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The phase diagrams of chains and ladders of (La, Y, Sr, Ca)14Cu24O41

The phase diagrams of the intrinsically hole doped spin-ladder and chain composites (La, Y, Sr, Ca)14Cu24O41 will be reviewed [1]. In the underdoped (La, Y)y(Sr, Ca)14-yCu24O41 (hole count =6-y per formula unit, f.u.) the ladders contain no holes and have a spin-gapped ground state, while the holes are found only in the chains. When most, or all, of the holes are removed (for 5≤ y≤ 6) an antiferromagnetic (AF) order of ferromagnetic chains may be established. For higher hole counts, short range AF correlations persist up to y≤ 2, and give way to an AF dimers phase which exists at short scales for 0.1≤ y≤ 2. The chains in (La, Y)y(Sr, Ca)14-yCu24O41 behave as one-dimensional disorder-driven insulators, as the number of holes in chains probably can not be over 5. Only in the chains of the fully doped Sr14-xCaxCu24O41, (at lowest temperatures, where there can be more than 5 holes in the chains) AF dimers establish two-dimensional (2D) long-range ordered pattern, to which a charge-ordered ground state is associated (CO-AF). Since Sr14-xCaxCu24O41 have, in total, 6 holes per f.u., around one hole (already for x=0 and increasing for x>0) is found in the ladders. The spin-gap of the undoped ladders persists with this hole transfer and seems to be present when superconductivity (SC) appears under pressure in x>10 materials. Early theory for ladders based a mechanism for SC on the spin-gap. However, since the pressure induces 2D charge transport in the ladder layers, it is also debatable that SC in ladders may be related to SC in layered cuprates. At ambient pressure charge-density wave (CDW) was identified as a ground state for 0≤ x≤ 9. It is striking that CO-AF in the chains, and CDW in the ladders are suppressed with increasing x by the similar rate, and, finally, for highest x both subsystems undergo separate AF-orderings. Rather unconventional nature of CDW in the strongly correlated system of the ladders is recognized in the observation of an insulator-to-insulator phase transition and of CDW 2D anisotropic response in ac-fields. In addition contradictory results are reported regarding nonlinear effects in dc-fields.

spin ladders; spin chains; charge.density wave; gapped spin liquid; superconductivity

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