Showing papers on "Variable-range hopping published in 1980"
TL;DR: In this paper, the authors review experimental data on the low-temperature behavior of pure and cation-doped 1T-TaS2 and suggest that the transition into the commensurate phase is accompanied by Mottlocalization while below helium temperatures, conduction proceeds by variable range hopping in disorder-localized states lying in a region where the Mott-Hubbard subbands slightly overlap.
Abstract: We review experimental data on the low-temperature behaviour of pure, and cation-doped 1T-TaS2, This suggests that the transition into the commensurate phase is accompanied by Mott-localization while below helium temperatures, conduction proceeds by variable range hopping in disorder-localized states lying in a region where the Mott-Hubbard subbands slightly overlap. Many properties of doped samples can be understood by assuming that the star-shaped atomic clusters characteristic of pure material are preferentially centred on impurities.
135 citations
IBM1
TL;DR: In this article, the transport and magnetic properties of pyrolyzed Kapton are studied for different pryolysis temperatures and the effect of these temperatures on the carrier number and mobility has been investigated.
73 citations
TL;DR: In this paper, a lower bound to the frequency range over which the spectral density of noise increases with decreasing frequency is found, and application of the theory to other strongly inhomogenous systems with percolation-type conduction is proposed.
56 citations
TL;DR: In this paper, a microscopic theory for conductivity and dispersive carrier propagation in disordered systems where the transport is due to hopping between localized states is presented, where the microscopic rate equations (which are interpreted as random-walk equations for single-particle diffusion) are solved directly by standard Green function techniques.
Abstract: A microscopic theory is presented for d.c. and a.c. conductivity and dispersive carrier propagation in disordered systems where the transport is due to hopping between localized states. The microscopic rate equations (which are interpreted as random-walk equations for single-particle diffusion) are solved directly by standard Green function techniques. Exact expressions are given for the frequency-dependent diffusion coefficient D(ω) and the transient current I(t) in an infinite polymer chain and a Bethe lattice. These systems exhibit dispersive-like characteristics as a consequence of structural disorder. Approximate expressions for these quantities in an R-hopping system are derived from a decoupling approximation. If unrenormalized perturbation theory is used (which means that back-and-forth hopping effects are not correlated) our results become identical to the continuous-time random-walk theory of Scher and Lax (1973), and Scher and Montroll (1975). With the help of our renormalized expressi...
49 citations
TL;DR: In this paper, the d.c. conductivity σdc and the thermopower S of a number of polyacene quinone radical polymers obey the relationship σ dc = σ o exp (− T o / T ) 1 3 and S/T = a+bT − 1 3, typical for two-dimensional variable-range hopping.
Abstract: The d.c. conductivity σdc and the thermopower S of a number of polyacene quinone radical polymers obey the relationship σ dc = σ o exp (− T o / T ) 1 3 and S/T = a+bT − 1 3 , typical for two-dimensional variable-range hopping. The results can be explained by Mott's model modified, however, by allowing for a linearly varying density of states near the Fermi level, and by taking into account the form of the wave function of the localized states. Special attention is paid to the prefactor σo which contains important hopping parameters, such as a frequency factor νo, usually assumed to be not higher than a typical phonon frequency (∼ 1013 s−1). The original calculation of Miller and Abrahams (MA) of the hopping rate, however, gives νo values ranging from 1016 to 1021 s−1. For the PAQR polymers reasonable estimates of the parameters are obtained when using MA expressions adapted for two-dimensional systems.
27 citations
TL;DR: In this article, the electrical and optical properties of the chalcogenide semiconductor (Se 32 Te 32 As 4 Ge 32 ) 100− x Ni xitx have been studied.
Abstract: The electrical and optical properties of the chalcogenide semiconductor (Se 32 Te 32 As 4 Ge 32 ) 100− x Ni xitx have been studied. As the Ni concentration is increased the electrical dc conductivity is drastically increased and variable range hopping conduction becomes dominant even above room temperature. The optical energy gap decreases with the Ni concentration from 1.18–0.95 eV. Ni-atoms in the chalcogenide semiconductor donate free electrons which occupy the gap state. This occupation causes the shift of the Fermi level toward the conduction band. It is an effect of this shift that the thermal activation energy is decreased. The decrease in optical energy gap is independent of the shift of the Fermi level and is ascribable to the appearance of the additional level located at 0.95 eV above the top of the valence band. This level originates from the 3d-level of the Ni-atom.
19 citations
TL;DR: In this paper, a computer simulation of transport by hopping between energy-distributed sites is presented, where the dependences of the carrier mobility upon temperature, site energy spread, and site localisation parameter are established.
Abstract: Transport by hopping between energetically-distributed sites is examined by means of a computer simulation. The dependences of the carrier mobility upon temperature, site energy spread, and site localisation parameter are established. As in the case of hopping between spatially-random sites, carrier transit pulses are not found to be highly dispersive. However, it is shown that restriction of hopping freedom to two or one dimensions can generate the anomalous dispersion observed experimentally. The temperature dependence of transit pulse dispersion, and its use for diagnosing experimental transport mechanisms, are re-examined.
19 citations
TL;DR: In this paper, the longitudinal resistance of n-InSb has been measured down to 30 mK in magnetic fields up to 40 kG. Variable range hopping was observed below 0.3 K in the non-metallic region induced by the magnetic field.
18 citations
TL;DR: The results of the drift mobility experiments for disordered simple hydrocarbons are presented in this paper, where the carrier mobilities are of the order of 10 −3 and 10 −5 cm 2 V −1 s −1 for tetracene and pterphenyl, respectively.
Abstract: The results of the drift mobility experiments for disordered simple hydrocarbons are presented. The carrier mobilities are of the order of 10 −3 and 10 −5 cm 2 V −1 s −1 for tetracene and p-terphenyl, respectively. The activation energy of the mobility is very small (a few kT at room temperature). These results are interpreted in terms of the hopping transport among localized states situated near the Fermi level.
18 citations
TL;DR: In this paper, conductivity measurements were made on thin amorphous films of Te x Se 1−x of various thicknesses in the temperature range 100 −330 K. The effect of the addition of selenium to tellurium is to reduce the density of states near the Fermi level and to increase the conductivity activation energy.
17 citations
TL;DR: In this article, the effects of group-III and group-V doping on a-Si films prepared by high pressure rf sputtering were studied and three different types of doping effects on conductivity were observed for different elements.
Abstract: Doping effects of group-III and -V element on the a-Si films prepared by "high" pressure rf sputtering were studied. Three different types of doping effects on conductivity were observed for different elements. One is the increase of conductivity due to the shift of the Fermi level, another is the increase of conductivity due to the increase of hopping sites contributing to variable range hopping conduction and the third is that doping is less effective on conductivity. Distinct adsorption effects on conductance were observed for the cases of B- or Al-doping. These phenomena indicate that the band-bending is caused in surface regions by adsorbates because of a relatively small density of localized gap states, so the formation of built-in potential due to Schottky barrier or p-n junction would be possible for those a-Si films prepared by "high" pressure rf sputtering.
TL;DR: In this paper, the results of the temperature dependence of the d.c. electrical conductivity σ in thin (5-50A) and thick ( ≥ 500 A ) amorphous germanium films prepared in UHV and measured in situ are reported.
Abstract: The results of the temperature dependence of the d.c. electrical conductivity σ in thin (5–50A) and thick ( ≥ 500 A ) amorphous germanium films prepared in UHV and measured in situ are reported. The temperature T dependence of log10 σ as predicted by simple variable range hopping model for thick ( T −1 4 behaviour) and thin ( T −1 3 behaviour) films has not been observed, except in a certain special case. The consequences of these results to the concept of variable range hopping in amorphous germanium are pointed out.
TL;DR: In this paper, an analytic theory, developed to treat electronic hopping transport in disordered systems, is applied to the magnetoresistance of impurity conduction in doped semiconductors.
TL;DR: In this paper, the decay as a function of time of the mean drift velocity of photoexcited carriers hopping through a narrow band of localized states is considered, and lower bounds are placed on the drift velocity which are often far larger than the smallest velocity predicted by the SSA.
Abstract: The results of calculations of the short-time and infinite-time hopping conductivity of model disordered systems are used as the basis of a critical examination of the single-site approximation scheme (SSA) that has been employed extensively to study time-dependent transport phenomena in disordered insulators. The Scher and Montroll theory of dispersive transport, which has been used successfully to interpret transient photoconductivity experiments, invokes this approximation scheme to identify the hopping problem with a continuous-time random walk on an ordered lattice. Present results indicate that, while the SSA yields exact results in the short-time limit, it underestimates, sometimes seriously, the long-time or dc conductivity. In particular, the decay as a function of time of the mean drift velocity of photoexcited carriers hopping through a narrow band of localized states is considered here. Lower bounds are placed on the drift velocity which are often far larger than the smallest velocity predicted by the SSA. The implications of this failure of the SSA for the interpretation of experiment are also discussed.
TL;DR: In this article, the effect of electron-electron interactions on hopping transport is discussed in the low temperature limit, where it is shown that certain many-electron excitations are important in addition to those considered in earlier studies.
Abstract: The effect of electron-electron interactions on hopping transport is discussed in the low temperature limit It is shown that certain many-electron excitations are important in addition to those considered in earlier studies Mott's original method for the calculation of variable range hopping is adapted to correlated hopping This results in a power dependence of the conductivity on the temperature, below a certain temperature
TL;DR: In this article, the electrical resistivity of doped semiconductor Si:P in the intermediate concentration range was measured under various surface conditions, and the surface treatments were shown to have a great influence upon the low temperature electrical conduction.
TL;DR: In this article, the conductivity of a sample of reduced channel-plate lead glass over the temperature range 25-300 K was measured and the behavior was consistent with conduction by two-dimensional variable range hopping.
Abstract: We have measured the conductivity of a sample of reduced channel‐plate lead glass over the temperature range 25–300 K. The behavior is consistent with conduction by two‐dimensional variable range hopping. From measurements of the thickness of the conducting layer we estimate the wave‐function overlap parameter α−1⩾50 A and hence from the temperature dependence of the conduction we deduce that the density of states at the Fermi level is less than 4×1017 eV−1 cm−3.
TL;DR: In this paper, the resistivity of doped semiconductor Si: P in the intermidiate concentration range was measured in low temperature region down to 10 mK. In the lowest temperatures Mott's formula for the variable range hopping is shown to be not applicable and T -n -like temperature dependence of resistivity is confirmed.
01 Jan 1980
TL;DR: A concise overview of the theory of the small-polaron Hall effect is presented in this paper with emphasis being placed on the physical original of small polaron's distinctive properties.
Abstract: The Hall mobility associated with hopping motion differs qualitatively from that attributed to the notion of itinerant carriers. Particularly striking is the occurrence of Hall-effect sign anomalies: electrons and holes which circulate in a magnetic field in the sense of positive and negative charges, respectively. A concise overview of the theory of the small-polaron Hall effect is presented with emphasis being placed on the physical original of the small polaron’s distinctive properties.
TL;DR: In this paper, a continuous metal-nonmetal transition is observed in solid ArLa mixtures, where the d.c. conductivity starts at a La atomic fraction cLa ≅ 0.15 with σ = 2 × 10 -2 [Ω-cm] -1 and increases exponentially with cLa until σ reaches the minimum metallic conductivity σ min = 300 ± 100[Ω]-cm] −1 at cLa = 0.4.
TL;DR: In this paper, the temperature dependence of electrical resistivities of 1T-TaS 2, 1T -TaS 1, 2, and 3 T-Fe x TaS 1-S 2 x Se x and 1T Fe x Ta 1-X S 2 x S 2 is investigated.
Abstract: The temperature dependence of electrical resistivities θ( T ) of 1T-TaS 2 , 1T-TaS 2- x Se x and 1T-Fe x Ta 1- x S 2 is found to be θ( T ) ∝ exp( T 0 / T ) 1/n in the temperature range of 4 K to the measured lowest temperature, 0.1 K, showing the variable range hopping of Anderson localized states. The n -value is nearly 3 for selenium doping and nearly 2 for non-doping and iron doping. The positive magnetoresistance, which is sizable only in the temperature range of 2 K to 0.5 K in 1T-TaS 2 , is found to be remarkably enhanced by the selenium doping, while the tendency is reversed by the iron doping.
TL;DR: In this article, the low frequency hopping conductivity of disordered systems is calculated in a cluster approximation at low temperatures where the temperature dependence of the DC conductivity is described by the Mott law.
Abstract: The low-frequency hopping conductivity of disordered systems is calculated in a cluster approximation at low temperatures where the temperature dependence of the DC conductivity is described by the Mott law. Calculations show that in the high-frequency region sigma ( omega , T) approximately omega s(T) exp(-p(T0/T)p), where s(T) increases, tending to unity as the temperature T decreases, p=1/4 for a finite density of electronic states at the Fermi level. The results obtained agree with the available experimental data on frequency and temperature dependence of the impurity conductivity in doped semiconductors.
TL;DR: In this paper, a polycrystalline films of copper-doped tellurium were grown by co-evaporating copper and gold from separate boats, and measurements were made of the electrical transport properties of these films.
TL;DR: The optical gap decreases with the increase in transition element content and electrical conduction is suggested to occur mainly by variable range hopping through transition element sites for samples doped with more than 1 at.% as discussed by the authors.
Abstract: Electrical conductivity largely increases and the center density of ESR due to dangling bonds decreases by the addition of transition elements in a-Si. ESR signals due to transition elements are observed at low temperature, the center density of which are largest for about 1 at.% doped samples. In the case of Mn, a signal with hyperfine lines appears. The optical gap decreases with the increase in transition element content. The electrical conduction is suggested to occur mainly by variable range hopping through transition element sites for samples doped with more than 1 at.%.
TL;DR: In this article, the hopping rate involving electron-electron interaction is estimated for an impurity band and is found to exceed that involving electronphonon coupling, and it is shown that electron-phonons interaction is more stable than electron-electric interaction.
TL;DR: In this paper, the authors derived exact expressions for the frequency dependent conductivity of a system whose dynamics is described by a Master Equation in which the transition ratesΓab can be functions of other dynamical variables.
Abstract: We derive explicit exact expressions for the frequency dependent conductivity of a system whose dynamics is described by a Master Equation in which the transition ratesΓab can be functions of other dynamical variables. We apply this method to study the hopping of particles in a lattice with the hopping rate modulated by an additional harmonic variable. The results of this model are related to the observed microwave conductivity of superionic conductors. In particular we show that the modulation of the hopping rate can produce a structure similar to the one observed in AgI type systems.
TL;DR: In this paper, a charge transport by correlated hopping of electrons in moderately compensated semiconductors is studied, based on the two-electron single-phonon hopping model reveals an interference effect between the contributions of different electrons to the current.
Abstract: A charge transport by the correlated hopping of electrons in moderately compensated semiconductors is studied. The calculation based on the two-electron single-phonon hopping model reveals an interference effect between the contributions of different electrons to the current. This can lead to an increase of the conductivity. It is also shown that approximations which go beyond the Hartree method are required.
TL;DR: In this paper, the influence of impurity phonon modes, including local modes, on the electronic jump rate was investigated and a very general formalism was developed for studying this particular problem, which may provide a useful alternative to existing multiphonon jump-rate theories.
Abstract: Most of the previous work on the theory of thermally assisted hopping between localized electronic states has assumed a perfect Debye phonon spectrum. The purpose of the present contribution is to consider the influence of impurity phonon modes, including local modes, on the electronic jump rate. In this paper we develop a very general formalism which is not only ideal for studying this particular problem but which may provide a useful alternative to existing multiphononjump-rate theories. A numerical investigation into the effect of impurity phonon modes on the hopping rate is pursued in the accompanying paper.
TL;DR: A review of electrical, structural and magnetic properties of pure and doped IT-TaS2 has been carried out in this article, showing that the transition into the commensurate phase at 200 K is accompanied by the Mott localization of 1/13th of the Ta 5d electrons.
Abstract: A review of electrical, structural and magnetic properties of pure and doped IT-TaS2 suggests that the transition into the commensurate phase at 200 K is accompanied by the Mott localization of 1/13th of the Ta 5d electrons. States at the edges of the almost touching Hubbard subbands become Anderson-localized and, at helium temperatures, conduction proceeds by variable range hopping. The rest of the d-electrons occupies the bonding-type orbitals of the star-shaped atomic clusters. Many properties of the doped samples can be understood by assuming that these clusters are preferentially centred on less electronegative impurities.