Abstract
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 $\ensuremath{\lambda}$-(BETS)${}_{2}{\mathrm{GaCl}}_{4}$ [BETS = (${\mathrm{CH}}_{2}{)}_{2}{\mathrm{S}}_{2}{\mathrm{Se}}_{2}{\mathrm{C}}_{6}{\mathrm{Se}}_{2}{\mathrm{S}}_{2}$(${\mathrm{CH}}_{2}{)}_{2}$] 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 (${\ensuremath{\mu}}^{+}\mathrm{SR}$) spectroscopy on $\ensuremath{\lambda}$-(BETS)${}_{2}{\mathrm{GaCl}}_{4}$. Our analysis method of the ${\ensuremath{\mu}}^{+}\mathrm{SR}$ 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 $2\mathrm{\ensuremath{\Delta}}/{k}_{\text{B}}{T}_{c}$ = 3.9(2) and the in-plane penetration depth is ${\ensuremath{\lambda}}_{\text{ac}}$(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.
Highlights
The superconducting (SC) gap state appearing in unconventional superconductors is one of the hardest issues in condensed matter physics
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
We found that the amplitude of the SC gap is 2 /kBTc = 3.9(2) and the in-plane penetration depth is λac(0) = 560(5) nm
Summary
The superconducting (SC) gap state appearing in unconventional superconductors is one of the hardest issues in condensed matter physics. Since the first discovery of the d-wave SC state with nodal line in CeCu2Si2 [1], it has been reported in the Cu- and Fe-based [2,3,4,5,6,7] and organic κ-(ET)2Cu2(NCS)2 [ET = (CH2 )2S2S2C6S2S2(CH2 )2] superconductors [8]. Unconventional pairing mechanisms of these superconductors can be explained by the spin fluctuations mediating the Cooper pair [6]. The criterion of the d-wave nodal state as a consequence of the unconventional Cooper-pairing mechanism and suppressed superfluid density has been debated in the case of Fermi surface (FS) becoming an imperfect spherical or cylindrical shape due to the lower crystal symmetry.
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