Spin gap of Sr14-xAxCu24-yMyO41 (A = Ca,La: M = Zn,Ni) studied by thermal conductivity

We have investigated the spin gap and hole pairing in the ladder of Sr(14-x)A(x)Cu(24-y)M(y)O(41) (A = Ca,La: M = Zn,Ni) from the thermal conductivity measurements. The observed thermal conductivity kappa has been analyzed to be composed of kappa (ph), kappa (spin) and kappa (hole) due to phonons, spins and holes, respectively, kappa (ph) shows a peak or a shoulder at similar to 30 K in every direction of every crystal. kappa (spin) is observed only along the c-axis and exhibits a broad peak around 100-150 K. The broad peak is depressed through the Zn substitution more markedly than the Ni substitution. The spin gap in the ladder estimated from K-spin is similar to 400 K and independent of x. This corresponds to the spin excitation from spin-singlet to spin-triplet. kappa (hole) becomes marked at high temperatures for larger x(Ca) values. The spin gap in the ladder estimated from kappa (hole) decreases with increasing x(Ca). This corresponds to the gap due to the dissociation of spin-singlet pairs, i.e. the dissociation of holt: pairs. (C) 2000 Elsevier Science Ltd. All rights reserved.