Anomalous Edge Research Articles

Grazing incidence diffraction anomalous fine structure study of GaN/AlN quantum dots

We report on a general method that takes advantage of the full capability of anomalous diffraction and can be applied to the very interesting and challenging case of small size embedded nanostructures. We study GaN quantum dots (QDs) grown by MBE in the Modified-Stransky–Krastanow regime, and encapsulated by an AlN epilayer. We investigate the QDs strain and composition that are related to size, morphology, and cap layer thickness by means of anomalous diffraction in grazing incidence. The X-ray energy is tuned across the absorption Ga K-edge where the Ga atoms scattering power is strongly modified and diffraction becomes chemically selective, giving direct information on composition. hkl-scans close to the (30−30) Bragg reflection of AlN at several energies above and below the Ga K-edge, allow to extract the structure factor FA of the Ga atoms which gives the in-plane lattice parameter, a, of GaN. Quantitative analysis of the anomalous edge lineshape and of the oscillatory extended region above the edge give information on the composition and the out-of-plane strain of the dots. A two-step covering mechanism of QDs By AlN is suggested.

A Structural Study on Self-regenerative Catalyst for Automotive Emissions Control by XAFS and DANES

A mechanism of how the Palladium-composite Perovskite catalyst for automotive emissions control keeps its activity long-lived is investigated by using XAFS and DANES (Diffraction Anomalous Near Edge Structure), a family of an anomalous X-ray diffraction techniques. The conventional catalysts are finely divided precious metal particles dispersed on a solid support. Since they suffers from heat in the car, their activity deteriorates because of the growth and agglomeration of the particles. Recently, however, the Palladium-composite Perovskite has been found to have retention of high catalyst activity during long-term use and ageing. In order to investigate the mechanism of such an ideal property in the atomic revel, X-ray diffraction and absorption techniques were employed. We find that as the catalyst is cycled between oxidative and reductive atmospheres typically encountered in exhaust gas, palladium reversibly moves into and out of the perovskite lattice. This movement appears to suppress the growth of metallic palladium particles.

Suppression of Anomalous Edge Channel Effect for 0.15 µm DRAM Cell and Beyond

Dynamic random access memory (DRAM) cell for suppressing the anomalous threshold voltage (VT) lowering due to interlayer dielectric (ILD)-related edge channel effect has been intensively investigated. Our work verifies that anomalous edge channel effect is mainly responsible for the migration of ILD-contained residual hydrogen and moisture (H2O) into Si/SiO2. Thus, the cell structure to cut off migration path is suggested, which enables to realize the robust characteristics of retention time, regardless of the variation of ILD-related process integration in 0.15 µm-DRAM.

The divertor program in stellarators

Two significant problems that need to be solved for any future fusion device are heat removal and particle control. A very promising method to attack these problems in tokamaks and helical devices is the use of a divertor, providing a controlled interaction zone between plasma and wall. By carefully designing a divertor, conditions can be created in front of the divertor targets, which lead to a sufficient reduction of the power load on the targets by strong radiation redistribution. Any solution of course needs to allow for an energy confinement which is at least sufficient for the realization of a fusion reactor. Since energy confinement has been found to be strongly related to edge anomalous transport and edge plasma profiles, the ultimate aim is to find an integral solution which is optimum with respect to exhaust, heat load and energy confinement.Two different types of divertors are presently being investigated in helical devices: the `helical divertor' and the `island divertor'. So far divertor concepts have been investigated only in a few helical devices. Theoretical and experimental efforts have mainly concentrated on the suitability of divertor magnetic field structures, while detailed studies of the divertor plasma properties for the two types of divertor configurations have only recently begun. In the course of this exploration, a promising new high-density H-mode (HDH) plasma operational regime has been discovered on the Wendelstein stellarator W7-AS. It benefits from high-energy (up to twice the value of the International Stellarator Scaling ISS95) and low impurity confinement times, complemented by edge radiated power fractions of up to 90% in detached regimes. This allowed quasi-steady-state operation for up to 50 energy confinement times and so far was only constrained by machine operability.

Bias Dependent Subband Edges and the 0.7 Conductance Anomaly

The 0.7 (2e2/h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts by measuring the differential conductance G as a function of source-drain bias Vsd and gate-source bias Vgs as well as a function of temperature. In the Vgs- Vsd plane we use a grayscale plot of the transconductance dG/dVgs to map out the bias dependent transitions between the normal and anomalous conductance plateaus. Any given transition is interpreted as arising when the bias controlled chemical potential μd (μs) of the drain (source) reservoir crosses a subband edge εα in the point contact. From the grayscale plot we extract the constant normal subband edges ε0, ε1,... and most notably the bias dependent anomalous subband edge ε'0 (μd) split off from ε0. We show by applying a finite-bias version of the recently proposed BCF model, how the bias dependence of the anomalous subband edge is the key to analyze various experimental observations related to the 0.7 anomaly.

Overscreening Kondo effect induced by photoemission

We study how the overscreening Kondo effect is dynamically induced to affect the photoemission spectrum when a core electron in the Kondo impurity with higher spin is emitted by the photoemission. The photoemission spectrum thereby shows an anomalous power-law edge singularity characteristic of the overscreening model. We discuss this anomalous behavior by using the exact solution and boundary conformal field theory. We also apply the idea to a quantum spin system, for which the edge singularity is controlled by the dynamically induced overscreening effect with a mobile Kondo impurity.

Soft Edge Magnetoplasmons in 2D Circular Pools of He 4 ions

The recent experimental discovery of anomalous soft edge magnetoplasmons (EMP) in 2D charged systems 1,2 and the problem of their identification requires an analysis of their physical nature. We consider three different scenarios for the appearance of soft EMP modes proposed by different authors. Our analysis indicates that the anomalous soft modes reported in Ref. 2 have the smooth variation of the charge density near the edge of the 2D system as their origin, although other satellite modes remain unexplained.

Anomalous thresholds and edge singularities in electrical impedance tomography

Studies of models of current flow behaviour in electrical impedance tomography (EIT) have shown that the current density distribution varies extremely rapidly near the edge of the electrodes used in the technique. This behaviour imposes severe restrictions on the numerical techniques used in image reconstruction algorithms. In this paper we have considered a simple two dimensional case and we have shown how the theory of end point/pinch singularities which was developed for studying the anomalous thresholds encountered in elementary particle physics can be used to give a complete description of the analytic structure of the current density near to the edge of the electrodes. As a byproduct of this study it was possible to give a complete description of the Riemann sheet manifold of the eigenfunctions of the logarithmic kernel. These methods can be readily extended to other weakly singular kernels.

The Inner-Shell Hole of Alkali Metals Studied by the DV-XαMethod

The discrete variational (DV) Xα method is applied to investigate the outermost inner-shell hole of all alkali metals. Calculations are performed for the model clusters M 9 and M 59 of alkali metals. The hole is assumed to be at the center of the cluster in the bcc structure. The hole potential of Rb and Cs is found to be extended to the cluster size. The result is applied to investigate the anomalous edge of X-ray absorption spectrum and the asymmetric shape of X-ray photoemission spectrum, but we could not find any peculiar characteristics expected for Cs. The obtained singularity indices α 's and the edge exponents α l 's of all the elements are very similar to those obtained by predecessors, failing to obtain a small value of α 0 for Cs which is claimed by experiment.

Breakdown of the quantum Hall effect in InAs/AlSb quantum wells due to counterflowing edge channels.

We investigated magnetotransport in the two-dimensional electron gas (2DEG) present in InAs/AlSb quantum wells. The filling factor Ng underneath a gate electrode was reduced relative to the bulk filling factor Nb. For Ng<Nb the resistance at the minima of the Shubnikov–de Haas oscillations failed to vanish, and the corresponding plateaus in the Hall resistance deviated from their expected quantized values Rxy=h/(e2Ng). The results are explained by the presence of anomalous edge channels at the 2DEG boundaries, which flow in opposite direction compared to the regular ones. They result from the band bending due to the combination of Fermi level pinning at the exposed InAs surface and the electrostatic potential generated by the gate electrode. This picture is supported by a comparison between the measured resistances and the resistances calculated with an edge channel model.

Corrections by the Exchange Effects on the Exponents of the Anomalous Soft X-Ray Edge of Li, Na, Mg and Al

Numerical values of the exponents of Li, Na, Mg and Al in the theory of Nozieres and De Dominicis (Phys. Rev. 178 (1969) 1097) on the anomalous edge of soft X-ray spectra are calculated. The exchange effects are included explicitly as corrections to the previous values calculated by Ohmura and Sano (J. Phys. Soc. Jpn. 43 (1977) 875). By the corrections, all the positive values of α 0 for L holes of Na, Mg and Al decreased, while the negative value of α 1 for the K hole of Li increased approaching to zero. All these results could be compared more favorably with existing experimental results done recently than the one obtained previously. The results support, on reflection, the previous calculation of α 0 on K and Rb (J. Phys. Soc. Jpn. 58 (1989) 4635) done in the same way as in the present paper.

Electronic states in the presence of the inner-shell hole studied by the method of DV-Xα

The molecular orbital theory by the method of DV-Xα is applied to investigate electronic states of electrons in the presence of the inner-shell hole. The hole is assumed to be at the center of the cluster which is composed of nine ions in the bcc structure. Using the present model, the hole potential born by missing an outermost inner-shell hole is found not to be atomic but extended to the cluster size. The result is applied to investigate the anomalous edge of absorption spectra and the related problem of heavy alkali metals with the inner-shell hole in the final state. Phase shifts for l = 2 and l = 3 of wavefunctions of conduction electrons due to the presence of the inner-shell hole in the final state are found to be negative. For Cs, we could not obtain phase shifts that are consistent with both the results of experiments on X-ray absorption and XPS.

Edge turbulence

This review concentrates on tokamak results, summarizing some of the characteristics (e.g. amplitudes, frequencies, wavelengths) and comparing them with theoretical expectations. With ohmic heating the measured electrostatic fluctuations are shown to explain the total anomalous edge fluxes, but no useful scaling law yet exists. Magnetic turbulence may become important with additional heating (perhaps higher beta). Although an adequate theory to explain all the measurements does not exist, there is an understanding of some of the basic drives and damping mechanisms, and these are discussed. Finally, a test particle approach to including the effects of the electrostatic turbulence induced transport in complicated geometry (e.g. with stochastic magnetic fields) is presented.

Edge magnetoplasmons in a two-dimensional electron fluid confined to a half-plane.

The magnetoplasma modes of a two-dimensional electron fluid in a half-plane are studied with a hydrodynamic model. For general screening by parallel grounded planes, the dynamical equations are reduced to a single integral equation, whose smallest positive or negative eigenvalues correspond to anomalous edge modes. Numerical solutions are obtained by expanding in a complete set of Laguerre polynomials.

Effects of edge anomaly on the appearance-potential spectroscopy of metals.

The anomalous edge behavior in the appearance-potential spectroscopy of simple metals has been investigated. With the use of simple diagrams and a realistic pseudopotential for the core-hole interaction, the Nozi\`eres--De Dominicis--Laramore theory has been extended beyond the immediate neighborhood of the edge region. It is shown that close to the edge, this theory reduces to the same power-law expression as calculated by Laramore. Away from the edge, numerical results are presented for both 1s and 2p intensity spectra of Al. Derivative spectra are also calculated numerically. The resulting singularities and widths of the edge structures in both 1s and 2p derivative spectra are consistent with experimental data.

Self-absorption studies of the soft X-ray emission and absorption edges of K, Mg, Al and Be

A detailed study is presented on self-absorption of the K-M23, Mg-L23, and Al-L23 edges as well as new data on the anomalous Be-K edge which has already been presented by the author. The self-absorption mechanism is examined in some detail and it is concluded that under favourable circumstances, the absorption spectrum at the edge can be evaluated and its separation from the emission edge measured with a fair degree of accuracy. Broadening and other parameters have been determined for both emission and absorption and are discussed with reference to similar determinations by other workers.

Effect of edge anomaly on the valence-band Auger spectra of metals

The processes responsible for the anomalous edge behavior in the x-ray band spectra of metals are found to play important roles in modifying the intensity of $\mathrm{CVV}$ Auger-electron emission spectra. The dynamical effects related to the sudden switching of the core-hole potential are known to produce a power-law behavior in the neighborhood of the upper emission edge. Using simplified diagrams and a realistic pseudopotential for the core-hole interaction, we have extended the edge theory to energies well below the Auger emission edge. A significant shift in the position of the maximum of the ${L}_{23}\mathrm{VV}$ Auger emission intensity of Al is calculated.

X-ray edge singularities of relaxed-out bound states and relaxed-in band states

Anomalous X-ray edge singularity exponents are obtained in the two cases of (i) relaxed-out bound states and (ii) relaxed-in band states, where the number of conduction band states is changed, and simple phase-shift arguments are given which consistently explain all the exponents.

Injection luminescence in N-ion implanted ZnSe

Injection type electroluminescence in the N-ion implanted ZnSe diode has been observed. The red band can be attributed to the selenium vacancies and the green band to the zinc vacancies or their complex. The anomalous edge emission intensity enhancement can be ascribed to the combined effect from radiative recombinations related to the nitrogen substituted for selenium sites and the intrinsic edge emission. The magnitude of this effect suggests a very efficient injection in the diode.

Anomalous absorption edge shift in magnetic HgCr 2S 4

The absorption edge and the diffuse reflectance spectrum of HgCr 2S 4 have been measured between 20°K and 400°K. The anomalous edge shifts observed below the Curie-Weiss temperature in all ferromagnetic chromium spinels can be qualitatively understood in terms of magnetoelastic volume strain.