bluuebunny/crossref_metadata_2025 huggingface dataset:
Stark and Zeeman effect in the [18.6]3.5 – X(1)4.5 transition of uranium monofluoride, UF
Authors: Linton C. , Adam A. G. , Steimle T. C.
Published: June 2014

High resolution spectra of the 0-0 band of the [18.6]3.5 – X(1)4.5 transition of uranium monofluoride, UF, obtained using a laser ablation spectrometer, showed a perturbation in the upper state. Examination of the Stark and Zeeman effects yielded permanent electric dipole moments of 2.01 and 1.88 D and magnetic g-factors of 3.28 and 3.26 for the ground and excited states, respectively. Both the dipole moment and g-factor of the ground state are in good agreement with ab initio calculations [I. O. Antonov and M. C. Heaven, J. Phys. Chem. A 117, 9684 (2013)]. The Zeeman effect results confirm that the ground state arises primarily from the U+(5f 37s2 4I4.5)F− configuration and suggest several possible configurations for the upper state.
Band-gap nonlinear optical generation: The structure of internal optical field and the structural light focusing
Authors: Zaytsev Kirill I. , Katyba Gleb M. , Yakovlev Egor V. , Gorelik Vladimir S. , Yurchenko Stanislav O.
Published: June 2014

A novel approach for the enhancement of nonlinear optical effects inside globular photonic crystals (PCs) is proposed and systematically studied via numerical simulations. The enhanced optical harmonic generation is associated with two- and three-dimensional PC pumping with the wavelength corresponding to different PC band-gaps. The interactions between light and the PC are numerically simulated using the finite-difference time-domain technique for solving the Maxwell's equations. Both empty and infiltrated two-dimensional PC structures are considered. A significant enhancement of harmonic generation is predicted owing to the highly efficient PC pumping based on the structural light focusing effect inside the PC structure. It is shown that a highly efficient harmonic generation could be attained for both the empty and infiltrated two- and three-dimensional PCs. We are demonstrating the ability for two times enhancement of the parametric decay efficiency, one order enhancement of the second harmonic generation, and two order enhancement of the third harmonic generation in PC structures in comparison to the nonlinear generations in appropriate homogenous media. Obviously, the nonlinear processes should be allowed by the molecular symmetry. The criteria of the nonlinear process efficiency are specified and calculated as a function of pumping wavelength position towards the PC globule diameter. Obtained criterion curves exhibit oscillating characteristics, which indicates that the highly efficient generation corresponds to the various PC band-gap pumping. The highest efficiency of nonlinear conversions could be reached for PC pumping with femtosecond optical pulses; thus, the local peak intensity would be maximized. Possible applications of the observed phenomenon are also discussed.
Impact of surface and residual stresses and electro-/magnetostatic axial loading on the suspended nanomechanical based mass sensors: A theoretical study
Authors: Stachiv Ivo
Published: June 2014

Suspended nanomechanical mass sensors are capable to detect the attached molecules or particles through the shifts in the resonant frequencies. However, surface and residual stresses can as well cause a shift of the sensor resonances. As result, understanding the impact of stresses in an accuracy and sensitivity of the mass sensors is a fundamental requirement for a rigorous analysis of experimental data. Here, we present a detailed theoretical study of the suspended nanomechanical resonators and mass sensors under axial load created by surface (residual) stresses or electrostatic (magnetostatic) forces. Easily accessible formulas allowing one either to accurately predict the resonant frequencies of the beam under tension/compression or to disentangle the effects of stresses (axial forces) and the molecule mass on the frequency shift of the suspended mass sensors have been derived. A dimensionless parameter enabling us a simple characterization of the device vibrational regime (i.e., beam, string, or beam-to-string transition) has been identified. Based on the results, the applicability limits of the classical beam theory with and without axial loading have been found. We also show that tuning the beam resonant frequencies enhances the mass sensitivity.
Dynamics of oxygen Rydberg atom generation following O 1s inner-shell excitation of H2O
Authors: Gejo T. , Ikegami T. , Honma K. , Takahashi O. , Shigemasa E. , Hikosaka Y. , Tamenori Y.
Published: June 2014

The emission of low-energy electrons from H2O has been investigated at photon excitation energies in the vicinity of the O 1s ionization threshold. Neutral oxygen Rydberg atoms (O*) were found to form, and the correlation between the initial inner-shell excited state of H2O and the Rydberg state of O* was determined. The initially excited electron in a Rydberg orbital is shown to remain associated with O* even after the cleavage of two O-H bonds. We also show that the energy discrepancy between two Rydberg states of H2O and O* can be explained by the influence of the post-collision interaction, which becomes stronger as the excitation energy approaches the 1s ionization threshold.
Numerical study of drop motion on a surface with stepwise wettability gradient and contact angle hysteresis
Authors: Huang Jun-Jie , Huang Haibo , Wang Xinzhu
Published: June 2014

In this work, the motion of a two-dimensional drop on a surface with stepwise wettability gradient (WG) is studied numerically by a hybrid lattice-Boltzmann finite-difference method. We incorporate the geometric wetting boundary condition that allows accurate implementation of a contact angle hysteresis (CAH) model. The method is first validated through a series of tests that check different constituents of the numerical model. Then, simulations of a drop on a wall with given stepwise WG are performed under different conditions. The effects of the Reynolds number, the viscosity ratio, the WG, as well as the CAH on the drop motion are investigated in detail. It was discovered that the shape of the drop in steady motion may be fitted by two arcs that give two apparent contact angles, which are related to the respective contact line velocities and the relevant contact angles (that specify the WG and CAH) through the relation derived by Cox [“The dynamics of the spreading of liquids on a solid surface. Part 1. viscous flow,” J. Fluid Mech. 168, 169–194 (1986)] if the slip length in simulation is defined according to Yue et al. [“Sharp-interface limit of the Cahn-Hilliard model for moving contact lines,” J. Fluid Mech. 645, 279–294 (2010)]. It was also found that the steady capillary number of the drop is significantly affected by the viscosity ratio, the magnitudes of the WG, and the CAH, whereas it almost shows no dependence on the Reynolds number.
Loss and thermal noise in plasmonic waveguides
Authors: Syms R. R. A. , Solymar L.
Published: June 2014

Rytov's theory of thermally generated radiation is used to find the noise in two-dimensional passive guides based on an arbitrary distribution of lossy isotropic dielectric. To simplify calculations, the Maxwell curl equations are approximated using difference equations that also permit a transmission-line analogy, and material losses are assumed to be low enough for modal losses to be estimated using perturbation theory. It is shown that an effective medium representation of each mode is valid for both loss and noise and, hence, that a one-dimensional model can be used to estimate the best achievable noise factor when a given mode is used in a communications link. This model only requires knowledge of the real and imaginary parts of the modal dielectric constant. The former can be found by solving the lossless eigenvalue problem, while the latter can be estimated using perturbation theory. Because of their high loss, the theory is most relevant to plasmonic waveguides, and its application is demonstrated using single interface, slab, and slot guide examples. The best noise performance is offered by the long-range plasmon supported by the slab guide.
Achieving clean epitaxial graphene surfaces suitable for device applications by improved lithographic process
Authors: Nath A. , Koehler A. D. , Jernigan G. G. , Wheeler V. D. , Hite J. K. , Hernández S. C. , Robinson Z. R. , Garces N. Y. , Myers-Ward R. L. , Eddy C. R. , Gaskill D. K. , Rao M. V.
Published: June 2014

It is well-known that the performance of graphene electronic devices is often limited by extrinsic scattering related to resist residue from transfer, lithography, and other processes. Here, we report a polymer-assisted fabrication procedure that produces a clean graphene surface following device fabrication by a standard lithography process. The effectiveness of this improved lithography process is demonstrated by examining the temperature dependence of epitaxial graphene-metal contact resistance using the transfer length method for Ti/Au (10 nm/50 nm) metallization. The Landauer-Buttiker model was used to explain carrier transport at the graphene-metal interface as a function of temperature. At room temperature, a contact resistance of 140 Ω-μm was obtained after a thermal anneal at 523 K for 2 hr under vacuum, which is comparable to state-of-the-art values.
Electrodynamics of superconducting pnictide superlattices
Authors: Perucchi A. , Capitani F. , Pietro P. Di , Lupi S. , Lee S. , Kang J. H. , Jiang J. , Weiss J. D. , Hellstrom E. E. , Eom C. B. , Dore P.
Published: June 2014

It was recently shown that superlattices where layers of the 8% Co-doped BaFe2As2 superconducting pnictide are intercalated with non superconducting ultrathin layers of either SrTiO3 or of oxygen-rich BaFe2As2, can be used to control flux pinning, thereby increasing critical fields and currents, without significantly affecting the critical temperature of the pristine superconducting material. However, little is known about the electron properties of these systems. Here, we investigate the electrodynamics of these superconducting pnictide superlattices in the normal and superconducting state by using infrared reflectivity, from THz to visible range. We find that multigap structure of these superlattices is preserved, whereas some significant changes are observed in their electronic structure with respect to those of the original pnictide. Our results suggest that possible attempts to further increase the flux pinning may lead to a breakdown of the pnictide superconducting properties.
Routes to fractality and entropy in Liesegang systems
Authors: Kalash Leen , Sultan Rabih
Published: June 2014

Liesegang bands are formed when solutions of co-precipitate ions interdiffuse in a 1D gel matrix. In a recent study [R. F. Sultan, Acta. Mech. Sin. 27, 119 (2011)], Liesegang patterns have been characterized as fractal structures. In addition to experimentally obtained patterns, geometric Liesegang patterns were constructed in conformity with the well-known empirical laws. Both mathematical fractal dimensions and box count dimensions for images of PbF2 and PbI2 Liesegang patterns have been calculated. Liesegang patterns can also be described by the entropy state function, and categorized as more or less ordered structures. We revisit the relation between entropy and fractal dimension, and apply it to simulated geometrical Liesegang patterns. We have resort to three different routes for the estimation of the entropy of a Liesegang pattern. The HarFA software enabled the calculation of the Hausdorff dimension and the topological entropy, then the information dimension and the Shannon entropy. In a third pathway, analytical calculations were carried out by estimating the probability of occurrence of a fractal element or coverage. The product of Shannon entropy and Boltzmann constant yields the thermodynamic entropy. The values for PbF2 and PbI2 Liesegang patterns attained the order of magnitude of the reported Third Law entropies, but yet remained lower, in conformity with the more ordered Liesegang structures.
Effect of damping on the laser induced ultrafast switching in rare earth-transition metal alloys
Authors: Oniciuc Eugen , Stoleriu Laurentiu , Cimpoesu Dorin , Stancu Alexandru
Published: June 2014

In this paper, we present simulations of thermally induced magnetic switching in ferrimagnetic systems performed with a Landau-Lifshitz-Bloch (LLB) equation for damping constant in a wide range of values. We have systematically studied the GdFeCo ferrimagnet with various concentrations of Gd and compared for some values of parameters the LLB results with atomistic simulations. The agreement is remarkably good, which shows that the dynamics described by the ferrimagnetic LLB is a reasonable approximation of this complex physical phenomenon. As an important element, we show that the LLB is able to also describe the intermediate formation of a ferromagnetic state which seems to be essential to understand laser induced ultrafast switching. The study reveals the fundamental role of damping during the switching process.
Intrinsically switchable thin film bulk acoustic wave resonators
Authors: Vorobiev A. , Gevorgian S.
Published: June 2014

The concept of the frequency switching in the composite bulk acoustic wave (BAW) resonators based on the thin films of paraelectric phase ferroelectrics is demonstrated experimentally. The composite BAW resonators based on the Ba0.25Sr0.75TiO3/SrRuO3/Ba0.25Sr0.75TiO3 multilayer structure are fabricated and characterized. It is shown that the resonance frequency of the BAW resonators can be switched more than two times (from 3.6 GHz to 7.7 GHz) by changing polarity of the 5 V dc bias voltage at one of the ferroelectric layers. The composite BAW resonators performance is analyzed using the theory of the dc field induced piezoelectric effect in the paraelectric phase ferroelectrics.
Voltage-controlled switching and thermal effects in VO2 nano-gap junctions
Authors: Joushaghani Arash , Jeong Junho , Paradis Suzanne , Alain David , Stewart Aitchison J. , Poon Joyce K. S.
Published: June 2014

Voltage-controlled switching in lateral VO2 nano-gap junctions with different gap lengths and thermal properties was investigated. The effect of Joule heating on the phase transition was found to be strongly influenced by the device geometry, the contact material, and the current. Our results indicate that the VO2 phase transition was likely initiated electronically, which was sometimes followed by a secondary thermally induced transition.
Carrier behavior in special multilayer device composed of different transition metal oxide-based intermediate connectors
Authors: Deng Yan-Hong , Chen Xiang-Yu , Ou Qing-Dong , Wang Qian-Kun , Jiang Xiao-Cheng , Zhang Dan-Dan , Li Yan-Qing
Published: June 2014

The impact of illumination on the connection part of the tandem organic light-emitting diodes was studied by using a special organic multilayer sample consisted of two organic active layers coupled with different transition metal oxide (TMO)-based intermediate connectors (ICs). Through measuring the current density-voltage characteristic, interfacial electronic structures, and capacitance-voltage characteristic, we observe an unsymmetrical phenomenon in current density-voltage and capacitance-voltage curves of Mg:Alq3/MoO3 and MoO3 composed devices, which was induced by the charge spouting zone near the ICs region and the recombination state in the MoO3 layer. Moreover, Mg:Alq3/MoO3 composed device displays a photovoltaic effect and the Voc shifts to forward bias under illumination. Our results demonstrate that the TMO-based IC structure coupled with photovoltaic effect can be a good approach for the study of photodetector, light sensor, and so on.
Continuous operation of a monolithic semiconductor terahertz source at room temperature
Authors: Lu Q. Y. , Bandyopadhyay N. , Slivken S. , Bai Y. , Razeghi M.
Published: June 2014

We demonstrate room temperature continuous wave THz sources based on intracavity difference-frequency generation from mid-infrared quantum cascade lasers. Buried ridge, buried composite distributed-feedback waveguide with Čerenkov phase-matching scheme is used to reduce the waveguide loss and enhance the heat dissipation for continuous wave operation. Continuous emission at 3.6 THz with a side-mode suppression ratio of 20 dB and output power up to 3 μW are achieved, respectively. THz peak power is further scaled up to 1.4 mW in pulsed mode by increasing the mid-infrared power through increasing the active region doping and device area.
Optically pumped whispering-gallery mode lasing from 2-μm GaN micro-disks pivoted on Si
Authors: Zhang Yiyun , Ma Zetao , Zhang Xuhui , Wang T. , Choi H. W.
Published: June 2014

2-μm micro-disks containing InGaN/GaN quantum wells supported on a tiny Si nanotip are fabricated via microsphere lithography followed by dry and wet etch processes. The micro-disks are studied by photoluminescence at both room-temperature and 10 K. Optically pumped blue lasing at room-temperature is observed via whispering-gallery modes (WGMs) with a lasing threshold as low as 8.43 mJ/cm2. Optical resonances in the micro-disks are studied through numerical computations and finite-difference time-domain simulations. The WGMs are further confirmed through the measured broadband transmission spectrum, whose transmission minima coincide well with predicted WGM frequencies.
Broadband impedance-matched electromagnetic structured ferrite composite in the megahertz range
Authors: Parke L. , Youngs I. J. , Hibbins A. P. , Sambles J. R.
Published: June 2014

A high refractive-index structured ferrite composite is designed to experimentally demonstrate broadband impedance matching to free-space. It consists of an array of ferrite cubes that are anisotropically spaced, thereby allowing for independent control of the effective complex permeability and permittivity. Despite having a refractive index of 9.5, the array gives less than 1% reflection and over 90% transmission of normally incident radiation up to 70 MHz for one of the orthogonal linear polarisations lying in a symmetry plane of the array. This result presents a route to the design of MHz-frequency ferrite composites with bespoke electromagnetic parameters for antenna miniaturisation.
Variation of critical point of aging transition in a networked oscillators system
Authors: Huang Wenwen , Zhang Xiyun , Hu Xin , Zou Yong , Liu Zonghua , Guan Shuguang
Published: June 2014

In this work, we study the variation of critical point in aging transition in a networked system consisting of both active and inactive oscillators. By theoretical analysis and numerical simulations, we show that the critical point of aging transition actually is determined by the (normalized) cross links between active and inactive subpopulations of oscillators. This reveals how specific configuration of active and inactive oscillators in the network can lead to the variation of transition point. In particular, we investigate how different strategies of targeted inactivation influence the transition point based on the theory. Our results theoretically explain why the low-degree nodes are crucial regarding dynamical robustness in such systems.
Broadband antireflection and absorption enhancement of ultrathin silicon solar microcells enabled with density-graded surface nanostructures
Authors: Chan Lesley , Kang Dongseok , Lee Sung-Min , Li Weigu , Hunter Hajirah , Yoon Jongseung
Published: June 2014

Density-graded surface nanostructures are implemented on ultrathin silicon solar microcells by silver-nanoparticle-catalyzed wet chemical etching to enable near-zero surface reflection over a broad wavelength range of incident solar spectrum as well as non-zeroth order diffraction and light trapping for longer wavelength photons, thereby achieving augmented photon absorption for ultrathin silicon microcells in a simple, cost-effective manner. The increase of absorbed photon flux through the “black silicon (b-Si)” surface translates directly into the corresponding enhancement of photovoltaic performance, where 5.7-μm b-Si microcells with the rational design of device configuration exhibit improved energy conversion efficiency by 148% and 50% with and without a diffuse backside reflector, respectively, compared to devices from the bare silicon without b-Si implementation. Systematic studies on nanostructured morphology, optical and electrical properties of b-Si microcells, together with semi-empirical numerical modeling of photon absorption, provide key aspects of underlying materials science and physics.
A microfabricated fringing field capacitive pH sensor with an integrated readout circuit
Authors: Arefin Md Shamsul , Bulut Coskun M. , Alan Tuncay , Redoute Jean-Michel , Neild Adrian , Rasit Yuce Mehmet
Published: June 2014

This work presents a microfabricated fringe-field capacitive pH sensor using interdigitated electrodes and an integrated modulation-based readout circuit. The changes in capacitance of the sensor result from the permittivity changes due to pH variations and are converted to frequency shifts using a crossed-coupled voltage controlled oscillator readout circuit. The shift in resonant frequency of the readout circuit is 30.96 MHz for a change in pH of 1.0–5.0. The sensor can be used for the measurement of low pH levels, such as gastric acid, and can be integrated with electronic pills. The measurement results show high repeatability, low noise, and a stable output.
Evanescent-wave pumped room-temperature single-mode GaAs/AlGaAs core-shell nanowire lasers
Authors: Wei Wei , Liu Yange , Zhang Xia , Wang Zhi , Ren Xiaomin
Published: June 2014

Evanescent-wave pumped room-temperature single-mode GaAs/AlGaAs core-shell nanowire lasers are proposed and demonstrated. The nanowires are axially excited by evanescent wave outside a microfiber with a diameter about 10 μm via a ns-pulse laser. The lasing emission with a low effective threshold less than 90 nJ is achieved at 868.62 nm along with a linewidth of ∼1.8 nm. Moreover, multiple lasing lines in a wavelength range from 852.56 nm to 882.48 nm are observed. The mechanism of diverse lasing wavelengths is revealed. Furthermore, the proposed GaAs/AlGaAs nanowire laser has advantages such as simple structure, easy to operate, and controllable lasing wavelength, tending to be practical in optical communications and integrated photonic circuits.
The intermediate temperature T* revealed in relaxor polymers
Authors: Liu B. L. , Tian B. B. , Geiger S. , Hu Z. G. , Zhao X. L. , Zou Y. H. , Wang J. L. , Sun J. L. , Sun S. , Dkhil B. , Meng X. J. , Chu J. H.
Published: June 2014

The temperature dependence of the dielectric and optical properties, crystal structure, and infrared spectra of the relaxor poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) terpolymer films obtained from Langmuir-Blodgett method have been comprehensively investigated. All the results suggest that there exists a peculiar point at ∼370 K, which is attributed to the intermediate temperature T* recently discovered in inorganic relaxors. Here, T* results from the change in the growth rate of the trans gauche T3GT3G′ chain conformations with temperature, which is similar to the transformation from dynamic to static behavior of the so-called polar nano-regions in inorganic relaxors.
Robust superconductivity with large upper critical field in Nb2PdS5
Authors: Jha Rajveer , Tiwari Brajesh , Rani Poonam , Kishan Hari , Awana V. P. S.
Published: June 2014

We report synthesis, structural details, and complete superconducting characterization of very recently discovered [Q. Zhang, Sci. Rep. 3, 1446 (2013)] Nb2PdS5 new superconductor. The synthesized compound is crystallized in monoclinic structure with C2/m (#12) space group. Bulk superconductivity is seen in both ac/dc magnetic susceptibility and electrical resistivity measurements with superconducting transition temperature (Tc) at 6 K. The upper critical field (Hc2) being estimated from high field magneto-transport [ρ(T)H] measurements is above 240 kOe. The estimated Hc2(0) is clearly above the Pauli paramagnetic limit of ∼1.84Tc. Heat capacity (Cp) measurements show clear transition with well defined peak at Tc, but with lower jump than as expected for a Bardeen–Cooper–Schrieffer type superconductor. The Sommerfeld constant (γ) and Debye temperature (ΘD) as determined from low temperature fitting of CP(T) data are 32 mJ/mole-K2 and 263 K, respectively. Hall coefficients and resistivity in conjugation with electronic heat capacity indicates multiple gap superconductivity signatures in Nb2PdS5. We also studied the impact of hydrostatic pressure (0–1.97 Gpa) on superconductivity of Nb2PdS5 and found nearly no change in Tc for the given pressure range.
Structural disorder versus spin canting in monodisperse maghemite nanocrystals
Authors: Kubickova S. , Niznansky D. , Morales Herrero M. P. , Salas G. , Vejpravova J.
Published: June 2014

Monodisperse maghemite nanoparticles with diameter ranging from 7 to 20 nm were examined by the In-field Mössbauer Spectroscopy (IFMS) in varying external magnetic field up to 6 T. Surprisingly, the small-sized particles (7 nm) exhibit nearly no spin canting in contrast to the larger particles with lower surface-to-volume ratio. We demonstrate that the observed phenomenon is originated by lower relative crystallinity of the larger particles with different internal structure. Hence, the persistence of the 2nd and 5th absorption lines in the IFMS cannot be unambiguously assigned to the surface spins.
Ultra broadband phase measurements on nanostructured metasurfaces
Authors: Pshenay-Severin E. , Falkner M. , Helgert C. , Pertsch T.
Published: June 2014

We report on an interferometric method developed for ultra broadband (from λ=0.65 μm to λ=1.7 μm) phase measurements on metasurfaces in transmission and reflection. Due to a unique performance of our method in terms of the accessible spectral range, accuracy (±0.02 rad), and flexibility with respect to the sample arrangement, this technique can be broadly used as a versatile tool for the comprehensive characterization of a broad class of dispersive optical materials. We compare our experimental technique with an indirect approach and based on the Kramers-Kronig transformation analysis, establish a rule for the use of the indirect method.
Communication: Integral equation theory for pair correlation functions in a crystal
Authors: Jaiswal Anubha , Bharadwaj Atul S. , Singh Yashwant
Published: June 2014

A method for calculating pair correlation functions in a crystal is developed. The method is based on separating the one- and two-particle correlation functions into the symmetry conserving and the symmetry broken parts. The conserving parts are calculated using the integral equation theory of homogeneous fluids. The symmetry broken part of the direct pair correlation function is calculated from a series written in powers of order parameters and that of the total pair correlation function from the Ornstein-Zernike equation. The results found for a two-dimensional hexagonal lattice show that the method provides accurate and detailed informations about the pair correlation functions in a crystal.
Donor impurity states and related terahertz range nonlinear optical response in GaN cylindrical quantum wires: Effects of external electric and magnetic fields
Authors: Correa J. D. , Mora-Ramos M. E. , Duque C. A.
Published: June 2014

We report a study on the optical absorption coefficient associated to hydrogenic impurity interstate transitions in zinc-blende GaN quantum wires of cylindrical shape taking into account the effects of externally applied static electric and magnetic fields. The electron states emerge within the effective mass approximation, via the exact diagonalization of the donor-impurity Hamiltonian with parabolic confinement and external field effects. The nonlinear optical absorption is calculated using a recently derived expression for the dielectric susceptibility, obtained via a nonperturbative solution of the density-matrix Bloch equation. Our results show that this treatment eliminates not only the intensity-dependent bleaching effect but also the change in sign of the nonlinear contribution due to the combined effect of asymmetric impurity location and the applied electric field.
Influence of beam-loaded effects on phase-locking in the high power microwave oscillator
Authors: Li Zhenghong , Zhou Zhigang , Qiu Rong
Published: June 2014

Owing to the power limitation of a single device, much more attentions are focused on developing high power microwave (HPM) oscillators that can be phase-locked to the external signal in the recent HPM researches. Although the phase-locking is proved to be feasible in the conventional devices (such as magnetrons), challenges still exist in the HPM devices due to beam-loaded effects, which are more obvious in HPM devices because of its high current and the low Q-factor of the device. A simple structured HPM oscillator (Bitron) is introduced to study such effects on the phase-locking in the HPM oscillator. The self-consistent analysis is carried out to study such effects together with particle in cell simulations. Then the modified Adler equation is established for the phase-locking HPM oscillator. Finally, conditions for the phase-locking in the HPM oscillator are given.
No electrostatic supersolitons in two-component plasmas
Authors: Verheest Frank , Lakhina Gurbax S. , Hellberg Manfred A.
Published: June 2014

The concept of acoustic supersolitons was introduced for a very specific plasma with five constituents, and discussed only for a single set of plasma parameters. Supersolitons are characterized by having subsidiary extrema on the sides of a typical bipolar electric field signature, or by association with a root beyond double layers in the fully nonlinear Sagdeev pseudopotential description. It was subsequently found that supersolitons could exist in several plasma models having three constituent species, rather than four or five. In the present paper, it is proved that standard two-component plasma models cannot generate supersolitons, by recalling and extending results already in the literature, and by establishing the necessary properties of a more recent model.
Relationship of edge localized mode burst times with divertor flux loop signal phase in JET
Authors: Chapman S. C. , Dendy R. O. , Todd T. N. , Watkins N. W. , Webster A. J. , Calderon F. A. , Morris J. ,   JET EFDA Contributors
Published: June 2014

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM.
High aspect ratio x-ray waveguide channels fabricated by e-beam lithography and wafer bonding
Authors: Neubauer H. , Hoffmann S. , Kanbach M. , Haber J. , Kalbfleisch S. , Krüger S. P. , Salditt T.
Published: June 2014

We report on the fabrication and characterization of hard x-ray waveguide channels manufactured by e-beam lithography, reactive ion etching and wafer bonding. The guiding layer consists of air or vacuum and the cladding material of silicon, which is favorable in view of minimizing absorption losses. The specifications for waveguide channels which have to be met in the hard x-ray range to achieve a suitable beam confinement in two orthogonal directions are extremely demanding. First, high aspect ratios up to 106 have to be achieved between lateral structure size and length of the guides. Second, the channels have to be deeply embedded in material to warrant the guiding of the desired modes while absorbing all other (radiative) modes in the cladding material. We give a detailed report on device fabrication with the respective protocols and parameter optimization, the inspection and the optical characterization.
Multiphoton-induced luminescence contrast between antiparallel ferroelectric domains in Mg-doped LiNbO3
Authors: Reichenbach P. , Kämpfe T. , Thiessen A. , Schröder M. , Haußmann A. , Woike T. , Eng L. M.
Published: June 2014

We report on differentiating antiparallel ferroelectric domains in congruent Mg-doped LiNbO3 (Mg:LNO) single crystals through a multiphoton photoluminescence technique. Sample illumination with femtosecond laser pulses at λ = 790 nm results in a broad multiphoton emission spectrum revealing a domain contrast of >3% between virgin and inverted domains. The contrast decreases via annealing and shows an exponential decay in the temperature range from 80 to 150 °C. Our findings give clear ground of a thermally induced structural change by surpassing a specific activation energy. Hence, the reported contrast dynamics must be closely connected to the thermal activation of charged defects, which dramatically alters the internal bias field of these defects. This explanation is also supported when using single crystal LNO of different Mg doping levels showing much lower multiphoton effects for a < 5% Mg concentration. Based on this effect of multiphoton luminescence, it becomes easy to microscopically monitor and quantify virgin and switched domains in LNO and other samples.
Adsorption structure of water molecules on the Be(0001) surface
Authors: Yang Yu , Li Yanfang , Wang Shuangxi , Zhang Ping
Published: June 2014

By using density functional theory calculations, we systematically investigate the adsorption of water molecules at different coverages on the Be(0001) surface. The coverage dependence of the prototype water structures and energetics for water adlayer growth are systematically studied. The structures, energetics, and electronic properties are calculated and compared with other available studies. Through our systematic investigations, we find that water molecules form clusters or chains on the Be(0001) surface at low coverages. When increasing the water coverage, water molecules tend to form a 2 × 2 hexagonal network on the Be(0001) surface.
Magnetism in undoped ZnS studied from density functional theory
Authors: Xiao Wen-Zhi , Wang Ling-ling , Rong Qing-Yan , Xiao Gang , Meng Bo
Published: June 2014

The magnetic property induced by the native defects in ZnS bulk, thin film, and quantum dots are investigated comprehensively based on density functional theory within the generalized gradient approximation + Hubbard U (GGA + U) approach. We find the origin of magnetism is closely related to the introduction of hole into ZnS systems. The relative localization of S-3p orbitals is another key to resulting in unpaired p-electron, due to Hund's rule. For almost all the ZnS systems under study, the magnetic moment arises from the S-dangling bonds generated by Zn vacancies. The charge-neutral Zn vacancy, Zn vacancy in 1− charge sate, and S vacancy in the 1+ charge sate produce a local magnetic moment of 2.0, 1.0, and 1.0 μB, respectively. The Zn vacancy in the neutral and 1− charge sates are the important cause for the ferromagnetism in ZnS bulk, with a Curie temperature (TC) above room temperature. For ZnS thin film with clean (111) surfaces, the spins on each surface are ferromagnetically coupled but antiferromagnetically coupled between two surfaces, which is attributable to the internal electric field between the two polar (111) surfaces of the thin film. Only surface Zn vacancies can yield local magnetic moment for ZnS thin film and quantum dot, which is ascribed to the surface effect. Interactions between magnetic moments on S-3p states induced by hole-doping are responsible for the ferromagnetism observed experimentally in various ZnS samples.
Structure, magnetic, and transport properties of epitaxial ZnFe2O4 films: An experimental and first-principles study
Authors: Jin Chao , Li Peng , Mi Wenbo , Bai Haili
Published: June 2014

We investigated the structure, magnetic, and transport properties of ZnFe2O4 (ZFO) by both experimental and first-principles study. The epitaxial ZFO films prepared with various oxygen partial pressures show clear semiconducting behavior and room-temperature ferrimagnetism. A large magnetoresistance of −21.2% was observed at 75 K. The room-temperature ferrimagnetism is induced by the cation disordering. The calculated results indicate that under cation disordering, the ZFO with various oxygen vacancies is a half-metal semiconductor with both possible positive and negative signs of the spin polarization, while ZFO with no oxygen vacancies is an insulator and can be acted as the spin filter layer in spintronic devises.
Magnetization jumps and exchange bias induced by a partially disordered antiferromagnetic state in (FeTiO3)0.9-(Fe2O3)0.1
Authors: Song P. , Li G. K. , Ma L. , Zhen C. M. , Hou D. L. , Wang W. H. , Liu E. K. , Chen J. L. , Wu G. H.
Published: June 2014

Magnetization jumps (MJs) and the exchange bias (EB) effect are simultaneously observed in the mixed-spin oxide (FeTiO3)0.9-(Fe2O3)0.1 at 2.0 K. Dc and ac susceptibility measurements confirm a reentrant spin glass phase with a partially disordered antiferromagnetic (PDA) state below the irreversibility temperature (Tir = 60 K). Antiferromagnetic (AFM) Fe3+ clusters are nested in AFM Fe2+ lattices forming a triangular lattice, in which 2/3 of the magnetic moments order antiferromagnetically with each other leaving the remaining 1/3 “confused.” This geometric frustration in the triangular lattice leads to a PDA state that is the ground state of the AFM triangular configuration. The PDA state, in the presence of a critical trigger field, evolves into a ferromagnetic (FM) state, and induces the AFM spins of the Fe2+ ions to enter a FM state, resulting in the MJs. Meanwhile, the FM spins of Fe2+ can serve as the pinned phase, and the AFM spins of Fe3+ can serve as the pinning phase, resulting in the EB effect. Thus, we point out that the PDA state is very likely to be at the origin of the MJs and the EB effect.
Coexistence of toroidal and polar domains in ferroelectric systems: A strategy for switching ferroelectric vortex
Authors: Chen W. J. , Zheng Yue , Wang Biao , Liu J. Y.
Published: June 2014

Phase field simulations have been conducted to investigate the vortex domain structure in ferroelectric nanodots epitaxially grown on ferroelectric nanofilm. It is found that the nanodot region has great impact on the domain evolution of the whole system. At suitable conditions (e.g., geometry, size, temperature, external strain, and electric field), the nanodot adopts a vortex domain pattern while the nanofilm maintains a polar domain. Interestingly, the vortex domain pattern of the nanodot can be switched by controlling the direction of the adjacent polar domain. Our study indicates an alternative strategy for controlling the chirality of vortex domain structure in nano-ferroelectrics.
Optical properties of strain-free AlN nanowires grown by molecular beam epitaxy on Si substrates
Authors: Wang Q. , Zhao S. , Connie A. T. , Shih I. , Mi Z. , Gonzalez T. , Andrews M. P. , Du X. Z. , Lin J. Y. , Jiang H. X.
Published: June 2014

The optical properties of catalyst-free AlN nanowires grown on Si substrates by molecular beam epitaxy were investigated. Such nanowires are nearly free of strain, with strong free exciton emission measured at room temperature. The photoluminescence intensity is significantly enhanced, compared to previously reported AlN epilayer. Moreover, the presence of phonon replicas with an energy separation of ∼100 meV was identified to be associated with the surface-optical phonon rather than the commonly reported longitudinal-optical phonon, which is further supported by the micro-Raman scattering experiments.
Spin switches for compact implementation of neuron and synapse
Authors: Quang Diep Vinh , Sutton Brian , Behin-Aein Behtash , Datta Supriyo
Published: June 2014

Nanomagnets driven by spin currents provide a natural implementation for a neuron and a synapse: currents allow convenient summation of multiple inputs, while the magnet provides the threshold function. The objective of this paper is to explore the possibility of a hardware neural network implementation using a spin switch (SS) as its basic building block. SS is a recently proposed device based on established technology with a transistor-like gain and input-output isolation. This allows neural networks to be constructed with purely passive interconnections without intervening clocks or amplifiers. The weights for the neural network are conveniently adjusted through analog voltages that can be stored in a non-volatile manner in an underlying CMOS layer using a floating gate low dropout voltage regulator. The operation of a multi-layer SS neural network designed for character recognition is demonstrated using a standard simulation model based on coupled Landau-Lifshitz-Gilbert equations, one for each magnet in the network.
Multistage effect in enhancing the field emission behaviour of ZnO branched nanostructures
Authors: Sugavaneshwar R. P. , Nanda Karuna Kar
Published: June 2014

We report the synthesis of branched ZnO nanostructures by vapour phase transport and their multistage effect in enhancing the field emission behaviour. First, the ZnO nanowires (first generation) are grown and second generation nanowires are grown on first one and so on to obtain the branched structures. The number of branches increases and the diameter of the branches decreases till the third generation nanowires. Fourth generation onwards, dense branched structures are obtained eventually yielding nanoforest-like morphology. The field emission behaviour is found to improve till the third generation and is assigned to smaller diameter of the branches.
Mechanical stress effects on Pb(Zr,Ti)O3 thin-film ferroelectric capacitors embedded in a standard complementary metal-oxide-semiconductor process
Authors: Acosta Antonio G. , Rodriguez John A. , Nishida Toshikazu
Published: June 2014

We report experimental investigations of externally applied mechanical stress on 70 nm Pb(Zr,Ti)O3 ferroelectric capacitors embedded within a 130 nm complementary metal-oxide-semiconductor manufacturing process. An average increase in the remnant polarization of 3.37% per 100 MPa compressive uniaxial stress was observed. The maximum polarization increased by 2.68% per 100 MPa, while the cycling endurance was not affected by stress. The significant difference between experiment and the lattice distortion model suggests that two mechanisms are responsible for the polarization change. These results indicate that stress engineering may be used to enhance the signal margin in ferroelectric random access memory and enable technology scaling.
Strong piezoelectricity exhibited by large-grained BaTiO3 ceramics
Authors: Zhang J. L. , Ji P. F. , Wu Y. Q. , Zhao X. , Tan Y. Q. , Wang C. L.
Published: June 2014

A good understanding of the strong piezoelectric mechanism of BaTiO3 ceramics is very important from both scientific and technological viewpoints. This Letter reports an unusual piezoelectric phenomenon observed in a group of large-grained dense BaTiO3 ceramics with average grain sizes ranging from 5.1 to 16.3 μm, which are prepared with a hydrothermally synthesized BaTiO3 fine powder by hot-press sintering. Among these BaTiO3 ceramics, the one possessing an average grain size of 5.6 μm exhibits a particularly prominent piezoelectricity with piezoelectric constants d33 = 485 pC/N and d33* = 725 pm/V at room temperature and a peak d33 value of 574 pC/N around 8.5 °C. This ceramic is further investigated from the aspects of domain patterns and polarization versus electric field (P-E) loops and compared with a conventionally sintered BaTiO3 ceramic prepared using a solid-state reaction method. The investigation shows that it has the domain patterns of simple parallel stripes inside the grains and the square-like P-E loops and differs significantly from the conventionally sintered ceramic, and indicates that the observed strong piezoelectricity is strongly related to the large remanent polarization.
Coexistence of epitaxial lattice rotation and twinning tilt induced by surface symmetry mismatch
Authors: Qiao L. , Xiao H. Y. , Weber W. J. , Biegalski M. D.
Published: June 2014

Combined x-ray diffraction and first-principles studies of various epitaxial rutile-type metal dioxide films on Al2O3(0001) substrates reveal an unexpected rectangle-on-parallelogram heteroepitaxy. Unique matching of particular lattice spacings and crystal angles between the oxygen sublattices of Al2O3(0001) and the film(100) result in coexisted crystal rotation and lattice twinning inside the film. We demonstrate that, besides symmetry and lattice mismatch, angular mismatch along a specific crystal direction is also an important factor determining epitaxy. A generalized theorem has been proposed to explain epitaxial behaviors for tetragonal metal dioxides on Al2O3(0001).
Oscillatory motion of water droplets in kerosene above co-planar electrodes in microfluidic chips
Authors: Beránek Pavel , Flittner Rudolf , Hrobař Vlastimil , Ethgen Pauline , Přibyl Michal
Published: June 2014

We experimentally observed oscillatory motion of water droplets in microfluidic systems with coplanar microelectrodes under imposed DC electric fields. Two-electrode arrangement with no bipolar electrode and eight-electrode arrangement with six bipolar microelectrodes were investigated. Kerosene was used as the continuous phase. We studied the dependences of the oscillation frequency on the electric field intensity and ionic strength of the water phase. We found that the electric field dependence is strongly nonlinear and discussed possible reasons of this phenomenon, e.g., the droplet deformation at electrode edges that affects the charge transfer between the electrode and droplet or the interplay between the Coulomb force on free charge and the dielectrophoretic force. Our experiments further revealed that the oscillation frequency decreases with growing salt concentration in the two-electrode arrangement, but increases in the eight-electrode arrangement, which was attributed to surface tension related processes and electrochemical processes on the bipolar electrodes. Finally, we analyzed the effects of the electric field on the oscillatory motion by means of a simplified mathematical model. It was shown that the electric force imposed on the droplet charge is the key factor to induce the oscillations and the dielectrophoretic force significantly contributes to the momentum transfer at the electrode edges. For the same electric field strength, the model is able to predict the same oscillation frequency as that observed in the experiments.