58 research outputs found
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
Electric Field Effect in Ultrathin Films near the Superconductor-Insulator Transition
The effect of an electric field on the conductance of ultrathin films of
metals deposited on substrates coated with a thin layer of amorphous Ge was
investigated. A contribution to the conductance modulation symmetric with
respect to the polarity of the applied electric field was found in regimes in
which there was no sign of glassy behavior. For films with thicknesses that put
them on the insulating side of the superconductor-insulator transition, the
conductance increased with electric field, whereas for films that were becoming
superconducting it decreased. Application of magnetic fields to the latter,
which reduce the transition temperature and ultimately quench
superconductivity, changed the sign of the reponse of the conductance to
electric field back to that found for insulators. We propose that this
symmetric response to capacitive charging is a consequence of changes in the
conductance of the a-Ge layer, and is not a fundamental property of the physics
of the superconductor-insulator transition as previously suggested.Comment: 4 pages text, 4 figure
Anomalous Hopping Exponents of Ultrathin Films of Metals
The temperature dependence of the resistance R(T) of ultrathin
quench-condensed films of Ag, Bi, Pb and Pd has been investigated. In the most
resistive films, R(T)=Roexp(To/T)^x, where x=0.75. Surprisingly, the exponent x
was found to be constant for a wide range of Ro and To in all four materials,
possibly implying a consistent underlying conduction mechanism. The results are
discussed in terms of several different models of hopping conduction.Comment: 6 pages, 5 figure
Time Dependent Development of the Coulomb Gap
We show that the time development of the Coulomb gap in a Coulomb glass can
involve very long relaxation times due to electron rearrangement and hopping.
We find that an applied magnetic field reduces the rate of electron hopping
and, hence, Coulomb gap formation. These results are consistent with recent
conductance experiments on thin semiconducting and metallic films.Comment: 4 pages, Latex, 3 encapsulated postscript figure
Electrical transport studies of quench condensed Bi films at the initial stage of film growth: Structural transition and the possible formation of electron droplets
The electrical transport properties of amorphous Bi films prepared by
sequential quench deposition have been studied in situ. A
superconductor-insulator (S-I) transition was observed as the film was made
increasingly thicker, consistent with previous studies. Unexpected behavior was
found at the initial stage of film growth, a regime not explored in detail
prior to the present work. As the temperature was lowered, a positive
temperature coefficient of resistance (dR/dT > 0) emerged, with the resistance
reaching a minimum before the dR/dT became negative again. This behavior was
accompanied by a non-linear and asymmetric I-V characteristic. As the film
became thicker, conventional variable-range hopping (VRH) was recovered. We
attribute the observed crossover in the electrical transport properties to an
amorphous to granular structural transition. The positive dR/dT found in the
amorphous phase of Bi formed at the initial stage of film growth was
qualitatively explained by the formation of metallic droplets within the
electron glass.Comment: 7 pages, 6 figure
Manifestation of ageing in the low temperature conductance of disordered insulators
We are interested in the out of equilibrium phenomena observed in the
electrical conductance of disordered insulators at low temperature, which may
be signatures of the electron coulomb glass state. The present work is devoted
to the occurrence of ageing, a benchmark phenomenon for the glassy state. It is
the fact that the dynamical properties of a glass depend on its age, i.e. on
the time elapsed since it was quench-cooled. We first critically analyse
previous studies on disordered insulators and question their interpretation in
terms of ageing. We then present new measurements on insulating granular
aluminium thin films which demonstrate that the dynamics is indeed age
dependent. We also show that the results of different relaxation protocols are
related by a superposition principle. The implications of our findings for the
mechanism of the conductance slow relaxations are then discussed
Electron Glass in Ultrathin Granular Al Films at Low Temperatures
Quench-condensed granular Al films, with normal-state sheet resistance close
to 10 k, display strong hysteresis and ultraslow, non-exponential
relaxation in the resistance when temperature is varied below 300 mK. The
hysteresis is nonlinear and can be suppressed by a dc bias voltage. The
relaxation time does not obey the Arrhenius form, indicating the existence of a
broad distribution of low energy barriers. Furthermore, large resistance
fluctuations, having a 1/f-type power spectrum with a low-frequency cut-off,
are observed at low temperatures. With decreasing temperature, the amplitude of
the fluctuation increases and the cut-off frequency decreases. These
observations combine to provide a coherent picture that there exists a new
glassy electron state in ultrathin granular Al films, with a growing
correlation length at low temperatures.Comment: RevTeX 3.1, 4 pages, 4 figures (EPS files) (Minor Additions
Evidence of Vortices on the Insulating Side of the Superconductor-Insulator Transition
The magnetoresistance of ultrathin insulating films of Bi has been studied
with magnetic fields applied parallel and perpendicular to the plane of the
sample. Deep in the strongly localized regime, the magnetoresistance is
negative and independent of field orientation. As film thicknesses increase,
the magnetoresistance becomes positive, and a difference between values
measured in perpendicular and parallel fields appears, which is a linear
function of the magnetic field and is positive. This is not consistent with the
quantum interference picture. We suggest that it is due to vortices present on
the insulating side of the superconductor-insulator transition.Comment: 4 pages, 3 figure
Electron-Assisted Hopping in Two Dimensions
We have studied the non-ohmic effects in the conductivity of a
two-dimensional system which undergoes the crossover from weak to strong
localization with decreasing electron concentration. When the electrons are
removed from equilibrium with phonons, the hopping conductivity depends only on
the electron temperature. This indicates that the hopping transport in a system
with a large localization length is assisted by electron-electron interactions
rather than by the phonons.Comment: 5 pages, 4 figure
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