It was 1980 when the first discovery of superconductivity in organic materials was reported. The TMTSF-based molecular conductors [Fig.1], the first organic superconductors, have been extensively studied because of their interesting superconducting as well as normal-state properties. However, the structure of the superconducting gap, which is one of the most fundamental pieces of information in researches of a superconductor, has been unresolved for more than 30 years in the case of the TMTSF-salts. This is mainly because of several experimental difficulties such as their relatively low Tc and limitation in availability of large single crystals.
We developed a sensitive calorimeter for measurements of heat capacity of small samples. With this calorimeter, we succeeded in the first mapping of the positions of the zeros on the gap of (TMTSF)2ClO4 from our precise field-angle resolved calorimetory [Figs.2(a) and 2(b)]. We then propose possible gap structures [Fig.2(d)]. Generally speaking, the structure of the gap is closely related to the mechanism of superconductivity in a material. Thus, our mapping provides a new clue to unveil the mechanism of unconventional superconductivity in the TMTSF systems, and to investigate universal relationship between superconductivity and magnetism observed in other superconductors such as high-transition-temperature copper oxides or iron pnictides.
In addition, we determined the thermodynamic phase diagram of this material for the first time for all three principle axes. The result propose existence of two regions: a long-range ordered superconducting state in low magnetic fields and an interesting short-range ordered or fluctuating superconducting state in higher fields.