Spin gap and hole pairing in the spin-ladder cuprate Sr14-xAxCu24O41 (A=Ca and La) studied by the thermal conductivity

We have systematically measured the thermal conductivity kappa of Sr(14-x)A(x)Cu(24)O(41) (A=Ca, La: 0 less than or equal to x(Ca) less than or equal to 9: 0 less than or equal to x(La) less than or equal to 5) single crystals, in order to investigate the spin gap and hole pairing. The observed kappa has been analyzed to he composed of kappa (ph),kappa (spin) and kappa (hole) due to phonons, spins and holes, respectively. The contribution of kappa (spin) is observed only along the c-axis except for x(Ca) greater than or equal to 6. The spin gap in the ladder estimated from kappa (spin) is similar to 400 K and independent of x. This is regarded as corresponding to the spin excitation from spin-singlet to spin-triplet, as those estimated from the neutron scattering and Raman scattering measurements. The contribution of kappa (hole) is observed at high temperatures for larger x(Ca) values. The activation energy of holes in the ladder estimated from kappa (hole) decreases with increasing x(Ca). This is regarded as corresponding to the dissociation of hole pairs, as that estimated from the electrical resistivity measurements. These results are consistent with the recent suggestion that most of holes are paired and localized in the ladder at low temperatures.