Volume 118, Issue 8, 28 August 2015
Index of content:
A combinatorial approach where doped bulk scintillator materials can be rapidly optimized for their properties through concurrent extrinsic doping/co-doping strategies is presented. The concept that makes use of design of experiment, rapid growth, and evaluation techniques, and multivariable regression analysis, has been successfully applied to the engineering of NaI performance, a historical but mediocre performer in scintillation detection. Using this approach, we identified a three-element doping/co-doping strategy that significantly improves the material performance. The composition was uncovered by simultaneously screening for a beneficial co-dopant ion among the alkaline earth metal family and by optimizing its concentration and that of Tl+ and Eu2+ ions. The composition with the best performance was identified as 0.1% mol Tl+, 0.1% mol Eu2+, and 0.2% mol Ca2+. This formulation shows enhancement of energy resolution and light output at 662 keV, from 6.3 to 4.9%, and from 44 000 to 52 000 ph/MeV, respectively. The method, in addition to improving NaI performance, provides a versatile framework for rapidly unveiling complex and concealed correlations between material composition and performance, and should be broadly applicable to optimization of other material properties.
- Photonics, Plasmonics, Lasers, and Optical Phenomena
118(2015); http://dx.doi.org/10.1063/1.4928853View Description Hide Description
A bright green photoluminescence (PL) from 4S3∕2 → 4I15∕2 emission band in Er3+:YVO4 single crystal has been observed with the excitation of an argon laser at 488.0 nm. More than two orders of PL enhancement have been obtained under the effect of magnetic fields, and the enhancement factor f reaches 170 when the applied magnetic field is 7.7 T under the sample temperature of 4.2 K. Unusually, the PL enhancements only happen at some certain magnetic fields (Bc s), and a decrease of sample temperature will lead to the increase of f and decrease of Bc . The results confirm that this PL enhancement originates from the resonance excitation of the electron transitions induced by the cross of the laser energy and the absorption energy modulated by both the magnetic field and temperature. This special PL enhancement in Er3+:YVO4 single crystal can be applied in the calibration of pulsed high magnetic field, detection of material fine energy structures, and modulation of magneto-optical devices.
118(2015); http://dx.doi.org/10.1063/1.4929500View Description Hide Description
We investigate the far-infrared thermal radiation properties of a heterojunction bipolar transistor. The device conveniently provides a high electric field for electrons to heat the lattice and the electron gas in a background with ions embedded. Because of very high effective temperature of the electron gas in the collector, the electron-ion bremsstrahlung makes efficient the thermal radiation in the far-infrared region. The transistor can yield a radiation power of 0.1 mW with the spectral region between 2 and 75 THz and a power conversion efficiency of 6 × 10−4. Such output contains a power of 20 μW in the low-frequency part (2–20 THz) of the spectrum.
118(2015); http://dx.doi.org/10.1063/1.4929449View Description Hide Description
We report the simulation, fabrication, and experimental characterization of a multichannel metamaterial absorber with the aim to be used as a label-free sensing platform in the terahertz regime. The topology of the investigated resonators deposited on a thin flexible polymer by means of optical lithography is capable of supporting multiple resonances over a broad frequency range due to the individual contribution of each sub-element of the unit cell. In order to explore the performance of the chosen structure in terms of sensing phenomenon, the reflection feature is monitored upon variation of the refractive index and the thickness of the analyte. We achieve numerically maximum frequency sensitivity of about 139.2 GHz/refractive index unit. Measurements carried out using terahertz time-domain spectroscopy show good agreement with the numerical predictions. The results are very promising, suggesting a potential use of the metamaterial absorber in wide variety of multispectral terahertz sensing applications.
Improving hole injection and carrier distribution in InGaN light-emitting diodes by removing the electron blocking layer and including a unique last quantum barrier118(2015); http://dx.doi.org/10.1063/1.4929451View Description Hide Description
The effects of removing the AlGaN electron blocking layer (EBL), and using a last quantum barrier (LQB) with a unique design in conventional blue InGaN light-emitting diodes (LEDs), were investigated through simulations. Compared with the conventional LED design that contained a GaN LQB and an AlGaN EBL, the LED that contained an AlGaN LQB with a graded-composition and no EBL exhibited enhanced optical performance and less efficiency droop. This effect was caused by an enhanced electron confinement and hole injection efficiency. Furthermore, when the AlGaN LQB was replaced with a triangular graded-composition, the performance improved further and the efficiency droop was lowered. The simulation results indicated that the enhanced hole injection efficiency and uniform distribution of carriers observed in the quantum wells were caused by the smoothing and thinning of the potential barrier for the holes. This allowed a greater number of holes to tunnel into the quantum wells from the p-type regions in the proposed LED structure.
118(2015); http://dx.doi.org/10.1063/1.4928675View Description Hide Description
A Monte Carlo model is developed and implemented to calculate the characteristics of x-ray induced secondary electron (SE) emission from a CsI photocathode used in an x-ray streak camera. Time distributions of emitted SEs are investigated with an incident x-ray energy range from 1 to 30 keV and a CsI thickness range from 100 to 1000 nm. Simulation results indicate that SE time distribution curves have little dependence on the incident x-ray energy and CsI thickness. The calculated time dispersion within the CsI photocathode is about 70 fs, which should be the temporal resolution limit of x-ray streak cameras that use CsI as the photocathode material.
118(2015); http://dx.doi.org/10.1063/1.4929651View Description Hide Description
Here we propose a multi-scaled reflective grating with excellent blazed performance (nearly perfect blazed effect at the well-predicted frequency and orientation). The blazed grating consists of a periodical array of metallic super-cells, each made of several equal-distant subwavelength slits with linearly reduced depth. A simple model based on Huygens-Fresnel principle is established to forecast the microwave response for the incidence of different polarizations: for transverse-electric polarization, the structure provides only the ordinary total reflection (i.e., without orientation deflected); for transverse-magnetic (TM) polarization, the waves are deflected to specific orientation due to the linear phase delay of the slit exits. Similar design route can be extended to acoustic systems, considering the mathematic similarity between the acoustic wave and the electromagnetic wave of TM-polarization.
- Electrical Discharges, Plasmas, and Plasma-Surface Interactions
Energy deposition characteristics of nanosecond dielectric barrier discharge plasma actuators: Influence of dielectric material118(2015); http://dx.doi.org/10.1063/1.4929362View Description Hide Description
An experimental study aimed at the characterization of energy deposition of nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuators was carried out. Special attention was given on the effect of the thickness and material used for dielectric barrier. The selected materials for this study were polyimide film (Kapton), polyamide based nylon (PA2200), and silicone rubber. Schlieren measurements were carried out in quiescent air conditions in order to observe density gradients induced by energy deposited. Size of heated area was used to qualify the energy deposition coupled with electrical power measurements performed using the back-current shunt technique. Additionally, light intensity measurements showed a different nature of discharge based upon the material used for barrier, for a fixed thickness and frequency of discharge. Finally, a characterisation study was performed for the three tested materials. Dielectric constant, volume resistivity, and thermal conductivity were measured. Strong trends between the control parameters and the energy deposited into the fluid during the discharge were observed. Results indicate that efficiency of energy deposition mechanism relative to the thickness of the barrier strongly depends upon the material used for the dielectric barrier itself. In general, a high dielectric strength and a low volumetric resistivity are preferred for a barrier, together with a high heat capacitance and a low thermal conductivity coefficient in order to maximize the efficiency of the thermal energy deposition induced by an ns-DBD plasma actuator.
118(2015); http://dx.doi.org/10.1063/1.4929364View Description Hide Description
Although Fowler and Nordheim developed the basics of field emission nearly one century ago with their introduction of the Fowler-Nordheim equation (FNE), the topic continues to attract research interest particularly with the development of new materials that have been proposed as field emitters. The first order analysis of experiments typically relies upon the FNE for at minimum a basic understand of the physical emission process and its parameters of emission. The three key parameters in the FNE are the work function, emission area, and field enhancement factor, all of which can be difficult to determine under experimental conditions. This paper focuses in particular, on the field enhancement factor β. It is generally understood that β provides an indication of the surface roughness or sharpness of a field emitter cathode. However, in this paper, we experimentally and computationally demonstrate that cathodes with highly similar surface morphologies can manifest quite different field enhancements solely through having different emission regions. This fact can cause one to re-interpret results in which a single sharp emitter is proposed to dominate the emission from a field emitting cathode.
118(2015); http://dx.doi.org/10.1063/1.4928870View Description Hide Description
The control of electron energy probability functions (EEPFs) in low pressure partially ionized plasmas is typically accomplished through the format of the applied power. For example, through the use of pulse power, the EEPF can be modulated to produce shapes not possible under continuous wave excitation. This technique uses internal control. In this paper, we discuss a method for external control of EEPFs by transport of electrons between separately powered inductively coupled plasmas (ICPs). The reactor incorporates dual ICP sources (main and auxiliary) in a tandem geometry whose plasma volumes are separated by a grid. The auxiliary ICP is continuously powered while the main ICP is pulsed. Langmuir probe measurements of the EEPFs during the afterglow of the main ICP suggests that transport of hot electrons from the auxiliary plasma provided what is effectively an external source of energetic electrons. The tail of the EEPF and bulk electron temperature were then elevated in the afterglow of the main ICP by this external source of power. Results from a computer simulation for the evolution of the EEPFs concur with measured trends.
Nanosecond Nd-YAG laser induced plasma emission characteristics in low pressure CO2 ambient gas for spectrochemical application on Mars118(2015); http://dx.doi.org/10.1063/1.4929570View Description Hide Description
An experimental study is conducted on the possibility and viability of performing spectrochemical analysis of carbon and other elements in trace amount in Mars, in particular, the clean detection of C, which is indispensible for tracking the sign of life in Mars. For this study, a nanosecond Nd-YAG laser is employed to generate plasma emission from a pure copper target in CO2 ambient gas of reduced pressure simulating the atmospheric condition of Mars. It is shown that the same shock wave excitation mechanism also works this case while exhibiting remarkably long cooling stage. The highest Cu emission intensities induced by 4 mJ laser ablation energy is attained in 600 Pa CO2 ambient gas. Meanwhile the considerably weaker carbon emission from the CO2 gas appears relatively featureless over the entire range of pressure variation, posing a serious problem for sensitive trace analysis of C contained in a solid sample. Our time resolved intensity measurement nevertheless reveals earlier appearance of C emission from the CO2 gas with a limited duration from 50 ns to 400 ns after the laser irradiation, well before the initial appearance of the long lasting C emission from the solid target at about 1 μs, due to the different C-releasing processes from their different host materials. The unwanted C emission from the ambient gas can thus be eliminated from the detected spectrum by a proper time gated detection window. The excellent spectra of carbon, aluminum, calcium, sodium, hydrogen, and oxygen obtained from an agate sample are presented to further demonstrate and verify merit of this special time gated LIBS using CO2 ambient gas and suggesting its viability for broad ranging in-situ applications in Mars.
- Magnetism, Spintronics, and Superconductivity
Growth condition dependence of photoluminescence polarization in (100) GaAs/AlGaAs quantum wells at room temperature118(2015); http://dx.doi.org/10.1063/1.4928325View Description Hide Description
We conducted systematic measurements on the carrier lifetime (τc ), spin relaxation time (τs ), and circular polarization of photoluminescence (P circ) in (100) GaAs/AlGaAs multiple quantum wells grown by molecular beam epitaxy (MBE). The τc values are strongly affected by MBE growth conditions (0.4–9 ns), whereas the τs are almost constant at about 0.13 ns. The result suggests that spin detection efficiency [τs /(τc + τs )], which is expected to be proportional to a steady-state P circ, is largely dependent on growth condition. We confirmed that the P circ has similar dependence on growth condition to those of τs /(τc + τs ) values. The study thus indicates that choosing the appropriate growth condition of the QW is indispensable for obtaining a high P circ from a spin-polarized light-emitting diode (spin-LED).
118(2015); http://dx.doi.org/10.1063/1.4928809View Description Hide Description
The non-linear, transient operation of DC electromagnetic launchers (EMLs) complicates their theoretical understanding and prevents scaling studies and performance comparisons without the aid of detailed numerical models. This paper presents a general theory for DC electromagnetic launchers that has simplified these tasks by identifying critical EML parameters and relationships affecting the EML's voltage, current, and power scaling, as well as its performance and energy conversion efficiency. EML parameters and relationships discussed in this paper include the specific force, the operating mode, the launcher constant, the launcher characteristic velocity, the contact characteristic velocity, the energy conversion efficiency, and the kinetic power and voltage-current scaling relationship. The concepts of the ideal EML, same-scale comparisons, and EML impedance are discussed. This paper defines conditions needed for the EML to operate in the steady-state. A comparison of the general theory with experimental results of several different types of DC (i.e., non-induction) electromagnetic launchers ranging from medium velocity (100's m/s) to high velocity (1000's m/s) is performed. There is good agreement between the general theory and the experimental results.
118(2015); http://dx.doi.org/10.1063/1.4929634View Description Hide Description
We provide compelling experimental evidence that the low-temperature transition in natural non-stoichiometric Fe7S8, a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials, is a phenomenon caused by magnetic coupling between epitaxially intergrown superstructures. The two superstructures differ in their defect distribution, and consequently in their magnetic anisotropy. At T < 30 K, the magnetic moments of the superstructures become strongly coupled, resulting in a 12-fold anisotropy symmetry, which is reflected in the anisotropic magneto-resistance.
- Dielectrics, Ferroelectrics, and Multiferroics
118(2015); http://dx.doi.org/10.1063/1.4929420View Description Hide Description
Dielectrics with high electrostatic energy storage must have exceptionally high dielectric breakdown strength at elevated temperatures. Another important consideration in designing a high performance dielectric is understanding the thickness and temperature dependence of breakdown strengths. Here, we develop a numerical model which assumes a coupled ionic redistribution and electronic breakdown is applied to predict the breakdown strength of low-alkali glass. The ionic charge transport of three likely charge carriers (Na +, H+/H3O+, Ba2+) was used to calculate the ionic depletion width in low-alkali boroaluminosilicate which can further be used for the breakdown modeling. This model predicts the breakdown strengths in the 108–109 V/m range and also accounts for the experimentally observed two distinct thickness dependent regions for breakdown. Moreover, the model successfully predicts the temperature dependent breakdown strength for low-alkali glass from room temperature up to 150 °C. This model showed that breakdown strengths were governed by minority charge carriers in the form of ionic transport (mostly sodium) in these glasses.
118(2015); http://dx.doi.org/10.1063/1.4929646View Description Hide Description
The class of RMn2O5 (R = Ho, Tb, Y, Eu) compounds offers multiferroic properties where the refined magnetic zig-zag order breaks the inversion symmetry. Varying the temperature, the system undergoes a magnetic and a subsequent ferroelectric phase transition where the ferroelectricity is magnetically induced. We propose a modified anisotropic Heisenberg model that can be used as a tractable analytical model studying the properties of those antiferromagnetic zig-zag spin chains. Based on a finite temperature Green's function method, it is shown that the polarization is induced solely by different exchange couplings of the two different Mn4+ and Mn3+ magnetic ions. We calculate the excitation energy of the spin system for finite temperatures, which for its part determines the temperature dependent magnetization and polarization. The ferroelectric phase transition is manifested as a kink in the excitation energy. The variation of the polarization by an external magnetic field depends strongly on the direction of that field. Whereas, the polarization in b-direction increases with an external magnetic field as well in b-direction it can be switched for strong fields in a-direction. The results based on that modified Heisenberg model are in qualitative agreement with experimental data.
- Physics of Nanoscale, Mesoscale, and Low-Dimensional Systems
Helicity sensitive terahertz radiation detection by dual-grating-gate high electron mobility transistors118(2015); http://dx.doi.org/10.1063/1.4928969View Description Hide Description
We report on the observation of a radiation helicity sensitive photocurrent excited by terahertz (THz) radiation in dual-grating-gate (DGG) InAlAs/InGaAs/InAlAs/InP high electron mobility transistors (HEMT). For a circular polarization, the current measured between source and drain contacts changes its sign with the inversion of the radiation helicity. For elliptically polarized radiation, the total current is described by superposition of the Stokes parameters with different weights. Moreover, by variation of gate voltages applied to individual gratings, the photocurrent can be defined either by the Stokes parameter defining the radiation helicity or those for linear polarization. We show that artificial non-centrosymmetric microperiodic structures with a two-dimensional electron system excited by THz radiation exhibit a dc photocurrent caused by the combined action of a spatially periodic in-plane potential and spatially modulated light. The results provide a proof of principle for the application of DGG HEMT for all-electric detection of the radiation's polarization state.
A comparative study of magnetic training effect in bulk and nanocrystalline La0.46Ca0.54MnO3 compound118(2015); http://dx.doi.org/10.1063/1.4929465View Description Hide Description
Transport, magneto-transport, and magnetic properties of La0.46Ca0.54MnO3 compounds having average grain size down to ∼15 nm have been studied. A magnetic training effect due to the external magnetic field cycling was distinctly observed in charge ordered antiferromagnetic bulk compound. Our present study indicates that the training effect was markedly modified along with the modification of the charge ordering due to the reduction of the grain size, and eventually both phenomenons disappeared in case of our lowest particle size sample (∼15 nm). Enhanced ferromagnetic correlation with the reduction of particle size plays the key role for the gradual diminishing of the training effect in the region of nanometer length scale.
Hydrogen adsorption and desorption with 3D silicon nanotube-network and film-network structures: Monte Carlo simulations118(2015); http://dx.doi.org/10.1063/1.4929365View Description Hide Description
Hydrogen is clean, sustainable, and renewable, thus is viewed as promising energy carrier. However, its industrial utilization is greatly hampered by the lack of effective hydrogen storage and release method. Carbon nanotubes (CNTs) were viewed as one of the potential hydrogen containers, but it has been proved that pure CNTs cannot attain the desired target capacity of hydrogen storage. In this paper, we present a numerical study on the material-driven and structure-driven hydrogen adsorption of 3D silicon networks and propose a deformation-driven hydrogen desorption approach based on molecular simulations. Two types of 3D nanostructures, silicon nanotube-network (Si-NN) and silicon film-network (Si-FN), are first investigated in terms of hydrogen adsorption and desorption capacity with grand canonical Monte Carlo simulations. It is revealed that the hydrogen storage capacity is determined by the lithium doping ratio and geometrical parameters, and the maximum hydrogen uptake can be achieved by a 3D nanostructure with optimal configuration and doping ratio obtained through design optimization technique. For hydrogen desorption, a mechanical-deformation-driven-hydrogen-release approach is proposed. Compared with temperature/pressure change-induced hydrogen desorption method, the proposed approach is so effective that nearly complete hydrogen desorption can be achieved by Si-FN nanostructures under sufficient compression but without structural failure observed. The approach is also reversible since the mechanical deformation in Si-FN nanostructures can be elastically recovered, which suggests a good reusability. This study may shed light on the mechanism of hydrogen adsorption and desorption and thus provide useful guidance toward engineering design of microstructural hydrogen (or other gas) adsorption materials.
A direct route for the investigation of crystalline field effects on the spectral properties of Eu3+ ions in designed nanostructures118(2015); http://dx.doi.org/10.1063/1.4928957View Description Hide Description
The spectral properties of lanthanide ions doped in a solid-state matrix, especially the Stark splitting and emission shifts, are strongly influenced by the local crystalline field. Here, we use a direct approach to quantitatively investigate the crystalline field effect on these spectral properties of Eu3+ ions in our designed β-PbF2:Eu3+ and BaF2:Eu3+ nanostructures. The extremely similar structures in which the central Eu3+ ions have the same site symmetry allow the local crystalline field to be simplified as the same point charge electrostatic field model. In this model, the direction and intensity of the electrostatic field are related to the distortion and expansion degree of the charge cloud of the luminescent center, respectively, and further determine the Stark splitting and emission shift. The theoretical analysis and charge-cloud stimulations were in good agreement with the experimental results. The direction and intensity of the crystalline field were calculated, and showed that the emission spectra shift to red with the increase in intensity. This work provides a comprehensive understanding of the spectral changes induced by a crystalline field, which is of great significance for the design of materials with the desired spectral properties.
118(2015); http://dx.doi.org/10.1063/1.4928835View Description Hide Description
The deep ultraviolet luminescence (hν ≥ 5 eV) of multiwall boron nitride nanotubes (BNNTs) is studied with time- and energy-resolved photoluminescence spectroscopy. Two luminescence bands are observed at 5.35 and 5.54 eV. Both emissions undergo a large blue shift of several tens of meV with a linear slope < 1 as the excitation energy Eexc increases. When eV, the spectral band positions become fixed, which marks the transition between the excitation of donor-acceptor pairs and creation of free charge carriers. We assign the 5.35 eV band to quasi donor-acceptor pair transitions and the band at 5.54 eV to free-bound transitions. Boron and nitrogen atoms distributed along characteristic defect lines in BNNTs should be involved in the luminescence process. The presented results permit a revision of previous assignments of electronic transitions in BNNTs.