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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Predicted boron-carbide compounds: A first-principles study

By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B{sub 5}C indicates its high stability. The density of states of B{sub 5}C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B{sub 4}C, BC, and BC{sub 4}, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Grain boundary barrier modification due to coupling effect of crystal polar field and water molecular dipole in ZnO-based structures

Surface water molecules induced grain boundaries (GBs) barrier modification was investigated in ZnO and ZnMgO/ZnO films. Tunable electronic transport properties of the samples by water were characterized via a field effect transistor (FET) device structure. The FETs fabricated from polar C-plane ZnO and ZnMgO/ZnO films that have lots of GBs exhibited obvious double Schottky-like current-voltage property, whereas that fabricated from nonpolar M-plane samples with GBs and ZnO bulk single-crystal had no obvious conduction modulation effects. Physically, these hallmark properties are supposed to be caused by the electrostatical coupling effect of crystal polar field and molecular dipole on GBs barrier.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Band-selective shaped pulse for high fidelity quantum control in diamond

High fidelity quantum control of qubits is crucially important for realistic quantum computing, and it becomes more challenging when there are inevitable interactions between qubits. We introduce a band-selective shaped pulse, refocusing BURP (REBURP) pulse, to cope with the problems. The electron spin of nitrogen-vacancy centers in diamond is flipped with high fidelity by the REBURP pulse. In contrast with traditional rectangular pulses, the shaped pulse has almost equal excitation effect in a sharply edged region (in frequency domain). So the three sublevels of host {sup 14}N nuclear spin can be flipped accurately simultaneously, while unwanted excitations of other sublevels (e.g., of a nearby {sup 13}C nuclear spin) is well suppressed. Our scheme can be used for various applications such as quantum metrology, quantum sensing, and quantum information process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Near infrared emission from molecule-like silver clusters confined in zeolite A assisted by thermal activation

Strong and broad near infrared (NIR) emission peaked at ~855 nm upon optimal excitation at 342 nm has been observed from molecule-like silver clusters (MLSCs) confined in zeolite A assisted by thermal activation. To the best of our knowledge, this is the first observation of NIR emission peaked at longer than 800 nm from MLSCs confined in solid matrices. The decay time of the NIR emission is over 10 μs, which indicates that it is a spin-forbidden transition. The ~855 nm NIR emission shows strong dependence on the silver loading concentration and the thermal activation temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electronic structure and thermoelectric properties of orthorhombic SrLiAs

The electronic structure and the transport properties of orthorhombic SrLiAs were investigated using first-principles calculations and the semiclassical Boltzmann theory. It is found that the electrical conductivity along the y-direction is higher than those along other two directions, which is most likely originated from the covalent ladder-like structure formed by the Li and As atoms. Moreover, the transport properties of n-type SrLiAs are better than those of p-type one, due to the large band dispersion along the y-direction near the Fermi level. Further, the value of power factor with respect to relaxation time achieves 9.2 × 10{sup 11} W K{sup −2} m{sup −1} s{sup −1} for n-type SrLiAs along the y-direction at 1000 K with an optimal carrier concentration of 6.5 × 10{sup 20 }cm{sup −3}. The obtained minimum lattice thermal conductivity is comparable to those of other Zintl phase compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Numerical investigation of optical Tamm states in two-dimensional hybrid plasmonic-photonic crystal nanobeams

Optical Tamm states (OTSs) in analogy with its electronic counterpart confined at the surface of crystals are optical surface modes at the interfaces between uniform metallic films and distributed Bragg reflectors. In this paper, OTSs are numerically investigated in two-dimensional hybrid plasmonic-photonic crystal nanobeams (HPPCN), which are constructed by inserting a metallic nanoparticle into a photonic crystal nanobeam formed by periodically etching square air holes into dielectric waveguides. The evidences of OTSs can be verified by transmission spectra and the field distribution at resonant frequency. Similar to OTSs in one-dimensional multilayer structures OTSs in HPPCN can be excited by both TE and TM polarization. The physical origin of OTSs in HPPCN is due to the combined contribution of strong reflection imposed by the photonic band gap (PBG) of the photonic crystal (PC) nanobeam and strong backward scattering exerted by the nanoparticle. For TE, incidence OTSs can be obtained at the frequency near the center of the photonic band gap. The transmissivity and the resonant frequency can be finely tuned by the dimension of nanoparticles. While for TM incidence OTSs are observed for relatively larger metallic nanoparticles compared with TE polarization. The differences between TE and TM polarization can be explained by two reasons. For one reason stronger backward scattering of nanoparticles for TE polarization can be achieved by the excitation of localized surface plasmon polariton of nanoparticles. This assumption has been proved by examining the scattering, absorption, and extinction cross section of the metallic nanoparticle. The other can be attributed to the deep and wide PBG available for TE polarization with less number of air holes compared with TM polarization. Our results show great promise in extending the application scope of OTSs from one-dimensional structures to practical integrated photonic devices and circuits.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

High-performance zero-bias ultraviolet photodetector based on p-GaN/n-ZnO heterojunction

Lattice-match p-GaN and n-ZnO bilayers were heteroepitaxially grown on the c-sapphire substrate by metal organic chemical vapor deposition and molecular beam epitaxy technique, respectively. X-ray diffraction and photoluminescence investigations revealed the high crystal quality of the bilayer films. Subsequently, a p-GaN/n-ZnO heterojunction photodetector was fabricated. The p-n junction exhibited a clear rectifying I-V characteristic with a turn-on voltage of 3.7 V. At zero-bias voltage, the peak responsivity was 0.68 mA/W at 358 nm, which is one of the best performances reported for p-GaN/n-ZnO heterojunction detectors due to the excellent crystal quality of the bilayer films. These show that the high-performance p-GaN/n-ZnO heterojunction diode is potential for applications of portable UV detectors without driving power.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Dynamic Fano-like resonances in erbium-doped whispering-gallery-mode microresonators

We report Fano-like asymmetric resonances modulated by optical gain in a whispering-gallery-mode resonator fabricated from erbium-doped silica. A time-dependent gain profile leads to dynamically varying sharp asymmetric resonances with features similar to Fano resonances. Depending on the scan speed of the frequency of the probe laser and the pump-probe power ratio, transmission spectra of the active microcavity exhibit a resonance dip, a resonance peak, or a Fano-like resonance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Mitigating impact of thermal and rectified radio-frequency sheath potentials on edge localized modes

The mitigating impact of thermal and rectified radio frequency (RF) sheath potentials on the peeling-ballooning modes is studied non-linearly by employing a two-fluid three-field simulation model based on the BOUT++ framework. Additional shear flow and the Kelvin-Helmholtz effect due to the thermal and rectified RF sheath potential are induced. It is found that the shear flow increases the growth rate while the K-H effect decreases the growth rate slightly when there is a density gradient, but the energy loss of these cases is suppressed in the nonlinear phase. The stronger external electrostatic field due to the sheaths has a more significant effect on the energy loss suppression. From this study, it is found the growth rate in the linear phase mainly determines the onset of edge-localized modes, while the mode spectrum width in the nonlinear phase has an important impact on the turbulent transport. The wider mode spectrum leads to weaker turbulent transport and results in a smaller energy loss. Due to the thermal sheath and rectified RF sheath potential in the scrape-off-layer, the modified shear flow tears apart the peeling-ballooning filament and makes the mode spectrum wider, resulting in less energy loss. The perturbed electric potential and the parallel current near the sheath region is also suppressed locally due to the sheath boundary condition.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

The elastic and thermoelectric properties of the Zintl compound Ca{sub 5}Al{sub 2}Sb{sub 6} under high pressure

The elastic and thermoelectric properties of Ca{sub 5}Al{sub 2}Sb{sub 6} under pressure are studied using ab initio calculation and semiclassical Boltzmann theory. The calculated elastic constants and minimum thermal conductivity indicate that Ca{sub 5}Al{sub 2}Sb{sub 6} exhibits an anisotropic structure and high thermoelectric performance. The size of the band gap shows a nonlinear change with increasing pressure. Based on the electronic structure, the calculated thermoelectric parameters show that the Seebeck coefficient has no obvious change under pressure, whereas the electrical conductivity improves with increasing pressure. Therefore, the power factor increases at an appropriate pressure of P = 2.6 GPa. P-type doping of Ca{sub 5}Al{sub 2}Sb{sub 6} may achieve better thermoelectric performance than n-type doping, in agreement with experiment. The anisotropic thermoelectric properties of Ca{sub 5}Al{sub 2}Sb{sub 6} indicate that the thermoelectric performance along the z-direction is superior to other directions. This is attributed to the combination of the large dispersion and high band degeneracy along the Γ-Z direction in the band structure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Structure and magnetic properties of the perovskite YCo{sub 0.5}Fe{sub 0.5}O{sub 3}

Y Co{sub 0.5}Fe{sub 0.5}O{sub 3}, in a structure of perovskite, has been successfully prepared with citrate precursors at 950-1100 °C in air by the sol-gel method. The X-ray diffraction patterns show that the samples are orthorhombic within the space group Pnma, where the Co and Fe ions are disordered at the 4b crystallographic sites. The crystal structure refinement undertaken by the Rietveld method has shown that the distortion of Co(Fe)O{sub 6} octahedra are large, where the ratio of Co/Fe-O bonding length along a axis to that in the bc plane is about 1.07. Such a large crystal lattice distortion implies a strong lattice-magnetism coupling, which may be utilized in the magnetoelectric devices. Magnetic measurement indicates that the Y Co{sub 0.5}Fe{sub 0.5}O{sub 3} is antiferromagnetic but showing weak ferromagnetism. We find that Fe{sup 3+} ions are in high-spin states, while Co{sup 3+} ions are in low-spin states which do not contribute to the magnetism. Both Fe{sup 3+} and Co{sup 3+} ions are not Jahn-Teller activated although the lattice distortion is large.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Rapid characterizing of ferromagnetic materials using spin rectification

Spin rectification is a powerful tool for dc electric detections of spin dynamics and electromagnetic waves. Technically, elaborately designed on-chip microwave devices are needed in order to realize that effect. In this letter, we propose a rapid characterizing approach based on spin rectification. By directly sending dynamic current into ferromagnetic films with stripe shape, resonant dc voltages can be detected along the longitudinal or transversal directions. As an example, Fe (010) films with precise crystalline structure and magnetic parameters were used to testify the reliability of such method. We investigated not only the dynamic parameters and the precise anisotropy constants of the Fe crystals but also the principle of spin rectification in this method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Angular dependent study on ferromagnetic resonance and spin excitations by spin rectification

We report angular dependent spin rectification spectra which are applied to studying spin excitations in single permalloy stripe. Based on planar Hall effect, those spin excitations generate special resonant dc Hall voltages, which have been characterized as functions of the amplitude and direction of applied magnetic field. Through high angular resolution 2D mappings, the evolutions of different spin excitation can be directly presented, and the dynamic magnetic parameters such as the gyromagnetic ratio, effective exchange field, as well as the quantized numbers of standing spin waves can be accurately determined through fitting the angular evolution of each resonance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Probing flexible conformations in molecular junctions by inelastic electron tunneling spectroscopy

The probe of flexible molecular conformation is crucial for the electric application of molecular systems. We have developed a theoretical procedure to analyze the couplings of molecular local vibrations with the electron transportation process, which enables us to evaluate the structural fingerprints of some vibrational modes in the inelastic electron tunneling spectroscopy (IETS). Based on a model molecule of Bis-(4-mercaptophenyl)-ether with a flexible center angle, we have revealed and validated a simple mathematical relationship between IETS signals and molecular angles. Our results might open a route to quantitatively measure key geometrical parameters of molecular junctions, which helps to achieve precise control of molecular devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microwave meta-atom enhanced spintronic rectification

An artificial meta-atom (MA), or alternatively, a plasmonic antenna, has been demonstrated to significantly enhance the microwave spin rectifying photovoltage by more than two orders in magnitude (∼280) in the ferromagnetic resonance regime. The large enhancement is attributed to the unique structure of the MA which magnifies both microwave electric (∼5) and magnetic (∼56) fields in the same near-field spatial region. Our work develops the interdisciplinary direction with artificial and natural magnetism and may find promising applications in high-frequency or opto-spintronic devices and wireless microwave energy harvesting.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Photoelectron imaging spectroscopy of MoC{sup −} and NbN{sup −} diatomic anions: A comparative study

The isoeletronic diatomic MoC{sup −} and NbN{sup −} anions have been prepared by laser ablation and studied by photoelectron imaging spectroscopy combined with quantum chemistry calculations. The photoelectron spectra of NbN{sup −} can be very well assigned on the basis of literature reported optical spectroscopy of NbN. In contrast, the photoelectron spectra of MoC{sup −} are rather complex and the assignments suffered from the presence of many electronically hot bands and limited information from the reported optical spectroscopy of MoC. The electron affinities of NbN and MoC have been determined to be 1.450 ± 0.003 eV and 1.360 ± 0.003 eV, respectively. The good resolution of the imaging spectroscopy provided a chance to resolve the Ω splittings of the X{sup 3}Σ{sup −} (Ω = 0 and 1) state of MoC and the X{sup 4}Σ{sup −} (Ω = 1/2 and 3/2) state of MoC{sup −} for the first time. The spin-orbit splittings of the X{sup 2}Δ state of NbN{sup −} and the a{sup 2}Δ state of MoC{sup −} were also determined. The similarities and differences between the electronic structures of the NbN and MoC systems were discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

High sensitivity microwave detection using a magnetic tunnel junction in the absence of an external applied magnetic field

In the absence of any external applied magnetic field, we have found that a magnetic tunnel junction (MTJ) can produce a significant output direct voltage under microwave radiation at frequencies, which are far from the ferromagnetic resonance condition, and this voltage signal can be increase by at least an order of magnitude by applying a direct current bias. The enhancement of the microwave detection can be explained by the nonlinear resistance/conductance of the MTJs. Our estimation suggests that optimized MTJs should achieve sensitivities for non-resonant broadband microwave detection of about 5000 mV/mW.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Direct numerical simulation of turbulent flow in a rotating square duct

A fully developed turbulent flow in a rotating straight square duct is simulated by direct numerical simulations at Re{sub τ} = 300 and 0 ≤ Ro{sub τ} ≤ 40. The rotating axis is parallel to two opposite walls of the duct and normal to the main flow. Variations of the turbulence statistics with the rotation rate are presented, and a comparison with the rotating turbulent channel flow is discussed. Rich secondary flow patterns in the cross section are observed by varying the rotation rate. The appearance of a pair of additional vortices above the pressure wall is carefully examined, and the underlying mechanism is explained according to the budget analysis of the mean momentum equations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗