bluuebunny/crossref_metadata_2025 huggingface dataset:
Explosion energy of the 2004 eruption of the Asama Volcano, central Japan, inferred from ionospheric disturbances
Authors: Heki Kosuke
Published: July 2006

The Japanese dense array of Global Positioning System recorded ionospheric disturbances as changes in Total Electron Content ∼12 minutes after the September 1 2004 eruption of the Asama Volcano, Central Japan. The disturbance had a period of one and a quarter minutes and propagated as fast as ∼1.1 km/s, suggesting its origin as the acoustic wave generated by the explosion. By comparing the disturbance amplitudes with those by a surface mine blast with a known energy, the overall Asama explosion energy is inferred to be about 2 × 1014 J.
Heliospheric magnetic field polarity inversions driven by radial velocity field structures
Authors: Landi Simone , Hellinger Petr , Velli Marco
Published: July 2006

Magnetic field polarity inversions embedded in the predominantly unipolar fast solar wind have been observed by the Ulysses spacecraft at high latitudes. Such reversals have the nature of folded back field lines which we suggest are generated by the interaction of standard large amplitude, low frequency, Alfvénic turbulence with velocity shears in the fast solar wind. We present 2D magnetohydrodynamic simulations of a very low frequency and high amplitude Alfvén wave propagating away from the sun embedded in a velocity shear structure such as a microstream and show how reversals in the magnetic field lines are generated naturally on a time‐scale consistent with their observation at Ulysses. The generated magnetic field and plasma signals are similar to those observed. We discuss the role turbulence‐stream shear interactions might play in limiting differential velocities in the asymptotic high speed solar wind.
Pre‐ and post‐fire behavioral trends revealed in satellite NDVI time series
Authors: Telesca Luciano , Lasaponara Rosa
Published: July 2006

Pre‐ and post‐fire vegetational dynamical behaviours in a forest site of Northern Italy were investigated, using the 1998 to 2003 time series of Normalized Difference Vegetation Index (NDVI) from SPOT‐VEGETATION sensor. The detrended fluctuation analysis (DFA) was adopted to detect persistent behaviour, in order to characterize the stability/instability properties of vegetation dynamics. Our analysis points to the following results: i) both pre‐ and post‐fire vegetation dynamics are persistent; ii) post‐fire dynamical behaviour is characterized by a larger degree of persistence; iii) a link between persistence and resilience can be identified.
Low‐temperature cooling behavior of single‐domain magnetite: Forcing of the crystallographic axes and interactions
Authors: Muxworthy Adrian R. , Williams Wyn
Published: July 2006

Low‐temperature cycling (LTC) of magnetic remanences carried by rocks has become a standard technique in paleomagnetism, rock magnetism, and environmental magnetism as a means of identifying mineralogy and grain size. LTC usually involves measuring low‐temperature thermomagnetic curves on cooling through crystallographic transitions, such as magnetite's Verwey transition. Historically, it has been assumed that remanence carried by single‐domain (SD) magnetite grains is not affected by cooling through the cubic/monoclinic Verwey transition, whereas larger multidomain (MD) magnetite grains partially demagnetize. However, it has been recently pointed out that the shape anisotropy even for an infinitely long cylinder is approximately 3 times smaller than the monoclinic magnetocrystalline anisotropy along the hardest axis, i.e., SD remanences are not impervious to LTC. Using a micromagnetic algorithm we simulate LTC curves for assemblages of effectively elongated SD magnetite grains and consider the contribution of magnetostatic interactions. Initially, we assume that relationship between the cubic and monoclinic symmetry is chosen randomly; however, there are key experimental features, which this model does not explain. A new “controlled switching” model is developed; the orientation of the low‐temperature monoclinic axes are not chosen randomly, but instead are controlled by the direction of the magnetic moment on cooling through the Verwey transition. This new model correctly predicts experimentally observed low‐temperature trends that the “random” model does not. We therefore propose a new model for the mechanism controlling the behavior of SD grains at the Verwey transition and show that the low‐temperature behavior of SD and MD grains can yield ambiguously similar behavior.
The feasibility of reintroducing Wild Boar (Sus scrofa) to Scotland
Authors: LEAPER R. , MASSEI G. , GORMAN M. L. , ASPINALL R.
Published: July 2006

The feasibility of reintroducing Wild Boar Sus scrofa to Scotland was investigated using the predictions of a Geographical information System (GIS) and a Population Viability Analysis (PVA). In the first stage of this study we summarized information from published literature on the biology, habitat requirements, home range use and population densities of Wild Boar, and using a GIS we identified four suitable sites of significant size, all located in Highlands. In the second stage we used a Population Viability Analysis (RAMAS/age), parameterized with published information on survival and fecundity of European Wild Boar populations, to calculate the number of animals required in a ‘release population’ if it were to survive on average 50 years. At the two largest sites that we considered, a release population of 50 animals would be likely to produce a viable population. The study concluded that biologically, the reintroduction of the Wild Boar is possible. However, a complementary study examining both the environmental and the potential wider ecological and socio‐economic impacts of Wild Boar will be required before reintroduction is seriously considered.
Infrared spectroscopy of Li(NH3)n clusters for n=4–7
Authors: Salter Tom E. , Mikhailov Victor A. , Evans Corey J. , Ellis Andrew M.
Published: July 2006

Infrared spectra of Li(NH3)n clusters as a function of size are reported for the first time. Spectra have been recorded in the N–H stretching region for n=4→7 using a mass-selective photodissociation technique. For the n=4 cluster, three distinct IR absorption bands are seen over a relatively narrow region, whereas the larger clusters yield additional features at higher frequencies. Ab initio calculations have been carried out in support of these experiments for the specific cases of n=4 and 5 for various isomers of these clusters. The bands observed in the spectrum for Li(NH3)4 can all be attributed to N–H stretching vibrations from solvent molecules in the first solvation shell. The appearance of higher frequency N–H stretching bands for n⩾5 is assigned to the presence of ammonia molecules located in a second solvent shell. These data provide strong support for previous suggestions, based on gas phase photoionization measurements, that the first solvation shell for Li(NH3)n is complete at n=4. They are also consistent with neutron diffraction studies of concentrated lithium/liquid ammonia solutions, where Li(NH3)4 is found to be the basic structural motif.
Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(π,π*) excited state
Authors: Yang Juan , Wagner Martin , Okuyama Katsuhiko , Morris Kevin , Arp Zane , Choo Jaebum , Meinander Niklas , Kwon Ohyun , Laane Jaan
Published: July 2006

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(π,π*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (ν45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (ν43) and ring-twisting (ν44) vibrations. Despite some interaction with ν43 and ν44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(π,π*) state coumaran is quasiplanar with a barrier to planarity of 34cm−1 and with energy minima at puckering angles of ±14°. The corresponding ground state (S0) values are 154cm−1 and ±25°. As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the π→π* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21cm−1 for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(π,π*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.
A new ligand field approach to linear transition metal dihalides
Authors: Vogel M. , Weber W. , Wenzel W.
Published: July 2006

We have performed theoretical multiconfigurational calculations of the molecular energy levels based on two axial ligand field models and determined the model parameters to reproduce ab initio energies of TiCl2, TiF2, VCl2, and VF2. We develop two extensions to ligand field theory (LFT) for linear transition metal dihalides, which are incorrectly described by standard LFT. The standard LFT is augmented (1) by including the ligand induced hybridization of the d(σ) orbital with the 4s orbital or (2) by using a different radial part for the d(σ) orbital. Both models reproduce the energies of the first electronic states in very good agreement with numerical multireference configuration interaction results. Furthermore the model parameters are very close to experimental known Racah parameters describing the term energies of Ti2+ and V2+.
Anharmonic vibrational levels of the two cyclic isomers of SiC3
Authors: Linguerri Roberto , Rosmus Pavel , Carter Stuart
Published: July 2006

Using coupled-cluster approach full six-dimensional analytic potential energy surfaces for two cyclic SiC3 isomers [C–C transannular bond (I) and Si–C transannular bond (II)] have been generated and used to calculate anharmonic vibrational wave functions. Several strong low-lying anharmonic resonances have been found. In both isomers already some of the fundamental transitions cannot be described within the harmonic approximation. Adiabatic electron affinities and ionization energies have been calculated as well. The Franck-Condon factors for the photodetachment processes c-SiC3−(I)→c-SiC3(I) and c-SiC3−(II)→c-SiC3(II) are reported.
Relativistic correction to the 1sσ and 2pσ electronic states of the H2+ molecular ion and the moleculelike states of the antiprotonic helium He+p¯
Authors: Tsogbayar Ts. , Korobov V. I.
Published: July 2006

Effective potentials of the relativistic Breit-Pauli corrections for the 1sσg and 2pσu electron states of the H2+ molecular ion and the 1sσ, 2sσ, and 3pσ states of the antiprotonic helium atom He+p¯ are calculated within the Born-Oppenheimer approximation. The variational expansion with randomly chosen exponents has been used for numerical studies. The results obtained for the Breit-Pauli effective potentials are accurate up to ten significant digits for the H2+ molecular ion and eight digits for the He+p¯ atom.
Heteronuclear isotropic mixing separated local field NMR spectroscopy
Authors: Dvinskikh Sergey V. , Yamamoto Kazutoshi , Ramamoorthy Ayyalusamy
Published: July 2006

This paper presents a theoretical, numerical, and experimental study of a new class of separated local field (SLF) techniques. These techniques are based on the heteronuclear isotropic mixing leading to spin exchange via the local field (HIMSELF). It is shown that highly efficient and robust SLF experiments can be designed based on double channel windowless homonuclear decoupling sequences. Compared to rotating frame techniques based on Hartmann-Hahn cross polarization, the new approach is less susceptible to the frequency offset and chemical shift interaction and can be applied in the structural studies of macromolecules that are uniformly labeled with isotopes such as C13 and N15. Furthermore, isotropic mixing sequences allow for transfer of any magnetization component of one nucleus to the corresponding component of its dipolar coupled partner. The performance of HIMSELF is studied by analysis of the average Hamiltonian and numerical simulation and is experimentally demonstrated on a single crystalline sample of a dipeptide and a liquid crystalline sample exhibiting motionally averaged dipolar couplings.
Laser control of reactions of photoswitching functional molecules
Authors: Tamura Hiroyuki , Nanbu Shinkoh , Ishida Toshimasa , Nakamura Hiroki
Published: July 2006

Laser control schemes of reactions of photoswitching functional molecules are proposed based on the quantum mechanical wave-packet dynamics and the design of laser parameters. The appropriately designed quadratically chirped laser pulses can achieve nearly complete transitions of wave packet among electronic states. The laser parameters can be optimized by using the Zhu-Nakamura theory of nonadiabatic transition. This method is effective not only for the initial photoexcitation process but also for the pump and dump scheme in the middle of the overall photoswitching process. The effects of momentum of the wave packet crossing a conical intersection on the branching ratio of products have also been clarified. These control schemes mentioned above are successfully applied to the cyclohexadiene/hexatriene photoisomerization (ring-opening) process which is the reaction center of practical photoswitching molecules such as diarylethenes. The overall efficiency of the ring opening can be appreciably increased by using the appropriately designed laser pulses compared to that of the natural photoisomerization without any control schemes.
Vapor-liquid equilibria from the triple point up to the critical point for the new generation of TIP4P-like models: TIP4P/Ew, TIP4P/2005, and TIP4P/ice
Authors: Vega C. , Abascal J. L. F. , Nezbeda I.
Published: July 2006

The vapor-liquid equilibria of three recently proposed water models have been computed using Gibbs-Duhem simulations. These models are TIP4P/Ew, TIP4P/2005, and TIP4P/ice and can be considered as modified versions of the TIP4P model. By design TIP4P reproduces the vaporization enthalpy of water at room temperature, whereas TIP4P/Ew and TIP4P/2005 match the temperature of maximum density and TIP4P/ice the melting temperature of water. Recently, the melting point for each of these models has been computed, making it possible for the first time to compute the complete vapor-liquid equilibria curve from the triple point to the critical point. From the coexistence results at high temperature, it is possible to estimate the critical properties of these models. None of them is capable of reproducing accurately the critical pressure or the vapor pressures and densities. Additionally, in the cases of TIP4P and TIP4P/ice the critical temperatures are too low and too high, respectively, compared to the experimental value. However, models accounting for the density maximum of water, such as TIP4P/Ew and TIP4P/2005 provide a better estimate of the critical temperature. In particular, TIP4P/2005 provides a critical temperature just 7K below the experimental result as well as an extraordinarily good description of the liquid densities from the triple point to the critical point. All TIP4P-like models present a ratio of the triple point temperature to the critical point temperature of about 0.39, compared with the experimental value of 0.42. As is the case for any effective potential neglecting many body forces, TIP4P/2005 fails in describing simultaneously the vapor and the liquid phases of water. However, it can be considered as one of the best effective potentials of water for describing condensed phases, both liquid and solid. In fact, it provides a completely coherent view of the phase diagram of water including fluid-solid, solid-solid, and vapor-liquid equilibria.
Relativistic plasma dielectric tensor evaluation based on the exact plasma dispersion functions concept
Authors: Castejón F. , Pavlov S. S.
Published: July 2006

The fully relativistic plasma dielectric tensor for any wave and plasma parameter is estimated on the basis of the exact plasma dispersion functions concept. The inclusion of this concept allows one to write the tensor in a closed and compact form and to reduce the tensor evaluation to the calculation of those functions. The main analytical properties of these functions are studied and two methods are given for their evaluation. The comparison between the exact dielectric tensor with the weakly relativistic approximation, widely used presently in plasma waves calculations, is given as well as the range of plasma temperature, harmonic number, and propagation angle in which the weakly relativistic approximation is valid.
Testing a model for triggering sawtooth oscillations in tokamaks
Authors: Bateman Glenn , Nguyen Canh N. , Kritz Arnold H. , Porcelli Franco
Published: July 2006

Simulations of JET and TFTR discharges with the BALDUR integrated modeling code are used to test a sawtooth model that consists of sawtooth triggering mechanisms [Porcelli et al., Plasma Phys. Contolled Fusion 38, 2163 (1996)] together with a modified version of the Kadomtsev sawtooth reconnection model [Kadomtsev, Sov. J. Plasma Phys. 1, 389 (1975)]. In simulations of 12 TFTR low confinement (L-mode) and 5 JET high confinement (H-mode) discharges, sawtooth triggering conditions are examined in order to determine which physical mechanisms are responsible for the crashes. It is found that most sawtooth crashes in the simulations are triggered by the m=1 resistive internal kink instability in plasmas that are in the semicollisional regime. However, in some discharges, some of the sawtooth crashes are triggered as a consequence of the driving force for the internal kink instability overcoming the fast ion stabilization. In rare instances, a sawtooth crash is triggered when the driving force for the internal kink instability overcomes the stabilization produced by the diamagnetic rotation of thermal ions. Generally, the median sawtooth period is found to increase as the magnetic reconnection fraction is increased. Optimal agreement with experimental data for the discharges considered is obtained with a magnetic reconnection fraction of approximately 37% although there is considerable scatter in the sawtooth periods observed in each discharge, both in the experimental data and in the simulations.
Intermediate state polarization in multiphoton ionization of HCl
Authors: Chichinin A. I. , Shternin P. S. , Gödecke N. , Kauczok S. , Maul C. , Vasyutinskii O. S. , Gericke K.-H.
Published: July 2006

The paper presents the detailed theoretical description of the intermediate state polarization and photofragment angular distribution in resonance enhanced multiphoton ionization (REMPI) of molecules and the experimental investigation of these effects in the EΣ+1 and VΣ+1 states of HCl populated by two-photon transitions. It is shown that the intermediate state polarization can be characterized by the universal parameter b which is in general a complex number containing information about the symmetry of the two-photon excitation and possible phase shifts. The photofragment angular distribution produced by one- or multiphoton excitation of the polarized intermediate state is presented as a product of the intermediate state axis spatial distribution and the angular distribution of the photofragments from an unpolarized intermediate state. Experiments have been carried out by two complementary methods: REMPI absorption spectroscopy of rotationally resolved (E,v′=0←X,v″=0) and (V,v′=12←X,v″=0) transitions and REMPI via the Q(0) and Q(1) rotational transitions followed by three-dimensional ion imaging detection. The values of the parameter b determined from experiment manifest the mostly perpendicular nature of the initial two-photon transition. The experimentally obtained H+ -ion fragment angular distributions produced via the Q(1) rotational transition show good agreement with theoretical prediction.
Switching characteristics of coupled quantum wires with tunable coupling strength
Authors: Ramamoorthy A. , Bird J. P. , Reno J. L.
Published: July 2006

We explore the switching characteristics of coupled quantum wires with a controllable interwire coupling. We perform extensive experimental and theoretical characterization of the coupling region between the wires, evaluating the local barrier that is formed in this region. With strong coupling between the wires, a nonmonotonic switching of their currents is obtained in a magnetic field, suggestive of quantum interference of electrons in their coupling region.
Highly tunable, high-throughput nanolithography based on strained regioregular conducting polymer films
Authors: Jones Alexandra G. , Balocco Claudio , King Rosemary , Song Aimin M.
Published: July 2006

Atomic force microscope (AFM) is now a standard imaging tool in laboratories but has displayed limited capability of nanolithography. We discover that an internal tensile strain exists in poly(3-hexylthiophene-2,5-diyl) (P3HT) films, and the physical effect is utilized to achieve highly tunable and high-throughput nanolithography. Trenches with widths spanning nearly two orders of magnitude from 40nmto2.3μm are fabricated. We show that P3HT is also excellent for pattern transfer to inorganic materials. Furthermore, a lithography speed of 0.5mm∕s is achieved, which is a few orders of magnitude higher than other known methods of AFM-based nanolithography.
Nitrogen incorporation in GaNAs layers grown by molecular beam epitaxy
Authors: Zhao Q. X. , Wang S. M. , Sadeghi M. , Larsson A. , Friesel M. , Willander M.
Published: July 2006

Ga N As ∕ Ga As quantum wells with high N concentrations, grown by molecular beam epitaxy, have been investigated by secondary-ion mass spectrometry (SIMS), high resolution x-ray diffraction (XRD), and photoluminescence (PL) measurements. The substitutional N concentration in an 18nm thick strained GaNAs layer varies from 1.4% to 5.9% when the growth rate is reduced from 1to0.2μm∕h. By further reducing the growth rate, more N can be incorporated but relaxation occurs. Both the total N concentration, deduced from SIMS measurements, and the substitutional N concentration, deduced from XRD measurements, increase with reduced growth rate. By comparing the SIMS and XRD results, we found that a large amount of N was not in substitutional position when the substitutional N concentration is high (>4%). The experimental results also show that there is no detectable change of total and substitutional N concentrations, within the instrument resolutions, after rapid thermal annealing at 700°C for 30s. However, PL measurements show a strong blueshift of the emission wavelength after annealing and the PL intensity increases by more than one order of magnitude.
Multifilament YBa2Cu3O6+x-coated conductors with minimized coupling losses
Authors: Levin G. A. , Barnes P. N. , Kell J. W. , Amemiya N. , Jiang Z. , Yoda K. , Kimura F.
Published: July 2006

We report an experimental approach to making multifilament coated conductors with low losses in applied time-varying magnetic field. Previously, the multifilament conductors obtained for that purpose by laser ablation suffered from high coupling losses. Here we report how this problem can be solved. When the substrate metal in the grooves segregating the filaments is exposed to oxygen, it forms high resistivity oxides that electrically insulate the stripes from each other and from the substrate. As the result, the coupling loss has become negligible over the entire range of tested parameters (magnetic field amplitudes B and frequencies f).
Facility for combined in situ magnetron sputtering and soft x-ray magnetic circular dichroism
Authors: Telling N. D. , van der Laan G. , Georgieva M. T. , Farley N. R. S.
Published: July 2006

An ultrahigh vacuum chamber that enables the in situ growth of thin films and multilayers by magnetron sputtering techniques is described. Following film preparation, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements are performed by utilizing an in vacuum electromagnet. XMCD measurements on sputtered thin films of Fe and Co yield spin and orbital moments that are consistent with those obtained previously on films measured in transmission geometry and grown in situ by evaporation methods. Thin films of FeN prepared by reactive sputtering are also examined and reveal an apparent enhancement in the orbital moment for low N content samples. The advantages of producing samples for in situ XAS and XMCD studies by magnetron sputtering are discussed.
Study on the resistive switching time of TiO2 thin films
Authors: Choi Byung Joon , Choi Seol , Kim Kyung Min , Shin Yong Cheol , Hwang Cheol Seong , Hwang Sung-Yeon , Cho Sung-sil , Park Sanghyun , Hong Suk-Kyoung
Published: July 2006

The required time for voltage-pulse-induced resistive switching of 40-nm-thick TiO2 thin films integrated in a contact-type structure (Pt top and TiN bottom contact, contact area ∼0.07μm2) was studied as a function of pulse voltage. For off→on switching at least 2V was necessary and the minimum switching times were ∼20ns at 2V and ∼10ns at 3V. For on→off switching, a minimum switching time of 5μs was obtained at 2.5V. The resistance of the on-state device was also dependent on the switching voltage and time.
Sample probe to measure resistivity and thermopower in the temperature range of 300–1000K
Authors: Ponnambalam V. , Lindsey S. , Hickman N. S. , Tritt Terry M.
Published: July 2006

We have fabricated and tested a sample probe that can measure resistivity (ρ) and thermopower (α) on either bar or rod shaped samples over a temperature span of 300–1000K. The design allows us to perform measurements both in vacuum as well as in inert atmosphere and hence suitable for any oxygen sensitive bulk samples. The main feature is the spring assisted pressure contacts between the sample and the two silver blocks, on which the electrical leads as well as thermocouples are mounted. This enables us to establish good thermal contacts over the entire temperature range of measurements between the sample and the thermocouples. To measure the thermopower, a slowly varying temperature difference (ΔT) is generated across the sample by using a small heater. The resulting slope of thermo-emf (ΔV) vs temperature difference (ΔT) plot is used to obtain the thermopower at any temperature. Resistivity is measured in sequence by a linear four-probe method at the same temperature. Hence in a single experiment, both resistivity and thermopower can be measured. Test measurements carried out on Ni and W standards yielded an accuracy of about few percent for both ρ and α.
Correlating thalamocortical connectivity and activity
Authors: da Fontoura Costa Luciano , Sporns Olaf
Published: July 2006

The segregated regions of the mammalian cerebral cortex and thalamus form an extensive and complex network, whose structure and function are still only incompletely understood. The present letter describes an application of the concepts of complex networks and random walks that allows the identification of nonrandom, highly structured features of thalamocortical connections and their potential effects on dynamic interactions between cortical areas in the cat brain. Utilizing large-scale anatomical data sets of this thalamocortical system, we investigate uniform random walks in such a network by considering the steady state eigenvector of the respective stochastic matrix. It is shown that thalamocortical connections are organized in such a way as to guarantee strong correlation between the outdegree and occupancy rate (a stochastic measure potentially related to activation) of each cortical area. Possible organizational principles underlying this effect are identified and discussed.
Insights into the origin of threading dislocations in GaN∕Al2O3 from atomic force microscopy
Authors: Oliver Rachel A. , Kappers Menno J. , Humphreys Colin J.
Published: July 2006

Threading dislocations in GaN∕Al2O3 are widely assumed to arise at the coalescence boundaries of misoriented islands. We have used atomic force microscopy to study a large number of coalescence boundaries and found no significant difference between the threading dislocation density in the boundary regions and the overall threading dislocation density for the large islands observed on partially coalesced samples. A secondary population of smaller islands is observed with a significantly higher dislocation density, and we suggest that when these small islands are overgrown by large islands, these threading dislocations are incorporated into the large islands, resulting in dislocation clusters.
Electrical characterization of GaAs metal bonded to Si
Authors: Bickford Justin R. , Qiao D. , Yu P. K. L. , Lau S. S.
Published: July 2006

Wafer bonding has traditionally focused on producing either insulating or nonlinear heterostructure interfaces; low-resistance Ohmic interfaces would offer the advantage of more efficient current delivery. In this study, doped GaAs was bonded to doped Si using indium and palladium interlayers. During heating above the lower melting point metal indium, a solid alloy is formed bonding the GaAs and Si together, this process is typically referred to as isothermal solidification. This method universally created Ohmic bond interfaces for all doping types. A metric was devised to measure the bond resistivity revealing a resistivity of 1.03×10−5Ωcm2 for n-GaAs bonded to p-Si.
A supersonic molecular beam study of the reaction of tetrakis(dimethylamido)titanium with self-assembled alkyltrichlorosilane monolayers
Authors: Ma P. F. , Dube A. , Killampalli A. S. , Engstrom J. R.
Published: July 2006

The reaction of a transition metal coordination complex, Ti[N(CH3)2]4, with self-assembled monolayers (SAMs) possessing–OH, –NH2, and –CH3 terminations has been examined using supersonic molecular beam techniques. The emphasis here is on how the reaction probability varies with incident kinetic energy (Ei=0.4–2.07eV) and angle of incidence (θi=0°–60°). The most reactive surface is the substrate underlying the SAMs–SiO2 with a high density of –OH(a) (>5×1014cm−2), “chemical oxide.” On chemical oxide, the dynamics of adsorption are well described by trapping, precursor-mediated adsorption, and the initial probability of adsorption depends only weakly on Ei and θi. The dependence of the reaction probability on substrate temperature is well described by a model involving an intrinsic precursor state, where the barrier for dissociation is approximately 0.2–0.5eV below the vacuum level. Reaction with the SAMs is more complicated. On the SAM with the unreactive, –CH3, termination, reactivity decreases continuously with increasing Ei while increasing with increasing θi. The data are best interpreted by a model where the Ti[N(CH3)2]4 must first be trapped on the surface, followed by diffusion through the SAM and reaction at the SAM∕SiO2 interface with residual –OH(a). This process is not activated by Ei and most likely occurs in defective areas of the SAM. On the SAMs with reactive end groups, the situation is quite different. On both the–OH and –NH2 SAMs, the reaction with the Ti[N(CH3)2]4 as a function of Ei passes through a minimum near Ei∼1.0eV. Two explanations for this intriguing finding are made—one involves the participation of a direct dissociation channel at sufficiently high Ei. A second explanation involves a new mechanism for trapping, which could be termed penetration facilitated trapping, where the Ti[N(CH3)2]4 penetrates the near surface layers, a process that is activated as the molecules in the SAM must be displaced from their equilibrium positions.
Formation of nanovoids in high-dose hydrogen implanted GaN
Authors: Radu I. , Singh R. , Scholz R. , Gösele U. , Christiansen S. , Brüderl G. , Eichler C. , Härle V.
Published: July 2006

The formation of nanovoids upon high-dose hydrogen implantation and subsequent annealing in GaN is investigated by transmission electron microscopy. The epitaxial GaN layers on sapphire were implanted at room temperature with H2+ ions at 100keV with a dose of 13×1016cm−2. Cross section transmission electron microscopy investigations revealed that nanovoids about 2nm in diameter had formed during hydrogen implantation at room temperature while large microcracks (∼150–200nm long) occurred upon annealing (1h at 700°C) leading to surface blistering. The nanovoids serve as precursors to the microcrack formation and are essential for the blistering process.
Second harmonic scattered light from a transparent glass-ceramic containing sodium niobate nanocrystals
Authors: Valdez Ernesto , de Araújo Cid B. , Lipovskii A. A.
Published: July 2006

A study of the incoherent second harmonic radiation generated in a transparent glass-ceramic which consists of sodium niobate nanocrystals embedded in a silica-niobate glass is reported. A nanosecond laser operating at 1064nm was used, and measurements were performed for samples with nanocrystal filling fractions up to 37%. The results allow the measurements of the first hyperpolarizability per nanocrystal, β=166×10−30esu.
Interdigital transducers with control gates on AlGaN∕GaN heterostructures
Authors: Shigekawa Naoteru , Nishimura Kazumi , Suemitsu Tetsuya , Yokoyama Haruki , Hohkawa Kohji
Published: July 2006

We investigated surface acoustic wave (SAW) filters composed of interdigital transducers with control gates formed on AlGaN∕GaN heterostructures. The contribution of SAWs appeared in the rf characteristics of the SAW filters when reverse-bias voltages were applied to the control gates and the GaN channels were depleted. We also observed higher-frequency signals due to SAWs in transducers with positively-biased control gates. These results suggest that SAW-based functional devices are achievable by employing transducers in which rf signals for exciting SAWs and control signals for modulating SAW properties are applied to different terminals.
Lasing in optically pumped Ga(NAsP)∕GaP heterostructures
Authors: Borck S. , Chatterjee S. , Kunert B. , Volz K. , Stolz W. , Heber J. , Rühle W. W. , Gerhardt N. C. , Hofmann M. R.
Published: July 2006

We experimentally investigate the characteristics of light emission of optically excited Ga(NAsP) multiple quantum-well structures grown pseudomorphically on a GaP substrate by metal-organic vapor-phase epitaxy. The emission power as a function of excitation power shows at temperatures from 15to200K a clear threshold after excitation with short laser pulses. The emission spectra become narrow at threshold and shift to higher energies. A well defined mode structure is observed above threshold. Complementary, quasi-steady-state gain measurements using the stripe-length method yield positive modal gain values of up to 10cm−1 at room temperature, thus validating that the structures show laser action.
High-field magnetic force microscopy as susceptibility imaging
Authors: Israel Casey , Wu Weida , Lozanne Alex de
Published: July 2006

We describe an extension of variable-temperature magnetic force microscopy (MFM) that allows spatial discrimination between the different states that exist in magnetically phase-separated materials. Some manganites exhibit a micrometer-scale separation of phases that are either ferromagnetic, paramagnetic, or antiferromagnetic. In an applied field large enough to saturate the ferromagnetic phase, any MFM contrast arising from the variation of the magnetization (domain walls, domains of differing orientation) is eliminated, while the nonferromagnetic phases are magnetized according to their susceptibilities. The different phases can then be discerned by their respective contrast levels in the MFM images.
Low temperature cell for cavity ring down absorption studies
Authors: Lewis Ernest K. , Moehnke Craig J. , Navea Juan G. , Manzanares Carlos E.
Published: July 2006

Phase shift cavity ring down is a technique that due to its long optical path length is an ideal method to detect weak absorptions. Coupling the method to a custom fitted cryostat allows gas phase molecules to be studied at cryogenic temperatures in a thermally isolated vacuum chamber. A novel design is described to construct the complete instrument. With optical cavities of length 10⩽ℓ⩽43cm, optical path lengths between 200m and 6km have been achieved. High vibrational overtones C–H (Δυ=5) are measured at 130K (methane), 150K (ethylene), and 155K (ethane). Oscillator strengths of each molecule calculated at different temperatures are in excellent agreement. The experimental setup can be used to study kinetics and spectroscopy of atmospheric molecules, planetary atmospheres, and molecular complexes in the gas phase. Low temperatures can be obtained using liquid He or liquid N2 as cryogens.
Practical realization of Nyquist’s gedanken experiment
Authors: Callegaro Luca , Pisani Marco
Published: July 2006

If two noisy electrical dipoles are connected with an electric line, noise power can flow through the line. Such power can be measured, in the audio frequency range, with a wattmeter with spectral display capabilities. If the dipoles are two equal resistors in thermodynamic equilibrium, the power depends exclusively on the temperature difference, giving a differential noise thermometry method. If the dipoles are at the same temperature, but one is out of thermodynamic equilibrium (because of a flowing dc current), noise power depends on excess noise only, and it is possible to measure excess noise far below Johnson noise threshold.
Low electrode induced optical losses in organic active single layer polyfluorene waveguides with two indium tin oxide electrodes deposited by pulsed laser deposition
Authors: Lattante S. , Romano F. , Caricato A. P. , Martino M. , Anni M.
Published: July 2006

We demonstrate that 20nm thick indium tin oxide (ITO) layers deposited by pulsed laser deposition present sheet resistance as low as 130Ω∕◻ and very uniform morphology, with an average roughness of about 0.4nm, and peak-to-valley roughness as low as 8.2nm. This good uniformity allowed us to realize a single layer polyfluorene active waveguide with both top and bottom ITO electrodes showing clear amplified spontaneous emission and electrode induced losses as low as 3.0cm−1. We investigated the effects of hole injection in the ASE intensity concluding that complete gain suppression due to polaron absorption would take place for current density of about 360mAcm−2.
Single-electron transistors in GaN∕AlGaN heterostructures
Authors: Chou H. T. , Goldhaber-Gordon D. , Schmult S. , Manfra M. J. , Sergent A. M. , Molnar R. J.
Published: July 2006

We report transport properties of two single-electron transistors (SETs) on a GaN∕AlGaN heterostructure. The first SET formed accidentally in a quantum point contact near pinchoff. Its small size produces large energy scales (a charging energy of 7.5meV and well-resolved excited states). The second, intentionally fabricated SET is much larger. More than 100 uniformly spaced Coulomb oscillations yield a charging energy of 0.85meV. Excited states are not resolvable in Coulomb diamonds, and Coulomb blockade peak height remains constant with increasing temperature, indicating that transport is through multiple quantum levels even at the 450mK base electron temperature of our measurements. Coulomb oscillations of both SETs are highly stable, comparable to the best GaAs SETs.
Direct observation of resonant energy transfer between quantum dots of two different sizes in a single water droplet
Authors: Xu Ling , Xu Jun , Ma Zhongyuan , Li Wei , Huang Xinfan , Chen Kunji
Published: July 2006

Water soluble CdTe nanocrystals (NCs) of two different sizes capped by thioglycolic acid were synthesized and were dispersed in pure water. We have observed that the emission color of CdTe NCs has been changed in a single droplet of water during the evaporation of the solvent. This is attributed to Förster resonance energy transfer between CdTe NCs, when their concentration becomes high enough to ensure close proximity. By combining our results of photoluminescence with quantitative analysis we conclude that the coupling between different sized CdTe NCs via long-range dipole-dipole interactions was enhanced in aqueous solution due to Brownian motion.
Microstructures on soluble polymer surfaces via drop deposition of solvent mixtures
Authors: Karabasheva Svetlana , Baluschev Stanislav , Graf Karlheinz
Published: July 2006

We report a single step procedure for aspheric shaped microlens formation. By means of the ink-jet printing technique, microdroplets of mixtures of solvents with different volatilities are deposited on a soluble polymer substrate. The generated microstructures (diameter in range of about 200μm–5mm) exhibit a pronounced focusing effect. The method can be extended to one-dimensional and two-dimensional microlens arrays.
Phase locking and spectral linewidth of a two-mode terahertz quantum cascade laser
Authors: Baryshev A. , Hovenier J. N. , Adam A. J. L. , Kašalynas I. , Gao J. R. , Klaassen T. O. , Williams B. S. , Kumar S. , Hu Q. , Reno J. L.
Published: July 2006

We have studied the phase locking and spectral linewidth of an ∼2.7THz quantum cascade laser by mixing its two lateral lasing modes. The beat signal at about 8GHz is compared with a microwave reference by applying conventional phase lock loop circuitry with feedback to the laser bias current. Phase locking has been demonstrated, resulting in a narrow beat linewidth of less than 10Hz. Under frequency stabilization we find that the terahertz line profile is essentially Lorentzian with a minimum linewidth of ∼6.3kHz. Power dependent measurements suggest that this linewidth does not approach the Schawlow-Townes limit.
Note on the physical meaning of averaged amplitudes and intensities in parametric instabilities driven by a stochastic pump
Authors: Mounaix Philippe , Posta Gustavo
Published: July 2006

The problem of the physical meaning of averaged amplitudes and intensities in parametric instabilities driven by a stochastic pump is addressed. The physical meaning of an averaged quantity is understood as its ability to yield a good estimate of the median of this quantity. In the limit of a purely temporal problem with a Gaussian white noise pump, rigorous results can be obtained which show that no physical meaning can be bestowed a priori either on the averaged amplitudes or on the averaged intensities. A conjecture is proposed which enlarges this result to the general case. The statistics of the wave amplitudes is obtained by constructing a tractable weak solution to the parametric problem.
Reexploring Transport Geography and Networks: A Case Study of Container Shipments to the West Coast of the United States
Authors: Fowler Christopher S
Published: July 2006

After a meteoric rise to prominence in the 1990s, the practice of conceptualizing the relationships between places in terms of networks has recently come under fire within economic geography. Its critics claim that networks are ‘fuzzy’, ‘anemic’, and ‘overendowed’. Others have criticized the network approach for its insensitivity to the way in which power is transmitted unevenly across networks. However, these are not necessarily failings of the network metaphor but, rather, of its employment to date within economic geography. Where most economic geographers have come to see the network metaphor as a way of representing connections between the nodes that they study (frequently cities or firms), transport geographers have long taken the network itself as the principal object of study. In the research of many transport geographers, networks appear as powerful agents shaping the places they connect. Incorporation of lessons drawn from transport geography into the burgeoning literature on networks in economic geography can overcome some of the growing criticism of network methodologies. The author uses an analysis of container flows to the West Coast of the United States to demonstrate the advantages of a network methodology based on transport networks.
Port–FEZ Bundles as Spaces of Global Articulation: The Case of Tianjin, China
Authors: Wang James J , Olivier Daniel
Published: July 2006

The authors examine possible ways to enhance interdisciplinary dialogue between transport geography and economic geography, its parent discipline. In the context of emerging paradigms of global production seeking to capture the outsourcing process with developing countries, the port–free-trade zone bundle is suggested as a valid intersect between spaces of production and spaces of circulation. Such zones act as interfaces between local and global spaces: they are spaces of global articulation. A case study of how such global articulation spaces are created in China is presented from the example of the Tianjin Economic Development Area (TEDA). Evidence suggests that, as the world factory, China purposely set up port–FEZs (free economic zone) in pairs and the use of containerized transport has been highly restricted to port-related production places; hence a dualism has been created in China's transport. It is found that TEDA consists of a continuum of customs-regulated spaces which act as a transition zone between domestic and international ‘status' commodities following a global production logic. At the same time, the creation of such enclaves consolidates a spatial ‘splintering’ effect that alienates such enclaves from their surroundings.
Temporary Migration and HIV Risk Behaviors in China
Authors: Yang Xiushi
Published: July 2006

The link between migration and HIV/AIDS is well documented. Theoretical work to understand the behavioral mechanisms underlying migrants' HIV risk behaviors is limited. In most studies migration is viewed as a virus carrier and population mixer. In this paper I present a framework conceptualizing the link between temporary migration and migrants' heightened HIV risk behaviors. Data from a large and population-based survey conducted in 2003 in southwestern China are used to empirically test the proposed framework. Results suggest that temporary migrants are more likely to use drugs than nonmigrants; they are also more likely to have had unprotected casual sex and a greater number of casual-sex partners than comparable nonmigrants. Postmigration economic marginalization, social isolation, and lax social control contribute to migrants' elevated risk behaviors, so do migration selectivity and social influences from family and peers. Temporary migrants are at risk of HIV; intervention needs to target migrants' postmigration milieus and their social network of family and peers.
Physical surrogates for macrofaunal distributions and abundance in a tropical gulf
Authors: Post Alexandra L. , Wassenberg Ted J. , Passlow Vicki
Published: July 2006

The characterisation of benthic habitats based on their abiotic (physical and chemical) attributes remains poorly defined in the marine environment, but is becoming increasingly central in the development of marine management plans in Australia and elsewhere in the world. The current study tested this link between physical and biological datasets for the southern Gulf of Carpentaria, Australia. The results presented were based on a range of physical factors, including the sediment composition (grain size and carbonate content), sediment mobility, water depth and organic carbon flux, and their relationship to the distribution and diversity of benthic macrofauna was tested. The results reveal the importance of process-based indices, such as sediment mobility, in addition to other environmental factors in defining the distribution of the benthic macrofauna. The distribution of the benthic macrofauna changes gradationally across the south-eastern Gulf, associated with changes in the per cent mud and gravel, the seabed exposure and the water depth. Patterns of diversity also reveal the importance of physical processes such as sediment mobility in defining benthic habitats. The species–environment relationships observed at the small scale of the current study are consistent with broader associations observed for other organisms within the Gulf.