Index of content:
Volume 118, Issue 20, 28 November 2015
Analytical studies on the size effects of a simply-shaped beam fixed at both ends have successfully explained the sudden changes of effective Young's modulus as its diameter decreases below 100 nm. Yet they are invalid for complex nanostructures ubiquitously existing in nature. In accordance with a generalized Young-Laplace equation, one of the representative size effects is transferred to non-uniformly distributed pressure against an external surface due to the imbalance of inward and outward loads. Because the magnitude of pressure depends on the principal curvatures, iterative steps have to be adopted to gradually stabilize the structure in finite element analysis. Computational results are in good agreement with both experiment data and theoretical prediction. Furthermore, the investigation on strengthened and softened Young's modulus for two complex nanostructures demonstrates that the proposed computational method provides a general and effective approach to analyze the size effects for nanostructures in arbitrary shape.
- Photonics, Plasmonics, Lasers, and Optical Phenomena
Omnidirectional polarization-insensitive tunable absorption in graphene metamaterial of nanodisk structure118(2015); http://dx.doi.org/10.1063/1.4936222View Description Hide Description
Tunable absorption based on graphenemetamaterial with nanodisk structure at near-infrared frequency was investigated using the finite difference time domain method. The absorption of the nanodisk structure which consisting of Au-MgF2-graphene-Au-polyimide (from bottom to top) can be tuned by the chemical potential of graphene at certain diameter of nanodisk. The permittivity of graphene is discussed with different chemical potential to obtain tunable absorption. It is shown that the increased value of the chemical potential of graphene can lead to blue-shifted of the absorption peaks and the values decreased. Moreover, dual-band and triple-band absorption can be achieved for resonance frequencies at normal incidence. Compared with diameter of nanodisks, the multilayerstructure shows multi-band absorber, and an omnidirectional absorption at 195.25 THz is insensitive to TE/TM polarization. This omnidirectional polarization insensitive absorption may be applied by optical communications such as optical absorber, near infrared stealth, and filter.
118(2015); http://dx.doi.org/10.1063/1.4936200View Description Hide Description
Needle-like electromagnetic field has various advantages for the applications in high-resolution imaging, Raman spectroscopy, as well as long-distance optical transportation. The realization of such field often requires high numerical aperture (NA) objective lens and the transmission masks. We demonstrate an ultralong needle-like focus in the optical range produced with an ordinary lens. This is achieved by focusing a symmetric Airy beam (SAB) generated via binary spectral modulation with a digital micromirror device. Such amplitude modulation technique is able to shape traditional Airy beams, SABs, as well as the dynamic transition modes between the one-dimensional and two-dimensional (2D) symmetric Airy modes. The created 2D SAB was characterized through measurement of the propagating fields with one of the four main lobes blocked by an opaque mask. The 2D SAB was verified to exhibit self-healing property against propagation with the obstructed major lobe reconstructed after a certain distance. We further produced an elongated focal line by concentrating the SAB via lenses with different NAs and achieved an ultralong longitudinal needle focus. The produced long needle focus will be applied in optical, chemical, and biological sciences.
118(2015); http://dx.doi.org/10.1063/1.4936322View Description Hide Description
We have developed a spectralmodel for describing the shape of the emission spectrum of InGaAlP-based red light-emitting diodes(LEDs) with quantum-well structure. The model is based on Maxwell-Boltzmann distribution with junctiontemperatureTj and an experimental two-dimensional joint density of states (DOS). We model the DOS with a sum of two exponentially broadened step functions describing the two lowest sub-bands in semiconductor quantum well. The relative locations = 0 meV and = 112.7 meV above the band gapenergyEg = 1.983 eV and the ratio 2.13 of the step heights were fixed using an experimental DOS extracted from a LEDspectrummeasured at known Tj and driving current I. The model can then be fitted to other spectra of other LED samples at varied Tj and I by varying the fitting parameters , and the broadening of the sub-band edges. The model was tested for three LED samples over I = 200–370 mA and Tj = 303–398 K. Junctiontemperatures obtained by modeling were compared with calibrated Tj obtained by the forward voltage method. The mean absolute difference was about 2.9 K (0.8%) over the whole region studied and the maximum difference was 8.5 K. The thermal coefficient measured for Eg was −0.509 meV K−1. For the first and second sub-band edges, the thermal broadening coefficients were 18 μeV K−1 and 37 μeV K−1, respectively.
Surface-enhanced Raman scattering activity of niobium surface after irradiation with femtosecond laser pulses118(2015); http://dx.doi.org/10.1063/1.4936363View Description Hide Description
The chemical modification of the niobium(Nb)surface after irradiation with femtosecond laser pulses was investigated by scanning electron microscopy coupled with energy dispersive spectroscopy, atomic force microscopy, grazing incidence X-ray diffraction, and micro-Raman spectroscopy. The physical-chemical analyses indicated that the laser treatment results in oxidation of the Nbsurface, as well as in the formation of Nb hydrides. Remarkably, after the samples' washing in ethanol, a strong Surface-Enhanced Raman Scattering(SERS) signal originating from the toluene residual traces was evidenced. Further, it was observed that the laser irradiatedNbsurface is able to provide a SERS enhancement of ∼1.3 × 103 times for rhodamine 6G solutions. Thus, for the first time it was shown that Nb/Nb oxide surfaces could exhibit SERS functionality, and so one can expect applications in biological/biochemical screening or for sensing of dangerous environmental substances.
- Electrical Discharges, Plasmas, and Plasma-Surface Interactions
Propagation characteristics of atmospheric-pressure He+O2 plasmas inside a simulated endoscope channel118(2015); http://dx.doi.org/10.1063/1.4936301View Description Hide Description
Cold atmospheric-pressure plasmas have potential to be used for endoscope sterilization. In this study, a long quartz tube was used as the simulated endoscope channel, and an array of electrodes was warped one by one along the tube. Plasmas were generated in the inner channel of the tube, and their propagation characteristics in He+O2 feedstock gases were studied as a function of the oxygen concentration. It is found that each of the plasmas originates at the edge of an instantaneous cathode, and then it propagates bidirectionally. Interestingly, a plasma head with bright spots is formed in the hollow instantaneous cathode and moves towards its center part, and a plasma tail expands through the electrode gap and then forms a swallow tail in the instantaneous anode. The plasmas are in good axisymmetry when [O2] ≤ 0.3%, but not for [O2] ≥ 1%, and even behave in a stochastic manner when [O2] = 3%. The antibacterial agents are charged species and reactive oxygen species, so their wall fluxes represent the “plasma dosage” for the sterilization. Such fluxes mainly act on the inner wall in the hollow electrode rather than that in the electrode gap, and they get to the maximum efficiency when the oxygen concentration is around 0.3%. It is estimated that one can reduce the electrode gap and enlarge the electrode width to achieve more homogenous and efficient antibacterial effect, which have benefits for sterilization applications.
- Magnetism, Spintronics, and Superconductivity
118(2015); http://dx.doi.org/10.1063/1.4936178View Description Hide Description
Superconducting magnetic bearings are normally built with bulk superconductors. Since coated conductors properties are far superior, we have proposed in a previous work the replacement of bulks for stacks of 2G wires in magnetic levitation devices. A major limitation of this replacement lies in the fact that the induced current is constrained in narrow loops along the available commercial widths of 2G wires. This work presents a technique to achieve wider loops of persistent current without the need of increasing the coated conductors width. As a result, the use of 2G wires in magnetic bearings took a step towards its economical feasibility.
Effect of oxygen content of Nd–Fe–B sintered magnet on grain boundary diffusion process of DyH2 dip-coating118(2015); http://dx.doi.org/10.1063/1.4936172View Description Hide Description
We investigated the effect of oxygen content on the microstructural and magnetic properties of a DyH2 dip-coated Nd–Fe–B sinteredmagnet. When the magnet had a low oxygen content (1500 ppm), the volume and size of the rare-earth-rich oxide (Nd–Dy–O) phase was reduced, and a uniform and continuous thin Nd-rich grain boundary phase (GBP) was well developed. The grain boundarydiffusion depth of Dy increased from 200 to 350 μm with decreasing oxygen content from ∼3000 to 1500 ppm. The coercivity of the low-oxygen magnet increased from 19.98 to 23.59 kOe after grain boundarydiffusion process (GBDP) while the remanence reduction was minimized. The formation of an fcc-NdOx Nd-rich phase in the high-oxygen magnet hindered the formation of a Nd-rich triple-junction phase and GBP. In contrast, a metallic dhcp-Nd phase, which was closely related to coercivity enhancement after GBDP, was formed in the low-oxygen magnet.
118(2015); http://dx.doi.org/10.1063/1.4936199View Description Hide Description
We report the static and dynamic magnetic characteristics of a high-layer-number NiFe/FeMn multilayer test structure with potential applications in broadband absorber and filter devices. To allow fine control over the absorption linewidths and to understand the mechanisms governing the resonances in a tailored structure similar to that expected to be used in real world applications, the multilayer was intentionally designed to have layer thickness and interface roughness variations. Magnetometry measurements show that the sample has complex hysteresis loops with features consistent with single ferromagnetic film reversals. Characterisation by transmission electron microscopy allows us to correlate the magnetic properties with structural features, including the film widths and interface roughnesses. Analysis of resonance frequencies from broadband ferromagnetic resonance measurements as a function of field magnitude and orientation provide values of the local exchange bias, rotatable anisotropy, and uniaxial anisotropy fields for specific layers in the stack and explain the observed mode softening. The linewidths of the multilayer are adjustable around the bias field, approaching twice that seen at larger fields, allowing control over the bandwidth of devices formed from the structure.
118(2015); http://dx.doi.org/10.1063/1.4936219View Description Hide Description
We investigate the non-linear dynamics of superparamagnetic beads moving around the periphery of patterned magnetic disks in the presence of an in-plane rotating magnetic field. Three different dynamical regimes are observed in experiments, including (1) phase-locked motion at low driving frequencies, (2) phase-slipping motion above the first critical frequency fc1, and (3) phase-insulated motion above the second critical frequency fc2. Experiments with Janus particles were used to confirm that the beads move by sliding rather than rolling. The rest of the experiments were conducted on spherical, isotropic magnetic beads, in which automated particle position tracking algorithms were used to analyze the bead dynamics. Experimental results in the phase-locked and phase-slipping regimes correlate well with numerical simulations. Additional assumptions are required to predict the onset of the phase-insulated regime, in which the beads are trapped in closed orbits; however, the origin of the phase-insulated state appears to result from local magnetization defects. These results indicate that these three dynamical states are universal properties of bead motion in non-uniform oscillators.
118(2015); http://dx.doi.org/10.1063/1.4936175View Description Hide Description
Temperature, current-direction, and film-thickness dependent anisotropic magnetoresistance measurements were performed on single-crystalline face-centered-cubic nickelfilms. An additional four-fold symmetry was confirmed besides the typical two-fold term even at room temperature. The angular-dependent longitudinal resistivity resolves into a two-fold term, which varies as a function of current direction, and a four-fold term, which is isotropically independent of current direction. The experimental results are interpreted well using an expression based on the phenomenological model. Both the two- and four-fold terms vary inversely proportional to film thickness, indicating that interfacial scattering can significantly influence the spin-dependent transport properties.
Brillouin light scattering study of transverse mode coupling in confined yttrium iron garnet/barium strontium titanate multiferroic118(2015); http://dx.doi.org/10.1063/1.4936320View Description Hide Description
Using the space-resolved Brillouin light scattering spectroscopy we study the transformation of dynamic magnetization patterns in a bilayer multiferroic structure. We show that in the comparison with a single yttriumirongarnet (YIG) filmmagnetization distribution is transformed in the bilayer structure due to the coupling of waves propagating both in an YIG film(magnetic layer) and in a barium strontium titanate slab (ferroelectric layer). We present a simple electrodynamic model using the numerical finite element method to show the transformation of eigenmode spectrum of confined multiferroic. In particular, we demonstrate that the control over the dynamic magnetization and the transformation of spatial profiles of transverse modes in magnetic film of the bilayer structure can be performed by the tuning of the wavevectors of transverse modes. The studied confined multiferroic stripe can be utilized for fabrication of integrated dual tunable functional devices for magnonic applications.
118(2015); http://dx.doi.org/10.1063/1.4936365View Description Hide Description
The effect of N incorporation on the structure and magnetic properties of CoPt thin filmsdeposited on glass substrates with TiN seed layers has been investigated. During the deposition of CoPt, introducing 20% N2 into Ar atmosphere promotes the (001) texture and enhances the perpendicular coercivity of CoPt film compared with the filmdeposited in pure Ar and post-annealed under the same conditions. From the in situx-ray diffraction results, it is confirmed that N incorporation expands the lattice parameter of CoPt, which favors the epitaxialgrowth of CoPt on TiN. During the post-annealing process, N releases from CoPt film and promotes the L10 ordering transformation of CoPt.
Site preference and magnetic properties of Ga/In-substituted strontium hexaferrite: An ab initio study118(2015); http://dx.doi.org/10.1063/1.4936368View Description Hide Description
The first-principles density functional theory has been used to study Ga/In-substituted strontium hexaferrite (SrFe12O19). Based on the calculation of the substitution energy of Ga and In in SrFe12O19 and the formation probability analysis, we conclude that in SrFe12−xGaxO19 the substituted Ga atoms prefer to occupy the 12k, 2a, and 4f1 sites, while In atoms in SrFe12−xInxO19 occupy the 12k, 4f2, and 4f1 sites. We used the site occupation probabilities to calculate the magnetic properties of the substituted SrFe12O19. It was found that as the fraction of Ga atoms in SrFe12−xGaxO19 increases, the saturation magnetization (Ms) as well as magnetic anisotropyenergy (MAE) decrease, while the anisotropy field (Ha) increases. In the case of SrFe12−xInxO19, Ms, MAE, and Ha decrease with an increase of the concentration of In atoms.
Critical current density and microstructure of iron sheathed multifilamentary Sr1−xKxFe2As2/Ag composite conductors118(2015); http://dx.doi.org/10.1063/1.4936370View Description Hide Description
Iron-based superconductors have been considered to be very promising in high-field applications for which multifilamentary wire and tape conductors with high mechanical strength are essential. In this work, 7-,19-, and 114-filament Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires and tapes with silver as matrix and iron as outer reinforcing sheath were produced by the ex situ powder-in-tube method. The mass densities of Sr-122 phase in 7- and 19-filament conductors were investigated by microhardness characterization, which revealed a positive correlation between hardness and transport critical current density (Jc) in round wires and flat tapes with various thicknesses. For Sr-122/Ag/Fe 114-filament conductors, in which an average cross-sectional filament size smaller than 50 μm was achieved by drawing into round wires of 2.0 mm in diameter, the transport Jc can be significantly enhanced by flat rolling, as for the 7- and 19-filament conductors. The highest transport Jc for Sr-122 filaments in the 7-, 19-, and 114-filament Sr-122/Ag/Fe tapes reached 1.4 × 104, 8.4 × 103, and 6.3 × 103 A cm−2 (4.2 K, 10 T), respectively, showing a Jc degradation with the increase of filament number. This Jc degradation can be ascribed to the sausage effect for SR-122 filaments in longitudinal direction and the grain refinement in these very fine filaments.
- Dielectrics, Ferroelectrics, and Multiferroics
Self-consistent drift-diffusion-reaction model for the electron beam interaction with dielectric samples118(2015); http://dx.doi.org/10.1063/1.4936201View Description Hide Description
The charging of insulating samples degrades the quality and complicates the interpretation of images in scanning electron microscopy and is important in other applications, such as particle detectors. In this paper, we analyze this nontrivial phenomenon on different time scales employing the drift-diffusion-reaction approach augmented with the trapping rate equations and a realistic semi-empirical source function describing the pulsed nature of the electron beam. We consider both the fast processes following the impact of a single primary electron, the slower dynamics resulting from the continuous bombardment of a sample, and the eventual approach to the steady-state regime.
Device level optimization of poly(vinylidene fluoride-trifluoroethylene)–zinc oxide polymer nanocomposite thin films for ferroelectric applications118(2015); http://dx.doi.org/10.1063/1.4936203View Description Hide Description
Polymernanocomposite was prepared using poly(vinylidene fluoride-trifluoroethylene) and zinc oxide (ZnO) nanopowder, which are ferroelectric in nature. Nanocomposite was prepared in various concentrations(0.2, 0.4, 0.8, and 1 wt. %) using probe ultra-sonication, followed by spin coating and annealing at 120 °C for 2 h to improve the formation of β-phase. Metal-ferroelectric-metal capacitor was fabricated using this optimized thin film as a ferroelectric layer. Device level optimization was carried out by polarization-electric field (P-E) hysteresis studies of this film, which shows polarization enhancement of composite. Various characterization techniques like atomic force microscopy, Fourier transform infra-red spectroscopy (FT-IR), Differential scanning calorimetry, and X-ray diffraction were used to study the β-phase formation of nancomposite. The capacitance–voltage (C-V) and current-voltage (I-V) characteristics were studied through varying frequency and temperature. C-V measurements show an increase of 79% in the capacitance of polymernanocomposite, which can be used for the fabrication of ferroelectric devices.
118(2015); http://dx.doi.org/10.1063/1.4936306View Description Hide Description
An intuitively persistent enhancement of the local surface potential of BiFeO3 layers in both heterostructures of BiFeO3/SrRuO3/SrTiO3 and BiFeO3/Sr0.09Nb0.01TiO3 was observed by the Kelvin probe force microscopy technique after the illumination of 375 nm laser. This photo-induced enhanced surface potential can maintain as long as 15 h after the illumination. We attributed this super-long-time relaxation of photo-induced influence to a photo-induced depolarization in the BiFeO3 thin films, and our first-principles calculation of double-potential well further provides an instinct understanding on this depolarization process. Our findings provide a peculiar understanding into the photo-induced phenomena on the widely researched ferroelectric systems and offer an approach to tune their multifunctionality of the magnetization and polarization not only by applied magnetic and electric fields but also by optical filed.
Ca doping dependence of resistive switching characteristics in ferroelectric capacitors comprising Ca-doped BiFeO3118(2015); http://dx.doi.org/10.1063/1.4936308View Description Hide Description
We have investigated the transport and ferroelectricproperties of ferroelectriccapacitors comprising Ca-doped BiFeO3 (BFO) to elucidate the correlation between resistive switching and ferroelectricity. A capacitor consisting of Ca-doped (3.6 at. %) BFO film exhibited polarization–voltage hysteresis, indicating ferroelectricity of the film. As the Ca-doping ratio was increased, the leakage current increased, and zero-crossing hysteretic current–voltage characteristics, i.e., bipolar resistive switching, were observed in capacitors consisting of Ca-doped BFO films with doping ratios of 6.4–13 at. %. A capacitor consisting of a highly Ca-doped BFO (23 at. %) film showed neither resistive switching nor ferroelectric behavior. Distinct changes in the retention and pulsed-voltage-induced resistive switching characteristics were observed around a Ca-doping ratio of 9.0 at. %. The Ca-doping dependence of the resistive switching appeared to correlate with the ferroelectric phase diagram of the Ca-doped BFO films [Yang et al., Nat. Mater. 8, 485 (2009)].
Interfacial synthesis of polypyrrole microparticles for effective dissipation of electromagnetic waves118(2015); http://dx.doi.org/10.1063/1.4936549View Description Hide Description
A strategy has been adopted to regulate the dielectric properties of polypyrrole microparticles for good electromagnetic absorption performance through an interfacial synthesis process. Classical Debye relaxation theory and resistor-capacitor model have been employed to illustrate the electromagnetic dissipation mechanism of polypyrrole microparticles. The prepared polypyrrole microparticles exhibit an effective electromagnetic absorption bandwidth 5.48 GHz (deeper than −10 dB) from 12.52 to 18 GHz with a filler loading of 15 wt. % in paraffin. It was demonstrated that the morphologies of conducting polymers can significantly affect the dissipation of electromagnetic waves, supplying a strategy for the design of effective electromagnetic absorption materials.
Lorentz factor determination for local electric fields in semiconductor devices utilizing hyper-thin dielectrics118(2015); http://dx.doi.org/10.1063/1.4936271View Description Hide Description
The local electric field (the field that distorts, polarizes, and weakens polar molecular bonds in dielectrics) has been investigated for hyper-thin dielectrics. Hyper-thin dielectrics are currently required for advanced semiconductor devices. In the work presented, it is shown that the common practice of using a Lorentz factor of L = 1/3, to describe the local electric field in a dielectric layer, remains valid for hyper-thin dielectrics. However, at the very edge of device structures, a rise in the macroscopic/Maxwell electric fieldEdiel occurs and this causes a sharp rise in the effective Lorentz factor Leff. At capacitor and transistor edges, Leff is found to increase to a value 2/3 < Leff < 1. The increase in Leff results in a local electric field, at device edge, that is 50%–100% greater than in the bulk of the dielectric. This increase in local electric field serves to weaken polar bonds thus making them more susceptible to breakage by standard Boltzmann and/or current-driven processes. This has important time-dependent dielectric breakdown (TDDB) implications for all electronic devices utilizing polar materials, including GaN devices that suffer from device-edge TDDB.