Pressure dependent phase diagram of MX chains

We investigate the phase diagram of halogen-bridged, mixed-valence, transition-metal linear chain complexes (MX chains) near the crossover from charge-density-wave (CDW) to metallic or antiferromagnetic (AF) behavior - i.e., in the regime where both electron-electron and electron-phonon interactions are important. We discuss spin/charge separation found near the phase boundary, analogous to that seen in 2-D high TC models, and novel long-period states driven by electron-phonon induced spin frustration. We also compare the theoretical predictions with recent experimental evidence for a pressure-induced CDW/AF crossover in the neutral chain [PtBr2][PtBr4(NH3)2]. We compare the experimental pressure dependent behavior of the frequency of the Raman ν1 mode with the predicted dependence in our two-band, tight-binding, Peierls-Hubbard model for MX chains, and discuss the relative importance of the various terms to reproduce the experimental behavior.