Abstract
The presence of macroscopic phase separation into superconducting and magnetic phases in LaFeAsO1−xFx and CaFe1−xCoxAsF is demonstrated by muon spin rotation (μSR) measurement across their phase boundaries (x=0.06 for LaFeAsO1−xFx and x=0.075–0.15 for CaFe1−xCoxAsF). In LaFeAsO0.94F0.06, both magnetism and superconductivity develop simultaneously below a common critical temperature, Tm≃Tc≃18 K, where the magnetism is characterized by strong randomness. A similar, but more distinct segregation of these two phases is observed in CaFe1−xCoxAsF, where the magnetic phase retains Tm close to that of the parent compound (Tc≪Tm≃80–120 K) and the superconducting volume fraction is mostly proportional to the Co content x. The close relationship between magnetism and superconductivity is discussed based on these experimental observations. Concerning the superconducting phase, an assessment is made on the anisotropy of the order parameter in the superconducting state of LaFeAsO1−xFx, CaFe1−xCoxAsF and Ba1−xKxFe2As2 (x=0.4) based on the temperature dependence of superfluid density [ns(T)] measured by μSR. The gap parameter, 2Δ/kBTc, determined from ns(T) exhibits a tendency that values in the hole-doped pnictides (Ba1−xKxFe2As2) are much greater than those in electron-doped ones (LaFeAsO1−xFx and CaFe1−xCoxAsF), suggesting a difference in the coupling to bosons mediating the Cooper pairs between relevant d electron bands.
Highlights
The recent discovery of the iron pnictide superconductor LaFeAsO1−xFx (LFAO-F) over a fluorine concentration of 0.05 ≤ x ≤ 0.2 with the maximal critical temperature (Tc) of 26 K [1] and the following revelation of much increased Tc upon the substitution of La for other rare-earth elements (Ce, Pr, Nd, Sm,... leading to a maxium Tc of 55 K [2, 3, 4]) or the application of pressure for LFAO-F (∼43 K [5]) have triggered broad interest in the mechanism yielding a relatively high Tc in this new class of compounds
Recent results of the muon spin rotation/relaxation experiment on a variety of iron pnictide superconductors showed that the superfluid density ns may fall on the empirical line on the ns vs Tc diagram observed for the underdoped cuprates [19, 24], from which possibility of the common mechanism of superconductivity is argued between oxypnictides and cuprates
Recent investigations in electron-doped (n type) cuprates strongly suggest that such an electron-hole “asymmetry” is a manifestation of difference in the fundamental properties of underlying electronic states between these two cases, where the n type cuprates are much more like normal Fermi liquids rather than doped Mott insulators [26]. This might be readily illustrated by pointing out that, given all the doped carriers participate in the Cooper pairs, the insensitivity of Tc against the variation of ns (∝ x) observed in LFAO-F [1] cannot be reconciled with the above-mentioned empirical linear relation, while it is reasonably understood from the conventional BCS theory where condensation energy is predicted to be independent of carrier concentration
Summary
The recent discovery of the iron pnictide superconductor LaFeAsO1−xFx (LFAO-F) over a fluorine concentration of 0.05 ≤ x ≤ 0.2 with the maximal critical temperature (Tc) of 26 K [1] and the following revelation of much increased Tc upon the substitution of La for other rare-earth elements (Ce, Pr, Nd, Sm,... leading to a maxium Tc of 55 K [2, 3, 4]) or the application of pressure for LFAO-F (∼43 K [5]) have triggered broad interest in the mechanism yielding a relatively high Tc in this new class of compounds. Recent investigations in electron-doped (n type) cuprates strongly suggest that such an electron-hole “asymmetry” is a manifestation of difference in the fundamental properties of underlying electronic states between these two cases, where the n type cuprates are much more like normal Fermi liquids rather than doped Mott insulators [26] This might be readily illustrated by pointing out that, given all the doped carriers participate in the Cooper pairs (as suggested by experiment), the insensitivity of Tc against the variation of ns (∝ x) observed in LFAO-F [1] cannot be reconciled with the above-mentioned empirical linear relation, while it is reasonably understood from the conventional BCS theory where condensation energy is predicted to be independent of carrier concentration. Our μSR measurement in a LFAO-F sample with x = 0.06 (Tc ≃ 18 K) reveals that these two phases coexist in the form of macroscopic phase separation, and more interestingly, that a spin glass-like magnetic phase develops in conjunction with superconductivity in the paramagnetic phase [27] This accordance strongly suggests a common origin of the electronic correlation between these two competing phases. We examine the temperature dependence of ns in LFAO-F, CFCAF, and in Ba1−xKxFe2As2 (BKFA) [29], and discuss the degree of anisotropy in their superconducting order parameters and the strength of coupling to bosons that mediate the Cooper pairs
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