Complete bond-operator theory of the two-chain spin ladder

The discovery of the almost ideal, two-chain spin-ladder material (C${}_5$H${}_{12}$N)${}_2$CuBr${}_4$ has once again focused attention on this most fundamental problem in low-dimensional quantum magnetism. With experimental data now available at all temperatures and magnetic fields, a correspondingly systematic theoretical description is required. This is obtained for the gapped regime within the bond-operator framework, by implementing three qualitative advances that account for triplet correlation effects, their evolution in a field, and the magnetization induced when both field and temperature are finite. Quantitative and parameter-free results are presented as experimental comparisons with the measured specific heat and as predictions for thermal renormalization of the triplet magnon excitations. DOI: http://dx.doi.org/10.1103/PhysRevB.83.054415 ©2011 American Physical Society