Electron Glass Research Articles

Anomalous electric‐field effect and glassy behaviour in granular aluminium: electron glass?

AbstractWe briefly review our study of non‐equilibrium phenomena observed in the electrical conductance of insulating granular aluminium thin films at low temperature. In MOS like structures, with granular aluminium channels, an anomalous field effect and its slow relaxation have been studied in some detail, the phenomenology of which is very similar to the one also observed in indium oxide. The interpretation of the experimental findings is discussed. To date, in granular systems, the present experiments can qualitatively be interpreted along two different lines. One (the “extrinsic” scenario) relies on a slow polarisation in the dielectric surrounding the metallic islands. The other one is a purely electronic mechanism: the formation of an electron Coulomb glass in the granular met al. Some experimental facts seem difficult to reconcile with the extrinsic scenario, but more selective experiments and/or quantitative theoretical predictions are still needed to definitely distinguish between the two scenarii. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preface: phys. stat. sol. (c) 5/3

AbstractThe following papers were presented at the 12th International Conference on Transport in Interacting and Disordered Systems (TIDS12) held 6–10 August 2007 in Marburg, Germany.It followed previous meetings in Egmond aan Zee (2005), Trieste (2003), Shefayim (2001), Murcia (1999), Rackeve (1997), Jerusalem (1995), Glasgow (1993), Marburg (1991), Chapel Hill (1989), Bratislava (1987), and Trieste (1985). The conference was organized by the Faculty of Physics, Philipps‐University Marburg. It was co‐sponsored by the Deutsche Forschungsgemeinschaft, by the International Research Training Group “Electron–Electron Interactions in Solids” (Marburg/Budapest), by Isabellenhuette Heusler GmbH KG, Dillenburg, by Marburger Universitätsbund e.V., and by Ursula Kuhlmann‐Fond.About 70 scientists from 17 countries attended the meeting. The program counted 12 invited talks, 30 oral and 30 poster contributions. They covered traditional topics for TIDS meetings. With respect to semiconductor inorganic materials, most emphasis was put on many‐particle interactions, electron glass, slow relaxations, metal–insulator transition, magnetoresistance, quantum Hall effect, and noise. In many talks transport processes in organic materials and polymers were considered, which has become a tradition for TIDS meetings. Also various aspects of transport in biological systems, such as membrane channels, DNA molecules, and bacteria were discussed in several talks.Not every talk and poster given at the TIDS12 is present with a paper in these Proceedings. However, the reader should be able to come up with a general picture of the TIDS12. Furthermore, the proceedings illustrate that rather similar physical ideas are applicable to charge transport properties of chemically different materials. It is this generality of the fundamental transport mechanisms in disordered interacting systems that allows researchers from different fields to understand each other and to discuss at the TIDS meetings various aspects of transport processes in chemically very different materials: inorganic, organic and biological systems. Examples for the ideas invoked in the conference and related to these issues can be found among the papers of the proceedings.The Guest Editor is highly indebted to Dr. Oleg Rubel and Dr. Kakhaber Jandieri, who contributed decisively to editing the TIDS12 Proceedings.November 2007

Relaxation Dynamics in Quantum Electron Glasses

It is experimentally shown that, depending on the carrier concentration of the system n, the dynamics of electron glasses either slows down with increasing temperature or it is independent of it. This also correlates with the dependence of a typical relaxation time (or "viscosity") on n. These linked features are argued to be consistent with a model for dissipative tunneling. The slow relaxation of the electron glass may emerge then as a manifestation of friction in a many-body quantum system. Our considerations may also explain why strongly localized granular metals are likely to show electron-glass effects while semiconductors are not.

Electron glass in samples approaching the mesoscopic regime

We study the dependence of the glassy properties of strongly localized ${\mathrm{In}}_{2}{\mathrm{O}}_{3\ensuremath{-}x}$ films on the sample lateral dimensions. Characteristic mesoscopic effects such as reproducible conductance fluctuations (CFs) are readily observable in gated structures for sample size smaller than $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ measured at $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the relative amplitude of the CF decreases with the sample volume as does the flicker noise. By contrast, down to sample size of a few microns, the nonequilibrium features that are attributed to the electron glass are indistinguishable from those observed in macroscopic samples, and in particular, the relaxation dynamics is independent of sample size down to $2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. In addition, the usual features that characterize the electron glass including slow relaxation, memory effects, and full-aging behavior are all observed in the ``mesoscopic'' regime, and they appear to be independent of the conductance fluctuations.

Anomalous electric-field effect and glassy behaviour in granular aluminium thin films: electron glass?

We present a study of non-equilibrium phenomena observed in the electrical conductance of insulating granular aluminium thin films. An anomalous field effect and its slow relaxation are studied in some detail. The phenomenology is very similar to the one already observed in indium oxide. The origin of the phenomena is discussed. In granular systems, the present experiments can naturally be interpreted along two different lines. One relies on a slow polarisation in the dielectric surrounding the metallic islands. The other one relies on a purely electronic mechanism: the formation of an electron Coulomb glass in the granular metal. More selective experiments and/or quantitative predictions about the Coulomb glass properties are still needed to definitely distinguish between the two scenarios.

An Intrinsic Bond-Centered Electronic Glass with Unidirectional Domains in Underdoped Cuprates

Removing electrons from the CuO2 plane of cuprates alters the electronic correlations sufficiently to produce high-temperature superconductivity. Associated with these changes are spectral-weight transfers from the high-energy states of the insulator to low energies. In theory, these should be detectable as an imbalance between the tunneling rate for electron injection and extraction-a tunneling asymmetry. We introduce atomic-resolution tunneling-asymmetry imaging, finding virtually identical phenomena in two lightly hole-doped cuprates: Ca(1.88)Na(0.12)CuO(2)Cl2 and Bi2Sr2Dy(0.2)Ca(0.8)Cu2O(8+delta). Intense spatial variations in tunneling asymmetry occur primarily at the planar oxygen sites; their spatial arrangement forms a Cu-O-Cu bond-centered electronic pattern without long-range order but with 4a(0)-wide unidirectional electronic domains dispersed throughout (a(0): the Cu-O-Cu distance). The emerging picture is then of a partial hole localization within an intrinsic electronic glass evolving, at higher hole densities, into complete delocalization and highest-temperature superconductivity.

Electronic bonding transition in compressedSiO2glass

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near $K$-edge spectra of ${\mathrm{SiO}}_{2}$ glass to $51\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ are consistent with those of quartz and coesite, whereas the spectra above $\ensuremath{\sim}22\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in ${\mathrm{SiO}}_{2}$ glass between $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $22\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed ${\mathrm{SiO}}_{2}$ glass.

Influence of A-site disorder on the half-doped manganites

The electronic transport and magnetism in half-doped Nd0.50Ca0.25Sr0.25MnO3 manganites have been investigated. Contrary to general half-doped system, it only displays a paramagnetic-ferromagnetic phase transition associated with an insulator-metal transition instead of with any features of charge ordering. With the decrease of temperature, an electronic phase separation and spin glass state occur in low temperature. We suggest that the A-site cation disorder induced by the size mismatch between Sr2+ ion and Ca2+ ion is mainly responsible for this phenomenon.

Axiomatic theories of ideal stretched exponential relaxation (SER)

Minimalist theories of complex systems are broadly of two kinds: mean-field (for example, hydrodynamic mode-coupling, random nucleation, lattice, or percolative fractal) and axiomatic. So far only axiomatic theories of properties absent from simple systems and intrinsic to complex systems, such as SER, have been quantitatively successful. SER is the prototypical complex temporal property of glasses, discovered by Kohlrausch 150 years ago, and now observed almost universally in microscopically homogeneous, complex non-equilibrium materials (strong and fragile network and fragile molecular glasses, polymers and copolymers, even electronic glasses). The Scher-Lax trap model (1973) is paradigmatic for electronic SER; for molecular SER Phillips (3IDMRCS 1995) identified two ‘magic’ shape fractions β=3/5 and 3/7, as confirmed by many later experiments here reviewed. In the dielectric SER frequency domain involving ion conduction, there are also special beta values for fused salts and glasses, slightly, but distinguishably, different because of the presence of a forcing electric field.

Temperature- and field-dependence of dynamics in electron glasses

We describe several experimental methods to quantify dynamics in electron glasses and illustrate their use in the glassy phase of crystalline indium-oxide films. These methods are applied to study the dependence of dynamics on temperature and on non-ohmic electric fields at liquid helium temperatures. It is shown that over a certain range of temperature the dynamics becomes slower with temperature or upon increasing an applied non-ohmic field, a behavior suggestive of a quantum-glass. It is demonstrated that non-ohmic fields produce qualitatively similar results as raising the system temperature. Quantitatively however, their effect may differ marekdly. The experimental advantages of using fields to mimic higher temperature are pointed out and illustrated.

Quench-cooling procedure compared with the gate protocol for aging experiments in electron glasses

Anderson-insulating indium-oxide films excited far from equilibrium exhibit a variety of memory effects including aging. Full-aging has been recently demonstrated in this system using two different experimental protocols. The first, (gate-protocol) employed a MOSFET structure and involved switching between two gate voltages. In a different procedure, the system was subjected to a non-ohmic longitudinal field $F$ for a waiting-time ${t}_{w}$, and the relaxation of $G$ was monitored after the field was switched back to its linear response value. In this paper we describe yet another protocol that involves measuring the response of the system that has been ``aged'' at some low temperature ${T}_{L}$ for a duration ${t}_{w}$ after it was quench-cooled from high temperature ${T}_{H}$. As in the previous protocols, this procedure results in full-aging behavior. The advantages and shortcomings of the quench-cooling protocol are pointed out. The results of aging experiments based on the better-controlled, gate-protocol performed with different systems are compared and discussed.

Slow dynamics in glasses: A comparison between theory and experiment

Minimalist theories of complex systems are broadly of two kinds: mean-field and axiomatic. So far all theories of complex properties absent from simple systems and intrinsic to glasses are axiomatic. Stretched Exponential Relaxation (SER) is the prototypical complex temporal property of glasses, discovered by Kohlrausch 150 years ago, and now observed almost universally in microscopically homogeneous, complex non-equilibrium materials, including luminescent electronic (Coulomb) glasses. Critical comparison of alternative axiomatic theories with both numerical simulations and experiments strongly favors dynamical trap models over static percolative or energy landscape models. PACS: 61.20.Lc; 67.40.Fd

A model for the electron glass

AbstractA model for the glassy behavior of an electron glass is described and analyzed. The model is applied to conduction experiments where the electrons are excited by a change of the gate voltage. The predictions of the model are compatible with results of such experiments. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

1/f noise near the “metal‐to‐insulator transition” in the 2DEG in a Si‐MOSFET

AbstractContrary to some recent observations of a gradual increase of 1/f ‐noise with decreasing carrier density in a 2DEG in Si and a 2DHG in GaAs, we have observed a non‐monotonic variation of noise power in a 2DEG in Si. We discuss the role of the following effects in the noise near the carrier density where ∂R /∂T changes its sign: i) formation of an electron glass; ii) percolation transition; iii) existence of a narrow impurity band at the Si‐SiO2 interface. We also analyse the effect of temperature fluctuations in the observed non‐monotonic behaviour of the noise power. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comment on “The electron glass in a switchable mirror: relaxation, ageing anduniversality”

A simple explanation for the logarithmic ageing of the photoconductivity inYH3−δ isproposed. We show that the scaling (‘simple’ ageing) of the relaxation response with the illuminationtime tw is consistent with the superposition of independently relaxingexcitations with time offsets distributed over a window of widthtw.

Waste E-glass particles used in cementitious mixtures

The properties of concretes containing various waste E-glass particle contents were investigated in this study. Waste E-glass particles were obtained from electronic grade glass yarn scrap by grinding to small particle size. The size distribution of cylindrical glass particle was from 38 to 300 μm and about 40% of E-glass particle was less than 150 μm. The E-glass mainly consists of SiO 2, Al 2O 3, CaO and MgO, and is indicated as amorphous by X-ray diffraction (XRD) technique. Compressive strength and resistance of sulfate attack and chloride ion penetration were significantly improved by utilizing proper amount of waste E-glass in concrete. The compressive strength of specimen with 40 wt.% E-glass content was 17%, 27% and 43% higher than that of control specimen at age of 28, 91 and 365 days, respectively. E-glass can be used in concrete as cementitious material as well as inert filler, which depending upon the particle size, and the dividing size appears to be 75 μm. The workability decreased as the glass content increased due to reduction of fineness modulus, and the addition of high-range water reducers was needed to obtain a uniform mix. Little difference was observed in ASR testing results between control and E-glass specimens. Based on the properties of hardened concrete, optimum E-glass content was found to be 40–50 wt.%.

Monte Carlo method for relaxation in electron glasses

We have developed a kinetic Monte Carlo algorithm to simulate nonequilibrium relaxation of Coulomb glasses at low temperatures. It is much faster than present procedures and can qualitatively change our scopes in the simulation of slowly relaxing systems. The algorithm chooses a transition according to its total probability, including the spatial and energy factors involved in the transition rates of hopping processes. It incorporates two important advantages. First, it can find all the relevant transitions restricting the search to only a small subset. Second, it can integrate out exactly soft, low energy transitions and other repetitive hops characteristic of metastable states. This is a general procedure that can be extended to most slow relaxation systems. We have applied the method to study energy relaxation and the distribution of the relaxation times up to ${10}^{10}$ Monte Carlo steps. We found that this distribution is roughly constant, in logarithmic scale, for long times.

History dependence, memory and metastability in electron glasses

We discuss the history dependence and memory effects which are observed in the out-of-equilibrium conductivity of electron glasses. The experiments can be understood by assuming that the local density of states retains a memory of the sample history. We provide analytical arguments for the consistency of this assumption, and discuss the saturation of the memory effect with increasing gate voltage change. This picture is bolstered by numerical simulations at zero temperature, which moreover demonstrate the incompressibility of the Coulomb glass on short timescales.

Memory effect in electron glasses: Theoretical analysis via a percolation approach

We present a theory for the memory effect in electron glasses. In fast gate voltage sweeps it is manifested as a dip in the conductivity around the equilibration gate voltage. We show that this feature, also known as anomalous field effect, arises from the long-time persistence of correlations in the electronic configuration. We argue that the gate voltage at which the memory dip saturates is related to an instability caused by the injection of a critical number of excess carriers. This saturation threshold naturally increases with temperature. On the other hand, we argue that the gate voltage beyond which memory is erased, is temperature independent. Using standard percolation arguments, we calculate the anomalous field effect as a function of gate voltage, temperature, carrier density and disorder. Our results are consistent with experiments, and in particular, they reproduce the observed scaling of the width of the memory dip with various parameters.

Nonohmic quasi-2D hopping conductance and the kinetics of its relaxation

The nonohmic properties of a quasi-2D hopping-conductance channel are studied. The channel is formed in a p-Si layer by a field effect in the region where the Fermi level crosses the impurity band. It is shown that the dependence of conductance σ on a longitudinal electric field E has a threshold character and obeys the law lnσ(E) ∝ E1/2. The dependences of the conductance of the quasi-2D channel on temperature and the electric field are satisfactorily explained using the concepts of nonlinear screening and of nonohmic properties of disordered systems with a random Coulomb potential. This mechanism of nonlinearity is confirmed by specific features of the mesoscopic fluctuations in the off-diagonal component of resistance, which reflect the reconstruction of a percolation cluster under the action of the longitudinal electric field. A long-term relaxation in conductance is observed during transition from a nonohmic to ohmic mode, which indicates that the system exhibits properties corresponding to electronic glass and that the current paths are significantly modified in a strong electric field.