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

An extended investigation of the electronic phase diagram of FeSe1−x up to pressures of p 2.4 GPa by means of ac and dc magnetization, zero field muon spin rotation (ZF µSR), and neutron diffraction is presented. ZF µSR indicates that at pressures p ≥ 0.8 GPa static magnetic order occurs in FeSe1−x and occupies the full sample volume for p 1.2 GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above p ≥ 1.2 GPa both the superconducting transition temperature Tc and the magnetic ordering temperature TN increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe1−x, located on the line connecting the Se atoms along the c-direction. Different magnetic structures are proposed and checked by combining the muon stopping calculations with a symmetry analysis, leading to a similar structure as in the LaFeAsO family of Fe-based superconductors. Furthermore, it is shown that the magnetic moment is pressure dependent and with a rather small value of µ ≈ 0.2 µB at p 2.4 GPa.

Polarized Raman-scattering spectra of superconducting, single-crystalline FeSe evidence pronounced phonon anomalies with temperature reduction. A large (∼ 6.5%) hardening of the B1g(Fe) phonon mode is attributed to the suppression of local fluctuations of the iron spin state with the gradual decrease of the iron paramagnetic moment. The ab-initio lattice dynamic calculations support this conclusion. The enhancement of the low-frequency spectral weight above the structural phase transition temperature Ts and its change below Ts is discussed in relation with the opening of an energy gap between low (S = 0) and higher spin states which prevents magnetic order in FeSe. The very narrow phonon line widths compared to observations in FeTe suggests the absence of intermediate spin states in the fluctuating spin state manifold in FeSe.

The topological insulator Bi 2 Se 3 shows a Raman scattering response related to topologically protected surface states amplified by a resonant interband transition. Most significantly this signal has a characteristic Lorentzian lineshape and spin-helical symmetry due to collision dominated scattering of Dirac states at the Fermi level E F on bulk valence states. Its resonance energy, temperature and doping dependence points to a high selectivity of this process. Its scattering rate (Γ ≈ 40 cm -1 ≈ 5 meV) is comparable to earlier observations, e.g. in spin-polaron systems.Although the observation of topological surface states in Raman scattering is limited to resonance conditions, this study represents a quite clean case which is not polluted by symmetry forbidden contributions from the bulk. PACS: 73.43.Lp, 73.20.At, 63.22.-m A 2u (1) 137.29 129 * A 2u (2) 161.35 160 *

Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered