bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence for low temperature glassy behavior in La0.5Sr0.5CoO3
Authors: Samal D. , Anil Kumar P. S.
Published: February 2012

We report the observed low temperature spin glass like feature in the optimally doped La0.5Sr0.5CoO3 ferromagnetic system. The characteristic of glassy behavior has been identified by noting (i) the frequency-dependent shift of the low temperature hump position in the out of phase ac susceptibility component; (ii) evolution of freezing temperature with dc biasing field that adheres to de Almeida–Thouless relation; (iii) memory effect; and (iv) sluggish magnetic relaxation. The results of magnetic measurements demonstrate that neither the inter-cluster interaction nor the spin disorder at the interface between ferromagnetic clusters is responsible for the manifestation of such collective glassy behavior. Rather, it is believed to arise from a distinct cluster glass like phase that possibly coexists with the dominant ferromagnetic phase.
Hybrid-like 2/1 flux-pumping and magnetic island evolution due to edge localized mode-neoclassical tearing mode coupling in DIII-D
Authors: King J. D. , La Haye R. J. , Petty C. C. , Osborne T. H. , Lasnier C. J. , Groebner R. J. , Volpe F. A. , Lanctot M. J. , Makowski M. A. , Holcomb C. T. , Solomon W. M. , Allen S. L. , Luce T. C. , Austin M. E. , Meyer W. H. , Morse E. C.
Published: February 2012

Direct analysis of internal magnetic field pitch angles measured using the motional Stark effect diagnostic shows m/n=2/1 neoclassical tearing modes exhibit stronger poloidal magnetic flux-pumping than typical hybrids containing m/n=3/2 modes. This flux-pumping causes the avoidance of sawteeth, and is present during partial electron cyclotron current drive suppression of the tearing mode. This finding could lead to hybrid discharges with higher normalized fusion performance at lower q95. The degree of edge localized mode-neoclassical tearing mode (ELM-NTM) coupling and the strength of flux-pumping increase with beta and the proximity of the modes to the ELMing pedestal. Flux-pumping appears independent of magnetic island width. Individual ELM-NTM coupling events show a rapid timescale drop in the island width followed by a resistive recovery that is successfully modeled using the modified Rutherford equation. The fast transient drop in island width increases with ELM size.
Electrically switchable random to photonic band-edge laser emission in chiral nematic liquid crystals
Authors: Morris Stephen M. , Gardiner Damian J. , Hands Philip J. W. , Qasim Malik M. , Wilkinson Timothy D. , White Ian H. , Coles Harry J.
Published: February 2012

Using a chiral nematic liquid crystal with a negative dielectric anisotropy, it is possible to switch between band-edge laser emission and random laser emission with an electric field. At low frequencies (<1 kHz), random laser emission is observed as a result of scattering due to electro-hydrodynamic instabilities. However, band-edge laser emission is found to occur at higher frequencies (>5 kHz), where the helix is stabilized due to dielectric coupling. These results demonstrate a method by which the linewidth of the laser source can be readily controlled externally (from 4 nm to 0.5 nm) using electric fields.
Properties of InxGa1−xN films in terahertz range
Authors: Gauthier-Brun A. , Teng J. H. , Dogheche E. , Liu W. , Gokarna A. , Tonouchi M. , Chua S. J. , Decoster D.
Published: February 2012

In this letter, we report the characterization of the refractive indices and complex conductivities of a set of GaN films with different carrier concentrations, InN film, and InxGa1−xN films with indium content varying from x = 0.07 to x = 0.14 grown by metalorganic chemical vapor deposition for frequencies ranging from 0.3 to 3 THz using terahertz time-domain spectroscopy (THz-TDS). The refractive indices of InxGa1−xN films at THz range are reported. The carrier density and mobility determined using THz-TDS method show good agreement with four-probe Hall measurements.
Hard x-ray monochromator with milli-electron volt bandwidth for high-resolution diffraction studies of diamond crystals
Authors: Stoupin Stanislav , Shvyd'ko Yuri , Shu Deming , Khachatryan Ruben , Xiao Xianghui , DeCarlo Francesco , Goetze Kurt , Roberts Timothy , Roehrig Christian , Deriy Alexey
Published: February 2012

We report on design and performance of a high-resolution x-ray monochromator with a spectral bandwidth of ΔEX ≃ 1.5 meV, which operates at x-ray energies in the vicinity of the backscattering (Bragg) energy EH = 13.903 keV of the (008) reflection in diamond. The monochromator is utilized for high-energy-resolution diffraction characterization of diamond crystals as elements of advanced x-ray crystal optics for synchrotrons and x-ray free-electron lasers. The monochromator and the related controls are made portable such that they can be installed and operated at any appropriate synchrotron beamline equipped with a pre-monochromator.
Theory and simulations of quantum glass forming liquids
Authors: Markland Thomas E. , Morrone Joseph A. , Miyazaki Kunimasa , Berne B. J. , Reichman David R. , Rabani Eran
Published: February 2012

A comprehensive microscopic dynamical theory is presented for the description of quantum fluids as they transform into glasses. The theory is based on a quantum extension of mode-coupling theory. Novel effects are predicted, such as reentrant behavior of dynamical relaxation times. These predictions are supported by path integral ring polymer molecular dynamics simulations. The simulations provide detailed insight into the factors that govern slow dynamics in glassy quantum fluids. Connection to other recent work on both quantum glasses as well as quantum optimization problems is presented.
Curvature in sub-Riemannian geometry
Authors: Bejancu Aurel
Published: February 2012

We study curvature problems on a nearly Riemannian manifold, which is a sub-Riemannian manifold (M, HM, g, VM) whose adapted tensor field given by (2.2) vanishes identically. First, we prove the existence and uniqueness of what we call horizontal Riemannian connection, which is a torsion-free and metric linear connection ∇ on the horizontal distribution HM. Then we define the horizontal sectional curvature of M and study nearly Riemannian space forms, which are nearly Riemannian manifolds of constant horizontal sectional curvature. Several horizontal geometric objects are introduced and studied, for example, horizontal Ricci tensor, horizontal scalar curvature, horizontal Laplacian. Finally, as a prelude to applications to Kaluza–Klein theories we define the horizontal Einstein gravitational tensor field and show that its horizontal divergence vanishes identically on M. Some examples are presented to illustrate the theory developed in the paper.
X-ray diffraction, dielectric, conduction and Raman studies in Na0.925Bi0.075Nb0.925Mn0.075O3 ceramic
Authors: Chaker Chiheb , Gagou Y. , Abdelmoula N. , Dellis J.-L. , Masquelier C. , Khemakhem H. , El Marssi M.
Published: February 2012

Ceramic with composition Na0.925Bi0.075Nb0.925Mn0.075O3 (NNBM0075) was synthesized by high temperature solid state reaction technique. It was studied using X-ray diffraction (XRD), dielectric measurements and Raman spectroscopy. The sample crystallizes in orthorhombic perovskite structure with space group Pbma at room temperature. Dielectric properties of the ceramic was investigated in a broad range of temperatures (−150 to 450 °C) and frequencies (0.1–103 kHz), and show two different anomalies connected to the symmetry change and electrical conductivity. Dielectric frequency dispersion phenomena in the NNBM0075 ceramic was analyzed by impedance spectroscopy in the temperature range from 55 to 425 °C. The Cole-Cole analysis based on electrical circuit and least square method was used to characterize the conduction phenomenon. A separation of the grain and grain boundary properties was achieved using an equivalent circuit model. The different parameters of this circuit were determined using impedance studies. Four conduction ranges, with different activation energies, were determined using the Arrhenius model. Raman spectra were studied as a function of temperatures and confirmed the X-ray and dielectric results. This composition is of interest for applications due to his physical properties and environmentally friendly character.
Faster radial strain relaxation in InAs–GaAs core–shell heterowires
Authors: Kavanagh Karen L. , Saveliev Igor , Blumin Marina , Swadener Greg , Ruda Harry E.
Published: February 2012

The structure of wurtzite and zinc blende InAs–GaAs (001) core–shell nanowires grown by molecular beam epitaxy on GaAs (001) substrates has been investigated by transmission electron microscopy. Heterowires with InAs core radii exceeding 11 nm, strain relax through the generation of misfit dislocations, given a GaAs shell thickness greater than 2.5 nm. Strain relaxation is larger in radial directions than axial, particularly for shell thicknesses greater than 5.0 nm, consistent with molecular statics calculations that predict a large shear stress concentration at each interface corner.
Understanding the operation of quantum dot intermediate band solar cells
Authors: Luque A. , Linares P. G. , Antolín E. , Ramiro I. , Farmer C. D. , Hernández E. , Tobías I. , Stanley C. R. , Martí A.
Published: February 2012

In this paper, a model for intermediate band solar cells is built based on the generally understood physical concepts ruling semiconductor device operation, with special emphasis on the behavior at low temperature. The model is compared to JL-VOC measurements at concentrations up to about 1000 suns and at temperatures down to 20 K, as well as measurements of the radiative recombination obtained from electroluminescence. The agreement is reasonable. It is found that the main reason for the reduction of open circuit voltage is an operational reduction of the bandgap, but this effect disappears at high concentrations or at low temperatures.
Investigation of the swirl flow on anode surface in high-current vacuum arcs
Authors: Jia Shenli , Yang Dingge , Wang Lijun , Shi Zongqian
Published: February 2012

The anode activities are critical for high-current vacuum arc characteristics, especially the interruption performance of vacuum interrupters. The serious anode melting and sputter of liquid droplets into arc column often lead to interruption failure. In our previous work, the obvious anode melting and swirl flow of melted anode metal were detected at the center of anode surface when arc current exceeded a critical value under axial magnetic field (AMF). It is found that the AMF has great influence on the anode swirl flow, no swirl flow is found on the butt plate anode without AMF, but obvious swirl flow can be found when a moderate AMF is applied. Meanwhile, the swirl flow direction reversed if the AMF direction also reversed. The electromagnetic forcej×Bin anode melting pool and the impact force of ions coming from cathode plasma jets which are inclined to the arc axis on anode surface were thought to be two main possible reasons. In order to discover the physics behind the anode swirl flow phenomenon, special experiments have been conducted in this paper. Both the theoretical analysis and experimental results indicate that it is not the electromagnetic force but the interaction between ions from cathode plasma jets and anode melting pool that leads to the observed anode swirl flow. The inclination direction of cathode plasma jets, which is consistent with the direction of magnetic field vector, determines the direction of swirl flow.
Current density-voltage analyses and interface characterization in Ag/DNA/p-InP structures
Authors: Güllü Ö. , Pakma O. , Türüt A.
Published: February 2012

The current density-voltage (J-V) characteristics of Ag/DNA/p-InP metal-insulator-semiconductor (MIS) structures have been investigated in room temperature. We have observed that the Ag/DNA/p-InP structure shows an excellent rectifying behavior and that this structure increases the barrier height (φb0). The main electrical parameters of these structures, such as ideality factor (n), barrier height, and average series resistance values were found to be 1.087, 0.726 eV, and 66.92 Ω. This value of n was attributed to the presence of an interfacial insulator layer at the Ag/p-InP interface and the density of interface states (Nss) localized at the InP/DNA interface. The values of Nss localized at the InP/DNA interface were found at 0.675-Ev in the 1.38 × 1012 eV−1 cm−2.
Phase segregation of a symmetric diblock copolymer in constrained space with a square-pillar array
Authors: Zou Zhixiang , He Xuehao , Wang Long
Published: February 2012

In this study, we apply a self-consistent field theory of polymers to study the structures of a symmetric diblock copolymer in parallel substrates filled with square-pillar arrays in which the substrates and pillars exhibit a weak preference for one block of the copolymer. Three classes of structures, i.e., lamellae, perpendicular cylinders, and bicontinuous structures, are achieved by varying the polymer film thickness, the pillar pitch (the distance between two centers of the nearest neighboring pillars), the gap and rotation of the pillars. Because of the confinement along horizontal directions imposed by the pillar array, eight novel types of perpendicular lamellar structures and eight novel types of cylindrical structures with various shapes and distributions occur. In the hybridization states of the parallel and perpendicular lamellar structures, several novel bicontinuous structures such as the double-cylinder network, pseudo-lamellae, and perforated lamellar structure are also found. By comparing the free energies of the various possible structures, the antisymmetric parallel lamellae are observed to be stable with the larger pillar gap at a certain film thickness. The structural transformations between the alternating cylindrical structures (alternating cross-shaped, square-shaped, and octagonal perpendicular cylinders) and parallel lamellae with increasing film thickness or pillar gap are well explained by the modified strong separation theory. Our results indicate that array confinement can be an effective method to prepare novel polymeric nanopattern structures.
A rare event sampling method for diffusion Monte Carlo using smart darting
Authors: Roberts K. , Sebsebie R. , Curotto E.
Published: February 2012

We identify a set of multidimensional potential energy surfaces sufficiently complex to cause both the classical parallel tempering and the guided or unguided diffusion Monte Carlo methods to converge too inefficiently for practical applications. The mathematical model is constructed as a linear combination of decoupled Double Wells [(DDW)n]. We show that the set (DDW)n provides a serious test for new methods aimed at addressing rare event sampling in stochastic simulations. Unlike the typical numerical tests used in these cases, the thermodynamics and the quantum dynamics for (DDW)n can be solved deterministically. We use the potential energy set (DDW)n to explore and identify methods that can enhance the diffusion Monte Carlo algorithm. We demonstrate that the smart darting method succeeds at reducing quasiergodicity for n ≫ 100 using just 1 × 106 moves in classical simulations (DDW)n. Finally, we prove that smart darting, when incorporated into the regular or the guided diffusion Monte Carlo algorithm, drastically improves its convergence. The new method promises to significantly extend the range of systems computationally tractable by the diffusion Monte Carlo algorithm.
Flux pinning and vortex transitions in doped BaFe2As2 single crystals
Authors: Ghorbani S. R. , Wang X. L. , Shabazi M. , Dou S. X. , Choi K. Y. , Lin C. T.
Published: February 2012

The vortex liquid-to-glass transition has been studied in Ba0.72K0.28Fe2As2 (BaK-122), Ba(Fe0.91Co0.09)2As2(BaCo-122), and Ba(Fe0.95Ni0.05)2As2(BaNi-122) single crystal with superconducting transition temperature, Tc = 31.7, 17.3, and 18 K, respectively, by magnetoresistance measurements. For temperatures below Tc, the resistivity curves were measured in magnetic fields within the range of 0 ≤ B ≤ 13 T, and the pinning potential was scaled according to a modified model for vortex liquid resistivity. Good scaling of the resistivity ρ(B, T) and the effective pinning energy U0(B,T) were obtained. The vortex state is three-dimensional at temperatures lower than a characteristic temperature T*. The vortex phase diagram was determined based on the evolution of the vortex-glass transition temperature Tg with magnetic field and the upper critical field, Hc2. We found that non-magnetic K doping results in a high glass line close to the Hc2, while magnetic Ni and Co doping causes a low glass line which is far away from the Hc2. Our results suggest that non-magnetic induced disorder is more favourable for enhancement of pinning strength compared to magnetic induced disorder. Our results show that the pinning potential is responsible for the difference in the glass states.
Kinetic criteria of glass formation and the pressure dependence of the glass transition temperature
Authors: Schmelzer Jürn W. P.
Published: February 2012

An overview on different attempts of formulation of kinetic criteria of glass formation is given. It is analyzed which of the characteristic time scales—time of observation, time of relaxation, and time of change of external parameters—have to be employed to appropriately develop such criteria. Based on this analysis, a general model-independent kinetic criterion for glass formation is formulated. As a first consequence, it is shown that it is not—as often claimed—the Deborah number which governs glass formation. Based on this general kinetic criterion for glass formation, general expressions for the dependence of the glass transition temperature on pressure (and vice versa) are obtained being essentially ratios of the partial derivatives of the appropriate relaxation times with respect to pressure and temperature, respectively. Employing, as examples, further two different (free volume and entropy based) models for the description of viscous flow and relaxation, respectively, relations similar but, in general, not identical to the classical Ehrenfest relations describing second-order equilibrium phase transitions are obtained. In this way, it can be explained why one of the Ehrenfest's relations is usually fulfilled in glass transition and the other not and why the Prigogine–Defay ratio in glass transition is not equal to one as this is the case with Ehrenfest's ratio in second-order equilibrium phase transitions.
Long-time stability of a low-energy electron diffraction spin polarization analyzer for magnetic imaging
Authors: Lofink F. , Hankemeier S. , Frömter R. , Kirschner J. , Oepen H. P.
Published: February 2012

The time stability of a polarization analyzer that is used for imaging of magnetic structures in a scanning electron microscope with spin polarization analysis (spin-SEM or SEMPA) is investigated. The detector is based on the diffraction of low-energy electrons at a W(100) crystal at 104.5 eV (LEED detector). Due to the adsorption of hydrogen from residual gas, a change of the scattering conditions is found that causes an angular shift of the LEED beams as well as changes of intensity. The quality factor, which describes the efficiency of the detector in SEMPA application, however, is found to be almost constant up to a hydrogen coverage of θ ≈ 0.25. This gives stable working conditions within roughly 1 h at vacuum conditions of 10−10 mbar.
Sub-hertz relaxation process in chiral smectic mixtures doped with silver nanoparticles
Authors: Mandal Pradip Kr. , Lapanik Artsiom , Wipf Robert , Stuehn Bernd , Haase Wolfgang
Published: February 2012

A thermal activated sub-hertz frequency dielectric absorption process in both SmC* and SmA* phases along with characteristic Goldstone mode have been observed in both pure and silver nano particle doped liquid crystal mixtures. This process is well resolved, in particular, in the composites probably due to capturing of ions/charges inside the thiol group layer around the silver particles surface, thereby increasing the resistivity of the composites. The process is interpreted as space charge accumulation of ions/charges at the interface between liquid crystals and the cell polymer layer close to the electrodes. Improved switching characteristics and reduced spontaneous polarization have been detected.
Fabrication of single-walled carbon nanohorns containing iodine and cesium
Authors: Cho J. H. , Lim S. T. , Huh S. R. , Kim G. H.
Published: February 2012

Iodine and cesium atoms were encapsulated in single-walled carbon nanohorns (SWCNHs). Atom encapsulation was carried out with sequential plasma aided procedures which consisted of opening SWCNH tips with an oxygen plasma, atom insertion in an iodine-mixed or cesium-mixed argon plasma, and closing the open tip in an argon plasma. Results reveal that oxidation plays a role in the tip opening procedure, and capillary forces are the driving force for the permeation of the atoms through the open tip of the SWCNHs. The open tip of the atom inserted SWCNH can be closed under the ion irradiation. It demonstrated the fabrication process of encapsulating atoms in SWCNH by using the sequential plasma assisted processes.
Ionization enhanced ion collection by a small floating grain in plasmas
Authors: Khrapak Sergey A. , Morfill Gregor E.
Published: February 2012

It is demonstrated that the ionization events in the vicinity of a small floating grain can increase the ion flux to its surface. In this respect, the effect of electron impact ionization is fully analogous to that of the ion-neutral resonant charge exchange collisions. Both processes create slow ion which cannot overcome electrical attraction of the grain and eventually fall onto its surface. The relative importance of ionization and ion-neutral collisions is roughly given by the ratio of the corresponding frequencies. We have evaluated this ratio for neon and argon plasmas to demonstrate that ionization enhanced ion collection can indeed be an important factor affecting grain charging in realistic experimental conditions.
Impact of incomplete ionization of dopants on the electrical properties of compensated p-type silicon
Authors: Forster M. , Cuevas A. , Fourmond E. , Rougieux F. E. , Lemiti M.
Published: February 2012

This paper investigates the importance of incomplete ionization of dopants in compensated p-type Si and its impact on the majority-carrier density and mobility and thus on the resistivity. Both theoretical calculations and temperature-dependent Hall-effect measurements demonstrate that the carrier density is more strongly affected by incomplete ionization in compensated Si than in uncompensated Si with the same net doping. The previously suggested existence of a compensation-specific scattering mechanism to explain the reduction of mobility in compensated Si is shown not to be consistent with the T-dependence of the measured carrier mobility. The experiment also shows that, in the vicinity of 300 K, the resistivity of compensated Si has a much weaker dependence on temperature than that of uncompensated silicon.
Characterization of thin-film GaAs diodes grown on germanium-on-insulator on Si substrate
Authors: Xu Z. , Yoon S. F. , Yeo Y. C. , Chia C. K. , Cheng Y. B. , Dalapati G. K.
Published: February 2012

In this study, we report the characterization of thin-film GaAs grown on germanium-on-insulator (GeOI) on Si substrate. A GaAs/GeOI diode with a 600 nm buffer layer showed a rectification of 1.0 × 107 at ±2 V and had an electrical performance similar to that of the reference sample grown on GaAs substrate. We demonstrate two thin diodes (<350 nm in thickness) that still showed high forward densities and rectification properties. The electrical performances of the diodes degraded as the diode active regions were grown closer to the GaAs/Ge interface due to the increase of defects propagating into the active regions. The experimental results were fitted with the thermionic emission equation and the Frenkel-Poole model.
Green and red emission for (K0.5Na0.5)NbO3:Pr ceramics
Authors: Sun Haiqin , Peng Dengfeng , Wang Xusheng , Tang Mianmian , Zhang Qiwei , Yao Xi
Published: February 2012

The photoluminescence properties of Pr-doped K0.5Na0.5NbO3 (KNN) ceramics were prepared by the solid-state reaction method and investigated as a function of Pr concentration and sintering temperature. The excitation band ranging from 430 to 500 nm covers the emission wavelength of all commercial blue light-emitting diode chips. The photoluminescence spectra of KNN:Pr exhibit strong green (528 nm) and red (617 nm and 650 nm) emissions. The optimum doping concentration of Pr in KNN is 0.5 mol. %, and the optimized sintering temperature is 1170 °C. In addition to the admirable intrinsic piezoelectric properties of KNN, Pr-doped KNN may take an important role in many fields as a multifunctional material.
Analysis and experimental demonstration of an active acoustic metamaterial cell
Authors: Akl Wael , Baz Amr
Published: February 2012

Active acoustic metamaterials (AAMM) have been developed to overcome the limited frequency bandwidth characteristics of passive acoustic metamaterials. The AAMM rely in their operation on using piezoelectric active ingredients in a fluid-solid composite structure forming the basic building block of a larger metamaterial periodic arrangement. A prototype of AAMM composite cell is manufactured and active control strategies are implemented on the piezoelectric elements to vary its stiffness in order to control the effective dynamic density of the cell. Acoustic characterization of the developed AAMM cell is carried out by measuring its acoustic impedance and transmission loss and comparing the results with the predictions of a finite element model. The obtained experimental measurements and the predictions of the finite element model are in very good agreement for the considered frequency range. The transfer functions between the reference microphone in the impedance tube and the piezoelectric elements demonstrate the coupling nature inside the cell rendering it to a system of single acoustic properties acting as a single degree of freedom system. The proposed AAMM can be useful in manufacturing the next generation of acoustic cloaks and metamaterials with controllable directivity and dispersion characteristics.
Energy transfer in CaYAlO4: Ce3+, Pr3+ for sensitization of quantum-cutting with the Pr3+-Yb3+ couple
Authors: Guille A. , Pereira A. , Breton G. , Bensalah-Ledoux A. , Moine B.
Published: February 2012

Silicon based solar cells show a poor yield in the UV-blue range of the solar spectrum. The thermalization of the charge carriers is one major loss mechanism, which could be overcome by depositing a quantum-cutting material in front of the solar cell. This phenomenon has recently been demonstrated in materials doped with the rare-earth couple Pr3+-Yb3+, where Pr3+ is able to sensitize two Yb3+ ion for one absorbed blue photon. Unfortunately, the weak absorption cross-section of Pr3+ ion makes it not suitable for thin films applications. Therefore, it is necessary to sensitize it to obtain a significant light yield. In that respect, Ce3+ ion could be used as a sensitizer of Pr3+ because its 4f-5d transition shows a high cross-section absorption. The purpose of this article is to study energy transfers between Ce3+ and Pr3+ in CaYAlO4 with a view to evaluate the potential of Ce3+ as a sensitizer of quantum-cutting with Pr3+-Yb3+ couple. We present results obtained in the bulk material and in a thin film deposited by pulsed laser deposition and we demonstrate that, in CaYAlO4, Ce3+ ion sensitizes the 3Pj levels of Pr3+ ion, but also that this may be followed by a cross-relaxation involving the 4f levels of Ce3+ ion, which would be in competition with energy transfer toward Yb3+ ion.
Material dependence of Casimir forces: Gradient expansion beyond proximity
Authors: Bimonte Giuseppe , Emig Thorsten , Kardar Mehran
Published: February 2012

A widely used method for estimating Casimir interactions [H. B. G. Casimir, Proc. K. Ned. Akad. Wet. 51, 793 (1948)] between gently curved material surfaces at short distances is the proximity force approximation (PFA). While this approximation is asymptotically exact at vanishing separations, quantifying corrections to PFA has been notoriously difficult. Here, we use a derivative expansion to compute the leading curvature correction to PFA for metals (gold) at room temperature. We derive an explicit expression for the amplitude θ̂1 of the PFA correction to the force gradient for axially symmetric surfaces. In the non-retarded limit, the corrections to the Casimir free energy are found to scale logarithmically with distance. For gold, θ̂1 has an unusually large temperature dependence.
Fe/MgO/Fe (100) textured tunnel junctions exhibiting spin polarization features of single crystal junctions
Authors: Duluard A. , Negulescu B. , Bellouard C. , Hehn M. , Lacour D. , Lu Y. , Lengaigne G. , Montaigne F. , Robert S. , Suire S. , Tiusan C.
Published: February 2012

Crystallographic and spin polarized transport properties of (100) textured and (100) epitaxial Fe/MgO/Fe magnetic tunnel junctions are compared. Strong similarities in the transport properties show that structural coherence and magnetic quality at the 25 nm grain scale in textured junctions are sufficient to issue signatures of the spin polarized transport specific to a single crystal junction. This demonstrates that the lateral coherence of the Bloch tunneling wave function is identically limited in both systems. Our analysis leads to model the textured tunnel junction as a juxtaposition of nanometer sized single crystal junctions, placed in parallel.
p-f hybridization in the ferromagnetic semiconductor HoN
Authors: Brown J. D. , Downes J. E. , McMahon C. J. , Cowie B. C. C. , Tadich A. , Thomsen L. , Guo J. H. , Glans P. A.
Published: February 2012

The electronic structure of thin film HoN has been studied using soft x-ray spectroscopy. The combination of soft x-ray emission, x-ray absorption, and photoemission techniques yields direct evidence for hybridization between the N 2p and the Ho 4f states, previously unseen in this or related rare earth nitride systems. The N 2p states extend up to 10 eV below the Fermi level to nearly twice the binding energy as previously believed. Optical spectroscopy yields a minimum direct gap of 1.48 eV. In light of these results, we identify HoN as a prime candidate for spin-diffusion and spintronics applications.
A statistical exploration of multiple exciton generation in silicon quantum dots and optoelectronic application
Authors: Su W. A. , Shen W. Z.
Published: February 2012

We have carried out an investigation of multiple exciton generation (MEG) in Si quantum dots (QDs) and its application in optoelectronic devices. A simple yet effective statistical model has been proposed based on Fermi statistical theory and impact ionization mechanism. It is demonstrated that the MEG efficiency depends on both the radius of Si QDs and the energy of incident photons, with the MEG threshold energy in the range of ∼2.2–3.1 Eg depending on the dot radius. While limited improvement has been observed in power conversion efficiency of single stage solar cells, MEG in Si QDs exhibits prospective for application in ultraviolet detectors due to the high internal quantum efficiency under short incident light.
Temperature-dependent decay dynamics in highly mismatched ZnSe1−xTex alloy
Authors: Lin Yan-Cheng , Jiang Wei-Shi , Chou Wu-Ching , Chen Wei-Kuo , Chang Wen-Hao , Chia Chin-Hau , Chen Cheng-Yu , Chyi Jen-Inn
Published: February 2012

This study investigates the temperature-dependent decay dynamics in highly mismatched ZnSe0.950Te0.050 alloy using photoluminescence (PL) and time-resolved PL spectroscopy. The PL peak energy exhibits a V-shaped dependence on temperature (10–300 K), indicating strong carrier localization. Kohlrausch’s stretched exponential law, in which the deduced stretching exponent β is highly consistent with the V-shaped PL peak shift, closely corresponds to the complex decay curves over a wide temperature range. Additionally, the PL lifetime τ initially increases and then monotonically declines as the temperature increases. These findings agree excellently with the low electron-hole binding energy upon thermal ionization of weakly bound electrons.
Tunneling current modulation by Ge incorporation into Si oxide films for flash memory applications
Authors: Ito Toshihide , Mitani Yuuichiro , Nakasaki Yasushi , Koike Masahiro , Konno Takuya , Matsuba Hiroshi , Kai Tetsuya , Kaneko Wakana , Ozawa Yoshio
Published: February 2012

Current-voltage characteristic for a Ge-incorporated Si oxide was investigated. Current enhancement was observed for the electric field larger than 10 MV/cm. Such a current enhancement only under high electric field is expected to improve programming performance without deteriorating reading performance. From secondary ion mass spectrometry and hard x-ray photoelectron spectroscopy analyses and current simulation, it is concluded that the Ge impurity in Ge4+ state around the tunnel oxide/substrate interface enhances the current by trap-assisted tunneling. The programming current enhancement induced by the Ge incorporation is expected to be one of the promising solutions for the next-generation flash memory.
GaAs/GaInNAs quantum well and superlattice solar cell
Authors: Courel Maykel , Rimada Julio C. , Hernández Luis
Published: February 2012

A theoretical study of GaAs/GaInNAs solar cells based on multiple-quantum well solar cells (MQWSCs) and superlattice solar cell (SLSC) configuration is presented. The conversion efficiency as a function of the quantum well width and depth is modeled for MQWSC, reaching high values. A study of the SLSC viability is also presented. The influence of the cluster width on the conversion efficiency is researched showing a better performance when width and the cluster number are increased. The SLSC conversion efficiency is compared with the maximum conversion efficiency obtained for the MQWSC showing that it is reached an amazing increment of 4%.
Counting statistics in an InAs nanowire quantum dot with a vertically coupled charge detector
Authors: Choi T. , Ihn T. , Schön S. , Ensslin K.
Published: February 2012

A gate-defined quantum dot (QD) in an InAs nanowire is fabricated on top of a quantum point contact realized in a two-dimensional electron gas. The strong coupling between these two quantum devices is used to perform time-averaged as well as time-resolved charge detection experiments for electron flow through the quantum dot. We demonstrate that the Fano factor describing shot noise or time-correlations in single-electron transport depends in the theoretically expected way on the asymmetry of the tunneling barriers even in a regime where the thermal energy kBT is comparable to the single-particle level spacing in the dot.
High-k GaAs metal insulator semiconductor capacitors passivated by ex-situ plasma-enhanced atomic layer deposited AlN for Fermi-level unpinning
Authors: Jussila H. , Mattila P. , Oksanen J. , Perros A. , Riikonen J. , Bosund M. , Varpula A. , Huhtio T. , Lipsanen H. , Sopanen M.
Published: February 2012

This paper examines the utilization of plasma-enhanced atomic layer deposition grown AlN in the fabrication of a high-k insulator layer on GaAs. It is shown that high-k GaAs MIS capacitors with an unpinned Fermi level can be fabricated utilizing a thin ex-situ deposited AlN passivation layer. The illumination and temperature induced changes in the inversion side capacitance, and the maximum band bending of 1.2 eV indicates that the MIS capacitor reaches inversion. Removal of surface oxide is not required in contrast to many common ex-situ approaches.
Communication: Variational many-body expansion: Accounting for exchange repulsion, charge delocalization, and dispersion in the fragment-based explicit polarization method
Authors: Gao Jiali , Wang Yingjie
Published: February 2012

A fragment-based variational many-body (VMB) expansion method is described to directly account for exchange repulsion, charge delocalization (charge transfer) and dispersion interactions in the explicit polarization (X-Pol) method. The present VMB/X-Pol approach differs from other fragment molecular orbital (FMO) techniques in two major aspects. First, the wave function for the monomeric system is variationally optimized using standard X-Pol method, as opposed to the iterative update procedure adopted in FMO. Second, the mutual polarizations in the dimeric terms are also variationally determined, whereas single-point energy calculations of the individual dimers embedded in a static monomer field are used in FMO. The second-order (two-body) VMB (VMB2) expansion method is illustrated on a series of water hexamer complexes and one decamer cluster, making use of Hartree-Fock theory, MP2, and the PBE1 and M06 density functionals to represent the monomer and dimer fragments. The computed binding energies are within 2 kcal/mol of the corresponding results from fully delocalized calculations. Energy decomposition analyses reveal specific dimeric contributions to exchange repulsion, charge delocalization, and dispersion. Since the wave functions for one-body and all two-body terms are variationally optimized in VMB2 and X-Pol, it is straightforward to obtain analytic gradient without the additional coupled-perturbed Hartree-Fock step. Thus, the method can be useful for molecular dynamics simulations.
Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament
Authors: Forestier B. , Houard A. , Revel I. , Durand M. , André Y. B. , Prade B. , Jarnac A. , Carbonnel J. , Le Nevé M. , de Miscault J. C. , Esmiller B. , Chapuis D. , Mysyrowicz A.
Published: February 2012

In the perspective of the laser lightning rod, the ability of femtosecond filaments to trigger and to guide large scale discharges has been studied for several years. The present paper reports recent experimental results showing for the first time that filaments are able not only to trigger and guide but also to divert an electric discharge from its normal path. Laser filaments are also able to divert the spark without contact between laser and electrodes at large distance from the laser. A comparison between negative and positive discharge polarities also reveals important discrepancies in the guiding mechanism.
Analytic behavior of the QED polarizability function at finite temperature
Authors: Bernal A. , Pérez A.
Published: February 2012

We revisit the analytical properties of the static quasi-photon polarizability function for an electron gas at finite temperature, in connection with the existence of Friedel oscillations in the potential created by an impurity. In contrast with the zero temperature case, where the polarizability is an analytical function, except for the two branch cuts which are responsible for Friedel oscillations, at finite temperature the corresponding function is non analytical, in spite of becoming continuous everywhere on the complex plane. This effect produces, as a result, the survival of the oscillatory behavior of the potential. We calculate the potential at large distances, and relate the calculation to the non-analytical properties of the polarizability.
A photo-defined membrane for precisely patterned cellular and microparticle arrays
Authors: McPherson A. L. , Walker G. M.
Published: February 2012

The ability to pattern particles in well-defined arrays enhances microfluidic devices. A low-fluorescence optically transparent photo-curable resist (1002F) was characterized for use as a mechanical sieve in a microfluidic chip. Films of thickness 10 μm and 25 μm were created containing pores 6–10 μm in diameter with pitches ranging from 5–300 μm. The uniform photo-defined pores had diameters with standard deviations of 3%. Integrated with microfluidic devices, the films were used to trap polystyrene microspheres, and in a different experiment, MCF7 human epithelial adenocarcinoma cells (ATCC HTB-22). A mechanical sieve was used to trap two types of fluorescent particles and, separately MCF7 cells with NIH/3T3 murine fibroblast cells (ATCC CRL-1658) as a proof-of-concept for striated cellular co-culture.
Helical insertion of peptidoglycan produces chiral ordering of the bacterial cell wall
Authors: Wang Siyuan , Furchtgott Leon , Huang Kerwyn Casey , Shaevitz Joshua W.
Published: February 2012

The regulation of cell shape is a common challenge faced by organisms across all biological kingdoms. In nearly all bacteria, cell shape is determined by the architecture of the peptidoglycan cell wall, a macromolecule consisting of glycan strands crosslinked by peptides. In addition to shape, cell growth must also maintain the wall structural integrity to prevent lysis due to large turgor pressures. Robustness can be accomplished by establishing a globally ordered cell-wall network, although how a bacterium generates and maintains peptidoglycan order on the micron scale using nanometer-sized proteins remains a mystery. Here, we demonstrate that left-handed chirality of the MreB cytoskeleton in the rod-shaped bacterium
Escherichia coli
gives rise to a global, right-handed chiral ordering of the cell wall. Local, MreB-guided insertion of material into the peptidoglycan network naturally orders the glycan strands and causes cells to twist left-handedly during elongational growth. Through comparison with the right-handed twisting of
Bacillus subtilis
cells, our work supports a common mechanism linking helical insertion and chiral cell-wall ordering in rod-shaped bacteria. These physical principles of cell growth link the molecular structure of the bacterial cytoskeleton, mechanisms of wall synthesis, and the coordination of cell-wall architecture.
Yeast mitochondrial threonyl-tRNA synthetase recognizes tRNA isoacceptors by distinct mechanisms and promotes CUN codon reassignment
Authors: Ling Jiqiang , Peterson Kaitlyn M. , Simonović Ivana , Cho Chris , Söll Dieter , Simonović Miljan
Published: February 2012

Aminoacyl-tRNA synthetases (aaRSs) ensure faithful translation of mRNA into protein by coupling an amino acid to a set of tRNAs with conserved anticodon sequences. Here, we show that in mitochondria of
Saccharomyces cerevisiae
, a single aaRS (MST1) recognizes and aminoacylates two natural tRNAs that contain anticodon loops of different size and sequence. Besides a regular

with a threonine (Thr) anticodon, MST1 also recognizes an unusual

, which contains an enlarged anticodon loop and an anticodon triplet that reassigns the CUN codons from leucine to threonine. Our data show that MST1 recognizes the anticodon loop in both tRNAs, but employs distinct recognition mechanisms. The size but not the sequence of the anticodon loop is critical for

recognition, whereas the anticodon sequence is essential for aminoacylation of

. The crystal structure of MST1 reveals that, while lacking the N-terminal editing domain, the enzyme closely resembles the bacterial threonyl-tRNA synthetase (ThrRS). A detailed structural comparison with
Escherichia coli
ThrRS, which is unable to aminoacylate

, reveals differences in the anticodon-binding domain that probably allow recognition of the distinct anticodon loops. Finally, our mutational and modeling analyses identify the structural elements in MST1 (e.g., helix α11) that define tRNA selectivity. Thus, MTS1 exemplifies that a single aaRS can recognize completely divergent anticodon loops of natural isoacceptor tRNAs and that in doing so it facilitates the reassignment of the genetic code in yeast mitochondria.
Wnt5a–Ror–Dishevelled signaling constitutes a core developmental pathway that controls tissue morphogenesis
Authors: Ho Hsin-Yi Henry , Susman Michael W. , Bikoff Jay B. , Ryu Yun Kyoung , Jonas Andrea M. , Hu Linda , Kuruvilla Rejji , Greenberg Michael Eldon
Published: February 2012

Wnts make up a large family of extracellular signaling molecules that play crucial roles in development and disease. A subset of noncanonical Wnts signal independently of the transcription factor β-catenin by a mechanism that regulates key morphogenetic movements during embryogenesis. The best characterized noncanonical Wnt, Wnt5a, has been suggested to signal via a variety of different receptors, including the Ror family of receptor tyrosine kinases, the Ryk receptor tyrosine kinase, and the Frizzled seven-transmembrane receptors. Whether one or several of these receptors mediates the effects of Wnt5a in vivo is not known. Through loss-of-function experiments in mice, we provide conclusive evidence that Ror receptors mediate Wnt5a-dependent processes in vivo and identify Dishevelled phosphorylation as a physiological target of Wnt5a–Ror signaling. The absence of Ror signaling leads to defects that mirror phenotypes observed in
Wnt5a
null mutant mice, including decreased branching of sympathetic neuron axons and major defects in aspects of embryonic development that are dependent upon morphogenetic movements, such as severe truncation of the caudal axis, the limbs, and facial structures. These findings suggest that Wnt5a–Ror–Dishevelled signaling constitutes a core noncanonical Wnt pathway that is conserved through evolution and is crucial during embryonic development.
Osteoclasts promote the formation of hematopoietic stem cell niches in the bone marrow
Authors: Mansour Anna , Abou-Ezzi Grazia , Sitnicka Ewa , W. Jacobsen Sten Eirik , Wakkach Abdelilah , Blin-Wakkach Claudine
Published: February 2012

Formation of the hematopoietic stem cell (HSC) niche in bone marrow (BM) is tightly associated with endochondral ossification, but little is known about the mechanisms involved. We used the oc/oc mouse, a mouse model with impaired endochondral ossification caused by a loss of osteoclast (OCL) activity, to investigate the role of osteoblasts (OBLs) and OCLs in the HSC niche formation. The absence of OCL activity resulted in a defective HSC niche associated with an increased proportion of mesenchymal progenitors but reduced osteoblastic differentiation, leading to impaired HSC homing to the BM. Restoration of OCL activity reversed the defect in HSC niche formation. Our data demonstrate that OBLs are required for establishing HSC niches and that osteoblastic development is induced by OCLs. These findings broaden our knowledge of the HSC niche formation, which is critical for understanding normal and pathological hematopoiesis.
Lethal myelofibrosis induced by Bmi1-deficient hematopoietic cells unveils a tumor suppressor function of the polycomb group genes
Authors: Oguro Hideyuki , Yuan Jin , Tanaka Satomi , Miyagi Satoru , Mochizuki-Kashio Makiko , Ichikawa Hitoshi , Yamazaki Satoshi , Koseki Haruhiko , Nakauchi Hiromitsu , Iwama Atsushi
Published: February 2012

Polycomb-group (PcG) proteins form the multiprotein polycomb repressive complexes (PRC) 1 and 2, and function as transcriptional repressors through histone modifications. They maintain the proliferative capacity of hematopoietic stem and progenitor cells by repressing the transcription of tumor suppressor genes, namely Ink4a and Arf, and thus have been characterized as oncogenes. However, the identification of inactivating mutations in the PcG gene, EZH2, unveiled a tumor suppressor function in myeloid malignancies, including primary myelofibrosis (PMF). Here, we show that loss of another PcG gene, Bmi1, causes pathological hematopoiesis similar to PMF. In a mouse model, loss of Bmi1 in Ink4a-Arf−/− hematopoietic cells induced abnormal megakaryocytopoiesis accompanied by marked extramedullary hematopoiesis, which eventually resulted in lethal myelofibrosis. Absence of Bmi1 caused derepression of a cohort of genes, including Hmga2, which is an oncogene overexpressed in PMF. Chromatin immunoprecipitation assays revealed that Bmi1 directly represses the transcription of Hmga2. Overexpression of Hmga2 in hematopoietic stem cells induced a myeloproliferative state with enhanced megakaryocytopoiesis in mice, implicating Hmga2 in the development of pathological hematopoiesis in the absence of Bmi1. Our findings provide the first genetic evidence of a tumor suppressor function of Bmi1 and uncover the role of PcG proteins in restricting growth by silencing oncogenes.