Electronic Properties of Layered Systems

We are a BESSY internal research group performing spectroscopy experiments with synchrotron radiation and microscopy experiments with a scanning tunneling microscope. Our research is concerned with the electronic and magnetic properties of three-dimensional systems, surfaces and quantum films.
The experimental methods we use are scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, spin- and angle-resolved photoelectron spectroscopy, Fermiology including angle-resolved constant-initial-state spectroscopy, x-ray absorption and x-ray magnetic circular dichroism.

Current projects include:

Nanowires electronically decoupled using carbide, graphite, and graphene

The first isolation of graphene, i. e., monolayer graphite, in 2004 and the measurement of its unusual electrical transport properties has been producing tremendous scientific activity since. We have for a long time been preparing graphite on SiC and graphene on Ni, and we experimented with nanostructuring of graphene on regularly stepped SiC and Ni substrates.

On Ni, graphene can be produced catalytically. The resulting graphene layer interacts strongly with the Ni substrate, and the properties are very different from freestanding graphene. After insertion of a single atomic layer of Au, we obtain neutral undoped graphene with the expected characteristic linear E(k) band dispersion.

Figure 1 shows this property as constant-energy surfaces measured up to the Fermi energy (EF) by angle-resolved photoelectron spectroscopy.

Figure 2 shows an example for the self-organization capabilities of tungsten surface carbides. Two different surface carbides which are seen as a fine grid in the STM images, the 15x12 structure (top left) and the 15x3 structure (top right), are both able to order deposited Au atoms into nanoclusters. The nanoclusters, which are seen as large white areas, are exactly 1 atomic layer high on the 15x12 structure and 2 atomic layers high on the 15x3 structure.

Electronic structure at the border between ferro- and antiferromagnetism

In the periodic system, the border between ferro- and antiferromagnetic elements runs between Mn and Fe. Therefore, these two elements are particularly suited for studying the interrelation between electronic structure and magnetic order. There are important unresolved problems with the prototype ferromagnetic semiconductor Ga_1-xMn_xAs which contains the otherwise antiferromagnetic element Mn. Despite important contributions from electron spectroscopy the reason for the ferromagnetic interaction has not yet been established, and there are still contesting theories.
Even Fe can become antiferromagnetic. A thickness-driven phase transition occurs with Fe films grown on Cu(100). If the Fe is 1 to 4 atomic layers thick, it is ferromagnetic but for 5 to 11 atomic layers, it is antiferromagnetic. Some 200 studies on this system have been published but were not able to explain the reason for this dramatic change.

Figure 3 shows the Fermi surface of 8 atomic layers of Fe on Cu(100) measured by photoelectron spectroscopy at 138 eV.

Spin-orbit effects at interfaces and in nanostructures

The so-called "spintronics" attempts to employ the electron spin for electronic devices The spin originates usually from ferromagnetic components. We want to establish instead the use of the spin-orbit interaction to create spin-polarized electronic states. Heavy, nonmagnetic elements can lead to spin-polarized electronic states when the inversion symmetry is broken. The occurs, e. g., at solid surfaces and leads to spin splittings. About a dozen papers has been published in this field so far but all of these rely on a solid-vacuum interface where the spin-polarized spin-orbit split electronic states reside. This is incompatible with devices which must be based on volume properties. We have for the first time found that metal quantum films lead to spin-orbit split electronic states. These films consist of several atomic layers of gold which are grown on a tungsten substrate.

Figure 4 shows the discrete energy levels of electronic states in quantum films (left: gold, right: silver). The stacked spectra correspond to film thickness increasing from bottom to top. The maxima in each spectrum correspond to discrete energy levels.

Figure 5 shows spin-resolved photoelectron spectra. Energy levels for spin up (blue) and spin down (red) are split by the spin-orbit interaction.