Majorana quantization and half-integer thermal quantum Hall effect in a Kitaev spin liquid
Majorana quantization and half-integer thermal quantum Hall effect in a Kitaev spin liquid
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DOI：
10.1038/s41586-018-0274-0
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发表时间：
2018-07-12
期刊：
影响因子：
64.8
通讯作者：
Matsuda, Y.
中科院分区：
文献类型：
--
作者：
Kasahara, Y.;Ohnishi, T.;Matsuda, Y.
二维电子气中的量子霍尔效应涉及拓扑保护的无耗散充电电流沿着样品边缘的流动。整数或分数电导分别与带有分数电荷的电子或准粒子的边缘电流相关。据预测，量子霍尔现象也可以由具有根本不同起源的边缘电流产生：量子自旋的分裂。然而，这种量子化尚未被观察到。在这里，我们报告了在绝缘二维量子磁体 (1) α-RuCl3 中观察到的这种霍尔效应量子化，在二维蜂窝晶格 (2-7) 上具有主导的基塔耶夫相互作用（依赖于键的伊辛型相互作用）。我们发现，施加与样品平行的磁场会破坏长程磁序，从而产生场诱导的量子自旋液体基态，并具有大量局部自旋纠缠(8-12)。在此状态的低温状态下，二维热霍尔电导达到作为所施加磁场的函数的量子平台，并且其量子化值恰好是整数量子霍尔效应的二维热霍尔电导的一半。大块材料中热霍尔电导的半整数量子化是电荷中性马约拉纳费米子（粒子本身是反粒子）的拓扑保护手性边缘电流的特征，其自由度是传统费米子的一半 (13-16)。这些结果表明，自旋分裂为流动的马约拉纳费米子和 Z(2) 通量，预计这会发生在 Kitaev 量子自旋液体 (1,3) 中。在临界磁场之上，量子化消失，热霍尔电导迅速变为零，表明具有和不具有手性马约拉纳边缘模式的状态之间存在拓扑量子相变。量子磁体中涌现的马约拉纳费米子预计将对强相关量子物质产生巨大影响，从而开启在相对高温下拓扑量子计算的可能性。
The quantum Hall effect in two-dimensional electron gases involves the flow of topologically protected dissipationless charge currents along the edges of a sample. Integer or fractional electrical conductance is associated with edge currents of electrons or quasiparticles with fractional charges, respectively. It has been predicted that quantum Hall phenomena can also be created by edge currents with a fundamentally different origin: the fractionalization of quantum spins. However, such quantization has not yet been observed. Here we report the observation of this type of quantization of the Hall effect in an insulating two-dimensional quantum magnet(1), alpha-RuCl3, with a dominant Kitaev interaction (a bond-dependent Ising-type interaction) on a two-dimensional honeycomb lattice(2-7). We find that the application of a magnetic field parallel to the sample destroys long-range magnetic order, leading to a field-induced quantum-spin-liquid ground state with substantial entanglement of local spins(8-12). In the low-temperature regime of this state, the two-dimensional thermal Hall conductance reaches a quantum plateau as a function of the applied magnetic field and has a quantization value that is exactly half of the two-dimensional thermal Hall conductance of the integer quantum Hall effect. This half-integer quantization of the thermal Hall conductance in a bulk material is a signature of topologically protected chiral edge currents of charge-neutral Majorana fermions (particles that are their own antiparticles), which have half the degrees of freedom of conventional fermions(13-16). These results demonstrate the fractionalization of spins into itinerant Majorana fermions and Z(2) fluxes, which is predicted to occur in Kitaev quantum spin liquids(1,3.) Above a critical magnetic field, the quantization disappears and the thermal Hall conductance goes to zero rapidly, indicating a topological quantum phase transition between the states with and without chiral Majorana edge modes. Emergent Majorana fermions in a quantum magnet are expected to have a great impact on strongly correlated quantum matter, opening up the possibility of topological quantum computing at relatively high temperatures.