bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrically controlled optical bandgap in a twisted photonic liquid crystal
Authors: Molina Ismael , Adrián Reyes J. , Avendaño Carlos G.
Published: June 2011

We consider a one-dimensional twisted photonic liquid crystal, which consists of N nematic liquid crystal slabs in a twisted configuration alternated by N isotropic dielectric layers under the action of a dc electric field (Edc) aligned along the periodicity axis. We write and solve numerically the corresponding Euler-Lagrange equations describing the nematic layer configuration. We express Maxwell’s equations in a 4×4 matrix representation, and by using the transfer matrix formalism, we obtain the optical band structures at arbitrary incidence angles and different external electric fields. We have found that there exists a strong dependence of electric field on the transmission and reflection spectra in enhancing and extinguishing bandgaps. The analysis presented here allow us to propose an electrically shiftable universal rejection filter for incident waves of left- and right-circular polarization. It is observed that by increasing the electric field we can highly enhance the cross-polarized reflection bandgaps and suppress the co-polarized ones. We analyzed the optical spectra for different values of twist angle, different ratios between dielectric and nematic layer thicknesses and number of layers N. Also, we showed that the cross-polarized bandgaps are blue-shifted as the incidence angle gets larger.
Charge transfer energy for Y2O3:Eu3+ nanophosphor
Authors: Shang Chun-Yu , Wang Xiu-Qin , Kang Hui , Han De-Ming
Published: June 2011

Based on the Kronig–Penney model, the changing tendency of the bandgap or of a particular level with the volume deformation in crystalline materials has been derived. On the basis of this changing tendency, the zero-phonon charge transfer (CT) energy is deduced to be decreased when the size of Y2O3:Eu3+ phosphor decreases into the nanoscale. In addition, the rigidity decrease of the lattice environment in Y2O3:Eu3+ nanophosphor leads to the enlargement of the CT state coordinate offset of the optical centers; this means that an optical center would reach a higher vibration level in CT excitation. The increasing magnitude of the vibration energy is smaller than the decreasing magnitude of the zero-phonon CT energy when the size of the Y2O3:Eu3+ phosphor decreases into the nanoscale. As a result, the CT energy is decreased, and the CT excitation spectrum shifts to a lower energy.
Towards a programmable microfluidic valve: Formation dynamics of two-dimensional magnetic bead arrays in transient magnetic fields
Authors: Wittbracht F. , Eickenberg B. , Weddemann A. , Hütten A.
Published: June 2011

The induction of dipolar coupling has proven to allow for the initiation of self-assembled, reconfigurable particle clusters of superparamagnetic microbeads suspended in a carrier liquid. The adjustment of the interplay between magnetic and hydrodynamic forces opens various possibilities for guiding strategies of these superstructures within microfluidic devices. In this work, the formation dynamics of such particle clusters under the influence of a rotating magnetic field are studied. Different agglomeration regimes are characterized by the dimensionality of the confined objects. The growth dynamics of the obtained agglomerates are analyzed quantitatively in order to deduce the microscopic growth mechanisms. The growth of two-dimensional clusters is governed by the addition of bead chains to previously formed agglomerates. Time scales for the cluster growth are characterized by the chain dissociation rate. Based on the experimental findings, we may conclude to a linear dependence of the chain dissociation rate on the rotation frequency of the applied magnetic field.
High thermoelectric performance of Cu substituted type-VIII clathrate Ba8Ga16–xCuxSn30 single crystals
Authors: Deng Shukang , Saiga Yuta , Kajisa Kousuke , Takabatake Toshiro
Published: June 2011

Single crystalline samples of type-VIII clathrate Ba8Ga16–xCuxSn30 (0 ≤ x ≤ 0.033) were prepared by the Sn-flux method. Upon substituting Cu for Ga, the carrier mobility at 300 K increases twice while the carrier density stays in the range 3.1−4.2×1019/cm3. Consequently, the electrical resistivity is decreased from 5.3 mΩcm for x = 0 to 3.2 mΩcm for x = 0.033. Irrespective of x, the Seebeck coefficient is largely negative and linearly changes with temperature in the range 300 < T < 600 K. The thermal conductivity is in the range 0.68−0.74 W/Km at 300 K for all samples. The dimensionless figure of merit ZT for x = 0.033 reaches the maximum of 1.35 at 540 K.
Determination of the anisotropies and reversal process in exchange-bias bilayers using a rotational magnetization curve approach
Authors: Sui Wenbo , Zhu Jingyi , Li Jinyun , Chai Guozhi , Jiang Changjun , Fan Xiaolong , Xue Desheng
Published: June 2011

Rotational magnetization curves of the exchange-bias bilayers were investigated based on the Stoner-Wohlfarth model, which can be grouped into three cases according to the magnetization reversal process. The unidirectional anisotropic field HE = 41.4 Oe, the uniaxial anisotropic field Hk = 4.2 Oe and the accurate direction of the easy axis of our FeNi/FeMn exchange-bias bilayers were obtained by fitting their experimental rotational magnetization curves. During the rotational process the magnetization reversal of the bilayers is a coherent rotation with a critical magnetization reversal field H1 = 41.372 Oe.
Effective anisotropy field in the free layer of patterned spin-valve resistors
Authors: Qian Zhenghong , Bai Ru , Yang Changmao , Li Qiliang , Sun Yucheng , Huo Dexuan , Li Lingwei , Zhan Hongliang , Li Yuan , Zhu Jianguo
Published: June 2011

In this work, the effective anisotropy Heff in the free layer of patterned spin-valve resistor has been investigated. A magnetic analysis is first conducted to explain the effective anisotropy, a mixed effect of the uniaxial anisotropy and the shape anisotropy. The experiment is then performed to verify the model analysis. The effective anisotropy Heff is found to be inversely proportional to the resistor linewidth, and can be modified by controlling the orientation of the axis of the uniaxial anisotropy relative to the resistor length, therefore providing a method of modifying the device sensitivity. The sensitivity is higher when spin valve is patterned with the free layer easy axis parallel rather than perpendicular to the resistor length.
Annealing behavior of the EB-centers and M-center in low-energy electron irradiated n-type 4H-SiC
Authors: Beyer F. C. , Hemmingsson C. , Pedersen H. , Henry A. , Janzén E. , Isoya J. , Morishita N. , Ohshima T.
Published: June 2011

After low-energy electron irradiation of epitaxial n-type 4H-SiC with a dose of 5×1016 cm−2, the bistable M-center, previously reported in high-energy proton implanted 4H-SiC, is detected in the deep level transient spectroscopy (DLTS) spectrum. The annealing behavior of the M-center is confirmed, and an enhanced recombination process is suggested. The annihilation process is coincidental with the evolvement of the bistable EB-centers in the low temperature range of the DLTS spectrum. The annealing energy of the M-center is similar to the generation energy of the EB-centers, thus partial transformation of the M-center to the EB-centers is suggested. The EB-centers completely disappeared after annealing temperatures higher than 700 ∘C without the formation of new defects in the observed DLTS scanning range. The threshold energy for moving Si atom in SiC is higher than the applied irradiation energy, and the annihilation temperatures are relatively low, therefore the M-center, EH1 and EH3, as well as the EB-centers are attributed to defects related to the C atom in SiC, most probably to carbon interstitials and their complexes.
Electron streams formation and secondary two stream instability onset in the post-saturation regime of the classical Weibel instability
Authors: Innocenti M. E. , Lazar M. , Markidis S. , Lapenta G. , Poedts S.
Published: June 2011

The electrostatic activity in the post-saturation regime of the velocity anisotropy driven Weibel instability is investigated by means of 1D 3V particle in cell simulations. Two different initial simulation configurations have been chosen to characterize the electrostatic activity in the post-saturation stage. A secondary two stream instability arises in both cases. However, significant differences occur in the thickness of the electron streams, in their initial locations, and in their effects on the bulk electron phase space distribution. An Hamiltonian description of particle motion in a 1D setting explains these differences in terms of the effective potential experienced by particles as a function of their initial perpendicular velocity. The different roles of the longitudinal electric field and the Lorentz force in the formation of electron streams are discussed.
Mono-energetic ion beam acceleration in solitary waves during relativistic transparency using high-contrast circularly polarized short-pulse laser and nanoscale targets
Authors: Yin L. , Albright B. J. , Jung D. , Bowers K. J. , Shah R. C. , Palaniyappan S. , Fernández J. C. , Hegelich B. M.
Published: June 2011

In recent experiments at the Trident laser facility, quasi-monoenergetic ion beams have been obtained from the interaction of an ultraintense, circularly polarized laser with a diamond-like carbon target of nm-scale thickness under conditions of ultrahigh laser pulse contrast. Kinetic simulations of this experiment under realistic laser and plasma conditions show that relativistic transparency occurs before significant radiation pressure acceleration and that the main ion acceleration occurs after the onset of relativistic transparency. Associated with this transition are a period of intense ion acceleration and the generation of a new class of ion solitons that naturally give rise to quasi-monoenergetic ion beams. An analytic theory has been derived for the properties of these solitons that reproduces the behavior observed in kinetic simulations and the experiments.
The dependence of barrier heights of a-C: Fe/n-Si heterojunctions on film-depositing temperatures
Authors: Wan Caihua , Zhang Xiaozhong , Vanacken Johan , Gao Xili , Moshchalkov Victor V.
Published: June 2011

Iron doped amorphous carbon (a-C: Fe) films on n-Si substrates were fabricated by pulse laser deposition technique. Barrier heights of the heterojunctions, measured in the electrical methods, were found to decrease gently below 300  °C and then increase anomalously thereafter. An interface energy band model was proposed to interpret the band structure of the heterojunctions as well as to calculate bandgaps of the a-C: Fe films. The abnormal increase in the barrier heights above 300  °C was attributed to the narrowing of π and π* bands due to the increased ordering degree of the sp2 clusters in a-C films, which was verified in Raman spectra and electron energy loss spectroscopy.
Designing a metallic nanoconcentrator for a lateral multijunction photovoltaic cell
Authors: Wang Trudie , Peumans Peter
Published: June 2011

A lateral multijunction photovoltaic (PV) concept is introduced that explores the unique ability of plasmonic nanoantennas to locally concentrate optical energy and spectrally filter incoming light at the subwavelength level. This electromagnetic field enhancement near the localized plasmon resonance modes of the metallic nanoantennas can be used to selectively increase light absorption in semiconductor nanowires at specific spectral and spatial regions. In our geometry, we take advantage of the ring antenna’s ability to excite two distinct plasmon modes in order to carry out spectral splitting and concentration of the electromagnetic field. A localized dipolar surface plasmon mode near the material resonance of the silver nanoantenna results from the ring behaving as an effective disk in the visible region and focuses the field on the external surface of the ring while a dipolar bonding resonance mode dependent on the coupling of modes excited on the inner and outer surface of the ring geometry in the near infrared (NIR) region focuses energy in the cavity of the ring. Using finite difference time domain (FDTD) simulations, we describe the basic mechanisms at work and demonstrate that the subwavelength ring antennas can couple incident light into semiconductor nanowires placed both inside and outside the ring through the two modes with minimal loss in the metal. The modes are used to laterally split different spectral regions of broadband incident light optimized to the material bandgap of the nanowires located in the regions of field enhancement to produce the lateral multijunction effect. We demonstrate that, for example, a ring antenna with both an internal diameter and a thickness of 40 nm can enhance absorption by 6x in the visible region for a 100 nm tall AlAs nanowire placed just outside the ring and by 380x in the NIR region for a geometrically similar GaAs nanowire placed inside the ring. Both enhancements occur just above the material band gaps of the nanowires. These findings, hence, show promise for the design of multijunction nanowire- based PV cells since the enhanced absorption allows for shorter nanowires, reducing both surface recombination and material cost. The dipolar bonding mode is particularly dependent on the geometry and size of the antenna, allowing the incoupling cross section to be spectrally tuned and enhanced across a wide range of wavelengths. Enhancement suffers, however, if the resonance peak is redshifted too far into the NIR region. We also explored the effects of periodicity and incident angle on absorption and find that there is a trade-off between several different resonant phenomena that can change both the spectral features and coupling efficiency of the geometry.
Admittance spectroscopy of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) based organic light emitting diodes
Authors: Petrosino Mario , Rubino Alfredo
Published: June 2011

Admittance spectroscopy of Indium Tin Oxide (ITO)–poly(3,4-ethylenedioxythiophene) (PEDOT)–polyfluorene–Al organic light emitting diodes is analyzed by varying PEDOT dispersion. Loss–frequency diagrams show one or two peaks. The results have been interpreted by using the Nicollian and Brews admittance model. Single level and multi level interface state distributions are found for each kind of PEDOT. Surface state density is about 1011 eV−1cm−2 for all distributions while the interface time constant varies between 471 μs and 220 ns. Good agreement is found between experimental data and analytical model. The combined effect of PEDOT conductivity and the inferred interface state density is analyzed and discussed.
Limits to mode-localized sensing using micro- and nanomechanical resonator arrays
Authors: Thiruvenkatanathan P. , Woodhouse J. , Yan J. , Seshia A. A.
Published: June 2011

In recent years, the concept of utilizing the phenomenon of vibration mode-localization as a paradigm of mechanical sensing has made profound impact in the design and development of highly sensitive micro- and nanomechanical sensors. Unprecedented enhancements in sensor response exceeding three orders of magnitude relative to the more conventional resonant frequency shift based technique have been both theoretically and experimentally demonstrated using this new sensing approach. However, the ultimate limits of detection and in consequence, the minimum attainable resolution in such mode-localized sensors still remain uncertain. This paper aims to fill this gap by investigating the limits to sensitivity enhancement imposed on such sensors, by some of the fundamental physical noise processes, the bandwidth of operation and the noise from the electronic interfacial circuits. Our analyses indicate that such mode-localized sensors offer tremendous potential for highly sensitive mass and stiffness detection with ultimate resolutions that may be orders of magnitude better than most conventional micro- and nanomechanical resonant sensors.
Sub-monolayer nucleation and growth of complex oxides at high supersaturation and rapid flux modulation
Authors: Kareev M. , Prosandeev S. , Gray B. , Liu Jian , Ryan P. , Kareev A. , Ju Moon Eun , Chakhalian J.
Published: June 2011

We report on the unusual nanoscale kinetics of the layer-by-layer deposition of complex oxide heterostructures composed of a unit-cell thick correlated metal LaNiO3 and dielectric LaAlO3. The ultrathin multilayers demonstrate extremely good crystallinity and surface morphology maintained over the large number of layers, as confirmed by combination of AFM, RHEED, and synchrotron x-ray diffraction. To elucidate the physics behind the growth, the temperature of the substrate and the deposition rate are varied over a wide range and the results are treated in the framework of a two-layer model. The results demonstrate that, in addition to temperature and vapor supersaturation, modulated flux can be employed as a powerful kinetic handle for the fine-tuned manipulation of sub-monolayer growth regimes of multicomponent materials.
Low frequency dielectric investigation of Rb0.5(ND4)0.5D2PO4 dipolar glass: Comparison with nuclear magnetic resonance investigations
Authors: Banys J. , Macutkevic J. , Lapinskas S. , Pirc R. , Kutnjak Z. , Blinc R.
Published: June 2011

The dielectric response of Rb0.5(ND4)0.5D2PO4 (DRADP-50) dipolar glass has been studied at low temperatures. From the experimental results, the distribution of the relaxation times and of the local polarization at various temperatures is calculated. It is shown that the local-polarization distribution function obtained from the dielectric response ɛ*(ν,T) is consistent with that obtained from NMR results. The results also show that the dielectric dispersion is due to diffusion, creation, and annihilation of Takagi groups.
High current-induced degradation of AlGaN ultraviolet light emitting diodes
Authors: Pinos A. , Marcinkevičius S. , Shur M. S.
Published: June 2011

Degradation under high current stress of AlGaN quantum well based light emitting diodes emitting at 285 and 310 nm has been studied using electroluminescence, time-resolved photoluminescence and current-voltage experimental techniques. The measurements have revealed that during aging decrease of the emission intensity is accompanied by increase of the tunneling current, increase of the nitrogen vacancy concentration and partial compensation of the p-doping. The main role in the device degradation has been ascribed to formation of tunneling conductivity channels, probably, via activation of the closed core screw dislocations with the help of nitrogen vacancies. Carrier lifetimes in the quantum wells and the p-cladding were found to be unaffected by the aging process, suggesting that the nonradiative recombination has a lesser influence on the device degradation.
Transformations of head-to-head domain walls in (La,Sr)MnO3 thin films
Authors: Uspenskaya L. S. , Tikhomirov O. A. , Nurgaliev T.
Published: June 2011

Kinetics of the in-plane magnetization reversal in a (La,Sr)MnO3 thin film has been studied at different temperatures using the magneto-optic visualization technique. In addition to changes of the domain wall coercivity and mobility, dramatic transformations of the domain wall structure have been observed. Motion of a single zig-zag domain wall is typical for high temperature magnetization reversal, whereas complicated diffuse transient structure between the two principal domains is observed at low temperature. Generation of secondary bubble domains in front of a moving wall has been observed for the first time in a quasistatic regime, contrary to previous superfast experiments.
Internal structure of 0.9 GHz microplasma
Authors: Miura Naoto , Hopwood Jeffrey
Published: June 2011

Argon microplasmas generated at 0.9 GHz by a split-ring resonator form a single filament-shaped nonequilibrium glow discharge. The internal structure of these microplasmas is probed using laser diode absorption and imaging emission spectrometry. These two-dimensional diagnostics are then Abel inverted to reveal that the core of the filament has a strongly depleted central metastable argon population. The microdischarges are approximately 0.2 mm in width at 1 atm and expand with increasing input power between 0.05–1.5 W. The relative electron density and the Ar(4p) density are estimated from the emission detected by a CCD camera through various bandpass filters. Absolute Ar(4s) densities, on the other hand, are determined by Ar 801.4 nm absorption. The Ar(4s) profile transitions from center-peaked at low power (0.05 W) to center-depleted above 0.25 W, saturating at 1019 m-3. The electron density profile within the microplasma, however, remains center-peaked regardless of the power. The spatially-resolved gas temperature was estimated from the broadening of the Ar 801.4 nm absorption profile. The error in this gas temperature measurement due to the depletion of the metastable atoms is corrected using numerical heat transfer models and shown to be 1650 K if the plasma power is 1.2 W. Differences between the peak and average temperatures based on the nitrogen rotational spectrum are also explained using nitrogen emission imaging.
Measurement of spin diffusion in semi-insulating GaAs
Authors: Weber C. P. , Benko Craig A. , Hiew Stanley C.
Published: June 2011

We use optical transient-grating spectroscopy to measure the spin diffusion of optically oriented electrons in bulk, semi-insulating GaAs(100). Trapping and recombination do not quickly deplete the photoexcited population. The spin diffusion coefficient of 88±12 cm2/s is roughly constant at temperatures from 15 K to 150 K, and the spin diffusion length is at least 450 nm. We show that it is possible to use spin diffusion to estimate the electron diffusion coefficient. Due to electron-electron interactions, the electron diffusion is 1.4 times larger than the spin diffusion.
Raman, Brillouin, and nuclear magnetic resonance spectroscopic studies on shocked borosilicate glass
Authors: Manghnani Murli H. , Hushur Anwar , Sekine Toshimori , Wu Jingshi , Stebbins Jonathan F. , Williams Quentin
Published: June 2011

Using Brillouin and Raman scattering and NMR techniques, we have investigated the elastic and structural properties of four post-shocked specimens of borosilicate glass, recovered from peak pressures of 19.8, 31.3, 40.3, and 49.1 GPa. The Raman spectra of shock-wave compressed borosilicate glass for peak pressures of 19.8 and 31.3 GPa show two new peaks at 606 cm−1 and near 1600 cm−1, while a peak at ∼923 cm−1 disappears in these glasses following shock-loading. The mode at 606 cm−1 is correlated with four-membered rings, composed of one BO4 and three SiO4 tetrahedra (a reedmergneritelike configuration). Modes near ∼1600 cm−1 are of uncertain origin, while that at 923 cm−1 may associated with silica tetrahedra with two nonbridging oxygens, although standard models of this type of glass suggest that total nonbridging oxygen contents should be low. The Raman spectra for the shocked samples at 40.3 and 49.1 GPa are similar to that of the unshocked sample, suggesting that much of the irreversible density and structural changes are recoverable following adiabatic decompression and thermal relaxation. This reversibility for the highest pressure shocked samples is in accord with the Brillouin results, which show an increase in the product of sound velocity and index of refraction at pressures up to 20 GPa. The Raman band initially at 450 cm−1, which corresponds to the bending vibration mode of the Si–O–Si, Si–O–B (with primarily six-membered rings in the network) reaches a maximum frequency of 470 cm−1 and narrowing at a peak shock pressure of 31.3 GPa, and then also decreases to its initial values for samples shocked at 40.3 and 49.1 GPa. This shift toward higher frequency under shock-wave compression indicates the average Si–O–Si, Si-O-B angles decrease with pressure. The narrowing of this band suggests a narrower distribution of Si–O–Si angles in the shocked samples for peak pressures of 19.8 and 31.3 GPa. 11B NMR spectra for all four shocked glasses are similar, and indicate ratios of BO3 to BO4 that are not greatly changed from the starting material. However, changes in peak shapes suggest significant changes in the connectivity of the B and Si components of the network, with more silicon neighbors surrounding BO4 tetrahedra in the shocked glasses, and a modest increase in the number of nonring related BO3 groups following shock-loading. Thus, the irreversible effects of shock-loading appear to be to generate smaller rings of tetrahedra (hence decreasing the average T–O–T bond angle), and to increase the average number of neighbors of Si around boron tetrahedra.
Investigation of Ca3TaGa3Si2O14 piezoelectric crystals for high temperature sensors
Authors: Yu Fapeng , Zhang Shujun , Zhao Xian , Yuan Duorong , Qin Lifeng , Wang Qing-ming , Shrout Thomas R.
Published: June 2011

The dielectric and electromechanical properties of fully ordered Ca3TaGa3Si2O14 (CTGS) crystals were investigated over the temperature range of −60∼700 °C. The highest electromechanical coupling factor, k26 (18.9%) and piezoelectric coefficient, d26 (−11.5 pC/N) were obtained for (YXl)-25° cuts. The temperature dependent behavior of resonance frequency (fr) was investigated in single-rotated thickness shear mode (TSM) (YXl)θ cuts (θ = −35°∼10°). The turnover temperatures of resonance frequency were found to increase from 20 °C to 330 °C, as the rotation angle θ varied from −22.5° to −35°. Bulk acoustic wave (BAW) resonators based on Y(−30°) monolithic disks with a fundamental frequency ∼2.87 MHz were fabricated, where the in air mechanical quality factor Q was found to be on the order of 24000 and 10000 at 20 °C and 700 °C, respectively. The high coupling k26, high mechanical Q, and high electrical resistivity (16 MΩ·cm) at 700 °C, together with the near zero TCF characteristics at elevated temperatures, demonstrate the potential of CTGS crystals for high temperature sensor applications.
Correcting model energies by numerically integrating along an adiabatic connection and a link to density functional approximations
Authors: Savin Andreas
Published: June 2011

Model Hamiltonians are considered for which electrons interact via long-range forces. It is assumed that their eigenvalues can be obtained with satisfying accuracy. Extrapolation techniques using asymptotic behavior considerations provide estimates for the energy of the physical system. Results for the uniform electron gas and some two-electron systems show that very few quadrature points can already produce good quality results. Connections to the density functional theory are discussed.
Carrier repopulation process for spatially-ordered InAs/InAlGaAs quantum dots
Authors: Lee Kwang Jae , Jo Byounggu , Lee Cheul-Ro , Lee In-Hwan , Kim Jin Soo , Oh Dae Kon , Kim Jong Su , Lee Sang Jun , Noh Sam Kyu , Leem Jae-Young , Ryu Mee-Yi
Published: June 2011

From the transmission electron microscope image, the seven-stacked InAs/InAlGaAs QDs on an InP substrate were spatially ordered instead of usual on-top vertical alignment. The increasing rate of the QD size became almost saturated by increasing the number of layers. The photoluminescence (PL) intensity for the seven-stacked InAs/InAlGaAs QDs was decreased up to 60K and remained almost stable at the temperature range from 60 to 220K. And then, the intensity was again drastically decreased with further increasing temperature. The emission peak was first red-shifted at the ratio of 0.446 meV/K from 20 to 60K. However, the degree of the red-shift in the emission peak from 60 to 220K was decreased at the negligibly small ratio of 0.028 meV/K. Above 220K, the emission peak was again significantly decreased at the ratio of 0.323 meV/K. While increasing the temperature, the carrier lifetimes obtained from the PL decay profiles for the seven-stacked QDs initially enhanced and then, almost stable at a certain temperature range. Finally, the PL decay time was decreased with further increasing temperature. These behaviors can be explained by the partial repopulation process of thermally excited carriers among QDs.
L1 FePt nanoparticles with distinct perpendicular magnetic anisotropy prepared on Au buffer layers by a micellar method
Authors: Gao Y. , Zhang X. W. , Yin Z. G. , Si F. T. , Bai Y. M. , Zhang X. L. , Qu S. , Wang Z. G.
Published: June 2011

FePt nanoparticles were self-assembled on a MgO (001) substrate by a micellar method. We introduced an Au buffer layer to control the lattice orientation and the magnetic alignment of FePt nanoparticles. A distinct c-axis preferred orientation of the FePt nanoparticles was achieved during the thermal annealing treatment. The driving force of lattice reorientation is considered to be the result of the stress caused by the lattice misfit between Au and FePt. The degree of c-axis orientation is significantly enhanced with increasing Au thickness, which is attributed to the decrease of the in-plane lattice and the improved crystal quality of the Au layer. Perpendicular magnetic anisotropy was observed for the FePt samples with the Au buffer layer. The out-of-plane coercivity and remanence ratio are 3.1 kOe and 0.8, respectively, which far exceed the in-plane values.
Counting the exponents of single transfer matrices
Authors: Molinari Luca Guido , Lacagnina Giuseppe
Published: June 2011

The eigenvalue equation of a band or a block tridiagonal matrix, the tight binding model for a crystal, a molecule, or a particle in a lattice with random potential or hopping amplitudes, and other problems lead to three-term recursive relations for (multicomponent) amplitudes. Amplitudes n steps apart are linearly related by a transfer matrix, which is the product of n matrices. Its exponents describe the decay lengths of the amplitudes. A formula is obtained for the counting function of the exponents, based on a duality relation and the Argument Principle for the zeros of analytic functions. It involves the corner blocks of the inverse of the associated Hamiltonian matrix. As an illustration, numerical evaluations of the counting function of quasi 1D Anderson model are shown.
Magnetostructural phase transition in electroless-plated Ni nanoarrays
Authors: Huang Chun-Chao , Lo Chih-Chieh , Tseng Yuan-Chieh , Liu Chien-Min , Chen Chih
Published: June 2011

Ni nanoarrays were synthesized by electroless-plating and shaped by an anodic aluminum oxide template. The as-plated arrays exhibited superparamagnetic (SM) ordering resulting from nanocrystalline microstructure. Ferromagnetic (FM) ordering was found to be restored as the arrays’ crystallinity was enhanced upon post-annealing. The microstructure (crystallinity) and the FM ordering are strongly coupled, revealing a magneto-structural correlation for the arrays. The magnetostructural properties of the arrays can be controlled by post-annealing, where the magnetization is proportional to the annealing temperature. The electroless-plated arrays synthesized in this work display magnetic anisotropy not found in electroplated ones. This is likely attributed to the nature of the clusterlike microstructure, whose cluster-boundaries may confine the FM rotation within the cluster. The spin-polarization was probed by x-ray magnetic circular dichroism while the arrays underwent the SM→FM phase transition. The sum-rules results reveal that the total magnetization of the arrays is dominated by spin moment (mspin). The change in mspin is responsible for the SM→FM phase transition upon annealing, as well as for the loss of magnetization upon temperature increase that we observed macroscopically.
Strain and piezoelectric potential effects on optical properties in CdSe/CdS core/shell quantum dots
Authors: Park Seoung-Hwan , Cho Yong-Hoon
Published: June 2011

Strain and piezoelectric potential effects on optical properties in CdSe/CdS core/shell quantum dots (QDs) were investigated theoretically using an eight-band strain-dependent k·p Hamiltonian. The strain effect on the shift of the subband energies is found to be larger than the piezoelectric field effect. As a result, interband transition energies are blueshifted with the inclusion of strain and piezoelectric field effects. We know that the theoretical interband transition energy shows a reasonable agreement with the experimental result. The absolute value of the hydrostatic strain in the QD increases with decreasing QD size, whereas that in the barrier decreases with decreasing QD size.
A smoothly decoupled particle interface: New methods for coupling explicit and implicit solvent
Authors: Wagoner Jason A. , Pande Vijay S.
Published: June 2011

A common theme of studies using molecular simulation is a necessary compromise between computational efficiency and resolution of the forcefield that is used. Significant efforts have been directed at combining multiple levels of granularity within a single simulation in order to maintain the efficiency of coarse-grained models, while using finer resolution in regions where such details are expected to play an important role. A specific example of this paradigm is the development of hybrid solvent models, which explicitly sample the solvent degrees of freedom within a specified domain while utilizing a continuum description elsewhere. Unfortunately, these models are complicated by the presence of structural artifacts at or near the explicit/implicit boundary. The presence of these artifacts significantly complicates the use of such models, both undermining the accuracy obtained and necessitating the parameterization of effective potentials to counteract the artificial interactions. In this work, we introduce a novel hybrid solvent model that employs a smoothly decoupled particle interface (SDPI), a switching region that gradually transitions from fully interacting particles to a continuum solvent. The resulting SDPI model allows for the use of an implicit solvent model based on a simple theory that needs to only reproduce the behavior of bulk solvent rather than the more complex features of local interactions. In this study, the SDPI model is tested on spherical hybrid domains using a coarse-grained representation of water that includes only Lennard-Jones interactions. The results demonstrate that this model is capable of reproducing solvent configurations absent of boundary artifacts, as if they were taken from full explicit simulations.
Hybridization-induced oscillatory magnetic polarization of C60 orbitals at the C60/Fe(001) interface
Authors: Tran T. L. A. , Wong P. K. J. , de Jong M. P. , van der Wiel W. G. , Zhan Y. Q. , Fahlman M.
Published: June 2011

We have studied the electronic and magnetic properties of the interface between C60 molecules and a Fe(001) surface. X-ray absorption spectroscopy and x-ray magnetic circular dichroism studies of C60 monolayers on Fe(001) surfaces show that hybridization between the frontier orbitals of C60 and continuum states of Fe leads to a significant magnetic polarization of C60 π∗-derived orbitals. The magnitude and also the sign of this polarization were found to depend markedly on the excitation energy. These observations underline the importance of tailoring the interfacial spin polarization at the Fermi level of ferromagnet/organic semiconductor interfaces for applications in organic spintronics.
Maternal endometrial oedema may increase perinatal mortality of cloned and transgenic piglets
Authors: Schmidt M. , Winter K. D. , Dantzer V. , Li J. , Kragh P. M. , Du Y. , Lin L. , Liu Y. , Vajta G. , Sangild P. T. , Callesen H. , Agerholm J. S.
Published: June 2011

The perinatal mortality of cloned animals is a well-known problem. In the present retrospective study, we report on mortality of cloned transgenic or non-transgenic piglets produced as part of several investigations. Large White (LW) sows (n = 105) received hand-made cloned LW or minipig blastocysts and delivered either spontaneously or after prostaglandin induction followed by either Caesarean section or vaginal birth. The overall pregnancy rate was 62%, with 26% of pregnancies terminating before term. This resulted in 48 deliveries. The terminated pregnancies consisted of 12 abortions that occurred at 35 ± 2 days gestation and five sows that went to term without returning to heat and then by surgery showed the uterus without fetal content. The gestation length was for sows with LW piglets that delivered by Caesarean section or vaginally was 115.7 ± 0.3 and 117.6 ± 0.4 days, respectively. In sows with minipiglets, the gestation length for those delivered by Caesarean section or vaginally 114.4 ± 0.2 and 115.5 ± 0.3 days, respectively. Of the 34 sows that delivered vaginally, 28 gave birth after induction, whereas 6 farrowed spontaneously. Of the 14 sows that delivered after Caesarean section and in the five empty sows, the endometrium and placenta showed severe oedema. Piglet mortality following vaginal delivery was higher than after Caesarean section (31% v. 10%, respectively; P < 0.001). When vaginal delivery occurred spontaneously, the stillborn rate was greater than after induced delivery (56% v. 24%, respectively; P < 0.0001). Internal organ weights were recorded for seven cloned LW piglets and six normal piglets. The relative weight of the heart, liver, kidneys and small intestine was found to be reduced in the cloned piglets (P < 0.05). The present study demonstrates extensive endometrial oedema in sows pregnant with cloned and transgenic piglets, as well as in empty recipients, at term. The growth of certain organs in some of the cloned piglets was reduced and the rate of stillborn piglets was greater in cloned and transgenic piglets delivered vaginally, possibly because of oedema of the fetal–maternal interface.
Loss of the pregnancy-induced rise in cortisol concentrations in the ewe impairs the fetal insulin-like growth factor axis
Authors: Jensen Ellen C. , Bennet Laura , Wood Charles , Vickers Mark , Breier Bernhard , Gunn Alistair J. , Keller-Wood Maureen
Published: June 2011

Maternal cortisol levels increase during pregnancy. Although this change is important for optimal fetal growth, the mechanisms of the changes in growth remain unclear. The hypothesis examined was that alterations in maternal plasma cortisol concentrations are associated with changes in the fetal insulin-like growth factor (IGF) axis. Pregnant ewes in late gestation (115 ± 0.4 days) were studied: six control animals, five ewes given 1 mg kg–1 day–1 cortisol (high cortisol) and five adrenalectomised ewes given 0.5–0.6 mg kg–1 day–1 cortisol (low cortisol). Blood samples were taken throughout the experiment and at necropsy (130 ± 0.2 days) and fetal liver was frozen for mRNA analysis. Fetal IGF-I and insulin plasma concentrations were lower and insulin-like growth factor-binding protein-1 (IGFBP-1) concentrations were higher in the low cortisol group compared with those in the control group (P < 0.05). Fetal liver IGF-II and IGFBP-3 mRNA were decreased in low cortisol animals compared with controls (P < 0.05). There were no significant changes in these parameters in the high cortisol group, and there were no changes in fetal liver IGF-I, growth hormone receptor, IGF-I receptor, IGF-II receptor, IGFBP-1 or IGFBP-2 mRNA levels between the groups. These data suggest that reduced fetal IGF availability contributes to reduced fetal growth when maternal cortisol secretion is impaired, but not during exposure to moderate increases in cortisol.
Identification of a β1 integrin isoform with restricted tissue expression in a teleost fish
Authors: Castillo-Briceño Patricia , Cabas Isabel , Arizcun Marta , Meseguer Jose , Mulero Victoriano , García-Ayala Alfonsa
Published: June 2011

The composition and organisation of extracellular matrix (ECM)-related molecules change during development. These components interact with different cell surface receptors to modulate the transduction of signals for cell growth, differentiation, migration, proliferation and apoptosis. Previous findings in the teleost fish gilthead seabream (Sparus aurata L., Teleostei), a marine protandrous hermaphrodite fish, showed that endocrine and immune stimuli are able to modulate the expression of ECM-related molecules, as well as specific correlations between them. In the present study, quantitative reverse transcription–polymerase chain reaction was used to examine the gene expression profile of β1 integrin isoform b (ITGB1b) and its possible role in reproductive physiology, especially in relation to spermatogenesis. Expression profiles were analysed in the context of the reproductive cycle (RC) and in relation with other ECM-related molecules, including matrix metalloproteinase (MMP)-2, MMP-9, MMP-13, tissue-specific inhibitor of metalloproteinase (TIMP)-2a, TIMP-2b, collagen (COL1A1) and ITGB1a. Expression of ITGB1b was found in the testis and brain and, to some extent, in endothelial cells. In contrast, ITGB1a was expressed ubiquitously. In the testis, the ITGB1b expression peaked during spermatogenesis, whereas the expression of the other ECM-related molecules is induced mainly during the post-spawning stage, both stages of marked tissue remodelling during the first and second RC in males. In addition, in fish exposed to the endocrine disruptor 17α-ethynyloestradiol (at 5 and 50 μg g–1 food during 7, 14 and 21 days), ITGB1b expression in the testis was inhibited in a dose- and time-dependent manner and was related to reduced serum levels of testosterone. Together, these results suggest a different functionality for the two ITGB1 isoforms in the gilthead seabream, where ITGB1b is more specifically involved in reproduction. This is the first report of an ITGB1 gene isoform whose expression is restricted to endocrine-related tissues in vertebrates.
Cyclic regulation of apoptotic gene expression in the mouse oviduct
Authors: Jeoung Myoungkun , Bridges Phillip J.
Published: June 2011

The oviduct is a dynamic structure whose function relies upon cyclic changes in the morphology of both ciliated and secretory luminal epithelial cells. Unfortunately, infection of these epithelial cells by sexually transmitted pathogens can lead to pelvic inflammatory disease, ectopic pregnancies and infertility. The disruption of normal, cyclic apoptosis in the oviducal epithelium appears to be a causal factor of oviducal pathology and therefore, these pathways represent a potential target for diagnosis and therapeutic intervention. The objective of this study was to determine the pattern of expression for apoptotic genes in the oviduct of the naturally cycling mouse, generating fundamental information that can be applied to the development of animal models for research and the identification of targets for disease intervention. Whole oviducts were collected from regular cycling mice killed at 1 p.m. on each day of the oestrous cycle and the expression of 84 apoptotic genes determined by targeted PCR super-array. Intact and cleaved caspases were then evaluated by western blotting. The expression of mRNA for genes classified as pro-apoptotic (Bad, Bak1 and Bok) and anti-apoptotic (Bag3, Bnip2 and Xiap) was regulated by day (P < 0.05). Differences in the temporal expression of several p53-related genes (Trp53bp2, Trp53inp1 and Trp73), those specific to the TNF superfamily (Tnfrsf10 and Tnfsf10b) and one caspase (Casp14) were also observed (P < 0.05). The cleaved forms of Caspases-3, -6 and -12 were all detected throughout the oestrous cycle. These results represent the first pathway-wide analysis of apoptotic gene expression in the murine oviduct.
Structural basis for pregnenolone biosynthesis by the mitochondrial monooxygenase system
Authors: Strushkevich Natallia , MacKenzie Farrell , Cherkesova Tatyana , Grabovec Irina , Usanov Sergey , Park Hee-Won
Published: June 2011

In humans, the precursor to all steroid hormones, pregnenolone, is synthesized from cholesterol by an enzyme complex comprising adrenodoxin reductase (AdR), adrenodoxin (Adx), and a cytochrome P450 (P450scc or CYP11A1). This complex not only plays a key role in steroidogenesis, but also has long been a model to study electron transfer, multistep catalysis, and C–C bond cleavage performed by monooxygenases. Detailed mechanistic understanding of these processes has been hindered by a lack of structural information. Here we present the crystal structure of the complex of human Adx and CYP11A1—the first of a complex between a eukaryotic CYP and its redox partner. The structures with substrate and a series of reaction intermediates allow us to define the mechanism underlying sequential hydroxylations of the cholesterol and suggest the mechanism of C–C bond cleavage. In the complex the [2Fe-2S] cluster of Adx is positioned 17.4 Å away from the heme iron of CYP11A1. This structure suggests that after an initial protein–protein association driven by electrostatic forces, the complex adopts an optimized geometry between the redox centers. Conservation of the interaction interface suggests that this mechanism is common for all mitochondrial P450s.
Negative feedback in the bone morphogenetic protein 4 (BMP4) synexpression group governs its dynamic signaling range and canalizes development
Authors: Paulsen Malte , Legewie Stefan , Eils Roland , Karaulanov Emil , Niehrs Christof
Published: June 2011

What makes embryogenesis a robust and canalized process is an important question in developmental biology. A bone morphogenetic protein (BMP) morphogen gradient plays a key role in embryonic development, and we are beginning to understand how the self-regulating properties of its signaling circuitry ensure robust embryonic patterning. An unexplored question is why the BMP signaling circuit is organized as a modular synexpression group, with a prevalence of feedback inhibitors. Here, we provide evidence from direct experimentation and mathematical modeling that the synexpressed feedback inhibitors BAMBI, SMAD6, and SMAD7 (
i
) expand the dynamic BMP signaling range essential for proper embryonic patterning and (
ii
) reduce interindividual phenotypic and molecular variability in
Xenopus
embryos. Thereby, negative feedback linearizes signaling responses and confers robust patterning, thus promoting canalized development. The presence of negative feedback inhibitors in other growth factor synexpression groups suggests that these properties may constitute a general principle.
Steroidogenic factor 1 directs programs regulating diet-induced thermogenesis and leptin action in the ventral medial hypothalamic nucleus
Authors: Kim Ki Woo , Zhao Liping , Donato Jose , Kohno Daisuke , Xu Yong , Elias Carol F. , Lee Charlotte , Parker Keith L. , Elmquist Joel K.
Published: June 2011

The transcription factor steroidogenic factor 1 (SF-1) is exclusively expressed in the brain in the ventral medial hypothalamic nucleus (VMH) and is required for the development of this nucleus. However, the physiological importance of transcriptional programs regulated by SF-1 in the VMH is not well defined. To delineate the functional significance of SF-1 itself in the brain, we generated pre- and postnatal VMH-specific SF-1 KO mice. Both models of VMH-specific SF-1 KO were susceptible to high fat diet-induced obesity and displayed impaired thermogenesis after acute exposure to high fat diet. Furthermore, VMH-specific SF-1 KO mice showed significantly decreased LepR expression specifically in the VMH, leading to leptin resistance. Collectively, these results indicate that SF-1 directs transcriptional programs in the hypothalamus relevant to coordinated control of energy homeostasis, especially after excess caloric intake.