bluuebunny/crossref_metadata_2025 huggingface dataset:
Adsorption of amino acids and nucleic acid bases onto minerals: a few suggestions for prebiotic chemistry experiments
Authors: Zaia Dimas A.M.
Published: June 2012

Amino acids and nucleic acid bases are very important for the living organisms. Thus, their protection from decomposition, selection, pre-concentration and formation of biopolymers are important issues for understanding the origin of life on the Earth. Minerals could have played all of these roles. This paper discusses several aspects involving the adsorption of amino acids and nucleic acid bases onto minerals under conditions that could have been found on the prebiotic Earth; in particular, we recommend the use of minerals, amino acids, nucleic acid bases and seawater ions in prebiotic chemistry experiments. Several experiments involving amino acids, nucleic acid bases, minerals and seawater ions are also suggested, including: (a) using well-characterized minerals and the standardization of the mineral synthesis methods; (b) using primary chondrite minerals (olivine, pyroxene, etc.) and clays modified with metals (Cu, Fe, Ni, Mo, Zn, etc.); (c) determination of the possible products of decomposition due to interactions of amino acids and nucleic acid bases with minerals; (d) using minerals with more organophilic characteristics; (e) using seawaters with different concentrations of ions (i.e. Na+, Ca2+, Mg2+, SO42− and Cl−); (f) using non-protein amino acids (AIB, α-ABA, β-ABA, γ-ABA and β-Ala and g) using nucleic acid bases other than adenine, thymine, uracil and cytosine. These experiments could be useful to clarify the role played by minerals in the origin of life on the Earth.
Raman spectroscopy and the search for life signatures in the ExoMars Mission
Authors: Edwards Howell G.M. , Hutchinson Ian B. , Ingley Richard
Published: June 2012

The survival strategies of extremophilic organisms in terrestrially stressed locations and habitats are critically dependent on the production of protective chemicals in response to desiccation, low wavelength radiation insolation, temperature and the availability of nutrients. The adaptation of life to these harsh prevailing conditions involves the control of the substratal geology; the interaction between the rock and the organisms is critical and the biological modification of the geological matrix plays a very significant role in the overall survival strategy. Identification of these biological and biogeological chemical molecular signatures in the geological record is necessary for the recognition of the presence of extinct or extant life in terrestrial and extraterrestrial scenarios. Raman spectroscopic techniques have been identified as valuable instrumentation for the detection of life extra-terrestrially because of the use of non-invasive laser-based excitation of organic and inorganic molecules, and molecular ions with high discrimination characteristics; the interactions effected between biological organisms and their environments are detectable through the molecular entities produced at the interfaces, for which the vibrational spectroscopic band signatures are unique. A very important attribute of Raman spectroscopy is the acquisition of molecular experimental data non-destructively without the need for chemical or mechanical pre-treatment of the specimen; this has been a major factor in the proposal for the adoption of Raman instrumentation on robotic landers and rovers for planetary exploration, particularly for the forthcoming European Space Agency (ESA)/National Aeronautics and Space Administration (NASA) ExoMars mission. In this paper, the merits of using Raman spectroscopy for the recognition of key molecular biosignatures from several terrestrial extremophile specimens will be illustrated. The data and specimens used in this presentation have been acquired from Arctic and Antarctic cold deserts and a meteorite crater, from which it will be possible to assess spectral data relevant for the detection of extra-terrestrial extremophilic life signatures.
A resonating rainfall and evaporation recorder
Authors: Stewart Ryan D. , Hut Rolf , Rupp David E. , Gupta Hoshin , Selker John S.
Published: June 2012

We propose a novel, accurate quantification of precipitation and evaporation, as needed to understand fundamental hydrologic processes. Our system uses a collection vessel placed on top of a slender rod that is securely fixed at its base. As the vessel is deflected, either by manual perturbation or ambient forcing (for example, wind), its oscillatory response is measured, here by a miniature accelerometer. This response can be modeled as a damped mass‐spring system. As the mass of water within the collection vessel changes, either through the addition of precipitation or by evaporative loss, the resonant frequency experiences an inverse shift. This shift can be measured and used to estimate the change in the mass of water. We tested this concept by creating a simple prototype which was used in field conditions for a period of 1 month. The instrument was able to detect changes in mass due to precipitation with an accuracy of approximately 1 mm.
Satellite mission monitors ocean surface salinity
Authors: Lagerloef Gary
Published: June 2012

Understanding the links between ocean circulation, the global water cycle, and climate variations requires knowledge of ocean surface salinity. NASA's Aquarius satellite mission (http://aquarius.nasa.gov), which monitors the global open ocean surface salinity field, embarked on its science operations phase after completing the in‐orbit performance assessment on 1 December 2011. The data (Figure 1) are already showing new and interesting information.
AGU's response to the Next Generation Science Standards
Authors: Adamec Bethany Holm , Passow Michael , Asher Pranoti
Published: June 2012

A team of expert writers and lead states, headed by the nonproft education reform organization Achieve (http://www.achieve.org), is in the process of developing the Next Generation Science Standards (NGSS). As a critical stakeholder with a strategic interest in talent pool development, AGU has been interested and involved in the development of the standards since early in the process. AGU members such as Michael Wysession and Ramon Lopez are on the writing team. AGU members and the public are encouraged to read and comment on the standards at http://www.nextgenscience.org/. The frst draft comment period recently closed; however, a comment period on the second draft will open in fall 2012.
Dunne receives 2011 G. K. Gilbert Award: Response
Authors: Dunne Thomas
Published: June 2012

Thomas Dunne received the 2011 G. K. Gilbert Award at the 2011 AGU Fall Meeting, held 5–9 December in San Francisco, Calif. The award recognizes “a scientist who has either made a single significant advance or sustained significant contributions to the field of Earth and planetary surface processes, and who has in addition promoted an environment of unselfish cooperation in research and the inclusion of young scientists into the field.”
High‐resolution record of geomagnetic excursions in the Matuyama chron constrains the ages of the Feiliang and Lanpo Paleolithic sites in the Nihewan Basin, North China
Authors: Ao Hong , An Zhisheng , Dekkers Mark J. , Wei Qi , Pei Shuwen , Zhao Hui , Zhao Hongli , Xiao Guoqiao , Qiang Xiaoke , Wu Dacheng , Chang Hong
Published: June 2012

The Nihewan Basin (40°N) in North China is a rich source of Early Pleistocene Paleolithic sites and thus a key area for studying early human evolution in high‐latitude (from an early human perspective) East Asia. Here a high‐resolution magnetostratigraphic investigation is carried out on a fluvio‐lacustrine section in the northeastern Nihewan Basin, which contains the Feiliang and Lanpo Paleolithic sites. Paleomagnetic results suggest that this section records the lower portion of the Brunhes polarity chron and the upper Matuyama polarity chron. Furthermore, the Jaramillo polarity subchron and seven of the nine validated geomagnetic excursions within the Matuyama polarity chron are identified, including the Kamikatsura, Santa Rosa, Intra‐Jaramillo, Cobb Mountain, Bjorn, Gardar and Gilsa excursions. The Feiliang artifact layer is located just at the bottom of the Cobb Mountain excursion, thus its age is estimated to be ∼1.2 Ma. The Lanpo artifact layer appears to be coeval with the Gilsa excursion, yielding an estimated age of ∼1.6 Ma. This study provides new evidence for the presence of early humans in North China before 1.5 Ma and documents the powerful role of geomagnetic excursions: they provide valuable age control points for ongoing efforts to date the early Paleolithic sites.
Joint analysis of displacement time series retrieved from SAR phase and amplitude: Impact on the estimation of volcanic source parameters
Authors: Manconi Andrea , Casu Francesco
Published: June 2012

We study the impact of jointly analyzing the phase and amplitude information of a SAR dataset on the interpretation of the surface displacement over time in areas characterized by large dynamics. In particular, we compute the ground displacement time series over Fernandina and Sierra Negra calderas (Galápagos islands), by applying the Small BAseline Subset (SBAS) and the Pixel Offset (PO)‐SBAS techniques to 25 ENVISAT SAR images acquired between 2003 and 2007, when both volcanoes experienced catastrophic eruptions. By merging the SBAS and PO‐SBAS time series results the spatial density of measurements increased substantially. In addition, the joint analysis of the time series allows us to better constrain the temporal evolution of the magmatic source volume changes. Our results show that the joint use of the SBAS and PO‐SBAS approaches may help for a more correct evaluation of large deformation affecting the Earth's surface caused by eruptions, earthquakes, and landslide phenomena.
An analysis of trends in the boreal winter mean tropospheric circulation during the second half of the 20th century
Authors: Greatbatch R. J. , Gollan G. , Jung T.
Published: June 2012

A relaxation technique applied to the ECMWF model is used to analyse 11, 21 and 31 year trends in the boreal winter mean 500 hPa North Atlantic Oscillation (NAO), Pacific North America pattern (PNA) and Southern Annular Mode (SAM) indices. For the PNA, the results indicate a strong influence from the tropics on all time scales, whereas for the NAO, the stratosphere is important on time scales of 11 and 21 but with an indication of feedback from extratropical sea surface temperature and sea‐ice (SSTSI) anomalies on the 11 year time scale. For the SAM, the tropics emerge as the most important influence. We find an influence from the stratosphere consistent with expectations based on ozone depletion, although no clear role for stratospheric forcing of the SAM is found in these experiments.
A 45‐year time series of dune mobility indicating constant windiness over the central Sahara
Authors: Vermeesch P. , Leprince S.
Published: June 2012

Although evidence is mounting that links global warming to changes in atmospheric dynamics over the Atlantic realm, similar studies over the African continent are lacking. And even if such models would exist, it would be difficult to verify their validity due to the paucity of meteorological observations and anemometers in the central Sahara. A pragmatic way around this problem is to monitor barchan dune velocity as a proxy for the windiness of desert areas. Dune migration rates are a measure of the amount of work done by the wind which does not require field measurements but can be observed from space instead. This paper presents a novel application of the remote sensing tool COSI‐Corr for the construction of time series of dune mobility from sequences of optical satellite imagery. The technique has been applied to the Bodélé Depression in northern Chad, to demonstrate that dune migration rates in the central Sahara have been remarkably constant for nearly half a century, leading us to conclude that wind velocities have not changed more than 0.2% per year over that period. It is therefore unlikely that the frequency and intensity of dust storms originating from this ‘hot spot’ has significantly changed over the past decades either.
Quasilinear theory of anisotropy‐beta relation for combined mirror and proton cyclotron instabilities
Authors: Yoon Peter H. , Seough Jungjoon
Published: June 2012

The solar wind and the Earth's magnetosheath are often characterized by proton temperature anisotropies that cannot be discussed by adiabatic fluid theory. An excessive perpendicular temperature anisotropy may result when the plasma undergoes compression. The proton temperature anisotropy, T⊥/T∥ > 1, leads to the proton cyclotron and mirror instabilities. Marginal stability conditions for these instabilities may be expressed as inverse correlations between T⊥/T∥ and parallel beta, β∥. In the literature, these correlations are constructed on the basis of linear theory, hybrid simulations, or observational fitting. The present paper makes use of quasilinear theory for the proton cyclotron and mirror instabilities. In such an approach the inverse correlation naturally emerges as the time‐asymptotic states of self‐consistent evolution. The inverse correlation thus constructed shows the predominance of proton cyclotron instability for lowβ∥ regime, while for high β∥ values, the mirror instability dictates the inverse correlation.
Plasma wake simulations and object charging in a shadowed lunar crater during a solar storm
Authors: Zimmerman M. I. , Jackson T. L. , Farrell W. M. , Stubbs T. J.
Published: June 2012

Within a permanently shadowed lunar crater the horizontal flow of solar wind is obstructed by upstream topography, forming a plasma wake that electrostatically diverts ions toward the crater floor and generates a surface potential that can reach kilovolts. In the present work kinetic plasma simulations are employed to investigate the morphology of a lunar crater wake during passage of a solar storm. Results are cast in terms of leading dimensionless ratios including the ion Mach number, ratio of crater depth to plasma Debye length, peak secondary electron yield, and electron temperature versus electron impact energy at peak secondary yield. This small set of ratios allows generalization to a much wider range of scenarios. The kinetic simulation results are fed forward into an equivalent‐circuit model of a roving astronaut. In very low‐plasma‐current environments triboelectric charging of the astronaut suit becomes effectively perpetual, representing a critical engineering concern for roving within shadowed lunar regions. Finally, simulated ion fluxes are used to explore sputtering and implantation processes within an idealized crater. It is suggested that the physics of plasma miniwakes formed in the vicinity of permanently shadowed topography may play a critical role in modulating the enigmatic spatial distribution of volatiles at the lunar poles.
Dimensionality effects on the magnetization reversal in narrow FePt nanowires
Authors: Nguyen V. D. , Vila L. , Marty A. , Pillet J. C. , Notin L. , Beigné C. , Pizzini S. , Attané J. P.
Published: June 2012

The magnetization reversal of 10 nm thick FePt nanowires has been studied for widths down to 30 nm. Above 500 nm, the magnetic domains grow within a dendritic structure. Below 300 nm, the reversal takes place by propagation of a single domain wall (DW), and the coercivity increases. Below 50 nm, the coercivity increase is such that a mix of nucleation and DW propagation appears. These results suggest that the reversal process is determined by the comparison of the wire dimensions with four characteristic lengths: the dendrite width, the disorder length, the mean edge roughness, and the nucleation distance.
Wavelength dependence on the space charge collection in CdZnTe detectors
Authors: Washington Aaron L. , Teague Lucile C. , Duff Martine C. , Burger Arnold , Groza Michael , Buliga Vladimir
Published: June 2012

The distribution of the internal electric field in Cd1−xZnxTe (CZT) materials has significant effects on the charge collection ability. Light exposure at various wavelengths is a relatively unexplored process that alters charge collection at the anode contact. The use of multiple wavelengths can target charge carriers at various trap energies and positions throughout the crystal. The controlled illumination increases charge collection by releasing trapped electron and hole carriers in the crystal despite differences in light energy. Our study presents the results from our investigation of the effect of external illumination of CZT on the internal electric field via the Pockels effect. The space charge collection is further analyzed based on location and intensity relative to the specific wavelength of illumination.
Structure and properties of superelastic hard carbon phase created in fullerene-metal composites by high temperature-high pressure treatment
Authors: Chernogorova O. , Drozdova E. , Ovchinnikova I. , Soldatov A. V. , Ekimov E.
Published: June 2012

Treatment of a fullerene soot extract and metal (Co) powder mixture under pressure of 5 and 8 GPa at 1000 °C leads to the transformation of fullerites into superelastic hard phase (SHP) and to simultaneous sintering of the powder mixture to nonporous composite material reinforced by the SHP particles. The structure of the SHP particles reveals a topological relation to the initial fullerite crystal morphology. Upon indentation, the SHP particles demonstrate an elastic recovery of up to 96%. The universal microhardness of the SHP particles HU = 26 GPa, and their microhardness HV = 35 GPa. A high ratio between the microhardness and elastic modulus (HV/E = 0.19-0.21) of the SHP particles makes them perspective candidates for design of materials with superior wear resistance and tribological properties.
High performance dye-sensitized solar cell based on hydrothermally deposited multiwall carbon nanotube counter electrode
Authors: Siriroj Sumeth , Pimanpang Samuk , Towannang Madsakorn , Maiaugree Wasan , Phumying Santi , Jarernboon Wirat , Amornkitbamrung Vittaya
Published: June 2012

Conductive glass was coated with multiwall carbon nanotubes (MWCNTs) by a hydrothermal method. MWCNTs films were subsequently used as dye-sensitized solar cell (DSSC) counter electrodes. The performance of hydrothermal MWCNT DSSC was ∼2.37%. After film annealing in an Ar atmosphere, annealed-hydrothermal MWCNT (AHT-CNT) DSSC efficiency was significantly increased to ∼7.66%, in comparison to ∼8.01% for sputtered-Pt DSSC. Improvement of AHT-CNT DSSC performance is attributed to a decrease in charge-transfer resistance from 1500 Ω to 30 Ω as observed by electrochemical impedance spectroscopy.
Thermal conductivity of argon at high pressures and high temperatures
Authors: Goncharov Alexander F. , Wong Michael , Allen Dalton D. , Ojwang J. G. O. , Struzhkin Viktor V. , Konôpková Zuzana , Lazor Peter
Published: June 2012

Knowledge of the thermal conductivity of Ar under conditions of high pressures and temperatures (P-T) is important for model calculations of heat transfer in the laser heated diamond anvil cell (DAC) as it is commonly used as a pressure transmitting medium and for thermal insulation. We used a modified transient heating technique utilizing microsecond laser pulses in a symmetric DAC to determine the P-T dependent thermal conductivity of solid Ar up to 50 GPa and 2500 K. The temperature dependent thermal conductivity of Ar was obtained by fitting the results of finite element calculations to the experimentally determined time dependent temperature of a thin Ir foil surrounded by Ar. Our data for the thermal conductivity of Ar are larger than that theoretically calculated using the Green-Kubo formalism, but they agree well with those based on kinetic theory. These results are important for ongoing studies of the thermal transport properties of minerals at pressures and temperatures native to the mantle and core.
Pressure induced structural transitions in KH, RbH, and CsH
Authors: Hooper James , Baettig Pio , Zurek Eva
Published: June 2012

The heavier alkali metal hydrides MH (M = K, Rb, Cs) undergo a series of pressure induced structural phase transitions: B1 (NaCl) → B2 (CsCl) → CrB. Experiments reveal that the latter occurs at 85 and 17.5 GPa for RbH and CsH, but it has not yet been observed for KH. Herein, evolutionary algorithms coupled with density functional theory calculations are employed to explore the potential energy surface of the aforementioned hydrides up to pressures of 300 GPa. The computations support previous theoretical work which predicts that KH will adopt the CrB structure when compressed. In addition, for KH and RbH we find configurations with Pnma and I41/amd symmetry that are thermodynamically competitive with the CrB structure at 300 GPa. Between 100–150 GPa, a Pnma structure which is analogous to a high-pressure form of CsI is found to be the most stable phase for the heaviest alkali hydride considered. At higher pressures a hitherto unknown CsH–P63/mmc arrangement becomes thermodynamically preferred up to at least 400 GPa. A detailed analysis of the geometric and electronic structures of the various phases is provided.
Pressure-induced phase transition in potassium azide up to 55 GPa
Authors: Ji Cheng , Zheng Richard , Hou Dongbin , Zhu Hongyang , Wu Jianzhe , Chyu Ming-Chien , Ma Yanzhang
Published: June 2012

Potassium azide was investigated by Raman scattering spectroscopy up to a pressure of 55.0 GPa by use of diamond anvil cell at room temperature. A pressure-induced reversible phase transition was revealed. The onset of the phase transition was characterized by the hardening of a previously soft lattice mode at 13.6 GPa. This transition is considered a structural phase transition. Compression induces a symmetry reduction, which is indicated by the splitting of the librational modes, the development of infrared active vibrational modes, and the appearance of other new modes in the external mode region. The new high-pressure phase, with azide ions still in a molecular state, can be preserved down to 1.2 GPa. The Grüneisen parameters for the parent phase were calculated.
The structural stability of AlPO4-5 zeolite under pressure: Effect of the pressure transmission medium
Authors: Lv Hang , Yao Mingguang , Li Quanjun , Liu Ran , Liu Bo , Lu Shuangchen , Jiang Linhai , Cui Wen , Liu Zhaodong , Liu Jing , Chen Zhiqiang , Zou Bo , Cui Tian , Liu Bingbing
Published: June 2012

The structural stability of AlPO4-5 zeolite (AFI) has been studied as a function of pressure up to 34.4 GPa in a diamond anvil cell by using synchrotron x-ray diffraction. It is found that the AFI structural stability can be enhanced significantly when a mixture of silicone oil and liquid nitrogen is used as pressure transmission medium (PTM). In this case, the crystalline-to-amorphous transition pressure for AFI increased to be 15.9 GPa, much higher than that of 8.5 GPa observed in the experiment by using silicone oil as PTM. We found that the average distance of the interplanar crystal spacing along to most planes was expanded obviously when liquid nitrogen is used as one component in the PTM. The presence of liquid nitrogen in the PTM also affects the structural evolution of the AFI channel under pressure. The results demonstrated that nitrogen molecules can be inserted into the channels of porous zeolite AFI single crystals, exerting a supporting effect against the structure collapse of AFI and thus improving their structural stability.
Tetragonal to orthorhombic phase transition of ammonia borane at low temperature and high pressure
Authors: Najiba Shah , Chen Jiuhua , Drozd Vadym , Durygin Andriy , Sun Yongzhou
Published: June 2012

The effect of pressure on the low temperature tetragonal (I4mm) to orthorhombic (Pmn21) phase transition of a potential hydrogen storage compound ammonia borane (NH3BH3) was investigated in diamond anvil cell using Raman spectroscopy. With applied pressure, the transition occurs at higher temperature, which indicates that pressure enhances the ordering of the structure. The positive Clapeyron slope of the transition was determined to be dP/dT = ∼25.7 MPa/K, indicating the transformation is of exothermic. Appearance of some of the characteristic Raman modes of orthorhombic phase requires undercooling of around ∼15 K below the transition, indicating possible existence of an intermediate phase.
Maintaining viability of white clover under very high pressure
Authors: Nishihira N. , Iwasaki T. , Shinpou R. , Hara A. , Ono F. , Hada Y. , Mori Y. , Takarabe K. , Saigusa M. , Matsushima Y. , Saini N. L. , Yamashita M.
Published: June 2012

The high pressure technique developed in physics may give a new possibility if it is applied to a biological study. We have been studying the tolerance of small living samples such as planktons and mosses, and found that all of them were alive after exposed to extremely high hydrostatic pressure of 7.5 GPa. This technique has been extended to a higher plant Trifolium lepens L. (white clover). A few seeds of white clover were exposed to 7.5 GPa for up to 6 days. After the pressure was released, they were seeded on agar, or directly on sowing soil. Seventeen out of the total 22 seeds exposed to the high pressure were found to be alive. Those exposed for up to 1 day and seeded on agar germinated roots. Those exposed for up to 1 h and seeded on soil germinated stems and leaves. The present technique has the possibility of being applied to improve breed of plants and to discover a very strong species that stands against very severe environmental conditions.
Similar behavior of thermoelectric properties of lanthanides under strong compression up to 20 GPa
Authors: Shchennikov Vladimir V. , Morozova Natalia V. , Ovsyannikov Sergey V.
Published: June 2012

We report results of investigations of thermoelectric properties of dozen of the lanthanides at ambient temperature under strong compression up to 15-20 GPa. We present data of pressure evolution of Seebeck coefficient in cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), and ytterbium (Yb) and review literature data for lanthanum (La). Peculiarities detected at pressure dependencies of thermoelectric power were addressed to the known structural transitions. Therefore, thermopower values of all high-pressure phases below 20 GPa were established. Seebeck effect in different lanthanides subjected to compression demonstrated similar behavior: monotonic rising from near-zero values, ±(1-2) μV/K to magnitudes of ∼+(8-10) μV/K beyond about 5 GPa. This tendency was noticed in those rare-earth elements that follow a commonly accepted sequence of structural transformations for this group, and hence the s→d electron transfer is considered as a possible reason for this similar behavior of thermoelectric properties. Distinctive behavior of Seebeck effect in cerium and ytterbium may be related to peculiar structural phase diagrams of these elements. Possible implications from the findings of this thermoelectric study are discussed.
Adsorption of selected gases on metal-organic frameworks and covalent organic frameworks: A comparative grand canonical Monte Carlo simulation
Authors: Wang Lili , Wang Lu , Zhao Jijun , Yan Tianying
Published: June 2012

The adsorption properties of H2, CO, NO, and NO2 in several typical nanoporous materials (covalent organic framework (COF)-105, COF-108, metal-organic framework (MOF)-5, and MOF-177) at 298 K were investigated by grand canonical Monte Carlo simulations. Good agreement between simulated results and experimental data has been achieved for H2 adsorption on MOF-5 and MOF-177, indicating the reliability of the theoretical approach. The simulated adsorption isotherms for these four gases show analogical trend, i.e., increasing nearly linearly with pressure. Among the four host materials, COF-108 exhibits the highest hydrogen uptake (∼0.89 wt. % at 100 bars) owing to its low densities and high surface area. The adsorption amounts of NO2 in these materials are higher than those of the other three gases because of the stronger gas-sorbent interaction. In particular, NO2 adsorption amount in MOF-177 can reach as high as 10.7 mmol/g at 298 K and 10 bars. The interaction between the four gases (H2, CO, NO, and NO2) and the COF/MOF adsorbents is further discussed in terms of the isosteric heat.
Non-contact characterization of graphene surface impedance at micro and millimeter waves
Authors: Gómez-Díaz J. S. , Perruisseau-Carrier J. , Sharma P. , Ionescu A.
Published: June 2012

The experimental characterization of the surface impedance of monolayer graphene at micro and millimeter wave frequencies is addressed. Monolayer graphene is transferred on a substrate stack, which is placed in the cross-section of a rectangular waveguide. In the fundamental mode, this setup is equivalent to a TE-polarized plane wave impinging under oblique incidence on an infinite graphene sheet, and similarly, the surface impedance of the graphene is a simple lumped element in a transmission-line model, that exactly represents the electromagnetic problem under study. Using this model, we propose a technique based on transmission matrices to accurately extract the surface impedance. The method is able to relax the influence of the substrates tolerances by taking advantage of the graphene infinitesimally small electrical thickness. It can also account for any gap between the sample and the test waveguide, thereby allowing to disregard graphene-metal contact resistance issues. The approach has been successfully applied to characterize graphene samples at X and Ka bands. The extracted surface impedances fit the Drude conductivity model in the absence of interband transitions, and provides a wideband frequency independent resistance (in the range of KΩs) and a weak inductive behavior (in the range of pHs).
Deep-level photoluminescence due to dislocations and oxygen precipitates in multicrystalline Si
Authors: Tajima Michio , Iwata Yasuaki , Okayama Futoshi , Toyota Hiroyuki , Onodera Hisashi , Sekiguchi Takashi
Published: June 2012

We have demonstrated the presence of a dislocation-related component and a component due to oxygen precipitates in a broad deep-level photoluminescence (PL) band in multicrystalline Si at room temperature. In PL intensity mapping, the lower-energy side of the deep-level PL band at about 0.79 eV appeared as a dark line along a small-angle grain boundary (SA-GB) surrounded by a bright band on either side, while the higher-energy side at about 0.87 eV as a bright line along the SA-GB. These intensity variations agree with the low-temperature PL intensity patterns for well-established dislocation-related lines of D1/D2 and those for oxygen precipitates, respectively. These patterns were observed around SA-GBs with a misorientation angle of 1–2°, and were assumed to be due to the distribution of secondary defects or impurities trapped by the strain field around dislocation clusters forming SA-GBs and that of preferential oxygen precipitation on the dislocations. A spectral component associated with the D3/D4 lines was also extractable from the deep-level PL at about 0.94 eV. The intensity increased on SA-GBs with the angle of <1°, where oxygen precipitation did not occur. This corresponds to the generally accepted idea that the D3/D4 lines are related to the intrinsic nature of dislocations.
Optical switching and related structural properties of epitaxial Ge2Sb2Te5 films
Authors: Gericke F. , Flissikowski T. , Lähnemann J. , Katmis F. , Braun W. , Riechert H. , Grahn H. T.
Published: June 2012

We investigate the optical switching process and the related structural properties of (GeTe)(Sb2Te3) epitaxial films close to Ge2Sb2Te5 composition on GaSb(001). While the amorphization process can take place in a single or in multiple steps, the re-crystallization process always takes place in multiple steps. Intermediate stages of the re-crystallization process are characterized by small crystalline islands within the amorphous area. The structural properties are investigated by optical microscopy and electron backscatter diffraction (EBSD) in a scanning electron microscope. The analysis of the EBSD pattern demonstrates that the crystalline islands at intermediate stages of the re-crystallization process exhibit different orientations. We conclude that the re-crystallization process is driven by nucleation without any orientation information from the substrate.
Thermoelectric properties of n-type PbSe revisited
Authors: Parker David , Singh David J. , Zhang Qinyong , Ren Zhifeng
Published: June 2012

It was recently predicted [D. Parker D. J. Singh, Phys. Rev. B 82, 035204 (2010)], contrary to then-prevalent opinion, that p-type PbSe would be a good thermoelectric material. This prediction was confirmed in Wang et al. [Adv. Mater. 23, 1367 (2011)], and recent experimental work (Zhang et al. Energy Env. Sci. 5, 5246 (2012)] now indicates that n-type PbSe may show good thermoelectric performance. In light of this work, we now re-examine the thermoelectric performance of n-type PbSe with a revised approximation (not available at the time of the original work) which improves band gap accuracy. We now find that n-type PbSe has large thermopowers characteristic of a high performance material, with values as high in magnitude as 250 μV/K at 1000 K and 300 μV/K at 800 K. We further find that optimal 1000 K n-type doping ranges are between 2 × 1019 cm−3 and 7 × 1019 cm−3, while at 800 K, the corresponding range is from 7 × 1018 to 4 × 1019 cm−3.
Coupling enhancement between monopole-type resonators using metamaterial cavity
Authors: Liou Chong-Yi , Kuo Chi-Jung , Lee Ming-Lung , Mao Shau-Gang
Published: June 2012

This work presents the strong coupling of monopole-type resonators using a metamaterial cavity with artificial magnetic conductor (AMC) surfaces and metal planes. The AMC surface is constructed by an array of metal patches on a double-layered dielectric substrate backed by a metallic plane, and its reflection phase with respect to the angle and the polarization of obliquely incident plane waves are characterized by the transmission-line model. The measured transmission power is −2 dB when the resonator spacing is 0.4λ. This demonstrates that the power transfer is enhanced significantly by metamaterial cavity compared to the metal cavity and free space.
Enhanced optical transmission through tapered metallic gratings
Authors: Shen Honghui , Maes Bjorn
Published: June 2012

To achieve non-resonant behavior, we propose a metallic grating device with linearly tapered slits. The tapering provides a gradual impedance variation from the entrance to the exit of the slits, leading to broadband and wide-angle enhanced transmission in the infrared. In addition, the light is strongly localized and enhanced at the slit exits, in contrast with straight slits. We describe the phenomenon with a transmission line model, which is in accordance with rigorous simulations.
InGaN/GaN single-quantum-well microdisks
Authors: Hsu Yu-Chi , Lo Ikai , Shih Cheng-Hung , Pang Wen-Yuan , Hu Chia-Hsuan , Wang Ying-Chieh , Chou Mitch M. C.
Published: June 2012

We have grown InxGa1−xN/GaN quantum wells atop GaN microdisk with γ-LiAlO2 substrate by using plasma-assisted molecular beam epitaxy. The structural and optical properties of the samples were analyzed by transmission electron microscopy, x-ray diffraction, cathodoluminescence, and photoluminescence measurements. Based on the measured results, we obtained the indium concentration of the InxGa1−xN/GaN single quantum well to be x = 0.25 with a band-gap energy of 2.31 eV, which is consistent with the bowing effect of bulk InxGa1−xN: Eg(x) = [3.42 − x * 2.65 − x * (1 − x) * 2.4] eV.
Very high open-circuit voltage of 5.89 V in organic solar cells with 10-fold-tandem structure
Authors: Zou Ye , Deng Zhenbo , Potscavage William J. , Hirade Masaya , Zheng Yanqiong , Adachi Chihaya
Published: June 2012

Organic solar cells (OSCs) based on chloroaluminum phthalocyanine (ClAlPc) as donor and fullerene C60 as acceptor with a multi-tandem structure were fabricated. We demonstrated very high open-circuit voltage (VOC) and enhanced power conversion efficiency (PCE) for the multi-tandem OSCs. Using a fivefold structure, we obtained PCE of 2.49% with a VOC of 3.50 V, in comparison with PCE of ∼2% and VOC of 0.72–0.81 V for the single device. We also fabricated a tenfold-stacked OSC showing an extremely high VOC of 5.89 V. The multi-tandem OSCs with very high VOC have great potential for applications in limited-area low-power electronics.
Efficient generation and guiding of megaampere relativistic electron current by silicon nanowires
Authors: Kumar Singh Prashant , Chatterjee Gourab , Lad Amit D. , Adak Amitava , Ahmed Saima , Khorasaninejad M. , Adachi M. M. , Karim K. S. , Saini S. S. , Sood A. K. , Ravindra Kumar G.
Published: June 2012

We demonstrate 30 times enhanced flux of relativistic electrons by a silicon nanowire coated target excited by 30 fs, 800 nm laser pulses at an intensity of 3 × 1018 W cm−2. A measurement of the megaampere electron current via induced megagauss magnetic field supports the enhancement feature observed in the electron energy spectrum. The relativistic electrons generated at the front of nanowire coated surface are shown to travel efficiently over 500 μm in the insulating substrate. The enhanced hot electron temperature is explained using a simple model and is supported by recent simulations.
Fabrication and characterization of InAlN/GaN-based double-channel high electron mobility transistors for electronic applications
Authors: Xue JunShuai , Zhang JinCheng , Zhang Kai , Zhao Yi , Zhang LinXia , Ma XiaoHua , Li XiaoGang , Meng FanNa , Hao Yue
Published: June 2012

In our previous work [J. S. Xue et al., Appl. Phys. Lett. 100, 013507 (2012)], superior electron-transport properties are obtained in InAlN/GaN/InAlN/GaN double-channel (DC) heterostructures grown by pulsed metal organic chemical vapor deposition (PMOCVD). In this paper, we present a detailed fabrication and systematic characterization of high electron mobility transistors (HEMTs) fabricated on these heterostructures. The device exhibits distinct DC behavior concerning with both static-output and small-signal performance, demonstrating an improved maximum drain current density of 1059 mA/mm and an enhanced transconductance of 223 mS/mm. Such enhancement of device performance is attributed to the achieved low Ohmic contact resistance as low as 0.33 ± 0.05 Ω·mm. Moreover, very low gate diode reverse leakage current is observed due to the high quality of InAlN barrier layer deposited by PMOCVD. A current gain frequency of 10 GHz and a maximum oscillation frequency 21 GHz are also observed, which are comparable to the state-of-the-art AlGaN/GaN-based DC HEMT found in the literature. The results demonstrate the great potential of PMOCVD for application in InAlN-related device’s epitaxy.
Electronic and quantum phase coherence properties of bismuth thin films
Authors: Rudolph M. , Heremans J. J.
Published: June 2012

We present a method to deposit bulk-like Bi films by thermal evaporation and study the electrical, quantum coherence, and physical properties. A two stage growth procedure was found to optimize the film properties, with an initial wetting layer deposited at lower temperature followed by an active layer at higher temperature. Transport measurements indicate carrier properties comparable to molecular beam epitaxial films and display weak-antilocalization, from which the quantum phase coherence lengths are deduced. A 76 nm film is found to optimally exhibit both bulk-like Bi characteristics and the 2-dimensional quantum coherence properties desired for Bi-based quantum devices.
Spin dynamics of magnetic nanostructures investigated by micromagnetic simulations
Authors: Rückriem R. , Krone P. , Schrefl T. , Albrecht M.
Published: June 2012

Micromagnetic simulations were performed to investigate the spin dynamics of cylindrical magnetic nanostructures of various sizes down to 20 nm. The presented simulation technique provides information on the equilibrium states, magnetization precession, and spatial distributions of excited spin wave modes of individual nanostructures. Larger cylindrical nanostructures reveal a flower state magnetization configuration and show rather complex edge and center modes, which depend strongly on the saturation magnetization. This behavior allows controlling the precession frequencies, which is important in microwave-assisted three-dimensional magnetic recording, where layers of different resonance frequency need to be addressed and switched individually.
A unique arrangement of atoms for the homologous compounds InMO3(ZnO)m (M = Al, Fe, Ga, and In)
Authors: Wen Jing , Wu Lili , Zhang Xitian
Published: June 2012

In this paper, a 2m×2m supercell with a unique arrangement of the atoms is taken as the unit cell of the InMO3(ZnO)m systems. A new formation rule of the atomic structure for these systems is suggested. The In atoms occupy the octahedral sites to form InO2− layer. The M atoms in the MO(ZnO)m+ (M/Zn-O) slab located at the tetrahedral or trigonal bipyramidal sites are aligned along the [1¯10] direction of the hexagonal lattice, forming a stepped configuration with a bilateral symmetry. The zigzag boundary can be clearly identified in our simulated high-resolution transmission electron microscopy (HRTEM) images based on this structure, which shows a good agreement with the experimental results. This structure has the minimum formation energy compared with other reported models based on the total-energy calculations. All of the HRTEM experimental data can be well explained based on the model. It gives us a new point of view to understand and study these materials.
Terraces at ohmic contact in SiC electronics: Structure and electronic states
Authors: Wang Zhongchang , Saito Mitsuhiro , Tsukimoto Susumu , Ikuhara Yuichi
Published: June 2012

Combining transmission electron microscopy with first-principles calculations, atomic structures of terraces at the contacts in silicon carbide devices are investigated and bridged to their electronic properties at an atomic scale. Experimentally, newly formed carbide Ti3SiC2 is demonstrated to bond directly to silicon carbide in the terrace region in an epitaxial and atomically ordered fashion, regardless of dimension of terraces. Further first-principles calculations reveal gap states in the semiconductor layers and a substantial charge accumulation around terraces in a connected and broadly distributed manner. The presence of gap states at Fermi energy and the likelihood to serve as electron conduction channels to allow current flow over the semiconductor identify the terraces as one of the origins underlying the ohmic contact in silicon carbide electronics. Such a combined experimental and theoretical investigation provides insight into the complex atomic and electronic structures of buried terraces, which should be applicable to addressing contact issues of interest in other electronic devices.
Lamb waves in double-layered nanoplates
Authors: Liu H. , Yang J. L.
Published: June 2012

This paper is concerned with the propagation of Lamb waves in double-layered nanoplates and a nonlocal continuum model is presented. Each layer of the double-layered nanoplates is coupled through the van der Waals interaction. The equations of wave motion are derived and dispersion relations are obtained. The present model predicts some notable features of the dispersion relations in double-layered nanoplates, in comparison with Lamb waves in classic plates in vacuo. By considering the van der Waals interaction, some modes appear in pairs at the same frequency, their phase velocities are very close at a given frequency. Some modes only propagate in a very narrow frequency band and may disappear above certain frequencies. The small-scale has significant influence on the propagation characteristics of Lamb waves. Larger values of the scale coefficient result in lower escape frequencies and phase velocities of Lamb waves.
Intense monochromatic terahertz electromagnetic waves from coherent GaAs-like longitudinal optical phonons in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells
Authors: Takeuchi Hideo , Asai Souta , Tsuruta Syuichi , Nakayama Masaaki
Published: June 2012

We demonstrate that, in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells, the terahertz electromagnetic wave from the coherent GaAs-like longitudinal optical (LO) phonon is enhanced by a piezoelectric field originating from a tensile strain in the GaAs layer. The presence of the tensile strain is confirmed using Raman scattering spectroscopy. The Fourier power spectrum of the terahertz waveform shows that the intensity of the terahertz band of the coherent GaAs-like LO phonon increases as the index n approaches 1. The amplitude of the GaAs-like LO phonon is proportional to the piezoelectric field in the strained GaAs layer.
High-performance photodetectors for visible and near-infrared lights based on individual WS2 nanotubes
Authors: Zhang Chaoying , Wang Sheng , Yang Leijing , Liu Yang , Xu Tingting , Ning Zhiyuan , Zak Alla , Zhang Zhiyong , Tenne Reshef , Chen Qing
Published: June 2012

We propose that a photodetector based on nanotubes formed from layered structure may have a faster response than nanowires or nanobelts. The layered compound tungsten disulfide (WS2) can absorb visible and near-infrared lights. We fabricated photodetectors based on individual WS2 nanotubes. The photodetectors exhibited a remarkable response to excitation with 633 and 785 nm light. The nanotube-based photodetectors exhibited short rise and decay times of a few hundred μs, high on/off ratio, and high spectral responsivity and external quantum efficiency. Our results imply that WS2 nanotubes are prospective candidates for high-performance nanoscale optoelectronic devices.
Quantum theory of rotational isomerism and Hill equation
Authors: Ugulava A. , Toklikishvili Z. , Chkhaidze S. , Abramishvili R. , Chotorlishvili L.
Published: June 2012

The process of rotational isomerism of linear triatomic molecules is described by the potential with two different-depth minima and one barrier between them. The corresponding quantum-mechanical equation is represented in the form that is a special case of the Hill equation. It is shown that the Hill-Schrödinger equation has a Klein's quadratic group symmetry which, in its turn, contains three invariant subgroups. The presence of these subgroups makes it possible to create a picture of energy spectrum which depends on a parameter and has many merging and branch points. The parameter-dependent energy spectrum of the Hill-Schrödinger equation, like Mathieu-characteristics, contains branch points from the left and from the right of the demarcation line. However, compared to the Mathieu-characteristics, in the Hill-Schrödinger equation spectrum the “right” points are moved away even further for some distance that is the bigger, the bigger is the less deep well. The asymptotic wave functions of the Hill-Schrödinger equation for the energy values near the potential minimum contain two isolated sharp peaks indicating a possibility of the presence of two stable isomers. At high energy values near the potential maximum, the height of two peaks decreases, and between them there appear chaotic oscillations. This form of the wave functions corresponds to the process of isomerization.
Effects of biaxial strains on the magnetic properties of Co-graphene heterojunctions
Authors: Lee Sangho , Choi Heechae , Chung Yong-Chae
Published: June 2012

Using the ab initio calculations, the changes of the magnetic properties of Co atoms on graphene for biaxial strains on graphene were investigated. Our calculation results showed that the compression of graphene changed the magnetic moments of Co adatoms more significantly than the elongation of graphene. From the 3d-electron density of states of Co atoms, the changes of magnetic properties of Co atoms on graphene were found to result from the significant hybridization between Co 3dyz and C 2pz orbitals. It was demonstrated that this 3d-electron hybridization of Co/graphene system originated from the strain-induced change of the local atomic structure.