Highly Correlated Electron Systems
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trons in the material is expected to be valid. With increasing temperature, the coherent Bloch-state description will evolve smoothly into the disordered-local-moment-state description in a temperature range of the order of the Kondo temperature for the single moment. The high-temperature spindegeneracy entropy evolves from the electronic entropy of the coherent ground state. This gives the simple estimate for the electronic specific heat entropy: y ~ fc/tln(2/ + l)/T* per spin.
Introduction The study of materials which have electronic phase transitions is a very active area. Such phase transitions include charge and spin density formation, as well the superconducting condensation in a rapidly expanding variety of materials. It is now common to lump these phenomena under the heading of correlated electron physics, involving as they do the essential role of electron-electron interactions in their occurrence. There are also materials in which there is found no electronic phase transition, but whose properties indicate strong electron-electron effects, such as a number of the so-called heavy fermion compounds. The part of condensed matter theory which addresses the particular physics of such materials is generally known as many-body physics. How to effectively treat strong electronic interactions theoretically is very much an unsolved problem, and theory does not give much more than limited guidance to the experimental research in this area. External magnetic fields have proved to be effective experimental probes of the properties of such systems, and the advent of increasingly strong pulsed fields is opening new possibilities for exposing and pulling apart the underlying electronic ground state of many such materials.
Heavy Fermion Materials Loss of Moment and the Coherent State It is well known that localized/electron magnetic moments can exist on rare earth and actinide elements in many intermetallic and other compounds. In certain of these elements, most notably Ce and U where the 4/ and 5/ inner shells are just forming, the / levels in many intermetallic compounds lie near the Fermi level. Due to mixing of the / levels with conduction electron states, the / level is broadened, forming a virtual bound state. It is known that in this situation, there is a net effective antiferromagnetic interaction between an / and a conduction electron. This antiferromagnetic interaction results in the Kondo effect in the impurity case: the con-
MRS BULLETIN/AUGUST 1993
duction electron spins compensate the local / moment. In an atomically ordered metallic compound, where the ratio between the number of / moments and conduction electrons is of order unity, this same compensation still appears to take place. It can be appreciated that this "dense" Kondo problem involves highly correlated electronic behavior. At the heart of this so-called heavy fermion problem, then, lies the simply formulated question of loss of moment. A Na atom, for example, has a single outer s electron with a spin of 1/2. When Na metal forms, this spin dis
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