ALBERT

Notes: A way of calculating the electron correlation energy for the ground state of large organic molecules is presented. It is demonstrated that various contributions to the correlation energy may be described by simple analytic expressions. In the case of interatomic correlations, they depend only on the bond length, its kind (e.g., σ or π bond) and the atoms involved in the bonding. Intraatomic correlations for a given atom are shown to be determined by its total charge and the fraction of p electrons. The method is developed by starting from semiempirical (self-consistent field) calculations and building correlations into it. It is straightforward and simple to apply. Moreover, it provides considerable physical insight into the phenomenon of electron correlations. A number of tests of its accuracy are presented by considering small molecules for which a comparison with other calculations can be made. An overall satisfactory agreement is found.

Notes: Material properties of low-dimensional organic donor–acceptor (DA) metals are comparatively discussed. It is shown that the low-temperature superconductivity in organic solids and the nonconventional temperature dependence of the dc electrical conductivity σ have the same microscopic precursor, softening of lattice modes caused by a divergence of the generalized susceptibility on effectively flat Fermi surfaces (FS) (T dependence of σ), as well as the resulting enhancement of the electron–phonon coupling (superconductivity). Prerequisite for this behavior are sizeable electronic charge fluctuations 〈(Δn2i)〉 in the strongly correlated organic DA salts at any temperature. Nonvanishing fluctuations, which are maintained also in the limit of almost perfect interatomic correlations, are possible as a result of nonintegral effective electron densities n¯i≠1.0. The electronic charge fluctuations at T=0 K and finite temperature are studied by an analytic many-body model. The T dependence of the fluctuations is discussed as a function of the accessible electron density n¯i.Charge densities n¯i≠1.0 as realized in the highly conducting organic metals attenuate a possible temperature control on the fluctuations. For the integral 1:1 charge transfer salts of D+A− stoichiometry, negligible fluctuations (insulating Mott configurations) for temperatures 〈500 K are predicted. In addition to the T dependence of the electronic fluctuations, the mean-squared atomic fluctuations are studied as a function of temperature. The fluctuation amplitude diverges with increasing mode softening. Experimental consequences of this behavior are touched. A Kubo–Mori projector technique in a single-particle approximation is adopted to quantify the strong interrelation between the T variation of σ and FS properties (mode softening). Boundary expressions following a T−2 and T−1 law of σ(T) are formulated. The first relation is valid for systems with sizeable frequency renormalization; the second one for conventional metals with vanishing mode softening. The theoretical results suggest a new subclassification of the DA salts into two classes; FS properties serve as underlying criterion. The validity of the transport theory is demonstrated for the two-dimensional (BEDT-TTF)2ClO4 system. A two-mode model in the strong-coupling approximation is used to calculate superconducting transition temperatures TC of organic superconductors in the presence of Kohn mode softening. A remarkable influence of the frequency renormalization on the magnitude of TC is evaluated. An additional enhancement of TC is possible due to the divergence of the mean-squared atomic (lattice) displacement. Calculated TC numbers of (BEDT-TTF)2A superconductors are in fair agreement with experimental results.

Notes: The material properties of low-dimensional 2:1 donor (D)–acceptor (A) salts with integral mutual charge transfer (CT) are exceptional in the class of the synthetic metals. The electronic charge fluctuations in the majority component of D+2A− and A−2D+ compounds are of extremal character in the presence of strong electronic correlations. In the minority component the fluctuations are extensively suppressed. The corresponding solids are one-chain conductors. The charge fluctuations are investigated by a simple analytic model formulated on the basis of the local approach for the many-particle problem and the bond-orbital approximation for the independent-particle wave function. The Fermi surfaces of the D+2A− and A+2D− CT salts are effectively flat also for smaller anisotropy ratios. Structural phase transitions of the Peierls type leading to one-dimensional as well as two- and/or three-dimensional ordering are analyzed in a tight-binding framework. There exists a large range, where the one-dimensional dimerization is suppressed, but where n-dimensional (n=2, 3) ordering can still exist. Fermi-surface nesting in materials with smaller anisotropy ratios allows for the conservation of metastable solid-state configurations, i.e., configurations where the electron–lattice interaction is enhanced, but where a metal–insulator transition is suppressed. For the quasi-one-dimensional metals with D+2A− and A−2D+ stoichiometry it is shown that electronic correlations lead to a remarkable enhancement of the metal–insulator transition temperature TP. Experimental data available for the respective 2:1 CT salts can be rationalized by simple analytic models. A possible violation of particle–hole symmetry between D+2A− and A−2D+ compounds in their ability to stabilize a low-temperature superconducting ground state is suggested.

Notes: The dc electrical conductivity above the Peierls-transition temperature Tc of quasi-one-dimensional (1D) organic metals is calculated by an ab initio single-particle theory based on the Kubo–Mori formalism. The respective inverse relaxation time in this approach is calculated numerically. The theory allows for a reliable reproduction of experimentally derived normalized conductivity curves of highly anisotropic organic metals. The applicability of the model is reduced with decreasing anisotropy. Phenomenologically it can be shown that a T−2 law of decay of the dc electrical conductivity above Tc is conventionally connected with large anisotropies. Decreasing anisotropy leads to T−n curves, where n is sizeably smaller than 2. The theoretical approach reproduces a T−n, n≈2, law of decay in the framework of one-phonon electron scattering processes. Therefore it is suggested that the deviations from the T−1 behavior of conventional three-dimensional metals is caused by the strong Kohn anomaly in 1D systems. The influence of the electron–phonon coupling and the Debye temperature on the analytic structure of the normalized dc electrical conductivity curves has been studied. A general theoretical description is suggested to account for the influence of electronic correlations on the magnitude of the electron–phonon coupling as well as on the Peierls-transition temperature. It is shown qualitatively that the organic metals belong to a class of valence fluctuating systems where charge fluctuations are conserved also in the limit of strong electronic correlations.