Quantized Hall effect and a new field-induced phase transition in the organic superconductor (TMTSF)2PF6

We report a Hall effect and magnetoresistance study of sin- gle crystals of the organic superconductor tetramethyl-tetraselenafulvalinium hexafluorophosphate, (TMTSF)2PF6, under hydrostatic pressure at temperatures down to 0.5 K in magnetic fields up to 25 T. There is clear evidence for a quantized Hall effect, with plateaus labeled by the integers n=5 to 1, up to 17 T. A new field-induced phase transition to a very resistive state occurs at 18 T. We believe that this is the n=0 spin-density-wave state predicted by the quantized nesting theory.