The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

We report on the synthesis of large single crystals of a new FeSe layer superconductor Cs(0.8)(FeSe(0.98))(2). X-ray powder diffraction, neutron powder diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs(0.8)(FeSe(0.98))(2) with those of the recently discovered potassium intercalated system K(x)Fe(2)Se(2). The new compound, Cs(0.8)(FeSe(0.98))(2), shows a slightly lower superconducting transition temperature (T(c) = 27.4 K) in comparison to 29.5 in (K(0.8)(FeSe(0.98))(2)). The volume of the crystal unit cell increases by replacing K by Cs-the c parameter grows from 14.1353(13) to 15.2846(11) Å. For the alkali metal intercalated layered compounds known so far, (K(0.8)Fe(2)Se(2) and Cs(0.8)(FeSe(0.98))(2)), the T(c) dependence on the anion height (distance between Fe layers and Se layers) was found to be analogous to those reported for As-containing Fe superconductors and Fe(Se(1 - x)Ch(x)), where Ch = Te, S.

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 present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1−xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57 Fe Mössbauer spectroscopy and muon spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5) µB at the iron site below TN = 138 K, well separated from a structural phase transition at TS = 156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below TN are reproduced. PACS numbers: 76.75.+i, 76.80.+y, 75.30.Fv, The recently discovered Fe-based superconductors LaFeAsO 1−x F x [1] and the related materials in which La is substituted by Sm, Ce, Nd, Pr, and Gd, respectively [2,3,4,5,6,7] has triggered an intense research in the oxypnictides. Besides the high critical temperature above 50 K there are further striking similarities to the properties of the high-T C cuprates. The oxypnictides have a layered crystal structure with alternating FeAs and LaO sheets, where the Fe atoms are arranged on a simple square lattice [1]. Theoretical studies reveal a two-dimensional electronic structure [8] and it is believed that conductivity takes place mainly in the FeAs layers while the LaO layers provide the charge reservoir when doped with F ions. Again similar as in the cuprates, superconductivity emerges when doping a magnetic mother compound with electrons or holes and thereby supressing the magnetic order [9]. This suggests an interesting interplay between magnetism and superconductivity and, indeed, a recent theoretical work suggests that magnetic fluctuations associated with quantum critical point are essential for superconductivity in the electron doped LaFeAsO 1−x F x superconductors [10].However, in contrast to the cuprates, the magnetic mother compound is not a Mott-Hubbard insulator but a poor metal. A large covalency in the FeAs layers was found [8,11], which in the case of tetragonal LaFePO, i.e. the compound where As is replaced by P, leads to a non-magnetic ground state [12,13]. In contrast, in LaFeAsO there is an additional structural distortion at elevated temperatures [14,15] and a long range spin density wave (SDW) antiferromagnetic order has been observed in neutron scattering experiments on powder samples below ∼150 K. [15]. First principle calculations yield antiferromagnetic order with Fe magnetic moments ranging from 1.5 µ B to 2.3 µ B [10,16,17,18], while the neutron scattering experiments indicate a much smaller value. Assuming that the full sample volume is contributing to the magnetic scattering an ordered moment of ∼ 0.35 µ B [15] is inferred from the weak superlattice reflections in powder neutron diffraction. A local probe measurement, which could verify...

We report on a detailed investigation of the electronic phase diagram of FeSe1-x under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure 0.8 GPa the nonmagnetic and superconducting FeSe1-x enters a region where static magnetic order is realized above Tc and bulk superconductivity coexists and competes on short length scales with the magnetic order below Tc. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.