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Spin ladders: new high-temperature superconductors

Recent studies of the structural and magnetic properties of spin-ladder compounds will be reviewed. The spin ladders of the composition (M2Cu2O3)m(CuO2)n (where M are divalent and trivalent cations) have both CuO2 ribbons and Cu2O3 two-leg ladders. Both of the building blocks show spin gaps in their excitation spectra which originate from a dimerized state. In the reviewed systems it is possible to change the number of hole carriers by changing the ratio Ca/Sr. The hole carriers are considered to be localized in the chains (ribbons) and are coupled with the copper spins forming the Zhang-Rice singlets. The dimers are formed between next-nearest neighbor copper ions. The charge ordering involving both dimerization and the Zhang-Rice singlet formation is suggested to exist in (M2Cu2O3)5(CuO2)7 compounds. In some compounds with high Ca/Sr ratio and with substantial hole transfer from ribbons to ladders the superconducting state is created with a high critical temperature (of about 80 K).