The understanding of the collective macroscopic behaviour of vast assemblies of interacting quantum objects has been of central concern for physicists for a long time. The new states of electronic matter, discovered in the last three decades, have continued to challenge our understanding of collective behaviour and have driven the development of new conceptional frameworks, the implications of which have spread far beyond the realm of condensed matter physics. With the discovery of phenomena such as heavy Fermions, superconductivity in transition-metal oxides and organic charge-transfer salts, and the colossal magnetoresistance, it has become clear that most spectacular forms of collective behaviour arise in electronic matter when the strength of the interactions becomes comparable to, or greater than the kinetic energy. Exploring the conditions giving rise to such exciting phenomena and understanding their microscopic origin is hampered, however, by the systems' high degree of complexity. The latter results not only from the large number of atomic species involved in the relevant materials, but also from the simultaneous action of several degrees of freedom such as the particles' charge, their spin, or orbital moments as well as the coupling to the lattice.
In the Transregional Collaborative Research Centre "Collective behaviour of condensed matter systems with variable many-body interaction", we propose to advance our understanding of collective behaviour of interacting bosonic and fermionic many-body systems by studying selected phenomena in a broader, less material-specific context. The issues to be addressed will include cooperative phenomena such as the Mott metal-insulator transition and superconductivity/superfluidity in the presence of strong interactions as well as Bose-Einstein condensation under diverse conditions. Beside the ground-state properties, excitations and interactions of the many-body system will be explored including the dynamical aspects of correlation and coherence.
The novelty of our approach is to carry out comparative investigations on various classes of materials, ranging from simple model systems to complex real substances. Using a wide basis of materials sharing a similar phenomenology, while showing quite different degrees of complexity, will help to separate materials-related issues from generic properties. Following the achievement of Bose-Einstein condensation, ultracold quantum gases have emerged as simple model systems for exploring and simulating fundamental aspects of interacting many-body systems. As an example, we mention the phase transition from a superfluid to a Mott insulator, recently observed in a gas of ultracold atoms by one of the proposers. Further model systems to be included in this initiative are magnons -- the quasiparticles of magnetic excitations -- in ferromagnetic thin films. The recently observed Bose-Einstein condensation of magnons at room temperature by members of this initiative underscores the model character of the magnon systems. The combination of these comparatively simple systems with real materials such as bulk quantum magnets or organic charge-transfer salts, forms the basis of the TR 49. Apart from a strong interaction of theory and experiment both in physics and chemistry, our initiative attempts to connect the hitherto distinct areas of quantum optics and solid state science and may thus be considered truly interdisciplinary.
In order to reach the scientific goals, the TR 49 -- besides the above new concept -- also excels in the areas of materials, theoretical approaches and experimental methods. We will confine ourselves to well-characterised materials, or material classes, notably those where a "building block scheme" applies, with particular emphasis placed on the tunability of the systems by physical or chemical means. We will take advantage of the combined efforts from synthetic and theoretical chemistry offering various possibilities of tuning the systems by deliberately introducing chemical modifications. Advanced theoretical methods will be employed ranging from ab initio density functional theory to many-body analytical (functional RG, RMFT) and numerical (DMRG, TM-DMRG, DMFT(NRG), DMFT(QMC)) techniques. Concerning experimental methods, we will combine forefront high-resolution thermodynamic measurements with advanced spectroscopic techniques. Among these are, for example, various photoemission-spectroscopy techniques and space-, time-, and phase-resolved Brillouin light-scattering spectroscopy. The former also includes the high-energy variant providing true bulk sensitivity. For ultracold quantum gases, state-of-the-art techniques for trapping, manipulating, controlling and detection will be employed.
The TR 49 also includes new and high-potential techniques which will set the stage for next generation experiments on interacting many-body systems. Using time- and angular-resolved two-photon photoemission spectroscopy, we will attempt to gain access to the ultrafast dynamics of correlated electronic matter. In the case of ultracold gases, we plan to realise quantum gases in an optical lattice with spatial addressability, opening the perspective for manipulating a many-body system on a microscopic length scale and for studying the ensuing dynamics.
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Druckversion: 14. Oktober 2008, 12:01