arXiv subjects

Z. Ovadyahu

Publications and source records attributed to Z. Ovadyahu.

At least 19 recordsLinked to original sources

arXiv2024

Onset of nonequilibrium in a driven Anderson insulator

The onset of nonequilibrium in a driven Anderson-insulator is identified by monitoring the system with two-thermometers. Features of nonequilibrium appear at surprisingly weak drive intensity demonstrating, among other things, that conductivity may not be a reliable thermometer for ensuring linear-response conditions. In addition, the spectral contents of the applied field could be more important to take the system out of equilibrium than its absorbed power. Ensuing hot-electron transport effects and the nontrivial role phonons play in driven quantum systems are pointed out.

cond-mat.dis-nn↗

arXiv2022

On the relation between disorder and homogeneity in an amorphous metal

Disorder and homogeneity are two concepts that refer to spatial variation of the system potential. In condensed-matter systems disorder is typically divided into two types; those with local parameters varying from site to site (diagonal disorder) and those characterized by random transfer-integral values (off-diagonal disorder). Amorphous systems in particular exhibit off-diagonal disorder due to random positions of their constituents. In real systems diagonal and off-diagonal disorder may be interconnected. The formal depiction of disorder as local deviations from a common value focuses attention on the short-range components of the potential-landscape. However, long range potential fluctuation are quite common in real systems. In this work we seek to find a correlation between disorder and homogeneity using amorphous indium-oxide films with different carrier-concentrations and with different degree of disorder. Thermal treatment is used as a means of fine tuning the system disorder. In this process the resistance of the sample decreases while its amorphous structure and chemical composition is preserved. The reduced resistivity affects the Ioffe-Regel parameter that is taken as a relative measure of disorder in a given sample. The homogeneity of the system was monitored using inelastic light-scattering. This is based on collecting the Raman signal from micron-size spots across the sample. The statistics of these low-energy data are compared with the sample disorder independently estimated from transport measurements. The analysis establishes that heterogeneity and disorder are correlated.

cond-mat.dis-nn↗

arXiv2022

Interaction induced spatial correlations in a Disordered Glass

A consequence of the disorder and Coulomb interaction competition is the electron-glass phase observed in several Anderson-insulators. The disorder in these systems, typically degenerate semiconductors, is stronger than the interaction, more so the higher is the carrier-concentration N of the system. Here we report on a new feature observed in the electron-glass phase of In_{x}O with the lowest N yet studied. The feature, resolved as a broad peak in field-effect measurements, has not been recognized in previously studied Anderson-insulators. Several empirical facts associated with the phenomenon are consistent with the conjecture that it reflects a correlated charge-distribution. In particular, the feature may be turned on and off by gate-voltage maneuvering, suggesting the relevance of charge-arrangements. It may also be suppressed by either; temperature, non-ohmic field, or exposure to infrared illumination. After being washed-out, the feature reappears when the system is allowed to relax for sufficiently long time. A puzzling aspect that arises is the apparent absence of the phenomenon when the carrier-concentration increases above a certain value. This is reminiscent of the glass-transition conundrum except that the role of temperature in the latter is played by disorder. Analysis of these findings highlights several issues that challenge our understanding of the disorder-interaction interplay in Anderson insulators.

cond-mat.dis-nn↗

arXiv2021

Microstructure and the Boson-peak in thermally-treated In_{x}O films

We report on the correlation between the boson-peak and structural changes associated with thermally-treating amorphous indium-oxide films. In this process, the resistance of a given sample may decrease by a considerable margin while its amorphous structure is preserved. In the present study, we focus on the changes that result from the heat-treatment by employing electron-microscopy, X-ray, and Raman spectroscopy. These techniques were used on films with different stoichiometry and thus different carrier-concentration. The main effect of heat-treatment is material densification, which presumably results from elimination of micro-voids. The densified system presents better wavefunction-overlap and more efficient connectivity for the current flow. X-ray, and electron-beam diffraction experiments indicate that the heat-treated samples show significantly less spatial heterogeneity with only a moderate change of the radial-distribution function metrics. These results are consistent with the changes that occur in the boson-peak characteristics due to annealing as observed in their Raman spectra.

cond-mat.dis-nn↗

arXiv2021

Suppressing quantum effects by optically driven nonequilibrium phonons

Optically-generated nonequilibrium phonon-distribution is used for exploring the origin of a nonlocal adiabatic response in an interacting Anderson insulator. Exposing the system to weak infrared radiation is shown to effectively suppress a long-range effect observed in field-effect experiments while producing little heating and barely changing the system conductance. These effects are shown to be consistent with the quantum nature of the effect and therefore are peculiar to disordered systems that are quantum-coherent.

cond-mat.mes-hall↗

arXiv2020

Structural dynamics in thermal-treatment of amorphous indium-oxide films

Thermally-treating amorphous indium-oxide films is used in various basic studies as a means of tuning the system disorder. In this process the resistance of a given sample decreases while its amorphous structure and chemical composition is preserved. The main effect of the process is an increase in the system density which in turn leads to improved interatomic overlap which is easily detected as improved conductivity. A similar effect has been observed in studies of other amorphous systems that were subjected to pressure. In the current work we show that the Raman spectra of amorphous indium-oxide change in response to thermal-treatment in a similar way as in pressure experiments performed on other disordered and amorphous systems. We present a study of how thermal-treatment changes the system dynamics by monitoring the resistance versus time of indium-oxide films following various stages of thermal-treatment. The time dependence of the sample resistance fits the stretched exponential law with parameters that change systematically with further annealing. Implication of these results to slow dynamics phenomena that are governed by the Kohlrausch's law are discussed.

cond-mat.dis-nn↗

arXiv2020

Long-range influence of manipulating disordered-insulators locally

Localization of wavefunctions is arguably the most familiar effect of disorder in quantum systems. It has been recently argued [[V. Khemani, R. Nandkishore, and S. L. Sondhi, Nature Physics, 11, 560 (2015)] that, contrary to naive expectation, manipulation of a localized-site in the disordered medium may produce a disturbance over a length-scale much larger than the localization-length $ξ$. Here we report on the observation of this nonlocal phenomenon in electronic transport experiment. Being a wave property, visibility of this effect hinges upon quantum-coherence, and its spatial-scale may be ultimately limited by the phase-coherent length of the disordered insulator. Evidence for quantum coherence in the Anderson-insulating phase may be obtained from magneto-resistance measurements which however are useful mainly in thin-films. The technique used in this work offers an empirical method to measure this fundamental aspect of Anderson-insulators even in relatively thick samples.

cond-mat.dis-nn↗

arXiv2019

Screening the Coulomb interaction and thermalization of Anderson insulators

Long range interactions are relevant for a wide range of phenomena in physics where they often present a challenge to theory. In condensed matter, the interplay of Coulomb interaction and disorder remains largely an unsolved problem. In two dimensional films the long-range part of the Coulomb interaction may be screened by a nearby metallic overlay. This technique is employed in this work to present experimental evidence for its effectiveness in limiting the spatial range of the Coulomb interaction. We use this approach to study the effects of the long-range Coulomb interaction on the out-of-equilibrium dynamics of electron-glasses using amorphous indium-oxide films. The results demonstrate that electronic relaxation times, extending over thousands of seconds, do not hinge on the long-range Coulomb interaction nor on the presence of a real gap in the density of states. Rather, they emphasize the dominant role played by disorder in controlling the slow thermalization processes of Anderson insulators taken far from equilibrium.

cond-mat.dis-nn↗

arXiv2018

Transition to exponential relaxation in weakly-disordered electron-glasses

The out-of-equilibrium excess conductance of electron-glasses typically relaxes with a logarithmic time-dependence. Here it is shown that the log(t) relaxation of a weakly-disordered amorphous indium-oxide films crosses-over asymptotically to an exponential dependence. This allows assigning a well-defined relaxation-time t' for a given system-disorder (characterized by the Ioffe-Regel parameter). Near the metal-insulator transition, t' obeys the scaling relation with the same critical disorder where the zero-temperature conductivity of this system vanishes. The latter defines the position of the disorder-driven metal-to-insulator transition (MIT) which is a quantum-phase-transition. In this regard the electron-glass differs from classical-glasses such as the structural-glass and spin-glass. The ability to experimentally assign an unambiguous relaxation-time allows us to demonstrate the steep dependence of the electron-glass dynamics on carrier-concentration.

cond-mat.dis-nn↗

arXiv2018

Conductance relaxation in GeBiTe - slow thermalization in an open quantum system

This work describes the microstructure and transport properties of GeBiTe films with emphasis on their out-of-equilibrium behavior. Persistent-photoconductivity (PPC), previously studied in the phase-change compound GeSbTe is also quite prominent in this system. Much weaker PPC response is observed in the pure GeTe compound and when alloying GeTe with either In or Mn. Films made from these compounds share the same crystallographic structure, the same p-type conductivity, a similar compositional disorder extending over mesoscopic scales, and similar mosaic morphology. The enhanced PPC response exhibited by the Sb and Bi alloys may therefore be related to their common chemistry. PPC is observable in GeBiTe films at the entire range of sheet resistances studied in this work. The excess conductance produced by a brief exposure to infrared illumination decays with time as a stretched-exponential (Kohlrausch law). Intrinsic electron-glass effects on the other hand, are observable in thin films of GeBiTe only for samples that are strongly-localized just like it was noted with the seven electron-glasses previously studied. These include a memory-dip which is the defining attribute of the phenomenon. The memory-dip in GeBiTe is the widest among the germanium-telluride alloys studied to date consistent with the high carrier-concentration of this compound. The thermalization process exhibited in either, the PPC-state or in the electron-glass regime is sluggish but the temporal law of the relaxation from the out-of-equilibrium state is distinctly different. Coexistence of the two phenomena give rise to some non-trivial effects, in particular, the visibility of the memory-dip is enhanced in the PPC-state. The relation between this effect and the dependence of the memory-effect magnitude on the ratio between the interparticle-interaction and quench-disorder is discussed.

cond-mat.mtrl-sci↗

arXiv2017

Slow Dynamics of the Electron-Glasses; the Role of Disorder

We examine in this work the role of disorder in contributing to the sluggish relaxation observed in intrinsic electron-glasses. Our approach is guided by several empirical observations: First and foremost, Anderson localization is a pre-requisite for observing these nonequilibrium phenomena. Secondly, sluggish relaxation appears to favor Anderson-insulators with relatively large Fermi-energies (hence proportionally large disorder). These observations motivated us to consider a way to measure the underlying disorder in a realistic Anderson insulator. Optical study using a series of amorphous indium-oxide (In_{x}O) establish a simple connection between carrier-concentration and the disorder necessary to approach the metal-insulator transition from the insulating side. This is used to estimate the typical magnitude of the quenched potential-fluctuation in the electron-glass phase of this system. The implications of our findings on the slow dynamics of Anderson-insulators are discussed. In particular, the reason for the absence of a memory-dip and the accompanying electron-glass effects in lightly-doped semiconductors emerges as a natural consequence of their weak disorder.

cond-mat.dis-nn↗

arXiv2017

Memory vs. irreversibility in thermal densification of amorphous glasses

We report on dynamic effects associated with thermally-annealing amorphous indium-oxide films. In this process the resistance of a given sample may decrease by several orders of magnitude at room-temperatures, while its amorphous structure is preserved. The main effect of the process is densification - increased system density. The study includes the evolution of the system resistivity during and after the thermal-treatment, the changes in the conductance-noise, and accompanying changes in the optical properties. The sample resistance is used to monitor the system dynamics during the annealing period as well as the relaxation that ensues after its termination. These reveal slow processes that fit well a stretched-exponential law, a behavior that is commonly observed in structural glasses. There is an intriguing similarity between these effects and those obtained in high-pressure densification experiments. Both protocols exhibit the "slow spring-back" effect, a familiar response of memory-foams. A heuristic picture based on a modified Lennard-Jones potential for the effective interparticle interaction is argued to qualitatively account for these densification-rarefaction phenomena in amorphous materials whether affected by thermal-treatment or by application of high-pressure.

cond-mat.dis-nn↗

arXiv2017

Nonequilibrium transport and Electron-Glass effects in thin GexTe films

We report on results of nonequilibrium transport measurements made on thin films of germanium-telluride (Ge_xTe) at cryogenic temperatures. Owing to a rather large deviation from stoichiometry (app. 10% of Ge vacancies), these films exhibit p-type conductivity with carrier-concentration N>10^20cm^(-3) and can be made either in the diffusive or strongly-localized regime by a judicious choice of preparation and post-treatment conditions. In both regimes the system shows persistent photoconductivity following excitation by a brief exposure to infrared radiation. Persistent photoconductivity is also observed in GexTe samples alloyed with Mn. However, in both Ge_xTe and GeMn_xTe_y the effect is much weaker than that observable in GeSb_xTe_y alloys suggesting that antimony plays an important role in the phenomenon. Structural studies of these films reveal an unusual degree of texture that is rarely realized in strongly-disordered systems with high carrier-concentrations. Anderson-localized samples of Ge_xTe exhibit non-ergodic transport which are characteristic of intrinsic electron-glasses, including a well developed memory-dip and slow relaxation of the excess conductance created in the excited state. These results support the conjecture that electron-glass effects with inherently long relaxation times is a generic property of all Anderson-localized systems with large carrier-concentration.

cond-mat.dis-nn↗

arXiv2015

Infrared-Induced Sluggish Dynamics in the GeSbTe Electron Glass

The electron-glass dynamics of Anderson-localized GeSbTe films is dramatically slowed-down following a brief infrared illumination that increases the system carrier-concentration (and thus its conductance). These results demonstrate that the dynamics exhibited by electron-glasses is more sensitive to carrier-concentration than to disorder. In turn, this seems to imply that many-body effects such as the Orthogonality Catastrophe must play a role in the sluggish dynamics observed in the intrinsic electron-glasses.

cond-mat.dis-nn↗

arXiv2015

Coexistence of Electron-Glass Phase and Persistent Photoconductivity in GeSbTe Compounds

It is demonstrated that persistent-photoconductivity (PPC), well-studied in lightly-doped semiconductors, is observable in GeSbTe compounds using infrared excitation at cryogenic temperatures. The low level of energy-flux necessary to induce an appreciable effect seems surprising given the high carrier-concentration n of these ternary alloys. On the other hand, their high density of carriers makes GeSbTe films favorable candidates for exhibiting intrinsic electron-glass effects with long relaxation times. These are indeed observed in GeSbTe thin-films that are Anderson-localized. In particular, a memory-dip is observed in samples with sheet resistances larger than app. 100 kOhms at T=4K with similar characteristics as in other systems that exhibit intrinsic electron-glass effects. Persistent-photoconductivity however is observable in GeSbTe films even for sheet resistances of the order of 1 kOhm, well below the range of disorder required for observing electron-glass effects. These two non-equilibrium phenomena, PPC and electron-glass, are shown to be of different nature in terms of other aspects as well. In particular, their relaxation dynamics is qualitatively different; the excess conductance dG/G associated with PPC decays with time as a stretched exponential whereas a logarithmic relaxation law characterizes dG(t) of all electron-glasses studied to date. Surprisingly, the magnitude of the memory-dip is enhanced when the system is in the PPC state. This counter-intuitive result may be related to the compositional disorder in these materials extending over mesoscopic scales. Evidence in support of this scenario is presented and discussed.

cond-mat.str-el↗

arXiv2015

Thermalization Processes in Interacting Anderson Insulators

This paper describes experiments utilizing a unique property of electron-glasses to gain information on the fundamental nature of the interacting Anderson-localized phase. The methodology is based on measuring the energy absorbed by the electronic system from alternating electromagnetic fields as function of their frequency. Experiments on three-dimensional (3D) amorphous indium-oxide films suggest that, in the strongly localized regime, the energy spectrum is discrete and inelastic electron-electron events are strongly suppressed. These results imply that, at low temperatures, electron thermalization and finite conductivity depend on coupling to the phonon bath. The situation is different for samples nearing the metal-insulator transition; in insulating samples that are close to the mobility-edge, energy absorption persists to much higher frequencies. Comparing these results with previously studied 2D samples [Ovadyahu, Phys. Rev. Lett., 108, 156602 (2012)] demonstrates that the mean-level spacing (on a single-particle basis) is not the only relevant scale in this problem. The possibility of de-localization by many-body effects and the relevance of a nearby mobility-edge (which may be a many-body edge) are discussed.

cond-mat.str-el↗

arXiv2014

Electron Glass in a three-dimensional system

We report on non-equilibrium transport features observed in experiments using three-dimensional amorphous indium-oxide films. It is demonstrated that all the features that characterize intrinsic electron-glasses which heretofore were seen in two-dimensional samples are also observed in field-effect measurements of systems that exhibit three-dimensional variable-range-hopping. In particular, a memory-dip is observed in samples configured with gate. The memory-dip width and magnitude support models that associate the phenomenon with the Coulomb-gap. The memory-dip and the glassy effects disappear once the quenched disorder in the system is reduced and the system becomes diffusive. This happens when the Ioffe-Regel dimensional parameter k_{F}l exceeds 0.3 which is the critical value for the metal-to-insulator transition in all versions of the amorphous indium-oxides [Phys. Rev. B 86, 165101 (2012)]. This confirms that being in the Anderson localized phase is a pre-requisite for observing the memory-dip and the associated glassy effects. The results of the gating experiments suggest that the out-of equilibrium effect caused by inserted charge extend over spatial scales considerably larger than the screening length.

cond-mat.dis-nn↗

arXiv2013

Intrinsic electron-glass effects in strongly-localized thallium-oxide films

Transport measurements made on films of thallium-oxide (n-type semiconductor) are presented and discussed. The focus in this work is on the strongly-localized regime where charge transport is by variable-range-hopping. It is demonstrated that, at liquid-helium temperatures, these films exhibit all the characteristic features of intrinsic electron-glasses. These include a slow (logarithmic in time) conductance-relaxation that may be induced by any of the following protocols: Quench-cooling from high temperatures, sudden change of gate-voltage, exposure to infrared radiation, and stressing the system with a non-Ohmic field. The microstructure of the films are characterized by electron microscopy and their carrier-concentration are measured by Hall effect. Field-effect experiments reveal a memory-dip that has a width compatible with the carrier-concentration of the system as compared with previously studied electron-glasses. It is observed that the common ingredient in all the systems that exhibit electron-glass effects is high carrier-concentration suggesting that their localized sites may be multi-occupied even when deep into the insulating regime. That lightly-doped semiconductors do not show intrinsic electron-glass effects is consistent with this empirical observation. The connection between the memory-dip and the Coulomb-gap is discussed in light of these findings.

cond-mat.mes-hall↗