Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou, China.
Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, USA.
Nature. 2020 Mar;579(7797):51-55. doi: 10.1038/s41586-020-2052-z. Epub 2020 Mar 4.
A wide range of metals exhibit anomalous electrical and thermodynamic properties when tuned to a quantum critical point (QCP), although the origins of such strange metals have posed a long-standing mystery. The frequent association of strange metals with unconventional superconductivity and antiferromagnetic QCPs has led to the belief that they are highly entangled quantum states. By contrast, ferromagnets are regarded as an unlikely setting for strange metals, because they are weakly entangled and their QCPs are often interrupted by competing phases or first-order phase transitions. Here we provide evidence that the pure ferromagnetic Kondo lattice CeRhGe becomes a strange metal at a pressure-induced QCP. Measurements of the specific heat and resistivity under pressure demonstrate that the ferromagnetic transition is continuously suppressed to zero temperature, revealing a strange-metal behaviour around the QCP. We argue that strong magnetic anisotropy has a key role in this process, injecting entanglement in the form of triplet resonating valence bonds into the ordered ferromagnet. We show that a singular transformation in the patterns of the entanglement between local moments and conduction electrons, from triplet resonating valence bonds to Kondo-entangled singlet pairs at the QCP, causes a jump in the Fermi surface volume-a key driver of strange-metallic behaviour. Our results open up a direction for research into ferromagnetic quantum criticality and establish an alternative setting for the strange-metal phenomenon. Most importantly, strange-metal behaviour at a ferromagnetic QCP suggests that quantum entanglement-not the destruction of antiferromagnetism-is the common driver of the varied behaviours of strange metals.
当调整到量子临界点 (QCP) 时，各种金属都会表现出异常的电和热力学性质，尽管这些奇异金属的起源一直是个谜。奇异金属经常与非常规超导和反铁磁 QCP 相关联，这导致人们相信它们是高度纠缠的量子态。相比之下，铁磁体被认为不太可能是奇异金属的环境，因为它们的纠缠较弱，其 QCP 经常被竞争相或一级相变打断。在这里，我们提供的证据表明，纯铁磁 Kondo 晶格 CeRhGe 在压力诱导的 QCP 下成为奇异金属。在压力下对比热和电阻率的测量表明，铁磁转变被连续抑制到零温度，在 QCP 附近表现出奇异金属行为。我们认为，强磁各向异性在这个过程中起着关键作用，以三重态共振价态键的形式注入纠缠到有序铁磁体中。我们表明，局域磁矩和传导电子之间的纠缠模式发生了奇异的转变，从三重态共振价态键到 QCP 处的 Kondo 纠缠单态对，导致费米面体积的跳跃——这是奇异金属行为的关键驱动因素。我们的结果为铁磁量子临界性的研究开辟了一个方向，并为奇异金属现象建立了一个替代环境。最重要的是，铁磁 QCP 处的奇异金属行为表明，量子纠缠——而不是反铁磁性的破坏——是奇异金属各种行为的共同驱动因素。
