We motivate the descriptionof heavy-electron metals in terms ofconcepts from the Kondo problem. These concepts are used to discuss magnetism and superconductivity in heavy-electron systems. Particularattention is given to what weview as the principal outstanding questions in this field and directionsin which the field is developing. This will include consideration of thedifferences between Ce and U heavy-electron compounds as well as the occurrence ofvery small ordered magnetic moments. © The Japan Society of Applied Physics.

Inelastic neutron scattering from the antiferromagnetic (TN=9.7 K) heavy-fermion system U2Zn17 reveals magnetic fluctuations which are highly localized in reciprocal space and broad in frequency. At higher energies, the fluctuations persist almost unchanged to temperatures well above Tn. Analysis of the neutron and bulk susceptibility data in terms of Kondo-type single-ion response functions modified by Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between ions indicates that it is the temperature dependence anticipated theoretically of the effective RKKY coupling which drives the magnetic phase transition in U2Zn17. © 1987 The American Physical Society.

We report the first infrared and optical measurements of the absolute conductivity of FeSi. Above 200 K the ac conductivity of FeSi resembles that of a dirty metal, while at low temperature most of the conductivity below 60 meV is depleted, consistent with an energy gap of that magnitude. Thus in contrast to what is expected from a picture based on simple thermal activation, the gap disappears at a temperature which is low relative to its size. Moreover, the spectral weight lost below the gap does not reappear just above the gap, but is instead distributed over an energy range of order eV. © 1993 The American Physical Society.

Neutron-scattering and resistivity experiments on single crystals of La1.95Ba0.05CuO4 are described. On warming to as high as 250 K, the correlation length does not change, and the temperature dependence of the generalized susceptibility (Q,) is due solely to the temperature-dependence of the local response. We show that the associated relaxation rate in the metallic regime is Latin small letter h with stroke1/2kBT, in accord with the marginal-Fermi-liquid hypothesis, and relate our results to nuclear spin-lattice relaxation data. © 1991 The American Physical Society.

The magnetic dynamics of La2CuO4 have been studied by means of inelastic neutron scattering in the energy range 0Latin small letter h with stroke140 meV. Experiments were carried out using a wide range of incident neutron energies so as to optimize the experimental resolution in each energy range studied. For energy transfers, Latin small letter h with stroke10 meV, and temperatures T in the range 5T320 K, we find that conventional spin-wave theory gives a good description of the magnetic dynamics. We obtain a low-temperature spin-wave velocity Latin small letter h with strokec=85030 meV. For low energies, Latin small letter h with stroke10 meV, and temperatures corresponding to the paramagnetic phase, our data indicate a response function consisting of overdamped spin waves and a quasielastic peak. © 1990 The American Physical Society.

We have used high energy inelastic neutron scattering to study the magnetic fluctuation spectrum of La2CuO4 in the energy and momentum ranges 30≤h{stroke};σ≤140 meV and |q|≤0.2 Å-1, respectively. Our results are consistent, within the experimental error, with conventional spin wave theory the low temperature spin wave velocity, being 850±30 meV Å-1. © 1990.

The results of recent inelastic neutron scattering investigations of the magnetic excitations in La2CuO4 are reviewed. Experiments were performed using a wide range of incident neutron energies. Our study of the paramagnetic phase is consistent with a picture for the magnetic dynamics which shows conventional spin waves at higher energies and an overdamped response due to spin wave interactions at lower energies. The evolution of the magnetic dynamics with doping in the La2-x(Ba, Sr)xCuO4 system is discussed. © 1991, American Institute of Physics. All rights reserved.

The phase transition of U2Zn17 at 9.7 K has been investigated by neutron powder diffraction. The transition corresponds to the onset of antiferromagnetic order where the U moments are oriented antiparallel to their nearest neighbors within the basal planes and the near neighbor along the c^ axis of this rhombohedral compound. At 5 K, the ordered moments lie within the basal planes and are of magnitude (0.8±0.1)1/4B, which is substantially below the paramagnetic moment of 2.251/4B/U atom given by high-temperature susceptibility data. © 1986 The American Physical Society.

Heat capacity, resistance, and magnetic susceptibility have been measured on FeSi, Fe0.90Co0.10Si, Fe0.10Co0.10Si, and CoSi single crystals. From resistance experiments, the activation energy (Δ) shows a small variation from Δ≈310 K in FeSi to Δ≈325 K for the Fe0.90Co0.10Si compound. For the 90% Co compound the gap signature disappears completely. Heat capacity experiments performed in the temperature range 1.5 K

We use inelastic neutron scattering to establish the modulation vectors and correlation lengths for the incommensurate magnetic fluctuations in metallic samples of La2-xSrxCuO4 with x=0.075 and 0.14. In notation appropriate for a square lattice where the magnetic instability in the undoped case occurs at (,), the vectors are along (,0) and (0,).The correlation length is larger than the distance between carriers, is weakly dependent on x, and changes significantly between 12 and 100 K for both compositions. © 1991 The American Physical Society.