Coexistence of two different spin density wave phases at high magnetic fields for (TMTSF)2ClO4

Resistance measurements for the quasi-one-dimensional conductor (TMTSF)2ClO4 have been performed in the high magnetic field spin-density-wave (SDW) phase up to 30 T. In the main SDW phase, we found two distinct rapid oscillations (ROs) in the a and c axis resistance. Both ROs have the same frequency, but the different phases. The results suggest the coexistence of two different SDW phases in a wide field region, where a first order phase boundary is expected to be present.