bluuebunny/crossref_metadata_2025 huggingface dataset:
Laboratory investigation of Martian water ice cloud formation using dust aerosol simulants
Authors: Ladino L. A. , Abbatt J. P. D.
Published: November 2012

The ice nucleation abilities of submicron aerosol particles of two Martian regolith analogs, the Mojave Mars simulant and Johnson Space Center Mars‐1, were investigated with the University of Toronto continuous flow diffusion chamber. The temperature range studied (> 200 K) is relevant to low‐altitude water ice cloud formation in the Martian atmosphere and the aerosol particles were suspended in air, in contrast to previous experiments. Both simulants were found to be active ice nuclei in the deposition nucleation mode between 223 K and 203 K. The Mojave Mars simulant particles were found to be better ice nuclei than the Johnson Space Center Mars‐1 particles requiring lower supersaturations to nucleate ice. It was observed that the critical supersaturation (Scrit) to activate 1% of the particles increased with decreasing temperature in accord with previous low‐temperature studies, rising to a value of above 1.7 at 203 K. This corroborates literature results that there is a substantial barrier to ice nucleation at low temperatures, underlining the need for incorporating this effect in Martian cloud microphysical models. It was also found that Scrit did not change when the size of the Mojave Mars simulant particles was increased from 240 to 400 nm. Comparison of the Martian simulants with other mineral dusts shows that the Johnson Space Center Mars‐1 analog behaves similarly to the well‐known terrestrial ice nuclei such as kaolinite and Arizona test dust particles, whereas the Mojave Mars simulant behaves closer to another clay, montmorillonite. The wettability parameter, m, was calculated to range from 0.955 to 0.959.
A comparison of the iddingsite alteration products in two terrestrial basalts and the Allan Hills 77005 martian meteorite using Raman spectroscopy and electron microprobe analyses
Authors: Kuebler Karla E.
Published: November 2012

We document the secondary mineral assemblages in two occurrences of terrestrial iddingsite, Lunar Crater, Nevada (LC) and Mauna Kea, Hawaii (MK), and compare these with the iddingsite in Allan Hills (ALHA) 77005. Short Raman spectroscopic traverses across olivine alteration fronts provide information about changes in mineralogy with alteration. Data from the Raman traverses are combined with electron microprobe (EMP) traverses at the same locations which provide information regarding element mobility and confirm mineral identifications made by Raman spectroscopy. This information is used with petrographic observations to argue for the martian origin of the iddingsite and jarosite, infer the sequence of alteration, and deliberate on the conditions and settings of alteration. Raman spectra indicate the presence of different iron oxides/oxyhydroxides in each sample (goethite in LC, maghemite in MK, and akaganéite in ALHA), and the terrestrial samples show different element mobility trends (loss of MgO and SiO2, retention of FeO) than ALHA (loss of MgO and FeO, influx of SiO2), whose trends reflect the deposition of jarosite. Altered olivine occur throughout the LC samples but only in the exteriors of the MK samples. The LC and MK alteration products formed by surface alteration, but ALHA 77005 is a lherzolite, and the olivine hosting the iddingsite are enclosed by orthopyroxene (appear to be restricted to the light lithology), suggesting that it formed at depth during magma consolidation. The ALHA iddingsite is an example of “deuteric alteration” (reaction with fluids that separated from the magma as crystallization progressed towards completion).
Recent acceleration of carbon accumulation in a boreal peatland, south central Alaska
Authors: Loisel Julie , Yu Zicheng
Published: November 2012

The ongoing warming in high‐latitude regions may be causing rapid changes in the structure and functioning of terrestrial ecosystems. Of particular concern is the fate of belowground soil organic carbon stored in peat‐accumulating wetlands, as these large carbon pools are sensitive to temperature and moisture conditions. Despite their important role in the global carbon cycle, considerable uncertainty remains over the carbon balance of northern peatlands in a changing climate. Here we examine the response of vegetation and carbon dynamics in a wet boreal peatland to recent climate warming using empirical peat core data and a new modeling approach. We observed a widespread shift from herbaceous Carex fen peat to Sphagnum moss peat around 100 years ago that was accompanied by a sharp increase in carbon accumulation rate. The observed apparent carbon accumulation rates over the past 100 years (96.8 g C m‐2 yr‐1) were almost 10 times greater than those over the past 4000 years (11.5 g C m‐2 yr‐1). Once differential decomposition history was considered using three modeling approaches, the expected long‐term accumulation rate of recent peat was still 2–6 times greater than that of the past 4000 years. We propose that recent warming has led to Sphagnum establishment, which rapidly altered the peatland surface chemistry and hydrology, further promoting Sphagnum growth and enhancing the carbon sink capacity of this peatland. Longer and warmer growing seasons could also have stimulated plant growth. Our results imply that accelerated carbon accumulation under global warming in some wet peatlands might offset some of the carbon losses experienced from other peatland types.
Global dust simulations in the multiscale modeling framework
Authors: Hsieh W. C. , Rosa D. , Collins W. D.
Published: November 2012

Key Points

Vertical transport in CAM lift tracers more efficiently than SPCAM in low layers

The dust implementation method is crucial for the simulated dust profiles

Treatments of convection processes can significantly affect dust budgets
Parameter estimation using data assimilation in an atmospheric general circulation model: From a perfect toward the real world
Authors: Schirber Sebastian , Klocke Daniel , Pincus Robert , Quaas Johannes , Anderson Jeffrey L.
Published: November 2012

Key Points

Objective parameter estimation using Ensemble Kalman Filter techniques

Cumulus‐convection scheme of an atmospheric general circulation model

Applying numerical weather prediction techniques to a climate model
Forecast performance assessment of a kinematic and a magnetohydrodynamic solar wind model
Authors: Norquist Donald C.
Published: November 2012

Performance evaluation of operational models guides forecasters on use of their products, focuses model developers in making improvements, and informs other modelers considering use of the output for forcing data. Two operational solar wind models, the Enlil magnetohydrodynamic model, and the Wang‐Sheeley‐Arge kinematic model, were executed daily in 2007–2011 from solar photosphere magnetograms compiled from the Global Oscillation Network Group telescope system. The original (uncorrected) magnetic field specification and a zero point‐corrected (ZPC) version were used as inner boundary conditions (IBCs) in separate 7 day forecast executions. Forecasts of solar wind radial speed (Vsw) and interplanetary magnetic field (IMF) polarity were compared with observations from the Advanced Composition Explorer satellite. Forecast verification metrics were computed by forecast day, year, and uncorrected or corrected IBCs. High speed events (HSEs) and IMF polarity changes (IPCs) predicted and observed were compared. Neither model exhibited a significant systematic error except in 2009, when both failed to represent the slow solar wind. Using the ZPC initial conditions resulted in smaller forecast‐observation differences in the years with greater Vsw variance. This was due to in part to reduced variance in the Vsw predictions from the ZPC IBCs. Differences were nil or worse in the other years. The time‐varying component of the forecast‐observation differences was smallest at forecast days 3 to 5, followed by a sharp rise. Impact of ZPCs on IMF polarity predictions was small. HSE prediction performance depended on detection algorithm used. Both models under predicted the number of forecast periods having IPCs.
Oxygen Consumption Rates during Three Different Neuronal Activity States in the Hippocampal CA3 Network
Authors: Huchzermeyer Christine , Berndt Nikolaus , Holzhütter Hermann-Georg , Kann Oliver
Published: November 2012

The brain is an organ with high metabolic rate. However, little is known about energy utilization during different activity states of neuronal networks. We addressed this issue in area CA3 of hippocampal slice cultures under well-defined recording conditions using a 20% O2 gas mixture. We combined recordings of local field potential and interstitial partial oxygen pressure (pO2) during three different activity states, namely fast network oscillations in the gamma-frequency band (30 to 100 Hz), spontaneous network activity and absence of spiking (action potentials). Oxygen consumption rates were determined by pO2 depth profiles with high spatial resolution and a mathematical model that considers convective transport, diffusion, and activity-dependent consumption of oxygen. We show that: (1) Relative oxygen consumption rate during cholinergic gamma oscillations was 2.2-fold and 5.3-fold higher compared with spontaneous activity and absence of spiking, respectively. (2) Gamma oscillations were associated with a similar large decrease in pO2 as observed previously with a 95% O2 gas mixture. (3) Sufficient oxygenation during fast network oscillations in vivo is ensured by the calculated critical radius of 30 to 40 μm around a capillary. We conclude that the structural and biophysical features of brain tissue permit variations in local oxygen consumption by a factor of about five.
Engulfment adaptor phosphotyrosine-binding-domain-containing 1 (GULP1) is a nucleocytoplasmic shuttling protein and is transactivationally active together with low-density lipoprotein receptor-related protein 1 (LRP1)
Authors: Wahler Anke , Beyer Anja-Silke , Keller Ilona E. , Schnack Cathrin , von Einem Björn , Pröpper Christian , Boeckers Tobias M. , Peltan Ithan D. , Strickland Dudley K. , Hyman Bradley T. , von Arnim Christine A. F.
Published: November 2012

APP (amyloid precursor protein) and LRP1 (low-density lipoprotein receptor-related protein 1) have been implicated in the pathogenesis of AD (Alzheimer's disease). They are functionally linked by Fe65, a PTB (phosphotyrosine-binding)-domain-containing adaptor protein that binds to intracellular NPxY-motifs of APP and LRP1, thereby influencing expression levels, cellular trafficking and processing. Additionally, Fe65 has been reported to mediate nuclear signalling in combination with intracellular domains of APP and LRP1. We have previously identified another adaptor protein, GULP1 (engulfment adaptor PTB-domain-containing 1). In the present study we characterize and compare nuclear trafficking and transactivation of GULP1 and Fe65 together with APP and LRP1 and report differential nuclear trafficking of adaptors when APP or LRP1 are co-expressed. The observed effects were additionally supported by a reporter-plasmid-based transactivation assay. The results from the present study indicate that Fe65 might have signalling properties together with APP and LRP1, whereas GULP1 only mediates LRP1 transactivation.
The small heat-shock protein αB-crystallin uses different mechanisms of chaperone action to prevent the amorphous versus fibrillar aggregation of α-lactalbumin
Authors: Kulig Melissa , Ecroyd Heath
Published: November 2012

Stress conditions can destabilize proteins, promoting them to unfold and adopt intermediately folded states. Partially folded protein intermediates are unstable and prone to aggregation down off-folding pathways leading to the formation of either amorphous or amyloid fibril aggregates. The sHsp (small heat-shock protein) αB-crystallin acts as a molecular chaperone to prevent both amorphous and fibrillar protein aggregation; however, the precise molecular mechanisms behind its chaperone action are incompletely understood. To investigate whether the chaperone activity of αB-crystallin is dependent upon the form of aggregation (amorphous compared with fibrillar), bovine α-lactalbumin was developed as a model target protein that could be induced to aggregate down either off-folding pathway using comparable buffer conditions. Thus when α-lactalbumin was reduced it aggregated amorphously, whereas a reduced and carboxymethylated form aggregated to form amyloid fibrils. Using this model, αB-crystallin was shown to be a more efficient chaperone against amorphously aggregating α-lactalbumin than when it aggregated to form fibrils. Moreover, αB-crystallin forms high molecular mass complexes with α-lactalbumin to prevent its amorphous aggregation, but prevents fibril formation via weak transient interactions. Thus, the conformational stability of the protein intermediate, which is a precursor to aggregation, plays a critical role in modulating the chaperone mechanism of αB-crystallin.
The plasma membrane transporter SLC5A8 suppresses tumour progression through depletion of survivin without involving its transport function
Authors: Coothankandaswamy Veena , Elangovan Selvakumar , Singh Nagendra , Prasad Puttur D. , Thangaraju Muthusamy , Ganapathy Vadivel
Published: November 2012

SLC5A8 (solute carrier gene family 5A, member 8) is a sodium-coupled transporter for monocarboxylates. Among its substrates are the HDAC (histone deacetylase) inhibitors butyrate, propionate and pyruvate. Expression of SLC5A8 is silenced in cancers via DNA methylation, and ectopic expression of SLC5A8 in cancer cells induces apoptosis in the presence of its substrates that are HDAC inhibitors. In the present study we show that ectopic expression of SLC5A8 in cancer cells translocates the anti-apoptotic protein survivin to the plasma membrane through protein–protein interaction resulting in depletion of nuclear survivin and also decreases cellular levels of survivin through inhibition of transcription. These SLC5A8-induced changes in the location and levels of survivin result in cell-cycle arrest, disruption of the chromosome passenger complex involved in mitosis, induction of apoptosis and enhancement in chemosensitivity. These effects are seen independently of the transport function of SLC5A8 and histone acetylation status of the cell; in the presence of pyruvate, a SLC5A8 substrate and also an HDAC inhibitor, these effects are amplified. Ectopic expression of SLC5A8 in the breast cancer cell line MB231 inhibits the ability of cells to form colonies in vitro and to form tumours in mouse xenografts in vivo. The suppression of survivin transcription occurs independently of HDAC inhibition, and the underlying mechanism is associated with decreased phosphorylation of STAT3 (signal transducer and activator of transcription 3). The observed effects are specific for survivin with no apparent changes in expression of other inhibitor-of-apoptosis proteins. The present study unravels a novel, hitherto unrecognized, mechanism for the tumour-suppressive role of a plasma membrane transporter independent of its transport function.
A computational study of graphene silicon contact
Authors: Chauhan Jyotsna , Rinzler Andrew , Guo Jing
Published: November 2012

Graphene, which is mechanically flexible, electrically conductively, and optically nearly transparent, is a promising contact material in semiconductor devices such as solar cells and touch screen sensors. We present a method of obtaining the barrier height and transport properties of graphene-silicon contacts by self-consistently solving the Poisson equation and carrier transport equation. It is found that the contact barrier height is sensitive to the doping density of silicon and can be modulated by gating, in contrast to conventional metal-semiconductor contacts. Despite of being a continuous film, the contact resistance of a monolayer graphene to silicon can be modulated by orders of magnitude by using a bottom gate. The modulation of the contact resistance decreases significantly as the number of graphene layers increases and becomes negligible when the number of the graphene layers is larger than about 6. The results indicate the unique properties of graphene-semiconductor contacts.
Effect of calcinations on electrical properties of TiO2 nanotubes
Authors: Vijayan Pournami P. , Thomas Marykutty , George K. C.
Published: November 2012

Nanotubes of TiO2 synthesized by hydrothermal method are treated at three different temperatures (400, 600, and 800 °C). Calcinations cause phase change as evidenced from x-ray diffraction patterns. AC conductivity studies done on as prepared TiO2 and calcined samples in the frequency range 100Hz–1MHz and at various temperatures between 303 and 423 K. AC conductivity increases with the calcination followed by a decrease which can be attributed to rutile formation. Surface defects are responsible for the ac conductivity change. PL spectrum shows the presence of surface oxygen vacancies and self trapped excitons in TiO2 nanotubes and annealed samples. The variable range hopping dominates the mode of electrical transport at low temperature and thermally activated process at high temperature. Conductivity is at its maximum for the sample calcined at 400 °C. There is a transition temperature in between the variable range hopping and thermally activated process, which differs with calcinations.
ZnO-nanorod-array/p-GaN high-performance ultra-violet light emitting devices prepared by simple solution synthesis
Authors: Jha Shrawan Kumar , Luan Chunyan , To Chap Hang , Kutsay Oleksandr , Kováč Jaroslav , Zapien Juan Antonio , Bello Igor , Lee Shuit-Tong
Published: November 2012

Pure ultra-violet (UV) (378 nm) electroluminescence (EL) from zinc oxide (ZnO)-nanorod-array/p-gallium nitride (GaN) light emitting devices (LEDs) is demonstrated at low bias-voltages (∼4.3 V). Devices were prepared merely by solution-synthesis, without any involvement of sophisticated material growth techniques or preparation methods. Three different luminescence characterization techniques, i.e., photo-luminescence, cathodo-luminescence, and EL, provided insight into the nature of the UV emission mechanism in solution-synthesized LEDs. Bias dependent EL behaviour revealed blue-shift of EL peaks and increased peak sharpness, with increasing the operating voltage. Accelerated bias stress tests showed very stable and repeatable electrical and EL performance of the solution-synthesized nanorod LEDs.
Optical characteristics of GaAs nanowire solar cells
Authors: Hu Y. , LaPierre R. R. , Li M. , Chen K. , He J.-J.
Published: November 2012

The reflectance, transmittance, and absorptance of GaAs nanowire (NW) arrays are calculated by solving Maxwell's equations using the finite element method. The model is compared with measurement results from well-ordered periodic GaAs NW arrays fabricated by dry etching. The model results are also compared with the reflectance measured from NWs grown by the Au-assisted vapor-liquid-solid method. The optimum NW diameter, periodicity (spacing between NWs), and length are determined to maximize absorptance of the AM1.5G solar spectrum and short circuit current density in a NW array solar cell. The optimum NW diameter, period, and length were 180 nm, 350 nm, and 5 μm, respectively, giving a photocurrent density from the NW of 27.3 mA/cm2 and corresponding to 91.3% absorption of the AM1.5 G solar spectrum. The photocurrent density saturated for NW lengths greater than 5 μm. A gold nanoparticle at the top of the NWs (used in the vapor-liquid-solid NW growth process) substantially reduced the optimum photocurrent density, while a polymer filling the space between NWs and a planar indium tin oxide contact had a relatively minor influence.
Multi-continuum Fano resonance in coupled quantum point contacts: A manifestation of the “integral” Fano formula
Authors: Mourokh L. , Ivanushkin P. , Yoon Y. , Aoki N. , Ochiai Y. , Bird J. P.
Published: November 2012

We discuss how a pair of quantum point contacts (QPCs), which are coupled to each other via their mutual wavefunction overlap with a common continuum, can be used to provide a realization of a multi-continuum Fano resonance. This behavior arises from the multi-subband character of the QPCs, each of whose transverse subbands may be viewed as providing a unique continuum. Reminiscent of the original analysis of Fano, we show that the resonance exhibited by this system can be defined in terms of an asymmetry parameter (q) and characteristic level broadenings (Γ & Γ0), although these parameters now determine the resonance lineshape through their inclusion in energy integrals, a result that we refer to as the “integral” Fano formula. We also demonstrate how, dependent upon the effective dimensionality of the “detector” QPC that exhibits the Fano resonance, the resonance amplitude can significantly exceed the one-dimensional conductance quantum (2e2/h). Our experimental and theoretical results, therefore, provide further support for the scenario of spontaneous bound-state formation in QPCs near pinch-off and suggest that this bound state may be used to study new aspects of Fano-resonance phenomenology.
Understanding the formation of ultrafine spinel CoFe2O4 nanoplatelets and their magnetic properties
Authors: Liu Wenchao , Chan Yukkwan , Cai Jinzhu , Leung Chiwah , Mak Cheeleung , Wong Kinhung , Zhang Fengming , Wu Xiaoshan , Qi X. D.
Published: November 2012

Cobalt ferrite (CoFe2O4, CFO) nanoplatelets with a rhomboidal shape were synthesized through a facile hydrothermal route using a very low concentration precursor at low temperatures. The effects of reaction temperature and time on the morphologies as well as the sizes of the products were systemically investigated. The as-synthesized CFO nanocrystals showed a special rhomboidal shape with crystal growth along the (111) direction. The very low concentration precursor plays an important role in forming CFO nanocrystals with such special shape and ultrafine size. The single-phase CFO nanoplatelets synthesized at 180 °C with size of 17 nm present high saturation magnetization (79.7 emu/g) and high coercivity (3100 Oe). The preparation conditions have significant effects on the crystal size and shape, magnetization and relaxation activation of the CFO nanoplatelets.
Chain extension of a confined polymer in steady shear flow
Authors: Bhattacharyya Pinaki , Cherayil Binny J.
Published: November 2012

The growing importance of microfluidic and nanofluidic devices to the study of biological processes has highlighted the need to better understand how confinement affects the behavior of polymers in flow. In this paper we explore one aspect of this question by calculating the steady-state extension of a long polymer chain in a narrow capillary tube in the presence of simple shear. The calculation is carried out within the framework of the Rouse-Zimm approach to chain dynamics, using a variant of a nonlinear elastic model to enforce finite extensibility of the chain, and assuming that the only effect of the confining surface is to modify the pre-averaged hydrodynamic interaction. The results, along with results from the corresponding calculations of finitely extensible versions of both the Rouse and Rouse-Zimm models, are compared with data from experiments on the flow-induced stretching of λ-phage DNA near a non-adsorbing glass surface [L. Fang, H. Hu, and R. G. Larson, J. Rheol. 49, 127 (2005)10.1122/1.1822930]. The comparison suggests that close to a surface hydrodynamic screening is significant, and causes the chains to become effectively free-draining.
Van der Waals interactions: Accuracy of pair potential approximations
Authors: Cole Milton W. , Kim Hye-Young , Liebrecht Michael
Published: November 2012

Van der Waals interactions between single atoms and solids are discussed for the regime of large separation. A commonly employed approximation is to evaluate this interaction as a sum of two-body interactions between the adatom and the constituent atoms of the solid. The resulting potentials are here compared with known results in various geometries. Analogous comparisons are made for diatomic molecules near either single atoms or semi-infinite surfaces and for triatomic molecules’ interactions with single atoms.
Nanoscale lubricating film formation by linear polymer in aqueous solution
Authors: Liu Shuhai , Guo Dan , Xie Guoxin
Published: November 2012

Film-forming properties of polymer in aqueous solution flowing through a nanogap have been investigated by using a thin film interferometry. The film properties of linear polymer in aqueous solution flowing through a confined nanogap depend on the ratio of water film thickness to averaged radius of polymer chains H0/RPolymer. It was found that the lubrication film thickness of linear polymer in aqueous solution decreases as the polymer molecular weight increasing when H0/RPolymer < 2 ∼ 3. A new lubrication map was proposed, which includes the lubrication regime of weak confinement influence, the lubrication regime of strong confinement influence (LRSCI), and the transition regime of confinement influence. It is very difficult to increase the lubrication film thickness using the higher molecule weight in the LRSCI regime. The lubrication mechanism inferred from our experimental results may help to better understand the dynamic film properties of linear polymer in aqueous solution flowing through a nanogap.
Complete analytical solution of electromagnetic field problem of high-speed spinning ball
Authors: Reichert T. , Nussbaumer T. , Kolar J. W.
Published: November 2012

In this article, a small sphere spinning in a rotating magnetic field is analyzed in terms of the resulting magnetic flux density distribution and the current density distribution inside the ball. From these densities, the motor torque and the eddy current losses can be calculated. An analytical model is derived, and its results are compared to a 3D finite element analysis. The model gives insight into the torque and loss characteristics of a solid rotor induction machine setup, which aims at rotating the sphere beyond 25 Mrpm.
Self-assembly of binary space-tessellating compounds
Authors: Khadilkar Mihir R. , Escobedo Fernando A.
Published: November 2012

The self-assembly of polyhedral particles has been a topic of interest in both experimental and simulation studies due to its potential to help engineer novel materials from colloidal nanoparticles. An important extension to the study of single species of polyhedral particles is the case of binary mixtures. Mixtures that tessellate space are particularly interesting because they are expected to form high-pressure ordered structures. Here, we study three such binary tessellating mixtures; namely, cuboctahedra + octahedra (Mixture 1), octahedra + tetrahedra (Mixture 2), and truncated cubes + octahedra (Mixture 3). We use Monte Carlo methods to first determine their phase behavior when driven by hard-core interactions (i.e., entropic self-assembly). We observe that upon gradual compression of the isotropic system, none of the three cases exhibits a spontaneous ordering into the expected tessellated structure: Mixtures 1 and 2 form a glassy disordered state that is shown to be metastable with respect to the tessellated phase via interfacial simulations; Mixture 3 demixes into a disordered phase and an unusual ordered phase where truncated cubes arrange in a cubic lattice while the octahedra remain disordered occupying interstitial pockets. Using polybead models for Mixtures 1 and 2, we show that the large free-energy barrier that precludes the spontaneous nucleation of the tessellating structure from the isotropic state can be overcome by introducing favorable enthalpic interactions. Our results allow identifying some relations between properties of individual species and the phase behavior of their mixtures, providing a first step toward a “chemistry” of polyhedral compounds, while also raising key questions regarding the kinetics of the pseudo “reactions” involved.
Raman scattering study of bulk and nanocrystalline PbMoO4 at high pressures
Authors: Vilaplana R. , Gomis O. , Manjón F. J. , Rodríguez-Hernández P. , Muñoz A. , Errandonea D. , Achary S. N. , Tyagi A. K.
Published: November 2012

High-pressure Raman scattering measurements have been performed in wulfenite (PbMoO4) for both bulk and nanocrystalline powders up to 22 GPa. Our Raman scattering measurements evidence the phase transition from the tetragonal scheelite structure to the monoclinic M-fergusonite structure in both bulk and nanocrystalline powders above 10.8 and 13.4 GPa, respectively. The pressure dependences of the Raman active modes in both structures were compared and discussed based on our theoretical results obtained from lattice dynamics ab initio calculations.
The impact of the Pb(Zr,Ti)O3-ZnO interface quality on the hysteretic properties of a metal-ferroelectric-semiconductor structure
Authors: Pintilie I. , Pasuk I. , Ibanescu G. A. , Negrea R. , Chirila C. , Vasile E. , Pintilie L.
Published: November 2012

The hysteretic properties of metal-ferroelectric-semiconductor (MFS) structures based on Pb(Zr0.2Ti0.8)O3 (PZT) and ZnO films were studied with respect of the quality of the PZT-ZnO interface. The films were grown by pulsed laser deposition (PLD) on platinized silicon (Pt/Si) substrate and on single crystal, (001) oriented SrTiO3 (STO) substrates. The structural analysis has revealed that the PZT-ZnO stack grown on single crystal STO is epitaxial, while the structure grown on Pt/Si has columnar texture. The temperature change of the capacitance-voltage (C-V) hysteresis direction, from clockwise at low temperatures to counter clockwise at high temperatures, was observed at around 300 K in the case of the MFS structure grown by PLD on Pt/Si substrate. This temperature is lower than the one reported for the case of the PZT-ZnO structure grown by sol-gel on Pt/Si substrate (Pintilie et al., Appl. Phys. Lett. 96, 012903 (2010)). In the fully epitaxial structures the C-V hysteresis is counter clockwise even at 100 K. These findings strongly points out that the quality of the PZT-ZnO interface is essential for having a C-V hysteresis of ferroelectric nature, with negligible influence from the part of the interface states and with a memory window of about 5 V at room temperature.
Electron paramagnetic resonance and photo-electron paramagnetic resonance investigation on the recharging of the substitutional nitrogen acceptor in ZnO
Authors: Stehr J. E. , Hofmann D. M. , Meyer B. K.
Published: November 2012

We investigated the substitutional nitrogen center in ZnO single crystals by electron paramagnetic resonance (EPR) and photo-EPR spectroscopy. Aside the three principle hyperfine lines due to the interaction of the N0 (2p5) electron spin with the nitrogen nucleus (I = 1, natural abundance 99.6%), we identify additional satellite lines which arise from ΔmS = ±1 and ΔmI = ±1, ±2 transitions becoming allowed due to quadrupole interaction. The quadrupole coupling constant e2qQ/h is determined to −5.9 MHz with an asymmetry parameter of η = 0.05. These values are somewhat different from those obtained for the nitrogen center in ZnO powders, but are closer to the theoretical calculations of Gallino et al. We further carefully investigated the photon induced recharging of the N centers. We determine the energy hυ required for the process NO− + hυ → NO0 + ecb− to 2.1 ± 0.05 eV, the dependence of the EPR signal intensity on the illumination time shows a mono-exponential behavior which gives evidence that a direct ionization process is monitored.
Studies on temperature dependent semiconductor to metal transitions in ZnO thin films sparsely doped with Al
Authors: Das Amit K. , Misra P. , Ajimsha R. S. , Bose A. , Joshi S. C. , Phase D. M. , Kukreja L. M.
Published: November 2012

For a detailed study on the semiconductor to metal transition (SMT) in ZnO thin films doped with Al in the concentration range from 0.02% to 2%, we grew these films on (0001) sapphire substrates using sequential pulsed laser deposition. It was found that the Al concentration in the films increased monotonically with the ratio of ablation durations of the alumina and ZnO targets used during the deposition. Using x-ray photo electron spectroscopy, it was found that while most of the Al atoms occupy the Zn sites in the ZnO lattice, a small fraction of the Al also gets into the grain boundaries present in the films. The observed SMT temperature decreased from ∼270 to ∼50 K with increase in the Al concentration from 0.02% to 0.25%. In the Al concentration range of ∼0.5% to 2%, these doped ZnO films showed metallic behavior at all the temperatures without undergoing any SMT. A theoretical model based on thermal activation of electrons and electron scatterings due to the grain boundaries, ionic impurities, and phonons has been developed to explain the observed concentration and temperature dependent SMT.
Induced additional anisotropy influences on magnetostriction of giant magnetostrictive materials
Authors: Zhang Changsheng , Ma Tianyu , Yan Mi
Published: November 2012

Initial magnetization realignments induced by a uniaxial compressive pre-stress or transverse magnetic annealing can enhance effectively strain output of giant magnetostrictive materials. However, differing from the use of stress that induces in-plane anisotropy, it is found that magnetic annealing is capable of resulting in a uniaxial magnetic anisotropy in 〈110〉 grain-aligned TbDyFe polycrystals. We find the critical induced additional anisotropy to generate a uniaxial magnetization alignment along only one easy axis that is essential to realize 90° domain evolution during initial magnetization stage, hence yielding a large magnetostriction under modest switching fields. By a phenomenological approach based on magnetization rotation, we explain quantitatively how the induced additional anisotropy energy influences the magnetostrictive behaviors under coupled magnetomechanical loadings. Consequently, distinctive approaches to further enhance the saturation magnetostriction are suggested through the combination use of induced additional anisotropy and compressive pre-stress.
Inhibition of plasmonically enhanced interdot energy transfer in quantum dot solids via photo-oxidation
Authors: Sadeghi S. M. , Nejat A. , West R. G.
Published: November 2012

We studied the impact of photophysical and photochemical processes on the interdot Forster energy transfer in monodisperse CdSe/ZnS quantum dot solids. For this, we investigated emission spectra of CdSe/ZnS quantum dot solids in the vicinity of gold metallic nanoparticles coated with chromium oxide. The metallic nanoparticles were used to enhance the rate of the energy transfer between the quantum dots, while the chromium oxide coating led to significant increase of their photo-oxidation rates. Our results showed that irradiation of such solids with a laser beam can lead to unique spectral changes, including narrowing and blue shift. We investigate these effects in terms of inhibition of the plasmonically enhanced interdot energy transfer between quantum dots via the chromium-oxide accelerated photo-oxidation process. We demonstrate this considering energy-dependent rate of the interdot energy transfer process, plasmonic effects, and the way photo-oxidation enhances non-radiative decay rates of quantum dots with different sizes.
Communication: Excited states, dynamic correlation functions and spectral properties from full configuration interaction quantum Monte Carlo
Authors: Booth George H. , Chan Garnet Kin-Lic
Published: November 2012

In this communication, we propose a method for obtaining isolated excited states within the full configuration interaction quantum Monte Carlo framework. This method allows for stable sampling with respect to collapse to lower energy states and requires no uncontrolled approximations. In contrast with most previous methods to extract excited state information from quantum Monte Carlo methods, this results from a modification to the underlying propagator, and does not require explicit orthogonalization, analytic continuation, transient estimators, or restriction of the Hilbert space via a trial wavefunction. Furthermore, we show that the propagator can directly yield frequency-domain correlation functions and spectral functions such as the density of states which are difficult to obtain within a traditional quantum Monte Carlo framework. We demonstrate this approach with pilot applications to the neon atom and beryllium dimer.
Calibration of the photoluminescence technique for measuring concentrations of shallow dopants in Ge
Authors: Allardt M. , Kolkovsky Vl. , Irmscher K. , Weber J.
Published: November 2012

A systematic study of the photoluminescence (PL) from excitons bound to shallow donors or acceptors in Al-, As-, B-, Ga-, In-, P-, and Sb-doped Ge is presented. The results of the PL measurements are correlated with those from photothermal ionization spectroscopy and Hall effect measurements. The dissociation energy of the excitons is shown to satisfy Haynes rule. A calibration is presented, which allows a determination of the donor and acceptor concentrations from their respective bound exciton intensities related to the free exciton intensity.
Structural analysis of silicon co-implanted with carbon and high energy proton for the formation of the lasing G-centre
Authors: Berhanuddin D. D. , Lourenço M. A. , Jeynes C. , Milosavljević M. , Gwilliam R. M. , Homewood K. P.
Published: November 2012

We investigate a new approach for efficient generation of the lasing G-centre (carbon substitutional-silicon self-interstitial complex) which crucially is fully compatible with standard silicon ultra-large-scale integration technology. Silicon wafers were implanted with carbon and irradiated with high energy protons to produce self-interstitials that are crucial in the formation of the G-centre. Rutherford backscattering spectrometry (RBS) and transmission electron microscopy were used to study the structure of the post-implanted silicon samples and to investigate the behaviour of the self-interstitials and damage introduced by the carbon and proton implantation. The effect of substrate pre-amorphisation on the G-centre luminescence intensity and formation properties was also investigated by implanting Ge prior to the carbon and proton irradiation. Photoluminescence measurements and RBS results show a significantly higher G-centre peak intensity and silicon yield, respectively, in samples without pre-amorphisation.
Unipolar resistive switching characteristics of pnictogen oxide films: Case study of Sb2O5
Authors: Ahn Youngbae , Wook Ryu Seung , Ho Lee Jong , Woon Park Ji , Hwan Kim Gun , Seok Kim Young , Heo Jaeyeong , Seong Hwang Cheol , Joon Kim Hyeong
Published: November 2012

Resistive switching (RS) and electrical endurance characteristics of amorphous Sb2O5 films in Pt/Sb2O5/Pt and Sb/Sb2O5/Pt structures were examined. Although both structures showed unipolar RS characteristics with power consumption smaller than those of other various transition metal oxides by ∼ two orders of magnitude, enhanced electrical endurance was obtained when Sb was employed as the top electrode than when Pt was employed as the top electrode. This improvement was explained by the creation of Sb clusters in the Sb2O5 films through the diffusion of oxygen from Sb2O5 to the Sb layer and the suppression of excessive oxygen loss during conducting filament formation process by the Sb top electrode. Metallic Sb clusters in pristine Sb2O5 films were confirmed by X-ray photoelectron spectroscopy depth profiling measurements and Auger electron spectroscopy. From the results of current density variation as a function of sample area, it was confirmed that the RS properties of Sb2O5 were controlled by the localized conducting filament in Sb2O5; this finding was in agreement with the result of high resolution transmission electron microscopy. The RS properties of this pnictogen oxide material are discussed with the RS properties of other transition metal oxide materials.
Investigation of plasma-doped fin structure and characterization of dopants by atom probe tomography
Authors: Kim B. H. , Park S. M. , Park S. W. , Park Y. B. , Kim H. J. , Park C. G.
Published: November 2012

As and P dopants in a plasma-doped Si-based fin structure were analyzed using atom probe tomography. The distributions and concentrations of As and P atoms in various regions of the fin structure and the oxidation levels for different dopants were determined. Most dopants were segregated at the fin boundary, and the As and P concentrations exceeded 9 × 1020 atoms/cm3 and 2 × 1020 atoms/cm3, respectively. The atomic oxygen and SiO2 concentrations depended on the dopant type. The larger and heavier As dopant severely damaged the surface of the fin structure and could cause more severe oxidation.
Estimating time-correlation functions by sampling and unbiasing dynamically activated events
Authors: Athènes Manuel , Marinica Mihai-Cosmin , Jourdan Thomas
Published: November 2012

Transition path sampling is a rare-event method that estimates state-to-state time-correlation functions in many-body systems from samples of short trajectories. In this framework, it is proposed to bias the importance function using the lowest Jacobian eigenvalue moduli along the dynamical trajectory. A lowest eigenvalue modulus is related to the lowest eigenvalue of the Hessian matrix and is evaluated here using the Lanczos algorithm as in activation-relaxation techniques. This results in favoring the sampling of activated trajectories and enhancing the occurrence of the rare reactive trajectories of interest, those corresponding to transitions between locally stable states. Estimating the time-correlation functions involves unbiasing the sample of simulated trajectories which is done using the multi-state Bennett acceptance ratio (MBAR) method. To assess the performance of our procedure, we compute the time-correlation function associated with the migration of a vacancy in α-iron. The derivative of the estimated time-correlation function yields a migration rate in agreement with the one given by transition state theory. Besides, we show that the information relative to rejected trajectories can be recycled within MBAR, resulting in a substantial speed-up. Unlike original transition path-sampling, our approach does not require computing the reversible work to confine the trajectory endpoints to a reactive state.
Aging and stiction dynamics in confined films of a star polymer melt
Authors: Yamada Shinji
Published: November 2012

The stiction properties of a star polyisoprene (PIP) melt (having 22 arms and an arm molecular weight of around 5000, Mw ≈ 110 000) confined between mica surfaces were investigated using the surface forces apparatus. Stop-start experiments were carried out and the stiction spike was measured as a function of surface stopping (aging) time t and applied pressure P; the time constants of the phase transitions in the stiction dynamics (freezing on stopping and melting on starting) were obtained from the force relaxation behaviors. The results were compared with those of a confined linear-PIP melt (Mw ≈ 48 000) and other confined fluid systems; the effect of star architecture on the phase transitions in confinement during aging is discussed. Estimation of the molecular size gives that the confined star-PIP films consist of three molecular layers; a non-adsorbed layer sandwiched between two layers adsorbed on opposed mica surfaces. There are (at least) four time constants in the freezing transition of the confined star-PIP melt; fast (τ1) and slow (τ2) time constants for lateral force relaxation on stopping, critical aging time for freezing (τf), and the logarithmic increase of the spike height against t. The three time constants on stopping, τ1, τ2, and τf, increase with the increase of P (decrease of the thickness D). As regards the melting transition on starting, spike force decay was fitted by a single exponential function and one time constant was obtained, which is insensitive to P (D). Comparison of the time constants between freezing and melting, and also with the results of linear-PIP reveals that the stiction dynamics of the star-PIP system involves the relaxation and rearrangement of segmental-level and whole molecular motions. Lateral force relaxation on stopping is governed by the individual and cooperative rearrangements of local PIP segments and chain ends of the star, which do not directly lead to the freezing of the system. Instead, geometrical rearrangements of the soft star-PIP spheres into dense packing between surfaces (analogous to the concept of a colloidal glass transition) are the major mechanism of the freezing transition (stiction) after aging. Interdigitation of PIP segments/chain ends between neighboring star molecules also contributes to the spike growth along with aging, and the melting transition on starting.
Experimental evidence of superionic conduction in H2O ice
Authors: Sugimura Emiko , Komabayashi Tetsuya , Ohta Kenji , Hirose Kei , Ohishi Yasuo , Dubrovinsky Leonid S.
Published: November 2012

Ionic conductivity and molar volume measurements were performed on H2O ice at high pressure (P) and temperature (T) in a resistive-heated diamond anvil cell. The conductivity data obtained at P = 20–62 GPa, T = 304–930 K are well fitted with a single Arrhenius equation. Isothermal volume measurements at T = 873 K, P = 30–101 GPa indicate that H2O ice undergoes phase transitions at P = 50 GPa and 53 GPa due to hydrogen-bond symmetrization. Combining these results, we suggest that the conduction mechanism does not change with pressure-induced hydrogen-bond symmetrization. Along the Arrhenius behavior of conductivity data, the experimental evidence for superionic conduction (>10−1 S/cm) was found at T = 739 K, P = 56 GPa and T = 749 K, P = 62 GPa, which is significantly low temperature compared with earlier theoretical estimates resorted to the observation of a drastic rise of the melting curve. We infer that the sudden increase of the melting temperature is not related to the onset of superionic conduction, but is attributed to the phase change regarding to the symmetrization.
(Zn,Cd)O thin films for the application in heterostructures: Structural and optical properties
Authors: Lange M. , Dietrich C. P. , Brachwitz K. , Böntgen T. , Lorenz M. , Grundmann M.
Published: November 2012

We report on (Zn,Cd)O thin films, grown by pulsed laser deposition on a-plane sapphire substrates with high Cd-contents up to 0.25. By incorporating Cd in ZnO and by applying a low growth temperature of about 300°C, the (Zn,Cd)O related luminescence redshifts to an energy of 2.46 eV as a result of the large Cd-content of 0.25. The redshift of the bandgap energy was additionally proven by transmission measurements. By fitting the transmission curves, the spectra of the absorption coefficient and the index of refraction are calculated. The (Zn,Cd)O thin films are single phase and exhibit the wurtzite crystal structure. An increasing a- and c-lattice constant is observed with increasing Cd-content.
Calculations of potential energy surfaces using Monte Carlo configuration interaction
Authors: Coe Jeremy P. , Taylor Daniel J. , Paterson Martin J.
Published: November 2012

We apply the method of Monte Carlo configuration interaction (MCCI) to calculate ground-state potential energy curves for a range of small molecules and compare the results with full configuration interaction. We show that the MCCI potential energy curve can be calculated to relatively good accuracy, as quantified using the non-parallelity error, using only a very small fraction of the full configuration interaction space. In most cases the potential curve is of better accuracy than its constituent single-point energies. We finally test the MCCI program on systems with basis sets beyond full configuration interaction: a lattice of 50 hydrogen atoms and ethylene. The results for ethylene agree fairly well with other computational work while for the lattice of 50 hydrogens we find that the fraction of the full configuration interaction space we were able to consider appears to be too small as, although some qualitative features are reproduced, the potential curve is less accurate.