Charge Fluctuation Mode Research Articles

Collective modes and quantum effects in two-dimensional nanofluidic channels.

Nanoscale fluid transport is typically pictured in terms of atomic-scale dynamics, as is natural in the real-space framework of molecular simulations. An alternative Fourier-space picture, that involves the collective charge fluctuation modes of both the liquid and the confining wall, has recently been successful at predicting new nanofluidic phenomena such as quantum friction and near-field heat transfer, that rely on the coupling of those fluctuations. Here, we study the charge fluctuation modes of a two-dimensional (planar) nanofluidic channel. Introducing confined response functions that generalize the notion of surface response function, we show that the channel walls exhibit coupled plasmon modes as soon as the confinement is comparable to the plasmon wavelength. Conversely, the water fluctuations remain remarkably bulk-like, with significant confinement effects arising only when the wall spacing is reduced to 7 Å. We apply the confined response formalism to predict the dependence of the solid-water quantum friction and thermal boundary conductance on channel width for model channel wall materials. Our results provide a general framework for Coulomb interactions of fluctuating matter under nanoscale confinement.

Role of Dust Grains in Beam-Driven Instabilities

The interaction of beam with magnetized plasma can lead to the excitation of many instabilities which are useful for understanding the stability, confinement and heating of the plasma. In the present work, we investigate the electromagnetic surface plasma waves, lower hybrid waves and ion cyclotron waves in three component dusty plasmas. The presence of charged dust grains modifies the dispersion relation of plasma waves. The charge and number density of dust grains and external magnetic field significantly modify the dispersion properties of these waves. It is shown that in the absence of beam, the plasma mode damps due to dust charge fluctuations and an additional damping dust charge fluctuation mode is induced in plasma. The beam propagating parallel or perpendicular to the magnetic field drives the plasma waves to instability via Cerenkov and cyclotron interaction. Using the analytical and numerical results the influence of the number density of negatively charged dust grains on growth rate of instabilities is studied.

Lattice effects of orbital and charge orderings in the perovskite manganitePr1−xCaxMnO3

We have made ultrasonic measurements of ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ $(x=0.35,$0.40,0.50) with the colossal magnetoresistance in order to investigate the lattice effects due to orbital and charge degrees of freedom in the systems. As a result, we observe three types of elastic softening. The first one is the softening of ${(C}_{11}\ensuremath{-}{C}_{12})/2$ in the $x=0.35,$ 0.40, and 0.50 compounds above the charge-ordering point ${T}_{\mathrm{co}},$ which originates from the interaction of the quadrupoles ${O}_{2}^{0}$ and ${O}_{2}^{2}$ with ${\ensuremath{\Gamma}}_{3}$ symmetry of ${e}_{g}$ orbital in ${\mathrm{Mn}}^{3+}$ ions to elastic strains. The second one is the softening of ${C}_{44}$ in the $x=0.50$ just above ${T}_{\mathrm{co}},$ which is caused by a coupling of the charge-fluctuation mode with ${\ensuremath{\Gamma}}_{5}$ symmetry in the coexistence ratio $N({\mathrm{Mn}}^{3+}):$$N({\mathrm{Mn}}^{4+})=1:1$ to the appropriate elastic strain. Employing a group-theoretical analysis based on the Landau theory for the phase transition, we have successfully deduced the ${Q}_{\mathrm{xy}}$-type charge-fluctuation mode with ${\ensuremath{\Gamma}}_{5}$ symmetry for the order parameter below ${T}_{\mathrm{co}},$ there being alternative alignment on Mn${}^{3+}$ and Mn${}^{4+}$ ions in $a\ensuremath{-}c$ $(x\ensuremath{-}y)$ plane and the same arrangement along $b(z)$ direction. The last one is the softening of ${(C}_{11}\ensuremath{-}{C}_{12})/2$ just above ${T}_{\mathrm{co}}$ due to the charge fluctuation.

Charge ordering, charge fluctuations and lattice effects in strongly correlated electron systems

Charge fluctuation and charge-ordering phenomena in compounds based on the 3d electrons of transition-metal ions and 4f electrons of rare-earth ions are reviewed with particular emphasis on the mutual coupling of charge and lattice degrees of freedom. For the description of charge ordering in inhomogeneously mixed-valence compounds of localized 3d or 4f-electron systems, we employ Landau's phenomenological theory for second-order phase transitions. By use of the group-theoretical method, the charge fluctuation mode corresponding to the active representation for the second-order transition is determined. The localization of 3d and 4f electrons makes the valence for the specific ions an integer number of charge units e in the ordered phases at low temperatures. The elastic soft mode observed by the ultrasonic method is often a useful indication for the charge fluctuation mode that is frozen below the charge-ordering point T C. The transverse c44 mode exhibiting a considerable softening in the rare-earth com...

Magnetic Field Dependence of Elastic Constants in Pr1-xCaxMnO3

We have investigated charge and orbital (quadrupolar) ordering in perovskite manganites Pr 1- x Ca x MnO 3 ( x =0.35, 0.40, 0.50) by ultrasonic measurements. Only transverse ( C 11 - C 12 )/2 mode of the x =0.35, 0.40, and 0.50 compounds shows a softening from 400 K down to the charge ordering point T co =220-240 K. This softening is originated from the bilinear coupling of the quadrupole moment of the e g orbital in Mn 3+ ion to the elastic strain. Furthermore, the elastic softening in both ( C 11 - C 12 )/2 and C 44 modes due to the coupling of the charge fluctuation mode to elastic strain is observed just above T co , especially for the x =0.50 compound. The considerable magnetic field dependence of the elastic constants is obseved. In particular, the magnetic fields up to 12 T releases completely the elastic softening of ( C 11 - C 12 )/2 mode due to the fact that the charge fluctuation is suppressed under 12 T in the x =0.40 compound.

Effects of dust charge fluctuations on current-driven dust-ion-acoustic waves.

The effects of fluctuating grain charge on current-driven dust-ion-acoustic modes are investigated in the presence of various physical parameters which are varied in order to describe the behavior of the mode in detail. Variation of the charge clearly influences the ion-acoustic mode by changing of its frequency and growth rate. We report here the existence of a dust charge fluctuation mode in the system, and it is shown that the mode is unstable for a negative charge on the dust per unit volume not exceeding some critical value.

Numerical simulation investigation of dust charging effects on electrostatic ion waves in dusty plasmas

A numerical model is described that may be used to investigate important effects of dust charging on ion waves and instabilities in dusty plasmas. The model considers a three species system with fluid electrons and ions and Particle-In-Cell PIC charged dust grains in which the dust charge on the grains varies in time according to the standard charging model. Waves in magnetized or unmagnetized dusty plasmas can be considered. Fundamental effects of dust charging on wave modes which include the dust charge fluctuation mode and damping of ion waves are considered. Also important effects of dust charging on waves produced by the expansion of dust into a background plasma are discussed. Finally an ion acoustic instability that is due to dust charging is investigated. These examples show that there is good agreement with past theoretical predictions on dust charge fluctuation effects and also numerical simulations provide useful additional insight into the physical processes involved.

Collective order-parameter modes for hypotheticalp-wave superconducting states inSr2RuO4

We have calculated all order-parameter collective modes and their contributions to the spin and charge susceptibilities for possible p-wave pairing states in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ The susceptibilities are calculated for pairing states having gaps without and with nodes, and for wave vectors $\mathbf{q}=0$ and nesting vector $\mathbf{q}=\mathbf{Q}$ associated with the \ensuremath{\alpha} and \ensuremath{\beta} bands of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ Important for the observability of the spin-fluctuation modes, for example, by spin resonance or neutron scattering, and of the charge-fluctuation modes by ultrasound, is the effect of quasiparticle damping. This effect is taken into account and discussed in connection with recent neutron-scattering data on ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$

Ultrasonic study of the charge-fluctuation compoundSm3Te4

We have measured the ultrasonic velocity and attenuation coefficient of the rare-earth chalcogenide ${\mathrm{Sm}}_{3}{\mathrm{Te}}_{4}$ to examine the valence-fluctuation effect due to the coexistence of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions in the ratio of 1:2. The ultrasonic dispersion around 120 K indicates that the charge-fluctuation time obeys the activation-type temperature dependence $\ensuremath{\tau}={\ensuremath{\tau}}_{0}\mathrm{exp}{(E/k}_{B}T)$ with a characteristic time $2\ensuremath{\pi}{\ensuremath{\tau}}_{0}=2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\mathrm{sec}$ and an activation energy $E=0.136\mathrm{eV}.$ The absence of the phase transition due to the charge ordering in ${\mathrm{Sm}}_{3}{\mathrm{Te}}_{4}$ means a freezing of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions in random distribution at low temperatures. We find an elastic softening with $\mathrm{ln}T$ dependence below 15 K down to a spin glass transition around 1.3 K. This striking behavior is attributed to a two-level system due to the tunneling of $4f$ electrons among randomly distributed ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions. This result is similar to the ionic tunneling in amorphous glass compounds. Employing the group-theoretical analysis, we show some aspects of charge-fluctuation modes in ${\mathrm{Sm}}_{3}{\mathrm{Te}}_{4}$ and a possible mechanism for the charge glass state of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions.

Oral Presentation: Charge ordering and elastic constants

We show results of the elastic constant measurements as a function of temperature for NaV 2O 5, Sr 14Cu 24O 41 and Fe 3O 4. In all these substances we see clearly evidence for the coupling of certain elastic modes to charge fluctuations. In NaV 2O 5, the c 66 mode exhibits a strong softening of 12% indicating a bilinear coupling to a charge fluctuation mode of B 1g symmetry. Similar charge ordering effects were observed for c 44 modes in Fe 3O 4 and Yb 4As 3. In Sr 14Cu 24O 41 we have slightly different indications of charge ordering in the Cu–O 2 chains.

Numerical model for low-frequency ion waves in magnetized dusty plasmas

A two-dimensional (2-D) numerical model is presented to study low-frequency ion waves and instabilities in magnetized dusty plasmas. Fundamental differences exist between dusty plasmas and electron-ion plasmas because of dust charging processes. Therefore, a primary goal of this investigation is to consider the unique effects of dust charging on collective effects in dusty plasmas. The background plasma electrons and ions are treated as two interpenetrating fluids whose densities are self-consistently reduced by dust charging. The dust is treated with a particle-in-cell (PIC) model in which the dust charge varies with time according to the standard dust charging model. Fourier spectral methods with a predictor-corrector time advance are used to temporally evolve the background plasma electron and ion equations. The dust charge fluctuation mode and the damping of lower hybrid oscillations due to dust charging, as well as plasma instabilities associated with dust expansion into a background plasma, are investigated using our numerical model. The numerical simulation results show good agreement with previous theoretical predictions and provide further insight into dust charging effects on wave modes and instabilities in dusty plasmas.

Quadrupolar Effect in Strongly Correlaed Systems.

We have performed the ultrasonic measurement on the Kondo compound CexLa1-xB6 and the charge fluctuation compound Yb4As3 to examine the quadrupolar effect. The magnetic phase diagram of CexLa1-xB6 is discussed in connection with the competition of the intersite interactions of the quadrupolar and magnetic moments to the Kondo coupling. The elastic softening of the C44 mode associated with the charge ordering in Yb4As3 is explained in terms of the susceptibility for the charge fluctuation mode with Γ5 symmetry.

Elastic anomalies of charge ordering in Yb 4As 3

Elastic constants of the mixed valence compound Yb 4As 3 with extremely low carrier 0.001 per Yb atom have been measured by the ultrasonic method. We found that the transverse C 44 mode exhibits a pronounced softening around the structural phase transition point at T c = 292 K due to the charge ordering of Yb 3+ ions along <111 direction. We interpret the softening of C 44 with the coupling of the strain ε yz , ε zx and ε xy to charge fluctuation mode processing the electric quadrupole moment in Yb 3+ linear chain.

Dynamical screening in the scanning tunneling microscope and metal-insulator-metal junctions

We investigate electron tunneling in a system consisting of two curved metal surfaces separated by insulator or vacuum. In particular, we calculate the modifications of the tunneling barrier due to dynamical screening, i.e., interaction with charge fluctuations. We apply our general results to the planar metal-insulator-metal (MIM) junction, and to the scanning tunneling microscope (STM), describing the tip and the sample surface in STM by two rotational hyperboloids. We analyze the influence of the shape, dielectric properties, and work functions of both metals on the tunneling characteristics in the MIM and STM systems. For metals with different plasma frequencies, charge-fluctuation modes are effectively decoupled, and the electron interaction with these modes is significantly different than in the case of like metals, causing asymmetry in the barrier and also in the tunneling currents and conductivities. We also show that, for geometrical reasons, the tunneling barrier in the STM is lowered near the tip apex, which leads to focusing of the tunneling current and increased lateral resolution of STM.

Fluctuation-assisted transport in polymers

We consider for the first time the explicit role that multi-mode fluctuations play in determining the transport characteristics of inhomogeneous conductors and insulators. Basing ourselves on rigorous analytical and thermodynamic calculations and recent Monte Carlo simulations, we have been able to identify experimentally the novel exp( aT k ), k>0, behaviour in the low-temperature transport regime of pyrrole-based polymers. The variable range hopping and fluctuation-assisted granular transport regimes can be clearly differentiated from each other at very low temperatures using different pyrrole compounds. The apparent universality of exp[ - ( T o/ T) s ]-type laws can be related to the fact that variable range hopping and variable range tunnelling give rise to similar laws in intermediate temperature regimes. The excitation modes which assist the carriers to cross the barriers at low temperatures can be identified as acoustic phonon modes rather than the charge fluctuation modes invoked in doped polyacetylene.

Charge fluctuations in the copper-oxide planes and the electron-phonon couplings

Using the slave boson approach to treat the U d → ∞ limit for the Coulomb repulsion on the copper site in the p- d model we give a short account on various charge fluctuation modes. The effect of the slave boson field adjustment on the strength and the Fermi surface effects for k = 0 and k = (π, π) charge transfer modes is discussed. The implications for the screening of the corresponding electron-phonon couplings is also pointed out.

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.

Mode-mode coupling effects on dynamic spin and charge susceptibilities in high-T c superconductors

The self-consistent and conserving approximation scheme of Kadanoff and Baym is used to derive general equations for the dynamic spin and charge susceptibilities of the 2D Hubbard hamiltonoperator. The resulting vertex corrections to the RPA susceptibilities describe coupling of a spin or charge fluctuation mode with one or two other modes. In the presence of the electronphonon interaction the vertex corrections are relatively small and thus our previous results for various physical quantities in high- T c superconductors are still valid. For the pure 2D Hubbard hamiltonoperator we calculate the averages of the dynamic susceptibilities over all wave vectors as functions of the coupling constant U/ t. The vertex corrections give rise to an enhancement of the spectral density of the charge susceptibility and to a suppression of the spectral density of the spin susceptibility. However, this effect alone is too small to yield a finite T c.

Model for an extra phonon mode in La2CuO4 and La2NiO4.

Phonon dispersion curves for two-dimensional analogs of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$ are modeled by adding a scalar degree of freedom to mimic a low-lying charge-fluctuation mode. The resulting model bears a close resemblance to the ``extra phonon mode'' seen in neutron scattering by Pintschovius et al. In particular, the mode is invisible to neutrons at Q=0 because scalar symmetry prevents admixture with vibrational modes at Q=0. Possible interpretations of this new mode are suggested.

Superconducting pairing of the extended multiband Hubbard model: A weak-coupling study.

Pairing mediated by charge and spin fluctuations is examined within the weak-coupling approximation for a three-band Hubbard model for the cuparate superconductors with both intrasite ({ital U}{sub {ital d}},{ital U}{sub {ital p}}) and intersite repulsion ({ital U}{sub {ital pd}}). Like in the one-band situation, paramagnon exchange near a spin-density-wave instability gives rise to a singlet {ital d}-wave pairing interaction. On the other hand, the exchange of charge-fluctuation modes due to {ital U}{sub {ital pd}} leads to {ital s}-wave coupling near an interband instability and to {ital d}-wave coupling near the intraband charge-density-wave instability. When simultaneously on-site interactions are included the {ital s}-wave pairing is shifted to large dopings away from half-filling and again replaced by {ital d}-wave coupling for smaller dopings.