Studies on Effects of Impurity Doping and NMR Measurements of La 1111 and/or Nd 1111 Fe-Pnictide Superconductors

Abstract

Measurements of the electrical resistivities ρ, Hall coefficients R H , thermoelectric powers S, electronic specific heat coefficients γ have been carried out for samples of LnFe 1-y M y AsO 1-x F x (Ln=La, Nd; M=Co, Mn; x=0.11) obtained by M atom dopings to the superconducting LnFeAsO 1-x F x (Ln1111) system. The NMR longitudinal relaxation rates 1/T 1 have also been measured for samples of LaFe 1-y Co y AsO 1-x F x with various x values. Co atoms doped to the superconducting LnFeAsO 1-x F x are nonmagnetic, and the T c -suppression rate ⎪dT c /dx⎪ by the Co atoms has been found to be too small to understand by the pair breaking effect expected for the S ± superconducting symmetry proposed as the most probable one for the system. It throws a serious doubt whether the symmetry is realized in the system. Instead of the pair breaking, two mechanisms of the T c -suppression by the doped impurities have been found: One is the electron localization, which appears when the sheet resistance R exceeds h/4e 2 =6.45 kΩ, and another is the disappearance (or reduction) of the hole-Fermi-surfaces around the Γ point in the reciprocal space. The latter mechanism has been observed, when the electron number increases with increasing Co-doping level and the system changes from an "anomalous metal" to an ordinary one. On the two distinct T dependences of the NMR longitudinal relaxation rate 1/T 1 of LaFeAsO 1-x F x , 1/T 1 ∝T 6 reported by our group in the T region from T c to ~0.4 T c for samples with the highest T c values with varying x, and 1/T 1 ∝T 2.5-3.0 observed by many groups in the almost entire T region studied below T c , we discuss what the origin of the difference is, and show that, at least, the T 2.5-3.0 -like dependence of 1/T 1 cannot be considered as the experimental evidence for the S ± symmetry of ∆.KEYWORDS: LaFe 1-y M y AsO 1-x F x (M=Co, Mn), NMR relaxation rate, 1/T 1 , transport properties, specific heats

Unusual disorder effects in superconductingLaFeAs1−δO0.9F0.1as revealed byHammerath1, Drechsler2, Grafe3 et al. 2010Phys. Rev. B

“…3 we show the normalized T −1 1 (T ) curves for our As-deficient sample and the samples from Ref. 15. Their nominal clean sample as well as the Co-doped one exhibit nearly the same T −1 1 (T )-dependence as our As-deficient sample, whereas our clean sample exhibits the T 3 -dependence (see Fig 2).…”

“…[11][12][13][14][15][16][17] Also the available weak coupling fits of the upper critical fields B c2 (T ) for the Nd-1111, the Sm-1111, and the La-1111 systems, [18][19][20] all closely related to the ones considered here, do not show a dominant interband pairing interaction generic for the intended s ± -scenario but result at most in comparable intraband and interband coupling strengths or even in dominant intraband ones. Finally, for the two-gap system FeSe 1−x only a tiny interband coupling has been derived from the T -dependence of the penetration depth.…”

“…To the best of our knowledge, no exponential but powerlaw dependencies ∝ T n have been observed 15,29,[33][34][35][36][37][38][39][40][41][42][43] for all Fe-based pnictide superconductors, with n in between 1.5 and 6, indicating unconventional superconductivity. These power law dependencies have been discussed within different models, such as s ± -and dwave symmetries, including the possibility of multiple SC gaps.…”

“…[38][39][40][41] Within the 122 family heavily overdoped compounds such as KFe 2 As 2 exhibit the lowest value observed so far whereas optimally or slightly underdoped compounds show the largest values. Recently, it has been suggested, 15,33 that the frequently observed T 3 powerlaw in the NSLRR should not be considered as an intrinsic effect but instead be attributed to some unspecified inhomogeneities in view of the missing correlation between the T c -value and the NSLRR exponent, while higher exponents would occur for cleaner samples. However, we find just the opposite behavior.…”

Strange Inter-Layer Properties of Ba(Fe1-xCox)2As2 Appearing in Ultrasonic Measurements

“…[6] However, Sato et al and Lee et al rejected this theory with their experimental results on the impurity effect for several iron-based superconductors. [7,8] On the other hand, Kontani et al and Yanagi et al proposed a conventional s-wave state without sign reversal mediated by orbital fluctuation. [9,10] Figure 1 shows the crystal structure of BaFe 2 As 2 , which is tetragonal at room temperature.…”