Comment on "Perturbation theory of scattering for grazing-incidence fast-atom diffraction", by W. Allison, S. Miret-Artés and E. Pollak, Phys. Chem. Chem. Phys., 2022, 24, 15851.

In this comment we discuss some aspects of Phys. Chem. Chem. Phys., 2022, 24, 15851, by Allison et al., an article intensely motivated by our study of grazing incidence fast atom diffraction (GIFAD) for He-KCl(001) [G. A. Bocan, H. Breiss, S. Szilasi, A. Momeni, M. E. Staicu Casagrande, M. S. Gravielle, E. A. Sánchez and H. Khemliche, Phys. Rev. Lett., 2020, 125, 096101; G. A. Bocan, H. Breiss, S. Szilasi, A. Momeni, M. E. Staicu Casagrande, E. A. Sánchez, M. S. Gravielle and H. Khemliche, Phys. Rev. B, 2021, 104, 235401]. In particular, (a) we show that, contrary to first order perturbation prediction, the surface corrugation is not proportional to the tangent of the rainbow angle and, (b) we analyze whether a Morse-like formula, like the one Allison et al. use, is able to reproduce the atom-surface potential derived from density functional theory (DFT) calculations. In addition, we give some clarifications regarding specific remarks the authors made about our articles.

Long Carbon Fibers for Microwave Absorption: Effect of Fiber Length on Absorption Frequency Band.

This work presents lightweight epoxy foams loaded with very low weight percentages (≤0.5 wt.%) of carbon fibers (CFs) with different lengths (3 mm, 6 mm, and 12 mm) as broadband microwave absorbing materials for anechoic chamber application. The effect of CF length on microwave absorption, especially on the absorption frequency band, is investigated for frequencies between 1 and 15 GHz. For the elaboration of composites, three different methods-spatula, shear mixing, and ultrasounds-are used for the dispersion of CFs. The observation of these CFs, after the dispersion step, shows a high fiber breakage rate when shear mixing is used, unlike when spatula or ultrasounds methods are used. On the other hand, the characterization of the elaborated composites highlights a correlation between the mixing methods, hence the fiber brakeage, and the measured reflection coefficient (reflection loss) of the composites. As a result, the minimum value of the reflection coefficient is shifted toward the high frequencies when the fiber breakage is observed, suggesting that short CFs absorb at high frequencies while long CFs absorb at low frequencies. Dielectric properties, extracted from the measurement in free space, of composites elaborated with different fiber lengths (3 mm, 6 mm, and 12 mm) confirm that short CFs (3 mm) show maximum losses at high frequencies (around 15 GHz) while long CFs (12 mm) show maximum dielectric losses at low frequencies (below 4 GHz). However, no significant variation is observed on the real part of the relative permittivity, as a function of fiber length, for these porous composites loaded with very low CF rates. A hybrid composite, with a mix of different CF lengths, is prepared and characterized. The simulation of the absorption performance of a pyramidal absorber, based on this hybrid composite, is compared to the one of pyramidal absorber based on composites loaded with a single length of carbon fibers. The pyramidal absorber-based hybrid composite predicts the best absorption performance, especially at the low frequency band. The simulated reflection coefficient of this absorber is less than -12 dB in all the studied frequency range, and less than -40 dB for frequencies higher than 3 GHz. This result confirms the interest of using a mix of carbon fiber lengths to achieve a broadband microwave absorber.

Toward an Ultra-Wideband Hybrid Metamaterial Based Microwave Absorber.

In this paper, we propose a novel design of an ultra-wideband hybrid microwave absorber operating in the frequency range between 2 GHz and 18 GHz. This proposed hybrid absorber is composed of two different layers that integrate a multiband metamaterial absorber and a lossy dielectric layer. The metamaterial absorber consists of a periodic pattern that is composed of an arrangement of different scales of coupled resonators and a metallic ground plane, and the dielectric layer is made of epoxy foam composite loaded with low weight percentage (0.075 wt.%) of 12 mm length carbon fibers. The numerical results show a largely expanded absorption bandwidth that ranges from 2.6 GHz to 18 GHz with incident angles between 0° and 45° and for both transverse electric and transverse magnetic waves. The measurements confirm that absorption of this hybrid based metamaterial absorber exceeds 90% within the above-mentioned frequency range and it may reach an absorption rate of 99% for certain frequency ranges. The proposed idea offers a further step in developing new electromagnetic absorbers, which will impact a broad range of applications.