme is based on the calculation of the mean force in MD simulations in which the height of a chosen molecule above the surface is constrained and subsequent integration of the mean force to obtain the potential of mean force and hence the chemical potential. The scheme is valid at any coverage and temperature, so that in principle it allows the calculation of the chemical potential as a function of coverage and temperature. It avoids all statistical mechanical approximations, except for the use of classical statistical mechanics for the nuclei, and assumes nothing in advance about the adsorption sites. From the chemical potential, the absolute desorption rate of the molecules can be computed, provided that the equilibration rate on the surface is faster than the desorption rate. We apply the theory by ab initio MD simulation to the case of H2O on MgO (001) in the low-coverage limit, using the Perdew-Burke-Ernzerhof (PBE) form of exchange correlation. The calculations yield an ab initio value of the Polanyi-Wigner frequency prefactor, which is more than two orders of magnitude greater than the value of 1013s−1 often assumed in the past. Provisional comparison with experiment suggests that the PBE adsorption energy may be too low, but the extension of the calculations to higher coverages is needed before firm conclusions can be drawn. The possibility of including quantum nuclear effects by using path-integral simulations is noted.
Laminar flow in a microchannel with hydrophobic surface patterned microribs oriented parallel to the flow direction
Authors: Maynes D. , Jeffs K. , Woolford B. , Webb B. W.
Published: September 2007

This paper reports results of an analytical and experimental investigation of the laminar flow in a parallel-plate microchannel with ultrahydrophobic top and bottom walls. The walls are fabricated with microribs and cavities that are oriented parallel to the flow direction. The channel walls are modeled in an idealized fashion, with the shape of the liquid-vapor meniscus approximated as flat. An analytical model of the vapor cavity flow is employed and coupled with a numerical model of the liquid flow by matching the local liquid and vapor phase velocity and shear stress at the interface. The numerical predictions show that the effective slip length and the reduction in the classical friction factor-Reynolds number product increase with increasing relative cavity width, increasing relative cavity depth, and decreasing relative microrib/cavity module length. Comparisons were also made between the zero shear interface model and the liquid-vapor cavity coupled model. The results illustrate that the zero shear interface model underpredicts the overall flow resistance. Further, the deviation between the two models was found to be significantly larger for increasing values of both the relative rib/cavity module width and the cavity fraction. The trends in the frictional pressure drop predictions are in good agreement with experimental measurements made at similar conditions, with greater deviation observed at increasing size of the cavity fraction. Based on the numerical predictions, an expression is proposed in which the friction factor-Reynolds number product may be estimated in terms of the important variables.
Selective suppression of high order axial modes of the gyrotron backward-wave oscillator
Authors: Pao K. F. , Fan C. T. , Chang T. H. , Chiu C. C. , Chu K. R.
Published: September 2007

Selective suppression of high order axial modes of the gyrotron backward-wave oscillator (gyro-BWO) is investigated in theory and in experiment. The gyro-BWO interaction is much more efficient in a down-tapered interaction structure, while it is also more susceptible to the problem of axial mode competition in such a structure. Because higher order axial modes (at a higher oscillation frequency) penetrate deeper into the interaction structure, application of distributed wall loss at the downstream end of the interaction structure is shown to be effective for selective suppression of these modes with minor effects on the efficiency of the desired fundamental axial mode. A stable gyro-BWO operating in a single mode throughout the entire beam pulse is demonstrated on the basis of this principle. Theoretical and experimental results are found to be in good agreement.
Preferential attachment during the evolution of a potential energy landscape
Authors: Massen Claire P. , Doye Jonathan P. K.
Published: September 2007

It has previously been shown that the network of connected minima on a potential energy landscape is scale-free, and that this reflects a power-law distribution for the areas of the basins of attraction surrounding the minima. Here, the aim is to understand more about the physical origins of these puzzling properties by examining how the potential energy landscape of a 13-atom cluster evolves with the range of the potential. In particular, on decreasing the range of the potential the number of stationary points increases and thus the landscape becomes rougher and the network gets larger. Thus, the evolution of the potential energy landscape can be followed from one with just a single minimum to a complex landscape with many minima and a scale-free pattern of connections. It is found that during this growth process, new edges in the network of connected minima preferentially attach to more highly connected minima, thus leading to the scale-free character. Furthermore, minima that appear when the range of the potential is shorter and the network is larger have smaller basins of attraction. As there are many of these smaller basins because the network grows exponentially, the observed growth process thus also gives rise to a power-law distribution for the hyperareas of the basins.
Linearized semiclassical initial value time correlation functions using the thermal Gaussian approximation: Applications to condensed phase systems
Authors: Liu Jian , Miller William H.
Published: September 2007

The linearized approximation to the semiclassical initial value representation (LSC-IVR) has been used together with the thermal Gaussian approximation (TGA) (TGA/LSC-IVR) [J. Liu and W. H. Miller, J. Chem. Phys. 125, 224104 (2006)] to simulate quantum dynamical effects in realistic models of two condensed phase systems. This represents the first study of dynamical properties of the Ne13 Lennard-Jones cluster in its liquid-solid phase transition region (temperature from 4 to 14 K). Calculation of the force autocorrelation function shows considerable differences from that given by classical mechanics, namely that the cluster is much more mobile (liquidlike) than in the classical case. Liquid para-hydrogen at two thermodynamic state points (25 and 14 K under nearly zero external pressure) has also been studied. The momentum autocorrelation function obtained from the TGA/LSC-IVR approach shows very good agreement with recent accurate path integral Monte Carlo results at 25 K [A. Nakayama and N. Makri, J. Chem. Phys. 125, 024503 (2006)]. The self-diffusion constants calculated by the TGA/LSC-IVR are in reasonable agreement with those from experiment and from other theoretical calculations. These applications demonstrate the TGA/LSC-IVR to be a practical and versatile method for quantum dynamics simulations of condensed phase systems.
IR signature of the photoionization-induced hydrophobic→hydrophilic site switching in phenol-Arn clusters
Authors: Ishiuchi Shun-ichi , Sakai Makoto , Tsuchida Yuji , Takeda Akihiro , Kawashima Yasutake , Dopfer Otto , Müller-Dethlefs Klaus , Fujii Masaaki
Published: September 2007

IR spectra of phenol-Arn (PhOH–Arn) clusters with n=1 and 2 were measured in the neutral and cationic electronic ground states in order to determine the preferential intermolecular ligand binding motifs, hydrogen bonding (hydrophilic interaction) versus π bonding (hydrophobic interaction). Analysis of the vibrational frequencies of the OH stretching motion, νOH, observed in nanosecond IR spectra demonstrates that neutral PhOH-Ar and PhOH–Ar2 as well as cationic PhOH+–Ar have a π-bound structure, in which the Ar atoms bind to the aromatic ring. In contrast, the PhOH+–Ar2 cluster cation is concluded to have a H-bound structure, in which one Ar atom is hydrogen-bonded to the OH group. This π→H binding site switching induced by ionization was directly monitored in real time by picosecond time-resolved IR spectroscopy. The π-bound νOH band is observed just after the ionization and disappears simultaneously with the appearance of the H-bound νOH band. The analysis of the picosecond IR spectra demonstrates that (i) the π→H site switching is an elementary reaction with a time constant of ∼7ps, which is roughly independent of the available internal vibrational energy, (ii) the barrier for the isomerization reaction is rather low(<100cm−1), (iii) both the position and the width of the H-bound νOH band change with the delay time, and the time evolution of these spectral changes can be rationalized by intracluster vibrational energy redistribution occurring after the site switching. The observation of the ionization-induced switch from π bonding to H bonding in the PhOH+–Ar2 cation corresponds to the first manifestation of an intermolecular isomerization reaction in a charged aggregate.
In situ observation of oxygen gettering by titanium overlayer on HfO2∕SiO2∕Si using high-resolution Rutherford backscattering spectroscopy
Authors: Nakajima Kaoru , Fujiyoshi Akira , Ming Zhao , Suzuki Motofumi , Kimura Kenji
Published: September 2007

Oxygen gettering by Ti overlayer (2–8nm) on a HfO2(3nm)∕SiO2(1.5nm)∕Si(001) structure was investigated using high-resolution Rutherford backscattering spectroscopy. After deposition of a thin Ti layer, the interfacial SiO2 layer is reduced by ∼0.2nm and the released oxygen is incorporated in Ti layer. Subsequent annealing at 330°C in UHV causes further reduction by 0.1–0.8nm depending on the Ti layer thickness. In addition to the reduction of the SiO2 layer, significant oxygen depletion in the HfO2 layer was observed for thicker Ti layers after annealing.
Unbiased auxiliary basis sets for accurate two-electron integral approximations
Authors: Aquilante Francesco , Lindh Roland , Bondo Pedersen Thomas
Published: September 2007

We propose Cholesky decomposition (CD) of the atomic two-electron integral matrix as a robust and general technique for generating auxiliary basis sets for the density fitting approximation. The atomic CD (aCD) auxiliary basis set is calculated on the fly and is not biased toward a particular quantum chemical method. Moreover, the accuracy of the aCD basis set can be controlled with a single parameter.
Thermoelectric properties of BaSi2, SrSi2, and LaSi
Authors: Hashimoto Kohsuke , Kurosaki Ken , Imamura Yasushi , Muta Hiroaki , Yamanaka Shinsuke
Published: September 2007

We studied the thermoelectric properties of BaSi2, SrSi2, and LaSi. The polycrystalline samples were prepared by spark plasma sintering (SPS). The electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were measured above room temperature. The power factor (S2∕ρ) is quite low (below 10−5 Wm−1 K−2 over the whole temperature range) for BaSi2 and LaSi, while relatively high (1.19×10−3 Wm−1 K−2 at 331 K) for SrSi2. BaSi2 exhibits quite low κ. The κ values at room temperature are 1.56, 5.25, and 6.71 Wm−1 K−1 for BaSi2, SrSi2, and LaSi, respectively. The maximum values of the dimensionless figure of merit, ZT=S2T∕ρ∕κ, are 0.01 at 954 K for BaSi2, 0.09 at 417 K for SrSi2, and 0.002 at 957 K for LaSi.
Characterization of ferromagnetic nanowires by partial first order reversal curves
Authors: Clime L. , Zhao S.-Y. , Normandin F. , Veres T.
Published: September 2007

A nonparametric approach to the identification of magnetic anisotropies in ferromagnetic nanostructures is applied in order to study the influence of the bath acidity on the induced magnetic anisotropies in electrodeposited ferromagnetic nanowires. By using partial first order reversal curves (pFORC), we find a two-peak probability density function whose topology changes with the acidity of the electrodeposition bath. Numerical micromagnetic simulations suggest that this particular distribution originates in two populations of nonidentical nanowires with different magneto-crystalline anisotropies. Bath acidity values influence the balance between these two populations rather than the intrinsic properties of individual nanowires.
Mg
B
2
radio-frequency superconducting quantum interference device prepared by atomic force microscope lithography
Authors: Gregor M. , Plecenik T. , Praščák M. , Mičunek R. , Kubinec M. , Gašparík V. , Grajcar M. , Kúš P. , Plecenik A.
Published: September 2007

A new method of preparation of radio-frequency superconducting quantum interference devices on MgB2 thin films is presented. The variable-thickness bridge was prepared by a combination of optical lithography and of the scratching by an atomic force microscope. The critical current of the nanobridge was 0.35μA at 4.2K. Noncontact measurements of the current-phase characteristics and of the critical current versus temperature have been investigated on the authors’ structures.
First-principles study of electronic properties of La2Hf2O7 and Gd2Hf2O7
Authors: Li N. , Xiao H. Y. , Zu X. T. , Wang L. M. , Ewing Rodney C. , Lian Jie , Gao Fei
Published: September 2007

The structural and electronic properties of A2Hf2O7 (A=La and Gd) pyrochlore compounds are investigated by means of first-principles total energy calculations. Also, the formation energies of defects are calculated, and the results can be used to explain the stability of pyrochlores. Hybridizations between A 5p and O 2s and between A 5d and O 2p states are observed, but the interaction between A 5p and O 2s orbitals is much stronger in Gd2Hf2O7 than that in La2Hf2O7. Gd2Hf2O7 shows a density of state distribution much different from that of La2Hf2O7. Mulliken overlap population analysis shows that the A–O48f and A–O8b bonds in Gd2Hf2O7 are more ionic than the corresponding bonds in La2Hf2O7, while the Hf–O48f bond in Gd2Hf2O7 is more covalent. These calculations suggest that A–O48f and A–O8b bonds may play important roles in their response to irradiation-induced amorphization observed experimentally.
Conversion process of the dominant electroluminescence mechanism in a molecularly doped organic light-emitting device with only electron trapping
Authors: Zhou Liang , Zhang Hongjie , Deng Ruiping , Li Zhefeng , Yu Jiangbo , Guo Zhiyong
Published: September 2007

In this work, the detailed conversion process of the dominant electroluminescence (EL) mechanism in a device with Eu(TTA)3phen (TTA=thenoyltrifluoroacetone, phen=1,10-phenanthroline) doped CBP (4,4′-N,N′-dicarbazole-biphenyl) film as the emitting layer was investigated by analyzing the evolution of carrier distribution on dye and host molecules with increasing voltage. Firstly, it was confirmed that only electrons can be trapped in Eu(TTA)3phen doped CBP. As a result, holes and electrons would be situated on CBP and Eu(TTA)3phen molecules, respectively, and thus creates an unbalanced carrier distribution on both dye and host molecules. With the help of EL and photoluminescence spectra, the distribution of holes and electrons on both Eu(TTA)3phen and CBP molecules was demonstrated to change gradually with increasing voltage. Therefore, the dominant EL mechanism in this device changes gradually from carrier trapping at relatively low voltage to Förster energy transfer at relatively high voltage.
Electronic properties of hydrogenated silicon nanowires with surface defects
Authors: Migas D. B.
Published: September 2007

Our ab initio results indicate that some defects, such as atomic vacancies, on the surface of hydrogenated silicon nanowires in the ⟨001⟩ orientation can preserve the direct character of the gap and enhance oscillator strength of the first direct transition. This finding extends a possible scenario of the efficient photoluminescence from porous silicon recently published [D. B. Migas, J. Appl. Phys. 98, 054310 (2005)].
Structural origin of the mobility enhancement in a pentacene thin-film transistor with a photocrosslinking insulator
Authors: Bae Jin-Hyuk , Kim Won-Ho , Kim Hyeok , Lee Changhee , Lee Sin-Doo
Published: September 2007

We present the underlying mechanism for the mobility enhancement in a pentacene thin-film transistor (TFT) with a photocrosslinking polymeric insulator, poly(vinyl cinnamate) (PVCi). Experimental results for the optical anisotropy, x-ray diffraction, and microscopic layer-by-layer coverage of the pentacene film on the photocrosslinked PVCi layer, exposed to a linearly polarized ultraviolet (LPUV) light, clearly demonstrate the importance of the structural packing of pentacene grains rather than the directional alignment of the pentacene molecules for the mobility enhancement. The packing density of pentacene grains is directly related to the number of photocrosslinking sites of the PVCi insulator produced by the LPUV. It is found that the mobility in the pentacene TFT is linearly proportional to the number of photocrosslinked sites of the PVCi insulator serving as interaction sites for the layer-by-layer coverage of the pentacene molecules with no preferred orientation.
Low frequency noise in InAlAs/InGaAs modulation doped field effect transistors with 50-nm gate length
Authors: Levinshtein M. E. , Rumyantsev S. L. , Tauk R. , Boubanga S. , Dyakonova N. , Knap W. , Shchepetov A. , Bollaert S. , Rollens Y. , Shur M. S.
Published: September 2007

We report on the low frequency [1/f and generation-recombination (GR)] noise in InAlAs/InGaAs modulation doped field effect transistors with a 50-nm gate length. The characteristic capture and emission times of the GR noise depended on the gate voltage. Measurements of the noise as a function of the gate voltage showed that the gate leakage current, contacts, and ungated sections of the channel did not contribute to the 1/f noise. The gate voltage dependence of the 1/f noise agreed well with the model of number of carriers fluctuations as a source of the 1/f noise. An effective density of traps responsible for the 1/f noise was found to be Deff≈2.7×1012 cm−2 eV−1.
Quantitative analysis of the guidance of vortices in superconducting films with magnetic dots
Authors: González M. P. , Hollmann E. , Wördenweber R.
Published: September 2007

The guidance of vortices in superconducting Nb films with periodic arrangements of magnetic Ni dots is studied via simultaneous four-probe measurements in standard and Hall configuration. Ni dots are fabricated via e-beam lithography and lift-off technology. Arrays with periodicity down to 400 nm and dot size of 200 nm are obtained. Subsequently, the arrays are covered with a thin superconducting Nb layer. Simultaneous Hall and standard resistive measurements of the flux dynamic in Nb bridges with and without magnetic dots demonstrate and quantify the guidance of flux via the magnetic dots. Guidance of vortices is only possible in a limited current regime; guidance improves with decreasing temperature. Within the identified current regime most of the vortices (>40% for zero field) and in the optimum case all of the vortices are guided by the magnetic dots.
CoFe
2
O
4
∕
Pb
(
Zr
052
Ti
0.48
)
O
3
disk-ring magnetoelectric composite structures
Authors: Li Lei , Lin Yi Qi , Chen Xiang Ming
Published: September 2007

Magnetoelectric composite structures of CFO (CoFe2O4)-disk/PZT (Pb(Zr052Ti0.48)O3)-ring and PZT-disk/CFO-ring were prepared and evaluated, where the magnetostrictive and piezoelectric phases were coupled through normal stresses. Under bias magnetic fields around 3400 Oe, the peak magnetoelectric coefficients reached 18.1 and 102.6 mV cm−1 Oe−1 at 1 kHz for CFO-disk/PZT-ring and PZT-disk/CFO-ring structures, respectively, which were much higher than that of the corresponding layered composite. Prediction for magnetoelectric coefficients at low frequency was conducted, and the improved interfacial mechanical coupling through normal stresses was responsible for the enhanced magnetoelectric properties of the present composite structures. Moreover, the frequency dependence of the magnetoelectric coefficient was investigated, and the resonant magnetoelectric coefficients were up to 4120 and 6430 mV cm−1 Oe−1 for CFO-disk/PZT-ring and PZT-disk/CFO-ring composite structures, respectively.
Experimental characterization of a 6.7 GHz coaxial Bragg reflector
Authors: Barroso Joaquim J. , Castro Pedro J. , Leite Neto Joaquim P. , Pimentel Guilherme L.
Published: September 2007

A coaxial line periodically loaded by circular disks on the inner conductor is designed and constructed to act as Bragg reflector in the 4.0–9.5 GHz frequency range. This is achieved by placing 10 disks (0.35 cm thick and 6.22 cm in diameter) equally spaced by the periodic distance of 1.8 cm on a coaxial waveguide with inner and outer diameters of 6.90 and 4.22 cm. Experiments on a periodic structure made from stainless steel demonstrate a band gap of 5.6 GHz centered at the 6.7 GHz design frequency in close agreement with 3D microwave computer simulations.
Controlled synthesis of lanthanide-doped NaYF4 upconversion nanocrystals via ligand induced crystal phase transition and silica coating
Authors: Shan Jingning , Ju Yiguang
Published: September 2007

Upconversion nanocrystals (NCs) of NaYF4:Yb,Ln (Ln=Er and Tm) in cubic and hexagonal phases were synthesized in oleic acid (OA) and/or trioctylphosphine (TOP) with octadecene (ODE) via a thermolysis method. Only cubic-phase NCs were obtained in OA/ODE and TOP/ODE solvents; on the other hand, the hexagonal-phase NCs resulted in OA/TOP/ODE. The phase transition from cubic to hexagonal was induced by a ligand formed between oleate and TOP at high temperature. Moreover, the addition of OA/TOP in high and low ratios produced hexagonal nanoparticles and nanorods, respectively. The formed NCs were coated by SiO2 using microemulsion reactions for further biofunctionalization in biological applications.
One-dimensional bubble model of pulsed discharge in water
Authors: Lu XinPei
Published: September 2007

In this paper, a one-dimensional bubble model of pulsed discharge in water is presented. With a total input energy of 0.63J, the simulation results show that when the bubble collapses at the center of the bubble, the plasma pressure oscillates strongly. It oscillates between 800 and 1150atm with an oscillation frequency of about 6.9MHz, while at r=R∕2 (R: bubble radius), the gas velocity oscillates intensely at the same frequency. It oscillates between −235 and 229m∕s when the bubble radius reaches its minimum. But it does not oscillate at r=R because of the inertia of the surrounding water. The bubble collapses and reexpands with almost the same speed as that of the zero-dimensional (0D) model. This further confirms why the shock wave pressure from the 0D mode has a good agreement with the experimental results since the shock wave pressure is only determined by the bubble wall velocity v(R).
Significant increase of the ferroelectric phase transition temperature in partially deuterated KH2PO4 by proton irradiation
Authors: Kim Se Hun , Lee Kyu Won , Oh B. H. , Kweon J. J. , Lee Cheol Eui
Published: September 2007

The ferroelectric phase transition temperature was significantly raised by 5K in partially deuterated KH2PO4 irradiated by a proton beam. Increase in the hydrogen bond length was indicated by the dielectric constant analysis. Deuteron nuclear magnetic resonance (NMR) measurements of the electric field gradient tensor showed atomic displacement after the proton irradiation, and P31 NMR measurements of the chemical shift tensor revealed phosphorous displacement in the hydrogen-bonded direction and the PO4 tetrahedral distortion. Increase of the phase transition temperature can be closely related to the structural modification involving the hydrogen-bond geometry.
Electrical conduction of Ge nanodot arrays formed on an oxidized Si surface
Authors: Nakayama Yasuo , Yamazaki Shiro , Okino Hiroyuki , Hirahara Toru , Matsuda Iwao , Hasegawa Shuji , Ichikawa Masakazu
Published: September 2007

Carrier transport mechanism on Ge nanodot arrays formed on SiO2 monolayer covering over the Si surface is investigated by microscopic four-point-probe measurements combined with core-level photoemission spectroscopy and scanning tunneling microscopy. Different conduction natures are found depending on whether or not the nanodots and the substrate are directly connected by subnanometer-sized voids penetrating the SiO2 layer. In the presence of the voids, conductivity is regulated by the dot-size through quantum-size effect.
A nonstandard geometric quantization of the harmonic oscillator
Authors: Lim Adrian P. C.
Published: September 2007

In the standard geometric quantization of the harmonic oscillator in (R2,dp∧dx), using standard holomorphic coordinate z=x−ip induces a standard complex structure which Kähler polarizes the prequantum line bundle. Hence, the quantum Hilbert space maybe identified with holomorphic functions which are L2 integrable with respect to Gaussian measure, denoted by H. The corresponding quantized Hamiltonian is then given by ℏz(∂∕∂z). We propose using a different almost complex structure J (under some restrictions) compatible with dp∧dx and construct a quantum Hilbert space HJ containing nontrivial L2 sections and a quantized Hamiltonian QJ. The main result is that (HJ,QJ) is unitarily equivalent to (H,ℏz(∂∕∂z)).
The impact of thermal effects on emission characteristics of asymmetrical AlGAO-waveguide microdisks based on quantum dots
Authors: Blokhin S. A. , Sakharov A. V. , Nadtochy A. M. , Kulagina M. M. , Zadiranov Yu. M. , Gordeev N. Yu. , Maximov M. V. , Ustinov V. M. , Ledentsov N. N. , Stock E. , Warming T. , Bimberg D.
Published: September 2007

Microphotoluminescence studies of AlGaO-based asymmetric waveguide microdisk structures incorporating a single sheet of self-organized InAs quantum dots (QD) under photoexcitation are performed. The authors have observed a strong redshift and broadening of the QD optical transition energy at high excitation densities due to excitation-density-induced overheating and many-particle effects and monitored the impact of these effects on the whispering gallery modes (WGMs) in spontaneous and stimulated emission regimes. They have observed strongly nonmonotonic dependencies of the linewidth and intensity of the WGMs with excitation density increase and underline possible reasons for the observed behavior.
Nonlinear Peltier effect in semiconductors
Authors: Zebarjadi Mona , Esfarjani Keivan , Shakouri Ali
Published: September 2007

Nonlinear Peltier coefficient of a doped InGaAs semiconductor is calculated numerically using the Monte Carlo technique. The Peltier coefficient is also obtained analytically for single parabolic band semiconductors assuming a shifted Fermi-Dirac electronic distribution under an applied bias. Analytical results are in agreement with numerical simulations. Key material parameters affecting the nonlinear behavior are doping concentration, effective mass, and electron-phonon coupling. Current density thresholds at which nonlinear behavior is observable are extracted from numerical data. It is shown that the nonlinear Peltier effect can be used to enhance cooling of thin film microrefrigerator devices especially at low temperatures.
Dielectric anisotropy and phonon modes of ordered indirect-gap Al0.52In0.48P studied by far-infrared ellipsometry
Authors: Hofmann T. , Gottschalch V. , Schubert M.
Published: September 2007

The infrared (100–600cm−1) optical properties of partially CuPt-type ordered Al0.52In0.48P deposited lattice matched on GaAs are studied by ellipsometry. The authors determine the ordinary and extraordinary dielectric functions and report on the evolution of the optical phonon mode frequencies of Al0.52In0.48P as a function of the degree of ordering. In addition to the InP- and AlP-like phonon modes, they observe two alloy-induced phonon modes which are anisotropic upon CuPt ordering. The observed modes are associated to vibrations with E and A1 symmetries. The alloy-induced phonon modes are useful for classifying the degree of ordering in this indirect band gap alloy.
Low-temperature charge transport in Ga-acceptor nanowires implanted by focused-ion beams
Authors: Robinson S. J. , Perkins C. L. , Shen T.-C. , Tucker J. R. , Schenkel T. , Wang X. W. , Ma T. P.
Published: September 2007

Ga-acceptor nanowires were embedded in crystalline Si using focused-ion beams. The dc current-voltage characteristics of these wires after annealing are highly nonlinear at low temperatures. A conductance threshold of less than 50mV is observed independent of Ga+ dosage and implant beam overlap. These features suggest a Coulomb blockade transport mechanism presumably caused by a network of Ga precipitates in the substrate. This granular scenario is further supported by measurements of gated nanowires. Nanowires with metallic conductance at low temperatures could be achieved by reducing the current density of the focused-ion beams.
Molecular-packing-enhanced charge transport in organic field-effect transistors based on semiconducting porphyrin crystals
Authors: Minari Takeo , Seto Mari , Nemoto Takashi , Isoda Seiji , Tsukagoshi Kazuhito , Aoyagi Yoshinobu
Published: September 2007

The effect of semiconductor crystal structure on the performance of organic field-effect transistors is investigated through the fabrication and characterization of devices based on single porphyrin crystals with different center metals. The field-effect mobility of the transistors is found to increase with decreasing intermolecular distance, attributable to greater overlap of π orbitals among close-packed molecules and the consequent promotion of charge transport.
Fluorescence intermittency of silicon nanocrystals and other quantum dots: A unified two-dimensional diffusion-controlled reaction model
Authors: Tang Jau
Published: September 2007

We present a unified model involving two-dimensional diffusion-controlled reactions of both slow and fast reaction coordinates to elucidate the dynamic origin of fluorescence intermittency observed not just in quantum dots but also in organic chromphores and biomolecules. This improved model also solves the puzzling behavior of Si nanocrystals which display an unusually large m (exceeding 2) for the power-law decay of t−m and provides remedies for the deficiencies in existing models.
Changing and reversing the exchange bias in a current-in-plane spin valve by means of an electric current
Authors: Tang Xiao-Li , Zhang Huai-Wu , Su Hua , Zhong Zhi-Yong , Jing Yu-Lan
Published: September 2007

The authors show that a current flowing in a direction not perpendicular to the layer planes of an exchange-biased spin valve systematically changes the exchange bias. The direction of the exchange bias can be completely reversed when a large enough current is applied. This effect occurs only when the direction of the initial sweeping magnetic field is antiparallel to the exchange-bias field. The effect is attributed to the recently predicted current-induced torque in an antiferromagnet and provides evidence to support the prediction that the critical current is smaller in an antiferromagnet than the typical value for current switching in a ferromagnet.
Microwave-assisted magnetization reversal in 0.36-μm-wide Permalloy wires
Authors: Nozaki Y. , Tateishi K. , Taharazako S. , Ohta M. , Yoshimura S. , Matsuyama K.
Published: September 2007

Microwave-assisted magnetization reversal in a 0.36μm wide Ni80Fe20 wire with a natural ferromagnetic resonance frequency of 8.42GHz was investigated experimentally. Magnetization switching could be detected from a discontinuous change in the ferromagnetic resonance frequency of the wire. The application of microwaves with a frequency of 7.5GHz and a power of 19.2dBm reduced the switching field from 217to142Oe. While high-power microwave-assisted switching reduced the dc switching field, it substantially broadened the switching field distribution. This is mainly attributed to inhomogeneous resonance properties of the nucleation sites in the wire.
Fluorescence tuning of confined molecules in porous silicon mirrors
Authors: Palestino A. G. , de la Mora M. B. , del Río J. A. , Gergely C. , Pérez E.
Published: September 2007

Emission signal from fluorescent molecules (fluorescein-5-maleimide) in a porous silicon mirror is enhanced by tuning the pore size and reflectance spectrum of the porous silicon multilayer structure. This is achieved when the reflectance spectrum of the silicon mirror overlaps the fluorescent excitation and emission wavelengths of the fluorescent molecule, and chemical linkers assure the molecular confinement.
Nanostructured optical fiber by the sol-gel process in the SiO2–ZrO2 system
Authors: Brasse G. , Restoin C. , Auguste J.-L. , Hautreux S. , Blondy J.-M. , Lecomte A.
Published: September 2007

A SiO2–ZrO2 nanostructured optical fiber developed using the chemical sol-gel method is fabricated and its waveguiding properties are studied. The refractive index difference between the core and the cladding is 0.021 for a (30%ZrO2–70%SiO2) molar core composition. Illumination of the fiber with a supercontinuum source (350–1750nm) shows that the light is guided over this entire range. The measured attenuation losses are 0.7dB∕m at a 1064nm wavelength and the calculated cutoff wavelength is 714nm.
Parasitic sub-band-gap emission originating from compensating native defects in Si doped AlGaN
Authors: Chen K. X. , Dai Q. , Lee W. , Kim J. K. , Schubert E. F. , Liu W. , Wu S. , Li X. , Smart J. A.
Published: September 2007

The authors investigate the dependence of the photoluminescence intensity ratio between the near-band-edge and the parasitic blue emission as a function of silane flow in n-type Al0.3Ga0.7N. It is found that when the silane flow rate is higher than 1.13×10−9mol∕min, the UV-to-blue ratio is a constant. When the silane flow rate is lower than 1.13×10−9mol∕min, the UV-to-blue ratio increases rapidly as the silane flow rate decreases. A theoretical model is proposed, which assumes that the parasitic blue emission is caused by an acceptorlike compensating native defect. The model is fully consistent with the experimental results.
Atomic force microscope cantilever based microcoordinate measuring probe for true three-dimensional measurements of microstructures
Authors: Dai Gaoliang , Wolff Helmut , Danzebrink Hans-Ulrich
Published: September 2007

An atomic force microscope (AFM) cantilever based coordinate measuring probe applicable for true three dimensional measurements of microstructures is presented. The probe has a shaft glued to a commercial AFM cantilever and a probing sphere located at the free end of the shaft. A conventional optical lever method is applied to detect the bending, torsion, and the dynamic behavior of the AFM cantilever which will change dramatically when the probe sphere is in proximity to the object to be measured. The probe has advantages such as small probing force (<1μN), high probing sensitivity allowing nanometer measurement resolution, easy exchange of probing elements, and low cost. Measurements of a microgear and a fuel injection nozzle using the developed probe are demonstrated, indicating its promising application potential.
Intriguing relations of interaction energy components in stacked nucleic acids
Authors: Langner Karol M. , Sokalski W. Andrzej , Leszczynski J.
Published: September 2007

Major components of the interaction energy that define several approximate levels starting from second order Möller-Plesset theory were studied for 58 stacked nucleic acid dimers. They included typical B-DNA and A-DNA structures, and selected published geometries. A survey of the various terms yields an unexpected correlation between the Pauli exchange and dispersion or correlation terms, which holds for each class of similar planar geometries and for various basis sets. The geometries that exhibit these correlations span a specific range of molecular overlaps when compared to a model benzene-pyridine stacked dimer. Also, the relationship between electrostatic interactions and MP2 stabilization energies reported earlier is confirmed and a prediction interval of practical relevance is estimated.
Ultrafast limited-angle-type x-ray tomography
Authors: Bieberle M. , Fischer F. , Schleicher E. , Hampel U. , Koch D. , Aktay K. S. d. C. , Menz H.-J. , Mayer H.-G.
Published: September 2007

The authors present an ultrafast electron beam x-ray computed tomography technique usable for imaging of fast processes, such as multiphase flows or moving parts in technical or biological objects. The setup consists of an electron beam unit with fast deflection capability and an ultrafast multielement x-ray detector and achieves 10000frames∕s image rate. Since full sampling of the Radon space requires an angular overlap of source path and detector which strongly decreases axial resolution, the authors devised a limited-angle-type tomography. As a demonstration they visualized the movement of particles and gas bubbles rising in a stagnant liquid.
Room temperature spin filtering in epitaxial cobalt-ferrite tunnel barriers
Authors: Ramos A. V. , Guittet M.-J. , Moussy J.-B. , Mattana R. , Deranlot C. , Petroff F. , Gatel C.
Published: September 2007

The authors report direct experimental evidence of room temperature spin filtering in magnetic tunnel junctions (MTJs) containing CoFe2O4 tunnel barriers via tunneling magnetoresistance (TMR) measurements. Pt(111)∕CoFe2O4(111)∕γ-Al2O3(111)∕Co(0001) fully epitaxial MTJs were grown in order to obtain a high quality system, capable of functioning at room temperature. Spin-polarized transport measurements reveal significant TMR values of −18% at 2K and −3% at 290K. In addition, the TMR ratio follows a unique bias voltage dependence that has been theoretically predicted to be the signature of spin filtering in MTJs containing magnetic barriers. CoFe2O4 tunnel barriers therefore provide a model system to investigate spin filtering in a wide range of temperatures.
Fabrication and electrical characteristics of Si nanocrystal/c-Si heterojunctions
Authors: Song Dengyuan , Cho Eun-Chel , Conibeer Gavin , Huang Yidan , Green Martin A.
Published: September 2007

Heterojunctions (HJs) were fabricated from p-type Si nanocrystals (Si NCs) embedded in a SiC matrix on an n-type crystalline Si substrate. Transmission electron microscopy revealed that Si NCs are clearly established, with sizes in the range of 3–5nm. The HJ diodes showed a good rectification ratio of 1.0×104 at ±1.0V at 298K. The ideality factor, junction built-in potential, and open-circuit voltage are ∼1.24, 0.72V, and 0.48V, respectively. Measurement of temperature-dependent I-V curves in forward conduction suggests that, in the medium voltage range, junction interface recombination can be described as the dominant current transport mechanism.
Elastic deformation of silicon (001) wafers due to thin film stresses
Authors: Janssen G. C. A. M. , Pujada B. R.
Published: September 2007

Single crystal silicon is an elastically anisotropic material. The in-plane elastic modulus of Si (001) wafers varies 30% with the orientation with respect to the crystal axes. The line tension exerted on a Si (001) wafer by a uniform isotropic film under compressive stress leads to a nonuniform deformation, reflecting the elastic anisotropy of the silicon single crystal. For low line tensions, this deformation has a fourfold symmetry, with larger deformation in the less stiff directions. For larger line tensions, a saddle point deformation superimposed on a cylindrical symmetric deformation is observed.
Atomic force microscope based nanofabrication of master pattern molds for use in soft lithography
Authors: Johannes Matthew S. , Cole Daniel G. , Clark Robert L.
Published: September 2007

The authors have developed a technique that couples nanolithographic patterning using an atomic force microscope with the parallel patterning abilities of soft lithography. Master pattern generation is accomplished using local anodic oxidation as a mask pattern for anisotropic wet etching of Si(110). The resulting nanostructures are then used as master patterns for the molding of polymeric stamps to be used for microcontact printing of alkanethiols. Analysis of the resulting patterns demonstrates the validity of this method as a simple, effective, and low cost alternative to conduct and prototype nanoscale patterning in a parallel fashion.
Investigation on the stabilization of the median resistance state for phase change memory cell with doublelayer chalcogenide films
Authors: Rao Feng , Song Zhitang , Wu Liangcai , Liu Bo , Feng Songlin , Chen Bomy
Published: September 2007

The triple-level data storage capability of the phase change memory with doublelayer chalcogenide films (Sb2Te3 and Ge2Sb2Te5) was investigated. Differences in physical properties of these two chalcogenide materials contribute to the appearance of a median resistance state. However, the stable median resistance state can only be achieved by selecting appropriate programming parameters. Voltage pulses with moderate magnitude and pulse width for both set and reset operations are favorable for forming a stable median resistance stage. Interdiffusion between Sb2Te3 and Ge2Sb2Te5 layers degrades the stabilization of the median resistance state.
Superconducting thin films of heavy-fermion compound CeCoIn5 prepared by molecular beam epitaxy
Authors: Izaki M. , Shishido H. , Kato T. , Shibauchi T. , Matsuda Y. , Terashima T.
Published: September 2007

Highly c-axis oriented thin films of Ce-based heavy-fermion superconductor CeCoIn5 were grown on Cr∕MgO substrates by molecular beam epitaxy. The films consist of micrograins which are strongly oriented in c axis but are random in the film plane. Kondo temperature, superconducting transition temperature, and upper critical fields of the films are very close to those of single crystals.
All semiconductor swept laser source utilizing quantum dots
Authors: Stevens B. J. , Childs D. T. D. , Groom K. M. , Hopkinson M. , Hogg R. A.
Published: September 2007

An all semiconductor swept laser source demonstrating continuous sweeping over a >11nm range with a linewidth suitable for optical coherence tomography is proposed and demonstrated. The operation of this device relies upon state filling in a multiple contact laser utilizing quantum dots with strongly overlapping ground and excited states.
Spectral sensitivity of two-dimensional nanohole array surface plasmon polariton resonance sensor
Authors: Pang Lin , Hwang Grace M. , Slutsky Boris , Fainman Yeshaiahu
Published: September 2007

An analytical expression of spectral sensitivity derived from a surface plasmon polariton dispersion relation for a two-dimensional nanohole array surface plasmon polariton resonance sensor is presented. The sensitivity of the nanohole array sensor depends on the periodicity of the array and the order of the excited surface plasmon polariton modes. The analytical expression is further confirmed by rigorous electromagnetic simulation and validated by experimental results. Real-time monitoring of protein-protein specific bonding is performed to demonstrate the integrated microfluidic nanohole array surface plasmon resonance biosensor.