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A growing number of metals have been found to show systematic deviations from the predictions of Landau Fermi-liquid theory. The most straightforward examples are magnetic metals in which the Curie ( T C ) or Neel ( T N ) temperature has been suppressed to 0 K by the application of hydrostatic pressure. Such systems are discussed here within a spin fluctuation framework which provides quantitatively accurate descriptions of the d-electron systems MnSi and ZrZn 2 near their ‘quantum critical points’, but which appears to be less successful (in its simplest form) in antiferromagnetic f-electron systems, particularly CePd 2 Si 2 , which at its critical pressure has a resistivity of the form Δρ ∝ T 1.2 = 0.1 over a temperature range extending from around 40 K to below 1 K. CeNi 2 Ge 2 , which is believed to be close to a quantum critical point at ambient pressure, shows similar behaviour. CeIn 3 , CePd 2 Si 2 and CeNi 2 Ge 2 exhibit superconductivity, in the first two cases limited to a narrow region near the critical pressure, making these the first unambiguous examples of magnetically mediated superconductivity.

Recent studies of properties of heavy quasiparticles have helped to establish thatf electrons and conduction electrons in the normal states of certain U and Ce compounds are characterised by Fermi volumes consistent with Luttingeris Theorem and by integrated qnasiparticle masses which agree with measured linear coefficients of the specific heat. Evidence is presented that this Fermi Liquid picture may, however, break down when the ground state is near pressure or magnetic field induced phase transitions.

We report on the observation of a superconducting phase in the itinerant electron ferromagnet UGe 2 . The superconductivity observed below 1 K and in a limited pressure range immediately below the critical pressure where ferromagnetism is abruptly suppressed, would seem to arise from the same electrons that produce band magnetism. This superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions and via a spin-triplet rather than a spin-singlet pairing normally associated with nearly anti-ferromagnetic metals.

We have measured the temperature and field dependence of the resistivity of the unconventional superconductor Sr2RuO4 at pressures up to 3.3 GPa. Using the Shubnikov-de Haas effect, we find that the Fermi surface sheet believed to be primarily responsible for superconductivity becomes more two-dimensional with increasing pressure, a surprising result that is, however, consistent with a recent model of orbital-dependent superconductivity in this system. Many-body enhancements and the superconducting transition temperature all fall gradually with increasing pressure, contrary to previous suggestions of a ferromagnetic quantum critical point at approximately 3 GPa.

The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

The magnetic ordering temperature of some rare earth based heavy fermion compounds is strongly pressure-dependent and can be completely suppressed at a critical pressure, p$_c$, making way for novel correlated electron states close to this quantum critical point. We have studied the clean heavy fermion antiferromagnets CePd$_2$Si$_2$ and CeIn$_3$ in a series of resistivity measurements at high pressures up to 3.2 GPa and down to temperatures in the mK region. In both materials, superconductivity appears in a small window of a few tenths of a GPa on either side of p$_c$. We present detailed measurements of the superconducting and magnetic temperature-pressure phase diagram, which indicate that superconductivity in these materials is enhanced, rather than suppressed, by the closeness to magnetic order., Comment: 11 pages, including 9 figures

Metals on the border of magnetism have attracted great attention in recent times, as a number of them exhibit a superconducting phase at low temperatures in the vicinity of the critical pressure at which the magnetic transition temperature is suppressed to zero. We present a brief overview and comparison of the body centred tetragonal compounds CePd2Si2 and CeNi2Ge2, and the simple cubic CeIn3. In spite of the differences in structure and the detailed behaviour in the unconventional normal state near the critical pressure, these compounds share the existence of superconductivity at the border of antiferromagnetic order, and therefore are prime candidates for the study of a possible magnetic pairing mechanism. In this paper, we will solely present a comparison of the normal state properties of these materials.

Apparatus which can be used to demonstrate the Josephson effect is described. It is simple enough to be assembled in an afternoon and is dependable enough to use in a lecture demonstration. Using a few pieces of niobium wire, an oscilloscope and signal generator, a storage dewar of liquid helium, and some easily obtainable parts, it is possible to see many of the effects which have made Josephson junctions such interesting and useful devices. These include lossless currents (the dc Josephson effect), hysteresis, and macroscopic quantum interference. Phenomena arising from the production of microwave energy when voltages are applied across a Josephson junction (the ac Josephson effect) can also be observed.