Fractional quantized nesting for the spin‐density waves induced by a magnetic field

Recent experiments on magnetic field‐induced spin‐density waves (SDW) in the highly anisotropic organic conductor (TMTSF)2ClO4 have led us to build from thermodynamic data a phase diagram with a series of magnetic subphases. This series of transitions results from the jump of the nesting vector of the SDW. In this paper we relate our investigations of the phase diagram by magnetocaloric experiments. The measurement of the entropy variations were performed at fixed temperature by sweeping the magnetic field on an oriented single crystal. A vanishing latent heat is observed at the transition between different subphases, but this experiment reveals that each subphase is split by several phase transitions. This new complexity explains the discrepancy between phase diagrams obtained from various techniques. It is proposed that a third characteristic length, the lattice periodicity, is responsible for this complexity, thus implying a fractional quantization of the SDW wave vector.