Distorted superconducting nodal line on single Fermi surface in anisotropic organic superconductor $\lambda-(BETS){2}GaCl{4}$

The superconducting (SC) gap structure that appeared in systems with a single Fermi surface (FS) is generally described by the single gap function with s- or d-wave symmetry. Among strongly correlated electron systems, the organic superconductor $\lambda-(BETS)_{2}GaCl_{4}$ $(BETS = (CH_{2})_{2}S_{2}Se_{2}C_{6}Se_{2}S_{2}(CH_{2})_{2})$ endeavors to examine a novel SC gap structure on the distorted single cylindrical FS (Fig. 1). This high-anisotropic superconductor is a type-II with SC transition Tc = 5.3 K. The stacking of BETS dimer in the conducting plane constructs an alternation of triangular and squared magnetic sub-lattice along a-direction, shown in Fig. 1(b), with t’/t = 0.8 and t’/t = 0.2. The electronic state of this ½-filled system can be described by the Hubbard model with the strong Coulomb interaction, U, inside the BETS dimer. The relatively narrow bandwidth, W, maintains near the Fermi level and partially nest FS, as plotted numerically by Density Functional Theory (DFT) calculation in Fig. 1(b). The U/W of $\lambda-(BETS)_{2}GaCl_{4}$ is 0.89 [1]. The folded Brillouin zone is no longer rectangular cuboid and becomes parallelepiped distorting the shape of FS to be the elliptic cylinder.

Here, we show the first example of the formation of the distorted SC nodal line using the positive muon spin rotation ($\mu^{+}SR$) spectroscopy on $\lambda-(BETS)_{2}GaCl_{4}$. Our new analysis method of the $\mu^{+}SR$ data reveals that the SC gap structure exhibits the newly invented “h-wave” symmetry. This is characterized by a distorted nodal line structure which contains four nodes on a single FS with a narrower width than that of the traditional d-wave symmetry by the steepness factor of 4.6(2.1) on average, alternating with a flat gap where the maximum gap exists with amplitude of $\Delta_{h}/k_{B}T_{c}$ = 3.9(2). In the presentation, we will show the gap structure in detail, introduce the SC parameters obtained by the $\mu^{+}SR$ consistent with other experimental results, and discuss the insight into the relation of the FS distortion and the unusual Cooper pair formation mediated by the anisotropic spin fluctuations. This work has been done during the Ph.D. project at Osaka University and the JRA program at RIKEN of the corresponding author.