Experiments on ladders reveal a complex interplay between a spin-gapped normal state and superconductivity

In recent years, the study of ladder materials has developed into a well-es tablished area of research within the general context of strongly correlate d electrons. This effort has been triggered by an unusual cross-fertilizati on between theory and experiments. In this paper, the main experimental res ults obtained in the context of ladders are reviewed from the perspective o f a theorist. Emphasis is given to the many similarities between the two-di mensional high-T-c cuprates and the two-leg ladder compounds, including Sr1 4-xCaxCu24O41 ([14-24-41]) which has a superconducting phase at high pressu re and a small hole density. Examples of these similarities include regimes of linear resistivity versus temperature in metallic ladders and a normal state with spin gap or pseudogap characteristics. It is remarked that the l adder [14-24-41] is the first superconducting Cu 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 theoretica l developments in this field is also included, as well as a brief descripti on of the properties of non-Cu-oxide ladders. Suggestions by the author on possible experiments are described in the text. Overall, it is concluded th at the enormous experimental effort carried out on ladders has already unve iled quite challenging and interesting physics that adds to the rich behavi our of electrons in transition metal-oxides, and in addition contributes to the understanding of the two-dimensional cuprates. However, considerable w ork still needs to be carried out to fully understand the interplay between charge and spin degrees of freedom in these materials.