Education and Scientific Development

On this page you'll find a short review of my scientific development (last update: June 30^th. 2000).

Short link to public results:
• Diploma Thesis
• Doctoral Thesis (PhD)
• List of Publications

Scientific Development:

In September 1988 I began to study physics at the University of Rostock. I continued my studies at the University of Marburg ('89) and finished at the FU Berlin ('95).

My scientific career began with the Diploma thesis in the group of Prof. Kaindl at the Freie Universität Berlin (Germany). I was occupied with surface phenomena of lanthanide metals. On the one hand we considered the topics on the changed chemical environment of the surface atoms, i.e. the impact on the binding energies of 4f core electrons, additional surface states and a possible change of magnetic ordering temperatures. With the means of inverse photoemission (IPE) I mainly investigated the unoccupied electronic structure, but we also studied the occupied part of surface states with photoemission (PE). The prominent results of my diploma thesis are:

• We first analyzed using combined PE and IPE data the surface states on the ferromagnetic lanthanide metals Gadolinium and Terbium. With temperature-dependent measurements we could first show that the surface state on Gd(0001) exhibits mainly a Stoner-like exchange splitting (see ref. 2/3 of publication list). In the case of Tb the surface state rather performs a depolarization behavior (Hubbard-like, see ref. 1 of publication list). This is in particular supported by the IPE data, while the PE data on this surface state allows Hubbard-like as well as Stoner-like behavior. Here we have a still open question.
• We also accurately determined the surface core-level shift (SCS) of the 4f¹ state of single-crystalline lanthanum metal films (see diploma thesis). The value of the SCS strongly depends on the intensity of the surface state on La(0001). This could be related to so-called final-state effects, whereas an intense surface the additional 4f electron (of the IPE final state) better screens. Furthermore the analysis of 4f IPE spectra from different k-ranges (k wave vector of electron inside of the solid) yielded a dependence of the surface-to-bulk intensity ratio. This is most probably caused by electron diffraction. An further influence of an eventually k-dependent electron mean free path could not be excluded.

After passing the diploma exams I accepted the offer of an doctoral thesis in the group of Prof. Kaindl. Thereby we first intended to equip our IPE spectrometer with a spin-polarized electron gun. With that it would have been possible to clarify the (up to now) open questions regarding the magnetic behavior of the surface states on Gd and Tb. In the first year of my dissertation I (theoretically) constructed an transversal spin-polarized electron gun, having in the energy range from 10 to 150 eV a divergence of less than 1(!) degree. The drawing are finalized in my desk, but could not be realized because of financial reasons.
Thereupon we first realized surface-sensitive electron-excited x-ray emission measurements on elemental lanthanides in cooperation with Prof. A. S. Shulakov from St. Petersburg State University (Russia). With this usually bulk-sensitive method we could show that an experimental separation of the valence electronic structure of the surface atomic layer from the bulk one is possible.
The second main topic of my thesis arose due to a publication in Physical Review Letters in 1997. The authors claimed that the changed chemical environment of the La atoms in LaSe causes a binding energy shift of the unoccupied 4f¹-level by more than 2.5 eV (closer to E_F) with respect to pure La metal. In contrary with the help of IPE measurements we have now evidence that it is not the case in LaS, LaSe und LaTe. In fact we could qualitatively explain a step-like shift of the 4f-level each with about 0.15 eV from La to LaS over LaSe to LaTe.

The main results of my doctoral thesis are:

• We could for the first time realize a separate (experimental) determination of the surface and bulk s-d-like valence density of states of La, Sm and Lu. Therefore we had to perform additional PE measurements of the 5p-levels; moreover (analytical) calculations of the transition probabilities of the valence electrons into the once unoccupied 5p-levels were necessary. A quite well agreement of experimental partial DOS with theoretical (bulk) band structures was obtained (see chap. 4 doctoral thesis).
• We could first experimentally obtain the occupied and unoccupied electronic structure of the lanthanum monochalcogenides (LaX with X=S,Se,Te) with the means of PE and IPE. The chemical shift of the 4f and 5p core levels were experimentally determined. We could qualitatively explain the large differences between the 5p and 4f shifts within a simple (self-made) initial-final-state model. The measured 4f binding energies are in excellent agreement with a thermo chemical model from other authors. Our IPE result provoked the authors from the above mentioned PRL paper to repeat their MOKE measurements. More recent MOKE results are now in tendential agreement with our IPE data, in contrast to their own old results.

After defending my thesis I performed IPE on the Kondo peak in CeSi in collaboration with the group of Prof. Fink from Technical University of Dresden (Germany).

During my scientific work at the Freie Universität Berlin I also acquired the following knowledge’s:

• I got a lot of experience in attaining ultra-high vacuum, which is a basic necessity for surface physics and all kinds of spectroscopy. As well as the creation of single-crystalline metal films (from construction of evaporators until the generation of the appropriate evaporate-annealing-procedure) is a field of detailed knowledge.
• Even in the presentation of my results (in talks, pictures and papers) I’ve found a well-understandable style, as I already demonstrated on some conferences.
• Computer handling is essential for every physicist. I have extended knowledge in writing converting programs and measure as well as controlling programs with BASIC, FORTRAN, Turbo Pascal, Borland Delphi and C++. I also cared several time for our group intranet (LAN, Win9x / WinNT4.0).
• Already during my basic and advanced university lessons I was interested in computer science; I thus selected it as subsidiary subject.

My further scientific interests regarding the area of material science, as for instance thin magnetic layers, semiconductor metal contacts, .. . I think that the facilities of surface-sensitive x-ray emission and spin-polarized PE as well as scanning tunneling microscopy are still not exhausted. In these fields I am interested in both fundamental research and in applications into information techniques (MRAM, GMR-Heads, ..).

If you are interested in further information’s on me, please use the given contact possibilities on my homepage (http://www.huebinger.de/index_e.html).
 
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