Kitaev-Heisenberg候选材料Na$_2$Co$_2$TeO$_6$中热霍尔效应的声子-磁性二分法
Phononic-magnetic dichotomy of the thermal Hall effect in the Kitaev-Heisenberg candidate material Na$_2$Co$_2$TeO$_6$
AI总结：
本文研究了Kitaev量子材料Na$_2$Co$_2$TeO$_6$的热霍尔效应，发现其热霍尔信号由声子和磁性激发共同产生，指出揭示真正的量子化磁性热霍尔效应需分离声子与磁性贡献。
摘要：
作为Kitaev量子自旋液体基态涌现激发的马约拉纳费米子，构成了容错量子计算中极具前景的概念。在实验上，近期报道的Kitaev材料$α$-RuCl$_3$中的拓扑半量子化热霍尔效应，似乎证实了该材料磁性激发的马约拉纳本质。然而，有观点认为$α$-RuCl$_3$中的热霍尔信号更可能源于声子或拓扑磁振子，而非马约拉纳费米子。在此，我们研究了密切相关的Kitaev量子材料Na$_2$Co$_2$TeO$_6$的热霍尔效应，并表明其热霍尔信号至少由声子和磁性激发两个成分产生。这一二分法源于我们的发现：横向热导率$κ_{xy}$带有声子$κ_{xx}$的清晰特征，但在进入低温磁有序相时发生符号改变。我们通过详细考虑$κ_{xy}$和$κ_{xx}$对温度和磁场的依赖关系，系统性地分离了这两个成分。我们的结果表明，要在Kitaev拓扑量子自旋液体中揭示真正量子化的磁性热霍尔效应，必须解开声子与磁性贡献的纠缠，其中后者包括预期的马约拉纳费米子等潜在的分数化激发。
英文摘要：
Majorana fermions as emergent excitations of the Kitaev quantum spin liquid ground state constitute a promising concept in fault tolerant quantum computation. Experimentally, the recently reported topological half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ seems to confirm the Majorana nature of the material's magnetic excitations. It has been argued, however, that the thermal Hall signal in $α$-RuCl$_3$ rather stems from phonons or topological magnons than from Majorana fermions. Here we investigate the thermal Hall effect of the closely related Kitaev quantum material Na$_2$Co$_2$TeO$_6$, and we show that the thermal Hall signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the transversal heat conductivity $κ_{xy}$ carries clear signatures of the phononic $κ_{xx}$, but changes sign upon entering the low-temperature, magnetically ordered phase. We systematically resolve the two components by considering the detailed temperature and field dependence of both $κ_{xy}$ and $κ_{xx}$. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in a Kitaev topological quantum spin liquid requires to disentangle phonon vs. magnetic contributions where the latter include potentially fractionalized excitations such as the expected Majorana fermions.
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