Spin gap and hole pairing of Sr(14-x)A(x)Cu(24)O(41)(A = Ca and La) singlecrystals studied by the electrical resistivity and thermal conductivity

We have measured the electrical resistivity rho and the thermal conductivit y kappa of Sr(14-x)A(x) Cu24O41 (A = Ca and La) single crystals. The Arrhen ius plot of ln rho vs T-1 gives two kinds of activation energy with a bound ary temperature T-rho. The activation energy at T < T-rho is in approximate agreement with the spin gap in the ladder estimated from the NMR measureme nts, suggesting that holes in the ladder are paired and localized at T < T- rho. The observed kappa has been analyzed to be composed of kappa(ph), kapp a(spin) and kappa(hole) due to phonons, spins and holes, respectively. The kappa(ph) exhibits a small peak at similar to 30 K in every direction of ev ery single-crystal. The contribution of kappa(spin) is observed along the c -axis except for x(Ca) greater than or equal to 6, and the spin gap, which corresponds to the spin excitation from spin-singlet to spin-triplet, has b een estimated to be similar to 420 K. For x(Ca) greater than or equal to 6, the spin gap, which corresponds to the destruction of spin-singlet pairs i .e. the dissociation of hole pairs, has been estimated from kappa along the c-axis at T > T-rho to decrease with increasing x(Ca).