limited by insufficient knowledge of the combinatorial possibilities of the different oxides, of the physics at the interfaces, of limitations set by strain effects or layer thickness, or of the possible compatibility with Si-technology. Although success has been obtained in the last years with heterostructures employed for electric field-effect studies, most efforts in the last years (worldwide) have gone into investigating single films of the different classes of materials (superconducting, ferroelectric, ferromagnetic / magnetoresistive) and simple combinations involving an insulating (tunnelling) barrier oxide with one type of material on both sides. In the latter case, the example of manganite-based ferromagnetic tunnel junctions, which perform much less well at high temperatures than might be expected, already shows that the materials science and the interface physics are not fully understood. To come to a comprehensive understanding of these issues, the THIOX programme focuses on five topics:

The focus topics are used to break down walls between different areas of expertise. It should allow groups working on junctions to also produce samples dedicated for growth studies; or groups working on magnetic layers to become familiar with ferroelectrics. Especially, it should allow for increased collaboration in structure analysis of samples by transmission electron microscopy, electron energy loss, and synchrotron radiation. In addition, it allows availability of well-characterized substrates, the combination of the advantages of different growth techniques for special purposes, and comparison of samples made with different deposition techniques. These techniques (pulsed laser deposition, molecular beam epitaxy, sputtering) have seen rapid improvements with increased possibilities for in-situ characterization, making growth of high-quality heterostructures much more feasible. Finally, it should allow easier access to and testing of submicron pattern technologies either for device applications or for the creation of artificial defects.

Introduction

Artificially layered complex oxide structures may well play a key role in the development of novel devices and device concepts. The potential of oxide thin films in device applications has already been demonstrated by the use of Bi-Sr-Ta-oxides in ferroelectric memory elements. Equally interesting and innovative combinations are superconductors with ferromagnets (spin injection) or superconductors / ferromagnets with ferroelectrics (allowing a change in the doping level upon electric polarisation reversal). However, the possibilities are still

Illustration of the zero-temperature behaviour of various correlated materials as a function of sheet charge density. Silicon is shown as a reference. The examples for high-T_c superconductors and for colossal magnetoresistive (CMR) manganites reflect YBa₂Cu₃O_7-δ and (La,Sr)MnO₃, respectively. The top bar has been drawn to illustrate schematically the richness of materials available for field-effect tuning and the spectrum of their phases. AF, antiferromagnetic; FM, ferromagnetic; I, insulator; M, metal; SC, superconductor; FQHE, fractional quantum Hall effect; Wigner, Wigner crystal.