Unconventional thermal metallic state of charge-neutral fermions in an insulator
Unconventional thermal metallic state of charge-neutral fermions in an insulator
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DOI：
10.1038/s41567-019-0552-2
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发表时间：
2019-09-01
期刊：
影响因子：
19.6
通讯作者：
Matsuda, Y.
中科院分区：
文献类型：
--
作者：
Sato, Y.;Xiang, Z.;Matsuda, Y.
在高磁场下，输运和热力学参数中的量子振荡是费米面的明确标志，费米面是金属的定义特征。因此，最近对绝缘SMB(6)和YbB12中的量子振荡的观察令人大吃一惊--尽管从电阻率的绝缘行为推断出了巨大的电荷间隙，但这些化合物似乎在高磁场下拥有费米表面。然而，零场中基态的性质却鲜有人研究。在这里，我们报告了使用低温热输运测量来发现YbB12基态中的无间隙、巡回、电荷中性的激发。在零场下，热容和导热系数中相当大的线性温度相关项在零温极限内被清晰地分辨出来，表明存在具有巡游性质的无缝费米子激发。值得注意的是，与温度有关的线性导热系数导致了对Wiedemann-Franz定律的惊人违反：洛伦兹比比传统金属中预期的大10(4)-10(5)倍，这表明YbB12是电荷绝缘体和热金属。此外，我们发现这些费米子与磁场耦合，尽管它们的电荷是中性的。我们的发现揭示了这种非传统量子态中的新准粒子。
Quantum oscillations in transport and thermodynamic parameters at high magnetic fields are an unambiguous signature of the Fermi surface, the defining characteristic of a metal. Recent observations of quantum oscillations in insulating SmB(6 )and YbB12, therefore, have been a big surprise-despite the large charge gap inferred from the insulating behaviour of the resistivity, these compounds seemingly host a Fermi surface at high magnetic fields. However, the nature of the ground state in zero field has been little explored. Here, we report the use of low-temperature heat-transport measurements to discover gapless, itinerant, charge-neutral excitations in the ground state of YbB12. At zero field, sizeable linear temperature-dependent terms in the heat capacity and thermal conductivity are clearly resolved in the zero-temperature limit, indicating the presence of gapless fermionic excitations with an itinerant character. Remarkably, linear temperature-dependent thermal conductivity leads to a spectacular violation of the Wiedemann-Franz law: the Lorenz ratio is 10(4) -10(5) times larger than that expected in conventional metals, indicating that YbB12 is a charge insulator and a thermal metal. Moreover, we find that these fermions couple to magnetic fields, despite their charge neutrality. Our findings expose novel quasiparticles in this unconventional quantum state.