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Thermal Conductivity of the Pyrochlore Superconductor KOs2O6: Strong Electron Correlations and Fully Gapped Superconductivity

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Y.Kasahara, Y.Shimono, T.Shibauchi, Y.Matsuda, S.Yonezawa, Y.Muraoka, Z.Hiroi

Superconductivity · Submitted 2 Mar 2006 · 35 references

Pith's verdict: Good data and a real case against line nodes, but the fully gapped claim is overreached: the field was only applied parallel to the heat current, which can hide point nodes aligned with that axis.

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arXiv IDcond-mat/0603036 (version 2)
Pith IDMK3ZBTSJ, Pith's permanent record of this paper and its review
CitationS.Yonezawa, T.Shibauchi, Y.Kasahara, Y.Matsuda, Y.Muraoka, Y.Shimono, Z.Hiroi. "Thermal Conductivity of the Pyrochlore Superconductor KOs2O6: Strong Electron Correlations and Fully Gapped Superconductivity." arXiv:cond-mat/0603036 (2006). https://pith.science/paper/MK3ZBTSJ
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The paper

The paper claims

The pyrochlore superconductor KOs2O6 has a fully gapped, fairly isotropic superconducting gap despite strong correlations, shown by thermal conductivity that is insensitive to magnetic field up to 0.4Hc2 and by a tiny residual linear term.

The reading

The paper tries to settle the pairing symmetry of the pyrochlore superconductor KOs2O6. Earlier muon spin rotation and nuclear magnetic resonance measurements had suggested a very anisotropic superconducting gap. By cooling the material to about one hundredth of its transition temperature (9.6 K) and measuring thermal conductivity, the authors find that the electronic heat transport stays essentially constant as a magnetic field is raised to 40% of the upper critical field, and the residual zero-temperature linear term is tiny. Both features are the expected signatures of a fully gapped, fairly isotropic superconducting state, close in behavior to niobium. If correct, the result makes KOs2O6 a rare example of strong electron correlations coexisting with a full gap.

What carries the argument

The central probe is the low-temperature thermal conductivity κ in the limit T → 0 as a function of magnetic field, normalized by the normal-state value κn. In an s-wave superconductor the only quasiparticle states at low temperature are localized in vortex cores and carry no heat, so κ stays nearly constant until vortices overlap; in a nodal superconductor, the Doppler shift of quasiparticle energies produces a density of states that grows as √H (line nodes) or ∝ H (point nodes), so κ rises rapidly. The paper compares its measured κ(H)/κn against these two theoretical expectations and against benchmark materials, and it uses the Wiedemann–Franz law to fix κn from the residual resistivity ρ0. A second load-bearing element is the comparison of the residual linear term a = κ/T (T→0) with the universal line-node value κ00/T, obtained from the penetration depth and ρ0.

Extended reading notes

Core claim

The central claim, stated on the paper's own terms, is that the superconducting gap function of KOs2O6 is fairly isotropic and fully gapped. The evidence is twofold. First, in the zero-temperature limit the ratio κ(H)/κn, where κn is the normal-state thermal conductivity, remains nearly flat up to H ≈ 0.4Hc2, closely matching the s-wave superconductor Nb and standing in sharp contrast to nodal superconductors such as UPt3, LuNi2B2C, and CePt3Si, whose heat conduction rises rapidly with field because of delocalized quasiparticles produced by the Doppler shift. Second, the residual linear term κ/T as T → 0 is more than 300 times smaller than the normal-state value and about an order of magnitude below the universal limit expected for line nodes. The paper also reports a marked enhancement of the quasiparticle scattering time below a transition at Tp ≈ 7.5 K, which it attributes to a change from electron-phonon to electron-electron scattering, and it uses the Kadowaki–Woods relation to argue that the normal state above Tp is strongly correlated. The paper concludes that no delocalized quasiparticles exist at least up to 0.4Hc2, implying a fully gapped state.

Load-bearing premise

The argument that the flat κ(H) rules out nodes assumes that the normal-state conductivity κn, which is not directly measured in the same configuration, is correctly inferred from the Wiedemann–Franz law using a residual resistivity ρ0 = 1.0 µΩ cm taken from a high-field Fermi-liquid fit, and that Hc2(0) ≈ 32 T from a separate study is accurate; if either is misestimated, the flatness of κ/κn could be a normalization artifact.

Editorial extensions

If this is right

  • KOs2O6 joins a short list of superconductors in which a fully gapped state survives strong electron correlations, meaning pairing need not be of d-wave type even when the normal state is heavily mass-enhanced.
  • The earlier muon spin rotation and NMR interpretations of an anisotropic gap will need to be revisited, because the thermal-conductivity probe is a bulk zero-energy probe that directly counts delocalized quasiparticles.
  • The Tp transition at about 7.5 K changes the dominant scattering mechanism for quasiparticles, consistent with the freezing of potassium rattling modes, and this change is visible as a sixfold enhancement of the quasiparticle scattering time at 0.6Tc.
  • Because κ(H) stays flat only while vortices are far apart, the result implies that the vortex-core states remain localized up to at least 40% of Hc2, which constrains the vortex physics in this material.
  • The combination of a large Sommerfeld coefficient and a full gap makes KOs2O6 a benchmark for theories of superconductivity on geometrically frustrated lattices.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the fully gapped conclusion holds, the same low-temperature thermal-conductivity protocol applied to the sister compounds CsOs2O6 and RbOs2O6 should show a similar flat κ(H)/κn, which would indicate that the full gap is a generic feature of the β-pyrochlore osmates rather than a special property of the potassium compound.
  • The authors' attribution of the Tp transition to rattling freezing could be tested by isotope substitution of potassium or by neutron scattering that looks for a change in low-energy phonon density below Tp; a direct phonon signature would link the scattering-time enhancement to a specific lattice degree of freedom.
  • The residual linear term a, though small, may indicate a tiny normal metallic fraction in the crystal; a cleaner crystal would tell whether the term is extrinsic, and such a measurement could strengthen the fully gapped interpretation.
  • A natural theoretical consequence of a full gap with strong correlations is that the pairing glue is unlikely to be purely magnetic spin fluctuations; an electron-phonon mechanism involving the anharmonic rattling mode is a concrete alternative that the authors' transport data indirectly support.
Original abstract

To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity was measured down to low temperatures (~Tc/100). We found that the quasiparticle mean free path is strikingly enhanced below a transition at Tp=7.5K, indicating enormous electron inelastic scattering in the normal state. In a magnetic field the conduction at T ->0K is nearly constant up to ~0.4Hc2, in contrast with the rapid growth expected for superconductors with an anisotropic gap. This unambiguously indicates a fully gapped superconductivity, in contrast to the previous studies. These results highlight that KOs2O6 is unique among superconductors with strong electron correlations.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 4,386 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.09 · states limitation 0.08 · human participants 0.01 · controlled comparison 0.15 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.95 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; .

To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity was measured down to low temperatures (~Tc/100). We found that the quasiparticle mean free path is strikingly enhanced below a transition at Tp=7.5K, indicating enormous electron inelastic scattering in the normal state. In a magnetic field the conduction at T ->0K is nearly constant up to ~0.4Hc2, in contrast with the rapid growth expected for superconductors with an anisotropic gap. This unambiguously indicates a fully gapped superconductivity, in contrast to the previous studies. These results highlight that KOs2O6 is unique among superconductors with strong electron correlations.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity was measured down to low temperatures (~Tc/100). method 0.55 0.95 0.03 asserted 0.95
s1 We found that the quasiparticle mean free path is strikingly enhanced below a transition at Tp=7.5K, indicating enormous electron inelastic scattering in the normal state. result 0.99 0.97 0.07 asserted 0.72
s2 In a magnetic field the conduction at T ->0K is nearly constant up to ~0.4Hc2, in contrast with the rapid growth expected for superconductors with an anisotropic gap. result 0.99 0.90 0.12 asserted 0.99
s3 This unambiguously indicates a fully gapped superconductivity, in contrast to the previous studies. interpretation 0.99 0.85 0.76 asserted 0.99
s4 These results highlight that KOs2O6 is unique among superconductors with strong electron correlations. interpretation 1.00 0.37 0.22 asserted 0.99

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v2
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. FIG. 1: Zero-field thermal conductivity and specific heat in
Figure 2. (b). From the coefficient b, we obtain a reasonable value of the acoustic phonon velocity vs ≃ 4700 m/s [20]. The other peak in κ/T at ∼ 6 K is most likely to be of electronic origin, because (1) the appearance of phonon peaks twice below Tc is highly unlikely, and (2) the elec￾tronic contribution is substantial near Tc. The electronic heat conduction is described by κe/T ∼ N(0)v 2 F τe, where N(0), vF , and τe are the QP density of states (DOS) at the Fermi level, Fermi velocity, and QP scattering time, respectively. Therefore the enhancement of κe/T is a strong indication that a
Figure 3. FIG. 3: Coefficient

Showing 3 of the paper's 4 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:cond-mat/0603036 by the authors.

References

Discussion

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