Superconductivity and the Metal-Semiconductor Transition

Abstract

Ceramic superconductors based on transition metal oxides have existed for many years but have attracted little attention due to their relatively low critical temperatures (T{sub c}). The perovskite structure has played a major role in the history of oxide superconductors. The first example of a perovskite superconductor was strontium titanate. Insulating behavior is normally found for SrTiO{sub 3}, but it can be reduced or doped to become a degenerate semiconductor which is superconducting. However, T{sub c} for strontium titanate superconductors is always below 1K. It was later found that a great variety of tungsten bronzes, A{sub x}WO{sub 3} where A = Na, K, Rb, Cs, NH{sub 4}, Ca, Sr, Ba, etc., are superconducting with T{sub c}'s as high as 6K. These structures may also be regarded as perovskite related. Analogous molybdenum bronzes, A{sub x}MoO{sub 3}, and rhenium bronzes, A{sub x}ReO{sub 3}, have also been prepared and found to be superconducting. A T{sub c} of 13K was discovered in the Ba(Pb,Bi)O{sub 3} perovskite system. This was particularly significant because 13K is the highest T{sub c} ever observed for a material not containing a transition metal. Ba(Pb,Bi)O{sub 3} was also the first superconductor discovered that could be processed by conventional ceramic processingmore » techniques, i.e. sintering in air. Finally, the authors have the discovery of high temperature superconductivity in the perovskite related systems La{sub 2-x}A{sub x}CuO{sub 4} (A = Ca, Sr or Ba) and RBa{sub 2}Cu{sub 3}O{sub 7} (R = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu or Y).« less