75As-NQR/NMR Studies on Oxygen-Deficient Iron-Based Oxypnictide Superconductors LaFeAsO1-y ( y = 0, 0.25, 0.4) and NdFeAsO0.6

Mukuda, H.; Terasaki, Nobuyuki; Kinouchi, Hiroaki; Yashima, M.; Kitaoka, Y.; Suzuki, Shigenori; Miyasaka, S.; Tajima, S.; Miyazawa, Keisuke; Shirage, Parasharam M.; Kitô, Hijiri; Eisaki, H.; Iyo, A.

doi:10.1143/jpsj.77.093704

Cited by 133 publications

(76 citation statements)

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“…Our penetration depth result of full gap is in good correspondence with such an extended s-wave state (or s ± state) with no nodes in both gaps in these two bands. In addition, such a sign change would suppress the coherence peak in the NMR relaxation rate, in agreement with the experiments [10,11,13,14]. We note that other theories [40,41,42] are also consistent with the fully gapped superconductivity.…”

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confidence: 89%

Microwave Penetration Depth and Quasiparticle Conductivity ofPrFeAsO1−ySingle Crystals: Evidence for a Full-Gap Superconductor

et al. 2009

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In-plane microwave penetration depth λ ab and quaiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor PrFeAsO1−y (Tc ≈ 35 K) by using a sensitive superconducting cavity resonator. λ ab (T ) shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap ∆(k). The temperature dependence of the superfluid density demonstrates that the gap is non-zero (∆/kBTc 1.6) all over the Fermi surface. The microwave conductivity below Tc exhibits an enhancement larger than the coherence peak, reminiscent of high-Tc cuprate superconductors.PACS numbers: 74.25. Nf, 74.20.Rp, 74.25.Fy Since the discovery of superconductivity in, high transition temperatures (T c ) up to 56 K have been reported in the doped Fe-based oxypnictides [2,3,4,5,6,7,8,9]. The nature of superconductivity and the pairing mechanism in this system are fundamental physical problem of crucial importance. The first experimental task to this problem is to elucidate the superconducting pairing symmetry, which is intimately related to the pairing interaction.The NMR Knight-shift measurements appear to indicate the spin-singlet pairing [10,11]. However, the superconducting gap structure, particulary the presence or absence of nodes in the gap, is highly controversial. The specific heat shows a nonlinear magnetic field dependence [12]. The NMR relaxation rate shows the absence of the coherence peak and the T 3 -dependence below T c [10,11,13,14]. The lower critical field exhibits a T -linear dependence at low temperatures [15]. The µSR experiments report an unusual field-dependence of the penetration depth [16]. In the point-contact spectroscopy, a zero-bias conductance peak is reported [17,18]. These results have been interpretated as an indication of unconvensional superconductivity with line nodes. On the other hand, the Andreev reflection data are found to be consistent with an isotropic gap [19]. All of these experiments have been performed by using polycrystalline samples. Definitely, measurements using single crystals are highly desired to obtain unambiguous conclusions on the superconducting gap structure.In this paper, we report on the measurements of the complex surface impedance in underdoped single crystals of the oxypnictide superconductor PrFeAsO 1−y (T c ≈ 35 K), from which properties of thermally excited quasiparticles can be directly deduced. Since the recent NMR experiments of the Pr-based iron oxypnictide suggest the non-magnetic state in the superconducting samples [11], PrFeAsO 1−y seems suitable for the penetration depth study [20,21]. Moreover, PrFeAsO 1−y has a higher T c than that of La-compounds, which enables the measurements in a wider temperature range. We observe flat temperature dependence of the in-plane penetration depth λ ab (T ) at low temperatures, indicating exponentially small quasiparticle excitations, which clearly contradicts the presence of nodes in the gap. The quasiparticle conductivity is enhanced compared with ...

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confidence: 89%

Microwave Penetration Depth and Quasiparticle Conductivity ofPrFeAsO1−ySingle Crystals: Evidence for a Full-Gap Superconductor

et al. 2009

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“…This excess NMR relaxation rate is generally a clear signature of line nodes, inconsistent with a simple s (including s ± ) gap. [106,107,108,109,110] A similar conclusion is reached from penetration depth measurements, i.e. that a non-exponential density of states is present.…”

Section: Superconductivitysupporting

confidence: 58%

Electronic structure of Fe-based superconductors

Singh

2009

Physica C: Superconductivity

139

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“…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.…”

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confidence: 99%

Unusual disorder effects in superconductingLaFeAs1−δO0.9F0.1as revealed by
Hammerath
¹
,
Drechsler
²
,
Grafe
³

et al. 2010
Phys. Rev. B

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We report 75 As NMR measurements of the spin-lattice relaxation in the superconducting state of LaFeAsO0.9F0.1 and As-deficient LaFeAs 1−δ O0.9F0.1. The temperature behavior of 1/T1 below Tc changes drastically from a T 3 -dependence for LaFeAsO0.9F0.1 to a T 5 -dependence for the Asdeficient sample. These results, together with the previously reported unexpected increase of Tc and the slope of the upper critical field near Tc for the As-deficient sample, are discussed in terms of non-universal SC gaps in Fe-pnictides and the effect of As deficiency as an exotic case where nonmagnetic 'smart' impurities even stabilize an s±−wave superconductor or within a scenario of a disorder-driven change to s++-superconductivity.PACS numbers: 74.70. Xa, 74.20.Rp The symmetry of the order parameter and the underlying Cooper-pairing mechanism in the newly discovered Fe-based superconductors 1 are one of the most challenging problems in contemporary solid state physics. Historically, nuclear magnetic resonance (NMR) studies showing up the so called Hebel-Slichter peak in the nuclear spin-lattice relaxation rate (NSLRR) played a significant role in establishing the BCS theory as the first microscopic description of conventional (weakly coupled) superconductors. 2 Physically, this behavior is caused by the coherence factors and the symmetry of a single nodeless superconducting (SC) gap. 3 Nowadays, within a simplified approach (ignoring damping, strong coupling, anisotropy, impurity, and inhomogeneity effects 4,5 ) its presence or absence together with the T -dependence of the NSLRR, 1/T 1 , below T c are frequently used to discriminate tentatively conventional from unconventional pairing. For a single Fermi surface (FS) sheet and superconductivity in the clean limit T 3 -and T 5 -dependencies would be regarded as evidence for line-and point-node SC order parameters, respectively, which for singlet pairing correspond to the d-and a special s + g-wave state. Recently it has been realized that the situation in multibands and especially in Fe-pnictides with impurities is far from being that simple, in particular, there is no universal behavior for the growing number of related compounds. 6 The s ± -scenario proposed 7-10 at the early stages of the Fe-pnictide research at present is still the most popular one. Due to the vicinity of a competing spin density wave state in the phase diagram, it is tempting to assume that antiferromagnetic (AFM) spin fluctuations might be the dominant pairing glue. Then, from the FS topology given by small hole (electron) pockets centered * Present address: Department of Physics, University of Florida, Gainesville,Florida 32611, USA around the Γ = (0, 0) (M = (π, π))-points of the Brillouin zone, a nodeless gap with opposite signs on each of the disconnected FS pockets separated by the wave vector Q = (π, π) is naturally suggested.With respect to pair-breaking interband impurity scattering some doubts about this sign-reversed s ± -scenario have been put forward. 11-17 Also the available weak couplin...

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⁷⁵As-NQR/NMR Studies on Oxygen-Deficient Iron-Based Oxypnictide Superconductors LaFeAsO_1-y ( y = 0, 0.25, 0.4) and NdFeAsO_0.6

Cited by 133 publications

References 9 publications

Microwave Penetration Depth and Quasiparticle Conductivity ofPrFeAsO1−ySingle Crystals: Evidence for a Full-Gap Superconductor

Microwave Penetration Depth and Quasiparticle Conductivity ofPrFeAsO1−ySingle Crystals: Evidence for a Full-Gap Superconductor

Electronic structure of Fe-based superconductors

Unusual disorder effects in superconductingLaFeAs1−δO0.9F0.1as revealed by
Hammerath
¹
,
Drechsler
²
,
Grafe
³

et al. 2010
Phys. Rev. B

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75As-NQR/NMR Studies on Oxygen-Deficient Iron-Based Oxypnictide Superconductors LaFeAsO1-y ( y = 0, 0.25, 0.4) and NdFeAsO0.6

Cited by 133 publications

References 9 publications

Microwave Penetration Depth and Quasiparticle Conductivity ofPrFeAsO1−ySingle Crystals: Evidence for a Full-Gap Superconductor

Microwave Penetration Depth and Quasiparticle Conductivity ofPrFeAsO1−ySingle Crystals: Evidence for a Full-Gap Superconductor

Electronic structure of Fe-based superconductors

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

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⁷⁵As-NQR/NMR Studies on Oxygen-Deficient Iron-Based Oxypnictide Superconductors LaFeAsO_1-y ( y = 0, 0.25, 0.4) and NdFeAsO_0.6

Unusual disorder effects in superconductingLaFeAs1−δO0.9F0.1as revealed by
Hammerath
¹
,
Drechsler
²
,
Grafe
³

et al. 2010
Phys. Rev. B