Frequency-dependent Complex Dielectric Constant Research Articles

Magnetic and electrical transport properties of YbFe2O4

We present magnetic and electrical transport properties such as resistivity, magnetoresistance, dielectric, and polarization of polycrystalline ${\mathrm{YbFe}}_{2}{\mathrm{O}}_{4}$. The ferrimagnetic transition temperature is measured at 243 K, followed by the two low-temperature transitions at $\ensuremath{\sim}190$ and $\ensuremath{\sim}65$ K, respectively. The magnetic properties including the $M\ensuremath{-}H$ hysteresis loops exhibit a strong temperature dependence and possibly indicate a spin-glass state below 65 K for ${\mathrm{YbFe}}_{2}{\mathrm{O}}_{4}$. The iron M\"ossbauer measurement at 295 K confirms the presence of two Fe sites. The measured resistivity can be modeled with the Mott's variable-range hopping model, $\ensuremath{\rho}\ensuremath{\propto}exp{({T}_{0}/T)}^{1/4}$, indicating the electron hopping between ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ sites. The magnetoresistance effects up to 6% at 8 T were observed and the effects could be caused by the field-induced changes in the electron-hopping processes. The frequency-dependent complex dielectric constant has been found to be strongly influenced by the contact effects, and the polarization of polycrystalline ${\mathrm{YbFe}}_{2}{\mathrm{O}}_{4}$ does not show ferroelectricity.

Magnetoresistance, magnetic, and dielectric properties of LuFe2O4 prepared by ebeam-assisted solid state reaction

We present structural, magnetoresistance, magnetic, Mössbauer, and dielectric properties of polycrystalline LuFe2O4 prepared by an electron-beam assisted solid state reaction. The x-ray diffraction pattern shows the single phase LuFe2O4 sample, and the ferrimagnetic transition temperature is measured at 240 K, followed by the two low-temperature transitions at 210 K and 140 K, respectively. The magnetic properties including the M-H hysteresis loops exhibit a strong temperature dependence and possibly indicate that LuFe2O4 enters a spin-glass state below 100 K. The iron Mössbauer measurement at 300 K indicates two Fe sites. The resistivity follows Mott’s variable-range hopping model, ρ∝exp(T0/T)1/4, indicating the electron hopping between Fe2+ and Fe3+. The magnetoresistance effects up to 2.5% at 5 T in the ferrimagnetic state were observed, and the effects could be caused by the field-induced changes in the electron hopping processes. The frequency-dependent complex dielectric constant has been found to be strongly influenced by the contact effects, and the intrinsic ferroelectricity of LuFe2O4 could not be ascertained.

Deterministic role of frequency, amplitude and concentration regimes on the complex dielectric relaxation of colloidal complex fluids

The present article explores the governing roles played by concentration regimes, frequency and amplitude of the forcing electric field on the dynamic dielectric responses of nanocolloidal non-polar suspensions. Frequency dependent complex dielectric constants were determined in the band of 1 to 107 Hz at constant temperature via dielectric spectroscopy. In addition to prevalant literature measurements, influence of field amplitude has also been performed to investigate the role of electric field intensity upon the relaxation behaviors. The experimental observations reveal that variation of frequency, as well as choice of particle material significantly influences the dielectric responses of nanocolloids. Further, the amplitude of field is found to induce differences in the dielectric response. It is also observed that the relaxation responses are grossly different for concentrated dispersions than the dilute counterparts. The concentration regimes are observed to bring about overlapping relaxation responses in the colloids. Good degree of quantitative agreement has been found between the experimental observations and the generic Havriliak-Negami relaxation model. It has been shown that for a given nanomaterial, reduction of relaxation timescale is a direct consequence of enhancement in concentration and the colloids transit from predominantly storage to leaky systems with concentration regimes. Additionally, the reported volume diffusion mechanism (VDM) is found to be in compliance with the experimental findings. The occurrence and shift of peaks and valleys in the dielectric spectra has been explained based on dielectric response theories for electrically heterogeneous media.

Reconstruction of annular bi-layered media in cylindrical waveguide section

We consider a radial transverse resonance model for a circular cylindrical waveguide composed into two layers with different frequency dependent complex dielectric constants. An inverse problem with four unknowns - three physical material parameters and one dimensional dielectric layer thickness parameter - is solved by employing TE110 and TE210 modes with different radial field distribution. First the resonance frequencies and quality factors are found fitting a Lorentzian function to the ‘measured’ data, using the method of least squares. Then found resonance frequencies and quality factors are used in a second inverse Newton-Raphson algorithm which solves four transverse resonance equations in order to get four unknown parameters. The use of TE110 and TE210 models offers one-dimensional radial tomographic capability. An open ended coaxial waveguide quarter-wave resonator is added to the sensor topology, and the effect on the convergence of numerical method is investigated.

Using off-specular ellipsometry spectra of dielectric grating-coupled plasmon mode for biosensing

We performed spectroscopic ellipsometry measurements of the specular and off-specular reflections of a one-dimensional polymethyl methacrylate grating on a Au-coated glass substrate and analyzed the data with efficient theoretical modeling. Very good agreement is obtained between calculation and experiment for both specular and off-specular reflections. It is found that the resonance peak of the off-specular reflection is much sharper than specular reflection at a wavelength that matches the condition for exciting the interface (surface plasmon) mode. When different media are used as the ambient, we found that the off-specular reflection near the surface-plasmon resonance can give significant enhancement in detection sensitivity compared with the corresponding specular reflection. This study may open a new way for biosensing. To analyze the data, we also developed a method to compute quasi-photonic band structures for periodic structures with frequency-dependent complex dielectric constants.

Dielectric relaxation of electrolyte solutions using terahertz transmission spectroscopy

We use terahertz (THz) transmission spectroscopy to obtain the frequency dependent complex dielectric constants of water, methanol, and propylene carbonate, and solutions of lithium salts in these solvents. The behavior of the pure solvents is modeled with either two (water) or three (methanol and propylene carbonate) Debye relaxations. We discuss the effects of ionic solvation on the relaxation behavior of the solvents in terms of modifications to the values of the Debye parameters of the pure solvents. In this way we obtain estimates for numbers of irrotationally bound solvent molecules, the numbers of bonds broken or formed, and the effects of ions on the higher-frequency relaxations.

Glassy properties of 9/65/35 PLZT ceramics

The freezing process in 9/65/35 PLZT ceramics has been investigated by measurements of the quasistatic field cooled and zero-field cooled dielectric susceptibilities and of the frequency dependent complex dielectric constant. The temperature dependence of the third harmonic dielectric nonlinearity a3 = ε3/ε1 4 has been determined in 9/65/35 PLZT ceramics. In zero electric field a deep in the temperature dependence of the static a3 indicates a crossover from paraelectric-like to glass-like behavior. The temperature dependence of the linear and nonlinear static dielectric response can be described with the modified spherical random-bond-random-field model of relaxor ferroelectrics.

Exact calculation of Maxwell equations for a tip-metallic interface configuration: Application to atomic resolution by photon emission.

We present an exact calculation of Maxwell's equations for a tip-metallic interface configuration by using a nonlocal boundary condition. The system is two dimensional, and consists of a cylinder at subwavelength distance from a corrugated surface of Ag or W. Both media are described by a frequency-dependent complex dielectric constant. The plasmon intensity has a huge enhancement, which is maximum when the tip-surface distance is about 1 nm, and it is sensitive to variations of 0.1 nm. This can account for recent experimental data on atomic resolution in photon emission by scanning tunneling microscopy operating at a constant current mode by Berndt et al. \textcopyright{} 1996 The American Physical Society.

Cluster description of water-in-oil microemulsions near the critical and percolation points

The three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behavior in the vicinity of room temperature. The phase diagram in the temperature-volume fraction (of the dispersed phase) plane exhibits a lower consolute critical point at about 40 degrees centigrades and 10% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to high volume fraction side at progressively lower temperatures. In this paper we review the evidence that allows to interpret the phase behavior of our system in terms of interacting spherical droplets. We also investigate the dynamics of droplets, below and approaching the critical point by dynamic light scattering. The first cumulant and time evolution of the droplet density correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena observed in an extensive set of measurements of electrical conductivity and permittivity close to percolation is also reviewed and interpreted through the same cluster-forming mechanism, which reproduces the most relevant features of the frequency-dependent complex dielectric constant of this system.

Mixed phase of deuteron glass and ferroelectric ordering in Rb1-x(ND4)xD2AsO4

Abstract Temperature and frequency dependent complex dielectric constants have been measured on 80% deuterated Rb1-x(ND4)xD2AsO4 single crystals for the concentrations x = 0.07 and x = 0.30. For samples of x = 0.07, the glassy state coexisted with ferroelectric long range ordering has been found. On the contrary, in x = 0.3 samples, deuteron (and partially proton) glass phase has been observed. Through the loss scaling using the Vogel-Fulcher law, we study the distinction between the coexistent glassy state and the pure glass state. The glassy state coexisted with ferroelectric long range ordering is assumed as a mixed phase with distinctive character.

Scattering of electromagnetic plane waves from rough periodic multilayered films

I use the Rayleigh–Fano theory to calculate the optical response of a stack of N thin films with rough interfaces, defined by arbitrary periodic functions of the same period, bounded by two semi-infinite media. Each medium is assumed to be homogeneous, isotropic, local, and linear and characterized by a frequency-dependent complex dielectric constant. I derive two matrix equations that relate the reflected and the transmitted fields to the incident wave. These matrices can be calculated in a recursive way. The solution involves Fourier coefficients of functions that are dependent on the roughness profiles. This treatment is valid for both TM(p) or TE(s) polarizations. I apply this formulation to some particular cases.

A study of the relaxation of discommensurations in K2ZnCl4 III. Measurements of the complex dielectric constant

For pt.II see ibid., vol.4, p.6899 (1992). Dynamics of discommensurations (DC) in the incommensurate-commensurate transition at TC in K2ZnCl4 were studied. Slow relaxation processes are revealed by an investigation of the frequency-dependent complex dielectric constant. The occurrence of a polydispersion indicates that in the vicinity of TC there exists a transient chaotic state with an inhomogeneous distribution of DC. By fitting the experimental data, the authors found that there exist two characteristic relaxation frequencies during the C-I transition. One corresponds to the regions of the frozen-in state of the C phase, the other to the regions with DC newly created from the C domains of the crystal. This indicates the coexistence of the C and I phases near TC. The results reveal that the phase transformation proceeds through the nucleation of DC stripples and through the rearrangement of both the newly created DC and the 'frozen-in' ones.

Technique for measuring the frequency-dependent complex dielectric constants of liquids up to 20 GHz

A technique for the measurement of the frequency-dependent complex dielectric constant ε=ε′−jε″ of liquids applicable to frequencies up to at least 20 GHz is described. The technique utilizes a coaxial probe dipped into the liquid. The reflection coefficient of the probe was measured using a network analyzer between 45 MHz and 20 GHz. A new de-imbedding procedure for eliminating the connector and coax mismatches is described, during which the impedance Z(ε) of the probe end was determined. The dielectric constant ε was determined from Z(ε) by modeling the coax–liquid interface as a capacitance. The de-imbedding procedure, which utilizes three calibrations, directly eliminates the (unknown) fringe-field impedance. Radiation effects were minimized by using narrow (0.047-in.) semirigid coax. The technique yields accurate results for ε′ and ε″ of liquids such as methanol and water over the entire frequency range up to 20 GHz, and can be used to determine the relaxation spectra of liquid and liquidlike samples.