bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-dimensional global fluid simulations of cylindrical magnetized plasmas
Authors: Naulin V. , Windisch T. , Grulke O.
Published: January 2008

Plasma dynamics in cylindrical geometry, with many well diagnosed experiments in operation worldwide, is of fundamental interest. These linear machines can provide an unique testing ground for direct and detailed comparisons of numerical simulations of nonlinear plasma dynamics with experiments. Thus, it is possible to assess the reproductive and predictive capabilities of plasma simulations in unprecedented detail. Here, three-dimensional global fluid simulations of a cylindrical magnetized plasma are presented. This plasma is characterized by the existence of spatially localized sources and sinks. The traditional scale separation paradigm is not applied in the simulation model to account for the important evolution of the background profiles due to the dynamics of turbulent fluctuations. Furthermore, the fluid modeling of sheath boundary conditions, which determine the plasma conditions, are an important ingredient to the code presented here. The linear properties of the model equations are studied and are shown to agree well with experimental observations of linear drift modes. The fully nonlinear simulations are characterized by turbulent fluctuations, which are dominated by low mode numbers in the large radial pressure gradient region. In the far plasma edge, the fluctuations display an intermittent character due to convection within radially extended spatiotemporal potential fluctuations. This paper reports on the model and general code results, while the detailed comparison to a specific experiment is left to a follow-up paper.
Pressure-induced phase transition in Ho0.8Dy0.2MnO3 multiferroic compound
Authors: Feng S. M. , Wang L. J. , Zhu J. L. , Li F. Y. , Yu R. C. , Jin C. Q. , Wang X. H. , Li L. T.
Published: January 2008

The multiferroic Ho0.8Dy0.2MnO3 compound crystallizing into hexagonal perovskite was investigated by using an in situ high pressure Raman scattering method at room temperature. It is found that with increasing pressure, the compound started to transform from hexagonal-type into an orthorhombic-type perovskite near 9.8GPa. The phase transition process is analyzed.
Enhancement of band gap and photoconductivity in gamma indium selenide due to swift heavy ion irradiation
Authors: Sreekumar R. , Jayakrishnan R. , Sudha Kartha C. , Vijayakumar K. P. , Khan S. A. , Avasthi D. K.
Published: January 2008

γ - In 2 Se 3 thin films prepared at different annealing temperatures ranging from 100to400°C were irradiated using 90MeV Si ions with a fluence of 2×1013ions∕cm2. X-ray diffraction analysis proved that there is no considerable variation in structural properties of the films due to the swift heavy ion irradiation. However, photosensitivity and sheet resistance of the samples increased due to irradiation. It was observed that the sample, which had negative photoconductivity, exhibited positive photoconductivity, after irradiation. The negative photoconductivity was due to the combined effect of trapping of photoexcited electrons, at traps 1.42 and 1.26eV, above the valence band along with destruction of the minority carriers, created during illumination, through recombination. Photoluminescence study revealed that the emission was due to the transition to a recombination center, which was 180meV above the valence band. Optical absorption study proved that the defects present at 1.42 and 1.26eV were annealed out by the ion beam irradiation. This allowed photoexcited carriers to reach conduction band, which resulted in positive photoconductivity. Optical absorption study also revealed that the band gap of the material could be increased by ion beam irradiation. The sample prepared at 400°C had a band gap of 2eV and this increased to 2.8eV, after irradiation. The increase in optical band gap was attributed to the annihilation of localized defect bands, near the conduction and valence band edges, on irradiation. Thus, by ion beam irradiation, one could enhance photosensitivity as well as the optical band gap of γ-In2Se3, making the material suitable for applications such as window layer in solar cells.
Electron transport through quantum dots with indirect coupling parameter
Authors: Chen Kuo-Wei , Chang Ching-Ray
Published: January 2008

We report on the analysis of electron transport through an Aharonov–Bohm interferometer with laterally coupled triple quantum dots. Analytical expressions for the linear conductance and density of states with an indirect coupling parameter α are obtained. The conductance spectrum shows the Breit–Wigner and the Fano resonances. The suppression of the resonance at α=1 is ascribed to the existence of a bound state in the continuum. This resonance is well resolved and clearly observed as α decreases. For particular values of the magnetic flux, two antiresonances are observed due to the destructive quantum interference. They disappear due to the localization of the bonding and antibonding states.
Nature of magnetism in copper-doped oxides: ZrO2, TiO2, MgO, SiO2, Al2O3, and ZnO
Authors: Dutta P. , Seehra M. S. , Zhang Y. , Wender I.
Published: January 2008

The nature of magnetism in 10% Cu-doped ZrO2, TiO2, MgO, SiO2, and Al2O3 is investigated using superconducting quantum interference device magnetometry and electron magnetic resonance (EMR). The doping was done by the incipient wetness impregnation technique. X-ray diffraction studies showed the presence of some CuO notably in TiO2, SiO2, and Al2O3. However, EMR yielded typical Cu2+ spectra in all the samples, resulting from some doping. Magnetic field (H) and temperature (T) dependences of the magnetization (M) show only paramagnetism in these oxides in contrast to ferromagnetism with Tc=380K observed in Cu∕ZnO. Concentrations x of Cu2+ doped into the oxides are determined from the M vs T and M vs H data, showing Cu∕ZrO2 with the largest x and hence largest magnetization.
Phase transitions in NdNiPb and Nd5NiPb3
Authors: Goruganti V. , Rathnayaka K. D. D. , Ross Joseph H. , Öner Y.
Published: January 2008

We report magnetic and thermodynamic measurements for NdNiPb and Nd5NiPb3. High-temperature Curie-Weiss fits yield effective moments per Nd of 3.59μB for NdNiPb and 3.70μB for Nd5NiPb3. These are close to the Nd3+ ionic moment, 3.62μB, showing that Ni is nonmagnetic in both cases. For NdNiPb a peak seen in both the magnetization and specific heat at 3.5K indicates an antiferromagnetic transition at that temperature. Specific heat measurements show this transition to be formed from crystal-field-split Nd magnetic levels, and a fit yields values for crystal field splitting. Nd5NiPb3 exhibits two magnetic transitions, an antiferromagnetic transition at 42K, and an apparently weak-ferromagnetic canting transition at 8K. These are confirmed by specific heat measurements. M-H curves show metamagnetism at temperatures between the two magnetic transitions.
The critical size between single domain and multidomain in L1-FePt particles
Authors: Kikuchi N. , Okamoto S. , Kitakami O.
Published: January 2008

The critical diameter between single domain and multidomain in single crystal L10-FePt particles has been studied. The particles are fabricated by depositing FePt on MgO(100) at 1173K and the diameter was ranged from a few to over a hundred nanometers by changing nominal deposition thickness in the range of 2–10nm. Initial anomalous Hall effect (AHE) curves were measured for as-deposited state and demagnetized state with in-plane field. The critical diameter for single domain obtained by analyzing initial curves coincides for both initial states and dc∼55nm for nominal thickness of 10nm.
Control of interlayer magnetostatic coupling in submicron-sized Fe∕Au∕Fe rings
Authors: Miyawaki T. , Kohda M. , Fujita A. , Nitta J.
Published: January 2008

We have investigated the inner diameter dependence of magnetostatic coupling in Fe∕Au∕Fe pseudospin valve (PSV) rings by the magneto-optical Kerr effect. Hysteresis loops of the soft layer with smaller inner diameter are considerably different from those found in single-layer rings maintaining the outer diameter. This is explained by the attenuation of circumferential shape anisotropy as the inner diameter decreases. The magnetostatic coupling is enhanced between the two Fe layers due to an increase of a stray field from ring edges. These results indicate that the magnetostatic coupling in the PSV rings is controlled by changing the inner diameter.
Theory of a quasihomogeneous field-effect transistor
Authors: Gribnikov Z. S. , Haddad G. I. , Eizenkop J.
Published: January 2008

We consider the stationary characteristics and high-frequency conductance of dual-gate field-effect transistors with a substantially inhomogeneous capacitive connection between gates and a current-conducting channel for an electron enhancement control mode. In most cases, such transistors are quasihomogeneous because their stationary JV-characteristics are not different from characteristics of perfectly homogeneous devices, which can be described by a certain effective channel length. But a nonstationary small-signal conductance of such transistors with the same stationary characteristics can be noticeably different since it is dependent on detailed space distributions of the capacitive inhomogeneity. In particular, a high-frequency gate current can depend on the stationary current direction in channels with symmetrical stationary characteristics. We discuss in detail the cases when a specific channel-gate capacitance increases or decreases monotonically along the channel current direction. Then high-frequency gate currents depend substantially on the stationary channel current direction (in spite of the same stationary JV-characteristics for both directions).
Free-carrier compensation in ferromagnetic ion-implanted SnO2:Co
Authors: Menzel D. , Awada A. , Dierke H. , Schoenes J. , Ludwig F. , Schilling M.
Published: January 2008

Magnetron sputtered SnO2 thin films on fused silica substrates were doped with 3, 5, and 7at.% Co by the ion-implantation technique leading to ferromagnetic order at room temperature. Transmission electron microscopy combined with energy loss spectroscopy confirms the absence of Co clusters which are larger than the resolution of ∼1nm. Electrical transport measurements using the Van der Pauw method show a high resistivity of the undoped SnO2 which increases with the Co concentration. The lower valence of Co with respect to Sn causes a compensation of donor electrons by localized holes.
Ultrafast carrier capture dynamics in InGaAs∕GaAs quantum wires
Authors: Cooke D. G. , Hegmann F. A. , Mazur Yu. I. , Wang Zh. M. , Black W. , Wen H. , Salamo G. J. , Mishima T. D. , Lian G. D. , Johnson M. B.
Published: January 2008

We use time-resolved terahertz-pulse spectroscopy to study the ultrafast carrier dynamics in InGaAs∕GaAs (311)A quantum wires. Anisotropy in the photoconductive dynamics is observed when aligning the terahertz probe polarization parallel versus perpendicular to the wire direction. The origin of this anisotropy is the carrier capture into localized quantum-wire states from delocalized wetting layer or barrier regions over time scales from 6to30ps. The capture efficiency is found to be strongly temperature dependent, with thermal emission dominating above 125K, while state-filling effects within the wires influence the capture rate below 125K. Transient spectroscopy reveals a Drude-like carrier conductivity.
Large amplitude dust-acoustic solitary waves and double layers in nonthermal plasmas
Authors: Verheest F. , Pillay S. R.
Published: January 2008

A systematic investigation has been made of large amplitude dust-acoustic solitary waves in plasmas consisting of negatively charged cold dust in the presence of either nonthermally distributed ions or electrons, with a particular emphasis on the delimitation of the compositional parameter space where such structures can be generated. The occurrence of negative electrostatic potential solitary waves is for higher Mach numbers limited by infinite dust compression. On the positive potential side solitary waves are possible until double layers are encountered, provided two distinct aspects of the dusty plasma compositions are fulfilled: Sufficient nonthermality in the ions and sufficient negative charge on the dust, the electron presence not really being required. There is then a region in parameter space where both negative and positive solitary structures can coexist. When electrons are the nonthermal plasma constituent, only negative potential waves are possible.
Sources of unintentional conductivity in InN
Authors: Janotti Anderson , Van de Walle Chris G.
Published: January 2008

Using first-principles methods, we investigate the effects of monatomic hydrogen in InN. We find that hydrogen can occupy interstitial and substitutional sites. Interstitial hydrogen is stable in the bond-center configuration and acts exclusively as a shallow donor, with a H–N stretching vibration at 3050cm−1. Hydrogen can also substitute for nitrogen in InN, bonding equally to the four In nearest neighbors in a multicenter-bond configuration. Substitutional hydrogen has low formation energy and, counterintuitively, is a double donor. Our results suggest that monatomic hydrogen is a plausible cause of the unintentional n-type conductivity that is often observed in as-grown InN.
Fabrication and characterization of FeRh nanoparticles
Authors: Ko Hnin Yu Yu , Suzuki Takao , Phuoc Nguyen N. , Cao Jiangwei
Published: January 2008

The magnetic and structural properties of FexRh100−x (x=35–80) nanoparticles fabricated by a solution phase chemical method have been studied systematically. From x-ray diffraction results, as-deposited nanoparticles of FeRh reveal a chemically disordered fcc structure which can be transformed into CsCl-type structure through thermal annealing. The high-resolution transmission electron microscopy observations of FeRh nanoparticles indicate that the average size of 3–5nm in as-deposited state and larger size (10–15nm) particles are also distributed in the annealed sate. Nanoparticles with x=35–55 exhibit the behaviors of magnetization with temperature, which suggests the transition from the AFM to FM phases at around 100–200°C, whereas those with x=80 do very little change of magnetization with temperature. It is also found that there seems an effect of applied magnetic field on transition, that is, the transition temperature is shifted to a lower side upon the application of magnetic field during the transition process.
A Riccati equation in radiative stellar collapse
Authors: Rajah S. S. , Maharaj S. D.
Published: January 2008

We model the behavior of a relativistic spherically symmetric shearing fluid undergoing gravitational collapse with heat flux. It is demonstrated that the governing equation for the gravitational behavior is a Riccati equation. We show that the Riccati equation admits two classes of new solutions in closed form. We regain particular models, obtained in previous investigations, as special cases. A significant feature of our solutions is the general spatial dependence in the metric functions which allows for a wider study of the physical features of the model, such as the behavior of the causal temperature in inhomogeneous space-times.
Quantum critical behavior of the (Cr86Ru14)1−xVx alloy system
Authors: Reddy L. , Alberts H. L. , Strydom A. M. , Prinsloo A. R. E. , Venter A. M.
Published: January 2008

Electrical resistivity, Hall effect, specific heat, and neutron diffraction measurements are used to investigate magnetic and quantum critical behavior in a commensurate spin-density-wave (CSDW) antiferromagnetic (Cr86Ru14)1−xVx alloy system. The x−T magnetic phase diagram obtained from the measurements depicts a critical point, occuring at T=0K for a critical V concentration xc=0.104. This point is classified as a CSDW-type quantum critical point using recently suggested methods for classification of quantum critical points from Hall measurements.
Phase transformation and superconducting properties of MgB2 using ball-milled low purity boron
Authors: Xu X. , Kim J. H. , Hossain M. S. A. , Park J. S. , Zhao Y. , Dou S. X. , Yeoh W. K. , Rindfleisch M. , Tomsic M.
Published: January 2008

MgB 2 samples were prepared by using 96% boron (B) powder with strong crystalline phase that had been ball milled for various times. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field critical current density (Jc) due to small grain size and better reactivity. Specifically, many grain boundaries for MgB2 could be acting as strong flux pinning centers. Based on Rowell connectivity analysis, when the ball-milling time increased, the connectivity factor, described as the active cross-sectional area fraction (AF), was decreased. This implies that the intergrain connectivity became worse. These properties could lead to poor Jc in low field. However, the pinning force strength, Jc1/2×B1/4, of samples using ball-milled 96% B is larger than that of the reference sample using commercial 96% B powder. These results accompany enhanced irreversibility (Hirr) and upper critical fields (Hc2).
Spin wave dynamics in two- and three-dimensional superlattices of nanosized ferromagnetic spheres
Authors: Tartakovskaya Elena , Kreuzpaintner Wolfgang , Schreyer Andreas
Published: January 2008

The dispersion of spin wave modes which due to the dipolar interactions propagate along different directions of ordered superlattices of nanospheres is investigated. For this purpose a procedure similar to the well-known method of linear combination of atomic orbitals is applied. Different geometries of two-dimensional (triangular and square) and three-dimensional (simple cubic and hexagonal-close-packed) arrangements are considered and the influence of dimensionality on the spin wave dynamics is analyzed. A phase transition which is caused by the competition between dipolar and uniaxial anisotropy interactions is predicted by the investigation of the dispersion of the uniform Kittel mode for the superlattice of the hexagonal order. In conclusion, it is shown how the weak dipolar interaction enhances or decreases the relaxation time in the samples with a controlled direction of the easy axis.
High-efficient ultraviolet emission in phonon-reduced ZnO films: The role of germanium
Authors: Kim Sung , Lee Do Kyu , Hong Seung Hui , Eom Sung Hwan , Oh Hyoung Taek , Choi Suk-Ho , Hwang Han Na , Hwang Chan Cuk
Published: January 2008

Photoluminescence (PL) properties have been studied for Ge-doped ZnO films grown on Si wafers by RF-magnetron sputtering. A PL line, named as G line, appears at 3.324eV by Ge doping and is attributed to Ge suboxide states including GeO color centers. As Ge concentration (nGe) increases, the intensities of free-exciton-, and neutral-donor-bound-exciton-, two-electron-satellite-, and G-PL lines increase, while those of their phonon replicas decrease. By Ge doping, no-phonon line deconvoluted from the near-band-edge (NBE) PL at 300K is enhanced, but its LO phonon replicas are reduced, resulting in the enhancement of the NBE PL with its reduced bandwidth. It is suggested that these results are due to the increase of the Ge suboxide states with increasing nGe, which is also confirmed by the analysis of the Ge 3d core-level spectra by x-ray photoelectron spectroscopy.
Modification and expansion of the generalized soft-sphere model to high temperature based on collision integrals
Authors: Kim Jae Gang , Kwon Oh Joon , Park Chul
Published: January 2008

In the present study, modification and expansion of the collision parameters for the general soft-sphere model [J. Fan, Phys. Fluids 12, 4399 (2002)] were made for use in the direct simulation Monte Carlo calculation of hypersonic flows in the temperature range of 300–50 000K. The collision integrals were expressed as a two-term function in a form of the inverse power of temperature, which was cast in terms of the soft-sphere scattering parameters and the four total cross-section parameters. Next, the most recent available data for the diffusion and viscosity collision integrals were collected and fitted into a function of temperature in the same form. By equating these expressions for the diffusion and viscosity collision integrals simultaneously, the five collision parameters were deduced as functions of species combinations. The resulting collision parameters for the general soft-sphere model were tabulated for 191 collision pairs involving 22 species. It was shown that the transport properties calculated by using the present collision parameters are much closer to experiments, theoretical data, and the values obtained by the ab initio calculations from quantum-mechanically derived potential energy surfaces than existing elastic collision models. The direct simulation Monte Carlo calculation of flow around a circular cylinder confirmed that discernible differences exist between the results based on the present study and those of the existing models.
High coercivity Gd-substituted Ba hexaferrites, prepared by chemical coprecipitation
Authors: Litsardakis G. , Manolakis I. , Serletis C. , Efthimiadis K. G.
Published: January 2008

A series of Gd-substituted Ba hexaferrites with nominal formula (Ba1−xGdx)O⋅5.25 Fe2O3 (x=0–0.30) were prepared by the chemical coprecipitation method from nitrate precursors and heating at T=800–1200°C for 2h. The samples have been examined by x-ray diffraction, vibrating-sample magnetometer, and scanning electron microscopy methods. Gd substituted samples form single phase materials with the M-type hexaferrite structure at all heating temperatures, in the range of x⩽0.10–0.20. The saturation magnetization (at 1.8T) varies slightly with x in most cases and, for x=0.05–0.10, it increases up to 66.7Am2∕kg, exceeding the value of the unsubstituted hexaferrite. A strong enhancement of the coercivity is observed for all substituted samples, with maximum values Hc=457kA∕m for the single-phase x=0.10 sample annealed at 1000°C and Hc=477kA∕m for the x=0.25 sample annealed at 1100°C which contains Fe2O3 and GdFeO3 impurities. As the variation of coercivity with either substitution rate (x) or annealing temperature is not monotonic, three different factors may account for the high coercivities that are obtained: (a) an inhibition of grain growth due to the presence of Gd, (b) a possible inherent effect on magnetocrystalline anisotropy, especially for single phase samples, and (c) a microstructural effect of secondary phases.
Effects of stoichiometry on defect formation in ZnO epilayers grown by molecular-beam epitaxy: An optically detected magnetic resonance study
Authors: Wang X. J. , Buyanova I. A. , Chen W. M. , Pan C. J. , Tu C. W.
Published: January 2008

Photoluminescence (PL) and optically detected magnetic resonance are employed to study effects of nonstoichiometry during the growth on defect formation in ZnO epilayers grown by molecular-beam epitaxy (MBE). Several defects are revealed via monitoring the yellow PL emission (∼2.17eV) and their magnetic resonance signatures are obtained. The defects are concluded to be common for the MBE growth and are facilitated during the off-stoichiometric growth conditions, especially under excess of oxygen.
Pump excited state absorption in holmium-doped fluoride glass
Authors: Librantz André Felipe Henriques , Jackson Stuart D. , Gomes Laércio , Ribeiro Sidney José Lima , Messaddeq Younes
Published: January 2008

The primary excited state absorption processes relating to the I65→I75 3μm laser transition in singly Ho3+-doped fluoride glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the I65 and I75 energy levels established the occurrence of two excited state absorption transitions from these energy levels that compete with previously described energy transfer upconversion processes. The I75→I45 excited state absorption transition has peak cross sections at 1216nm (σesa=2.8×10−21cm2), 1174nm (σesa=1×10−21cm2), and 1134nm (σesa=7.4×10−22cm2) which have a strong overlap with the I85→I65 ground state absorption. On the other hand, it was established that the excited state absorption transition I65→S25 had a weak overlap with ground state absorption. Using numerical solution of the rate equations, we show that Ho3+-doped fluoride fiber lasers employing pumping at 1100nm rely on excited state absorption from the lowest excited state of Ho3+ to maintain a population inversion and that energy transfer upconversion processes compete detrimentally with the excited state absorption processes in concentrated Ho3+-doped fluoride glass.
High-resolution liquid- and solid-state nuclear magnetic resonance of nanoliter sample volumes using microcoil detectors
Authors: Kentgens A. P. M. , Bart J. , van Bentum P. J. M. , Brinkmann A. , van Eck E. R. H. , Gardeniers J. G. E. , Janssen J. W. G. , Knijn P. , Vasa S. , Verkuijlen M. H. W.
Published: January 2008

The predominant means to detect nuclear magnetic resonance (NMR) is to monitor the voltage induced in a radiofrequency coil by the precessing magnetization. To address the sensitivity of NMR for mass-limited samples it is worthwhile to miniaturize this detector coil. Although making smaller coils seems a trivial step, the challenges in the design of microcoil probeheads are to get the highest possible sensitivity while maintaining high resolution and keeping the versatility to apply all known NMR experiments. This means that the coils have to be optimized for a given sample geometry, circuit losses should be avoided, susceptibility broadening due to probe materials has to be minimized, and finally the B1-fields generated by the rf coils should be homogeneous over the sample volume. This contribution compares three designs that have been miniaturized for NMR detection: solenoid coils, flat helical coils, and the novel stripline and microslot designs. So far most emphasis in microcoil research was in liquid-state NMR. This contribution gives an overview of the state of the art of microcoil solid-state NMR by reviewing literature data and showing the latest results in the development of static and micro magic angle spinning (microMAS) solenoid-based probeheads. Besides their mass sensitivity, microcoils can also generate tremendously high rf fields which are very useful in various solid-state NMR experiments. The benefits of the stripline geometry for studying thin films are shown. This geometry also proves to be a superior solution for microfluidic NMR implementations in terms of sensitivity and resolution.
Low frequency electrostatic and electromagnetic modes in nonuniform cold quantum plasmas
Authors: Saleem H. , Ahmad Ali , Khan S. A.
Published: January 2008

The low frequency electrostatic and electromagnetic linear modes in a nonuniform cold quantum electron-ion plasma are studied. The effect of stationary dust on an electrostatic mode is also investigated. The quantum corrections in the linear dispersion relations of a cold dense plasma are presented with possible applications.
Time-resolved photoluminescence of type-II quantum dots and isoelectronic centers in Zn–Se–Te superlattice structures
Authors: Cheung M. C.-K. , Cartwright A. N. , Sellers I. R. , McCombe B. D. , Kuskovsky I. L.
Published: January 2008

Spectrally and time-resolved photoluminescence of a ZnTe∕ZnSe superlattice reveals a smooth transition of the photoluminescence (PL) lifetime from ∼100ns at 2.35eV to less than a few nanoseconds at 2.8eV. The significant increase of the lifetime in the low energy region is strong evidence to support the formation of type-II quantum dots (QDs), since in these nanostructures the spatial separation of carriers is increased. The shorter lived emission above 2.5eV is attributed to excitons bound to Te isoelectronic centers in the ZnSe matrix. The smooth transition of the PL lifetime confirms that clusters of these Te atoms evolve into type-II ZnTe∕ZnSe QDs.
Multiple exciton generation and electrical extraction from a PbSe quantum dot photoconductor
Authors: Kim Sung Jin , Kim Won Jin , Sahoo Yudhisthira , Cartwright Alexander N. , Prasad Paras N.
Published: January 2008

Multiple exciton generation and subsequent electrical extraction from a thin film photoconductive device constructed from PbSe nanocrystal quantum dots are demonstrated. The hydrazine treatment of the PbSe film drastically improves the conductivity of the film while maintaining excellent optical and structural film quality. The effects of multiple exciton generation and electrical extraction (electrons collected per photon absorbed) were quantified as a function of incident photon energy from 1.55to3.1eV. The multiple carrier extraction (>100%) was observed at photon energies greater than 2.8 times of the quantum dot bandgap with ∼210% measured at 4.4 times the bandgap.
Measuring the electrical characteristics of individual junctions in the SnO2 capped ZnO nanowire arrays on Zn substrate
Authors: Liu Y. , Wang S. , Zhang Z. Y. , Peng L.-M. , Shi L. , Li Quan
Published: January 2008

Direct measurements on electrical characteristics have been carried out in situ inside a scanning electron microscope using a multiple nanoprobe system on individual SnO2 capped ZnO nanowires (NWs) within a NW film on a Zn substrate. It is shown that while good Ohmic contacts can be made at Zn–ZnO NW and ZnO NW–SnO2 cap (when heavily doped with Zn) junctions, the overall I-V characteristics of the Zn–ZnO–SnO2 junction system differ significantly among different NWs, suggesting doping inhomogeneity in the NW film.
Magnetic molecules made of nitrogen or boron-doped fullerenes
Authors: Yang Chih-Kai
Published: January 2008

By using density functional theory, we investigate the electronic structure of a fullerene C60 molecule doped with a nitrogen atom on its surface. We find that the impurity is strongly bonded to the carbon cage and the C60N molecule has a large magnetic moment of 3.00μB. We also study the adsorption of a boron atom on the fullerene. The bonding is not as strong but still provides a fairly stable structure. Most importantly, the C60B molecule possesses a magnetic moment of 0.99μB. Both molecular magnets should be useful for the application in magnetic detection, quantum information, and spintronics.
Terahertz detection in single wall carbon nanotubes
Authors: Fu K. , Zannoni R. , Chan C. , Adams S. H. , Nicholson J. , Polizzi E. , Yngvesson K. S.
Published: January 2008

It is reported that terahertz radiation from 0.69to2.54THz has been sensitively detected in a device consisting of bundles of carbon nanotubes containing single wall metallic carbon nanotubes, quasioptically coupled through a lithographically fabricated antenna, and a silicon lens. The measured data are consistent with a bolometric detection process in the metallic tubes and the devices show promise for operation well above 4.2K.
Investigation of Zr–Si–N∕Zr bilayered film as diffusion barrier for Cu ultralarge scale integration metallization
Authors: Wang Ying , Cao Fei , Liu Yun-tao , Ding Ming-Hui
Published: January 2008

The effectiveness of ZrSiN∕Zr bilayered films to serve as diffusion barriers in Cu∕Si contacts has been investigated. Annealing studies for Cu∕ZrSiN∕Zr∕Si contact systems were carried out in N2∕H2(10%) ambient. X-ray diffraction data suggest that Cu film has preferential (111) crystal orientation and Cu silicide cannot be observed up to 700°C. Scanning electron microscopy micrographs show that the Cu film was integrated and free from agglomeration after annealing at 700°C. Auger electron spectroscopy depth profiles of the Cu∕ZrSiN(10nm)∕Zr(20nm)∕Si samples have no noticeable change except Zr silicide grows with annealing temperature up to 700°C. The results indicate excellent barrier property for ZrSiN(10nm)∕Zr(20nm) bilayer structure for Cu metallization.
Electron tomography of embedded semiconductor quantum dot
Authors: Inoue Tomoya , Kita Takashi , Wada Osamu , Konno Mitsuru , Yaguchi Toshie , Kamino Takeo
Published: January 2008

We performed an electron tomography for a single InAs quantum dot (QD) embedded in GaAs. A comprehensive three-dimensional image of indium distribution has been reconstructed by using a high-angle annular dark-field scanning transmission electron microscope. This was achieved by using a special nanopillar specimen prepared by a focused ion beam technique. The real structure of the embedded single QD has been found to have a complicated anisotropic structure reflecting the QD structure before being capped.
A visual indication of environmental humidity using a color changing hologram recorded in a self-developing photopolymer
Authors: Naydenova Izabela , Jallapuram Raghavendra , Toal Vincent , Martin Suzanne
Published: January 2008

A reflection hologram for visual indication of environmental humidity has been studied. The hologram is recorded in a self-developing photopolymer and changes color when exposed to a change in humidity and is fully reversible. The range of color change, reversibility, and the response time of the hologram have been studied in a controlled humidity environment. Fully reversible holograms with response times from few seconds to tens of minutes have been designed. Extremely sensitive bright visual humidity indicators, capable of dramatic color change within a few seconds of breathing on them are demonstrated.
Wavefront Engineering for Photolithography
Authors: Levenson Marc D.
Published: January 2008

Physicists have always been fascinated by the very small. These days, the inner structures of everyday items such as the personal computer fall into that size category and warrant our interest. The critical dimensions of individual features of state-of-the-art memory chips are now as small as 500 nanometers and are getting smaller. The microwave transistors in some satellite dish receivers require gates smaller than a quarter of a micron.
Multiparticle Interferometry and the Superposition Principle
Authors: Greenberger Daniel M. , Horne Michael A. , Zeilinger Anton
Published: January 2008

Discussing the particle analog of Thomas Young's classic double-slit experiment, Richard Feynman wrote in 1964 that it “has in it the heart of quantum mechanics. In reality, it contains the only mystery.” That mystery is the one-particle superposition principle. But Feynman's discussion and statement have to be generalized. Superposition may be the only true quantum mystery, but in multiparticle systems the principle yields phenomena that are much richer and more interesting than anything that can be seen in one-particle systems.
Photovoltaics: Unlimited Electrical Energy from the Sun
Authors: Stone Jack L.
Published: January 2008

The first practical solar cell was developed at Bell Laboratories in 1954. With the advent of the space program, photovoltaic cells made from semiconductor-grade silicon quickly became the power source of choice for use on satellites. The systems were very reliable, and cost was of little concern. In the early 1970s, the disruption of oil supplies to the industrialized world led to serious consideration of photovoltaics as a terrestrial power source. This application focused research attention on improving performance, lowering costs and increasing reliability. These three issues remain important today even though researchers have made extraordinary progress over the years. This article details that progress.
Boltzmann's Entropy and Time's Arrow
Authors: Lebowitz Joel L.
Published: January 2008

Given the success of Ludwig Boltzmann's statistical approach in explaining the observed irreversible behavior of macroscopic systems in a manner consistent with their reversible microscopic dynamics, it is quite surprising that there is still so much confusion about the problem of irreversibility. (See figure 1.) I attribute this confusion to the originality of Boltzmann's ideas: It made them difficult for some of his contemporaries to grasp. The controversies generated by the misunderstandings of Ernst Zermelo and others have been perpetuated by various authors. There is really no excuse for this, considering the clarity of Boltzmann's later writings. Since next year, 1994, is the 150th anniversary of Boltzmann's birth, this is a fitting moment to review his ideas on the arrow of time. In Erwin Schrödinger's words, “Boltzmann's ideas really give an understanding” of the origin of macroscopic behavior. All claims of inconsistencies that I know of are, in my opinion, wrong; I see no need for alternate explanations. for further reading I highly recommend Boltzmann's works as well as references 2–7. (See also PHYSICS TODAY, January 1992, page 44, for a marvelous description by Boltzmann of his visit to California in 1906.)
The 1993 International Physics Olympiad
Authors: Levi Barbara Goss , Eisenkraft Arthur
Published: January 2008

The US played host in July to the XXIV International Physics Olympiad, in which 201 top high school physics students from 41 nations competed in theoretical and experimental exams. Although the weeklong event, held this year in Williamsburg, Virginia, naturally fosters some international rivalry, its purpose is just as much to build friendships around a common interest in physics. The US organizing committee worked particularly hard to promote these friendships (see the summary on pages 42–43 by Arthur Eisenkraft, the executive director of the XXIV Olympiad.)
Where Can One Hope to Profitably Apply the Ideas of Chaos?
Authors: Ruelle David
Published: January 2008

The success of the ideas of chaos has led to attempts to apply them to a great variety of situations. This is in principle a good strategy, but the results are not always up to expectations. In some cases the results are predictably of little interest. Suppose you have concocted a mathematical model in biology or economics; you put this model on your computer and you discover a Feigenbaum period-doubling cascade, which is often a sign that chaos is present. Is this result interesting? Well, probably not. One reason is that the detailed dynamical properties of your model may not have anything to do with the properties of the real-life system. Another reason why your discovery may be without interest is that the occurrence of a Feigenbaum cascade need not have any particular biological or economic significance: You still have to address the problem of the relevance of your finding for biology or economics.
Advanced Accelerator Concepts
Authors: Wurtele Jonathan S.
Published: January 2008

High-energy accelerators have been physicists’ main tools for exploring the building blocks of matter for more than 60 years. During this time the particle energy has increased exponentially as a result of a combination of improvements in existing machines and the invention of new acceleration techniques. Historically, whenever a given type of accelerator has reached the limit of its performance, an innovative idea for particle manipulation, storage, cooling or acceleration has made possible experiments at ever higher energies. The tremendous increase in the energy of accelerators has not, however, been without an increase in capital costs. The cancellation of the Superconducting Super Collider makes timely an examination of possible alternative concepts for investigating some of the same physics.
Recharging the Energy Laboratories
Authors: Shank Charles V.
Published: January 2008

Despite their legacy of research achievement, our national energy laboratories are at the threshold of a sea change, brought on by the end of the cold war and by domestic economic pressures. New attention is being paid to America's ability to compete economically in the world marketplace, and questions are naturally being raised about how the nation invests its research dollars. As industrial labs, including those of IBM, AT&T, Exxon and General Electric, continue to be eliminated, downsized or refocused to meet corporate financial objectives, new expectations are being raised that would have national laboratories work in partnership with the private sector to improve the competitiveness of American industry. The challenge is no less than to reverse what some see as an erosion of the US technology base, using the investment made over the past decades in the national laboratories.
Of Love, Physics and Other Passions: The Letters of Albert and Mileva
Authors: Holton Gerald
Published: January 2008

The popular image of the scientist as “dispassionate,” and therefore somehow less than human, is undoubtedly bolstered by the convention according to which modern scientists remove discussion of their personal hopes and desires as much as possible from the publication of their research results. There is at least one good reason for the appearance of impersonality in the scientific literature: It minimizes obstacles against building consensus and thus hastens the conversion of private science into public science.
New Ways of Looking at US Science and Technology
Authors: Brown George E.
Published: January 2008

It is curious how I have gained a reputation as a curmudgeonly commentator on science and technology issues. I like to think that the term “curmudgeon” is inappropriate, because I believe I'm just more outspoken and possibly more realistic than most self-appointed gurus in the embattled field of science and technology policy. So to maintain my status, I will stick my neck out to speculate on some trends and patterns in science and technology as well as in higher education.
Of Love, Physics and Other Passions: The Letters of Albert and Mileva (Part 2)
Authors: Holton Gerald
Published: January 2008

Few plans can be more romantic than when two young lovers agree to meet in springtime at sunny Lake Como for a long excursion into the Alps above. The year was 1901. Albert Einstein, 22 years of age, and Mileva Marić, some three years older, had first met as fellow students at the Swiss Federal Polytechnic in Zurich. Now they were leaving their troubles behind them—Mileva having failed her important examination, Albert still without the certainty of finding a job, least of all at a university. But they were united in the promise of a life devoted to each other and to the study of physics. (See part 1 of this article, in last month's issue, page 23.)
Adaptive Optics in Astronomy
Authors: Thompson Laird A.
Published: January 2008

Optical observations by ground-based astronomers have long been limited by the distorting effects of the Earth's atmosphere. Primary mirrors have been polished to exquisite accuracy for telescopes with apertures as large as 10 meters, but at optical wavelengths these can deliver an angular resolution typically no better than that of a 25-cm telescope, as atmospheric turbulence deforms the image on a millisecond time scale. One (highly expensive) approach to overcome this problem has been to loft instruments such as the Hubble Space Telescope above the atmosphere. Another approach, pursued by instrument builders in the astronomy community and their counterparts in the military, has been to design electro-optical systems that measure and undo the effects of clear-air turbulence in real time. (See figure 1.) A number of such adaptive optic devices have already been built and operated on large ground-based telescopes, delivering near-diffraction-limited performance at infrared and visible wavelengths. With the first significant astronomical images beginning to appear from these adaptive optics systems (see the cover of this issue), the level of interest in this work is rising very rapidly in the astronomy community.
The Primary Steps of Photosynthesis
Authors: Fleming Graham R. , Grondelle Rienk van
Published: January 2008

Photosynthesis, the process by which plants convert solar energy into chemical energy, results in about 10 billion tons of carbon entering the biosphere annually as carbohydrate—equivalent to about eight times mankind's energy consumption in 1990. The apparatus used by plants to perform this conversion is both complex and highly efficient. Two initial steps of photosynthesis—energy transfer and electron transfer—are essential to its efficiency: Molecules of the light-harvesting system transfer electronic excitation energy to special chlorophyll molecules, whose role is to initiate the directional transfer of electrons across a biological membrane; the electron transfer, which takes place in a pigment-protein complex called the reaction center, then creates a potential difference that drives the subsequent biochemical reactions that store the energy. (Higher plants use two different reaction centers, called photosystems I and II, while purple bacteria make do with a single reaction center. The difference is that the bacteria do not generate oxygen in the photosynthetic process.) Both the elementary energy transfer and the primary electron transfer are ultrafast (occurring between 10−13 and 10−12 seconds), leading to the trapping of excitation energy at the reaction center (on a 100-picosecond timescale) and subsequent electron transfer in about 3 picoseconds with almost 100% quantum yield.
Relativistic Heavy-Ion Physics Without Nuclear Contact
Authors: Bertulani Carlos , Baur Gerhard
Published: January 2008

An increasing number of physicists are investigating nuclear collisions at relativistic energies. (See figure 1.) Accelerators completely devoted to the study of these collisions (such as the Relativistic Heavy Ion Collider at Brookhaven National Laboratory) are under construction. So are hadron colliders (such as the Large Hadron Collider at CERN), which can also be used to accelerate heavy ions. The aim of these projects is to study what happens to nuclear matter at high pressures and temperatures. The expectation is that such experiments will access information that can test important predictions of quantum chromodynamics—for example, a nuclear matter transition from a mixture of quarks and gluons to hadrons, as occurred in the first moments of the universe according to the Big Bang theory.
Is Physics Education Adapting to a Changing World?
Authors: Wolf Werner P.
Published: January 2008

The future of education, research and employment in physics has been under much discussion lately. Contrary to many glowing predictions in the mid-1980s, physicists graduating today face a severe shortage of jobs. Recalling comments such as one made by Malcolm Beasley and Lawrence Jones (PHYSICS TODAY, June 1986, page 36) that “It appears that the overall demand will more and more exceed the supply in the coming decade” reminds us just how much the situation has changed. Many solutions are being proposed. Articles relating to jobs and offering employment advice proliferate in the publications of our scientific societies. The situation is similar in other countries.
Special Issue: Physics and the Environment
Authors: Socolow Robert H.
Published: January 2008

Planet Earth is too small. Our activities as a species are constrained by our environment in more complex and subtle ways than we understood only a few decades ago. We have been living within fences and only now have found these fences. What a loss of innocence!
Clouds and Their Effects on the Climate System
Authors: Kiehl Jeffrey T.
Published: January 2008

The balance between incoming solar radiation and outgoing infrared radiation determines to a significant degree the Earth's climate. Clouds play a major role in determining the net radiative balance, so any change in cloud coverage or optical properties leads to a new climate state. To study the present and future climate states, the most comprehensive numerical tools currently available are global climate models. A major challenge in these models of Earth's environment is an accurate representation of clouds. (See figure 1.)
Nonlinear Optics in Semiconductors
Authors: Garmire Elsa
Published: January 2008

The dream of the all-optical processing of information has motivated efforts over the past decade to create sensitive semiconductors that have large optical nonlinearities. Semiconductors have been the medium of choice in part because a well-developed materials technology has already been built around them. One can design semiconductor structures easily and predict their performance accurately.