Extreme quantum limit in a quasi-two-dimensional organic conductor

We have measured magnetoresistance, Hall resistance, and magnetization of tetramethyltetraselenafulvalenium perchlorate [(TMTSF)2ClO4] at low temperatures in fields up to 31 T. Above the cascade of field-induced spin-density-wave transitions, the system remains in a remarkably stable semimetallic state, with constant Landau filling factor, from 8 to 27 T. The measurements indicate that this state is related to the (1/3 fractional quantum Hall effect. At 27 T there is a new transition, to a novel state, distinct in its behavior from the lower-field transitions.