Showing papers on "Variable-range hopping published in 1990"
TL;DR: In this paper, the electronic, magnetic, and vibrational properties of pure and intercalate complexes of the layer type transition metal dichalcogenides such as ZrS2, NbSe2 and MoS2 are discussed in relation to their highly anisotropic or "two-dimensional" character.
51 citations
TL;DR: In this article, a new material, CrTe2, was prepared by using a rapid precipitation metathesis reaction between the Zintl material K2Te2 and CrCl2 in solution.
28 citations
TL;DR: In this paper, a negative magnetoresistance in the regime of variable-range-hopping conductivity based on the dependence of the activation energy of the hop on magnetic field is proposed.
23 citations
TL;DR: The first measurement for the electrical properties of the precursor glasses in the high-temperature superconducting system Bi-Sr-Ca-Cu-O was reported in this paper.
Abstract: The first measurement is reported for the electrical properties of the precursor glasses in the high-temperature superconducting system Bi-Sr-Ca-Cu-O. The phonon-assisted hopping of small polarons is consistent with the DC conductivity in the high temperature region; however, at lower temperatures a variable range hopping mechanism dominates the conductivity. The correlated barrier hopping model is consistent with the temperature and frequency dependence of the AC conductivity and its frequency exponent.
23 citations
TL;DR: In this article, the electrical conductivity of the system CrSi(O,N) was investigated in the temperature range from 4 K (50 mK) to 430 K. The composition of the alloys was in the range of 55/45 to 85/15 (Si/Cr atomic ratio) with oxygen or nitrogen concentrations of up to 50%.
15 citations
TL;DR: In this paper, the electrical resistance (temperature and pressure dependence) and room temperature compressibility measurements for electrochemically synthesized polypyrrole are reported, and an expression for the pressure dependence of the conductivity, σ(P ) = σ ( T )(1 + βP ) exp[(P / P o ) n ], where n ⋍ 3 4.
15 citations
TL;DR: In this article, the preparation of a new phase, BiSrY1−xCexCuO5+y, obtained for 0.2 < x < 0.6, is described.
Abstract: The preparation of a new phase, BiSrY1−xCexCuO5+y, obtained for 0.2 < x < 0.6, is described. The X-ray powder diffra ction pattern can be indexed on the basis of an orthorhombic cell clearly related to those of the BSCCO family. A structural model is proposed on the basis of cell dimensions, symmetry and composition. According to this model the new phase can be thought of as being derived from Bi2Sr2CaCu2O8 by substitution of the Ca layer with an (Y,Ce)2O2 double layer. Electrical conductivity measu rements are performed between 15 and 300 K evidencing a semiconducting behaviour, where a thermally activated process and a three-dimensional variable range hopping conduction successively take place. For Ce concentrations higher than 30% the resistivity-temperature characteristics exhibit a progressive reduction of the resistivity trend, indicating an insulator-metal transition. The parameters of the transport processes are evaluated, discussed and compared with those reported for similar compounds.
14 citations
TL;DR: In this paper, the authors report on the results of electrical conductivity measurements carried out on coconut-shell activated carbon and suggest that the charge carriers moved by variable-range hopping below 200 K, by hopping among localized energy states between 200 K and 385 K, and by percolation through energy states close to the mobility edge above 385 K.
Abstract: This work reports on the result of electrical conductivity measurements carried out on coconut‐shell activated carbon. The results suggest that the charge carriers moved by variable‐range hopping below 200 K, by hopping among localized energy states between 200 K and 385 K, and by percolation through energy states close to the mobility edge above 385 K. In the latter two conduction processes, significant additional frequency‐dependent contributions to the conductivity were observed and are tentatively attributed to polarization of unremovable lignin and/or ionic impurities. These explanations are based on the present trends of the electrical transport theory for amorphous semiconductors.
12 citations
TL;DR: In this paper, the effects of sodium and molybdenum in sodium-molybenium phosphate glasses have been studied by dc and ac conductivity measurements over a wide temperature range.
Abstract: The effects of sodium and molybdenum in sodium-molybdenum phosphate glasses have been studied by dc and ac conductivity measurements over a wide temperature range. Results indicate that the contribution of electronic conduction to dc conductivity increases with molybdenum concentration. It is difficult to separate the ionic from electronic conduction. Greaves variable range hopping for dc conductivity has been observed to be valid at intermediate temperatures. The value of the electron wave function decay constant α has been evaluated to be 1.05 A -1 (assuming N ( E F ≈ 10 19 eV −1 cm −3 ). These values are in agreement with those estimated using a tunnelling model for ac conductivity at 77 K. Values of s evaluated from the relation σ ac ( ω ) = Aω s are comparable to those evaluated from the hopping over barrier model. The dielectric relaxation frequency for these glasses has been observed to be about 1.5 kHz in the temperature range of 100–200 K.
12 citations
TL;DR: In this paper, the conductivity of a commercial thick-film resistor is measured between 4 K and 15 mK and in magnetic fields up to 7 Tesla using the variable-range hopping mechanism with a Coulomb gap in the density of states.
Abstract: The conductivity of a commercial thick-film resistor is measured between 4 K and 15 mK and in magnetic fields up to 7 Tesla. The data can be described by the variable-range hopping mechanism with a Coulomb gap in the density of states. The negative magnetoresistance may be attributed to quantum-interference effects in the strongly localized regime.
11 citations
TL;DR: In this article, the AC electrical conductivity of the semiconducting glasses in the system CuO-Bi2O3-P2O5 is reported for the first time over the temperature range 80-420 K. The electrical data have been analyzed in the light of the theoretical models of polaronic conduction.
Abstract: DC electrical conductivity of the semiconducting glasses in the system CuO-Bi2O3-P2O5 is reported for the first time over the temperature range 80-420 K. The electrical data have been analysed in the light of the theoretical models of polaronic conduction. The analysis shows that the adiabatic hopping theory is the most appropriate to describe the polaronic conduction in the high-temperature region. At lower temperatures, a variable range hopping mechanism dominates the conduction. Various parameters such as decay constant, density of states at the Fermi level, etc., have been obtained from analysis of the electrical conductivity data.
TL;DR: In this article, PPPP samples were implanted with alkali metal ions using different energies and characterized and the evolution of thermo electric power versus temperature was followed in terms of a variable range hopping process between localized states induced near the Fermi level through implantation.
Abstract: Polyparaphenylene (PPP) samples were implanted with alkali metal ions using different energies. They were characterized and the evolution of thermo electric power versus temperature was followed. The results may be analyzed in terms of a variable range hopping process between localized states induced near the Fermi level through implantation. Furthermore with samples implanted at low energy, a transition toward values characteristic of the expected doping shows up (n type doping with alkali metals).
TL;DR: In this paper, the conductivity of Cadmium telluride thin films was characterized by X-ray diffraction, scanning electron microscopy and optical absorption measurements, which indicated a direct band gap around 1.55 eV.
Abstract: Cadmium telluride thin films were prepared by electron beam evaporation on glass substrates kept at different temperatures in the range 30 - 300 °C. The films were characterized by X-ray diffraction, scanning electron microscopy and optical absorption measurements. The conductivity of the films was measured in the temperature range 100 - 300 K. While the low temperature data (100 - 200 K) could be explained by the variable range hopping process, the high temperature data (200 - 300 K) could be explained on the basis of Seto's model for thermionic emission of the carriers over the grain boundaries. Transmission spectra have indicated a direct band gap around 1.55 eV.
TL;DR: In this paper, the Coulomb interaction model for low-temperature conductivity in disordered systems was used to explain the residual radiation damage induced by the ion implantation process.
Abstract: Resistivity measurements carried out in the temperature range 80 mK to 1 K on silicon bolometers-phosphorus doped by ion implantation-at concentrations near the metal-insulator transition, exhibit variable range hopping (VRH) conduction in the whole observed temperature range, with a T-1/2 dependence, in accordance with the Coulomb interaction model for low-temperature conductivity in disordered systems. Samples which apparently show a different behaviour, intermediate between VRH and metallic conduction, can be modeled by a metallic resistance in parallel with an active layer which follows the classic exp(T0/T)1/2 law. The observed behaviour can be explained in terms of residual radiation damage induced by the ion implantation process.
TL;DR: The temperature dependent resistance of ion-implanted Si:As bolometers has been measured in the variable range hopping regime, with R(T) = CT 2m eχp(T m T ) m, for 1.6 > T > 0.2 K.
Abstract: The temperature dependent resistance R(T) of ion-implanted Si:As bolometers has been measured in the variable range hopping regime, with R(T) = CT 2m eχp( T m T ) m , for 1.6 > T > 0.2 K . The exponent m changed from 0.5 to 0.28 as the dopant concentration approached the metal-insulator transition. These sensitive bolometers, with an implant depth of 0.1 microns, are limited by the thermal contact between electrons and phonons below 0.1K.
TL;DR: In this article, the dc conductivity of amorphous germanium films, prepared by thermal evaporation in high vacuum, was investigated between 77 and 300 K as a function of applied electric field.
Abstract: The dc conductivity of amorphous germanium films, prepared by thermal evaporation in high vacuum, was investigated between 77 and 300 K as a function of applied electric field. The dc conductivity of these films obeys Mott's T − 1 4 variable range hopping conductivity law below 200 K for low ( F 3 V/cm) applied electric fields. The electric field dependent variable range hopping conductivity data was analyzed using the models developed by Apsley and Hughes (1975) and by Pollak and Riess (1976). The two important parameters of hopping conduction, the localized wave function exponent α −1 and the density of localized states at the Fermi level N ( E F ), were calculated by using these models.
TL;DR: In this paper, an expression for the heat conductivity of a two-dimensional system in the quantum Hall regime within the high-field percolation model was derived in the effective medium approximation Mott type formulas.
Abstract: An expression for the heat conductivity κ
xx
is derived in the effective medium approximation Mott type formulas are obtained for κ
xx
and the Peltier coefficient β
xx
Using percolation theory in a three-dimensional system the Wiedemann-Franz ratio was found to depend on the temperature like
$$1/\sqrt T $$
The Mott type formulas were evaluated in a similar way for a two-dimensional system in the quantum Hall regime within the high-field percolation model In contrast to previous calculations of the high field hopping conductivity σ
xx
, the results are fully consistent with the experimental data on σ
xx
and the density of states at the Fermi level Finally, β
xx
is estimated which together with σ
xx
and σ
xy
=ie
2/h(i=0,1,2,), determines both thermopower coefficients α
xx
and ασ
xy
TL;DR: Using an extension of the Gutzwiller approximation, the density of states, magnetic moment, and the antiferromagnetic ground-state energy of high-{Tc} copper oxides is calculated.
Abstract: Using an extension of the Gutzwiller approximation, we calculated the density of states, magnetic moment, and the antiferromagnetic ground-state energy of high-{Tc} copper oxides. The Hubbard Hamiltonian used for the calculations includes hopping between oxygen atoms as well as Cu-O hopping. It is shown that hopping between oxygen atoms is particularly significant for the magnetic properties. The effect of doping, on-site hole repulsion, and charge-transfer energy on the local density of states, magnetic moment of Cu, and antiferromagnetic ground-state energy is studied in detail.
TL;DR: In this article, generalized charge carrier equations for hopping transport in semiconductors are derived which include also the widely used Van Roosbroeck equations, based on a microscopic stochastic interacting particle system which models the hopping of electrons on a random set of states.
Abstract: Generalized charge carrier equations for hopping transport in semiconductors are derived which include also the widely used Van Roosbroeck equations. The approach is based on a microscopic stochastic interacting particle system which models the hopping of electrons on a random set of states.
TL;DR: In this article, an electron localization in poly(o-toluidine was investigated by electric field dependence of conductivity σ(E), and the authors found lnσ(E) = KE 1/2, with K varying as square root of temperature (K ∞ T −1/2).
Abstract: Electron localization in the methyl ring-substituted derivative of polyaniline, poly(o-toluidine), has been investigated by electric field dependence of conductivity σ(E). We found lnσ(E) = KE 1/2, with K varying as square root of temperature (K ∞ T −1/2). This result is in accord with quasi-one-dimensional variable range hopping of charge carriers localized along a one-dimensional chain, with an effective Coulomb barrier between adjacent sites. The measured non-ohmic conductivity is modeled by the Poole-Frenkel effect where the Coulomb potential between localized sites is lowered by the electric field. Model analysis yields an estimated effective barrier height of ∼0.1 eV, corresponding to an effective charge carrier mass of the order of that of an electron. In contrast, the charging energy limited tunneling model predicts lnσ ∞ E −1, which is not consistent with the experimental results.
TL;DR: In this article, the a.c. conductivity of undoped and iodine or bromine doped poly(tetrathiafulvalenes) is described by the Extended Pair Approximation (EPA).
Abstract: Results of conductivity measurements on undoped and iodine or bromine doped poly(tetrathiafulvalenes) are reported. At higher temperatures the temperature dependence of the d.c. conductivity can be best described by σd.c. = °o. exp(T/To)1/4, typical for variable range hopping. The a.c. conductivity follows the general law °(ω) ∼ ω2 for frequencies higher than a critical frequency. A description of the frequency dependent conductivity is given by the extended pair approximation (EPA). At lower temperatures a deviation from linearity of the function in σ versus T−1/4 is observed.
TL;DR: In this article, the hopping rate of localized defects interacting with a two-dimensional electron system is studied, and it is shown that, at low temperatures, hopping rate is an oscillatory function of the inverse of the magnetic field.
01 Jan 1990
TL;DR: In this article, the temperature dependence of the electrical conductivity for oriented polyacetylene (PA) doped with iodine, molybdene chloride and sulfuric acid is studied.
TL;DR: In this paper, structural and electrical properties of thin granular Pd x C 1−x films (10 ⩽ t 70 nm) in the vicinity of the metal-insulator transition (MIT) were studied.
TL;DR: In this paper, the origins of the magnetoresistance in granular metals with emphasis on the quantum coherence effects arising from the interference of electrons traveling by different paths are discussed, as well as the maximum in the magnetoreduction expected from scaling arguments and observed in the granular aluminum.
Abstract: We review the origins of the magnetoresistance in granular metals with emphasis on the quantum coherence effects arising from the interference of electrons traveling by different paths. We discuss the maximum in the magnetoresistance expected from scaling arguments and observed in granular aluminum. Two theories of the magnetoresistance in the variable range hopping regime, one based on the magnetic-field induced shift of the mobility edge and the other dealing with ‘directed’ hopping, are compared with experiments.
01 Jan 1990
01 Jan 1990