Conventional s-Wave Superconductivity in BiS₂-Based NdO_0.71F_0.29BiS₂ Revealed by Thermal Transport Measurements

Abstract

Conventional s-wave superconductivity in BiS 2 -based NdO 0.71 F 0.29 BiS 2 revealed by thermal transport measurements To study the superconducting gap structure of BiS2-based layered compound NdO0.71F0.29BiS2 (T c = 5 K), we measured the thermal conductivity κ, which is a sensitive probe of low-energy quasiparticle spectrum. In the absence of a magnetic field, residual linear term in the thermal conductivity κ0/T at T → 0 is vanishingly small, indicating that the residual normal fluid, which is expected for nodal superconductors, is absent. Moreover, the applied magnetic field hardly affects thermal conductivity in wide range of the vortex state, indicating the absence of Doppler shifted quasiparticles. These results provide evidence that NdO0.71F0.29BiS2 is a fully gapped superconductor. The obtained gap structure, along with the robustness of the superconductivity against the impurity, suggest a conventional s-wave superconducting state in NdO0.71F0.29BiS2.

Unconventional Superconductivity in the BiS2 -Based Layered Superconductor NdO0.71Ota1, Okazaki2, Yamamoto3 et al. 2017Phys. Rev. Lett.Self Cite

“…According to random phase approximation calculations, a similar FS topology can produce a d-or g-wave SC gap symmetry and hence a nodal SC gap anisotropy, and its anisotropy is similar to our results that the nodes are located at the short edge of the rectangular FSs [10,18,29]. On the other hand, magnetic penetration depth measurements and thermal transport measurements have indicated nodeless superconductivity in NdO 1−x F x BiS 2 (x = 0.3 and 0.5) [30,31]. These results may seem to be inconsistent with our results.…”

Extremely high upper critical field in BiCh2-based (Ch: S and Se) layered superconductor LaO0.5F0.5BiS2−xSex (x = 0.22 and 0.69)

“…Finally, we briefly discuss the pairing symmetry of the superconductivity of the BiCh 2 -based superconductor. Many experiments are consistent with fully gapped s -wave superconductivity 26 – 28 , while the ARPES study found an anisotropic superconducting gap 29 . The breaking of local inversion symmetry may be a key factor in the solution of the puzzle because spin-singlet and spin-triplet superconductivity can be mixed in the local inversion symmetry breaking.…”

“…Several theoretical calculations suggest that anisotropic superconductivity, such as extended s -wave, d -wave, and g -wave states, are realised in BiCh 2 -based superconductors 22 – 25 . However, early-stage experimental results have reported conventional fully gapped s -wave superconductivity; thermal conductivity, specific heat, and magnetic penetration depth measurements support fully gapped s -wave superconductivity 26 – 28 . In contrast, an anisotropic superconducting gap was observed in a laser angle-resolved photoemission spectroscopy (ARPES) apparatus 29 .…”

Possible pairing mechanism switching driven by structural symmetry breaking in BiS2-based layered superconductors

“…Regarding the mechanisms of superconductivity in the BiCh 2 -based family, the pairing mechanisms of the BiCh 2 -based superconductor family are still controversial 19 , owing to superconducting properties that are tunable by external and/or chemical pressure effects, which sometimes causes scattered results. Although earlier theoretical and experimental studies suggested conventional mechanisms with fully gapped s-wave pairing states 21 – 23 , recent theoretical calculations of T c indicated that a T c of an order of several K to 10 K in BiS 2 -based superconductors with a tetragonal structure cannot be explained within phonon-mediated models 24 . Furthermore, angle-resolved photoemission spectroscopy (ARPES) proposed unconventional pairing mechanisms owing to the observation of a highly anisotropic superconducting gap in NdO 0.71 F 0.29 BiS 2 25 .…”