Assisted Hopping Research Articles

Hopping conduction in calcium vanadate semiconducting glasses

The temperature and compositional dependences of the electrical conductivity of semiconducting calcium vanadate glasses have been reported in the temperature range 80-500 K. It has been observed that the multiphonon assisted hopping model of small polarons in the nonadiabatic regime, which considers the strong interaction of electrons with both the optical and acoustical phonons, can interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. On the other hand, Mott's nearest neighbour hopping model at high temperatures yields smaller values of the localization length. Moreover, Mott's variable range hopping model is consistent with the low temperature data.

Semiconducting properties of magnesium vanadate glasses

The electrical conductivity of semiconducting magnesium vanadate glasses has been reported for a wide composition range in the temperature range of 80–500 K. The experimental results have been analyzed in the framework of different hopping models. It has been observed that the multiphonon assisted hopping model of small polarons in the nonadiabatic regime, proposed by Emin, can interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. On the other hand, Mott’s optical phonon assisted hopping model at high temperatures provides smaller values of the localization length. However, Mott’s variable range hopping model is consistent with the low temperature data. Schnakenberg’s model yields higher values of the hopping and the disorder energies than the activation energy obtained at the highest and the lowest temperature ranges.

Far infrared photoconductivity studies in silicon blocked impurity band structures

We report two novel features in the photoconductive response of a boron doped Blocked Impurity Band (BIB) structure. In this structure which comprises a pure and a very doped photoconductive layer between ohmic contacts, spectra of impurities at an interface with the pure layer were evidenced in a process involving field assisted hopping among excited states. In addition, a peculiar photovoltaic response, with open circuit photovoltage surprisingly independent of the incident photon flux was measured and analysed.

Multiphonon assisted hopping in strontium vanadate semiconducting glasses

The temperature and compositional dependence of the electrical conductivity of semiconducting strontium vanadate glasses have been reported in the temperature range 80-500 K. It has been observed that the multiphonon assisted hopping model of small polarons in the nonadiabatic regime, which considers the strong interaction of electrons with both the optical and acoustical phonons, can interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. Mott's optical phonon assisted hopping model at high temperatures yields smaller values of the localization length. However, Mott's variable range hopping model can predict the low temperature data. Schnakenberg's model provided higher values of the hopping energy than the activation energy obtained at the highest temperature range.

Hopping transport in multiple-dot silicon single electron MOSFET

The transport properties in multiple-dot single electron MOSFETs have been investigated by numerical calculation solving the master equation. In multiple-dot single electron devices, the capacitive coupling effects play an important role in the electron transport. The calculation shows that the transport in the multiple-dot system is governed by the phonon assisted hopping if the single electron charging energy and coupling energy are larger than the thermal energy. The result is in good agreement with the experimental results of silicon narrow channel MOSFETs and confirms the hopping transport model in the multiple-dot system we proposed in our previous paper.

Effect of Er-substitution on superconducting and metal-insulator transitions in 2212 bismuth cuprate

Results of a systematic study of lattice parameters and resistivity on Bi 2Sr 2Ca 1− x Er x Cu 2O 8+δ, with value of x in the range 0.0 ⩽ x ⩽ 1.0 are reported. The quality of samples has been examined by XRD and resistivity measurements. The suppression of superconductivity is attributed to the effect of disorder. The resistivity behaviour in the insulating regime is analyzed in the hopping conduction approach. The nature of the density of states as determined using the Ortuno and Pollak model is found to be concave near E F. At the MIT, the localization length diverges and the metallicity vanishes following a scaling law with the critical exponent, μ = 1. The high value of differential activation energy suggests multiphonon assisted hopping at low temperatures.

Electron localization in mixed-valence manganites.

Mixed-valence manganites $({{A}^{3+}}_{0.7}{{B}^{2+}}_{0.3})\mathrm{Mn}{\mathrm{O}}_{3}$ exhibit a resistivity peak with giant negative magnetoresistance below the Curie point. Residual resistivity ranges from 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ to 8\ifmmode\times\else\texttimes\fi{}${10}^{3}$ \ensuremath{\Omega} m, but the electronic heat capacity when $A=\mathrm{Y}\mathrm{or}\mathrm{La}$ is that of a normal $d$-band metal; $\ensuremath{\gamma}\ensuremath{\approx}6$ mJ ${\mathrm{mole}}^{\ensuremath{-}1}$${\mathrm{K}}^{\ensuremath{-}2}$. Spatial fluctuations in the Coulomb and spin-dependent potentials localize the ${e}_{g}\ensuremath{\uparrow}({\ensuremath{\sigma}}^{*})$ electrons in wave packets that are large on the scale of the Mn-Mn separation. Transport in the magnetically localized state involves zero-point-energy assisted hopping or tunneling of electrons from one weakly localized wave packet to another.

Origin of intrinsic Josephson coupling in the cuprates and its relation to order parameter symmetry: An incoherent hopping model

Experiments on the cuprate superconductors demonstrate that these materials may be viewed as a stack of Josephson junctions along the direction normal to the CuO 2 planes (the c-axis). In this paper, we present a model which describes this intrinsic Josephson coupling in terms of incherent quasiparticle hopping along the c-axis arising from wave-function overlap, impurity-assisted hopping, and boson-assised hopping. We use this model to compute the magnitude and temperature T dependence of the resulting Josephson critical current j c( T) for s- and d-wave superconductors. Contrary to other approaches, d-wave pairing in this model is compatible with an intrinsic Josephson effect at all hole concentrations and leads to j c( T) α T at low T. By parameterizing our theory with c-axis resistivity data from YBa 2Cu 3O 7−δ (YBCO), we estimate j c( T) for optimally doped and underdoped members of this family. j c( T) can be measured either directly or indirectly through microwave penetration depth experiments, and current measurements on Bi 2Sr 2CaCu 2O 8 and La 2− x Sr x CuO 4 are found to be consistent with s-wave pairing and the dominance of assisted hopping processes. The situation in YBCO is still unclear, but our estimates suggest that further experiments on this compound would be of great help in elucidating the validity of our model in general and the pairing symmetry in particular.

Theory of the orbital Kondo effect with assisted hopping in strongly correlated electron systems: Parquet equations, superconductivity, and mass enhancement.

The orbital Kondo effect is treated in a model where, additional to the conduction band, there are localized orbitals close to the Fermi energy. If the hopping between the conduction band and the localized heavy orbitals depends on the occupation of the atomic orbitals in the conduction band, then orbital Kondo correlation occurs. The noncommutative nature of the coupling required for the Kondo effect is formally due to the form factors associated with the assisted hopping, which in the momentum representation depends on the momenta of the conduction electrons involved. The leading logarithmic vertex corrections are due to the local Coulomb interaction between the electrons on the heavy orbital and in the conduction band. The renormalized vertex functions are obtained as a solution of a closed set of differential equations and they show power behavior. The amplitude of large renormalization is determined by an infrared cutoff due to finite energy and dispersion of the heavy particles. The enhanced assisted hopping rate results in mass enhancement and attractive interaction in the conduction band. The superconductivity transition temperature calculated is largest for the intermediate mass enhancement, ${\mathit{m}}^{\mathrm{*}}$/m\ensuremath{\approxeq}2--3. For larger mass enhancement the small one-particle weight (Z) in the Green's function reduces the transition temperature, which may be characteristic for other models as well. The theory is developed for different one-dimensional and square-lattice models, but the applicability is not limited to them. In the one-dimensional case charge- and spin-density susceptibilities are also discussed. Good candidates for the heavy orbital are f bands in the heavy fermionic systems and nonbonding oxygen orbitals in high-temperature superconductors and different flatbands in the quasi-one-dimensional organic conductors.

A kinetic lattice-gas model for the triangular lattice with strong dynamic correlations. I. Self-diffusion

Self-diffusion in a lattice-gas model with two-vacancy assisted hopping on the triangular lattice is investigated, by both Monte Carlo simulation and analytical calculation. A very rapid decrease of the tracer-correlation factor and marked size effects in finite lattices give evidence for strong dynamic correlations in both space and time at high particle concentration. Although the decrease of the self-diffusion coefficient over 3.5 decades for concentrations up to c=0.77 is best fitted by a power law (0.835-c)3.54, it is argued that the model does not have a sharp dynamical phase transition with a critical concentration lower than one. The argument is based on a proof of absence of permanently blocked particles in infinite lattices at all concentrations below one. The self-diffusion coefficient is calculated analytically within a pair approximation which gives good results for lower concentrations, but fails at the higher concentrations. The approximation is in qualitative agreement with the Monte Carlo data for the tracer-correlation factor at all concentrations for a variant of the model with one-vacancy assisted hopping, in which the dynamic correlations are less pronounced.

A kinetic lattice-gas model for the triangular lattice with strong dynamic correlations. II. Collective diffusion

The density-density correlation function and the site-occupation autocorrelation function are calculated for the lattice-gas model with two-vacancy assisted hopping on the triangular lattice, by both Monte Carlo simulation and analytical approximations. The non-exponential time dependence of the density-density correlation function for short wavelengths and high concentrations is explained by the strong spatial inhomogeneity of the collective diffusion process. The coefficient of collective diffusion decreases very rapidly at the highest concentrations, similarly to the self-diffusion coefficient. The analytical calculations for a two-variable approximation and a mode-coupling approximation give good results for lower concentrations. At higher concentrations the mode-coupling approximation is unstable, leading to a diverging rather than decaying correlation function for concentrations higher than 0.34.

Investigations on Bi-Sr-Ca-Cu-O Glasses: Thermal, Magnetic, and Transport Properties

Glasses of composition $Bi_2Sr_2CaCu_nO_{6+n} (n = 2 \hspace{2}to\hspace{2} S)$ have been prepared hy the melt quenching technique. Thermal, electrical, and magnetic properties of tlicsc glasses linvc been examined. Electrical conductivities suggest the validity of the multiphonon assisted hopping model. Susceptibilities indicate weak antiferromagnetic coupling. However, the electron spin resonance (ESR) studies suggest a large proportion of spins to be ESR inactive. A structural model of the glasses has been proposed that considers both $Bi_2O_3$ and CuO as glass formers. It is shown that the properties of the glasses investigated in this work are consistent with the model.

Classical analogs of mesoscopic quantum phenomena

An important aspect of mesoscopic electronic systems is that the electrons maintain their phase coherence and propagate as waves. The resulting behavior is complicated by disorder, time-dependent scattering sites and many-electron effects (possibly modeled with a nonlinear wave equation). Studying the latter effects theoretically or experimentally in mesoscopic electronic systems is difficult. However, it is possible to contrive classical wave systems which precisely duplicate the salient features of the quantum electron problem, and which may be readily studied experimentally. This paper will describe the use of such systems to study phonon assisted hopping, the effects of nonlinearity on Anderson localization, the effects of Penrose tile symmetry, noise in mesoscopic electronics, ‘superdiffusion’, and to study an aspect of the discrepancy between theory and experiment for normal electron persistent currents in mesoscopic rings.

Generalized Hamiltonian for tunneling centers in metallic systems

The electron assisted hopping rate of an atom is calculated for an assymmetric double well potential. It is shown that the intuitive results of Vladár and Zawadowski based on barrier fluctuation agree with the matrix elements obtained by direct evaluation of the electron scattering amplitude due to the atom. However, the assisted hopping rate can not be interpreted as a simple consequence of the change in the Gamow factor but the prefactor is also affected due to potential fluctuations. Carrying out a one-dimensional calculation we show that the assisted hopping to higher energy levels may be quite large. This might strongly influence the evolution of the effective coupling constants and thus the characteristic temperature of the orbital Kondo effect.

Röntgenographische, elektrische und spektroskopische Untersuchungen an den Palladiumchalkogeniden PdS1-xSex / Roentgenographic, Electrical and Spectroscopic Investigations of the Palladium Chalcogenides PdS1-xSex

Silvery reflecting melting cakes of PdS1-xSex (0.0 ≤ x ≤ 1.0) have been prepared by annealing mixtures of Pd, S and Se at 873 K in evacuated quartz tubes. The PdS-type mixed crystals are stable only in the ranges 0.0 ≤ .x ≤ 0.2 and 0.9 ≤ x ≤ 1.0, whereas a new phase appears in the intermediate region. The new phase PdS1-xSex (0.2 < x < 0.9) crystallizes in the space group P42/mbc and Z = 24. Lattice constants are a = 1126.2(7), c = 683.2(7) pm (PdS0.6Se0.4) and a = 1130.8(9), c = 685.4(8) pm (PdS0.4Se0.6). The relationships between the new structure type and the PtS and PdS structure types are discussed. The infrared reflection spectra of polycrystalline samples of PdS were measured between 10 and 300 K in the frequency range from 30 to 400 cm-1. Both phonons and plasmons contribute to the spectra. The temperature dependence of the plasmon contribution shows an unusual behaviour which is explained by the assumption of a phonon assisted hopping conductivity mechanism at low temperatures and a Drude-like transport at high temperatures. The normal mode which may be involved in the hopping process is proposed from a normal coordinate calculation using a three-parameter short range valence force field model. The electrical transport data are in general agreement with the optical ones. The infrared spectra of the mixed crystals PdS1-xSex (0.2 < x ≤ 1) are discussed.

Excimer Laser Induced Electrical Conductivity in Polymers

ABSTRACTThe electrical conductivity of high temperature polymers (i.e. polyimide) has been changedpermanently from 10−17 Ω−1 cm−1 to 10 Ω−1 cm−1 by excimer laser irradiation. The conduction mechanism is found to be phonon assisted variable range hopping between small (∼10 nm) carbon clusters that form a macroscopic percolation cluster. The critical concentration of carbon clusters is found to be Pc = 0.24 ± 0.02 and the critical exponent of the insulator-conductor transition is t = 1.97 ± 0.25.

Correlation of time-resolved electroluminescence and cathodoluminescence measurements on quantum well light emitters with varying barrier widths

We present results of a comparative study of the time-dependent luminescence properties of multiple quantum well structures with varying barrier widths which are embedded in the active area of a light-emitting device. The carrier kinetics is investigated by different experimental approaches: Cathodoluminescence and electroluminescence experiments where excitation is on/off-modulated for the purpose of time-resolved measurements and time-resolved electroluminescence experiments in the small signal regime which allow for observation of the carrier kinetics under flatband conditions. Due to the exact determination of the excess carrier density the latter technique provides a sensitive tool for a precise estimation of the mono- and bimolecular recombination coefficients. Comparison with light output data yields radiative and nonradiative parts. We find that coupling of quantum wells dramatically favors nonradiative interface recombination as expected from a theoretical model accounting for the superlattice wavefunctions. On the other hand, the bimolecular recombination rate remains unaltered even when the barrier width is lowered from 18 to 0.9 nm. In contrast, on/off modulated experiments reveal that luminescence decay is strongly influenced by carrier drift out of the active area. A barrier width dependent carrier mobility in growth direction accounts for these results if phonon assisted hopping rather than Bloch transport is presumed. Thus, an estimation of device quality of quantum well light emitters by conventional time-resolved cathodo- (or photo-) luminescence experiments is found to be possible if internal field induced carrier drift processes are taken into account.

The change of electrical conduction in the valence/conduction band to the impurity band in CdSexTe1−x thin films

Reported here are the results of investigations carried out on the mechanism of electrical conduction in CdSexTe1−x thin films between 300 and 125 K in vacuum. All the films showed a transition from grain boundary limited conduction in the conduction/valence band to phonon assisted hopping via the impurity band (impurity band conduction) at around 280–290 K. The grain boundary limited conduction showed an activation energy equal to about 0.14 eV and conduction via impurity band showed an activation energy of about 0.02 eV, which is characteristic of phonon assisted hopping conduction.

Influence of fast neutrons on electrical properties in neutron transmutation doped GaAs: New annealing stage

In neutron transmutation doping (NTD) to the undoped semi-insulating GaAs, a new annealing stage related to the tunneling assisted hopping conduction was found around 400 °C for fast neutron fluences of ≥7.0×1017 n/cm2. It is suggested that this stage is based on the enhancement in the hopping conduction by the activated dopant in the NTD process. The stage was not observed for the irradiation with a small amount of fast neutrons. The activation energy for the annihilation of As antisite defects (AsGa) was found to be 0.9 eV. The annealing temperature to achieve the desired carried concentration increased with the fast neutron fluence.

Linear response theory revisited. IV. Applications

The one-body linear response expressions (diagonal and nondiagonal) for the dc electrical conductivity, obtained in a previous paper, are applied to specific situations. In the case of ‘‘no collisional current’’ we evaluate the relaxation times which enter an expression (diagonal) for the longitudinal magnetoconductivity; in the case of ‘‘collisional current’’ another expression (diagonal) for the transverse magnetoconductivity is evaluated. The calculations are carried out in the framework of electron–phonon interaction in crystalline materials; various kinds of phonons are considered. The formula for ‘‘collisional current’’ is also used for an analytic evaluation of the phonon assisted hopping conductivity in crystalline and amorphous materials. The results are in harmony with those of the literature or are new. Further, the nondiagonal expression leads to a result for the oscillatory Hall effect, which, in contrast with previous results, is independent of the interaction (e.g., interaction with impurities).