Mott Variable Range Hopping Research Articles

Influence of non-magnetic ion In3+ on the magneto-transport properties in La0.7Bi0.05Sr0.15Ca0.1Mn1-xInxO3 (0 ≤ x ≤ 0.3) perovskite

In this work, the polycrystalline La0.7Bi0.05Sr0.15Ca0.1Mn1-xInxO3 (0 ≤ x ≤ 0.3) have been prepared by the Pechini-sol-gel method. A significant improvement in magnetoresistance (MR) has been observed from 64% for x = 0–84% for x = 0.3 at a magnetic applied field of 5T. Analysis on the measured electrical resistivity data indicates that for T below metal –semiconducting transition temperature (TM-Sc), the resistivity was described by a phenomenological percolation model which is based on theory on electron-electron, electron-phonon scattering. However, for T above TM-Sc, two conduction models are explored to describe the transport behaviors: the Mott's variable range hopping (VRH) model and the small polaron hopping model (SPH).

Magnetism and magnetoresistance in Al half doped LaNiO3

Abstract The rhombohedral perovskite LaAl0.5Ni0.5O3 (or La2AlNiO6), synthesized by a combustion method is found to contain Al3+ and Ni3+ cations in the rhombohedral lattice. The material exhibits a temperature dependent (thermally excited) paramagnetic susceptibility and follows the modified Curie-Weiss law implying an antiferromagnetic ordering at very low temperatures. A non-saturating hysteresis loop with a coercivity of 108 Oe is observed at 2 K which is arising from the magnetic contribution from Ni in the lattice. The conduction mechanism in LaAl0.5Ni0.5O3 for 80–300 K follows the Mott's variable range hopping mechanism. A negative magnetoresistance of 1.75% is observed at 2 K under 90 kOe field which decreases linearly by increasing the temperature up to 18 K and then changes to positive with comparatively low value. Both the negative and positive magnetoresistance increases linearly with an applied magnetic field.

Magnetotransport and conductivity mechanisms in Cu2ZnSnxGe1\u2212xS4 single crystals

Resistivity, ρ(T), and magnetoresistance (MR) are investigated in the Cu2ZnSnxGe1−xS4 single crystals, obtained by the chemical vapor transport method, between x = 0–0.70, in the temperature range of T ~ 50–300 K in pulsed magnetic field of B up to 20 T. The Mott variable-range hopping (VRH) conductivity is observed within broad temperature intervals, lying inside that of T ~ 80–180 K for different x. The nearest-neighbor hopping conductivity and the charge transfer, connected to activation of holes into the delocalized states of the acceptor band, are identified above and below the Mott VRH conduction domain, respectively. The microscopic electronic parameters, including width of the acceptor band, the localization radius and the density of the localized states at the Fermi level, as well as the acceptor concentration and the critical concentration of the metal-insulator transition, are obtained with the analysis of the ρ(T) and MR data. All the parameters above exhibit extremums near x = 0.13, which are attributable mainly to the transition from the stannite crystal structure at x = 0 to the kesterite-like structure near x = 0.13. The detailed analysis of the activation energy in the low-temperature interval permitted estimations of contributions from different crystal phases of the border compounds into the alloy structure at different compositions.

Transport Properties in InAs Thin Films with Different Degrees of Disorder Near the Metal–Insulator Transition

Transport properties of Indium Arsenide (InAs) films deposited at different working pressures are investigated in the temperature interval of T ∼ 2–250 K. Electrical transport of InAs films is related to the degree of disorder, which increases as working pressure increases. Activated conductivity models satisfactorily describe conduction in the localized band-tails at high temperatures, while conductivity shows metallic characteristic at low temperatures for μc-InAs samples. The critical conductivity exponent of μc-InAs is close to 1, which indicates that μc-InAs samples exhibit a close proximity to the metal–insulator transition. The product of aB and n1/3 for samples S1 and S2 is 0.236 and 0.207, respectively, which is roughly consistent with the Mott criterion. But for α-InAs samples in the high temperature region, electron transport is described by conduction in extended states and in localized states near the Fermi level. Furthermore, a crossover from Mott variable range hopping (VRH) to the Efros–Shklovskii VRH mechanism is observed in α-InAs thin films at low temperatures.

Structural and electrical properties of HCl–polyaniline–Ag composites synthesized by polymerization using Ag-coated (NH4)2S2O8 powder

Ag nanoparticles were sputter-deposited on ammonium persulfate ((NH4)2S2O8) powder to obtain (NH4)2S2O8−Ag powder, which was used to synthesize the HCl-doped polyaniline−Ag (HCl−PANI−Ag) composite via a polymerization procedure. The Ag nanoparticles were dispersed in the HCl−PANI matrix, and their sizes mainly ranged from 3 to 6 nm. The Ag nanoparticles did not affect the structure of emeraldine salt in the composite, and they increased the ordered crystalline regions in the HCl−PANI matrix. The HCl−PANI−Ag composite had a conductivity of (6.8 ± 0.1) S/cm, which is about four times larger than that of the HCl−PANI. The charge transport mechanism in the composite is explained by the three-dimensional Mott variable-range hopping (3D-Mott-VRH).

Effect of Fe and N co-doping on structural, electrical and optical properties of tin oxide thin films

The physical and electrical properties of magnetron sputtering fabricated (Fe, N) co-doping tin oxide thin films were investigated. The deposited films are in polycrystalline tetragonal structure. No Fe metal clusters or secondary phases are formed. All the films display n-type conductivity and the carrier concentration of (Sn1−xFex)O1.982N0.018 films, x = 0, 0.023, 0.042, 0.075 and Sn0.987Fe0.013(O2−yNy) films, y = 0, 0.038, 0.091, 0.130 decrease monotonically with Fe and N co-doping. Transport mechanism of the films can be attributed to the Mott variable range hopping (VRH) model. The SnO1.982N0.018 films showed maximum optical transmittance 90% in the visible region. The red-shift in the band gap energy can be attributed to the sp-d exchange interaction between the SnO2 band electrons and the substitutional N for O in SnO2 lattice.

Structural, magnetic, and magnetotransport properties of LaFe0.5Ni0.5O3

The perovskite-type LaFe0.5Ni0.5O3 belonging to the rhombohedral (space group R-3c) crystal structure has been synthesized for which we have identified a magnetic transition at T1 = 8 K corresponding to the minimum observed in the derivative of temperature dependent magnetization. A bifurcation in the ZFC and FC curves is observed below T1 that suggests a frustrated magnetic behavior. The non-zero moment above T1 hints the possibility of the presence of a high-temperature magnetic transition in the material. The resistivity of LaFe0.5Ni0.5O3 evolves as a function of temperature similar to that of a semiconductor. Mott's variable range hopping governs the conduction mechanism of the material. Presence of various anisotropy terms and inhomogeneous magnetic interactions lead to the presence of antiferromagnetic and ferromagnetic interfaces, which eventually causes a magnetic exchange bias and magnetic hysteresis in resistivity. We have also observed direction dependent magnetoresistance in the material.

Effect of V2O5 concentration on the structural and optical properties and DC electrical conductivity of ternary semiconducting glassy nanocomposites

Transition metal oxide–doped glassy nanocomposites of the form xV2O5-(1 − x) (0.05CdO-0.95ZnO) with x = 0.3, 0.5, 0.7, and 0.9 were prepared by the melt quenching route. Their microstructure was studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy, transmission electron microscopy, and optical absorption spectroscopy. The absorption bands at around 640–756 cm−1 are attributed to the antisymmetric stretching vibrations of VO2 groups. Physical attributes such as density, molar volume, oxygen molar volume, oxygen packing density, number of bonds per unit volume, and the average stretching force constant were evaluated. The XRD patterns reveal distinct diffraction peaks indicating crystallinity in the sample with x = 0.3, whereas significantly broadened patterns are observed in samples with x = 0.5, 0.7, and 0.9, reflecting a dominant amorphous nature. Different peaks in the XRD patterns indicate the formation of Zn2V2O7, ZnV2O6, CdV2O6, Cd2V2O7, and CdVO3 nanocrystallites, which are dispersed in the as-prepared glassy matrices. The optical absorption spectra show indirect transitions. With a rise in the concentration of V2O5, the optical transmission cutoff wavelength moves to higher wavelength. DC electrical conductivity was studied at various temperatures from 60–650 °C. The establishment of small polaron hopping was anticipated from the DC electrical conductivity data. DC conductivity was inferred with use of Mott's and Greaves's variable range hopping models.

Influence of nano-dimensionality on magnetotransport, magnetic and electrical properties of Nd1-xSrxMnO3-δ (0.3 ≤ x ≤ 0.7)

Magneto-electric and electronic properties of Nd1-xSrxMnO3-δ (0.3 ≤ x ≤ 0.7) (NSMO) nanoparticles are investigated. NSMO nanoparticles are synthesized by glycine assisted auto combustion method. The structural and morphological properties are probed using X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). NSMO crystallizes in orthorhombic structure and nearly spherical agglomerated morphology is observed. Resistivity of the order of few MOhm-cm is recorded for all the compositions and the transport mechanism is interpreted by using Mott variable range hopping (VRH) model. In absence of magnetic field all samples show insulating nature over the studied temperature region (300- 5 K), while Metal to Insulator transition is observed in the presence of magnetic field (8T). Large Colossal Magnetoresistance (CMR) is noticed below 50 K for NSMO.Observed Curie temperature is in the range of 102–115 K and it is found to be independent of Strontium doping concentration. Bifurcation in FC-ZFC, for all the samples, is at higher temperature than blocking temperature (∼72 K) indicates glassy state. We have calculated thickness of magnetic dead layer by two different ways and it lies between 1.6 and 2.9 nm. Concentration of divalent element and oxygen non-stoichiometry affects the relative fraction of Mn3+/Mn4+ ions. Mn3+/Mn4+ in our samples is determined from areas under the deconvoluted X-Ray Photoelectron Spectra (XPS) of Mn core level. The Mn3+/Mn4+ ratio is higher than the value expected from nominal composition, which indicates oxygen non-stoichiometry. Oxygen non-stoichiometry is also confirmed by chemical titration method.

Influence of hopping conduction on the thermopower of indium oxide thick films

We have studied the temperature (T) dependent behaviors of thermopower (S) and resistivity (ρ) of a series of indium oxide (In2O3) thick films prepared at different oxygen partial pressures. For the films deposited at lower oxygen partial pressures, the ρ − T curves display metallic characteristics, and the relation between thermopower and temperature is dominated by the electron diffusion process. For the films deposited at higher oxygen partial pressures, the Mott type and Efros-Shklovskii type variable-range-hopping (VRH) processes are observed in the resistivity data. The thermopower obeys S ∝ T1∕2 from ∼50 to ∼200 K, which can be explained by Mott VRH process. At lower temperatures, the existence of phonon-drag causes S to change from negative to positive with decreasing temperature and then present a peak value with further decreasing temperature. Our results demonstrate the validity of theoretical predictions concerning three dimensional Mott VRH thermopower.

The velocity of 1d Mott variable-range hopping with external field

Mott variable-range hopping is a fundamental mechanism for low–temperature electron conduction in disordered solids in the regime of Anderson localization. In a mean field approximation, it reduces to a random walk (shortly, Mott random walk) on a random marked point process with possible long-range jumps. We consider here the one-dimensional Mott random walk and we add an external field (or a bias to the right). We show that the bias makes the walk transient, and investigate its linear speed. Our main results are conditions for ballisticity (positive linear speed) and for sub-ballisticity (zero linear speed), and the existence in the ballistic regime of an invariant distribution for the environment viewed from the walker, which is mutually absolutely continuous with respect to the original law of the environment. If the point process is a renewal process, the aforementioned conditions result in a sharp criterion for ballisticity. Interestingly, the speed is not always continuous as a function of the bias. Keywords: random walk in random environment, disordered media, ballisticity, environment viewed from the walker, electron transport in disordered solids.

Influences of Oxygen Partial Pressure on the Cu Metal Clustering and Room Ferromagnetism of the (In0.89Fe0.06Cu0.05)2O3 Films

The influences of oxygen partial pressure on the local structure, magnetic, and transport properties of (In0.89Fe0.06Cu0.05)2O3 films deposited by different Ar–O2 flow rate (10:0, 10:0.4, and 10:3.0) were studied systematically by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), Hall effect, room temperature (RT) magnetic measurements, and first-principles calculations. The detailed structural analyses indicate that the doped Fe atoms substitute for In3+ sites of the In2O3 lattices with +2 and +3 mixture valences for all the films. However, the majority of codoped Cu atoms form the Cu metal clusters in the film deposited at oxygen flow rate of 0 sccm. With increasing oxygen partial pressure, the Cu metal clusters completely disappear and all the Cu atoms are incorporated substitutionally into the In2O3 lattices at In sites. All the films exhibit intrinsic RT ferromagnetism, and Mott variable range hopping (VRH) transport behavior. The saturation magneti...

Synergistic conductivity increase in polypyrrole/molybdenum disulfide composite

Polypyrrole/molybdenum disulfide (PPy/MoS2) composites were synthesized by in-situ chemical polymerization of pyrrole in the presence of MoS2 flakes. The conductivity of the composite with a moderate content of PPy (15–30 wt%) reached 13 S cm−1, which is markedly higher than the conductivity of both the pristine PPy and MoS2, 1 and 10−6 S cm−1, respectively. The improved conductivity was explained by the formation of ordered thin PPy films with high conductivity at the MoS2 surface. At higher pyrrole content, globular PPy was formed in the bulk of the composite resulting in an increase of a disordered polymer fraction, and the conductivity decreased. The composite conductivity is thus controlled not only by content of PPy but also by the proportions between ordered and disordered PPy phases. The structural and morphological characterization of composite materials is based on Fourier-transform infrared and Raman spectroscopies, wide-angle X-ray diffraction, and scanning and transmission electron microscopies. The charge-carrier transport in the composites fits the Mott variable-range hopping mechanism.

Structural, Magnetic, and Electrical Transport Properties of Nanocrystalline SrCo0.5Mn0.5O<inline-formula> <tex-math notation="LaTeX">$_{3-\delta}$ </tex-math> </inline-formula>

We report the structural, magnetic, and electrical transport response of nanocrystalline SrCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-δ</sub> compound synthesized by the sol-gel method. X-ray diffraction pattern shows that the sample crystallizes in the hexagonal crystal structure with cubic stacking at room temperature. The compound retains its structural symmetry down to 20 K. The transmission electron microscopy image confirms the surface morphology of the nanocrystalline sample, and the average particle size was found to be ~90 nm. Temperature dependence magnetization data show bifurcation in the zero field cooled (ZFC) and field cooled (FC) magnetization measured under the applied magnetic field of 500 Oe with onset irreversible temperature Tirr ~ 242 K. At low temperature, the small peak has been observed in ZFC curve, suggesting the spin blocking below TB ~ 35 K. The peak has disappeared on the application of higher magnetic field, and the magnitude of bifurcation has also been reduced. The electrical transport data reveal semiconducting such as behavior and follow Mott's variable range hopping (VRH) conduction mechanism in the temperature range 260-300 K. The density of states in the vicinity of Fermi energy is found to be N(EF) ~ 5.702 χ 1018 eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> using the VRH fit. The density of state value is in good agreement with the other reported semiconducting oxides. It shows maximum positive magnetoresistance of ~7% at 200 K for the magnetic field change of 3 T.

Effect of source temperature on the electrical transport, microstructural and optical properties of tin monosulfide thin films prepared by close spaced sublimation method

Tin monosulfide (SnS) thin films were prepared by close spaced sublimation method at three different source temperatures of 550, 575 and 600°C. Structural, textural and compositional features were investigated by XRD, SEM, EDS and Raman spectroscopy. From absorption spectra, direct band gap increased from 1.18 to 1.23 eV with increasing source temperature. Electrical resistivity, Hall constant, carrier concentration and mobility values of the films prepared at source temperature of 600°C were 0.54 Ω.cm, 40.76 cm3/C, 1.537#x00D7;1017 and 75.48 cm2/(V.s), respectively. The temperature-dependent conductivity was performed in the temperature range of 50-300 K. It was shown that three types of conduction mechanisms can be expected: thermionic emission passing through grain boundary at high temperature range (240-300 K), the Mott variable-range hopping (Mott-VRH) at low temperature range (135-235 K) and the Efros-Shklovskii variable-range hopping (ES-VRH) at very low temperature range (80-130 K).

Mott variable range hopping and bad-metal in lightly doped spin-orbit Mott insulator BaIrO3

The Mott transition and the vicinity of a Mott insulating phase have constantly been a fertile ground for exploring exotic quantum states, most notably the high-${T}_{c}$ cuprates. The layered iridate represents another intriguing example. The nature of the Mott insulator phase and transition mechanism to a metallic state is still under debate. Much of the challenge originates from a series of energy scales involved in the electronics phases. Here, we report synthetization, characterization, and transport measurements on doped and undoped $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{Ir}{\mathrm{O}}_{3}$ films grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$. The films are fully strained up to 70 nm thick and have a tetragonal lattice structure. For a doping level of $x=0.1$, a bad-metal state with linear temperature dependence of the resistivity beyond the Mott-Ioffe-Regel limit emerges in a wide temperature range down to a critical temperature ${T}_{c}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, below which the system shows Mott variable range hopping (Mott VRH) conduction behavior. The ground state is confirmed to be insulating by the Mobius criterion. A strong correlation between the bad-metal state and Mott VRH localized state is found as the slope of the linear resistivity is inversely proportional to film thickness and the size of the Mott VRH activation energy $\mathrm{\ensuremath{\Delta}}$ is linearly proportional to ${T}_{c}$. We further show that upon doping the spin-orbit Mott insulator, itinerant metallic regions coexist with localized regions within a nanoscale phase-separated ground state with a small activation energy. Our results shed light on the nature of the metallic state and a crossover to a bad-metal phase for doping the spin-orbit Mott insulator.

Enhanced dielectric and piezoelectric properties of BNT-KNNG piezoelectric ceramics

The structural, morphological and electric properties of (1-x) Bi0.5Na0.5TiO3 – x K0.5Na0.5NbO3 + 1 wt. % Gd2O3 (BNT-KNNG; x = 0–0.02) ceramics were investigated in detail. XRD studies revealed the rhombohedral phase, which is also confirmed by HRTEM. The diffuse phase transition of BNT-KNNG samples exhibited a relaxor behaviour supported by modified Curie Weiss law and Vogel–Fulcher law. The AC-Conductivity of BNT-KNNG ceramics in terms of hopping length, hopping energy and density of states across the Fermi level was analyzed by Mott's variable range hopping mechanism. The addition of KNNG effectively reduced grain size, transition temperature, leakage current and improved dielectric (ε′ = 1074 and tanδ = 0.059 @ 1 kHz) and piezoelectric properties; electromechanical coupling factors (k33, k31, kt ∼ 48% and kp = 53%), and piezoelectric constants (d33 = 108 pC/N and g33 = 1.14 × 10−2 Vm/N) are observed for x = 0.01 sample, which is a potential candidate for high power electromechanical applications.

Electron transport in surface modified TiO2 nanoparticles

Surface modified nanoparticles of P25 Evonik Degussa TiO2 with silane coupling agent 3-(2-aminoethylamino) propyltrimethoxysilane (AAPTMS) were prepared at low temperature conditions. The samples were characterized using the BET technique. Their temperature dependent dc electrical conductivity was investigated in the temperature range 195–440 K. In order to investigate the influence of the environment, measurements were conducted in vacuum and in air. A change of conductivity with the molar ratio of AAPTMS/P25 is shown. Moreover, a decrease of conductivity is exhibited when compared to that of P25 nanopowder, attributed to the presence of silica in AAPTMS. The modified samples show a significant decrease of conductivity in air. In addition, the conductivity dependence on temperature differs from that in vacuum, suggesting the competition between the adsorption of oxygen and water from the surrounding atmosphere. It was demonstrated that the electrical conduction was controlled by the grain boundaries and the small polaron hopping at the high and intermediate temperatures whereas Mott's variable range hopping was the main conduction mechanism at the lower temperature regime.

Magnetoresistance mechanisms in carbon-nanotube yarns

Carbon nanotube (CNT) yarns are novel CNT-based materials that extend the advantages of CNT from the nanoscale to macroscale applications. Herein we have modeled the electrical properties of carbon nanotube yarn as a function of temperature and magnetic field. The conductivity was well explained by 3D Mott Variable Range Hopping (VRH) law at T < 100 K. The hopping effective dimension is reduced with increasing of magnetic field. Negative magnetoresistance (MR) was observed at different magnetic field range. A quadratic MR was found at small magnetic fields up to BD2, where a deviation occurred from the quadratic magnetic field dependence of the MR. In the intermediate magnetic field region, the negative MR shows linear magnetic field dependence up to the magnetic crossover field of BD1. It was found that physical parameters such as the localization length and the density of states at the Fermi level are temperature and magnetic field dependent. In addition, a general scaling behavior was found for the MR as a function of (B/B*)1/3, where B* is a crossover magnetic field.

Variable range hopping conduction in fully dense calcium cobalt oxide textured ceramics

Fully dense hot-pressed calcium cobalt oxide (Ca3Co4O9) pellet exhibits textures with Lotgering factor of unity and supports the ideally aligned (00 l), l = 2, 3, 4, 5 crystallographic planes associated in the ceramics. The DC electrical conductivity of the Ca3Co4O9 pellets connected to metal/Ca3Co4O9-pellet/metal device structures in vertical and lateral configurations follow Mott's variable range hopping. The exhibition of anisotropic ratio of ~ 10 between the room temperature conductivity values in both the structures, reveal the presence of partially oriented textured grains in the currently developed Ca3Co4O9 ceramics.