SIGN REVERSAL OF THE QUANTUM HALL NUMBER IN (TMTSF)(2)PF6

The quantum Hall effect is investigated in the organic conductor (TMTS F)(2)PF6 (TMTSF is tetramethyltetraselenafulvalene) under hydrostatic pressure. Under a pressure of p = 9 kbar, the sequence of the quantum Hall numbers L = sigma(xy)/(-2e(2)/h) with increasing magnetic field i s regular: L = ...,3,2,1,... as reported earlier. At p = 8.5 kbar, how ever, the irregular sequence L = ...,3, -2, 2, 1,... is found. The tem perature-magnetic field phase diagram is investigated in detail. We di scuss our findings in the framework of recent theories of the quantum Hall effect in magnetic-field-induced spin-density waves.