Rectangular Dots Research Articles

Binary square spiral zone plates trimmed with quasi-random dots for single-order focusing

How to realize a square spiral zone plates with a single focus is still plagued by the trade-off between structural complexity and fabrication difficulty. Here, we propose a new single optical element, named Quasi-random dots Square Spiral Zone Plates (QDSSZPs), to suppress high-order foci. The key idea of this design is to reshape the structure of a square spiral zone plates using a quasi-random distribution of rectangular dots to make it sinusoidal in transmittance. Numerical simulations and experimental results show that the QDSSZPs has a good single-order focus characteristic and can effectively suppress undesired high-order foci, which is almost consistent with the theoretical prediction. The characteristic of suppressing high-order foci in the QDSSZPs has broad applications in imaging and microscopy.

A Streamline Dot Flow Field Design for Proton Exchange Membrane Fuel Cell

Flow field optimization plays a vital role in improving the output performance of proton exchange membrane fuel cells (PEMFCs), especially at high current density. A streamline dot flow field is proposed in this study to enhance the mass transfer of reactant gas and compared it with parallel, rectangular dot, and rhombic dot flow fields. The influence of the tail length of streamline dot block on the performance of fuel cells is further analyzed. The results show that the uniform distributions of pressure and oxygen molar fraction are significantly increased in dot flow fields compared with parallel flow field. In dot flow fields, vortices produced behind the dot blocks could increase the flow resistance and pressure drop. Among six models, the dot flow field with 2 mm tail has the most uniform current density distribution at PEM-CL cross section, and its index of uniformity current density (IUCD) is the smallest with the value of 0.012991. And the parallel flow field model has the largest value of IUCD. Compared with parallel flow field, the current density of streamline dot flow field model with 2 mm tail increased by 4.9%.

A vision system for pose estimation of an underwater robot

This paper presents a vision system designed for an underwater robot to facilitate measuring the distance to an inspected surface and its orientation with respect to the robot. The system uses a set of laser modules to project a rectangular dot pattern on the inspected surface. The vision system is used to measure the dots’ locations in the 3D space and calculate the distance to the surface, together with the pitch angle and the yaw angle of the camera. A novel calibration procedure is presented, which allows for constructing high resolution camera-laser systems, without requiring a perfect alignment of laser beams and camera optical axis. An underwater vision system simulation environment, based on open-source software, is also introduced and extensively used to test the developed algorithms. The experimental results, obtained in a laboratory tank and a pool, confirm high performance of the system.

Heat-assisted magnetic recording media materials

Abstract

Resonant Scattering and Energy Relaxation in Quantum Dot Systems

We propose a new Auger-like mechanism for energy relaxation in quantum dots (QD) driven by resonant scattering of delocalized wetting layer (WL) carriers. It is demonstrated that resonant scattering leads to a considerable increase in the relaxation rate that can explain experimentally obtained relaxation rates. Analytical results for the relaxation rate are obtained for rectangular dots revealing a weak logarithmic dependence on the dot depth and level density. Comparing results for a rectangular and a parabolic QD model we conclude that the relaxation rate is not very sensitive to a chosen model.

Interrelation of Aromaticity and Conductivity of Graphene Dots/Antidots and Related Nanostructures.

It is illustrated and computationally verified by ab initio density functional theory and simple but powerful order-of-magnitude arguments, based on deformation energy ΔEdef in relation to the uncertainty principle, that the conductivity and aromaticity of graphene and graphene-based structures, such as graphene dots, antidots, and nanoribbons, are negatively interrelated for π aromatic structures, in agreement with recent experimental data. However, for σ aromaticity, the interrelation could be positive, especially for extended periodic structures. We predict that the conductivity of rectangular graphene dots and antidots, is anisotropic with much larger magnitude along the direction perpendicular to the zigzag edges, compared to the conductivity in direction parallel to them. The same is true for the polarizability and electron mobility. This is directly connected with the much higher aromaticity around the armchair edges compared to the aromaticity near the zigzag edges. Furthermore, contrary to what would be expected on the basis of simple arguments for defect states, we predict that antidot patterning could significantly improve the conductivity (sometimes by 1 order of magnitude) in one or both directions, depending on their number, arrangement, and passivation. For narrow atomically precise armchair nanoribbons (AGNRs) of finite length, both conductivity and energy gaps are dominated by lateral and longitudinal quantum confinement, which decrease with increasing length (for a given width), leading to a peculiar behavior of monotonically increasing “maximum conductivity” as the band gaps monotonically decrease. The electron distribution at the band edges of the AGNRs, in agreement with recent experimental data are well-localized at the zigzag edges. Using the concept of gap-determining LUMO–HOMO frontier states to avoid HOMOs and LUMOs localized at the zigzag edges, we can predict with very high accuracy the recently measured band gaps of AGNRs of widths N = 7 and N = 13. Both the smallest (10–3–10–4) and the largest (a few 2) calculated values of conductance and conductivity for the smaller structures and the larger nanographenes, respectively, are in full accord with the corresponding experimental values of single-molecule junction conductance and the measured minimum conductivity of graphene at 1.6 K.

Achromatic Talbot lithography with partially coherent extreme ultraviolet radiation: process window analysis

The main purpose of this work is the experimental determination of the process window for achromatic Talbot lithography with partially coherent extreme ultraviolet (EUV) radiation. This work has been performed using the EUV laboratory exposure tool. It consists of a discharge produced plasma source with a direct beam path to a phase-shifting transmission mask, avoiding losses due to additional optical components, the photoresist-coated wafer, and a positioning system for each component. Both the source and the mask are optimized for 11-nm wavelength. The process window has been identified by a systematic analysis of several exposure series. The optimization of exposure parameters resulted in 50-nm half-pitch of the wafer features using a transmission mask with a rectangular dot array of 70-nm half-pitch. The depth of field is found to be 20 μm, and it can be extended by spatial filtering. The exposure dose and mask–wafer distance are varied around their optimal values to estimate the process window, using defectivity of the pattern as a control parameter.

Actomyosin contractility and RhoGTPases affect cell-polarity and directional migration during haptotaxis.

Although much is known about chemotaxis- induced by gradients of soluble chemical cues - the molecular mechanisms involved in haptotaxis (migration induced by substrate-bound protein gradients) are largely unknown. We used micropatterning to produce discontinuous gradients consisting of μm-sized fibronectin-dots arranged at constant lateral but continuously decreasing axial spacing. Parameters like gradient slope, protein concentration and size or shape of the fibronectin dots were modified to determine optimal conditions for directional cell migration in gradient patterns. We demonstrate that fibroblasts predominantly migrate uphill towards a higher fibronectin density in gradients with a dot size of 2 × 2 μm, a 2% and 6% slope, and a low fibronectin concentration of 1 μg ml-1. Increasing dot size to 3.5 × 3.5 μm resulted in stationary cells, whereas rectangular dots (2 × 3 μm) orientated perpendicular to the gradient axis preferentially induce lateral migration. During haptotaxis, the Golgi apparatus reorients to a posterior position between the nucleus and the trailing edge. Using pharmacological inhibitors, we demonstrate that actomyosin contractility and microtubule dynamics are a prerequisite for gradient recognition indicating that asymmetric intracellular forces are necessary to read the axis of adhesive gradients. In the haptotaxis signalling cascade, RhoA and Cdc42, and the atypical protein kinase C zeta (aPKCζ), but not Rac, are located upstream of actomyosin contractility.

Experimental and theoretical analysis of Landauer erasure in nano-magnetic switches of different sizes

Bistable nano-magnetic switches are extensively used in storage media and magnetic memories, associating each logic state to a different equilibrium orientation of the magnetization. Here we consider the issue of the minimum energy required to change the information content of nano-magnetic switches, a crucial topic to face fundamental challenges of current technology, such as power dissipation and limits of scaling. The energy dissipated during a reset operation, also known as “Landauer erasure”, has been accurately measured at room temperature by vectorial magneto-optical measurements in arrays of elongated Permalloy nanodots. Both elliptical and rectangular dots were analysed, with lateral sizes ranging from several hundreds to a few tens of nanometers and thickness of either 10nm or 5nm. The experimental results show a nearly linear decrease of the dissipated energy with the dot volume, ranging from three to one orders of magnitude above the theoretical Landauer limit of kBT×ln(2). These experimental findings are corroborated by micromagnetic simulations showing that the significant deviations from the ideal macrospin behavior are caused by both inhomogeneous magnetization distribution and edge effects, leading to an average produced heat which is appreciably larger than that expected for ideal nanoswitches.

Spin wave eigenmodes in single and coupled sub-150 nm rectangular permalloy dots

We present the results of a Brillouin light scattering investigation of thermally excited spin wave eigenmodes in square arrays of either isolated rectangular dots of permalloy or twins of dipolarly coupled elements, placed side-by-side or head-to-tail. The nanodots, fabricated by e-beam lithography and lift-off, are 20 nm thick and have the major size D in the range between 90 nm and 150 nm. The experimental spectra show the presence of two main peaks, corresponding to modes localized either at the edges or in the center of the dots. Their frequency dependence on the dot size and on the interaction with adjacent elements has been measured and successfully interpreted on the basis of dynamical micromagnetic simulations. The latter enabled us also to describe the spatial profile of the eigenmodes, putting in evidence the effects induced by the dipolar interaction between coupled dots. In particular, in twinned dots the demagnetizing field is appreciably modified in proximity of the “internal edges” if compared to the “external” ones, leading to a splitting of the edge mode. These results can be relevant for the exploitation of sub-150 nm magnetic dots in new applications, such as magnonic metamaterials, bit-patterned storage media, and nano-magnetic logic devices.

Micromagnetic study of minimum-energy dissipation during Landauer erasure of either isolated or coupled nanomagnetic switches

The question of the minimum energy required to change the information content of physical switches is attracting a growing interest because current computer technology is facing fundamental challenges, such as increased power dissipation, and rapidly approaching the limits of scaling. In this context, bistable nanomagnetic switches can be used to store bits of information, associating each logic state to a different equilibrium orientation of the magnetization. Here we present the results of virtual experiments implemented by micromagnetic simulations on elongated dots of Permalloy, performing a reset to 1 operation, also known as Landauer erasure. We show that for isolated, realistic, elliptical dots with sub-100 nm lateral dimension, the minimum-energy consumption turns out to be consistent with the value expected for an ideal bistable switch, i.e., $E={k}_{B}T\ifmmode\times\else\texttimes\fi{}ln(2)$, known as the Landauer limit. The only condition to achieve the correct limit is that the adopted erasure protocol includes a proper time interval where the probability distribution of the magnetization has the same amplitude in the two possible states. However, it is shown that significant deviations from the theoretical limit can be found either increasing the lateral dot dimension above about 100 nm or considering a rectangular (rather than elliptical) dot shape, where the presence of straight edges promotes a nonuniform behavior of the dot magnetization. Finally, we show that if the magnetic switch is not isolated, as it happens in potential devices where a regular array of dots is present, the dipolar interaction among adjacent switches can lead to either a reduction or an increase of the minimum dissipated energy, depending on the specific geometric arrangement of the magnetic dots.

Tunable spin wave dynamics in two-dimensional Ni80Fe20 nanodot lattices by varying dot shape

We demonstrate tunable spin wave spectrum in two-dimensional Ni80Fe20 nanodot lattices by varying dot shape. A single collective mode in elliptical dot lattices transforms into three distinct modes for the half-elliptical, rectangular, and diamond dot lattices, albeit with different peak frequencies and intensities. A drastic change is observed for the triangular dots, where eight modes covering a broad band are observed. Using micromagnetic simulations, we characterized the modes as different localized, extended, and quantized modes, whose frequencies and spatial profiles are determined by a combination of internal field profiles within the nanodots and the stray magnetic field within the lattice.

P 64. Cartography of causal contributions of human frontal cortex to visual attention

It is widely known that the human frontal cortex is involved in the allocation of visual attention, however, the exact causal functional contributions of individual subregions are not well understood. In the present study, we used a simple visual localization task and applied rTMS pulses to map frontal cortical subregions likely to generate significant visuo-spatial biases during the spatial deployment of attention prior to perception. A group of nine adult healthy subjects executed a task based on the localization of small rectangular black dots. Targets were displayed unilaterally (left or right) or bilaterally (left and right) for a brief period (40 ms). In a systematic mapping approach, a stimulation grid of 9 (3 × 3) sites was anchored 2 cm rostral to the motor hand area. Three pulses of real or sham 10 Hz rTMS were delivered in a counterbalanced manner, at each of the grid locations 50 ms post target onset to interfere with the ongoing neural processing. As a main finding, significant deterioration of detection performance, as measured by effective reaction time, for stimuli in the contralateral hemifield and increased performance for ipsilateral targets were observed for two grid regions anatomically associated with the right FEF and right middle frontal gyrus. We conclude that the disruptive effects of TMS on a simple spatial localization task for which a well-balanced deployment of attention is required are exquisitely spatially selective, and are found in specific frontal cortical subregions.

Micromagnetic Exchange Interaction Tensor and Magnetization Reversal of hcp Co-Based Alloy Thin Film Nano-Structures

We investigate the effect of the micromagnetic exchange tensor on the magnetization switching in bi-layer rectangular dots. The micromagnetic model adopted in this work is used to calculate the in-plane and out-of-plane hysteresis loops based on the fundamental magnetic parameters of hcp CoCrPt for a wide range of elemental compositions corresponding to magnetically hard and semi-hard cases. The exchange interaction term in micromagnetic energy functional is constructed to capture the results of the first principles calculations that show large difference between the in-plane and out-of-plane constants of the micromagnetic exchange tensor for hcp Co. In our calculations we compare two cases for in-plane and perpendicular exchange constants: (a) A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">||</sub> = A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> and (b) A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">||</sub> = 0.5 A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> , and find that the athermal coercive force for 30×30 nm bi-layer rectangular dots differs for these two cases. Further, we find that the magnetization switching changes from curling mode in the case (a) to the vortex formation in the case (b). We also find that these differences in magnetization switching become more pronounced if interface exchange between hard and soft layers is increased to the bulk value.

Effect of Interdot Separation on Collective Magnonic Modes in Chains of Rectangular Dots

The behavior of collective spin excitations in chains of rectangular NiFe dots is studied as a function of interdot separation. Dots have thickness of 40 nm and lateral dimensions of 715 × 450 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . They are put side by side along the major axis and the interdot separation is varied in the range 55-625 nm. Brillouin light scattering experiments have been performed at normal incidence (exchanged wave vector q = 0) and with the external magnetic field applied along the chain length. A satisfactory interpretation of the experimental data is achieved by magnonic bands calculations based on the dynamical matrix method. Such calculations have been performed at both the center and the border of the first Brillouin zone, in the case of Bloch wave vector q parallel to the applied field. In this way we can predict the amplitude of modes frequency oscillation (magnonic band), which is an important property to identify the behavior of a one-dimensional magnonic meta-material.

Magnetostatic Green's functions for the description of spin waves in finite rectangular magnetic dots and stripes

We present derivation of the magnetostatic Green's functions used in calculations of spin-wave spectra of finite-size non-ellipsoidal (rectangular) magnetic elements. The elements (dots) are assumed to be single domain particles having uniform static magnetization. We consider the case of flat dots, when the in-plane dot size is much larger than the dot height (film thickness), and assume the uniform distribution of the variable magnetization along the dot height. The limiting cases of magnetic waveguides with rectangular cross-section and thin magnetic stripes are also considered. The developed method of tensorial Green's functions is used to solve the Maxwell equations in the magnetostatic limit, and to represent the Landau–Lifshitz equation of motion for the magnetization of a magnetic element in a closed integro-differential form.

Collective spin modes in chains of dipolarly interacting rectangular magnetic dots

We present a combined experimental and micromagnetic study of spin excitations in chains of dense magnetic dots. The samples consist of long chains of rectangular dots with rounded corners having lateral dimensions of 715 \ifmmode\times\else\texttimes\fi{} 450 nm${}^{2}$ and 1025 \ifmmode\times\else\texttimes\fi{} 450 nm${}^{2}$, respectively. Chains are composed of magnetic elements put side by side along either their major or minor axis with edge-to-edge separation below 100 nm. The frequency dispersion of the spin-wave excitations was measured by Brillouin light-scattering technique as a function of the transferred wave vector directed along the chains of dots and for an external magnetic field applied perpendicularly to the transferred wave vector in the dots plane. Evidence is given of collective excitations in the form of Bloch waves propagating through the chains characterized by magnonic energy bands. Micromagnetic calculations, performed by using the dynamical matrix method, enable us to satisfactorily reproduce the frequency dispersion of collective spin modes as well as to visualize the spatial profile of the dynamic magnetization inside the dots. We also propose a general rule to understand the frequency dispersion of collective modes starting from the relative phase of dynamic magnetization in the facing sides of adjacent dots.

Coupling of Surface and Lowest Landau Level States in a Rectangular Graphene Dot

A rectangular graphene dot with two zigzag edges and two armchair edges have electronic states in the presence of a magnetic field that are localized on the zigzag edges with non zero values of the wavefunction inside the dot. We have investigated the dependence of these wavefunctions on the size of the dot, and explain the physical origin of them in terms of surface and the lowest Landau level (LLL) states of infinitely long nanoribbons. We find that the armchair edges play a crucial role by coupling the surface and LLL states.

States near Dirac points of a rectangular graphene dot in a magnetic field

In neutral graphene dots, the Fermi level coincides with the Dirac points. We have investigated in the presence of a magnetic field several unusual properties of single electron states near the Fermi level of such a rectangular-shaped graphene dot with two zigzag and two armchair edges. We find that a quasidegenerate level forms near zero energy and the number of states in this level can be tuned by the magnetic field. The wave functions of states in this level are all peaked on the zigzag edges with or without some weight inside the dot. Some of these states are magnetic field-independent surface states while the others are field-dependent. We have found a scaling result from which the number of magnetic field-dependent states of large dots can be inferred from those of smaller dots.

Dipolar interaction in dense chains of submicrometric rectangular dots

Dense chains of rectangular dots (715×450 nm) put side by side, with interdot spacing variable in the range between about 55 and 625 nm, have been patterned by deep UV lithography, starting from a 40 nm thick permalloy film. A magneto-optical Kerr effect study, assisted by magnetic force microscopy, revealed that the magnetization inversion proceeds through the nucleation and the annihilation of two vortices, with the nucleation and annihilation fields appreciably affected by the coupling. The latter also modifies the frequency of the fundamental resonant mode of precession of the magnetization, measured by Brillouin light scattering from spin-wave. Micromagnetic simulations enabled us to account for the main features observed in the experiment.