Magnetothermal Transport in the Spin-1/2 Chains of Copper Pyrazine Dinitrate

We present experiments on the thermal transport in the spin-1/2 chain compound copper pyrazine dinitrate Cu(C₄H₄N₂)(NO₃)₂. The heat conductivity shows a surprisingly strong dependence on the applied magnetic field B, characterized at low temperatures by two main features. The first one appearing at low B is a characteristic dip located at μ_BB∼k_BT, that may arise from umklapp scattering. The second one is a plateaulike feature in the quantum critical regime, μ_B|B-B_c|<k_BT, where B_c is the saturation field at T=0. The latter feature clearly points towards a momentum and field-independent mean free path of the spin excitations, contrary to theoretical expectations.