Isotropic Defects Research Articles

Anomalous polarization-sensitive Fabry-Perot resonance in a one-dimensional photonic crystal containing an all-dielectric metamaterial defect.

Owing to polarization-independent property of propagating phases inside isotropic dielectric layers, Fabry-Perot resonances in metal-dielectric-metal sandwich structures and one-dimensional (1-D) photonic crystals (PhCs) with isotropic dielectric defects are polarization-insensitive. Herein, we introduce an all-dielectric elliptical metamaterial (EMM) defect into a 1-D PhC to realize an anomalous polarization-sensitive Fabry-Perot resonance empowered by the polarization-sensitive property of the propagating phase inside the all-dielectric EMM layer. The wavelength difference of the Fabry-Perot resonance between transverse magnetic and transverse electric polarizations is larger than 100 nm at the incident angle of 45 degrees. Enabled by the polarization-sensitive property of the Fabry-Perot resonance, high-performance polarization selectivity can be achieved in a broad angle range. Our work offers a viable recipe, well within the reach of current fabrication technique, to explore polarization-dependent physical phenomena and devices.

Modulated structure and hopping transport mechanism involving a defect-induced localization–delocalization transition in a Ca–Ce(La)–Nb–W–O system

CaCeNbWO8-δ (CCNWO) oxide is a three-dimensional semiconductor with potential applications in electronic devices. It has been reported that the temperature dependence of the resistivity of CCNWO deviates from that predicted by the Arrhenius equation over a wide temperature range. However, its complex hopping transport mechanism has remained unclear due to unknown defect structures. A combination of transmission electron microscopy and the density functional theory showed that CaCeNbWO8-δ and CaCe0.5La0.5NbWO8-δ have a modulated defect structure, and the carriers mainly undergo a Nb4d→W5d transition. At low temperatures, anisotropic defects promote the diversification of transport properties, variable-range hopping, and nearest neighbor hopping. With increasing temperature, isotropic defects promote simplification of the transport property. Compared with CCNWO, CaCe0.5La0.5NbWO8-δ, with a more stable structure, has stronger electron–electron interactions and shifts the localization–delocalization transition point to a higher temperature. The proposed hopping transport mechanism involving a defect-induced localization–delocalization transition allows semiconductor design and device integration schemes, thus impacting a variety of applications.

Non-specular scattering of carriers from surface defects in thin metal interconnects

Non-specular scattering of carriers, despite being one of the key contributors to metal film resistivity, is conveniently characterized by a single parameter p (referred to as specularity parameter) in the literature without providing a means to separate out relative contributions of surface defects to it. In this work, we report a theoretical formalism to predict contributions to (non-specular) reflection (scattering) of carriers from different types of chemical and structural defects on the film surface. Establishing specular reflection as a special case of the more generalized reflection from the surface, we show that elastic scatterings contribute to the non-specular reflections by randomizing the forward momentum along the transport direction. We predict density and angle-dependent specularity coefficient for various types of defects and show that large angle reflections tend to be non-specular in the presence of isotropic surface defects. Using this formalism, we predict the probability of specular reflection for technologically relevant metal–metal and metal–insulator interfaces. Calculated specularity coefficients are in agreement with values obtained from experimental measurements. Our methodology will help in designing next generation interconnects and engineering the interfaces with minimal diffuse reflection.

Intrinsic anisotropy and pinning anisotropy in nanostructured YBa2Cu3O7−δ from microwave measurements

Anisotropy is an intrinsic factor that dictates the magnetic properties of YBa2Cu3O7−δ and therefore has a great impact on many applications. Artificial pinning centres are often introduced in an attempt to mitigate its effect, resulting in less anisotropic electrical and magnetic properties. However, the nanoengineering of superconductors makes the quantification of the anisotropy itself uncertain: the intrinsic anisotropy due to the layered structure, quantified by the anisotropy factor γ, mixes with the additional anisotropy due to pinning. As a consequence, there is no consensus on the experimental anisotropy factor γ that can result in YBa2Cu3O7−δ when directional (twin planes, nanorods) or isotropic defects are present. We present measurements of the magnetic field and angular dependent surface impedance in very different nanostructured YBa2Cu3O7−δ films, grown by a chemical route and by pulsed laser deposition with different kind of defects (nanorods, twin planes, nanoparticles). We show that the surface impedance measurements are able to disentangle the intrinsic anisotropy from the directional pinning anisotropy, thanks to the possibility of extracting the true anisotropic flux–flow resistivity and by correctly exploiting the angular scaling. We find in all films that the intrinsic anisotropy γ = 5.3 ± 0.7. By contrast, the pinning anisotropy determines a much more complex, feature–rich and nonuniversal, sample–dependent angular landscape.

A Marker of Cerebral Ischemia in Solid State Structures of Blood Serum.

Reversible cerebral ischemia of medium severity was reproduced in male Wistar rats by bilateral occlusion of the common carotid arteries. Solid-phase structures (anisomorphons) were obtained by marginal dehydration of the serum. Small focal isotropic defects in the serum anisomorphon texture were found in 100% cases during occlusion of the carotid arteries. Similar signs were detected in all patients with chronic cerebral ischemia, which proved specificity of this morphological marker of the disease.

Photoionization cross-section of isotropic defects or impurity centers in isolators

An analytical expression to calculate the photoionization cross-section of isotropic defects or impurity centers is being proposed by using the time-dependent perturbation theory. The ground-state wave function of the electron captured in the impurity state is described by a three-dimensional isotropic harmonic oscillator and the electron excited state in the continuum conduction band is described by a plane wave. The expression has been obtained considering all multipoles terms in the Hamiltonian, and that the radiation field which interacts with electrons is semi-classical and linearly polarized. This approximation is assumed because the effects of the linear contribution are dominant. The available data of the Al2O3:C and Lu2SiO5:Ce systems are in good agreement with our predictions. Such satisfactory comparison is a strong indication that the present model can be used to provide good predictions of the photoionization cross-section in several areas.

Effects of introducing isotropic artificial defects on the superconducting properties of differently doped Ba-122 based single crystals.

The effects of isotropic artifical defects, introduced via fast neutron (E > 0.1 MeV) irradiation, on the physical properties of differently (Co, P and K) doped BaFe2As2 superconducting single crystals were studied. The Co- and P-doped single crystals showed a second peak in the magnetization curve (fishtail effect) in the pristine state. Significant variations in the radiation-induced changes in the critical current density Jc were observed in the different types of crystal, while the irreversibility fields did not change remarkably. The highest Jcs were obtained for the K-doped crystal, exceeding 3 × 1010 Am−2 (T = 5 K, B = 4 T) and remaining above 8.5 × 109 Am−2 at 30 K and 1 T. The pinning force was analyzed to compare the pinning mechanisms of the individual samples. While distinct differences were found before the irradiation, the same pinning behavior prevails afterwards. The pinning efficiency η = Jc/Jd was estimated from the depairing current density Jd. η was similar in all irradiated crystals and comparable to the value in neutron irradiated cuprates, suggesting that the huge critical current densities measured in the irradiated K-doped crystal are due to its large depairing current density, making this compound the most promising for applications.

Microalloying and the mechanical properties of amorphous solids

The mechanical properties of amorphous solids like metallic glasses can be dramatically changed by adding small concentrations (as low as 0.1%) of foreign elements. The glass-forming-ability, the ductility, the yield stress and the elastic moduli can all be greatly effected. This paper presents theoretical considerations with the aim of explaining the magnitude of these changes in light of the small concentrations involved. The theory is built around the experimental evidence that the microalloying elements organise around them a neighbourhood that differs from both the crystalline and the glassy phases of the material in the absence of the additional elements. These regions act as isotropic defects that in unstressed systems modify the shear moduli. When strained, these defects interact with the incipient plastic responses which are quadrupolar in nature. It will be shown that this interaction interferes with the creation of system-spanning shear bands and increases the yield strain. We offer experimentally testable estimates of the lengths of nano-shear bands in the presence of the additional elements.

Interaction of vortices in anisotropic superconductors with isotropic defects

To assess a superconductor’s suitability for high-current applications, it is imperative to know how its critical current density () depends on magnetic field and its orientation. We present a comprehensive study of the anisotropy of the in-plane in iron-based superconducting single crystals carried out by an advanced magnetometry technique. As is governed by material defects capable of vortex pinning, we investigated three samples with three essentially different defect landscapes representing weak, strong and intermediate pinning. In the latter case, a second maximum in the field dependence of the critical current (fishtail effect) occured, allowing us to investigate the influence of this effect on the angular dependence of in an intermediate pinning regime for the first time in a quantitative way. For weak and strong pinning, the influence of the field orientation on the in-plane can be described by the anisotropic scaling theory. For weak pinning, we find the predicted scaling for the field () and for the in-plane current (unity). For strong pinning, we find the same scaling for the field, but show for the first time that an additional scaling factor () emerges for the in-plane critical current density. We attribute this new scaling rule (double scaling) to the presence of defects which are larger than the vortex core.

Numerical simulation of flow and structure in nematic liquid crystalline materials between eccentric cylinders

In this paper the time transient isothermal flow of lyotropic nematic liquid crystals (LC) between two eccentric cylinders was studied numerically by applying the Landau-de Gennes (LdG) theory. The start-up flow induced by the steady rotation of the inner cylinder was used to model the lubrication problem inside a journal bearing. The ability of liquid crystalline materials to form ordered boundary layers with good load-carrying capacity, outstanding lubricating properties, has been widely demonstrated. The LdG theory for the microstructure of the molecules along with continuity and momentum equations were solved simultaneously using General PDE and Laminar Flow packages of the COMSOL Multiphysics. Flow properties and structure were investigated as a function of simulation time, the Reynolds number and the Energy ratio, R. Interconnection and impact of the texture and defects formation/evolution on velocity profile and pressure distribution was observed; nucleation and evolution of disclination line and defect points were detected and pursued over the simulation time. For high Reynolds numbers, the flow reached quasi-stationary condition while the microstructure evolution was at unsteady state. The influence of the Energy ratio and the Reynolds number on the formation and evolution of polydomains was studied. Indeed, it was found that coupling the evolution of liquid crystalline microstructure with the hydrodynamics has a quantitative and qualitative impact on the macro-scale attributes of the flow and on the structure. The standard procedure to classify defects has been applied, oblate and isotropic defects have been identified.

Nanostrain induced pinning in YBa2Cu3O7−x nanocomposites even close to the irreversibility line

It has been recently shown that solution-derived YBa2Cu3O7−x (YBCO) nanocomposites can reach extraordinary pinning forces at 77 K, thanks to the isotropic nanostrain introduced by nanoparticles. In this paper we demonstrate that nanostrain pinning is even effective very close to the irreversibility line (IL). Ac susceptibility and transport measurements are used to determine the isotropic and anisotropic contributions to the activation energy for thermal depinning in different (BaZrO3, Y2O3, Ba2YTaO6) YBCO nanocomposites. Results demonstrate that the activation energy associated with isotropic defects correlates linearly with the nanostrain.

Lattice Defects Diffuse Scattering from Thin Films of a Ge-Si System with Low-Energy Ar+ and Xe+ Bombardment During Molecular Beam Epitaxy (MBE) Growth

Homo–Ge, homo–Si, and hetero–Si0.2Ge0.8 alloy epitaxial layers, using molecular beam epitaxy (MBE), were grown on Ge and Si (001) substrates in order to study development of crystalline strains caused by ion bombardment during the growth of materials. Ion energies and ion/atom fluxes were used in the epitaxial growth, and significant lattice distortions along the growth direction developed. Using high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM), the form of distortion, caused by low-energy argon (Ar+) and xenon (Xe+) bombardment of the thin epitaxial films grown on the (001) substrates, were investigated. The isotropic point defects case (of spherical distortions) occurs in epitaxial thin films “as-grown” processes. The intensity distribution has two maxima, one from the distorted layer and the other from the original unaffected matrix. The significant changes in the 2θ location, peak broadening and integrated intensity from the secondary (004)* reflections were obtained as a function of aging temperatures in the grown layers. Defects-induced diffuse scattering close to and between Bragg reflections supplies information on the strain and symmetry of the distortions fields and yields the atomic structure of point defects (self-interstitial, vacancies, and small clusters). First, aging heat treatment affects the distribution of distortions obtained in local regions at the “as-grown” layer, which develops to a special topography of continued distortions at higher aging temperatures. At aging temperatures above 923 K (650 °C), this extra diffraction peak disappears. The TEM observations reveal the appearance of dislocation lines with dark and bright contrasts around them, interdislocation strain contrasts, and disordered point defects atoms in the silicon region with semicoherent interfaces. The ion bombardment-induced formations and injection of the different types of pointlike defects and defects clusters.

Diffuse scattering anisotropy and inhomogeneous lattice deformations in the lead magnoniobate relaxor PMN above the Burns temperature

Neutron diffuse scattering measurements on single crystal PbMg${}_{1/3}$Nb${}_{2/3}$O${}_{3}$ above the Burns temperature are reported. The high temperature elastic diffuse component is highly asymmetric near reciprocal lattice points not corresponding to high-symmetry directions. This asymmetry can be described using Huang scattering formalism assuming that the scattering originates from mesoscopic lattice deformations due to elastic defects. Qualitative agreement between this model and the experimental data is achieved with simple, isotropic defects. It is demonstrated that the weak satellite maxima near Bragg reflections can be interpreted in terms of a finite resolution effect.

Isotropic and anisotropic pinning in TFA-grownYBa2Cu3O7 − x filmswith BaZrO3 nanoparticles

YBCO films grown by the trifluoroacetate (TFA) method with increasing number ofBaZrO3 (BZO) nanoparticles have been measured by in-field angular transportmeasurements to investigate changes in the pinning landscape. Theisotropic and anisotropic contributions to the critical current density,Jc(H), with the magnetic field applied in and orientation have been determined, allowing us to characterizethe population of isotropic and correlated defects along thec axisand ab planes. First, the influence of the YBCO oxygenation process on the formation of differentsorts of anisotropic defects in standard films is demonstrated. Next, we show that theaddition of non-coherent BZO nanoparticles to the YBCO matrix produces an expansion ofthe single-vortex pinning regime toward higher fields, due to the presence of isotropicpinning centers. Moreover, by increasing the amount of isotropic defects in the BZOnanocomposites it is possible to extend the region dominated by strong isotropicpinning centers to large magnetic fields and thus enhance the irreversibility line.

Angular properties of pure and Ca-substituted YBa 2Cu 3O 7− δ superconducting thin films grown on SrTiO 3 and CeO 2 buffered Al 2O 3 substrates

In this work transport properties of superconducting 10 at.% Ca-substituted YBCO thin films grown on (1 0 0)-SrTiO 3 single crystal substrate (STO) and superconducting pure and 10 at.% Ca-substituted YBCO thin films grown on CeO 2 buffered Al 2O 3 substrates (CAO) have been analyzed as a function of the temperature, applied magnetic field and angle between magnetic field direction and the direction normal to the film surfaces. Particularly, the angular analysis provides an easy way to discriminate between isotropic point defects and correlated pinning sites. Despite the intragrain pinning mechanisms remained unaffected by Ca substitution, a detrimental effect on grain boundary properties clearly emerged for 10 at.% Ca concentration. This effect is enhanced in sample grown on CeO 2 buffered sapphire where a more disturbed grain boundary is expected resulting in an enhancement of the correlated pinning, already observed in pure YBCO films grown on CAO, and in a reduction of the intrinsic pinning efficiency.

Modification of intergrain connectivity, upper critical field anisotropy and critical current density in ion irradiated MgB2 films

We compare the effect of isotropic point defects vis á vis extended defects on the inter and intra grain properties of superconducting MgB2 thin films. In a recent paper Gandikota et al. [Appl. Phys. Lett. 86 (2005) 012508] reported that after 200MeV α particle irradiation intergrain connectivity remains unaffected. Our results on the contrary indicate that connectivity does depend on irradiation dose and type of ions used. We ascertain that extended defects alter the σ band properties of this two-band superconductor more effectively than the point defects. The improvement in upper critical field and critical current density is intricately related to the type and density of defects created.

A comparative study of the structure and energetics of elementary defects in 3C- and4H-SiC

The potential non-equivalent defects in both 3C- and 4H-SiC are classified by a new methodthat is based on symmetry considerations. In 4H-SiC their number is considerably higherthan in 3C-SiC, since the hexagonal symmetry leads to diversification. The differenttheoretical methods hitherto used to investigate defects in 3C-SiC are critically reviewed.Classical MD simulations with a recently developed interatomic potential are employed toinvestigate the stability, structure and energetics of the large number of non-equivalentdefects that may exist in 4H-SiC. Most of the potential defect configurations in 4H-SiCare found to be stable. The interstitials between hexagonal and trigonal rings,which do not exist in 3C-SiC, are characteristic for 4H-SiC and other hexagonalpolytypes. The structure and energetics of some complex and anisotropic dumbbellsdepend strongly on the polytype. On the other hand, polytypism does not have asignificant influence on the properties of the more compact and isotropic defects,such as vacancies, antisites, hexagonal interstitials, and many dumbbells. Theresults allow conclusions to be drawn about the energy hierarchy of the defects.

Structures and energetics of defects: a comparative study of 3C- and 4H-SiC

The structures, formation energies and stable configurations of elementary defects (vacancies, antisite defects and self-interstitials) in 3C- and 4H-SiC are studied using classical molecular dynamics simulation with a recently developed interatomic potential. The defect structures in 3C-SiC are relatively simple, but those in 4H-SiC are more complex. The interstitials between hexagonal and trigonal rings are characteristic for 4H-SiC and other hexagonal polytypes, but not for 3C-SiC. The number of non-equivalent defects in 4H-SiC is much higher than that in 3C-SiC, and a considerable difference is found for some complex and anisotropic defects, in particular for the dumbbells D1Si–Si, D1Si–C and D2Si–C. The lattice deformation beyond the first nearest neighbor shell, which depends strongly on the polytype structure, plays an important role on these effects. However, the polytypism does not have a significant influence on the structure and energetics of the more compact and isotropic defects, such as vacancies and antisite defects. Despite the complexity of defect configurations, the tetrahedral interstitials have very similar properties in 3C- and 4H-SiC because their first, second and third nearest neighbor shells are identical.

Supermodes of chiral photonic filters with combined twist and layer defects.

We consider the circularly polarized localized modes of chiral photonic structures with combined central twist and isotropic layer defects. The general filter is shown to suffer from anomalous remittance and saturation of linewidth as the thickness of the structure is increased. However, by choosing parameters that phase match the elements of the round-trip matrix of the isotropic layer defect, we demonstrate the existence of supermodes that maintain exceptional purity of polarization state and exponential decrease in linewidth as the thickness is increased.

Elastic interaction between defects in thin and 2D films

Elastic interactions between defects is investigated at the surface of thin layers, a question on which we have given a brief account [P. Peyla et al. Phys. Rev. Lett. 82, 787 (1999)]. Two isotropic defects do not interact in an unlimited medium, regardless of the spatial dimension, a result which can be shown on the basis of the Gauss theorem in electrostatics. Within isotropic elasticity theory, defects interact only (i) if they are, for example, at a surface (or at least if they feel a boundary), or if their action on the material is anisotropic (e.g. they create a non central force distribution, though the material elasticity is isotropic). It is known that two identical isotropic defects on the surface of a semi-infinite material repel each other. The repulsion law behaves as 1/r 3(r = defects separation). We first revisit the Lau-Kohn theory and extend it to anisotropic defects. Anisotropy is found to lead to attraction. We show that in thin films defects may either attract or repel each other depending on the local geometric force distribution caused by the defect. It is shown that the force distribution (or more precisely the forces configuration symmetry) fixes the exponent in the power law 1/r n (e.g. for a four-fold symmetry n = 4). We discuss the implication of this behaviour in various situations. We treat the interactions in terms of the symmetries associated with the defect. We argue that if the defects are isotropic, then their effective interaction in an unlimited 2D (or a thin film) medium arises from the induced interaction, which behaves as 1/r 4 for any defect symmetry. We shall also comment on the contribution to the interaction which arises from flexion of thin films.