Experiments on Ladders Reveal a Complex Interplay between a Spin-Gapped Normal State and Superconductivity

Abstract

In recent years, the study of ladder materials has developed into a well-established area of research within the general context of Strongly Correlated Electrons. This effort has been triggered by an unusual cross-fertilization between theory and experiments. In this paper, the main experimental results obtained in the context of ladders are reviewed from the perspective of a theorist. Emphasis is given to the many similarities between the two-dimensional high-$\rm T_c$ cuprates and the two-leg ladder compounds, including Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ (14-24-41) which has a superconducting phase at high pressure and a small hole density. Examples of these similarities include regimes of linear resistivity vs temperature in metallic ladders and a normal state with spin-gap or pseudogap characteristics. Some controversial results in this context are also discussed. It is remarked that the ladder 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. A short summary of the main theoretical developments in this field is also included, as well as a brief description of the properties of non-copper-oxide ladders. Suggestions by the author on possible experiments are described in the text. Overall, it is concluded that the enormous experimental effort carried out on ladders has already unveiled quite 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, still considerable work needs to be carried out to fully understand the interplay between charge and spin degrees of freedom in these materials.