Electron Glass Research Articles

Frequency-dependent conductivity in glasses

The Barton-Nakajima-Namikawa relationship between the static dielectric constant epsilon (0), and the DC conductivity sigma (0), observed in many glasses, sigma (0) varies as omega c epsilon (0), with omega c the loss peak frequency, has been generally assumed to signify a connection between the low frequency AC and the DC conduction processes. The author argues that the connection is that both are due to non-local relaxation processes, the relaxation time of the DC process being effectively infinite. Above the loss peak, the relation sigma ( omega ) varies as omega s is compatible with the pair approximation, i.e. a local relaxation theory. Below the loss peak, the role of Coulomb interactions is critical. If the effects of Coulomb interactions may be neglected, or treated perturbatively, the relaxation time of a process spanning a linear dimension x is proportional to x2. But if Coulomb interactions dominate, the relaxation time may be proportional to x. Only the latter condition is compatible with the BNN relation, and is believed to be one of the most important distinctions between the ionic glasses and the Fermi (electronic) glass.

Mixed glass former effect in AgI-Ag 2O-V 2O 5-P 2O 5 quaternary amorphous solid electrolytes

In the process of investigating electrical conduction in ternary and quaternary amorphous solid electrolytes, interesting results have been obtained in the AgI-Ag 2O-V 2O 5-P 2O 5 quaternary system. The present investigation reports the mixed glass former effect on the ionic and electronic conductivities and glass transition temperatures of the electrolyte compositions mol% 66.67 AgI-22.22 Ag 2 O-11.11((1− x) V 2 O 5- xP 2 O 5). A double maxima was observed in the ionic conductivity at x = 0.2 and 0.7 where the electronic conductivity assumes minimum values. The glass transition temperature was observed to increase with x in analogy with amorphous semiconducting (1−x)V 2O 5-xP 2O 5- oxides. These results are discussed on the basis of the entropy of mixing two glass formers and the possible structural changes that might occur in the quaternary amorphous network due to the presence of a second glass former. The observed properties are compared to the ternary amorphous systems AgI-Ag 2O-V 2O 5 and AgI-Ag 2O-P 2O 5 emphasising the mixed glass former effect in light of their covalency and field strength.

Long-lived Coulomb gap in a compensated semiconductor-the electron glass.

We have studied the injection of excess charge into the lightly p-doped compensated GaAs semiconductor layer of a metal-insulator-semiconductor heterostructure. Below a few Kelvin, the length over which excess carriers screen an applied voltage step grows dramatically as the step size is decreased. This nonlinear screening reflects the reduced density of states near the Fermi level caused by electron-electron interactions (the Coulomb gap). The nonlinearities persist for minutes or longer, demonstrating the existence of an electron-glass regime.

A glow discharge unit to render electron microscope grids and other surfaces hydrophilic.

We describe the design, construction, and operation of a simple glow discharge unit that can be used to make surfaces such as carbon-coated electron microscopy grids and glass coverslips hydrophilic. The use of a vacuum leak detector (Tesla coil) in place of a conventional high-voltage power supply and a small plastic desiccator for the vacuum chamber make the unit very inexpensive. Owing to the small volume of the chamber and the simplicity of the unit, the whole glow discharge process can be carried out in only 2 to 3 min, a time considerably shorter than that required for conventional vacuum evaporators. The hydrophilic surface improves adsorption of particles by several orders of magnitude in preparation for negative staining, freeze-drying, and other procedures.

A new percolation theory for many-electron excitations at low temperatures

A new percolation theory for n-electron hopping in the low-temperature (large-n) limit is presented. Applied to the DC conductivity sigma the theory yields sigma varies as TP. While p can vary from system to system, it is typically large, of the order of 20. In another application the glass temperature, Te, of the electron glass is calculated. Here Te is the minimum temperature for which the ergodic hypothesis may be satisfied within reasonable experimental times. For the specific case of impurity conduction systems in germanium (concentration 1016 cm-3, compensation 1/2) Te=0.6 K. A related calculation of the entropy yields a low-temperature behaviour S varies as T2, and shows that the time evolution of the entropy is smooth.

Critical behaviour in electron glass

Abstract In strongly localized systems one expects non-ergodic behaviour below a ‘glass temperature’, T e. In this paper it is shown that for systems both with and without Coulomb interactions the critical behaviour of the dielectric susceptibility, X, is the same, in particular X∝(T e–T)−2·4. The critical behaviour of the entropy, S, is also the same in both systems ΔS S−S c∝T−T e. The low-temperature behaviour of the specific heat c is also discussed, with c∝T 2 in the interacting system and c∝T in the non-interacting system.

Ultra-slow processes in disordered insulators

We consider some consequences of very slow transition rates in Anderson localized disordered systems. Primary focus is on the glassy state of the electronic system. The problem is considered without, and with Coulomb interactions. It is found that the entropy changes smoothly through the glass temperature, while the poarizability changes sharply. Some possible applications of the electron glass are discussed.

Topological disorder and Bi-level states in the 2-D electron glass

We have investigated the topological disorder and the inherent structure of an electron glass on a corrugated surface by the molecular dynamics method. Dislocations and grain-boundary loops are found to be the dominant defects in the system. Direct evidence for two-level states is obtained in the electron glass and the transitions between them are found to be due to co-operative positional changes in a small number of electrons.

Very slow relaxation in systems lacking translational symmetry, with emphasis on disordered insulators

Abstract The paper addresses the problem of very slow relaxations of electrons observed in a large variety of materials. Three aspects of the slow relaxations are explicitly considered: a.c. conductivity, dispersive transport, and the electron glass. Interactions have previously been proposed as the primary cause for the very slow relaxation process. It is argued here that while interactions are important in making slow rates even slower, the primary cause for the very slow relaxations is localization, and thus lack of translational symmetry. Specifically, it is shown that the appropriate a.c. conductivity, dispersive transport, as well as the electron glass can exist without interactions. The time evolution of the entropy and of the polarizability are calculated for non-interacting electrons in a disordered insulator. It is found that a temperature exists where the polarizability changes dramatically, but the entropy varies smoothly. The effects of interactions are discussed on a qualitative level.

Phase transitions in a disordered extended Hubbard model and in the random field Blume-Capel model

The phase transitions in a half-filled random-site-energy extended Hubbard model (RSEHM) and in the random field Blume-Capel model (RFBCM) are studied with the mean-field approximation and with the renormalization group approach. The two-delta, the Gaussian and the square distribution of the random site energy are considered. The phase diagram includes the high charge order, the low charge order, the electron glass and the Mott phase. Depending on the form of distribution function, one may find lines of tricritical points, isolated critical points and triple points, as well as the heat charge order phenomenon. These results are then transformed in order to study the RFBCM, which exhibits a similar phase diagram but without heat magnetization. The effect of fluctuations on the mean-field phase diagram is briefly discussed using the renormalization group method. We conjecture that the tricritical point in a system with random field is different from that in a pure system.

Effects of temperature, and single and double dopings on the electromechanical properties of quaternary glasses

Pure, single doped (Cr 2O 3) and double doped (Cr 2O 3/MnO 2) quaternary (alkali alumino—borosilicate) glasses have been prepared. Several measurements of the temperature dependence of the DC electrical conductivity and the corresponding activation energies for conduction were evaluated in each case. Furthermore, the Vickers hardness numbers were examined as a function of glass composition and extent of doping. The results obtained were discussed and correlated in detail. Finally, a correlation between the electrical and mechanical properties of the test glasses was deduced, and the optimum composition was recommended for the electronic glass industry.

Microscopic Structure of Two-Dimensional Electron Glass

Structural properties of a classical two-dimensional system of electrons on a corrugated surface are investigated by the molecular dynamics method. Rapid quenching of the electron liquid leads to glass formation. Direct evidence for two-level states is obtained in the electron glass and the transitions between them are found to arise from displacements of a small number of spatially localized electrons.

Electronic Structure and Glass Formability in Metallic Glasses

ABSTRACTRecent developments in theory and experiment seem to enable us to relate the electronic structure to the formability of glassy states in transition metal-metalloid base alloys. Especially stressed is the role of the antibonding states in the glass formation.

Effects of temperature, titanium content and laser irradiation on the electrosonic properties of some related silicate glasses

In the present communication, pure and various titania-containing silicate glasses have been synthesized and identified. Extensive studies of the temperature dependence of the DC electrical conductivity, activation energy for conduction, ultrasonic attenuation, and attenuation coefficient as a function of temperature of these specimens were examined and discussed in detail. Promising results were obtained and the role of titanium and laser irradiation were clearly defined. Finally, a correlation for the electrical log σ, Δ E and ultrasonic (α) properties has been proposed for the first time for the electronic glass industry.

Insulating phase of a disordered system: Fermi glass versus electron glass

The authors examine the critical divergence of the low-frequency conductivity of the noninteracting Fermi glass and interacting electron glass models of the insulating phase of a disordered system as the metallic phase is approached. Results for the two are found to be rather different, which can be tested experimentally. In particular, for the electron glass, there exists a nonvanishing contribution to the dielectric constants from the low-frequency (hopping) conductivity even at low temperatures, which scales with the high-frequency (optical) contribution, and diverges with the same exponent at the insulator-metal transition.

Properties of the electron glass

We have carried out numerical calculations on two- and three-dimensional models of highly localized electrons interacting by an unscreened Coulomb potential. A Coulomb gap was found in the density of states for bare single-particle excitations. This "soft" gap was deeper than the power-law gap seen in earlier simulations, but for three-dimensional systems it was fitted well by an exponential form proposed by Efros. The gap fills as the temperature is raised. We also found an unexpected clustering of states with the same occupation and energies close to the chemical potential. The density of states for dressed single-particle excitations ("electronic polarons") showed a Coulomb gap too, but a narrower one than that for the bare excitations. There is a clear analogy between this model of interacting electrons and an Ising spin-glass with $\frac{1}{r}$ antiferromagnetic interactions and a random field on each site. We have exploited this analogy to search for a glass transition in the electronic system by calculating the specific heat, susceptibility, and a modified Edwards-Anderson order parameter.

Input and display registration in a stereoscopic workstation

Several techniques are described to obtain spatial registration between three-dimensional input devices and a stereoscopic video display. A video monitor is viewed through a half-silvered mirror, with left and right eye views time-multiplexed through video interlace and viewed with electronic shutter glasses. A magnetic six-degree-of-freedom digitizer and an ordinary magnetostrictive graphics tablet were used as input devices.

Mean field approach to the electron glass: The Coulomb gap

We investigate the static electronic properties of a disordered system of localised electrons interacting via long-ranged Coulomb forces. Using a meanfield (MF) approach a soft Coulomb gap around the Fermi level is found for the single particle density of states in the thermodynymic limit for T=0 K. We suggest a novel definition of an order parameter q AP which overcomes shortcomings of the Edwards-Anderson order parameter q EA for the system at hand with site dependent quenched disorder. q AP considers the relative importance of disorder and interaction properly.

Critical Scaling of the Conductance in a Disordered Insulator

A critical scaling of the real and imaginary parts of the low-frequency ac conductance of insulating phosphorus-doped silicon near the metal-insulator transition has been observed. The results are interpreted as evidence of an electron glass, i.e., glasslike behavior, intimately connected with the scaling description of the transition, in which Coulomb interactions play a significant role.

Dielectric relaxation and modulus of V2O5—TeO2glasses

Abstract The dielectric behaviour of V2O5—TeO2 glasses in the composition range 10 to 80 mol.% V2O5 is reported for the temperature region 77 to 500 K and the frequency region 0.1 to 100 kHz. The variation of dielectric constant έ’ with temperature and frequency in the region where the measured a.c. conductivity approaches the d.c. conductivity indicates a bulk dielectric relaxation. The data have been analysed in terms of conventional dipolar dielectric relaxation with a distribution of dielectric relaxation times and in terms of conductivity relaxation using dielectric modulus M* = 1/e* plots. The d.c. conductivity, dielectric relaxation and conductivity relaxation have the same activation energy of 0.21–0.42 eV for decreasing V2O5 content and a frequency factor v 0 of 5 × 1010 Hz. The difference between this value and the 1012 Hz expected for phonon frequencies is attributed to a tunnelling contribution. The dielectric modulus approach is satisfactory for electronic glasses with respect to the calculat...