Quantum Hall Transitions in (TMTSF)$_2$PF$_6$

We have studied the temperature dependence of the integer quantum Hall transitions in the molecular crystal (TMTSF)$_2$PF$_6$. We find that the transition width between the quantum Hall plateaus does not exhibit the universal power-law scaling behavior of the integer quantum Hall effect observed in semiconducting devices. Instead, the slope of the $rho_{xy}$ risers, $drho_{xy}/dB$, and the (inverse) width of the $rho_{xx}$ peaks, $(Delta B)^{-1}$, show a BCS-like energy gap temperature dependence. We discuss these results in terms of the field-induced spin-density wave gap and order parameter of the system.