Quasiparticle Excitations outside the Vortex Cores in MgB<sub>2</sub>Probed by Muon Spin Rotation

Ohishi, Kazuki; Muranaka, Toshiya; Akimitsu, Jun; Koda, A.; Higemoto, Wataru; Kadono, R.

doi:10.1143/jpsj.72.29

Cited by 36 publications

(51 citation statements)

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“…On the other hand, we note that there are at least six low-T experiments showing or at least suggesting power-law behaviors which seem to indicate gap nodes (Panagopoulos et al 2001, Zhukov et al 2001, Pronin et al 2001, Ohishi et al 2003. [Admittedly, the lowest temperatures in some of these experiments are too high to be convincing.…”

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confidence: 76%

Strongly anisotropic s-wave gaps in exotic superconductors

Brandow

2003

Philosophical Magazine

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The exotic superconductors, defined as those which follow the phenomenological trend of Uemura (T c approximately µ l L -2 ), presently constitute the most broad and general class of superconductors which can reasonably be considered "similar to the high-T c cuprates". It is therefore of much interest to determine the forms of their pairing gap functions. We examine evidence for the gap forms in non-cuprate exotics, and demonstrate some general features. The cubic materials often have highly anisotropic gaps. The planar materials tend to have even stronger gap anisotropy, to the extent that they often have gap nodes, but nevertheless they usually have an s-wave-like gap symmetry. There is good evidence for the latter even in the controversial cases of planar organics and nickel borocarbides. Exceptions to these generalizations are also pointed out and discussed. §1. INTRODUCTIONThere are now many unusual superconductors which are recognized to share some of the novel features of the high-T c cuprate superconductors. The possibility of further similarity to the cuprates, in particular the possibility of pairing by exchange of a virtual spin fluctuation or paramagnon, has motivated suggestions of d-wave gap forms in some of these other superconductors, in cases where there is evidence for gap nodes. This argument has been applied especially to the planar organics and sometimes also to the 2 nickel borocarbides, in addition to the well known cases of heavy-fermion materials. This line of reasoning calls for a careful examination of the evidence for the gap forms in these and other unusual superconductors.In this paper we focus on the exotic superconductors of Uemura and co-workers. 1 These exotics include a number of the unusual superconductors of earlier (pre-cuprate) interest, as well as the cuprates themselves and others discovered after the cuprates, and they include most of the materials previously suggested as being similar to the cuprates. 2 As discussed below, they now constitute the most broad and general class of superconductors that can reasonably be considered similar to the cuprates. (The variety of crystal structures and material chemistries in this class of materials is remarkably extensive.2 ) It is therefore very reasonable to focus on these materials. We now examine many of the non-cuprate exotic materials and material families, those for which there is reasonable gap-form evidence, in order to establish two general conclusions about their superconducting phenomenology: (1) In the cubic and nearly-cubic materials, which are conventionally expected to have nearly isotropic gaps, there is often evidence of very strong gap anisotropy. Sometimes there even appear to be gap nodes, although the gap symmetry remains s-wave-like. (2) In the planar (quasi-two-dimensional) materials there are often gap nodes, but nevertheless they typically still have an s-wave-like gap symmetry.[Here s-wave-like means having the full point-group symmetry of the crystal structure, a condition which does allow the p...

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confidence: 76%

Strongly anisotropic s-wave gaps in exotic superconductors

Brandow

2003

Philosophical Magazine

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“…Besides this, it has turned out that MgB 2 exhibits η = 1.27 at 0.26T c . 22 In this case, it is suggested that the presence of two superconducting energy gaps, one being relatively small (∼1 meV), is responsible for the field dependence of λ eff observed at a finite temperature. In this sense, we cannot exclude the multiple-gap scenario in KOs 2 O 6 , since the measurement was performed at a finite temperature of 2 K. The present result suggests the presence of a small energy gap having a magnitude of less than ∼0.2 meV in the multiple-gap scenario.…”

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Possible Anisotropic Order Parameter in Pyrochlore Superconductor KOs₂O₆ Probed by Muon Spin Rotation

et al. 2005

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Magnetic penetration depth (λ eff ) in a pyrochlore superconductor KOs 2 O 6 was studied by means of muon spin rotation (µSR). The temperature dependence of the transverse spin relaxation rate, which is proportional to λ −2 eff , exhibits a considerable deviation from that expected for the empirical two-fluid model (∝ T 4 ), obeying a T 2 tendency. Furthermore, λ eff apparently increases with increasing applied field, suggesting the presence of nonlinear and nonlocal effects. The present result strongly suggests that the superconducting order parameter in KOs 2 O 6 is anisotropic or multigapped with a small energy gap. In this letter, we report the results of a muon spin rotation (µSR) experiment on KOs 2 O 6 under several transverse fields (TF) up to 6 T. The observed temperature dependence of the magnetic penetration depth λ eff deduced from the TF-µSR measurement cannot be explained by the empirical two-fluid model. Furthermore, the field dependence of λ eff strongly suggests the existence of low-lying quasiparticle excitations, which are attributed to either an anisotropic superconducting order parameter or a multigap structure in KOs 2 O 6 . KEYWORDSThe µSR experiment was conducted at the M15 beamline of TRIUMF, Canada. Timedependent muon polarization under a transverse field was measured using a high-timeresolution µSR spectrometer furnished with a superconducting magnet. For each µSR measurement, the magnetic field was applied at 20 K, which is well above T c , and subsequently the specimen was cooled to the lowest temperature of 2 K. The measured KOs 2 O 6 sample was prepared as a pellet of polycrystalline powder in a square shape of 8 × 8 mm 2 . Positive 2/8 J. Phys. Soc. Jpn.Letter muons with a beam momentum of 29 MeV/c, whose polarizations were normal to the beam axis, were implanted. The µ + -e + decay asymmetry and associated Larmor precession with a frequency ω ≡ γ µ B (where γ µ = 2π × 135.54 MHz/T is the muon gyromagnetic ratio) were measured to deduce the local field distribution. The decay events in which muons missed the sample were eliminated with an anticoincidence electronics logic circuit, resulting in quite low background positron spectra.

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“…We examined the consistency of our data with the ARPES result by employing a phenomenological doublegap model with s-wave symmetry, 28,29) s ðTÞ ¼ ð0Þ À w Á ðÁ 1 ; TÞ À ð1 À wÞ Á ðÁ 2 ; TÞ; ðÁ; TÞ ¼ 2ð0Þ…”

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Full Gap Superconductivity in Ba_0.6K_0.4Fe₂As₂ Probed by Muon Spin Rotation

et al. 2009

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Quasiparticle Excitations outside the Vortex Cores in MgB₂Probed by Muon Spin Rotation

Cited by 36 publications

References 35 publications

Strongly anisotropic s-wave gaps in exotic superconductors

Strongly anisotropic s-wave gaps in exotic superconductors

Possible Anisotropic Order Parameter in Pyrochlore Superconductor KOs₂O₆ Probed by Muon Spin Rotation

Full Gap Superconductivity in Ba_0.6K_0.4Fe₂As₂ Probed by Muon Spin Rotation

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Quasiparticle Excitations outside the Vortex Cores in MgB2Probed by Muon Spin Rotation

Cited by 36 publications

References 35 publications

Strongly anisotropic s-wave gaps in exotic superconductors

Strongly anisotropic s-wave gaps in exotic superconductors

Possible Anisotropic Order Parameter in Pyrochlore Superconductor KOs2O6 Probed by Muon Spin Rotation

Full Gap Superconductivity in Ba0.6K0.4Fe2As2 Probed by Muon Spin Rotation

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Quasiparticle Excitations outside the Vortex Cores in MgB₂Probed by Muon Spin Rotation

Possible Anisotropic Order Parameter in Pyrochlore Superconductor KOs₂O₆ Probed by Muon Spin Rotation

Full Gap Superconductivity in Ba_0.6K_0.4Fe₂As₂ Probed by Muon Spin Rotation