Volume 115, Issue 13, 07 April 2014
Index of content:
We report a first principles theoretical investigation of spin polarized quantum transport in Mn2Ga/MgO/Mn2Ga and Mn3Ga/MgO/Mn3Ga magnetic tunneling junctions (MTJs) with the consideration of metal(Mg, Co, Cr) insertion layer effect. By changing the concentration of Mn, our calculation shows a considerable disparity in transport properties: A tunneling magnetoresistance (TMR) ratio of 852% was obtained for Mn2Ga-based MTJs, however, only a 5% TMR ratio for Mn3Ga-based MTJs. In addition, the influence of insertion layer has been considered in our calculation. We found the Co insertion layer can increase the TMR of Mn2Ga-based MTJ to 904%; however, the Cr insertion layer can decrease the TMR by 668%; A negative TMR ratio can be obtained with Mg insertion layer. Our work gives a comprehensive understanding of the influence of different insertion layer in Mn-Ga based MTJs. It is proved that, due to the transmission can be modulated by the interfacial electronic structure of insertion, the magnetoresistance ratio of Mn2Ga/MgO/Mn2Ga MTJ can be improved by inserting Co layer.
- Lasers, Optics, and Optoelectronics
115(2014); http://dx.doi.org/10.1063/1.4869555View Description Hide Description
The point defects of single ZnO microwires grown by carbothermal reduction were studied by microphotoluminescence, photoresistance excitation spectra, and resistance as a function of the temperature. We found the deep level defect density profile along the microwire showing that the concentration of defects decreases from the base to the tip of the microwires and this effect correlates with a band gap narrowing. The results show a characteristic deep defect levels inside the gap at 0.88 eV from the top of the VB. The resistance as a function of the temperature shows defect levels next to the bottom of the CB at 110 meV and a mean defect concentration of 4 × 1018 cm−3. This combination of techniques allows us to study the band gap values and defects states inside the gap in single ZnO microwires and opens the possibility to be used as a defect spectroscopy method.
Simulation analysis of GaN microdomes with broadband omnidirectional antireflection for concentrator photovoltaics115(2014); http://dx.doi.org/10.1063/1.4870714View Description Hide Description
Microdome structures are analyzed as surface topology to reduce surface reflection over a broad spectral range and wide light incidence angle for concentrator photovoltaics application. Three dimensional finite difference time domain method was used to accurately calculate the surface reflection and transmission for surface topologies with different feature sizes and aspect ratios. Studies show that the use of GaN microdomes will lead to a significant reduction of the surface reflection over a broad wavelength range and wide incidence angle range. The surface reflection significantly depends on the surface structure feature size and geometrical shape. The design of the GaN microdomes provides flexibility to tune the structure in order to obtain the minimum surface reflection for different designs of concentrator optical systems. The surface reflections of the GaN microdomes are compared with that of the conventional flat surface as well as the one with antireflection coating.
- Plasmas and Electrical Discharges
Three-dimensional simulations of discharge plasma evolution on a dielectric barrier discharge plasma actuator115(2014); http://dx.doi.org/10.1063/1.4870384View Description Hide Description
To develop simulation techniques for reconstructing microdischarges in a dielectric barrier discharge (DBD) plasma actuator and analyze spanwise non-uniformity in a body force field, three-dimensional discharge plasma simulations of a DBD plasma actuator were conducted assuming step-like positive and negative applied voltages. Our study showed that to break the spanwise uniformity, some disturbances were required in the computational conditions to reconstruct the three-dimensional microdischarges, and the attachment of some minute bumps (several tens of micrometers in size) on the electrode edge allowed for the successful reconstruction of glow-type microdischarges and streamer-type filamentary discharges in the negative and positive applied voltage cases, respectively. The tentative body force field has strong spanwise non-uniformity corresponding to the plasma structure, and in addition, a spanwise directional body force also exists, especially in the streamer discharge. However, the spanwise averaged body force has the same spatial-distribution and time-evolution characteristics obtained with the two-dimensional simulation.
Measurement of OH (X 2Σ) in immediate vicinity of dielectric surface under pulsed dielectric barrier discharge at atmospheric pressure using two geometries of laser-induced fluorescence115(2014); http://dx.doi.org/10.1063/1.4870598View Description Hide Description
The behavior of the ground state OH radical was studied in humid air using pulsed surface dielectric barrier discharge. The validity of OH measurement in immediate vicinity of the dielectric surface was discussed using two geometries of laser-induced fluorescence (LIF) measurement, and surface distribution and temporal profiles of OH were examined. A comparison of parallel- and perpendicular-LIF techniques for OH measurement revealed that OH was mostly produced in a region several tens of μm above the dielectric surface. The surface distribution profile of OH agreed well with that of N2(C), and in both cases, the total production generated by a discharge pulse was proportional to the discharge energy. The OH local density in surface streamer channels was estimated to be 1.3 × 1015 cm−3 according to the decay rate of OH, which was similar to that in corona discharge. Additionally, the local density of OH remained constant regardless of discharge energies. These results indicate that the yield of OH increased with the discharge energy owing to expansion of the surface streamer region, increase in streamer radius or thickness or the number of branches, whereas the local density of OH in the streamer channel remained constant regardless of discharge energy.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
115(2014); http://dx.doi.org/10.1063/1.4870137View Description Hide Description
In this study, we propose a phononic plate model with periodically corrugated surfaces and investigate the propagation behavior of Lamb waves in it. In contrast to the traditional phononic crystals which are composed of isotropic materials, the electroelastic coupling is taken into account in this investigation. Based on the Floquet's theorem, the Lamb wave solution is expanded in the series form of spatially harmonic plane waves. Band structures in the first Brillouin zone are presented graphically. The band gaps can be tuned by adjusting the structural parameters. We find that the structural symmetry of the plate has significant influence on the band gap properties. The symmetry breaking of the plate structure leads to more band gaps appearing in the first Brillouin zone. Moreover, the band gaps are also sensitive to the symmetry of the electric boundary conditions, which provides a feasible way to electrically control the acoustic band structure.
115(2014); http://dx.doi.org/10.1063/1.4870140View Description Hide Description
An acoustic sensor with composited transmission and reflecting structure is investigated for detection in liquid medium, in which a temperature compensational layer is deposited on the Si wafer to create a low temperature coefficient of frequency. This type of structure regulates most acoustic energy into the substrate, i.e., Si wafer and the compensational layer. Consequently, the standing resonant vibrations in the substrate are induced, as its thickness is integer multiples of a half wavelength. The acoustic sensor shows a low frequency shift with temperature, and the temperature coefficient of frequency can be controlled to about zero at the room temperature. Additionally, the acoustic sensor for the liquid detection on the viscosity and mass can be estimated 35 kHz/Pa · s and 23.25 kHz cm2/ng, respectively.
115(2014); http://dx.doi.org/10.1063/1.4870284View Description Hide Description
High-resolution photoluminescence studies on m-plane (1-100) homoepitaxial films grown by metalorganic chemical vapor deposition on AlN revealed several sharp donor-bound exciton (DBX) peaks with a full width at half maximum as narrow as 550 μeV. Power dependent photoluminescence distinguished DBXs tied to the Γ5 free exciton (FX) from those tied to the Γ1 FX. Both the n = 2 and n = 1 excited states of the Γ5 and Γ1 were resolved, giving binding energies of 52 meV and 55 meV, respectively. The DBX transition at 6.006 eV was identified as originating from the neutral-donor-oxygen (O0X). This assignment was based on secondary ion mass spectroscopy measurements, peak position with respect to the Si0X, and deep defect luminescence peaks located at 3.25 eV and 3.58 eV.
Position of fermi level on Al0.2Ga0.8N surface and distribution of electric field in Al0.2Ga0.8N/GaN heterostructures without and with AlN layer115(2014); http://dx.doi.org/10.1063/1.4870442View Description Hide Description
Position of Fermi level on Al0.2Ga0.8N surface and distribution of electric field in Al0.2Ga0.8N/GaN transistor heterostructures without and with AlN layer were studied experimentally using contactless electroreflectance and theoretically solving Schrodinger-Poisson equation with various surface boundary conditions. It has been observed that the thin AlN layer changes very strongly the distribution of electric field in this heterostructure but the Fermi level position on Al0.2Ga0.8N surface does not change significantly. Its position is the same within experimental uncertainly (i.e., ∼0.5 eV below conduction band) for both bulk Al0.2Ga0.8N and Al0.2Ga0.8N/GaN heterostructures.
Ellipsometry characterization of polycrystalline ZnO layers with the modeling of carrier concentration gradient: Effects of grain boundary, humidity, and surface texture115(2014); http://dx.doi.org/10.1063/1.4870443View Description Hide Description
Spectroscopic ellipsometry (SE) has been applied to study the effects of grain boundary, humidity, and surface texture on the carrier transport properties of Al-doped ZnO layers fabricated by dc and rf magnetron sputtering. In the SE analysis, the variation in the free carrier absorption toward the growth direction, induced by the ZnO grain growth on foreign substrates, has been modeled explicitly by adopting a multilayer model in which the optical carrier concentration (Nopt ) varies continuously with a constant optical mobility (μopt ). The effect of the grain boundary has been studied by comparing μopt with Hall mobility (μHall ). The change in μHall /μopt indicates a sharp structural transition of the ZnO polycrystalline layer at a thickness of d ∼ 500 nm, which correlates very well with the structure confirmed by transmission electron microscopy. In particular, below the transition thickness, the formation of the high density grain boundary leads to the reduction in the μHall /μopt ratio as well as Nopt . As a result, we find that the thickness dependence of the carrier transport properties is almost completely governed by the grain boundary formation. On the other hand, when the ZnO layer is exposed to wet air at 85 °C, μHall reduces drastically with a minor variation of μopt due to the enhanced grain boundary scattering. We have also characterized textured ZnO:Al layers prepared by HCl wet etching by SE. The analysis revealed that the near-surface carrier concentration increases slightly after the etching. We demonstrate that the SE technique can be applied to distinguish various rough textured structures (size ∼ 1 μm) of the ZnO layers prepared by the HCl etching.
115(2014); http://dx.doi.org/10.1063/1.4870475View Description Hide Description
Infrared and Raman-scattering spectroscopies were employed to explore the lattice and spin dynamics of Bi1– x Dy x FeO3 nanoparticles. With increasing substitution of Bi by Dy in the range of 0.00 ≤ x ≤ 0.40, we observe (1) evidence for an increase of local lattice distortion of the FeO6 octahedra, and significant changes of phonon parameters in the x = 0.15 sample, corresponding to the structural transformation from rhombohedral to orthorhombic, (2) the chemically substituted enhanced dielectric constant of 43.4 in the x = 0.20 sample, (3) the development of two-magnon excitations in Dy substituted samples, which sensitively gauges the modification of magnetic structures from a cycloidal spin to a homogeneous magnetized state, and (4) Dy substitution disrupts the lattice-spin interactions at high temperatures. These findings extend our understanding of tailoring the structural and magnetic properties of chemically substituted multiferroic nanoparticles and advance the technologically important development of these materials.
115(2014); http://dx.doi.org/10.1063/1.4870577View Description Hide Description
Structural stability of a transparent conducting oxide CuLaO2 at high pressures is investigated using in-situ Raman spectroscopy, electrical resistance, and x-ray diffraction techniques. The present Raman investigations indicate a sequence of structural phase transitions at 1.8 GPa and 7 GPa. The compound remains in the first high pressure phase when pressure is released. Electrical resistance measurements carried out at high pressures confirm the second phase transition. These observations are further supported by powder x-ray diffraction at high pressures which also showed that a-axis is more compressible than c-axis in this compound. Fitting the pressure dependence of unit cell volume to 3rd order Birch-Murnaghan equation of state, zero pressure bulk modulus of CuLaO2 is determined to be 154(25) GPa. The vibrational properties in the ambient delafossite phase of CuLaO2 are investigated using ab-initio calculations of phonon frequencies to complement the Raman spectroscopic measurements. Temperature dependence of the Raman modes of CuLaO2 is investigated to estimate the anharmonicity of Raman modes.
115(2014); http://dx.doi.org/10.1063/1.4870456View Description Hide Description
We generate silicon vacancy related defects in high-quality epitaxial silicon carbide layers by means of electron irradiation. By controlling the irradiation fluence, the defect concentration is varied over several orders of magnitude. We establish the excitation profile for optical pumping of these defects and evaluate the optimum excitation wavelength of 770 nm. We also measure the photoluminescence dynamics at room temperature and find a monoexponential decay with a characteristic lifetime of 6.1 ns. The integrated photoluminescence intensity depends linear on the excitation power density up to 20 kW/cm2, indicating a relatively small absorption cross section of these defects.
115(2014); http://dx.doi.org/10.1063/1.4870460View Description Hide Description
We report experimental and theoretical investigation of anisotropy in optical properties and their origin in the ferroelectric and paraelectric phases of bismuth titanate. Room temperature ellipsometric measurements performed on pulsed laser deposited bismuth titanate thin films of different orientations show anisotropy in the dielectric and optical constants. Subsequent first-principles calculations performed on the ground state structures of ferroelectric and high temperature paraelectric phases of bismuth titanate show that the material demonstrates anisotropic optical behavior in both ferroelectric and paraelectric phases. We further show that O 2p to Ti 3d transition is the primary origin of optical property of the material while optical anisotropy results from the asymmetrically oriented Ti-O bonds in TiO6 octahedra in the unit cell.
Discrimination between energy transfer and back transfer processes for GaAs host and Er luminescent dopants using electric response analysis115(2014); http://dx.doi.org/10.1063/1.4870808View Description Hide Description
The energy transfer and back transfer processes of GaAs co-doped with Er and O (GaAs:Er,O) were experimentally distinguished by using a frequency response analysis of the AC photocurrent. The results were achieved by using the difference in the frequency dispersion between (1) the dispersion of the energy transfer, which is triggered by the trapping of free charges in the GaAs host and is represented with the Debye relaxation response and (2) the dispersion of the energy back transfer, which is induced by non-radiative transition of 4f bound electrons in the Er dopants and is described with a Lorentzian. The Debye relaxation response found in GaAs:Er,O provided a charge trapping time that was dependent on temperature, which was well correlated with the thermal quenching property of intense intra-4f-shell luminescence. The spectral shape of the Lorentzian dependence on the temperature was explained with the thermal excitation of Er 4f electrons and release of trapped charges in GaAs. The thermal excitation and release of charges consistently explained the characteristics of weak 4f luminescence in low- and high-temperature regions, respectively.
Spectroscopic ellipsometry determination of the optical constants of titanium-doped WO3 films made by co-sputter deposition115(2014); http://dx.doi.org/10.1063/1.4869665View Description Hide Description
Titanium (Ti) doped tungsten oxide (WO3) thin films were grown by co-sputter deposition of W and Ti metal targets. The sputtering powers to the W and Ti were kept constant at 100 W and 50 W, respectively, while varying the growth temperature (Ts) in the range of 25–400 °C. The structural quality of Ti-doped WO3 films is dependent on Ts. Ti-doped WO3 films grown at Ts < 400 °C were amorphous. A temperature of 400 °C is critical to promote the structural order and formation of monoclinic, nanocrystalline films. The optical constants and their dispersion profiles determined from spectroscopic ellipsometry indicate that there is no significant inter-diffusion at the film-substrate interface for W-Ti oxide film growth of ∼40 nm. The index refraction (n) at λ = 550 nm varies in the range of 2.15–2.40 with a gradual increase in Ts. Lorentz-Lorenz analysis (n(λ) = 550 nm) of the data indicates the gradual improvement in the packing density coupled with structural transformation accounts for the observed optical quality of the Ti-doped WO3 films as a function of Ts. A correlation between the growth conditions and optical constants is discussed.
- Electronic Structure and Transport
115(2014); http://dx.doi.org/10.1063/1.4870115View Description Hide Description
Based on first-principles quantum transport calculations, we design a graphene-based biosensor device, which is composed of graphene nanoribbons electrodes and a biomolecule. It is found that when different nucleobases or poly nucleobase chains are located in the nanogap, the device presents completely different transport properties, showing different current informations. And the change of currents from 2 to 5 orders of magnitude for four different nucleobases suggests a great ability of discrimination by utilizing such a device. The physical mechanism of this phenomenon originates from their different chemical composition and structure. Moreover, we also explore the coupling effect of several neighboring bases and the size effect of the nanogap on transport properties. Our results show the possibility of rapid sequencing DNA by measuring such a transverse-current of the device, and provide a new idea for sequencing DNA.
115(2014); http://dx.doi.org/10.1063/1.4870230View Description Hide Description
We synthesize a scandium-doped PbTe single-crystal ingot and investigate the phase and the elemental composition as well as galvanomagnetic properties of Pb 1-yScyTe alloys in weak magnetic fields (4.2 K ≤ T ≤ 300 K, B ≤ 0.07 T) upon varying the scandium content (y ≤ 0.02). We find that all investigated samples are single-phase and n-type. The distribution of scandium impurities along the axis of the ingot is estimated to be exponential. An increase of scandium impurity content leads to a monotonous growth of the free electron concentration by four orders of magnitude (approximately from 1016 cm−3 to 1020 cm−3). In heavily doped alloys (y > 0.01), the free electron concentration at the liquid-helium temperature tends to saturation, indicating the pinning of the Fermi energy by the scandium resonant impurity level located on the background of the conduction band. Using the two-band Kane and six-band Dimmock dispersion relations for IV-VI semiconductors, dependences of the Fermi energy measured from the bottom of the conduction band Ec on the scandium impurity content are calculated and the energy of the resonant scandium level is estimated to be E Sc ≈ Ec + 280 meV. Diagrams of electronic structure rearrangement of Pb 1-yScyTe alloys upon doping are proposed.
115(2014); http://dx.doi.org/10.1063/1.4870290View Description Hide Description
We investigate theoretically quantum transport and Goos-Hänchen (GH) effect of electrons in a p-n-p junction on monolayers of MoS2. We find that the transmission properties of spin-up (spin-down) electrons in K valley are the same with spin-down (spin-up) electrons in valley due to the time-reversal symmetry. The GH shifts for the transmitted K and beams in the n-p interface are in the opposite direction, and GH shifts for the spin-up and spin-down electron beams at the same valley have different values in the same direction due to the different group velocities. Therefore, the spin-up and spin-down electrons can be separated after passing a sufficiently long channel created by a p-n-p junction. These features provide us a new way to generate a fully spin- and valley-polarized current in monolayers of MoS2.
115(2014); http://dx.doi.org/10.1063/1.4870578View Description Hide Description
Fumed silica A-300 was carbonized by means of pyrolysis of CH2Cl2. The obtained initial SiO2:C nanopowders of black color, with an average diameter of 14–16 nm and carbon (C) concentration 7 wt. %, subjected to the oxidation and passivation treatment were studied by electron paramagnetic resonance (EPR) in the temperature range 4–400 K. Two EPR signals of Lorentzian lineshape with nearly equal g-factors and different linewidth were observed in the initial, oxidized, and passivated SiO2:C nanopowders. The two-component EPR spectrum was explained by the presence of C in two electronic states. The intensive narrow EPR signal, which has a temperature-dependent intensity, linewidth, and resonance field position, was attributed to the carbon-related defect with non-localized electron hopping between neighboring C-dangling bonds. The striking effect is that the temperature dependence of the EPR linewidth demonstrates the motional narrowing of the EPR signal at very low temperatures from 4 K to 20 K, which is not typically for nonmetallic materials and was explained by the quantum character of C layer conductivity in the SiO2:C. The observed peaks in the temperature dependence of the conduction electron EPR signal integral intensity in the high-temperature range 200–440 K was explained by the presence of the C nanodots at the surface of SiO2 nanoparticles and the ejection of electrons from the confinement energy levels of C quantum dot when the temperature becomes comparable to the confinement energy.
Spin orbit splitting of the photon induced Fano resonance in an oscillating graphene electrostatic barrier115(2014); http://dx.doi.org/10.1063/1.4869846View Description Hide Description
We investigate theoretically the effect of a time dependent oscillating potential on the transport property of the Dirac Fermion through a monolayer graphene electrostatic barrier under the influence of the Rashba spin orbit interaction. The time dependent problem is solved in the frame work of the non-perturbative Floquet approach. It is noted that the dynamic condition of the barrier may be controlled by tuning the Rashba parameter. Introduction of the spin orbit interaction causes splitting of the Fano resonance (FR), a characteristic feature in photon assisted tunneling. The separation between the spin split FR's gives an indirect measure of the fine structure of the quasi-hole bound state inside the barrier. The present findings on the Rashba splitting of the FR and its external control by tuning the oscillating field parameters might have potential for applications in spintronic devices, especially in the spin field effect transistors. The spin polarization of different Floquet sidebands is found to be quite sensitive to the spin-pseudospin interaction.