Inhomogeneous Dielectrics Research Articles

Driving Forces of Protein Association: The Dimer-Octamer Equilibrium in Arylsulfatase A

The enzyme arylsulfatase A (ASA) occurs in solution as dimer ( α 2) above pH 6 and associates to octamers ( α 2) 4 below pH 6. The crystal structure of ASA suggests that the ( α 2)-( α 2) 4 equilibrium is regulated by protonation/deprotonation of Glu-424 located at the interface between ( α 2) dimers in the octamer. The reason for this assumption is that Glu-424 can be in two different conformers where it forms an intra or intermolecular hydrogen bond, respectively. In the present study we investigate this protein association process theoretically. The electrostatic energies are evaluated by solving the Poisson-Boltzmann equation for the inhomogeneous dielectric of the protein-water system for the dimer and octamer configurations. If a conventional surface energy term is used for the nonelectrostatic interactions, the absolute value of free energy of association fails to agree with experiment. A more detailed treatment that explicitly accounts for hydrophilic and hydrophobic character of the amino acids in the dimer-dimer interface of the octamer can explain this discrepancy qualitatively. The pH dependence of the computed association energy clearly demonstrates that the octamer is more stable at low pH if Glu-424 becomes protonated and forms an intermolecular hydrogen bond. We found a slight preference of Glu-424 to be in a conformation where its acidic group is fully solvent-exposed in the dimer state to form hydrogen bonds with water molecules. Application of the proton linkage model to calculate the association energy from the simulated data yielded results identical to the one obtained from the corresponding direct method.

Stress-induced electron emission from nanocomposite amorphous carbon thin films

Traditionally, the emission of electrons from materials have been explained using either the Fowler–Nordheim emission mechanism where high electric fields are used to extract electrons from surfaces or using conventional thermal emission where high currents are used to “boil” off electrons to vacuum. In this letter, we propose an alternative mechanism for electron emission from highly compressive thin films based on stress-induced “band structure” modification of nano-ordered sp2 regions in the thin films. Experimental results are recorded which show that the localized compressive stress governs electron emission in the amorphous carbon thin films studied here rather than the surface nanostructures/features or the diamond-like sp3 hybridized bond component. This analysis is in agreement with the concept of an internal or nongeometric field enhancement from sp2 nanostructures giving rise to high dielectric inhomogeneity within the carbon thin film. The results presented could be extended to explain the anomalous field emission behavior of carbon nanotubes.

Numerical analysis of uniform rectangular waveguides filled by inhomogeneous dielectrics

AbstractA numerical analysis of a uniform rectangular waveguide filled by an inhomogeneous dielectric with a continuous spatial variation of the relative permittivity is presented in this Letter. The proposed method, based on a method‐of‐moments (MoM) solution of the differential equations for TE, LSE, and LSM modes, is applied to find out the cutoff frequencies of the waveguide. Some numerical examples are also presented, considering both continuous and stepped variations of the permittivity along a transverse direction. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 313–316, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10446

Chaotic spectrum of a cavity resonator filled with randomly located sapphire particles.

We propose a new approach to the study of correlation properties of electromagnetic waves of a millimeter wave band after multiple passage through a random medium. The approach consists of an investigation of a spectrum of a cavity resonator, filled with randomly located dielectric inhomogeneities. As an object for the approach implementation, the spherical cavity resonator filled with sapphire particles, whose sizes are comparable to a wavelength, was chosen. It is revealed that the spectrum of this resonator in the frequency range 26 GHz-38 GHz is chaotic. A number of correlation effects, such as the effect of "repulsion" of frequencies, the shape of the nearest-neighbor frequency spacing distribution close to the Wigner distribution, the characteristic curve of spectral rigidity, and the correlation of resonance lines on intensity and frequency were found in this spectrum. A superwide frequency band effect of the long retention of an energy of a short microwave impulse in a resonator is exposed and studied. We analyze features of the investigated chaotic spectrum on the basis of the conception of a spatial dispersion of effective dielectric permeability for a medial field in a medium filling the resonator. It is shown that this conception allows us to explain the complete elimination of the degeneration of a spectrum and its chaotization, and also the essential broadening of the resonance lines at the expense of the transfer of energy of dominant transverse modes to strongly damping longitudinal oscillations.

Retarded field calculation of electron energy loss in inhomogeneous dielectrics

The exact solution of Maxwell's equations in the presence of arbitrarily shaped dielectrics is expressed in terms of surface-integral equations evaluated at the interfaces. The electromagnetic field induced by the passage of an external electron is then calculated in terms of self-consistently obtained boundary charges and currents. This procedure is shown to be suitable for the simulation of electron energy loss spectra when the materials under consideration are described by local frequency-dependent response functions. The particular cases of translationally invariant interfaces and axially symmetric interfaces are discussed in detail. The versatility of this method is emphasized by examples of energy loss spectra for electrons passing near metallic and dielectric wedges, coupled cylinders, spheres, and tori, and other complex geometries, where retardation aspects and Cherenkov losses can sometimes be significant.

Electromagnetic barrier penetration in a dispersive medium: Tunneling times and dispersion relations

We consider a scattering process inside a dispersive and absorptive medium, and derive an analytical expression for the scattering-time parameter that characterizes the duration of this process. We consider a microwave wave packet that scatters from a dielectric inhomogeneity inside an otherwise homogeneous waveguide. The waveguide is filled with a dielectric medium with a finite inhomogeneity region in the direction of the wave-packet propagation. The permittivity of the entire medium is described by a complex function of the frequency. Hence both absorption and dispersion affect the propagation of the wave packet. We define the scattering-time parameter to be the difference in the ``average arrival time'' between the free and actual propagation. We show that this time difference (in the limit of a narrow-band wave packet) is given by a linear combination of the real and imaginary parts of the ``complex tunneling time.'' In the special case where there is no dissipation or attenuation outside the scatterer, we recover the well-known result that the scattering-time parameter is identical to the real part of the complex tunneling time. The contribution of the imaginary part of the complex tunneling time to the scattering-time parameter can, therefore, be attributed to absorption and attenuation outside the scatterer. We further show that the real and imaginary-time parameters satisfy dispersion relations. Causality in this process is briefly discussed.

Inhomogeneous Dielectric Media: Wave Propagation and Dielectric Permittivity Reconstruction in the Case of a Rectangular Waveguide

In this paper we study the theoretical part of a novel non-destructive testing method for the reconstruction of the dielectric profile of an inhomogeneous dielectric material. We consider the inhomogeneity to be described by a polynomial of third order, i.e., εr(z)= ε0 + ε1z2 + ε3z3 and we utilize a system of three rectangular waveguides to solve the direct and the inverse problem. We apply Frobenius method to solve the underlying differential equations. We give special attention to the first cut-off frequency on the main mode TE10 and we propose a good estimation of it by using the mean value of the dielectric profile.

Dielectric Relaxation in Disordered Polar Dielectrics

A review of the current understanding of the effect of crystal lattice defects on the low-frequency dielectric relaxation properties in single crystals and polycrystalline polar dielectrics is given. Special attention is paid to different long-term phenomena, found in dielectric experiments, such as a low frequency relaxation polarization, a very slow relaxation of metastable states, a dielectric dispersion with a broad spectrum of distribution of relaxation times, a nonergodic behavior of the remanent polarization, etc. Mechanisms for dielectric loss and permittivity taking into account kinetics of defects rearrangement and their interactions in disordered inhomogeneous polar dielectrics are assessed.

Dielectric Relaxation in Disordered Polar Dielectrics

A review of the current understanding of the effect of crystal lattice defects on the low-frequency dielectric relaxation properties in single crystals and polycrystalline polar dielectrics is given. Special attention is paid to different long-term phenomena, found in dielectric experiments, such as a low frequency relaxation polarization, a very slow relaxation of metastable states, a dielectric dispersion with a broad spectrum of distribution of relaxation times, a nonergodic behavior of the remanent polarization, etc. Mechanisms for dielectric loss and permittivity taking into account kinetics of defects rearrangement and their interactions in disordered inhomogeneous polar dielectrics are assessed.

Recovery of small electromagnetic inhomogeneities from partial boundary measurements

We consider for the inverse problem of identifying locations and certain properties of the shapes of small dielectric inhomogeneities in a homogeneous background medium from boundary measurements on part of the boundary or dynamic boundary measurements for a finite time interval. Using as weights particular background solutions we develop asymptotic methods based on appropriate averaging of the data. To cite this article: H. Ammari, A.G. Ramm, C. R. Mecanique 330 (2002) 199–205.

Axial electromagnetic wave propagation in inhomogeneous dielectrics

We review the mathematical description of axial propagation in inhomogeneous dielectrics, focusing on two canonical examples: scalar Bragg gratings and thin film helicoidal bianisotropic media (TFHBM). Coupled wave analysis is reviewed for the former, and exact methods based on rotational transforms are discussed for the latter. There are many similarities between the physical descriptions that are emphasised in reviewing the modelling techniques, although the transform which allows the TFHBM case to be solved exactly is shown to be inapplicable in the scalar case.

Lorentz-invariant superluminal tunneling.

It is shown that superluminal optical signaling is possible without violating Lorentz invariance and causality via tunneling through photonic band gaps in inhomogeneous dielectrics of a special kind.

Origin of electric field enhancement in field emission from amorphous carbon thin films

The observation of electron emission from amorphous carbon thin films at low applied electric fields is explained in terms of an enhancement of the field brought about by dielectric inhomogeneities within the film. These inhomogeneities originate from the differences between conductive, spatially localized sp2 C clusters surrounded by a more insulating sp3 matrix. By a more complete understanding of the concentration and distribution of the clusters, a generic model for field emission from amorphous carbon thin films can be developed. Extensions of this model to explain the emission properties of carbon nanotubes and carbon nanocomposite materials are also presented.

Expanding high reflection range in a dielectric multilayer reflectorby disorder and inhomogeneity

A systematical study is presented on the impact of disorder andinhomogeneity on the high reflection range (HRR) in a one-dimensionalbinary dielectric multilayer reflector for various incident angles. It isfound that the introduction of disorder can expand the HRR for bothtransverse electric (TE) and transverse magnetic (TM) waves, irrespectiveof the incident angle. The HRR is found to become wider as the degree ofdisorder increases, until the effect becomes saturated. On the other hand,for the TE wave only, the introduction of inhomogeneity may broaden thewidth of the HRR regardless of the incident angle. While for the TM wave,the effect of inhomogeneity depends on the angle of incidence, and it canexpand the HRR only for the case of a small incident angle. The phenomenonis expected to find significant applications in the optimal design ofbroadband binary dielectric omnidirectional reflectors.

Dielectric resonances and B1 field inhomogeneity in UHFMRI: computational analysis and experimental findings

B1 Field inhomogeneity and the relative effects of dielectric resonances are analyzed within the context of ultra high field MRI. This is accomplished by calculating the electromagnetic fields inside spherical phantoms and within a human head model in the presence and absence of an RF coil. These calculations are then compared to gradient echo and RARE images, respectively. For the spherical phantoms, plane incident wave analyses are initially presented followed by full wave finite difference time domain (FDTD) calculations. The FDTD methods are then utilized to examine the electromagnetic interactions between the TEM resonator and an anatomically detailed human head model. The results at 340 MHz reveal that dielectric resonances are most strongly excited in objects similar in size to the human head when the conducting medium has a high dielectric constant and a low conductivity. It is concluded that in clinical UFHMRI, the most important determinants of B1 field homogeneity consist of 1) the RF coil design, 2) the interaction between the RF coil, the excitation source and the sample, and finally 3) the geometry and electrical properties of the sample.

Generation of high frequency gravitational waves

An electric charge shaken in an homogeneous stationary magnetic field produces both electromagnetic and gravitational waves (Gertsenshtein waves). We first study this process in a homogeneous medium with a refractive index n, expressing the gravitational wave as a simple functional of the electromagnetic field. Then, an inhomogeneous dielectric is considered: under certain conditions, the diffraction of a plane electromagnetic wave by a small dielectric sphere produces a promising gravitational energy flux.

Acoustic and electromagnetic wave interaction: analytical formulation for acousto-electromagnetic scattering behavior of a dielectric cylinder

An analytical solution for the bistatic electromagnetic (EM) scattering from an acoustically excited vibrating dielectric circular cylinder is presented. The incident acoustic wave causes a boundary deformation as well as a dielectric inhomogeneity within the dielectric cylinder. First, a perturbation method is developed to calculate the EM scattering from a slightly deformed and inhomogeneous dielectric cylinder. Then, assuming the vibration frequency is much smaller than the frequency of the incident EM wave, a closed form expression for the time-frequency response of the bistatic scattered field is obtained. The solution for acoustic scattering from an elastic cylinder is applied to give the displacement on the surface as well as the compression and dilation within the cylinder. Both the surface displacement and the variation in material density (dielectric constant fluctuation) within the cylinder contribute to the Doppler component of the EM scattered field. Results indicate that the Doppler frequencies correspond to the mechanical vibration frequencies of the cylinder and that the Doppler components only become sizeable near frequencies corresponding to the natural modes of free vibration in the cylinder. These resonances depend only on the object properties and are independent of the surrounding medium. Thus, utilizing the information in the Doppler spectrum scattered by an acoustically excited object vibrating at resonance could provide a means for buried object identification.

Microstrip Disk Antennas With Inhomogeneous Artificial Dielectrics

A novel kind of microstrip patch antenna is proposed in this paper. The antenna layout consists of a probe-fed circular patch integrated on a grounded slab of inhomogeneous artificial dielectric. A rigorous full wave analysis of this radiation component is here developed starting from the determination of new expressions for the spectral dyadic Green's function that take into account the inhomogeneity presence. The spectral Green's dyad is also obtained in the Hankel domain and a complete and accurate Method of Moments (MoM) is presented. Finally, several numerical results, obtained by means of a self produced code based on the theory here developed are presented in order to show the main capabilities of the novel antenna. Particularly, the comparison between the radiation pattern and the input impedance of such an antenna and those obtained for a similar one with a homogeneous substrate shows an improvement of both the directivity and bandwidth.

Synthesis of Inhomogeneous Dielectric, Dispersionless TEM Lenses for High-Power Application

A recent result showed that guided transverse electromagnetic (TEM) waves of a very general form could be propagated in an inhomogeneous isotropic dielectric medium with permittivity proportional to Psi-2, where Psi is the cylindrical radius in the usual (Psi,phi,z) cylindrical coordinate system. This has the important property that the permeability mu can be taken as mu0, a constant. Moreover, very general conductor cross sections in (z ,Psi)coordinates can be specified. This allows for the construction of dispersionless bends in high-voltage transmission systems in which the pulse rise time is small compared to transit times across the transmission-line (e.g., coax) cross section. This is important for high-power pulse sources and antennas. The analysis shows that while the permeability can be allowed to be variable, the choice of mu=mu0 allows one to specify purely dielectric lenses. Thus the generalized form of a TEM plane wave involving inhomogeneous constitutive parameters can be used for generating lens designs based on the scaling.

Influence of hydrogen on the volt-ampere and volt-farad characteristics of mis silicon tunnel diodes

The results of a joint analysis of volt-ampere and volt-farad characteristics of aPd−SiO x -n-Si structure with a thin (3.7 nm) oxide tunnel layer are presented. It is shown that the forward-bias region of the volt-ampere characteristic can be used to identify the dependence of the surface potential ϕδ inSi on the voltage with consideration of the dielectric layer inhomogeneity as a function of the dielectric thickness. The probability of tunneling through theSiO x layer is estimated for local regions with the least thickness equal to 1.3 nm. When the voltage increases from 0.1 to 0.7 V, the probability of tunneling decreases from 0.78 to 0.40, whereas the potential barrier height increases from 0.04 to 0.08 eV with consideration of the mirror image forces (given that the relativeSiO x permittivity is equal to 3.9). In going from the room atmosphere to a gas mixture comprising 50 vol.% of hydrogen, the positive-charge density in the dielectric increases by 8·10−7 C/cm2. Moreover, the voltage on flat bands and j s decrease for local regions of a specimen with thin dielectric layers much more weakly than for most of the field electrode.