Standing Wave Conditions Research Articles

Structural and magnetic asymmetry at the interfaces of MgO/FeCoB/MgO trilayer: Precise study under x-ray standing wave conditions

Magnetic tunnel junctions based on FeCoB as a magnetic electrode and MgO as a tunneling barrier gained much attention because of their applications in random access memories and magnetic sensors in disk drives. In this work, the structural and magnetic properties of the MgO/FeCoB/MgO trilayer have been studied precisely under x-ray standing wave (XSW) conditions, where XSW is generated through a high-density (Pt) waveguide structure. The combined x-ray scattering and fluorescence data obtained under XSW conditions revealed the formation of a high-density FeCoB layer at the MgO/FeCoB interface (FeCoB-on-MgO) in the as-deposited trilayer. Diffusion of B from the FeCoB layer into MgO is attributed to the formation of Fe- and Co-rich high-density layer (B-deficient FeCoB layer) at the interface. Angular-dependent magnetism of the trilayer structure revealed the presence of in-plane magnetic anisotropy (IMA), which disappeared with thermal annealing at a temperature of 450 °C. Stress in B-deficient FeCoB layer at the interface is attributed to the origin of IMA through magneto-elastic anisotropy energy minimization. The disappearance of anisotropy after annealing is mainly due to the removal of long-range stress and the formation of crystalline bcc-FeCo phase.

Quantification of the Field Enhancement of Surface Plasmon Under Standing Wave Conditions

Quantification of the Field Enhancement of Surface Plasmon Under Standing Wave Conditions

Unravelling the Acoustic and Thermal Responses of Perfluorocarbon Liquid Droplets Stabilized with Cellulose Nanofibers.

The attractive colloidal and physicochemical properties of cellulose nanofibers (CNFs) at interfaces have recently been exploited in the facile production of a number of environmentally benign materials, e.g. foams, emulsions, and capsules. Herein, these unique properties are exploited in a new type of CNF-stabilized perfluoropentane droplets produced via a straightforward and simple mixing protocol. Droplets with a comparatively narrow size distribution (ca. 1-5 μm in diameter) were fabricated, and their potential in the acoustic droplet vaporization process was evaluated. For this, the particle-stabilized droplets were assessed in three independent experimental examinations, namely temperature, acoustic, and ultrasonic standing wave tests. During the acoustic droplet vaporization (ADV) process, droplets were converted to gas-filled microbubbles, offering enhanced visualization by ultrasound. The acoustic pressure threshold of about 0.62 MPa was identified for the cellulose-stabilized droplets. A phase transition temperature of about 22 °C was observed, at which a significant fraction of larger droplets (above ca. 3 μm in diameter) were converted into bubbles, whereas a large part of the population of smaller droplets were stable up to higher temperatures (temperatures up to 45 °C tested). Moreover, under ultrasound standing wave conditions, droplets were relocated to antinodes demonstrating the behavior associated with the negative contrast particles. The combined results make the CNF-stabilized droplets interesting in cell-droplet interaction experiments and ultrasound imaging.

Pitting Corrosion in Steel Controlled By the Electrochemical Potential Combined with Synchrotron X-Rays

Corrosion of metal alloys such as steel is a crucial field of study [1-4] due to its direct implication in various aqueous environments (seawater, rain, surface or ground water, etc.). The pitting corrosion of stainless steel (SS), occurring at the atomic level, critically weakens structures [5] locally, eventually leading to their failure. A variety of pit shapes have been reported to be related to the surface chemistry of the alloy/electrolyte interface and the presence of structural defects [6]. However, the very first stage of the pit formation, shown to occur suddenly, is not clear yet [7]. Stainless steel (SS) is a complex alloy which can incorporate several metallic (Fe, Cr, Ni, Mo, Mn, etc.) and non-metallic (C, Si, N, P, S, etc.) elements. Those elements can potentially favour both the corrosion propagation (e.g. Fe oxidation leading to rust formation) and prevention (e.g. Cr oxidation giving rise to passivating Cr2O3) [8-9]. Hence, a combination of experimental methods involving synchrotron radiation was utilised in electrochemical conditions to study the early stage of the SS disintegration. Thin films of SS304L (low carbon, Mo free alloy) of different thicknesses have been grown in India, using DC Magnetron Sputtering [10], on two different substrates, i.e. (Si (100) and Ru/B4C multilayer; 39.1Å bilayer on Si). Thin films are used to enhance the sensitivity of the different experimental techniques to the very surface of the samples and reduce the bulk signal. We first report on this process, using X-Ray Powder Diffraction (XRPD) and X-Ray Reflectivity (XRR) to characterise the structural composition and thickness of the film respectively. The chemical composition of the films was then characterised using X-ray Fluorescence Spectroscopy (XFS). The evolution of the films deposited on Si (100) and Ru/B4C multilayer was investigated using X-ray Absorption Near Edge Structure (XANES) and X-ray Fluorescence Spectroscopy in X-ray Standing Wave conditions (XFS-XSW) respectively. The evolution was obtained by comparing the XANES and XFS-XSW spectrums of the pristine films to the same films after electrochemical treatments in an alkaline electrolyte (0.1M KCl). XANES comparison between the pristine SS 304L thin film (500Å on Si (100)) and references (SS 304 and Fe foils) revealed that the spectrum is well fitted by a linear combination of 56% SS 304 in a fcc structure, 29% Fe in a bcc structure and 15% of Fe3O4 oxide layer. The Ru/B4C multilayer substrate is used to generate a XSW with its planes parallel to the surface, probing the very few atomic layers of the surface, the SS thin film in this case. The relative position of the XSW normal to the surface can be shifted, thus allowing to probe the depth distribution of metal components like Fe, Ni and Cr with respect to SS thin film surface with sub-submonolayer resolution. From the XFS-XSW on SS 304L (30Å on multilayer) before and after electrochemical treatments, we observe some significant change in composition in terms of positions and coherent fraction [11] obtained from XSW analysis. We will present first results of in-situ experiments were the thin film / electrolyte interface is probed continuously whilst the electrochemical treatment is applied. The aim is to further comprehend the chemical and structural processes initiating the pitting corrosion of stainless steel.

Emergence of Novel Multipactor Modes Under Standing Wave Conditions in a Coaxial Line With an RF Window

The multipactor (MP) phenomenon can generate exponentially increasing electron populations in RF vacuum systems, leading to components becoming damaged. This paper uses numerical methods to analyze an RF vacuum window in a coaxial line under various standing wave conditions. Our methods are first benchmarked against the analytic and experimental results for the traveling wave and the numerical results for standing waves that were found in previous work. Second, the effect of standing waves on electron motion in the axial direction is discussed and related to electron trajectories in the presence of an RF window. It is found that standing waves with the magnetic maximum located behind the RF window lead to window collisions that can affect MP depending on the power, frequency, and conductor radii. For a low number of window collisions, MP was highly predictable by earlier work, but as the percentage of collisions occurring on the window surpassed approximately 50%, a novel mode of MP emerged to dominate the phenomenon.

Relationship of scattering phase shifts to special radiation force conditions for spheres in axisymmetric wave-fields.

When investigating the radiation forces on spheres in complicated wave-fields, the interpretation of analytical results can be simplified by retaining the s-function notation and associated phase shifts imported into acoustics from quantum scattering theory. For situations in which dissipation is negligible, as taken to be the case in the present investigation, there is an additional simplification in that partial-wave phase shifts become real numbers that vanish when the partial-wave index becomes large and when the wave-number-sphere-radius product vanishes. By restricting attention to monopole and dipole phase shifts, transitions in the axial radiation force for axisymmetric wave-fields are found to be related to wave-field parameters for traveling and standing Bessel wave-fields by considering the ratio of the phase shifts. For traveling waves, the special force conditions concern negative forces while for standing waves, the special force conditions concern vanishing radiation forces. An intermediate step involves considering the functional dependence on phase shifts. An appendix gives an approximation for zero-force plane standing wave conditions. Connections with early investigations of acoustic levitation are mentioned and some complications associated with viscosity are briefly noted.

Positron wave interference and Klein tunnel during the production of pairs in the double-well potential

In this paper, the electron-positron creation process in a double well scheme is investigated. A series of simulations is conducted by solving the quantized Dirac equation numerically. Here the split operator scheme is used to solve the Dirac equation, and the Fourier analysis is adopted to study the evolution of the wave function. The evolution starts from the state that all the negative energy eigenstates are occupied. By projecting the time dependent wave function to the positive energy eigenstates, the distributions of electrons and positrons in coordinate space and momentum space would be calculated. The total number of the electrons and positrons can be obtained by integrating the momentum distributions, and the number of the positrons in different parts of coordinate space can be achieved by integrating the space distributions. At first the electron-positron is created at the double-well edge, and positrons are emitted from the edges of double-well potential and propagate out while the electrons are bounded by the barriers. It is found that when the positron waves from different double-well edges encounter in the double-well for the first time, there occurs no positron wave interference phenomenon. The wave interference emerges after the positron no indent wave is reflected by the barriers. At the same time, because of Klein tunneling the number of positrons outside the double well begin to surpass the positrons inside the double well. After a piece of time, the amplitude of interference wave would reach its peak, and then collapses since Klein tunneling. If the double-well potential meets the standing-wave conditions, a stationary wave would be found before the interference wave reaches its peak if the distance between the double wells is short, and a stationary wave would be found after the interference wave has reached its peak if the distance between the double wells is long. And the stationary wave occurs when the positron wave is reflected by the barriers for the second time. The occurring of the stationary wave would affect the pairs producing process by making the number of pairs fluctuate. Because of Klein tunneling, the wave packages close to the double-well would disappear first, and the others can last for a longer time when the standing-wave condition is fulfilled, but all of the stationary wave packages disappear in the double well finally. And there is barely no positrons left inside the double well to the end since Klein tunneling.

Microwave Discharge as an Effective Tool for Surface Treatment of Small Samples

Plasma created in different gases using conventional microwave (MW) discharge is presented. The discharge chamber is a quartz tube mounted into a rectangular waveguide. Standing-wave conditions established in the MW cavity allow for creation of plasma in a small volume. The discharge power up to 1200 W allows for treatment of small samples with rather intense plasma. Samples mounted on sample holders with poor thermal contact with cold materials are heated to elevated temperature that allows for intensive surface chemical reactions. Plasma color depends on the type of gas and impurities released from samples as a result of plasma treatment.

Communication through the substrate as a solution for small plant‐dwelling insects.

High‐frequency airborne sound is quickly absorbed in vegetation, and outside the plant shelter such signals attract predators. Furthermore, small insects cannot emit efficiently low‐frequency airborne sound when their diameter is smaller than a third of the radiated wavelength. For plant‐dwelling insects, substrate‐borne sound signaling remains the best solution for communication in dense vegetation typical for the tropics. Communication range in such an environment depends on tuning of vibratory signal properties with mechanical characteristics of plants and sensory abilities to extract the information from the environmental noise. Nezara viridula and other until now investigated species of the stink bug family Pentatomidae represent the model for solutions optimizing long‐range communication through green plants. They communicate with vibratory signals of the dominant frequency around 100 Hz, which travel through plants with low attenuation, creating standing wave conditions in the plant’s rod‐like stru...

Discovery of outcrop-scale fine-grained sediment waves in the lower Halang Formation, an upper Miocene submarine-fan succession in West Java

We report the first discovery of outcrop-scale, fine-grained sediment waves in the left-side muddy overbank deposits relative to the down-current direction of the adjacent channel deposits in the lower Halang Formation turbidite system in a late Miocene back-arc basin, West Java. The present outcrop examples are characterized by an average wavelength of 10.7 m and an average wave height of 0.13 m, and the aspect ratio is similar to that of modern large-scale, fine-grained sediment waves. The dimension and asymmetry of waveforms decrease in the down-current direction in association with the increase in wave steepness. In particular, aggradation and up-current migrating of waveforms are commonly observed in the proximal part of overbank deposits. Furthermore, the stoss-side deposits locally contain thicker and coarser sandstone and coarse siltstone beds (beds up to 5 cm thick) than those of the lee-side deposits. The basal part of individual waveforms does not contain any sandy bedform and/or irregularity as a precursor for developing asymmetrical waveforms from fine-grained turbidity currents. These spatial variations in geometry and component deposits of the waveforms are interpreted to have been formed under antidune or cyclic-step and standing-wave conditions from the proximal to distal overbank environments, respectively. The present outcrop-scale examples can fill the gap in dimension and formative processes between laboratory-scale bedforms and modern large-scale, fine-grained sediment waves for elucidating scale-independent processes, which are responsible for the development of fine-grained waveforms in a deep-water environment.

The impact of standing wave effects on transcranial focused ultrasound disruption of the blood–brain barrier in a rat model

Microbubble-mediated disruption of the blood–brain barrier (BBB) for targeted drug delivery using focused ultrasound shows great potential as a therapy for a wide range of brain disorders. This technique is currently at the pre-clinical stage and important work is being conducted in animal models. Measurements of standing waves in ex vivo rat skulls were conducted using an optical hydrophone and a geometry dependence was identified. Standing waves could not be eliminated through the use of swept frequencies, which have been suggested to eliminate standing waves. Definitive standing wave patterns were detected in over 25% of animals used in a single study. Standing waves were successfully eliminated using a wideband composite sharply focused transducer and a reduced duty cycle. The modified pulse parameters were used in vivo to disrupt the BBB in a rat indicating that, unlike some other bioeffects, BBB disruption is not dependent on standing wave conditions. Due to the high variability of standing waves and the inability to correctly estimate in situ pressures given standing wave conditions, attempts to minimize standing waves should be made in all future work in this field to ensure that results are clinically translatable.

Low-Dimensionality Effects in the Melting of a Langmuir−Blodgett Multilayer

Low-dimensionality effects in the melting behavior of a cadmium arachidate Langmuir-Blodgett multilayer have been studied. Depth resolved information about structural changes occurring with temperature is obtained using in-plane X-ray diffraction under standing wave conditions. The surface region exhibits a distinctly different melting behavior as compared to the bulk of the film. While in the bulk of a 13-monolayer cadmium arachidate multilayer, the crystalline phase directly transforms to a tilted hexaticlike phase at 360 K, in the near surface region transformation occurs via an intermediate smectic phase. This behavior of the surface region is similar to that observed in two-dimensional crystals. Thus even in a thick Langmuir-Blodgett multilayer, the surface region exhibits low-dimensionality effects.

Study of nano-scale diffusion in thin films and multilayers

It is shown that x-ray reflectivity and x-ray fluorescence under standing wave conditions can be used to study atomic diffusion with an accuracy of a fraction of a nanometer. Both the techniques can be made isotope selective by making use of nuclear resonance scattering from a Mossbauer active isotope. The techniques have been used to study self-diffusion of Fe in amorphous and nano-crystalline alloys of FeZr and FeN. The observed correlation of the activation energy E with pre-exponent factor D0 confirms that in amorphous FeZr alloy diffusion takes place via collective motion of a group of atoms. Even in nanocrystalline alloys it is found that atomic diffusion occurs mainly through grain boundaries which are amorphous or highly disordered in nature.

Depth resolved x-ray absorption fine structure study in magnetic multilayers using x-ray standing waves

X-ray standing waves generated using total external reflection from Au buffer layer has been used to get information about the structure of the interfaces in a Tb∕Fe∕Tb trilayer. X-ray absorption fine structure (XAFS) measurements done under standing wave conditions have sufficient depth resolution in order to differentiate the center of the Fe layer from the interfacial region. XAFS measurements are done in two geometries, with plane of polarization of the incident x-ray lying either in the film plane or perpendicular to it. In these two geometries information is obtained about the bonds lying in the film plane or perpendicular to it, respectively. These measurements provide clear evidence that Fe–Tb bonds in the interfacial region lie preferentially normal to the film plane. This asymmetry of Fe–Tb bonds is the cause of perpendicular magnetic anisotropy in the system. Present studies demonstrate that use of x-ray standing waves can provide sufficient depth resolution in order to elucidate the interface structure in magnetic multilayers.

Compact Waveguide-Based Power Divider Feeding Independently Any Number of Coaxial Lines

The device described in this paper has been designed to enable the feeding of many individual plasma sources from a single microwave generator, providing a noninterfering and constant supply of power to each coaxial line driving these plasma sources. The power coming from the generator flows through a waveguide under standing-wave conditions provided by the presence of a conducting plane located at the waveguide end opposite that linked to the generator. Power is extracted from the waveguide, at the maximum of intensity of the E-field standing wave, by a waveguide-to-coaxial-line transition designated as a probe. One or two probes can be set at each such maximum of field intensity (and this on both sides of the waveguide wide wall), yielding a compact power divider. Each coaxial line feeds a microwave field applicator, sustaining plasma, through a matching circuit comprising a tuning means and a ferrite isolator (circulator with a matched load), the latter ensuring that whatever happens to the plasma source, the other feeding lines are not affected. The conditions required for a perfect match of the microwave generator to the power divider are elaborated and examples of actual designs are presented

Bubble-based acoustic radiation force using chirp insonation to reduce standing wave effects

Bubble-based acoustic radiation force can measure local viscoelastic properties of tissue. High intensity acoustic waves applied to laser-generated bubbles induce displacements inversely proportional to local Young’s modulus. In certain instances, long pulse durations are desirable but are susceptible to standing wave artifacts, which corrupt displacement measurements. Chirp pulse acoustic radiation force was investigated as a method to reduce standing wave artifacts. Chirp pulses with linear frequency sweep magnitudes of 100, 200 and 300 kHz centered around 1.5 MHz were applied to glass beads within gelatin phantoms and laser-generated bubbles within porcine lenses. The ultrasound transducer was translated axially to vary standing wave conditions, while comparing displacements using chirp pulses and 1.5 MHz tone burst pulses of the same duration and peak rarefactional pressure. Results demonstrated significant reduction in standing wave effects using chirp pulses, with displacement proportional to acoustic intensity and bubble size. (E-mail: terpeldi@umich.edu)

Depth Resolved Structural Studies in Multilayers Using X-ray Standing Waves

X-ray based characterization techniques are powerful tools for the study of atomic scale structure of materials. However, high penetrating power of X-rays make them less suitable for depth selective studies, as required in the characterization of multilayer structures. In the present work, it is shown that depth selectivity of the techniques like, X-ray fluorescence, X-ray absorption spectroscopy and nuclear resonance fluorescence can be greatly enhanced by generating X-ray standing waves inside the multilayer structure. The concentration profiles of various elements can be obtained with a depth resolution of the order of 0.1 nm. Depth dependent information about the local structure around a given atom can be obtained from XAFS under standing wave conditions. It is demonstrated that detection of nuclear resonance fluorescence by tuning the energy of the incident X-rays to a Mossbauer transition can yield depth profile of a particular isotope, and can be used for self-diffusion studies. The techniques of X-ray reflectivity and conversion electron Mossbauer spectroscopy are used to provide useful complementary information.

Depth selective XANES study of swift heavy ion irradiation effects in metal/Si systems

Swift heavy ion induced intermixing of thin metallic layers of Ti, Fe and W with Si has been studied. A single specimen containing 3.0 nm thick layers of the three metals at different depths in Si, ensures a more reliable comparison of the mixing efficiencies of the three systems. X-ray reflectivity and X-ray standing wave analysis has been used to obtain the concentration profile of the three metals before and after irradiation with 100 MeV Au ions. While Ti and Fe exhibit considerable mixing with Si after an irradiation fluence of 1 × 10 13 ions/cm 2, W profile is only slightly broadened. XANES measurements under X-ray standing wave conditions have been able to differentiate between the structure of the center of a layer from that of the interfacial region. It is found that irradiation results in only a further disordering of the central as well as interfacial regions of the W layer. In contrast, the Fe layer gets completely mixed with Si and forms an amorphous silicide.

Dynamics of Buried Polymer -polymer Interfaces in Thin Films

ABSTRACTWe have performed x-ray photon correlation spectroscopy (XPCS) measurements on a polymer-bilayer system comprised of 100 nm polystyrene film on top of an 80 nm polybromostyrene film, supported on a Si substrate. In order to distinguish the dynamics at the top interface from that at the polymer-polymer interface we have performed the measurement at grazing incidence. In this geometry, a standing wave is set up in the film. We derive a relation for the intensity of the standing wave and the resulting diffuse scattering. This model is compared with the measured diffuse scattering from which we extract a value of 0.7±0.4 dyne/cm for the surface tension between PS and PBrS at 180C. XPCS was then measured in each of two standing wave conditions, first where diffuse scattering only occurs at the polymer-vacuum interface and then where it only occurs at the interior polymer-polymer interface. The measured time correlation functions for each of the two interfaces show clear differences, with the polymer-polymer interface exhibiting much slower dynamics.

Communication with substrate-borne signals in small plant-dwelling insects.

Vibratory signals of plant-dwelling insects, such as land bugs of the families Cydnidae and Pentatomidae, are produced mainly by stridulation and/or vibration of some body part. Signals emitted by the vibratory mechanisms have low-frequency characteristics with a relatively narrow frequency peak dominant around 100 Hz and differently expressed frequency modulation and higher harmonics. Such spectral characteristics are well tuned to the transmission properties of plants, and the low attenuation enables long-range communication on the same plant under standing wave conditions. Frequencies of stridulatory signals extend up to 10 kHz. In some groups, vibratory and stridulatory mechanisms may be used simultaneously to produce broadband signals. The subgenual organ, joint chordotonal organs, campaniform sensilla and mechanoreceptors, such as the Johnston's organ in antennae, are used to detect these vibratory signals. Species-specific songs facilitate mate location and recognition, and less species-specific signals provide information about enemies or rival mates.