Spin gap and hole pairing in the spin-ladder cuprate Sr_{14-cursive Greek chi}A_{cursive Greek chi}Cu₂₄O₄₁ (A=Ca and La) studied by the thermal conductivity

We have systematically measured the thermal conductivity κ of Sr_{14-cursive Greek chi}A_{cursive Greek chi}Cu₂₄O₄₁ (A=Ca, La: 0 ≤ cursive Greek chi(Ca) ≤ 9: 0 ≤ cursive Greek chi(La) ≤ 5) single crystals, in order to investigate the spin gap and hole pairing. The observed κ has been analyzed to be composed of κ_ph, κ_spin and κ_hole due to phonons, spins and holes, respectively. The contribution of κ_spin is observed only along the c-axis except for cursive Greek chi(Ca) ≥ 6. The spin gap in the ladder estimated from κ_spin is ∼400K and independent of cursive Greek chi. 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 κ_hole is observed at high temperatures for larger cursive Greek chi(Ca) values. The activation energy of holes in the ladder estimated from κ_hole decreases with increasing cursive Greek chi(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.