Microwave spectroscopy on heavy‐fermion systems: Probing the dynamics of charges and magnetic moments

Abstract

Investigating solids with light gives direct access to charge dynamics, electronic and magnetic excitations. For heavy fermions, one has to adjust the frequency of the probing light to the small characteristic energy scales, leading to spectroscopy with microwaves. We review general concepts of the frequency-dependent conductivity of heavy fermions, including the slow Drude relaxation and the transition to a superconducting state, which we also demonstrate with experimental data taken on UPd 2 Al 3 . We discuss the optical response of a Fermi liquid and how it might be observed in heavy fermions. Microwave studies with focus on quantum criticality in heavy fermions concern the charge response, but also the magnetic moments can be addressed via electron spin resonance (ESR). We discuss the case of YbRh 2 Si 2 , the open questions concerning ESR of heavy fermions, and how these might be addressed in the future. This includes an overview of the presently available experimental techniques for microwave studies on heavy fermions, with a focus on broadband studies using the Corbino approach and on planar superconducting resonators.

Surface-resistance measurements using superconducting stripline resonators

“…4 Many of these materials display features worth taking a closer look in this frequency and temperature region, making our method a useful tool in an active field of present research. …”

“…75 Candidate materials with even higher T c are NbN and NbTiN, 76,77 and these feature also higher critical magnetic field, which is required for the study of magnetic-field induced phase transitions. 4 …”

“…48,49 While coplanar resonators have advantages compared to stripline and microwave resonators in terms of achievable quality factors, scalability, and ease of fabrication, it is difficult to combine them with bulk samples if one is interested in quantitative information about the sample's microwave properties. 4 Therefore, we employ superconducting stripline resonators. 50 While previous applications of this technique focused on thin film samples, [35][36][37][38][39] DiIorio et al also demonstrated measurements on a bulk sample of gold as a reference.…”

Metallic coplanar resonators optimized for low-temperature measurements

“…Planar structures (microstrip, stripline and coplanar) have already been employed for ESR experiments [15,[36][37][38][39][40][41][42][43][44][45][46]. Although microstrip and stripline resonators have also been successfully applied in ESR measurements [36-38, 41, 45, 47], coplanar configurations might be more appropriate candidates since they carry the signal non-dispersively in contrast to a microstrip configuration [45,48].…”

“…ESR studies have already been done on this material using conventional and superconducting planar resonators [45,49]. In figure 5(c) we plot the ESR lines obtained at 1.7 K for two different resonance fields.…”