Showing papers in "Journal of Applied Physics in 1993"
TL;DR: A set of 61 ion polarizabilities has been derived from the dielectric constants of 129 oxides and 25 fluorides using a least squares refinement technique in conjunction with the Clausius-Mosotti equation as mentioned in this paper.
Abstract: A set of 61 ion polarizabilities has been derived from the dielectric constants of 129 oxides and 25 fluorides using a least squares refinement technique in conjunction with the Clausius–Mosotti equation. These polarizabilities can be used to estimate mean dielectric constants of ‘‘well‐behaved’’ compounds. They should be particularly useful in calculation of mean dielectric constants of compounds whose dielectric constants have not been determined. They can also be used as a framework for distinguishing unusual dielectric behavior from normal dielectric behavior where deviations can frequently be attributed to piezo‐ or ferroelectricity, conductivity (ionic or electronic), ‘‘rattling’’ or ‘‘compressed’’ cations with correspondingly high or low polarizabilities, or the presence of dipolar impurities. Deviations observed from calculated dielectric constants can be used to search for unusual physical behavior.
2,146 citations
TL;DR: The literature on quantum-well infrared photodetectors (QWIPs) is reviewed in this paper, where a detailed discussion is given on the device physics of the intersubband absorption and hot-carrier transport processes for individual detectors, as well as the high performance which has been achieved for large staring arrays.
Abstract: The extensive literature on quantum‐well infrared photodetectors (QWIPs) is reviewed. A detailed discussion is given on the device physics of the intersubband absorption and hot‐carrier transport processes for individual detectors, as well as the high performance which has been achieved for large staring arrays. QWIPs having widely different structures, materials, and spectral responses are covered, as is the optimization of the quantum‐well parameters for maximum performance.
1,471 citations
TL;DR: In this article, two mechanisms triggered by electron heating in the oxide conduction band are discussed: trap creation and band gap ionization by carriers with energies exceeding 2 and 9 eV, respectively.
Abstract: Degradation of silicon dioxide films is shown to occur primarily near interfaces with contacting metals or semiconductors. This deterioration is shown to be accountable through two mechanisms triggered by electron heating in the oxide conduction band. These mechanisms are trap creation and band‐gap ionization by carriers with energies exceeding 2 and 9 eV with respect to the bottom of the oxide conduction band, respectively. The relationship of band‐gap ionization to defect production and subsequent degradation is emphasized. The dependence of the generated sites on electric field, oxide thickness, temperature, voltage polarity, and processing for each mechanism is discussed. A procedure for separating and studying these two generation modes is also discussed. A unified model from simple kinetic relationships is developed and compared to the experimental results. Destructive breakdown of the oxide is shown to be correlated with ‘‘effective’’ interface softening due to the total defect generation caused by both mechanisms.
682 citations
TL;DR: In this paper, solutions for the differential equation governing the evolution of back stresses are presented for several representative cases, and the solutions are discussed in the light of experimental as well as theoretical developments from the literature.
Abstract: Electromigration is an important concern in very large scale integrated circuits. In narrow, confined metal interconnects used at the chip level, the electromigration flux is resisted by the evolution of mechanical stresses in the interconnects. Solutions for the differential equation governing the evolution of back stresses are presented for several representative cases, and the solutions are discussed in the light of experimental as well as theoretical developments from the literature.
661 citations
TL;DR: In this paper, a new model for the formation of heterojunctions in polycrystalline CuInSe2 thin films on the basis of surface analysis experiments is presented, and the existence of an In-rich n-type surface layer on samples relevant for solarcell devices is shown.
Abstract: A new model for the formation of heterojunctions in polycrystalline CuInSe2 thin films on the basis of surface analysis experiments is presented. In situ photoemission measurements of CuInSe2 clearly show the existence of an In‐rich n‐type surface layer on samples relevant for solar‐cell devices. Furthermore, this layer has been identified as an ordered vacancy compound (OVC) with a band gap of about 1.3 eV. The previous model of the CuInSe2/CdS solar cell with a p‐n heterojunction between p‐type CuInSe2 and n‐type CdS is replaced by the model of a chalcopyrite/defect chalcopyrite heterojunction between p‐type bulk CuInSe2 and the In‐rich n‐type OVC. The existence of this junction was proven directly by evaporating an ohmic metal contact onto the surface n‐type layer and measuring the spectral quantum efficiency and electron‐beam‐induced current of this device. The band offsets of CuInSe2‐based devices have been determined.
586 citations
TL;DR: In this article, a revised nomenclature for defects in MOS devices was developed, which clearly distinguishes the language used to describe the physical location of defects from that used to describing their electrical response.
Abstract: We have identified several features of the 1/f noise and radiation response of metal‐oxide‐semiconductor (MOS) devices that are difficult to explain with standard defect models. To address this issue, and in response to ambiguities in the literature, we have developed a revised nomenclature for defects in MOS devices that clearly distinguishes the language used to describe the physical location of defects from that used to describe their electrical response. In this nomenclature, ‘‘oxide traps’’ are simply defects in the SiO2 layer of the MOS structure, and ‘‘interface traps’’ are defects at the Si/SiO2 interface. Nothing is presumed about how either type of defect communicates with the underlying Si. Electrically, ‘‘fixed states’’ are defined as trap levels that do not communicate with the Si on the time scale of the measurements, but ‘‘switching states’’ can exchange charge with the Si. Fixed states presumably are oxide traps in most types of measurements, but switching states can either be interface tr...
444 citations
TL;DR: In this paper, the nucleation and growth of isolated nickel disilicide precipitates in amorphous Si thin films and the subsequent low-temperature silicide-mediated crystallization of Si was studied using in situ transmission electron microscopy.
Abstract: The nucleation and growth of isolated nickel disilicide precipitates in Ni‐implanted amorphous Si thin films and the subsequent low‐temperature silicide‐mediated crystallization of Si was studied using in situ transmission electron microscopy. Analysis of the spatial distribution of the NiSi2 precipitates strongly suggested the occurrence of site saturation during nucleation. NiSi2 precipitates were observed in situ to migrate through the amorphous Si thin films leaving a trail of crystalline Si at temperatures as low as ∼484 °C. Initially, a thin region of epitaxial Si formed on {111} faces of the octahedral NiSi2 precipitates with a coherent interface which was shown by high‐resolution electron microscopy to be Type A. Migration of the NiSi2 precipitates led to the growth of needles of Si which were parallel to 〈111〉 directions. The growth rate of the crystalline Si was limited by diffusion through the NiSi2 precipitates, and an effective diffusivity was determined at 507 and 660 °C. A mechanism for the enhanced growth rate of crystalline Si is proposed.
422 citations
TL;DR: In this article, an equation of phonon radiative transfer (EPRT) was developed which shows the correct limiting behavior for both purely ballistic and diffusive transport, and the solution of the EPRT for diamond thin films not only produces wall temperature jumps under ballistic transport but shows markedly different transient response from that of the Fourier law and the hyperbolic heat equation.
Abstract: Ballistic and diffusive phonon transport under small time and spatial scales are important in fast‐switching electronic devices and pulsed‐laser processing of materials. The Fourier law represents only diffusive transport and yields an infinite speed for heat waves. Although the hyperbolic heat equation involves a finite heat wave speed, it cannot model ballistic phonon transport in short spatial scales, which under steady state follows the Casimir limit of phonon radiation. An equation of phonon radiative transfer (EPRT) is developed which shows the correct limiting behavior for both purely ballistic and diffusive transport. The solution of the EPRT for diamond thin films not only produces wall temperature jumps under ballistic transport but shows markedly different transient response from that of the Fourier law and the hyperbolic heat equation even for predominantly diffusive transport. For sudden temperature rise at one film boundary, the results show that the Fourier law and the hyperbolic heat equat...
392 citations
TL;DR: In this article, an analysis was carried out of optical heterodyne conversion with an interdigitated-electrode photomixer made from low-temperature-grown (LTG) GaAs and pumped by two continuous-wave, frequency offset pump lasers.
Abstract: An analysis has been carried out of optical heterodyne conversion with an interdigitated‐electrode photomixer made from low‐temperature‐grown (LTG) GaAs and pumped by two continuous‐wave, frequency‐offset pump lasers. The analytic prediction is in excellent agreement with the experimental results obtained recently on a photomixer having 1.0‐μm‐wide electrodes and gaps. The analysis predicts that a superior photomixer having 0.2‐μm‐wide electrodes and gaps would have a temperature‐limited conversion efficiency of 2.0% at a low difference frequency, 1.6% at 94 GHz, and 0.5% at 300 GHz when connected to a broadband 100 Ω load resistance and pumped at hν=2.0 eV by a total optical power of 50 mW. The predicted 3‐dB bandwidth (193 GHz) of this photomixer is limited by both the electron‐hole recombination time (0.6 ps) of the LTG‐GaAs material and the RC time constant (0.5 ps) of the photomixer circuit.
391 citations
TL;DR: In this paper, the effects of strain energy on the preferred orientation of TiN thin films were investigated, and it is considered that this phenomenon is due to the increase of strain energies with its thickness.
Abstract: The effects of strain energy on the preferred orientation of TiN thin films were investigated. In the TiN film deposited by plasma‐enhanced chemical‐vapor deposition with a power of 50 W, the overall energy of the film mainly depended on the surface energy because its strain energy was relatively small. The preferred orientation of the film corresponded to the plane with the lowest surface energy, i.e., (200). However, in the TiN film deposited by rf sputtering with a power of 200 W, the overall energy of the film was largely controlled by strain energy due to its large strain energy, and its growth orientation corresponded to the plane with the lowest strain energy, i.e., (111). Furthermore, the preferred orientation of the TiN film was changed from (200) to (111) with the film thickness. It is considered that this phenomenon is due to the increase of strain energy with its thickness.
377 citations
TL;DR: In this paper, a combination of in situ reflection high-energy electron diffraction, double-crystal x-ray diffraction and cross-sectional transmission electron microscopy was used to determine the film/substrate epitaxial relationships.
Abstract: Reactive‐ion molecular‐beam epitaxy has been used to grow epitaxial hexagonal‐structure α‐GaN on Al2O3(0001) and Al2O3(0112) substrates and metastable zinc‐blende‐structure β‐GaN on MgO(001) under the following conditions: growth temperature Ts=450–800 °C; incident N+2/Ga flux ratio JN+2/JGa=1–5; and N+2 kinetic energy EN+2=35–90 eV The surface structure of the α‐GaN films was (1×1), with an ≊3% contraction in the in‐plane lattice constant for films grown on Al2O3(0001), while the β‐GaN films exhibited a 90°‐rotated two‐domain (4×1) reconstruction Using a combination of in situ reflection high‐energy electron diffraction, double‐crystal x‐ray diffraction, and cross‐sectional transmission electron microscopy, the film/substrate epitaxial relationships were determined to be: (0001)GaN∥ (0001)Al2O3 with [2110]GaN∥[1100]Al2O3 and [1100]GaN∥[1210]Al2O3, (2110)GaN∥(0112)Al2O3 with [0001]GaN∥[0111]Al2O3 and [0110]GaN∥[2110]Al2O3, and (001)GaN∥(001)MgO with [001]GaN∥[001]MgOFilms with the lowest e
TL;DR: In this article, a quantitative model which describes the aging of the material properties in acceptor doped ferroelectric ceramics as a result of the reorientation of defects in time is proposed.
Abstract: A quantitative model which describes the aging of the material properties in acceptor doped ferroelectric ceramics as a result of the reorientation of defects in time is proposed. By this reorientation the domain walls are clamped by a time dependent force leading to the observed decrease of the material properties during aging. The model is experimentally confirmed with aging and deaging measurements on barium titanate and lead zirconate titanate ceramics.
TL;DR: In this article, the dielectric behavior and structure of the Sr(Zn1/3Nb2/3)O3•Ba (Zn 1/3 Nb 2/3 )O3 solid solution have been investigated with the intention of understanding the relationship between the structural changes and the temperature coefficient of the relative permittivity τe.
Abstract: The dielectric behavior and structure of the Sr(Zn1/3Nb2/3)O3‐Ba(Zn1/3Nb2/3)O3 solid solution have been investigated with the intention of understanding the relationship between the structural changes and the temperature coefficient of the relative permittivity τe. A correlation between the value of τe and the occurrence of O‐octahedra tilts has been established. The occurrence of ferroelastic domains and their influence on τe has also been investigated. It is proposed that the results obtained can be used to generalize about the structure‐property relationships in compounds of the same class.
TL;DR: In this article, the results of an ensemble Monte Carlo simulation of the electron transport in gallium nitride (GaN) were presented, which showed that intervalley electron transfer plays a dominant role in GaN in high electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance.
Abstract: The results of an ensemble Monte Carlo simulation of the electron transport in gallium nitride (GaN) are presented. The calculation shows that intervalley electron transfer plays a dominant role in GaN in high electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. An analytic expression for the polar optical momentum relaxation time for phonon energies larger than the thermal energy is also derived. This expression applies to many wide‐gap semiconductors, such as GaN and SiC, at room temperature since these semiconductors have large polar optical‐phonon energies (on the order of 100 meV). The calculated mobility agrees well with the results of the Monte Carlo calculation.
TL;DR: In this article, an analytic expression for the dielectric constant of a binary piezoelectric 0-3 composite by modifying the well known Kerner expression [Proc Phys Soc London Sec B 69, 802 (1956)] to include interactions was presented.
Abstract: In this article an analytic expression is presented for the dielectric constant of a binary piezoelectric 0‐3 composite by modifying the well known Kerner expression [Proc Phys Soc London Sec B 69, 802 (1956)] to include interactions The developed expression is a function of the constituent dielectric constants and their volume fractions only and compares very favorably with available experimental data for piezoelectric ceramic inclusions in a dielectric continuum having widely differing dielectric constants
TL;DR: In this article, a one-dimensional fluid simulation of a 13.56 MHz argon glow discharge including metastable species was performed as an example of a coupled glow discharge/neutral transport reaction system.
Abstract: A one‐dimensional fluid simulation of a 13.56 MHz argon glow discharge including metastable species was performed as an example of a coupled glow‐discharge/neutral‐transport‐reaction system. Due to the slow response time of metastables (∼10 ms) direct time integration of the coupled system requires ∼105 rf cycles to converge. This translates to prohibitively long computation time. An ‘‘acceleration’’ scheme was employed using the Newton–Raphson method to speed up convergence, thereby reducing the computation time by orders of magnitude. For a pressure of 1 Torr, metastables were found to play a major role in the discharge despite the fact that their mole fraction was less than 10−5. In particular, metastable (two‐step) ionization was the main mechanism for electron production to sustain the discharge. Bulk electric field and electron energy were lower, and a smaller fraction of power was dissipated in the bulk plasma when compared to the case without metastables. These results suggest that neutral transport and reaction must be considered in a self‐consistent manner in glow discharge simulations, even in noble gas discharges.
TL;DR: In this paper, the dependence of the speed of two elastic waves on the applied electric field is measured yielding one electroelastic constant g13133=(95±10)NV−1 m−1.
Abstract: The elastic, piezoelectric, dielectric, elasto‐optic, and electro‐optic tensors have been determined by numerically evaluating the measurements published until now and using the additional measurements presented in this work. The dependence of the speed of two elastic waves on the applied electric field is measured yielding one electroelastic constant g13133=(95±10)NV−1 m−1. The complete set of parameters consisting of the low‐frequency clamped dielectric constants eSij, refractive indices, elastic stiffness constants at constant electric field CEijkl, piezoelectric coefficients eijk, elasto‐optic tensor at constant electric field pEijkl, and clamped electro‐optic coefficients rSijk is used to calculate effective electro‐optic coefficients and effective dielectric constants that have to be used in photorefractive experiments where the elastic deformations associated with a periodic space‐charge field have to be considered.
TL;DR: In this article, the authors derived a general expression for the lattice strain which corresponds to the x-ray diffraction measurement on the polycrystalline specimen (cubic system) compressed nonhydrostatically in an opposed anvil device.
Abstract: A general expression has been derived using anisotropic elasticity theory for the lattice strain which corresponds to the x‐ray diffraction measurement on the polycrystalline specimen (cubic system) compressed nonhydrostatically in an opposed anvil device. The expressions for the various diffraction geometries emerge as the special cases of this equation. The strain calculated using isotropic elasticity theory corresponds to the macroscopic strain in the specimen, and can be obtained from the present equation by letting the anisotropy factor 2(S11−S12)/S44=1. Further, it is shown that the ratio of the lattice strain to the macroscopic strain (in the direction of the lattice strain) produced by the deviatoric stress component depends on the Miller indices (hkl) of the lattice planes and the elastic anisotropy factor. This ratio is unity only if the crystallites constituting the specimen are elastically isotropic, and increases with increasing anisotropy of the crystallites.
TL;DR: In this article, the equations describing static and dynamic fluxon solutions are derived in a system of thin alternating layers of superconductors and insulators, represented by a useful compact matrix form, which is intended to describe systems fabricated for example of niobium or Niobium-nitride thin films; in the limit of ultrathin superconductor films it may give a model for describing fluxon motion in layered high-Tc supercondors.
Abstract: In a system of thin alternating layers of superconductors and insulators the equations describing static and dynamic fluxon solutions are derived. The approach, represented by a useful compact matrix form, is intended to describe systems fabricated for example of niobium or niobium‐nitride thin films; in the limit of ultrathin superconductor films it may give a model for describing fluxon motion in layered high‐Tc superconductors. Numerical examples of current versus voltage curves to be expected in such an experiment are presented.
TL;DR: In this article, a variable mass, cylindrical plasma model (VMCPM) is developed for sparks created by electrical discharge in a liquid media, which consists of three differential equations: fluid dynamics, an energy balance, and the radiation equation combined with a plasma equation of state.
Abstract: A variable mass, cylindrical plasma model (VMCPM) is developed for sparks created by electrical discharge in a liquid media. The model consist of three differential equations—one each from fluid dynamics, an energy balance, and the radiation equation—combined with a plasma equation of state. A thermophysical property subroutine allows realistic estimation of plasma enthalpy, mass density, and particle fractions by inclusion of the heats of dissociation and ionization for a plasma created from deionized water. Problems with the zero‐time boundary conditions are overcome by an electron balance procedure. Numerical solution of the model provides plasma radius, temperature, pressure, and mass as a function of pulse time for fixed current, electrode gap, and power fraction remaining in the plasma. Moderately high temperatures (≳5000 K) and pressures (≳4 bar) persist in the sparks even after long pulse times (to ∼500 μs). Quantitative proof that superheating is the dominant mechanism for electrical discharge ma...
TL;DR: In this paper, the electron energy distribution function (EEDF) was measured in a 13.56MHz capacitive argon rf discharge at two gas pressures: p=0.03 Torr and Druyvesteyn-like at 0.3 Torr.
Abstract: Various probe diagnostic methods have been applied to rf plasmas with non‐Maxwellian electron energy distribution functions (EEDF) and the results of these diagnostic methods have been compared. Plasma density and electron temperature were obtained using standard procedures from the electron retardation region (classic Langmuir method), the ion saturation region, and the electron saturation region of the measured probe I/V characteristic. Measurements were made in a 13.56‐MHz capacitive argon rf discharge at two gas pressures: p=0.03 Torr, where stochastic electron heating is dominant, and p=0.3 Torr, where collisional electron heating dominates. Thus, the measured EEDF at each gas pressure manifests a distinct departure from thermodynamic equilibrium being bi‐Maxwellian at 0.03 Torr and Druyvesteyn‐like at 0.3 Torr. Considerable differences in electron density and temperature were obtained from the different parts of the probe characteristic and these values differ dramatically in many cases from those f...
TL;DR: In this paper, it is shown that the GMR is due to magnetic scattering of the conduction electrons by the nonaligned magnetic entities, and the essential contribution to the resistivity is ρm[1−F(M/Ms), where F(M)/Ms is the spin disorder from ferromagnetic alignment and ρ is the magnetic resistivity that defines the size of the giant negative magnetoresistance.
Abstract: Giant negative magnetoresistance (GMR) has been observed in a number of granular ferromagnetic systems [Co–Ag, Co–Cu, Fe–Cu, Fe–Ag, and (Fe–Ni)–Ag] with effect sizes as much as 85% at 5 K and 25% at 300 K. It is shown that the GMR is isotropic and is due to magnetic scattering of the conduction electrons by the nonaligned magnetic entities. The essential contribution to the resistivity is ρm[1−F(M/Ms)], where F(M/Ms) measures the spin disorder from ferromagnetic alignment and ρm is the magnetic resistivity that defines the size of the GMR. The magnitude of GMR is affected by the size and density of the magnetic entities which can be controlled by varying the composition and the process conditions. When the composition is varied, the maximum GMR is realized in systems with magnetic constituents of about 25%.
TL;DR: In this paper, the transport properties in magnetic granular films are modeled by considering the spin-dependent impurity scattering within the granules and the interface roughness scattering at the boundaries of granules.
Abstract: The transport properties in magnetic granular films are modeled by considering the spin‐dependent impurity scattering within the granules and the interface roughness scattering at the boundaries of the granules The magnetoresistance for these films is derived by using the formalism developed for layered structures with currents perpendicular to the plane of the layers and which is applicable to random systems With this model, various features of the magnetoresistance observed in recent experiments can be explained and the optimal choice of parameters to maximize the magnetoresistance can be determined
TL;DR: In this article, the field-induced adiabatic magnetic entropy change of a series of iron-substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 was investigated.
Abstract: The working refrigerant material in the majority of magnetic refrigerators has been Gd3Ga5O12 (GGG) which has an upper temperature limit near 15 K. In this paper we report on the field‐induced adiabatic magnetic entropy change, ΔSm(H,T), of a series of iron‐substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 which have the potential to increase the working temperature range or to reduce the field requirements of cryogenic magnetic refrigeration. Depending on Fe concentration, x, the entropy change of these materials at applied fields of 0.9 and 5.0 T is much greater than that of GGG, especially at temperatures above 15 K. At low Fe concentrations, the results are consistent with formation of magnetically ordered clusters of spins at low temperatures. Room temperature electron paramagnetic resonance measurements show that Fe3+ ions mediate exchange interactions which are responsible for clustering at low temperatures.
TL;DR: The behavior of linear arrays of cells composed of quantum dots is examined in this paper, where the electrons in the cell tend to align along one of two axes resulting in a cell 'polarization' which can be used to encode binary information.
Abstract: The behavior of linear arrays of cells composed of quantum dots is examined. Each cell holds two electrons and interacts Coulombically with neighboring cells. The electrons in the cell tend to align along one of two axes resulting in a cell ‘‘polarization’’ which can be used to encode binary information. The ground‐state polarization of a cell is a highly nonlinear function of the polarization of its neighbors. The resulting bistable saturation can be used to transmit binary information along the line of cells, thus forming a binary wire.
TL;DR: In this paper, an elastic-plastic finite element method numerical model has been formulated to study residual stresses developed at graded ceramic-metal interfaces during cooling, and the results were compared with those obtained for sharp (nongraded) interfaces to assess the potential for achieving residual stress reductions.
Abstract: An elastic‐plastic finite element method numerical model has been formulated to study residual stresses developed at graded ceramic‐metal interfaces during cooling. The results were compared with those obtained for sharp (nongraded) interfaces to assess the potential for achieving residual stress reductions. Analyses were conducted for various axisymmetric cylindrical specimen geometries relevant to structural joining, coating, and thick film applications. The graded microstructure was treated as a series of perfectly bonded layers, each having slightly different properties. Constitutive relations for the interlayers were estimated using a modified rule‐of‐mixtures approximation, and strain and stress distributions were calculated for simulated cooling from an assumed fabrication temperature. The results demonstrate the importance of accounting for plasticity when comparing graded and nongraded interfaces. Significant geometrical effects on peak stresses were observed in the graded materials. It is shown ...
TL;DR: In this paper, a detailed theoretical analysis of the static deflection of cantilever plates is presented, which is of fundamental importance in application to the atomic force microscope (AFM).
Abstract: The analysis of the static deflection of cantilever plates is of fundamental importance in application to the atomic force microscope (AFM). In this paper we present a detailed theoretical study of the deflection of such cantilevers. This shall incorporate the presentation of approximate analytical methods applicable in the analysis of arbitrary cantilevers, and a discussion of their limitations and accuracies. Furthermore, we present results of a detailed finite element analysis for a current AFM cantilever, which will be of value to the users of the AFM.
TL;DR: In this article, a zero-field spontaneous relaxor-ferroelectric transition is reported in Pb(Sc 0.5Ta0.5)O3 (PST), with and without defects.
Abstract: A zero-field spontaneous relaxor-ferroelectric transition is reported in Pb(Sc0.5Ta0.5)O3 (PST). This behavior is different from that of other relaxors, where such transitions occur only under the field. A highly disordered PST that has the wide relaxation spectrum typical of relaxors is shown to transform spontaneously into a macroscopic ferroelectric state. Introduction of defects (lead vacancies) into the material impedes the transition resulting in the usual relaxor behavior. Dielectric properties of PST, with and without defects, are analyzed. For the interpretation of the observed properties, a model invoking an additional nonpolar phase is proposed. This model does not imply a freezing in the system. At the low-frequency limit, it is possible to account for the Vogel-Fulcher (VF) law for the temperature of the maximum of the dielectric constant, using only the commonly accepted assumption of an exponentially wide relaxation time spectrum that shrinks on heating. The presented approach interprets the observed proximity between the ferroelectric phase transition temperature and that of the freezing temperature obtained from the VF relation.
TL;DR: In this paper, the electronic structure of coaxial, graphene double-layer tubules is predicted for various combinations of metallic and insulating constituent inner and outer monolayers, depending on the diameter and chirality of the tubule.
Abstract: The electronic structure of coaxial, graphene double‐layer tubules is predicted for various combinations of metallic and insulating constituent inner and outer monolayers, depending on the diameter and chirality of the tubule. For the examples chosen, some of the energy bands of the inner and outer tubules are coupled to each other by commensurate interlayer interactions. Nevertheless, because of symmetry, the energy bands of metallic monolayer tubules remain metallic even after interlayer interactions are considered. The possible implications of these results on molecular metal‐insulator devices are discussed.
TL;DR: In this article, the dynamics of photoexcitation of defect centers in germanium-doped silica glass optical fibers and fiber preforms were analyzed by examining the photoluminescence and absorption spectra, and two models were proposed to explain the observed behavior.
Abstract: We present data concerning the dynamics of photoexcitation of defect centers in germanium‐doped silica glass optical fibers and fiber preforms. It is shown that two‐photon absorption of mode‐locked and Q‐switched light at 527 nm results in partial ionization of the germanium oxygen deficiency center (GODC) 400‐nm luminescence band. This bleaching induces loss in the 320–600‐nm wavelength range, which we argue is due to the introduction of Ge(1) centers and charges trapped at structural defects in the glass. The excitation and relaxation of dynamics of the GODC are analyzed by examining the photoluminescence and absorption spectra, and two models are proposed to explain the observed behavior.