Phononless hopping in the mobility gap of amorphous semiconductors

Two different theories are analyzed and shown to yield the same result for the phononless contribution to the dc hopping conductivity due to electrons in localized states (with single-electron energies broadened by the electron-phonon interaction) in the mobility gap of amorphous semiconductors. Numerical estimate is then used to show that this mechanism cannot explain experimental data on elemental covalent amorphous semiconductors.     

ac Impurity hopping conducting in p-InSb

We report the first measurement of the ac conductivity of p-InSb at low temperatures in the fréquency range 10 −10¹⁰ Hz. The conductivity has been determined from the gathering of three sets of data, the dc conductivity curve, the conductivity data obtained from an ultrasonic experiment and the microwave conductivity. Frequency effects have only been observed in the hopping conduction regime. It was found that the ac conductivity varies as ω (S=0.75). The results are discussed in relation with the single-phonon hopping theory.     

Ac and Dc conductivity in amorphous silicon-hydrogen films

The ac conductivity of glow discharge deposited amorphous silicon-hydrogen films with metal, semiconducting, or insulating contacts have been measured in the frequency range 2 Hz – 30 MHz at temperatures between 170 and 300 K. In addition, the dc conductivity has been determined for most samples. The frequency and temperature dependences of the measured conductivity are compared quantitatively with the predictions of the quantum-mechanical tunneling theory and with the results obtained from a numerical analysis of a more recent selfconsistent theory of the ac and dc conductivity in disordered hopping systems.     

Alternative generalized master equation approach to the dc phonon-assisted hopping

As the momentum operator has no diagonal elements between localized states, the hopping conduction theory should be formulated in terms of the linear response of the site-off-diagonal elements of the single-electron density matrix to an external field. A theory of this kind, starting from generalized master equations and yielding the dc phonon-assisted hopping conductivity and thermopower is formulated. This is an alternative to the usual approach treating the current conduction via a time-derivative of the electric dipole momentum.Stimulating discussions with Dr. B. Velický turning the present author's attention to his own old ideas about the hopping conduction via off-diagonal elements of the single-electron density matrix are appreciated.     

Spin—Dependent Scattering Effects and Dimensional Crossover in a Quasi—Two—Dimensional Disordered Electron System

Two kinds of spin-dependent scattering effects (magnetic-impurity and spin-orbit scatterings) are investigated theoretically in a quasi-tow-dimensional (quasi-2D) disordered electron system.By making use of the diagrammatic techniques in perturbation theory,we have calculated the dc conductivity and magnetoresistance due to weak-localization effects,the analytical expressions of them are obtained as functions of the interlayer hopping energy and the characteristic times:elastic,inelastic,magnetic and spin-orbit scattering times.The relevant dimensional crossover behavior from 3D to 2D with decreasing the interlayer coupling is discussed,and the condition for the crossover is shown to be dependent on the aforementioned scattering times.At low temperature there exists a spin-dependent-scattering-induced dimensional crossover in this system.     

Ultrasonic attenuation due to hopping of silver ions in superionic conductors

The conductivity and ultrasonic absorption in superionic conductors are condsidered. The hopping motion of silver ions exhibit collective properties because of the Fermi type site occupation numbers. The calculated a.c. conductivity is different from the ordinary Drude expression. The ultrasonic wave modulate the hopping motion of the silver ions and produce space charge. The dissipation of the wave energy is due to this production and is calculated as Joule heating. The absorption coefficient is proportional to the conductivity and the square power of the frequency in agreement with the experimental results.     

Non-adiabatic polaron hopping conduction in CaMn1−xCrxO3 (0?x?0.3)

DC electrical conductivity (σ_dc) of electron-doped antiferromagnetic CaMn_1−xCr_xO₃ (0?x?0.3) has been discussed elaborately in the light of polaron hopping conduction. The increase in Cr doping concentration increases the conductivity and decreases the activation energy. Non-adiabatic polaron hopping conduction is observed in all the manganites at high temperatures. The analysis of σ_dc data shows that small polarons are formed at lower concentrations (?5%) of Cr doping and undoped samples. However, large polarons are materialized at higher doping (?10%) concentrations. This is consistent with the fact that doped Cr³⁺ has larger ionic size compared to that of Mn⁴⁺. Again, strong electron-phonon (e-ph) interaction is perceived in undoped and 5% Cr-doped samples but not in manganites with larger doping concentration. This also confirms the formation of larger polarons with the increase of x. Mott's variable range hopping (VRH) model can elucidate the dc conductivity at very low temperatures. It has been detected that single phonon-assisted hopping is responsible for the dc conduction in the Cr-doped CaMnO₃ manganites.     

Generalization of iterative perturbation theory and coherent potential approximation (ITP+CPA) to double exchange model with orbital degeneracy

A combination of the iterative perturbation theory (ITP) of the dynamical mean field theory (DMFT) and coherent-potential approximation (CPA) is generalized to the double exchange model with orbital degeneracy. The Hubbard interaction and the off-diagonal components for the hopping matrix $t_{ij}^{mn} (m \ne n)$ are considered in our calculation of spectrum and optical conductivity. The numerical results show that the effects of the non-diagonal hopping matrix elements are important.     

Compositional dependence of the electrical conductivity of calcium vanadate glassy semiconductors

Scanning electron microscopy (SEM), X- ray diffraction (XRD), density (d), oxygen molar volume (V_m) and dc conductivity of different compositions of calcium vanadate glasses are reported. SEM exhibits a surface without any presence of a microstructure which is a characteristic of the amorphous phase. The overall features of these XRD curves confirm the amorphous nature of the present glasses. Density was observed to decrease with an increase in V₂O₅ content. The experimental results were analyzed with reference to theoretical models existing in the literature. It has been observed that the high-temperature conductivity data are consistent with Mott's nearest-neighbor hopping model. However, both Mott variable-range hopping (VRH) and Greaves intermediate range hopping models are found to be applicable. The hopping at high temperatures in the calcium vanadate glasses occurs by non-adiabatic process in contrast to the vanadate glasses formed with conventional network formers. The hopping model of Schnakenberg can predict the temperature dependence of the conductivity data. The percolation model of Triberis and Friedman applied to the small polaron hopping (SPH) regime is also consistent with data. The various model parameters such as density of states, hopping energy, etc., obtained from the best fits were found to be consistent with the glass compositions.     

Direct current hopping conductance in one-dimensional diagonal disordered systems

Based on a tight-binding disordered model describing a single electron band, we establish a direct current (dc) electronic hopping transport conductance model of one-dimensional diagonal disordered systems, and also derive a dc conductance formula. By calculating the dc conductivity, the relationships between electric field and conductivity and between temperature and conductivity are analysed, and the role played by the degree of disorder in electronic transport is studied. The results indicate the conductivity of systems decreasing with the increase of the degree of disorder, characteristics of negative differential dependence of resistance on temperature at low temperatures in diagonal disordered systems, and the conductivity of systems decreasing with the increase of electric field, featuring the non-Ohm's law conductivity.     

Local atomic ordering in nickel based Ir and Rh alloys

Experimental measurements of the diffuse X-ray scattering are performed on alloys of Ni with Rh and Ir. The atomic short range order (SRO) parameters α_i are calculated from the measured intensity. The existence of SRO is established in the two systems. The values of α₁ are observed to have anomalously large negative values for all the samples.The experimental data so obtained is interpreted theoretically by calculating the interaction energies on the basis of electronic theory of ordering. Theoretically calculated values of interaction energies are found to be in agreement with the experimentally determined type of order in these alloys.     

Polaron effect and the dc phonon-assisted hopping conductivity in amorphous semiconductors

A new small parameterW of off-diagonal electron-phonon matrix elements with respect to diagonal ones is introduced. Limiting ourselves to terms of the second order inW, the dc phonon assisted hopping conductivity is calculated to the infinite order in the electron-phonon coupling constant. Result clearly exhibits the polaron effect which is discussed in detail.     

Rate equations in the hopping transport III. A completely new interpretation of the dc phonon-assisted hopping in the mobility gap

A new interpretation of the low-frequency phonon-assisted hopping conduction in the mobility gap of disordered semiconductors which is inherently connected with the effect of broadening of electronic levels due to electron-phonon coupling is suggested. This interpretation is fully compatible with our previous reported mathematical theory of the hopping conduction. It is argued that the current understanding of the hopping transport is based on a dubious neglect of the above mentioned broadening which makes the usual theories of the dc phonon-assisted hopping conduction inconsistent.     

Single- and multigrain nanojunctions with a self-assembled monolayer of conjugated molecules

Systematic conductivity measurements in nanoscale junctions containing a self-assembled monolayer of conjugated molecules are reported. Different conductivity mechanisms are identified depending on the granularity of the metal used as a substrate for assembling the monolayer. Unexpectedly, the energy scale controlling the dominant conductance channels is quite low in comparison with the molecular level spacing. In single-grain junctions, the dominant conductance mechanism is hopping with an energy scale of the order of 10-100 meV determined by the nature of the metal contacts. In the case of multigrain junctions, additional tunnel conductance is observed with low-energy Coulomb-blockade features.     

Mechanism of dc conduction in polyaniline doped with sulfuric acid

A temperature variation of dc conductivity in the range 77–300 K has been carried out in order to explore the mechanism of charge transport in polyaniline (PAN) doped with sulfuric acid. The variable range hopping (VRH) exponent changes as the transition of the PAN lattice takes place in a narrow pH range thereby indicating that the charge transport is crucially composition dependent. A decrease in activation energy has been observed as the doping level is increased. Spin concentration of charge carriers determined by electron spin resonance spectroscopy has also been found to depend on the doping level of the specimen. Polarons and bipolarons formed during the doping process are the charge carriers in this system. The temperature dependence of dc conductivity and activation energy data are indicative of existence of both VRH and mixed conduction for various doping levels in these samples.     

Transport and dielectric properties of V2O5-MnO-TeO2 glasses

The frequency (1-10 kHz) and temperature (80-350 K) dependences of the ac conductivity and dielectric constant of the V₂O₅-MnO-TeO₂ system, containing two transition-metal ions, have been measured. The dc conductivity _dc measured in the high-temperature range (200-450 K) decreases with addition of the oxide MnO. This is considered to be due to the formation of bonds such as V--O--Mn and Mn--O--Mn in the glass. The conductivity arises mainly from polaron hopping between V^4+M and V⁵⁺ ions. It is found that a mechanism of adiabatic small-polaron hopping is the most appropriate conduction model for these glasses. This is in sharp contrast with the behaviour of the Mn-free V₂O₅-TeO₂ glass, in which non-adiabatic hopping takes place. High-temperature conductivity data satisfy Mott's small-polaron hopping model and also a model proposed by Schnakenberg in 1968. A power-law behaviour ( _ac = Aω^s , with s < 1) is well exhibited by the ac conductivity σ_ac data of these glasses. Analysis of dielectric data indicates a Debye-type relaxation behaviour with a distribution of relaxation times. The MnO-concentration-dependent σ_ac data follow an overlapping large-polaron tunnelling model over the entire range of temperatures studied. The estimated model parameters are reasonable and consistent with changes in composition.     

Anisotropic properties and conduction mechanism of TlInSe2 chain semiconductor

TlInSe₂ chain crystals were prepared using the modification of the Bridgman technique. The grown crystals were identified by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). We investigate the anisotropy of transport properties for the first time for TlInSe₂ crystals. Temperature dependence of the dc electrical conductivity, Hall coefficient, Hall mobility, and charge carrier concentration were investigated in the temperature range 184–455 K. The conduction mechanism of TlInSe₂ crystals was studied, and measurements revealed that the dc behavior of the grown crystals can be described by Mott’s variable range hopping (VRH) model in the low temperature range, while it is due to thermoionic emission of charge carriers over the chain boundaries above 369 K. The Mott temperature, the density of states at the Fermi level, and the average hopping distance are estimated in the two crystallographic directions. The temperature dependence of the ac conductivity and the frequency exponent, s, is reasonably well interpreted in terms of the correlated barrier-hopping CBH model.     

The influence of exciton motion on spin-resonance absorption

We use a stochastic model for the exciton motion which comprises both the coherent and the incoherent motion. The incoherent part is taken care of by a stochastic process which allows the local excitation energy and the transition matrix element to fluctuate by means of a Markovian process. The interaction between the spins and their surroundings is described by the usual spin-Hamiltonian which is, however, simplified to a spin 1/2 particle (instead of the triplet state). In the present paper we solve exactly the two limiting cases of completely coherent and incoherent motion (for two molecules). In the incoherent case the influence of the exchange interaction integral is taken into account by perturbation theory. We find expressions which are immediately comparable with ESR-experimental data. This comparison and additional information derived from optical absorption measurements allow us to determine all free parameters of our model uniquely. In particular, the fluctuations of the exchange interaction integral (with strength γ₁) play an important role. From these parameters we may furthermore calculate the correlation time of the proton spin resonance in agreement with experimental data. The results show clearly that at room temperature in anthracene crystals the exciton undergoes a hopping process.     

Optical and electrical properties of binary WO3–Pb3O4 glasses

A glass system of the composition xWO₃+(100−x)Pb₃O₄, with x=5, 10, 20 and 30 mol.% was prepared. The optical absorption, ac and dc conductivities are the subject of the present work. The optical absorption indicates that the electronic transition is indirect and is associated with phonon assisted transition. The exponential dependence of the absorption coefficient as a function of the incident photon energy suggests that the Urbach rule is obeyed, and indicates the formation of a band tail. On the other hand, ac conductivity measurements are performed in the frequency range 0.1–100 kHz, and in the temperature range 300–600 K. The results of the electrical conductivity are discussed on the basis of electronic glass conduction models. Correlated narrow-band limit for random sites and single polaron hopping model are found to describe the experimental results effectively. The dielectric constant was correlated to the optical band gap and a satisfactory relation was found. It was also possible to calculate the thermochromic properties from independent ac and dc measurements, and it was possible to evaluate the optical gap at 0 K by extrapolation.     

Charge transport in the insulating state of (DMe-DCNQI)2Li above T(SP): a possible fractional charge soliton conduction with +/-1 / 2e

A spin-Peierls system (DMe-DCNQI)2Li is studied with W-band electron paramagnetic resonance (EPR) ( approximately 94 GHz) to unveil a charge transport mechanism in the insulating 4k(F) charge density wave state above T(SP). The electron hopping between the neighbor DCNQI columns provides an additional broadening of the EPR linewidth, since the neighbor columns are generally nonequivalent to each other with respect to g shift. The obtained intercolumn hopping rates lead us to the conclusion that the electron hopping to a hole soliton carrying a fractional charge of e / 2 in the neighbor column dominates the intercolumn charge transport.