Volume 118, Issue 9, 07 September 2015
Index of content:
The segregation behavior of carbon and oxygen atoms at various silicon grain boundaries was studied using a combination of atomistic simulation and analytical modeling. First, quasi-lattice Grand Canonical Monte Carlo simulations were used to compute segregation isotherms as a function of grain boundary type, impurity atom loading level, and temperature. Next, the atomistic results were employed to regress different analytical segregation models and extract thermodynamic and structural properties. The multilayer Brunauer–Emmett–Teller (BET) isotherm was found to quantitatively capture all the simulation conditions probed in this work, while simpler, single layer models such as the Langmuir-McLean model did not. Some of the BET parameters, namely, the binding free energy of the first adsorption layer and the impurity holding capacity of each layer, were tested for correlation with various measures of grain boundary structure and/or mechanical properties. It was found that certain measures of the atomistic stress distribution correlate strongly with the first-layer binding free energy for substitutional carbon atoms, while common grain boundary identifiers such as sigma value and energy density are not useful in this regard. Preliminary analysis of the more complex case of interstitial oxygen segregation showed that similar measures based on atomistic stress also may be useful here, but more systematic correlative studies are needed to develop a comprehensive picture.
- Photonics, Plasmonics, Lasers, and Optical Phenomena
Measurements and simulations of the optical gain and anti-reflection coating modal reflectivity in quantum cascade lasers with multiple active region stacks118(2015); http://dx.doi.org/10.1063/1.4929810View Description Hide Description
We report spectrally resolved gain measurements and simulations for quantum cascade lasers (QCLs) composed of multiple heterogeneous stacks designed for broadband emission in the mid-infrared. The measurement method is first demonstrated on a reference single active region QCL based on a double-phonon resonance design emitting at 7.8 μm. It is then extended to a three-stack active region based on bound-to-continuum designs with a broadband emission range from 7.5 to 10.5 μm. A tight agreement is found with simulations based on a density matrix model. The latter implements exhaustive microscopic scattering and dephasing sources with virtually no fitting parameters. The quantitative agreement is furthermore assessed by measuring gain coefficients obtained by studying the threshold current dependence with the cavity length. These results are particularly relevant to understand fundamental gain mechanisms in complex semiconductor heterostructure QCLs and to move towards efficient gain engineering. Finally, the method is extended to the measurement of the modal reflectivity of an anti-reflection coating deposited on the front facet of the broadband QCL.
Experimentally generating any desired partially coherent Schell-model source using phase-only control118(2015); http://dx.doi.org/10.1063/1.4929811View Description Hide Description
A technique is presented to produce any desired partially coherent Schell-model source using a single phase-only liquid-crystal spatial light modulator (SLM). Existing methods use SLMs in combination with amplitude filters to manipulate the phase and amplitude of an initially coherent source. The technique presented here controls both the phase and amplitude using a single SLM, thereby making the amplitude filters unnecessary. This simplifies the optical setup and significantly increases the utility and flexibility of the resulting system. The analytical development of the technique is presented and discussed. To validate the proposed approach, experimental results of three partially coherent Schell-model sources are presented and analyzed. A brief discussion of possible applications is provided in closing.
118(2015); http://dx.doi.org/10.1063/1.4929825View Description Hide Description
We demonstrate the integration of Nd3+ doped barium-titanium-silicate microsphere lasers with a silicon nitride photonic platform. Devices with two different geometrical configurations for extracting the laser light to buried waveguides have been fabricated and characterized. The first configuration relies on a standard coupling scheme, where the microspheres are placed over strip waveguides. The second is based on a buried elliptical geometry whose working principle is that of an elliptical mirror. In the latter case, the input of a strip waveguide is placed on one focus of the ellipse, while a lasing microsphere is placed on top of the other focus. The fabricated elliptical geometry (ellipticity = 0.9) presents a light collecting capacity that is 50% greater than that of the standard waveguide coupling configuration and could be further improved by increasing the ellipticity. Moreover, since the dimensions of the spheres are much smaller than those of the ellipses, surface planarization is not required. On the contrary, we show that the absence of a planarization step strongly damages the microsphere lasing performance in the standard configuration.
118(2015); http://dx.doi.org/10.1063/1.4928813View Description Hide Description
Far-field directional radiation of a single dipole in a cuboid slot is investigated in the presence of a dielectric substrate. Due to strong near field coupling between the dipole source and the surfaces of the slot and the dielectric, the far-field radiation shows strongly anisotropic pattern depending on the dipole radiation energy. By tuning local resonances within the air-slot interface or the substrate-slot interface, highly directional radiation either to free space or to the substrate space can be obtained. In the visible spectrum ranging from 1.2 eV to 3.5 eV, up to 18 fold directivity can be obtained. The up-to-down ratio can be tuned from −7.5 dB to 10 dB. We identify induced eigenmodes responsible for highly unidirectional radiations as a function of the emitter spectrum and slot thickness to assess controllability of radiation power and direction.
118(2015); http://dx.doi.org/10.1063/1.4929643View Description Hide Description
Through computer simulations and surface plasmon resonance (SPR) measurements, we establish optimum parameters for the design and fabrication of SPR sensors of high sensitivity, resolution, stability, and long decay-length evanescent fields. We present simulations and experimental SPR data for variety of sensors fabricated by using bimetal (Ag/Au) and multilayer waveguide-coupled Ag/Si3N4/Au structures. The simulations were carried out by using the transfer matrix method in MATLAB environment. Results are presented as functions of the thickness of the metal (Ag or Au) and the waveguide dielectric used in Ag/Si3N4/Au structures. Excellent agreement is observed between the simulations and experiments. For optimized thickness of the Si3N4 waveguide (150 nm), the sensor exhibits very high sensitivity to changes in the refractive index of analytes, , extremely high resolution , and long penetration depth of evanescent fields .
118(2015); http://dx.doi.org/10.1063/1.4929649View Description Hide Description
Extended birefringent waveguiding microchannels up to 15 mm long were created inside fused silica by single-pulse irradiation with femtosecond Bessel beams. The birefringent refractive index change of 2–4 × 10−4 is attributed to residual mechanical stress. The microchannels were chemically etched in KOH solution to produce 15 mm long microcapillaries with smooth walls and a high aspect ratio of 1:250. Bessel beams provide higher speed of material processing compared to conventional multipulse femtosecond laser micromachining techniques and permit simple control of the optical axis direction of the birefringent waveguides, which is important for practical applications [Corrielli et al., “Rotated waveplates in integrated waveguide optics,” Nat. Commun. 5, 4249 (2014)].
118(2015); http://dx.doi.org/10.1063/1.4930090View Description Hide Description
Fano interference is predicted in a structure consisting of a periodic array of uncoupled gold nano-disks (AuNDs) supported by a Bragg stack. The phenomenon occurs due to resonant interference between the reflection spectrum of the Bragg stack and localized surface plasmon resonances (LSPRs) on the AuNDs. Fitting the computational results to the Fano lineshape formula yields the Fano parameters of the resonances including their linewidth. One of the most important advantages of the structure is that the linewidth of the Fano resonances (∼9 nm) is much narrower than the bandwidth of the LSPR nano-disks (∼80 nm) and the reflection bandwidth of the Bragg stack on which the disks are supported (∼100 nm). The narrow Fano linewidth and the ease with which they can be interrogated using perpendicularly incident light makes the structure attractive for biosensing applications. The bulk sensitivities and figure of merit of Fano resonances in a proposed design are 44 nm/RIU (redshift) and −52 nm/RIU (blueshift), and ∼4 RIU−1, respectively.
- Electrical Discharges, Plasmas, and Plasma-Surface Interactions
High yield neutron generator based on a high-current gasdynamic electron cyclotron resonance ion source118(2015); http://dx.doi.org/10.1063/1.4929955View Description Hide Description
In present paper, an approach for high yield compact D-D neutron generator based on a high current gasdynamic electron cyclotron resonance ion source is suggested. Results on dense pulsed deuteron beam production with current up to 500 mA and current density up to 750 mA/cm2 are demonstrated. Neutron yield from D2O and TiD2 targets was measured in case of its bombardment by pulsed 300 mA D+ beam with 45 keV energy. Neutron yield density at target surface of 109 s−1 cm−2 was detected with a system of two 3He proportional counters. Estimations based on obtained experimental results show that neutron yield from a high quality TiD2 target bombarded by D+ beam demonstrated in present work accelerated to 100 keV could reach 6 × 1010 s−1 cm−2. It is discussed that compact neutron generator with such characteristics could be perspective for a number of applications like boron neutron capture therapy, security systems based on neutron scanning, and neutronography.
- Magnetism, Spintronics, and Superconductivity
Isothermal tuning of magnetic coercivity in NiFe/NiO/[Co/Pt] heterostructures with orthogonal easy axes118(2015); http://dx.doi.org/10.1063/1.4929760View Description Hide Description
Heterostructures of NiFe/NiO/[Co/Pt] with mutually orthogonal easy axes allow for isothermal tuning of the magnetic coercivity at room temperature with no associated shift in the hysteresis loop along the applied field axis. This is in contrast to what is typically seen in exchange biased heterostructures. The NiFe coercivity is enhanced from 14.5 to 105 Oe through the application of moderate dc magnetic fields of <3 kOe. This enhancement is completely reset with the application of a similarly sized dc magnetic field perpendicular to the film. The pinning of the antiferromagnetic NiO interlayer (i.e., blocking temperature, which is expected to be well below 50 K at this thickness in the absence of adjacent magnetic layers) is greatly enhanced and influenced by the in-plane magnetization of both the NiFe and [Co/Pt]. In addition, these heterostructures show unique high and low-field training effects due to alignment of [Co/Pt] stripe domains. This dynamic, yet predictable, behavior where the coercivity is isothermally tuned without any permanent structural/chemical modifications has potential uses in advanced magnetic logic/storage, as well as tuning the interfacial coupling in spintronic applications.
118(2015); http://dx.doi.org/10.1063/1.4929820View Description Hide Description
Magnetic tunnel junction (MTJ) is an important device element for many practical spintronic systems. In this paper, we propose and theoretically investigate a very attractive MTJ Fe(001)/O/NaCl(001)/O/Fe(001) as a two-terminal transport junction. By density functional theory total energy methods, we establish two viable device models: one with and the other without mirror symmetry across the center plane of the structure. Large tunnel magnetoresistance ratio (TMR) is predicted from first principles, at over 1800% and 3600% depending on the symmetry. Microscopically, a spin filtering effect is responsible for the large TMR. This effect essentially filters out all the minority spin channels (spin-down) from contributing to the tunnelling current. On the other hand, transport of the majority spin channel (spin-up) having and symmetry is enhanced by the FeO buffer layer in the MTJ.
Dual control of ferromagnetic resonance frequency in multiferroic heterostructures by oblique deposition and electrical field118(2015); http://dx.doi.org/10.1063/1.4929835View Description Hide Description
By using oblique deposition technique to fabricate FeCo/MnIr/[Pb(Mg1 / 3Nb2 / 3)O3]0.68-[PbTiO3]0.32 (011) multiferroic heterostructures, we experimentally demonstrate that it is feasible to dually control the ferromagnetic resonance frequency by changing an oblique deposition angle and an applied electrical field. In particular, by changing the oblique angle, the resonance frequency of the samples in unpoled state can be fixed from 2.39 GHz to 9 GHz. Upon the application of the electrical field varied from 0 kV/cm to 6 kV/cm, the resonance frequency of each sample shows different tunability range depending on the oblique angle and it can be tailored beyond 10 GHz. The result can be interpreted in terms of the enhancement of the magnetic anisotropy arising from the inverse magnetostriction effect. Our demonstration suggests a great implication for tunable microwave applications with ultra-wide band requirement.
118(2015); http://dx.doi.org/10.1063/1.4929573View Description Hide Description
We introduce an implementation of magnetophoresis to measure the absolute magnetization of ferromagnetic nanorods dispersed in fluids, by analyzing the velocity of single nanorods under an applied magnetic field gradient. A microfluidic guideway prevents aggregation of nanorods, isolates them, and confines their motion for analysis. We use a three-dimensional imaging system to precisely track nanorod velocity and particle-surface proximity. We test the effect of the guideway on nanorod velocity under field gradient application, finding that it guides magnetophoresis, but imposes insignificant drag beyond that of a planar surface. This result provides insight into the transport of magnetic nanorods at microstructured interfaces and allows the use of an analytical model to accurately determine the reacted viscous drag in the force balance needed for quantitative magnetometry. We also estimate the confining potential of the guideway with Brownian motion measurements and Boltzmann statistics. We use our technique to measure the magnetization of ferromagnetic nanorods with a noise floor of 8.5 × 10−20 A·m2·Hz−½. Our technique is quantitative, rapid, and scalable for determining the absolute magnetization of ferromagnetic nanoparticles with high throughput.
- Dielectrics, Ferroelectrics, and Multiferroics
High temperature phase stability in Li0.12Na0.88NbO3: A combined powder X-ray and neutron diffraction study118(2015); http://dx.doi.org/10.1063/1.4929645View Description Hide Description
The phase stabilities of ecofriendly piezoelectric material of lithium doped sodium niobate for composition Li0.12 Na 0.88NbO3 (LNN12) have been investigated by a combination of powder X-ray and neutron diffraction techniques in the temperature range of 300–1100 K. We observed interesting changes with appearance or disappearance of the super-lattice reflections in the powder diffraction patterns. Unambiguous experimental evidence is shown for coexistence of paraelectric and ferroelectric orthorhombic phases in the temperature range of 525 K to 675 K. We identified the correct crystal structure of LNN12 with temperature and correlated it with observed anomaly in the physical properties. Identification of crystal structure also helps in the mode assignments in Raman and infrared spectroscopies. We argued that application of chemical pressure as a result of Li substitution in NaNbO3 matrix favors the freezing of zone centre phonons in contrast to the freezing of zone boundary phonons in pure NaNbO3 with the variation of temperature.
118(2015); http://dx.doi.org/10.1063/1.4929956View Description Hide Description
We report the magnetic properties and the magnetoelectric effect in Y-type hexaferrite polycrystalline sample of Ba0.5Sr1.5Co2Fe12O22 (BSCFO). The magnetic critical temperature of BSCFO is 620 K, and the low-temperature helical magnetic order can hold up to 255 K. The isothermal magnetization curves sustain ferromagnetic hysteresis from 10 K to 300 K. Under external magnetic fields, the magnetic field induced electric polarization is observed up to 150 K. The magnitude and the sign of the electric polarization can be controlled by the applied magnetic field. These magnetic and magnetoelectric results provide the promise of the studied new hexaferrite as a potential multiferroic material.
118(2015); http://dx.doi.org/10.1063/1.4930034View Description Hide Description
A new lithium-niobate (LiNbO3)-type polar compound, namely, ZnPbO3 (a = 5.41605(7) Å and c = 14.33151(3) Å), with closed-shell ions only was synthesized under high pressure and high temperature (8 GPa and 1273 K). A point-charge-model calculation based on atomic positions refined by Rietveld analysis of synchrotron X-ray diffraction data gave an electrical ionic polarization of 77 μC/cm2 along the hexagonal c-axis. Detailed structural analysis indicated that the contribution of Pb 4+ in ZnPbO3 to the polarization was almost twice as large as that of Sn4+ in ZnSnO3. Transport measurement showed metallic behavior of ZnPbO3 from room temperature to low temperature despite the fact that both cations are closed-shell ions.
118(2015); http://dx.doi.org/10.1063/1.4930035View Description Hide Description
In this study, the influences of the adsorption of water molecules on the changes in the atomic and electric structures of BaTiO3 surface were investigated using ab initio calculation. Water molecules are molecularly and dissociatively adsorbed on the BaTiO3 surface, which makes electrons transfer from water molecules to the BaTiO3 surface. The redistribution of electrons in the BaTiO3 surface layers weakens the Ba-O interactions and strengthens the Ti-O interactions, so that the Ti atom shifts in TiO2 plane, i.e., an in-plane domain switching. The adsorption of water molecules on BaTiO3 surfaces also results in a reduction in the surface rumpling.
- Physics of Nanoscale, Mesoscale, and Low-Dimensional Systems
A framework for solving atomistic phonon-structure scattering problems in the frequency domain using perfectly matched layer boundaries118(2015); http://dx.doi.org/10.1063/1.4929780View Description Hide Description
We present a numerical approach to the solution of elastic phonon-interface and phonon-nanostructure scattering problems based on a frequency-domain decomposition of the atomistic equations of motion and the use of perfectly matched layer (PML) boundaries. Unlike molecular dynamic wavepacket analysis, the current approach provides the ability to simulate scattering from individual phonon modes, including wavevectors in highly dispersive regimes. Like the atomistic Green's function method, the technique reduces scattering problems to a system of linear algebraic equations via a sparse, tightly banded matrix regardless of dimensionality. However, the use of PML boundaries enables rapid absorption of scattered wave energies at the boundaries and provides a simple and inexpensive interpretation of the scattered phonon energy flux calculated from the energy dissipation rate in the PML. The accuracy of the method is demonstrated on connected monoatomic chains, for which an analytic solution is known. The parameters defining the PML are found to affect the performance and guidelines for selecting optimal parameters are given. The method is used to study the energy transmission coefficient for connected diatomic chains over all available wavevectors for both optical and longitudinal phonons; it is found that when there is discontinuity between sublattices, even connected chains of equivalent acoustic impedance have near-zero transmission coefficient for short wavelengths. The phonon scattering cross section of an embedded nanocylinder is calculated in 2D for a wide range of frequencies to demonstrate the extension of the method to high dimensions. The calculations match continuum theory for long-wavelength phonons and large cylinder radii, but otherwise show complex physics associated with discreteness of the lattice. Examples include Mie oscillations which terminate when incident phonon frequencies exceed the maximum available frequency in the embedded nanocylinder, and scattering efficiencies larger than two near the Brillouin zone edge.
118(2015); http://dx.doi.org/10.1063/1.4929875View Description Hide Description
This study investigates the structural transformations and properties of silica glass nanowires under tensile loading via molecular dynamics simulations using the BKS (Beest-Kramer-Santen) interatomic potential. Surface states of the elongated nanowires were quantified using radial density distributions, while structural transformations were evaluated via ring size distribution analysis. The radial density distributions indicate that the surface states of these silica nanowires are significantly different than those of their interior. Ring size analysis shows that the ring size distributions remain mainly unchanged within the elastic region during tensile deformation, however they vary drastically beyond the onset of plastic behavior and reach plateaus when the nanowires break. The silica nanowires undergo structural changes which correlate with strain energy and ring size distribution variations. It is also found that the ring size distribution (and strain energy) variations are dependent on the diameter of the silica nanowires. Interestingly, for ultrathin nanowires (diameters < 5 .0 nm), the variation of ring size distributions shows a distinct trend with respect to tensile strain, indicating that the surface states play a key role in both modifying the mechanical properties and structural characteristics. These results for ultrathin nanowires are consistent with prior theoretical and simulation predictions. The overall findings in this study provide key insights into the novel properties of nano-sized amorphous materials, and are aimed to inspire further experiments.
Investigation of single-layer/multilayer self-assembled InAs quantum dots on GaAs1-xSbx/GaAs composite substrates118(2015); http://dx.doi.org/10.1063/1.4929639View Description Hide Description
The structure-performance properties of single-layered and multi-layered InAs/GaAs1−xSbx quantum dot (QD) system, grown by molecular beam epitaxy on GaAs (001) substrates, have been investigated as a function of Sb concentration. Electron microscopy observations showed no significant crystalline defects for the single-layered InAs QDs (Sb 20%). X-ray diffraction analysis revealed that the increase of Sb concentration from 7.3% to 10.2% for the multi-layered QDs increased the strain relaxation from 0% to ∼23% and the dislocation density of GaAsSb layers went up to 3.6 × 109 cm−2. The peak energy of QD luminescence was red-shifted with increasing Sb concentration due to reduced strain inside QDs. Moreover, the carrier lifetime of the QDs was highly improved from 1.7 to 36.7 ns due to weak hole confinement as the Sb concentration was increased from 7.3% to 10.2%. These structures should be highly promising as the basis for photovoltaic solar-cell applications. Finally, the increased Sb concentration increased the thermal activation energy of electrons confined in the QDs from 163.7 to 206.8 meV, which was indicative of the improved thermal stability with Sb concentration.
118(2015); http://dx.doi.org/10.1063/1.4929640View Description Hide Description
Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases—Mo2TiAlC2 and Mo 2Ti2AlC3—synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m)Ti:1.1Al:2C with 1.5 ≤ m ≤ 2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600 °C for 4 h under Ar flow. In general, for m ≥ 2 an ordered 312 phase, (Mo 2Ti)AlC2, was the majority phase; for m < 2, an ordered 413 phase (Mo 2Ti2)AlC3, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo 2TiAlC1.7 and Mo 2Ti1.9Al0.9C2.5, respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo 2TiAlC2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo 2Ti2AlC3, with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M layer ordering is mostly driven by the high penalty paid in energy by having the Mo atoms surrounded by C in a face-centered configuration, i.e., in the center of the Mn+1Xn blocks. At 331 GPa and 367 GPa, respectively, the Young's moduli of the ordered Mo 2TiAlC2 and Mo 2Ti2AlC3 are predicted to be higher than those calculated for their ternary end members. Like most other MAX phases, because of the high density of states at the Fermi level, the resistivity measurement over 300 to 10 K for both phases showed metallic behavior.