Non-ohmic variable-range hopping transport in one-dimensional conductors

We investigate theoretically the effect of a finite electric field on the resistivity of a disordered one-dimensional system in the variable-range hopping regime. We find that at low fields the transport is inhibited by rare fluctuations in the random distribution of localized states that create high-resistance breaks in the hopping network. As the field increases, the breaks become less resistive. In strong fields the breaks are overrun and the electron distribution function is driven far from equilibrium. The logarithm of the resistance initially shows a simple exponential drop with the field, followed by a logarithmic dependence, and finally, by an inverse square-root law.     

Static dielectric constant of disordered organic quasi one-dimensional conductors: Localization by defects

We have measured the low temperature dielectric constant ? of two similar quasi one-dimensional organic conductors, N-Me-iso Qn(TCNQ)₂ and Qn(TCNQ)₂. For N-Me-iso Qn(TCNQ)₂ below 10 K, ? is independent of temperature and is frequency independent in the range 5 × 10⁵ Hz to 9 × 10⁹ Hz, within the 50% experimental uncertainty. Thus we believe the low temperature microwave dielectric constant to be a good approximation of the static value in this salt. For Qn(TCNQ)₂ at low temperatures, the relation ? ∝ (c+c₀)^-2 holds, where c is the defect concentration and c₀ is an effective defect concentration of the nominally pure material. This relation is predicted by the model of interrupted metallic strands with energy spacings larger than kT, and it indicates that electrons are strongly localized by defects along the conducting chains.     

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.     

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.     

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.     

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.     

The Fermi gas model of one-dimensional conductors

The Fermi gas model of one-dimensional conductors is reviewed. The exact solutions known for particular values of the coupling constants in a single chain problem (Tomonaga model, Luther-Emery model) are discussed. Renormalization group arguments are used to extend these solutions to arbitrary values of the couplings. The instabilities and possible ground states are studied by investigating the behaviour of the response functions. The relationship between this model and others is discussed and is used to obtain further information about the behaviour of the system. The model is generalized to a set of coupled chains to describe quasi-one-dimensional systems. The crossover from one-dimensional to three-dimensional behaviour and the type of ordering are discussed.     

Standard components of polar and axial vectors for quasi one-dimensional systems

The standard components of axial and polar vectors for line groups are found. Applications of these results to the theory of phase transitions and to the derivation of selection rules for quasi-one-dimensional systems are briefly outlined.     

Muon studies of organic ferromagnets and conductors

Muon spin rotation (μSR) experiments can be especially helpful in the study of various organic molecular materials. The technique has been used to study the magnetization in nitronyl nitroxide organic ferromagnets, the spin-density-wave ground state in various charge transfer salts, the dynamics of carriers in organic conductors and the vortex structures in organic superconductors. I outline the motivation for studying these new materials and review and discuss the results of recent work.     

Electrooptic properties of one-dimensional photonic crystals based on organic ferroelectric and dye

We present the results of our study of the electrooptic properties of a one-dimensional photonic crystal created from alternating thin layers of two organic compounds, a ferroelectric and a dye. We show that the ferroelectric subsystem of molecular layers in such a heterostructure can be polarized by an external electric field, leading to a linear electrooptic effect and bistability of the electrooptic response in the spectral region of the photonic stop band. On the other hand, a quasi-linear electrooptic effect attributable to the quadratic Stark effect and the built-in spatially periodic static electric field is observed in the spectral region of the absorption of dye molecules. The emergence of this built-in field is explained in terms of the continuity of the electric displacement vector in a spatially periodic dielectric heterostructure with a macroscopically polarized subsystem of ferroelectric layers.     

Electron scattering in a point contact of one-dimensional conductors

The scattering matrix of a point contact between one-dimensional coherent conductors is considered. It is shown that the flux conservation law, time-reversal symmetry, and an hypothesis of continuity of the wave function lead to parametrization of the scattering matrix by a single real parameter, regardless of the number of conductors connected by the contact. The condition of maximum transmission fixes this parameter and thereby uniquely defines the scattering matrix. The condition of flux conservation then reduces to the condition that the sum of the derivatives of the wave function with respect to the directions of the conductors vanish. Possible applications of the model considered to experimentally feasible arrays of one-dimensional elements are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 2070–2074 (November 1999)     

Non-equilibrium and band tailing in organic conductors

The concept of band tailing with focal point and width of the tail from IR absorption spectra of different organic conductors is found valid even for thermal and elastic changes. The experimental situations like change of solvents, method of preparation, applied pressure and pressure cycle apart from compositions is analyzed within the framework of tailing of states. Non-equilibrium due to coupling between applied energy and free electrons can be responsible for the exponential relaxation from non-equilibrium to equilibrium     

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.     

The Fano anti-resonance and IR-properties of organic conductors

At high temperatures, i.e. in the metallic state excitation of the IR-forbidden symmetric intramolecular modes is shown to become effective. Near the threshold of emission for such a phonon there appears a fine structure as a result of the acoustic phonon assisted excitation of the intramolecular mode by energetic band electrons. The absorption reveals the behaviour typical of the Fano anti-resonance. The infrared frequency dependence of the organic compounds conductivity is discussed.     

Magnetic field dependence of the Peierls instability in one-dimensional conductors

We show that an applied magnetic field will suppress the Peierls instability in one-dimensional conductors. The effect is big enough to be observable. In sufficiently strong fields the transition between the metallic and insulating state is due to the condensation of a phonon with wave vectorq≠2p _F, but close to 2p _F. The role of spinorbit interactions is also discussed.