Band Electronic Structure of One- and Two-Dimensional Pentacene Molecular Crystals

We report EHT calculations of the band electronic structure of substituted pentacene derivatives and the polymorphs of the parent compound. The results show that there are wide disparities among the bandwidths and electronic dimensionalities of these compounds. The parent pentacene polymorphs are 2-dimensional in their band electronic structure with moderate dispersions; the bandwidths in the 14.1 Å d-spacing polymorph are noticeably larger than for the 14.5 Å d-spacing polymorph, reported by Campbell. Whereas the parent pentacene polymorphs adopt the well-known herringbone packing, the new, substituted pentacenes are noticeably different in their solid state structures and this is reflected in the band electronic structures. TMS adopts a highly 1-dimensional structure that leads to a large bandwidth along the stacking direction; TIPS also adopts a stacked structure, but because the molecules are laterally interleaved in the fashion of bricks in a wall, this compound is strongly 2-dimensional.

Variable-range hopping in quasi-one-dimensional electron crystals

We study the effect of impurities on the ground state and the low-temperature dc transport in a 1D chain and quasi-1D systems of many parallel chains. We assume that strong interactions impose a short-range periodicicity of the electron positions. The long-range order of such an electron crystal (or equivalently, a $4 k_F$ charge-density wave) is destroyed by impurities. The 3D array of chains behaves differently at large and at small impurity concentrations $N$. At large $N$, impurities divide the chains into metallic rods. The low-temperature conductivity is due to the variable-range hopping of electrons between the rods. It obeys the Efros-Shklovskii (ES) law and increases exponentially as $N$ decreases. When $N$ is small, the metallic-rod picture of the ground state survives only in the form of rare clusters of atypically short rods. They are the source of low-energy charge excitations. In the bulk the charge excitations are gapped and the electron crystal is pinned collectively. A strongly anisotropic screening of the Coulomb potential produces an unconventional linear in energy Coulomb gap and a new law of the variable-range hopping $-\ln\sigma \sim (T_1 / T)^{2/5}$. $T_1$ remains constant over a finite range of impurity concentrations. At smaller $N$ the 2/5-law is replaced by the Mott law, where the conductivity gets suppressed as $N$ goes down. Thus, the overall dependence of $\sigma$ on $N$ is nonmonotonic. In 1D, the granular-rod picture and the ES apply at all $N$. The conductivity decreases exponentially with $N$. Our theory provides a qualitative explanation for the transport in organic charge-density wave compounds.Comment: 20 pages, 7 figures. (v1) The abstract is abridged to 24 lines. For the full abstract, see the manuscript (v2) several changes in presentation per referee's comments. No change in result

Spatial distribution of nonlinear current in NbSe3

Spatial distribution of the electric potential has been studied in NbSe3 in both linear and nonlinear regimes. In the nonlinear regime two characteristic distances were found. The larger one corresponds to a long-scale smooth spatial variation of the nonlinear current and is close to the sample length. A simple mode1 of dislocation motion accounted for such behaviour is offered. the smaller one depends on the nonlinear current and may be related with injection of CDW excitations having the life-time of a few milliseconds

Contact and bulk electromodulated transmission spectra of blue bronze

We have used electromodulated infrared transmission spectroscopy to search for states associated with soliton excitations of the CDW phase in blue bronze with tunable diode lasers. In particular, we have searched the spectral region near the edge of the CDW gap and compared the spectra associated with contact polarization (i.e. current injection) with that associated with bulk CDW polarization. No new states are observed, implying that the lifetime of injected solitons is < 0.5 µs. Both bulk and contact polarization are observed to shift the frequency of high energy phonons, suggesting that the density of electronic states in the gap, present because of CDW fluctuations, depends on CDW polarization

Spatial distribution of charge-density-wave excitations due to current conversion near contacts

The non uniform spatial distribution of the electric field E along the quasi-one-dimensional conductor (o-TaS3) has been studied in the low-temperature range (T VT the electric field increases near both contacts and spatial distribution of E becomes more symmetrical