Probing the enigmatic Fermi surface of SmB6 by low-energy electrodynamics

在相关电子物理领域，六硼化钐（SmB6）是研究最多但也最令人困惑的材料之一。传统上被认为是近藤绝缘体，SmB6最近被解释为第一个具有电子相关性的拓扑绝缘体。这个概念需要一个由拓扑表面态的电荷引起的二维费米表面，但是最近在SmB6上备受瞩目的量子振荡结果严重矛盾：一项研究报告了二维费米表面的特征，而另一项研究表明了三维费米表面。后一种结果不能与SmB6在低温下真正绝缘的事实相一致，它引发了大量新的理论概念，其中一些概念引入了与固体中常规电子完全不同的外来准粒子。在这个项目中，我们想用低能电动力学的方法来阐明SmB6的费米表面的性质。特别是，我们想要检测和表征SmB6的回旋共振。这将是对费米曲面性质的独立实验，不管后者是二维的还是三维的。在传统固体中，回旋共振是通过在静态磁场中对样品施加微波电场来探测的。对于SmB6，对相关准粒子的有效质量和散射率的数值没有一致的看法，甚至不清楚它们是载流子还是中性粒子。因此，在这个阶段，我们无法预测在哪个磁场和激励频率的组合以及在哪个温度以下会观察到SmB6的回旋共振。因此，我们将使用各种实验技术（频率在45 MHz至1.3太赫兹之间，磁场高达8 T，温度低至20 mK，并可选择用电或磁高频场选择性激发）来揭示SmB6的电动力学响应。这些极宽参数范围的实验将通过回旋共振探测揭示费米表面的关键信息，此外，它们还将帮助我们回答SmB6性质中的另一个难题，即先前报道的SmB6的太赫兹电导率（远高于已建立的直流电导率的预期）是否由可能产生SmB6三维费米表面的相同神秘准粒子引起。

In the field of correlated electron physics, samarium hexaboride (SmB6) is one of the most studied and yet most puzzling materials. Traditionally considered a Kondo insulator, SmB6 was more recently interpreted as the first topological insulator with electronic correlations. This notion calls for a two-dimensional Fermi surface caused by the charges of the topological surface states, but recent high-profile quantum-oscillation results on SmB6 are gravely conflicting: one study reported signatures for a two-dimensional Fermi surface whereas another indicated a three-dimensional Fermi surface. The latter result cannot be reconciled with SmB6 being truly insulating in the bulk at low temperature, and it has sparked a plethora of new theoretical concepts, some of them invoking exotic quasiparticles quite different from regular electrons in solids.In this project we want to elucidate the nature of the Fermi surface of SmB6 by means of low-energy electrodynamics. In particular, we want to detect and characterize the cyclotron resonance of SmB6. This will be an independent experimental access to the properties of the Fermi surface, regardless of whether the latter is two- or three-dimensional. In conventional solids, cyclotron resonance is probed by applying a microwave electric field to a sample in a static magnetic field. For the case of SmB6, there is no consensus on the values of effective mass and scattering rate of the relevant mobile quasiparticles, and it is not even clear whether they are charge carriers or neutral. Therefore, at this stage, we cannot predict at which combination of magnetic field and excitation frequency and below which temperature the cyclotron resonance of SmB6 will be observable. Hence we will use a variety of experimental techniques (at frequencies between 45 MHz and 1.3 THz, at magnetic fields up to 8 T, at temperatures down to 20 mK, and with option to selectively excite with either electric or magnetic high-frequency field) to unriddle the electrodynamic response of SmB6. These experiments in extremely wide parameter ranges will reveal key information on the Fermi surface via detection of cyclotron resonance, and in addition they will help us to answer another puzzle in the properties of SmB6, namely whether the previously reported THz conductivity of SmB6, which is much higher than expected from the established dc conductivity, is caused by the same enigmatic quasiparticles that possibly generate the three-dimensional Fermi surface of SmB6.