arXiv subjects

Z. Ovadyahu

Publications and source records attributed to Z. Ovadyahu.

At least 37 records · Page 2Linked to original sources

arXiv2012

Compositional disorder and tranport peculiarities in the amorphous indium-oxides

(abridged) We present results of the disorder-induced metal-insulator-transition (MIT) in three-dimensional amorphous indium-oxide films. The amorphous version studied here differs from the one reported earlier [PRB 46, 10917 (1992)] in that it has a much lower carrier concentration. As a measure of the static disorder we use the dimensionless parameter kFl. Thermal annealing is employed as the experimental handle to tune the disorder. On the metallic side of the transition, the low temperature transport exhibits weak-localization and electron-electron correlation effects characteristic of disordered electronic systems. The MIT occurs at a kFl~0.3 for both versions of the amorphous material. However, in contrast with the results obtained on the electron-rich version of this system, no sign of superconductivity is seen down to ~0.3K even for the most metallic sample used in the current study. This demonstrates that using kFl as a disorder parameter for the superconductor-insulator-transition (SIT) is an ill defined procedure. A microstructural study of the films, employing high resolution chemical analysis, gives evidence for spatial fluctuations of the stoichiometry. This brings to light that, while the films are amorphous and show excellent uniformity in transport measurements of macroscopic samples, they contain compositional fluctuations that extend over mesoscopic scales. It is argued that this compositional disorder may be the reason for the apparent violation of the Ioffe-Regel criterion in the two versions of the amorphous indium-oxide. However, more dramatic effects due to this disorder are expected when superconductivity sets in, which are in fact consistent with the prominent transport anomalies observed in the electron-rich version of indium-oxide. The relevance of compositional disorder to other systems near their SIT is discussed.

cond-mat.mtrl-sci↗

arXiv2012

Suppression of inelastic electron-electron scattering in Anderson Insulators

We report on measurements of absorption from applied ac fields in Anderson-localized indium-oxide films. The absorption shows a roll-off at a frequency that is much smaller than the electron-electron scattering rate measured at the same temperature in diffusive samples of this material. These results are interpreted as evidence for discreteness of the energy spectrum.

cond-mat.str-el↗

arXiv2011

Microwave-Enhanced hopping-conductivity; a non-Ohmic Effect

Hopping conductivity is enhanced when exposed to microwave fields (Phys. Rev. Lett., 102, 206601, 2009). Data taken on a variety of Anderson-localized systems are presented to illustrate the generality of the phenomenon. Specific features of these results lead us to conjecture that the effect is due to a field-enhanced hopping, which is the high frequency version of the non-Ohmic effect, well known in the dc transport regime. Experimental evidence in support of this scenario is presented and discussed. It is pointed out that existing models for non-Ohmic behavior in the hopping regime may, at best, offer a qualitative explanation to experiments. In particular, they cannot account for the extremely low values of the threshold fields that mark the onset of non-Ohmic behavior.

cond-mat.dis-nn↗

arXiv2011

Optical excitation of Electron-Glasses

Electron-glasses can be readily driven far from equilibrium by a variety of means. Several mechanisms to excite the system and their relative merits are reviewed. In this study we focus on the process of exciting electron-glasses by interaction with near infrared radiation. The efficiency of this protocol varies considerably among different electron-glasses, but it only weakly depends on their resistance at liquid helium temperatures. A dramatic enhancement of the excitation efficiency is observed upon doping crystalline indium-oxide with Au. Some enhancement is observed also in samples doped with Pb but this enhancement fades away with time unlike the situation in the Au-doped samples. Several structural and analytical tools are used to characterize the changes in the materials that may be responsible for these effects. Possible routes by which high-frequency electromagnetic fields take the system far from equilibrium are discussed.

cond-mat.str-el↗

arXiv2010

Intrinsic electron-glassiness in strongly-localized Be films

We present results of out--of-equilibrium transport measurements made on strongly-localized Beryllium films and demonstrate that these films exhibit all the earmarks of intrinsic electron-glasses. These include slow (logarithmic) relaxation, memory effects, and more importantly, the observation of a memory dip that has a characteristic width compatible with the carrier-concentration of beryllium. The latter is an empirical signature of the electron-glass. Comparing various non-equilibrium attributes of the beryllium films with other systems that exhibit intrinsic electron-glasses behavior reveals that high carrier-concentration is their only common feature rather than the specifics of the disorder that rendered them insulating. It is suggested that this should be taken as an important hint for any theory that attempts to account for the surprisingly slow relaxation times observed in these systems.

cond-mat.str-el↗

arXiv2009

Conductance relaxation in the Electron-Glass; Microwaves versus infra-red response

We study the time-dependent conductance of electron-glasses excited by electromagnetic radiation at microwaves (MW) and infra-red frequencies. In either case the conductance G is enhanced during exposure but its time dependence after the radiation is turned off is qualitatively different depending on the frequency. For comparison, results of excitation produced by a gate-voltage and temperature changes are also shown. The glassy nature of the system allows us to demonstrate that the MW-enhanced conductance is not due to heating. These findings are discussed in terms of an energy E_{c} that characterizes the equilibrium charge distribution of the electron-glass.

cond-mat.dis-nn↗

arXiv2008

Slow conductance relaxations; Distinguishing the Electron Glass from extrinsic mechanisms

Slow conductance relaxations are observable in a many condensed matter systems. These are sometimes described as manifestations of a glassy phase. The underlying mechanisms responsible for the slow dynamics are often due to structural changes which modify the potential landscape experienced by the charge-carriers and thus are reflected in the conductance. Sluggish conductance dynamics may however originate from the interplay between electron-electron interactions and quenched disorder. Examples for both scenarios and the experimental features that should help to distinguish between them are shown and discussed. In particular, it is suggested that the `memory-dip' observable through field-effect measurements is a characteristic signature of the inherent electron-glass provided it obeys certain conditions.

cond-mat.dis-nn↗

arXiv2007

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 \textit{slows down }with increasing temperature or it is \textit{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.

cond-mat.str-el↗

arXiv2007

Scale dependent superconductor-insulator transition

We study the disorder driven superconductor to insulator transition in amorphous films of high carrier-concentration indium-oxide. Using thin films with various sizes and aspect ratios we show that the `critical' sheet-resistance $R_{\small \square}$ depends systematically on sample geometry; superconductivity disappears when $R_{\small \square}$ exceeds $\approx6 $k$Ω$ in large samples. On the other hand, wide and sufficiently short samples of the same batch exhibit superconductivity (judged by conductivity versus temperature) up to $R_{\small \square}$ which is considerably larger. These results support the inhomogeneous scenario for the superconductor-insulator transition.

cond-mat.dis-nn↗

arXiv2007

The Electron-Glass in samples approaching the Mesoscopic regime

We study the dependence of the glassy properties of strongly localized indium-oxide films on the sample lateral dimensions. Characteristic mesoscopic effects such as reproducible conductance fluctuations (CF) are readily observable in gated structures for sample size smaller than 100 microns measured at 4K, and the relative amplitude of the CF decreases with the sample volume as does the flicker noise. By contrast, down to sample size of few microns, the non-equilibrium 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 microns. 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.

cond-mat.dis-nn↗

arXiv2006

A quench-cooling procedure compared with the gate-protocol for aging experiments in the Electron-Glass

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 tw, 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 TL for a duration tw after it was quench-cooled from high temperature TH. Like 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

cond-mat.dis-nn↗

arXiv2006

Temperature and field dependence of Dynamics in the Electron-Glass

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.

cond-mat.dis-nn↗

arXiv2006

Non-Gaussian conductance noise in disordered electronic systems due to a non-linear mechanism

We present results of conductance-noise experiments on disordered films of crystalline indium oxide with lateral dimensions 2microns to 1mm. The power-spectrum of the noise has the usual 1/f form, and its magnitude increases with inverse sample-volume down to sample size of 2microns, a behavior consistent with un-correlated fluctuators. A colored second spectrum is only occasionally encountered (in samples smaller than 40microns), and the lack of systematic dependence of non-Gaussianity on sample parameters persisted down to the smallest samples studied (2microns). Moreover, it turns out that the degree of non-Gaussianity exhibits a non-trivial dependence on the bias V used in the measurements; it initially increases with V then, when the bias is deeper into the non-linear transport regime it decreases with V. We describe a model that reproduces the main observed features and argue that such a behavior arises from a non-linear effect inherent to electronic transport in a hopping system and should be observed whether or not the system is glassy.

cond-mat.dis-nn↗

arXiv2005

Conductance noise in interacting Anderson insulators driven far from equilibrium

The combination of strong disorder and many-body interactions in Anderson insulators lead to a variety of intriguing non-equilibrium transport phenomena. These include slow relaxation and a variety of memory effects characteristic of glasses. Here we show that when such systems are driven with sufficiently high current, and in liquid helium bath, a peculiar type of conductance noise can be observed. This noise appears in the conductance versus time traces as downward-going spikes. The characteristic features of the spikes (such as typical width) and the threshold current at which they appear are controlled by the sample parameters. We show that this phenomenon is peculiar to hopping transport and does not exist in the diffusive regime. Observation of conductance spikes hinges also on the sample being in direct contact with the normal phase of liquid helium; when this is not the case, the noise exhibits the usual 1/f characteristics independent of the current drive. A model based on the percolative nature of hopping conductance explains why the onset of the effect is controlled by current density. It also predicts the dependence on disorder as confirmed by our experiments. To account for the role of the bath, the hopping transport model is augmented by a heuristic assumption involving nucleation of cavities in the liquid helium in which the sample is immersed. The suggested scenario is analogous to the way high-energy particles are detected in a Glaser's bubble chamber.

cond-mat.dis-nn↗

arXiv2003

Decoherence in Disordered Conductors at Low Temperatures, the effect of Soft Local Excitations

The conduction electrons' dephasing rate, $τ_ϕ^{-1}$, is expected to vanish with the temperature. A very intriguing apparent saturation of this dephasing rate in several systems was recently reported at very low temperatures. The suggestion that this represents dephasing by zero-point fluctuations has generated both theoretical and experimental controversies. We start by proving that the dephasing rate must vanish at the $T\to 0$ limit, unless a large ground state degeneracy exists. This thermodynamic proof includes most systems of relevance and it is valid for any determination of $τ_ϕ$ from {\em linear} transport measurements. In fact, our experiments demonstrate unequivocally that indeed when strictly linear transport is used, the apparent low-temperature saturation of $τ_ϕ$ is eliminated. However, the conditions to be in the linear transport regime are more strict than hitherto expected. Another novel result of the experiments is that introducing heavy nonmagnetic impurities (gold) in our samples produces, even in linear transport, a shoulder in the dephasing rate at very low temperatures. We then show theoretically that low-lying local defects may produce a relatively large dephasing rate at low temperatures. However, as expected, this rate in fact vanishes when $T \to 0$, in agreement with our experimental observations.

cond-mat.mes-hall↗

arXiv2003

Stress-aging in the electron-glass

A new protocol for an aging experiment is studied in the electron-glass phase of indium-oxide films. In this protocol, the sample is exposed to a non-ohmic electric field F for a waiting time t_{w} during which the system attempts to reach a steady state (rather than relax towards equilibrium). The relaxation of the excess conductance dG after ohmic conditions are restored exhibit simple aging as long as F is not too large.

cond-mat.dis-nn↗

arXiv2003

History-dependent relaxation and the energy scale of correlation in the Electron-Glass

We present an experimental study of the energy-relaxation in Anderson-insulating indium-oxide films excited far from equilibrium. In particular, we focus on the effects of history on the relaxation of the excess conductance dG. The natural relaxation law of dG is logarithmic, namely dG=-log(t). This may be observed over more than five decades following, for example, cool-quenching the sample from high temperatures. On the other hand, when the system is excited from a state S_{o} in which it has not fully reached equilibrium to a state S_{n}, the ensuing relaxation law is logarithmic only over time t shorter than the time t_{w} it spent in S_{o}. For times t>t_{w} dG(t) show systematic deviation from the logarithmic dependence. It was previously shown that when the energy imparted to the system in the excitation process is small, this leads to dG=P(t/t_{w}) (simple-aging). Here we test the conjecture that `simple-aging' is related to a symmetry in the relaxation dynamics in S_{o} and S_{n}. This is done by using a new experimental procedure that is more sensitive to deviations in the relaxation dynamics. It is shown that simple-aging may still be obeyed (albeit with a modified P(t/t_{w})) even when the symmetry of relaxation in S_{o} and S_{n} is perturbed by a certain degree. The implications of these findings to the question of aging, and the energy scale associated with correlations are discussed.

cond-mat↗

arXiv2002

Impairing the memory of an electron-glass by IR excitation

We study the influence of various excitations on the anomalous field effect observed in insulating indium-oxide films. In conductance G versus gate-voltage Vg measurements one observes a characteristic cusp around the Vg at which the system has equilibrated. In the absence of any disturbance this cusp may persist for a long time after a new gate voltage was imposed on the sample and hence reflects a memory of the previous equilibrium state. This memory is believed to be related to the correlations between electrons. Here we show that exciting the conduction electrons by exposing the sample to IR light degrades this memory. We argue that any excitation that randomizes the system destroys the correlations and therefore impairs the memory.

cond-mat.str-el↗