Abstract: We investigate the low temperature electron transport properties of chemically reduced graphene oxide (RGO) sheets with different carbon sp2 fractions of 55 to 80 %. We show that in the low bias (Ohmic) regime, the temperature (T) dependent resistance (R) of all the devices follow Efros-Shklovskii variable range hopping (ES-VRH) R ~ exp[(T(ES)/T)^1/2] with T(ES) decreasing from 30976 to 4225 K and electron localization length increasing from 0.46 to 3.21 nm with increasing sp2 fraction. From our data, we predict that for the temperature range used in our study, Mott-VRH may not be observed even at 100 % sp2 fraction samples due to residual topological defects and structural disorders. From the localization length, we calculate a bandgap variation of our RGO from 1.43 to 0.21 eV with increasing sp2 fraction from 55 to 80 % which agrees remarkably well with theoretical prediction. We also show that, in the high bias regime, the hopping is field driven and the data follow R ~ exp[(E(0)/E)^1/2] providing further evidence of ES-VRH.

Abstract: For two-dimensional lattices in a tight-binding description, the intrinsic spin-orbit coupling, acting as a complex next-nearest-neighbor hopping, opens gaps that exhibit the quantum spin Hall effect. In this paper, we study the effect of a real next-nearest-neighbor hopping term on the band structure of several Dirac systems. In our model, the spin is conserved, which allows us to analyze the spin Chern numbers. We show that in the Lieb, kagome, and ${T}_{3}$ lattices, variation of the amplitude of the real next-nearest-neighbor hopping term drives interesting topological phase transitions. These transitions may be experimentally realized in optical lattices under shaking, when the ratio between the nearest- and next-nearest-neighbor hopping parameters can be tuned to any possible value. Finally, we show that in the honeycomb lattice, next-nearest-neighbor hopping only drives topological phase transitions in the presence of a magnetic field, leading to the conjecture that these transitions can only occur in multigap systems.

Abstract: This paper reviews ferroelectric oxides in the unusual condition where the concentration of electronic carriers is close to a metal–insulator transition; in certain structures and compositions these materials have properties of interest for oxide based thermoelectric applications. In relaxor ferroelectrics, nanopolar regions associated with intrinsic localized phonon modes provide glass-like phonon characteristics due to the large levels of phonon scattering. The (Sr1−xBax)Nb2O6−δ relaxor ferroelectric single crystals have a high thermoelectric power factor, S2σ ∼ 40 μW/cm K2 at 277 °C along the c-axis, which is competitive with the best thermoelectrics. In the heavily reduced, nonstoichiometric n-type perovskite BaTiO3−δ and tungsten bronze (Sr1−xBax)Nb2O6−δ, it is shown that metallic-like conductivity occurs in the paraelectric phase and the onset of ferroelectricity stabilizes semiconducting character. Both the phase transition temperature dependence on the carrier concentration and evidence for polarization coupling to the conductivity mechanism will be discussed.

Abstract: A La2NiMnO6 polycrystalline sample prepared by the sol–gel method showed monoclinic crystal structure with the P21/n space group and a saturation magnetization of 4.63 μB/f.u. at 5 K. Impedance spectroscopy results in the temperature range of 10 K < T < 300 K have revealed a distinct conduction process at grains and grain boundaries, where the grains followed the variable range hopping mechanism and the grain boundaries obeyed Arrhenius thermal activation. A negative magnetoresistance of 2.5% was observed at the paramagnetic to ferromagnetic transition, and this became temperature independent below the magnetic ordering. A marginal positive magnetodielectric (MD) effect that followed the dielectric relaxation was observed and its magnitude was found to decrease with increase of the frequency. A systematic study on the magnetic field induced dielectric properties, dc transport and dc bias effect on the dielectric permittivity has revealed the extrinsic origin of the MD effect in the bulk sample of La2NiMnO6.

Abstract: We investigate the mechanism of charge transport in indium gallium zinc oxide (a-IGZO), an amorphous metal-oxide semiconductor. We measured the field-effect mobility and the Seebeck coefficient (S=ΔV/ΔT) of a-IGZO in thin-film transistors as a function of charge-carrier density for different temperatures. Using these transistors, we further employed a scanning Kelvin probe-based technique to determine the density of states of a-IGZO that is used as the basis for the modeling. After comparing two commonly used models, the band transport percolation model and a mobility edge model, we find that both cannot describe the full properties of the charge transport in the a-IGZO semiconductor. We, therefore, propose a model that extends the mobility edge model to allow for variable range hopping below the mobility edge. The extended mobility edge model gives a superior description of the experimental results. We show that the charge transport is dominated by variable range hopping below, rather than by bandlike transport above the mobility edge. © 2012 American Physical Society.

Abstract: A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up ``spin-blocked'' pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.

Abstract: Spectral properties of the two-dimensional Hubbard model near the Mott transition are investigated by using cluster perturbation theory. The Mott transition is characterized by freezing of the charge degrees of freedom in a single-particle excitation that leads continuously to the magnetic excitation of the Mott insulator. Various anomalous spectral features observed in cuprate high-temperature superconductors are explained in a unified manner as properties near the Mott transition.

Abstract: We study the electrolyte-gate-induced conductance at the surface of SrTiO(3)(001). We find two distinct transport regimes as a function of gate voltage. At high carrier densities, a percolative metallic state is induced in which, at low temperatures, clear signatures of a Kondo effect are observed. At lower carrier densities, the resistance diverges at low temperatures and can be well described by a 2D variable range hopping model. We postulate that this derives from nonpercolative transport due to inhomogeneous electric fields from imperfectly ordered ions at the electrolyte-oxide interface.

Abstract: A recently introduced percolative theory of unipolar organic magnetoresistance is generalized by treating the hyperfine interaction semiclassically for an arbitrary hopping rate. Compact analytic results for the magnetoresistance are achievable when carrier hopping occurs much more frequently than the hyperfine field precession period. In other regimes, the magnetoresistance can be straightforwardly evaluated numerically. Slow and fast hopping magnetoresistance are found to be uniquely characterized by their lineshapes. We find that the threshold hopping distance is analogous a phenomenological two-site model's branching parameter, and that the distinction between slow and fast hopping is contingent on the threshold hopping distance.

Abstract: Polycrystalline samples of YxAlyB14 (x ∼ 0.57) with different fractional occupancies y (0.41 ≤ y ≤ 0.63) were synthesized and their thermoelectric properties investigated. Electrical conductivities generally followed three-dimensional variable range hopping with a rapid delocalization indicated as electrons were increased. Positive Seebeck coefficients were obtained for the Al-poor sample, y = 0.41, which was shifted in the negative direction with increase of y. Maximum Seebeck coefficient values were approximately 400 μV K−1 at 850 K and −200 μV K−1 at 1000 K, for p-type and n-type, respectively. Excellent control of p-n characteristics was achieved in a system with the same crystal structure and consisting of the same elements.

Abstract: LuFe2O4 often is considered as a prototypical multiferroic with polar order arising from the electronic degrees of freedom only (“electronic ferroelectricity”). In the present work, we check the intrinsic nature of the dielectric response of this material by performing dielectric measurements of polycrystalline samples with different types of contact materials and with different grain sizes. In addition, frequency-dependent measurements of the electric-field dependent polarization are provided. The obtained results unequivocally prove that the reported colossal dielectric constants in LuFe2O4, which were interpreted in terms of electronic ferroelectricity, are of non-intrinsic surface-related origin. The intrinsic dielectric properties of this material show no indications of any ferroelectric order and, thus, LuFe2O4 is not multiferroic. Its intrinsic dielectric constant is close to 20 and its dielectric loss is dominated by charge transport via variable range hopping.

Abstract: Ternary Cu–Bi–S based compounds have been thought to be alternative materials for well-known CuInS2 because of their abundance. Cu–Bi–S based nanomaterials have been less studied. We here report the synthesis, optical and electrical properties of single crystal Cu4Bi4S9 nanowires. High-quality Cu4Bi4S9 nanowires were synthesized through a modified solvothermal route by controlling the reaction sources and temperature. The optical bandgap for Cu4Bi4S9 nanowires were determined by using UV-vis-NIR and cyclic voltammetry techniques. Single nanowire devices were fabricated by using lithographic techniques. The devices exhibit photoconductive response with high external quantum efficiency (2.9 × 108%). Temperature-dependent electrical transport properties were also investigated. We observed that the transport properties of Cu4Bi4S9 nanowire show typical semiconductor behaviour in the temperature region 10–140 K and metal-like character in the temperature region of 150–300 K. The carrier transport in Cu4Bi4S9 nanowires can be described by the small polaron model in temperature region of 60–140 K and the variable range hopping mechanism in temperature region of 10–50 K. We further studied the properties of Cu4Bi4S9 nanowires in field-emission devices. The devices exhibit a relatively low turn-on field (6.9 V μm−1). The potential applications of Cu4Bi4S9 nanowires as field emitting materials and light absorbers in detectors are indicated.

Abstract: Polycrystalline Pr0.8Ca0.2MnO3 has been synthesized through solid state reaction route and phase purity is analyzed using synchrotron XRD. Magnetization M(T) showed Mn spin alignment due to ferromagnetic ordering around 125 K (TC), whereas M(H) showed an onset of non-linear behavior from 200 K and with temperature re-orientation of magnetic moment is discussed. The modulation of relaxation processes revealed different formations of impedance plane plots with temperatures. A change in equivalent circuit models from (R1C1)(R2Q2) to (R1C1)(R2Q2)(R3Q3) at 100 K is being reported. The analysis of the impedance data is carried out by calculating impedance of grains and grain boundaries. Fitted parameters derived from these equivalent circuit parameters showed a change in conduction mechanism from small polaronic hopping model (SPH) to Mott's variable range hopping (MVRH) model, around 125 K (TC). Carriers hop to larger distance with multiple activation energies and are described by MVRH below TC. Above this temperature, different trap centers start facilitating these carriers through SPH. Dielectric relaxation shows dispersion around 125 K and the origin of this polarization lies close to the relaxation derived from the grains and their interfaces.

Abstract: LuFe2O4 often is considered as a prototypical multiferroic with polar order arising from the electronic degrees of freedom only ("electronic ferroelectricity"). In the present work, we check the intrinsic nature of the dielectric response of this material by performing dielectric measurements of polycrystalline samples with different types of contact materials and with different grain sizes. In addition, frequency-dependent measurements of the electric-field dependent polarization are provided. The obtained results unequivocally prove that the reported colossal dielectric constants in LuFe2O4, which were interpreted in terms of electronic ferroelectricity, are of non-intrinsic surface-related origin. The intrinsic dielectric properties of this material show no indications of any ferroelectric order and, thus, LuFe2O4 is not multiferroic. Its intrinsic dielectric constant is close to 20 and its dielectric loss is dominated by charge transport via variable range hopping.

Abstract: We propose a new conduction model associated with thermionic emission of carriers over potential barriers at grain boundaries, which is applicable to non-degenerate through degenerate polycrystalline semiconductors. We first assume the grain-boundary potential barriers of a uniform height and derive an analytical expression of the electrical conductivity on the basis of an energy filtering model with fully incorporating the Fermi-Dirac distribution of carriers. We then introduce fluctuations of the potential barrier height as a Gaussian distribution into the model. It is shown that the non-linear curves of Arrhenius plots of electrical conductivity in various polycrystalline semiconductors (Si, CdS, CdSe, ZnO, WO3, In2Se3) in literature are well fitted using the model with the effect of fluctuations taken into account, being regardless of whether carriers are degenerate or non-degenerate, without assuming other conduction mechanisms such as tunneling and variable range hopping.

Abstract: La0.65Sr0.35MnO 3-δ (LSMO) films (δ ≈ 0, 0.07, 0.12) were studied by hard x-ray photoelectron spectroscopy. It is found that the Mn 3d valence band states shift to higher binding energies for oxygen-deficient samples, while their overall width decreases as a result of the reduced density of charge carriers. The concurrent disappearance of the well-screened state at the Mn 2p level indicates a decrease in hybridization of the Mn 3d and the doping-induced states. The lack of clear band gap formation for oxygen-deficient LSMO above the metal/insulator transition is compatible not with polaron formation but, rather, with a Mott variable range hopping mechanism, which is also supported by our transport data. The large electron probing depth of hard x-ray photoelectron spectroscopy is crucial for this study because modifications to the electronic structure may occur in the nearsurface region.

Abstract: The electrical and electroluminescence (EL) properties at room and high temperatures of oxide/nitride/oxide (ONO)-based light emitting capacitors are studied. The ONO multidielectric layer is enriched with silicon by means of ion implantation. The exceeding silicon distribution follows a Gaussian profile with a maximum of 19%, centered close to the lower oxide/nitride interface. The electrical measurements performed at room and high temperatures allowed to unambiguously identify variable range hopping (VRH) as the dominant electrical conduction mechanism at low voltages, whereas at moderate and high voltages, a hybrid conduction formed by means of variable range hopping and space charge-limited current enhanced by Poole-Frenkel effect predominates. The EL spectra at different temperatures are also recorded, and the correlation between charge transport mechanisms and EL properties is discussed.