Highly-correlated low-dimensional systems on metal surfaces

A fundamental issue in materials design is to understand and control the relationship between microscopic interactions, mesoscopic structure and macroscopic properties. Low-dimensional electronic systems are particularly promising in this respect, because their groundstate is determined by a delicate balance between different interactions: Electron-electron as well as electron-phonon interactions are strongly enhanced in low-dimensional systems. The former gives rise e.g. to anomalous transport properties (Mott transition, giant magneto resistance) and magnetically ordered states (antiferromagnetism, spin density waves), the latter to spontaneous symmetry breaking (Peierls transition, Charge Density Wave: CDW). In low-dimensional systems on surfaces the interactions can be easily controlled by small amounts of suitable adsorbates. Hence, the balance can be tilted in various directions to stabilise different groundstates. Thus, different structures and even macroscopic properties may be obtained. In the previous project (P13657-PHY) it was found that halogens on Pt(110) can form self-assembled quasi-1D structures with anomalous properties in photoemission (attributed to electron-electron interaction) and a surface phase transition induced by minute amounts of doping atoms (attributed to a surface-CDW). The aim of the present project is therefore the detailed characterisation of the various groundstates of the system. This comprises a determination of the geometric structure by STM, LEED I-V measurements and low-energy ion scattering for both, the "normal" groundstate (c(2x2)-Br/Pt(110) and (2x1)-Cl/Pt(110), respectively) and the (3x1) charge-ordered groundstate. Similarly, the electronic structure of both phases has to be determined. Major goals are the mapping of the Fermi surface and an analysis of the spectral function in order to quantify many-body effects and compare them with other highly correlated compounds such as the Mn-oxides and high-Tc superconductors. Furthermore, the possibility of manipulating the charge-ordered groundstate by adsorbates, especially via shifting the Fermi surface, will be explored. The issue of long-range adsorbate-adsorbate interactions will be examined, because anomalously long-range interactions are expected for quasi-1D materials and may give rise to unusual order- disorder phenomena in the adlayer. If it is possible to continuously tune through commensurate and incommensurate periodicities, the Frenkel-Kontorova model predicts the formation of regular and eventually chaotic soliton lattices. Direct observation of these processes by STM would provide exciting insights into the dynamics of atomic-scale systems with competing periodicities.

Recently, materials were discovered, which exhibit superconductivity at moderately low temperatures or a colossal dependence of electrical resistivity on magnetic fields. These interesting properties result from a non-metallic behaviour of electrons. Whereas in normal metals electrons can freely move quite independently from each other, this is not the case in the above mentioned so-called high-temperature superconductors or colossal-magneto- resistance materials. Here the electrons coordinate each other`s movements like the dancers in a classical ballet. As one electron makes its turn, all the others respond in a highly correlated fashion. For this reason, such materials are called correlated systems. Just how in detail the correlation is brought about is not yet understood and therefore high temperature superconductivity, for instance, is still an unsolved mystery. The present project looked for electronic correlation in a material, where, by definition, one would not expect to find it: in metals. Under certain circumstances some electrons are confined to the surface of metals. If, furthermore, the surface is structured in such a way, as to allow electronic movement only in one direction, then the electrons can`t help taking notice of each other and hence they correlate their movements. Imagine a young man and a pretty girl strolling around the crowded St. Markus` square, how easily they can get away without even noticing each other. However, if police would allow people to move only between barriers defining a narrow stripe across the square, they would sooner or later meet each other and - time and space being favourable - form a couple. Obviously from then on their movements would be correlated. This is a reasonably close analogy of the pairing process required for electrons of opposite spin to make superconductivity happen in the high-temperature superconductors. In the present project we made use of self-assembly to provide one-dimensional structures on metal surfaces for electron confinement. We succeeded in demonstrating correlation among the electrons on such surfaces and we were able to tune the strength of the electronic correlation by both, temperature and deposition of various atoms or molecules onto the surface. In particular, our results suggest the possibility of establishing a magnetic order on the surface of platinum, which previously was thought to be non-magnetic. These results will improve the understanding of correlation mechanisms and eventually help to design materials with even higher superconducting transition temperature or unprecedented sensitivity for detecting magnetic fields etc.