bluuebunny/crossref_metadata_2025 huggingface dataset:
Nitric acid in cirrus clouds
Authors: Voigt C. , Schlager H. , Ziereis H. , Kärcher B. , Luo B. P. , Schiller C. , Krämer M. , Popp P. J. , Irie H. , Kondo Y.
Published: March 2006

Uptake of nitric acid (HNO3) in Arctic cirrus ice crystals was observed on 11 February 2003 by in‐situ instruments onboard the M55 Geophysica aircraft. The cirrus cloud with a mean ice water content of 5.4 mg m−3 covered northern Scandinavia for several hours and extended up to the thermal tropopause at 12.3 km. Within the cirrus region, on average 9% of the total HNO3 measured as reactive nitrogen (NOy) is present in ice particles, increasing to 19% at temperatures below 205 K. In contrast to previous studies, we discuss the HNO3 uptake in ice in terms of HNO3/H2O molar ratios in ice crystals. The HNO3 content of the ice increases with increasing gas phase HNO3 concentrations and decreasing temperatures. Enhanced uptake of HNO3 in ice and heterogeneous chemistry on cold cirrus clouds may disturb the upper tropospheric ozone budget.
Zonal intermediate currents in the equatorial Atlantic Ocean
Authors: Ollitrault Michel , Lankhorst Matthias , Fratantoni David , Richardson Philip , Zenk Walter
Published: March 2006

Acoustic float data collected near 800 m depth, are used to map zonal mean currents within the Antarctic Intermediate Water (AAIW) tongue in the equatorial Atlantic. Alternating zonal jets of 2° latitudinal width are revealed between 6°S and 6°N. Displacements from profiling floats drifting near 1000 m depth, also reveal similar zonal jets at the base of the AAIW layer. The strongest jets (15 cm s−1 peak) are found at 4°S, 2°S, 0°, 2°N and 4°N. They are coherent longitudinally over order of 3000 km and, poleward of 1°S and 1°N, generally coherent vertically between 800 m and 1000 m. Large seasonal fluctuations exist at both levels: within 1° of equator, AAIW at 800 m flows westward (8 cm s−1 mean) in boreal summer and fall but eastward (3 cm s−1 mean) in winter, whereas the flow at 1000 m is eastward in late fall and winter.
Tidal modulation of infragravity waves via nonlinear energy losses in the surfzone
Authors: Thomson Jim , Elgar Steve , Raubenheimer Britt , Herbers T. H. C. , Guza R. T.
Published: February 2006

The strong tidal modulation of infragravity (200 to 20 s period) waves observed on the southern California shelf is shown to be the result of nonlinear transfers of energy from these low‐frequency long waves to higher‐frequency motions. The energy loss occurs in the surfzone, and is stronger as waves propagate over the convex low‐tide beach profile than over the concave high‐tide profile, resulting in a tidal modulation of seaward‐radiated infragravity energy. Although previous studies have attributed infragravity energy losses in the surfzone to bottom drag and turbulence, theoretical estimates using both observations and numerical simulations suggest nonlinear transfers dominate. The observed beach profiles and energy transfers are similar along several km of the southern California coast, providing a mechanism for the tidal modulation of infragravity waves observed in bottom‐pressure and seismic records on the continental shelf and in the deep ocean.
The 2005 hurricane season: An echo of the past or a harbinger of the future?
Authors: Virmani Jyotika I. , Weisberg Robert H.
Published: March 2006

Recent Atlantic hurricane activity raises several questions. For example, why was the 2005 season so early and active and are there similarities with the past? We show that parallels exist between 2005 and previously active years. Tropical Atlantic sea surface temperature (SST) anomalies beginning in winter aided the formation of early storms. On longer timescales SST, sea level pressure and zonal winds exhibit multidecadal variability; weaker easterlies during the positive phase of the Atlantic Multidecadal Oscillation (AMO) result (via ocean‐atmosphere interactions) in warmer water and increased hurricane activity. However, individual active years appear independent of the AMO phase.
Wavelet power spectra of aeolian sand transport by boundary layer turbulence
Authors: Baas Andreas C. W.
Published: March 2006

This letter reports on wavelet spectral analysis of collocated time‐series of streamwise wind speed and aeolian sand transport collected at 0.04 m above the bed in a field experiment on top of a large sand mound in a desert environment. Results suggest three distinct spatio‐temporal scales of interaction between boundary layer turbulence and sediment transport: a long‐term scale associated with the regional mixing layer, an integral time‐scale associated with the internal boundary layer, and a streamer scale associated with the inertial subrange (eddy cascade) and saltation inertia.
August 2005 intense rainfall event in Switzerland: Not necessarily an analog for strong convective events in a greenhouse climate
Authors: Beniston Martin
Published: February 2006

The intense convective storms that affected the Swiss Alps in late August 2005 resulted in what has been referred to as the “floods of the century” (i.e., the past 100 years). While exceptional in terms of their intensity, the 2005 storms do not appear to be anchored within any long‐term trends; there are no more intense storms today than a century ago. Despite uncertainties related to regional climate simulations of precipitation in complex terrain, projections for a “greenhouse climate” by 2100 suggest that extreme rainfall events may undergo a seasonal shift, with a rise in the number of episodes in the spring and autumn. Paradoxically, associated impacts may be reduced because the buffering effects of snowfall on runoff may be greater in future springs and autumns than during current summers, implying the 2005 floods are not necessarily an analog for such events by the end of the 21st century.
A linearized approach to the radiative budget of the stratosphere: influence of the ozone distribution
Authors: Cariolle D. , Morcrette J.‐J.
Published: March 2006

The influence of the ozone distribution on stratospheric temperatures is studied using a linearized version of a detailed radiative code. The response of the linear radiation model to ozone change is found to be very consistent with the sensitivity of the ECMWF general circulation model (GCM). In particular, the lower stratospheric temperatures of the linear model and of the GCM respond similarly to changes in the vertical ozone gradient in the upper troposphere‐lower stratosphere. It is concluded that the GCM sensitivity to ozone is mostly a linear response via changes in the solar and long wave radiation absorption, combined with the very long radiative damping times that prevail in the lower stratosphere. The present linearized approach to radiative transfer can therefore constitute a valuable means to obtain at low computational cost a first guess of the stratospheric temperature response of coupled GCM‐Chemical Transport Models to ozone distribution changes.
Temporal variations of mineral dust, biogenic tracers, and anthropogenic species during the past two centuries from Belukha ice core, Siberian Altai
Authors: Olivier S. , Blaser C. , Brütsch S. , Frolova N. , Gäggeler H. W. , Henderson K. A. , Palmer A. S. , Papina T. , Schwikowski M.
Published: March 2006

In July 2001, a 140 m long ice core was recovered from the Belukha glacier (49°48′26″N, 86°34′43″E, 4062 m a.s.l.) in the Siberian Altai. The ion chemistry of the upper 86 m, covering the last two centuries, is characterized by biogenic emissions (ammonium and formate), aeolian dust (calcium, magnesium, chloride, and sodium) and anthropogenic species (sulfate, nitrate, and ammonium). Particularly high ammonium and formate concentrations indicate pronounced emissions from Siberian forests. The inferred fire frequency does not show a long‐term trend but distinct periods of enhanced activity. Sulfate has the highest industrial to preindustrial ratio and an anthropogenic contribution of more than 80%. Variations in this record reflect sulfur dioxide emissions in Siberia and Kazakhstan. Sulfate concentrations remained low until 1950, then sharply increased and peaked in the 1970s. The decrease in the 1980s is attributed to the economic, political, and social crises and to the replacement of coal with gas. Rising nitrate concentrations since 1960 reflect traffic growth and enhanced fertilizer application. Increasing ammonium concentrations since the 1950s are attributable to population inflow in southern Siberia with the associated enhancement of agricultural activity. A nitrate peak of short duration in 1908 is thought to be the atmospheric signature from the Tunguska event on 30 June 1908.
A numerical study of aerosol effects on the dynamics and microphysics of a deep convective cloud in a continental environment
Authors: Cui Zhiqiang , Carslaw Kenneth S. , Yin Yan , Davies Stewart
Published: February 2006

The effects of aerosols on a deep convective cloud in a midlatitude continental environment are studied using an axisymmetric cloud model with a sectional treatment of aerosol and hydrometeor microphysical processes. Simulations are conducted using observations from the Cooperative Convective Precipitation Experiments (CCOPE). The isolated cloud occurred in an environment with low wind shear and with relatively dry air in the midtroposphere and upper troposphere. By varying the concentration of aerosol particles in the accumulation mode within realistic limits for a continental environment, the simulated cloud exhibited different properties. The overall impact as the aerosol concentration increased is that (1) the cloud development was inhibited; (2) the precipitation was suppressed; (3) the maximum values of liquid water content decreased, but the maximum values of droplet number concentration increased before the dissipating stage; (4) a clear tendency was found for ice crystals to be larger and less numerous in the anvil cloud; and (5) there was a significant reduction of the inflow in the lower 2 km of the atmosphere. In the relatively dry environment in the midtroposphere, the latent heat changes associated with the Wegener‐Bergeron‐Findeisen mechanism played an important role in the upper part of the cloud at altitudes below the homogeneous freezing level. In particular, immersion freezing and latent heat release were much more rapid in the base simulation than in the increased aerosol simulation. Less latent heat release and insufficient inflow together impeded the development of the cloud with the higher aerosol loading. Our simulations suggest that continental clouds existing below the homogeneous freezing level could show an opposite response of cloud top height and anvil crystal concentrations to changes in aerosol to what has previously been reported for clouds ascending to higher levels.
Column‐integrated aerosol optical properties in Sodankylä (Finland) during the Solar Induced Fluorescence Experiment (SIFLEX‐2002)
Authors: Gómez‐Amo José L. , Martínez‐Lozano José A. , Utrillas María P. , Pedrós Roberto , Estellés Victor
Published: March 2006

A study has been made of the column aerosols using solar irradiance extinction measurements at ground level in a boreal region (Sodankylä, Finland) during spring 2002. The aerosol properties have been related to air mass origin. In general, the aerosol levels were observed to be very low, independent of the air mass origin, with an aerosol optical depth (AOD) value at 500 nm of less than 0.09 ± 0.03. Two characteristic patterns were observed depending on whether the air masses originated in the north and west or from the south and east. In the first case (north and west origins) the aerosol load was very small, with very low optical depths in the range 0.03 ± 0.02 to 0.09 ± 0.03 for 500 nm wavelengths. The size distribution usually showed two modes, with a strong contribution from the large‐particle mode, probably a consequence of the presence of maritime‐type particles originating in the ocean. When the air masses originated from a south or east direction, the behavior was the opposite to that noted before. In these cases the AOD was rather larger, above all for air masses originating in central Europe and Russia with an average value at 500 nm of 0.14 ± 0.02.
Variation of column‐integrated aerosol properties in a Chinese urban region
Authors: Xia X. A. , Chen H. B. , Wang P. C. , Zhang W. X. , Goloub P. , Chatenet B. , Eck T. F. , Holben B. N.
Published: March 2006

Thirty‐three months of aerosol data in Beijing are presented in this paper. Aerosol optical thickness (AOT) increases from January to June and then decreases gradually. However, airborne particulate matter with diameter less than 10 μm (PM10) concentration exhibits higher values in winter and spring and lower concentration in summer. For the same PM10 concentration, AOT in summer is approximately two, three, and four times that in autumn, winter, and spring, respectively. AOT increases persistently during daytime, and the diurnal variation varies from about 15% in summer to about 45% in winter. The seasonal and diurnal variation of AOT is quite different from that of surface particle concentration. This is partly attributed to the variation of atmospheric mixing layer height. Aerosol volume concentrations increase with AOT by nearly identical magnitude for fine and coarse mode except in spring. The volume concentration of coarse mode in spring increases by a magnitude of more than two times that derived in remaining seasons. Aerosol fine mode radius increases with AOT, whereas coarse mode radius keeps relatively invariable with AOT. Mean aerosol single‐scattering albedo at 440 nm is about 0.90 and decreases slightly with wavelength. Aerosol single‐scattering albedos increase and their spectral dependence reverses during dust periods. Aerosol size and absorption in Beijing are close to results derived in Mexico City and Kanpur, but they are quite different from those in Maryland and Paris. Therefore different urban aerosol models should be created and used in satellite remote sensing in different urban regions.
Size‐resolved particulate matter composition in Beijing during pollution and dust events
Authors: Dillner Ann M. , Schauer James J. , Zhang Yuanhang , Zeng Limin , Cass Glen R.
Published: March 2006

Each spring, Beijing, China, experiences dust storms which cause high particulate matter concentrations. Beijing also has many anthropogenic sources of particulate matter including the large Capitol Steel Company. On the basis of measured size segregated, speciated particulate matter concentrations, and calculated back trajectories, three types of pollution events occurred in Beijing from 22 March to 1 April 2001: dust storms, urban pollution events, and an industrial pollution event. For each event type, the source of each measured element is determined to be soil or anthropogenic and profiles are created that characterize the particulate matter composition. Dust storms are associated with winds traveling from desert regions and high total suspended particle (TSP) and PM2.5 concentrations. Sixty‐two percent of TSP is due to elements with oxides and 98% of that is from soil. Urban pollution events have smaller particulate concentrations but 49% of the TSP is from soil, indicating that dust is a major component of the particulate matter even when there is not an active dust storm. The industrial pollution event is characterized by winds from the southwest, the location of the Capitol Steel Company, and high particulate concentrations. PM2.5 mass and acidic ion concentrations are highest during the industrial pollution event as are Mn, Zn, As, Rb, Cd, Cs and Pb concentrations. These elements can be used as tracers for industrial pollution from the steel mill complex. The industrial pollution is potentially more detrimental to human health than dust storms due to higher PM2.5 concentrations and higher acidic ion and toxic particulate matter concentrations.
Dust and pollution aerosols over the Negev desert, Israel: Properties, transport, and radiative effect
Authors: Derimian Yevgeny , Karnieli Arnon , Kaufman Yoram J. , Andreae Meinrat O. , Andreae Tracey W. , Dubovik Oleg , Maenhaut Willy , Koren Ilan , Holben Brent N.
Published: March 2006

The complex spatial, temporal, and optical characteristics of atmospheric aerosols cause large uncertainties in the estimation of aerosol effects on climate. Analysis of long‐term measurements from key regions can provide a better understanding of the role of atmospheric aerosols in the climate system. In the current study, observations of aerosol optical properties and mass concentrations were carried out during 1995–2003 in the Israeli Negev desert. The measurement site is relatively remote from local pollution sources; however, it lies at the crossroad between dust from the Sahara and the Arabian peninsula and pollution from Europe. The instruments employed were a Sun/sky photometer, a stacked filter unit sampler, and an integrating nephelometer. We analyzed the data for seasonal variability, general vertical aerosol structure, and radiative climate effect by dust and anthropogenic aerosol. The intra‐annual variability of aerosol optical properties was found to be closely related to seasonally varying synoptic conditions. Two seasonal peaks of aerosol optical thickness were noted: The first maximum related to dust particle activity and the second to anthropogenic aerosol. Similar maximums were noted in aerosol light scattering at the surface; however, their relative importance is reversed and is related to differences in the vertical distribution of dust and anthropogenic aerosols. The calculated aerosol radiative effect shows cooling both at the top of the atmosphere and at the surface during the whole year. The radiative effect of the airborne dust is the dominating forcing component during most of the time in the study area.
Reproducibility of geochemical and climatic signals in the Atlantic coral Montastraea faveolata
Authors: Smith Jennifer M. , Quinn Terrence M. , Helmle Kevin P. , Halley Robert B.
Published: March 2006

Monthly resolved, 41‐year‐long stable isotopic and elemental ratio time series were generated from two separate heads of Montastraea faveolata from Looe Key, Florida, to assess the fidelity of using geochemical variations in Montastraea, the dominant reef‐building coral of the Atlantic, to reconstruct sea surface environmental conditions at this site. The stable isotope time series of the two corals replicate well; mean values of δ18O and δ13C are indistinguishable between cores (compare 0.70‰ versus 0.68‰ for δ13C and −3.90‰ versus −3.94‰ for δ18O). Mean values from the Sr/Ca time series differ by 0.037 mmol/mol, which is outside of analytical error and indicates that nonenvironmental factors are influencing the coral Sr/Ca records at Looe Key. We have generated significant δ18O–sea surface temperature (SST) (R = −0.84) and Sr/Ca‐SST (R = −0.86) calibration equations at Looe Key; however, these equations are different from previously published equations for Montastraea. Variations in growth parameters or kinetic effects are not sufficient to explain either the observed differences in the mean offset between Sr/Ca time series or the disagreement between previous calibrations and our calculated δ18O‐SST and Sr/Ca‐SST relationships. Calibration differences are most likely due to variations in seawater chemistry in the continentally influenced waters at Looe Key. Additional geochemical replication studies of Montastraea are needed and should include multiple coral heads from open ocean localities complemented whenever possible by seawater chemistry determinations.
Prediction of discharge from water surface width in a braided river with implications for at‐a‐station hydraulic geometry
Authors: Ashmore Peter , Sauks Emilie
Published: March 2006

The total discharge of small braided rivers can be monitored nearly continuously using ground‐based, orthorectified oblique images and a locally derived relationship between mean water surface width in a reach and total discharge with accuracy of the order of 10%. The width‐discharge relationship is nearly linear over the normal range of summer flows measured in this proglacial stream. The width‐discharge relation for a given location and channel configuration is transferable to adjacent reaches with different morphology with little loss in accuracy. However, the prospect of a more universal relation for braided rivers is remote because of differences in channel geometry and hydraulics between rivers. The classical at‐a‐station hydraulic geometry exponent for width in this case is approximately 1.05, which is unique, well above that observed in the few other braided rivers for which this has been derived, and implies almost no change in mean velocity and depth with changing stage.
Application of digital filtering for minimizing aliasing effects in atmospheric turbulent surface layer spectra
Authors: Gobbi M. F. , Chamecki M. , Dias N. L.
Published: March 2006

We propose a very simple and straightforward technique to reduce aliasing effects on the digital spectra of turbulent data from field experiments. A detailed analysis of the technique is done, and it is tested and compared with more traditional low‐pass filtering and with oversampling. The proposed method shows satisfactory results for all tests performed.
Climate model biases in seasonality of continental water storage revealed by satellite gravimetry
Authors: Swenson Sean Claude , Milly P. C. D.
Published: February 2006

Satellite gravimetric observations of monthly changes in continental water storage are compared with outputs from five climate models. All models qualitatively reproduce the global pattern of annual storage amplitude, and the seasonal cycle of global average storage is reproduced well, consistent with earlier studies. However, global average agreements mask systematic model biases in low latitudes. Seasonal extrema of low‐latitude, hemispheric storage generally occur too early in the models, and model‐specific errors in amplitude of the low‐latitude annual variations are substantial. These errors are potentially explicable in terms of neglected or suboptimally parameterized water stores in the land models and precipitation biases in the climate models.
OD
vibrations and hydration structure in an Al3+(aq) solution from a Car-Parrinello molecular-dynamics simulation
Authors: Amira Sami , Spångberg Daniel , Hermansson Kersti
Published: March 2006

The optimized geometry, energetics, and vibrational properties of Al(D2O)n3+ clusters, with n=1,2,4, and 6, have been studied using plane waves, different local basis sets, different methodologies [density-functional theory, MP2, CCSD(T)], and different functionals (BLYP, PBE). Moreover, Car-Parrinello molecular-dynamics (MD) simulations using the BLYP functional, plane waves, and the Vanderbilt ultrasoft pseudopotentials have been performed for an aqueous Al3+ solution with 1 ion and 32 D2O molecules in a periodic box at room temperature, studied for 10ps. The cluster calculations were performed to pinpoint possible shortcomings of the electronic structure description used in the Car-Parinello MD (CPMD) simulation. For the clusters, the hydration structure and interaction energies calculated with the ‘BLYP/plane-wave’ approach agree well with high-level ab initio methods but the exchange-correlation functional introduces errors in the OD stretching frequencies (both in the absolute values and in the ion-induced shifts). For the aqueous solution, the CPMD simulation yields structural properties in good agreement with experimental data. The CPMD-simulated OD stretching vibrational band for the first-shell water molecules around Al3+ is strongly downshifted by the influence of the ion and is compared with experimental data from the literature. To make such a comparison meaningful, the influences of a number of systematic effects have been addressed, such as the exchange-correlation functional, the fictitious electron mass, anharmonicity effects, and the small box size in the simulation. Each of these factors (except the last one) is found to affect the OD frequency by 100cm−1 or more. The final “corrected” frequencies agree with experiment within ∼30cm−1 for bulk water but are too little downshifted for the first-shell Al3+(aq) water molecules (by ∼200cm−1).
Theoretical studies of transition states by the multioverlap molecular dynamics methods
Authors: Itoh Satoru G. , Okamoto Yuko
Published: March 2006

The multioverlap molecular dynamics method gives a flat probability distribution in the multidimensional dihedral-angle-distance space, where the dihedral-angle distance of a configuration with respect to a reference state gives a measure for structural similarity. Hence, this method realizes a random walk among specific configurations in the multidimensional dihedral-angle-distance space at a constant temperature and explores widely in the configurational space. We applied the multioverlap molecular dynamics method to a pentapeptide, Met-enkephalin, in gas phase as a test system. Comparing the results of this method with those of the conventional canonical and multicanonical algorithms, we demonstrate its effectiveness. Furthermore, from the detailed free-energy landscape obtained from the results of the multioverlap molecular dynamics simulation, we obtain the transition state between two specific reference configurations of Met-enkephalin. We also deduce the transition pathway between the two specific reference configurations.
Resistive studies of the pseudogap in YBCO films with consideration of the transition from BCS to Bose–Einstein condensation
Authors: Solovjov A. L. , Dmitriev V. M.
Published: February 2006

A new approach is proposed for analysis of the excess conductivity σ′(T) arising below a characteristic temperature T*⪢Tc in YBa2Cu3O7−y (YBCO) films with different oxygen concentrations. The approach is based on the idea that σ′(T) is formed as a result of the formation at T⩽T* of local pairs (tightly bound bosons) obeying Bose–Einstein statistics in the temperature interval Tm<T<T*. At temperatures Tcmf<T<Tm the pairs obey BCS theory (Tcmf is the critical temperature separating the phase transition region from the region of critical fluctuations). Thus in Y123 systems a transition from Bose–Einstein condensation to condensation of the BCS type occurs with decreasing temperature. An equation in which the dynamics of formation of the tightly bound bosons is taken into account is proposed which gives a good description of the temperature dependence σ′(T) and in which the parameter Δ*, identified with the pseudogap in high-Tc superconductors, is contained in explicit form. The temperature dependence Δ*(T) is obtained for all the films studied.
Incoherent interlayer electron hopping as a possible reason for enhanced magnetic quantum oscillations in the mixed state of a layered organic superconductor
Authors: Gvozdikov V. M. , Wosnitza J.
Published: February 2006

We present a theory which is able to explain enhanced magnetic quantum-oscillation amplitudes in the superconducting state of a layered organic metal with incoherent electronic transport across the layers. The incoherence acts through the deformation of the layer-stacking factor, which becomes complex and decreases the total scattering rate in the mixed state. This novel mechanism restores the coherence by establishing a long-range order across the layers and can compensate the usual decrease of the Dingle factor below the upper critical magnetic field caused by the intralayer scattering.
Nucleation processes at spin-reorientation phase transitions in real crystals
Authors: Vakhitov R. M. , Gareeva E. R. , Vakhitova M. M.
Published: February 2006

The possible mechanisms of nucleation at a phase transition of the spin reorientation type in a real magnet are investigated by a phenomenological approach. The greatest attention is devoted to the fluctuation mechanism, in which the new-phase nuclei are modeled on the basis of ideas about magnetic inhomogeneities of the zero-degree domain wall type, and the sample considered is a cubic ferromagnet with uniaxial anisotropy induced along the [011] axis. It is shown that such inhomogeneities can represent stable formations if the finite size of the sample and the presence of defects in it are taken into account. The behavior of zero-degree domain walls is investigated for different values of the material parameters and defect characteristics, and their role in the kinetics of the spin-reorientation phase transition is elucidated.
Magnetic and transport properties of charge ordered La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films
Authors: Prokhorov V. G. , Komashko V. A. , Kaminsky G. G. , Lee Y. P. , Park S. Y. , Hyun Y. H. , Svetchnikov V. L. , Kim K. W. , Rhee J. Y.
Published: February 2006

The magnetic and transport properties of La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films with different thickness, prepared by rf-magnetron sputtering with the use of a so-called “soft” (or powder) target on a LaAlO3 substrate, are investigated. Electron-diffraction and high-resolution electron microscopy (HREM) studies show that the charge-ordered phase is observed at room temperature for all films. Both the paramagnetic-to-ferromagnetic transition at TC≈250K upon cooling and the appearance of an antiferromagnetic (AFM) phase at TN≲140K are observed in the La0.5Ca0.5MnO3 films, while the La0.4Ca0.6MnO3 films exhibit the AFM transition only, at the same temperature, except for a small ferromagnetic (FM) response from a “dead” layer. It is shown that the volume fraction of the FM phase in the La0.5Ca0.5MnO3 film does not exceed 30% and that the FM phase coexists with the AFM phase at low temperature. All films manifest an exponential temperature dependence of the resistance, with no evidence of the metal-insulator transition. This is explained by the scarcity of the FM phase for the formation of an infinite percolating cluster and by the existence of a charge-ordered phase. The field-dependent magnetoresistance at low temperature is described in terms of the spin-assisted polaron-hopping model.
Hydrogen-bonded complexes of 2-aminopyrimidine–parabenzoquinone in an argon matrix
Authors: Plokhotnichenko A. M. , Stepanian S. G. , Karachevtsev V. A. , Adamowicz L.
Published: February 2006

The H-bonded complexes of 2-aminopyrimidine (NH2Py) with parabenzoquinone (Qu) in a low-temperature argon matrix are investigated by the method of IR spectroscopy. The IR absorption spectra in the spectral range 400–3600cm−1 are obtained for different concentration ratios of these compounds at a temperature of 11K. The molar integrated absorption coefficients in the bands of the stretching modes of the free and H-bonded NH2 group are determined. Quantum-mechanical calculations of the IR spectra of the NH2Py and Qu molecules and their dimers are carried out. A comparison of the experimental and calculated results permits the conclusion that NH2Py–Qu dimers in an argon matrix have a planar structure with two weak hydrogen bonds, NH⋯O and CH⋯N.
Low-temperature mixed spin state of Co3+ in LaCoO3 evidenced from Jahn–Teller lattice distortions
Authors: Gnezdilov V. , Fomin V. , Yeremenko A. V. , Choi K.-Y. , Pashkevich Yu. , Lemmens P. , Shiryaev S. , Bychkov G. , Barilo S.
Published: February 2006

One-phonon and multiphonon excitations of the single-crystalline LaCoO3 were studied using Raman spectroscopy in the temperature region 5–300K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn–Teller (JT) activity of the intermediate-spin (IS) state of the Co3+ ions. In particular, the 608-cm−1 stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a function of temperature. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck–Condon mechanism that is characteristic for a JT orbital order.
Dynamic elastic moduli of niobium at low temperatures: their temperature dependence in the normal state, the influence of the superconducting transition, and dislocation effects
Authors: Pal-Val P. P. , Natsik V. D. , Pal-Val L. N.
Published: February 2006

The low-temperature elastic properties of niobium crystals of different purity and orientation are investigated in the normal (N) and superconducting (S) states. The experiments are done by the composite vibrator technique at frequencies ∼90kHz. The temperature dependence of Young’s modulus E(T) and of the shear modulus G(T) are obtained in the temperature range 2–12K in the N and S states. It is found that both the temperature dependence of the individual components of the elastic moduli in the normal state, MN(T), and the temperature dependence of the difference ΔMNS(T)=MN(T)−MS(T) can in some cases have anomalies that cannot be satisfactorily explained in a theoretical description of a direct interaction of the acoustic deformations of the crystal lattice of the metal with the electronic and phonon excitations. In particular, it is found that for fast-cooled samples there exists a temperature region in which an anomalous increase of the elastic moduli at the N–S transition is observed. These anomalies are interpreted in terms of a resonant interaction of the acoustic vibrations with low-energy dislocation excitations. It is shown that they are a consequence of a superposition of a low-temperature dynamic dislocation relaxation and quasistatic thermodynamic variation of the electronic contribution to the elastic moduli at temperatures below Tc. The data agree with known results of high-frequency measurements at frequencies of 10–30MHz.
Ergodicity breaking and conformational hysteresis in the dynamics of a polymer tethered at a surface stagnation point
Authors: Beck Victor A. , Shaqfeh Eric S. G.
Published: February 2006

We study the dynamics of long chain polymer molecules tethered to a plane wall and subjected to a stagnation point flow. Using a combination of theory and numerical techniques, including Brownian dynamics (BD), we demonstrate that a chain conformation hysteresis exists even for freely draining (FD) chains. Hydrodynamic interactions (HI) between the polymer and the wall are included in the BD simulations. We find qualitative agreement between the FD and HI simulations, with both exhibiting simultaneous coiled and stretched states for a wide range of fixed flow strengths. The range of state coexistence is understood by considering an equivalent projected equilibrium problem of a two state reaction. Using this formalism, we construct Kramers rate theory (from the inverse mean first passage time for a Markov process) for the hopping transition from coil to stretch and stretch to coil. The activation energy for this rate is found to scale proportionally to chain length or Kuhn step number. Thus, in the limit of infinite chain size the hopping rates at a fixed value of the suitably defined Deborah number approach zero and the states are “frozen.” We present the results that demonstrate this “ergodicity breaking.”
Simulation of x-ray absorption near-edge spectra and x-ray fluorescence spectra of optically excited molecules
Authors: Pandey R. K. , Mukamel Shaul
Published: February 2006

The x-ray absorption near-edge spectra (XANES) and fluorescence spectra of molecules in the ground state and optically excited states are computed using time-dependent density functional theory and time-dependent Hartree-Fock theory. The calculated XANES spectra of optically excited methanol, benzonitrile, hydrogen sulphide, and titanium tetrachloride and the fluorescence spectra of optically excited methanol can be used to simulate ultrafast optical pump/x-ray probe experiments.
Geometries and properties of excited states in the gas phase and in solution: Theory and application of a time-dependent density functional theory polarizable continuum model
Authors: Scalmani Giovanni , Frisch Michael J. , Mennucci Benedetta , Tomasi Jacopo , Cammi Roberto , Barone Vincenzo
Published: February 2006

In this paper we present the theory and implementation of analytic derivatives of time-dependent density functional theory (TDDFT) excited states energies, both in vacuo and including solvent effects by means of the polarizable continuum model. The method is applied to two case studies: p-nitroaniline and 4-(dimethyl)aminobenzonitrile. For both molecules PCM-TDDFT is shown to be successful in supporting the analysis of experimental data with useful insights for a better understanding of photophysical and photochemical pathways in solution.
Time-dependent density functional theory calculations for core-excited states: Assessment of standard exchange-correlation functionals and development of a novel hybrid functional
Authors: Nakata Ayako , Imamura Yutaka , Otsuka Takao , Nakai Hiromi
Published: February 2006

A new hybrid functional for accurate descriptions of core and valence excitations, the core-valence Becke's three-parameter exchange (B3)+Lee-Yang-Paar (LYP) correlation functional (CV-B3LYP), is proposed. The construction of the new hybrid functional is based on the assessment that B3LYP performs well for properties concerning valence electrons and Becke's half-and-half exchange+LYP functional (BHHLYP), which includes 50% portion of Hartree-Fock exchange, performs well for core excitations. By using the appropriate portions of Hartree-Fock exchange for core and valence regions separately, CV-B3LYP overcomes the disadvantages of BHHLYP and B3LYP, which give inferior descriptions of valence and core excitations, respectively. Density functional theory (DFT) calculations with the CV-B3LYP functional reproduce core- and valence-orbital energies close to those of BHHLYP and B3LYP, respectively. Time-dependent DFT calculations with the CV-B3LYP functional yield both core- and valence-excitation energies with reasonable accuracy.
Intrinsic spectral geometry of the Kerr-Newman event horizon
Authors: Engman Martin , Cordero-Soto Ricardo
Published: March 2006

We uniquely and explicitly reconstruct the instantaneous intrinsic metric of the Kerr-Newman event horizon from the spectrum of its Laplacian. In the process we find that the angular momentum parameter, radius, area; and in the uncharged case, mass, can be written in terms of these eigenvalues. In the uncharged case this immediately leads to the unique and explicit determination of the Kerr metric in terms of the spectrum of the event horizon. Robinson’s “no hair” theorem now yields the corollary: One can “hear the shape” of noncharged stationary axially symmetric black hole space-times by listening to the vibrational frequencies of its event horizon only.
Pressure effects on the α and α′ relaxations in polymethylphenylsiloxane
Authors: Kriegs H. , Gapinski J. , Meier G. , Paluch M. , Pawlus S. , Patkowski A.
Published: March 2006

In some polymers, in addition to the usual structural α relaxation, a slower α′ relaxation is observed with a non-Arrhenius temperature dependence. In order to understand better the molecular origin of this α′ relaxation in poly(methylphenylsiloxane) (PMPS) we have studied, for the first time, the pressure dependence of its relaxation time, together with the usual temperature dependence, by means of dynamic light scattering (DLS). For the same material the α relaxation was also studied by means of DLS and dielectric spectroscopy (DS) in broad temperature and pressure ranges. We find that the temperature dependence of both α and α′ relaxation times, at all pressures studied, can be described by a double Vogel-Fulcher-Tammann (VFT) law. The pressure dependence of the characteristic temperatures Tg (glass transition temperature) and T0 (Vogel temperature) as well as the activation volumes for both α and α′ processes are very similar, indicating, that both relaxation processes originate from similar local molecular dynamics. Additionally, for both α and α′ relaxations the combined temperature and pressure dependences of the relaxation times can be described using a parameter Γ=ρn∕T with the same value of the exponent n.
Interactions of polarizable media in water: A molecular dynamics approach
Authors: Wynveen A. , Bresme F.
Published: March 2006

We investigate the interactions of polarizable solutes in water as a function of the solute permittivity. A generic and computationally efficient simulation methodology for the investigation of systems involving dielectric discontinuities is introduced. We report results for interactions between two polarizable cylindrical solutes of nanometer dimensions, which demonstrate that the interactions between the solutes strongly depend on the solute permittivity ε. For low permittivity, ε∼1–2, the interactions are dominated by surface tension forces whose origin lies in the formation of a vapor cavity between the two hydrophobic solutes. This effect leads to a drying transition, where the intersolute force changes discontinuously at a specific solute-solute separation. We find that a moderate permittivity, ε∼20, enhances the solvation of the polarizable objects inhibiting this drying transition. In the limit of moderately high permittivity, the interactions are dominated by solvation forces. These forces are much larger than those calculated using macroscopic models of dielectrics, which consider water as a continuum dielectric medium. Our results emphasize the importance of including the solvent explicitly to investigate dielectric discontinuities and interactions between polarizable media in water.
On the orientation of lamellar block copolymer phases under shear
Authors: Fraser Blair , Denniston Colin , Müser Martin H.
Published: March 2006

Self-assembled lamellar structures composed of block copolymers are simulated by molecular dynamics. The response of a bulk system to external shear is investigated, in particular, the average energy, the entropy production, and the stability of the lamellae’s orientation. We distinguish two orientations, a parallel orientation in which the normal to the lamellae sheets lies in the direction of the shear gradient, and a perpendicular orientation in which the normal lies perpendicular to the shear gradient and shear direction. The perpendicular phase is stable throughout all shear rates. The parallel phase has higher internal energy and larger entropy production than the perpendicular phase and moreover becomes unstable at relatively small shear rates. The perpendicular orientation should therefore be more stable at any finite shear rate. Surface effects are probably responsible for the stability of the parallel phase observed experimentally at small shear rates.
In situ photoelectron emission microscopy of a thermally induced martensitic transformation in a CuZnAl shape memory alloy
Authors: Xiong Gang , Joly Alan G. , Beck Kenneth M. , Hess Wayne P. , Cai Mingdong , Langford Stephen C. , Dickinson J. Thomas
Published: February 2006

We report photoelectron emission microscope observations of the thermal martensitic transformation in a CuZnAl shape memory alloy. The phase transformation appears at 48°C during heating and at 42°C upon cooling. The transformation is marked by a sharp change in photoelectron intensity, as well as a significant displacement and reorientation of surface features. The difference in the photoelectron intensity before and after the transformation is attributed to a change in work function of about 0.2eV. Photoemission electron microscopy provides real-time information on microstructural changes and phase-dependent electronic properties.
Photoluminescence and electroluminescence of a new blue-emitting homoleptic iridium complex
Authors: Chew Siewling , Lee Chun Sing , Lee Shuit-Tong , Wang Pengfei , He Jian , Li Wen , Pan Jie , Zhang Xiaohong , Kwong Hoilun
Published: February 2006

A new electrophosphorescent material, iridium(III)tris(5-(2,4-difluoro-phenyl)-10,10- dimethyl-4-aza-tricycloundeca-2,4,6-triene), Ir(F2-mppy)3, has been synthesized and characterized. Ir(F2-mppy)3 shows strong blue emission peaking at 475nm with a high phosphorescent quantum yield (ΦPL) of 0.72 in degassed toluene. Electroluminescent devices with a structure of indium tin oxide (ITO)/NPB (40nm)∕Ir(F2-mppy)3(x):CBP (30nm)/BCP (6nm)∕Alq3 (20nm)∕Mg:Ag (10:1) and 15% (10%) exhibit a maximum luminous power efficiency of 9.58lm∕W (7.68lm∕W), CIE coordinates (x=0.18,y=0.36) (x=0.17,y=0.29), and a maximum luminance of ∼9000cd∕m2 at 12.5V (∼6000cd∕m2 at 11.5V).
Homojunction photodiodes based on Sb-doped p-type ZnO for ultraviolet detection
Authors: Mandalapu L. J. , Yang Z. , Xiu F. X. , Zhao D. T. , Liu J. L.
Published: February 2006

ZnO -based p-n homojunctions were grown using molecular-beam epitaxy. Sb and Ga were used as dopants to achieve the p-type and n-type ZnO, respectively. The mesa devices were fabricated by employing wet etching and standard photolithography techniques. Al∕Ti metal was deposited by electron-beam evaporation and annealed to form Ohmic contacts. Current-voltage measurements of the device showed good rectifying behavior, from which a turn-on voltage of about 2V was obtained. Very good response to ultraviolet light illumination was observed from photocurrent measurements.
Controlling defect and Si nanoparticle luminescence from silicon oxynitride films with CO2 laser annealing
Authors: Tewary Anuranjita , Kekatpure Rohan D. , Brongersma Mark L.
Published: February 2006

We demonstrate that a focused CO2 laser beam (λ=10.6μm) can be employed to locally synthesize light emitting defects and Si nanoparticles in silicon rich oxynitride thin films. Films with a stoichiometry of SiO1.08N0.32 were prepared by plasma enhanced chemical vapor deposition with N2O and SiH4. Strongly absorbing CO2 laser light was then used to induce local heating in the films in air ambient using power densities in the range from 0 to 580W∕cm2 and times of 5s to 60min. High-resolution cross sectional transmission electron microscopy (TEM) images of the irradiated region revealed the presence of crystalline Si nanoparticles. Photoluminescence (PL) spectra taken from irradiated areas showed two distinct peaks around 570 and 800nm. From a combined TEM, Rutherford back scattering (RBS), forming gas annealing (FGA), PL, and PL lifetime study it is concluded that the 570nm peak with a short PL lifetime (<10ns) is related to defects characteristic of silicon suboxides and that the 800nm peak is due to exciton recombination inside the Si nanoparticles. The appearance of an isosbestic point in the PL spectra suggests that upon CO2 laser heating Si nanoparticles are formed at the expense of the luminescent defect structures, which are annealed out.
Field-emission enhancement from change of printed carbon nanotube morphology by an elastomer
Authors: Lee H. J. , Lee Y. D. , Cho W. S. , Ju B. K. , Lee Yun-Hi , Han J. H. , Kim J. K.
Published: February 2006

The surface morphology of screen-printed carbon nanotube films was modified by using the poly-dimethylsiloxane (PDMS) elastomer. The entangled carbon nanotube (CNT) bundles were broken up into individual free standing nanotubes to remarkably improve the field-emission characteristics over the as-deposited CNT film. In addition, the cathode film morphology of top-gated triode structures can be treated by using the proposed surface treatment technique, which is a low-cost process.
Composition-dependent structures and properties of (1-x)BaTiO3-xNaNbO3 thin films
Authors: Zhang Shan-Tao , Lu Ming-Hui , Chen Yan-Feng , Liu Zhi-Guo , Ming Nai-Ben , Wang Jia , Cheng Guang-Xu
Published: February 2006

( 1 - x ) BaTiO 3 - x NaNbO 3 (x=0.025, 0.050, and 0.075) thin films were prepared on Pt∕Ti∕SiO2∕Si substrates by pulsed laser deposition. The structures, ferroelectric and dielectric properties were found to be strongly dependent on the composition wherein the ferroelectric polarization, tunability and temperature coefficient of capacitor (TCC) decrease with increasing x. At an applied field of 390kV∕cm, the room-temperature tunability are 67%, 38%, and 7% for the films with x=0.025, 0.050, and 0.075, respectively. Especially, the tunability reaches 78% with a field of 610kV∕cm for the films with x=0.025. The measured negative TCC values of these films are 1.7×10−3∕°C, 1.0×10−3∕°C, and 8.9×10−4∕°C, respectively.