Quantum Hall effect anomaly and collective modes in the magnetic-field-induced spin-density-wave phases of quasi-one-dimensional conductors

We study the collective modes in the magnetic-field-induced spin-density-wa ve (FISDW) phases experimentally observed in organic conductors of the Bech gaard salts family. In phases that exhibit a sign reversal of the quantum H all effect (Ribault anomaly), the coexistence of two spin-density waves giv es rise to additional long-wavelength collective modes besides the Goldston e modes due to spontaneous translation and rotation symmetry breaking. Thes e modes strongly affect the charge and spin response functions. We discuss some experimental consequences for the Bechgaard salts.