Structural instabilities in lanthanum cuprate superconductors

Subtle but well characterized structural phase transformations occur at low temperatures in some lanthanum cuprate superconductors. These transformations, which involve collective tilting of the CuO6 octahedra, have a strong influence on superconducting and normal state properties. Experiments suggest that for a given hole doping the following hierarchy exists for superconductivity in these materials: Tc(HTT)>Tc(LTO)>Tc(LTT), where HTT represents the untilted structure and LTO and LTT are the two limiting low temperature tilted structures. An additional very strong suppression of Tc, suggestive of a competing electronic groundstate, occurs in the LTT phase for a hole to Cu ratio of 1/8. Neither of these effects are understood at present. This paper reviews both the evidence upon which these conclusions are based and some outstanding unanswered questions.