Molecular materials play an increasingly important role in safe and sustainable applications owing to their variety of physical properties and high degree of arrangement in material design. They exhibit a fascinating range of properties, including superconductivity, formation of Dirac electron systems, spin- and charge-density waves, photoinduced phase transitions, altermagnetism and so on. k-(BEDT-TTF)₂Cu(NCS)₂ is known as a superconductor (T_c = 10.4 K) with an anisotropic superconducting gap that depends on the mechanism of superconductivity. However, the gap symmetry is still under debate.

Photoemission spectroscopy (PES) is a powerful tool for the direct observation of electronic structure, which reveals the superconducting gap symmetry of inorganic superconductors. However, experiments to obtain the intrinsic spectra of molecular superconductors using PES are extremely difficult due to problems such as relatively weaker photoelectron signals than those of inorganic materials, radiation damage caused by the excitation light, cracks, and disappearance of the metal-superconducting transition during rapid cooling. Previous studies have not shown even Fermi edge above T_c, which should be observed for two-dimensional metals.

We developed a PES apparatus with a low-energy excitation photon source (6-eV laser) and a cryostat (down to 3 K) with a precise temperature control system. The 6-eV laser can minimize radiation damage and provide a high photoionization cross-section for the s- and p-orbital electrons. Therefore, we expect to obtain the intrinsic PES spectra of molecular conductors with sufficient statistics.

The PES spectra above T_c (15 K) show a clear Fermi edge that represents the signature of two-dimensional conductivity (Fermi liquid) and not one-dimensional conductivity (Tomonaga-Luttinger liquid), as implied from the previous PES results. Below T_c, the leading edge of the spectrum shifted slightly but clearly to the higher binding energy, indicating the first observation of a superconducting gap opening in molecular superconductors by PES. From the Dynes’fitting (normal: s-wave, extended: d-wave) of the observed spectrum in the superconducting phase, gap size (D) and broadening (G) are 0.9 and 0.7 meV, respectively, for s-wave, and 1.4 and 0.45 meV, respectively, for d-wave. In the case of s-wave symmetry, the gap size is quite smaller than the weak coupling limit expected from the conventional BCS theory. In addition, the G value is significantly larger than that of conventional s-wave superconductors. These are not reasonable to understand. On the other hand, the d-wave values are both reasonable, indicating d-wave superconducting gap anisotropy for k-(BEDT-TTF)₂Cu(NCS)₂.

The results show that PES can be applicable to observe the electronic structure around the Fermi level of molecular conductors and open the door for angle-resolved photoemission spectroscopy (ARPES), which can directly provide the spectral function A(k,w) modulo matrix elements effects (energy(w)-momentum(k)-dependent electronic structure = band dispersion, Fermi surface, and gap size mapping).

(Written by Takayuki Kiss on behalf of all authors.)

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