Isotropic Angular Dependence Research Articles

Angular Dependence of Twisted Soldered-Stacked-Square (3S) HTS Wire

Angular dependence of second generation (2G) high-temperature superconducting (HTS) tape is a crucial parameter for its industrial applications. Known as soldered-stacked-square (3S) HTS wire, a novel structure of HTS wire has been proposed in our previous research. In this paper, angular dependence of 1 mm HTS tape, 4 mm tape, 1 mm 3S HTS wire, and 1 mm 3S HTS twisted wire were measured in liquid nitrogen bath, respectively. It can be found that the 3S HTS wire would not enhance the critical current density under external magnetic field, while an isotropic angular dependence is obtained when 3S HTS wire is twisted. The test results obtained in this paper provides useful information for the future applications of 3S HTS wire.

Measurement of angularly dependent spectra of betatron gamma-rays from a laser plasma accelerator with quadrant-sectored range filters

Measurement of angularly dependent spectra of betatron gamma-rays radiated by GeV electron beams from laser wakefield accelerators (LWFAs) are presented. The angle-resolved spectrum of betatron radiation was deconvolved from the position dependent data measured for a single laser shot with a broadband gamma-ray spectrometer comprising four-quadrant sectored range filters and an unfolding algorithm, based on the Monte Carlo code GEANT4. The unfolded gamma-ray spectra in the photon energy range of 0.1–10 MeV revealed an approximately isotropic angular dependence of the peak photon energy and photon energy-integrated fluence. As expected by the analysis of betatron radiation from LWFAs, the results indicate that unpolarized gamma-rays are emitted by electrons undergoing betatron motion in isotropically distributed orbit planes.

Microstructural characterization of GdBa2Cu3O7-δ superconductor films with BaHfO3 artificial pinning centers by scanning transmission electron microscopy.

Critical current (IC) of superconductor films under magnetic field is strongly influenced by dispersions and morphologies of artificial pinning centers (APCs) in general [1]. BaHfO3 (BHO) is acknowledged as the best candidates of APCs for REBCO films, which shows utmost thickness dependence and isotropic angular dependence of IC values for REBCO films [2]. Moreover, several researchers have focused on the nanostrains caused by the lattice mismatch at the interface between APCs and REBCO matrix, which are also the source for enhanced vortex pinning of the REBCO films [3]. In this study, we investigated to examine the nanostrain at the interface using spherical aberration (CS) corrected scanning transmission electron microscopy (STEM).BHO introduced GdBa2Cu3O7-δ (GdBCO) film was fabricated by pulsed laser deposition (PLD) method. TEM samples were prepared by focused ion beam (FIB; Quanta 3D 200i, FEI) method followed by Ar ion thinning (NanoMill, Fischione) method. Atomic scale imaging was performed by spherical aberration corrected STEM (JEM-ARM200F, JEOL), then microstructures of BHO/GdBCO interface was then examined by Fourier transformation (FFT).BHO nanorods and nanoparticles were found dispersed in the GdBCO matrix, where {100} and {110} facets were present at BHO/GdBCO interfaces, as shown in Fig.1. In the case of PLD process, most favorable growth direction of BHO is [001] direction, so that the regular quadrangular prism shaped BHO with {100} facets would be grown along [001] direction of GdBCO matrix [4]. {110} facets of BHO were formed to maintain the minimum surface area at BHO/GdBCO to reduce the interfacial energy.jmicro;63/suppl_1/i27-a/DFU082F1F1DFU082F1Fig. 1.Plan view HAADF-STEM image and FFT image showing facets at BHO/GdBCO interfaces. This work was supported by the Ministry of Economy, Trade and Industry (METI) as "Development of Fundamental Technologies for HTS Coils" and the JSPS KAKENHI (26600046).

Nodal synthetic kernel (N-SKN) method for solving neutron transport equation in one- and two-dimensional X–Y geometries

In this study, a nodal method based on the synthetic kernel (SKN) approximation is presented for solving the neutron transport equation in one- and two-dimensional cartesian geometries. The two-dimensional neutron transport equation for a node is transformed to one-dimensional transport equation based on the face-averaged scalar flux and the current. At the node interfaces, DP1 expansion is employed to the surface angular fluxes in conjunction with isotropic angular dependence of the transverse leakage term. The one-dimensional integral transport equation is obtained in terms of the node-face-averaged incoming/outgoing neutron flux and the currents. The synthetic kernel approximation is employed to the transport kernels and nodal-face contributions. The resulting SKN equations are solved analytically. One-dimensional interface-coupling nodal SK1 and SK2 equations (incoming/outgoing flux and current) are derived for the small nodal-mesh limit. These equations have simple recursive forms which do not pose burden on either the memory or the computational time. The method was applied to one- and two-dimensional benchmark problems and compared with the solutions obtained with nodal integral method.

The effects of density and size of BaMO3 (M=Zr, Nb, Sn) nanoparticles on the vortex glassy and liquid phase in (Y,Gd)Ba2Cu3Oy coated conductors

We studied the effect of systematically-controlled size (22–83 nm) and density (0.1–13 × 1021 m−3) of strong pinning random BaMO3 (M=Zr, Nb, Sn) nanoparticles (NPs) by engineering of their inclusion in (Y0.77Gd0.23)Ba2Cu3Oy ((Y,Gd)BCO) coated conductors. The critical current density (Jc) gradually increases from that of pure (Y,Gd)BCO to that of 3 wt% BZO with increasing density of NPs for H ‖ c and H ‖ 45°. Moreover, at low/intermediate fields the films with higher densities of isotropic pinning centers show nearly isotropic angular dependence at both 77 and 65 K, indicating that a high density of spherical nanoparticles effectively pins vortices over a broad angular range. We find that the enhancement of Jc depends mainly on the density of the NPs and not on their size. The vortex melting transition (characterized by the critical exponent (s) of the resistive transition) changes, particularly at H ‖ 45° in films with higher densities of NPs. This change is reflected in a decrease of s to a value very close to that observed for H ‖ c, similar to that of a Bose-glass. Thus, the density and morphology of the pinning centers are important factors determining not only Jc but also the character of the solid–liquid transition.

Flux pinning properties of TFA-MOD(Y,Gd)Ba2Cu3Ox tapeswith BaZrO3 nanoparticles

A high critical current density Jc with a nearly isotropic angular dependence at 77.3 K was recently obtained by the introduction of nanoparticlesto (Y,RE)Ba2Cu3Oy (RE123, RE = Sm, Gd)-coated conductors prepared by trifluoroacetate metal–organic deposition (TFA-MOD).The flux pinning properties were evaluated on the basis of detailed measurements of theupper critical field, the irreversibility field and the critical current density of theTFA-MOD RE123-coated conductor. We found that strong random pinning by theBaZrO3 nanoparticlesand weak c-axis correlated pinning by twin boundaries functioned cooperatively in nanoparticles introducedto (Y, Gd)123-coated conductors prepared by TFA-MOD. The coexistence of two differentpinning centers played an important role in the nearly isotropic angular dependence ofJc.

Effect of a dimensional crossover on the upper critical field of practical Nb–Ti alloy superconductors

Multilayers of a Nb0.37Ti0.63 alloy, a chief material of superconducting magnet technology, and a Cu0.95Sn0.05 or Cu0.70Ni0.30 alloy exhibit a dimensional crossover with a decreasing bilayer period Λ=dN+dS, where dS=3dN. Cusps of Hc2(θ) and square root Hc2∥(T) develop when Λ⩽40 nm, which indicate a crossover to 2D behavior from 3D behavior seen at Λ=60 nm. Full proximity coupling of Cu–Sn layers for Λ=13 nm restores isotropic angular dependence, but with sharply lower Hc2 values. By contrast, proximity coupling was suppressed by magnetic Cu–Ni layers, and 2D behavior was retained while Tc fell below 4 K for Λ<20 nm. The data are consistent with numeric results obtained by Takahashi and Tachiki [Phys. Rev. B 33, 4620 (1986)] when the variation of the Bardeen–Cooper–Schrieffer pairing potential is the primary cause of the dimensional crossover. Since practical Nb–Ti conductors have a layered nanostructure, this result suggests that a dimensional crossover should also be found in wires. However, the 3D–2D crossover occurs when Λ is much greater than the separation of the flux lines at high field (10–20 nm) and above the range where optimum flux pinning is found. This implies that a 2D state (for insulating or magnetic layers) or a 2D strongly coupled state (for normal metals) exists when flux pinning is strongest. These implications are discussed in the context of practical Nb–Ti wires used in superconducting magnet technology.

Space- and time-resolved optical spectroscopy of plumes generated by laser ablation of NiO in a vacuum

Laser ablation of NiO at 266 nm has been studied by measuring absorption spectra and emission spectra which are resolved both temporally and spatially. The spectra were measured at several points 1 mm apart from the laser spot and with several detection angles. Ablated species in different electronic states such as the ground state, lower lying states, and excited states are distinguished from absorption and emission spectroscopy. Time-of-flight spectra of these species and their angular dependence are obtained. It is revealed that excited Ni neutrals are generated mainly by the recombination of Ni+ ions with electrons, and Ni+ ions ejected nearly normal to the surface are exclusively excited due to multiple collisional effect. The isotropic angular dependence of most probable velocities of Ni+ ions and Ni neutrals are explained by ambipolar diffusion of the plume. The mechanism of species ejection is also discussed.

EPR and luminescence studies of LaF 3 and CeF 3 under X-ray and laser irradiation

After X-ray irradiation below 70 K, LaF 3 exhibits a 3-line EPR resonance consistent with self-trapped holes (V k centers) on the fluorine sublattice. Specifically, we conclude that the V k centers are formed on pairs of fluorine ions making 15 ° angles to the c-axis. At temperatures below 200 K, the spectrum of X-ray excited luminescence is dominated by 5d-4f transitions of Ce 3+ impurity present at 4 ppm. Near room temperature, the transfer to the Ce 3+ ceases within detectability, revealing a weak band suggested in other work to be self-trapped exciton luminescence. Above 400 K, the 5d-4f transitions of Ce 3+ again dominate the spectrum, implying resumption of efficient energy transfer from the uniformly distributed X-ray excitations in the LaF 3 crystal to the dilute impurity. In CeF 3, a 7-line hyperfine EPR pattern at g = 2, with isotropic angular dependence, is shown to be composed of independent 3-line and 4-line patterns possessing different thermal and microwave power saturation properties. Such patterns arise from a single electron (or hole) interacting with 2 and 3 equivalent spin- 1 2 nuclei, respectively.

Propagation angle dependence of radar auroral E-region irregularities

The Sweden and Britain Radar aurora Experiment (SABRE) measures the phase velocity and backscatter amplitude of 1 m wavelength irregularities in the auroral zone E-region. These irregularities are produced by two types of plasma instabilities and theory predicts that the variation in backscatter amplitude with the angle between the electron drift velocity V d and the radar vector k, the so called propagation angle, is different for the two cases. An isotropic angular dependence is expected for gradient drift instabilities, but for the alternative two stream instability process a minimum is produced in the backscatter amplitude for directions orthogonal to the flow. The SABRE data have been averaged to remove small scale fluctuations and for drift velocities less than ~200 ms −1 the backscatter dependence on the propagation angle is isotropic, strongly suggesting that gradient drift processes are dominant. At higher drift velocities anisotropies develop in the angular dependence with deep minima at approximately 90° to the flow and a subsidiary minimum at 0° to this direction. Although these characteristics suggest a two stream instability mechanism, there are features in the experimental data that are not predicted by this simple theory. It is suggested that ion drifts may in part be responsible for these additional features.

Azimuthal dependence of impact scattering in electron energy loss spectroscopy

The azimuthal dependence of electron energy loss spectroscopy (EELS) dipole and impact scattering intensity has been measured. Spectra for a saturation coverage of H adsorbed on W(110) exhibit loss peaks due to impact scattering from adsorbate vibrational modes. The intensity of the 160 meV loss peak has been measured as a function of the azimuthal angle between the scattering plane and a mirror plane of the s urface. The angular pattern has strong maxima oriented perpendicular to the <111 > rows of atoms on the surface, and has the C 2v symmetry of the W(110) surface. This azimuthal dependence is strikingly different from the nearly isotropic angular dependence of dipole scattering from Cl adsorbed on W(110). Selection rules for impact scattering account for the general features of the angular pattern based on asymmetric stretch modes associated with bridge site H atoms. We have shown that the azimuthal dependence of the 36 meV Cl/W(110) dipole scattering loss peak is isotropic and that the 160 meV H/W(110) impact scattering loss peak exhibits a striking azimuthal pattern with C 2v symmetry. The symmetry and deep minima suggest that selection rules play a central role in determining the azimuthal pattern. Application of these rules to two orthogonal directions (as in ref. 6) may be misleading, as is clear from Fig. 2, because essential features of the pattern will not be observed. Our analysis of the full pattern has suggested two bridge sites may be occupied at saturation coverage, but has still not resolved certain questions about the H/W(110) system. 1 Impact scattering selection rules for potential adsorbate sites. The listed directions are the intersections of the planes with the (110) surface for the mirror planes and the scattering planes, and the displacement directions for the adsorbate vibrational modes. Modes are assumed to be strictly parallel to the surface. The long bridge site is between two W atoms along the <001 > direction, the short bridge site is between two W atoms along the < 111 - > direction, and the distorted bridge site is displaced from the long bridge site along the < 110 - > direction (ref. 6) The asterisks (*) denote that the scattering amplitude is zero for all directions in the scattering plane, otherwise it is zero only in the specular direction. The < 110 - > mode of the distorted bridge is not covered by the selection rules of ref. 2. SITE LONG BRIDGE SHORT BRIDGE DISTORTED BRIDGE MIRROR PLANES [001], [1 1 - 0] NONE [1 1 - 0] 2-FOLD ABOUT Z YES YES NO PARALLEL MODES [001] [1 1 - 0] [1 1 - 1] [ 1 - 12] [001] [1 1 - 0] DIRECTIONS OF ZERO SCATTERING [001] * [1 1 - 0] [001] * [1 1 - 0] [1 1 - 1] [ 1 - 12] [1 1 - 1] [ 1 - 12] [001] * [1 1 - 0] NA

Threshold resonances in vibrational excitation of HBr molecules by electron impact

Vibrational excitation of HBr molecules by electron impact has been studied in a differential scattering experiment at threshold. The cross section is characterised by a narrow peak close to threshold followed by a broader maximum at higher energies, both of essentially isotropic angular dependence. This behaviour can be observed up to nu =5 excitations. The integral cross section for the nu =1 excitation in the threshold peak has an absolute value of several 10-15 cm2. The results are compared with threshold effects in the stronger polar hydrogen halides, HF and HCl.