Metal-Insulator Transition and Antiferromagnetism in a One-Dimensional Organic Solid

Abstract

An experimental study of the organic charge-transfer salt $N$-methyl-phenazinium tetracyanoquinodimethan (NMP-TCNQ) is presented. Magnetic-susceptibility, specific-heat, spin-resonance, and conductivity measurements indicate a metallic state above 200 °K with a continuous transition to a small-band-gap magnetic Mott insulator below 200 °K. The ground-state and low-lying excitations indicate that this system can be quantitatively described in terms of the one-dimensional Hubbard model with a transfer integral of 2.1×${10}^{-2\mathrm{}}$ eV and an effective Coulomb interaction of 0.17 eV. These values are discussed in terms of the fundamental molecular physics of the TCN${\mathrm{Q}}^{-}$ anion in the NMP-TCNQ crystal. It is concluded that in addition to the Heitler-London correlation which reduces the interaction between two excess electrons on a TCNQ molecule, the NMP cation polarizability plays a significant role in reducing the effective interaction. The transition to the metallic state is attributed to electron-hole correlations which become important when the number of excitations is large. These correlations persist into the metallic state where the electronic system behaves as a quasi-free-electron Fermi liquid as indicated by the unenhanced Pauli susceptibility and simple transport properties. The low-temperature one-dimensional antiferromagnetic state is studied using spin-resonance and specific-heat techniques. The linear temperature dependence of the specific heat predicted for the one-dimensional antiferromagnet has been observed. Electron-spin-resonance linewidth studies indicate motionally narrowed dipolar widths with the correlation time determined by the Fermi velocity in the metallic state and by exchange in the insulating state. The large fluctuations expected for a one-dimensional "phase transition" show up as a maximum in the correlation time, which however never exceeds ${10}^{-13\mathrm{}}$ sec. The spin-lattice relaxation of the conduction electrons via phonons and molecular vibrations in the metallic state has been observed. The results are consistent with Elliot's theory in which $T_1^{-1\mathrm{}}\sim {\left(\frac{\Delta g}{g}\right)}^2{\tau }_R^{-1\mathrm{}}$, where ${\tau }_R$ is the scattering time as determined from the resistivity.