Kim Carneiro

Organic superconductors: synthesis, structure, conductivity, and magnetic properties

Publisher Summary This chapter presents a study on synthesis, structure, conductivity, and magnetic properties of organic superconductors. Useful insight is gained by analyzing the crystallographic structures of synthetic metals and correlating them with their solid-state properties as exemplified by their electrical conductivities and magnetic susceptibilities. The chapter focuses only on TMTSF (tetramethyltetraselenafulvalene) and ET salts because they form the basis for all known organic superconductors, and because they display, more or less, all of the structural features and properties of earlier synthetic conductors. The chapter demonstrates that it is possible to give several ingredients that contribute to the recipe for the synthesis of an improved synthetic metal or superconductor. Synthesizing a molecular conductor generally falls into two separate parts. First, the starting material, the organic donor “conducting molecule” has to be prepared, and secondly, single crystals of the desired conducting salts must be grown. From structural and transport studies of the organic conductors based on (TMTSF) 2 X and (ET) 2 X, their varied properties appear both rich and complex. A unique feature is, however, the very small role that Coulomb interactions play, in particular when considering their stoichiometry.

Characterisation of the transport properties of conducting polypyrrole films

Abstract The conducting polypyrrole film, prepared electrochemically in aqueous solution has a dense, homogenous non-crystalline structure. Temperature dependent studies of thermopower and of DC and 35GHz conductivities have been performed on a group of such films. In order to interpret the results, a model is suggested by extending the Motts Variable Range Hopping (VRH) model to a system of mobile localized states. In this model a parameter l is introduced, which stands for the mean distance that a mobile localized state carrying an electron can be displaced before the electron hops away. In the case of conducting polypyrrole these states are referred to those localized on polarons and bipolarons. This extended VRH model reproduces the measured DC-conductivity and thermopower, and the fitted value for l is of the order of several pyrrole units. The finite value of l indicates the that the polaron and bipolaron defects in the polypyrrole films are mobile and they can move along the polymer chain as charge carriers.

X-ray scattering from Co0.83[Pt(C2O4]2]·6H2O: A system with two coupled order parameters

Abstract X-ray studies of the quasi-one-dimensional conductor Co0.83 [Pt(C2O4)2]·6H2O, Co(def)OP, show both a Peierls distortion with a period of 2k F = 1.70·2π/c and a separate (earlier reported) three-dimensional superlattice. The two distortions couple, so that the drastic decrease in electrical conductivity is accompanied by the occurence of the superlattice spots with a common critical temperature, T = (297 ± 4)K. The striking differences between Co(def)OP and Mg(def)OP are discussed and explained in terms of the different atomic structures of the cations.

New Results on Two Synthetic Conductors (TMTSF)2BRO4 and (BEDT-TTF)2I3

Abstract We present experimental studies of transport properties of (TMTSF)2BrO4 and (BEDTTTF)2I3. The behaviour of the TMTSF salt suggests transport via variable-range hopping among localized states. The BEDTTTF-salt exhibits thermopower which indicates almost isotropic metallic behaviour in the ab-plane.

A comparison of the electronic and the chemical and structural properties of quasi-1D anion deficient tetracyanoplatinates

Abstract The temperature dependences of the electrical conductivity of seven structurally similar quasi-one-dimensional anion deficient tetracyanoplatinates have been compared and discussed in terms of their chemical and structural properties. It is demonstrated that the variation from compound to compound of both the experimentally determined (κF, Δ) and calculated 1D parameters (ϵF, λ, Tp) can be understood in terms of the influence of the anions, cations, and water of hydration on the intra-chain PtPt distance and the degree of partial oxidation. The transverse electron coupling constant, ν, is calculated using the experimentally determined 3D ordering temperature T3D, and is also shown to be dependent on the constituents of the crystal lattice.

The Synthesis, Structure, Electrical Conduction Properties, and Theory of Divalent, Tetravalent, and One-Dimensional Partially Oxidized Tetracyanoplatinate Complexes

The most novel features of partially oxidized tetracyanoplatinate (POTCP) complexes containing platinum atoms in a nonintegral oxidation state are their highly anisotropic physical properties. The structural basis for these properties is the stacking of square-planar tetracyanoplatinate, “TCP” = [Pt(CN)4], groups, 1, as illustrated in Figure 1. While many square-planar TCP salts of Pt2.0+ are known which form columnar stacks containing infinite chains of Pt atoms, e.g., Rb2[Pt(CN)4] · 1.5H2O, (intrachain Pt-Pt separation, d Pt-Pt is 3.42 A), 13 striking changes in these complexes can occur if the intrachain separation is ~ 3.0 A or less (d Pt-Pt in Pt metal = 2.78 A).66 These changes include the appearance of salts with brilliant metallic lusters ranging from copper to bronze to gold and the onset of quasi-one-dimensional (1-D) metallic conductivity. These properties arise from electron delocalization along the overlapped Pt 5d z2 orbitals (Figure 1) Measurements of the electrical conductivity parallel to the Pt chain reveal values which may be 105 greater than the conductivity perpendicular to the chain direction.

On the Peierls transition in a periodic potential

Abstract The properties of a one-dimensional conductor in the presence of an external periodic potential V Q have been investigated. The low temperature gap is enhanced, the temperature dependence of the gap Δ(T) is smeared at T PF , the Peierls transition temperature in the absence of V Q , and hence there is no longer a real phase transition. We also relate the phase pinning frequency ω o to the size of V Q .

Surface area measurements on polypyrrole using SANS, AC impedance, and STM

Abstract Electrochemically prepared polypyrrole films grown on a platinum electrode have been studied and there surface areas have been measured using the following techniques: AC Impedance measurements, Small angle neutron scattering (SANS), and Scanning tunnelling microscopy. The results of the measurements show that the films have a very large surface area. The surface area derived from AC impedance and SANS gives an increase of 40–50 times that of a bare platinum electrode. The STM results only gives an 2.2 fold increase in surface area.

Diffuse X-ray scattering from several platinum chain compounds

Abstract Values of the Fermi wavevector for several platinum based one-dimensional conductors were determined from diffuse X-ray scattering measurements. The values were compared with those expected from the chemical compositions. The importance of conclusive values of this parameter is stressed and the coupling between Peierls instability and other structural instabilities is discussed.

The Peierls instability in one-dimensional conductors with arbitrary bandfilling

Abstract Mean field calculations of the Peierls instability have been carried out for arbitrary bandfilling both in the case of free electrons as well as for the tightbinding approximation. New expressions for Δ(0) and Tp are derived which change the experimentally derived λ to a more reasonable value. Δ(0)/kBTp may differ significantly from the BCS value 1.76 in the tightbinding approximation.