Charge Fluctuation Spectra Research Articles

Charge dynamics in strongly-correlated electronic systems

We consider the dynamic charge susceptibility and the charge density waves in strongly-correlated electronic systems within the two-dimensional t-J-V model. Using the equation of motion method for the relaxation functions in terms of the Hubbard operators, we calculate the static susceptibility and the spectrum of charge fluctuations as functions of doped hole concentrations and temperature. Charge density waves emerge for a sufficiently strong intersite Coulomb interaction. Calculation of the dynamic charge susceptibility reveals a strong damping of charge density waves for a small hole doping and propagating high-energy charge excitations at large doping.

Majorana approach to the stochastic theory of line shapes

Motivated by recent Mossbauer experiments on strongly correlated mixed-valence systems, we revisit the Kubo-Anderson stochastic theory of spectral lineshapes. Using a Majorana representation for the nuclear spin we demonstrate how to recast the classic lineshape theory in a field-theoretic and diagrammatic language. We show that the leading contribution to the self-energy can reproduce most of the observed lineshape features including splitting and lineshape narrowing, while the vertex and the self-consistency corrections can be systematically included in the calculation. This new approach permits us to predict the line-shape produced by an arbitrary bulk charge fluctuation spectrum providing a model-independent way to extract the local charge fluctuation spectrum of the surrounding medium. We also derive an inverse formula to extract the charge fluctuation from the measured lineshape.

Interlayerc-axis transport in the normal state of cuprates

A theoretical model of c-axis transport properties in cuprates is proposed. Inter-plane and in-plane charge fluctuations make hopping between planes incoherent and diffusive (the in-plane momentum is not conserved after tunneling). The non-Drude optical conductivity $\sigma_c(\omega)$ and the power-law temperature dependence of the {\it dc} conductivity are generically explained by the strong fluctuations excited in the process of tunneling. Several microscopic models of the charge fluctuation spectrum are considered.

A molecular dynamics investigation of longitudinal collective modes in metal-salt solutions

Results are reported of extensive molecular dynamics (MD) simulations of a simple model of the prototype metal-salt solution Kx(KCl)1-x. The Coulomb interactions between ions are assumed to be screened by the degenerate valence electrons, within the Thomas-Fermi approximation. The main emphasis is on the concentration (x) dependence of the density and charge fluctuation spectra. The MD data for these spectra are analysed in terms of the extended mode concept, by fitting the data to superpositions of Lorentzians, with wavenumber-dependent coefficients. This analysis clearly shows the emergence, in addition to an extended acoustic (sound) mode, of a high-frequency optic mode at intermediate and low metal concentrations x. This mode may be considered as a remnant of the well known propagating charge fluctuation (or plasmon) mode in molten salts; at finite x the optic mode is strongly affected by electron screening. The subtle interplay between acoustic and optic modes is in qualitative agreement with the predictions of a recent generalized hydrodynamics analysis.

Spin- and charge-fluctuation effects on dynamic conductivity in Cu-O plane superconductors

A self-consistent microscopic theory for the 2D Hubbard Hamiltonian and additional electron-phonon interaction yields below {Tc} a gap in the quasiparticle decay rate as well as in the spin and charge fluctuation spectra. The mass enhancement and the quasiparticle decay rate exhibit in the normal state marginal' Fermi liquid behavior, while in the superconducting state the quasi particle character is restored. These properties lead to anomalous behavior of the dynamic conductivity: considered as a function of temperature, a peak evolves at {Tc} as the frequency is decreased well below 2{Delta}. In the optical conductivity, considered as a function of frequency, a gap between 2{Delta} and 4{Delta} opens rapidly as T is decreased below {Tc}. Results are in qualitative agreement with experiments on high-{Tc} superconductors.

Mean field kinetic theory of a classical electron gas in a periodic potential. III. The high-temperature limit in two dimensions

The Vlasov-like mean field kinetic equation for a classical electron plasma in the periodic field of an ionic lattice is solved in the high-temperature limit in two dimensions. The predictions for the one-electron density, the static structure factor, and the long-wavelength charge fluctuation spectrum are compared to the results of extensive molecular dynamics simulations. The predicted shift and damping of the plasma oscillation mode are in reasonable agreement with the simulation data at intermediate couplings (Γ=e2/kBT≃1), but the agreement deteriorates as the temperature or the density is lowered, because mean field theory does not lead to the expected Kosterlitz-Thouless transition to the dielectric phase where electrons are localized.

Frequency sum rules and charge fluctuation spectra in binary ionic mixtures

Abstract The explicit analytical expressions of the fourth and sixth frequency moments of the number density correlation functions of binary ionic mixtures in a uniform background are given for any concentration. Using the memory function formalism, we analyse the molecular dynamics results (MD) for charge fluctuation spectra of equimolar H + − He ++ mixtures for different coupling parameters. The use of the frequency moments allows us to determine the memory function in a systematic manner. The agreement is good except for long wavelength.

Dynamic correlations in a charged lattice gas.

The many-particle hopping problem in a one-component lattice gas of charged particles is investigated. A method is pointed out which allows the simultaneous treatment of correlations induced by the lattice and by the Coulomb interaction. Results are obtained for the concentration and temperature dependence of the tracer correlation factor and the quasielastic width of the incoherent scattering function. In addition, we discuss the long-wavelength charge fluctuation spectrum and possible anisotropy effects in layered systems.

Investigations of metal-insulator-semiconductor structure inhomogeneities using a small-size mercury probe

Capacitance and (under feedback mode of operation) bias voltage of HgSiO 2Si structures were measured using a 7 μm diam. mercury probe as a metal contact. The measurements were carried out both with separate points and during continuous scanning of the investigated surface. The inhomogeneities of insulator capacitance, built-in charge, surface state charge and doping level could be measured separately. The accuracy of measuring was approximately 1%, the resolving power was better than 1 μm. The inhomogeneity of the electrophysical state of the interface was found to be connected mainly with inhomogeneity of built-in charge. The charge inhomogeneities looked like peaks or steps on a relatively homogeneous background. The charge distribution was not a Gaussian. The more the deviation of the varying physical parameter from its mean value the larger was the space size λ of this inhomogeneity. The results touch upon the assumption that the idea about a geometrical spectrum of charge fluctuations must be corrected to emphasize large λ components (like a 1/ƒ frequency spectrum of noise).

Charge fluctuations in SiO-2Si interface

On the homogeneous (as indicated by a small scanning mercury probe) regions of an SiO2-Si structure the dependences of MIS conductance on frequency were measured. It was found that the equivalent parallel conductance due to surface states has a narrow peak in the Gt/tuω-ωplot. This result contradicts the generally accepted view of small (50-500 A) surface potential fluctuations in SiO2-Si structures, enabling us to suppose that real MIS structures have separate (more than 10 μm) homogeneous regions approaching the ideal interface model. Therefore the concept of a geometrical spectrum of charge fluctuations has to be revised to emphasize large A components (like a 1/f frequency spectrum of noise).

Self-diffusion, conductivity, and long-wavelength plasma oscillations in strongly coupled two-component plasmas

The autocorrelation functions of the microscopic electric current $J(t)$ and the electron velocity ${Z}_{2}(t)$ are calculated for strongly coupled, semiclassical two-component plasmas. The corresponding memory functions are expressed in terms of mode-coupling integrals involving density- and energy-correlation function in the framework of a microscopic kinetic theory which preserves the exact statics. Through a sequence of well-defined approximations, the expressions for the memory functions are made self-consistent, the resulting equations are solved iteratively with the interaction potentials, and the static-partial-structure factors as the only input. The theory is then applied to weakly degenerate hydrogen and carbon plasmas for values of the plasma parameter of order 1. The resulting correlational functions $J(t)$ and ${Z}_{2}(t)$ and their integrals, the electrical conductivity, and the electron self-diffusion constant, agree reasonably well with the "molecular-dynamics" data of Hansen and McDonald and with additional simulation results presented here. The "long-time tail" in $J(t)$ observed in the simulations is interpreted in terms of mode-coupling effects. The damping and frequency shift of the plasmon peak in the dynamical charge-fluctuation spectrum are explicity evaluated in the long-wavelength limit; the frequency shift above the plasma frequency is shown to be nonnegligible for strong coupling.

Mode coupling theory of charge fluctuation spectrum in a binary ionic liquid

The paper reports a mode coupling calculation of the memory function for the dynamics of long-wave-length charge fluctuations in a symmetric binary ionic liquid, as a simple model for molten alkali halides. Symmetry under charge conjugation is exploited to derive selection rules for the decay of single-mode excitations into multimode excitations. An estimation of the two-mode decay channels indicates that they can be approximately separated into fast processes and slow processes. The former involve a pair of outcoming longitudinal modes, one of acoustic character and the other of optical character, and are the main cause of broadening of the high-frequency peak in the van Hove structure factor for charge density fluctuations. The slow-decay channels involve either a longitudinal-optic and a transverse-acoustic mode or a longitudinal-acoustic and a transverse-optic mode and are dominant in the low-frequency region of the spectrum where Drude-lile conduction is occurring. The calculated shape of the van Hove function is in reasonable agreement with the available computer simulation data, after an empiricla scaling of the spectrum of fluctuating forces.

Statistical mechanics of dense ionized matter. VIII. Dynamical properties of binary ionic mixtures

Molecular-dynamics calculations are reported for binary mixtures of ions of like charge in a uniform neutralizing background. For a system simulating the mixture ${\mathrm{H}}^{+}$-${\mathrm{He}}^{2+}$ the calculations show that the spectrum of charge fluctuations is dominated by a propagating plasmon mode with a lifetime which is finite even in the long-wavelength limit. At high charge densities there is an additional low-frequency contribution to the spectrum; this is shown to arise from a coupling between charge fluctuations and fluctuations in mass density. The results are in satisfactory agreement with a simple hydrodynamic calculation, though it appears that in practice the hydrodynamic limit is never fully attained; it is also found that a memory-function approach incorporating the coupling of mass and charge gives a good description of the collective dynamical modes at high charge densities.

Charge fluctuation spectra at very long wave-length in ionic fluids

Charge fluctuation spectra at very long wave-length in ionic fluids