Fermi surface of the stage-1 potassium graphite intercalation compound

De Haas–van Alphen (dHvA) experiments with angular-dependence measurements for the stage-1 potassium graphite intercalation compound ${\mathrm{C}}_8$K have been done. Two dHvA frequencies, 3126 and 4250 T, were measured with the magnetic field parallel to the c axis. The angular dependence of the dHvA frequencies for directions within ±18° of the c axis showed that there are both three-dimensional and two-dimensional parts of the Fermi surface in ${\mathrm{C}}_8$K. When the c axis is parallel to the magnetic field, the three-dimensional Fermi surface has a cross-sectional area corresponding to the dHvA frequency of 3126 T and the two-dimensional cylinder gives rise to the frequency of 4250 T. The charge transfer per potassium atom measured directly from the dHvA effect is 0.97. This implies that the potassium is ionized completely. In addition, the corresponding effective masses 0.86${\mathit{m}}_0$ and 0.92${\mathit{m}}_0$ were obtained for ${\mathrm{C}}_8$K from the temperature dependence of the dHvA amplitude.