Outermost Hole Research Articles

Atom sieve for nanometer resolution neutral helium microscopy

Neutral helium microscopy is a new tool for imaging fragile and/or insulating structures as well as structures with large aspect ratios. In one configuration of the microscope, neutral helium atoms are focused as de Broglie matter waves using a Fresnel zone plate. The ultimate resolution is determined by the width of the outermost zone. Due to the low-energy beam (typically less than 0.1 eV), the neutral helium atoms do not penetrate solid materials and the Fresnel zone plate therefore has to be a free-standing structure. This creates particular fabrication challenges. The so-called Fresnel photon sieve structure is especially attractive in this context, as it consists merely of holes. Holes are easier to fabricate than the free-standing rings required in a standard Fresnel zone plate for helium microscopy, and the diameter of the outermost holes can be larger than the width of the zone that they cover. Recently, a photon sieve structure was used for the first time, as an atom sieve, to focus a beam of helium atoms down to a few micrometers. The holes were randomly distributed along the Fresnel zones to suppress higher order foci and side lobes. Here, the authors present a new atom sieve design with holes distributed along the Fresnel zones with a fixed gap. This design gives higher transmission and higher intensity in the first order focus. The authors present an alternative electron beam lithography fabrication procedure that can be used for making high transmission atom sieves with a very high resolution, potentially smaller than 10 nm. The atom sieves were patterned on a 35 nm or a 50 nm thick silicon nitride membrane. The smallest hole is 35 nm, and the largest hole is 376 nm. In a separate experiment, patterning micrometer-scale areas with hole sizes down to 15 nm is demonstrated. The smallest gap between neighboring holes in the atom sieves is 40 nm. They have 47011 holes each and are 23.58 μm in diameter. The opening ratio is 22.60%, and the Fresnel zone coverage of the innermost zones is as high as 0.68. This high-density pattern comes with certain fabrication challenges, which the authors discuss.

Strongly three-dimensional electronic structure and Fermi surfaces ofSrFe2(As0.65P0.35)2: Comparison withBaFe2(As1−xPx)2

The isovalent-substituted iron-pnictide superconductor ${\mathrm{SrFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$ ($x=0.35$) has a slightly higher optimum critical temperature than the similar system ${\mathrm{BaFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$, and its parent compound ${\mathrm{SrFe}}_{2}$${\mathrm{As}}_{2}$ has a much higher N\'eel temperature than ${\mathrm{BaFe}}_{2}$${\mathrm{As}}_{2}$. We have studied the band structure and the Fermi surfaces of optimally doped ${\mathrm{SrFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$ by angle-resolved photoemission spectroscopy (ARPES). Three holelike Fermi surfaces (FSs) around $(0,0)$ and two electronlike FSs around $(\ensuremath{\pi},\ensuremath{\pi})$ have been observed as in the case of ${\mathrm{BaFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$. Measurements with different photon energies have revealed that the outermost hole FS is more strongly warped along the ${k}_{z}$ direction than the corresponding one in BaFe(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$, and that the innermost one is an ellipsoidal pocket. The electron FSs are almost cylindrical, unlike the corrugated ones in BaFe(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$. A comparison of the ARPES data with a first-principles band-structure calculation revealed that the quasiparticle mass renormalization factors are different not only between bands of different orbital character, but also between the hole and electron FSs of the same orbital character. By examining the nesting conditions between the hole and electron FSs, we conclude that magnetic interactions between FeAs layers rather than FS nesting play an important role in stabilizing the antiferromagnetic order. The insensitivity of superconductivity to the FS nesting can be explained if only the ${d}_{xy}$ and/or ${d}_{xz/yz}$ orbitals are active in inducing superconductivity or if FS nesting is not important for superconductivity.

Stress Mitigation Design of a Tubesheet by Considering the Thermal Stress Inducement Mechanism

Adoption of double-wall straight-tube steam generators (SGs) made of Mod.9Cr-1Mo steel is planned for next-generation fast breeder reactors (FBRs) in Japan. One of the major concerns with the SG is the structural integrity of the tubesheet. During a transient event, a maximum thermal stress may be induced by the temperature distribution in the tubesheet, and the magnitude of the stress depends on the configuration of the tubesheet. Therefore, the stress generation mechanism of a tubesheet was studied through finite element (FE) analysis. Semispherical tubesheet models were investigated for the first survey of the thermal stress mechanism. The calculated results of the semispherical tubesheet model indicated an extensive peak stress around the outermost hole. The recognized thermal stress mechanism of a semispherical tubesheet is as follows: (1) The dominant thermal stress is hoop stress caused by the temperature difference between the perforated and surrounding regions. (2) The thermal stress is insensitive to the size of the specific portion, although it is dominated by an interaction mechanism between the perforated and surrounding regions. (3) The stress concentration around the edge of the holes generates a peak stress. (4) The amplitude of the peak stress depends on the tubesheet penetration angle, and the stress concentration becomes greatest near the outermost hole. Based on the above stress generation mechanism, we proposed a stress-mitigated tubesheet, a center-flattened spherical tubesheet (CFST), as an improved configuration. The calculated peak stress of the CFST was smaller than that of the semispherical tubesheet. Further investigation revealed the detailed stress generation mechanism of the CFST during a thermal transient. There were, in fact, two different comparable thermal peak stress mechanisms observed for the CFST. Both the location and magnitude of the maximum peak stress depended on the steam temperature histories during the thermal transient. The radial stress caused by structural discontinuity, which was located at the outermost hole, depended on the rate (dT/dt) of the steam temperature change. The hoop stress caused by the interaction between the perforated and surrounding regions, which occurred at the first inner layer hole (with respect to the outermost layer holes) depended on the range (ΔT) of the steam temperature change.

Design of an Oscillating Sprinkler

(1985). Design of an Oscillating Sprinkler. Mathematics Magazine: Vol. 58, No. 1, pp. 29-38.