772 research outputs found
Knockouts, Robustness and Cell Cycles
The response to a knockout of a node is a characteristic feature of a
networked dynamical system. Knockout resilience in the dynamics of the
remaining nodes is a sign of robustness. Here we study the effect of knockouts
for binary state sequences and their implementations in terms of Boolean
threshold networks. Beside random sequences with biologically plausible
constraints, we analyze the cell cycle sequence of the species Saccharomyces
cerevisiae and the Boolean networks implementing it. Comparing with an
appropriate null model we do not find evidence that the yeast wildtype network
is optimized for high knockout resilience. Our notion of knockout resilience
weakly correlates with the size of the basin of attraction, which has also been
considered a measure of robustness.Comment: 11 pages, 3 figures, 3 table
Analysis of the Temporal Organization of Sleep Spindles in the Human Sleep EEG Using a Phenomenological Modeling Approach
The sleep electroencephalogram (EEG) is characterized by typical oscillatory patterns such as sleep spindles and slow waves. Recently, we proposed a method to detect and analyze these patterns using linear autoregressive models for short (≈ 1 s) data segments. We analyzed the temporal organization of sleep spindles and discuss to what extent the observed interevent intervals correspond to properties of stationary stochastic processes and whether additional slow processes, such as slow oscillations, have to be assumed. We have found evidence for such an additional slow process, most pronounced in sleep stage 2
Chaos or Noise - Difficulties of a Distinction
In experiments, the dynamical behavior of systems is reflected in time
series. Due to the finiteness of the observational data set it is not possible
to reconstruct the invariant measure up to arbitrary fine resolution and
arbitrary high embedding dimension. These restrictions limit our ability to
distinguish between signals generated by different systems, such as regular,
chaotic or stochastic ones, when analyzed from a time series point of view. We
propose to classify the signal behavior, without referring to any specific
model, as stochastic or deterministic on a certain scale of the resolution
, according to the dependence of the -entropy,
, and of the finite size Lyapunov exponent,
, on .Comment: 24 pages RevTeX, 9 eps figures included, two references added, minor
corrections, one section has been split in two (submitted to PRE
Spin currents in diluted magnetic semiconductors (extended version)
Spin currents resulting in the zero-bias spin separation have been observed
in unbiased diluted magnetic semiconductor structures (Cd,Mn)Te/(Cd,Mg)Te. The
pure spin current generated due to the electron gas heating by terahertz
radiation is converted into a net electric current by application of an
external magnetic field. We demonstrate that polarization of the magnetic ion
system enhances drastically the conversion due to the spin-dependent scattering
by localized Mn(2+) ions and the giant Zeeman splitting.Comment: 6 pages, 4 figure
Spin polarized electric currents in semiconductor heterostructures induced by microwave radiation
We report on microwave (mw) radiation induced electric currents in
(Cd,Mn)Te/(Cd,Mg)Te and InAs/(In,Ga)As quantum wells subjected to an external
in-plane magnetic field. The current generation is attributed to the
spin-dependent energy relaxation of electrons heated by mw radiation. The
relaxation produces equal and oppositely directed electron flows in the spin-up
and spin-down subbands yielding a pure spin current. The Zeeman splitting of
the subbands in the magnetic field leads to the conversion of the spin flow
into a spin-polarized electric current.Comment: 3 pages, 4 figure
Detecting Determinism in High Dimensional Chaotic Systems
A method based upon the statistical evaluation of the differentiability of
the measure along the trajectory is used to identify in high dimensional
systems. The results show that the method is suitable for discriminating
stochastic from deterministic systems even if the dimension of the latter is as
high as 13. The method is shown to succeed in identifying determinism in
electro-encephalogram signals simulated by means of a high dimensional system.Comment: 8 pages (RevTeX 3 style), 5 EPS figures, submitted to Phys. Rev. E
(25 apr 2001
Entropic descriptor of a complex behaviour
We propose a new type of entropic descriptor that is able to quantify the
statistical complexity (a measure of complex behaviour) by taking
simultaneously into account the average departures of a system's entropy S from
both its maximum possible value Smax and its minimum possible value Smin. When
these two departures are similar to each other, the statistical complexity is
maximal. We apply the new concept to the variability, over a range of length
scales, of spatial or grey-level pattern arrangements in simple models. The
pertinent results confirm the fact that a highly non-trivial, length-scale
dependence of the entropic descriptor makes it an adequate complexity-measure,
able to distinguish between structurally distinct configurational macrostates
with the same degree of disorder.Comment: 14 pages, 7 figures, extended versio
Spin-polarized electric currents in diluted magnetic semiconductor heterostructures induced by terahertz and microwave radiation
We report on the study of spin-polarized electric currents in diluted
magnetic semiconductor (DMS) quantum wells subjected to an in-plane external
magnetic field and illuminated by microwave or terahertz radiation. The effect
is studied in (Cd,Mn)Te/(Cd,Mg)Te quantum wells (QWs) and (In,Ga)As/InAlAs:Mn
QWs belonging to the well known II-VI and III-V DMS material systems, as well
as, in heterovalent AlSb/InAs/(Zn,Mn)Te QWs which represent a promising
combination of II-VI and III-V semiconductors. Experimental data and developed
theory demonstrate that the photocurrent originates from a spin-dependent
scattering of free carriers by static defects or phonons in the Drude
absorption of radiation and subsequent relaxation of carriers. We show that in
DMS structures the efficiency of the current generation is drastically enhanced
compared to non-magnetic semiconductors. The enhancement is caused by the
exchange interaction of carrier spins with localized spins of magnetic ions
resulting, on the one hand, in the giant Zeeman spin-splitting, and, on the
other hand, in the spin-dependent carrier scattering by localized Mn2+ ions
polarized by an external magnetic field.Comment: 14 pages, 13 figure
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