A comparative study of magnetic relaxation in YBa2Cu3−x Fe x O7−y single crystals

The zero-field-cooled magnetization relaxation has been studied over wide temperature and field ranges on iron-doped YBa₂Cu₃O_7–y single crystals. The relationship between the twin boundary density, which depends on the iron composition, and the pinning strength of the flux lines is determined systematically. For magnetic fields up to 1 T the inductive critical current density decreases as the iron composition increases. The Bean critical state model and the Anderson-Kim thermally activated flux-creep model are utilized to analyze and interpret the experimental results.     

The effect of out-of-plane motion on 2D and 3D digital image correlation measurements

The effect of out-of-plane motion (including out-of-plane translation and rotation) on two-dimensional (2D) and three-dimensional (3D) digital image correlation measurements is demonstrated using basic theoretical pinhole image equations and experimentally through synchronized, multi-system measurements. Full-field results obtained during rigid body, out-of-plane motion using a single-camera vision system with (a-1) a standard f55mm Nikon lens and (a-2) a single Schneider–Kreuznach Xenoplan telecentric lens are compared with data obtained using a two-camera stereovision system with standard f55mm Nikon lenses.Results confirm that the theoretical equations are in excellent agreement with experimental measurements. Specifically, results show that (a) a single-camera, 2D imaging system is sensitive to out-of-plane motion, with in-plane strain errors (a-1) due to out-of-plane translation being proportional to ΔZ/Z, where Z is the distance from the object to the pin hole and ΔZ the out-of-plane translation displacement, and (a-2) due to out-of-plane rotation are shown to be a function of both rotation angle and the image distance Z; (b) the telecentric lens has an effective object distance, Z_eff, that is 50× larger than the 55 mm standard lens, with a corresponding reduction in strain errors from 1250 μs/mm of out-of-plane motion to 25 μs/mm; and (c) a stereovision system measures all components of displacement without introducing measurable, full-field, strain errors, even though an object may undergo appreciable out-of-plane translation and rotation.     

Modified nodal point method for the measurement of the large focal length of lenses

Some modifications have been suggested to the nodal slide and microscope method to suit the focal length measurement of large focal length lenses. In this procedure, the experimental set-up has to be accommodated on the two usual optical benches in line, and the position of the nodal point of the lens under test is easily located by introducing an extra lens of comparatively shorter focal length between the test lens and microscope. The results of the measurement of ‘f’ for a few lenses, and the experimental parameters, are presented.     

High magnetic field properties of mixed terbium-yttrium ferrite garnets

Magnetization measurements in d.c. magnetic fields up to 180 kOe have been performed on polycrystalline specimens and on single crystals of mixed terbium-yttrium iron garnets: Tb _x Y_3–x Fe₅O₁₂ in the 4.2–300 K temperature range. On polycrystals (x=2.5; 2; 1; 0.37) the concentration dependence of both compensation point and spontaneous magnetization has been determined. In the single crystals (x=2; 1; 0.37) a strong magnetic anisotropy has been observed with a change of the easy direction of magnetization from [111] to [100] whenx decreases; abrupt field-induced transitions have been observed at low temperatures when the external field is applied along the [100] direction.     

Anomalous plasma phenomena of hollow cathode arc discharge

Magnetically confined argon plasma produced by hollow cathode arc discharge has been studied in different experimental conditions, with discharge current from 10–50 A, vessel argon pressure between 10^–3 and 10^–4 torr (1 torr=133·32 Pa) and axial magnetic field up to 0·12 T. The plasma density measured by a cylindrical Langmuir probe is found to be 10¹⁹ to 4 × 10¹⁹ m^–3 and the electron temperatureT _e varies between 2·5 and 4·8 eV. When an external axial magnetic field is applied the plasma temperature decreases with the increase in the magnetic field intensity until it reaches a minimum value at 0·075T and then increases with the same rate. This has been interpreted as high frequency waves excitation due to electron beam-plasma interaction, which explains the electron density jumps with the magnetic field intensity. Enhanced plasma transport across the magnetic field is studied and classified as anomalous diffusion.     

Design of a hybrid diffractive/refractive achromatized telecentric f·θ lens

The design of a hybrid diffractive/refractive achromatized telecentric f·θ lens with a field of view (FOV) 50° and an effective focal length of 750 mm is presented. The optical stop is placed at the front focal plane so that it is a telecentric system. The design is based on a traditional refractive counterpart, and the designed system consists of a hybrid diffractive/refractive lens and four refractive lenses. The designed f·θ lens shows a considerable reduction in weight with a simplified structure, and exhibits superior performance compared to the refractive system. It is emphasized that the designed f·θ lens can be applied to modern color scanning systems that operate in the visible wavelength range with high performance. It can also be applied to high-energy scanning systems. When applying the designed hybrid diffractive/refractive f·θ lens to the high-energy scanning system, a big laser operating in one longitudinal mode can be replaced by a small multi-mode laser, and the scanning system can be simplified greatly with the accuracy improved.     

Electron drag in intermediate magnetic fields with low electron density

Electron drag between two two-dimensional electron systems has been measured in intermediate magnetic fields (/τ<ω_ck_BT) with a relatively low electron density. We explore, in this sample, the unusual increase of drag in intermediate magnetic fields which was well characterized by a nearly temperature independent B³ dependence. The anomalous behavior of electron drag observed in higher density samples is found to persist for low sample density.     

Neutron diffraction study of the U(Pd1−xFex)2Ge2 magnetic structure

The neutron diffraction and magnetic susceptibility studies have shown that the magnetic structure of UPd₂Ge₂ changes dramatically even under very low iron doping. Though the general magnetic structure of pure UPd₂Ge₂ and of 1%Fe-doped samples is the same, the temperature intervals of existence of different magnetic phases are different. The values of transition temperatures, where (i) the ‘square’ modulated longitudinal spin-density wave (LSDW) structure with the propagation vector k=(0; 0; ) starts to transform into the sinusoidal modulated LSDW structure and (ii) the commensurate phase transforms into incommensurate one, shift under the 1%Fe doping to the higher temperatures (from 50 to 65 K and from 80 to 90 K, respectively). In the pure and 1%Fe-doped UPd₂Ge₂, the magnetic transition from the commensurate to incommensurate phase is accompanied by the drastic decrease of the propagation vector k_z. In the 2%Fe-doped sample, besides the Néel point of T_N=135 K, we have found two additional characteristic temperatures of 65 and 93 K. Below 65 K, the material has a simple antiferromagnetic (AF) structure with the propagation vector k=(0; 0; 1) and, at 65 K<T<T_N, the magnetic structure is LSDW with sinusoidal modulation. Over almost the total region 65 K<T<T_N, the LSDW magnetic structure is incommensurate. Only at about 93 K, the propagation vector passes the commensurate value of , whereas at 65<T<93 K and at 93 K<T<T_N. We have found that the magnetic susceptibility and the uranium magnetic moment are sensitive to the transition. With increasing iron concentration to x0.15, the simple AF structure with k=(0; 0; 1) develops over all temperature region up to the Néel point. Below T_N, the uranium magnetic moments are always parallel to the tetragonal c-axis.     

Electrical transport, magnetic and thermal properties of icosahedral Al–Pd–Mn quasicrystals

We report measurements of electrical resistivity (ρ), Hall coefficient (R_H), magnetization (M) and specific heat (C_p(T)) of high-quality icosahedral Al_70.4Pd_20.8Mn_8.8 phases with different thermal treatment. An improvement in the quasi-crystallinity upon the annealing treatment caused a drastic increase in ρ up to 7000 μΩ cm accompanied by a very small electronic specific heat coefficient γ. The low temperature ρ(T) data has been analyzed in terms of weak localization and electron–electron interaction effects. The Hall resistivity (ρ_H) is found to be strongly temperature-dependent and varies linearly with the magnetization (M) for the same field and temperature. Magnetization measurement reveals that more conductive samples are more magnetic and vice versa. Magnetic susceptibility (χ) data of all the annealed samples agrees with the Curie–Weiss-like behavior implying the existence of localized moments. The negative Curie–Weiss temperature (θ) indicates strong antiferromagnetic coupling between individual Mn atoms. The magnetic Mn concentration is found to be small, ranging from 1.73×10^-4 for the less magnetic sample studied up to 3×10^-3 for the more magnetic one. The small electronic specific heat coefficient obtained for all the samples suggests a significant reduction in the electronic density of states (DOS) at the Fermi level (E_F) upon thermal annealing treatment.     

Fabrication of gradient refractive index ball lenses

The theoretical model of the Maxwell fish eye sphere lens has long been regarded as an absolute optical instrument without practical application since it was proposed. But the study on the theory of Maxwell fish eye micro-spherical lens shows that at certain condition it has fine image formation ability and coupling efficiency. Based on the Fick's diffusion equations, the distribution function of refractive index of gradient refractive index (GRIN) ball lenses is derived. Lithium-containing silicate glass is fabricated and GRIN ball lenses whose diameters are from 0.3 to 3.0 mm are made by ion exchange and the sagging method in sodium nitrate. Radial refractive index profiles of these GRIN ball lens are measured by interferometer. Results show that the distribution of the index of refraction is a parabolic curve and in concordance with the computational consequence and its Δn is 0.0002.     

Electromagnetic properties of bulk Bi-Pb-Sr-Ca-Cu-O superconductor at low magnetic fields

Magnetization and critical current density measurements have been performed on a Bi_1.7Pb_0.3Sr₂Ca₂Cu₅O _y bulk material as the functions of magnetic field and temperature. The diamagnetic shielding effect has been discussed from the initial magnetization curve. The lower critical field of bulk granular superconductor (H _g) and the lower critical field of superconducting grains (H _c1) are estimated from magnetization curves. They are both linearly decreasing functions of temperature: (dH _g/dT) and (dH _c1/dT) are –0.4 and –1.8 G/K respectively. The transport critical current density drops drastically by a factor of 4 at a magnetic field of about 20G. The magnetization J _c of superconducting grains derived from the remanent magnetization is about 10⁶ A/cm² at 77 K, zero field, much greater than the transport J _c. The experimental results reveal that the transport J _c is dominated by weaklink current between grains. The magnetization J _c versus temperature has been obtained from the remanent flux at zero magnetic field and is a linearly decreasing function of temperature within experimental error.     

Near-field magneto-photoluminescence of quantum-dot-like composition fluctuations in GaAsN and InGaAsN alloys

Near-field magneto-photoluminescence scanning microscopy has been used to investigate the structural and optical properties of quantum-dot (QD)-like compositional fluctuations in GaAsN and InGaAsN alloys. Sharp spectral lines (halfwidth 0.5–2 meV) from these QDs are observed at T<70 K, and their Zeeman splitting and diamagnetic shifts are used to determine the size (r6–18 nm), density (100 μm⁻³), and nitrogen composition. Near-field scanning images reveal phase separation effects in the distribution of nitrogen, but little effect appears from the presence or absence of indium. Indium does have a strong effect on the exciton g-factor for observations in a magnetic field.     

Fermi’s golden rule in a mesoscopic metal ring

We examine the time-dependent nonequilibrium current in a mesoscopic metal ring threaded by a static magnetic flux φ that is generated by a time-dependent electric field oscillating with frequency ω. We show that in quadratic order in the field there are three fundamentally different contributions to the current. (a) A time-independent contribution which can be obtained from a thermodynamic derivative. (b) A term increasing linearly in time that can be understood in terms of Fermi’s golden rule. The derivation of this term requires a careful treatment of the infinitesimal imaginary parts that are added to the real frequency ω when the electric field is adiabatically switched on. (c) Finally, there is also a time-dependent current oscillating with frequency 2 ω. We suggest an experiment to test our results.     

Effect of quartic-phase aberrations on the focal switch of Hermite–Gaussian beams

The focal switch of Hermite–Gaussian beams diffracted at an aperture and subsequently focused by a spherically aberrated lens is studied. Our main attention is focused on the effect of quartic-phase aberrations on the behavior of the focal switch. It is shown that quartic-phase aberrations affect the relative focal shift Δz_f, turning position s_1,t, and relative transition height Δz_sw. Apart from a critical maximum truncation parameter α_c,max, there is a critical minimum truncation parameter α_c,min. Within the region α_c,min<α<α_c,max the focal switch can take place, but quartic-phase aberrations give rise to a decrease of α_c,max-α_c,min in comparison with the aberration-free case.     

Magnetic responses in 1D mesoscopic rings and cylinders

I investigated a detailed study of persistent current and low-field magnetic susceptibility in one-dimensional mesoscopic rings and cylinders threaded by slowly varying magnetic flux φ in the tight-binding model. In perfect rings described by constant number of electrons N_e, current shows only saw-tooth variation with φ, while for those rings described by constant chemical potential μ, current varies saw-tooth like for some special choices of μ, but in all other cases it shows kink-like structures. On the other hand, in perfect cylinders I get both saw-tooth and kink-like structures in persistent current whether these cylinders are described by constant N_e or μ. In presence of impurity, current gets a continuous variation with φ only for the rings described by constant N_e, while in all other cases it depends on the choice of μ. My exact calculation predicts that the diamagnetic and paramagnetic sign of the low-field currents can be determined exactly for the rings described by constant N_e. In perfect rings, I get only diamagnetic currents both for odd and even N_e, while in presence of impurity current always shows diamagnetic sign for the rings with odd N_e and paramagnetic sign for the rings with even N_e. Both for the perfect and disordered rings described by constant μ the sign of the current cannot be mentioned exactly since it depends on the choice of μ and disordered configurations. Similar arguments are also true for the cylinders those are described either by constant N_e or by constant μ since the sign of the current in these systems depends on N_e, μ and disordered configurations.     

Design of a neutron polarizer using polarizing super mirrors for the TOF-SANS instrument at the J-PARC

Small-angle neutron scattering technique using polarized neutrons is powerful for studying structures in the range between nm and μm of magnetic materials. In addition, they have been used for the incident beam of focusing-geometry SANS instruments using a magnetic neutron lens, where a high polarization degree of about 99.9% is necessary because the imperfectness of the neutron polarization increases the background level. We are going to install such a magnetic focusing system on the new time-of-flight SANS (TOF-SANS) instrument at the J-PARC so as to make q_min smaller than 10⁻³ Å⁻¹ and improve the resolution of the conventional TOF-SANS at low q. As a polarizing device of the instrument, two V-shaped polarizing super mirrors arranged in crossed geometry to enhance the polarization degree has been considered. In this paper, we present the concept and the detailed design of this device and its performance estimated by Monte Carlo simulations.     

Ab initio study of structural, electronic and optical properties of Ca1−xSrxS compounds

Full-potential linearized augmented plane wave method (FP-LAPW) within density functional theory has been used to calculate structural, electronic and optical properties of Ca_1−xSr_xS, an alkali earth chalcogenide, with varying compositional parameter x in the range 0<x<1. Whereas the structural properties are discussed in terms of charge transfer between the two cations, calculated electronic band structure and density of states have been analyzed in terms of contribution from the S p, Ca d and Sr d states. Furthermore, we have calculated some optical properties such as real and imaginary parts of dielectric constant, ε(ω), and the calculated results have been discussed in comparison with the existing experimental data and other theoretical calculations.     

A new metallic state in two dimensions

We report the discovery of a new and unexpected kind of metal in two dimensions (2D), which exists in the presence of scattering by local magnetic moments. The experiment was carried out on a 2D electron system in silicon, where the local magnetic moments have been induced by disorder and their number was varied using substrate bias (V_sub). In the new metal, the conductivity decreases as σ(n_s,T)=σ(n_s,T=0)+A(n_s)T² (n_s – carrier density) to a non-zero value as temperature T→0. In three dimensions, this T² dependence is well known, and results from Kondo scattering by local magnetic moments. In 2D, however, the existence of a metal with dσ/dT>0 is very surprising. As the number of local moments is reduced, the range of temperatures [T<T_m(V_sub)] where they dominate transport becomes smaller. For T>T_m, we observe the usual 2D metallic behavior with dσ/dT<0.     

Negative refractive index, perfect lenses and checkerboards: Trapping and imaging effects in folded optical spaces

Newly discovered metamaterials have opened new vistas for better control of light via negative refraction, whereby light refracts in the “wrong” manner. These are dielectric and metallic composite materials structured at subwavelength lengthscales. Their building blocks consist of local resonators such as conducting thin bars and split rings driving the material parameters such as the dielectric permittivity and magnetic permeability to negative (complex) values. Combined together, these structural elements can bring about a (complex valued) negative effective refractive index for the Snell–Descartes law and result in negative refraction of radiation. Negative refractive index materials can support a host of surface plasmon states for both polarizations of light. This makes possible unique effects such as imaging with subwavelength image resolution through the Pendry–Veselago slab lens. Other geometries have also been investigated, such as cylindrical or spherical lenses that enable a magnification of images with subwavelength resolution. Superlenses of three-fold (equilateral triangle), four-fold (square) and six-fold (hexagonal) geometry allow for multiple images, respectively two, three, and five. Generalization to rectangular and triangular checkerboards consisting of alternating cells of positive and negative refractive index represents a very singular situation in which the density of modes diverges at the corners, with an infinity of images. Sine-cosecant anisotropic heterogeneous square and triangular checkerboards can be respectively mapped onto three-dimensional cubic and icosahedral corner lenses consisting of alternating positive and negative refractive regions. All such systems with corners between negative and positive refractive media display very singular behavior with the local density of states becoming infinitely large at the corner, in the limit of no dissipation. We investigate all of these, using the unifying viewpoint of transformation optics. To cite this article: S. Guenneau, S.A. Ramakrishna, C. R. Physique 10 (2009).     

On the theory of temporal aberrations for cathode lenses

A new approach to the theory of temporal aberration for cathode lenses is given in the present paper. A definition of temporal aberration is given in which a certain initial energy of electron emission along the axial direction ε_z1 (0ε_z1ε_0max) is considered. A new method to calculate the temporal aberration coefficients of cathode lenses named “direct integral method” is also presented. The “direct integral method” gives new expressions of the temporal aberration coefficients which are expressed in integral forms. The difference between “direct integral method” and “τ-variation method” is that the “τ-variation method” needs to solve the differential equations for the three of temporal geometrical aberration coefficients of second order, while the “direct integral method” only needs to carry out the integral calculation for all of these temporal aberration coefficients of second order.All of the formulae of the temporal aberration coefficients deduced from “direct integral method” and “τ-variation method” have been verified by an electrostatic concentric spherical system model, and contrasted with the analytical solutions. Results show that these two methods have got identical solutions and the solutions of temporal aberration coefficients of the first and second order are the same as with the analytical solutions. Although some forms of the results seem different, but they can be transformed into the same form. Thus, it can be concluded these two methods given by us are equivalent and correct, but the “direct integral method” is related to solve integral equations, which is more convenient for computation and could be suggested for use in practical design.