Generation of spin currents from one-dimensional quantum spin liquid

Spin-Seebeck effects (SSEs) in a one-dimensional quantum spin liquid (QSL) system have been investigated in a Sr₂CuO₃/Pt hybrid structure. Sr₂CuO₃ contains one-dimensional spin- 1 2 chains in which typical spinons in QSL have been confirmed. Heat-induced voltage measured in a clean Pt/Sr₂CuO₃ exhibits anomalous sign reversal with decreasing temperature, the negative component of which can be attributed to the spinon-induced SSE. However, the SSE was found to be critically decreased upon the exposure of Sr₂CuO₃ to air, which can be associated with the chemical degradation of the interface of Sr₂CuO₃. Despite the drastic change in the SSE signals, properties of the one-dimensional QSL are little changed in the spin susceptibility as well as the thermal conductivity of Sr₂CuO₃. The SSE signal is also sensitive to the purity of Sr₂CuO₃; it is suppressed with a decrease in the purity of the primary compounds of the Sr₂CuO₃. The result indicates that the spinon-induced SSE in Sr₂CuO₃ is sensitive to the bulk condition due to the one-dimensional atomic channel for spin transport in Sr₂CuO₃. In a carefully prepared Sr₂CuO₃/Pt sample, we found that the spinon-induced SSE signal is tolerant to magnetic fields; it increases linearly with the field even up to 9 T. In contrast, SSEs are suppressed under such a high field in ferrimagnetic insulators Y₃Fe₅O₁₂ or paramagnetic insulators Gd₃Ga₅O₁₂, which is caused by the Zeeman gap in the spin-wave or paramagnetic spin excitations. The robustness of the spinon-induced SSE is consistent with the Tomonaga-Luttinger liquid theories.