Thermal-transport study of spin excitations in quantum spin liquid state

量子自旋液体(QSL)是量子自旋的类流体状态，其长程有序态被量子涨落破坏。QSL的基态及其奇异现象几十年来一直被广泛讨论，但至今仍未确定。我们对新发现的QSL候选体进行了热输运测量，发现在零温极限下热导率与温度有相当大的线性关系，表现出平均自由程较长(~1000晶格距离)的无间隙激发。如此长的平均自由程显示了QSL作为具有长程相干性的量子凝聚态的新特征。

Quantum spin liquids(QSLs) are fluid-like states of quantum spins where its long-range ordered state is destroyed by quantum fluctuations. The ground state of QSL and its exotic phenomena, which have been extensively discussed for decades, have yet to be identified. We employ thermal transport measurements on newly discovered QSL candidates, and find a sizable linear temperature dependence of thermal conductivity in the zero-temperature limit, showing gapless excitation with a long mean free path(~ 1, 000 lattice distances). Such a long mean free path demonstrates a novel feature of QSL as a quantum-condensed state with long-distance coherence.