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

Abstract

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.

Superconductivity of the Filled Skutterudite ThPt4Ge12Investigated by Muon Spin Rotation

“…It is worth noting that the derived value of n is close to n = 2 theoretically predicted for nodal superconductors, 10) and very close to the experimental values reported for KOs 2 O 6 (n = 2.39), 11) PrRu 4 Sb 12 (n =1.44), 12) and PrOs 4 Sb 12 (n =2.9). 13) According to Ref., 14) the muon-spin relaxation rate in clean-limit superconductors is given by where…”

Rattling-Induced Superconductiviy in the β-Pyrochlore Oxides AOs2O6

“…found a strong magneticfield dependence of penetration depth and concluded the existence of anisotropic s-wave pairing or multigaps with a very small minimum gap of less than 0.2 meV. 19) In addition, very recent scanning tunneling spectroscopy measurements on a KOs 2 O 6 single crystal revealed fully gapped superconductivity with a larger gap anisotropy of 30%. 68) Although all the results are consistent with each other regarding the anisotropy of s-wave superconductivity, the magnitude of anisotropy in the gap remains a controversial issue.…”

“…Extensive study has been carried out on the three -pyrochlore oxides in order to elucidate the nature of their superconductivity. For the superconducting gap symmetry, most experimental results indicate that it is of the s-wave BCS type, [12][13][14][15][16][17][18][19][20] suggesting conventional phonon-mediated superconductivity; there is no evidence of an alternative ''glue'' such as magnetic fluctuations, which are sometimes found in an exotic superconductor with a nodal gap. Interestingly, pronounced variations of various properties have been observed over the series.…”

Crystal growth, structure, and superconducting properties of theβ-pyrochloreKOs2O6

“…Based on the results of the specific heat measurements, 5 pressure influence on the magnetic field penetration depth, 6 and nuclear magnetic resonance, 7 RbOs 2 O 6 is suggested to be a weak coupled isotropic BCS superconductor. On the other hand, upper critical field H c2 [8], muon spin rotation, 9 and specific heat experiments in KOs 2 O 6 [10], as well as specific heat experiments in RbOs 2 O 6 [2] and CsOs 2 O 6 [11], suggest an unconventional type of paring in these compounds. Most of experiments concerning KOs 2 O 6 have been performed on polycrystalline samples.…”