We study the acoustic scattering properties of a phononic crystal designed to behave as a gradient index lens in water, both experimentally and theoretically. The gradient index lens is designed using a square lattice of stainless-steel cylinders based on a multiple scattering approach in the homogenization limit. We experimentally demonstrate that the lens follows the graded index equations derived for optics by mapping the pressure intensity generated from a spherical source at 20 kHz. We find good agreement between the experimental result and theoretical modeling based on multiple scattering theory.

We study the Zeeman spin-splitting in hole quantum wires oriented along the [011] and [011] crystallographic axes of a high mobility undoped (100)-oriented AlGaAs/GaAs heterostructure. Our data shows that the spin-splitting can be switched 'on' (finite g * ) or 'off' (zero g * ) by rotating the field from a parallel to a perpendicular orientation with respect to the wire, and the properties of the wire are identical for the two orientations with respect to the crystallographic axes. We also find that the g-factor in the parallel orientation decreases as the wire is narrowed. This is in contrast to electron quantum wires, where the g-factor is enhanced by exchange effects as the wire is narrowed. This is evidence for a k-dependent Zeeman splitting that arises from the spin-3 2 nature of holes.

Pentamode metamaterials are a class of acoustic metafluids that are characterized by a divergence free modified stress tensor. Such materials have an unconventional anisotropic stiffness and isotropic mass density, which allow themselves to mimic other fluid domains. Here we present a pentamode design formed by an oblique honeycomb lattice and producing customizable anisotropic properties. It is shown that anisotropy in the stiffness can exceed three orders of magnitude, and that it can be realistically tailored for transformation acoustic applications. [16][17][18][19]. Such devices are defined as metafluids, which are effective materials with unconventional fluid-like properties whose particular bulk realization typically requires an anisotropic mass density. Experimental demonstration has been sparse, with most studies relying on a superlattice approach of alternating isotropic layers [20]. However, such an approach is difficult to realize and limited by the so-called mass catastrophe, which requires infinite mass density in the effective material profile.An alternative approach is to generalize the conventional stress strain relationship to include pentamode metamaterials. Pentamode materials [21][22][23] are metafluids that support five easy infinitesimal strains (i.e. there is only one non-zero eigenvalue of the elasticity tensor which is of a pure pressure type), and satisfies the invariance of the governing equations by virtue of maintaining a harmonic transformation. Pure pentamodes, in general, have an isotropic density and anisotropic stiffness with a negligible shear modulus. Recently, isotropic pentamode materials have become experimental reality, and it was shown that the structure's effective bulk modulus exceeded the shear modulus by three orders of magnitude [24]. However, it has yet to be reported that anisotropic pentamode metamaterials can be realistically implemented for specific applications, since an elastic solid with a zero shear modulus would have no stability and immediately flow away.In this Letter, we show that an oblique honeycomb lattice can be utilized as a simple yet versatile building block for pentamode device construction, which exerts highly anisotropic control over sound waves. The method presents a distinctly different approach to acoustic metamaterials, in that it does not require the difficult to achieve high value anisotropy in the effective mass density in addition to removing frequency bandwidth problems associated with inertial metafluids. Potential applications include extraordinary scattering reduction and arbitrary wave manipulation, low loss acoustic delay lines [25], and phase controlled logic gates [26].Anisotropy in pentamode metafluids. We consider elastic wave propagation in a microstructure having the general characteristics presented in Fig. 1. For simplicity we present our results in a two-dimensional (2D) plain strain space, however, the analysis can straightforwardly be extended to three dimensions. Only wavelengths much larger than the lattice constant...

Owing to the prevalence of fractal patterns in natural scenery and their growing impact on cultures around the world, fractals constitute a common feature of our daily visual experiences, raising an important question: what responses do fractals induce in the observer? We monitored subjects' EEG while they were viewing fractals with different fractal dimensions, and the results show that significant effects could be found in the EEG even by employing relatively simple silhouette images. Patterns with a fractal dimension of 1.3 elicited the most interesting EEG, with the highest alpha in the frontal lobes but also the highest beta in the parietal area, pointing to a complicated interplay between different parts of the brain when experiencing this pattern.

No abstract

The strength of the Zeeman splitting induced by an applied magnetic field is an important factor for the realization of spin-resolved transport in mesoscopic devices. We measure the Zeeman splitting for a quantum point contact etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to the transport direction. We observe an enhancement of the Lande g-factor from |g*|=3.8 +/- 0.2 for the third subband to |g*|=5.8 +/- 0.6 for the first subband, six times larger than in GaAs. We report subband spacings in excess of 10 meV, which facilitates quantum transport at higher temperatures.Comment: [Version 2] Revtex4, 11 pages, 3 figures, accepted for publication in Applied Physics Letter

We study the Zeeman spin splitting in a quantum point contact ͑QPC͒ etched into an InGaAs/InP heterostructure for three orthogonal magnetic field orientations with respect to the QPC. For the two in-plane orientations we observe an isotropic Zeeman spin splitting, which becomes stronger as the system is made more one dimensional. The Lande g factor is enhanced by up to a factor of two compared to two-dimensional electron systems in InGaAs/InP. A much larger Zeeman splitting is observed when the field is oriented perpendicular to the heterostructure, resulting in a g factor of 15.7 in the one-dimensional limit.

Gradient index media, which are designed by varying local element properties in given geometry, have been utilized to manipulate acoustic waves for a variety of devices. This study presents a cylindrical, two-dimensional acoustic “black hole” design that functions as an omnidirectional absorber for underwater applications. The design features a metamaterial shell that focuses acoustic energy into the shell's core. Multiple scattering theory was used to design layers of rubber cylinders with varying filling fractions to produce a linearly graded sound speed profile through the structure. Measured pressure intensity agreed with predicted results over a range of frequencies within the homogenization limit.