bluuebunny/crossref_metadata_2025 huggingface dataset:
Dielectric relaxation study of amorphous TiTaO thin films in a large operating temperature range
Authors: Rouahi A. , Kahouli A. , Challali F. , Besland M. P. , Vallée C. , Pairis S. , Yangui B. , Salimy S. , Goullet A. , Sylvestre A.
Published: November 2012

Two relaxation processes have been identified in amorphous TiTaO thin films deposited by reactive magnetron sputtering. The parallel angle resolved x-ray photoelectron spectroscopy and field emission scanning electron microscopy analyses have shown that this material is composed of an agglomerates mixture of TiO2, Ta2O5, and Ti-Ta bonds. The first relaxation process appears at low temperature with activation energy of about 0.26 eV and is related to the first ionisation of oxygen vacancies and/or the reduction of Ti4+ to Ti3+. The second relaxation process occurs at high temperature with activation energy of 0.95 eV. This last peak is associated to the diffusion of the doubly ionized oxygen vacancies VÖ. The dispersion phenomena observed at high temperature can be attributed to the development of complex defect such as (VÖ − 2Ti3+).
Enhanced room temperature electronic and thermoelectric properties of the dilute bismuthide InGaBiAs
Authors: Dongmo Pernell , Zhong Yujun , Attia Peter , Bomberger Cory , Cheaito Ramez , Ihlefeld Jon F. , Hopkins Patrick E. , Zide Joshua
Published: November 2012

We report room temperature electronic and thermoelectric properties of Si-doped In0.52Ga0.48BiyAs1−y with varying Bi concentrations. These films were grown epitaxially on a semi-insulating InP substrate by molecular beam epitaxy. We show that low Bi concentrations are optimal in improving the conductivity, Seebeck coefficient, and thermoelectric power factor, possibly due to the surfactant effects of bismuth. We observed a reduction in thermal conductivity with increasing Bi concentration, which is expected because of alloy scattering. We report a peak ZT of 0.23 at 300 K.
Rotational spectroscopic study of hydrogen cyanide embedded in small 4He clusters
Authors: Dempster Steve P. , Sukhorukov Oleksandr , Lei Qi-Yi , Jäger Wolfgang
Published: November 2012

High resolution microwave spectra of the a-type, J = 1–0, transitions of HeN = 1–6–H12C14N, HeN = 1–6–H13C14N, HeN = 1–6–H12C15N, HeN = 1–7–D12C14N, and HeN = 1–6–D13C14N clusters produced in a supersonic jet expansion were measured and analyzed. The resulting effective rotational constants, Beff, initially decrease with the number of the attached helium atoms before reaching a minimum at N = 3 helium atoms for all isotopologues. The subsequent increase in Beff for N ≥ 4 is indicative of the onset of microscopic superfluidity. Comparison of our experimental Beff constants with those from quantum Monte Carlo simulations [A. A. Mikosz, J. A. Ramilowski, and D. Farrelly, J. Chem. Phys. 125, 014312 (2006)10.1063/1.2213253] reveals a nearly congruent trend in Beff for N up to 6. Analysis of the hyperfine structure of the 14N containing isotopologues yielded a gradual incremental increase in the magnitude of χaa and ⟨P2(cos θ)⟩ for N = 1–6, which suggests the internal rotation of the HCN molecule is becoming increasingly hindered.
The equation-of-motion coupled cluster method for triple electron attached states
Authors: Musiał Monika , Olszówka Marta , Lyakh Dmitry I. , Bartlett Rodney J.
Published: November 2012

The initial implementation of the triple electron attachment (TEA) equation-of-motion (EOM) coupled cluster (CC) method is presented, aiming at the description of electronic states with three open shell electrons outside a suitably chosen closed shell vacuum. In particular, such an approach can be used for describing dissociation of chemical bonds predominantly formed by three valence electrons, for example, in LiC and NaC molecules. Both ground and excited states are considered while rigorously maintaining the correct spin value. The preliminary results show a correct asymptotic behavior of the dissociation curves. At the same time, we emphasize that a chemically accurate description will require an extension of the minimal TEA-EOM-CC model introduced here, analogous to those already used in the double ionization potential and double electron attachment methods.
Doping level dependent space charge limited conduction in polyaniline nanoparticles
Authors: Nath Chandrani , Kumar A.
Published: November 2012

Spherical shaped polyaniline nanoparticles of average diameter ∼4 nm were doped with different concentration of hydrochloric acid. The x-ray diffraction studies reveal an increase in crystallinity with doping. Temperature dependent current-voltage measurements on the different nanoparticle samples indicate the prevalence of bulk-controlled space charge limited conduction (SCLC) mechanism in the high voltage (>1 V) region while the ohmic conduction dominates at the lower voltage (<1 V) region. With increasing doping the nature of SCLC changes from trap-free SCLC to Child-Langmuir type SCLC via exponential trap-limited SCLC. Moreover, the field and temperature dependence of mobility exhibits the universal Poole-Frenkel behavior. The energetic disorder parameter, spatial disorder parameter, inter-site distance, and localization length have been extracted employing the uncorrelated and the correlated Gaussian disorder model.
Third harmonic generation in intraband transitions of spherical silicon quantum dots
Authors: Anchala  , Purohit S. P. , Mathur K. C.
Published: November 2012

A theoretical study of the third harmonic generation (THG) is reported involving intraband transitions in the conduction band of spherical Si semiconductor quantum dot surrounded by SiO2, Si3N4, and SiC matrix. The wave function and energies of a singly charged Si dot are calculated using the effective mass approximation. A finite barrier height is considered at the interface of the dot and the surrounding matrix. The results show that the THG coefficient of the silicon quantum dot strongly depends on the radius of the dot and the surrounding matrix.
Phononic band gap engineering in graphene
Authors: Sgouros A. , Sigalas M. M. , Kalosakas G. , Papagelis K. , Papanicolaou N. I.
Published: November 2012

Using ab initio and molecular dynamics simulations with semi-empirical potentials, the phonon density of states (PnDOS) of graphene with different types of defects such as substitution atoms (Si), carbon isotopes (12C and 14C), and vacancies was calculated. The main interest was to investigate the possibility to generate phononic band gaps (PBGs) in the PnDOS of graphene, since the derived structures may have sufficiently low thermal conductivity and find applications in improved thermoelectric materials. From all the studied defect types, the silicon substitution is the only one that creates PBGs.
Hydrogen stability in hydrogenated amorphous carbon films with polymer-like and diamond-like structure
Authors: Buijnsters J. G. , Gago R. , Redondo-Cubero A. , Jiménez I.
Published: November 2012

Hydrogen (H) stability in hydrogenated amorphous carbon (a-C:H) films with different structures grown by (biased) electron-cyclotron-resonance chemical vapor deposition has been studied against thermal annealing and swift-ion impact (2 MeV He+). For this purpose, a-C:H films with either polymer-like (PLCH) or diamond-like (DLCH) character grown on grounded or biased (−200 V) substrates, respectively, were annealed up to 450 °C. The local-order structural evolution around C sites was analyzed by x-ray absorption near-edge spectroscopy (XANES) and the H content and radiation-induced release were determined by successive elastic recoil detection analysis (ERDA) acquisitions. A relatively high H content is measured for both as-grown PLCH (∼45 at. %) and DLCH films (∼33 at. %). Upon annealing, PLCH films suffer thermal-induced surface decomposition resulting in a thickness reduction and only above 350 °C the H content in the film matrix decreases. PLCH films also display a pronounced H loss rate during ERDA measurements, whereas H is stable in DLCH. These results indicate that H bonding differs in both structures (i.e., weaker C-H bonds in PLCH). XANES shows that upon annealing both structures suffer H loss at the near surface region together with a graphitization process, although the impact is more pronounced in PLCH. XANES fine-structure reveals that aromatic clusters are formed upon annealing in PLCH due to H loss, whereas this process is partially inhibited in DLCH due to the thermal stability of the C-H bonds.
Structure and magnetic properties of highly dispersed Ni-Mn-Ga powders prepared by spark-erosion
Authors: Perekos A. E. , Chernenko V. A. , Bunyaev S. A. , Zalutskiy V. P. , Ruzhitskaya T. V. , Boitsov O. F. , Kakazei G. N.
Published: November 2012

Highly dispersed powders of Ni50,3Mn30Ga19,7 were prepared by spark-erosion in ethanol, water, and kerosene. Powder particles have mostly spherical shape and broad size distribution, with a maximum around 1 μm. In the as-prepared state, two series of peaks are observed by x-ray diffraction. They are associated with the disordering of two ordered phases, existed in bulk Ni-Mn-Ga: the high-temperature L21 austenitic phase and the low-temperature L10 martensitic one. Annealing decreases the half-width of the peaks and increases the L10/L21 intensity ratio. Magnetically as-prepared powders demonstrate a clear superparamagnetic behavior that changes to ferromagnetic one after annealing. These properties are quite different from the ones of mechanically dispersed powders and rapidly quenched ribbons of the same composition. The spark-erosion conditions, particularly cooling rates up to 109 K/s, lead to the formation of the mixed phase state, much higher atomic disorder, and to the appearance of different types of structural inhomogeneities.
Low field permittivity of ferroelectric-ferrite ceramic composites: Experiment and modeling
Authors: Elena Ciomaga Cristina , Stefania Olariu Cristina , Padurariu Leontin , Victor Sandu Andrei , Galassi Carmen , Mitoseriu Liliana
Published: November 2012

The dielectric properties of xNiFe2O4-(1−x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 ceramic composites with different volume filling factors (x = 2, 5, 10, 20, 30, 40, 50, 60, 70 wt. %) prepared by sol-gel method have been investigated in the frequency range (106 ÷ 109) Hz. A reducing effective permittivity with increasing the ferrite amount was observed as result of the “sum property.” The experimental results have been compared with the effective permittivity values predicted by effective medium approximation (EMA) models and finite element method (FEM) calculations. The comparison was aimed to evaluate the appropriateness of the results of the two approaches in describing the effective dielectric properties of the composite with contrasting permittivities and to understand the role of microstructural characteristics on the dielectric response in a broad filling factor range. The best description of the experimental data in overall filling factor range is given by the results of FEM calculations which used realistic microstructures and among the EMA models, by the Lichtenecker's formula.
Nonequilibrium optical phonon effect on high-field electron transport in InN
Authors: Reklaitis Antanas
Published: November 2012

The effect of nonequilibrium longitudinal optical (LO) phonons on electron transport in wurtzite InN in high electric fields is studied by Monte Carlo simulations. The novel and efficient single-particle Monte Carlo method for a self-consistent simulation of nonequilibrium electron and phonon systems is developed. The effect of nonequilibrium LO phonons on electron drift velocity, valley population, and small-signal ac mobility in InN is investigated. It is found that the effect of nonequilibrium LO phonons on electron transport characteristics in InN becomes pronounced when electron density exceeds 1017 cm−3.
Oscillation frequency of magnetic vortex induced by spin-polarized current in a confined nanocontact structure
Authors: Liu Yan , Li Huanan , Hu Yong , Du An
Published: November 2012

We studied the oscillation frequency associated with gyrotropic motion of magnetic vortex in a Permalloy nanodot driven by an out-of-plane spin-polarized current injected through a nanocontact by micromagnetic simulations and analytical calculations. The analytical results were calculated by the Thiele's equation, where both the forces corresponding to the spin-transfer-torque and the Oersted field accompanying the current were taken into account. Variation curves of oscillation frequency with vortex core position, nanocontact radius, and current density were given. Good agreement with analytical calculations and micromagnetic simulations was found.
An ab-initio study of electronic and optical properties of corundum Al2O3 doped with Sc, Y, Zr, and Nb
Authors: Lima A. F. , Dantas J. M. , Lalic M. V.
Published: November 2012

Ab-initio calculations based on density functional theory have been employed to study the structural, electronic, and optical properties of yttrium (Y), scandium (Sc), zirconium (Zr), and niobium (Nb) doped α-Al2O3 with corundum structure. Exchange and correlation effects between electrons have been treated by generalized gradient approximation within the Perdew-Burk-Ezrenhof parameterization and by recently developed Tran-Blaha modified Becke-Johnson approach. Most attention in the work has been paid to the impurity d states, whose energy splitting has been analyzed in terms of the crystal field theory and whose influence on the gap size and the offset of the bands around it has been carefully evaluated. The influence of these states on modification of the optical absorption edge and the static dielectric constant of the doped systems has been also studied. It is concluded that only the Y doped α-Al2O3 (1) preserves the size of the band gap of the pure alumina, (2) does not change significantly the band offset around it, and (3) elevates the value of the static dielectric constant of the compound. These three conditions, necessary for usability of the doped material as a high-ε dielectric gate, are not satisfied by the Sc-, Zr-, and Nb-doped alumina compounds. Therefore, only the Y-doped α-Al2O3 exhibits potential to be further explored for employment in the semiconductor industry.
Young’s modulus measurement of two-photon polymerized micro-cantilevers by using nanoindentation equipment
Authors: Cicha Klaus , Koch Thomas , Torgersen Jan , Li Zhiquan , Liska Robert , Stampfl Jürgen
Published: November 2012

Due to its high feature resolution and the capability to produce 3D-structures without the need of support-structures, two-photon polymerization (2PP) is one of the fastest growing technologies in the field of additive manufacturing. To ensure good quality of the fabricated parts, proper methods for evaluating the suitability of different material systems for 2PP are necessary. In this paper, we present a fast screening method based on measuring the Young's modulus of micro-cantilevers giving a quantifiable measure and representing the material- and fabrication-properties in one experiment.
Communication: Application of state-specific multireference coupled cluster methods to core-level excitations
Authors: Brabec Jiří , Bhaskaran-Nair Kiran , Govind Niranjan , Pittner Jiří , Kowalski Karol
Published: November 2012

The concept of the model space underlying multireference coupled-cluster (MRCC) formulations is a powerful tool to deal with complex correlation effects for various electronic states. Here, we demonstrate that iterative state-specific MRCC methods (SS-MRCC) based on properly defined model spaces can be used to describe core-level excited states even when Hartree-Fock orbitals are utilized. We show that the SS-MRCC models with single and double excitations are comparable in accuracy to high-level single reference equation-of-motion coupled cluster (EOMCC) formalism.
Rectified Brownian transport in corrugated channels: Fractional Brownian motion and Lévy flights
Authors: Ai Bao-quan , Shao Zhi-gang , Zhong Wei-rong
Published: November 2012

We study fractional Brownian motion and Lévy flights in periodic corrugated channels without any external driving forces. From numerical simulations, we find that both fractional Gaussian noise and Lévy-stable noise in asymmetric corrugated channels can break thermodynamical equilibrium and induce directed transport. The rectified mechanisms for fractional Brownian motion and Lévy flights are different. The former is caused by non-uniform spectral distribution (low or high frequencies) of fractional Gaussian noise, while the latter is due to the nonthermal character (occasional long jumps) of the Lévy-stable noise. For fractional Brownian motion, average velocity increases with the Hurst exponent for the persistent case, while for the antipersistent case there exists an optimal value of Hurst exponent at which average velocity takes its maximal value. For Lévy flights, the group velocity decreases monotonically as the Lévy index increases. In addition, for both cases, the optimized periodicity and radius at the bottleneck can facilitate the directed transport. Our results could be implemented in constrained structures with narrow channels and pores where the particles undergo anomalous diffusion.
A little ribbing: Flux starvation engineering for rippled indium tin oxide nanotree branches
Authors: Tucker Ryan T. , Beaudry Allan L. , LaForge Joshua M. , Taschuk Michael T. , Brett Michael J.
Published: November 2012

Combining vapour-liquid-solid growth with glancing angle deposition (VLS-GLAD) facilitates fabrication of branched nanowires not possible with either technique alone. Indium tin oxide (ITO) nanostructures grown by VLS-GLAD produce extremely porous nanotree structures, where periodic branch diameter oscillations are sometimes observed. We explain this rippled branch growth with a simple model linking the physics governing branch growth to the process variables controlled in VLS-GLAD. The model is verified by inducing specific, aperiodic ripples onto growing ITO branches through macroscopic vapour flux control and manipulation of local shadowing.
Dynamic photoacoustic spectroscopy for trace gas detection
Authors: Wynn C. M. , Palmacci S. , Clark M. L. , Kunz R. R.
Published: November 2012

We present a method of photoacoustic spectroscopy in which a laser beam tuned to an absorption feature of a gas is swept through its plume at the speed of sound. The resulting coherent addition of acoustic waves leads to an amplification of the signal without the need for a resonant chamber, thus enhancing the ability to remotely sense the gas. We demonstrate the concept using a tunable CO2 laser and SF6 gas in conjunction with a microphone. Sound pressure levels of 83 dB (relative to 20 μPa) are generated from a 15-ppm plume.
High pressure optical cell for synthesis and in situ Raman spectroscopy of hydrogen clathrate hydrates
Authors: Celli Milva , Zoppi Marco , Zaghloul Mohamed A. S. , Ulivi Lorenzo
Published: November 2012

We report the design, realization, and test of a high-pressure optical cell that we have used to measure the Raman spectra of hydrogen clathrate hydrates, synthesized in situ by the application of 200-300 MPa of gas pressure on solid water. The optical apparatus is mounted on a cryogenic system so to allow measurements and sample treatment at any temperature between 300 and 20 K. A capillary pipe is connected to the inside of the cell to allow the gas flow into and out of the cell, and to regulate the internal pressure at any value from 0 to 300 MPa. In the experimental test described in this paper, the cell has been partly filled, at room temperature, with a small amount of water, then frozen at 263 K before injecting hydrogen gas, at pressure of 150 MPa, into the cell. This procedure has permitted to study hydrogen clathrate formation, by measuring Raman spectra as a function of time.
An unusual variation of stability and hardness in molybdenum borides
Authors: Liang Yongcheng , Yuan Xun , Fu Zhao , Li Yuan , Zhong Zheng
Published: November 2012

Molybdenum borides are currently raising great expectations for superhard materials, but their crystal structures and mechanical behaviors are still under discussion. Here, we report an unexpected reduction of stability and hardness from porous hP16-MoB3 and hR18-MoB2 to dense hP20-MoB4 and hR21-Mo2B5, respectively. Furthermore, we demonstrate that this anomalous variation has its electronic origin. These findings not only manifest that the long-recognized hP20-MoB4 (hP3-MoB2) and hR21-Mo2B5 should be hP16-MoB3 and hR18-MoB2, respectively, but also challenge the general design principle for ultrahard materials only pursuing the dense transition-metal borides with high boron content.
Chaos in an ion-channel free-electron laser with realistic helical wiggler
Authors: Esmaeilzadeh Mahdi , Taghavi Amin
Published: November 2012

Chaotic behavior of an electron motion in a free-electron laser with realistic helical wiggler and ion-channel guiding is studied using Poincaré surface-of-section maps. The effects of a realistic electron beam density on chaotic electron dynamics are investigated by considering an electron beam with Gaussian density profile in radial distance. The effects of self-fields on chaotic electron dynamics are investigated for different Gaussian beam parameters, and the results are compared with those of uniform electron beam. It is shown that the electron chaotic behavior can be controlled by changing the Gaussian beam parameter. Also, the chaotic behavior can be controlled by increasing the ion-channel and/or the electron beam densities.
Note: Position self-sensing for piezoelectric actuators in the presence of creep and rate-dependent hysteresis
Authors: Islam M. N. , Seethaler R. J.
Published: November 2012

Sensorless position signal can be obtained from the charge inflow to the piezoelectric actuator or effective capacitance variation due to stroke. However, the first technique imposes sophisticated hardware requirement and it is prone to drift due to current offset. The latter is limited to slow operations. In this note, a hybrid position observer is used to fuse these two self-sensing signals to obtain a high quality position signal which compensates for both creep (slow operations) and rate-dependent hysteresis (fast operations). Experimental results are presented to demonstrate the performance of the observer.
Effect of self-magnetic fields on the nonlinear dynamics of relativistic electron beam with virtual cathode
Authors: Hramov A. E. , Kurkin S. A. , Koronovskii A. A. , Filatova A. E.
Published: November 2012

The report is devoted to the results of the numerical study of the virtual cathode (VC) formation conditions in the relativistic electron beam (REB) under the influence of the self-magnetic and external axial magnetic fields. The azimuthal instability of the relativistic electron beam leading to the formation of the vortex electron structure in the system was found out. This instability is determined by the influence of the self-magnetic fields of the relativistic electron beam, and it leads to the decrease of the critical value of the electron beam current (current when the non-stationary virtual cathode is formed in the drift space). The typical dependencies of the critical current on the external uniform magnetic field value were discovered. The effect of the beam thickness on the virtual cathode formation conditions was also analyzed.
Omnidirectional photonic band gap enlarged by one-dimensional ternary unmagnetized plasma photonic crystals based on a new Fibonacci quasiperiodic structure
Authors: Zhang Hai-Feng , Liu Shao-Bin , Kong Xiang-Kun , Bian Bo-Rui , Dai Yi
Published: November 2012

In this paper, an omnidirectional photonic band gap realized by one-dimensional ternary unmagnetized plasma photonic crystals based on a new Fibonacci quasiperiodic structure, which is composed of homogeneous unmagnetized plasma and two kinds of isotropic dielectric, is theoretically studied by the transfer matrix method. It has been shown that such an omnidirectional photonic band gap originates from Bragg gap in contrast to zero-n¯ gap or single negative (negative permittivity or negative permeability) gap, and it is insensitive to the incidence angle and the polarization of electromagnetic wave. From the numerical results, the frequency range and central frequency of omnidirectional photonic band gap can be tuned by the thickness and density of the plasma but cease to change with increasing Fibonacci order. The bandwidth of omnidirectional photonic band gap can be notably enlarged. Moreover, the plasma collision frequency has no effect on the bandwidth of omnidirectional photonic band gap. It is shown that such new structure Fibonacci quasiperiodic one-dimensional ternary plasma photonic crystals have a superior feature in the enhancement of frequency range of omnidirectional photonic band gap compared with the conventional ternary and conventional Fibonacci quasiperiodic ternary plasma photonic crystals.
Effective large-area free-standing graphene field emitters by electrophoretic deposition
Authors: Koh Angel T. T. , Foong Y. M. , Pan Likun , Sun Zhuo , Chua Daniel H. C.
Published: November 2012

Large area graphene field emitters were fabricated using electrophoretic deposition. By varying the deposition time, we were able to fabricate emitters with varied field enhancement factors. The turn-on field increased when the deposition time increased, and it was observed that a monolayer of graphene flakes covered on the substrate gave the best results. The low emission turn-on field obtained from the graphene field emitters was attributed to the random orientation of graphene flakes which give protruded edges that acted as field enhancing spots. Graphene emitters produced by this method present a low cost cold cathode that can be mass produced.
Distribution of switching fields in magnetic granular materials
Authors: Hovorka O. , Pressesky J. , Ju G. , Berger A. , Chantrell R. W.
Published: November 2012

We present analytical calculations and kinetic Monte-Carlo modeling of rate-dependent behavior of switching field distributions (SFDs) in an ensemble of Stoner-Wohfarth particles, assuming distributions of anisotropies and volumes, and thermal activation included by the Néel-Brown theory. By applying probabilistic arguments, we show that the SFD can be self-consistently separated into the contribution from distributions of intrinsic properties of particles and the (irreducible) contribution resulting solely from thermal fluctuations, which is shown to become a significant effect at sweep rates relevant to the recording process. This provides a unifying framework for systematic analysis of different classes of systems.
Highly efficient indium tin oxide-free organic photovoltaics using inkjet-printed silver nanoparticle current collecting grids
Authors: Neophytou M. , Hermerschmidt F. , Savva A. , Georgiou E. , Choulis S. A.
Published: November 2012

We report an in-depth investigation of an inkjet-printed silver (Ag) nanoparticle grid combined with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) of different conductivities as an alternative to an indium tin oxide (ITO)-based transparent anode for organic solar cell applications. The reported measurements revealed higher transparency of the inkjet-printed Ag nanoparticle-based grid when compared to different thicknesses of ITO on glass substrates. Based on the proposed current collecting grid, a record power conversion efficiency of 2% is achieved for ITO-free organic solar cells.
Influence of p-doping on the temperature dependence of InAs/GaAs quantum dot excited state radiative lifetime
Authors: Harbord Edmund , Iwamoto S. , Arakawa Y. , Spencer P. , Clarke E. , Murray R.
Published: November 2012

The radiative lifetime of the excited state transition of undoped and p-doped InAs/GaAs quantum dots (QDs) is estimated from measurements of time-integrated and time-resolved luminescence from both ground and excited states. The radiative lifetime of the undoped QDs increases from 500 ps at 10 K to almost 3 ns at room temperature, consistent with a Boltzmann redistribution of holes over the available energy states. The rate of increase can be suppressed by a factor of ∼2 by p-doping the QDs to maintain a hole population in the lowest confined dot states to high temperatures.
High magnetoresistance in graphene nanoribbon heterojunction
Authors: Bala Kumar S. , Jalil M. B. A. , Tan S. G.
Published: November 2012

We show a large magnetoresistance (MR) effect in a graphene heterostructure consisting of a metallic and semiconductor-type armchair-graphene-nanoribbon. In the heterostructure, the transmission across the first subband of the semiconducting armchair-graphene-nanoribbon and metallic armchair-graphene-nanoribbon is forbidden under zero magnetic-field, due to the orthogonality of the wavefunctions. A finite magnetic-field introduces the quantum hall-like effect, which distorts the wavefunctions. Thus, a finite transmission occurs across the heterojunction, giving rise to a large MR effect. We study the dependence of this MR on temperature and electron energy. Finally, we design a magnetic-field-effect-transistor which yields a MR of close to 100% (85%) at low (room) temperature.
A novel spin modulation of work function for C adsorbed Cr/Fe(001) metal gate
Authors: Zhong Kehua , Xu Guigui , Cheng Yanmin , Tang Keqin , Chen Zhigao , Huang Zhigao
Published: November 2012

Work functions and magnetic moments of C adsorbed Cr/Fe(001) surfaces with different C coverages θ and magnetic alignments (parallel or antiparallel) between Cr and Fe atom moments are investigated using first-principles methods based on density functional theory. The calculated results reveal that the spin configuration plays a significant role in determining the work function of the systems. The work functions of the systems with parallel states are evidently larger than those with antiparallel states. Moreover, for θ≤0.5 ML, with increasing value of θ, the work function increases from 4.23 eV to 5.13 eV for antiparallel states and from 4.47 eV to 5.44 eV for parallel states. While for θ>0.5 ML, the work function decreases with increasing value of θ. It can be also found that, for θ≤0.5 ML, the smaller the Cr and Fe magnetic moments are, the lower the Fermi energy EF is and the larger the work functions of the systems are. Based on analysis and discussion, we conclude that the changes of the work functions and magnetic moments are mainly determined by those of Fermi level and density of state (DOS) induced by the spin polarization, the electron transfer and the surface structure. Our work strongly suggests that controlling the magnetic states is a promising way for modulating the work function of magnetic metal gate.
On the origin of inter band gap radiative emission in crystalline silicon
Authors: Burud I. , Flø A. S. , Olsen E.
Published: November 2012

Crystal imperfections degrade the quality of multicrystalline silicon wafers by introducing alternative recombination mechanisms. Here we use non-destructive hyperspectral imaging to detect photoluminescence signals from radiatively active recombination processes over the wafer with a highly resolved spectral third dimension. We demonstrate that band-to-band recombination can be visually separated from recombination through traps across the whole surface of a wafer using hyperspectral imaging. Our samples are studied in the near infrared wavelength region, 900-1700 nm, which includes the region of the so called D-band emission lines. These constitute four resolved emission lines found in the photoluminescence spectrum of silicon, commonly related to recombination through shallow inter-band gap energy levels near the conduction- and valence band edges. The shape and structure of these emissions from our measurements suggest that all the D-lines have different origins.
The structure and magnetism of graphone
Authors: Feng L. , Zhang W. X.
Published: November 2012

Graphone is a half-hydrogenated graphene. The structure of graphone is illustrated as trigonal adsorption of hydrogen atoms on graphene at first. However, we found the trigonal adsorption is unstable. We present an illustration in detail to explain how a trigonal adsorption geometry evolves into a rectangular adsorption geometry. We check the change of magnetism during the evolution of geometry by evaluating the spin polarization of the intermediate geometries. We prove and clarify that the rectangular adsorption of hydrogen atoms on graphene is the most stable geometry of graphone and graphone is actually antiferromagnetic.
The marsupial pouch: implications for reproductive success and mammalian evolution
Authors: Edwards Melanie J. , Deakin Janine E.
Published: November 2012

Extant mammals are divided into sub- and infraclasses that are distinguished by their mode of reproduction. The monotremes lay eggs, the marsupials give birth to altricial young that typically develop in a pouch, and the eutherians have prolonged in utero development, resulting in well developed young at birth. The three groups exhibit what appears to be a nice progression of evolution towards the well developed newborn young of eutherian mammals. However, marsupials do not represent a step in the progression of producing well developed young, but maintain a reproductive strategy that has evolved to prosper in their specific niche. The production of undeveloped young with increased development in the pouch (or counterpart) provides specific advantages to those species living in diverse environments. The evolution of this reproductive strategy provides a clever solution to the uncertain and often adverse conditions encountered by many species, and the survival of the developing young in a pouch containing potentially harmful microorganisms is truly remarkable. In this review, we explore the unique features of the pouch, highlight the research questions that remain unanswered regarding this unique marsupial attribute and discuss the advantages of the marsupial reproductive strategy and the potential role of the pouch in mammalian diversification.
Genetic evidence that brain-derived neurotrophic factor mediates competitive interactions between individual cortical neurons
Authors: English Christopher N. , Vigers Alison J. , Jones Kevin R.
Published: November 2012

Brain-derived neurotrophic factor (BDNF) is a secreted protein important for development and function of neocortical circuitry. Although it is well established that BDNF contributes to the sculpting of dendrite structure and modulation of synapse strength, the range and directionality of BDNF signaling underlying these functions are incompletely understood. To gain insights into the role of BDNF at the level of individual neurons, we tested the cell-autonomous requirements forBdnfin visual cortical layer 2/3 neurons. We found that the number of functionalBdnfalleles a neuron carries relative to the prevailing genotype determines its density of dendritic spines, the structures at which most excitatory synapses are made. This requirement forBdnfexists both during postnatal development and in adulthood, suggesting that the amount of BDNF a neuron is capable of producing determines its success in ongoing competition in the environment of the neocortex. Our results suggest that BDNF may perform a long-sought function for a secreted growth factor in mediating the competitive events that shape individual neurons and their circuits.
SGTA antagonizes BAG6-mediated protein triage
Authors: Leznicki Pawel , High Stephen
Published: November 2012

The BAG6 complex was first identified as an upstream loading factor for tail-anchored membrane proteins entering the TRC40-dependent pathway for posttranslational delivery to the endoplasmic reticulum. Subsequently, BAG6 was shown to enhance the proteasomal degradation of mislocalized proteins by selectively promoting their ubiquitination. We now show that the BAG6-dependent ubiquitination of mislocalized proteins is completely reversible and identify a pivotal role for the small glutamine-rich tetratricopeptide repeat-containing protein α (SGTA) in specifically antagonizing this process. SGTA does not simply mask the exposed hydrophobic transmembrane domain of a mislocalized protein, thereby preventing BAG6 recruitment. Rather, SGTA actively promotes the deubiquitination of mislocalized proteins that are already covalently modified, thus reversing the actions of BAG6 and inhibiting its capacity to promote substrate-specific degradation. This SGTA-mediated effect is independent of its tetratricopeptide motifs, suggesting it does not require the actions of Hsp70 and Hsp90 chaperones. These data reveal that, in a cellular context, mislocalized protein ubiquitination is the result of a dynamic equilibrium reflecting competition between pathways that promote either protein maturation or degradation. The targeted perturbation of this equilibrium, achieved by increasing steady-state SGTA levels, results in a specific stabilization of a model mislocalized protein derived from the amyloid precursor protein, an effect that is completely negated by ensuring efficient precursor delivery to the endoplasmic reticulum. We speculate that a BAG6/SGTA cycle operates during protein maturation and quality control in the cytosol and that together these components dictate the fate of a specific subset of newly synthesized proteins.
A mouse model for human deafness DFNB22 reveals that hearing impairment is due to a loss of inner hair cell stimulation
Authors: Lukashkin Andrei N. , Legan P. Kevin , Weddell Thomas D. , Lukashkina Victoria A. , Goodyear Richard J. , Welstead Lindsey J. , Petit Christine , Russell Ian J. , Richardson Guy P.
Published: November 2012

The gene causative for the human nonsyndromic recessive form of deafness DFNB22 encodes otoancorin, a 120-kDa inner ear-specific protein that is expressed on the surface of the spiral limbus in the cochlea. Gene targeting in ES cells was used to create an EGFP knock-in, otoancorin KO (
Otoa

EGFP/EGFP

) mouse. In the
Otoa

EGFP/EGFP

mouse, the tectorial membrane (TM), a ribbon-like strip of ECM that is normally anchored by one edge to the spiral limbus and lies over the organ of Corti, retains its general form, and remains in close proximity to the organ of Corti, but is detached from the limbal surface. Measurements of cochlear microphonic potentials, distortion product otoacoustic emissions, and basilar membrane motion indicate that the TM remains functionally attached to the electromotile, sensorimotor outer hair cells of the organ of Corti, and that the amplification and frequency tuning of the basilar membrane responses to sounds are almost normal. The compound action potential masker tuning curves, a measure of the tuning of the sensory inner hair cells, are also sharply tuned, but the thresholds of the compound action potentials, a measure of inner hair cell sensitivity, are significantly elevated. These results indicate that the hearing loss in patients with
Otoa
mutations is caused by a defect in inner hair cell stimulation, and reveal the limbal attachment of the TM plays a critical role in this process.
Non-Bloom syndrome-associated partial and total loss-of-function variants of BLM helicase
Authors: Mirzaei Hamed , Schmidt Kristina H.
Published: November 2012

Bloom syndrome (BS) is an autosomal recessive disorder caused by mutations in the RecQ-like DNA helicase BLM, which functions in the maintenance of genome stability. Using a humanized model of
Saccharomyces cerevisiae
that expresses a chimera of the N terminus of yeast Sgs1 and the C terminus of human BLM from the chromosomal
SGS1
locus, we have functionally evaluated 27
BLM
alleles that are not currently known to be associated with BS. We identified nine alleles with impaired function when assessed for hypersensitivity to the DNA-damaging agent hydroxyurea (HU). Six of these alleles (
P690L
,
R717T
,
W803R
,
Y811C
,
F857L
,
G972V
) caused sensitivity to HU that was comparable to known BS-associated or helicase-dead alleles, suggesting that they may cause BS and, in the heterozygous state, act as risk factors for cancerogenesis. We also identified three alleles (
R791C
,
P868L
,
G1120R
) that caused intermediate sensitivity to HU; although unlikely to cause BS, these partial loss-of-function alleles may increase risk for cancers or other BS-associated complications if a person is homozygous or compound heterozygous for these alleles or if they carry a known BS-associated allele.
Rapid fragmentation of neuronal networks at the onset of propofol-induced unconsciousness
Authors: Lewis Laura D. , Weiner Veronica S. , Mukamel Eran A. , Donoghue Jacob A. , Eskandar Emad N. , Madsen Joseph R. , Anderson William S. , Hochberg Leigh R. , Cash Sydney S. , Brown Emery N. , Purdon Patrick L.
Published: November 2012

The neurophysiological mechanisms by which anesthetic drugs cause loss of consciousness are poorly understood. Anesthetic actions at the molecular, cellular, and systems levels have been studied in detail at steady states of deep general anesthesia. However, little is known about how anesthetics alter neural activity during the transition into unconsciousness. We recorded simultaneous multiscale neural activity from human cortex, including ensembles of single neurons, local field potentials, and intracranial electrocorticograms, during induction of general anesthesia. We analyzed local and global neuronal network changes that occurred simultaneously with loss of consciousness. We show that propofol-induced unconsciousness occurs within seconds of the abrupt onset of a slow (<1 Hz) oscillation in the local field potential. This oscillation marks a state in which cortical neurons maintain local patterns of network activity, but this activity is fragmented across both time and space. Local (<4 mm) neuronal populations maintain the millisecond-scale connectivity patterns observed in the awake state, and spike rates fluctuate and can reach baseline levels. However, neuronal spiking occurs only within a limited slow oscillation-phase window and is silent otherwise, fragmenting the time course of neural activity. Unexpectedly, we found that these slow oscillations occur asynchronously across cortex, disrupting functional connectivity between cortical areas. We conclude that the onset of slow oscillations is a neural correlate of propofol-induced loss of consciousness, marking a shift to cortical dynamics in which local neuronal networks remain intact but become functionally isolated in time and space.
Energy-dispersive X-ray emission spectroscopy using an X-ray free-electron laser in a shot-by-shot mode
Authors: Alonso-Mori Roberto , Kern Jan , Gildea Richard J. , Sokaras Dimosthenis , Weng Tsu-Chien , Lassalle-Kaiser Benedikt , Tran Rosalie , Hattne Johan , Laksmono Hartawan , Hellmich Julia , Glöckner Carina , Echols Nathaniel , Sierra Raymond G. , Schafer Donald W. , Sellberg Jonas , Kenney Christopher , Herbst Ryan , Pines Jack , Hart Philip , Herrmann Sven , Grosse-Kunstleve Ralf W. , Latimer Matthew J. , Fry Alan R. , Messerschmidt Marc M. , Miahnahri Alan , Seibert M. Marvin , Zwart Petrus H. , White William E. , Adams Paul D. , Bogan Michael J. , Boutet Sébastien , Williams Garth J. , Zouni Athina , Messinger Johannes , Glatzel Pieter , Sauter Nicholas K. , Yachandra Vittal K. , Yano Junko , Bergmann Uwe
Published: November 2012

The ultrabright femtosecond X-ray pulses provided by X-ray free-electron lasers open capabilities for studying the structure and dynamics of a wide variety of systems beyond what is possible with synchrotron sources. Recently, this “probe-before-destroy” approach has been demonstrated for atomic structure determination by serial X-ray diffraction of microcrystals. There has been the question whether a similar approach can be extended to probe the local electronic structure by X-ray spectroscopy. To address this, we have carried out femtosecond X-ray emission spectroscopy (XES) at the Linac Coherent Light Source using redox-active Mn complexes. XES probes the charge and spin states as well as the ligand environment, critical for understanding the functional role of redox-active metal sites. Kβ
1,3
XES spectra of Mn
II
and Mn
2
III,IV
complexes at room temperature were collected using a wavelength dispersive spectrometer and femtosecond X-ray pulses with an individual dose of up to >100 MGy. The spectra were found in agreement with undamaged spectra collected at low dose using synchrotron radiation. Our results demonstrate that the intact electronic structure of redox active transition metal compounds in different oxidation states can be characterized with this shot-by-shot method. This opens the door for studying the chemical dynamics of metal catalytic sites by following reactions under functional conditions. The technique can be combined with X-ray diffraction to simultaneously obtain the geometric structure of the overall protein and the local chemistry of active metal sites and is expected to prove valuable for understanding the mechanism of important metalloproteins, such as photosystem II.
Folding of the four-helix bundle FF domain from a compact on-pathway intermediate state is governed predominantly by water motion
Authors: Sekhar Ashok , Vallurupalli Pramodh , Kay Lewis E.
Published: November 2012

Friction plays a critical role in protein folding. Frictional forces originating from random solvent and protein fluctuations both retard motion along the folding pathway and activate protein molecules to cross free energy barriers. Studies of friction thus may provide insights into the driving forces underlying protein conformational dynamics. However, the molecular origin of friction in protein folding remains poorly understood because, with the exception of the native conformer, there generally is little detailed structural information on the other states participating in the folding process. Here, we study the folding of the four-helix bundle FF domain that proceeds via a transiently formed, sparsely populated compact on-pathway folding intermediate whose structure was elucidated previously. Because the intermediate is stabilized by both native and nonnative interactions, friction in the folding transition between intermediate and folded states is expected to arise from intrachain reorganization in the protein. However, the viscosity dependencies of rates of folding from or unfolding to the intermediate, as established by relaxation dispersion NMR spectroscopy, clearly indicate that contributions from internal friction are small relative to those from solvent, so solvent frictional forces drive the folding process. Our results emphasize the importance of solvent dynamics in mediating the interconversion between protein configurations, even those that are highly compact, and in equilibrium folding/unfolding fluctuations in general.