Quantized Hall effect in the organic superconductor (TMTSF)2ReO4(TMTSF=tetramethyltetraselenafulvalinium)

We report evidence for a well-defined quantum Hall effect in the field-induced spin-density-wave states of the organic superconductor (TMTSF${)}_2$ ${\mathrm{ReO}}_4$. Hall resistance plateaus at h/2${\mathit{e}}^2$ and h/4${\mathit{e}}^2$ were clearly identified and correspond to the n=1 and n=2 states of the standard quantized nesting model. Magnetic breakdown is proposed as a tentative interpretation for the change in behavior of the Hall voltage occurring around 16 T and the existence of fast magnetoresistance oscillations. It is found that even in the semiconducting n=0 phase the absolute value of ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$ remains of the order of h/${\mathit{e}}^2$. A tentative explanation for this effect in terms of incipient field-induced localization is proposed.