Core Photoemission

Physics is the paradigm of all scientific knowledge. Over the centuries it has evolved to a complexity that has resulted in a separation into various subfields, always connected with one another and very difficult to single out. Freeman Dyson, in his beautiful book 'Infinite in All Directions', distinguishes two aspects of physics and two types of physicists: the unifiers and the diversifiers. The unifiers look for the most general laws of nature, like the universal attraction between masses and electric charges, the laws of motion, relativity principles, the simplest elementary particles, the unification of all forces, symmetry violation and so on. The diversifiers consider the immense variety of natural phenomena, infinite in their extension, try to explain them on the basis of known general principles, and generate new phenomena and devices that do not exist in nature. Even at the beginning of modern science Galileo Galilei, besides studying the laws of motion and laying down the principle of relativity, was interested in the phenomenon of fluorescence and disproved the theories put forward at his time. He was both a unifier and a diversifier. The full explanation of fluorescence had to await the advent of quantum mechanics, as did the explanation of other basic phenomena like electrical conductivity and spectroscopy. The past century witnessed an explosive expansion in both aspects of physics. Relativity and quantum mechanics were discovered and the greatest of the unifiers, Albert Einstein, became convinced that all reality could be comprehended with a simple set of equations. On the other hand a wide range of complex phenomena was explained and numerous new phenomena were discovered. One of the great diversifiers, John Bardeen, explained superconductivity and invented the transistor. In physics today we encounter complex phenomena in the behavior of both natural and artificial complex systems, in matter constituted by many particles such as interacting atoms, in crystals, in classical and quantum fluids as well as in semiconductors and nanostructured materials. Furthermore, the complexity of biological matter and biological phenomena are now major areas of study as well as climate prediction on a global scale. All of this has evolved into what we now call ''condensed matter physics''. This is a more comprehensive term than ''solid state physics'' from which, when the electronic properties of crystals began to be understood in the thirties, it originated in some way. Condensed matter physics also includes aspects of atomic physics, particularly when the atoms are manipulated, as in Bose-Einstein condensation. It is now the largest part of physics and it is where the greatest number of physicists work. Furthermore, it is enhanced through its connections with technology and industry. In condensed matter physics new phenomena, new devices, and new principles, such as the quantum Hall effect, are constantly emerging. For this reason we think that condensed matter is now the liveliest subfield of physics, and have decided to address it in the present Encyclopedia. Our focus is to provide some definitive articles for graduate students who need a guide through this impenetrable forest, researchers who want a broader view into subjects related to their own, engineers who are interested in emerging and new technologies together with biologists who require a deeper insight into this fascinating and complex field that augments theirs. In this Encyclopedia we have selected key topics in the field of condensed matter physics, provided historical background to some of the major areas and directed the reader, through detailed references, to further reading resources. Authors were sought from those who have made major contributions and worke