Spin dynamics in Kondo lattices at low temperatures and frequencies

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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.