Effects of Ru Doping on the Transport Behavior and Superconducting Transition Temperature of NdFeAsO_0.89F_0.11

Abstract

The transport behavior and superconducting transition temperature T c of NdFe 1-y Ru y AsO 0.89 F 0.11 have been studied for various y values. Because Ru impurities are isoelectronic to host Fe atoms, we basically expect that the number of electrons does not change with y, at least in the region of small y values. The results indicate that the rate of T c suppression by Ru atoms is too small to be explained by the pair breaking effect of nonmagnetic impurities expected for the S ± symmetry, confirming our previous results for Co doping. , various studies have been carried out to identify the symmetry of their superconducting order parameter ∆. In many of these studies, much effort has been made to find experimental evidence for the S ± symmetry proposed theoretically at the early stage of the study.2, 3) For such a symmetry, reflecting the sign difference between the order parameters on disconnected Fermi surfaces around the Γ [= (0, 0)] and M [= (π, 0)] points in the reciprocal space, important features can be expected in several observable physical quantities: Neutron inelastic scattering measurements have been carried out to find the so-called "resonance peak" in the magnetic excitation spectra χ"(Q, ω)4-6) expected in the superconducting phase around a point in the scattering vector(Q)-energy(ω) space, 7,8) and an observed peak has been discussed in relation to the "resonance peak". After the confirmation of the singlet state of Cooper pairs by Knight-shift measurements, 9, 10) the temperature (T) dependence of the NMR longitudinal relaxation rate 1/T 1 has been extensively discussed, and the absence of the coherence peak has been pointed out by many research groups.11-15) The T dependence described by the relation 1/T 1 ∝T n with n∼3 has been reported below T c in almost the entire T region studied for LaFeAsO 1-x F x 11, 13,14) and LaFeAsO 1-δ .12) These results have been discussed to be favorable for the S ± symmetry.The effects of impurity doping can provide information on the relative signs of the order parameters on Fermi surfaces around the Γ and M points, and on the basis of Co doping studies, we have emphasized that the observed rate of T c decrease due to the doping of nonmagnetic impurities is too small to be explained by the pair breaking effect expected for the S ± symmetry of the order parameter. 9,[15][16][17][18][19] The result seems to be consistent with those of studies carried out by neutron 20) and α-particle 21) irradiations. Onari and Kontani have pointed out from the theoretical side that the above data of doping effects cannot be explained by the pair breaking of nonmagnetic impurities. 22) Regarding the magnetic excitation spectra χ"(Q, ω), it has been pointed 23) out that a "peak" in χ"(Q, ω) is also expected for the S symmetry of the order parameter, which has no sign difference between the Fermi surfaces around the Γ and M points, suggesting that we have to be careful in arguing whether the observed data really indicate the existence of the "resonance peak". On the T dependence of...

Tuning the magnetic and structural phase transitions of PrFeAsO via Fe/Ru spin dilution

“…The complete suppression of the SDW ordering is determined to be around x = 0.6 which is similar to the concentration inferred from previous resistivity measurements 8 . Perhaps coincidentally, this is very close to the disruption of superconductivity by Ru/Fe substitution in F-optimally doped 1111 [21][22][23][24] . This value may be very significant as discussed below.…”

Anomalous phonon redshift in K-dopedBaFe2As2iron pnictides

“…4 Indeed, on one hand in-and outof-plane dopings affect T c and the unpaired quasiparticle density in the BaFe 2 As 2 family, 5 but on the other hand, superconductivity is robust against impurities in 1111 materials. 6,7 Furthermore, a large iron isotope effect has been reported in SmFeAsO 1−x Fe x and Ba 1−x K x Fe 2 As 2 , 8 indicating that electron-phonon coupling plays some role in the paring mechanism. By taking these facts into account, Kontani et al proposed that electron-phonon coupling arising from the Fe-ion oscillation can induce orbital fluctuations, mediating the s ++ paring without sign reversal.…”

Enhancement of low-frequency fluctuations and superconductivity breakdown in Mn-dopedLa1−yYyFeAsO0.89F0.11<

“…If one concentrates on the LnFeAsO 1−z F z (Ln1111) family, with Ln a lanthanide ion, one notices that spinless impurities introduced by substituting Fe by Ru, cause a very weak effect both on the magnetic (z = 0) [3][4][5][6] and on the superconducting (z = 0.11) [7][8][9][10][11] ground state. One has to introduce almost 60% of Ru to quench either one of the two phases.…”