We report zero-field muon spin relaxation ͑SR͒ measurements on RFeAsO with R = La, Ce, Pr, and Sm. We study the interaction of the FeAs and R ͑rare-earth͒ electronic systems in the nonsuperconducting magnetically ordered parent compounds of RFeAsO 1−x F x superconductors via a detailed comparison of the local hyperfine fields at the muon site with available Mössbauer spectroscopy and neutron-scattering data. These studies provide microscopic evidence of long-range commensurate magnetic Fe order with the Fe moments not varying by more than 15% within the series RFeAsO with R = La, Ce, Pr, and Sm. At low temperatures, long-range R magnetic order is also observed. Different combined Fe and R magnetic structures are proposed for all compounds using the muon site in the crystal structure obtained by electronic potential calculations. Our data point to a strong effect of R order on the iron subsystem in the case of different symmetry of Fe and R order parameters resulting in a Fe spin reorientation in the R-ordered phase in PrFeAsO. Our symmetry analysis proves the absence of collinear Fe-R Heisenberg interactions in RFeAsO. A strong Fe-Ce coupling due to non-Heisenberg anisotropic exchange is found in CeFeAsO which results in a large staggered Ce magnetization induced by the magnetically ordered Fe sublattice far above T N Ce . Finally, we argue that the magnetic R-Fe interaction is probably not crucial for the observed enhanced superconductivity in RFeAsO 1−x F x with a magnetic R ion. LaFeAsO CeFeAsO PrFeAsO SmFeAsO FIG. 2. ͑Color online͒ Muon-spin precession frequency as a function of reduced temperature for RFeAsO with R = La, Ce, Pr, and Sm. Inset: Magnetic signal fraction for RFeAsO, with R = La, Ce, Pr, and Sm. Lines are guides to the eyes. INTERPLAY OF RARE EARTH AND IRON MAGNETISM… PHYSICAL REVIEW B 80, 094524 ͑2009͒ 094524-3 APPENDIX 1. Magnitude and symmetry of dipole fields created by the iron and rare-earth subsystems at the A-type muon site

We report upper critical field B c2 (T ) data for LaO 0.9 F 0.1 FeAs 1−δ in a wide temperature and field range up to 60 T. The large slope of B c2 ≈ -5.4 to -6.6T/K near an improved T c ≈ 28.5 K of the in-plane B c2 (T ) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Pauli-limited behavior (PLB) with B c2 (0) ≈ 63 to 68 T. We interpret a similar hitherto unexplained flattening of the B c2 (T ) curves reported for at least three other disordered closely related systems as the Co-doped BaFe 2 As 2 , the (Ba,K)Fe 2 As 2 , or the NdO 0.7 F 0.3 FeAs (all single crystals) for applied fields H (a, b) also as a manifestation of PLB. Their Maki parameters have been estimated analyzing their B c2 (T ) data within the Werthamer-Helfand-Hohenberg approach. The pronounced PLB of (Ba,K)Fe 2 As 2 single crystals obtained from a Sn-flux is attributed also to a significant As deficiency detected by wave length dispersive x-ray spectroscopy as reported by Ni N et al. 2008 Phys. Rev. B 78 014507. Consequences of our results are discussed in terms of disorder effects within conventional (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), e.g. p-and d-wave SC, can be discarded for our samples. The increase of dB c2 /dT | Tc by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity driven transition from s ± USC to CSC of an extended s ++ -wave state or to a stabilized s ± -state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with B c2 data from the literature which show often no PLB for fields below 60 to 70 T probed so far. A novel disorder related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.

For the purpose of using plasmonics in an integrated scheme where single emitters can be probed efficiently, we experimentally and theoretically study the scattering properties of single nanorod gold antennas as well as antenna arrays placed on one-dimensional dielectric silicon nitride waveguides. Using real space and Fourier microscopy correlated with waveguide transmission measurements, we quantify the spectral properties, absolute strength, and directivity of scattering. The scattering processes can be well understood in the framework of the physics of dipolar objects placed on a planar layered environment with a waveguiding layer. We use the single plasmonic structures on top of the waveguide as dipolar building blocks for new types of antennas where the waveguide enhances the coupling between antenna elements. We report on waveguide hybridized Yagi-Uda antennas which show directionality in out-coupling of guided modes as well as directionality for in-coupling into the waveguide of localized excitations positioned at the feed element. These measurements together with simulations demonstrate that this system is ideal as a platform for plasmon quantum optics schemes as well as for fluorescence lab-on-chip applications.

Using muon-spin rotation, we studied the in-plane (λ ab ) and the out of plane (λc) magnetic field penetration depth in SrFe1.75Co0.25As2 (Tc ≃ 13.3 K). Both λ ab (T ) and λc(T ) are consistent with the presence of two superconducting gaps with the gap to Tc ratios 2∆/kBTc = 7.2 and 2.7. The penetration depth anisotropy γ λ = λc/λ ab increases from γ λ ≃ 2.1 at Tc to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume.

We present a straightforward method to realize non-planar dielectric structures with a controlled height profile for use in calibration of fluorophores. Calibration of fluorescence quantum efficiency and intrinsic radiative and nonradiative decay rates of emitters is possible by using changes in the local density of optical states, provided one can control the emitter-surface distance with nanometer accuracy. We realize a method that is accurate yet fast to implement. We fabricate PMMA wedges (4 mm×4 mm×2 µm) by gray-tone UV-lithography of Shipley S1813G2. Its applicability as dielectric spacer is demonstrated in Drexhage experiments for three different emitters in the visible and nearinfrared wavelength regime. The decay-rate dependence of the fluorescent state of emitters on the distance to a silver mirror is observed and compared to calculations based on the local density of states. Quantitative values for (non)radiative decay rates and quantum efficiencies are extracted. Furthermore, we discuss how Drexhage experiments can help to scrutinize the validity of effective material parameters of metamaterials in the near field regime.