Spin-gap and superconductivity in ladder compounds

Abstract

Abstract The study of ladder materials is a relatively new but already well-established area of research within strongly correlated electrons. In this paper, a brief review of theoretical and experimental aspects of this family of compounds is presented. The main calculations that led to the theoretical prediction of a spin-gap in the undoped limit, as well as superconductivity upon doping, are discussed. Several real ladder materials with spin-gaps are mentioned. The famous compound Sr(14− x ) Ca( x ) Cu(24) O(41) (also known as ‘14-24-41’), with a superconducting phase at high pressure and small hole density, 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 that (14-24-41) is the first superconducting copper-oxide material with a non-square-lattice layered arrangement, and certainly much can be learned from a careful analysis of this compound. It is concluded that the recent enormous experimental effort on ladders has unveiled challenging and interesting physics that adds to the rich behavior of electrons in transition-metal-oxides, and in addition contributes to the understanding of the two-dimensional cuprates. However, considerable work still needs to be carried out to fully understand the interplay between charge and spin degrees of freedom in these materials.