Magnetic polarons and magnetoresistance in EuB6

EuB6 is a low carrier density ferromagnet which exhibits large magnetoresistance, positive or negative depending on temperature. The formation of magnetic polarons just above the magnetic critical temperature has been suggested by spin-flip Raman scattering experiments. We find that the fact that EuB6 is a semimetal has to be taken into account to explain its electronic properties, including magnetic polarons and magnetoresistance.Comment: 6 pages, 1 figur

Numerical calculations of effective elastic properties of two cellular structures

Young's moduli of regular two-dimensional truss-like and eye-shape-like structures are simulated by using the finite element method. The structures are the idealizations of soft polymeric materials used in the electret applications. In the simulations size of the representative smallest units are varied, which changes the dimensions of the cell-walls in the structures. A power-law expression with a quadratic as the exponential term is proposed for the effective Young's moduli of the systems as a function of the solid volume fraction. The data is divided into three regions with respect to the volume fraction; low, intermediate and high concentrations. The parameters of the proposed power-law expression in each region are later represented as a function of the structural parameters, unit-cell dimensions. The presented expression can be used to predict structure/property relationship in materials with similar cellular structures. It is observed that the structures with volume fractions of solid higher than 0.15 exhibit the importance of the cell-wall thickness contribution in the elastic properties. The cell-wall thickness is the most significant factor to predict the effective Young's modulus of regular cellular structures at high volume fractions of solid. At lower concentrations of solid, eye-like structure yields lower Young's modulus than the truss-like structure with the similar anisotropy. Comparison of the numerical results with those of experimental data of poly(propylene) show good aggreement regarding the influence of cell-wall thickness on elastic properties of thin cellular films.Comment: 7 figures and 2 table

Magnetic metamaterials at telecommunication and visible frequencies

Arrays of gold split-rings with 50-nm minimum feature size and with an LC resonance at 200-THz frequency (1500-nm wavelength) are fabricated. For normal incidence conditions, they exhibit a pronounced fundamental magnetic mode, arising from a coupling via the electric component of the incident light. For oblique incidence, a coupling via the magnetic component is demonstrated as well. Moreover, we identify a novel higher-order magnetic resonance at around 370 THz (800-nm wavelength) that evolves out of the Mie resonance for oblique incidence. Comparison with theory delivers good agreement and also shows that the structures allow for a negative magnetic permeability.Comment: 4 pages, 3 figure

Understanding the dynamics of photoionization-induced solitons in gas-filled hollow-core photonic crystal fibers

We present in detail our developed model [Saleh et al., Phys. Rev. Lett. 107] that governs pulse propagation in hollow-core photonic crystal fibers filled by an ionizing gas. By using perturbative methods, we find that the photoionization process induces the opposite phenomenon of the well-known Raman self-frequency red-shift of solitons in solid-core glass fibers, as was recently experimentally demonstrated [Hoelzer et al., Phys. Rev. Lett. 107]. This process is only limited by ionization losses, and leads to a constant acceleration of solitons in the time domain with a continuous blue-shift in the frequency domain. By applying the Gagnon-B\'{e}langer gauge transformation, multi-peak `inverted gravity-like' solitary waves are predicted. We also demonstrate that the pulse dynamics shows the ejection of solitons during propagation in such fibers, analogous to what happens in conventional solid-core fibers. Moreover, unconventional long-range non-local interactions between temporally distant solitons, unique of gas plasma systems, are predicted and studied. Finally, the effects of higher-order dispersion coefficients and the shock operator on the pulse dynamics are investigated, showing that the resonant radiation in the UV [Joly et al., Phys. Rev. Lett. 106] can be improved via plasma formation.Comment: 9 pages, 10 figure

High-Resolution Kinoform X-Ray Optics Printed via 405 nm 3D Laser Lithography

Efficient focusing of X-rays is essential for high-resolution X-ray microscopy. Diffractive X-ray optics called kinoforms offer the highest focusing efficiencies in theory. However, they have long remained unavailable due to their challenging nanofabrication. Recently, various X-ray optic geometries including kinoforms have been realized using 3D laser lithography at near-infrared wavelengths. As the smallest features (period) of the kinoform determines the resolving power, there is a natural drive to find ways to fabricate kinoforms with ever smaller features. Here, a custom-built 3D laser lithography setup with an excitation wavelength of 405 nm is used, which allows to half the smallest period of the kinoforms compared to previous work. A 40% improvement in scanning transmission X-ray microscopy image resolution, that is, a cutoff resolution of 145 nm, and an efficiency of 7.6% at 700 eV is achieved. A reconstructed pixel size of 18.5 nm, reaching the limit imposed by the design of the microscopy set-up, is demonstrated through ptychographic imaging of a magnetic sample which has a strongly reduced contrast mechanism. Moreover, X-ray lenses manufactured by 405 nm 3D laser lithography have the potential to become much less expensive than X-ray lenses made by other means

Observation of Chirality‐Induced Roton‐Like Dispersion in a 3D Micropolar Elastic Metamaterial

A theoretical paper based on chiral micropolar effective-medium theory suggested the possibility of unusual roton-like acoustical-phonon dispersion relations in 3D elastic materials. Here, as a first novelty, the corresponding inverse problem is solved, that is, a specific 3D chiral elastic metamaterial structure is designed, the behavior of which follows this effective-medium description. The metamaterial structure is based on a simple-cubic lattice of cubes, each of which not only has three translational but also three rotational degrees of freedom. The additional rotational degrees of freedom are crucial within micropolar elasticity. The cubes and their degrees of freedom are coupled by a chiral network of slender rods. As a second novelty, this complex metamaterial is manufactured in polymer form by 3D laser printing and its behavior is characterized experimentally by phonon-band-structure measurements. The results of these measurements, microstructure finite-element calculations, and solutions of micropolar effective-medium theory are in good agreement. The roton-like dispersion behavior of the lowest phonon branch results from two aspects. First, chirality splits the transverse acoustical branches as well as the transverse optical branches. Second, chirality leads to an ultrastrong coupling and hybridization of chiral acoustical and optical phonons at finite wavevectors

Fluctuation induced hopping and spin polaron transport

We study the motion of free magnetic polarons in a paramagnetic background of fluctuating local moments. The polaron can tunnel only to nearby regions of local moments when these fluctuate into alignment. We propose this fluctuation induced hopping as a new transport mechanism for the spin polaron. We calculate the diffusion constant for fluctuation induced hopping from the rate at which local moments fluctuate into alignment. The electrical resistivity is then obtained via the Einstein relation. We suggest that the proposed transport mechanism is relevant in the high temperature phase of the Mn pyrochlore colossal magneto resistance compounds and Europium hexaboride.Comment: 8 pages, 3 figure

High amp; 8208;Resolution Kinoform X Ray Optics Printed via 405 nm 3D Laser Lithography

Efficient focusing of X rays is essential for high resolution X ray microscopy. Diffractive X ray optics called kinoforms offer the highest focusing efficiencies in theory. However, they have long remained unavailable due to their challenging nanofabrication. Recently, various X ray optic geometries including kinoforms have been realized using 3D laser lithography at near infrared wavelengths. As the smallest features period of the kinoform determines the resolving power, there is a natural drive to find ways to fabricate kinoforms with ever smaller features. Here, a custom built 3D laser lithography setup with an excitation wavelength of 405 nm is used, which allows to half the smallest period of the kinoforms compared to previous work. A 40 improvement in scanning transmission X ray microscopy image resolution, that is, a cutoff resolution of 145 nm, and an efficiency of 7.6 at 700 eV is achieved. A reconstructed pixel size of 18.5 nm, reaching the limit imposed by the design of the microscopy set up, is demonstrated through ptychographic imaging of a magnetic sample which has a strongly reduced contrast mechanism. Moreover, X ray lenses manufactured by 405 nm 3D laser lithography have the potential to become much less expensive than X ray lenses made by other mean