Condensed Matter Seminars

The ‘telephone number compound’ family, (La,Sr,Ca) ₁₄ Cu ₂₄ O ₄₁ , is composed of copper oxide planes which form alternating layers of ladders and chains. In the absence of Sr these materials are intrinsically hole doped with a valence of +2.25 per Cu.In contrast, for La ₄ Sr ₁₀ Cu ₂₄ O ₄₁ the hole doping is much reduced and magnon heat conductivity measurements show that the ladders are free of holes [1]. A third member of the family, Sr _2.5 Ca _11.5 Cu ₂₄ O ₄₁ , is hole-doped with ~2 holes per seven rungs of the ladder (the exact doping is debated). This compound is of particular interest because it shows a number of phenomena in common with high-Tc cuprates, including linear temperature dependence of resistivity above 130K, charge ordering below 60K and superconductivity under applied pressure [2]. Our neutron scattering measurements on the undoped ladder La ₄ Sr ₁₀ Cu ₂₄ O ₄₁ reveal gapped one-magnon and multi-magnon excitations. The results have been modelled to find accurate values of the exchange constants [3]. The doped ladder Sr _2.5 Ca _11.5 Cu ₂₄ O ₄₁ has also been measured. The holes give rise to a number of changes in the excitation spectrum which will be described and discussed.