Nodeless electron pairing in CsV3Sb5-derived kagome superconductors
Result of the Month
Isotropic superconducting gap in Cs(V0.86Ta0.14)3Sb5 a, ARPES intensity integrated over ± 5 meV around Fermi level (E_F). The broken lines represent the Fermi surface (FS) contours. b, Temperature dependence of energy distributed curve (EDC) at Fermi momentum (k_F) in a cut marked as black line in a. Inset shows the temperature dependent superconducting (SC) gap amplitude determined by the fitting procedure based on the Bardeen-Cooper-Schrieffer (BCS) spectral function. The blue broken curve represents BCS-like temperature dependence. c-e, EDCs at k_F measured at T = 2 K and 7 K along with the a, b and d FSs, respectively. The k_F positions of these EDCs are summarized in f as black thick circles. The black lines are the curves fitted by BCS spectral function. The dashed lines mark the peak of the EDCs. g, SC gap magnitude estimated from the fits to EDCs shown in c-e. The shaded areas represent the error bars determined from the standard deviation of E_F. The square makers are the SC gap results from an independent sample and the corresponding k_F are shown as thin square in f.
In this work, we utilize an ultrahigh resolution and low temperature laser-ARPES, together with a chemical substitution of V in CsV3Sb5 that raises Tc, to precisely measure the gap structure in the superconducting state. CsV3Sb5 crystallizes in a layered structure with V atoms forming a two-dimensional kagome net. At low temperatures, the material exhibits a CDW transition at TCDW ~ 93 K, and eventually becomes superconducting at Tc ~ 3 K. To finely tune the competition between superconductivity and CDW, we take Nb and Ta to substitute V in CsV3Sb5. The Cs(V0.93Nb0.07)3Sb5 sample exhibits a Tc of 4.4 K and a TCDW of 58 K, while the Cs(V0.86Ta0.14)3Sb5 sample exhibits a higher Tc of 5.2 K, but no clear CDW transition. Strikingly, the gap structures of both samples are isotropic, regardless of the disappearance of CDW, hinting at a robust nodeless pairing in CsV3Sb5-derived kagome superconductors.
Ultrahigh-resolution ARPES measurements were performed in a laser-based ARPES setup at the ISSP, University of Tokyo, which consisted of a continuous wave laser (excitation energy = 5.8 eV) provided from OXIDE Corporation and a vacuum ultraviolet laser (excitation energy = 6.994 eV), a Scienta Omicron HR8000 hemispherical analyser, and a sample manipulator cooled by decompression-evaporative the liquid helium. The sample temperature was varied from 2 to 7 K, and the energy resolution for the superconducting gap measurements was better than 0.6 meV for 5.8-eV laser and 1.5 meV for 6.994-eV laser. High resolution of the hemispherical analyser was necessary to resolve the superconducting gaps in those materials with very low Tc.
https://doi.org/10.48550/arXiv.2303.00875
(The final published version of this work is available at https://doi.org/10.1038/s41586-023-05907-x)