bluuebunny/crossref_metadata_2025 huggingface dataset:
Atmospheric origin of the devastating coupled air‐sea event in the east Adriatic
Authors: Belušić Danijel , Grisogono Branko , Klaić Zvjezdana Bencetić
Published: September 2007

Enhanced sea level oscillations with devastating effects occurred on 26/27 June 2003 in the east Adriatic. The phenomenon was recognized as a resonantly coupled air‐sea interaction, where the sea wave is forced by the propagating atmospheric pressure disturbance. This study examines the dynamics and predictability of the atmospheric component of the coupled system. First, the initiation, amplification and maintenance of the system are examined. The source of the perturbation is traced back to the Alpine region, where the potentially unstable air in the westerly current lifts over the Alps resulting in convection and wave development. The wave and convective activity apparently couple in a wave‐CISK manner and propagate southeastward down the eastern Adriatic coast. Due to the potentially unstable air above the Adriatic, the system additionally amplifies along its track. The dissipation of the system occurs after the landfall. Second, the performance of the numerical model at the resolution that is similar to operational NWP model resolution is examined. This severe event is highly susceptible to the details of the model configuration, presumably due to the sensitivity in coupling of the wave and MCS. Thus although the model is able to reproduce and forecast this kind of event at a fairly coarse resolution, the details of the spatial structure, as well as the time of onset, are questionable. In this case, the greatest sensitivity is primarily in the treatment of moist and convective processes.
Ecology and spatial pattern of cyanobacterial community island patches in the Atacama Desert, Chile
Authors: Warren‐Rhodes Kimberley A. , Dungan Jennifer L. , Piatek Jennifer , Stubbs Kristin , Gómez‐Silva Benito , Chen Yong , McKay Christopher P.
Published: September 2007

Plant landscape ecology studies have been carried out for decades and are fundamental to biological research. In contrast, few corollary spatial landscape studies exist for microorganisms, particularly in extreme environments. To address this gap, we mapped the abundance and spatial distribution of photoautotrophs colonizing translucent rocks in several sites in the Atacama Desert, including the hyperarid core. Cyanobacterial communities at all sites are predominantly (≥75%) ‘perilithic’ (confined to the periphery of rocks) and occur in non‐random spatial patterns (“island patches”) at multiple scales. Cyanobacterial patches typically contain 1‐5 colonized rocks but in some cases support much higher numbers. A high resolution mapping of a single 18‐m2 rock cluster at the Aguas Calientes study site (25°S, 69°W) revealed colonization of 5.2% (49 of 948 quartz rocks) and showed colonized rocks to be much larger (∼2X) than the available mean rock size. Ripley's K and point pattern analyses show that quartz rocks are not “selected” or occupied by cyanobacteria randomly, but that non‐random processes distinct from those creating the background rock pattern must be invoked to explain microbial patchiness in the Atacama Desert. These processes include physical controls (rock size/orientation, microtopography) that reflect resource (water) limitations, and biological dispersal via rainfall, fog and wind.
Mid‐Holocene change in types of degassing volcanoes, using indium in Antarctic ice as a tracer of volcanic source type
Authors: Hinkley Todd , Matsumoto Akikazu
Published: September 2007

Proportions of trace metals in Antarctic ice samples indicate that the type of volcanoes that dominated atmospheric emissions changed at about the middle of the Holocene from relatively mafic, deep source volcanoes to more silicic, shallower‐source volcanoes. We base this inference on the strong contrast in the abundances of the trace metal indium (In), relative to other trace metals present in ice, deposited at different times in the past, and on contrasting In abundances in modern emissions of volcanoes of different types. Indium is more abundant in the emissions of deep‐source mafic volcanoes than in more felsic, shallower‐source volcanoes. Earlier workers have shown, on the basis of petrologic and some meteoritic evidence, that In may be partitioned to the interiors (stony mantles) of differentiated planets, or enriched in the liquids of partly crystallized mafic melts.
Asymmetry of tropical precipitation change under global warming
Authors: Chou Chia , Tu Jien‐Yi , Tan Pei‐Hua
Published: September 2007

A clear trend of tropical precipitation changes induced by global warming is found in hemispherical averages of most climate model simulations as well as from observation. It is observed that in response to global warming, an asymmetric pattern develops between tropical precipitation changes in the northern and southern hemispheres, and this asymmetry is locked with the seasonal cycle of tropical convection. In boreal summer (winter), the northern hemispherical average departure from tropical mean increases (decreases), while the departure of the southern hemispherical average decreases (increases). This implies an enhanced seasonal precipitation range between rainy and dry seasons and an increased precipitation difference between northern and southern hemispheres.
Are Chinese loess deposits essentially continuous?
Authors: Zhu Rixiang , Zhang Rui , Deng Chenglong , Pan Yongxin , Liu Qingsong , Sun Youbin
Published: September 2007

We conducted a paleomagnetic examination of the last glacial loess of three representative profiles along an east‐west transect in the central Loess Plateau in order to assess the continuity of Chinese loess. The results show that the Xifeng and Luochuan profiles record the Laschamp geomagnetic excursion but with different morphologies. Together with the published results from Weinan, southern plateau, our results suggest that sedimentation of the last glacial loess in the central‐southern plateau was continuous at the time scale equivalent to the duration of the Laschamp excursion (∼2 kyr), but probably episodic at finer time scales (<2 kyr). No geomagnetic excursion was found at the Yichuan profile near the Yellow River valley, where loess accumulation may be strongly affected by local environmental changes and thus may have been discontinuous. Both site location and time scale therefore need to be considered when considering continuity of Chinese loess.
Dry‐end surface soil moisture variability during NAFE'06
Authors: Teuling A. J. , Uijlenhoet R. , Hurkmans R. , Merlin O. , Panciera R. , Walker J. P. , Troch P. A.
Published: September 2007

Characterization of the space‐time variability of soil moisture is important for land surface and climate studies. Here we develop an analytical model to investigate how, at the dry‐end of the soil moisture range, the main characteristics of the soil moisture field (spatial mean and variability, steady state distribution) depend on the intermittent character of low intensity rain storms. Our model is in good agreement with data from the recent National Airborne Field Experiment (NAFE'06) held in the semiarid Australian Murrumbidgee catchment. We find a positive linear relationship between mean soil moisture and its associated variability, and a strong dependency of the temporal soil moisture distribution to the amount and structure of precipitation.
On the attribution of changing pan evaporation
Authors: Roderick Michael L. , Rotstayn Leon D. , Farquhar Graham D. , Hobbins Michael T.
Published: September 2007

Evaporative demand, measured by pan evaporation, has declined in many regions over the last several decades. It is important to understand why. Here we use a generic physical model based on mass and energy balances to attribute pan evaporation changes to changes in radiation, temperature, humidity and wind speed. We tested the approach at 41 Australian sites for the period 1975–2004. Changes in temperature and humidity regimes were generally too small to impact pan evaporation rates. The observed decreases in pan evaporation were mostly due to decreasing wind speed with some regional contributions from decreasing solar irradiance. Decreasing wind speeds of similar magnitude has been reported in the United States, China, the Tibetan Plateau and elsewhere. The pan evaporation record is invaluable in unraveling the aerodynamic and radiative drivers of the hydrologic cycle, and the attribution approach described here can be used for that purpose.
Spatio‐temporal characterization of the equatorial electrojet from CHAMP, Ørsted, and SAC‐C satellite magnetic measurements
Authors: Alken Patrick , Maus Stefan
Published: September 2007

The equatorial electrojet (EEJ) is an eastward electric current on the day‐side, flowing in a narrow band along the dip equator in the ionospheric E region. Recent magnetic observations from the CHAMP, Ørsted, and SAC‐C satellites, comprising more than 95,000 dip equator crossings from 1999 to 2006, have provided an unprecedented longitudinal coverage of the EEJ magnetic signature. We have used these data to construct an empirical model of the EEJ current climatological mean and day to day variability as a function of longitude, local time, season, and solar flux. Our model has been successfully verified against vertical drift data from the JULIA radar at Jicamarca. We have also used the EEJ observations to estimate the self‐correlation of the EEJ, confirming short longitudinal correlation lengths of 15° and finding a temporal correlation length of 2.4 h. Our model's predictions of the eastward electric field and its standard deviation may provide useful input to various kinds of ionospheric simulations. Coefficients and software are available online at http://models.geomag.us/EEJ.html and http://www.earthref.org.
Spatial patterns of intraseasonal variability of chlorophyll and sea surface temperature in the California Current
Authors: Legaard Kasey R. , Thomas Andrew C.
Published: September 2007

Six years of daily satellite data are used to quantify and map intraseasonal variability of chlorophyll and sea surface temperature (SST) in the California Current. We define intraseasonal variability as temporal variation remaining after removal of interannual variability and stationary seasonal cycles. Semivariograms are used to quantify the temporal structure of residual time series. Empirical orthogonal function (EOF) analyses of semivariograms calculated across the region isolate dominant scales and corresponding spatial patterns of intraseasonal variability. The mode 1 EOFs for both chlorophyll and SST semivariograms indicate a dominant timescale of ∼60 days. Spatial amplitudes and patterns of intraseasonal variance derived from mode 1 suggest dominant forcing of intraseasonal variability through distortion of large scale chlorophyll and SST gradients by mesoscale circulation. Intraseasonal SST variance is greatest off southern Baja and along southern Oregon and northern California. Chlorophyll variance is greatest over the shelf and slope, with elevated values closely confined to the Baja shelf and extending farthest from shore off California and the Pacific Northwest. Intraseasonal contributions to total SST variability are strongest near upwelling centers off southern Oregon and northern California, where seasonal contributions are weak. Intraseasonal variability accounts for the majority of total chlorophyll variance in most inshore areas save for southern Baja, where seasonal cycles dominate. Contributions of higher EOF modes to semivariogram structure indicate the degree to which intraseasonal variability is shifted to shorter timescales in certain areas. Comparisons of satellite‐derived SST semivariograms to those calculated from co‐located and concurrent buoy SST time series show similar features.
Carbon and oxygen dynamics of shallow aquatic systems: Process vectors and bacterial productivity
Authors: Torgersen Thomas , Branco Brett
Published: September 2007

In shallow aquatic systems subject to heavy allochthonous (terrestrial) organic loading, bacterial processing of organic matter can be a significant component in the ecosystem C‐cycle. Because this bacterial processing of organic matter also produces reduced species (Fe+2, Mn+2, S−2, NH3, etc.), these processes also create an additional chemical oxygen demand. Hence, net ecosystem operation can deviate significantly from the “Redfield Line” defined by the traditional photosynthesis/respiration reaction stoichiometry. Here, a 3D process vector concept is presented in terms of (gas‐exchange‐corrected) CO2 or O2‐TDIC‐time to characterize significant bacterial contributions to the net O2 and TDIC dynamics of the ecosystem. The direction of the process vector provides an important clue to the internal process dynamics and the biogeochemical pathways that govern the net ecosystem function. Using instrumented in situ measures of O2 and TDIC in a small pond (ca. 0.7 m deep), this ecosystem is shown to operate significantly off the “Redfield Line” with multiple periods producing a net increase in both (gas‐exchange‐corrected) O2 and CO2 and exporting of both to the atmosphere. A methodology for the quantification of the minimum bacterially produced TDIC in the ecosystem (MBP) is developed. The rates (38–91 mmoles C m−2 day−1) are consistent with measured carbon fluxes from shallow terrestrial aquatic systems as well as anticipated cell numbers in the sediment that likely contribute to this bacterially produced carbon. The process vector concept can be extended to additional dimensions (CH4, NO3−, NH4+, Fe+2, etc.) and may provide a tool for visualization of observatory data streams.
Flow path influence on an N:P ratio in two headwater streams: A paired watershed study
Authors: Green Mark B. , Nieber John L. , Johnson Greg , Magner Joe , Schaefer Brennon
Published: September 2007

Hydrologic flow paths were assessed and stream water quality was monitored in two adjacent watersheds to improve the understanding of how flow paths impact stream water nitrate nitrogen to total phosphorus ratios (NO3:TP). We hypothesized that a watershed with mostly subsurface flow paths will have high stream water NO3:TP ratios relative to a watershed with a greater tendency to have overland flow paths. Flow paths were assessed with end member mixing analysis (EMMA), stream water temperature, and stream discharge flashiness. The EMMA suggested that groundwater comprised a larger percentage of stream discharge in the west stream compared to the east stream. The analysis of stream water temperature and stream flashiness corroborated the results from the EMMA. Nitrate and TP concentrations were similar in both streams during low discharge. However, during storm flows nitrate decreased and TP increased in the east stream, while nitrate and TP concentrations remained similar during all discharge rates in the west stream. Thus, NO3:TP ratios in the east stream dramatically decreased during peak storm flows, while NO3:TP ratios remained relatively stable during all discharge rates in the west stream. Exploring hydrological processes helped explain the temporal variation within and spatial variation between the two streams, with regard to NO3:TP ratios. Further attention is necessary to understand whether hydrologic flow paths act as a control of other stream water N:P ratios and quantify flow path control relative to in‐stream processes.
Acute Astrocyte Activation in Brain Detected by Mri: New Insights into T1 Hypointensity
Authors: Sibson Nicola R , Lowe John P , Blamire Andrew M , Martin Matthew J , Obrenovitch Tiho P , Anthony Daniel C
Published: September 2007

Increases in the T1 of brain tissue, which give rise to dark or hypointense areas on T1-weighted images using magnetic resonance imaging (MRI), are common to a number of neuropathologies including multiple sclerosis (MS) and ischaemia. However, the biologic significance of T1 increases remains unclear. Using a multiparametric MRI approach and well-defined experimental models, we have experimentally induced increases in tissue T1 to determine the underlying cellular basis of such changes. We have shown that a rapid acute increase in T1 relaxation in the brain occurs in experimental models of both low-flow ischaemia induced by intrastriatal injection of endothelin-1 (ET-1), and excitotoxicity induced by intrastriatal injection of N-methyl-d-aspartate (NMDA). However, there appears to be no consistent correlation between increases in T1 relaxation and changes in other MRI parameters (apparent diffusion coefficient, T2 relaxation, or magnetisation transfer ratio of tissue water). Immunohistochemically, one common morphologic feature shared by the ET-1 and NMDA models is acute astrocyte activation, which was detectable within 2 h of intracerebral ET-1 injection. Pretreatment with an inhibitor of astrocyte activation, arundic acid, significantly reduced the spatial extent of the T1 signal change induced by intrastriatal ET-1 injection. These findings suggest that an increase in T1 relaxation may identify the acute development of reactive astrocytes within a central nervous system lesion. Early changes in T1 may, therefore, provide insight into acute and reversible injury processes in neurologic patients, such as those observed before contrast enhancement in MS.
Influence of aggregation, defects, and contaminant oxygen on water dissociation at Cu(110) surface: A theoretical study
Authors: Tang Qian-Lin , Chen Zhao-Xu
Published: September 2007

The DFT-PW91 slab model approach is employed to investigate the influence of aggregation, surface defects, and contaminant oxygen on water dissociation on Cu(110) at low temperatures. The dissociation barriers of water in various aggregate states are calculated in the range of 60–75kJ∕mol on the clean surfaces, in nice agreement with the experimentally determined values. It is revealed that the aggregation of water shows no propensity to reduce the activation barrier for the O–H bond breaking on Cu(110), at variance with the water chemistry on Ru(0001). The calculated activation energy on Cu(211) which is the most active stepped surface investigated is equal to the value on the (110) surface, indicating that the hydroxyl groups observed on Cu(110) at low temperatures may not stem from surface defects. The coadsorbed oxygen, whether as a “spectator” or a “participant,” facilitates the water dissociation both kinetically and thermodynamically.
Atomistic simulations of calcite nanoparticles and their interaction with water
Authors: Cooke David J. , Elliott James A.
Published: September 2007

Molecular dynamics (MD) simulations have been used to study the stability of calcite nanoparticles ranging in size from 18to324f.u., both in vacuo and in the presence of explicit water molecules. In vacuo, the smallest particles become highly disordered during the MD simulation due to rotation and translation of the undercoordinated CO32− anions at the edges of the particles. As the nanoparticle size increases, the influence of the fully coordinated bulk ions begins to dominate and long-range order is seen both in the Ca–C pair distribution functions and in the degree of rotational order of the CO32− anions. However, when explicit water is added to the system, the molecules in the first hydration layer complete the coordination shell of the surface ions, preserving structural order even in the smallest of the nanoparticles. Close to particle surface, the structure of the water itself shows features similar to those seen close to planar periodic (101¯4) surfaces, although the molecules are far less tightly bound.
Canonical transformation theory from extended normal ordering
Authors: Yanai Takeshi , Chan Garnet Kin-Lic
Published: September 2007

The canonical transformation theory of Yanai and Chan [J. Chem. Phys. 124, 194106 (2006)] provides a rigorously size-extensive description of dynamical correlation in multireference problems. Here we describe a new formulation of the theory based on the extended normal ordering procedure of Mukherjee and Kutzelnigg [J. Chem. Phys. 107, 432 (1997)]. On studies of the water, nitrogen, and iron oxide potential energy curves, the linearized canonical transformation singles and doubles theory is competitive in accuracy with some of the best multireference methods, such as the multireference averaged coupled pair functional, while computational timings (in the case of the iron oxide molecule) are two to three orders of magnitude faster and comparable to those of the complete active space second-order perturbation theory. The results presented here are greatly improved both in accuracy and in cost over our earlier study as the result of a new numerical algorithm for solving the amplitude equations.
Emission of Au nanoparticles with and without rhodamine 6G dye
Authors: Zhu G. , Gavrilenko V. I. , Noginov M. A.
Published: September 2007

We have observed Stokes and anti-Stokes emission of Au nanoparticles suspended in methanol and rhodamine 6G dye solution. Photoluminescence of Au nanoparticles is a three-step process involving single-photon or three-photon excitation of electron-hole pairs, relaxation of excited electrons and holes, and emission from electron-hole recombination, possibly enhanced by surface plasmons. In the presence of dye, the excitation of anti-Stokes emission of gold involves two-photon absorption in rhodamine 6G molecules followed by the energy transfer to Au nanoparticles with simultaneous absorption of one pumping photon by Au. This mechanism significantly enhances anti-Stokes emission of gold nanoparticles in the presence of dye.
On the simulation of photoelectron spectra in molecules with conical intersections and spin-orbit coupling: The vibronic spectrum of CH3S
Authors: Schuurman Michael S. , Weinberg Daniel E. , Yarkony David R.
Published: September 2007

A method to simulate photoelectron spectra for states coupled by conical intersections and the spin-orbit interaction is reported. The algorithm is based on the multimode vibronic coupling model and treats the spin-orbit interaction in a nonperturbative manner. Since the algorithm is not dependent on molecular symmetry, the approach is generally applicable to accidental conical intersections as well as the symmetry required intersections found in Jahn-Teller molecules. The method is also computationally efficient using energy gradient and derivative coupling information to limit the number of nuclear configurations at which ab initio data are required. This approach is applied to simulate the negative ion photoelectron spectrum of the methylthio radical. The two-state Hamiltonian employed to describe this system was determined employing ab initio gradients and derivative couplings at only 17 nuclear configurations.
Quasiclassical trajectory calculations of the OH+NO2 association reaction on a global potential energy surface
Authors: Chen Chao , Shepler Benjamin C. , Braams Bastiaan J. , Bowman Joel M.
Published: September 2007

We report a full-dimensional potential energy surface (PES) for the OH+NO2 reaction based on fitting more than 55 000 energies obtained with density functional theory-B3LYP∕6-311G(d,p) calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO2, and the fragments OH+NO2 and HO2+NO. Detailed comparison of the structures, energies, and harmonic frequencies of various stationary points on the PES are made with previous and present high-level ab initio calculations. Two hydrogen-bond complexes are found on the PES and confirmed by new ab initio CASPT2 calculations. Quasiclassical trajectory calculations of the cross sections for ground rovibrational OH+NO2 association reactions to form HOONO and HONO2 are done using this PES. The cross section to form HOONO is larger than the one to form HONO2 at low collision energies but the reverse is found at higher energies. The enhancement of the HOONO complex at low collision energies is shown to be due, in large part, to the transient formation of a H-bond complex, which decays preferentially to HOONO. The association cross sections are used to obtain rate constants for formation of HOONO and HONO2 for the ground rovibrational states in the high-pressure limit.
Stretching effects on the permeability of water molecules across a lipid bilayer
Authors: Gauthier Alain , Joós Béla
Published: September 2007

Using a coarse grained molecular dynamics model of a solvent-surfactant system, we study the effects of stretching on the permeability of water across a lipid bilayer. The density profile, free energy profile, diffusion profile, and tail ordering parameter were computed for a set of stretched membranes maintained at constant area. We computed the water permeability across each membrane using the inhomogeneous solubility-diffusion model first proposed by Marrink and Berendsen [J. Phys. Chem. 98, 4155 (1994)]. We find that even though the resistance to permeation profile shows a great deal of qualitative change as the membranes are stretched, the overall permeability remains nearly constant within the relevant range of stretching. This is explained by the fact that the main barrier to permeation, located in the densest section of the tails, is insensitive to increased area per lipid, as a result of competing effects. Expansion leads to thinning and a higher density in the tail region, the latter leading to an increase in the free energy barrier. However, this is compensated by the reduction in the transverse distance to cross and a larger diffusion coefficient due to increased disordering in the chains.
New communication schemes based on adaptive synchronization
Authors: Yu Wenwu , Cao Jinde , Wong Kwok-Wo , Lü Jinhu
Published: September 2007

In this paper, adaptive synchronization with unknown parameters is discussed for a unified chaotic system by using the Lyapunov method and the adaptive control approach. Some communication schemes, including chaotic masking, chaotic modulation, and chaotic shift key strategies, are then proposed based on the modified adaptive method. The transmitted signal is masked by chaotic signal or modulated into the system, which effectively blurs the constructed return map and can resist this return map attack. The driving system with unknown parameters and functions is almost completely unknown to the attackers, so it is more secure to apply this method into the communication. Finally, some simulation examples based on the proposed communication schemes and some cryptanalysis works are also given to verify the theoretical analysis in this paper.
Theoretical study of the stability of multiply charged C70 fullerenes
Authors: Zettergren Henning , Sánchez Goar , Díaz-Tendero Sergio , Alcamí Manuel , Martín Fernando
Published: September 2007

We have calculated the electronic energies and optimum geometries of C70q+ and C68q+ fullerenes (q=0–14) by means of density functional theory. The ionization energies for C70 and C68 fullerenes increase more or less linearly as functions of charge, consistent with the previously reported behavior for C60 and C58 [S. Díaz-Tendero et al., J. Chem. Phys. 123, 184306 (2005)]. The dissociation energies corresponding to the C70q+→C68q++C2, C70q+→C68(q−1)++C2+, C70q+→C68(q−2)++C++C+, C70q+→C68(q−3)++C2++C+, and C70q+→C68(q−4)++C2++C2+ decay channels show that C70q+ (like C60q+) is thermodynamically unstable for q⩾6. However, the slope of the dissociation energy as a function of charge for a given decay channel is different from that of C60q+ fullerenes. On the basis of these results, we predict q=17 to be the highest charge state for which a fission barrier exists for C70q+.
Two-body Coulomb explosion and hydrogen migration in methanol induced by intense 7 and 21fs laser pulses
Authors: Itakura Ryuji , Liu Peng , Furukawa Yusuke , Okino Tomoya , Yamanouchi Kaoru , Nakano Hidetoshi
Published: September 2007

Two-body Coulomb explosion with the C–O bond breaking of methanol induced by intense laser pulses with the duration of Δt=7 and 21fs is investigated by the coincidence momentum imaging method. When Δt=7 fs, the angular distribution of recoil vectors of the fragment ions for the direct C–O bond breaking pathway, CH3OH2+→CH3++OH+, exhibits a peak deflected from the laser polarization direction by 30°–45°, and the corresponding angular distribution for the migration pathway, CH2OH2+→CH2++H2O+, in which one hydrogen migrates from the carbon site to the oxygen site prior to the C–O bond breaking, exhibits almost the same profile. When the laser pulse duration is stretched to Δt=21fs, the angular distributions for the direct and migration pathways exhibit a broad peak along the laser polarization direction probably due to the dynamical alignment and/or the change in the double ionization mechanism; that is, from the nonsequential double ionization to the sequential double ionization. However, the extent of the anisotropy in the migration pathway is smaller than that in the direct pathway, exhibiting a substantial effect of hydrogen atom migration in the dissociative ionization of methanol interacting with the linearly polarized intense laser field.
Free energies of CO2∕H2 capture by p-tert-butylcalix[4]arene: A molecular dynamics study
Authors: Daschbach John L. , Thallapally Praveen K. , Atwood Jerry L. , McGrail B. Peter , Dang Liem X.
Published: September 2007

The interactions of CO2∕H2 with p-tert-butylcalix[4]arene (TBC4) were studied using potential of mean force (PMF) and free energy perturbation approaches. To the best of our knowledge, the present work is one of the first to employ the constrained mean force approach to evaluate solute selectivity by the TBC4 molecule. The computed PMFs for the interaction of CO2∕H2 with a single TBC4 molecule establish that the interaction of CO2 with the open end of the cage structure is attractive while the interaction with H2 is repulsive. Free energy perturbation calculations were performed for the same two guest molecules with a pair of facing TBC4 molecules used as a representative model as found in the TBC4 molecular solid. At low temperature, both CO2∕H2 have favorable interactions with the TBC4 pair, with the CO2 interaction being considerably greater. These results are in agreement with recent experimental data showing considerable CO2 uptake by TBC4 at moderate pressures.
Structure of incommensurate gold sulfide monolayer on Au(111)
Authors: Quek Su Ying , Biener Monika M. , Biener Juergen , Bhattacharjee Joydeep , Friend Cynthia M. , Waghmare Umesh V. , Kaxiras Efthimios
Published: September 2007

We develop an atomic-scale model for an ordered incommensurate gold sulfide (AuS) adlayer which has previously been demonstrated to exist on the Au(111) surface, following sulfur deposition and annealing to 450K. Our model reproduces experimental scanning tunneling microscopy images. Using state-of-the-art Wannier-function-based techniques, we analyze the nature of bonding in this structure and provide an interpretation of the unusual stoichiometry of the gold sulfide layer. The proposed structure and its chemistry have implications for related S–Au interfaces, as in those involved in self-assembled monolayers of thiols on Au substrates.
Surface tension and capillary waves at the nematic-isotropic interface in ternary mixtures of liquid crystal, colloids, and impurities
Authors: Popa-Nita V. , Oswald P.
Published: September 2007

In mixtures of thermotropic liquid crystals with spherical poly(methyl methacrylate) particles, self-supporting networklike structures are formed during slow cooling past the isotropic-to-nematic phase transition. Experimental results support the hypothesis that a third component, alkane remnants slowly liberated from the particles, plays a crucial role. A theoretical model, based on the phenomenological Landau–de Gennes, Carnahan-Starling, and hard-sphere crystal theories, is developed to describe the continuous phase separation in a ternary nematic-impurity-colloid mixture. The interfacial tension and the dispersion relation of the surface modes of the nematic-isotropic interface are determined. The colloids decrease the interfacial tension and the damping rate of surface waves, whereas impurities act in an opposite way. This should strongly influence the formation of abovementioned networklike structures and could help explain some of their rheological properties.
Vibrational structure, spin-orbit splitting, and bond dissociation energy of Cl2+(X̃Πg2) studied by zero kinetic energy photoelectron spectroscopy and ion-pair formation imaging method
Authors: Li Juan , Hao Yusong , Yang Jie , Zhou Chang , Mo Yuxiang
Published: September 2007

The isotopomer-resolved vibrational and spin-orbit energy structures of Cl2+(X̃Πg2) have been studied by one-photon zero kinetic energy photoelectron spectroscopy. The spin-orbit energy splitting for the ground vibrational state is determined as 717.7±1.5cm−1, which greatly improves on the accuracy of the previously reported data. This value is found to be in good agreement with the ab initio quantum chemical calculation taking account of the inner shell electron correlation. The first adiabatic ionization energy (IE) of Cl2 is determined as 92645.9±1.0cm−1. Using the ion-pair formation imaging method to discriminate signals of Cl+(D21) from those of Cl+(Pj3), the threshold for ion-pair (Etipp) production, Cl+(D21)+Cl−(S01)←Cl2(XΣg+1), is determined as 107096−2+8cm−1. By using the determined IE and Etipp for Cl2 and also the reported IE and electronic affinity for chlorine atom, the bond dissociation energies of Cl2(XΣg+1) and Cl2+(X̃Πg2) have been determined as 19990−2+8 and 31935.1−2+8, respectively.
Experimental determination of the inhomogeneous contribution to linewidth in Permalloy films using a time-resolved magneto-optic Kerr effect microprobe
Authors: Schneider M. L. , Gerrits Th. , Kos A. B. , Silva T. J.
Published: September 2007

We adapted a time-resolved magneto-optic microprobe for use with a pulsed inductive microwave magnetometer apparatus, to measure the magnetization dynamics of a thin Permalloy film at micrometer and millimeter length scales under exactly the same experimental conditions. The optical microprobe has a spatial resolution of 1 μm. We compare the data obtained over these different length scales to quantitatively determine the localized inhomogeneous contribution to magnetic damping. When measured directly with the magneto-optic technique, the inhomogeneous contributions are in agreement with the value 88±16 A/m (1.1±0.2 Oe) extrapolated from PIMM measurements of linewidth versus frequency.
Making four- and two-component relativistic density functional methods fully equivalent based on the idea of “from atoms to molecule”
Authors: Peng Daoling , Liu Wenjian , Xiao Yunlong , Cheng Lan
Published: September 2007

It is shown that four- and two-component relativistic Kohn-Sham methods of density functional theory can be made fully equivalent in all the aspects of simplicity, accuracy, and efficiency. In particular, this has been achieved based solely on physical arguments rather than on mathematical tricks. The central idea can be visualized as “from atoms to molecule,” reflecting that the atomic information is employed to “synthesize” the molecular no-pair relativistic Hamiltonian. That is, the molecular relativistic Hamiltonian can, without loss of accuracy, be projected onto the positive energy states of the isolated Dirac atoms with the projector approximated simply by the superposition of the atomic ones. The dimension of the four-component Hamiltonian matrix then becomes the same as that of a two-component one. Another essential ingredient is to formulate quasirelativistic theory on matrix form rather than on operator form. The resultant quasi-four-component, normalized elimination of the small component, and symmetrized elimination of the small component approaches are critically examined by taking the molecules of MH and M2 (M=At, E117) as examples.
Population transfer to excited vibrational levels of H2 molecule by stimulated hyper-Raman passage with chirped laser pulses
Authors: Sarkar Chitrakshya , Bhattacharya Rangana , Bhattacharyya S. S. , Saha Samir
Published: September 2007

We have theoretically investigated the population transfer from the initial ground rovibrational level vg=0, Jg=0 to the final rovibrational levels vf=1,2, Jf=0 of the ground electronic state XΣg+1 via the resonant intermediate level vi=6, Ji=0 of the excited electronic state EFΣg+1 of H2 molecule by (2+2)-photon stimulated hyper-Raman passage (STIHRP). The density matrix technique has been employed to evaluate the population transfer to the final target levels using linearly chirped pump and Stokes laser pulses with different chirp rates. Both the pulses are considered to have the same temporal shape, pulse width, and linear parallel polarizations. We have studied in detail the dependence of the population transfer on the set of laser parameters for pulse (peak) intensities in the ranges of 1.5×1011–1.0×1012 and 1.0×1012–7.0×1012W∕cm2. The corresponding pulse widths (full width at half maximum) are of the order of 115–200 and 15–30ps. We have found that the chirp rate parameters can be optimized to achieve almost complete population transfer from the ground (g) to the final (f) target levels. This, to our knowledge, is the first application of a (2+2)-photon STIHRP process with chirpings to a model molecular system (H2). The study demonstrates the suitability of the chirped (2+2)-photon STIHRP technique for selective and almost total inversion of vibrational population in a diatomic molecule.
Field dependence of the activation volumes in Co/Pd multilayered films
Authors: Ryu Kwang-Su , Choe Sug-Bong , Lee Kyeong-Dong , Shin Sung-Chul
Published: September 2007

The field dependence of the activation volumes of nucleation and wall-motion processes in magnetization reversal of Co/Pd multilayers is presented, as observed using a magneto-optical Kerr-effect microscope capable of real-time domain imaging in a wide time range of 10−5−103 s. The analytic forms of the activation volumes are derived from a theoretical consideration of equilibrium conditions of the two typical domain evolution processes in a ferromagnetic film with uniaxial perpendicular anisotropy; the theory is shown to be in good agreement with the experimental results. It was found that the field dependence of the activation volumes can be characterized by the difference between the domain wall energy and the dipolar energy.
Influence of traps on top and bottom contact field-effect transistors based on modified poly(phenylene-vinylene)
Authors: Herasimovich A. , Scheinert S. , Hörselmann I.
Published: September 2007

We report on the investigations of the differences in the measured current characteristics of source/drain top contact (TOC) and bottom contact (BOC) organic field-effect transistors (OFETs). The active layer was made from poly[1,4-phenylene-(4-methylphenyl)imino-4,48-diphenylene-(4-methylphenyl)imino-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] and a highly doped silicon wafer with a thermal oxide was used for the gate and the insulator, respectively. Both transistors show a good subthreshold behavior characterized by a threshold voltage of Vth≈−5 V and an inverse subthreshold increase of S≈0.25 V∕dec. The estimated mobility is in the range of 10−3 cm2∕V s. The main difference between the TOC and the BOC transistors is a nonlinear increase of the drain current at small drain voltages in the output characteristics of the TOC OFETs. Numerical two-dimensional simulations show that a high concentration of donor-like traps with a Gaussian or exponential distribution are the reason for this peculiarity. The investigation of the influence of air confirms the presence of such traps.
Effect of various physical factors on thin lubricant film migration on the flying head slider at the head-disk interface of hard disk drives
Authors: Kubotera Hiroyuki , Bogy David B.
Published: September 2007

The mechanism of lubricant migration on a flying slider’s air bearing surface in magnetic hard disk drives is examined in this paper. As the head-disk spacing diminishes, the lubricant contamination from the disk and its migration behavior on the flying head slider becomes an increasing concern. The spatiotemporal behavior of the thin lubricant film on a slider surface is complicated, and it is affected by many physical factors. We investigate the contributions and functionalities of the potentially related physical factors and parameters on the lubricant film dynamics by numerical simulation. The analysis results show that the air bearing shear stress is the primary driving force in the lubricant migration, while the air bearing pressure makes only a minor contribution. The relative strength and direction of the Couette and Poiseuille flow components of the air bearing shear stress govern the lubricant migration direction and the points of lubricant stagnation. The lubricant surface free energy plays an important role while the contribution of diffusion is relatively small through the Laplace pressure but relatively large through the lubricant disjoining pressure which is determined by the surface energy gradient. We also find that the lubricant viscosity affects the lubricant dynamics as a time scaling factor. These results reveal the underlying physical framework of the complex phenomena and provide useful insight for developing the head disk interface of magnetic disk drives.
Size effect in morphotropic phase boundary Pb(Mg1∕3Nb2∕3)O3–PbTiO3
Authors: Algueró M. , Ricote J. , Jiménez R. , Ramos P. , Carreaud J. , Dkhil B. , Kiat J. M. , Holc J. , Kosec M.
Published: September 2007

Phases, domain configuration, and properties of 0.65Pb(Mg1∕3Nb2∕3)O3–0.35PbTiO3 ceramics with grain sizes of 4 and 0.15μm have been studied. The average phase is monoclinic Pm in coexistence with tetragonal. An evolution from micron-sized lamellar domains towards submicron/nanometer sized crosshatched domains is found with the decrease in size, which results in electrical relaxor type behavior and hindered switching. This is proposed to be associated with the slowing down of the relaxor to ferroelectric transition that causes the long time presence of an intermediate domain configuration. Nevertheless, a high sensitivity piezoelectric submicron-structured material is obtained under tailored poling (d33∼300pCN−1).
Mass dependence of quantum energy inequality bounds
Authors: Eveson Simon P. , Fewster Christopher J.
Published: September 2007

In a recent paper [C. J. Fewster and M. J. Pfenning, J. Math. Phys. 47, 082303 (2006)], quantum energy inequalities were used to place simple geometrical bounds on the energy densities of quantum fields in Minkowskian space-time regions. Here, we refine this analysis for massive fields, obtaining more stringent bounds which decay exponentially in the mass. At the technical level, this involves the determination of the asymptotic behavior of the lowest eigenvalue of a family of polyharmonic differential equations, a result which may be of independent interest. We compare our resulting bounds with the known energy density of the ground state on a cylinder space-time. In addition, we generalize some of our technical results to general Lp spaces and draw comparisons with a similar result in the literature.
Three dimensional drop tracking flow chamber for coalescence studies
Authors: Grillet Anne M. , Brooks Carlton F. , Bourdon Chris J. , Gorby Allen D.
Published: September 2007

We have developed a novel flow chamber which imposes a controlled axisymmetric stagnation flow to enable the study of external flow effects on coalescence dynamics. This system allows for the first time the precise positioning of a drop in a three dimensional flow and additionally enforces a highly symmetric flow around the drop. We focus on the study of a single drop approaching a stationary flat plane as this is analogous to two drops approaching each other. A single drop is created and then guided along the unsteady center line of a stagnation flow. The real time computer control algorithm analyzes video images of the drop in two orthogonal planes and manipulates flow restricting valves along the four outlets of the flow. We demonstrate using particle image velocimetry that the computer control not only controls the drop position but also ensures a symmetric flow inside the flow chamber. This chamber will enable a detailed investigation of the drainage of the thin film between the drop and the lower surface in order to probe the effect of external flow on coalescence.
Altering low-bias transport in zigzag-edge graphene nanostrips with edge chemistry
Authors: Gunlycke D. , Li J. , Mintmire J. W. , White C. T.
Published: September 2007

Zigzag-edge graphene nanostrips could be terminated with a variety of atoms or functional groups. Presented local-density-functional calculations show that these different species have a significant impact on the electronic structure of these strips near the Fermi level. The terminations covered include hydrogen and oxygen atoms as well as hydroxyl and imine groups. Zigzag-edge nanostrips terminated with hydrogen atoms or hydroxyl groups exhibit spin polarization in equilibrium, while the nanostrips terminated with oxygen or imine groups are unpolarized, leading to much different low-bias transport properties.
Polarization control of 0.85μm vertical-cavity surface-emitting lasers integrated with gold nanorod arrays
Authors: Babu Dayal P. , Koyama F.
Published: September 2007

In this letter, the authors unambiguously demonstrate the polarization control of 0.85μm single transverse mode vertical-cavity surface-emitting lasers (VCSELs) using plasmonic nanorod arrays fabricated on the top surface of VCSELs. The strong direction dependent scattering of light from anisotropic gold nanorods enables stable polarization operations of VCSELs with suppressing the absorption in gold layer and leads to high differential quantum efficiencies. These nanorod arrays integrated VCSELs show negligible penalties in laser performances having the maximum output power of nearly 1mW with the side mode suppression ratio of 45dB and the orthogonal polarization suppression ratio of 21dB.
Demonstration of an ultraviolet ZnO-based optically pumped third order distributed feedback laser
Authors: Hofstetter Daniel , Bonetti Yargo , Giorgetta Fabrizio R. , El-Shaer Abdel-Hamid , Bakin Andrey , Waag Andreas , Schmidt-Grund Rüdiger , Schubert Mathias , Grundmann Marius
Published: September 2007

The authors demonstrate an optically pumped ZnO distributed feedback laser operating at 383nm. For a large temperature range between 10 and 270K, the device lased in a single longitudinal mode. Mode selection was accomplished via a third order diffraction grating, which was dry etched into a 120nm thick Si3N4 layer deposited on the ZnO active region. They observed a spectral linewidth of 0.4nm, a pump threshold intensity of 0.12MW∕cm2, and a peak output power of 14mW. From wavelength versus temperature measurements, they deduced a temperature tuning coefficient of the ZnO refractive index of 9×10−5K−1.
Strong enhancement of second-harmonic generation in all-dielectric resonant waveguide grating
Authors: Siltanen Mikael , Leivo Samuli , Voima Pauliina , Kauranen Martti , Karvinen Petri , Vahimaa Pasi , Kuittinen Markku
Published: September 2007

The authors demonstrate that a dielectric resonant waveguide grating can enhance optical second-harmonic generation by a factor of 550 compared to a similar flat surface. Their structure, which consists of purely dielectric and thereby transparent materials, has a low index silicon dioxide grating covered by a high index titanium dioxide layer and it is optimized for the fundamental wavelength of 1064nm. Polarization dependence of the second-harmonic response suggests that the enhancement arises from the favorable interaction of the resonant waveguide mode and its strong local field with the inherent surface nonlinearity of the dielectric materials.