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Neutral helium atom microscopy is a novel microscopy technique that offers strictly surface-sensitive, non-destructive imaging. Several experiments have been published in recent years where images are obtained by scanning a helium beam spot across a surface and recording the variation in scattered intensity at a fixed total scattering angle $\theta_{sd}$ and fixed incident angle $\theta_{i}$ relative to the overall surface normal. These experiments used a spot obtained by collimating the beam (referred to as helium pinhole microscopy). Alternatively, a beam spot can be created by focusing the beam with an atom optical element. However up till now imaging with a focused helium beam (referred to as helium zone plate microscopy) has only been demonstrated in transmission. Here we present the first reflection images obtained with a focused helium beam. Images are obtained with a spot size (FWHM) down to 4.7 $\mu$m $\pm$ 0.5 $\mu$m, and we demonstrate focusing down to a spot size of about 1 $\mu$m. Furthermore, we present the first experiments measuring the scattering distribution from a focused helium beam spot. The experiments are done by varying the incoming beam angle $\theta_{i}$ while keeping the beam-detector angle $\theta_{sd}$ and the point where the beam spot hits the surface fixed - in essence, a microscopy scale realization of a standard helium atom scattering experiment. Our experiments are done using an electron bombardment detector with adjustable signal accumulation, developed particularly for helium microscopy., Comment: 20 pages, 7 figures

The mechanical properties of two-dimensional (2D) materials are important for a wide range of applications including composite materials, van der Waals materials and flexible electronics. Several aspects are highly debated in the literature: For example, the theoretically predicted bending rigidity, $\kappa (T)$, of the most important 2D material, graphene, varies from 0.8~eV to 10~eV for predictions at 0 K and there are predictions that it could increase or decrease with temperature depending on the applied theory. Here we present the first systematic experimental study of the temperature dependence $\kappa(T)$ of graphene. Extracting values of $\kappa$ from the ZA-phonon dispersion relation measured with helium atom scattering, we find the value of $\kappa(T)$ to increase with sample temperature. A linear fit in the temperature range (120 K-480 K) gives a bending rigidity of $\kappa (T) = (1.1 \pm 0.1)[\textrm{eV}] + (0.0013 \pm 0.0001)[\textrm{eV/K}] \times T$.

The boson peak is an excess in the phonon density of states relative to the Debye Model, which occurs at frequencies below the Debye limit. It is present in most amorphous materials and, as was recently shown, can sometimes be found also in crystals. Here we present first experimental evidence of the boson peak in a 2D material, namely 2D silica (SiO$_2$). The measurements were obtained by helium atom scattering. A dispersionless boson peak is seen at $6 \pm 0.5$ meV (1.5$ \pm$ 0.15 THz) and $-6 \pm 1.5$ meV ($-1.5 \pm 0.4$ THz), with reasonable evidence for a double excitation at $\pm 12 \pm 2.5$ meV ($2.9 \pm 0.6$ THz).