Non-ohmic properties and electric breakdown of quasi one-dimensional organic conductors

The dependence of the d.c. conductivity of single crystals of TTF-TCNQ on the electric field strength has been investigated. At 4.2 K drastic deviations from Ohm's law are observed. At an electric field strength of 300–600 V/cm, dependent on the room temperature conductivity, a reversible breakdown occurs connected with a rise of the conductivity by about three orders of magnitude. Similar effects have been found in single crystals of (TTF)₇J₅ and (TTF)J₂ in the temperature range around 100 K. Possible mechanisms responsible for this large conductivity changes are discussed.     

Temperature dependent microwave dielectric constant of TTF-TCNQ and related one-dimensional conductors

The results of an experimental study of the low temperature (4.2–40 K) complex dielectric constant of the organic conductors (TTF)(TCNQ), (TMTTF)(TCNQ), (DSeDTF)(TCNQ), (TSeF)(TCNQ), and the alloy (TTF)_0.97(TSeF)_0.03(TCNQ) are described. The similar features observed in these different systems suggest that a common mechanism is involved.     

Nonlinear transport at low temperatures in peierls-distorted quasi one-dimensional conductors

The carrier mobility deduced from the experimental low-field conductivity σ and the carrier concentration (calculated from the known gap) is ～ 10⁴ cm²/Vsec in TTF-TCNQ (tetrathiafulvalene tetracyanoquinodimethane) and its Se analog TSeF-TCNQ. The observed low temperature increase in σ with field arises from heating of the carriers. This results in several nonlinear effects, e.g., increase in the mobility, surmounting of barriers (arising from charge density wave accommodation to defects) and creation of electron-hole pairs.     

Localization in a one-dimensional disordered model

Numerical investigation of a random, one dimensional Kronig-Penny-like model is performed using long chains and large ensembles. Dependence of the inverse localization length α on randomness, irreproducibility of resistance measurements and the dependence of the standard deviation of α on α and the length of the chain were studied. For energies, E=k² close to the zone boundary k=π, we have found α～(π-k).     

Constant dielectric loss in disordered ionic conductors: Theoretical aspects

Cooperative charge relaxation within a random system of electrostatically interacting defect centers provides a mechanism for a “nearly constant dielectric loss” (NCL) response in structurally disordered ionic conductors. Pertinent models based on statistical mechanics are reviewed briefly. In addition, we present a theoretical frame for the problem of how two kinds of ionic motion, hopping migration and NCL-type local charge relaxation, are superimposed in the total ac-response. Using renewal theory, the modification of NCL-spectra due to hopping is calculated in terms of the waiting time distribution for ionic hops. In the special case of Poissonian hops with average rate λ the modified complex dielectric susceptibility in the NCL-regime is obtained by analytic continuation from the corresponding susceptibility in the absence of hops. Implications with respect to the crossover between hopping transport and NCL-behavior are discussed.     

Localization properties of driven disordered one-dimensional systems

We generalize the definition of localization length to disordered systems driven by a time-periodic potential using a Floquet-Green function formalism. We study its dependence on the amplitude and frequency of the driving field in a one-dimensional tight-binding model with different amounts of disorder in the lattice. As compared to the autonomous system, the localization length for the driven system can increase or decrease depending on the frequency of the driving. We investigate the dependence of the localization length with the particle's energy and prove that it is always periodic. Its maximum is not necessarily at the band center as in the non-driven case. We study the adiabatic limit by introducing a phenomenological inelastic scattering rate which limits the delocalizing effect of low-frequency fields.     

High-frequency dielectric constant of a two-dimensionally disordered model medium

An exact solution is derived for the two-dimensional two-phase model of a disordered medium proposed by Morozovskii and Snarskii [Ukr. Fiz. Zh. (Russ. Ed.) 28, 1230 (1983)] that arises because of hierarchical mixing of phases with different dielectric constants. The problem reduces to a nonlinear recurrence relation for the dielectric constants. It is found that the expressions for the dielectric constants at the nth stage of the iteration process can be expressed in terms of elementary functions. It is also found that in the absence of absorption the high-frequency dielectric constant of a composite material with different signs of the initial dielectric constants ε does not converge to a limit and is an oscillatory function of the frequency and n. Finally, for massive and thin-film samples, the local plasmon frequencies in the medium are established. Zh. éksp. Teor. Fiz. 114, 669–675 (August 1998)     

Hall mobility in quasi one-dimensional conductors with charge density or spin density wave ground state

High mobilities found for free carriers below the phase transition in quasi one-dimensional crystals such as TTF-TCNQ and (TMTSF)₂PF₆ indicate that defect scattering is unimportant. We calculate the Hall mobility due to phonon scattering and find good agreement with the measured value of 10⁴cm²/Vsec for (TMTSF)₂PF₆ at 4K.     

Soliton model for the dielectric permeability of some quasi one-dimensional charge transfer salts

The dielectric permeability of bound soliton antisoliton pairs is considered. The model yields and explanation for the anomanously large dielectric constant, the conductivity at microwave frequencies and their low temperature behaviour observed in some quasi one-dimensional organic charge transfer.     

Growth,crystal structure and transport properties of quasi one-dimensional conductors NbS3

We report synthesis of quasi-one-dimensional conductor NbS₃, TEM studies and transport properties of the prepared samples. The grown NbS₃ whiskers are Peierls conductors known as phase II with the transitions at T_P1=365 K and T_P2=150 K. CDW1 is stable and not so sensitive to growth conditions. It can slide and be synchronized by the external microwave irradiation up to 16 GHz. CDW2 strongly depends on growth conditions. Nevertheless, it also shows sliding and synchronization.     

一维有机导体的Peierls相变研究

根据建立在电子-声子相互作用基础上的Peierls 相变理论，利用形变势模型和半经验晶体轨道方法计算的能带结构数据，对一维有机导体tetrathiafulvalene-tetracyanoquinodimethane的Peierls相变温度进行了计算.并对其金属-绝缘体相变机制进行了讨论. 关键词： 电子-声子相互作用 Peierls相变温度 一维有机导体     

Localization length of wave functions in one-dimensional disordered systems with periodic modulation

The localization length of wave functions in one-dimensional disordered systems with periodic modulation is studied. The role of spatial inhomogeneity in the problem is considered. We calculate the localization length varying with energy for the compositionally modulated systems with either the disorder of Anderson distributions or the randomness of periods. The results show that the non-uniformity of disordered systems leads to some different properties of the localized wave functions.     

Localization effects and dispersion of the dielectric response function in one-dimensional metals

We report the results of calculations and analysis concerning the dependence of the dielectric response function(, q) on frequency and wave numberq in one-dimensional conductors. The localization of electron states leads to unusually complicated dependences of(, q) on andq in low-frequency and long-wavelength regions, while the spatial and time dispersions become closely interwoven with each other. The effect of geometric resonance is discussed. It appears as quasiharmonical oscillations of complex susceptibility as function of andq, owing to the hopping nature of electron conductivity in a nonuniform electromagnetic field.     

Electrical resistivity studies of certain quasi one-dimensional organic systems at high pressures and low temperatures

We report the electrical resistivity studies of six quasi one-dimensional organic systems under high pressures up to 8 GPa and temperatures down to 77K. The room temperature resistivity of these complexes lies in the wide range from 10⁸ ohm cm to 0.05 ohm cm, but they have common features under high pressures. The possible interpretations of these behaviours have also been discussed.     

Anisotropic dielectric constant of TTF-TCNQ observed by dielectric resonance

Independent measurements at low temperatures of the anisotropic dielectric constant (∈₁) of TTF-TCNQ are reported. The measurements utilize dielectric resonance techniques to determine ∈^B₁ and ∈^a₁ on the same crystal. The results confirm the unusually large value for ∈^b₁(4.2 K) > 10³ and the anisotropic behavior ∈^b₁(4.2 K) > 10² found earlier in cavity perturbation studies of TTF-TCNQ.