The superconducting (SC) gap structure appearing in systems with single Fermi surface (FS) is generally described by the single gap function with s- or d-wave symmetry. The organic superconductor λ-(BETS)2GaCl4 [BETS = (CH2)2S2Se2C6Se2S2(CH2)2] endeavors to examine a novel SC gap structure on the distorted single cylindrical FS. Here, we show an example of the formation of the distorted SC nodal line by using the positive muon spin rotation (μ+SR) spectroscopy on λ-(BETS)2GaCl4. Our analysis method of the μ+SR data reveals that the nodal line has a narrower width than that of the traditional d-wave by the steepness factor of 4.6(2.1), and a flat part with the maximum gap exists. We found that the amplitude of the SC gap is 2Δ/kBTc = 3.9(2) and the in-plane penetration depth is λac(0) = 560(5) nm. Our present study gives insight into the relation of the FS distortion and the unusual Cooper pair formation mediated by the anisotropic spin fluctuations.
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