Perspectives in Condensed Matter Physics – serie

This volume contains reprints of 30 scientific articles which provide a record of some of the main advances in understanding which have lead us from Edwin Hall's discovery of the effect which bears his name to our current understanding of the quantum Hall efFect. In the process of deciding which articles were to be included in the collection and which were to be left out I became aware of the gaps in my knowledge of the relevant history, both very recent and more distant. The final choices are, I have no doubt, imperfect. I know that they reflect to too large a degree the aspects of, the subject on which I have 'worked myself, so that the collection represents mil perspective on the quantum Hall effect. It is my hope ,however, that the reader will find that these articles taken together have a rather interesting story to tell: a story which reminds us of the unpredictable consequences of scientific enquiry and reflects the impressive achievements of condensed matter physics in this century. The reprints are accompanied by an Introduction which can stand on its own as a brief explanation of the quantum Hall efFect or serve as an primer for the reprinted articles. The point of view taken in the introduction is again my own and is not the only useful one.

This volume contains reprints of 30 scientific articles which provide a record of some of the main advances in understanding which have lead us from Edwin Hall's discovery of the effect which bears his name to our current understanding of the quantum Hall efFect. In the process of deciding which articles were to be included in the collection and which were to be left out I became aware of the gaps in my knowledge of the relevant history, both very recent and more distant. The final choices are, I have no doubt, imperfect. I know that they reflect to too large a degree the aspects of, the subject on which I have 'worked myself, so that the collection represents mil perspective on the quantum Hall effect. It is my hope ,however, that the reader will find that these articles taken together have a rather interesting story to tell: a story which reminds us of the unpredictable consequences of scientific enquiry and reflects the impressive achievements of condensed matter physics in this century. The reprints are accompanied by an Introduction which can stand on its own as a brief explanation of the quantum Hall efFect or serve as an primer for the reprinted articles. The point of view taken in the introduction is again my own and is not the only useful one.

Interface and surface science have been important in the development of semicon­ ductor physics right from the beginning on. Modern device concepts are not only based on p-n junctions, which are interfaces between regions containing different types of dopants, but take advantage of the electronic properties of semiconductor­ insulator interfaces, heterojunctions between distinct semiconductors, and metal­ semiconductor contacts. The latter ones stood almost at the very beginning of semi­ conductor physics at the end of the last century. The rectifying properties of metal-semiconductor contacts were first described by Braun in 1874. A physically correct explanation of unilateral conduction, as this deviation from Ohm's law was called, could not be given at that time. A prerequisite was Wilson's quantum theory of electronic semi-conductors which he published in 1931. A few years later, in 1938, Schottky finally explained the rectification at metal-semiconductor contacts by a space-

The unexpected and therefore really amazing discovery of J.G. Bednorz and K.A. Muller, that certain oxide compounds enter a superconducting state at temperatures above 30 K, pushed research on superconductivity into the limelight of science in general in a way that seemed reserved only for high energy or particle physics. It may therefore be expected that this entire review would solely deal with superconductivity at high temperatures, i.e. above the boiling point of hydrogen. Any unexpance in general in a way that seemed reserved only for high energy or particle physics. It may therefore be expected that this entire review would solely deal with superconductivity at high temperatures, i.e. above the boiling point of hydrogen. Any unexpected occurrence of superconductivity is, however, a challenge to scientists interested either in the physics of this phenomenon or in its materials science aspects. In this respect, the eighties have been quite revolutionary in the sense that, on various occasions, superconductivity was discovered in materials whose physical properties were not obviously favourable for adopting this ground state.Therefore, apart from emphasizing the topic of oxide superconductors, this collection of reprints also contains a selection of papers that deal with other subjects, such as coexistence of magnetic order and super-conductivity, heavy electron and organic superconductors. This is all the more justified when we consider that fact that various aspects of superconductivity in high Tc oxide compounds are, or might be, connected with features that are also observed in these other materials. For non-specialists who might be interested in this collection of reprints the Editor briefly reviews the possibilities for identifying superconductivity and discusses some special features of the superconducting state.

E se non che di cid son vere prove A nd were it not for the true evidence Per piti e piti autori, che sa,ra. nno Of many authors who will be Per i miei versi nominati altrove, Mentioned elsewhere in my rhyme Non presterei alla penna 10. mana I would not lend my hand to the pen Per nota1' cid ch'io vidi, can temenza And describe my observations, for fear ehe non fosse do. altri casso e van 0; That they would be rejected and in vane; Mala lor chiara. e vera. esperienza But these authors' clear and true experience Mi assicura. nel dir, come persone Encourages me to report, since they Degne di fede ad ogni gra. n sentenza. Should always be trusted for their word. [From" Dittamondo", by Fazio degli UbertiJ Heterojunction interfaces, the interfaces between different semiconducting materi- als, have been extensively explored for over a quarter of a century. The justifica- tion for this effort is clear - these interfaces could become the building blocks of lllany novel solid-state devices. Other interfaces involving semiconductors are al- ready widely used in technology, These are, for example, metal-semiconductor and insulator-semiconductor junctions and hOll1ojunctions.In comparison, the present applications of heterojunction int. erfaces are limited, but they could potentially becOlne lnuch lllore ext. ensive in the neal' future. The path towards the widespread use of heterojunctions is obstructed by several obstacles.

E se non che di cid son vere prove A nd were it not for the true evidence Per piti e piti autori, che sa,ra. nno Of many authors who will be Per i miei versi nominati altrove, Mentioned elsewhere in my rhyme Non presterei alla penna 10. mana I would not lend my hand to the pen Per nota1' cid ch'io vidi, can temenza And describe my observations, for fear ehe non fosse do. altri casso e van 0; That they would be rejected and in vane; Mala lor chiara. e vera. esperienza But these authors' clear and true experience Mi assicura. nel dir, come persone Encourages me to report, since they Degne di fede ad ogni gra. n sentenza. Should always be trusted for their word. [From" Dittamondo", by Fazio degli UbertiJ Heterojunction interfaces, the interfaces between different semiconducting materi- als, have been extensively explored for over a quarter of a century. The justifica- tion for this effort is clear - these interfaces could become the building blocks of lllany novel solid-state devices. Other interfaces involving semiconductors are al- ready widely used in technology, These are, for example, metal-semiconductor and insulator-semiconductor junctions and hOll1ojunctions.In comparison, the present applications of heterojunction int. erfaces are limited, but they could potentially becOlne lnuch lllore ext. ensive in the neal' future. The path towards the widespread use of heterojunctions is obstructed by several obstacles.

The unexpected and therefore really amazing discovery of J. G. Bednorz and K. A. R32 Miiller , that certain oxide compounds enter a superconducting state at temperatures above 30 K pushed research on superconductivity into the limelight of science in general in a way that seemed reserved for a while for high-energy or particle physics only. The common interest was then even more aroused when subsequent work rather quickly established that in the same class of compounds (oxides), critical temperatures of superconductivity above R36 the boiling point of nitrogen could be achieved . It might therefore be expected, that this entire review would solely deal with superconductivity at high temperatures, i. e. , above the boiling point of hydrogen. From my point of view, however, any unexpected occurrence of superconductivity is a challenge to scientists interested either in the physics of this phenomenon or in its materials-science aspects. In this respect, the last ten years have been quite revolutionary in the sense that on various occasions, superconductivity was discovered in materials whose physical properties were not obviously favourable for adopting this ground state. This period started with the observation that homogeneous coexistence of superconductivity and magnetic order in the same material was possible. Later it was found that electrons whose effective mass was tremendously enhanced by magnetic interactions, may also form a superconducting state, namely in materials that were subsequently identified as heavy­ electron superconductors.

In 1988 physicists and chemists commemorated the centenary of the discovery of the first liquid crystals. Fora long period after this discovery, although many significant results were found, liquid crystal research remained a marginal topic of condensed matter physics. The situation changed in the sixties. At that time the remarkable electro-optical properties of liquid crystals were recognized and found soon widespread application in numeric displays. From a more fundamental point of view, the interest in disordered systems. increased in general at the same time. Liquid crystals represented an important dass of such systems. Among others, phase transitions, hydrodynamics and topological defects occurring in them attracted considerable attention. The connection between the liquid-crystalline state and the structure of biological membranes stimulated a Iot of works also. In the present volume we discuss a relatively new and rapidly developing branch of the fi. eld, namely nonlinear optical effects in liquid crystals. Optical studies have always played a signifi. cant role in liquid crystal science. Research of optical nonlinearities in liquid crystals began at the end of the sixties. Since then it became a powerful tool in the investigation of symmetry properties, interfacial phenomena or dynamic behaviour. Furthermore, several new aspects of nonlinear processes were demonstrated and studied extensively in liquid crystals. The subject covered in this book is therefore of importance both for liquid crystal research and for nonlinear optics itself. The term "nonlinear optics" is used here in a broad sense.