scispace - formally typeset
Search or ask a question

Showing papers on "Variable-range hopping published in 2015"


Journal ArticleDOI
TL;DR: In this paper, the impact of the trapped charges on the carrier transport of MoS2-based metal-oxide-semiconductor FETs is evaluated. And the trapped charge density and time constant at different temperatures are extracted.
Abstract: The field effect transistors (FETs) based on thin layer MoS2 often have large hysteresis and unstable threshold voltage in their transfer curves, mainly due to the charge trapping at the oxide-semiconductor interface. In this paper, the charge trapping and de-trapping processes at the SiO2-MoS2 interface are studied. The trapping charge density and time constant at different temperatures are extracted. Making use of the trapped charges, the threshold voltage of the MoS2 based metal-oxide-semiconductor FETs is adjusted from 4 V to −45 V. Furthermore, the impact of the trapped charges on the carrier transport is evaluated. The trapped charges are suggested to give rise to the unscreened Coulomb scattering and/or the variable range hopping in the carrier transport of the MoS2 sheet.

201 citations


Journal ArticleDOI
TL;DR: This work confirms the excellent potential of MoS2 as a possible channel-replacement material and highlights the role of multiple transport phenomena in governing its transistor action.
Abstract: We fabricate transistors from chemical vapor deposition-grown monolayer MoS2 crystals and demonstrate excellent current saturation at large drain voltages (Vd). The low-field characteristics of these devices indicate that the electron mobility is likely limited by scattering from charged impurities. The current-voltage characteristics exhibit variable range hopping at low Vd and evidence of velocity saturation at higher Vd. This work confirms the excellent potential of MoS2 as a possible channel-replacement material and highlights the role of multiple transport phenomena in governing its transistor action.

94 citations


Journal ArticleDOI
TL;DR: This work reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.
Abstract: Anderson localization, the absence of diffusive transport in disordered systems, has been manifested as hopping transport in numerous electronic systems, whereas in recently discovered topological insulators it has not been directly observed. Here, we report experimental demonstration of a crossover from diffusive transport in the weak antilocalization regime to variable range hopping transport in the Anderson localization regime with ultrathin $({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}{)}_{2}{\mathrm{Te}}_{3}$ films. As disorder becomes stronger, negative magnetoconductivity due to the weak antilocalization is gradually suppressed, and eventually, positive magnetoconductivity emerges when the electron system becomes strongly localized. This work reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.

87 citations


Journal ArticleDOI
Runchen Fang1, Wenhao Chen1, Ligang Gao1, Weijie Yu1, Shimeng Yu1 
TL;DR: In this paper, the authors investigated the low-temperature switching characteristics and conduction mechanism of the Pt/HfO x /TiN resistive random access memory devices.
Abstract: This letter investigates the low-temperature switching characteristics and conduction mechanism of the Pt/HfO x /TiN resistive random access memory devices. For the first time, Pt/HfO x /TiN devices were demonstrated to be well functional at ultralow temperature (4 K). The switching voltages slightly increase at lower temperature. The failure state in a breakdown sample shows a metallic behavior, while the normal low-resistance states and high-resistance states show a semiconducting behavior. The slope change in the 1/kT plot below 77 K indicates a transition from the nearest-neighboring hopping to the variable range hopping. Different slopes or activation energies are observed at the same resistance level in the same device but after different programming cycles, indicating a cycle-dependent variation of the filament configuration.

83 citations


Journal ArticleDOI
Abstract: The temperature dependence of electrical conductivity of the Cu2ZnSnS4 (CZTS) thin films has been measured in order to identify the dominant conduction mechanism. These CZTS film have been deposited by ultrasonic assisted chemical vapor deposition method at different substrate temperatures in a single step process. All the films exhibit p-type semiconducting behaviour. In the high temperature range (> 250 K), the dominance of thermally activated band conduction is observed, whereas in the lower temperature region (250−70 K), the hopping conduction is present. Detailed analysis of the temperature dependence of conductivity of the films reveals that in the temperature range from 250–170 K, nearest neighbour hopping dominates, whereas in the lower temperature region (170–70 K), the dominant hopping conduction is Mott's 3D variable range hopping and not Efros-Shklovskii variable range hopping. The value of Mott's temperature is found to decrease for the CZTS films deposited at higher temperature, which has be...

75 citations


Journal ArticleDOI
TL;DR: In this paper, the transport characteristics of ultrathin SrRuO3 films, deposited epitaxially on TiO2-terminated SrTiO3 (001) single-crystal substrates, were studied as a function of film thickness.
Abstract: Transport characteristics of ultrathin SrRuO3 films, deposited epitaxially on TiO2-terminated SrTiO3 (001) single-crystal substrates, were studied as a function of film thickness. Evolution from a metallic to an insulating behavior is observed as the film thickness decreases from 20 to 4 unit cells. In films thicker than 4 unit cells, the transport behavior obeys the Drude low temperature conductivity with quantum corrections, which can be attributed to weak localization. Fitting the data with 2-dimensional localization model indicates that electron-phonon collisions are the main inelastic relaxation mechanism. In the film of 4 unit cells in thickness, the transport behavior follows variable range hopping model, indicating a strongly localized state. Magnetoresistance measurements reveal a likely magnetic anisotropy with the magnetic easy axis along the out-of-plane direction.

62 citations


Journal ArticleDOI
TL;DR: In this paper, a polaron-based master equation is proposed to bridge the results from coherent band-like transport regime governed by the Redfield equation to incoherent hopping transport in the classical regime, where a non-monotonic dependence of diffusion coefficient is observed both as a function of temperature and system-phonon coupling strength.
Abstract: Quantum transport in disordered systems is studied using a polaron-based master equation. The polaron approach is capable of bridging the results from the coherent band-like transport regime governed by the Redfield equation to incoherent hopping transport in the classical regime. A non-monotonic dependence of the diffusion coefficient is observed both as a function of temperature and system-phonon coupling strength. In the band-like transport regime, the diffusion coefficient is shown to be linearly proportional to the system-phonon coupling strength and vanishes at zero coupling due to Anderson localization. In the opposite classical hopping regime, we correctly recover the dynamics described by the Fermi’s Golden Rule and establish that the scaling of the diffusion coefficient depends on the phonon bath relaxation time. In both the hopping and band-like transport regimes, it is demonstrated that at low temperature, the zero-point fluctuations of the bath lead to non-zero transport rates and hence a finite diffusion constant. Application to rubrene and other organic semiconductor materials shows a good agreement with experimental mobility data.

61 citations


Journal ArticleDOI
TL;DR: The m-GO bolometer effect identified in this study is different from that exhibited in vanadium oxides, which require added gold-black films that function as IR absorbers owing to their limited IR absorption capability.
Abstract: This study characterizes the effects of incident infrared (IR) radiation on the electrical conductivity of graphene oxide (GO) and examines its potential for mid-IR detection. Analysis of the mildly reduced GO (m-GO) transport mechanism near room temperature reveals variable range hopping (VRH) for the conduction of electrons. This VRH behavior causes the m-GO resistance to exhibit a strong temperature dependence, with a large negative temperature coefficient of resistance of approximately -2 to -4% K(-1). In addition to this hopping transport, the presence of various oxygen-related functional groups within GO enhances the absorption of IR radiation significantly. These two GO material properties are synergically coupled and provoke a remarkable photothermal effect within this material; specifically, a large resistance drop is exhibited by m-GO in response to the increase in temperature caused by the IR absorption. The m-GO bolometer effect identified in this study is different from that exhibited in vanadium oxides, which require added gold-black films that function as IR absorbers owing to their limited IR absorption capability.

54 citations


Journal ArticleDOI
TL;DR: A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials, which implies the relevance of a quantum interference effect.
Abstract: We have investigated the temperature evolution of magnetism and its interrelation with structural parameters in the perovskite-based layered compound Sr2IrO4, which is believed to be a Jeff = 1/2 Mott insulator. The structural distortion plays an important role in this material and induces a weak ferromagnetism in an otherwise antiferromagnetically ordered magnetic state with a transition temperature around 240 K. Interestingly, at low temperatures, below around 100 K, a change in the magnetic moment has been observed. Temperature dependent x-ray diffraction measurements show that sudden changes in structural parameters around 100 K are responsible for this. Resistivity measurements show insulating behavior throughout the temperature range across the magnetic phase transition. The electronic transport can be described with Mott's two-dimensional variable range hopping (VRH) mechanism, however, three different temperature ranges are found for VRH, which is a result of varying the localization length with temperature. A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials. The quadratic field dependence of MR implies the relevance of a quantum interference effect.

53 citations


Journal ArticleDOI
TL;DR: In this paper, the electric and dielectric properties of La0.5Ca 0.5−xAgxMnO3 (LCMO-Ag with x = 0 and x= 0.4) were investigated using the impedance spectroscopy technique.
Abstract: The electric and dielectric properties of La0.5Ca0.5−xAgxMnO3 (LCMO–Ag with x = 0 and x = 0.4) were investigated using the impedance spectroscopy technique. For the free compound, conductivity analysis proved the absence of a metal–semiconductor transition in the temperature range of 80 K to 700 K. When silver was introduced, such transition appears at around TMS = 200 K. We found that the conduction mechanism was governed by small polaron hopping (SPH) in the high temperature region and by variable range hopping (VRH) in the low temperature region. Several results were deduced from the complex impedance analysis (CIA). First, a non-Debye relaxation phenomenon in the system was observed. Second, the CIA permitted us to model the compounds in terms of an electrical equivalent circuit. Moreover, this study confirmed the contribution of the grain boundary in the transport properties. It was found that the activation energy deduced from the conductivity, the relaxation time and the grain boundary were close to each other. The dielectric constant and dielectric loss were investigated. For the free compound, the dielectric transition was not observed in the investigated temperature range but, for the doped compound, the dielectric transition appeared at Td = 200 K. The Td value was close to the TMS value. The thermal evolution of the real part of dielectric permittivity is described by the Curie–Weiss law. From the variation of the imaginary part of the dielectric permittivity and the dielectric loss versus temperature at different frequencies, we found that the free compound had a typical behavior of a relaxor but the doped one had a typical behavior of a normal dielectric.

53 citations


Journal ArticleDOI
TL;DR: In this paper, structural, dielectric, and magnetic properties of polycrystalline double perovskite Nd2NiMnO6 compound were investigated using the Maxwell-Wagner interfacial polarization model.
Abstract: We have investigated structural, dielectric, and magnetic properties of polycrystalline double perovskite Nd2NiMnO6 compound. The compound crystallizes in monoclinic P21∕n symmetry and is partially B-site disordered depending on the synthesis conditions. It undergoes second-order ferromagnetic transition at 192 K and shows glassy behaviour at low temperature. The glassy phase is due to anti-site disorder within the homogeneous sample. Temperature and frequency dependent dielectric measurements reveal colossal values of dielectric constant and is best interpreted using Maxwell-Wagner interfacial polarization model. Impedance spectroscopy has been used to analyse the intrinsic dielectric response. This enabled us to differentiate the conduction process at the grain and grain boundaries. Arrhenius behaviour is favoured at the grain boundary, while variable range hopping mechanism is considered most suitable within the grain region. dc conductivity measurements corroborate variable range hopping conduction.

Journal ArticleDOI
TL;DR: The photoelectric measurement demonstrates the potential for applications for photovoltaic devices and indicates that the semiconducting sample follows the mechanisms of variable range hopping (VRH) and adiabatic small polaron hopping (SPH).
Abstract: [Bi2O2]-containing tetragonal compounds have received enormous attention due to unique functions including ferroelectricity, photocatalysis, and superconductivity. Here, a new layered compound Bi9O7.5S6 was synthesized via a facile hydrothermal route. The compound, belonging to a new structure type crystallizes in a rhombohedral system with space group R3m (a = 4.0685(1) A, c = 31.029(5) A, V = 444.8(1) A3, Z = 1). The overall crystal structure consists of alternatively packed unique [Bi2O2] and [BiS2] layers along [001] which are combined with each other by van der Waals interaction. The phase purity of the product is confirmed by powder X-ray diffraction. XPS analyses indicate +3 for Bi and −2 for S atoms. The temperature dependence of resistivity ρ(T) indicates that the semiconducting sample follows the mechanisms of variable range hopping (VRH) and adiabatic small polaron hopping (SPH). The direct-transition band gap, Eg = 1.27 eV derived from optical absorption spectrum, falls in the optimal region ...

Journal ArticleDOI
TL;DR: A series of GdMn1−xCrxO3 (0,⩽ ǫ 0.02) has been synthesized by the conventional solid state reaction method mainly to understand the response of Cr substitution on structural, microstructural, magnetic and electrical transport properties.

Journal ArticleDOI
01 Aug 2015-Carbon
TL;DR: In this article, a systematic study on charge transport properties of thermally reduced graphene oxide (rGO) layers, from room temperature to 2 K and in the presence of magnetic fields up to 7 T, is presented.

Journal ArticleDOI
TL;DR: In this paper, the structural, magnetic, magneto-transport and thermoelectric properties of polycrystalline compounds of La 0.8− x Bi x Ca 0.2 MnO 3 (0≤ x ≤ 0.1) compounds are reported.

Journal ArticleDOI
TL;DR: In this paper, the impact of pronounced disorder on charge transport at low temperatures was investigated and the authors revealed the existence of a metal-to-insulator transition (MIT) where upon increasing order the zero-temperature limit of conductivity changes from zero (insulator) to nonzero values (metal).
Abstract: Disorder and its reduction upon annealing play a crucial role in understanding the electrical transport in the crystalline phase-change material Ge1Sb2Te4. Previous studies focus either on the impact of disorder at moderate temperatures or on the low-temperature properties of crystalline films with a low degree of disorder. The present investigation describes and discusses the impact of pronounced disorder on charge transport at low temperatures. The present data reveal the existence of a metal-to-insulator transition (MIT), where upon increasing order the zero-temperature limit of conductivity changes from zero (insulator) to nonzero values (metal). The position of the MIT is determined with respect to the control parameter, i.e., the disorder, which is modified through the annealing conditions. Disorder is shown to localize carriers for an exceptionally large density of states. In the most disordered films, variable range hopping is observed, enabling the determination of the localization length. At the lowest temperatures studied, deviations from Mott variable range hopping are observed, which can be explained by a transition to Efros–Shklovskii hopping due to the presence of a soft Coulomb gap.

Journal ArticleDOI
TL;DR: The influence of Cr doping on magnetic, magnetocaloric and electrical properties in a polycrystalline sample of Pr0.7Ca0.3MnO3 is investigated in this paper.

Journal ArticleDOI
TL;DR: In this article, the authors report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method.
Abstract: The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3 × 103 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 1012 to 1013 eV−1cm−2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~1011 eV−1cm−2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.

Journal ArticleDOI
TL;DR: The systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method suggest that the reduction of gap states below ~1011 eV−1cm−2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.
Abstract: The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3*10^3 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 10^12 to 10^13 eV^-1cm^-2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~10^11 eV^-1cm^-2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.

Journal ArticleDOI
Nianduan Lu1, Ling Li1, Writam Banerjee1, Pengxiao Sun1, Nan Gao1, Ming Liu1 
TL;DR: In this article, the authors improved the A-H method to investigate the charge carrier hopping transport by introducing polaron effect and electric field based on the Marcus theory and variable-range hopping theory.
Abstract: Charge carrier hopping transport is generally taken from Miller-Abrahams and Marcus transition rates. Based on the Miller-Abrahams theory and nearest-neighbour range hopping theory, Apsley and Hughes developed a concise calculation method (A-H method) to study the hopping conduction in disordered systems. Here, we improve the A-H method to investigate the charge carrier hopping transport by introducing polaron effect and electric field based on Marcus theory and variable-range hopping theory. This improved method can well describe the contribution of polaron effect, energetic disorder, carrier density, and electric field to the charge carrier transport in disordered organic semiconductor. In addition, the calculated results clearly show that the charge carrier mobility represents different polaron effect dependence with the polaron activation energy and decreases with increasing electric field strength for large fields.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated temperature-dependent charge transport behavior in thin reduced graphene oxide (RGO) film and showed that charges transport through two parallel percolating conducting pathways.
Abstract: We have investigated temperature-dependent charge transport behavior in thick reduced graphene oxide (RGO) film. Our results show that charges transport through two parallel percolating conducting pathways. One contains large disordered regions as one of its constituents, so its conductance is determined dominantly by variable range hopping (VRH). The other is composed of small and medium disordered regions and crystalline sp2 domains, so its conductance is determined by a serial connection of quantum tunneling and thermal activation. The more oxygen functional groups are removed from GO film upon progressive reduction, the lower the potential barriers between the crystalline sp2 domains and disordered regions become. The contribution of thermal activation to total conductance does not appear evidently for highly reduced GO film having low potential barriers, but thermal activation causes the conductance of moderately reduced film to change continuously, even at low temperatures where the VRH is almost fr...

Journal ArticleDOI
TL;DR: In this article, two powder samples of manganese ferrite (MnFe2O4) with different morphology and particle size 30-40nm, denoted by A and B, have been synthesized by different methods starting from MnCl2·4H2O and FeCl3·6H 2O. The results showed that the density of states at the Fermi level is constant over the investigated temperature range.
Abstract: Two powder samples of manganese ferrite (MnFe2O4) with different morphology and particle size 30–40 nm, denoted by A and B have been synthesized by different methods starting from MnCl2·4H2O and FeCl3·6H2O. Sample A was obtained by co-precipitation followed by calcination at 900 °C and sample B has been obtained by hydrothermal method. XRD analysis show that calcination leads to the occurrence of other phases than MnFe2O4, therefore the hydrothermal method gives better results. From the temperature dependence of the electrical resistivity, measured over the range 300–483 K, the activation energy, Δ E , of the investigated samples has been evaluated, resulting in 0.43 eV (for sample A) and 0.32 eV (for sample B). The conductivity mechanism in the samples was explained in terms of Mott's variable range hopping (VRH) model. The results showed that the density of states at the Fermi level is constant over the investigated temperature range, being in order of 0.788 × 10 17 eV − 1 cm − 3 (for sample A) and 2.05 × 10 17 eV − 1 cm − 3 (for sample B). The hopping distance, R and the hopping energy, W (parameters of VRH model) have also been computed. Room temperature values are R=27.08 nm and W=152 meV for sample A and R=21.29 nm and W=120 meV for sample B.

Journal ArticleDOI
TL;DR: In this paper, the microscopic switching and conducting mechanisms in TiW/Cu2O/Cu memristive devices have been thoroughly investigated and the bipolar resistive switching behaviors without an electro-forming process are ascribed to the formation and rupture of the conducting filaments composed of copper vacancies.
Abstract: P-type Cu2O is a promising CMOS-compatible candidate to fabricate memristive devices for next-generation memory, logic and neuromorphic computing. In this letter, the microscopic switching and conducting mechanisms in TiW/Cu2O/Cu memristive devices have been thoroughly investigated. The bipolar resistive switching behaviors without an electro-forming process are ascribed to the formation and rupture of the conducting filaments composed of copper vacancies. In the low resistive state, the transport of electrons in the filaments follows Mott's variable range hopping theory. When the devices switch back to high resistive state, the coexistence of Schottky emission at the Cu/Cu2O interface and electron hopping between the residual filaments is found to dominate the conducting process. Our results will contribute to the further understanding and optimization of p-type memristive materials.

Journal ArticleDOI
TL;DR: In this paper, the electric current of polyethylene-carbon nanotube composite, in the electrical percolation threshold region, over the absorbed dose under the applied bias voltage was investigated via the finite element method.
Abstract: In this research work, the electric current of polyethylene-carbon nanotube composite, in the electrical percolation threshold region, over the absorbed dose under the applied bias voltage was investigated via the finite element method. The investigated geometry was formed by a two-dimensional cross-sectional view of randomly orientated nanotubes as ellipses. The variable range hopping model developed by Mott and thermally activated hopping model was used to calculate the electrical conductivity of carbon nanotubes and polymer, respectively. Regarding the calorimetric approach, we considered the absorbed dose equal to heat capacity of polyethylene-carbon nanotube composite multiplied by temperature rise. Results showed that this kind of composite can be used for low dose rate applications for monitoring and radiation protection utilizations.

Journal ArticleDOI
TL;DR: An expression of the hopping rate between localized states in semiconducting disordered polymers that contain the most used rates in the literature as special cases is proposed and shows that, under the appropriate limits, this expression reduces to a single-phonon rate expression or the Miller-Abrahams rate.
Abstract: We propose an expression of the hopping rate between localized states in semiconducting disordered polymers that contain the most used rates in the literature as special cases. We stress that these rates cannot be obtained directly from electron transfer rate theories as it is not possible to define diabatic localized states if the localization is caused by disorder, as in most polymers, rather than nuclear polarization effects. After defining the separate classes of accepting and inducing nuclear modes in the system, we obtain a general expression of the hopping rate. We show that, under the appropriate limits, this expression reduces to (i) a single-phonon rate expression or (ii) the Miller-Abrahams rate or (iii) a multi-phonon expression. The description of these limits from a more general expression is useful to interpolate between them, to validate the assumptions of each limiting case, and to define the simplest rate expression that still captures the main features of the charge transport. When the rate expression is fed with a range of realistic parameters the deviation from the Miller-Abrahams rate is large or extremely large, especially for hopping toward lower energy states, due to the energy gap law.

Journal ArticleDOI
TL;DR: In this article, the phase purity, homogeneity, lattice structure and cell parameters of the Zn 1− x Ni x Fe 2 O 4 was checked by XRD technique using conductance spectroscopy.
Abstract: The phase purity, the homogeneity, lattice structure and cell parameters of the Zn 1− x Ni x Fe 2 O 4 was checked by XRD technique. The Ni doping effects on the electrical properties in Zn 1− x Ni x Fe 2 O 4 was investigated using conductance spectroscopy. When introducing nickel in Zn ferrite, the conductivity of the material increases. This effect disappears at high temperature ( T >600 K). The conductivity spectra can be splitted in three regions. In the first one, the conductivity is frequency independent. In the second region, the conductivity can be described by the ω n law. It decreases with increasing frequency in the third region. DC conductivity analysis proves that electronic conduction was dominated by thermally activated Hopping of Small Polaron (SPH) at high temperature and Variable Range Hopping (VRH) at low temperature. For T > T s , where T s is the temperature at which appears a change in the slope of σ dc T (1000/ T ) curves, the activation energy decreases when increasing Ni content (from E a =665 meV for x =0 to E a =283 for x =0.5). For a ll Ni compositions, the activation energy is equal to 280 meV when temperature is lower than T s . From AC-conductivity study, it is found that activation energy decreases with increasing Ni concentration and varies significantly when increasing frequency. Also, the variation of the exponent “ n ” confirms that hopping model is the dominating mechanism in the system.

Journal ArticleDOI
TL;DR: In this paper, the currentvoltage and the temperature dependence of the contact resistance [R(T)] of Au/YBa2Cu3O7−δ (optimally doped YBCO) interfaces have been studied at different resistance states.
Abstract: Current-voltage (IV) characteristics and the temperature dependence of the contact resistance [R(T)] of Au/YBa2Cu3O7−δ (optimally doped YBCO) interfaces have been studied at different resistance states. These states were produced by resistive switching after accumulating cyclic electrical pulses of increasing number and voltage amplitude. The IV characteristics and the R(T) dependence of the different states are consistent with a Poole-Frenkel (P-F) emission mechanism with trapping-energy levels Et in the 0.06–0.11 eV range. Et remains constant up to a number-of-pulses-dependent critical voltage and increases linearly with a further increase in the voltage amplitude of the pulses. The observation of a P-F mechanism reveals the existence of an oxygen-depleted layer of YBCO near the interface. A simple electrical transport scenario is discussed, where the degree of disorder, the trap energy level, and the temperature range determine an electrical conduction dominated by non-linear effects, either in a P-F e...

Journal ArticleDOI
TL;DR: In this paper, the electrical transport properties of several disordered systems including composites, amorphous/doped semiconductors, conducting polymers and manganites (both in single crystal and poly-crystalline phase) are highlighted from the standpoint of non-Ohmic direct current conduction.

Journal ArticleDOI
TL;DR: In this article, the mirage effect technique has been used to estimate the thermal conductivity of NiMn 2 O 4 ternary nickel manganese oxide thin films spinels.

Journal ArticleDOI
TL;DR: In this article, the theory of spin fluctuations was developed for an ensemble of localized electrons, taking into account both the hyperfine interaction of electron and nuclear spins and electron hopping between the sites.
Abstract: The theory of spin fluctuations is developed for an ensemble of localized electrons, taking into account both the hyperfine interaction of electron and nuclear spins and electron hopping between the sites. The analytical expression for the spin noise spectrum is derived for an arbitrary relation between the electron spin precession frequency in a field of nuclear fluctuations and the hopping rate. An increase in the hopping rate results in a drastic change in the spin noise spectrum. The effect of an external magnetic field is briefly addressed.