Showing papers on "Variable-range hopping published in 2004"
TL;DR: In this paper, the dc and ac conductivities of samples with different concentrations of PANI have been investigated as a function of frequency and temperature, and an electric modulus presentation is used to interpret the dielectric spectra.
Abstract: Nanocomposites of polyaniline (PANI)–titanium dioxide (PANI–TiO2) are prepared from a colloidal sol of TiO2 nanoparticles. The dc and ac conductivities of samples with different concentrations of PANI have been investigated as a function of frequency and temperature. The dc conductivity follows three-dimensional variable range hopping. The ac conductivity has been interpreted as a power law of frequency. The temperature variation of the frequency exponent suggests a correlated barrier hopping conduction process in the nanocomposites. A very large dielectric constant of about 3700 at room temperature has been observed. An electric modulus presentation is used to interpret the dielectric spectra. The interface between polyaniline and TiO2 plays an important role in yielding a large dielectric constant in the nanocomposite.
250 citations
TL;DR: In this article, the authors reported the observation of ferromagnetism at over 900k in Cr-GaN and Cr-AlN thin films, showing that 14% and 20% of the Cr atoms are magnetically active.
Abstract: We report the observation of ferromagnetism at over 900K in Cr-GaN and Cr-AlN thin films. The saturation magnetization moments in our best films of Cr-GaN and Cr-AlN at low temperatures are 0.42 and 0.6 u_B/Cr atom, respectively, indicating that 14% and 20%, of the Cr atoms, respectively, are magnetically active. While Cr-AlN is highly resistive, Cr-GaN exhibits thermally activated conduction that follows the exponential law expected for variable range hopping between localized states. Hall measurements on a Cr-GaN sample indicate a mobility of 0.06 cm^2/V.s, which falls in the range characteristic of hopping conduction, and a free carrier density (1.4E20/cm^3), which is similar in magnitude to the measured magnetically-active Cr concentration (4.9E19/cm^3). A large negative magnetoresistance is attributed to scattering from loose spins associated with non-ferromagnetic impurities. The results indicate that ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange mechanism as a result of hopping between near-midgap substitutional Cr impurity bands.
172 citations
TL;DR: In this article, it was shown that uncontrolled or intentional doping of the insulator around dots by donors leads to random charging of dots and finite bare density of states at the Fermi level.
Abstract: The conductivity of a granular metal or an array of quantum dots usually has the temperature dependence associated with variable range hopping within the soft Coulomb gap of density of states. This is difficult to explain because neutral dots have a hard charging gap at the Fermi level. We show that uncontrolled or intentional doping of the insulator around dots by donors leads to random charging of dots and finite bare density of states at the Fermi level. Then Coulomb interactions between electrons of distant dots results in a soft Coulomb gap. We show that in a sparse array of dots the bare density of states oscillates as a function of concentration of donors and causes periodic changes in the temperature dependence of conductivity. In a dense array of dots the bare density of states is totally smeared if there are several donors per dot in the insulator.
111 citations
TL;DR: In this article, the structure and electronic transport properties of polymer-derived pristine and boron-doped silicon carbonitride ceramics have been studied, with particular emphasis on understanding the effect of annealing treatments.
Abstract: The structure and electronic transport properties of polymer-derived pristine and boron-doped silicon carbonitride ceramics have been studied, with particular emphasis on understanding the effect of annealing treatments. Structural analysis using the radial distribution function formalism showed that the local structure is comprised of basic building blocks of Si tetrahedra with B, C, and N at the corners. Comparison of the electrical properties of pristine and boron-doped ceramics shows that boron doping leads to enhanced p-type conductivity, with a small positive thermopower. The postpyrolysis annealing treatments at elevated temperatures have a significant effect on the conductivity. The conductivity variation with temperature for these ceramics shows Mott's variable range hopping (VRH) behavior, characteristic of a highly defective semiconductor.
92 citations
TL;DR: In this paper, the mechanism of high conductivity and the change of the electronic structure by nitrogen incorporation is investigated by low-temperature conductivity, and it is shown that nitrogen induces percolative paths in the grain boundary regions and there is an increase in the density of states at the Fermi level that helps increase the conductivity.
Abstract: Dramatic enhancement of the $n$-type conductivity of nanocrystalline diamond films by introducing up to $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ of nitrogen into the film has been reported. Previously, there were some theoretical predictions about the change of electronic structure of the films by nitrogen incorporation, but the origin of the enhanced conductivity is still not clearly understood. In this article the mechanism of high conductivity and the change of the electronic structure by nitrogen incorporation is investigated by low-temperature conductivity and other supporting measurements. It is shown that nitrogen induces percolative paths in the grain boundary regions and there is an increase in the density of states at the Fermi level that helps increase the conductivity. Low-temperature conductivity has been explained from a change over from Arrhenious behavior to Efros-Shklovokii\char21{}Pollak\char21{}Mott variable range hopping conductivity. Using a model combining band and hopping conduction electrical conductivity of highly doped samples over wide range of temperature has been explained. This approach also helps us to improve the understanding of the electronic structure and transport of conducting amorphous carbon by resolving some typical problems in the analysis of temperature dependent conductivity.
75 citations
TL;DR: In this article, the Arrhenius relationship was used to evaluate the activation energy of conductivity of polypyrrole-polyvinyl alcohol copolymer composites.
72 citations
TL;DR: In this article, the authors identify a novel mechanism characterizing the hopping transport in the Mott insulating phase of Ca2-xSrxRuO4 near the metal-insulator transition.
Abstract: By using a combination of detailed experimental studies and simple theoretical arguments, we identify a novel mechanism characterizing the hopping transport in the Mott insulating phase of Ca2-xSrxRuO4 near the metal-insulator transition. The hopping exponent alpha shows a systematic evolution from a value of alpha=1/2 deeper in the insulator to the conventional Mott value alpha=1/3 closer to the transition. This behavior, which we argue to be a universal feature of disordered Mott systems close to the metal-insulator transition, is shown to reflect the gradual emergence of disorder-induced localized electronic states populating the Mott-Hubbard gap.
68 citations
TL;DR: In this article, Ba1?xLaxSnO3 (up to x? 0.15) has been synthesized via the solid state ceramic route and the ac electrical and dielectric properties of single phase samples have been studied in the frequency range 10?Hz?5?MHz for temperatures ranging from 300 to 550?K.
Abstract: A few samples in the system Ba1?xLaxSnO3 (up to x ? 0.15) have been synthesized via the solid state ceramic route. Powder x-ray diffraction pattern of the samples confirm the formation of a single phase solid solution only in the samples with x up to ?0.05. It was found that single phase compositions have a cubic crystal structure similar to that of undoped barium stannate. The ac electrical and dielectric properties of single phase samples have been studied in the frequency range 10?Hz?5?MHz for temperatures ranging from 300 to 550?K. The conductivities (both ac and dc) and relaxation time (?) exhibit two regions of temperature dependence, namely region I, which represents 300?420?K, and region II, which governs 420?550?K. Conduction and dielectric relaxation in both temperature regions are explained in terms of variable range hopping of electrons. Complex plane impedance analysis revealed that the specific conductivity of the grains is two orders of magnitude higher than that of the grain boundaries.
61 citations
TL;DR: In this paper, structural, magnetic and transport properties of Pr0.65Ca0.35−xSrxMnO3 ( x = 0 - 0.35 ) perovskites were reported.
55 citations
TL;DR: In this paper, temperature variation of DC electrical resistivity and magneto-resistivity have been measured on mono-valent, Ag-doped La 1− x Ag x MnO 3 compounds for x = 0.05-0.30.
Abstract: Temperature variation of DC electrical resistivity and magneto-resistivity have been measured on mono-valent, Ag- doped La 1− x Ag x MnO 3 compounds for x =0.05–0.30. All these samples exhibit metal–insulator transitions and the transition temperature increases with applied field. The resistivity data in the metallic region, i.e. below the transition temperature could be fitted to the empirical relation ρ = ρ 0 + ρ 1 T n . The value of exponent n , mostly varies from n =3 for zero field case to n =2 for 50 kOe field. The electrical transport in zero field is controlled by strong spin fluctuations scattering and in the presence of high magnetic field, it is mostly due to electron–electron scattering. The applied magnetic fields are found to suppress the scattering due to spin fluctuations. The resistivity data in the semiconducting region, i.e. above the transition temperature could be fitted to Mott variable range hopping model. The applied magnetic fields tend to delocalize the charge carriers in the semiconducting region and thereby decrease in hopping energy and increase in density of states has been observed. Colossal magneto-resistivity have been observed in all the above samples and the maximum value is found to be 73% for x =0.15 sample for 50 kOe field.
55 citations
TL;DR: In this article, temperature-dependent magnetization, magnetoresistance and magneto-thermoelectric power of the K doped La 1− x Ca x − y K y MnO 3 type samples with x = 0.3 and 0⩽ y ⩽0.15 has been studied.
TL;DR: In this article, the authors show that the mechanism of leakage current at high fields is that of space charge limited conduction (SCLC) in a regime with a continuous distribution of traps, according to the original model of Rose.
Abstract: We present current-voltage studies of very thin (ca. 77 nm) barium titanate single crystals up to 1.3 GV/m applied field. These show that the mechanism of leakage current at high fields is that of space charge limited conduction (SCLC) in a regime with a continuous distribution of traps, according to the original model of Rose [Phys. Rev. 97, 1537 (1955)]. This study represents a factor of x5 in field compared with the early studies of BaTiO3 conduction [A. Branwood et al., Proc. Phys. Soc. London 79, 1161 (1962)]. Comparison is also given with ceramic multilayer barium titanate capacitors, which show similar SCLC behaviour. The data are shown to be completely incompatible with variable range hopping [B. I. Shklovskii, Sov. Phys. Semicond. 6, 1964 (1973)], despite the recent report of such mechanism in SrTiO3 films [D. Fuchs, M. Adam, and R. Schneider, J. Phys. IV France 11, 71 (2001)].
TL;DR: In this article, the spin-singlet Mott phase and the nematic mott phase were investigated in high-dimensional bipartite lattices and it was shown that phase transitions are generally weakly first order.
Abstract: We present microscopic wave functions of spin-singlet Mott insulating states and nematic Mott insulating states. We also investigate quantum phase transitions between the spin-singlet Mott phase and the nematic Mott phase in both large-N limit and small-N limit ( N being the number of particles per site! in high-dimensional bipartite lattices. In the mean-field approximation employed in this article we find that phase transitions are generally weakly first order.
TL;DR: In this paper, the effect of Mn site substitution of Al on the structural, magnetic and transport properties of charge ordered Pr0.5Ca 0.5MnO3 was investigated.
Abstract: We report on the effect of Mn site substitution of Al on the structural, magnetic and transport properties of charge ordered Pr0.5Ca0.5MnO3. This substitution introduces non-magnetic impurities in the Mn–O–Mn network, causes the Mn3+/Mn4+ ratio to deviate away from unity and is seen to predominantly replace Mn4+ without introducing any appreciable change in the structure. The strength of charge ordering is suppressed with increasing Al doping, as is seen from AC susceptibility, the magnetocaloric effect and DC resistivity. The observation of traces of charge ordering over an extended range of impurity doping of 10%, besides being indicative of the robust nature of charge ordering in Pr0.5Ca0.5MnO3, also underscores the relatively insignificant effect that Al substitution has on the lattice distortions and the importance of non-magnetic substitutions. For all the samples, the conductivity in the temperature range above the charge ordering transition is observed to be due to the adiabatic hopping of small polarons, whereas below the charge ordering transition, Mott's variable range hopping mechanism is seen to be valid.
TL;DR: In this paper, structural, magnetic and transport studies have been carried out on (La07−2xEux)(Ca03Srx)MnO3(005⩽xﵵ025) compounds forming in a distorted orthorhombic structure (space group Pnma, No 62).
Abstract: Structural, magnetic and transport studies have been carried out on (La07−2xEux)(Ca03Srx)MnO3 (005⩽x⩽025) compounds forming in a distorted orthorhombic structure (space group Pnma, No 62) The Eu and Sr substitutions avoid any average A-site cation size disparity throughout the series However, increasing both the cation size mismatch at the A-site and carrier concentration induces interesting changes in structural, transport and magnetic properties Both insulator–metal transition temperature (Tp) and Curie temperature (TC) decrease with increasing x The resistivity of all the samples in the semiconducting regime fits to the Variable Range Hopping (VRH) of Mott type model Carrier localization length, L, obtained from VRH plots, decreases from 46 A for x=005 to 39 A for the x=020 sample In the metallic region, the n term in the resistivity fits to the Zener-Double exchange polynomial law (ρ=ρ0+ρ2T2+ρnTn) increases from n=55 for x=005 to n=75 for x=015 From magnetic susceptiblity measureme
01 Jan 2004
TL;DR: In this article, the electron-beam and focused-ion-beam (FIB)-induced deposition of Pt nanowires was investigated and the observed temperature dependence of the resistance of the EB-deposited wires indicates that the electron transport in the wires is dominated by variable range hopping (VRH).
Abstract: Pt nanowires were fabricated using electron-beam (EB)- and focused-ion-beam (FIB)-induced deposition. The resistance of the EB-deposited nanowires was high (≈107 Ω) as deposited and increased markedly when cooled down. The observed temperature dependence of the resistance of the EB-deposited wires indicates that the electron transport in the wires is dominated by variable range hopping (VRH). Coulomb oscillations were observed at temperatures up to ≈200 K for EB-deposited wires simultaneously with VRH. Postannealing was effective to reduce the resistance. The resistance of the FIB-deposited nanowires, in contrast, hardly depended on temperature. Positive magnetoresistance due to weak antilocalization was observed in the case of the FIB-deposited wires.
TL;DR: The alternating current conductivities of conjugated polymers such as doped polyaniline and Doped polypyrrole in the dielectric state have strong temperature and frequency dependencies.
TL;DR: In this article, the electrical properties of granular nanocomposites have been studied and the concentration dependences of electrical resistivity are S-shaped (in accordance with the percolation theory of conduction) with a threshold at a metallic component concentration of ∼41 at. %.
Abstract: The electrical properties of (Co45Fe45Zr10)x(Al2O3)1−x
granular nanocomposites have been studied. The concentration dependences of electrical resistivity are S-shaped (in accordance with the percolation theory of conduction) with a threshold at a metallic component concentration of ∼41 at. %. An analysis of the temperature behavior carried out in the range 300–973 K revealed that structural relaxation and crystallization of the amorphous phase are accompanied by a decrease in the electrical resistivity of the composites above the percolation threshold and by its increase below the percolation threshold. For metallic phase concentrations x<41 at. %, variable range hopping conduction over localized states near the Fermi level was found to be dominant at low temperatures (77–180 K). A further increase in temperature brings about a crossover of the conduction mechanism from Mott’s law ln(σ) ∝ (1/T)1/4 to ln(σ) ∝ (1/T)1/2. A model of inelastic resonance tunneling over a chain of localized states of the dielectric matrix was used to find the average number of localized states involved in the charge transport between metallic grains.
TL;DR: Lu2Pc2 and lithium hexylthio-phthalocyanine have been used to synthesize [Lu2(Pc)4] which exhibits variable range hopping conductivity as inorganic semiconductors and also observed electrochromic effect from cyclic voltammetric (CV) measurement.
TL;DR: In this paper, the effect of intra-atomic correlations on hopping anisotropies of 5f electrons was studied for two-and three-sites clusters and phase diagrams in the presence and absence of an external magnetic field were presented.
Abstract: We study the effect of intra-atomic correlations on hopping anisotropies of 5f electrons. Results are presented for two- and three-sites clusters. They include phase diagrams in the presence and absence of an external magnetic field as a function of anisotropic hopping. It is shown that intra-atomic correlations may considerably enhance hopping anisotropies to the extent that electrons in some of the 5f orbitals remain localized. This provides a microscopic basis for a previously made assumption that for some Uranium-based heavy-fermion materials the 5f electrons must be treated as partially localized when Fermi surfaces and effective masses are calculated.
TL;DR: In this article, the effect of the addition of Pb to Se-In alloys is to reduce the density of states near the Fermi level, and the results were analyzed in terms of the Davis-Mott model.
TL;DR: In this paper, a ternary ruthenium oxide Na2RuO4 was prepared and shown to crystallize with a new structure type, and single crystal X-ray diffraction measurements reveal that RuO4 chains made up of RuO5 trigonal bipyramids by sharing axial corners.
Abstract: A new ternary ruthenium oxide Na2RuO4 was prepared and shown to crystallize with a new structure type. Single crystal X-ray diffraction measurements reveal that Na2RuO4 consists of RuO4 chains made up of RuO5 trigonal bipyramids by sharing axial corners. Na2RuO4 is a magnetic semiconductor with a variable range hopping behavior, and its molar magnetic susceptibility χmol has a broad maximum at ∼74 K. The derivative d(χmol·T)/dT exhibits a peak at 37.7 K which has been confirmed by heat capacity measurement to be due to long-range antiferromagnetic ordering.
TL;DR: In this article, the structural transition temperature, Ts, decreases with decreasing V2+ content to a minimum value of 105 K. Magnetic susceptibility data show evidence for two phase transitions at 135 and 40 K.
TL;DR: In this paper, the authors investigated the properties of the germanium selenium system (GexSe100-x ) in the lower temperature region (room temperature) by correlated barrier hopping (CBH) model.
Abstract: In general, the conductivity in chalcogenide glasses at higher temperatures is dominated by band conduction (DC conduction). But, at lower temperatures, hopping conduction dominates over band conduction. A study at lower temperature can, eventually, provide useful information about the conduction mechanism and the defect states in the material. Therefore, the study of electrical properties of GexSe100-x
in the lower temperature region (room temperature) is interesting. Temperature and frequency dependence of GexSe100-x (x = 15, 20 and 25) have been studied over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggested that the behaviour of germanium selenium system (GexSe100-x
) have been successfully explained by correlated barrier hopping (CBH) model.
TL;DR: In this article, the authors present results of systematic studies of X-ray, electrical resistivity ( ρ ) and thermoelectric power (TEP, S ) on polycrystalline Sr 2 FeRe 1− x W x O 6 (x = 0.1−0.2) samples.
Abstract: In this report, we present results of systematic studies of X-ray, electrical resistivity ( ρ ) and thermoelectric power (TEP, S ) on polycrystalline Sr 2 FeRe 1− x W x O 6 ( x =0–1.0) samples. For the undoped Sr 2 FeReO 6 ( x =0) sample, analysis of the ρ ( T ) data suggests that the electron–electron scattering plays a major role in governing the conduction mechanism. The resistivity data of the low doped samples ( x =0.1–0.2) exhibits metal–insulator transition. For samples with x =0.3–1.0, the electrical transport at low temperatures behaves like that of a semiconductor/insulator, where ρ ( T ) data show close agreement with Mott's variable range hopping theory implying weak localization of the charge carriers. On the other hand, at high temperatures the charge conduction follows thermally activated semiconductor type character. For all the samples, S is found to be positive indicating hole conduction. The samples with x =0–0.2 show almost linear dependence of S on T . The Fermi energy estimated from the slope of the S ( T ) curves lies in the range 0.5–0.3 eV for samples with x =0–0.2. For samples with x =0.3–0.6, S ( T ) data show a broad peak which shifts to a low-temperature side as x increases. This feature has been qualitatively discussed in the light of percolation model.
TL;DR: In this article, the transport properties of A2BMoO6 (A=Ca, Sr, and Ba; B=Cr, Mn, and Fe) were investigated from studies of x-ray diffraction, thermoelectric power (TEP) S, and resistivity ρ measurements over a wide temperature range (20-900 K).
Abstract: The transport properties of A2BMoO6 (A=Ca, Sr, and Ba; B=Cr, Mn, and Fe) crystals are investigated from studies of x-ray diffraction, thermoelectric power (TEP) S, and resistivity ρ measurements over a wide temperature range (20–900 K). The resistivity of A2FeMoO6 (A=Ca, Sr, and Ba) crystals shows a metallic character and its analysis suggests that the electron-electron scattering is one of the contributions governing the conduction mechanism of these compounds. On the other hand, Ca2MnMoO6 shows an insulator-metal transition similar to that reported in rare earth nickelate [Phys. Rev. B 45, 8209 (1992)]. The conductivity σ(T) data of Sr2CrMoO6 can be best fitted in terms of simultaneous contributions to σ with an activated-type process and variable range hopping (VRH) mechanism. In the case of Sr2MnMoO6, ρ(T) behavior follows an activated-type hopping for T>400 K whereas for T<85 K it can be described satisfactorily by VRH mechanism. For all the compounds, TEP exhibits negative values (n type) except for...
TL;DR: In this article, Substoichiometric amorphous tungsten oxide films were prepared by DC-magnetron sputtering and the films were electrochemically intercalated with lithium.
TL;DR: In this article, the dielectric permittivity of tetrameric cobalt phthalo- cyanine sample was evaluated from the observed capaci- tance values in the frequency range 100 KHz to 5 MHz and in the temperature range of 300 to 383°K.
Abstract: Electrically conductive organic and metal- loorganic polymers are of great interest and they have ap- plications in electronic, optical, photonic, photoelectric, elec- trochemical, and dielectric devices. Tetrameric cobalt phtha- locyanine was prepared by conventional chemical method. The dielectric permittivity of the tetrameric cobalt phthalo- cyanine sample was evaluated from the observed capaci- tance values in the frequency range 100 KHz to 5 MHz and in the temperature range of 300 to 383°K. It is found that the system obeys the Maxwell Wagner relaxation of space charge phenomenon. Further, from the permittivity studies AC conductivity was evaluated. The values of AC conduc- tivity and DC conductivity were compared. Activation en- ergy was calculated. To understand the conduction mecha- nism Mott's variable range hopping model was applied to the system. The T 1/4 behavior of the DC conductivity along with the values of Mott's Temperature (T0), density of states at the Fermi energy N (EF), and range of hopping R and hopping energy W indicate that the transport of charge carriers are by three-dimensional variable range hopping. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2529 -2535, 2004
TL;DR: In this article, the conductivity mechanisms in polycrystalline CuInSe2 and the ordered vacancy compounds CuIn3Se5, CuIn5Se8 and CuIn7Se12 were studied by low temperature conductivity measurements and room temperature Hall and photoconductivity measurements.
Abstract: The conductivity mechanisms in polycrystalline CuInSe2 and the ordered vacancy compounds CuIn3Se5, CuIn5Se8 and CuIn7Se12 were studied by low temperature conductivity measurements and room temperature Hall and photoconductivity measurements. The polycrystalline films were structurally, morphologically and compositionally characterized using x-ray diffraction, atomic force microscopy and energy dispersive analysis by x-rays, respectively. In general, the films showed Mott's variable range hopping conduction in the temperature range below 200 K while above 200 K thermally activated conductivity due to grain boundary effects and defect levels was observed. To investigate the reason for the metallic behaviour shown by Cu-rich films, the films were etched and the change in behaviour was studied.
TL;DR: In this paper, X-ray analysis of thin AgIn5S8 thin films was carried out to reveal the polycrystalline nature of the films and the lattice parameter was calculated to be 10.784(5) A.
Abstract: AgIn5S8 thin films are deposited on glass substrates, kept at 300 K, by thermal evaporation of AgIn5S8 single crystals under the pressure of 10–5 Torr. The X-ray fluorescence analysis revealed that the films have a weight percentage of ∼11.5% Ag, 61.17% In, and 27.33% S which corresponds to 1:5:8 stoichiometric composition. X-ray analysis of the films reveals the polycrystalline nature of the films. The lattice parameter (a) of the films was calculated to be 10.784(5) A. The dark n-type electrical conductivity of the films was measured in the temperature range of 30–350 K. The conductivity data analysis shows that the thermionic emission of the charge carriers having activation energies of 147 and 224 meV in the temperature ranges of 130–230 and 240–350 K, respectively, are the dominant transport mechanism in the films. The variable range hopping transport mechanism is dominant below 130 K. The room temperature photocurrent–photon energy dependency predicts a band gap of 1.91 eV of the films. The illumination intensity-photocurrent dependency measured in the intensity range of 13–235 W cm−2 reveals monomolecular recombination (linear) in the films and bimolecular recombination (sublinear) at the film surface corresponding to low and high applied illumination intensities, respectively. The time-dependant photocurrent measured at fixed illumination intensity reveals a response time of 0.85, 2.66 and 10.0 s in the time periods of 0–0.5, 0.5–1.0, and 1.0–10.0 s, respectively.