Magnetic Susceptibilities of Spin-1/2 Antiferromagnetic Heisenberg Ladders and Applications to Ladder Oxide Compounds

摘要

A comprehensive theoretical and experimental study is presented of themagnetic susceptibility versus temperature \chi(T) of spin S = 1/2 two- andthree-leg Heisenberg ladders and ladder oxide compounds. Extensive quantumMonte Carlo simulations of \chi(T) were carried out for both isolated andcoupled two-leg ladders with spatially anisotropic intraladder exchange.Accurate fits to these and related literature QMC data were obtained. We havealso calculated the one- and two-magnon dispersion relations and the dynamicalspin structure factor for anisotropic isolated 2 x 12 ladders. The exchangeconstants in the two-leg ladder compound SrCu2O3 are estimated from LDA+Ucalculations. We report the detailed crystal structure of SrCu2O3 and of thethree-leg ladder compound Sr2Cu3O5. New experimental \chi(T) data are reportedfor the two-leg ladder cuprates SrCu2O3 and LaCuO_{2.5}, and for the(nominally) two-leg ladder vanadates CaV2O5 and MgV2O5. The new and literature\chi(T) data for these compounds and for Sr2Cu3O5 are modeled using our QMC\chi(T) simulation fits, and the exchange coupling constants between thespins-1/2 are thereby estimated for each material. The surpisingly strongspatial anisotropy of the bilinear intraladder exchange constants in thecuprate compounds is discussed together with the results of other experimentssensitive to this anisotropy. Recent theoretical predictions are discussedincluding those which indicate that a four-spin cyclic exchange interactionwithin a Cu4 plaquette is important to determining the magnetic properties andwhich can significantly influence the exchange interactions estimated from\chi(T) data assuming the presence of only bilinear exchange.