The effect of doping on the spin-Peierls (SP) transition in single crystals of CuGehas been investigated using neutron scanering techniques and susceptibility measurements. When the Cuz+ site is partly substituted by either ZnZ+ (4%) or Nilt (1.5%) the SP is suppressed. and a long-range N&l state develops below TN = 2.8 K (a) and T,, = 2.2 K (Ni) with a wave vector of (0.1, I). This observation contrasts with earlier reports of a spin glass m s i d o n in the doped compounds in the same temperature range. Our results also indicate that the reported N&l state in the nominally pure compound results from impurities. Preliminary mea~urements of the Zn-doped sample show that the gap that opens in the magnetic excitations below Tsp collapses in the doped compound.

The dispersion of the spin-waves in single crystals of Li2CuO2 has been investigated by means of inelastic neutron scattering. The results yield a single spin-wave branch characterized by a large gap ∆ = 1.4 meV at the zone centre due to easy-axis uniaxial anisotropy. Linear spin-wave theory is used to obtain the exchange integrals in this material. It turns out that the nearest-neighbor exchange interactions in the basal plane are antiferromagnetic leading to frustration between the magnetic moments. Our results show that Li2CuO2 is an S = 1/2 antiferromagnetic insulator with competing interactions.

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