  We theoretically study a spin-spin correlation enhanced by non-magnetic
impurities in a frustrated two-leg spin ladder. The frustration is introduced
by the next-nearest-neighbor antiferromagnetic interaction in the leg direction
in the antiferromagnetic two-leg spin ladder. The spin-spin correlation
function around impurity site is calculated by the density-matrix
renormalization-group method. We find that the spin-spin correlation is
enhanced around impurity site with the wavenumber reflecting the frustration.
As increasing the frustration, the wavenumber is shifted from commensurate to
incommensurate. We discuss several experimental results on BiCu2PO6 in the
light of our theory.
