General Information

The doping dependence of phase transitions and ordering phenomena in copper-oxygen superconductors

is the key towards the fascinating world of a material class which revolutionized our understanding of macroscopic quantum phenomena. In spite of enormous progress, however, we are still far from a microscopic description of superconductivity in the range above 100 K. Since a concerted effort became increasingly desirable, the DFG (German Research Foundation) estabished the Research Unit FOR 538 in February 2004. In January 2007 the second funding period was approved.

The purpose of the joint activities is to arrive at a detailed understanding of the phase diagram of copper-oxygen systems with superconducting transition temperatures T_c up to 160 K and to disentangle general and sample related properties. As an example, while the electronic structure is essentially the same in all material classes there is a difference by an order of magnitude in the maximal T_c.

Figure 1: Phase Diagram of the Cuprates

The Research Unit includes now three theory projects, sample preparation and seven experimental methods, i.e. angle-resolved photoemission (ARPES), neutron and Raman scattering, optical and dc conductivity, heat transport and nuclear magnetic resonance (NMR). The combination of single and two-particle methods allows a better understanding of the relevant interactions in highly correlated systems. One of the main innovations of the work is the use of a common basis of samples for all spectroscopic methods and the simultaneous analysis of the results.

The questions to be addressed include:

Properties of electron- and hole-doped cuprates as a function of carrier concentration
Separation of general and sample-specific properties
Study of the various phase and metal-insulator transitions
Charge and spin ordering
Competing order
Quantum criticality
Study of the interactions relevant for superconductivity

The substantial improvement of the sample quality and the innovations in the experimental and theoretical techniques over the last five years allow qualitatively new insights and a much more detailed understanding of the relevant physics. This development is presently revolutionalizing the view of the cuprates. Therefore, the Research Unit is being established very timely. The participating groups, which have shown their competence over an extended period of time, will profit very much from the focussed activities.

Purpose of Research Units

The DFG supports joint activities of research groups which have contributed successfully to a specific field. The main purpose is to combine and coordinate activities devoted to a better understanding of a complicated subject. In addition, young scientists should be attracted and get in touch with interesting problems of modern solid state physics. The DFG provides also funding for national and international workshops to communicate the results.