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pletely unexpected properties. Living matter and life itself are perhaps the most spectacular examples of emergent phenomenon; no matter how much we learn about individual atoms, life cannot be understood or explained in this purely reductionist manner. One of the biggest surprises of the last decade was high-temperature superconductivity. It is hard to imagine a less likely candidate for a superconductor than an insulating ceramic compound with properties similar to those of a china coffee cup. Yet when chemically doped to introduce charge carriers, such compounds not only superconduct, they do so at record high temperatures.

The characteristic energy scale for individual atoms is 1 to 10 electron volts (eV). However, as we look on larger length scales at collections of atoms, characteristic energies become smaller and smaller, and excitations become more and more collective. At low energies, the effective elementary degrees of freedom may be collective objects very different from individual electrons and atoms, and their effective interactions may be very different from the original ''bare" Coulomb interactions. These collective effects are the source of the surprises that emerge.

It is instructive to compare this situation with that in high-energy elementary particle physics. There we know the effective degrees of freedom and their interactions at low energies—it is the world of atoms around us. The intellectual challenge is to understand degrees of freedom at shorter and shorter length scales and higher and higher energy scales. This is done by constructing high-energy particle accelerators to act as microscopes with ever greater magnification, or by studying extreme conditions in astrophysical systems and the early universe. This approach is just the reverse of what is done in condensed-matter physics, where we strive to understand collective effects at longer and longer length scales. The analog of the particle accelerator is the refrigerator, which lowers thermal energy scales and increases the distance over which particles suffer inelastic collisions. The analog of an extreme astrophysical system is a sample in a dilution refrigerator. The intellectual challenge is the same in the two fields: to find correct descriptions of the physics that work over a wide range of scales.

Fifty years ago understanding a novel quantum object known as a "hole" (see Box 3.1) led to the invention of the transistor. In the past decade there has been tremendous progress in the discovery and study of a variety of novel quantum phenomena. This chapter presents brief descriptions of a few examples drawn from superfluidity, superconductivity, Bose-Einstein condensation, quantum magnetism, and the quantum Hall effect. It cannot cover many other fascinating areas of development in the last decade, including significant advances in our understanding of quantum critical phenomena, non-Fermi liquids, metal-insulator and superconductor-insulator transitions in two dimensions, quantum chaos and the role of interactions, coherence, and disorder in mesoscopic systems.

There has been particularly significant progress in this last area, both technologically and theoretically. For example, electron "wave guides" have been constructed, and the quantization of their conductance in units of e2/h has been



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