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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).

Supersonic helium beams are used in a wide range of applications, for example surface scattering experiments and, most recently, microscopy. The high ionization potential of neutral helium atoms makes it difficult to build efficient detectors. Therefore, it is important to develop beam sources with a high centre line intensity. Several approaches for predicting the centre line intensity exist, with the quitting surface model incorporating the largest amount of physical dependencies. However, until now only a limited amount of experimental data has been available. Here we present a study where we compare the quitting surface model with an extensive set of experimental data. In the quitting surface model the source is described as a sphere from where the particles leave in a molecular flow determined by Maxwell-Boltzmann statistics. We use numerical solutions of the Boltzmann equation to determine its properties. The centre-line intensity is then calculated using an analytical integral. This integral can be reduced to two cases, one which assumes a continuously expanding beam until the skimmer aperture, and another which assumes a quitting surface placed before the aperture. We compare the two cases to experimental data with a nozzle diameter of 10 micron, skimmer diameters ranging from 4 micron to 390 micron, a source pressure range from 2 to 190 bar, and nozzle-skimmer distances between 17.3 mm and 5.3 mm. To support the two analytical approaches, we have also performed equivalent ray tracing simulations. We conclude that the quitting surface model predicts the centre line intensity well for skimmers with a diameter larger than 120 micron when using a beam expanding until the skimmer aperture. For the case of smaller skimmers the trend is correct, but the absolute agreement not as good. We propose several explanations for this, and test the ones that can be implemented analytically.

Surface scattering of neutral helium beams created by supersonic expansion is an established technique for measuring structural and dynamical properties of surfaces on the atomic scale. Helium beams have also been used in Fraunhofer and Fresnel diffraction experiments. Due to the short wavelength of the atom beams of typically 0.1nm or less, Fraunhofer diffraction experiments in transmission have so far been limited to grating structures with a period (pitch) of up to 200nm. However, larger periods are of interest for several applications, for example for the characterization of photonic crystal membrane structures, where the period is typically in the micron/high sub-micron range. Here we present helium atom diffraction measurements of a photonic crystal membrane structure with a two dimensional square lattice of 100x100 circular holes. The nominal period and hole radius were 490nm and 100nm respectively. To our knowledge this is the largest period that has ever been measured with helium diffraction. The helium diffraction measurements are interpreted using a model based on the helium beam characteristics. It is demonstrated how to successfully extract values from the experimental data for the average period of the grating, the hole diameter and the width of the virtual source used to model the helium beam.