75As-NMR Studies of LaFeAsO1-xFxfor VariousxValues

Kobayashi, Yoshihiko; Satomi, Erika; Lee, Sang Chul; Sato, Masatoshi

doi:10.1143/jpsj.79.093709

Cited by 20 publications

(19 citation statements)

References 23 publications

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“…One is the role of AFSF. In cuprates, it has been believed that AFSF plays a crucial role to induce high-T c superconductivity, but the situation in LaFeAsO 1−x F x is still unclear [5,[12][13][14][15][16]. Some previous studies by NMR found no AFSF [12][13][14][15].…”

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confidence: 99%

“…The third unresolved issue is the superconducting gap symmetry. The spin-lattice relaxation rate (1/T 1 ) decreases sharply below T c , but the data were insufficient for distinguishing between d-wave from sign-reversal s-wave [5,[12][13][14][15][16]. From other experimental probes, some measurements suggested the existence of node [17], but the photoemission spectroscopy and the point contact Andreev reflection measurement suggested a nodeless gap [18,19].…”

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confidence: 99%

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Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAs
T
¹
,
Li
²
,
Kawasaki
³

et al. 2012
Phys. Rev. Lett.
75
11
79
5
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We report a systematic study by 75 As nuclear-quadrupole resonance in LaFeAsO1−xFx. The antiferromagnetic spin fluctuation (AFSF) found above the magnetic ordering temperature TN = 58 K for x = 0.03 persists in the regime 0.04 ≤ x ≤ 0.08 where superconductivity sets in. A domeshaped x-dependence of the superconducting transition temperature Tc is found, with the highest Tc = 27 K at x = 0.06 which is realized under significant AFSF. With increasing x further, the AFSF decreases, and so does Tc. These features resemble closely the cuprates La2−xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T1) below Tc decreases exponentially down to 0.13 Tc, which unambiguously indicates that the energy gaps are fully-opened. The temperature variation of 1/T1 below Tc is rendered nonexponential for other x by impurity scattering.

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confidence: 99%

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confidence: 99%

Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAs
T
¹
,
Li
²
,
Kawasaki
³

et al. 2012
Phys. Rev. Lett.
75
11
79
5
View full text Add to dashboard Cite

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“…In the moderately doped LaFeAs(O,F) and LiFeAs, superconductivity is realized in the absence of strong critical fluctuations. [18,19,28,38] The present perovskite systems possess about twofold higher T c than those of LaFeAs(O,F) and LiFeAs, however there is no clear indication that the spin fluctuation develops compared with them. This suggests that the development of the lowenergy spin fluctuation is not important for achieving high T c .…”

Section: Resultsmentioning

confidence: 66%

“…The pseudogap behavior in the similar temperature range has been seen for La1111 systems in the lightly or optimally doped region without the magnetic ordering: x = 0.07 (T c = 22.5 K) or x = 0.11 (T c = 21 K). [18,28] According to band calculations, the pseudogap behavior indicates the presence of the high density of states just below the Fermi level, [29] which mainly originates in the band at (π, π) in unfolded Brillouin zone (so-called γ pocket if it crosses the Fermi level). [30] If the γ pocket takes part in the Fermi surface, the nesting between the γ pocket and the electron-like pockets (so-called β) yields the lowenergy spin fluctuations which strongly develops towards low temperatures; however if it is absent, the pseudogap appears and the moderates temperature dependence of 1/T 1 T is realized.…”

Section: Resultsmentioning

confidence: 99%

NMR investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As
Tomita
¹
,
Kotegawa
²
,
Tao
³

et al. 2012
Phys. Rev. B

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75 As and 45 Sc NMR measurements unravel the electronic state for Fe-based superconductors with perovskite-type blocking layers Ca4(Mg,Ti)3Fe2As2O8−y (T onset c = 47 K) and Ca5(Sc,Ti)4Fe2As2O11−y (T onset c = 41 K). In Ca5(Sc,Ti)4Fe2As2O11−y, the nuclear spin relaxation rate 1/T1 shows pseudogap behavior below ∼ 80 K, suggesting that the electronic state is similar to that of LaFeAs(O,F) system with moderate electron doping. The presence of the pseudogap behavior gives an interpretation that the hole-like band (so-called γ pocket) is located just below the Fermi level from the analogy to LaFeAs(O,F) system and the disappearance of the γ pocket yields the suppression of the low-energy spin fluctuations. On the other hand, in Ca4(Mg,Ti)3Fe2As2O8−y satisfying the structural optimal condition for higher Tc among the perovskite systems, the extrinsic contribution, which presumably originates in the Ti moment, is observed in 1/T1T ; however, the moderate temperature dependence of 1/T1T appears by its suppression under high magnetic field. In both systems, the high Tc of ∼ 40 K is realized in the absence of the strong development of the low-energy spin fluctuations. The present results reveal that the structural optimization does not induce the strong development of the low-energy spin fluctuations. If we consider that superconductivity is mediated by spin fluctuations, the structural optimization is conjectured to provide a benefit to the development of the high-energy spin fluctuations irrespective to the low-energy part.

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“…First, effects of impurity-doping on the superconducting transition temperature T c of the systems have been mainly studied by several members of our group [4][5][6][7][8][9][10][11][12][13][14][15] for the following reasons. The spin-fluctuation mechanism predicts the S symmetry, 2,3) where the signs of the superconducting order parameters are opposite between the disconnected Fermi surfaces around the and M points in the reciprocal space.…”

Section: Introductionmentioning

confidence: 99%

On the Magnetic Excitation Spectra of Ba(Fe_0.9Co_0.1)₂As₂in the Superconducting State

et al. 2011

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Magnetic excitation spectra of Ba(Fe 0.9 Co 0.1 ) 2 As 2 obtained for aligned crystals with the superconducting transition temperature T c 23 K are presented after an overview of results of our preceding studies on the symmetry of the order parameter . Although many researchers have reported that their experimental results are consistently explained by the so-called S symmetry, and believe that the spin-fluctuation is relevant to the superconducting mechanism of Fe pnictide superconductors. However, the situation is not so simple, because almost all these data can be understood by the S ++ -symmetry, too. Furthermore, as we first pointed out and discussed in detail, the rates of the T c suppression by nonmagnetic impurities can hardly be explained by the S symmetry. Here, we show our own data of the magnetic excitation spectra of Ba(Fe 0.9 Co 0.1 ) 2 As 2 , and discuss which one of the S and S ++ -symmetries can better describe the characteristics of the peak observed there. These processes are important, because the S and S ++ symmetries are connected with the spin-fluctuation mechanism and a newly proposed one related to the orbital degrees of freedom of the systems, respectively.

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⁷⁵As-NMR Studies of LaFeAsO_1-xF_xfor VariousxValues

Cited by 20 publications

References 23 publications

Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAs
T
¹
,
Li
²
,
Kawasaki
³

et al. 2012
Phys. Rev. Lett.
75
11
79
5
View full text Add to dashboard Cite

Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAs
T
¹
,
Li
²
,
Kawasaki
³

et al. 2012
Phys. Rev. Lett.
75
11
79
5
View full text Add to dashboard Cite

NMR investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As
Tomita
¹
,
Kotegawa
²
,
Tao
³

et al. 2012
Phys. Rev. B

On the Magnetic Excitation Spectra of Ba(Fe_0.9Co_0.1)₂As₂in the Superconducting State

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75As-NMR Studies of LaFeAsO1-xFxfor VariousxValues

Cited by 20 publications

References 23 publications

Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAsT1, Li2, Kawasaki3 et al. 2012Phys. Rev. Lett.7511795View full textAdd to dashboardCite

Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAsT1, Li2, Kawasaki3 et al. 2012Phys. Rev. Lett.7511795View full textAdd to dashboardCite

NMR investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2AsTomita1, Kotegawa2, Tao3 et al. 2012Phys. Rev. B

On the Magnetic Excitation Spectra of Ba(Fe0.9Co0.1)2As2in the Superconducting State

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⁷⁵As-NMR Studies of LaFeAsO_1-xF_xfor VariousxValues

Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAs
T
¹
,
Li
²
,
Kawasaki
³

et al. 2012
Phys. Rev. Lett.
75
11
79
5
View full text Add to dashboard Cite

Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAs
T
¹
,
Li
²
,
Kawasaki
³

et al. 2012
Phys. Rev. Lett.
75
11
79
5
View full text Add to dashboard Cite

NMR investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As
Tomita
¹
,
Kotegawa
²
,
Tao
³

et al. 2012
Phys. Rev. B

On the Magnetic Excitation Spectra of Ba(Fe_0.9Co_0.1)₂As₂in the Superconducting State