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We have investigated planar metal-insulator-metal tunnel junctions with aluminium oxide as the dielectricum. These oxide barriers were grown on an aluminium electrode in pure oxygen at room temperature till saturation. By applying the Simmons model we derived discrete widths of the tunnelling barrier, separated by s ≈ 0.38 nm. This corresponds to the addition of single layers of oxygen atoms. The minimum thickness of s 0 ≈ 0.54 nm is then due to a double layer of oxygen. We found a strong and systematic dependence of the barrier height on the barrier thickness. Breakdown fields up to 5 GV m −1 were reached. They decreased strongly with increasing barrier thickness. Electrical breakdown could be described by a metal-insulator like transition of the dielectric barrier due to the large density of tunnelling electrons.

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We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type germanium, using tunneling spectroscopy on mechanically controllable break junctions. At low temperatures, the tunnel conductance shows a minimum at zero bias voltage due to the Coulomb gap. Above 1 K, the gap is filled by thermal excitations. This behavior is reflected in the variable-range hopping resistivity measured on the same samples: up to a few degrees Kelvin the Efros-Shklovskii lnR infinity T(-1/2) law is obeyed, whereas at higher temperatures deviations from this law occur. The type of crossover differs from that considered previously in the literature.

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A survey is given of typical phenomena, new materials and recent developments in heavy-fermion physics. In particular, the following topics are addressed: (i) YbNiAl, a new heavy-fermion local-moment antiferromagnet (LMM) with Neel temperature T(N) = 3 K, (ii) ''non-Fermi-liquid'' behavior at the magnetic instability in two heavy-fermion systems with intact from sublattice, i.e. orthorhombic CePt(Si1-xGex) and tetragonal U(Cu4+xAl8-x), (iii) the low-temperature properties of the anisotropic ''Kondo insulator'' CeNiSn, and (iv) some of the most unusual observations made on ''low-carrier-density'' rare-earth systems like Sm3Te4 and Sm3Se4. While the ezotic symmetry-broken (superconducting and magnetic) ground states of heavy-fermion metals are discussed in several other contributions to this volume, we focus in the remainder of this paper on the relationship between LMM ordering and heavy-fermion superconductivity: Firstly, the LMM ordered compound CeCu2Ge2 (T(N) = 4.1 K) is addressed which was recently found to become a non-magnetic heavy-fermion superconductor under high hydrostatic pressure, p greater-than-or-equal-to 70 kbar (D. Jaccard et al., Phys. Lett. A 163,475 (1992)). Point-contact spectroscopy is used to investigate in more detail the high-pressure superconducting phase of CeCU2Ge2. Secondly, we summarize high-pressure results on UPd2Al3, the first compound to show homogeneous coexistence between LMM ordering and heavy-fermion superconductivity

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