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.

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With increasing miniaturization in industry and medical technology, non-destructive testing techniques are an area of ever-increasing importance. In this framework, X-ray microscopy offers an efficient tool for the analysis, understanding, and quality assurance of microscopic samples, in particular as it allows reconstructing three-dimensional data sets of the whole sample's volume via computed tomography (CT). The following article describes a compact X-ray microscope in the hard X-ray regime around 9 keV, based on a highly brilliant liquid-metal-jet source. In comparison to commercially available instruments, it is a hybrid that works in two different modes. The first one is a micro-CT mode without optics, which uses a high-resolution detector to allow scans of samples in the millimeter range with a resolution of 1 μm. The second mode is a microscope, which contains an X-ray optical element to magnify the sample and allows resolving 150 nm features. Changing between the modes is possible without moving the sample. Thus, the instrument represents an important step towards establishing high-resolution laboratory-based multi-mode X-ray microscopy as a standard investigation method.

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Laboratory-based X-ray computed tomography (CT) with resolutions around 100nm per line pair is technically feasible since a few years [1]. However, this measurement principle is still not widely usedespecially not for industrial and medical applications. Reasons for this are high costs, the complex operation of systems based on optical elements, as well as the restricted flexibility of their field-of-view. As applied development center for X-ray technology, the Fraunhofer EZRT is currently developing an easy-to-handle, flexible setup for industrial use. The setup uses standard projection geometry, a small Xray source and large detector pixel; thus it operates using high geometric magnification. Due to significant advances made on the component side combined with a high stability of the setup, we are able to resolve in standard operation 150nm lines and spaces. Compared to modified SEM X-ray sources, the sample does not need to be placed in vacuum [3]. Moreover, due to the source's peak energy of 60kV, sample diameter and material can be chosen relatively flexible.For our setup we utilize an X-ray tube developed by Excillum in the framework of the NanoXCT Project, which is now commercially available as "NanoTube". In our acquisition geometry, the penumbral blurring of the source's focal spot defines the achievable resolution. Besides a thin transmission target combined with a suited design of the electron optics to guarantee a small source size, also high spatial spot stability is required for computed tomography. The latter is established by advanced temperature and noise control. In order to provide good heat dissipation, the transmission target is made of tungsten deposited on a 100µm diamond window. With this, we reach a source size diameter of a few hundred nanometers.Although the brightness of the spot is high, the total emitted flux is still relatively low, which is a major drawback compared to an optics-based setup [4]. Thus, we need an efficient detector and a large solid angle to achieve reasonably short exposure times. Therefore, we use 750µm CdTe as sensor material with a quantum efficiency greater than 80% over the full spectrum. The Hybrid Photon Counting "SANTIS" detector possesses 2048 x 514 pixel with 75µm edge length, and was provided by DECTRIS. Each pixel of a dedicated readout chip is directly bonded to a semiconducting X-ray sensor [2]. Besides the high efficiency due to the photon counting technique, it possesses virtually zero noise. Furthermore, two energy thresholds enable us to perform dual-energy computed tomography without additional filters. Moreover, the quasi-rectangular point spread function of the detector supersedes oversampling.Large solid angles are obtained by a short source-detector-distance of minimum 380mm. This also requires small distances between the source and sample. Therefore, an optical collision avoidance system is implemented to be able to adjust source-object-distances below 500µm without risking a damage of the source's window. In combination with this, the syst...