Thermal conductivity due to spins in the frustrated two-leg spin ladder system BiCu₂PO₆

We have measured the thermal conductivity in the frustrated two-leg spin ladder system of Bi(Cu₁−_xZn_x)₂PO₆ single crystals with x = 0 and 0.02 with the spin quantum number S = 1=2 to investigate the thermal conductivity due to spins, κ_spin. For x = 0, analogous temperature dependences of the thermal conductivity along the a-axis perpendicular to the spin ladders, κ_a, and that along the b-axis parallel to the spin ladders, κ_b, have been observed, indicating that the thermal conductivity due to phonons is dominant in both κ_a and κ_b. Moreover, it has been found that a small hump exists at ∼15K only in κb, which becomes clear upon the application of magnetic fields, and is suppressed by the partial substitution of nonmagnetic Zn for Cu. Thus, the hump has been concluded to be due to the contribution of κ_spin. The magnitude of κ_spin normalized by the square of the nearest-neighbor exchange interaction, J², as a function of the temperature normalized by the magnitude of the spin gap, Δ, has been found to be roughly the same as that in another two-leg S = 1=2 spin ladder system, Ca₉La₅Cu₂₄O₄₁. This means that κ_spin exists universally in two-leg S = 1=2 spin ladder systems and is roughly scaled by J² and Δ and that the spin frustration has little influence on κ_spin.