Spin dynamics in Kondo lattices at low temperatures and frequencies

近藤晶格（与传导电子相互作用的局部磁矩的晶格）是固体中强电子关联的模型系统。原则上，局域自旋的动力学可以用电子自旋共振（ESR）来研究。但是对于低温下的近藤晶格，人们认为局部磁矩被传导电子屏蔽，从而阻止了ESR。令人惊讶的是，重费米子材料YbRh2Si2在低温下表现出明显的ESR。这种ESR被解释为金属重费米子的共振，但它显然受到底层近藤晶格的局部矩的影响。考虑到YbRh2Si2的量子临界性质，对这种ESR的更深入的理解将增强我们对近藤晶格自旋动力学的一般知识。我们想进行低温ESR测量来研究YbRh2Si2的量子临界自旋动力学。这是第一次直接实验接近量子临界点的近藤晶格的自旋动力学。YbRh2Si2是一种已建立的量子临界模型材料，在这里，我们实验的相关范围是温度低于100 mK，磁场小于70 mT的量子临界场。我们采用超导平面谐振器，频率在1 GHz和15 GHz之间，在稀释冰箱中实现（温度低至约100 ℃）。30 mK）。我们新的实验技术开辟了一个以前无法访问的参数制度的ESR和他们的兴趣的低能量自旋动力学的研究，远远超出了我们的样品材料YbRh2Si2.Understanding ESR在非常低的温度一般需要新的理论方法，因为我们的特殊情况下YbRh2Si2接近量子临界点。在这里，我们将讨论近藤晶格中ESR的现有模型，但我们也将研究ESR的相关能量尺度（线宽）和其他实验（例如比热）之间的关系。结合新的实验ESR制度和理论方法将导致我们更深入地了解重费米子系统的量子临界性。

Kondo lattices (lattices of local magnetic moments that interact with conduction electrons) are model systems for strong electronic correlations in solids. In principle, the dynamics of local spins can be studied with electron spin resonance (ESR). But for the case of Kondo lattices at low temperatures, one expects that the local moments are shielded by conduction electrons, thus preventing ESR. Surprisingly, the heavy-fermion material YbRh2Si2 exhibits pronounced ESR at low temperatures. This ESR is interpreted as a resonance of the metallic heavy fermions, but it is clearly impacted by the local moments of the underlying Kondo lattice. A deeper understanding of this ESR, taking into account the quantum-critical nature of YbRh2Si2, will enhance our general knowledge concerning the spin dynamics of Kondo lattices.We want to perform low-temperature ESR measurements to study the quantum-critical spin dynamics of YbRh2Si2. This is the first ever direct experimental access to the spin dynamics of a Kondo lattice close to a quantum-critical point. YbRh2Si2 is an established model material of quantum criticality, and here the relevant regime for our experiments is temperatures below 100 mK and magnetic fields smaller than the quantum-critical field of 70 mT. We employ superconducting planar resonators at frequencies between 1 GHz and 15 GHz, implemented in a dilution refrigerator (for temperatures down to approx. 30 mK). Our novel experimental techniques open up a previously unaccessible parameter regime for ESR and they are of interest for low-energy studies of spin dynamics that go far beyond our sample material YbRh2Si2.Understanding ESR at very low temperatures in general requires new theoretical approaches, as does our particular case of YbRh2Si2 close to a quantum critical point. Here, we will address the existing models for ESR in Kondo lattices, but we will also study the relation between the relevant energy scales of ESR (line width) and other experiments (e.g. specific heat). Combining new experimental ESR regimes and theoretical approaches will lead us to a deeper insight of quantum criticality of heavy-fermion systems.