17 research outputs found
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
Hopping models and ac universality
Some general relations for hopping models are established. We proceed to
discuss the universality of the ac conductivity which arises in the extreme
disorder limit of the random barrier model. It is shown that the relevant
dimension entering into the diffusion cluster approximation (DCA) is the
harmonic (fracton) dimension of the diffusion cluster. The temperature scaling
of the dimensionless frequency entering into the DCA is discussed. Finally,
some open questions about ac universality are mentioned.Comment: 6 page
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.Comment: 10 figure
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.Comment: 15 pages 22 figure
Anomalous Noise in the Pseudogap Regime of YBaCuO
An unusual noise component is found near and below about 250 K in the normal
state of underdoped YBCO and Ca-YBCO films. This noise regime, unlike the more
typical noise above 250 K, has features expected for a symmetry-breaking
collective electronic state. These include large individual fluctuators, a
magnetic sensitivity, and aging effects. A possible interpretation in terms of
fluctuating charge nematic order is presented.Comment: 4 pages, 4 figure
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
Ageing memory and glassiness of a driven vortex system
Many systems in nature, glasses, interfaces and fractures being some
examples, cannot equilibrate with their environment, which gives rise to novel
and surprising behaviour such as memory effects, ageing and nonlinear dynamics.
Unlike their equilibrated counterparts, the dynamics of out-of- equilibrium
systems is generally too complex to be captured by simple macroscopic laws.
Here we investigate a system that straddles the boundary between glass and
crystal: a Bragg glass formed by vortices in a superconductor. We find that the
response to an applied force evolves according to a stretched exponential, with
the exponent reflecting the deviation from equilibrium. After the force is
removed, the system ages with time and its subsequent response time scales
linearly with its age (simple ageing), meaning that older systems are slower
than younger ones. We show that simple ageing can occur naturally in the
presence of sufficient quenched disorder. Moreover, the hierarchical
distribution of timescales, arising when chunks of loose vortices cannot move
before trapped ones become dislodged, leads to a stretched-exponential
response.Comment: 16 pages, 5 figure