of defects and surfaces.2 The importance of this work for the future of materials science should be obvious to readers of this volume. Other areas of research are excluded because they are discussed elsewhere in this volume. (See, for example, the discussion of icosahedral quasicrystals by Cahn and Gratias and the discussion of new techniques in surface science by Plummer et al.) The references cited in this chapter, and the experimental curves displayed in the text, are intended to be illustrative of their subjects. These are not necessarily the most important contributions in each case, and no attempt has been made to construct a balanced list of all the key references in the four subjects discussed. It is hoped that the references given will be sufficient to help the interested reader gain entry to the literature in these areas.

This chapter presents four particular examples where materials science and physics have combined to advance our understanding of nature. The fractional quantized Hall effect and the field of heavy-electron systems are two cases where experiments on newly developed materials or structures have yielded results so surprising as to change our understanding of the behavior of interacting electron systems in certain conditions. The two remaining examples, research on novel forms of structural order and on quantum interference effects in electron transport in ultrasmall structures and disordered systems, illustrate the larger symbiotic relationship that exists between physics and materials science. Here we shall find examples where theoretical predictions of unusual properties have led to the experimental investigation of novel materials and structures, and where experimentation has confounded previous expectations.

HEAVY-ELECTRON COMPOUNDS

One of the most fascinating subjects in condensed-matter physics is heavy-electron compounds.39 The conduction electrons in these metallic compounds have effective masses of 100 to 1,000 times the free-electron mass as opposed to values of about 10 for transition metals. It has been said that the carriers in heavy-electron compounds behave more like protons or helium atoms than electrons! The huge effective mass is manifest directly in the large electronic specific heats, at low temperatures, and in the similarly large Pauli paramagnetic susceptibilities. The Fermi degeneracy temperature, which marks the onset of the low-temperature regime for the specific heat and various properties, is on the order of 100 K in many of these compounds as opposed to 10,000 K and higher in ordinary metals. These heavy-electron compounds show many other amazing properties as well.

The table on page 133 lists selected heavy-electron compounds, together with their low-temperature specific-heat coefficients, γ=Cel/T. (The corresponding coefficient for free electrons would be about 1 mJ/mol-K2.) To illustrate the variety of properties of the heavy-electron compounds, we note



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