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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Standing Waves Induced by Valley-Mismatched Domains in Ferroelectric SnTe Monolayers

Two-dimensional (2D) quasiparticle standing waves originate from the interference of coherent quantum states and are usually created by the scattering off edges, atomic steps, or adatoms that induce large potential barriers. We report standing waves close to the valence band maximum (E V ), confined by electrically neutral domain walls of newly discovered ferroelectric SnTe monolayers, as revealed by spatially resolved scanning tunneling spectroscopy. Ab initio calculations show that this novel confinement arises from the polarization lifted hole valley degeneracy and a ~90° rotation of the Brillouin zones that render holes’ momentum mismatched across neighboring domains. These results show a potential for polarization-tuned valleytronics in 2D ferroelectrics.

2-dimensional systems↗
NASA NTRS1979

Effects of refraction by means flow velocity gradients on the standing wave pattern in three-dimensional, rectangular waveguides

The influence of a mean vortical flow on the connection between the standing wave pattern in a rectangular three dimensional waveguide and the corresponding duct axial impedance was determined analytically. The solution was derived using a perturbation scheme valid for low mean flow Mach numbers and plane wave sound frequencies. The results show that deviations of the standing wave pattern due to refraction by the mean flow gradients are small.

Hersh, A. S.↗
NASA NTRS2012

Reverberant Acoustic Testing and Direct Field Acoustic Testing Acoustic Standing Waves and their Impact on Structural Responses

The aerospace industry has been using two methods of acoustic testing to qualify flight hardware: (1) Reverberant Acoustic Test (RAT), (2) Direct Field Acoustic Test (DFAT). The acoustic field obtained by RAT is generally understood and assumed to be diffuse, expect below Schroeder cut-of frequencies. DFAT method of testing has some distinct advantages over RAT, however the acoustic field characteristics can be strongly affected by test setup such as the speaker layouts, number and location of control microphones and control schemes. In this paper the following are discussed based on DEMO tests performed at APL and JPL: (1) Acoustic wave interference patterns and acoustic standing waves, (2) The structural responses in RAT and DFAT.

vibro-acoustics↗
DOE OSTI2020

Hard x-ray standing-wave photoemission study of the interfaces in a BiFeO$_3$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ superlattice

Hybrid multiferroics such as BiFeO$_3$ (BFO) and La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) heterostructures are highly interesting functional systems due to their complex electronic and magnetic properties. One of the key parameters influencing the emergent properties is the quality of interfaces, where varying interdiffusion lengths can give rise to different chemistry and distinctive electronic states. Here we report high-resolution depth resolved chemical and electronic investigation of BFO/LSMO superlattice using standing-wave hard X-ray photoemission spectroscopy in the first-order Bragg as well as near-total-reflection geometry. Our results show that the interfaces of BFO on top of LSMO are atomically abrupt, while the LSMO on top of BFO interfaces show an interdiffusion length of around 1.2 unit cells. The two interfaces also exhibit different chemical gradients, with the BFO/LSMO interface being Sr-terminated by a spectroscopically distinctive high binding energy component in Sr 2p core-level spectra, which is spatially contained within 1 unit cell from the interface. From the electronic point of view, unique valence band features were observed for bulk-BFO, bulk-LSMO and their interfaces. Our X-ray optical analysis revealed a unique electronic signature at the BFO/LSMO interface, which we attribute to the coupling between those respective layers. Valence band decomposition based on the Bragg-reflection standing-wave measurement also revealed the band alignment between BFO and LSMO layers. Our work demonstrates that standing-wave hard x-ray photoemission is a reliable non-destructive technique for probing depth-resolved electronic structure of buried layers and interfaces with sub-unit-cell resolution. Equivalent investigations can be successfully applied to a broad class of material such as perovskite complex oxides with emergent interfacial phenomena.

Martins, HP↗
NASA NTRS1986

Electric and magnetic observations of the structure of standing waves in the magnetosphere

Electric and magnetic field instruments on the DE 1 spacecraft provided observations of toroidal standing wave oscillations of magnetic shells in 1981 and 1982. The amplitude and nodal structure of these waves, as a function of increasing magnetic latitude, was studied and compared with model calculations of these waves. Eight pulsation events were selected for study. The pulsations in these events ranged in period from 20 to 300 s. They were observed at L values between 2.8 and 9.7 and at magnetic latitudes from the equator to 48 deg. The duration of these pulsation events was 20 to 30 min, and they usually consisted of a 5- to 10-min growth in amplitude, a 5- to 10-min interval of fluctuating amplitude, and a 5- to 20-min interval of decreasing amplitude. The relative E and B amplitudes and the phase relations between the E and B waves could be qualitatively explained by the nodal structure of the waves predicted by model calculations. Detailed quantitative comparisons of E and B amplitudes were in agreement with more recent model calculations. In a few cases, electron densities were also available as derived from plasma frequency observations; in these cases the standing wave periods calculated were reasonably close to, but somewhat less than, the observed periods.

Cahill, L. J., Jr.↗
DOE OSTI2016

Radiation forces and torque on a rigid elliptical cylinder in acoustical plane progressive and (quasi)standing waves with arbitrary incidence

This paper presents two key contributions; the first concerns the development of analytical expressions for the axial and transverse acoustic radiation forces exerted on a 2D rigid elliptical cylinder placed in the field of plane progressive, quasi-standing, or standing waves with arbitrary incidence. The second emphasis is on the acoustic radiation torque per length. The rigid elliptical cylinder case is important to be considered as a first-order approximation of the behavior of a cylindrical fluid column trapped in air because of the significant acoustic impedance mismatch at the particle boundary. Based on the rigorous partial-wave series expansion method in cylindrical coordinates, non-dimensional acoustic radiation force and torque functions are derived and defined in terms of the scattering coefficients of the elliptic cylinder. A coupled system of linear equations is obtained after applying the Neumann boundary condition for an immovable surface in a non-viscous fluid and solved numerically by matrix inversion after performing a single numerical integration procedure. Computational results for the non-dimensional force components and torque, showing the transition from the progressive to the (equi-amplitude) standing wave behavior, are performed with particular emphasis on the aspect ratio a/b, where a and b are the semi-axes of the ellipse, the dimensionless size parameter, as well as the angle of incidence ranging from end-on to broadside incidence. The results show that the elliptical geometry has a direct influence on the radiation force and torque, so that the standard theory for circular cylinders (at normal incidence) leads to significant miscalculations when the cylinder cross section becomes non-circular. Moreover, the elliptical cylinder experiences, in addition to the acoustic radiation force, a radiation torque that vanishes for the circular cylinder case. The application of the formalism presented here may be extended to other 2D surfaces of arbitrary shape, such as Chebyshev cylindrical particles with a small deformation, stadiums (with oval shape), or other non-circular geometries.

42 ENGINEERING↗
DOE OSTI2019

Observation of Nonlinear Standing Waves Excited by Plasma-Series-Resonance-Enhanced Harmonics in Capacitive Discharges

We report the first experimental observation of nonlinear standing waves excited by plasma-series-resonance-enhanced harmonics in low pressure, very high frequency, parallel plate, capacitively coupled plasmas. Furthermore, spatial structures of the harmonics of the magnetic field, measured by a magnetic probe, are in very good agreement with simulations based on a nonlinear electromagnetics model. At relatively low pressure, the nonlinear sheath motion generates high-order harmonics that can be strongly enhanced near the series resonance frequencies. Satisfying certain conditions, such nonlinear harmonics induce radial standing waves, with voltage and current maxima on axis, resulting in center-high plasma density. Excitation of higher harmonics is suppressed at higher pressures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Slot-coupled CW standing wave accelerating cavity

A slot-coupled CW standing wave multi-cell accelerating cavity. To achieve high efficiency graded beta acceleration, each cell in the multi-cell cavity may include different cell lengths. Alternatively, to achieve high efficiency with acceleration for particles with beta equal to 1, each cell in the multi-cell cavity may include the same cell design. Coupling between the cells is achieved with a plurality of axially aligned kidney-shaped slots on the wall between cells. The slot-coupling method makes the design very compact. The shape of the cell, including the slots and the cone, are optimized to maximize the power efficiency and minimize the peak power density on the surface. The slots are non-resonant, thereby enabling shorter slots and less power loss.

43 PARTICLE ACCELERATORS↗
NASA NTRS1985

Acoustic radiation potential on a sphere in plane, cylindrical, and spherical standing wave fields

The method of Gor'kov is applied for deriving the acoustic radiation potential on a sphere in an arbitrary sound field. Generalized potential and force expressions are derived for arbitrary standing wave modes in rectangular, cylindrical, and spherical geometries for the case where the sphere radius is much smaller than the wavelength. Criteria for determining radiation-potential minima are derived and examples of characteristic spatial radiation-potential profiles are presented. Single modes that can sustain stable positioning are discussed for each geometry. The localizing force strengths for representative standing wave modes in the three geometries are also compared. The positioning of samples due to acoustic forces only are considered. However, the method developed is general and is extended to include gravity or other external forces.

Barmatz, M.↗
DOE OSTI2016

Discrimination and quantification of Fe and Ni abundances in Genesis solar wind implanted collectors using X-ray standing wave fluorescence yield depth profiling with internal referencing

In this paper, X-ray standing wave fluorescence yield depth profiling was used to determine the solar wind implanted Fe and Ni fluences in a silicon-on-sapphire (SoS) Genesis collector (60326). An internal reference standardization method was developed based on fluorescence from Si and Al in the collector materials. Measured Fe fluence agreed well with that measured previously by us on a sapphire collector (50722) as well as SIMS results by Jurewicz et al. Measured Ni fluence was higher than expected by a factor of two; neither instrumental errors nor solar wind fractionation effects are considered significant perturbations to this value. Impurity Ni within the epitaxial Si layer, if present, could explain the high Ni fluences and therefore needs further investigation. As they stand, these results are consistent with minor temporally-variable Fe and Ni fractionation on the timescale of a year.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characterization of free-standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy

Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS) and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with ångström precision. We determined that (i) the exposure to air induced the formation of an InAsO 4 layer on top of the stoichiometric InAs(QM); (ii) the top interface between the air-side InAsO 4 and the InAs(QM) is not sharp, indicating that interdiffusion occurs between these two layers; (iii) the bottom interface between the InAs(QM) and the native oxide SiO 2 on top of the (Si/Mo) substrate is abrupt. In addition, the valence band offset (VBO) between the InAs(QM) and the SiO 2 /(Si/Mo) substrate was determined by HXPS. The value of VBO = 0.2 ± 0.04 eV is in good agreement with literature results obtained by electrical characterization, giving a clear indication of the formation of a well-defined and abrupt InAs/SiO 2 heterojunction. We have demonstrated that HXPS and SW-HXPS are non-destructive, powerful methods for characterizing interfaces and for providing chemical depth profiles of nanostructures, quantum membranes, and 2D layered materials.

36 MATERIALS SCIENCE↗
NASA NTRS1984

Digital data-acquisition system for measuring the free decay of acoustical standing waves in a resonant tube

A low-cost digital system based on an 8-bit Apple II microcomputer has been designed to provide on-line control, data acquisition, and evaluation of sound absorption measurements in gases. The measurements are conducted in a resonant tube, in which an acoustical standing wave is excited, the excitation removed, and the sound absorption evaluated from the free decay envelope. The free decay is initiated from the computer keyboard after the standing wave is established, and the microphone response signal is the source of the analog signal for the A/D converter. The acquisition software is written in ASSEMBLY language and the evaluation software in BASIC. This paper describes the acoustical measurement, hardware, software, and system performance and presents measurements of sound absorption in air as an example.

Meredith, R. W.↗
NASA NTRS1981

Standing waves observed at the dayside magnetopause

Energetic (greater than 24 keV) ion distributions are used in remotely sounding an ion trapping boundary at the dayside magnetopause for two inbound crossings of ISEE-2 on days 347 and 312, 1977, at 0830 and 1100 LT. Applying single particle trajectory models to the observed temporal relationships between trapping boundary distance and orientation parameters, it is concluded that longitudinal standing waves occur on the boundary, having typical wavelengths of 1500-3000 km, amplitudes of 500 km, and periods of 200-400 sec. The standing wave nodes move along the node axis with velocities ranging from 1.5 to 6 km/sec. The data are found to be inconsistent with models of traveling waves moving tailward along the boundary.

Fahnenstiel, S. C.↗
DOE OSTI2023

Equidistant optimization of elliptical superconducting rf standing wave cavities

A record accelerating rate was achieved earlier in standing wave (SW) SRF cavities when their shape was optimized for a lower peak surface magnetic field sacrificing the peak surface electric field. In view of new materials with higher limiting magnetic fields, expected for SRF cavities, in the first line of the Nb 3 Sn, the approach to optimization of cavity shape should be revised. A method of equidistant optimization, offered earlier for traveling wave (TW) cavities is applied to SW cavities. It is shown here that without limitation by magnetic field, the maximal accelerating rate is defined not only by limitations of the electric field but to a significant degree by the cavity shape. For example, for a cavity with the aperture radius of $R$ $a$ = 35 mm, the minimal ratio of the peak surface electric field to the accelerating rate is about $E$ pk / $E$ acc = 1.54. So, with the maximal surface field experimentally achieved $E$ pk ~ 125 MV / m, the maximal achievable accelerating rate is about 80 MeV / m even if there are no restrictions by the magnetic field. Optimized cavity shapes with and without limitations by a magnetic field are presented. Another opportunity—optimization for a low magnetic field, is opening for the same material, Nb 3 Sn, with the purpose of having a high-quality factor and increased accelerating rate that can be used for industrial linacs with cryocooler-based cooling scheme.

43 PARTICLE ACCELERATORS↗
NASA NTRS1967

Standing waves in self-trapped light filaments.

Light waves traveling backward and forward in very small filaments of optical material result in standing waves producing visible spatial beats, allowing detection of changes in refractive index

STANDING WAVE↗
DOE OSTI2015

X-ray standing wave analysis of nanostructures using partially coherent radiation

The effect of longitudinal (or temporal) coherence on total reflection assisted x-ray standing wave (TR-XSW) analysis of nanoscale materials is quantitatively demonstrated by showing how the XSW fringe visibility can be strongly damped by decreasing the spectral resolution of the incident x-ray beam. The correction for nonzero wavelength dispersion (δλ ≠ 0) of the incident x-ray wave field is accounted for in the model computations of TR-XSW assisted angle dependent fluorescence yields of the nanostructure coatings on x-ray mirror surfaces. Given examples include 90 nm diameter Au nanospheres deposited on a Si(100) surface and a 3 nm thick Zn layer trapped on top a 100 nm Langmuir-Blodgett film coating on a Au mirror surface. Present method opens up important applications, such as enabling XSW studies of large dimensioned nanostructures using conventional laboratory based partially coherent x-ray sources.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1985

Force on a heated sphere in a horizontal plane acoustic standing wave field

The force on a heated sphere in a horizontal plane acoustic standing wave field is the subject of this investigation. The heated sphere produces a thermal gradient in the resonance chamber. The force on the sphere in a direction perpendicular to that of gravity is measured. This force is enhanced in the region near the pressure node, and is weakened in the region near the pressure antinode. Measurements of the force on a heated sphere with sound pressure levels between 148 and 156 dB are presented.

Leung, E. W.↗