EFFECT OF UMKLAPP SCATTERING ON THE MAGNETIC-FIELD-INDUCED SPIN-DENSITY WAVES IN QUASI-ONE-DIMENSIONAL ORGANIC CONDUCTORS

We study the effect of umklapp scattering on the magnetic-field-induce d spin-density-wave (FISDW) phases which are experimentally observed i n the quasi-one-dimensional organic conductors of the Bechgaard salts family. Within the framework of the quantized nesting model, we show t hat the transition temperature is determined by a modified Stoner crit erion which includes the effect of umklapp scattering. We determine th e SDW polarization (linear or circular) by analyzing the Ginzburg-Land au expansion of the free energy. We also study how umklapp processes m odify the quantum Hall effect (QHE) and the spectrum of the FISDW phas es. We find that umklapp scattering stabilizes phases which exhibit a sign reversal of the QHE, as experimentally observed in the Bechgaard salts. These ''negative'' phases are characterized by the simultaneous existence of two SDW's with comparable amplitudes. As the umklapp sca ttering strength increases, they may become helicoidal (circularly pol arized SDW's). The QHE vanishes in the helicoidal phases, but a magnet oelectric effect appears. These two characteristic properties may be u tilized to detect the magnetic-field-induced helicoidal SDW phases exp erimentally.