Introduction 175 4.2. Magnetization of small superconducting particles 2. Phenomenological description of small particles 178 4.3. Microscopic theory of small superconductors 2.1. Interaction with electromagnetic radiation 178 4.4. Fluctuations in small superconducting particles 2.2. Mie's theory 178 5. Preparation of small particles 2.3. The dielectric constant of a small metallic particle 184 5.1. Metal colloids 2.4. The effective dielectric constant of the embedding 5.2. Granular metal films medium 189 5.3. Particles prepared by the gas-evaporation technique 3. Quantum size effects in small metallic particles 192 5.4. Particles prepared by nucleation and growth in a 3.1. Microscopic description of electrons in small particles 192 matrix 273 3.2. Kubo's small particle 197 5.5. Impregnated porous materials 275 3.3. Level statistics 201 Appendix I. Thermodynamic calculations for Kubo's small 3.4. Spectroscopy of the small particle level structure 213 particle 3.5. Experiments 217 Appendix II. Thermodynamic calculations for the particle 4. Superconductivity in small particles 232 with equal level spacing 4.1. Characteristic lengths of superconductors 232 References

The energy diagram of Mn 12 -acetate is probed close to the top of the anisotropy barrier using a novel, high-sensitivity, electron paramagnetic resonance technique. Multiple resonances, and their temperature dependence, are observed from 35 to 115 GHz, for a single high-quality crystal. The data are sufficiently detailed to make extremely accurate comparisons with predictions based on a spin S 10 Hamiltonian. Although overall agreement is good, we find evidence for possible inadequacies of this model. [S0031-9007(98)05531-8]

The magnetization of a Mn 12 -acetate single crystal was measured with a cantilever magnetometer to temperatures below 60 mK. Contrary to expectations we did not observe steps in the hysteretic magnetization with indices higher than 11. The data suggest a significant degree of higher-order anisotropy. The fourth-order term appears to be about Ϫ3ϫ10 Ϫ4 k B . Both hysteresis in magnetization at different ramping rates of the field, and relaxation at different fixed values of the magnetic field were studied. The relaxation is found to be logarithmic below 1 K, and not single exponential. We contend that the process of magnetization reversal is initiated by tunneling across the anisotropy barrier from a weak population in excited levels from which they can easily tunnel across the barrier, and that the consequent emission and reabsorption of phonons helps to promote a significant fraction of the spins to these excited levels.

Angle-dependent magnetoresistance oscillations (AMROs) have been studied in the isostructural charge-transfer salts and (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) in steady fields of up to 30 T. The shapes of the approximately elliptical quasi-two-dimensional (Q2D) Fermi surfaces that these organic metals possess have been determined at 30 T and are found to be in broad agreement with recent band-structure calculations. The Fermi surface of the salt undergoes a reconstruction at low fields and temperatures, resulting in a change in the dimensionality of the AMROs from Q2D character to quasi-one-dimensional character. This change is associated with the kink transition that is observed in magnetic field sweeps and is attributed to the formation of a spin-density wave ground state. The phase boundary of the change in the AMRO dimensionality has been followed to both the low-temperature high-field (about 23 T) and low-field high-temperature (about 8 K) extremes. The data are compared with recently proposed models of the AMROs and Fermi surfaces for these materials.

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