bluuebunny/crossref_metadata_2025 huggingface dataset:
White‐light emission from Y2SiO5:Ce3+, Tb3+ and Sr2Si5N8:Eu2+ phosphor blends: a predictive model
Authors: Flores Dora‐Luz , Gutierrez Everardo , Cervantes David , Chacon Marco , Hirata Gustavo
Published: March 2017

In recent years, there has been an increasing interest in developing multifunctional materials that can be used in different areas, such as artificial lighting. White light‐emitting diodes offer the possibility of efficient, safe, and reliable solid‐state lighting. This study presents a synthesis and characterisation process of a white‐light emitting phosphor based on the combination of rare earth‐activated Y2SiO5:Ce3+, Tb3+ (YSO) and Sr2Si5N8:Eu2+ (SSN) phosphors. The YSO phosphor was prepared by combustion synthesis, whereas the SSN phosphor was prepared by solid‐state reaction under N2/H2 atmosphere. Photoluminescence measurements showed the contribution of a broad‐band emission of Ce3+ ions located in the blue region of the electromagnetic spectrum, a well‐defined green emission of Tb3+ ions with the main peak located at λEm = 545 nm in the YSO phosphor, and a broad‐band emission in the red region due to Eu2+ ions in the SSN phosphor. It is worth mentioning that excitation wavelength was in long UV radiation at 360 nm. In addition, a predictive model simulating and modelling the rare earths’ molar percentages to approximate to the D65 white light chromaticity coordinates is presented.
Sonochemistry as a General Procedure for the Synthesis of Coumarins­, Including Multigram Synthesis
Authors: da Silveira Pinto Ligia , de Souza Marcus
Published: March 2017

This study describes a general procedure for the synthesis of different coumarins via sonochemistry using active methylene compounds and 2-hydroxybenzaldehydes or resorcinol. The application of sonochemistry for the synthesis of these compounds was also very effective on a multigram scale with a higher yield, higher amount of crystalline compound, and shorter reaction time compared with the compounds obtained using the classical procedures.
Hemolysis, Elevated Liver Enzymes, Low Platelets Syndrome Superimposed on Hemolytic Uremic Syndrome
Authors: Conceição Madalena Gomes , Gomes Paulo Pereira , Clode Nuno , Martins Inês
Published: March 2017

A pregnancy complicated by typical hemolytic uremic syndrome (HUS) and hemolysis, elevated liver enzymes, low platelets (HELLP) syndrome is reported. At 20 weeks of gestation, a case of HUS was diagnosed, with Shiga toxin-producing Escherichia coli identified. Plasmapheresis allowed continuation of the pregnancy for 5 weeks. Superimposed preeclampsia and HELLP syndrome were diagnosed after the establishment of nephrotic range proteinuria, hypertension and recurrence of hemolysis. This is a singular case, as it demonstrates that HELLP syndrome can superimpose upon HUS, a fact that can impact future research on reproductive immunology. It also reminds clinicians that the overlapping of clinical and laboratory findings in HELLP syndrome makes the diagnosis of other thrombotic microangiopathies during pregnancy a clinical challenge.
Atypical Suprasellar Neurocytoma in a 31-Month-Old Child: A Case Report
Authors: Roels P. , De Beul P. , Herregods N.
Published: March 2017

We report a case of a 31-month-old girl with visual impairment. Magnetic resonance imaging showed a large cystic lesion with a mural nodule in the suprasellar region. Biopsy was performed, and histopathological examination demonstrated an atypical extraventricular neurocytoma (EVN). EVN is a rare neuroepithelial tumor with similar histological and biological characteristics in comparison to intraventricular neurocytoma. However, the morphological appearance of EVN can show wide variability with significant overlap of imaging findings compared with other neoplasms. The majority of EVNs are seen supratentorial involving the cerebral hemispheres. An EVN in the sellar or suprasellar region has only been reported twice in adults and to our knowledge never in children.
Myocardial Ischemia Caused by Subepicardial Hematoma
Authors: Grieshaber Philippe , Nef Holger , Böning Andreas , Niemann Bernd
Published: March 2017

Background Bleeding from bypass anastomosis leakage occurs early after coronary artery bypass grafting. Later, once the anastomosis is covered by intima, spontaneous bleeding is unlikely.

Case Description A 63-year-old male patient developed a pseudoaneurysm-like, subepicardial late-term bleeding resulting in a hematoma that compromised coronary artery flow by increasing extracoronary pressure. This resulted in severe angina pectoris (Canadian Cardiovascular Society IV) and myocardial ischemia within the affected area. After surgical removal of the hematoma and repair of the anastomosis, the patient's symptoms disappeared and no signs of myocardial ischemia were present.

Conclusion Surgical removal is an efficient therapy for subepicardial hematoma inducing myocardial ischemia.
Massive Urinary Protein Excretion Associated with Greater Neonatal Risk in Preeclampsia
Authors: Mateus Julio , Newman Roger , Sibai Baha , Li Qing , Barton John , Combs C. , Guzman Edwin , Boggess Kim , Gyamfi Cynthia , von Dadelszen Peter , Woelkers Doug
Published: March 2017

Objective The objective of this study was to compare clinical outcomes of preeclamptic pregnancies according to the proteinuria level.

Study Design Secondary analysis of a multicenter prospective cohort study of women with preeclampsia (PE) symptomatology. Nonproteinuria, mild-proteinuria, and massive-proteinuria PEs were defined as: < 165 mg in 12 hours or < 300 mg in 24 hours, 165 mg to 2.69 g in 12 hours or 300 mg to 4.99 g in 24 hours, and ≥ 2.7 g in 12 hours or ≥ 5.0 g in 24 hours, respectively. Individual and composite maternal, fetal, and neonatal outcomes were compared among the PE groups.

Results Of the 406 analyzed pregnancies, 36 (8.8%) had massive-proteinuria PE, 268 (66.0%) mild-proteinuria PE, and 102 (25.1%) nonproteinuria PE. Compared with the other groups, massive-proteinuria PE women had significantly higher blood pressures (p < 0.001), epigastric pain (p = 0.007), and uric acid serum levels (p < 0.001) prior to delivery. Composite maternal morbidity was similar across the groups. Delivery < 340/7 weeks occurred in 80.6, 49.3, and 22.5% of massive-proteinuria, mild-proteinuria, and nonproteinuria PE groups, respectively (p < 0.0001). Composite adverse neonatal outcomes were significantly higher in the massive-proteinuria PE compared with the other groups (p = 0.001).

Conclusion While potentially not important diagnostically, massive proteinuria is associated with more severe clinical manifestations of PE prompting earlier delivery.
Oxygen migration during resistance switching and failure of hafnium oxide memristors
Authors: Kumar Suhas , Wang Ziwen , Huang Xiaopeng , Kumari Niru , Davila Noraica , Strachan John Paul , Vine David , Kilcoyne A. L. David , Nishi Yoshio , Williams R. Stanley
Published: March 2017

While the recent establishment of the role of thermophoresis/diffusion-driven oxygen migration during resistance switching in metal oxide memristors provided critical insights required for memristor modeling, extended investigations of the role of oxygen migration during ageing and failure remain to be detailed. Such detailing will enable failure-tolerant design, which can lead to enhanced performance of memristor-based next-generation storage-class memory. Here, we directly observed lateral oxygen migration using in-situ synchrotron x-ray absorption spectromicroscopy of HfOx memristors during initial resistance switching, wear over millions of switching cycles, and eventual failure, through which we determined potential physical causes of failure. Using this information, we reengineered devices to mitigate three failure mechanisms and demonstrated an improvement in endurance of about three orders of magnitude.
Does fragility of glass formation determine the strength of T
g-nanoconfinement effects?
Authors: Mangalara Jayachandra Hari , Marvin Michael D. , Wiener Nicholas R. , Mackura Mark E. , Simmons David S.
Published: March 2017

Nanoscale confinement has been shown to alter the glass transition and associated mechanical and transport properties of glass-forming materials. Inspired by expected interrelations between nanoconfinement effects, cooperative dynamics in supercooled liquids, and the “fragility” (or temperature-abruptness) of the glass transition, it is commonly expected that nanoconfinement effects on Tg should be more pronounced for more fragile glass formers. Here we employ molecular dynamics simulations of glass formation in the bulk and under nanoconfinement of model polymers in which we systematically tune fragility by several routes. Results indicate that a correlation between fragility and the strength of nanoconfinement effects is weak to modest at best when considering all systems but can appear to be stronger when considering a subset of systems. This outcome is consistent with a reanalysis of the Adam-Gibbs theory of glass formation indicating that fragility does not necessarily track in a universal way with the scale of cooperative motion in glass-forming liquids. Finally, we find that factors such as composition gradients or variability in measurement sensitivity to different parts of the dynamic gradient have the potential to significantly confound efforts to identify trends in Tg-nanoconfinement effects with variables such as fragility, emphasizing the importance of employing diverse data sets and multiple metrologies in the study of this problem.
Thermal anisotropy enhanced by phonon size effects in nanoporous materials
Authors: Romano Giuseppe , Kolpak Alexie M.
Published: March 2017

While thermal anisotropy is a desirable materials property for many applications, including transverse thermoelectrics and thermal management in electronic devices, it remains elusive in practical natural compounds. In this work, we show how nanoporous materials with anisotropic pore lattices can be used as a platform for inducing strong heat transport directionality in isotropic materials. Using density functional theory and the phonon Boltzmann transport equation, we calculate the phonon-size effects and thermal conductivity of nanoporous silicon with different anisotropic pore lattices. Our calculations predict a strong directionality in the thermal conductivity, dictated by the difference in the pore-pore distances, i.e., the phonon bottleneck, along the two Cartesian axes. Using Fourier's law, we also compute the diffusive heat transport for the same geometries obtaining significantly smaller anisotropy, revealing the crucial role of phonon-size effects in tuning thermal transport directionality. Besides enhancing our understanding of nanoscale heat transport, our results demonstrate the promise of nanoporous materials for modulating anisotropy in thermal conductivity.
Surface intermixing by atomic scale roughening in Sb-terminated InAs
Authors: Anderson Evan M. , Lundquist Adam M. , Pearson Chris , Millunchick Joanna M.
Published: March 2017

The interaction between Sb and As is investigated by scanning tunneling microscopy of InAs (001) surfaces. Sb roughens the surface by increasing the coverage of divots (vacancy clusters) and two dimensional islands, which both possess a c(4 × 4) reconstruction. ab initio total energy calculations predict that the As-terminated β2(2 × 4) transforms into In-Sb heterodimer terminated c(4 × 4) configurations with increasing Sb chemical potential, which causes the roughening by pulling atoms from the terrace. Thus we propose an alternative model for interfacial broadening by intermixing rather than by surface segregation.
Effects of nitrogen substitution in amorphous carbon films on electronic structure and surface reactivity studied with x-ray and ultra-violet photoelectron spectroscopies
Authors: Murata Yuma , Nakayama Rempei , Ichihara Fumihiko , Ono Hiroshi , Choo Cheow-Keong , Tanaka Katsumi
Published: March 2017

We investigated the effects of incorporating a low percentage of nitrogen on the local and the electronic structures of amorphous carbon (a-C) using X-ray photoelectron spectroscopy and ultra-violet photoelectron spectroscopy (UPS). Nitrogen-doped amorphous carbon films (a-CNx) with varying nitrogen contents were prepared by a thermal decomposition method using a mixture of CH4 + NH3 under atmosphere. A slight shift of the C 1s core-level spectrum toward the higher binding energy side was detected in a-CNx as a function of nitrogen content, whereas a shift of the Fermi level (EF) cannot be confirmed from the UPS results. This was interpreted as a chemical shift between carbon and nitrogen atoms rather than as a shift of the EF. The C 1s peak shifts can be explained by the presence of two kinds of C–N local structures and the charge transferred bulk C–C components by nitrogen atoms. The two kinds of deconvoluted C 1s components could be well correlated with the two N 1s components. Two localized states were detected below the EF in UPS spectra of a-CNx, which could be assigned to defect bands. These defects played a significant role in the surface reactivity, and were stabilized in a-CNx. The adsorption and reaction of NO were carried out on a-CNx as well as a-C films. It was found that both defect sites and O2– species were responsible on a-C, while O2– species were selectively active for NO adsorption on a-CNx. We concluded that nitrogen doping reduces defect density to stabilize the surface of a-C, while at the same time inducing the selective adsorption capability of NO.
1H–2H cross-polarization NMR in fast spinning solids by adiabatic sweeps
Authors: Wi Sungsool , Schurko Robert , Frydman Lucio
Published: March 2017

Cross-polarization (CP) experiments employing frequency-swept radiofrequency (rf) pulses have been successfully used in static spin systems for obtaining broadband signal enhancements. These experiments have been recently extended to heteronuclear I, S = spin-1/2 nuclides under magic-angle spinning (MAS), by applying adiabatic inversion pulses along the S (low-γ) channel while simultaneously applying a conventional spin-locking pulse on the I-channel (1H). This study explores an extension of this adiabatic frequency sweep concept to quadrupolar nuclei, focusing on CP from 1H (I = 1/2) to 2H spins (S = 1) undergoing fast MAS (νr = 60 kHz). A number of new features emerge, including zero- and double-quantum polarization transfer phenomena that depend on the frequency offsets of the swept pulses, the rf pulse powers, and the MAS spinning rate. An additional mechanism found operational in the 1H–2H CP case that was absent in the spin-1/2 counterpart, concerns the onset of a pseudo-static zero-quantum CP mode, driven by a quadrupole-modulated rf/dipolar recoupling term arising under the action of MAS. The best CP conditions found at these fast spinning rates correspond to double-quantum transfers, involving weak 2H rf field strengths. At these easily attainable (ca. 10 kHz) rf field conditions, adiabatic level-crossings among the {|1⟩,|0⟩,|−1⟩} mS energy levels, which are known to complicate the CP MAS of quadrupolar nuclei, are avoided. Moreover, the CP line shapes generated in this manner are very close to the ideal 2H MAS spectral line shapes, facilitating the extraction of quadrupolar coupling parameters. All these features were corroborated with experiments on model compounds and justified using numerical simulations and average Hamiltonian theory models. Potential applications of these new phenomena, as well as extensions to higher spins S, are briefly discussed.
Improvement in color properties of copper mesh electrodes via electrophoretic coating with nano-structured carbon materials
Authors: Hwang Young-Jin , Kim Bu-Jong , Park Jin-Seok
Published: March 2017

In this study, the effects of coating with nano-structured carbon materials, such as carbon nanotube (CNT) and graphene, on the characteristics of transparent conductive electrodes based on copper (Cu) meshes, particularly on the visibility related to their color properties, were examined. The electrical sheet resistance of the Cu meshes remained almost unchanged regardless of the coating with CNT and graphene. Through the electrophoretic deposition method, the CNT and graphene layers were selectively used to coat only the regions where Cu mesh patterns had been formed, which helped minimize the transmittance loss caused by the coating with CNT and graphene. The reflectance of the Cu mesh was substantially reduced by the coating with CNT and graphene, meaning that the CNT or graphene coating layer played the role of suppressing the visible light reflected from the Cu mesh. In addition, the reflectance reduction effect was greater when the Cu mesh was coated with CNT rather than with graphene, which was attributed to the difference in particle size between the CNT suspension and the graphene suspension. Furthermore, the chromatic parameters (e.g., redness, yellowness) of the Cu meshes approached almost zero as the thickness of the CNT or graphene coating layer increased, meaning that the Cu meshes became nearly colorless, while the primitive Cu mesh was tinged with a red-orange color. The experiment results presented in this study verified that the combined technology with CNT and graphene coating contributed to the amelioration of the poor visibility caused by the high reflectance and color-tinted nature of the conventional Cu mesh.
Oxygen content tailored magnetic and electronic properties in cobaltite double perovskite thin films
Authors: Harrell Zach , Enriquez Erik , Chen Aiping , Dowden Paul , Mace Brennan , Lü Xujie , Jia Quanxi , Chen Chonglin
Published: March 2017

Oxygen content in transition metal oxides is one of the most important parameters to control for the desired physical properties. Recently, we have systematically studied the oxygen content and property relationship of the double perovskite PrBaCo2O5.5+δ (PBCO) thin films deposited on the LaAlO3 substrates. The oxygen content in the films was varied by in-situ annealing in a nitrogen, oxygen, or ozone environment. Associated with the oxygen content, the out-of-plane lattice parameter progressively decreases with increasing oxygen content in the films. The saturated magnetization shows a drastic increase and resistivity is significantly reduced in the ozone annealed samples, indicating the strong coupling between physical properties and oxygen content. These results demonstrate that the magnetic properties of PBCO films are highly dependent on the oxygen contents, or the film with higher oxygen uptake has the largest magnetization.
Local injection of pure spin current generates electric current vortices
Authors: Bazaliy Ya. B. , Ramazashvili R. R.
Published: March 2017

We show that local injection of pure spin current into an electrically disconnected ferromagnetic–normal-metal sandwich induces electric currents that run along the closed loops inside the device and are powered by the source of the spin injection. Such electric currents may significantly modify voltage distribution in spin-injection devices and induce long-range tails of spin accumulation.
Tunneling electroresistance effect in a Pt/Hf0.5Zr0.5O2/Pt structure
Authors: Ambriz-Vargas F. , Kolhatkar G. , Thomas R. , Nouar R. , Sarkissian A. , Gomez-Yáñez C. , Gauthier M. A. , Ruediger A.
Published: March 2017

The present work reports the fabrication of a ferroelectric tunnel junction based on a CMOS compatible 2.8 nm-thick Hf0.5Zr0.5O2 tunnel barrier. It presents a comprehensive study of the electronic properties of the Pt/Hf0.5Zr0.5O2/Pt system by X-ray photoelectron and UV-Visible spectroscopies. Furthermore, two different scanning probe techniques (Piezoresponse Force Microscopy and conductive-AFM) were used to demonstrate the ferroelectric behavior of the ultrathin Hf0.5Zr0.5O2 layer as well as the typical current-voltage characteristic of a ferroelectric tunnel junction device. Finally, a direct tunneling model across symmetric barriers was used to correlate electronic and electric transport properties of the ferroelectric tunnel junction system, demonstrating a large tunnel electroresistance effect with a tunneling electroresistance effect ratio of 20.
Near infrared overtone (vOH = 2 ← 0) spectroscopy of Ne–H2O clusters
Authors: Ziemkiewicz Michael P. , Pluetzer Christian , Wojcik Michael , Loreau Jérôme , van der Avoird Ad , Nesbitt David J.
Published: March 2017

Vibrationally state selective overtone spectroscopy and dynamics of weakly bound Ne–H2O complexes (D0(para) = 31.67 cm−1, D0(ortho) = 34.66 cm−1) are reported for the first time, based on near infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02−⟩←|00+⟩ and |02+⟩←|00+⟩) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level ab initio potential energy surface (CCSD(T)/VnZ-f12 (n = 3,4), corrected for basis set superposition error and extrapolated to the complete basis set limit) are employed for assignment of Σ←Σ,Π←Σ, and Σ←Π infrared bands in the overtone spectra, where Σ(K = 0) and Π (K = 1) represent approximate projections (K) of the body angular momentum along the Ne–H2O internuclear axis. End-over-end tumbling of the ortho Ne–H2O cluster is evident via rotational band contours observed, with band origins and rotational progressions in excellent agreement with ab initio frequency and intensity predictions. A clear Q branch in the corresponding |02+⟩fΠ(111)←eΣ(000) para Ne–H2O spectrum provides evidence for a novel e/f parity-dependent metastability in these weakly bound clusters, in agreement with ab initio bound state calculations and attributable to the symmetry blocking of an energetically allowed channel for internal rotor predissociation. Finally, Boltzmann analysis of the rotational spectra reveals anomalously low jet temperatures (Trot ≈ 4(1) K), which are attributed to “evaporative cooling” of weakly bound Ne–H2O clusters and provide support for similar cooling dynamics in rare gas-tagging studies.
Electrode material dependence of two-dimensional electron and vapour density distribution over vacuum arc discharge
Authors: Inada Y. , Matsuoka S. , Kumada A. , Ikeda H. , Hidaka K.
Published: March 2017

Electrode material dependence of intense-mode vacuum arc behaviour was systematically investigated by using the Shack-Hartmann method capable of simultaneously visualising two-dimensional electron and metal vapour density distributions from single-shot recordings. The electrode materials studied included Cu, CuCr (Cu75Cr25 wt. %), WC, and AgWC (Ag40WC60 wt. %). A comparison between the Cu and CuCr electrodes showed that the metal vapour densities for the CuCr decreased in an even shorter time scale than for the Cu. In the case of the WC electrodes, the widths of the electron density distributions became narrower as the arc current decreased although the electron densities hardly decreased in the decaying process of the arc current. The density measurements conducted at the late stage of the vacuum arcs demonstrated that the metal vapour densities around the anode were maintained at the highest value for the AgWC among the electrode materials in this study.
Using monomer vibrational wavefunctions as contracted basis functions to compute rovibrational levels of an H2O-atom complex in full dimensionality
Authors: Wang Xiao-Gang , Carrington Tucker
Published: March 2017

In this paper, we present new ideas for computing rovibrational energy levels of molecules composed of two components and apply them to H2O–Cl−. When both components are themselves molecules, Euler angles that specify their orientation with respect to an axis system attached to the inter-monomer vector are used as vibrational coordinates. For H2O–Cl−, there is only one set of Euler angles. Using Euler angles as intermolecular vibrational coordinates is advantageous because in many cases coupling between them and coordinates that describe the shape of the monomers is unimportant. The monomers are not assumed to be rigid. In the most efficient calculation, vibrational wavefunctions of the monomers are used as contracted basis functions. Energy levels are calculated using the Lanczos algorithm.
Degeneracy and bandgap narrowing in the semiconductor electron-hole product
Authors: Black Lachlan E.
Published: March 2017

It is shown that previously proposed expressions for the semiconductor electron-hole product, which purport to separate the influence of carrier degeneracy and bandgap narrowing, fail to properly delineate these effects. A reformulation of the expression in which the two effects are successfully separated is proposed, valid when the majority carrier concentration is independent of bandgap narrowing, as occurs in the common case of low injection and quasi-neutrality. It is shown that under other conditions, the two effects may not be treated in isolation but that only their combined effect or the marginal effect of one or the other on the total electron-hole product may be assessed. Convenient expressions are provided for the latter case. The revised expression provides both conceptual and computational advantages for semiconductor device modelling.
Photothermal radiometry parametric identifiability theory for reliable and unique nondestructive coating thickness and thermophysical measurements
Authors: Guo X. , Mandelis A. , Tolev J. , Tang K.
Published: March 2017

In this paper, we present a detailed reliability analysis of estimated parameters to a three-layer theoretical model of photothermal radiometry frequency domain signals by applying parameter identifiability conditions from two steel samples coated with ∼10 μm and 20 μm thick ceramic coating, to measure the thermophysical parameters of the coating, such as thermal diffusivity, thermal conductivity, and coating thickness. The three parameters are unique only when their sensitivity coefficients are linearly independent over the range of measurements. The study demonstrates the complexity of the identifiable experimental conditions through identifiability maps (calculated nonidentifiable locations) and sensitivity coefficient plots, even when the three separated parameters are grouped into two parameters. The validation of the reliability analysis theory by comparing the independently measured, with the fitted thicknesses of two coatings under random and optimized conditions, underscore the great importance of identifiability analysis (sensitivity coefficient plots) in the design of experiments for reliable parameter extractions, especially when the number of parameters is greater than the measurement data channels.
Weak ferromagnetism and short range polar order in NaMnF3 thin films
Authors: KC Amit , Borisov Pavel , Shvartsman Vladimir V. , Lederman David
Published: March 2017

The orthorhombically distorted perovskite NaMnF3 has been predicted to become ferroelectric if an a = c distortion of the bulk Pnma structure is imposed. In order to test this prediction, NaMnF3 thin films were grown on SrTiO3 (001) single crystal substrates via molecular beam epitaxy. The best films were smooth and single phase with four different twin domains. In-plane magnetization measurements revealed the presence of antiferromagnetic ordering with weak ferromagnetism below the Néel temperature TN = 66 K. For the dielectric studies, NaMnF3 films were grown on a 30 nm SrRuO3 (001) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics.
Gilbert damping of magnetostatic modes in a yttrium iron garnet sphere
Authors: Klingler S. , Maier-Flaig H. , Dubs C. , Surzhenko O. , Gross R. , Huebl H. , Goennenwein S. T. B. , Weiler M.
Published: March 2017

The magnetostatic mode (MSM) spectrum of a 300 μm diameter single crystalline sphere of yttrium iron garnet is investigated using broadband ferromagnetic resonance (FMR). The individual MSMs are identified via their characteristic dispersion relations, and the corresponding mode number tuples (nmr) are assigned. Taking FMR data over a broad frequency and magnetic field range allows one to analyze both the Gilbert damping parameter α and the inhomogeneous line broadening contribution to the total linewidth of the MSMs separately. The linewidth analysis shows that all MSMs share the same Gilbert damping parameter α = 2.7(5) × 10−5 irrespective of their mode index. In contrast, the inhomogeneous line broadening shows a pronounced mode dependence. This observation is modeled in terms of two-magnon scattering processes of the MSMs into the spin-wave manifold, mediated by surface and volume defects.
Layer number dependent optical properties of multilayer hexagonal BN epilayers
Authors: Du X. Z. , Uddin M. R. , Li J. , Lin J. Y. , Jiang H. X.
Published: March 2017

Deep ultraviolet photoluminescence emission spectroscopy has been employed to probe the layer number dependent near band-edge transitions above 5 eV in multilayer hexagonal boron nitride (h-BN) epilayers grown by metal-organic chemical vapor deposition. Two emission lines near 5.30 and 5.47 eV were resolved at 10 K. These two emission lines share similar spectroscopic features, and their energy peak separation is nearly independent of the number of layers. The observed energy separation of ∼172 meV coincides well with the in-plane phonon vibration mode, E2g, having an energy of 1370 cm−1 (∼172 meV). The results suggested that the emission line at ∼5.30 eV and ∼5.47 eV are a donor-acceptor-pair transition and its one E2g phonon replica, respectively. When the number of layers decreases from 100 to 8, the emission peak positions (Ep) of both emission lines blueshifted monotonically, indicating the dimensionality effects on the optical properties of h-BN. The layer number dependence of Ep can be described by an empirical formula, which accounts for the variations of the energy bandgap and activation energies of impurities with the number of layers. The results revealed that the impurity activation energies and the carrier-phonon coupling strength increase as the dimensionality of h-BN scales from thick layer to monolayer, suggesting that it is more difficult to achieve conductivity control through doping in monolayer or few-layer h-BN than in thick h-BN.
Magnetically controlled negative refraction of solitons in liquid crystals
Authors: Shvedov Vladlen G. , Izdebskaya Yana V. , Sheng Yan , Krolikowski Wieslaw
Published: March 2017

We demonstrate magnetically controlled refraction of a spatial soliton forming beam at the interface between an isotropic material and a uniaxial nematic liquid crystal, in which the optical axis together with an incident beam can make any desired angles with the interface. Depending on the direction of the applied magnetic field and incident beam angle, the refracted soliton forming beam experiences either positive or negative refraction.
Electron transport in Al-Cu co-doped ZnO thin films
Authors: Serin T. , Atilgan A. , Kara I. , Yildiz A.
Published: March 2017

To investigate the influence of varying Al content on structural, optical, and electrical properties of ZnO thin films, Al-Cu co-doped ZnO thin films with fixed Cu content at 1 wt. % and different Al contents (1, 3, and 5 wt. %) were successfully synthesized on glass substrates using a sol-gel process. The results indicated that the varying Al content affects not only the grain size and band gap but also the electrical conductivity of the films, and a linear relationship was found between the band gap and strain values of the films. The temperature-dependent electrical conductivity data of the films demonstrated that electron transport was mainly controlled by the grain boundaries at intermediate and high temperatures, whereas it was governed by Mott-variable range hopping at low temperatures. Additionally, 3 wt. % Al content improved the electrical conductivity of Al-Cu co-doped ZnO by lowering the trap density and enhancing the hopping probability.
Solid solution strengthening in GaSb/GaAs: A mode to reduce the TD density through Be-doping
Authors: Gutiérrez M. , Araujo D. , Jurczak P. , Wu J. , Liu H.
Published: March 2017

The need for a low bandgap semiconductor on a GaAs substrate for thermophotovoltaic applications has motivated research on GaSb alloys, in particular, the control of plastic relaxation of its active layer. Although interfacial misfit arrays offer a possibility of growing strain-free GaSb-based devices on GaAs substrates, a high density of threading dislocations is normally observed. Here, we present the effects of the combined influence of Be dopants and low growth temperature on the threading dislocation density observed by Transmission Electron Microscopy. The Be-related hardening mechanism, occurring at island coalescence, is shown to prevent dislocations to glide and hence reduce the threading dislocation density in these structures. The threading density in the doped GaSb layers reaches the values of seven times less than those observed in undoped samples, which confirms the proposed Be-related hardening mechanism.
Enhanced strain effect of aged acceptor-doped BaTiO3 ceramics with clamping domain structures
Authors: Wang Lei , Zhou Zhiyong , Zhao Xiaobo , Liu Zhen , Liang Ruihong , Dong Xianlin
Published: March 2017

A clamping domain structure is proposed to improve the amount of non-180° domain switching in BaTiO3 based piezoelectric ceramics. Experimental results show a large unipolar strain of 0.23% at 5 kV/mm in aged 0.5 mol. % Mn doped BaTiO3 ceramics with clamping domain structures, and the normalized strain (d33* = Smax/Emax) reaches 600 pm/V at low electric fields of 2 or 3 kV/mm. In contrast, pure BaTiO3 ceramics with clamping domain structures exhibit no clear polarization constriction or strain enhancement at 3 kV/mm. Electron paramagnetic resonance spectra verify the existence of titanium vacancies, Mn2+ and Mn4+, in 0.5 mol. % Mn doped BaTiO3 ceramics. These results indicate that the enhanced strain effect can be attributed to the combined effect of the clamping domain structure and stabilization of defect dipoles. This method provides a general way to obtain large strain in ferroelectrics.
Effect of excess lithium in LiMn2O4 and Li1.15Mn1.85O4 electrodes revealed by quantitative analysis of soft X-ray absorption spectroscopy
Authors: Zhuo Zengqing , Olalde-Velasco Paul. , Chin Timothy , Battaglia Vincent , Harris Stephen J. , Pan Feng , Yang Wanli
Published: March 2017

We performed a comparative study of the soft x-ray absorption spectroscopy of the LiMn2O4 and Li1.15Mn1.85O4 electrode materials with a quantitative analysis of Mn oxidation states. The revealed redox evolution of Mn upon electrochemical cycling clarifies the effect of excess Li in the materials, which naturally explains the different electrochemical performance. The spectral analysis perfectly agrees with different initial cycling capacities of the two materials. The results show unambiguously that Mn3+ starts to dominate the electrode surface after only one cycle. More importantly, the data show that, while LiMn2O4 electrodes follow the nominal Mn redox evolution, the formation of Mn3+ on the electrode surface is largely retarded for Li1.15Mn1.85O4 during most of the electrochemical processes. Such a different surface Mn redox behavior leads to differences in the detrimental effects of Mn2+ formation on the surface, which is observed directly after only two cycles. Our results provide strong evidence that a key effect of the (bulk) excess Li doping is actually due to processes on the electrode surfaces.
Method for characterizing bulk recombination using photoinduced absorption
Authors: Wilson Nora M. , Sandén Simon , Sandberg Oskar J. , Österbacka Ronald
Published: March 2017

The influence of reaction order and trap-assisted recombination on continuous-wave photoinduced absorption measurements is clarified through analytical calculations and numerical simulations. The results reveal the characteristic influence of different trap distributions and enable distinguishing between shallow exponential and Gaussian distributions and systems dominated by direct recombination by analyzing the temperature dependence of the in-phase and quadrature signals. The identifying features are the intensity dependence of the in-phase at high intensity, PAI∝IγHI, and the frequency dependence of the quadrature at low frequency, PAQ∝ωγLF. For direct recombination, γHI and γLF are temperature independent, and for an exponential distribution, they depend on the characteristic energy Ech as γHI=1/(1+Ech/kT) and γLF=kT/Ech, while a Gaussian distribution shows γHI and γLF as functions of I and ω, respectively.
Non-metal to metal transition in n-type ZnO single crystal materials
Authors: Brochen Stéphane , Feuillet Guy , Santailler Jean-Louis , Obrecht Rémy , Lafossas Matthieu , Ferret Pierre , Chauveau Jean-Michel , Pernot Julien
Published: March 2017

The electrical properties of ZnO mono-crystalline materials, either in the form of bulk crystals or epitaxial films, were investigated for a large range of un-intentional or intentional doping concentrations extending from 4.0×1015 cm−3 up to 1.3×1020 cm−3. Hall and resistivity measurements were carried out from 10 K to 300 K, yielding the temperature dependent carrier densities and carrier mobilities. This allowed for an unambiguous determination of the dopant ionization energies, taking into account the concentration of compensation centers. The ionization energy variation as a function of dopant concentration was found to follow Mott's law, being consistent with the hydrogenic behavior of all involved donors; an effective critical Mott's concentration for the insulator to metal transition was found to be around 4.2×1018 cm−3, while the apparent value of the isolated donor ionization energy was determined as being 60 meV.
Direct metal writing: Controlling the rheology through microstructure
Authors: Chen Wen , Thornley Luke , Coe Hannah G. , Tonneslan Samuel J. , Vericella John J. , Zhu Cheng , Duoss Eric B. , Hunt Ryan M. , Wight Michael J. , Apelian Diran , Pascall Andrew J. , Kuntz Joshua D. , Spadaccini Christopher M.
Published: March 2017

Most metal additive manufacturing approaches are based on powder-bed melting techniques such as laser selective melting or electron beam melting, which often yield uncontrolled microstructures with defects (e.g., pores or microcracks) and residual stresses. Here, we introduce a proof-of-concept prototype of a 3D metal freeform fabrication process by direct writing of metallic alloys in the semi-solid regime. This process is achieved through controlling the particular microstructure and the rheological behavior of semi-solid alloy slurries, which demonstrate a well suited viscosity and a shear thinning property to retain the shape upon printing. The ability to control the microstructure through this method yields a flexible manufacturing route to fabricating 3D metal parts with full density and complex geometries.
Optofluidic Fano resonance photonic crystal refractometric sensors
Authors: Wang Shuling , Liu Yonghao , Zhao Deyin , Yang Hongjun , Zhou Weidong , Sun Yuze
Published: March 2017

We report an ultra-compact surface-normal optofluidic refractometric sensor based on a two-dimensional silicon photonic crystal on insulator. In contrast to the conventional symmetric Lorentzian resonance that is prevalently used in the label-free sensors, the asymmetric lineshape and steep peak-to-dip transition of a Fano resonance enable the enhanced detection sensitivity. The detection limit of 1.3 × 10−6 refractive index units is achieved, which is among the lowest reported experimentally in the defect-free photonic crystal sensors.
Transverse spin relaxation and magnetic correlation in Pr1−x
Ca
x
MnO3: Influence of particle size variation and chemical doping
Authors: Shukla Vinay Kumar , Mukhopadhyay Soumik
Published: March 2017

The short ranged magnetic correlations and dynamics of hole doped Pr1–xCaxMnO3 (0.33 < x < 0.5) of different crystallite sizes have been investigated using electron spin resonance spectroscopy. The major contribution to the temperature dependence of paramagnetic line-width is attributed to the spin-lattice relaxation dominated by thermally activated hopping of small polarons with the typical activation energy of 20–50 meV. Irrespective of the crystallite size and dopant concentration, the transverse spin relaxation time (t2) follows a universal scaling behaviour of the type t2∼(T/T0)n in the paramagnetic regime, where T0 and n are the scaling parameters. Using the temperature dependence of t2, we construct a phase diagram which shows that near half-doping, the magnetic correlations associated with charge ordering not only survives even down to the crystallite size of 22 nm but is also actually enhanced. We conclude that the eventual suppression of charge ordering with reduction in the particle size is possibly more to do with the greater influence of chemical disorder than any intrinsic effect.
Effect of Sr-doping of LaMnO3 spacer on modulation-doped two-dimensional electron gases at oxide interfaces
Authors: Chen Y. Z. , Gan Y. L. , Christensen D. V. , Zhang Y. , Pryds N.
Published: March 2017

Modulation-doped oxide two-dimensional electron gas formed at the LaMnO3 (LMO) buffered disorderd-LaAlO3/SrTiO3 (d-LAO/LMO/STO) heterointerface provides new opportunities for electronics as well as quantum physics. Herein, we studied the dependence of Sr-doping of La1-xSrxMnO3 (LSMO, x = 0, 1/8, 1/3, ½, and 1) spacer on the transport properties of d-LAO/LSMO/STO in order to determine the effects of the filling of Mn eg subbands as well as the LSMO polarity on the modulation-doping. Upon increasing the LSMO film thickness from 1 unit cell (uc) to 2 uc, a sharp metal to insulator transition of interface conduction was observed, independent of x. The resultant electron mobility is higher than 1900 cm2 V−1 s−1 at 2 K, which increases upon decreasing x. The sheet carrier density, on the other hand, is in the range of 6.9 × 1012∼1.8 × 1013 cm−2 (0.01 ∼ 0.03 e/uc) and is largely independent on x for all the metallic d-LAO/LSMO (1 uc)/STO interfaces. These results are consistent with the charge transfer induced modulation doping scheme and clarify that the polarity of the buffer layer plays a trivial role on the modulation doping. The negligible tunability of the carrier density could result from the reduction of LSMO during the deposition of disordered LAO or that the energy levels of Mn 3d electrons at the interface of LSMO/STO are hardly varied even when changing the LSMO composition from LMO to SrMnO3.
Study of intersubband transitions in GaN-ZnGeN2 coupled quantum wells
Authors: Han Lu , Lieberman Colin , Zhao Hongping
Published: March 2017

In this work, we design and analyze a closely lattice-matched wide bandgap GaN-ZnGeN2 coupled quantum well (QW) structure targeting for near-infrared (IR) (λ ≤ 3 um) intersubband transition for quantum cascade laser applications. The coupled quantum well structure comprised two GaN wells separated by a thin ZnGeN2 barrier layer. The QW active region is surrounded by thick ZnGeN2 layers as barriers. The computations of the electron-phonon and electron-photon scattering rates are carried out by employing the Fermi's golden rule for transitions. The calculation takes into consideration the conservation of energy and momentum in scattering processes. The coupled QW structure is optimized through tuning the confined subband energy levels in the conduction band to achieve (1) electron-LO phonon resonant scattering when the energy separation between the first and second conduction subband levels matches the phonon energy of GaN (92 meV); and (2) dominant electron-photon transition in near-IR between the third and second conduction subband levels.
Molecular beam epitaxy of 2D-layered gallium selenide on GaN substrates
Authors: Lee Choong Hee , Krishnamoorthy Sriram , O'Hara Dante J. , Brenner Mark R. , Johnson Jared M. , Jamison John S. , Myers Roberto C. , Kawakami Roland K. , Hwang Jinwoo , Rajan Siddharth
Published: March 2017

Large area epitaxy of two-dimensional (2D) layered materials with high material quality is a crucial step in realizing novel device applications based on 2D materials. In this work, we report high-quality, crystalline, large-area gallium selenide (GaSe) films grown on bulk substrates such as c-plane sapphire and gallium nitride (GaN) using a valved cracker source for Se. (002)-Oriented GaSe with random in-plane orientation of domains was grown on sapphire and GaN substrates at a substrate temperature of 350–450 °C with complete surface coverage. Higher growth temperature (575 °C) resulted in the formation of single-crystalline ε-GaSe triangular domains with six-fold symmetry confirmed by in-situ reflection high electron energy diffraction and off-axis x-ray diffraction. A two-step growth method involving high temperature nucleation of single crystalline domains and low temperature growth to enhance coalescence was adopted to obtain continuous (002)-oriented GaSe with an epitaxial relationship with the substrate. While six-fold symmetry was maintained in the two step growth, β-GaSe phase was observed in addition to the dominant ε-GaSe in cross-sectional scanning transmission electron microscopy images. This work demonstrates the potential of growing high quality 2D-layered materials using molecular beam epitaxy and can be extended to the growth of other transition metal chalcogenides.
Graphene-based three-body amplification of photon heat tunneling
Authors: Simchi Hamidreza
Published: March 2017

We consider a three slabs configuration including two non-doped single layer graphene (SG) on insulating silicon dioxide (G/SiO2) substrates and one non-doped suspended single-layer graphene (SG). The suspended layer is placed between two G/SiO2 layers. Without the SG layer, the heat flux is maximum at the Plasmon frequency supported by the G/SiO2 slabs. In the three slabs configuration, the photon heat tunneling is amplified between two G/SiO2 layers significantly, only for a specific range of vacuum gap between the SG layer and G/SiO2 layers, and Plasmon frequency due to the coupling of modes between each G/SiO2 layer and SG layer. Since the SG layer is a single atomic layer, the photon heat tunneling assisted by this configuration does not depend on the thickness of the middle layer, and as a result, it can enable novel applications for nanoscale thermal management.
Computer simulations of the diffusion of Na+ and Cl− ions across POPC lipid bilayer membranes
Authors: Salih Rangeen , Matthai C. C.
Published: March 2017

We have carried out molecular dynamics simulations using NAMD to study the diffusivity of Na and Cl ions across a POPC lipid bilayer membrane. We show that an imbalance of positively and negatively charged ions on either side of the membrane leads to the diffusion of ions and water molecules. We considered the cases of both weak and very strong charge imbalance across the membrane. The diffusion coefficients of the ions have been determined from the mean square displacements of the particles as a function of time. We find that for strong electrochemical gradients, both the Na and Cl ions diffuse rapidly through pores in the membrane with diffusion coefficients up to ten times larger than in water. Rather surprisingly, we found that although the Na ions are the first to begin the permeation process due to the lower potential barrier that they experience compared to the Cl ions, the latter complete the permeation across the barrier more quickly due to their faster diffusion rates.