Role of anion ordering and effective pressure in the field-induced spin-density-wave phase of (TMTSF)₂X

Magnetoresistance and Hall resistance measurements were conducted in the field-induced spin-density-wave (FISDW) phase of (TMTSF)₂ReO₄ above 1.0 GPa, with an anion ordering specified by $Q_{\text{AO}}=(0,1/2,1/2)$. The quantized Hall resistance shows the sequence $N=0,1,2,-2,4,-4,-6,-8,\cdots$, with decreasing field that is successfully explained by the "extended standard model". Consequently, we demonstrate that the difference between the chemical and hydrostatic pressures is linked to the appearance of the peculiar FISDW phase of the TMTSF salts with $Q_{\text{AO}}=(0,1/2,\ast )$. (letter)