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.

To learn whether the N170 would be differentially affected by orientation inversion of same- and other-race faces, we recorded event-related brain potentials (ERPs) in Caucasian and locally resident Asian subjects viewing upright and upside-down front-view photographs of African, Asian, and Caucasian faces. Results show that the time-to-peak was significantly delayed for inverted Caucasian faces in both subject groups. The same-race N170 peaked later than either other-race N170 in the Caucasian, but showed no significant difference between Caucasian and Asian faces in the Asian participants. Inversion, therefore, appears to affect the N170’s latency predominantly for faces of familiar races. This conclusion gains support from a positive correlation between the latency of the N170 evoked by both upright and inverted Caucasian faces and the amount of time that the Asian participants had lived in Europe. The N170’s race- and inversion-dependent latency increase may, thus, express an impairment of processing that emphasizes individuation of familiar-race faces, and develops during familiarization.

Superconducting materials are of great interest both for the fundamental understanding of electrons in solids as well as for a range of different applications. Studying superconductors with microwaves offers a direct experimental access to the electrodynamic response of these materials, which in turn can reveal fundamental material properties such as the superconducting penetration depth. Here we describe two different techniques to study superconductors at microwave frequencies: the broadband Corbino approach can cover frequencies from the MHz range up to 50 GHz continuously but is limited to thin-film samples whereas the stripline resonators are sensitive enough to study low-loss single crystals but reveal data only at a set of roughly equidistant resonant frequencies. We document the applicability of these two techniques with data taken on an ultrathin TaN film and a single crystal of the heavy-fermion superconductor CeCu 2 Si 2 , respectively.

Broadband microwave spectroscopy can probe material properties in wide spectral and temperature ranges. The quality of such measurements crucially depends on the calibration, which also removes from the obtained spectra signatures of standing waves. Here we consider cryogenic Corbino-type reflection measurements on superconductors close to the critical temperature. We show that the non-linear sample response, which relates to sample heating, can lead to strong signatures of standing waves even in a well-calibrated Corbino spectrometer. We demonstrate our findings with microwave measurements as a function of frequency, power, and temperature and for different length of the microwave transmission line. Finally we note such non-linear effects beyond the case of superconductors by probing a VO 2 thin film at the insulator-metal transition. arXiv:1810.07784v3 [cond-mat.supr-con]

The electronic properties of heavy‐fermion metals are governed by mobile charge carriers with drastically enhanced effective masses. Correspondingly, the characteristic energy scales of their optical response are much lower than for conventional metals: the transport relaxation of heavy fermions can be as low as a few GHz. Signatures of heavy‐fermion superconductivity and quantum criticality are also found or at least expected in this frequency range. To probe these charge dynamics at extremely low frequencies, novel techniques of microwave spectroscopy have to be employed. As described by Scheffler et al. (), devices based on planar microwave transmission lines are a powerful approach for this goal. Such experiments reveal the frequency‐dependent conductivity (i.e. the charge response), but they can also be used to study spin dynamics via electron spin resonance (ESR). This opens new possibilities to experimentally explore the spin dynamics of the heavy‐fermion material YbRh2Si2 throughout its phase diagram including the vicinity of the quantum‐critical point.

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