Thermodynamic study of the superconducting gap structure of (TMTSF)$_2$ClO$_4$

We have studied the superconducting (SC) gap structure of the quasi-one-dimensional molecular conductor (TMTSF)$_2$ClO$_4$ based on our high-resolution heat capacity measurement. We developed a new calorimeter, which allowed us field-angle resolved calorimetry using one single crystal weighing as small as 76~$\mu$g. From our results, we investigate the SC gap structure in the $k$-space as well as in the spin space. Comparison between the SC phase diagram deduced from the present results and those obtained from previous resistivity measurements are also discussed.