Positron lifetime studies of 100-MeV oxygen irradiated Pb-doped Bi-2223 superconductors

Positron lifetime studies have been carried out for unirradiated and 100-MeV oxygen ion irradiated Pb-doped Bi-2223 superconductors. The analysis of positron lifetime spectra revealed three lifetime components: a short lifetime, τ1 = 153–196 ps; an intermediate lifetime, τ2 = 269–339 ps; and a long lifetime, τ3 = 616–812 ps. A decrease in all the lifetime components, τ1, τ2 and τ3 with an increase of the relative intensities of I2 and I3 has been observed with increasing fluence. The positron lifetime results obtained are contradictory to the expected trend for the irradiated samples. This variation has been attributed to the formation of cationic clusters and their segregation at the grain boundaries; and probably in between the Bi–O planes too, where the positron density distribution is a maximum. This is highly possible since the metal atoms have a fair chance to get displaced from their original positions as a result of swift heavy ion irradiation. The growth of the Bi-2212 phase with irradiation supports the positron lifetime results

Magnon-magnon interactions in the Spin-Peierls compound CuGeO_3

In a magnetic substance the gap in the Raman spectrum, Delta_R, is approximatively twice the value of the neutron scattering gap, Delta_S, if the the magnetic excitations (magnons) are only weakly interacting. But for CuGeO_3 the experimentally observed ratio Delta_R/Delta_S is approximatively 1.49-1.78, indicating attractive magnon-magnon interactions in the quasi-1D Spin-Peierls compound CuGe_3. We present numerical estimates for Delta_R/Delta_S from exact diagonalization studies for finite chains and find agreement with experiment for intermediate values of the frustration parameter alpha. An analysis of the numerical Raman intensity leads us to postulate a continuum of two-magnon bound states in the Spin-Peierls phase. We discuss in detail the numerical method used, the dependence of the results on the model parameters and a novel matrix-element effect due to the dimerization of the Raman-operator in the Spin-Peierls phase.Comment: submitted to PRB, Phys. Rev. B, in pres