Commensurability effect of magnetoresistance anisotropy in the quasi-one-dimensional conductor tetramethyltetraselenafulvalenium perchlorate, (TMTSF${)}_2$${\mathrm{ClO}}_4$

We have observed a clear series of fine structures superposed on the angular dependence of magnetoresistance in the metallic and magnetic-field-induced spin-density-wave phases in the low-dimensional organic superconductor (TMTSF${)}_2$${\mathrm{ClO}}_4$. They appear at the field angles whose tangent satisfies a fractional condition reflecting the crystal geometry. This new oscillatory phenomenon is considered to result from a resonance effect, i.e., commensurate versus incommensurate, between two periodicities in the electron motion along an open orbit under tilted magnetic fields.