Superconducting gap function in the organic superconductor (TMTSF)2ClO4 with anion ordering; First-principles calculations and quasiclassical analysis for angle-resolved heat capacity

We calculate angle-dependent heat capacity in a low magnetic field range on the basis of Kramer-Pesch approximation together with an electronic structure obtained by first-principles calculations to determine a superconducting gap function of (TMTSF)2ClO4 through its comparisons with experiments. The present comparative studies reveal that a nodal d-wave gap function consistently explains the experimental results for (TMTSF)2ClO4. Especially, it is emphasized that the observed unusual axis-asymmetry of the angle-dependence eliminates the possibility of s-wave and node-less d-wave functions. It is also found that the directional ordering of ClO4 anions does not have any significant effects on the Fermi surface structure contrary to the previous modelings since the two Fermi surfaces obtained by the band calculations almost cross within the present full accuracy in first-principles calculations.