J. Sichelschmidt

Relevance of Ferromagnetic Correlations for the Electron Spin Resonance in Kondo Lattice Systems

Electron spin resonance (ESR) measurements of the ferromagnetic (FM) Kondo lattice system CeRuPO show a well defined ESR signal which is related to the Ce3+ magnetism. In contrast, no ESR could be observed in the antiferromagnetic (AFM) homologue CeOsPO. Additionally, we detect an ESR signal in ferromagnetic YbRh while it was absent in a number of Ce or Yb intermetallic compounds with dominant AFM exchange. Thus, the observation of an ESR signal in a Kondo lattice is neither specific to Yb nor to the proximity to a quantum critical point, but seems to be connected to the presence of FM fluctuations. These conclusions not only provide a basic concept to understand the ESR in Kondo lattice systems even well below the Kondo temperature (as observed in YbRh2Si2) but point out ESR as a prime method to investigate directly the spin dynamics of the Kondo ion.

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

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 UPd2Al3. 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 YbRh2Si2, 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.

Sr2V3O9 and Ba2V3O9: Quasi-one-dimensional spin-systems with an anomalous low temperature susceptibility

The magnetic behavior of the low-dimensional vanadium oxides

Evolution of the Kondo state of YbRh2Si2 probed by high-field ESR.

{\mathrm{Sr}}_{2}{\mathrm{V}}_{3}{\mathrm{O}}_{9}

Electron spin resonance of YbIr2Si2 below the Kondo temperature

and

Strong electron correlations in FeSb2: An optical investigation and comparison with RuSb2

{\mathrm{Ba}}_{2}{\mathrm{V}}_{3}{\mathrm{O}}_{9}

Optical observation of non-Fermi-liquid behavior in the heavy fermion state of YbRh2Si2

was investigated by means of magnetic susceptibility and specific heat measurements. In both compounds, the results can be very well described by an

Hindered magnetic order from mixed dimensionalities in CuP2O6

S=\frac{1}{2}

Why could electron spin resonance be observed in a heavy fermion Kondo lattice

Heisenberg antiferromagnetic chain with an intrachain exchange of

Spin fluctuations with two-dimensional XY behavior in a frustrated S=1/2 square-lattice ferromagnet

J=82