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\nwell-established area of research within the general context of Strongly\nCorrelated Electrons. This effort has been triggered by an unusual\ncross-fertilization between theory and experiments. In this paper, the main\nexperimental results obtained in the context of ladders are reviewed from the\nperspective of a theorist. Emphasis is given to the many similarities between\nthe two-dimensional high-$\\rm T_c$ cuprates and the two-leg ladder compounds,\nincluding Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ (14-24-41) which has a\nsuperconducting phase at high pressure and a small hole density. Examples of\nthese similarities include regimes of linear resistivity vs temperature in\nmetallic ladders and a normal state with spin-gap or pseudogap characteristics.\nSome controversial results in this context are also discussed. It is remarked\nthat the ladder 14-24-41 is the first superconducting copper-oxide material\nwith a non-square-lattice layered arrangement, and certainly much can be\nlearned from a careful analysis of this compound. A short summary of the main\ntheoretical developments in this field is also included, as well as a brief\ndescription of the properties of non-copper-oxide ladders. Suggestions by the\nauthor on possible experiments are described in the text. Overall, it is\nconcluded that the enormous experimental effort carried out on ladders has\nalready unveiled quite challenging and interesting physics that adds to the\nrich behavior of electrons in transition-metal-oxides, and in addition\ncontributes to the understanding of the two-dimensional cuprates. However,\nstill considerable work needs to be carried out to fully understand the\ninterplay between charge and spin degrees of freedom in these materials.\n