bluuebunny/crossref_metadata_2025 huggingface dataset:
A magnetic-field-induced irreversible transition in geometrically frustrated antiferromagnetic TbBaCo4O7 with kagome and triangular lattices
Authors: Bludov A. N. , Gnatchenko S. L. , Szymczak R. , Szymczak H. , Barilo S. N. , Bychkov G. L. , Shiryaev S. V.
Published: January 2010

The temperature dependences M(T) and field dependences M(H) of the magnetization have been measured in single-crystal TbBaCo4O7 for two magnetic-field directions: H∥c and H⊥c. In the case of H∥c, a magnetic field-induced irreversible transition is detected at low temperatures, accompanied by magnetization jumps on the M(H) dependences. It is shown that the transition field depends on the rate at which the external magnetic field is introduced.
A high resolution Mirnov array for the Mega Ampere Spherical Tokamak
Authors: Hole M. J. , Appel L. C. , Martin R.
Published: January 2010

Over the past two decades, the increase in neutral-beam heating and α particle production in magnetically confined fusion plasmas has led to an increase in energetic particle driven mode activity, much of which has an electromagnetic signature which can be detected by the use of external Mirnov coils. Typically, the frequency and spatial wave number band of such oscillations increase with increasing injection energy, offering new challenges for diagnostic design. In particular, as the frequency approaches the megahertz range, care must be taken to model the stray capacitance of the coil, which limits the resonant frequency of the probe; model transmission line effects in the system, which if unchecked can produce system resonances; and minimize coil conductive shielding, so as to minimize skin currents which limit the frequency response of the coil. As well as optimizing the frequency response, the coils should also be positioned to confidently identify oscillations over a wide wave number band. This work, which draws on new techniques in stray capacitance modeling and coil positioning, is a case study of the outboard Mirnov array for high-frequency acquisition in the Mega Ampere Spherical Tokamak, and is intended as a roadmap for the design of high frequency, weak field strength magnetic diagnostics.
Relation between size dispersion and line shape in quantum dot ensembles
Authors: Nikolaev V. V. , Averkiev N. S.
Published: January 2010

We propose a method to model the density of optical transitions, absorption, and differential-absorption spectra of quantum dot ensembles. The developed approach combines physical straightforwardness of the conventional Gaussian-peak modeling with a more preside account of the influence of size dispersion on the line shape of quantum dots.
The incongruous observation of magnetic phase separation in La0.85Sr0.15CoO3 spin glass system
Authors: Samal D. , Shivakumara C. , Kumar P. S. Anil
Published: January 2010

Phase separation (PS) in hole-doped cobaltites (La1−xSrxCoxO3) is drawing renewed interest recently. In particular, the magnetic behavior of La0.85Sr0.15CoO3 has been subjected to a controversial debate for the past several years; while some groups show evidence for magnetic PS, others show spin glass (SG) behavior. Here, an attempt is made to resolve the controversy related to “PS versus SG” behavior in this compound. We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and the magnetotransport properties of La0.85Sr0.15CoO3 samples. We contemplate that the magnetic PS in La0.85Sr0.15CoO3 is neither intrinsic nor inherent, but it is a consequence of the preparation conditions. It is realized that a low temperature annealed (LTA) sample shows PS whereas the high temperature annealed (HTA) sample shows SG behavior. The Brillouin-like behavior of field cooled dc magnetization and apparently no frequency dependent peak shift in ac susceptibility for the LTA sample characterize it to be of ferromagneticlike whereas a kink in field cooled dc magnetization and a considerable amount (∼3 K) of frequency dependent peak shift in the ac susceptibility for the HTA sample characterize it to be of SG state. The magnetotransport properties show that the HTA sample is more semiconducting as compared to the LTA sample. This is interpreted in terms of the presence of isolated as well as coalescing metallic ferromagnetic clusters in the case of LTA sample. The magnetoresistance (MR) at 10 K for the HTA sample exhibits a huge value (∼65%) as compared to the LTA sample, and it monotonically decreases with the rise in temperature. Such a high value of MR in the case of HTA sample is strongly believed to be due to the spin dependent part of random potential distribution. Further, the slow decay of remnant magnetization with progress of time and the existence of hysteresis at higher temperatures (up to 200 K) in the case of LTA sample as compared to the HTA sample clearly unveil different magnetic states associated with them.
Three dimensional microstructuring of biopolymers by femtosecond laser irradiation
Authors: Oujja M. , Pérez S. , Fadeeva E. , Koch J. , Chichkov B. N. , Castillejo M.
Published: January 2010

A sequence of single femtosecond pulses is used to create a pattern of laser affected spots at increasing depths below the surface of transparent biopolymer samples. Materials with different water contents and mechanical strengths, gelatine, chitosan, synthetic polyvinyl pyrrolidone, and biopolymer-polymer blends, are irradiated near the edge of the sample with an amplified Titanium:Sapphire laser (800 nm) delivering 30 fs pulses through a 0.45 numerical aperture objective with energies of 100–3000 nJ. The micrometric modified region is observed by optical microscopy perpendicularly to irradiation. Self-focusing and optical aberration are major factors controlling morphology and size of the created spots.
Microscopic derivation of discrete hydrodynamics
Authors: Español Pep , Anero Jesús G. , Zúñiga Ignacio
Published: January 2010

By using the standard theory of coarse graining based on Zwanzig’s projection operator, we derive the dynamic equations for discrete hydrodynamic variables. These hydrodynamic variables are defined in terms of the Delaunay triangulation. The resulting microscopically derived equations can be understood, a posteriori, as a discretization on an arbitrary irregular grid of the Navier–Stokes equations. The microscopic derivation provides a set of discrete equations that exactly conserves mass, momentum, and energy and the dissipative part of the dynamics produces strict entropy increase. In addition, the microscopic derivation provides a practical implementation of thermal fluctuations in a way that the fluctuation-dissipation theorem is satisfied exactly. This paper points toward a close connection between coarse-graining procedures from microscopic dynamics and discretization schemes for partial differential equations.
A generalized statistical complexity measure: Applications to quantum systems
Authors: López-Ruiz R. , Nagy Á. , Romera E. , Sañudo J.
Published: January 2010

A two-parameter family of complexity measures C̃(α,β) based on the Rényi entropies is introduced and characterized by a detailed study of its mathematical properties. This family is the generalization of a continuous version of the Lopez-Ruiz–Mancini–Calbet complexity, which is recovered for α=1 and β=2. These complexity measures are obtained by multiplying two quantities bringing global information on the probability distribution defining the system. When one of the parameters, α or β, goes to infinity, one of the global factors becomes a local factor. For this special case, the complexity is calculated on different quantum systems: H-atom, harmonic oscillator, and square well.
Uncertainty of a hybrid surface temperature sensor for silicon wafers and comparison with an embedded thermocouple
Authors: Iuchi Tohru , Gogami Atsushi
Published: January 2010

We have developed a user-friendly hybrid surface temperature sensor. The uncertainties of temperature readings associated with this sensor and a thermocouple embedded in a silicon wafer are compared. The expanded uncertainties (k=2) of the hybrid temperature sensor and the embedded thermocouple are 2.11 and 2.37 K, respectively, in the temperature range between 600 and 1000 K. In the present paper, the uncertainty evaluation and the sources of uncertainty are described.
Mutual friction in helium II: a microscopic approach
Authors: Cataldo H. M.
Published: January 2010

We develop a microscopic model of mutual friction represented by the dissipative dynamics of a normal fluid flow which interacts with the helical normal modes of vortices comprising a lattice in thermal equilibrium. Such vortices are assumed to interact with the quasiparticles forming the normal fluid through a pseudomomentum-conserving scattering Hamiltonian. We study the approach to equilibrium of the normal fluid flow for temperatures below 1K, deriving an equation of motion for the quasiparticle pseudomomentum which leads to the expected form predicted by the Hall—Vinen—Bekharevich—Khalatnikov equations. We obtain an expression for the mutual friction coefficient in terms of microscopic parameters, which turns out to be practically independent of the vortex mass for values arising from diverse theories. By comparing our expression of B with previous theoretical estimates, we deduce interesting qualitative features about the excitation of Kelvin modes by the quasiparticle scattering.
An alternative view at the zero-bias anomaly of metallic point contacts
Authors: Gloos Kurt
Published: January 2010

We have studied the zero-bias anomaly of point contacts as function of contact size for a wide range of materials from simple normal metals, superconductors, and magnets to highly correlated heavy-fermion compounds. The size δR of the zero-bias anomaly varied in a universal manner proportional to the square of the contact resistance R between 1Ω and 10kΩ, that is for contact radii from about 15nm down to 0.2nm. Magnetic impurities, two-level systems, or a tunneling barrier are unlikely to be the main source of this anomaly. We suggest instead Kondo-type scattering of the conduction electrons due to a spontaneous electron spin polarization at the point contact.
The effect of sorbed hydrogen on low-temperature radial thermal expansion of single-walled carbon nanotube bundles
Authors: Dolbin A. V. , Esel’son V. B. , Gavrilko V. G. , Manzhelii V. G. , Popov S. N. , Vinnikov N. A. , Sundqvist B.
Published: January 2010

The effect of a normal H2 impurity upon the radial thermal expansion αr of single-walled carbon nanotube (SWNT) bundles is investigated in the interval T=2.2–27K using the dilatometric method. It is found that H2 saturation of SWNT bundles causes a shift of the temperature interval of the negative thermal expansion towards lower temperatures (as compared to pure carbon nanotubes) and a sharp increase in the magnitude of αr in the whole range of temperatures investigated. The low temperature desorption of H2 from a powder consisting of bundles of SWNTs, open and closed at the ends, is investigated.
Toward quantum interference of photons from independent quantum dots
Authors: Benyoucef M. , Wang L. , Rastelli A. , Schmidt O. G.
Published: January 2010

The authors present steps toward the experimental realization of indistinguishable single photon sources based on independent unstrained GaAs quantum dots (QDs), which are embedded in planar cavities to improve the light collection efficiency. The emission lines of two QDs are brought into resonance and overlapped at a beam splitter. The coherence properties of the emitted photons are investigated by measuring the first-order field correlation function. Despite the fact that the short dephasing time of the selected QDs prevents us to observe quantum interference between the two photons, the approach could be applied to other QDs.
The influence of As/III pressure ratio on nitrogen nearest-neighbor environments in as-grown GaInNAs quantum wells
Authors: Kudrawiec R. , Korpijärvi V.-M. , Poloczek P. , Misiewicz J. , Laukkanen P. , Pakarinen J. , Dumitrescu M. , Guina M. , Pessa M.
Published: January 2010

The energy fine structure, corresponding to different nitrogen nearest-neighbor environments, was observed in contactless electroreflectance (CER) spectra of as-grown GaInNAs quantum wells (QWs) obtained at various As/III pressure ratios. In the spectral range of the fundamental transition, two CER resonances were detected for samples grown at low As pressures whereas only one CER resonance was observed for samples obtained at higher As pressures. This resonance corresponds to the most favorable nitrogen nearest-neighbor environment in terms of the total crystal energy. It means that the nitrogen nearest-neighbor environment in GaInNAs QWs can be controlled in molecular beam epitaxy process by As/III pressure ratio.
The effect of temperature on the secondary electron emission yield from single crystal and polycrystalline diamond surfaces
Authors: Stacey A. , Prawer S. , Rubanov S. , Ahkvlediani R. , Michaelson Sh. , Hoffman A.
Published: January 2010

The effect of temperature in the 293–473 K range, on the secondary electron emission (SEE) yield of single crystal and polycrystalline diamond film surfaces is reported. For the polycrystalline films the SEE yield was found to decay as function of electron irradiation dose while for the single crystal an increase occurs first, followed by a decrease. For both surfaces, the SEE yield increases significantly upon heating and obtained a nearly constant value with electron dose at 473 K. These effects are explained as due to the temperature dependence of the electron beam induced hydrogen desorption and surface band bending.
Electric-field-controlled directional motion of ferroelectric domain walls in multiferroic BiFeO3 films
Authors: Kim T. H. , Baek S. H. , Yang S. M. , Jang S. Y. , Ortiz D. , Song T. K. , Chung J.-S. , Eom C. B. , Noh T. W. , Yoon J.-G.
Published: January 2010

We describe the directional ferroelectric domain wall motion in a multiferroic BiFeO3 thin film, which was grown epitaxially on a vicinal (001) SrTiO3 substrate. A structural analysis of the film shows that a strain gradient is developed in our film, which creates a symmetry breaking in a ferroelectric double-well potential. The asymmetric double-well potential can cause ferroelectric domain walls to move sideways with preferred directionality under a vertical electric field. Our results suggest the possibility of controlling the direction of domain growth with an electric field by imposing constraints on ferroelectric films, such as a strain gradient.
Second harmonic generation from graphene and graphitic films
Authors: Dean Jesse J. , van Driel Henry M.
Published: January 2010

Optical second harmonic generation (SHG) of 800 nm, 150 fs fundamental pulses is observed from exfoliated graphene and multilayer graphitic films mounted on an oxidized silicon (001) substrate. The SHG anisotropy is observed as a sample is rotated about the surface normal. For p-polarized fundamental and SHG light, the isotropic SHG from a graphene layer only slightly interferes with the fourfold symmetric response of the underlying substrate, while other samples show a threefold symmetry characteristic of significant SHG in the multilayer graphitic films. The dominance of the threefold anisotropy is maintained from bilayer graphene to bulk graphite.
Hysteresis of the bcc–hcp transition in a solid mixture of He3 in He4
Authors: Birchenko A. P. , Vekhov E. O. , Mikhin N. P. , Chishko K. A.
Published: January 2010

Hysteresis of the bcc–hcp transition is observed by precision barometry in samples of a solid mixture of 1% He3 in He4 grown by the capillary blocking method. It is shown that the lines of equilibrium bcc–hcp and hcp–bcc transitions on the P–T diagram on cooling and heating do not coincide (the cooling line corresponds to higher pressure). In the process of thermocycling in the two-phase region the system executes a closed thermodynamic cycle, two branches of which correspond to the “normal” bcc–hcp transformation, with a slope of dP∕dT=6–12bar∕K, and the other two, to an “anomalous” transformation with a slope of dP∕dT=2.5–7bar∕K (the slope increases with decreasing molar volume). This effect is not observed in crystals of pure He4, and it can therefore be attributed to properties of the He3 impurity subsystem. A hierarchy of relaxation times for the pressure in the system is established which indicates that the mechanisms of transformation on different branches of the cycle are different. An interpretation of the observed effects is proposed in the framework of a model of a heterophase polydomain superstructure consisting of second-phase domains localized at He3 impurities in the He4 host matrix.
Observation of exciton luminescence from icosahedral xenon-argon clusters
Authors: Vakula V. L. , Danylchenko O. G. , Doronin Yu. S. , Kovalenko S. I. , Libin M. Yu. , Samovarov V. N.
Published: January 2010

Exciton-impurity luminescence is observed for the first time in binary mixtures of solidified rare gases, in the xenon-argon system with argon as the impurity. An intense exciton-impurity emission band is observed in binary clusters having the structure of a multilayered icosahedron. The optical transition occurs from an energy level lying very close to the lowest bulk exciton level in bulk xenon samples. The results demonstrate the potential for probing exciton levels in disordered condensed rare gas media.
Polaronic effects in electron shuttling
Authors: Skorobagatko Gleb A. , Krive Ilya V. , Shekhter Robert I.
Published: January 2010

A shuttle-like mechanism of electron transport through a single level vibrating quantum dot is considered in the regime of strong electromechanical coupling. It is shown that the increment of shuttle instability is a nonmonotonic function of the driving voltage. The interplay of two oppositively acting effects—vibron-assisted electron tunneling and polaronic blockade—results in oscillations of the increment on the energy scale of vibron energy.
Linearly polarized light emission from InGaN light emitting diode with subwavelength metallic nanograting
Authors: Zhang Liang , Teng Jing Hua , Chua Soo Jin , Fitzgerald Eugene A.
Published: January 2010

Surface emitting linearly polarized InGaN/GaN light emitting diode (LED) is demonstrated using a subwavelength metallic nanograting. The aluminum based grating with a period of 150 nm is fabricated on top of the p-contact layer in a conventional InGaN LED structure grown on (0001) oriented sapphire substrate. Polarization ratio can reach 7:1, the highest ever reported polarization ratio directly from a light emitting diode. The polarization characteristics are studied in details both experimentally and theoretically, suggesting an effective way to make polarized light emission devices.
Zero point energy leakage in condensed phase dynamics: An assessment of quantum simulation methods for liquid water
Authors: Habershon Scott , Manolopoulos David E.
Published: January 2010

The approximate quantum mechanical ring polymer molecular dynamics (RPMD) and linearized semiclassical initial value representation (LSC-IVR) methods are compared and contrasted in a study of the dynamics of the flexible q-TIP4P/F water model at room temperature. For this water model, a RPMD simulation gives a diffusion coefficient that is only a few percent larger than the classical diffusion coefficient, whereas a LSC-IVR simulation gives a diffusion coefficient that is three times larger. We attribute this discrepancy to the unphysical leakage of initially quantized zero point energy (ZPE) from the intramolecular to the intermolecular modes of the liquid as the LSC-IVR simulation progresses. In spite of this problem, which is avoided by construction in RPMD, the LSC-IVR may still provide a useful approximation to certain short-time dynamical properties which are not so strongly affected by the ZPE leakage. We illustrate this with an application to the liquid water dipole absorption spectrum, for which the RPMD approximation breaks down at frequencies in the O–H stretching region owing to contamination from the internal modes of the ring polymer. The LSC-IVR does not suffer from this difficulty and it appears to provide quite a promising way to calculate condensed phase vibrational spectra.
Monte Carlo determination of mixed electrolytes next to a planar dielectric interface with different surface charge distributions
Authors: Wang Zhi-yong , Ma Yu-qiang
Published: January 2010

Employing canonical ensemble Monte Carlo simulations, we report a calculation of the distribution of small ions next to a planar negatively charged surface in the presence of mixed electrolytes of monovalent and trivalent salt ions within the framework of the primitive model under more realistic hydrated ion size conditions. The effects of surface charge discreteness and dielectric breakdown on charge inversion are discussed based on increasing concentration of both monovalent and trivalent salt. Moreover, a comparison of the simulation results for different discretization models is made along with the case of uniformly distributed charge in terms of the ionic density profiles as well as the integrated charge distribution function. For finite size charged groups located inside the lower dielectric region, a complete equivalence with the case of uniform distribution is observed if the quantities of interest are exclusively analyzed as a function of the distance to the charged interface. With protruding head groups into the aqueous solution, the excluded volume dominates over the correlation effect, therefore the ions are less accumulated in the vicinity of the charged surface, inducing that the onset position of charge inversion experiences an evident shift toward the aqueous environment. Overall, the effect of repulsive image forces on the diffuse double layer structure can be significant at low surface charge density irrespectively of surface charge distributions.
Blue-enhanced thin-film photodiode for dual-screen x-ray imaging
Authors: Vygranenko Y. , Sazonov A. , Heiler G. , Tredwell T. , Vieira M. , Nathan A.
Published: January 2010

This article reports on a-Si:H-based low-leakage blue-enhanced photodiodes for dual-screen x-ray imaging detectors. Doped nanocrystalline silicon was incorporated in both the n- and p-type regions to reduce absorption losses for light incoming from the top and bottom screens. The photodiode exhibits a dark current density of 900 pA/cm2 and an external quantum efficiency up to 90% at a reverse bias of 5 V. In the case of illumination through the tailored p-layer, the quantum efficiency of 60% at a 400 nm wavelength is almost double that for the conventional a-Si:H n-i-p photodiode.
Turbulence structure of open channel flows over permeable and impermeable beds: A comparative study
Authors: Manes Costantino , Pokrajac Dubravka , McEwan Ian , Nikora Vladimir
Published: January 2010

The behavior of turbulent open channel flows over permeable surfaces is not well understood. In particular, it is not clear how the surface and the subsurface flow within the permeable bed interact and influence each other. In order to clarify this issue we carried out two sets of experiments, one involving velocity measurements in open channel flows over an impermeable bed composed of a single layer of spheres, and another one where velocities were measured over and within a permeable bed made of five such layers. Comparison of surface flow velocity statistics between the two sets of experiments confirmed that bed permeability can significantly affect flow resistance. It was also confirmed that even in the hydraulically rough regime, the friction factors for the permeable bed increase with increasing Reynolds number. Such an increase in flow resistance implies a different distribution of normal form-induced stress between the permeable and impermeable bed cases. Subsurface flow measurements performed within the permeable bed revealed that there is an intense transport of turbulent kinetic energy (TKE) occurring from the surface to the subsurface flow. We provide evidence that the transport of TKE toward the lower bed levels is driven mainly by pressure fluctuations, whereas TKE transport due to turbulent velocity fluctuations is limited to a thinner layer placed in the upper part of the bed. It was also confirmed that the turbulence imposed by the surface flow gradually dissipates while penetrating within the porous medium. Dissipation occurs faster for the small scales than for the large ones, which instead are persistent, although weak, even at the lowest bed levels.
Adiabatic and diabatic invariants in ion-molecule reactions
Authors: Lorquet J. C.
Published: January 2010

A point charge interacting with a dipole (either induced or permanent) constitutes a completely integrable dynamical subsystem characterized by three first integrals of the motion (E, pφ, and either ℓ2 or a Hamilton–Jacobi separation constant β). An ion-molecule reaction (capture or fragmentation) can be seen as an interaction between such a subsystem and a bath of oscillators. This interaction is a perturbation that destroys some of the first integrals. However, the perturbation depends on the separation between the fragments and the destruction is gradual. The mathematical simplicity of the long-range electrostatic interaction potential leads to useful simplifications. A first-order perturbation treatment based on the structured and regular nature of the multipole expansion is presented. The separating integrals valid in the asymptotic limit are found to subsist at intermediate distances, although in a weaker form. As the reaction coordinate decreases, i.e., as the fragments approach, the asymptotic range is followed by an outer region where (i) the azimuthal momentum pφ remains a constant of the motion; (ii) the square angular momentum ℓ2 or the separation constant β transform into a diabatic invariant in regions of phase space characterized by a high value of the translational momentum pr; (iii) for low values of pr, it is advantageous to use the action integral ∮pθdθ, which is an adiabatic invariant. The conditions under which an effective potential obtained by adding centrifugal repulsion to an electrostatic attractive term can be validly constructed are specified. In short, the dynamics of ion-molecule interactions is still regular in parts of phase space corresponding to a range of the reaction coordinate where the interaction potential deviates from its asymptotic shape.
Enhanced spontaneous emission rate in annular plasmonic nanocavities
Authors: Kroekenstoel E. J. A. , Verhagen E. , Walters R. J. , Kuipers L. , Polman A.
Published: January 2010

The spontaneous emission rate of erbium ions is enhanced by coupling to localized plasmonic resonances in subwavelength annular apertures in a Au film. The Er3+ ions, embedded in SiO2, are selectively located inside the apertures. The annular apertures act as nanocavities, enhancing the local density of optical states at the Er emission wavelength of 1.54 μm when the cavities are tuned to that wavelength. We show that this leads to an eightfold increase of the photoluminescence intensity, in conjunction with a 2.4-fold enhancement of the spontaneous emission rate.
Trap-limited transport in rubrene transistors
Authors: Konezny S. J. , Bussac M. N. , Zuppiroli L.
Published: January 2010

The charge carrier mobility in the transport channel of an organic transistor is estimated within the framework of a trap-and-release model. The model accounts for the observed dependence of the mobility on the dielectric constant ε of the gate insulator. This dependence is attributed to both the effective mass of the carrier and the energetic depth of transport traps due to interface defects being functions of ε. These results are used to describe the critical role of the interface between the organic semiconductor and the dielectric material in governing charge transport in organic transistors.
Improvement of the poly-3-hexylthiophene transistor performance using small molecule contact functionalization
Authors: Winter Rebecca , Hammer Maria S. , Deibel Carsten , Pflaum Jens
Published: January 2010

We demonstrate an approach to improve poly-3-hexylthiophene field effect transistors by modifying the gold contacts with monolayer thick pentacenequinone (PQ) or naphthalene (NL). The effective contact resistance is reduced by a factor of two and sixteen for interlayers of PQ and NL, respectively. The observation is attributed to different injection barriers at the metal-organic interface caused by the functionalization and to an additional tunneling barrier enhancing the on/off ratios. This barrier yields to activation energies of 37 meV (NL) and 104 meV (PQ) below 190 K, which are smaller than without functionalization, 117 meV.
High-field Hall effect and magnetoresistance in Fe3O4 epitaxial thin films up to 30 Tesla
Authors: Fernández-Pacheco A. , Orna J. , De Teresa J. M. , Algarabel P. A. , Morellon L. , Pardo J. A. , Ibarra M. R. , Kampert E. , Zeitler U.
Published: January 2010

We have measured the Hall effect and the magnetoresistance of epitaxial Fe3O4 thin films grown on MgO (001) in magnetic fields up to 30 T. Using such high fields, it is possible to magnetically saturate films thicker than 40 nm, providing access to intrinsic conduction properties. We find an effective electron density corresponding to 1 electron per f.u. A smaller value is obtained for thinner films, caused by the increasing density of antiphase boundaries defects. The magnetoresistance is not saturating at 30 T, showing linear dependence at high fields, and peaks at the Verwey transition.
Solution-based patterned growth of rubrene nanocrystals for organic field effect transistors
Authors: Luo Liang , Liu Ge , Huang Liwei , Cao Xinqiang , Liu Ming , Fu Hongbing , Yao Jiannian
Published: January 2010

We report a solution-processed chemical reaction approach to patterned growth of rubrene polycrystalline films over Ag electrodes for organic field effect transistors. The reduction in cationic precursors of Ru•+ by Ag atoms generated neutral molecules with a concentration above the nucleation threshold near to the electrode surfaces. This initiates the site-specific nucleation followed by growth of rubrene nanocrystals on the electrodes. These rubrene nanocrystals were found in the triclinic crystal phase and formed a polycrystalline film across the source and drain electrodes. The mobilities of rubrene transistors can achieve 0.02 cm2 V−1 s−1 with an on/off ratio of 103.
Holographic assembly of semiconductor CdSe quantum dots in polymer for volume Bragg grating structures with diffraction efficiency near 100%
Authors: Liu Xiangming , Tomita Yasuo , Oshima Juro , Chikama Katsumi , Matsubara Koutatsu , Nakashima Takuya , Kawai Tsuyoshi
Published: January 2010

We report on the fabrication of centimeter-size transmission Bragg gratings in semiconductor CdSe quantum dots dispersed 50 μm thick photopolymer films. This was done by holographic assembly of CdSe quantum dots in a photopolymerizable monomer blend. Periodic patterning of CdSe quantum dots in polymer was confirmed by a fluorescence microscope and confocal Raman imaging. The diffraction efficiency from the grating of 1 μm spacing was near 100% in the green with 0.34 vol % CdSe quantum dots, giving the refractive index modulation as large as 5.1×10−3.
X-ray spectroscopy studies on the surface structural characteristics and electronic properties of platinum nanoparticles
Authors: Bayindir Z. , Duchesne P. N. , Cook S. C. , MacDonald M. A. , Zhang P.
Published: January 2010

The surface structural characteristics and electronic behavior of three platinum nanoparticle (NP) samples prepared with tertiary amine (Pt-TA), primary amine (Pt-PA), and thiol (Pt-SR) molecules were studied using Pt 4f, 5d, and S 2p x-ray photoelectron spectroscopy (XPS), Pt L3-edge x-ray absorption spectroscopy (XAS), and theoretical projected local density of states (l-DOS) calculations. Transmission electron microscopy and XPS composition analysis indicated that the three NPs were all very small (1–2 nm), the NP size decreasing in the order of Pt-TA>Pt-PA∼Pt-SR. All the three samples showed a positive Pt 4f binding energy (BE) shift relative to that of the bulk, in the order of bulk<Pt-TA<Pt-PA<Pt-SR. The origin of the BE shift was elucidated by XAS and deconvolution of the Pt 4f XPS peak, indicating that the observed BE shifts were largely associated with the initial state effect (i.e., nanosize and surface structure). The surface and size effects on the electronic behavior of Pt were further studied by valence band XPS and the results were interpreted with calculated d-DOS of three Pt55 model clusters with varied surface structures. Finally, the implication of these results on tuning the electronic properties of Pt NPs with size, surface, and alloying effects was discussed.
Recombination dynamics of photoluminescence in thiol-protected gold nanoclusters
Authors: Shu G. W. , Lin C. C. , Chung H. P. , Shen J. L. , Lin C. A. J. , Lee C. H. , Chang W. H. , Chan W. H. , Wang H. H. , Yeh H. I. , Yuan C. T. , Tang J.
Published: January 2010

Recombination dynamics of photoluminescence (PL) in Au nanoclusters (NCs) with different capping molecules were studied with time-resolved PL. Based on the emission-energy of carrier lifetimes; we suggest that the fast and slow PL decay of Au NCs originates from recombination of the linear Au–S bond and the staple motif, respectively. The effect of carrier localization in Au NCs was found to depend on the capping molecules. The zero-dimensionality of carriers in Au NCs was demonstrated by the temperature dependence of the time-resolved PL.
Synthesis and characterization of proton conducting inorganic-organic hybrid nanocomposite films from mixed phosphotungstic acid/phosphomolybdic acid/tetramethoxysilane/3-glycidoxypropyltrimethoxysilane/phosphoric acid for H2/O2 fuel cells
Authors: Lakshminarayana G. , Nogami Masayuki
Published: January 2010

Novel proton conductive inorganic-organic hybrid films doped with a mixture of phosphotungstic acid and phosphomolybdic acid were prepared by sol-gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), tetramethoxysilane (TMOS), and phosphoric acid as precursors. These hybrid membranes were studied with respect to their structural, thermal, elastic modulus, proton conductivity, and hydrogen permeability properties. The Fourier transform infrared spectroscopy and P31 nuclear magnetic resonance measurements have shown good chemical stability and complexation of H3PO4 with both PW12O403− and PMo12O403− Keggin anions in the studied hybrid films. The thermogravimetric analysis and differential thermal analysis measurements confirmed that the membranes were thermally stable up to 350 °C. The thermal stability of the membranes was enhanced by the presence of SiO2 framework. The effect of mixed heteropoly acid (HPA) concentration on the microstructure of the membranes was studied by scanning electron microscopy, energy dispersive x-ray analysis, and transmission electron microscopy micrographs and no phase separation at the surfaces of the TMOS-GPTMS-H3PO4-HPA membranes was observed, and also homogeneous distribution of all elements was confirmed, indicating that these membranes are homogeneous at nanoscale. High proton conductivity of 9.8×10−2 S/cm with composition of 20TMOS-50GPTMS-30H3PO4-3PMA-6PWA (mol %) was obtained at 130 °C and 90% relative humidity (RH). The hydrogen permeability was found to decrease from 1.12×10−10 to 8.5×10−11 cm2/s in the temperature range of 30–130 °C. For 3PMA/6PWA (mol %) doped hybrid film, membrane electrode assemblies were prepared and a maximum power density of 68 mW/cm2 at 140 mA/cm2 as well as a current density of 262 mA/cm2 were obtained at 130 °C and 50% RH when utilized in a H2/O2 fuel cell.
A new figure of merit for qualifying the fluorine-doped tin oxide glass used in dye-sensitized solar cells
Authors: Huang Xiaoming , Zhang Yiduo , Sun Huicheng , Li Dongmei , Luo Yanhong , Meng Qingbo
Published: January 2010

We have successfully obtained a new figure of merit for qualifying fluorine-doped tin oxide (FTO) glass in dye-sensitized solar cells (DSCs) through two equivalent testing methods. These methods are demonstrated and applied to change the equivalent transmittance and sheet resistance of FTO glass even after the glass is assembled in DSCs. By recording the I-V characteristic of a DSC with changed equivalent transmittance and sheet resistance of FTO glass, the dependent relations between the DSC performance (short circuit current density, open circuit voltage, and fill factor) and FTO properties (transmittance and sheet resistance) are found. With these relations, the new figure of merit MTC for FTO glass is successfully defined to be in linearly increasing dependence on the efficiency of DSCs. A series of DSCs with different FTO glasses is prepared to test the effectiveness of MTC and Haacke’s figure of merit ΦTC, which has been widely used for more than 30 years. The result shows that MTC is proportional to the efficiency of DSCs, while ΦTC is not. MTC could be very useful as a guideline to greatly simplify the process of optimizing the FTO glass to improve the efficiency of DSCs.
Role of germanium in the reduced temperature dependence of Ti-based nanocrystals formation for nonvolatile memory applications
Authors: Feng Li-Wei , Chang Chun-Yen , Chang Ting-Chang , Tu Chun-Hao , Wang Pai-Syuan , Chang Yao-Feng , Chen Min-Chen , Huang Hui-Chun
Published: January 2010

We investigated the physical and electrical characteristics of Ti-based nanocrystals (NCs) with composition of germanium fabricated by cosputtering titanium silicide and germanium targets for low temperature applications of nonvolatile memory. The addition of Ge significantly reduces the thermal budget necessary for Ti-based NCs formation to 500 °C in 2 min due to the rise of its morphological instability and agglomeration properties. Compositions characteristics were analyzed by x-ray photon-emission spectroscopy and formations of NCs were observed by transmission electron microscopy. Additionally, capacitance-voltage characteristics, data retention, and endurance properties are characterized to demonstrate its advantages for nonvolatile memory device applications.
Spin-wave propagation in a microstructured magnonic crystal
Authors: Chumak A. V. , Pirro P. , Serga A. A. , Kostylev M. P. , Stamps R. L. , Schultheiss H. , Vogt K. , Hermsdoerfer S. J. , Laegel B. , Beck P. A. , Hillebrands B.
Published: January 2010

Transmission of microwave spin waves through a microstructured magnonic crystal in the form of a Permalloy waveguide of a periodically varying width was studied experimentally and theoretically. The spin wave characteristics were measured by spatially resolved Brillouin light scattering microscopy. A rejection frequency band was clearly observed. The band gap frequency was controlled by the applied magnetic field. The measured spin-wave intensity as a function of frequency and propagation distance is in good agreement with a model calculation.
Strong enhancement of Raman-induced nonreciprocity in silicon waveguides by alignment with the crystallographic axes
Authors: Krause Michael , Renner Hagen , Brinkmeyer Ernst
Published: January 2010

Raman gain in silicon photonic wires depends on the relative propagation directions of the pump and Stokes waves due to strong longitudinal mode-field components. Here we show that this Raman-induced nonreciprocity can be varied in a wide range by changing the orientation of the waveguide with respect to the crystallographic axes. In a ⟨001⟩ orientation, the nonreciprocity reaches its maximum which is larger by more than two orders of magnitude as compared to the traditional ⟨011⟩ orientation. The waveguide is then practically Raman-inactive for copropagation, while the contradirectional Stokes wave may experience significant Raman gain.
Control of spin-wave phase and wavelength by electric current on the microscopic scale
Authors: Demidov Vladislav E. , Urazhdin Sergei , Demokritov Sergej O.
Published: January 2010

We demonstrate experimentally the ability to control by electric current the phase and the wavelength of spin waves propagating in submicrometer magnetic waveguides. The dependence of the spin-wave characteristics on the current and on the excitation frequency was determined by phase-resolved microfocus Brillouin light scattering spectroscopy. We show that moderately small currents are sufficient to induce nearly twofold changes of the wavelength and spin-wave phase shift of more than ±π radians over a propagation distance of several micrometers.
Size-dependent induced magnetism in carbon-doped ZnO nanostructures
Authors: Kwak Hyunwook , Chelikowsky James R.
Published: January 2010

We examine the role of quantum confinement on carbon doped ZnO nanocrystals and nanowires using first-principles real-space pseudopotential method. For these nanocrystals, we find carbon dopants become magnetic under confinement. Moreover, by examining dopant-dopant interactions in ZnO nanowires, we find the ferromagnetic exchange interactions to be strongly enhanced at the nanoscale. Our work offers strong theoretical evidence for nanoscale induced magnetism and direct exchange coupling interactions arising from “nonmagnetic dopants.”
Stochastic simulations of cargo transport by processive molecular motors
Authors: Korn Christian B. , Klumpp Stefan , Lipowsky Reinhard , Schwarz Ulrich S.
Published: January 2010

We use stochastic computer simulations to study the transport of a spherical cargo particle along a microtubule-like track on a planar substrate by several kinesin-like processive motors. Our newly developed adhesive motor dynamics algorithm combines the numerical integration of a Langevin equation for the motion of a sphere with kinetic rules for the molecular motors. The Langevin part includes diffusive motion, the action of the pulling motors, and hydrodynamic interactions between sphere and wall. The kinetic rules for the motors include binding to and unbinding from the filament as well as active motor steps. We find that the simulated mean transport length increases exponentially with the number of bound motors, in good agreement with earlier results. The number of motors in binding range to the motor track fluctuates in time with a Poissonian distribution, both for springs and cables being used as models for the linker mechanics. Cooperativity in the sense of equal load sharing only occurs for high values for viscosity and attachment time.
Comparison of interionic/intermolecular vibrational dynamics between ionic liquids and concentrated electrolyte solutions
Authors: Fujisawa Tomotsumi , Nishikawa Keiko , Shirota Hideaki
Published: January 2010

In this study, we have compared the interionic/intermolecular vibrational dynamics of ionic liquids (ILs) and concentrated electrolyte solutions measured by femtosecond optically heterodyne-detected Raman-induced Kerr effect spectroscopy. A typical anion in ILs, bis(trifluoromethanesulfonyl)amide ([NTf2]−), has been chosen as the anion for the sample ILs and concentrated electrolyte solutions. ILs used in this study are 1-butyl-3-methylimidazolium, 1-butylpyridinium, N-butyl-N,N,N-triethylammonium, and 1-butyl-1-methylpyrrolidinium with [NTf2]−. Li[NTf2] solutions (∼3.3M) of water, methanol, propylene carbonate, and poly(ethylene glycol) have been selected as control samples. Kerr transients of the ILs and electrolyte solutions show intra- and interionic/intermolecular vibrational dynamics followed by slow picosecond overdamped relaxation. Fourier transform Kerr spectra have shown a difference in the relative intensities of intraionic vibrational bands of [NTf2]− (280–350 cm−1) between the ILs and electrolyte solutions. The origin of the difference is attributed to the change in the conformational equilibrium between cisoid and transoid forms of [NTf2]−, which is caused by a favorable stabilization of dipolar cisoid form due to Li+ and dipolar solvent molecules in the electrolyte solutions. Low-frequency Kerr spectra (0–200 cm−1) exhibit unique features with the variation of cation and solvent species. The aromatic ILs have a prominent high-frequency librational motion at about 100 cm−1 in contrast to the case for the nonaromatic ones. The common structure of the spectra observed at about 20 cm−1 likely comes from an interionic motion of [NTf2]−. The nonaromatic ILs allow a fair comparison with the electrolyte solutions of propylene carbonate and poly(ethylene glycol) because of the structural similarities. The comparison based on the first moment of the interionic/intermolecular vibrational spectrum suggests the stronger interionic/intermolecular interaction in the concentrated electrolyte solutions than the ILs.
Investigation of compositional inhomogeneities in complex polycrystalline Cu(In,Ga)Se2 layers for solar cells
Authors: Fontané X. , Izquierdo-Roca V. , Calvo-Barrio L. , Pérez-Rodríguez A. , Morante J. R. , Guettler Dominik , Eicke A. , Tiwari A. N.
Published: January 2010

In-depth resolved composition inhomogeneities of polycrystalline Cu(In,Ga)Se2 (CIGS) complex layers for high efficiency solar cells were investigated with Raman scattering measurements. In-depth resolved analysis of the frequency of the main CIGS Raman mode in the spectra measured after sputtering of the layers at different depths lead to identification of different compositions across the layer thickness. These data are in good agreement at both qualitative and quantitative levels with the in-depth resolved composition analysis of the samples by sputtered neutral mass spectroscopy. In addition, Raman measurements also allow detection of additional phases as ordered vacancy compounds.
Nanoparticle aggregation in the presence of a block copolymer
Authors: Chen Houyang , Ruckenstein Eli
Published: January 2010

By employing dissipative particle dynamics simulations, we investigated the cooperative phase behavior of composites containing diblock copolymer molecules and nanoparticles. The nanoparticles and their aggregates are located either in the domains generated by each of the two kinds of segments of the block copolymer and/or at their interface. Various conditions, such as the interactions between nanoparticles, the interactions between them and the segments of the block copolymer, the interactions between the segments of the block copolymer, and the volume ratio of nanoparticles to block copolymer, have been changed to gather information about nanoparticle aggregation and block copolymer/nanoparticles morphology building. The dynamics of nanoparticles aggregation and the evolution of the morphology of the block copolymer have been followed. Our findings have identified two critical repulsive interactions regarding nanoparticle aggregation, one between nanoparticles and the other between the segments of the block copolymer and nanoparticles. Aggregates have been generated for repulsive interactions larger than the former critical value or smaller than the latter one.
Influence of Si precursor on Ge segregation during ultrathin Si reduced pressure chemical vapor deposition on Ge
Authors: Vincent B. , Vandervorst W. , Caymax M. , Loo R.
Published: January 2010

This letter reports on the Ge segregation mechanism occurring during ultrathin (few monolayers) Si cap growth on Ge substrates by reduced pressure chemical vapor deposition. Thanks to extremely low energy secondary ion mass spectroscopy, we have highlighted that Ge segregation in Si-covered Ge does not depend on the growth temperature (in the 350–500 °C range) or on the carrier gas (H2,N2) used during Si growth. Solely the Si precursor used, i.e., the Si incorporation mechanism, impacts the Ge segregation rate. A multi-Ge segregation model is proposed, considering probabilities of sites exchanges in between all Si–Ge stacked atoms within the first nanometer of the layers.