We studied the temperature-pressure phase diagram of EuFe 2 As 2 by electrical resistivity measurements. The spin-density-wave transition at T 0 associated with the FeAs-layers is continuously suppressed with increasing pressure, while the antiferromagnetic ordering temperature of the Eu 2+ moments seems to be nearly pressure independent up to 2.6 GPa. Above 2 GPa a sharp drop of the resistivity, ͑T͒, indicates the onset of superconductivity at T c Ϸ 29.5 K. Surprisingly, on further reducing the temperature, ͑T͒ is increasing again and exhibiting a maximum caused by the ordering of the Eu 2+ moments, a behavior which is reminiscent of reentrant superconductivity as it is observed in the ternary Chevrel phases or in the rare-earth nickel borocarbides.

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.

Neutron diffraction experiments have been performed on a magnetically ordered CeCu2Si2 single crystal exhibiting A-phase anomalies in specific heat and thermal expansion. Below T(N) approximately 0.8 K antiferromagnetic superstructure peaks have been detected. The propagation vector of the magnetic order appears to be determined by the topology of the Fermi surface of heavy quasiparticles as indicated by renormalized band-structure calculations. The observation of long-range incommensurate antiferromagnetic order as the nature of the A phase in CeCu2Si2 suggests that a spin-density-wave instability is the origin of the quantum critical point in CeCu2Si2.

The magnetic, transport and optical properties of EuFe2As2 single crystals have been investigated parallel and perpendicular to the ab-plane. The anisotropy ρc/ρ ab ≈ 8 depends only slightly on temperature. In both orientations, the spin-density wave transition at TSDW = 189 K shows up as a considerable increase in the dc resistivity. Susceptibility measurements evidence the magnetic order of the Eu 2+ moments at TN = 19 K with little influence on the electronic transport taking place in the FeAs layers. Polarization-dependent infrared spectroscopy reveals strongly anisotropic optical properties and yields a carrier density of only 4.2 × 10 21 cm −3 and a bandmass of m b = 2m0. A sizeable Drude contribution is present at all temperatures and narrows upon cooling. Below TSDW, the spin-density-wave gap develops in the in-plane optical conductivity; no appreciable change is detected for the perpendicular polarization. Modifications in the phonon features are associated with changes of the electronic properties at TSDW. The extended Drude analysis yields a linear behavior of the frequency-dependent scattering rate below TSDW, indicating an interaction between the charge carriers and spin fluctuations in the spin-density-wave state.PACS numbers: 75.30.

The magnetic structure of the Eu 2+ moments in the superconducting EuFe2(As1−xPx)2 sample with x = 0.15 has been determined using element specific x-ray resonant magnetic scattering. Combining magnetic, thermodynamic and scattering measurements, we conclude that the long range ferromagnetic order of the Eu 2+ moments aligned primarily along the c axis coexists with the bulk superconductivity at zero field. At an applied magnetic field ≥ 0.6 T, superconductivity still coexists with the ferromagnetic Eu 2+ moments which are polarized along the field direction. We propose a spontaneous vortex state for the coexistence of superconductivity and ferromagnetism in EuFe2(As0.85P0.15)2.