6.2 Spin-Peierls material CuGeO₃

The first spin Peierls materials were all organic. This may be attributed to the characteristics of organic materials that (1) the lattice is soft and its deformation is relatively easy and (2) the distance between the spin-chains is relatively large, so that they are magnetically well isolated from each other. As an inorganic material, CuGeO₃ was the first material identified to exhibit a spin Peierls transition [141].

  

Figure: Crystal structure of CuGeO₃. In the c-axis direction, CuO₂ chains are present. The lattice parameters are cited from Ref. [142]; the in-chain magnetic coupling parameters are from Ref. [143].

The crystal structure of CuGeO₃ is shown in Fig.52. A Cu²⁺ ion (S=1/2) is surrounded by six O^2- ions, which form a distorted CuO₆ octahedron. Cu²⁺ ions are bridged with four closer O^2- ions, making a CuO₂ chain (ribbon) in the c-axis direction. These chains are supported with Ge⁴⁺ ions, which locate at the tetrahedral site surrounded by four O^2- ions. Reflectivity and photoemission measurements [144] have shown that the $p\!-\!d$ hybridization of the CuO₂ chain is small. Hence the localized-moment picture of the Cu²⁺ spins should be good in CuGeO₃.

In the first report, Hase et al. [141] presented (1) an isotropic drop of susceptibility at $T_{\rm SP}$=14 K and (2) the field dependence of $T_{\rm SP}(H)\propto H^2$, as signatures of the spin Peierls transition in CuGeO₃. Since then, there have been many experiments performed on this material, and some of them report typical signatures of spin Peierls transition, as reviewed in the following.