bluuebunny/crossref_metadata_2025 huggingface dataset:
Clustering, randomness and regularity in cloud fields: 1. Theoretical considerations
Authors: Weger R. C. , Lee J. , Zhu Tianri , Welch R. M.
Published: February 2012

During the last decade, there has been considerable controversy in the literature concerning the nature of cloud distributions. Both theoretical and observational evidence has been presented to support cloud field clustering in some cases and cloud field regularity in other cases. Therefore considerable uncertainty exists. It is essential to resolve this issue since it impacts both cloud dynamical and global cloud radiative forcing issues. In the present investigation, analysis and simulation studies based upon nearest‐neighbor cumulative distribution statistics, when applied to a point process consisting of cloud centers, show that (1) the Poisson representation of random point processes is superior to pseudorandom number generated models; (2) pseudorandom number generated models bias the observed nearest‐neighbor statistics towards regularity; and (3) an upper bound on the errors caused by edge effects in finite areas is shown to be negligible in certain ranges of cloud number density. Interpretation of this nearest‐neighbor statistic is discussed for many cases of superpositions of clustering, randomness, and regularity. This is accomplished by plotting the observed versus random nearest‐neighbor cumulative distribution functions. Both the magnitude of the positive or negative deviations from the diagonal are important indicators of clustering and regularity. Alternative point‐to‐neighbor statistics are found to be useful for identifying the components of combinations of distribution types. Finally, and perhaps most importantly, the nature and magnitude of cloud size effects upon the nearest‐neighbor and point‐to‐neighbor cumulative distribution curves are investigated. It is found that even at only 15% cloud cover, size effects dominate the bias caused by the lack of randomness of pseudorandom number generators or by edge effects. Furthermore, cloud size effects shift the nearest‐neighbor cumulative distribution curves to mimic regularity.
Absolute absorption of ozone in the midinfrared
Authors: Pickett H. M. , Peterson D. B. , Margolis J. S.
Published: February 2012

The absolute absorption of ozone in the midinfrared has been measured relative to 253.7‐nm ultraviolet absorption to an accuracy of 1 % using a Bruker Fourier transform spectrometer. These new results agree with previous measurements by diode laser measurements after the latter are corrected for more recent infrared calculations and ultraviolet cross sections. They also agree with a determination based on refractive index measurements in the infrared.
Charge separation by ice and water drops during growth and evaporation
Authors: Dong Yayi , Hallett John
Published: February 2012

Experiments reveal complicated charging behavior during evaporation and vapor growth of ice and water in air. In the laboratory environment, the ice specimen acquires a positive charge during growth and a negative charge during evaporation, except in the temperature region between −4°C and 0°C, where ice behaves like water, acquiring a negative charge during condensation growth. The current is in excess of 10−16 amps cm−2 s−1. The ion concentration in the air has a strong effect on the charging direction. The analysis shows that the charging direction is not simply the result of evaporation or growth as suggested in earlier work but is determined by the properties of the charge carriers in ice (or water) and in the air. The new hypothesis can explain the charging behavior of both the growth of ice and water from the vapor and also evaporation observed in the present experiments. This new hypothesis also gives interpretation of some discrepancies in the earlier studies. The mechanism does not require absolute growth or evaporation to operate, and it is possibly an important contributory factor in charge separation both in thunderstorms and weakly electrified clouds when long growth or evaporation times are available.
Overview of atmospheric correction and radiometric calibration efforts during FIFE
Authors: Halthore Rangasayi N. , Markham Brian L.
Published: February 2012

The primary responsibility within the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) for understanding the radiometric properties of the remote sensing instrumentation and the atmospheric optical properties affecting measurements with these instruments rested within the atmospheric corrections and radiometric calibration subgroup of FIFE, called the correction/calibration group for short. Specific activities included (1) calibration of remote sensing instrumentation used on aircraft and surface platforms, (2) formulation of recommendations for calibrating data from satellite sensors, (3) measurement and analysis of atmospheric properties, (4) development and evaluation of radiative transfer algorithms which perform the atmospheric correction of remotely sensed data, and (5) derivation and evaluation of surface properties such as reflectances and temperatures. A review of the activities performed by this group and a discussion of the importance of these activities in retrieving surface parameters is presented here.
Interannual wind variability on the south and east coasts of South Africa
Authors: Schumann Eckart H.
Published: February 2012

An analysis of 38 years of wind data from three sites along the South African east and south coasts is made to determine interannual variability. Different period bands are investigated, and it is found that the northernmost site (Durban, at about 30°S) differs markedly from the two southern sites (Port Elizabeth and Cape Town, at about 34°S). At the latter sites interannual trends exist in the yearly averaged wind directions, while for Port Elizabeth the principal axis orientations including winds lying within the “weather band” also show such a trend. It appears that a major readjustment occurred at the two southern sites during the very strong El Niño Southern Oscillation (ENSO) event in 1982/1983, with abrupt changes of up to 30° in the wind directions being registered. On the other hand, there are no clearly identifiable trends in the wind speeds, and longer time series will be needed to establish correlations with weaker ENSO events.
Seasonal cycle and second‐moment statistics of a simple coupled climate system
Authors: Kim Kwang‐Yul , North Gerald R.
Published: February 2012

Simple one‐dimensional ocean models have been used to make projections of future temperature trends due to a greenhouse warming, aerosols, or emission reduction. This paper considers the seasonal cycle and the second‐moment statistics of a two‐dimensional energy balance model with a deep ocean. The surface component, a typical surface energy balance model including the ocean mixed layer, is coupled to an infinite‐depth ocean which is characterized by uniform vertical diffusion and upwelling. These processes transport heat energy vertically from the mixed layer to the depth of the ocean and may be important for long‐term climatic change of the Earth. The model has an explicit two‐dimensional geography. The model reproduces the January and July temperature fields reasonably. The amplitude and phase of the annual and semiannual cycles of the model compare favorably with those of observations. The second‐moment statistics of the response field are important measures of the sensitivity of the model. When forced by a noise forcing with spectrum white in space and time, the variance, temporal correlation, and spatial correlation of the surface temperature of the model look similar to those of observations. In the presence of a deep ocean the variance and spatial correlation scale of the model are reduced somewhat over the ocean compared with the mixed layer only case. The temporal correlation scale is shortened by about a month.
Time, evolution and physical reductionism
Authors: Rull Valentí
Published: February 2012

Understanding and explaining the nature of time remains a major challenge for science. The task is made harder by the living world, which seems to run on a different concept of time than the realm of physics.
Autophagy—alias self‐eating—appetite and ageing
Authors: Rubinsztein David C
Published: February 2012

Two articles—one published online in January and in the March issue EMBO reports—implicate autophagy in the control of appetite by regulating neuropeptide production in hypothalamic neurons. Autophagy decline with age in POMC neurons induces obesity and metabolic syndrome.
Lipid body biogenesis and the role of microtubules in lipid synthesis in Ornithogalum umbellatum lipotubuloids
Authors: Kwiatkowska Maria , Popłońska Katarzyna , Wojtczak Agnieszka , Stȩpiński Dariusz , Polit Justyna Teresa
Published: February 2012

Lipid bodies present in lipotubuloids of Ornithogalum umbellatum ovary epidermis take the form of a lens between leaflets of ER (endoplasmic reticulum) membrane filled with a highly osmiophilic substance. The two enzymes, DGAT1 [DAG (diacylglycerol) acyltransferase 1] and DGAT2 (DAG acyltransferase 2), involved in this process are synthesized on rough ER and localized in the ER near a monolayer surrounding entities like lipid bodies. After reaching the appropriate size, newly formed lipid bodies transform into mature spherical lipid bodies filled with less osmiophilic content. They appear to be surrounded by a half‐unit membrane, with numerous microtubules running adjacently in different directions. The ER, no longer continuous with lipid bodies, makes contact with them through microtubules. At this stage, lipid synthesis takes place at the periphery of lipid bodies. This presumption, and a hypothesis that microtubules are involved in lipid synthesis delivering necessary components to lipid bodies, is based on strong arguments: (i) silver grains first appear over microtubules after a short [3H]palmitic acid incubation and before they are observed over lipid bodies; (ii) blockade of [3H]palmitic acid incorporation into lipotubuloids by propyzamide, an inhibitor of microtubule function; and (iii) the presence of gold grains above the microtubules after DGAT1 and DGAT2 reactions, as also near microtubules after an immunogold method that identifies phospholipase D1.
Predictors of Second Language Acquisition in Latino Children With Specific Language Impairment
Authors: Gutiérrez-Clellen Vera , Simon-Cereijido Gabriela , Sweet Monica
Published: February 2012

Purpose

This study evaluated the extent to which the language of intervention, the child’s development in Spanish, and the effects of English vocabulary, use, proficiency, and exposure predict differences in the rates of acquisition of English in Latino children with specific language impairment (SLI).

Method

In this randomized controlled trial, 188 Latino preschoolers with SLI participated in a small-group academic enrichment program for 12 weeks and were followed up 3 and 5 months later. Children were randomly assigned to either a bilingual or an English-only program. Predictors of English growth included measures of Spanish language skills and English vocabulary, use, proficiency, and exposure. Performance on English outcomes (i.e., picture description and narrative sample) was assessed over time. A series of longitudinal models were tested via multilevel modeling with baseline and posttreatment measures nested within child.

Results

Children demonstrated growth on the English outcomes over time. The language of intervention, Spanish skills, English vocabulary, and English use significantly predicted differences in rates of growth across children for specific measures of English development.

Conclusions

This study underscores the role of the child’s first language skills, the child’s level of English vocabulary development, and level of English use for predicting differences in English acquisition in Latino preschoolers with SLI. These factors should be carefully considered in making clinical decisions.
Macrostructural Narrative Language of Adolescents and Young Adults With Down Syndrome or Fragile X Syndrome
Authors: Finestack Lizbeth H. , Palmer Meghan , Abbeduto Leonard
Published: February 2012

Purpose

To gain a better understanding of language abilities, the expressive macrostructural narrative language abilities of verbally expressive adolescents and young adults with Down syndrome (DS) and those with fragile X syndrome (FXS) were examined.

Method

The authors evaluated 24 adolescents and young adults with DS, 12 male adolescents and young adults with FXS, and 21 younger children with typical development (TD). Narrative samples were assessed at the macrostructural level using the narrative scoring scheme (Heilmann, Miller, Nockerts, & Dunaway, 2010). Three group comparisons were made using (a) the full sample matched on nonverbal mental age, (b) a subset of the participants individually matched on nonverbal mental age, and (c) a subset of participants individually matched on mean length of utterance.

Results

Study analyses revealed that the DS and FXS groups significantly outperformed the TD group on a limited number of narrative scoring scheme measures. No significant differences emerged between the DS and FXS groups.

Conclusions

The study’s results suggest that some aspects of macrostructural narrative language may be relative strengths for adolescents and young adults with DS and those with FXS. These results can be used to create more nuanced and informed approaches to assessment and intervention for these populations.
Effect of pulse-modulated microwaves on fullerene ion production with electron cyclotron resonance ion source
Authors: Asaji T. , Uchida T. , Minezaki H. , Oshima K. , Racz R. , Muramatsu M. , Biri S. , Kitagawa A. , Kato Y. , Yoshida Y.
Published: February 2012

Fullerene plasmas generated by pulse-modulated microwaves have been investigated under typical conditions at the Bio-Nano electron cyclotron resonance ion source. The effect of the pulse modulation is distinct from that of simply structured gases, and then the density of the fullerene plasmas increased as decreasing the duty ratio. The density for a pulse width of 10 μs at the period of 100 μs is 1.34 times higher than that for CW mode. We have studied the responses of fullerene and argon plasmas to pulsed microwaves. After the turnoff of microwave power, fullerene plasmas lasted ∼30 times longer than argon plasmas.
Electron cyclotron resonance ion source plasma chamber studies using a network analyzer as a loaded cavity probe
Authors: Toivanen V. , Tarvainen O. , Lyneis C. , Kauppinen J. , Komppula J. , Koivisto H.
Published: February 2012

A method and first results utilizing a network analyzer as a loaded cavity probe to study the resonance properties of a plasma filled electron cyclotron resonance ion source (ECRIS) plasma chamber are presented. The loaded cavity measurements have been performed using a dual port technique, in which two separate waveguides were used simultaneously. One port was used to ignite and sustain the plasma with a microwave source operating around 11 GHz and the other was used to probe the cavity properties with the network analyzer using a frequency range around 14 GHz. The first results obtained with the JYFL 14 GHz ECRIS demonstrate that the presence of plasma has significant effects on the resonance properties of the cavity. With plasma the frequency dependent behavior is strongly damped and this trend strengthens with increasing microwave power.
Advanced light ion source extraction system for a new electron cyclotron resonance ion source geometry at Saclay
Authors: Delferrière O. , Gobin R. , Harrault F. , Nyckees S. , Sauce Y. , Tuske O.
Published: February 2012

One of the main goal of intense light ion injector projects such as IPHI, IFMIF, or SPIRAL2, is to produce high current beams while keeping transverse emittance as low as possible. To prevent emittance growth induced in a dual solenoid low energy transfer line, its length has to be minimized. This can be performed with the advanced light ion source extraction system concept that we are developing: a new ECR 2.45 GHz type ion source based on the use of an additional low energy beam transport (LEBT) short length solenoid close to the extraction aperture to create the resonance in the plasma chamber. The geometry of the source has been considerably modified to allow easy maintenance of each component and to save space in front of the extraction. The source aims to be very flexible and to be able to extract high current ion beams at energy up to 100 kV. A specific experimental setup for this source is under installation on the BETSI test bench, to compare its performances with sources developed up to now in the laboratory, such as SILHI, IFMIF, or SPIRAL2 ECR sources. This original extraction source concept is presented, as well as electromagnetic simulations with OPERA-2D code. Ion beam extraction in space charge compensation regime with AXCEL, and beam dynamics simulation with SOLMAXP codes show the beam quality improvement at the end of the LEBT.
Results of RIKEN superconducting electron cyclotron resonance ion source with 28 GHz
Authors: Higurashi Y. , Ohnishi J. , Nakagawa T. , Haba H. , Tamura M. , Aihara T. , Fujimaki M. , Komiyama M. , Uchiyama A. , Kamigaito O.
Published: February 2012

We measured the beam intensity of highly charged heavy ions and x-ray heat load for RIKEN superconducting electron cyclotron resonance ion source with 28 GHz microwaves under the various conditions. The beam intensity of Xe20+ became maximum at Bmin ∼ 0.65 T, which was ∼65% of the magnetic field strength of electron cyclotron resonance (BECR) for 28 GHz microwaves. We observed that the heat load of x-ray increased with decreasing gas pressure and field gradient at resonance zone. It seems that the beam intensity of highly charged heavy ions with 28 GHz is higher than that with 18 GHz at same RF power.
Automatic generation of IGUN(C) input files for enhanced mesh resolution at the plasma meniscus
Authors: Becker Reinard
Published: February 2012

The plasma-beam interface (meniscus) is highly nonlinear and its correct simulation needs a mesh resolution of the order of the Debye length. In high intensity ion sources, the plasma density is usually too high and the Debye length is too small for a sufficient mesh resolution. A well established method to overcome this dilemma is the use of a field line and an equipotential line to be created in a first run, in order to dissect the simulation problem into a plasma part with much higher mesh resolution and a transport part with usual resolution. In the past many users of IGUN have found it difficult to perform this dissection. Therefore, a new feature has been added to IGUN to automatically write new input files for the dissected areas. For this a field line starting point needs to be defined as well as the potential of a pseudo electrode. The field line then is used for the plasma part as a slanted and curved Neumann boundary, while the pseudo electrode will act as the extraction electrode. The trajectory end data then are used in the automatically generated concatenating run as ion starting input without any need for the user to adjust for positions or different mesh resolutions. Here we show as an example the simulation of the well-known CHORDIS ion source, the calculated field line, the pseudo equipotential line, and the resulting simulations for the automatically generated input files for the plasma and the transport parts.
Generation of high charge state metal ion beams by electron cyclotron resonance heating of vacuum arc plasma in cusp trap
Authors: Nikolaev A. G. , Savkin K. P. , Oks E. M. , Vizir A. V. , Yushkov G. Yu. , Vodopyanov A. V. , Izotov I. V. , Mansfeld D. A.
Published: February 2012

A method for generating high charge state heavy metal ion beams based on high power microwave heating of vacuum arc plasma confined in a magnetic trap under electron cyclotron resonance conditions has been developed. A feature of the work described here is the use of a cusp magnetic field with inherent “minimum-B” structure as the confinement geometry, as opposed to a simple mirror device as we have reported on previously. The cusp configuration has been successfully used for microwave heating of gas discharge plasma and extraction from the plasma of highly charged, high current, gaseous ion beams. Now we use the trap for heavy metal ion beam generation. Two different approaches were used for injecting the vacuum arc metal plasma into the trap − axial injection from a miniature arc source located on-axis near the microwave window, and radial injection from sources mounted radially at the midplane of the trap. Here, we describe preliminary results of heating vacuum arc plasma in a cusp magnetic trap by pulsed (400 μs) high power (up to 100 kW) microwave radiation at 37.5 GHz for the generation of highly charged heavy metal ion beams.
Magnon excitation and temperature dependent transport properties in magnetic tunnel junctions with Heusler compound electrodes
Authors: Drewello Volker , Ebke Daniel , Schäfers Markus , Kugler Zoë , Reiss Günter , Thomas Andy
Published: February 2012

Magnetic tunnel junctions were prepared with the Heusler compounds Co2FeAl, Co2FeSi, and Co2MnSi as the soft magnetic electrode. The Co2MnSi electrodes had a multilayer design that used either the Co2FeAl or the Co2FeSi compound as a buffer material. Pinned Co-Fe was used as the hard reference electrode. The electronic transport characteristics were analyzed by tunneling spectroscopy. The dependence of sample properties on the buffer material was of interest, especially the gap in the minority density of states of the Heusler electrode. The temperature dependence of the transport properties was also investigated.
Effects of Ni addition on the magnetostriction and microstructures of Fe70−xPd30Nix high-temperature ferromagnetic shape memory alloys
Authors: Lin Yin-Chih , Lin Chien-Feng
Published: February 2012

This study investigated the effects of adding a third alloying element, Ni, to create Fe70−xPd30Nix (x = 2, 4, 6, and 8 at. % Ni) ferromagnetic shape memory alloys. The Ni replaced a portion of the Fe. The magnetostriction and microstructures of Fe70−xPd30Nix high-temperature ferromagnetic shape memory alloys were studied in detail. Investigation of the magnetostriction and microstructures indicated that the greater Ni amount in the Fe70−xPd30Nix alloys caused the less saturation magnetostriction at room temperature (RT); it was also observed that it was more difficult to generate an annealed recrystallization. However, greater Ni addition into the Fe70-xPd30Nix (x = 6 and 8 at. % Ni) alloys, the L10 + L1m twin phase decomposition into stoichiometric L10 + L1m + αbct structures could be suppressed when the alloys were aged at 500 °C for 100 h. The result was that the Fe70−xPd30Nix (x = 6 and 8 at. % Ni) alloys maintained a high magnetostriction and magnetostrictive susceptibility (Δλ‖s/ΔH) after the 500 °C/100 h aged treatment. This magnetic property of the Fe70−xPd30Nix (x = 6 and 8 at. % Ni) alloys is suitable for application in a high temperature (T > 500 °C) and high frequency environment.
Electric field control of interface magnetic anisotropy
Authors: Xu Lei , Zhang Shufeng
Published: February 2012

We present a model for determining the Rashba spin-orbit coupling (RSOC) at magnetic surfaces or interfaces by explicitly taking into account the interaction between the inversion-symmetry-broken potential and the spin-dependent electric dipoles of the Bloch states. We show that the RSOC alone can generate a perpendicular surface magnetic anisotropy comparable to the observed values in transition metals. When an external electric field is applied across the interface, the induced screening potential modifies the RSOC and thus controls the direction of the magnetization. Our results are consistent with the existing experiments.
Ferromagnetic resonance probed annealing effects on magnetic anisotropy of perpendicular CoFeB/MgO bilayer
Authors: Chen Y. S. , Cheng Chih-Wei , Chern G. , Wu W. F. , Lin J. G.
Published: February 2012

The annealing effects of perpendicular MgO/CoFeB/Ta structure are investigated with ferromagnetic resonance technique. The data of angular dependent resonance field, linewidth, and intensity are obtained for both as-grown and annealed samples. An increment of magnetic out-of-plane anisotropy field from 1800 to 3300 Oe with annealing is observed, which should be related to the improvement of crystalline at the interface. Ferromagnetic resonance linewidth significantly broadens after annealing, and this broadening is attributed to the enhancement of inhomogeneity of the internal field.
Performance of a fast digital integrator in on-field magnetic measurements for particle accelerators
Authors: Arpaia P. , Bottura L. , Fiscarelli L. , Walckiers L.
Published: February 2012

The fast digital integrator has been conceived to face most demanding magnet test requirements with a resolution of 10 ppm, a signal-to-noise ratio of 105 dB at 20 kHz, a time resolution of 50 ns, an offset of 10 ppm, and on-line processing. In this paper, the on-field achievements of the fast digital integrator are assessed by a specific measurement campaign at the European Organization for Nuclear Research (CERN). At first, the architecture and the metrological specifications of the instrument are reported. Then, the recent on-field achievements of (i) ±10 ppm of uncertainty in the measurement of the main field for superconducting magnets characterization, (ii) ±0.02 % of field uncertainty in quality assessment of small-aperture permanent magnets, and (iii) ±0.15 % of drift, in an excitation current measurement of 600 s under cryogenic conditions, are presented and discussed.
Active noise cancellation in a suspended interferometer
Authors: Driggers Jennifer C. , Evans Matthew , Pepper Keenan , Adhikari Rana
Published: February 2012

We demonstrate feed-forward vibration isolation on a suspended Fabry-Perot interferometer using Wiener filtering and a variant of the common least mean square adaptive filter algorithm. We compare the experimental results with theoretical estimates of the cancellation efficiency. Using data from the recent Laser Interferometer Gravitational Wave Observatory (LIGO) Science Run, we also estimate the impact of this technique on full scale gravitational wave interferometers. In the future, we expect to use this technique also to remove acoustic, magnetic, and gravitational noise perturbations from the LIGO interferometers. This noise cancellation technique is simple enough to implement in standard laboratory environments and can be used to improve signal-to-noise ratio for a variety of high precision experiments.
Quantum deformation of two four-dimensional spin foam models
Authors: Fairbairn Winston J. , Meusburger Catherine
Published: February 2012

We construct the q-deformed version of two four-dimensional spin foam models, the Euclidean and Lorentzian versions of the Engle, Pereira, Rovelli and Livine (EPRL) model. The q-deformed models are based on the representation theory of two copies of \documentclass[12pt]{minimal}\begin{document}$U_q(\mathfrak {su}(2))$\end{document}Uq(su(2)) at a root of unity and on the quantum Lorentz group with a real deformation parameter. For both models, we give a definition of the quantum EPRL intertwiners, study their convergence and braiding properties, and construct an amplitude for the four-simplexes. We find that both of the resulting models are convergent.
Excess conductivity of Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ superconductors
Authors: Khan Nawazish A. , Hasnain S. M.
Published: February 2012

Oxide high-Tc superconductors (HTSC) are anisotropic in character since the charge carriers have free moment in the conducting CuO2 planes [A. L. Solov’ev and V. M. Dmitriev, Fiz. Nizk. Temp. 35, 227 (2009) [Low Temp. Phys. 35, 169 (2009)]] whereas their motion is impeded by insulating/partially insulating MBa2O4−δ (M = Y, Bi, Hg, Tl, CuTl, etc.) charge reservoir layers. In the transport process the charge carriers have to tunnel across insulating/partially insulating barriers along the c-axis and across the grain boundaries, which promote a fluctuation in the order parameter and in turn to the conductivity of the carriers. The studies of such fluctuation conductivity (FIC) may help in understanding the intrinsic mechanism of superconductivity. Here the electrical resistivity ρ(T) versus temperature data of as-prepared and oxygen post-annealed Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y = 0, 0.5, 1.5, 2.5) samples is studied for FIC analyses in the temperature regime well above the critical temperature; such analyses have been carried out by employing Lawrence and Doniach (LD) and Maki–Thompson (MT) models. The coherence length, inter-plane coupling, exponent, dimensionality of fluctuations and the phase relaxation time of the carriers are determined from such analyses. It is observed that the crossover temperature associated with two distinct exponents fits very well with the two-dimensional (2D) and three-dimensional (3D) LD equations. The crossover temperature T0 is shifted to higher temperatures with enhanced Zn doping. The 3D LD region is shifted to higher temperature with the increased Zn doping. We have elucidated from these analyses that lower Tl content in the final compound may increase the charge carrier’s doping efficiency of MBa2O4−δ charge reservoir layer, resulting into an increase in the coherence length along the c-axis and superconductivity parameters. A small decrease in the coherence length along the c-axis ξc(0) is observed in the samples with Zn doping of y = 1.5 whereas ξc(0) increases in the samples y = 0.5, 2.5. In comparison with as-prepared samples, the ξc(0) decreases after post-annealing in oxygen atmosphere. It is most likely that a decrease in the density of charge carrier’s is promoted by oxygen diffusion in the unit cell may suppress the ξc(0). The increase oxygen diffusion is evidenced from the softening of phonon modes after post-annealing in oxygen atmosphere. The decreased population of small spins of Cu atoms induced by doping of Zn is viewed in the terms of suppression of spin gap and hence the pseudo-gap in Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y = 0, 0.5, 1.5, 2.5) samples.
Low-temperature heat capacity of fullerite C60 doped with deuteromethane
Authors: Bagatskii M. I. , Sumarokov V. V. , Dolbin A. V. , Sundqvist B.
Published: February 2012

The heat capacity C of fullerite doped with deuteromethane (CD4)0.4(C60) has been investigated in the temperature interval 1.2–120 K. The contribution ΔCCD4 of the CD4 molecules to the heat capacity C has been isolated. It is shown that at T ≈ 120 K the rotational motion of CD4 molecules in the octahedral voids of the C60 lattice is weakly hindered. When the temperature is lowered to 80 K, the rotational motion of the CD4 molecules changes from weakly hindered rotation to libration. In the range T = 1.2–30 K, ΔCCD4 is described quite accurately by the sum of contributions from the translational and librational vibrations and tunneling rotation of CD4 molecules. The contribution of tunneling rotation to the heat capacity ΔCCD4(T) is dominant below 5 K. The effect of nuclear-spin conversion of the CD4 molecules on the heat capacity has been observed and the characteristic times for nuclear spin conversion between the lowest levels of the A- and T-species of CD4 molecules at T < 5 K have been estimated. A feature observed in ΔCCD4(T) near T = 5.5 K is most likely a manifestation of a first-order phase transition in the orientational glass form of the solution.
Generalized four-point characterization method using capacitive and ohmic contacts
Authors: Kim Brian S. , Zhou Wang , Shah Yash D. , Zhou Chuanle , Işık N. , Grayson M.
Published: February 2012

In this paper, a four-point characterization method is developed for samples that have either capacitive or ohmic contacts. When capacitive contacts are used, capacitive current- and voltage-dividers result in a capacitive scaling factor not present in four-point measurements with only ohmic contacts. From a circuit equivalent of the complete measurement system, one can determine both the measurement frequency band and capacitive scaling factor for various four-point characterization configurations. This technique is first demonstrated with a discrete element four-point test device and then with a capacitively and ohmically contacted Hall bar sample over a wide frequency range (1 Hz–100 kHz) using lock-in measurement techniques. In all the cases, data fit well to a circuit simulation of the entire measurement system, and best results are achieved with large area capacitive contacts and a high input-impedance preamplifier stage. An undesirable asymmetry offset in the measurement signal is described which can arise due to asymmetric voltage contacts.
Geometric and dynamic perspectives on phase-coherent and noncoherent chaos
Authors: Zou Yong , Donner Reik V. , Kurths Jürgen
Published: February 2012

Statistically distinguishing between phase-coherent and noncoherent chaotic dynamics from time series is a contemporary problem in nonlinear sciences. In this work, we propose different measures based on recurrence properties of recorded trajectories, which characterize the underlying systems from both geometric and dynamic viewpoints. The potentials of the individual measures for discriminating phase-coherent and noncoherent chaotic oscillations are discussed. A detailed numerical analysis is performed for the chaotic Rössler system, which displays both types of chaos as one control parameter is varied, and the Mackey-Glass system as an example of a time-delay system with noncoherent chaos. Our results demonstrate that especially geometric measures from recurrence network analysis are well suited for tracing transitions between spiral- and screw-type chaos, a common route from phase-coherent to noncoherent chaos also found in other nonlinear oscillators. A detailed explanation of the observed behavior in terms of attractor geometry is given.
Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films
Authors: Anger F. , Ossó J. O. , Heinemeyer U. , Broch K. , Scholz R. , Gerlach A. , Schreiber F.
Published: February 2012

We report detailed temperature dependent photoluminescence (PL) spectra of pentacene (PEN), perfluoropentacene (PFP), and PEN:PFP mixed thin films grown on SiO2. PEN and PFP are particularly suitable for this study, since they are structurally compatible for good intermixing and form a model donor/acceptor system. The PL spectra of PEN are discussed in the context of existing literature and compared to the new findings for PFP. We analyze the optical transitions observed in the spectra of PEN and PFP using time-dependent density functional theory calculations. Importantly, for the mixed PEN:PFP film we observe an optical transition in PL at 1.4 eV providing evidence for coupling effects in the blend. We discuss a possible charge-transfer (CT) and provide a tentative scheme of the optical transitions in the blended films.
The influence of the epitaxial strain on the magnetic anisotropy in ferromagnetic (Ga,Mn)(As,P)/GaAs thin films
Authors: Yahyaoui M. , Testelin C. , Gourdon C. , Boujdaria K.
Published: February 2012

The magnetic anisotropy properties of the ferromagnetic semiconductor (Ga,Mn)(As,P) are studied in a material-specific microscopic k.p approach. We calculate the band diagram and the density of states of such quaternary ferromagnetic alloys using a 40 band k.p model as well as antiferro- magnetic s,p-d exchange interaction. Our simulations clearly show that the uniaxial and cubic magnetic anisotropy parameters reveal a pronounced dependence on the vertical strain. We also show that the microscopic calculations are in good agreement with the theoretical and experimental results especially in the description of the fourth-order in-plane and out-of-plane components of the magnetic anisotropy. The anisotropy constants obtained from the microscopic calculations allow us to construct a three-dimensional magnetic free energy surface, which clearly indicates that the easy axis of magnetization depends on the strain conditions.
Optical field-ionization of a neutral gas with inhomogeneous density for electron acceleration by a high-intensity laser
Authors: Nandan Gupta Devki , Pal Singh Kunwar , Suk Hyyong
Published: February 2012

A scheme for the laser-induced acceleration of an electron from a double-ionizing neutral gas is investigated, where an inhomogeneous neutral gas profile (resembling a gas-jet experiment) is considered in order to observe the actual electron energy gain during acceleration. Optical-field ionization of the neutral gas can defocus the laser pulse significantly, and an electron accelerates by being pushed in front of a laser pulse in vacuum, and then decelerates due to the defocused (quite low-intensity) tail part of the laser pulse. The reduction in electron deceleration incurred by defocusing the laser-induced double-ionization of the neutral gas makes the electron acceleration continuous. In this study, we introduced an inhomogeneous gas profile that resembles a laser gas-jet experiment. However, the inhomogeneity of the gas reduced the rate of tunnel ionization, which limited the defocusing of the laser pulse; thus, though the electron energy gain is reduced but this proposal is more feasible and realistic.
Nanoassembly control and optical absorption in CdTe-ZnO nanocomposite thin films
Authors: Potter B. G. , Beal R. J. , Allen C. G.
Published: February 2012

The spatial distribution of CdTe nanoparticles within a ZnO thin-film matrix was manipulated using a dual-source, sequential radio-frequency (RF)-sputter deposition technique to produce nanocomposite materials with tuned spectral absorption characteristics. The relative substrate exposure time to each sputtering source was used to control the semiconductor phase connectivity, both within the film plane and along the film growth direction, to influence the degree of photocarrier confinement and the resulting optical transition energies exhibited by the CdTe phase. Significant changes (up to ΔE ≈ 0.3 eV) in the absorption onset energy for the CdTe nanoparticle ensemble were produced through modification in the extended structure of the semiconductor phase. Raman spectroscopy, cross-sectional transmission electron microscopy, and x-ray diffraction were used to confirm the phase identity of the CdTe and ZnO and to characterize the nanostructures produced in these composite films. Isochronal annealing for 5 min at temperatures up to 800 °C further indicated the potential to improve film crystallinity as well as to establish the post-deposition thermal processing limits of stability for the semiconductor phase. The study highlights the significance of ensemble behavior as a means to influence quantum-scale semiconductor optical characteristics of import to the use of such materials as the basis for a variety of optoelectronic devices, including photosensitized heterojunction components in thin film photovoltaics.
Molecular dynamics simulation of the regrowth of nanometric multigate Si devices
Authors: Marqués Luis A. , Pelaz Lourdes , Santos Iván , López Pedro , Duffy Ray
Published: February 2012

We use molecular dynamics (MD) simulation techniques to study the regrowth of nanometric multigate Si devices, such as fins and nanowires, surrounded by free surfaces and interfaces with amorphous material. Our results indicate that atoms in amorphous regions close to lateral free surfaces or interfaces rearrange at a slower rate compared to those in bulk due to the discontinuity of the lateral crystalline template. Consequently, the recrystallization front which advances faster in the device center than at the interfaces adopts new orientations. Regrowth then proceeds depending on the particular orientation of the new amorphous/crystal interfaces. In the particular case of 〈110〉 oriented fins, the new amorphous/crystal interfaces are aligned along the 〈111〉 direction, which produces frequent twining during further regrowth. Based on our simulation results, we propose alternatives to overcome this defected recrystallization in multigate structures: device orientation along 〈100〉 to prevent the formation of limiting {111} amorphous/crystal interfaces and presence of a crystalline seed along the device body to favor regrowth perpendicular to the lateral surfaces/interfaces rather than parallel to them.
Enhanced indirect ferromagnetic p-d exchange coupling of Mn in oxygen rich ZnO:Mn nanoparticles synthesized by wet chemical method
Authors: Ilyas Usman , Rawat R. S. , Tan T. L. , Lee P. , Chen R. , Sun H. D. , Fengji Li , Zhang Sam
Published: February 2012

This paper investigates the ferromagnetism in ZnO:Mn powders and presents our findings about the role played by the doping concentration and the structural defects towards the ferromagnetic signal. The narrow-size-distributed ZnO:Mn nanoparticles based powders with oxygen rich stoichiometery were synthesized by wet chemical method using zinc acetate dihydrate and manganese acetate tetrahydrate as precursors. A consistent increase in the lattice cell volume, estimated from x-ray diffraction spectra and the presence of Mn 2p3/2 peak at ∼640.9 eV, in x-ray photoelectron spectroscopic spectra, confirmed a successful incorporation of manganese in its Mn2+ oxidation state in ZnO host matrix. Extended deep level emission spectra in Mn doped ZnO powders exhibited the signatures of oxygen interstitials and zinc vacancies except for the sample with 5 at. % Mn doping. The nanocrystalline powders with 2 and 5 at. % Mn doping concentration were ferromagnetic at room temperature while the 10 at. % Mn doped sample exhibited paramagnetic behavior. The maximum saturation magnetization of 0.05 emu/g in the nanocrystalline powder with 5 at. % Mn doping having minimum defects validated the ferromagnetic signal to be due to strong p-d hybridization of Mn ions.
Fitting properties from density functional theory based molecular dynamics simulations to parameterize a rigid water force field
Authors: Sala Jonàs , Guàrdia Elvira , Martí Jordi , Spångberg Daniel , Masia Marco
Published: February 2012

In the quest towards coarse-grained potentials and new water models, we present an extension of the force matching technique to parameterize an all-atom force field for rigid water. The methodology presented here allows to improve the matching procedure by first optimizing the weighting exponents present in the objective function. A new gauge for unambiguously evaluating the quality of the fit has been introduced; it is based on the root mean square difference of the distributions of target properties between reference data and fitted potentials. Four rigid water models have been parameterized; the matching procedure has been used to assess the role of the ghost atom in TIP4P-like models and of electrostatic damping. In the former case, burying the negative charge inside the molecule allows to fit better the torques. In the latter, since short-range interactions are damped, a better fit of the forces is obtained. Overall, the best performing model is the one with a ghost atom and with electrostatic damping. The approach shown in this paper is of general validity and could be applied to any matching algorithm and to any level of coarse graining, also for non-rigid molecules.
The study of conductance in magnetic tunnel junctions with a thin MgO barrier: The effect of Ar pressure on tunnel magnetoresistance and resistance area product
Authors: Zaleski A. , Wrona J. , Czapkiewicz M. , Skowroński W. , Kanak J. , Stobiecki T.
Published: February 2012

The thickness dependence of tunnel magnetoresistance and resistance area product in Co40Fe40B20/MgO wedge/Co40Fe40B20 magnetic tunnel junctions (MTJs) has been studied for multiple Ar partial pressure (PAr) values during MgO sputtering. The extension of the simple equivalent circuit model [B. Oliver et al., J. Appl. Phys. 91, 4348 (2002)] has been suggested in order to include different transport mechanism contributions to the overall conductance of the MTJ as a function of the MgO barrier thickness. Parameters of the model, used for quantitative description of the conductivity of unpatterned MTJ stacks, have been analyzed as a function of PAr.
Convergence of expansions in Schrödinger and Dirac eigenfunctions, with an application to the R-matrix theory
Authors: Stasińska Julia
Published: February 2012

Expansion of a wave function in a basis of eigenfunctions of a differential eigenvalue problem lies at the heart of the R-matrix methods for both the Schrödinger and Dirac particles. A central issue that should be carefully analyzed when functional series are applied is their convergence. In the present paper, we study the properties of the eigenfunction expansions appearing in nonrelativistic and relativistic R-matrix theories. In particular, we confirm the findings of Rosenthal [J. Phys. G 13, 491 (1987)] and Szmytkowski and Hinze [J. Phys. B 29, 761 (1996); Szmytkowski and Hinze J. Phys. A 29, 6125 (1996)] that in the most popular formulation of the R-matrix theory for Dirac particles, the functional series fails to converge to a limit claimed by other authors.
Coupled-cluster, Möller Plesset (MP2), density fitted local MP2, and density functional theory examination of the energetic and structural features of hydrophobic solvation: Water and pentane
Authors: Ghadar Yasaman , Clark Aurora E.
Published: February 2012

The interaction potentials between immiscible polar and non-polar solvents are a major driving force behind the formation of liquid:liquid interfaces. In this work, the interaction energy of water–pentane dimer has been determined using coupled-cluster theory with single double (triple) excitations [CCSD(T)], 2nd order Möller Plesset perturbation theory (MP2), density fitted local MP2 (DF-LMP2), as well as density functional theory using a wide variety of density functionals and several different basis sets. The M05-2X exchange correlation functionals exhibit excellent agreement with CCSD(T) and DF-LMP2 after taking into account basis set superposition error. The gas phase water–pentane interaction energy is found to be quite sensitive to the specific pentane isomer (2,2-dimethylpropane vs. n-pentane) and relative orientation of the monomeric constituents. Subsequent solution phase cluster calculations of 2,2-dimethylpropane and n-pentane solvated by water indicate a positive free energy of solvation that is in good agreement with available experimental data. Structural parameters are quite sensitive to the density functional employed and reflect differences in the two-body interaction energy calculated by each method. In contrast, cluster calculations of pentane solvation of H2O solute are found to be inadequate for describing the organic solvent, likely due to limitations associated with the functionals employed (B3LYP, BHandH, and M05-2X).
Surface depletion mediated control of inter-sub-band absorption in GaAs/AlAs semiconductor quantum well systems
Authors: Buchwald Walter R. , Cleary Justin W. , Hendrickson Joshua
Published: February 2012

The modification of quantum well inter-sub-band absorption properties due to surface depletion induced band bending is reported. Fourier transform infrared spectroscopy measurements of a GaAs/AlAs multiple quantum well system reveal a reduction in the characteristic absorption resonance in correlation with wet chemical etching. High resolution transmission electron microscopy confirms the presence of the quantum wells after etching, suggesting the quantum wells are positioned within the surface depletion region of the structure. This method of inter-sub-band absorption modification could be used for the formation of quantum dots from a quantum well system with the precise, deterministic control of their location.