Polarization Relaxation Time Research Articles

Creation of polarized ultracold neutrons and observation of Ramsey resonance for electric dipole moment measurement

Polarized UCNs have been created by selecting only one spin state passing through a magnetized Fe foil. Typical degree of polarization was about 90 %. The polarization relaxation time in the prototype Ramsey cell was \(T_1 =1100^{+800}_{-400}\) s. Clear Ramsey resonance spectra have been observed for two precession time settings, tc = 100 ms and 30 s. The transverse relaxation time T2 was about 50 s.

Time-Dependent Density Functional Theory of Polarization Relaxation under External Field

A new inhomogeneous time-dependent density functional theory is proposed to study an effect of external field on the polarization relaxation of molecular liquids. The effect of external field is considered by the number density of solvent molecules, and the homogeneous approximation introduced by many previous studies is not employed. To demonstrate the validity of the theory, the spatial distribution of the relaxation time is calculated in a polar solvent system with an ion. The calculated results show a notable effect of translational diffusion on the spatial distribution of the polarization relaxation time.

Electro-responsive and dielectric characteristics of graphene sheets decorated with TiO2nanorods

Titanium dioxide (TiO2) nanorod-decorated graphene sheets have been synthesized by a simple non-hydrolytic sol–gel approach and demonstrated to be a highly effective dispersing material for electrorheological fluids. Electron microscopy and X-ray diffraction analysis indicated that the TiO2 nanorods with a high crystallinity are well-dispersed and successfully anchored on the graphene sheet surface through the formation of covalent bonds between Ti and C atoms. Furthermore, electro-responsive properties of graphene sheets decorated with TiO2 nanorods are investigated on the basis of the dielectric loss model. The dielectric property analysis based on the dielectric loss model clarifies that an introduction of TiO2 nanorods has been coupled with the larger achievable polarizability and short relaxation time of interfacial polarization. Consequently, the TiO2 nanorod-decorated graphene sheets exhibit consistently higher ER efficiency than that of GO sheets and show unprecedented electro-responsive performance in extremely low concentration ER materials (<0.2 wt%).

Two-layer materials of polyethylene and a carbon nanotube/cyanate ester composite with high dielectric constant and extremely low dielectric loss

Double-layer materials with one layer being a polyethylene (PE) film and the other layer a multi-wall carbon nanotube (MWCNT)/cyanate ester (CE) resin composite, PE-MWCNT/CE, were prepared. They have high dielectric constant and extremely low dielectric loss. For comparison, MWCNT/CE composites with different contents of MWCNTs were also prepared. Results show that the two kinds of materials have greatly different dielectric properties. With the same content of MWCNTs, the PE-MWCNT/CE material shows a higher dielectric constant and much lower dielectric loss than the MWCNT/CE composite. More specifically, the dielectric constant and loss tangent at 10Hz of the PE-MWCNT/CE material with 0.5wt.% MWCNTs are respectively 168 and 0.006, about 1.4 and 2.5×10−5 times the values of the corresponding MWCNT/CE composite. The nature behinds these interesting data was detected from the space charge polarization effect and equivalent circuits. The mechanism for the unique dielectric behavior of the PE-MWCNT/CE materials is that the presence of PE film not only reinforces the space charge polarization, but also subdues the leakage current. On the other hand, based on the discussion on the Cole–Cole plots, an effective method is developed to accurately calculate the relaxation time of space charge polarization in electric conductor/polymer materials.

Determination of polarization relaxation times of atomic levels

Rotation of polarization plane of a probe signal under influence of a time-advanced intense pulse is investigated taking into account optical pumping of atoms in a resonant medium. A formula is derived for the angle of rotation of polarization plane of the probe signal. It follows from this formula that modification of the polarization plane is caused by two processes: radiative decay of atoms and collision-induced redistribution of atoms over magnetic sublevels. It is shown that this method enables direct measuring long times of atomic collisions on the background of short times of the radiation decay.

Gain-enhanced optical coherence in a high optical gain semiconductor

The propagation of ultrashort pulses in a traveling wave semiconductor amplifier is considered. It is demonstrated that the effective polarization relaxation time, which determines the coherence of the interaction of pulses within the medium, strongly depends on its optical gain. As a result, it is shown that at large optical gains the coherence time can exceed the transverse relaxation time T2 by an order of magnitude, this accounting for the strong femtosecond superradiant pulse generation commonly observed in semiconductor laser structures.

Is it the grain size or the characteristic pore size that controls the induced polarization relaxation time of clean sands and sandstones?

There is a wide range of evidence to suggest that permeability can be constrained through of induced polarization measurements. For clean sands and sandstones, current mechanistic models of induced polarization predict a relationship between the low‐frequency time constant inferred from induced polarization measurements and the grain diameter. A number of observations do, however, disagree with this and indicate that the observed relaxation behavior is rather governed by the so‐called dynamic pore radius Λ. To test this hypothesis, we have developed a set of new scaling relationships, which allow the relaxation time to be computed from the pore size and the permeability to be computed from both the Cole‐Cole time constant and the formation factor. Moreover, these new scaling relationships can be also used to predict the dependence of the Cole‐Cole time constant as a function of the water saturation under unsaturated conditions. Comparative tests of the proposed new relationships with regard to various published experimental results for saturated clean sands and sandstones as well as for partially saturated clean sandstones, do indeed confirm that the dynamic pore radius Λ is a much more reliable indicator of the observed relaxation behavior than grain‐size‐based models.

Dielectric properties of PEO–PVP–MMT nanocomposite hydrocolloids

The relative complex dielectric function, electric modulus, alternating current electrical conductivity and complex impedance behaviour of aqueous poly(ethylene oxide) (PEO) and poly(vinyl pyrrolidone) (PVP) blend with montmorillonite (MMT) clay nano-platelets as colloidal suspensions have been investigated over the frequency range 20 Hz to 1 MHz at 15, 30 and 45 °C. The direct current ionic conductivity and real part of permittivity of the hydrocolloids have increased whereas electrode polarization relaxation time has decreased with the increase of MMT concentration, and these dielectric parameters have significantly varied with change of the temperature. The hydrogen bond complex formation between PEO, PVP and dispersed MMT nano-platelets in aqueous solutions has been discussed. The correlation between complex structures and the dielectric parameters of these materials have been confirmed.

Dielectric properties of solid solutions (1 − x)(KBi)1/2TiO3-xNa1/4Bi3/4Fe1/2Ti1/2O3 near the morphotropic phase boundary

The dielectric properties of solid solution ceramics (1 − x)(KBi)1/2TiO3-(x/2)(NaBi)1/2TiO3-(x/2)BiFeO3 near the morphotropic phase boundary lying near x = 0.75 and separating the compositions with the orthorhombic (x 0.75) structures have been studied. The solid solutions are shown to undergo a diffuse phase transition with the Curie temperature TC = 640–650 K. Based on an analysis of the frequency dependence of the imaginary component of the electric modulus in the range T > TC, it is concluded that the transition of the system of solid solutions near the morphotropic phase boundary (x = 0.75) to the dipole-ordered state is accompanied by the formation of heterophase microregions with different relaxation times of the dielectric polarization. The activation energies of the relaxation of the dielectric polarization (ΔEM) in the aforementioned temperature range and of the dc charge carriers (ΔEdc) are determined. It is found that ΔEdc stepwise increases near 570 K in going toward higher temperatures. It is shown that the behavior of the characteristics of the dielectric response of the solid solutions below 400 K corresponds to a relaxor ferroelectric. For the compositions 0.7 ≤ x ≤ 0.8, the estimated freezing temperature of dipole clusters of the relaxation state is about 240 K, and their activation energy decreases with increasing x.

Dielectric spectroscopy and viscosity studies of aqueous poly(ethylene oxide) and poly(vinyl pyrrolidone) blend

The complex dielectric function, electric modulus, impedance and ac electrical conductivity behaviour of aqueous solutions of 5 wt% poly(ethylene oxide) (PEO) and poly(vinyl pyrrolidone) (PVP) and their different volume percent blends were investigated in the frequency range 20 Hz to 1 MHz at 15, 30 and 45 °C. It is found that the real part of dielectric function of these blends at 1 MHz decreases with the increase of PEO concentration and their dc electrical conductivity has strong correlation with the electrode polarization relaxation time. The static permittivity, ionic conductivity, electrode polarization relaxation time and apparent viscosity have linear behaviour with temperature variation at fixed volume concentration of the aqueous polymers blend. The viscosity of these aqueous polymeric blends increases with the increase of PEO concentration. The behaviour of hydrogen bond interactions between the polar segments of PEO and PVP were explored from the comparative change in dielectric parameters and viscosity of the two phase aqueous polymeric systems.

Dielectric spectroscopy and confirmation of ion conduction mechanism in direct melt compounded hot-press polymer nanocomposite electrolytes

Solid-type polymer nanocomposite electrolyte (PNCE) comprising poly(ethylene oxide) (PEO), lithium perchlorate (LiClO4) and montmorillonite (MMT) nano-platelets were synthesized by direct melt compounded hot-press technique at 70 °C under 3 tons of pressure. The spectra of complex dielectric function, electric modulus and alternating current (ac) electrical conductivity, and complex impedance plane plots of these materials were investigated in the frequency range 20 Hz to 1 MHz at ambient temperature. The variation of electrode polarization and ionic conduction relaxation times with MMT concentration up to 20 wt.% confirms their strong correlation with direct current ionic conductivity. The predominance of exfoliated MMT structures in PEO matrix and their effect on cation conduction mechanism and ion pairing were discussed by considering a supramolecular transient cross-linked structure. The normalized ac conductivity as a function of scaled frequency of these PNCE materials obey the universal time–concentration superposition behaviour alike the disordered solid ionic conductors.

Collisions of alkali K and Rb atoms in ground state: Modification of interaction potentials

The interaction of alkali K and Rb atoms that reside in the ground state is considered in the range of collision energies E = 10−4 to 10−2 au. The singlet (X1Σ+) and triplet (a3Σ+) interaction potentials available in the literature are analyzed and modified. For the KRb dimer in the range of interatomic distances 15–21a0, we chose analytical representations of the singlet and triplet potentials that more accurately describe the interaction of alkali Rb and K atoms in the ground state. Complex cross sections of the spin exchange are calculated for the first time that permit one to calculate the processes of polarization transfer and relaxation times, as well as shifts in the magnetic resonance frequencies caused by K-Rb spin exchange collisions.

Polariton-mode lasing and Bose condensate of dipolar excitons in heterostructures

The analysis of the polariton modes in 2D traps based on heterostructures with quantum wells for Bose-Einstein condensation of dipolar excitons is presented. The characteristic equation of such modes is derived with allowance for the polarization relaxation of excitons and radiative losses from the trap. The spectrum and structure of high-quality modes are analytically and numerically studied. It is demonstrated that several modes become unstable at a high enough density of excitons and a long relaxation time of the exciton polarization. In accordance with the estimations, such an instability can be reached in the experiments on the Bose-Einstein condensation of dipolar excitons and can be used to interpret the corresponding coherent emission.

Frequency‐Dependent Electric Properties of Microscale Rock Models for Frequencies from One Millihertz to Ten Kilohertz

In this study, spectral induced polarization (SIP) spectra were generated numerically to better understand how actual rock microstructure and electrolyte properties in rock pores affect the spectral pattern, i.e., the characteristic relaxation time of polarization as well as the polarization strength of a rock pore system. The dynamics of charge carriers in three‐dimensional pore systems were simulated using a frequency‐dependent formulation of the Nernst–Planck–Poisson (NPP) ion‐transport equations. Basically, a pore‐system model of alternating stacked cylinders of two different sizes was studied considering the electrical double layer (EDL). A reduced cationic mobility—resulting from increased adsorption of these cations at the rock–water interface—was assumed within the EDL. By solving the NPP equations using the finite element method, complex resistivity phase spectra were generated. Subsequently, the effect of pore structural properties and electrolyte conductivity on the magnitude and frequency position of the characteristic resistivity phase minimum of rocks was studied. The following results were found: First, regarding pore geometry, the characteristic frequency of the phase minimum fmin decreases with increasing pore length of the large pore. Second, both small pores having a radius of a few Debye lengths combined with larger pores are needed to ensure detectable phase amplitudes. Third, with regard to electrolyte concentration, the phase amplitude is inversely proportional to the concentration, whereas fmin remains constant. Because the studied model does not provide a direct and exclusive link between the simulated electrical properties and pore throat size, further research is needed here to specify a convincing SIP interpretation method for improved permeability estimation.

Ionic properties of liquid crystals dispersed with carbon nanotubes and montmorillonite nanoplatelets

The ionic properties of liquid crystals doped with one-dimensional carbon nanotubes, two-dimensional montmorillonite, and a mixture of both are investigated. The results indicate that the relaxation times of electrode polarization and ionic conductivity are time-dependent after the cell fabrication regardless of doping. While all of the dopants are effective in capturing impurity ions, the cells containing the hybrid dopant exhibit the feeblest ionic effect at room temperature.

Polarization Caging in Diffusion-Controlled Electron Transfer Reactions in Solution

In some bimolecular diffusion-controlled electron transfer (ET) reactions such as ion recombination (IR), both solvent polarization relaxation and the mutual diffusion of the reacting ion pair may determine the rate and even the yield of the reaction. However, a full treatment with these two reaction coordinates is a challenging task and has been left mostly unsolved. In this work, we address this problem by developing a dynamic theory by combining the ideas from ET reaction literature and barrierless chemical reactions. Two-dimensional coupled Smoluchowski equations are employed to compute the time evolution of joint probability distribution for the reactant (P((1))(X,R,t)) and the product (P((2))(X,R,t)), where X, as is usual in ET reactions, describes the solvent polarization coordinate and R is the distance between the reacting ion pair. The reaction is described by a reaction line (sink) which is a function of X and R obtained by imposing a condition of equal energy on the initial and final states of a reacting ion pair. The resulting two-dimensional coupled equations of motion have been solved numerically using an alternate direction implicit (ADI) scheme (Peaceman and Rachford, J. Soc. Ind. Appl. Math. 1955, 3, 28). The results reveal interesting interplay between polarization relaxation and translational dynamics. The following new results have been obtained. (i) For solvents with slow longitudinal polarization relaxation, the escape probability decreases drastically as the polarization relaxation time increases. We attribute this to caging by polarization of the surrounding solvent. As expected, for the solvents having fast polarization relaxation, the escape probability is independent of the polarization relaxation time. (ii) In the slow relaxation limit, there is a significant dependence of escape probability and average rate on the initial solvent polarization, again displaying the effects of polarization caging. Escape probability increases, and the average rate decreases on increasing the initial polarization. Again, in the fast polarization relaxation limit, there is no effect of initial polarization on the escape probability and the average rate of IR. (iii) For normal and barrierless regions the dependence of escape probability and the rate of IR on initial polarization is stronger than in the inverted region. (iv) Because of the involvement of dynamics along R coordinate, the asymmetrical parabolic (that is, non-Marcus) energy gap dependence of the rate is observed.

Electrical conductivity and dielectric spectroscopic studies of PVA–Ag nanocomposite films

Abstract Poly(vinyl alcohol)–silver (PVA–Ag) nanocomposite films were prepared by the soft chemical route. Effect of concentration of embedded Ag nanoparticles on conductivity and dielectric relaxation behaviour of these nanocomposite films has been studied. An increase in dc conductivity from 1.38 × 10−11 to 9.17 × 10−11 S/cm and decrease in frequency dependent dielectric constant (from 1.74 to 1.07 at 75 kHz) are observed with the increase in concentration of Ag nanoparticles in PVA from 0 to 1.32 wt%. The values of ac conductivity and polarization relaxation time, deduced from dielectric data, also corroborate towards the enhanced conducting behaviour of PVA matrix with increase in the concentration of embedded Ag nanoparticles.

Electron spin relaxation in graphene from a microscopic approach: Role of electron-electron interaction

Electron spin relaxation in graphene on a substrate is investigated from the microscopic kinetic spin Bloch equation approach. All the relevant scatterings, such as the electron impurity, electron-acoustic phonon, electron-optical phonon, electron-remote-interfacial phonon, as well as electron-electron Coulomb scatterings, are explicitly included. Our study concentrates on clean intrinsic graphene, where the spin-orbit coupling from the adatoms can be neglected. We discuss the effect of the electron-electron Coulomb interaction on spin relaxation under various conditions. It is shown that the electron-electron Coulomb scattering plays an important role in spin relaxation at high temperature. We also find a significant increase in the spin relaxation time for high spin polarization even at room temperature, which is due to the Coulomb Hartree-Fock contribution-induced effective longitudinal magnetic field. It is also discovered that the spin relaxation time increases with the in-plane electric field due to the hot-electron effect, which is different from the nonmonotonic behavior in semiconductors. Moreover, we show that the electron-electron Coulomb scattering in graphene is not strong enough to establish the steady-state hot-electron distribution widely used in the literature and an alternative approximate one is proposed based on our computation.

Effects of [SDS]/[H 2O] molar ratio on the electromagnetic properties of polyaniline/maghemite nanocomposites

Polyaniline (PANI)/maghemite (γ-Fe 2O 3) nanocomposites were prepared by using the reverse micelle polymerization, where aniline, ferrous and ferric salts and sodium dodecyl sulfate (SDS) act as monomer, precursor of γ-Fe 2O 3 and surfactant, respectively. The effect of the molar ratio of [SDS]/[H 2O] on the electromagnetic properties of PANI/γ-Fe 2O 3 nanocomposites was investigated by Fourier transform spectroscopy (FTIR), UV–visible spectroscopy (UV–vis), X-ray photoelectron spectroscopy (XPS), wide angle X-ray diffraction diffractometer (WAXD), impedance analysis analyzer, micro-ohmetry and superconductor quantum interference device (SQUID). The nanostructure of the PANI/γ-Fe 2O 3 nanocomposites was characterized by micrographs of transmission electron microscopy (TEM). Results showed that both the γ-Fe 2O 3 content and particle size in the nanocomposites decreased with molar ratio of [SDS]/[H 2O]. The γ-Fe 2O 3 phase is non-uniformly distributed in the PANI matrix, and exhibits a broader size distribution at higher [SDS]/[H 2O] molar ratio due to the reduced strength of PANI–γ-Fe 2O 3 interactions. In the presence of γ-Fe 2O 3, the growth rate of quinoid ring is markedly retarded. The retard effect is significantly reduced by increasing the [SDS]/[H 2O] molar ratio, leading to the improvement of crystallinity, conductivity and dielectric properties (i.e., permittivity and loss factor) of nanocomposites. Simultaneously, the ionic polarization relaxation time is shortened from 2.61 × 10 −9 to 1.04 × 10 −9 s. For SQUID analysis at room temperature, the typical superparamagnetic behavior is found with the saturation magnetization decreased with the [SDS]/[H 2O] molar ratio, resulting from the reduced γ-Fe 2O 3 content, smaller γ-Fe 2O 3 particle size, and the wider particle size distribution.

Effect of melt compounding temperature on dielectric relaxation and ionic conduction in PEO–NaClO4–MMT nanocomposite electrolytes

Polymer nanocomposite electrolytes (PNCEs) of poly(ethylene oxide) and sodium perchlorate monohydrate complexes with montmorillonite (MMT) clay up to 20 wt.% MMT concentration of poly(ethylene oxide) (PEO) are synthesized by melt compounding technique at melting temperature of PEO (∼70 °C) and NaClO4 monohydrate (∼140 °C). Complex dielectric function, electric modulus, alternating current (ac) electrical conductivity, and impedance properties of these PNCEs films are investigated in the frequency range 20 Hz to 1 MHz at ambient temperature. The direct current conductivity of these materials was determined by fitting the frequency-dependent ac conductivity spectra to the Jonscher power law. The PNCEs films synthesized at melting temperature of NaClO4 monohydrate have conductivity values lower than that of synthesized at PEO melting temperature. The complex impedance plane plots of these PNCEs films have a semicircular arc in upper frequency region corresponding to the bulk material properties and are followed by a spike in the lower frequency range owing to the electrode polarization phenomena. Relaxation times of electrode polarization and ionic conduction relaxation processes are determined from the frequency values corresponding to peaks in loss tangent and electric modulus loss spectra, respectively. A correlation is observed between the ionic conductivity and dielectric relaxation processes in the investigated PNCEs materials of varying MMT clay concentration. The scaled ac conductivity spectra of these PNCEs materials also obey the ac universality law.