QUANTIZED HALL CONDUCTANCE AND ITS SIGN REVERSAL IN FIELD-INDUCED SPIN-DENSITY WAVES

The sign-reversal phenomenon of the quantized Hall conductance as a function of an external magnetic field (H) observed in (TMTSF)2X is investigated theoretically. After giving a general Hall-conductance formula written in terms of order parameters of the field-induced spin-density wave (FISDW), we have done extensive mean-field calculations for a simplified standard model. It is shown that the many competing order parameters DELTA(n) (n = 0, +/-1, +/-2, ...) which coexist in a FISDW state can make the sign of the Hall constant change particularly near the subphase boundary of the FISDW and that numbering of the integer subphases (N = 0, 1, 2.... stabilized in this order from high fields) does not necessarily coincide with the Hall number L defined by L = sigma(xy)/(e2/h). We have found that the jumps of the Hall constant are accompanied by the spin-density wave gap closing when a FISDW is continuously evolving as H varies. In order for the sign reversal to occur a FISDW must contain many order parameters DELTAn with n being both signs.