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Thermal Conductivity in the Frustrated Two-Leg Spin-Ladder System BiCu2PO6

We have measured temperature and magnetic-field dependences of the thermal conductivity of single crystals of the frustrated two-leg spin-ladder system BiCu2PO6 in magnetic fields up to 14 T. It has been found that the temperature dependence of the thermal conductivity along every principal crystallographic axis shows two peaks in zero field but that the magnitude of the thermal conductivity along the b-axis parallel to spin ladders, $κ$b, is significantly larger than those of the thermal conductivity along the a-axis, $κ$a, and along the c-axis, $κ$c, at high temperatures above 7 K. These results suggest that the thermal conductivity due to spins probably exists only in $κ$b. Furthermore, it has been found that both magnetic-field dependences of $κ$a and $κ$b at 3 K show kinks at ~ 7 T and ~ 10 T, where the spin state may change.