  Exotic superconductivity has often been discovered in materials with a
layered (two-dimensional) crystal structure. The low dimensionality can affect
the electronic structure and can realize high transition temperatures (Tc)
and/or unconventional superconductivity mechanisms. As standard examples, we
now have two types of high-Tc superconductors. The first group is the Cu-oxide
superconductors whose crystal structure is basically composed of a stacking of
spacer (blocking) layers and superconducting CuO2 layers.1-4 The second group
is the Fe-based superconductors which also possess a stacking structure of
spacer layers and superconducting Fe2An2 (An = P, As, Se, Te) layers.5-13 In
both systems, dramatic enhancements of Tc are achieved by optimizing the spacer
layer structure, for instance, a variety of composing elements, spacer
thickness, and carrier doping levels with respect to the superconducting
layers. In this respect, to realize higher-Tc superconductivity, other than
Cu-oxide and Fe-based superconductors, the discovery of a new prototype of
layered superconductors needs to be achieved. Here we show superconductivity in
a new bismuth-oxysulfide layered compound Bi4O4S3. Crystal structure analysis
indicates that this superconductor has a layered structure composed of stacking
of Bi4O4(SO4)1-x and Bi2S4 layers; the parent compound (x = 0) is Bi6O8S5. Band
calculation suggests that Bi4O4S3 (x = 0.5) is metallic while Bi6O8S5 (x = 0)
is a band insulator with Bi3+. Furthermore, the Fermi level for Bi4O4S3 is just
on the peak position of the partial density of states of the Bi 6p orbital
within the BiS2 layer. The BiS2 layer is a basic structure which provides
another universality class for layered superconducting family, and this opens
up a new field in the physics and chemistry of low-dimensional superconductors.
