IFF
Scientific Report 2001/2002

In any condensed matter system, electrons are the "glue" that hold the atoms together. Therefore the electronic structure constitutes a microscopic base for all material properties. The electronic interactions determine whether a solid is metallic, insulating or semiconducting, whether it is transparent or exhibits a distinctive color, whether it is a magnet or a superconductor.

The research program of the Institute for Electronic Properties is devoted towards the investigation of the electronic structure of atoms, clusters, nonostructures, and solids. The ultimate goal is the development of an understanding and thus a base for the control of the properties of new materials. The research efforts of the institute are concentrated in the areas: Magnetism of thin films and nanostructures, clusters as new materials, and methods and instrumentation.

The major event in the year 2001 was the departure of Prof. Wolfgang Eberhardt from his position as an institute director. He left to take up a position as a scientific director of BESSY in conjunction with a professorship at the TU Berlin. Dr. H. Durr followed him to BESSY in the late summer. Since Dr. C. Carbone and Dr. S. Blugel had left the institute already in the year 2000 to take up positions as professors in Trieste and Osnabriick, respectively, many of the group leaders have left the institute. Several of the PhD students have followed or are following Prof. Eberhardt to BESSY. Major equipment, which has been bought by the BESSY GmbH, will be transferred to Berlin.

The investigation of the electronic properties is fundamental for any condensed matter research. Therefore, at the Forschungszentrum, it has been decided to keep an Institute for Electronic Properties within the condensed matter department IFF. As a consequence of the successful HGF magneto-electronic project within the research center, the research on magnetic nanostructures will be strengthened in the future.

Despite the fact that the institute is in a transition period, a lot of research activities were going on in 2001, leading to many exciting results. Some examples are given below and others in the attached detailed reports.

In the group magneto-electronics, the research efforts have concentrated on Fe/Al/Fe trilayers (see attached report), Fe/Al0/Fe TMR (tunnel magneto resistance) devices, and Fe/Si/Fe trilayers. The latter system, where a semiconductor is sandwiched between two metallic layers, shows a surprisingly high interlayer coupling of up to 8 mJ/m², which cannot be understood within the conventional models. Nanocontacts in Fe/Cr/Fe trilayers structures have been prepared with electron beam lithography to study a new effect, which can be described as the reverse of the GMR effect: the current-induced magnetic coupling. A switching between ferromagnetic and antiferromagnetic coupling can be achieved just by reversing the electron current. Finally, in the small theory group working in very close contact with the experimental magneto-electronic group, a novel ab-initio Green function formulation of the transfer matrix has been developed and applied to complex band-structures.

Within the cluster group, a new magnetron sputter source for the deposition of clusters has been constructed (see attached report). This continuous source is superior to pulsed cluster sources due to the more stable operation conditions and higher cluster intensities. The research efforts in the cluster group were concentrated on scanning tunneling spectroscopy of endohedral fullerenes and femtosecond time resolved studies. Small endohedral fullerenes Ce@C_n (n = 36,44,50,60) have been produced in a laser vaporization cluster source, mass selected and non-destructively deposited on a highly oriented pyrolytic graphite substrate. The density of states has been determined by scanning tunneling spectroscopy. A semiconductor-like density of states at the Fermi level is observed with energy gaps between 0.55 and 1.1 eV. Metal-adsorbate-clusters have been studied using femtosecond time resolved photoelectron spectroscopy. Information concerning the dissociation dynamics of these systems could be deduced. The photodisscociation process of Pt₂N₂ occurs on a time scale of 100(25) ps and involves different reaction pathways. Another example of the ongoing research involves the relaxation dynamics of opticly excited electrons in free Ni₃^- cluster. By means of femtosecond two-color pump-probe photoelectron spectroscopy, the electronic relaxation of excited electrons via inelastic electron-electronscattering could be observed in real-time. The inelastic electron-electron relaxation time was determined to be 190 fs.

One highlight in the development of methods and instrumentation was the successful commissioning of the spin-polarized photoelectron emission microscope (PEEM) with the application to magnetic nanostructures. Combining a femtosecond pump-probe laser with a photoelectron emission microscope, the spin dynamics in nanostructures can be studied on a femtosecond time scale. Magnetic sensitivity is obtained by analyzing the spin-polarization of the emitted photoelectrons. The technique has been successfully applied to Co/Pt nanoscale magnetic dots. The institute also operates two synchrotron beamlines. The beamline for soft x-rays at BESSY II has been commissioned. Within the entire energy range the experimental resolution exceeded the designresolution of E/DE = 10.000. With the linear or circular polarized radiation available, the electronic properties of novel materials are now being examined. The systems include nanotubes, magnetic multilayers or organic semiconductors. The VUV-beamline at the DELTA storage ring in Dortmund suffered in the past due to storage ring problems. Many of these problems, such as beam position fluctuations and beam losses, have now been solved and thus in 2001 a first stable operation of the beamline became possible. As a consequence, a first scientific result has been obtained: the electronic structure of Co-quantum-wires has been determined.

The successful scientific work of the institute is documented by the following personal awards and achievements:

Dr. S. Blügel has received an offer for a C4 professorship at the University of Kaiserslautern in the field of Theoretical Solid State Physics. Dr. D.E. Bdrgler has received an offer for a C4 professorship at the MartinLuther-University, Halle/Saale, in the field of Experimental Physics. Prof. Dr. W. Eberhardt and Dr. J. Morenzin have been awarded the first prize of the "Mitteldeutscher Rundfunk", MDR, for the invention of counterfeitprove magnetic stripes. The following students have been awarded their diploma or PhD degrees in 2001:

Diploma:

Doctorate:

I would like to close this report by thanking all members of the institute for their dedicated work during the past year.

Thomas Brückel

Publications in refereed journals