Abstract: The low-temperature charge transport in n-CuGaSe2 was investigated in zero and nonzero magnetic field Both the Mott as well as the Shklovskii–Efros regimes of the variable-range hopping are observed in different temperature intervals The complete set of the parameters describing the properties of the localized electrons (the localization radius, the dielectric permeability, the width of the Coulomb gap, and the values of density of states at the Fermi level) are obtained by analysis of the conductivity in zero field, on one hand, and the positive magnetoresistance in a small field, on the other hand The negative magnetoresistance in low fields is observed in all specimens in both hopping regimes Moreover, it is interpreted as a result of quantum interference between different paths of the tunneling electrons in conditions of scattering by intermediate centers

Abstract: Homogenous (Bi3Pb)Sr3Ca3 (Cu4−nCrn)Ox (n 4 0 to 020) type glassy precursors become high-Tc superconductors by annealing at 840 °C The suppression of Tc with increase of Cr concentration supports the pair-breaking mechanism The feeble semiconducting behavior shown by the doped samples above their respective Tc values followed Mott's variable range hopping conduction mechanism Like Ti- and Fe-doped samples, studied earlier, the thermoelectric power (TEP) of the present Cr-containing sample showed small positive peak above Tc, which was considered to be associated with the phonon-drag effect The linear part of the temperature-dependent TEP (above Tc) well fitted the two-band model

Abstract: Novel compounds T6MBi2, where T=Zr, Hf; M=Fe, Co, Ni, were synthesised by arc melting elemental combinations. Isotypism of these compounds with the ordered Fe2P-type was established from Rietveld X-ray powder data. Physical properties (electrical resistivity and thermoelectric power) were investigated for a series of ternary transition metal compounds with stoichiometries such as TM1−xX and T6MX2, where T is an electropositive transition metal (Zr, Hf and Nb), M is a metal from the iron group (Fe, Co, Ni) and X is Sb or Bi. Resistivities reveal a general metallic behaviour except for ZrCoSb and NbFeSb which are characterized by variable range hopping conductivity in a substantial temperature region. Thermoelectric power of these materials does not exceed 100 μV K−1.

Abstract: The resistance R of a “low-resistance” microchannel plate (MCP) electron multiplier was measured over the extended temperature range 4–293 K. The ratio R(T)/R(293 K) reached values of 102, 103 and 106 at temperatures T=59, 40 and 19 K, respectively. These results are interpreted in terms of variable range hopping conduction mechanisms in the reduced lead silicate MCP glass. The results may be important for cryogenic charged particle detection using MCPs, as, for example, in a recent proposal for a quantum computer.

Abstract: The preparation of vanadate glasses containing barium oxide and their electrical properties in the temperature range of 80–500 K have been reported in this article. Analysis of the electrical properties has been made in the light of different hopping models. The multiphonon assisted hopping model of small polarons in the nonadiabatic regime, proposed by D. Emin [Phys. Rev. Lett. 32, 303 (1974)] have been observed to describe the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with the glass composition. J. Schnakenberg’s model [Phys. Status Solidi 28, 623 (1968)] is also consistent with the temperature dependence of the conductivity data. Mott’s optical phonon assisted hopping model at high temperatures provides smaller values of the localization length. However, Mott’s variable range hopping model is valid at low temperatures.

Abstract: A model of hopping conduction between nearest neighbors is developed in which the majority and compensating dopant atoms are assumed to form a unified simple cubic lattice in a crystalline matrix. The hopping of carriers occurs when thermally activated “equalization” of majority impurity levels takes place, while the compensating impurities block the corresponding sites. The range of relatively high temperatures is considered in which the interactions giving rise to a Coulomb gap can be neglected and the density of states of the majority impurity band is Gaussian. The concentration dependences of the activation energy for hopping conductivity ɛ 3 (nonmonotonic and having a maximum) and the preexponential factor σ3 are found. The results are compared with experimental data obtained by different authors for neutron-doped Ge: Ga.

Abstract: The dc conductivity of molybdenum phosphate glasses with the batch composition [(MoO3)z(P2O5)z-1 where z = 0.55, 0.60, 0.65, 0.70, and 0.75 has been studied. The non-linearity of the Arrhenius plot (ln(σ) versus T-1>) may be due to processes involving several similar activation energies, conduction by small polarons, or variable range hopping of carriers. The electrical conductivity results for these glasses have been discussed by applying the two models suggested by Meunier et al. and it has been found that the use of the small polaron model yields physically plausible values for WH and WD within experimental error.

Abstract: Despite a large amount of data and numerous theoretical proposals, the microscopic mechanism of transport in thick-film resistors remains unclear. However, recent low-temperature measurements point toward a possible variable-range-hopping mechanism of transport. Here, we examine how such a mechanism affects the gauge factor of thick-film resistors. We find that at sufficiently low temperatures T, for which the resistivity follows the Mott’s law R(T)∼exp(T0/T)1/4, the gauge factor (GF) is proportional to (T0/T)1/4. Moreover, the inclusion of Coulomb gap effects leads to GF∼(T0′/T)1/2 at lower temperatures. In addition, we study a simple model which generalizes the variable-range-hopping mechanism by taking into account the finite mean intergrain spacing. Our results suggest a possible experimental verification of the validity of the variable-range hopping in thick-film resistors.

Abstract: The transient and steady-state electronic and optoelectronic properties of disordered semiconductors involve interactions of charge carriers with localized states distributed over a range of energies. Where the concentration of such states is sufficiently high, the transport mechanism can be dominated by hopping (quantum-mechanical tunnelling) transitions directly between them. In the analysis of the above situation, one easily applicable approach is to replace a continuous energy distribution with a 'ladder' of energetically discrete sets of traps. This can greatly simplify calculations, but a serious limitation is that the resulting properties depend extremely sensitively upon the slice width selected. In this letter, two variants of a new approach, based upon simple concepts of differential probability and detailed balance, and applicable to an arbitrary energy distribution of trapping centres, are introduced. The capabilities of the procedures are demonstrated by their application to the calculation of effective hopping rates in an exponential energy distribution of localized states. Finally, to illustrate the application of the techniques, the effective rates are coupled to the corresponding hopping distances. This allows the results to be employed (as a preliminary example) in the calculation of the time dependence of the transient photocurrent within an exponential band tail.

Abstract: Polycrystalline Cd doped InSe thin films were obtained by thermal co-evaporation of alpha-In2Se3 lumps and Cd onto glass substrates at a temperature of 150°C. The films were annealed at 150°C and 200°C. The films were found to contain around 46% In, 47% Se and 7% Cd in weight. The films exhibited p-type conductivity. The results of conductivity measurements have revealed that thermionic emission and variable range hopping are the two dominant conduction mechanisms, in the temperature ranges of 320-160 K and 150-40 K respectively. It was observed that above 240 K mobility is limited by the scattering at the grain boundaries. As the temperature decreases, thermal lattice scattering followed by the ionized impurity scattering dominate as the two main mechanisms controlling the mobility. Acceptor to donor concentration ratio was found to be slightly increasing due to annealing.

Abstract: We prepared high epitaxial thin films of the compound Sr2FeMoO6 with narrow rocking curves by pulsed laser deposition The diagonal and nondiagonal elements of the resistivity tensor were investigated at temperatures from 4 K up to room temperature in magnetic fields up to 8 T An electronlike ordinary Hall effect and a holelike anomalous Hall contribution are observed Both coefficients have reversed sign compared to the colossal magnetoresistive manganites We found at 300 K an ordinary Hall coefficent of −187×10−10 m3/A s, corresponding to a nominal charge carrier density of four electrons per formula unit At low temperature only a small negative magnetoresistance is observed which vanishes at higher temperatures The temperature coefficient of the resistivity is negative over the whole temperature range A Kondo like behavior is observed below 30 K while above 100 K variable range hopping like transport occurs

Abstract: Despite a large amount of data and numerous theoretical proposals, the microscopic mechanism of transport in thick film resistors remains unclear. However, recent low temperature measurements point toward a possible variable range hopping mechanism of transport. Here we examine how such a mechanism affects the gauge factor of thick film resistors. We find that at sufficiently low temperatures $T$, for which the resistivity follows the Mott's law $R(T)\sim \exp(T_0/T)^{1/4}$, the gauge factor GF is proportional to $(T_0/T)^{1/4}$. Moreover, the inclusion of Coulomb gap effects leads to ${\rm GF}\sim (T_0'/T)^{1/2}$ at lower temperatures. In addition, we study a simple model which generalizes the variable range hopping mechanism by taking into account the finite mean inter-grain spacing. Our results suggest a possible experimental verification of the validity of the variable range hopping in thick film resistors.

Abstract: The system VxCrS2 (0 < x ≤1) has been investigated by X-ray and neutron powder diffraction. Five single-phase regions have been identified. All single-phase materials adopt pseudolayered structures, in which octahedral sites between alternate pairs of close packed sulfide layers are fully and partially occupied by cations. With increasing vanadium content, rhombohedral M2S3, trigonal M2S3, monoclinic M3S4, trigonal 3C-M7S8, and hexagonal M1-xS structures are successively observed. All single-phase materials exhibit semiconducting behavior, consistent with a variable range hopping conduction mechanism. Magnetic properties show considerably greater variation. The phase with the lowest vanadium content (rhombohedral M2S3) exhibits ferrimagnetic ordering at 100(10) K. Phases with higher vanadium content also exhibit long-range magnetic ordering, with the exception of the monoclinic M3S4, which is a spin glass (Tg = 40(5) K).

Abstract: We present magnetoconductance (MC) measurements of homogeneously disordered Be films whose zero field sheet conductance ( G) is described by the Efros-Shklovskii hopping law G(T) = (2e(2)/h)exp-(T0/T)(1/2). The low field MC of the films is negative with G decreasing a factor of 2 below 1 T. In contrast the MC above 1 T is strongly positive. At 8 T, G increases tenfold in perpendicular field and fivefold in parallel field. In the simpler parallel case, we observe field enhanced variable range hopping characterized by an attenuation of T0 via the Zeeman interaction.

Abstract: Expressions for the screening length and the ambipolar diffusion length are derived, for the first time, for the case where hopping conduction and band conduction coexist in semiconductors with hydrogen-like impurities. A method is proposed for calculating the diffusion coefficient of electrons (holes) hopping between impurity atoms from data on the Hall effect, in the case where the hopping and band conductivities are equal. An interpretation is given of available experimental data on hopping photoconduction between acceptors (Ga) and donors (As) in p-Ge at T=4.2 K doped by a transmutation method. It is shown that the relative magnitude of the mobilities of electrons hopping between donors and holes hopping between acceptors can be found from the hopping photoconductivity measured as a function of the intensity of band-to-band optical carrier excitation.

Abstract: We report investigations of the hole transport in arrays of Ge quantum dots buried in Si. Based on measurements of the temperature dependence of the conductance, the charge-transfer mechanism is proposed to be due to variable-range hopping between the dots with the typical hopping energy determined by inter-dot Coulomb interaction. We find that putting a metal plane close to the dot layer causes a crossover from Efros-Shklovskii variable-range hopping conductance to two-dimensional Mott behavior as the temperature is reduced. At the crossover temperature the hopping activation energy is observed to fall off. The experimental results are explained by screening of long-range Coulomb potentials and give evidence for strong electrostatic interaction between dots in the absence of screening. Conductance oscillations with gate voltage resulting from successive loading of holes into the dots are observed in the field-effect structures.

Abstract: Mott type variable range hopping conduction has been observed in a wide temperature range (1.6 to 100 K) in several barely insulating single-crystalline samples of p-type compensated CuInTe2. In the less doped sample, a crossover to Efros-Shklovskii type variable range hopping conduction is found by lowering the temperature below 18 K, an unusually high temperature for this type of phenomenon. The crossover temperature Tc then deduced is not so far from that one can calculate.

Abstract: The electrical conductivity σ of amorphous Si 100- x Ti x alloys prepared by ion-beam sputtering has been measured over a temperature range between 4.2 and 290 K. The temperature dependence of σ is...

Abstract: Transport properties of n-type CuGaSe2 single crystals are investigated in the temperature range from 2 to 350 K. The effective donor concentration is varied between 2×1012 and 4.4×1017 cm-3 by co-doping with Ge and Zn. The charge transport properties are analysed and interpreted in the framework of an Anderson metal-insulator transition. A critical donor concentration Nc of 1.4×1017 cm-3 is estimated, which is in good agreement with the Mott criterion (Nc1/3≈0.25/aH). However, the second characteristic concentration, above which the Fermi level merges into the conduction band, was not observed experimentally. This is in accordance with an estimate of 7×1017 cm-3 according to the Matsubara-Toyozawa criterion, which exceeds the highest donor concentrations achieved in this material so far. The effective dopant density in n-type CuGaSe2 is limited by self-compensation due to intrinsic defects (mainly Cu vacancies). Furthermore, at low temperatures a crossover from Mott- to Efros-Shklovskii-type variable range hopping is observed on the dielectric side of the transition.

Abstract: We present electron spin resonance spectra and the electronic transport characteristics of multiwalled carbon nanotubes (CNTs) which were screen printed in a thick-film form for field emission displays Electron spin resonance spectra showed a Dysonian line due to conduction electrons, and the reduced temperature dependence of the g-value indicates the metallic properties of CNTs Zero-field resistivity and magnetoresistance were measured as a function of temperature T in the range 17-390 K and magnetic field, respectively The resistivity of nanotubes for temperatures of 10-390 K indicates that the system is intrinsically metallic and the characteristics are well described by Mott's T -1/4 law in temperatures above 10 K We found that the main contribution to the conductivity comes from carriers that hop directly between localized states via variable range hopping The temperature dependence above 10 K is in good agreement with that of an individual multiwalled CNT However, below 10 K, the resistivity is well fit to Efros' T -1/2 law, confirming the presence of a coulomb gap for the system With a decrease of the temperature below 10 K K the charge carriers in the system are localized by strong disorder, bringing a nearly insulating state Using a diode configuration, we measured the field electron emission characteristics and found that the CNT thick film appears to emit electrons with a density of 80 μA/cm 2 , accompanying highly bright light emission The emission current-voltage characteristics can be fitted to a straight line in agreement with the Fowler-Nordheim equation, which confirms that the emission current resulted from field emission of the CNT thick film

Abstract: Variable range hopping conduction in neutron-transmutation-doped, isotopically enriched 70Ge:Ga samples for the temperature range T = 20 to 250 mK has been studied in the critical regime for the metal–insulator transition. Eighteen samples investigated had Ga concentrations in the range N = 0.942Nc to 0.998Nc, where Nc is the critical Ga concentration for the metal–insulator transition. The low temperature resistivities ρ of all samples obey the variable range hopping conduction theory of Efros-Shklovskii type with an appropriate temperature dependence in the pre-factor; ρ = ρ0T–1/3 exp (T0/T)1/2. The critical exponent α of T0 as a function of N has been determined using the form T0 ∝ (1 — N/Nc)α. A clear crossover of α from α ≈ 1 to 3.5 has been observed at N ≈ 0.99Nc. The excitation of holes from the lower- to upper-Hubbard bands is the dominant conduction mechanism in the α ≈ 1 region, while the standard variable range hopping of holes from occupied to empty Ga sites takes place in the very vicinity of the transition where α ≈ 3.5. The important role of the doping compensation is proposed in connection with the critical behavior of the electrical conductivity.

Abstract: La0.5Sr0.5MnO3 thin films deposited on (001) SrTiO3 substrates at different temperatures are prepared using pulsed laser deposition, and their electro- and magnetotransport properties are experimentally evaluated. The structure analysis reveals that the thin films show amorphous, mixed amorphous/nanocrystalline as well as epitaxial microstructures, respectively, depending on the deposition temperature. While the amorphous thin film exhibits variable range hopping conduction, the epitaxial sample is metal like and ferromagnetic. Enhanced low-field magnetoresistance at low temperature for the microstructure in which the nanocrystalline phase and amorphous phase coexist is demonstrated. It is argued that the amorphous layer separating the neighboring nanocrystals behaves as barrier for the spin-dependent tunneling, resulting in enhanced magnetoresistance at low magnetic field. The modified two-channel model where the insulating conduction channel and the spin-ordered and metallic conduction channel coexist i...

Abstract: The effect of temporal and spatial potential-energy fluctuations on low-temperature dark dc conductivity in disordered materials is considered. Analytical models are formulated to treat the variable-range hopping in a disordered system of localized states whose energies are subjected to 1/f temporal fluctuations. Long-range spatial potential fluctuations are described as a random distribution of intrinsic electric field. Temporal 1/f fluctuations are assumed either to be independent of carrier hopping or to be caused by the latter process. Both spatial and independent 1/f temporal fluctuations are shown to yield temperature dependences of the conductivity much weaker than those predicted by the Mott law. Self-sustaining 1/f temporal fluctuations caused by the carrier random walk lead to the crossover from Mott's T -1/4 to T -1/3 dependence with increasing temperature.