Transverse Geometry Research Articles

Influence of microscopic defects in type-II superconducting thin films on the magnetic flux penetration

The magnetic flux penetration into thin type-II superconducting films with circular defects is investigated. The artificial circular defects (diameter $=40 \ensuremath{\mu}\mathrm{m})$ in an ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin film (thickness $\ensuremath{\approx}300 \mathrm{nm})$ were prepared by pulse-laser irradiation. The flux penetration into the zero-field-cooled superconducting film was visualized by means of the magneto-optic method. A stepwise increase of the external magnetic field allowed a detailed investigation of the influence of local defects on the flux penetration. For a magnetic field parallel to a long sample (longitudinal geometry) with a long cylindrical defect a single parabolic discontinuity line appears. Also in the case of a thin superconducting film exposed to a transverse magnetic field (transverse geometry), a single parabolic discontinuity line has been supposed in the vicinity of a local defect. On the contrary, our investigations show that the flux and current distribution around a single defect in a superconducting thin film can be determined not by a single, but by two discontinuity parabolas. In thin superconducting films in transverse geometry screening currents in the Meissner region $(j<{j}_{c})$ are present in contrast to extended infinitely long samples in the longitudinal geometry. We explain our experimental results by the influence of these Meissner screening currents on the temporal formation of the shape of an approaching flux front.

Linear nonreciprocal dichroism in boracite Co3B7O13I

Linear nonreciprocal dichroism that is odd in magnetic field B was observed in the transverse geometry and studied for cubic (symmetry class T d ) phase of boracite Co3B7O13I. Nonreciprocal dichroism was observed in the range of absorption bands corresponding to the transitions of the Co2+ ion in the energy range ΔE=1.2–3.2 eV. The sign and magnitude of nonreciprocal dichroism depend on the mutual orientation of the magnetic field and crystallographic axes. Nonreciprocal dichroism refers to the phenomena of magnetically induced spatial dispersion, and its anisotropy is typical for the manifestation of the second-order magnetoelectric susceptibility in the optical range.

Magnetoresistance in microwave synthesizedAg2+δSe(0.0<~δ<~0.2)

Compositional variation of magnetoresistance (MR) ratio with silver excess stoichiometry in ${\mathrm{Ag}}_{2+\ensuremath{\delta}}\mathrm{Se}$ samples has been studied employing a synthetic approach to prepare instantly a series of bulk silver selenide compounds using microwave energy. Bulk density g99% is achieved in situ, by means of volumetric heating/sintering in less than 10 min, which makes this process unique compared to other chemical and nonchemical approaches. We observe a systematic decrease in metal-insulator transition with increasing Ag. Magnetoresistance ratio observed in transverse geometry is 145%, greater than the longitudinal geometry (70%). This series shows linear dependence of MR with higher fields $(3lHl16\mathrm{T})$ with no saturation of MR ratio even at 16 T in transverse mode. However, a distinct saturation effect was noticed in almost all the compositions for magnetic field above 13 T in longitudinal mode. Electron probe microanalysis shows an elemental-rich selenium phase for $\ensuremath{\delta}=0.05$ composition which accounts for low resistivity, large magnetoresistance ratio and unsymmetrical MR curves when field is applied in both the directions. This is attributed to the charge density fluctuations in chemically inhomogeneous systems. At 300 K, MR ratios of 30 and 59 % for $\ensuremath{\delta}=0.2,$ in longitudinal and transverse mode is observed, respectively.

Figure of merit for transverse magneto-optical Kerr effect

An expression for the Fresnel coefficient r pp is established in the general case of an interface between two semi-infinite media and is extended to multilayers. By analogy with the well-known Θ K R for polar Kerr effect, a figure of merit is proposed for the transverse geometry. It is an alternative characterizing quantity to the relative change of reflectivity for p polarization, Δ R p/ R p.

The transverse geometry of $G$-manifolds and Riemannian foliations

Given a compact Riemannian manifold on which a compact Lie group acts by isometries, it is shown that there exists a Riemannian foliation whose leaf closure space is naturally isometric (as a metric space) to the orbit space of the group action. Furthermore, this isometry (and foliation) may be chosen so that a leaf closure is mapped to an orbit with the same volume, even though the dimension of the orbit may be different from the dimension of the leaf closure. Conversely, given a Riemannian foliation, there is a metric on the basic manifold (an $O(q)$-manifold associated to the foliation) such that the leaf closure space is isometric to the $O(q)$-orbit space of the basic manifold via an isometry that preserves the volume of the leaf closures of maximal dimension. Thus, the orbit space of any Riemannian G-manifold is isometric to the orbit space of a Riemannian $O(q)$-manifold via an isometry that preserves the volumes of orbits of maximal dimension. Consequently, the spectrum of the Laplacian restricted to invariant functions on any $G$-manifold may be identified with the spectrum of the Laplacian restricted to invariant functions on a Riemannian $O(q)$-manifold. Other similar results concerning the spectrum of differential operators on sections of vector bundles over Riemannian foliations and $G$-manifolds are discussed.

The geometry of ℝ–covered foliations

We study R-covered foliations of 3-manifolds from the point of view of their transverse geometry. For an R-covered foliation in an atoroidal 3-manifold M, we show that M-tilde can be partially compactified by a canonical cylinder S^1_univ x R on which pi_1(M) acts by elements of Homeo(S^1) x Homeo(R), where the S^1 factor is canonically identified with the circle at infinity of each leaf of F-tilde. We construct a pair of very full genuine laminations transverse to each other and to F, which bind every leaf of F. This pair of laminations can be blown down to give a transverse regulating pseudo-Anosov flow for F, analogous to Thurston's structure theorem for surface bundles over a circle with pseudo-Anosov monodromy. A corollary of the existence of this structure is that the underlying manifold M is homotopy rigid in the sense that a self-homeomorphism homotopic to the identity is isotopic to the identity. Furthermore, the product structures at infinity are rigid under deformations of the foliation F through R-covered foliations, in the sense that the representations of pi_1(M) in Homeo((S^1_univ)_t) are all conjugate for a family parameterized by t. Another corollary is that the ambient manifold has word-hyperbolic fundamental group. Finally we speculate on connections between these results and a program to prove the geometrization conjecture for tautly foliated 3-manifolds.

Dissipation Power of a Disk-Shaped Superconducting Thin Film Along Its Virgin Magnetization Curves

Based on the Bean critical state model j ⩽ jc for both longitudinal and transverse geometry, the total dissipation power P along the virgin magnetization curves is calculated. The dissipation power obtained from the corresponding magnetic measurements for a disk-shaped YBa2Cu3O7-δ thin film in a perpendicular magnetic field is verified. The virgin magnetization curves for the sample at different temperatures are calculated.

Phonon anomalies versus magnetic ordering in CuO

We report the first measurements of FIR and MIR reflectivity of single-crystal monoclinic antiferromagnetic copper monoxide (CuO) in a wide temperature range (7–300 K) when b -axis and ac -plane polarized modes are completely separated and excited in transverse geometry. Previously reported softening of the A u 3 mode at the Néel transition is confirmed and found to be up to two times more significant (about 10%). For the first time an emergence of several new IR-active lines at low temperatures is observed. The highest frequency new mode at 690 cm −1 strongly hardens and strengthens with cooling. The data are discussed in terms of strong spin–phonon coupling and possible charge disproportionation at low T.

Precision Lamb shift measurement in hydrogenic nitrogen by fast beam laser spectroscopy

A measurement of the 2S1/2–2P3/2 (fine structure–Lamb shift) transition in N6+ is under way. The technique involves fast beam spectroscopy with a transverse interaction geometry and uses an isotopic CO2 laser and a resonant build up cavity.

Magnetoresistance of magnetite

The resistivity and magnetoresistance of a magnetite single crystal and Fe3O4 films of various thicknesses were measured in the temperature range 70 K<T<300 K and in magnetic fields in the range -1 Tµ0H1 T. The magnetoresistance depends on both current and magnetic field direction. The anisotropic magnetoresistance is determined as the difference of the magnetoresistances in longitudinal and transverse geometry. The data were analysed within a phenomenological model above the Verwey temperature. The anisotropic magnetoresistance for currents along [100] was found to show a sign change simultaneously with that of the crystalline anisotropy constant K1. Whereas the magnetoresistance of the single crystal saturates above the anisotropy field, the Fe3O4 films show a significant high-field magnetoresistance depending linearly on the applied field. This behaviour was attributed to carrier transport across antiphase boundaries. A simple model was proposed that is in good qualitative agreement with the data. The single crystal shows a significant decrease of the Verwey transition in magnetic fields applied along [110]; this leads to a magnetoresistance of 70% in an external field of 1 T.

Lasers in dense gases pumped by low-energy electron beams

The use of low-energy (≈15 keV) electron beams for pumping laser systems in dense gases with high specific power deposition is described. Thin (300 nm) SiNx ceramic foils used as entrance window in a transverse geometry for the electron beam allows pressure differentials of several atmospheres with low percentage energy loss in transmission. The 1.73 μm XeI (5d[3/2]1–6p[5/2]2) infrared laser was used for a first demonstration of this concept. The laser operated between 130 and 650 mbar. A threshold pumping power of 5.3 W and a maximum output power of 6 mW were observed. The system can be scaled to high pumping power (≈MW/cm3) and short wavelength.

Measurements of angular and spatial characteristics of magnetostatic wave beams by an optical guided-wave probe

Optical measurements of the angular and spatial characteristics of magnetostatic forward volume wave (MSFVW) beams propagating both in a linear and in a non-linear regime in yttrium–iron–garnet (YIG) film have been performed. The MSFVW beams were excited by a single-element microstrip transducer, within the microwave frequency range 2–3 GHz. The measurements were conducted with the help of an optical probing technique based on the magneto-optical (MO) interaction between guided optical waves and the MSFVWs in the YIG film. The MO probing system was built on the principle of transverse Bragg diffraction geometry, allowing us to measure the spatial evolution of the partial plane waves making up the MSFVW beam. In the linear regime, the measurements of the longitudinal profiles of the partial waves have shown that the MSFVW attenuation coefficient can increase following a linear law, from 2 to 10 cm −1, as the MSFVW wave number increases from 200 to 2000 cm −1. Also, it was shown that MSFVW beams with different frequencies can be spatially separated at a small distance from the microstrip transducer (≈2 mm) by using a weak non-uniform magnetic bias field with a gradient of ≈30 Oe/cm. In the non-linear regime, there are, at least, three specific levels of the incident microwave power which correspond to different manifestations of non-linearity of the MSFVW beams. At a certain value of the power, the partial plane waves of the beam can periodically change the direction of propagation (we observed a periodic collision of these waves). A small increase of the power level can lead to a spatial-periodic energy exchange between the different groups of the partial plane waves. At high microwave powers, non-linear MSFVW can excite a discrete spectrum of new spin-wave modes, the in-plane wave number of which is less than the wave number of the MSFVW. These non-linearly excited waves exist within a small spatial region situated near the microstrip transducer.

Optical guided modes in sandwiches with ultrathin metallic films

Wave propagation in magneto–optical media is analyzed on the basis of eigenmode waves. Three basic configurations for magneto-optical effects are studied. In the transversal geometry the guided mode condition in sandwiches can be divided into s- and p-polarized waves. For longitudinal and polar configurations the waveguiding is composed of circular-like polarized eigenmodes.

Surface-induced transverse magneto-optical Kerr effect

We predict a magneto-optical Kerr rotation from a magnetized surface in the transverse geometry at normal incidence which is linear in the surface magnetization and is forbidden in the bulk. This surface-induced transverse magneto-optical Kerr effect (SITMOKE) is found to vanish at most bulk-terminated low-index faces of cubic media but is allowed at lower-symmetry planes so that SITMOKE is therefore suitable to study the magnetism of vicinal and reconstructed surfaces. The existence of SITMOKE is demonstrated by detection of an analogous strain-induced effect in a magnetic garnet film with a (210) crystallographic orientation.

Element-specific magnetocrystalline anisotropy energy studied by transverse magnetic circular X-ray dichroism

The magnetocrystalline anisotropy energy in 3d transition metal systems is related to the spin-subband orbital moments and the magnetic dipole operator which accounts for the spin-flip excitations. Magnetic circular X-ray dichroism measurements in a transverse geometry, where the light helicity is perpendicular to the magnetization direction, make it feasible to determine the easy-axis of magnetization.

Failure of Zircaloy cladding under transverse plane-strain deformation

Experiments have been performed to examine the ductility of Zircaloy 4 cladding tubes under conditions of near plane-strain deformation in the hoop direction (transverse to the tube axis) at temperatures of 25 and 300°C and at strain rates of 10 −3 and 10 2 s −1. To conduct these experiments, a specimen configuration was designed in which near plane-strain deformation is achieved, and a test methodology was established to determine two failure conditions: the limit strain at the onset of localized necking and the fracture strain. Experiments performed on cold-worked stress relieved material using the transverse plane-strain specimen geometry indicate major differences in failure behavior from that observed in uniaxial tension, although both test conditions result in failure by a localized necking process. The experimental results also indicate that while plane-strain fracture strains increase with temperature between 25 and 300°C, at a given temperature they are insensitive to strain rate. The limit strains at localized necking also increase with temperature but only at the high 10 2 s −1 strain rate. Finally, the failure data indicate a strong sensitivity to surface flaws, as predicted by localized necking theory.

Magnetically induced spatial dispersion in the cubic magnetic semiconductors Cd1−x MnxTe

In the transverse geometry we have detected birefringence that is linear in the magnetic field B and the light wave vector k in the cubic magnetic semiconductors Cd1−xMnxTe (0⩽x⩽0.52). The effect was found to be large, ∼1 (deg cm−1 T−1), and highly anisotropic, in contrast to the Faraday and Voigt effects. The phenomenon is represented by terms of type γijklBkkl in the permittivity tensor eij and can be described by two parameters, A and g. Spectral studies have shown that the normalized parameters A/x and g/x are independent of x, i.e., the effect can be related to the Mn2+ ions. Below the edge Eg of the forbidden band, the dispersion of A is described by a (Eg−E)−1.4-dependence, while the dispersion of g is nil. Theoretical analysis has shown that the spectral curves for A and g can be explained by the special features of the dispersion laws for electrons and holes (features related to the fact that there is no inversion center) and by the dependence of the parameters of the exchange interaction on the electron wave vector.

Magnetohydrodynamics of Cloud Collisions in a Multiphase Interstellar Medium

We extend previous studies of the physics of interstellar cloud collisions by beginning an investigation of the role of magnetic fields through two-dimensional magnetohydrodynamical (MHD) numerical simulations. In particular, we study head-on collisions between equal mass, mildly supersonic, diffuse clouds similar to those in our previous study. Here we include a moderate magnetic field, corresponding to β=pg/pb=4, and two limiting field geometries, with the field lines parallel (aligned) and perpendicular (transverse) to the colliding cloud motion. We explore both adiabatic and radiative (η=τrad/τcoll0.38) cases, and we simulate collisions between clouds evolved through prior motion in the intercloud medium. In addition to the collision of evolved identical clouds (symmetric cases), we also study collisions of clouds that are initially identical but have different evolutionary ages (asymmetric cases). Depending on their geometry, magnetic fields can significantly alter the outcome of the collisions compared to the hydrodynamic (HD) case. (1) In the aligned case, adiabatic collisions, like their HD counterparts, are very disruptive independently of the symmetry. However, when radiative processes are taken into account, partial coalescence takes place even in the asymmetric case, unlike the HD calculations. (2) In the transverse case, the effects of the magnetic field are even more dramatic, with remarkable differences between unevolved and evolved clouds. Collisions between (initially adjacent) unevolved clouds are almost unaffected by magnetic fields. However, the interaction with the magnetized intercloud gas during precollision evolution produces a region of very high magnetic energy in front of the cloud. In collisions between evolved clouds with transverse field geometry, this region acts like a bumper, preventing direct contact between the clouds and eventually reversing their motion. The elasticity, defined as the ratio of the final to the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we considered. This behavior is found in both adiabatic and radiative cases.

Soft x-ray resonant magnetic reflectivity study of thin films and multilayers

Soft x-ray resonant magnetic reflectivity measurements on thin films and multilayers in a transverse geometry using linear polarized photons are presented. Magneto-optic calculations taking into account the layer roughness allows us to reproduce all the experimental features of the angular and energy reflectivity curves as well as the asymmetry ratio in both cases. Application to FexMn1−x alloy films epitaxially grown on Ir(001) brings more insights on the magnetic transition occurring at x=0.75.

Magnetic anisotropy and exchange biasing in heterojunctions studied by transverse magnetic circular X-ray dichroism

The anisotropy of the orbital magnetic moment and the magnetic dipole term can be obtained directly with magnetic circular X-ray dichroism in a transverse geometry, where the spins are forced out of their easy direction of magnetization. Strong effects are expected at surfaces and interfaces. We demonstrate with the example of an iron monolayer on Ni(1 1 0) that non-collinear moments can give rise to large changes in the dichroism of the L 2,3 absorption spectrum. The ratio between orbital and spin moment can be used to determine the easy direction of magnetization.