Direct comparison between potential landscape and local density of states in a disordered two-dimensional electron system.

The local density of states (LDOS) of the adsorbate-induced two-dimensional electron system (2DES) on n-InAs(110) is studied by scanning tunneling spectroscopy. In contrast to a similar 3DES, the 2DES LDOS exhibits 20 times stronger corrugations and rather irregular structures. Both results are interpreted as consequences of weak localization. Fourier transforms of the LDOS reveal that the k values of the unperturbed 2DES still dominate the 2DES, but additional lower k values contribute. To clarify the origin of the LDOS patterns, we measure the potential landscape of the 2DES area. We use it to calculate the expected LDOS and find reasonable agreement between calculation and experiment.

Sharper images by focusing soft X-rays with photon sieves.

Fresnel zone plates consisting of alternating transmissive and opaque circular rings can be used to focus X-rays. The spatial resolution that can be achieved with these devices is of the order of the width of the outermost zone and is therefore limited by the smallest structure (20-40 nm) that can be fabricated by lithography today. Here we show that a large number of pinholes distributed appropriately over the Fresnel zones make it possible to focus soft X-rays to spot sizes smaller than the diameter of the smallest pinhole. In addition, higher orders of diffraction and secondary maxima can be suppressed by several orders of magnitude. In combination with the next generation of synchrotron light sources (free-electron lasers) these 'photon sieves' offer new opportunities for high-resolution X-ray microscopy and spectroscopy in physical and life sciences.

Tuning dimensionality by nanowire adsorption on layered materials.

The dimensionality of electronic states determines a number of physical phenomena such as phase transitions, transport, or superconductivity. Employing scanning tunneling microscopy combined with angle-resolved photoemission spectroscopy we demonstrate how the dimensionality of electronic states can be continuously tuned from three to two dimensions. This is achieved by adsorption of nanowires on surfaces of layered crystals without changing the chemical composition of the material. Exemplary results for Rb nanowires on TiTe2 are discussed with the help of electronic structure calculations.