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.     

Giant magnetoresistance in the variable-range hopping regime

We predict the universal power-law dependence of the localization length on the magnetic field in the strongly localized regime. This effect is due to the orbital quantum interference. Physically, this dependence shows up in an anomalously large negative magnetoresistance in the hopping regime. The reason for the universality is that the problem of the electron tunneling in a random media belongs to the same universality class as the directed polymer problem even in the case of wave functions of random sign. We present numerical simulations that prove this conjecture. We discuss the existing experiments that show anomalously large magnetoresistance. We also discuss the role of localized spins in real materials and the spin polarizing effect of the magnetic field.     

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.     

Percolation and variable-range hopping in random systems with spherical site symmetry

We show that the numerical work of Seager and Pike^1,2 suggests that the critical volume fraction (CVF) is a constant for sites of spherical symmetry in n dimensions, with $\text{CVF}\text{?nπ}{}^{\mathrm{1?n}}$ for small n. The average number of bonds per site, $\text{B}\text{?}{}_{\mathrm{c}}$, is calculated for a random distribution of site radii, and shown to agree with the Monte Carlo calculation. Analysis of a model having spherical site symmetry in (position-energy) space yields percolation constants C₂ = 2.1, C₃ = 2.6. This calculation indicates that there is an anomaly in some estimated values for the AHL percolation model. The physical significance of our model and its possible use in hard-core problems is discussed.     

Metal-insulator transition and variable-range hopping conductivity of n-CdSb in magnetic field

Resistivity, ρ, of a II-V group semiconductor n-CdSb doped with In is investigated in pulsed magnetic fields up to and at temperatures . The low-temperature resistivity ρ(T) increasing with T in the range of B<4 T is found to have an upturn around B∼4 T and strong activated behavior at further increase of B. These observations give evidence for magnetic-field-induced metal-insulator transition (MIT). In the insulating side of the MIT, Mott variable-range hopping (VRH) conductivity with two types of asymptotic behavior, ln ρ (T, B)∼T^−3/4B² and ln ρ (T, B)∼(B/T)^1/3, is established in low and high magnetic fields, respectively. The VRH conductivity is analyzed using a model of the near-edge electron energy spectrum established by investigations of the Hall effect. The VRH conductivity is shown to take place over the band tail states of one out of two impurity bands, which for T=0 and B=0 lie above the conduction band edge.     

Comment on two-dimensional variable-range hopping conduction in group IV amorphous elemental semiconductors

The temperature T and thickness t dependence of the d.c. conductivity σ in very thin films of amorphous germanium, silicon and carbon is discussed. It is shown that a simple two-dimensional variable-range hopping model in a homogeneous medium cannot account for the experimental results, although the predicted $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}$ dependence of ?n σ is often observed. The consequences of this result for hopping transport in group IV amorphous semiconductors are pointed out.     

Preexponential factor in variable-range hopping conduction in CuInTe2

We study the temperature dependence of the electrical resistivity in a single crystal of p-type uncompensated CuInTe₂ on the insulating side of the metal-insulator transition down to 0.4 K. We observe a crossover from Mott to Efros-Shklovskii variable-range hopping conduction. In Efros-Shklovskii-type conduction, the resistivity is best described by explicitly including a preexponential temperature dependence according to the general expression ρ=ρ₀T^αexp(T_ES/T)^1/2, with α≠0. A theory based on the resistor network model was developed to derive an explicit relation between α and the decay of the wavefunction of the localized states. A consistent correspondence between the asymptotic extension of the wavefunction and the conduction regime is proposed. The results indicate a new mechanism for a local resistivity maximum in insulators, not involving magnetic effects.     

Magnetoelectric effect in one-dimensional metallic conductors

The effect of a transverse magnetic field on a one-dimensional metallic current carrying conductor is considered. The field produces a transverse electric polarization which is wavevector dependent, and which causes a transverse magnetoelectric field. The magnitude of the effect is estimated for typical one dimensional conductors.     

Coulomb drag between one-dimensional conductors

We have analyzed Coulomb drag between currents of interacting electrons in two parallel one-dimensional conductors of finite length L attached to external reservoirs. For strong coupling, the relative fluctuations of electron density in the conductors acquire energy gap M. At energies larger than gamma = constxv(-)exp(-LM/v(-))/L+gamma(+), where gamma(+) is the impurity scattering rate, and, for L>v(-)/M, where v(-) is the fluctuation velocity, the gap leads to an "ideal" drag with almost equal currents in the conductors. At low energies the drag is suppressed by coherent instanton tunneling, and the zero-temperature transconductance vanishes, indicating the Fermi-liquid behavior.     

Effect of impurity scattering of a tunneling electron on variable-range hopping conduction

Variable-range hopping conduction in semiconductors is determined by the asymptotic behavior of impurity wave functions on distances much larger than mean interimpurity separation. Scattering of an impurity electron by the other impurities situated near its tunneling path is shown to result in a correction a to electron localization lengtha. This correction depends on the impurity scattering length and impurity concentrationN and may be of the order ofa(Na ³ ) ora(Na ³ )^1/2.     

Effect of magnetic field on Mott's variable-range hopping parameters in multiwall carbon nanotube mat

We report the temperature and magnetic field dependence of the conductivity of multiwall carbon nanotube mat in the temperature range 1.4-150 K and in magnetic fields up to 10 T. It is observed that charge transport in this system is governed by Mott’s variable-range hopping of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Mott’s various parameters, such as localization length, hopping length, hopping energy and density of states at the Fermi level are deduced from the variable-range hopping fit. The resistance of the sample decreases with the magnetic field applied in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states with the well-known consequence of a decrease in Mott’s T0 parameter in variable-range hopping. The application of magnetic field lowers the crossover temperature at which three-dimensional variable-range hopping turns to two-dimensional variable-range hopping. The conductivity on the lower temperature side is governed by the weak localization giving rise to positive magnetoconductance. Finally, a magnetic field-temperature diagram is proposed showing different regions for different kinds of transport mechanism.     

Raman scattering in one-dimensional ionic conductors Rb-priderites

Raman spectra of single crystals of RbAl and RbMg-priderites, which are known to be one-dimensional ionic conductors, have been measured and analyzed. In addition to the spectra due mainly to the vibrations of Ti(Al)-O or Ti(Mg)-O frameworks observed in the frequency region above 100 cm^?1, the Raman bands, which can be assigned to the vibration modes of Rb⁺ ions, have been observed in the frequency region below 100 cm^?1. Raman spectra of the solid solutions (Rb, K)-Al priderites have also been studied.     

Impurity-induced coherent current oscillations in one-dimensional conductors

The electronic transport through a single impurity in a repulsive Luttinger liquid (LL) has been theoretically studied. It has been found that the direct current Ī above the threshold voltage related to the strength of the impurity potential is accompanied by coherent oscillations with frequency f= Ī/e. There is an analogy with the Josephson junctions: the well-known regime of the power-law I−V curves in the LL corresponds to the damping of the Josephson current below the critical one, while the oscillatory regime in the LL can be compared with the Josephson oscillations above the critical current. The text was submitted by the authors in English.     

On thermopower in hopping transport

Certain general aspects of hopping transport in the context of thermopower are reinvestigated. Onsager symmetry of the macroscopic kinetic coefficients is derived from detailed balance of the microscopic coefficients of the kinetic equation by an expansion of the kinetic equation around local equilibrium in an external potential and temperature gradient. The resulting expression for the thermopower differs from expressions proposed in the literature. The difference however, seems to be small.     

Low frequency hopping conductivity in disordered one-dimensional lattice

We investigate classical hopping on a one-dimensional chain with random nearest-neighbour transfer rates. It is found, by studying corrections, that an Effective-Medium approach gives the exact asymptotic hopping conductivity at low frequencies for certain distributions of the transfer rates.     

Interfacial transport in lithium-ion conductors

Physical models of ion diffusion at different interfaces are reviewed. The use of impedance spectroscopy(IS), nuclear magnetic resonance(NMR), and secondary ion mass spectrometry(SIMS) techniques are also discussed. The diffusion of ions is fundamental to the operation of lithium-ion batteries, taking place not only within the grains but also across different interfaces. Interfacial ion transport usually contributes to the majority of the resistance in lithium-ion batteries. A greater understanding of the interfacial diffusion of ions is crucial to improving battery performance.     

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.