Secondary macroscopic quantum phenomena in neutral superfluids

Kostas Gavroglu, Yorgos Goudaroulis

Giuseppe Grioli

T. Tsuneto

A system composed of particles, whose quantum mechanical zero point energy is large compared to their interaction energy, does not solidify even at absolute zero temperature and remains a so-called quantum liquid. Typical examples of such systems are conduction electrons in metals and liquid helium. Many conductors undergo a phase transition at their critical temperature T c and become superconducting below it. Similarly, liquid 4 He under its vapour pressure becomes a superfluid at T c = 2.17 K ( T λ is often used instead of T c ), and liquid 3 He at T c = 0.9 mK. Although the phenomena are called superconductivity for a charged system like conduction electrons, and superfluidity for a neutral system like liquid helium, they are characterised by the same basic property that the wave nature of the particles manifests itself on a macroscopic scale, as we shall see below. Phenomena The most important factor in determining the properties of a quantum liquid is the statistics of the constituent particles. Liquid 3 He becomes a typical Fermi liquid below 0.1 K and a superfluid below a few mK, in contrast with the bosonic liquid 4 He mentioned above. The two superfluids are quite different both phenomenologically and microscopically. Table 1.1 shows systems which exhibit, or are predicted to exhibit (in brackets), superconductivity or superfluidity. In the case of fermion systems pair formation is responsible for the phenomena so that their properties differ according to the pairing type.

Kurt Alfred Georg Mendelssohn

Philip Warren Anderson

John H. Miller

Superconductivity and related macroscopic quantum phenomena John H. Miller, Jr., from the University of Houston, Dept. of Physics and Texas Center for Superconductivity, walks us through superconductivity and related macroscopic quantum phenomena. Quantum behavior applies not only to microscopic but also to macroscopic systems. Spin-½ particles, fermions, obey the Pauli Exclusion Principle, whereby no two can occupy the same quantum state. By contrast, many integer-spin particles, bosons, can condense into a single macroscopic state. In a superconductor (1), below its critical temperature or Tc , electrons condense into an electronic superfluid that flows with no resistance. For electrons (spin- ½) to condense, they must form pairs. There is one problem, however. Electrons normally repel each other.