  We used a spectroscopic microwave technique utilizing superconducting
stripline resonators at frequencies between 3 GHz and 15 GHz to examine the
charge dynamics of YbRh2Si2 at temperatures and magnetic fields close to the
quantum critical point. The different electronic phases of this heavy-fermion
compound, in particular the antiferromagnetic, Fermi-liquid, and
non-Fermi-liquid regimes, were probed with temperature-dependent microwave
measurements between 40 mK and 600 mK at a set of different magnetic fields up
to 140 mT. Signatures of phase transitions were observed, which give
information about the dynamic response of this peculiar material that exhibits
field-tuned quantum criticality and pronounced deviations from Fermi-liquid
theory.
