Spin Gap and Hole Pairing in the Spin-Ladder Cuprate Sr_14-xA_xCu₂₄O₄₁ (A=Ca and La) Studied by the Thermal Conductivity

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We have systematically measured the thermal conductivity κ of Sr_14-xA_xCu₂₄O₄₁ (A=Ca, La: 0≤ x(Ca) ≤ 9: 0 ≤ x(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 x(Ca) ≥ 6. The spin gap in the ladder estimated from κ_spin is ∼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 κ_hole is observed at high temperatures for larger x(Ca) values. The activation energy of holes in the ladder estimated from κ_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.