We have measured the thermal conductivity along the a-axis parallel to the one-dimensional (1D) spin chain, κa, of the Haldane gap system Y2 BaNiO5. As a result, we have observed thermal conductivity due to spins, κsp, as large as at least 10 W / K m around 100 K corresponding to the magnitude of the Haldane gap. The large κsp may be due to the large bandwidth of spin excitations. Otherwise, it may be attributed to the ballistic thermal conduction due to spins which is theoretically expected in Haldane gap systems with the spin quantum number S = 1.
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