Spin-gap and superconductivity in ladder compounds

The study of ladder materials is a relatively new but already well-establis hed area of research within strongly correlated electrons. In this paper, a brief review of theoretical and experimental aspects of this family of com pounds is presented. The main calculations that led to the theoretical pred iction of a spin-gap in the undoped limit, as well as superconductivity upo n doping, are discussed. Several real ladder materials with spin-gaps are m entioned. The famous compound Sr(14-x) Ca(x) Cu(24) O(41) (also known as '1 4-24-41') with a superconducting phase at high pressure and small hole dens ity, is described in detail, Similarities between the two-dimensional high- T-c cuprates and the two-leg ladder compounds are also discussed, including regimes of linear resistivity vs. temperature in ladders. It is remarked t hat (14-24-41) is the first superconducting copper-oxide material with a no n-square-lattice layered arrangement, and certainly much can be learned fro m a careful analysis of this compound. It is concluded that the recent enor mous experimental effort on ladders has unveiled challenging and interestin g physics that adds to the rich behavior of electrons in transition-metal-o xides, and in addition contributes to the understanding of the two-dimensio nal cuprates. However, considerable work still needs to be carried out to f ully understand the interplay between charge and spin degrees of freedom in these materials. (C) 2001 Elsevier Science B.V. All rights reserved.