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 the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation (µSR) measurements. Zero-field µSR reveals the occurrence of small spontaneous static magnetic fields with the onset of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field µSR is nearly flat approaching T = 0 K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal symmetry breaking in SrPtAs are allowed. Out of these, a dominantly d + id (chiral d-wave) order parameter is most consistent with our experimental data. Transition metal pnictides have attracted considerable scientific interest as they present the second largest family of superconductors after the cuprates [1]. All superconductors of this family share one common structural feature: superconductivity takes place in a square lattice formed by the transition metal elements. Very recently superconductivity with a T c of 2.4 K has been discovered in SrPtAs [2], which has a unique and attractive structural feature: It crystallizes in a hexagonal structure with weakly coupled PtAs layers forming a honeycomb lattice. SrPtAs supports three pairs of split Fermi surfaces, two of which are hole-like and centered around the Γ-point with a cylindrical shape extended along the k z direction and together host only about 30% of the density of states. The remaining 70% of the density of states are hosted by the third pair of split Fermi surfaces that is electron-like, centered around the K and K ′
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.
A study of the temperature and field dependence of the penetration depth of the superconductor RbFe2As2(Tc=2.52 K) was carried out by means of muon-spin rotation measurements. In addition to the zero-temperature value of the penetration depth (0)=267(5) nm, a determination of the upper critical field Bc2(0)=2.6(2) T was obtained. The temperature dependence of the superconducting carrier concentration is discussed within the framework of a multigap scenario. Compared to the other "122" systems which exhibit much higher Fermi level, a strong reduction in the large gap BCS ratio 2Δ/kBTc is observed. This is interpreted as a consequence of the absence of interband processes. Indications of possible pair-breaking effect are also discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.