Presence Of Free Charges Research Articles

Multimode vibrational coupling across the insulator-to-metal transition in 1T-TaS2 in THz cavities.

The use of optical cavities on resonance with material excitations allows controlling light-matter interaction in both the regimes of weak and strong coupling. We study here the multimode vibrational coupling of low energy phonons in the charge-density-wave material 1T-TaS2 across its insulator-to-metal phase transition. For this purpose, we embed 1T-TaS2 into THz Fabry-Pérot cryogenic cavities tunable in frequency within the spectral range of the vibrational modes of the insulating phase and track the linear response of the coupled phonons across the insulator-to-metal transition. In the low temperature dielectric state, we reveal the signatures of a multimode vibrational strong collective coupling. The observed polariton modes inherit character from all the vibrational resonances as a consequence of the cavity-mediated hybridization. We reveal that the vibrational strong collective coupling is suppressed across the insulator-to-metal transition as a consequence of the phonon-screening induced by the free charges. Our findings emphasize how the response of cavity-coupled vibrations can be modified by the presence of free charges, uncovering a new direction toward the tuning of coherent light-matter interaction in cavity-confined correlated materials.

Charge induced acceleration noise in the LISA gravitational reference sensor

The presence of free charge on isolated proof-masses, such as those within space-borne gravitational reference sensors, causes a number of spurious forces which will give rise to associated acceleration noise. A complete discusssion of each charge induced force and its linear acceleration noise is presented. The resulting charge acceleration noise contributions to the LISA mission are evaluated using the LISA Pathfinder performance and design. It is shown that one term is largely dominant but that a full budget should be maintained for LISA and future missions due to the large number of possible contributions and their dependence on different sensor parameters.

Detailed Description of Electro-Osmotic Effect on an Encroaching Fluid Column Inside a Narrow Channel

This paper uses eigenexpansion technique to describe electro-osmotic effect on unsteady intrusion of a viscous liquid driven by capillary action in a narrow channel. It shows how the dynamics can be manipulated by imposing an electric field along the flow direction in the presence of free charges. Similar manipulation can generate controlled transiency in motion of a complex fluid in a tube by nondestructive forcing leading to efficient rheological measurement. Existing theories analyze similar phenomena by accounting for all involved forces among which the viscous contribution is calculated assuming a steady velocity profile. However, if the transport is strongly transient, a new formulation without an underlying quasi-steady assumption is needed for accurate prediction of the time-dependent penetration. Such rigorous mathematical treatment is presented in this paper where an eigenfunction expansion is used to represent the unsteady flow. Then, a system of ordinary differential equations is derived from which the unknown time-dependent amplitudes of the expansion are determined along with the temporal variation in encroached length. The outlined methodology is applied to solve problems with both constant and periodically fluctuating electric field. In both cases, simplified and convenient analytical models are constructed to provide physical insight into numerical results obtained from the full solution scheme. The detailed computations and the simpler reduced model corroborate each other verifying accuracy of the former and assuring utility of the latter. Thus, the theoretical findings can render a new rheometric technology for effective determination of fluid properties.

Meta-screening and permanence of polar distortion in metallized ferroelectrics

Ferroelectric materials are characterized by a spontaneous polar distortion. The behavior of such distortion in the presence of free charge is the key to the physics of metallized ferroelectrics in particular, and of structurally polar metals more generally. Using first-principles simulations, here we show that a polar distortion resists metallization and the attendant suppression of long-range dipolar interactions in the vast majority of a sample of 11 representative ferroelectrics. We identify a meta-screening effect, occurring in the doped compounds as a consequence of the charge rearrangements associated to electrostatic screening, as the main factor determining the survival of a noncentrosymmetric phase. Our findings advance greatly our understanding of the essentials of structurally polar metals, and offer guidelines on the behavior of ferroelectrics upon field-effect charge injection or proximity to conductive device elements.

Existence, Uniqueness, and a Comparison of Nonintrusive Methods for the Stochastic Nonlinear Poisson--Boltzmann Equation

The stochastic nonlinear Poisson--Boltzmann equation describes the electrostatic potential in a random environment in the presence of free charges and has applications in many fields. We show the e...

Aggregates Promote Efficient Charge Transfer Doping of Poly(3-hexylthiophene).

The doping efficiencies of regioregular (r-Re) and regiorandom (r-Ra) poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) were studied in solution using electron paramagnetic resonance (EPR), (19)F NMR, optical absorption, and Raman spectroscopy. EPR spectra of doped r-Re P3HT possess significantly larger amounts of paramagnetic species than r-Ra P3HT with similar F4-TCNQ loadings (∼0.1% up to 50%), which is confirmed by corresponding optical absorption spectra. (19)F NMR also show a rapid disappearance of the pristine F4-TCNQ signal when small amounts of r-Re P3HT are added due to minority paramagnetic species acting as efficient spin relaxation channels. Raman spectra of both P3HT variants indicate strong interactions with F4-TCNQ, however, the presence of free charges is only detected in r-Re samples owing to its ability to aggregate and adopt ordered conformations allowing for delocalization of hole charges after initial contact with the dopant.

Lateral bending of tapered piezo-semiconductive nanostructures for ultra-sensitive mechanical force to voltage conversion

Quasi-1D piezoelectric nanostructures may offer unprecedented sensitivity for transducing minuscule input mechanical forces into high output voltages due to both scaling laws and increased piezoelectric coefficients. However, until now both theoretical and experimental studies have suggested that, for a given mechanical force, lateral bending of piezoelectric nanowires results in lower output electric potentials than vertical compression. Here we demonstrate that this result only applies to nanostructures with a constant cross-section. Moreover, though it is commonly believed that the output electric potential of a strained piezo-semiconductive device can only be reduced by the presence of free charges, we show that the output piezopotential of laterally bent tapered nanostructures, with typical doping levels and very small input forces, can be even increased up to two times by free charges. Our analyses confirm that, though not optimal for piezoelectric energy harvesting, lateral bending of tapered nanostructures with typical doping levels can be ideal for transducing tiny input mechanical forces into high and accessible piezopotentials. Our results provide guidelines for designing high-performance piezo-nano-devices for energy harvesting, mechanical sensing, piezotronics, piezo-phototronics, and piezo-controlled chemical reactions, among others.

Classification of topological symmetry sectors on anyon rings

The golden chain with antiferromagnetic interaction is an anyonic system of particular interest as when all anyons are confined to the chain, it is readily stabilized against fluctuations away from criticality. However, additional local scaling operators have recently been identified on the disk which may give rise to relevant fluctuations in the presence of free charges. Motivated by these results for Fibonacci anyons, this paper presents a systematic method of identifying all topological sectors of local scaling operators for critical anyon rings of arbitrary winding number on surfaces of arbitrary genus, extending the original classification scheme proposed by Feiguin et al. Using the new scheme, it is then shown that for the golden chain, additional relevant scaling operators exist on the torus which are equivalent to those detected on the disk and which may disrupt the stability of the critical system. Protection of criticality against perturbations generated by these additional scaling operators can be achieved by suppressing the exchange of charge between the anyon ring and the rest of the manifold.

The Use of Electric Force as a Replacement of Buoyancy in Two-phase Flow

When an electrostatic field is applied to a medium, an electric force arises, originated by polarization and by the possible presence of free charge. If the medium is a two-phase fluid, this force drives the vapor (of lower dielectric permittivity) towards the zone of weaker electric field, giving the opportunity of separating phases and enhancing boiling heat transfer in microgravity conditions, where buoyancy is lacking. Electric force is also responsible for bubble deformation. In this paper, after a comprehensive description of electro-hydro-dynamics (EHD) mechanisms in a two-phase fluid, some relevant literature data about EHD-induced phase separation are reported, confirming the effectiveness of the technique. The electric force on a growing and rising bubble is then evaluated numerically in some simple cases, with a fem technique. Finally, original experiments on gas bubble detachment are analyzed in the attempt to enucleate the role of different mechanisms contributing to EHD enhancement. In particular, it is stressed that the role of electric forces in altering bubble shape and increasing bubble internal pressure seems to be the most significant in promoting bubble detachment.

Analysis of Droplet Generation in Electrospray Using a Carbon Fiber Based Microfluidic Emitter

This work presents simulation of jet break up in electrospray ionization using a microfluidic emitter. The emitter comprises a pointed carbon fiber located coaxial with a fused silica capillary of 360 microns OD and 75 microns ID, with its sharp tip extending 30 microns beyond the capillary terminus. The numerical model employs leaky-dielectric formulations for solving the electrodynamics and volume-of-fluid method for tracking the liquid-air interface. The existing leaky-dielectric model is modified to account for the presence of free charges inside the bulk of the liquid as well as at the interface. A small velocity perturbation is used at the capillary inlet to emulate the natural disturbance necessary for the jet break up. First, the model is validated by comparing model predictions with experimental results for a conventional emitter reported in literature. Then, it is applied to simulate the electrospray performance of the Carbon Fiber (CF) emitter including the Taylor cone and jet break up processes. Model predictions for CF emitter are compared with experimental results in terms of jet-length and current-flow characteristics. The influence of emitter geometry, operating conditions and liquid properties on the electrospray performance are investigated. Droplet diameter is correlated with flow rate and liquid properties and the correlation results are compared with that reported in literature.

Electret-thermal and dielectric analyses of electrically active colloids

The nature and mechanism of interfacial electrical interaction in organic colloids containing an active solid phase and a polar liquid phase are studied. Such systems are shown to offer specific properties. Specifically, high electrical forces producing gradients of the potential of the self-electric internal field arise at the liquid-solid interface. In the presence of free charges and ionic conductivity channels, this field can induce currents in such systems. When investigating the colloid system of the human venous blood, it is found that the internal field causes mesoscopic inhomogeneity in the structure of the aqueous components of the plasma and cytoplasm. It is shown that the concentration of free water molecules in the liquid phase of the system is maximal in group I blood and the physically nonuniform properties of combined water show up to the greatest extent in group IV blood. This finding is corroborated by the dispersion of the dielectric polarization in blood of different groups.

Van der Waals forces in presence of free charges: An exact derivation from equilibrium quantum correlations

We study interatomic forces in a fluid consisting of a mixture of free charges and neutral atoms in the framework of the quantum many-body problem at nonzero temperature and nonzero density. Of central interest is the interplay between van der Waals forces and screening effects due to free charges. The analysis is carried out in a partially recombined hydrogen plasma in the Saha regime. The effective potentials in the medium between two atoms, or an atom and a charge, or two charges, are determined from the large-distance behavior of equilibrium proton-proton correlations. We show, in a proper low-temperature and low-density scaling limit, that those potentials all decay as r(-6) at large distance r, while the corresponding amplitudes are calculated exactly. In particular, the presence of free charges only causes a partial (nonexponential) screening of the atomic potential, and it does not modify its typical r(-6) decay. That potential reduces to the standard van der Waals form for two atoms in vacuum when the temperature is driven to zero. The analysis is based on first principles: it does not assume preformed atoms and takes into account in a coherent way all effects, quantum mechanical binding, ionization, and collective screening, which originate from the Coulomb potential. Our method relies on the path integral representation of the quantum Coulomb gas.

Electric Field Effects on Internal Conversion: An Alternative Mechanism for Field-Induced Fluorescence Quenching of MEH-PPV and Its Oligomers in the Low Concentration Limit

In a previously published study (J. Phys. Chem. B 2006, 110, 7732-7742), we reported field-induced fluorescence quenching in both poly[2-methoxy,5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in solvent glass matrices at high dilution (<0.1% by weight). The observed quenching is not readily explained by field-induced exciton dissociation or by the presence of free charges, two mechanisms that have been invoked to explain this phenomenon by previous authors. A model is developed here that ascribes the observed fluorescence quench in dilute samples to an energetic shift of the relaxed excited state caused by the electric field resulting in increased nonradiative relaxation. To determine whether the relevant nonradiative pathway is intersystem crossing or internal conversion, analytical expressions are derived for each of these two mechanisms. Only the expression derived for the Stark effect on the rate of internal conversion quantitatively predicts the magnitude of quench observed in MEH-PPV and in the oligomeric species.

Effects of Maxwell‐Wagner polarization on soil complex dielectric permittivity under variable temperature and electrical conductivity

The presence of free charges and numerous discontinuities separating liquid, gas, and solid phases in partially saturated soils give rise to Maxwell‐Wagner polarization that may significantly affect bulk dielectric permittivity measurements. Evidence suggests a complex interplay shaping the response of low‐frequency (&lt;100 MHz) dielectric permittivity to changes in ambient temperature and amount of ionic charges present in a wet soil. Model calculations based on the Maxwell‐Wagner‐Brugermann‐Hanai (MWBH) theory are supported by direct measurements using a network analyzer, showing an increase in soil bulk dielectric permittivity with increasing temperature and with higher bulk electrical conductivity at low frequency range (&lt;100 MHz). Beyond a certain frequency the decrease in permittivity of free water becomes dominant and results in a decrease in soil bulk dielectric permittivity with increasing temperature. The value of this crossover frequency can be predicted as a function of solution electrical conductivity (EC) as confirmed in limited tests. The dielectric permittivity inferred from TDR waveform travel time analysis is not significantly influenced by the low frequency range of the dielectric spectrum. In contrast, dielectric permittivity sensors operating at frequencies lower than 100 MHz are likely to show significant sensitivity to factors affecting the Maxwell‐Wagner effect (temperature and electrical conductivity), hence requiring special care in measurement interpretation.

Electrostatically induced submicron patterning of thin perfect and leaky dielectric films: A generalized linear stability analysis

Thin leaky and perfect dielectric films can be driven electrically to form well-ordered patterns typically of pillar arrays. The process, sometimes referred to as lithographically induced self-assembly, begins by spin coating a polymer onto a silicon wafer to generate an initially featureless film. A small gap, filled with air or another polymeric/organic liquid, is left between the first film and a second wafer called the mask, which may be patterned. This configuration is heated above the glass transition temperature of the polymer, upon which flow ensues in response to destabilizing electrical forces. The authors examined the initial stages of pillar formation under a pattern free mask by performing a linear stability analysis under the lubrication approximation [J. Non-Newtonian Fluid Mech. 102, 233 (2002)] and found the quest for smaller pillars to be assisted by the presence of free charge as described by the leaky dielectric. In practice, however, the lubrication approximation may not strictly apply in the situations of greatest interest. Contrasting the linear stability analysis with and without the lubrication approximation shows that the approximation fails where surface tension is small and electric fields are large, typical of experiments with a polymer/organic liquid instead of air in the gap—precisely the conditions that generate the smallest pillar arrays. This general linear stability analysis predicts conditions where pillars/holes pack more tightly, have smaller diameters, greater aspect ratios, and larger growth exponents for both perfect and leaky dielectric films, in which the smallest features reach deep into the submicron length scales. The analysis also highlights two approaches to smaller pillars by either making the polymer film conducting or choosing polymers with high dielectric contrast, with the former being more universally applicable.

Treatment of needle-like ferroelectric domain patterns in the presence of free charge

In many experimental situations, domains with antiparallel polarization are formed in ferroelectric samples in which a spatially nonhomogeneous free charge ρ(r) has developed whose relaxation time is considerably longer than that of domain patterns. Together with the applied field Ea it influences the domain structure corresponding to minimum energy. Here we analyze which domain patterns is stable, considering the values of Ea and PS and the location and magnitude of ρ(r) as independent variables. Calculations have been performed for planar distributions of charge, in the approximation of narrow needle-like domains. It is possible to construct synoptical phase diagrams which demonstrate the location of different domain patterns corresponding to local equilibrium, in the space of independent variables.

Breakdown of Debye screening in quantum Coulomb systems and van der Waals forces

Screening of the 1 r Coulomb potential is one of the most fundamental properties of systems made of charged particles at thermal equilibrium. According to the familiar Debye description, the effective potential between two charges surrounded by their polarization clouds decays exponentially fast. In the classical case, this mean-field prediction is confirmed by rigorous proofs. However, when quantum mechanics is taken into account, we show that exponential screening is lost. Indeed, the intrinsic quantum fluctuations of the charges generate residual multipole-like interactions which are only partially screened. The particle correlations then decay as 1 r 6 , i.e. the square of dipolar interactions. This mechanism is illustrated in the simple framework of a solvable model of two quantum charges immersed in a classical plasma. The corresponding effective interactions between the charges are quite similar to the usual van der Waals forces between two atoms in the vacuum. In fact, as strongly suggested by a path integral analysis of the Hydrogen plasma at low temperature and low density, the effective interactions between any neutral and/or charged entities in a partially ionized gas, all behave as 1 r 6 at large distances. Thus, the presence of free charges does not modify the 1 r 6 nature of van der Waals forces, contrarily to what is expected in a classical description of screening effects. Possible implications of the breakdown of Debye screening due to quantum fluctuations are briefly evocated.

Dielectric Properties of Polyelectrolyte Solutions

Dielectric properties of polyelectrolyte solutions have been studied experi­ mentally for more than thirty years, but progress in understanding these properties has been rather slow. A particularly unfavorable combination of experimental difficulties and unsettled theoretical problems has noticeably influenced both the extent of the work devoted to this subject and the pace in which explanation and prediction proceeds in this field. The latter is not surprising in view of the difficulties encountered in dielectric theory in general (see e.g. Refs. 1 , 2) and taking into account the additional problems arising through the presence of free charges in the solution. Moreover, in the complementary field of the dielectric properties of ordinary electrolyte solutions, the situation is hardly better, and even there many problems remain unsolved in spite of recent progress (3). In recent years new theoretical work directly or indirectly related to the dielectric properties of aqueous polyelectrolyte solutions has appeared (see the section on Theory), focusing attention again on the rather un­ satisfactory state of affairs in this field. Hence, the appearance of this review seems justified and we hope it will stimulate further investigations, theoretical as well as experimental, in which new possibilities available through developments in other areas might be exploited. A further justification is that in recent times, only a few review articles of limited extent in this field have appeared (4-7). We shall limit this survey to aqueous solutions of true polyelectrolytes,