Low-temperature polaronic relaxations with variable range hopping conductivity in FeTiMO(6) (M = Ta,Nb,Sb)

Low-temperature dielectric measurements on FeTiMO(6) (M = Ta,Nb,Sb) rutile-type oxides at frequencies from 0.1 Hz to 10 MHz revealed anomalous dielectric relaxations with frequency dispersion. Unlike the high-temperature relaxor response of these materials, the low-temperature relaxations are polaronic in nature. The relationship between frequency and temperature of dielectric loss peak follows T(-1/4) behavior. The frequency dependence of ac conductivity shows the well-known universal dielectric response, while the dc conductivity follows Mott variable range hopping (VRH) behavior, confirming the polaronic origin of the observed dielectric relaxations. The frequency domain analysis of the dielectric spectra shows evidence for two relaxations, with the high-frequency relaxations following Mott VRH behavior more closely. Significantly, the Cr- and Ga-based analogs, CrTiNbO(6) and GaTiMO(6) (M = Ta,Nb), that were also studied, did not show these anomalies.

Effect of magnetic field on Mott's variable-range hopping parameters in multiwall carbon nanotube mat

We report the temperature and magnetic field dependence of the conductivity of multiwall carbon nanotube mat in the temperature range 1.4-150 K and in magnetic fields up to 10 T. It is observed that charge transport in this system is governed by Mott's variable-range hopping of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Mott's various parameters, such as localization length, hopping length, hopping energy and density of states at the Fermi level are deduced from the variable-range hopping fit. The resistance of the sample decreases with the magnetic field applied in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states with the well-known consequence of a decrease in Mott's T-0 parameter in variable-range hopping. The application of magnetic field lowers the crossover temperature at which three-dimensional variable-range hopping turns to two-dimensional variable-range hopping. The conductivity on the lower temperature side is governed by the weak localization giving rise to positive magnetoconductance. Finally, a magnetic field-temperature diagram is proposed showing different regions for different kinds of transport mechanism.

Insulator-to-metal transition in vanadium supersaturated silicon: variable-range hopping and Kondo effect signatures

We report the observation of the insulator-to-metal transition in crystalline silicon samples supersaturated with vanadium. Ion implantation followed by pulsed laser melting and rapid resolidification produce high quality single-crystalline silicon samples with vanadium concentrations that exceed equilibrium values in more than 5 orders of magnitude. Temperature-dependent analysis of the conductivity and Hall mobility values for temperatures from 10K to 300K indicate that a transition from an insulating to a metallic phase is obtained at a vanadium concentration between 1.1 × 10^(20) and 1.3 × 10^(21) cm^(−3) . Samples in the insulating phase present a variable-range hopping transport mechanism with a Coulomb gap at the Fermi energy level. Electron wave function localization length increases from 61 to 82 nm as the vanadium concentration increases in the films, supporting the theory of impurity band merging from delocalization of levels states. On the metallic phase, electronic transport present a dispersion mechanism related with the Kondo effect, suggesting the presence of local magnetic moments in the vanadium supersaturated silicon material.

Low-field hopping conductivity in granular metals

The hopping conductivity of granular metals is known to be of the form sigma varies as exp (-(T0/T)12/) in the temperature range 20Kvariable range hopping mechanism of conduction in amorphous systems, is discussed. Use is made of the empirical fact that the ratio of intergrain separation and grain size is nearly constant for macroscopically homogeneous samples. It is also assumed that the hopping of electrons is predominantly between nearest neighbours. Extension of the treatment to thin films and comparison with previous theoretical work is briefly discussed.

Electron hopping i eui-xsrxfeO3 and ndi-xsrxcOO3

Mössbauer and electrical resistivity measurements on Eu1–xSrxFeO3(0.0 < x[less-than-or-eq] 0.4) show the presence of a time-averaged electron configuration of Fe in these solids at T > TN. Variable range hopping arising from Anderson localization seems to occur at T < TN indicating that the electron hopping time in this regime is likely to be greater than 10–7 s. Mössbauer studies on Nd1–xSrxCoO3 show that in the Anderson localization regime, the hopping time is greater than 10–7 s in this system as well.

Majorana fermions in superconducting wires: Effects of long-range hopping, broken time-reversal symmetry, and potential landscapes

We present a comprehensive study of two of the most experimentally relevant extensions of Kitaev's spinless model of a one-dimensional p-wave superconductor: those involving (i) longer-range hopping and superconductivity and (ii) inhomogeneous potentials. We commence with a pedagogical review of the spinless model and, as a means of characterizing topological phases exhibited by the systems studied here, we introduce bulk topological invariants as well as those derived from an explicit consideration of boundary modes. In time-reversal symmetric systems, we find that the longer range hopping leads to topological phases characterized by multiple Majorana modes. In particular, we investigate a spin model that respects a duality and maps to a fermionic model with multiple Majorana modes; we highlight the connection between these topological phases and the broken symmetry phases in the original spin model. In the presence of time-reversal symmetry breaking terms, we show that the topological phase diagram is characterized by an extended gapless regime. For the case of inhomogeneous potentials, we explore phase diagrams of periodic, quasiperiodic, and disordered systems. We present a detailed mapping between normal state localization properties of such systems and the topological phases of the corresponding superconducting systems. This powerful tool allows us to leverage the analyses of Hofstadter's butterfly and the vast literature on Anderson localization to the question of Majorana modes in superconducting quasiperiodic and disordered systems, respectively. We briefly touch upon the synergistic effects that can be expected in cases where long-range hopping and disorder are both present.

Kinetic study for hopping conduction through deoxyribonucleic acid molecules

Recent experiments indicated that disorder effect in deoxyribonucleic acid (DNA) may lead to a transition of the electronic hole transport mechanism from band resonant tunneling to thermally activated hopping. In this letter, based on Mott's variable-range hopping theory, we present a kinetic study for the hole transport properties of DNA molecules. Beyond the conventional argument in large-scale systems, our numerical study for finite-size DNA molecules reveals a number of unique features for: (i) the current-voltage characteristics, (ii) the temperature and length dependence, and (iii) the transition from conducting to insulating behaviors. (c) 2005 American Institute of Physics.

Hopping conduction and persistent photoconductivity in Cu2ZnSnS4 thin films

The temperature dependence of electrical conductivity and the photoconductivity of polycrystalline Cu2ZnSnS4 were investigated. It was found that at high temperatures the electrical conductivity was dominated by band conduction and nearest-neighbour hopping. However, at lower temperatures, both Mott variable-range hopping (VRH) and Efros–Shklovskii VRH were observed. The analysis of electrical transport showed high doping levels and a large compensation ratio, demonstrating large degree of disorder in Cu2ZnSnS4. Photoconductivity studies showed the presence of a persistent photoconductivity effect with decay time increasing with temperature, due to the presence of random local potential fluctuations in the Cu2ZnSnS4 thin film. These random local potential fluctuations cannot be attributed to grain boundaries but to the large disorder in Cu2ZnSnS4.

Procesos hopping a través del modelo difusional en materiales nanocristalinos usados para aplicaciones fotovoltaicas

Se presentan los modelos de hopping de rango variable (variable range hopping; VRH), vecinos cercanos (nearest neighbor hopping; NNH) y barreras de potencial presentes en las fronteras de grano; como mecanismos de transporte eléctrico predominantes en los materiales semiconductores para aplicaciones fotovoltaicas. Las medidas de conductividad a oscuras en función de temperatura fueron realizadas para región de bajas temperaturas entre 120 y 400 K con Si y compuestos Cu3BiS2 y Cu2ZnSnSe4. Siguiendo la teoría de percolación, se obtuvieron parámetros hopping y la densidad de estados cerca del nivel de Fermi, N(EF), para todas las muestras. A partir de los planteamientos dados por Mott para VRH, se presentó el modelo difusional, que permitió establecer la relación entre la conductividad y la densidad de estados de defecto o estados localizados en el gap del material. El análisis comparativo entre modelos, evidenció, que es posible obtener mejora hasta de un orden de magnitud en valores para cada uno de los parámetros hopping que caracterizan el material.

Low-temperature electrical conductivity of LaNi1-xFexO3

We report the electrical conductivity between 2 and 300 K for LaNi1-xFexO3 across the composition-controlled metal-insulator (m-i) transition. Using a method first suggested by Mobius, we identify the critical concentration x(c) to be 0.3 for the m-i transition. The negative temperature coefficient of resistivity observed at low temperatures in the metallic phase follows a temperature dependence characteristic of disorder effects. The semiconducting compositions (x greater than or equal to 0.3) do not show a simple activation energy but exhibit variable-range hopping at high temperatures confirming that the m-i transition in this system is driven by increasing disorder effects.

Phase conversions in calcium manganites with changing Ca/Mn ratios and their influence on the electrical transport properties

The phase-interconversions between the spinel-, brownmillerite-, defect rocksalt and perovskite-type structures have been investigated by way of (i) introducing deficiency in A-sites in CaxMn2-xO3 (0.05 <= x <= 1) i.e., by varying Ca/Mn ratio from 0.025 to 1 and (ii) nonstoichiometric CaMnO3-delta (CMO) with 0.02 <= delta <= 1. The temperature dependence of resistivity (rho-T) have been investigated on nonstoichiometric CaMnO3-delta (undoped) as well as the CMO substituted with donor impurities such as La3+, Y3+, Bi3+ or acceptor such as Na1+ ion at the Ca-site. The rho-T characteristics of nonstoichiometric CaMnO3-delta is strongly influenced by oxygen deficiency, which controls the concentration of Mn3+ ions and, in turn, affects the resistivity, rho. The results indicated that the substitution of aliovalent impurities at Ca-site in CaMnO3 has similar effects as of CaMnO3-delta ( undoped) annealed in atmospheres of varying partial pressures whereby electron or hole concentration can be altered, yet the doped samples can be processed in air or atmospheres of higher P-O2. The charge transport mechanisms of nonstoichiometric CaMnO3-delta as against the donor or acceptor doped CaMnO3 (sintered in air, P-O2 similar to 0.2 atm) have been predicted. The rho (T) curves of both donor doped CaMnO3 as well as non-stoichiometric CaMnO3-delta, is predictable by the small polaron hopping (SPH) model, which changes to the variable range hopping (VRH) at low temperatures whereas the acceptor doped CaMnO3 exhibited an activated semiconducting hopping ( ASH) throughout the measured range of temperature (10-500 K).

Colossal dielectric behavior of semiconducting Sr2TiMnO6 ceramics

Manganitelike double perovskite Sr2TiMnO6 (STMO) ceramics fabricated from the powders synthesized via the solid-state reaction route, exhibited dielectric constants as high as similar to 10(5) in the low frequency range (100 Hz-10 kHz) at room temperature. The Maxwell-Wagner type of relaxation mechanism was found to be more appropriate to rationalize such high dielectric constant values akin to that observed in materials such as KxTiyNi(1-x-y)O and CaCu3Ti4O12. The dielectric measurements carried out on the samples with different thicknesses and electrode materials reflected the influence of extrinsic effects. The impedance studies (100 Hz-10 MHz) in the 180-300 K temperature range revealed the presence of two dielectric relaxations corresponding to the grain boundary and the electrode. The dielectric response of the grain boundary was found to be weakly dependent on the dc bias field (up to 11 V/cm). However, owing to the electrode polarization, the applied ac/dc field had significant effect on the low frequency dielectric response. At low temperatures (100-180 K), the dc conductivity of STMO followed a variable range hopping behavior. Above 180 K, it followed the Arrhenius behavior because of the thermally activated conduction process. The bulk conductivity relaxation owing to the localized hopping of charge carriers obeyed the typical universal dielectric response.

Colossal dielectric behavior of semiconducting $Sr_{2}TiMnO_{6}$ ceramics

Manganitelike double perovskite Sr2TiMnO6 (STMO) ceramics fabricated from the powders synthesized via the solid-state reaction route, exhibited dielectric constants as high as similar to 10(5) in the low frequency range (100 Hz-10 kHz) at room temperature. The Maxwell-Wagner type of relaxation mechanism was found to be more appropriate to rationalize such high dielectric constant values akin to that observed in materials such as KxTiyNi(1-x-y)O and CaCu3Ti4O12. The dielectric measurements carried out on the samples with different thicknesses and electrode materials reflected the influence of extrinsic effects. The impedance studies (100 Hz-10 MHz) in the 180-300 K temperature range revealed the presence of two dielectric relaxations corresponding to the grain boundary and the electrode. The dielectric response of the grain boundary was found to be weakly dependent on the dc bias field (up to 11 V/cm). However, owing to the electrode polarization, the applied ac/dc field had significant effect on the low frequency dielectric response. At low temperatures (100-180 K), the dc conductivity of STMO followed a variable range hopping behavior. Above 180 K, it followed the Arrhenius behavior because of the thermally activated conduction process. The bulk conductivity relaxation owing to the localized hopping of charge carriers obeyed the typical universal dielectric response.

Low temperature electrical transport properties of carbon matrix containing iron nanoparticles

We present a comparative study of the low temperature electrical transport properties of the carbon matrix containing iron nanoparticles and the films. The conductivity of the nanoparticles located just below the metal-insulator transition exhibits metallic behavior with a logarithmic temperature dependence over a large temperature interval. The zero-field conductivity and the negative magnetoresistance, showing a characteristic upturn at liquid helium temperature, are consistently explained by incorporating the Kondo relation and the two dimensional electron-electron interaction. The films, in contrast, exhibit a crossover of the conductivity from power-law dependence at high temperatures to an activated hopping law dependence in the low temperature region. The transition is attributed to changes in the energy dependence of the density of states near the Fermi level. The observed magnetoresistance is discussed in terms of quantum interference effect on a three-dimensional variable range hopping mechanism.

Anderson transitions in Ln1-xSrxCoO3 (Ln=Pr or Nd)

Electrical conductivity measurements show that Ln1-xSrxCoO3, (Ln = Pr or Nd) undergoes a non-metal-metal transition when x-0 3. The d.c. conductivity of compositions with 0range 0.6-0.8. Surprisingly, however, \delta seems to increase with increasing temperature. Thermopower measurements show clear evidence for a change in the mechanism of conduction (around 350 K) from variable range hopping to transport at Ec. All these transport properties suggest that Co4+ ions in Ln1-xSrxCoO3 (0