804 research outputs found
Nanowire Acting as a Superconducting Quantum Interference Device
We present the results from an experimental study of the magneto-transport of
superconducting wires of amorphous Indium-Oxide, having widths in the range 40
- 120 nm. We find that, below the superconducting transition temperature, the
wires exhibit clear, reproducible, oscillations in their resistance as a
function of magnetic field. The oscillations are reminiscent of those which
underlie the operation of a superconducting quantum interference device.Comment: 4 pages, 4 figures, 1 tabl
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Improving the reliability of model-based decision-making estimates in the two-stage decision task with reaction-times and drift-diffusion modeling
A well-established notion in cognitive neuroscience proposes that multiple brain systems contribute to choice behaviour. These include: (1) a model-free system that uses values cached from the outcome history of alternative actions, and (2) a model-based system that considers action outcomes and the transition structure of the environment. The widespread use of this distinction, across a range of applications, renders it important to index their distinct influences with high reliability. Here we consider the two-stage task, widely considered as a gold standard measure for the contribution of model-based and model-free systems to human choice. We tested the internal/temporal stability of measures from this task, including those estimated via an established computational model, as well as an extended model using drift-diffusion. Drift-diffusion modeling suggested that both choice in the first stage, and RTs in the second stage, are directly affected by a model-based/free trade-off parameter. Both parameter recovery and the stability of model-based estimates were poor but improved substantially when both choice and RT were used (compared to choice only), and when more trials (than conventionally used in research practice) were included in our analysis. The findings have implications for interpretation of past and future studies based on the use of the two-stage task, as well as for characterising the contribution of model-based processes to choice behaviour
Universality at integer quantum Hall transitions
We report in this paper results of experimental and theoretical studies of
transitions between different integer quantum Hall phases, as well as
transition between the insulating phase and quantum Hall phases at high
magnetic fields. We focus mainly on universal properties of the transitions. We
demonstrate that properly defined conductivity tensor is universal at the
transitions. We also present numerical results of a non-interacting electron
model, which suggest that the Thouless conductance is universal at integer
quantum Hall transitions, just like the conductivity tensor. Finite temperature
and system size effects near the transition point are also studied.Comment: 20 pages, 15 figure
Survival probabilities in time-dependent random walks
We analyze the dynamics of random walks in which the jumping probabilities
are periodic {\it time-dependent} functions. In particular, we determine the
survival probability of biased walkers who are drifted towards an absorbing
boundary. The typical life-time of the walkers is found to decrease with an
increment of the oscillation amplitude of the jumping probabilities. We discuss
the applicability of the results in the context of complex adaptive systems.Comment: 4 pages, 3 figure
Survival Probabilities of History-Dependent Random Walks
We analyze the dynamics of random walks with long-term memory (binary chains
with long-range correlations) in the presence of an absorbing boundary. An
analytically solvable model is presented, in which a dynamical phase-transition
occurs when the correlation strength parameter \mu reaches a critical value
\mu_c. For strong positive correlations, \mu > \mu_c, the survival probability
is asymptotically finite, whereas for \mu < \mu_c it decays as a power-law in
time (chain length).Comment: 3 pages, 2 figure
The quantized Hall effect in the presence of resistance fluctuations
We present an experimental study of mesoscopic, two-dimensional electronic
systems at high magnetic fields. Our samples, prepared from a low-mobility
InGaAs/InAlAs wafer, exhibit reproducible, sample specific, resistance
fluctuations. Focusing on the lowest Landau level we find that, while the
diagonal resistivity displays strong fluctuations, the Hall resistivity is free
of fluctuations and remains quantized at its value, . This is
true also in the insulating phase that terminates the quantum Hall series.
These results extend the validity of the semicircle law of conductivity in the
quantum Hall effect to the mesoscopic regime.Comment: Includes more data, changed discussio
Phase Diagram of Integer Quantum Hall Effect
The phase diagram of integer quantum Hall effect is numerically determined in
the tight-binding model, which can account for overall features of recently
obtained experimental phase diagram. In particular, the quantum Hall plateaus
are terminated by two distinct insulating phases, characterized by the Hall
resistance with classic and quantized values, respectively, which is also in
good agreement with experiments.Comment: 4 pages, RevTex, 4 PostScript figures; one new figure is added; minor
modifications in the tex
Phase-Transition in Binary Sequences with Long-Range Correlations
Motivated by novel results in the theory of correlated sequences, we analyze
the dynamics of random walks with long-term memory (binary chains with
long-range correlations). In our model, the probability for a unit bit in a
binary string depends on the fraction of unities preceding it. We show that the
system undergoes a dynamical phase-transition from normal diffusion, in which
the variance D_L scales as the string's length L, into a super-diffusion phase
(D_L ~ L^{1+|alpha|}), when the correlation strength exceeds a critical value.
We demonstrate the generality of our results with respect to alternative
models, and discuss their applicability to various data, such as coarse-grained
DNA sequences, written texts, and financial data.Comment: 4 pages, 4 figure
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