ABSTRACT
Lattice vibrations in metals generally give rise to a finite electrical
resistivity, and it disappears only when the metal resumes perfectly
periodic ion potentials at absolute zero temperature. But there are
a number of metals the electrical resistivity of which vanishes at
finite temperatures together with the Meissner effect, Josephson
effect, and flux quantum. These basic properties are distinctive
for superconductivity. Fifty-seven elements are currently known
to exhibit superconductivity. To take a simple example, supercon-
ductivity under high pressure was observable at extremely low
temperature, even for the familiar gas oxygen (see Fig. 7.1) [1].
Historically, superconductivity for mercury after liquefying helium
was discovered in 1911. Since then, superconductivity has been
actively studied in many metals, alloys, and oxides. The mechanism
of superconductivity had remained unresolved for many years as
one of themost mysterious puzzles in physics until Bardeen, Cooper,
and Schrieffer (BCS) put forward the epoch-making theory, that
is, BCS theory, in 1957 [2]. They established the fundamentals on
the basis of the Cooper pair-mediated electron-phonon interaction.
According to their theory, the maximum superconductive transition
temperature (Tc) has been considered to be 30-40 K at most. In 1986, however, Bednorz and Mu¨ller reported the possibility
of superconductivity for La-Ba-Cu-O and won the Novel Prize in
physics in 1987 [3]. Motivated by their work, researchers in solid-
state physics have taken some important steps in high-Tc superconductors for cuprates. As a theoretical concept proposed soon
after the discovery, resonating valence band (RVB) theory suggested
by Anderson is famous, in which doublons and holons form bound
charge-neutral excitations and lead to zero electrical resistivity,
including, but not limited to, the square and triangular lattices [4].
In Section 7.1, Bose-Einstein condensation is introduced, which
would help one’s interpretation of superconductivity. In Section
7.2, high-Tc cuprate superconductors and digest applications are introduced historically. Superconductivity is also reported for other
transition metal oxides; superconductivity observed for water-
intercalated NaxCoO2 is discussed as a first example for cobalt oxide
superconductors in Section 7.3.
