bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of temporal laser profile on the emission spectra for underwater laser-induced breakdown spectroscopy: Study by short-interval double pulses with different pulse durations
Authors: Tamura Ayaka , Matsumoto Ayumu , Nakajima Takashi , Fukami Kazuhiro , Ogata Yukio H. , Nishi Naoya , Sakka Tetsuo
Published: January 2015

We investigate the effects of temporal laser profile on the emission spectra of laser ablation plasma in water. We use short-interval (76 ns) double pulses with different pulse durations of the composing two pulses for the irradiation of underwater target. Narrow atomic spectral lines in emission spectra are obtained by the irradiation, where the two pulses are wide enough to be merged into a single-pulse-like temporal profile, while deformed spectra are obtained when the two pulses are fully separated. The behavior of the atomic spectral lines for the different pulse durations is consistent with that of the temporal profiles of the optical emission intensities of the plasma. All these results suggest that continuous excitation of the plasma during the laser irradiation for ∼100 ns is a key to obtain narrow emission spectral lines.
Analysis of streamwise-oriented vortex interactions for two wings in close proximity
Authors: Barnes Caleb J. , Visbal Miguel R. , Gordnier Raymond E.
Published: January 2015

This investigation addresses the impingement of the trailing vortex provided by a leader-wing upon a follower-wing operating in close proximity. Exploration of the relative spacing between the two wings reveals several distinct flow regimes occur within a small range of lateral positions of the incident vortex. These changes effectively alter the evolution of the follower-wing wake via mutual induction between the incident and trailing vortices. Several unsteady mechanisms impact the general flow field in each regime. The incident vortex for an inboard impingement rapidly decays over the wing due to transition to turbulence. A tip-aligned vortex results in a highly unsteady interaction and generates enhanced surface pressure fluctuations beneath the tip vortex. Placing the incident vortex outboard elicits mutual instability between the leader and follower-wing trailing vortices. While lift-enhancement was found to be dominated by an inviscid increase in effective angle of attack, viscous effects in the near-tip region alter the local surface force distribution and influence the rolling moment coefficient. These flow variations which occur over a small range of lateral positions could generate buffeting loads in the presence of a wandering streamwise vortex.
Time-resolved x-ray diffraction and Raman studies of the phase transition mechanisms of methane hydrate
Authors: Hirai Hisako , Kadobayashi Hirokazu , Hirao Naohisa , Ohishi Yasuo , Ohtake Michika , Yamamoto Yoshitaka , Nakano Satoshi
Published: January 2015

The mechanisms by which methane hydrate transforms from an sI to sH structure and from an sH to filled-ice Ih structure were examined using time-resolved X-ray diffractometry (XRD) and Raman spectroscopy in conjunction with charge-coupled device camera observation under fixed pressure conditions. The XRD data obtained for the sI–sH transition at 0.8 GPa revealed an inverse correlation between sI and sH, suggesting that the sI structure is replaced by sH. Meanwhile, the Raman analysis demonstrated that although the 12-hedra of sI are retained, the 14-hedra are replaced sequentially by additional 12-hedra, modified 12-hedra, and 20-hedra cages of sH. With the sH to filled-ice Ih transition at 1.8 GPa, both the XRD and Raman data showed that this occurs through a sudden collapse of the sH structure and subsequent release of solid and fluid methane that is gradually incorporated into the filled-ice Ih to complete its structure. This therefore represents a typical reconstructive transition mechanism.
Unexpected methyl migrations of ethanol dimer under synchrotron VUV radiation
Authors: Xiao Weizhan , Hu Yongjun , Li Weixing , Guan Jiwen , Liu Fuyi , Shan Xiaobin , Sheng Liusi
Published: January 2015

While methyl transfer is well known to occur in the enzyme- and metal-catalyzed reactions, the methyl transfer in the metal-free organic molecules induced by the photon ionization has been less concerned. Herein, vacuum ultraviolet single photon ionization and dissociation of ethanol dimer are investigated with synchrotron radiation photoionization mass spectroscopy and theoretical methods. Besides the protonated clusters cation (C2H5OH) ⋅ H+ (m/z = 47) and the β-carbon-carbon bond cleavage fragment CH2O ⋅ (C2H5OH)H+ (m/z = 77), the measured mass spectra revealed that a new fragment (C2H5OH) ⋅ (CH3)+ (m/z = 61) appeared at the photon energy of 12.1 and 15.0 eV, where the neutral dimer could be vertically ionized to higher ionic state. Thereafter, the generated carbonium ions are followed by a Wagner-Meerwein rearrangement and then dissociate to produce this new fragment, which is considered to generate after surmounting a few barriers including intra- and inter-molecular methyl migrations by the aid of theoretical calculations. The appearance energy of this new fragment is measured as 11.55 ± 0.05 eV by scanning photoionization efficiency curve. While the signal intensity of fragment m/z = 61 starts to increase, the fragments m/z = 47 and 77 tend to slowly incline around 11.55 eV photon energy. This suggests that the additional fragment channels other than (C2H5OH) ⋅ H+ and CH2O ⋅ (C2H5OH)H+ have also been opened, which consume some dimer cations. The present report provides a clear description of the photoionization and dissociation processes of the ethanol dimer in the range of the photon energy 12-15 eV.
Orbital line-up at poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV)/poly(3-hexylthiophene) (P3HT) heterojunction
Authors: Wang Wenfeng , Schlaf Rudy
Published: January 2015

The poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV)/poly(3-hexylthiophene) (P3HT) heterojunction was investigated by photoemission spectroscopy. The orbital line-up at the MEH-PPV/P3HT heterojunction was determined based on the data of X-ray and ultra-violet photoemission measurements. The results were discussed with reference to the Induced Density of Interface States (IDIS) and integer charge transfer models. The predicted interface orbital line-up by IDIS is in agreement with measured energy alignment of the investigated interface. This suggests the IDIS model is also valid for polymer/polymer heterojunctions.
Surface-plasmon-enhanced lasing emission based on polymer distributed feedback laser
Authors: Zhang Dingke , Chen Shijian , Huang Yingzhou , Zhang Zhen , Wang Yanping , Ma Dongge
Published: January 2015

Optical losses associated with the metallic contacts necessary for charge injection are an obstacle to the development of electrically pumped organic lasers. In this work, we show that it is possible to overcome these losses by introducing surface plasmons (SPs) in a distributed feedback laser to enhance the lasing emission. We perform a detailed study of the SPs influence on the lasing emission. We experimentally show that enhanced lasing emission has been successfully achieved in the presence of a metal electrode. The laser emission is strongly dependent on the thickness of Ag layer. By optimizing the thickness of Ag layer, surface-plasmon-enhanced lasing emission has been achieved with much reduced thresholds and higher intensity. When the thickness of the Ag layer increases to 50 nm, the device exhibits ten-fold emission intensity and a fifth of excitation threshold comparing with Ag-free one. The finite-difference time-domain (FDTD) results show that large field intensity is built at the 4-(dicyanomethylene)-2-i-propyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran:/poly(9-vinylcarbazole)Ag interface, which could lead to a strong coupling between lasing and SPs, and consequently a much enhanced laser emission at the photon energy of around 2.02 eV (615 nm). Our FDTD simulations gave an explanation of the effects of the SPs on lasing operation in the periodic structures. The use of SPs would lead to a new class of highly efficient solid-state laser sources and provide a new path to achieve electrically pumped organic lasers.
Convergence of statistical moments of particle density time series in scrape-off layer plasmas
Authors: Kube R. , Garcia O. E.
Published: January 2015

Particle density fluctuations in the scrape-off layer of magnetically confined plasmas, as measured by gas-puff imaging or Langmuir probes, are modeled as the realization of a stochastic process in which a superposition of pulses with a fixed shape, an exponential distribution of waiting times, and amplitudes represents the radial motion of blob-like structures. With an analytic formulation of the process at hand, we derive expressions for the mean squared error on estimators of sample mean and sample variance as a function of sample length, sampling frequency, and the parameters of the stochastic process. Employing that the probability distribution function of a particularly relevant stochastic process is given by the gamma distribution, we derive estimators for sample skewness and kurtosis and expressions for the mean squared error on these estimators. Numerically, generated synthetic time series are used to verify the proposed estimators, the sample length dependency of their mean squared errors, and their performance. We find that estimators for sample skewness and kurtosis based on the gamma distribution are more precise and more accurate than common estimators based on the method of moments.
Ionospheric modifications in high frequency heating experiments
Authors: Kuo Spencer P.
Published: January 2015

Featured observations in high-frequency (HF) heating experiments conducted at Arecibo, EISCAT, and high frequency active auroral research program are discussed. These phenomena appearing in the F region of the ionosphere include high-frequency heater enhanced plasma lines, airglow enhancement, energetic electron flux, artificial ionization layers, artificial spread-F, ionization enhancement, artificial cusp, wideband absorption, short-scale (meters) density irregularities, and stimulated electromagnetic emissions, which were observed when the O-mode HF heater waves with frequencies below foF2 were applied. The implication and associated physical mechanism of each observation are discussed and explained. It is shown that these phenomena caused by the HF heating are all ascribed directly or indirectly to the excitation of parametric instabilities which instigate anomalous heating. Formulation and analysis of parametric instabilities are presented. The results show that oscillating two stream instability and parametric decay instability can be excited by the O-mode HF heater waves, transmitted from all three heating facilities, in the regions near the HF reflection height and near the upper hybrid resonance layer. The excited Langmuir waves, upper hybrid waves, ion acoustic waves, lower hybrid waves, and field-aligned density irregularities set off subsequent wave-wave and wave-electron interactions, giving rise to the observed phenomena.
Optimization methods of the net emission computation applied to cylindrical sodium vapor plasma
Authors: Hadj Salah S. , Hajji S. , Ben Hamida M. B. , Charrada K.
Published: January 2015

An optimization method based on a physical analysis of the temperature profile and different terms in the radiative transfer equation is developed to reduce the time computation of the net emission. This method has been applied for the cylindrical discharge in sodium vapor. Numerical results show a relative error of spectral flux density values lower than 5% with an exact solution, whereas the computation time is about 10 orders of magnitude less. This method is followed by a spectral method based on the rearrangement of the lines profile. Results are shown for Lorentzian profile and they demonstrated a relative error lower than 10% with the reference method and gain in computation time about 20 orders of magnitude.
Investigation of the cylinder wake under spanwise periodic forcing with a segmented plasma actuator
Authors: Bhattacharya S. , Gregory James W.
Published: January 2015

The wake response to three-dimensional forcing of flow over a circular cylinder was studied. Spanwise-segmented dielectric-barrier discharge plasma actuators were mounted on the cylinder in a square wave pattern for active forcing of the cylinder wake. The buried electrodes were placed periodically to create a spanwise-modulated blowing profile, with the aim of targeting three-dimensional instabilities in the wake. Considerable spanwise variation in the wake was achieved, which was a direct consequence of the difference in the location of shed spanwise vortices from the cylinder, along with the generation of streamwise vorticity. Two distinct power levels were used for forcing the flow, with different flow response observed between the two conditions. With low power, the segmented forcing caused the large-scale spanwise structures in the forcing region to lead those in the no-forcing region, with an accompanying shift away from the centerline and generation of streamwise vorticity. While vortex shedding was not substantially attenuated with low-power forcing, the shedding in the near wake was significantly attenuated with high-power forcing. This attenuation in the shedding strength was accompanied by a decrease in the peak shedding frequency, indicating an increase in the formation length. High-power forcing caused elongation of the Kármán vortices due to the induced strain field and strong differential development of the wake shedding frequency. In both forcing regimes, the wake three-dimensionality increased as shown by the increased width of the spectral peaks.
First-principles prediction of phononic thermal conductivity of silicene: A comparison with graphene
Authors: Gu Xiaokun , Yang Ronggui
Published: January 2015

There has been great interest in two-dimensional materials, beyond graphene, for both fundamental sciences and technological applications. Silicene, a silicon counterpart of graphene, has been shown to possess some better electronic properties than graphene. However, its thermal transport properties have not been fully studied. In this paper, we apply the first-principles-based phonon Boltzmann transport equation to investigate the thermal conductivity of silicene as well as the phonon scattering mechanisms. Although both graphene and silicene are two-dimensional crystals with similar crystal structure, we find that phonon transport in silicene is quite different from that in graphene. The thermal conductivity of silicene shows a logarithmic increase with respect to the sample size due to the small scattering rates of acoustic in-plane phonon modes, while that of graphene is finite. Detailed analysis of phonon scattering channels shows that the linear dispersion of the acoustic out-of-plane (ZA) phonon modes, which is induced by the buckled structure, makes the long-wavelength longitudinal acoustic phonon modes in silicene not as efficiently scattered as that in graphene. Compared with graphene, where most of the heat is carried by the acoustic out-of-plane (ZA) phonon modes, the ZA phonon modes in silicene only have ∼10% contribution to the total thermal conductivity, which can also be attributed to the buckled structure. This systematic comparison of phonon transport and thermal conductivity of silicene and graphene using the first-principle-based calculations shed some light on other two-dimensional materials, such as two-dimensional transition metal dichalcogenides.
Effect of the computational domain size and shape on the self-diffusion coefficient in a Lennard-Jones liquid
Authors: Kikugawa Gota , Ando Shotaro , Suzuki Jo , Naruke Yoichi , Nakano Takeo , Ohara Taku
Published: January 2015

In the present study, molecular dynamics (MD) simulations on the monatomic Lennard-Jones liquid in a periodic boundary system were performed in order to elucidate the effect of the computational domain size and shape on the self-diffusion coefficient measured by the system. So far, the system size dependence in cubic computational domains has been intensively investigated and these studies showed that the diffusion coefficient depends linearly on the inverse of the system size, which is theoretically predicted based on the hydrodynamic interaction. We examined the system size effect not only in the cubic cell systems but also in rectangular cell systems which were created by changing one side length of the cubic cell with the system density kept constant. As a result, the diffusion coefficient in the direction perpendicular to the long side of the rectangular cell significantly increases more or less linearly with the side length. On the other hand, the diffusion coefficient in the direction along the long side is almost constant or slightly decreases. Consequently, anisotropy of the diffusion coefficient emerges in a rectangular cell with periodic boundary conditions even in a bulk liquid simulation. This unexpected result is of critical importance because rectangular fluid systems confined in nanospace, which are present in realistic nanoscale technologies, have been widely studied in recent MD simulations. In order to elucidate the underlying mechanism for this serious system shape effect on the diffusion property, the correlation structures of particle velocities were examined.
Femtosecond laser-induced phase transformations in amorphous Cu77Ni6Sn10P7 alloy
Authors: Zhang Y. , Liu L. , Zou G. , Chen N. , Wu A. , Bai H. , Zhou Y.
Published: January 2015

In this study, the femtosecond laser-induced crystallization of CuNiSnP amorphous ribbons was investigated by utilizing an amplified Ti:sapphire laser system. X-ray diffraction and scanning electronic microscope were applied to examine the phase and morphology changes of the amorphous ribbons. Micromachining without crystallization, surface patterning, and selective crystallization were successfully achieved by changing laser parameters. Obvious crystallization occurred under the condition that the laser fluence was smaller than the ablation threshold, indicating that the structural evolution of the material depends strongly on the laser parameters. Back cooling method was used to inhibit heat accumulation; a reversible transformation between the disordered amorphous and crystalline phases can be achieved by using this method.
Two-dimensional control of field-driven magnetic bubble movement using Dzyaloshinskii–Moriya interactions
Authors: Petit Dorothée , Seem Peter R. , Tillette Marine , Mansell Rhodri , Cowburn Russell P.
Published: January 2015

The field-induced asymmetric growth of magnetic bubble domains in Pt/Co/Pt out-of-plane magnetized films with Dzyaloshinskii–Moriya interactions (DMI) is used to control the lateral displacement of bubbles. We demonstrate experimentally that we can laterally translate bubbles away from their nucleation site by applying a series of alternating 3-dimensional field pulses with a controlled relative sign between the out-of-plane and in-plane components. Using magneto optical Kerr effect imaging, the domain wall velocity as a function of applied field strength was measured from which the magnitude of the DMI field was estimated.
Peculiar long-range supercurrent in superconductor-ferromagnet-superconductor junction containing a noncollinear magnetic domain in the ferromagnetic region
Authors: Meng Hao , Wu Xiuqiang , Ren Yajie
Published: January 2015

We study the supercurrent in clean superconductor-ferromagnet-superconductor heterostructure containing a noncollinear magnetic domain in the ferromagnetic region. It is demonstrated that the magnetic domain can lead to a spin-flip scattering process, which reverses the spin orientations of the singlet Cooper pair and simultaneously changes the sign of the corresponding electronic momentum. If the ferromagnetic layers on both sides of magnetic domain have the same features, the long-range proximity effect will take place. That is because the singlet Cooper pair will create an exact phase-cancellation effect and gets an additional π phase shift as it passes through the entire ferromagnetic region. Then, the equal spin triplet pair only exists in the magnetic domain region and can not diffuse into the other two ferromagnetic layers. So, the supercurrent mostly arises from the singlet Cooper pairs, and the equal spin triplet pairs are not involved. This result can provide a approach for generating the long-range supercurrent.
A multi-cloak bifunctional device
Authors: Raza Muhammad , Liu Yichao , Ma Yungui
Published: January 2015

Invisibility cloak has attracted the attention of electromagnetic researchers due to its magical properties and marvelous potential applications in the field of applied physics and engineering. Recently, a multiphysics cloaking has put the new spirit into this field. In this paper, we introduce a device, composed of three shells and each shell works as an invisibility cloak for a specific physical phenomenon. Following this technique, a number of cloaks with different implementation approaches can be proposed for distinct physical phenomena in a single structure. Here, we restrict ourselves for the case of two physical behaviors: thermal and electrical conductivities. This type of multi-cloaking structure can be best used in mechanically designed structures to better control heating and electrical effects.
Growth and characterization of AlGaN/GaN/AlGaN double-heterojunction high-electron-mobility transistors on 100-mm Si(111) using ammonia-molecular beam epitaxy
Authors: Ravikiran L. , Dharmarasu N. , Radhakrishnan K. , Agrawal M. , Yiding Lin , Arulkumaran S. , Vicknesh S. , Ng G. I.
Published: January 2015

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrödinger equations. From simulations, an AlGaN buffer with “Al” mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (∼1013 cm−2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510 cm2/V.s and a sheet carrier concentration (ns) of 0.97 × 1013 cm−2 for the DH-HEMT structure, while they are 1310 cm2/V.s and 1.09 × 1013 cm−2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of 2DEG in the DH-HEMT heterostructure, which helped in the enhancement of its room temperature mobility. DH-HEMT showed 3 times higher buffer break-down voltage compared to SH-HEMT, while both devices showed almost similar drain current density. Small signal RF measurements on the DH-HEMT showed a unity current-gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of 22 and 25 GHz, respectively. Thus, overall, DH-HEMT heterostructure was found to be advantageous due to its higher buffer break-down voltages compared to SH-HEMT heterostructure.
Influence of metglas layer on nonlinear magnetoelectric effect for magnetic field detection by frequency modulation
Authors: Ma Jiashuai , Jiao Jie , Zhang Haiwu , Liu Yuting , Fang Cong , Lin Di , Zhao Xiangyong , Luo Haosu
Published: January 2015

Linear magnetoelectric (ME) and nonlinear magnetoelectric effects are considered systematically in this paper. We have prepared 3 multi-push-pull magnetoelectric laminated composites Metglas/PMNT/Metglas with n layers metglas (n = 1, 2, and 3). When n increases from 1 to 3, the linear magnetoelectric charge coefficients increase from 1900 to 2200 and then to 2600 pC/Oe at 2 Hz under their each optimal magnetic bias. By using these 3 ME composites, we have also prepared 3 ME sensors, the sensitivity of which are as low as 130, 168, and 199 pT/√Hz at 1 Hz for n = 1, 2, and 3, respectively, via nonlinear modulation method. Therefore, the nonlinear ME charge coefficients decrease as layer numbers of metglas increase, which is different with the ME charge linear coefficient's increasing trend, also demonstrated coupled with the finite element analysis method. The factors that result in the higher nonlinear ME charge coefficient in ME composites with fewer metglas layers are analyzed and discussed in detail.
Fast magnetic reconnection with large guide fields
Authors: Stanier A. , Simakov Andrei N. , Chacón L. , Daughton W.
Published: January 2015

In this letter, it is demonstrated using two-fluid simulations that low-β magnetic reconnection remains fast, regardless of the presence of fast dispersive waves, which have been previously suggested to play a critical role. To understand these results, a discrete model is constructed that offers scaling relationships for the reconnection rate and dissipation region (DR) thickness in terms of the upstream magnetic field and DR length. We verify these scalings numerically and show how the DR self-adjusts to process magnetic flux at the same rate that it is supplied to a larger region where two-fluid effects become important. The rate is therefore independent of the DR physics and is in good agreement with kinetic results.
Microturbulence study of the isotope effect
Authors: Bustos A. , Bañón Navarro A. , Görler T. , Jenko F. , Hidalgo C.
Published: January 2015

The influence of the ion mass on the dynamics of magnetized plasmas is an important challenge in fusion research. The discrepancies between the improvement of the magnetic confinement with the ion mass in tokamak experiments and diffusive turbulent transport predictions have remained unexplained for several decades. We refer to this phenomenon as the isotope effect. In this paper, we study this effect with gyrokinetic theory using the Gene code. We find several sets of plasma parameters that correspond to low wavenumber turbulence for which the isotope effect is present, although the intensity is smaller than the experimental observations. We also relate these results to the zonal flow intensity of the system, which is characterized by the average shear flow rate.
Comparisons between tokamak fueling of gas puffing and supersonic molecular beam injection in 2D simulations
Authors: Zhou Y. L. , Wang Z. H. , Xu X. Q. , Li H. D. , Feng H. , Sun W. G.
Published: January 2015

Plasma fueling with high efficiency and deep injection is very important to enable fusion power performance requirements. It is a powerful and efficient way to study neutral transport dynamics and find methods of improving the fueling performance by doing large scale simulations. Two basic fueling methods, gas puffing (GP) and supersonic molecular beam injection (SMBI), are simulated and compared in realistic divertor geometry of the HL-2A tokamak with a newly developed module, named trans-neut, within the framework of BOUT++ boundary plasma turbulence code [Z. H. Wang et al., Nucl. Fusion 54, 043019 (2014)]. The physical model includes plasma density, heat and momentum transport equations along with neutral density, and momentum transport equations. Transport dynamics and profile evolutions of both plasma and neutrals are simulated and compared between GP and SMBI in both poloidal and radial directions, which are quite different from one and the other. It finds that the neutrals can penetrate about four centimeters inside the last closed (magnetic) flux surface during SMBI, while they are all deposited outside of the LCF during GP. It is the radial convection and larger inflowing flux which lead to the deeper penetration depth of SMBI and higher fueling efficiency compared to GP.
Au-Cu2O core-shell nanowire photovoltaics
Authors: Oener S. Z. , Mann S. A. , Sciacca B. , Sfiligoj C. , Hoang J. , Garnett E. C.
Published: January 2015

Semiconductor nanowires are among the most promising candidates for next generation photovoltaics. This is due to their outstanding optical and electrical properties which provide large optical cross sections while simultaneously decoupling the photon absorption and charge carrier extraction length scales. These effects relax the requirements for both the minority carrier diffusion length and the amount of semiconductor needed. Metal-semiconductor core-shell nanowires have previously been predicted to show even better optical absorption than solid semiconductor nanowires and offer the additional advantage of a local metal core contact. Here, we fabricate and analyze such a geometry using a single Au-Cu2O core-shell nanowire photovoltaic cell as a model system. Spatially resolved photocurrent maps reveal that although the minority carrier diffusion length in the Cu2O shell is less than 1 μm, the radial contact geometry with the incorporated metal electrode still allows for photogenerated carrier collection along an entire nanowire. Current-voltage measurements yield an open-circuit voltage of 600 mV under laser illumination and a dark diode turn-on voltage of 1 V. This study suggests the metal-semiconductor core-shell nanowire concept could be extended to low-cost, large-scale photovoltaic devices, utilizing for example, metal nanowire electrode grids coated with epitaxially grown semiconductor shells.
Optical tomography of fluorophores in dense scattering media based on ultrasound-enhanced chemiluminescence
Authors: Kobayashi Masaki , Kikuchi Naoto , Sato Akihiro
Published: January 2015

This letter proposes and demonstrates ultrasound-combined optical imaging in dense scattering media. A peroxyoxalate chemiluminescence system that includes fluorophores to chemically excite the pigment is stimulated by ultrasound irradiation with power of less than 0.14 W/cm2. Using focused ultrasound, the chemiluminescence is selectively spatially enhanced, which leads to imaging of the pigment when embedded in a light-scattering medium via scanning of the focal point. The ultrasonically enhanced intensity of the chemiluminescence depends on the base intensity of the chemiluminescence without the applied ultrasound irradiation, which thereby enables quantitative determination of the fluorophore concentration. The authors demonstrate the potential of this method to resolve chemiluminescent targets in a dense scattering medium that is comparable to biological tissue. An image was acquired of a chemiluminescent target that included indocyanine green as the fluorophore embedded at a depth of 20 mm in an Intralipid-10% 200 ml/l solution scattering medium (the reduced scattering coefficient was estimated to be approximately 1.3 mm−1), indicating the potential for expansion of this technique for use in biological applications.
Stranski-Krastanov InN/InGaN quantum dots grown directly on Si(111)
Authors: Soto Rodriguez Paul E. D. , Aseev Pavel , Gómez Victor J. , Kumar Praveen , Ul Hassan Alvi Naveed , Calleja Enrique , Mánuel José M. , Morales Francisco M. , Jiménez Juan J. , García Rafael , Senichev Alexander , Lienau Christoph , Nötzel Richard
Published: January 2015

The authors discuss and demonstrate the growth of InN surface quantum dots on a high-In-content In0.73Ga0.27N layer, directly on a Si(111) substrate by plasma-assisted molecular beam epitaxy. Atomic force microscopy and transmission electron microscopy reveal uniformly distributed quantum dots with diameters of 10–40 nm, heights of 2–4 nm, and a relatively low density of ∼7 × 109 cm−2. A thin InN wetting layer below the quantum dots proves the Stranski-Krastanov growth mode. Near-field scanning optical microscopy shows distinct and spatially well localized near-infrared emission from single surface quantum dots. This holds promise for future telecommunication and sensing devices.
Communication: Theoretical prediction of free-energy landscapes for complex self-assembly
Authors: Jacobs William M. , Reinhardt Aleks , Frenkel Daan
Published: January 2015

We present a technique for calculating free-energy profiles for the nucleation of multicomponent structures that contain as many species as building blocks. We find that a key factor is the topology of the graph describing the connectivity of the target assembly. By considering the designed interactions separately from weaker, incidental interactions, our approach yields predictions for the equilibrium yield and nucleation barriers. These predictions are in good agreement with corresponding Monte Carlo simulations. We show that a few fundamental properties of the connectivity graph determine the most prominent features of the assembly thermodynamics. Surprisingly, we find that polydispersity in the strengths of the designed interactions stabilizes intermediate structures and can be used to sculpt the free-energy landscape for self-assembly. Finally, we demonstrate that weak incidental interactions can preclude assembly at equilibrium due to the combinatorial possibilities for incorrect association.
Thermally activated band conduction and variable range hopping conduction in Cu2ZnSnS4 thin films
Authors: Zubair Ansari Mohd , Khare Neeraj
Published: January 2015

The temperature dependence of electrical conductivity of the Cu2ZnSnS4 (CZTS) thin films has been measured in order to identify the dominant conduction mechanism. These CZTS film have been deposited by ultrasonic assisted chemical vapor deposition method at different substrate temperatures in a single step process. All the films exhibit p-type semiconducting behaviour. In the high temperature range (> 250 K), the dominance of thermally activated band conduction is observed, whereas in the lower temperature region (250−70 K), the hopping conduction is present. Detailed analysis of the temperature dependence of conductivity of the films reveals that in the temperature range from 250–170 K, nearest neighbour hopping dominates, whereas in the lower temperature region (170–70 K), the dominant hopping conduction is Mott's 3D variable range hopping and not Efros-Shklovskii variable range hopping. The value of Mott's temperature is found to decrease for the CZTS films deposited at higher temperature, which has been attributed to enhanced density of states at the Fermi level.
Externally controlled local magnetic field in a conducting mesoscopic ring coupled to a quantum wire
Authors: Maiti Santanu K.
Published: January 2015

In the present work, the possibility of regulating local magnetic field in a quantum ring is investigated theoretically. The ring is coupled to a quantum wire and subjected to an in-plane electric field. Under a finite bias voltage across the wire a net circulating current is established in the ring which produces a strong magnetic field at its centre. This magnetic field can be tuned externally in a wide range by regulating the in-plane electric field, and thus, our present system can be utilized to control magnetic field at a specific region. The feasibility of this quantum system in designing spin-based quantum devices is also analyzed.
Constraints for the aperiodic O-mode streaming instability
Authors: Lazar M. , Schlickeiser R. , Poedts S. , Stockem A. , Vafin S.
Published: January 2015

In plasmas, where the thermal energy density exceeds the magnetic energy density (β∥ > 1), the aperiodic ordinary mode (O-mode) instability is driven by an excess of parallel temperature A = T⊥/T∥ < 1 (where ∥ and ⊥ denote directions relative to the uniform magnetic field). When stimulated by parallel plasma streams, the instability conditions extend to low beta states, i.e., β∥ < 1, and recent studies have proven the existence of a new regime, where the anisotropy threshold decreases steeply with lowering β∥ → 0 if the streaming velocity is sufficiently high. However, the occurrence of this instability is questionable especially in the low-beta plasmas, where the electrostatic two-stream instabilities are expected to develop much faster in the process of relaxation of the counterstreams. It is therefore proposed here to identify the instability conditions for the O-mode below those required for the onset of the electrostatic instability. A hierarchy of these two instabilities is established for both the low β∥ < 1 and large β∥ > 1 plasmas. The conditions where the O-mode instability can operate efficiently are markedly constrained by the electrostatic instabilities especially in the low-beta plasmas.
Diffuse phase ferroelectric vs. Polomska transition in (1-x) BiFeO3-(x) Ba Zr0.025Ti0.975O3 (0.1 ≤ x ≤ 0.3) solid solutions
Authors: Jha Pardeep K. , Jha Priyanka A. , Singh Vikash , Kumar Pawan , Asokan K. , Dwivedi R. K.
Published: January 2015

Investigations on the solid solutions (1-x) BiFeO3 - (x) Ba Zr0.025Ti0.975O3 (0.1 ≤ x ≤ 0.3) in the temperature range 300–750 K show colossal permittivity behavior and the occurrence of diffuse phase ferroelectric transition along with frequency dependent anomaly which disappears at temperature ∼450 K. For x = 0.3, these anomalies have been verified through differential scanning calorimetry and dielectric/impedance/conductivity measurements. The occurrence of peak in pyrocurrent (dPs/dT) vs. T plots also supports phase transition. With the increasing x, transition temperature decreases and diffusivity increases. This anomaly is absent at high frequencies (>100 kHz) in conductivity plots, indicating Polomska like surface phase transition, which is supported by modulus study.
Mechanism of photochromic effect in Pb(Zr,Ti)O3 and (Pb,La)(Zr,Ti)O3 ceramics under violet/infrared light illumination
Authors: Xu Caixia , Zhang Jingwen , Xu Long , Zhao Hua
Published: January 2015

Obvious photochromic effects were observed in Pb(Zr,Ti)O3 and (Pb,La)(Zr,Ti)O3 (PLZT) ceramics, along with exponential responses to illumination power in both darkening and bleaching processes. An interesting anomalous dispersion in the transparent PLZT was observed and discussed. A tentative physical picture based on photoinduced electron stimulated processes and on structural change was proposed to explain all the interesting observations. Rate equations were established and solved in verifying the validity of the proposed model. This work may serve as guidance in designing tunable achromatic lenses, UV and IR light detectors and sensors.
Resistance noise at the metal–insulator transition in thermochromic VO2 films
Authors: Topalian Zareh , Li Shu-Yi , Niklasson Gunnar A. , Granqvist Claes G. , Kish Laszlo B.
Published: January 2015

Thermochromic VO2 films were prepared by reactive DC magnetron sputtering onto heated sapphire substrates and were used to make 100-nm-thick samples that were 10 μm wide and 100 μm long. The resistance of these samples changed by a factor ∼2000 in the 50 < Ts < 70 °C range of temperature Ts around the “critical” temperature Tc between a low-temperature semiconducting phase and a high-temperature metallic-like phase of VO2. Power density spectra S(f) were extracted for resistance noise around Tc and demonstrated unambiguous 1/f behavior. Data on S(10 Hz)/Rs2 scaled as Rsx, where Rs is sample resistance; the noise exponent x was −2.6 for Ts < Tc and +2.6 for Ts > Tc. These exponents can be reconciled with the Pennetta–Trefán–Reggiani theory [Pennetta et al., Phys. Rev. Lett. 85, 5238 (2000)] for lattice percolation with switching disorder ensuing from random defect generation and healing in steady state. Our work hence highlights the dynamic features of the percolating semiconducting and metallic-like regions around Tc in thermochromic VO2 films.
Thermal stability of Ni/Ti/Al ohmic contacts to p-type 4H-SiC
Authors: Yu Hailong , Zhang Xufang , Shen Huajun , Tang Yidan , Bai Yun , Wu Yudong , Liu Kean , Liu Xinyu
Published: January 2015

Low resistivity Ni/Ti/Al ohmic contacts on p-type 4H-SiC epilayer were developed, and their thermal stabilities were also experimentally investigated through high temperature storage at 600 °C for 100 h. The contact resistance of the Al/Ti/Ni/SiC contacts degraded in different degrees, and the contact morphology deteriorated with the increases of the average surface roughness and interface voids. X-ray spectra showed that Ni2Si and Ti3SiC2, which were formed during ohmic contact annealing and contributed to low contact resistivity, were stable under high temperature storage. The existence of the TiAl3 and NiAl3 intermetallic phases was helpful to prevent Al agglomeration on the interface and make the contacts thermally stable. Auger electron spectroscopy indicated that the incorporation of oxygen at the surface and interface led to the oxidation of Al or Ti resulting in increased contact resistance. Also, the formation of these oxides roughened the surface and interface. The temperature-dependence of the specific contact resistance indicated that a thermionic field emission mechanism dominates the current transport for contacts before and after the thermal treatment. It suggests that the Ni/Ti/Al composite ohmic contacts are promising for SiC devices to be used in high temperature applications.
Effect of SnO2 concentration on the tuning of optical and electrical properties of ZnO-SnO2 composite thin films
Authors: Rakshit Tamita , Manna Indranil , Ray Samit K.
Published: January 2015

ZnO-SnO2 composite thin films have been deposited at 400 °C on glass substrates using targets of different SnO2 content (1 to 40 wt. %) by pulsed laser deposition technique. The structural, optical, and electrical properties of the composite films have been studied as a function of SnO2 content. It is revealed from X-ray diffraction analysis that films are crystalline in nature and the crystallite size decreases from 20–23 nm to 5–7 nm with increase of SnO2 content. X-ray photoelectron spectroscopy analysis indicates that Sn is predominantly doped into the ZnO lattice upto a SnO2 content of 15 wt. % in the composite. For higher concentration, a separate SnO2 phase is segregated in the composite. The band gap energy as well as the electrical conductivity can be tuned by varying the SnO2 content in the composite. Low temperature electrical conductivity measurements show three dominant conduction mechanisms in the temperature range of 20–300 K. At high temperature range of 200–300 K, thermal activation conduction process is dominant. Nearest neighbor hopping conduction mechanism, which occurs in the shallow impurity bands, is dominant in the temperature range of 90–200 K. In the low temperature range of 20–90 K, the electronic transport occurs through Mott's variable range hopping conduction process.
Transverse wave propagation in viscoelastic single-walled carbon nanotubes with small scale and surface effects
Authors: Pang M. , Zhang Y. Q. , Chen W. Q.
Published: January 2015

The general governing equation of transverse wave motion in a viscoelastic single-walled carbon nanotube (SWCNT) adhered by surface material is formulated on the basis of the nonlocal elasticity theory and the Kelvin model. The properties of transverse wave propagation in the SWCNT are investigated. The explicit expressions are derived for the frequency and phase velocity of the wave motion. The small scale and surface effects and the influences of structural damping on the properties of wave propagation are elucidated. It is concluded that the frequency and phase velocity of transverse wave propagation in the viscoelastic SWCNT are related to the small scale, surface elasticity, residual surface tension, and structural damping. The small scale and surface effects and the impact of structural damping on the properties of transverse wave propagation are dependent upon the wave number and tube diameter.
Attosecond extreme ultraviolet generation in cluster by using spatially inhomogeneous field
Authors: Feng Liqiang , Liu Hang
Published: January 2015

A promising method to generate the attosecond extreme ultraviolet (XUV) sources has been theoretically investigated emerging from the two-dimensional Ar+ cluster driven by the spatially inhomogeneous field. The results show that with the introduction of the Ar+ cluster model, not only the harmonic cutoffs are enhanced, but also the harmonic yields are reinforced. Furthermore, by properly moderating the inhomogeneity as well as the laser parameters of the inhomogeneous field, the harmonic cutoff can be further extended. As a result, three almost linearly polarized XUV pulses with durations of 40 as, 42 as, and 45 as can be obtained.
Vacuum ultraviolet photochemical selective area atomic layer deposition of Al2O3 dielectrics
Authors: Chalker P. R. , Marshall P. A. , Dawson K. , Brunell I. F. , Sutcliffe C. J. , Potter R. J.
Published: January 2015

We report the photochemical atomic layer deposition of Al2O3 thin films and the use of this process to achieve area-selective film deposition. A shuttered vacuum ultraviolet (VUV) light source is used to excite molecular oxygen and trimethyl aluminum to deposit films at 60°C. In-situ QCM and post-deposition ellipsometric measurements both show that the deposition rate is saturative as a function of irradiation time. Selective area deposition was achieved by projecting the VUV light through a metalized magnesium fluoride photolithographic mask and the selectivity of deposition on the illuminated and masked regions of the substrate is a logarithmic function of the UV exposure time. The Al2O3 films exhibit dielectric constants of 8 – 10 at 1 MHz after forming gas annealing, similar to films deposited by conventional thermal ALD.
Minimal graphene thickness for wear protection of diamond
Authors: van Wijk M. M. , Fasolino A.
Published: January 2015

We show, by means of molecular dynamics simulations, that the transformation from diamond to amorphous carbon occurring while sliding under pressure can be prevented by having at least two graphene layers between the diamond slabs. The resulting reduction of wear makes this combination of materials suitable for new coatings and micro- and nanoelectromechanical devices. Grain boundaries, vacancies and steps on the diamond surface do not change this prediction. We attribute this behavior to the bonding in layered materials like graphene. The strong in-plane bonding and the weak interlayer interaction that evolves to a strong interlayer repulsion under pressure prevent the transition to amorphous carbon when more than one layer is present.
Highly sensitive sensors based on magneto-optical surface plasmon resonance in Ag/CeYIG heterostructures
Authors: Qin Jun , Deng Longjiang , Xie Jianliang , Tang Tingting , Bi Lei
Published: January 2015

In this paper we present a new magneto-optical surface plasmon resonance (MOSPR) sensor using a dielectric magneto-optical thin film of Ce1Y 2Fe5O12 (CeYIG) and a metallic film of Ag. We demonstrate that by combining the low optical loss of Ag (ε =-62.485+4.331i) and the strong magneto-optical effect of CeYIG (ΦF = 2 × 104 deg/cm) at ∼1000 nm wavelength, a high figure of merit (FoM) for index sensing can be achieved. The device shows a factor of 20 improvement in the index resolution in comparison with ferromagnetic metal based MOSPR sensors at the same wavelength. By carrying out both analytical and numerical simulations, an optimized Ag/CeYIG MOSPR structure with the highest index sensitivity is achieved at the incident light wavelength of 1160 nm and the CeYIG thickness of 25 nm. The enhanced resolution of the DMOSPR sensor has been attributed to both the field enhancement at the sensing interface due to the high index of CeYIG films and, the high magneto-optical figure of merit of the CeYIG near infrared wavelength range.