From fundamentals to gap nanoengineering of high-T c and related oxides Davor Pavuna * Department of Physics ± IPA, Ecole Polytechnique Federale de Lausanne, CH - 1015 Lausanne, Switzerland Received 28 August 2000; accepted 8 September 2000 Abstract Complete understanding of fundamentals of a given class of electronic materials often produces successful new technology. In analogy with a successful (Al)GaAs band-gap engineering, I discuss an equivalent nanotechnology of high-T c (and related) oxides. Controlled heteroepitaxy of layered oxides provides the opportunity to design the desired superconducting gap and/or insulating barrier. However, the progress is somewhat hindred by dicult local doping and non-homogeneous oxygen distribution in some oxides. I also brie¯y discuss puzzling electronic properties across the electronic phase diagram: the `pseudogap' controversy, metal± insulator transition and the anomalous transport. As we solve remaining obstacles we shall be able to tailor the electronic (and/or magnetic) properties of most layered oxides. New concepts and applications will emerge, yet for successful ambient nanelectronics of the 21st century we should ®rst learn to arti®cially `construct' colossal layered superconductors with T c  450 K. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.25; 74.70; 74.10 Keywords: High-T c superconductors 1. Introduction: tailoring the properties of layered oxides Complete understanding of fundamentals of a family of electronic materials often results in a successful new technology. That may require huge interdisciplinary ef- fort of the whole generation of scientists and engineers. Almost 15 years after the striking discovery of high-T c superconductivivity in cuprates [1], we are still trying to fully understand these versatile solids and control their properties. As the main topic of this workshop is the physics and potential applications of synthetic metals, it is useful to begin with applications of high-T c oxides and compare them to prominent electronic materials, like Si or GaAs. Pure Si is an electronically clean solid that, thanks to the skillful use of doping and SiO 2 insulating oxide, can be functionally nanoengineered (down to about 2 nm) and integrated into very large scale multi- transistor chips. GaAs is optically clean material in which the direct band-gap and desired AlGaAs hetero- structure properties can be modi®ed at will (and calcu- lated in advance) by a suitable MBE heteroepitaxy [2]. As a result, the bandgap engineering photonic technol- ogy produces its own original, `archetype' device ± the laser of a desired wavelength. Note that the successful Si-based technology has so far acumulated more than 10 7 men-years of know-how, III±V (GaAs) photonic technology 10 6 men-years, while all superconductivity has hardly reached 10 5 men-years. Superconducting oxides do have their own `archetype' devices (ultrafast Josephson switch, SQUIDs and/or RSFQ-logic) while the magnetic oxides (for example, manganites) provide some of the most versatile magnetic memories [4]. Moreover, there is no doubt that the in-depth under- standing of the fundamentals [3±7] of this ®eld (super- ¯uidity included) will be relevant to many branches of advanced science and technology in the 3rd millennium. To illustrate the concept of high-T c oxide `nanoengi- neering' let us use a direct analogy with the bandgap engineering of (Al)GaAs lasers [2]: the direct bandgap, and consequently the emitting wavelength, is altered by varying the Al content in an optically clean, epitaxially grown AlGaAs heterostructure. An equivalent concep- tual approach to layered high-T c (and related) oxides requires, at the very least, the following: Current Applied Physics 1 (2001) 9±14 www.elsevier.com/locate/cap * Tel.: +41-21-693-3301. E-mail address: davor.pavuna@ep¯.ch (D. Pavuna). 1567-1739/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 7 - 1 7 3 9 ( 0 0 ) 0 0 0 0 3 - 1