Role Of Electron-electron Interactions Research Articles

1H-NMR and charge transport in metallic polypyrrole at ultra-low temperatures and highmagnetic fields

The temperature dependence of conductivity, proton spin relaxation time(T1) and magnetoconductance (MC) in metallic polypyrrole (PPy) doped withPF6− have been carried out at mK temperatures and high magnetic fields. AtT<1 K both electron–electron interaction (EEI) and hopping contributesto conductivity. The temperature dependence of a protonT1 is classified in threeregimes: (a) for T<6 K—relaxation mechanism follows a modified Korringa relation due to EEI and disorder, (b) for6 K<T<50 K—relaxation mechanism is via spin diffusion to the paramagnetic centers and (c) forT>50 K—relaxation is due to the dipolar interaction modulated by the reorientation of the symmetricPF6 groups following the Bloembergen, Purcell and Pound (BPP) model. The data analysis showsthat the Korringa ratio is enhanced by an order of magnitude. The positive and negative MC atT<250 mK is due to the contributions from weak localization and Coulomb-correlated hoppingtransport, respectively. The role of EEI is observed to be consistent in conductivity,T1 and MC data,especially at T<1 K.

Use of low-temperature Hall effect to measure dopant activation: Role of electron-electron interactions

A complete understanding of the activation of phosphorus dopants in silicon as a function of doping density is important for the fabrication of silicon transistors and devices as they scale down in size to the atomic level. Here, we present a systematic study of the low-temperature Hall effect in Si:P $\ensuremath{\delta}$-doped layers with degenerate doping densities in the range $(\ensuremath{\sim}0.2--2)\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, from just above the transition to strong localization to the highest carrier densities achievable. Importantly, at low temperatures, we find a temperature dependent correction in the Hall coefficient consistent with that predicted by electron-electron interaction theory. This correction manifests as an apparent decrease in the carrier density determined from conventional Hall measurements of up to $\ensuremath{\sim}50%$ in low density samples and can be mistakenly interpreted as reduced dopant activation. After correcting for electron-electron interaction effects, we demonstrate that all P dopants in the Si:P $\ensuremath{\delta}$-doped layers are electrically active for the complete range of doping densities studied.

Electron-electron interaction effects in transport through open dots: Pinning of resonant levels

The role of electron-electron interaction in transport properties of open quantum dots is studied. The self-consistent full quantum-mechanical magnetotransport calculations within the Hartree, dens ...

Electronic properties of disordered two-dimensional carbon

Two-dimensional carbon, or graphene, is a semi-metal that presents unusual low-energy electronic excitations described in terms of Dirac fermions. We analyze in a self-consistent way the effects of localized (impurities or vacancies) and extended (edges or grain boundaries) defects on the electronic and transport properties of graphene. On the one hand, point defects induce a finite elastic lifetime at low energies with the enhancement of the electronic density of states close to the Fermi level. Localized disorder leads to a universal, disorder independent, electrical conductivity at low temperatures, of the order of the quantum of conductance. The static conductivity increases with temperature and shows oscillations in the presence of a magnetic field. The graphene magnetic susceptibility is temperature dependent (unlike an ordinary metal) and also increases with the amount of defects. Optical transport properties are also calculated in detail. On the other hand, extended defects induce localized states near the Fermi level. In the absence of electron-hole symmetry, these states lead to a transfer of charge between the defects and the bulk, the phenomenon we call self-doping. The role of electron-electron interactions in controlling self-doping is also analyzed. We also discuss the integer and fractional quantum Hall effect in graphene, the role played by the edge states induced by a magnetic field, and their relation to the almost field independent surface states induced at boundaries. The possibility of magnetism in graphene, in the presence of short-range electron-electron interactions and disorder is also analyzed.

Orbital magnetic properties of quantum dots: The role of electron-electron interactions

We study the magnetic orbital response of a system of N interacting electrons confined in a two-dimensional geometry and subjected to a perpendicular magnetic field in the finite-temperature Hartree-Fock approximation. The electron-electron interaction is modeled by a short-range Yukawa-type potential. We calculate the ground-state energy, magnetization, and magnetic susceptibility as a function of the temperature, potential range, and magnetic field. We show that the amplitude and period of oscillations in the magnetic susceptibility are strongly affected by the electron-electron interaction as evidenced in experimental results. The zero-field susceptibility displays both paramagnetic and diamagnetic phases as a function of temperature and the number of confined electrons.

Gilbert damping in ferromagnetic films due to adjacent normal-metal layers

Enhancement of the spin relaxation rate in a monolayer ferromagnetic sheet due to adjacent normal metal layers is studied using a dynamic version of the Ruderman-Kittel-Kasuya-Yosida (RKKY) theory. We follow the previously published approach [E. \ifmmode \check{S}\else \v{S}\fi{}im\'anek and B. Heinrich, Phys. Rev. B 67, 144418 (2003)] in which the damping comes from the time lag of the RKKY oscillations induced by $s\ensuremath{-}d$ exchange interaction with the precessing magnetization. The present work extends this approach by incorporating into the transverse dynamic susceptibility the effect of the static inhomogeneous spin polarization due to the longitudinal component of the magnetization. The calculated excess Gilbert damping constant is found to agree with the spin pumping approach of Tserkovnyak et al. [Phys. Rev. Lett. 88, 117601 (2002)]. Comparison with this approach allows predictions to be made about the role of electron-electron interaction in the present theory.

Self-interaction-free density-functional theoretical study of the electronic structure of spherical and vertical quantum dots

We study the electronic structure and shell-filling effects of both spherical and vertical quantum dots by means of the density functional theory ~DFT! with optimized effective potential ~OEP! and self-interactioncorrection ~SIC! recently developed. The OEP/SIC procedure allows the elimination of the spurious selfinteraction energy and the construction of accurate single-particle local potential with proper long-range Coulombic behavior. The OEP/SIC equations are discretized and solved accurately and efficiently by the generalized pseudospectral ~GPS! method. The highest occupied orbital energy of N-electron quantum dots provides a direct measure of the electron affinity or chemical potential. We apply the OEP/SIC method to the study of the capacitive energy of N-electron spherical dots for N up to 70. The results show the shell and subshell structure pattern and the electron filling pattern follows closely the Hund’s rule. We also consider the effect of including the vertical dimension in the quantum dot study. We perform a detailed study of the shell-filling effect and the angular and radial density distributions of vertical quantum dots. The calculated capacitive energy spectrum is in good agreement with the recent experimental results, providing physical insights regarding the origin of electron shells and the role of electron-electron interaction in quantum dots.

Fluctuations of Conductance Peak Spacings in the Coulomb Blockade Regime: Role of Electron-Electron Interaction

We study influence of electron-electron interaction on statistics of Coulomb blockade peak spacings in disordered quantum dots. It is shown that the interaction combined with fluctuations of eigenfunctions of the Fermi sea enhances the peak spacing fluctuations, in accordance with recent experiments. In addition, account of the spin degrees of freedom leads to a pronounced odd-even structure for weak interaction ( ${e}^{2}/\ensuremath{\epsilon}\ensuremath{\ll}{v}_{F}$); in the opposite case ( ${e}^{2}/\ensuremath{\epsilon}\ensuremath{\gtrsim}{v}_{F}$) this structure is washed out.

Semiclassical description of angle-dependent magnetoresistance oscillations in quasi-one-dimensional metals

We use a semiclassical model to calculate the angle-dependent magnetoresistance oscillations (AMROs) in quasi-one-dimensional (q1D) organic conductors. A number of contrasting models have been proposed to explain this effect, mainly in the context of the (TMTSF${)}_{2}$X (where TMTSF is tetramethyltetraselenafulvalene and X=${\mathrm{ClO}}_{4}$ or ${\mathrm{PF}}_{6}$) family; some of the models concentrate on the role of electron-electron interactions while others postulate Fermi-surface hotspots or even field-dependent hopping renormalization. Instead, we have used a more intuitive semiclassical approach to calculate the angle-dependent magnetoresistance oscillations for a completely general class of q1D Fermi surfaces. The model demonstrates how the details of the Fermi surface corrugation give particular features in the experimental data and illustrates the important roles played by both open and closed orbits. The AMRO observed in (TMTSF${)}_{2}$${\mathrm{ClO}}_{4}$ when the magnetic field is rotated close to the a axis are discussed in this context. The results are particularly applied to the organic charge transfer salt (ET${)}_{2}$KHg(SCN${)}_{4}$ [where ET is bis(ethylenedithio)tetrathiafulvalene]; this material is interesting because the Fermi surface undergoes a transition from predominantly q1D to quasi-two-dimensional (q2D) character at \ensuremath{\sim}22 T, a result which has been primarily established on the basis of AMRO experiments.

Pressure dependence of the conductivity and magnetoconductance in oriented iodine-doped polyacetylene.

The temperature dependences of conductivity and magnetoconductance (MC) have been measured, both parallel and perpendicular to the chain axis, in oriented polyacetylene films doped with iodine. The evolution of anisotropy in conductivity as a function of doping time has been studied. Hydrostatic pressure increases the conductivity for pressures up to 4 kbar; at higher pressure the conductivity gradually decreases. The reduced activation-energy function [W=-T[d(ln\ensuremath{\rho})/dT] is temperature independent in the intermediate temperature region (T\ensuremath{\approxeq}180--60 K) suggesting that the material is just on the metallic side of the critical regime of the disorder-induced metal-insulator transition. However, at 8 kbar, W(T) exhibits a positive temperature coefficient from 180 to 1.2 K, implying that the system is more metallic due to enhanced interchain transport. The ${\mathit{T}}^{1/2}$ dependence of conductivity and the enhanced negative MC, at very low temperatures, indicates that the role of electron-electron interactions is significant. MC measurements have been carried out in all the five different combinations of current with respect to the chain axis and field directions. The sign of MC is positive (negative) when the field is perpendicular (parallel) to the chain axis. The anisotropy in MC is attributed to the anisotropic diffusion coefficient. This is shown by the enhanced positive contribution to the MC at 8 kbar, when the field (current) is parallel (parallel) to the chain axis.At 1.2 K, when both the magnetic field and current are perpendicular to the chain axis, the field-induced crossover from positive to negative MC shows the subtle interplay of weak localization and electron-electron interaction contributions to MC. The inelastic scattering lengths, at 1.2 K, in parallel and perpendicular directions to the chain axis are found to be 1163 and 210 \AA{}, respectively. The ${\mathit{T}}^{\mathrm{\ensuremath{-}}3/4}$ dependence of the inelastic scattering length at low temperatures is consistent with the ${\mathit{T}}^{3/4}$ dependence of conductivity. This indicates that the inelastic electron-electron scattering known to be of importance in disordered metals is the dominant scattering mechanism in iodine-doped oriented polyacetylene.

Junctions between coplanar 2D gases: a probe of boundary effects in the quantized Hall regime

We discuss the behavior of junctions between two coplanar 2D electron gases with different densities in a perpendicular applied magnetic field. The presence of energy gaps in the density of states caused by Landau level quantization and spin splitting leads to behavior in these junctions qualitatively similar to what is observed in a semiconductor p-n junction. We will emphasize aspects of these junctions where the role of electron-electron interactions are important in understanding their properties.

Transport in polyaniline near the critical regime of the metal-insulator transition.

Critical behavior of the electrical conductivity near the disorder-induced metal-insulator (MI) transition has been observed in polyaniline (PANI) doped with camphor sulfonic acid (CSA). The temperature dependence of the resistivity (\ensuremath{\rho}) depends on the degree of disorder present in the PANI-CSA films. In the most metallic samples the resistivity is nearly temperature independent; whereas in the critical region of MI transition, \ensuremath{\rho}(T) is characterized by a power-law temperature dependence, \ensuremath{\rho}(T)\ensuremath{\propto}${\mathit{T}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\beta}}}$. Resistivity ratios \ensuremath{\rho}(1.4 K)/\ensuremath{\rho}(300 K) as low as 1.6 have been observed in the most metallic samples of PANI-CSA. In the metallic regime, the conductivity is characterized by \ensuremath{\sigma}(T)=\ensuremath{\sigma}(0)+${\mathit{mT}}^{1/2}$ at low temperatures; the dependence of m(H) on magnetic field provides information on the role of electron-electron interactions in the transport near the MI transition. The ${\mathit{T}}^{\mathrm{\ensuremath{-}}1}$ dependence found for inelastic-scattering time (${\mathrm{\ensuremath{\tau}}}_{\mathrm{in}}$) is in agreement with that predicted for metallic systems near the MI transition. For the samples initially in the critical regime, a magnetic field of 8--10 T induces the transition to variable-range hopping. The typical localization length in PANI-CSA just on the insulating side of the MI transition is about 80--130 \AA{}. The magnitude of the positive magnetoresistance increases considerably as the system moves from the metallic to the insulating regime. The magnitude and quasilinear temperature dependence of the thermopower near the critical regime of the MI transition is typical of that expected for a metal.

Orbital magnetic response of quantum dots: A signature of electron-electron interactions.

We study the linear orbital magnetic response of quantum dots as a function of the number of electrons in the dot assuming a two-dimensional parabolic confining potential. We discuss the orientational paramagnetism and precession diamagnetism and show that each provides insights into the role of electron-electron interactions in the dot. We also study the response of three electrons in a strong magnetic field. Treating the Coulomb interaction as a perturbation, we find that the total angular momentum undergoes a continuous transition as a function of the magnetic-field strength, which results in the oscillations of electron-spin magnetization. We find no evidence that the interaction leads to spin polarization without the account of the Zeeman term

The role of electron-electron interactions in conducting polymers

A few general concepts of the many-electron problem and some specific effects of Coulomb interactions in conjugated polymers are summarized. The emphasis is on key experiments, mostly on the best studied material polyacetylene.

Quantum dots in a magnetic field: Role of electron-electron interactions.

The eigenstates of electrons interacting in quantum dots in a magnetic field are studied. The interaction has important effects on the magnetic-field dependence of the energy spectrum. However, when the confinement potential is quadratic, the optical excitation energies of the many-body system are exactly the same as those of a single electron. This makes the interaction effects difficult to observe directly but they could be seen by measuring the thermodynamic properties of the electrons. This is illustrated with the calculations of the electronic heat capacity.

Effects of electron correlations in conjugated organic molecules and solids

The discrepancies between the non-interacting models and experimental results for conjugated systems is highlighted in this brief review. The interacting model hamiltonians correctly give the forbidden singlet state below the optical gap in polyenes and also explain both the nonvanishing optical gap in polyacetylenes and the vanishing optical gap in symmetric cyanine dyes. The negative spin densities in polyene radicals is also understood in terms of a correlated picture. The role of electron-electron interactions in other strongly correlated systems, such as polydiacetylene and mixed and segregated stack charge transfer solids, are also briefly discussed.

Monte Carlo analysis of electron transport in small semiconductor devices including band-structure and space-charge effects.

The physics of electron transport in Si and GaAs is investigated with use of a Monte Carlo technique which improves the state-of-the-art treatment of high-energy carrier dynamics. (1) The semiconductor is modeled beyond the effective-mass approximation by using the band structure obtained from empirical-pseudopotential calculations. (2) The electron-phonon, electron-impurity, and electron-electron scattering rates are computed in a way consistent with the full band structure of the solid, thus accounting for density-of-states and matrix-element effects more accurately than previous transport formulations. (3) The long-range carrier-carrier interaction and space-charge effects are included by coupling the Monte Carlo simulation to a self-consistent two-dimensional Poisson solution updated at a frequency large enough to resolve the plasma oscillations in highly doped regions. The technique is employed to study experimental submicrometer Si field-effect transistors with channel lengths as small as 60 nm operating at 77 and 300 K. Velocity overshoot and highly nonlocal, off-equilibrium phenomena are investigated together with the role of electron-electron interaction in these ultrasmall structures. In the systems considered, the inclusion of the full band structure has the effect of reducing the amount of velocity overshoot via electron transfer to upper conduction valleys, particularly at large biases and low temperatures. The reasonableness of the physical picture is supported by the close agreement of the results of the simulation to available experimental data.

Electron-electron and electron-phonon interactions in irradiated N-methyl pyridinium derivatives with TCNQ studied by optical spectroscopy

Electron irradiation of the organic semiconductors of NMePy(TCNQ) 2 type produces defects which change their vibrational and electronic spectra. The spectral changes are described and discussed. The electron-phonon coupling constant changes are deduced; the defect concentration as well as their volume are calculated from spectral measurements. The role of electron-electron interactions are emphasized and we suggest, that the N-methyl pyridinium derivatives with TCNQ are Mott insulators Etude experimentale et discussion des modifications des spectres d'absorption IR et electroniques en presence des defauts induits par irradiation electronique. Deduction des variations des constantes de couplage electron-phonon, de la concentration et du volume des defauts. Role des interactions electron-electron

Role of Electron-Electron Interaction in the Theory of Positron Annihilation in Metals. III

In our previous paper with the above title an equation of motion for the electron-positron pair was derived with some approximations; here a modified equation for the pair is derived. This has the feature that in the vertex appear in a symmetrical way. We also derive an expression for the positron self-energy and explicitly show that the electron-positron bound state channel does not appear in the positron self-energy and so the renormalization due to the bound state, if it exists, is not as dramatic as in the problems where the two-particle bound state channel occurs in the one-particle self-energy (e. g. superconductivity). Recently 1 )' 2 ) we derived an equation for positronium formation in a many­ electron system using a technique due to Martin and Schwinger. 3 ) In this deri­ vation the step from expression ( 4 · 9) to the expression ( 4 ·10) of I involved inversion of a complicated operator and ( 4 ·10) was arrived at by the simplest approximation to this inverse. By generalizing an identity given by Martin and Schwinger, 3 ) we here evaluate this inverse and after some approximations similar to the one given by Martin and Schwinger, we derive an equation which is more general than that given by ( 4 ·10) of I. This derivation has two features. The vertex functions occur in a symmetrical way in the most general form. Secondly, when further approximations corresponding to RPA are made for the vertex functions, the vertex for the electrons and positrons appear along with the one-electron and one-positron Green's functions in the kernel ( 4 ·12); this is more accurate than the one given by ( 4 ·12) because under certain conditions, one expects some cancellation of the one-particle wave function renormalizations by an appeal to Ward identities. 4 ) Of course ( 4 ·12) is a special case of the equation that will be derived here when the reduced vertex functions are replaced by the leading terms in the equations they obey. The analysis of our second paper 2 ) ·was for this special case and the conclusions reached there are thus unaltered. We will also explicitly display the appearance of the two-particle electron-positron Green's function in the positron self-energy. In particular we recover the self-energy of the positron given by (6 · 2) of I from this expression. In conclusion, we will make a few general remarks con

Review Lecture - Electrons in liquid metals and other disordered systems

In the past decade, a quantitative theory of the electrical properties of liquid metals has been built up, based upon a model of nearly free electrons scattered by the screened pseudopotentials of the assembly of ions. Within the uncertainties of our knowledge of the pseudopotentials, this simple theory agrees with the experimentally observed resistivity and thermoelectric power of non-transition metals and their alloys. It now seems that higher-order corrections to the n. f. e. formula in such systems are not large enough to be easily observed. Current interest is shifting to systems where the n. f. e. model should not be valid; liquid semiconductors, metallic vapours, metal-ammonia solutions, impurity band semiconductors, and semiconductivity glasses. The experimental situation is not reviewed, but attention is drawn to some basic theoretical questions, such as the nature of the atomic or molecular potentials, the role of electron-electron interaction, the character of the metal-insulator transition, and the quantum mechanical interpretation of such classical physical processes as the localization, percolation, and hopping of electrons in highly disordered materials.