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Register a new userTheory of the ferroelectric phase in organic conductors: optics and physics of solitons
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S. Brazovskii
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Recently the ferroelectric anomaly (Nad, Monceau, et al) followed by the charge disproportionation (Brown, et al) have been discovered in (TMTTF)2X compounds. The corresponding theory of the combined Mott-Hubbard state describes both effects by interference of the build-in nonequivalence of bonds and the spontaneous one of sites. The state gives rise to three types of solitons: pi solitons (holons) are observed via the activation energy Delta in the conductivity $G$; noninteger alpha solitons (the FE domain walls) provide the frequency dispersion of the ferroelectric response; combined spin-charge solitons determine G(T) below subsequent structural transitions of the tetramerisation. The photoconductivity gap 2Delta is determined by creations of soliton - antisoliton pairs. The optical edge lies well below, given by the collective ferroelectric mode which coexists with the combined electron-phonon resonance and the phonon antiresonance. The charge disproportionation and the ferroelectricity can exist hiddenly even in the Se subfamily giving rise to the unexplained yet low frequency optical peak, the enhanced pseudogap and traces of phonons activation.
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Comparing resistivity data of quasi-one dimensional superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4 along the least conducting c*-axis and along the high conductivity a -axis as a function of temperature and pressure, a low temperature regime is observed in which a unique scattering time governs transport along both directions of these anisotropic conductors. However, the pressure dependence of the anisotropy implies a large pressure dependence of the interlayer coupling. This is in agreement with the results of first-principles DFT calculations implying methyl group hyperconjugation in the TMTSF molecule. In this low temperature regime, both materials exhibit for rc a temperature dependence aT + bT^2. Taking into account the strong pressure dependence of the anisotropy, the T-linear rc is found to correlate with the suppression of the superconducting Tc, in close analogy with ra data. This work is revealing the domain of existence of the 3D coherent regime in the generic (TMTSF)_2X phase diagram and provides further support for the correlation between T-linear resistivity and superconductivity in non-conventional superconductors.
On the basis of an analysis of a 3/4-filled two-dimensional (2D) extended Hubbard model under the fluctuation-exchange approximation, we find Coulomb frustrated phase separation (PS) in a region of nonzero temperature, where the quantum critical phenomenon of charge ordering (CO) dominates. In quasi-2D organic conductors on the verge of CO, this frustrated PS provides a mechanism for generating spatial inhomogeneity, which is characterized by an extremely slow relaxation and an intermediate length scale.
We have investigated the magnetic field-induced metal-insulator transition in the tau-phase organic conductors, which occurs in fields above 35 T, and below 14 K, by magnetization, thermoelectric, and pressure dependent transport methods. Our results show that the transition is a bulk thermodynamic process where a magnetic field-dependent gap opens upon entry into the insulating state. We argue that the transition involves a magnetic field-induced change in the electronic structure.
We report on dc and microwave experiments of the low-dimensional organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$ClO$_4$ along the $a$, $b^{prime}$, and $c^*$ directions. In the normal state of (TMTSF)$_2$PF$_6$ below T=70 K, the dc resistivity follows a power-law with $rho_a$ and $rho_{b^{prime}}$ proportional to $T^2$ while $rho_{c^*}propto T$. Above $T = 100$ K the exponents extracted from the data for the $a$ and $c^*$ axes are consiste1nt with what is to be expected for a system of coupled one-dimensional chains (Luttinger liquid) and a dimensional crossover at a temperature of about 100 K. The $b^prime$ axis shows anomalous exponents that could be attributed to a large crossover between these two regimes. The contactless microwave measurements of single crystals along the $b^{prime}$-axis reveal an anomaly between 25 and 55 K which is not understood yet. The organic superconductor (TMTSF)$_2$ClO$_4$ is more a two-dimensional metal with an anisotropy $rho_a/rho_{b^{prime}}$ of approximately 2 at all temperatures. Such a low anisotropy is unexpected in view of the transfer integrals. Slight indications to one-dimensionality are found in the temperature dependent transport only above 200 K. Even along the least conducting $c^*$ direction no region with semiconducting behavior is revealed up to room temperature.
Low-dimensional organic conductors could establish themselves as model systems for the investigation of the physics in reduced dimensions. In the metallic state of a one-dimensional solid, Fermi-liquid theory breaks down and spin and charge degrees of freedom become separated. But the metallic phase is not stable in one dimension: as the temperature is reduced, the electronic charge and spin tend to arrange themselves in an ordered fashion due to strong correlations. The competition of the different interactions is responsible for which broken-symmetry ground state is eventually realized in a specific compound and which drives the system towards an insulating state. Here we review the various ordering phenomena and how they can be identified by optic and magnetic measurements. While the final results might look very similar in the case of a charge density wave and a charge-ordered metal, for instance, the physical cause is completely different. When density waves form, a gap opens in the density of states at the Fermi energy due to nesting of the one-dimension Fermi surface sheets. When a one-dimensional metal becomes a charge-ordered Mott insulator, on the other hand, the short-range Coulomb repulsion localizes the charge on the lattice sites and even causes certain charge patterns. We try to point out the similarities and conceptional differences of these phenomena and give an example for each of them. Particular emphasis will be put on collective phenomena which are inherently present as soon as ordering breaks the symmetry of the system.
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