Archivio istituzionale della ricerca dell'Università degli Studi di Palermohttps://iris.unipa.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Mon, 24 Jan 2022 03:50:26 GMT2022-01-24T03:50:26Z10291Monte Carlo Study of Electron Spin
Relaxation in n-type GaAs Bulkhttp://hdl.handle.net/10447/94811Titolo: Monte Carlo Study of Electron Spin
Relaxation in n-type GaAs Bulk
Mon, 16 Apr 2012 00:00:00 GMThttp://hdl.handle.net/10447/948112012-04-16T00:00:00ZEffect of a fluctuating electric field on electron spin dephasing in III-V semiconductorshttp://hdl.handle.net/10447/60090Titolo: Effect of a fluctuating electric field on electron spin dephasing in III-V semiconductors
Abstract: In the present work we investigate electron spin relaxation in low-doped n-type GaAs semiconductor bulks driven by a static electric field. The electron dynamics is simulated by a Monte Carlo procedure which keeps into account all the possible scattering phenomena of the hot electrons in the medium and includes the evolution of spin polarization. Spin relaxation lengths are computed through the D’yakonov-Perel process, which is the only relevant relaxation mechanism in zinc-blende semiconductors.
Since semiconductor based devices are always imbedded into a noisy environment that can strongly affect their performance, the decay of initial spin polarization of conduction electrons is calculated in the presence of a fluctuating component added to the static driving electric field. The starting point for our analysis is the computation of changes in the depolarization length caused by the addition of an external correlated noise source, at different values of field strength, lattice temperature, doping density, noise amplitude, noise correlation time, etc.
Our findings show that, only for values of noise correlation time comparable to the dephasing time, relaxation lengths decrease with the increasing of noise intensity. Moreover, for each value of the noise amplitude, a nonmonotonic behavior of spin depolarization length with the noise correlation time is found. The presence of a minimum is well explained by studying the effective mean electric field experienced by the electrons ensemble within the relaxation time.
Furthermore, our study reveals that the system receives a benefit in terms of weakening of the length reduction by the inclusion of the electron-electron scattering mechanism. This effect will be also discussed.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/10447/600902011-01-01T00:00:00ZRELAXATION OF ELECTRON SPIN DURING FIELD TRANSPORT IN GaAs BULKShttp://hdl.handle.net/10447/60091Titolo: RELAXATION OF ELECTRON SPIN DURING FIELD TRANSPORT IN GaAs BULKS
Abstract: The spin depolarization of drifting electrons in a n-type doped GaAs bulk semiconductor is studied, in a wide range of lattice temperature (40 K < TL < 300 K) and doping density (10^{13} cm^{−3} < n < 10^{16} cm^{−3}), by adopting a semiclassical Monte Carlo approach. The effect of the mechanism of Dyakonov-Perel (DP) on the spin depolarization of the conduction electrons is analyzed as a function of the amplitude of a static electric field, ranging between 0.1 and 6 kV cm^{−1}, by considering the spin dynamics of electrons in both the Γ-valley and the upper L-valleys of the semiconductor. Moreover, the role of the electron-electron scattering mechanism in the suppression of DP spin relaxation is discussed.
For high values of the electric field, the strong spin–orbit coupling of electrons in the Γ-valleys significantly reduces the average spin polarization lifetime, but, unexpectedly, for field amplitudes greater than 2.5 kV cm^{−1}, the spin lifetime increases with the lattice temperature. Our numerical findings are validated by a good agreement with the available experimental results and with calculations recently obtained on different semiconductor structures.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/10447/600912011-01-01T00:00:00ZNoise Enhanced Stability Phenomenon in Electron Spin Dynamicshttp://hdl.handle.net/10447/65978Titolo: Noise Enhanced Stability Phenomenon in Electron Spin Dynamics
Abstract: Possible utilization of the electron spin as an information carrier in electronic devices is an engaging challenge for future spin-based electronics. In
these new devices, the information stored in a system of polarized electron spins, is transferred by
applying an external electric field and finally detected. However, each initial non-equilibrium magnetization decays both in time and distance during
the transport. Because of increasing miniaturization, to avoid too much intense electric fields,
which could lead the system to exhibit a strongly
nonlinear physical behavior, applied voltages are
very low. Low voltages are subjected to the background noise; hence, it is mandatory to understand
the influence of fluctuations on the spin depolarization process in order to guarantee a reliable manipulation, control and detection of information in
spin-based devices.
The presence of noise is generally considered a
disturbance, since strong fluctuations affect the
performance of the devices. In the last decade,
however, an increasing interest has been directed
towards possible constructive aspects of noise in the dynamical response of non-linear systems.
Interesting theoretical works on the possibility to
improve the ultra-fast magnetization dynamics of
magnetic spin systems by including random fields
have been recently published. Instead, at the
best of our knowledge, the investigation of the role
of noise on the electron spin dynamics in semiconductors is still beginning. Preliminary findings
show that in semiconductor crystals a fluctuating
electric field can strongly modify the spin depolarization length, an essential design parameter in
spin-based electronic devices.
In this contribution, we investigate the effects of different types of external source of noise on the
spin relaxation process in low-doped n-type GaAs
crystals. The electron transport is simulated by a
Monte Carlo procedure which takes into account
all the possible scattering phenomena of hot electrons in the medium and includes the evolution of
the spin polarization vector. The effects caused
by the addition of external fluctuations are investigated by analyzing the modification of the spin
depolarization length. In the presence of a correlated source of noise and for electric field amplitudes greater than the Gunn field, an increase of
the spin relaxation length up to 20% is found. This result can be considered a Noise
Enhanced Stability (NES) consequence and can be
explained in terms of a decrease of the occupation
of the L-valleys, where the strength of spin-orbit
coupling felt by electrons is at least one order of
magnitude greater than that present in the lowest energy valley. If the random component of the
driving electric field is modeled with a dichotomous stochastic process (Random telegraph noise) characterized by two discrete levels, the spin depolarization process changes, in a way that critically depends on the jump rate of the stochastic process.
We investigate on a possible relationship between
the semiconductor characteristic time scales and
the noise characteristic time in order to find the
most favorable condition for the transmission of information by electron spin.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10447/659782012-01-01T00:00:00ZNoise effects in biological systemshttp://hdl.handle.net/10447/7347Titolo: Noise effects in biological systems
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10447/73472008-01-01T00:00:00ZEnvironmental Noise and Nonlinear Relaxation in Biological Systemshttp://hdl.handle.net/10447/60079Titolo: Environmental Noise and Nonlinear Relaxation in Biological Systems
Abstract: We analyse the effects of environmental noise in three different
biological systems: (i) mating behaviour of individuals of
'Nezara viridula' (L.) (Heteroptera Pentatomidae); (ii) polymer translocation in crowded
solution; (iii) an ecosystem described by a Verhulst model with a
multiplicative Lèvy noise. Specifically, we report on
experiments on the behavioural response of 'N. viridula'
individuals to sub-threshold deterministic signals in the presence
of noise. We analyse the insect response by directionality tests
performed on a group of male individuals at different noise
intensities. The percentage of insects which react to the
sub-threshold signal shows a non-monotonic behavior, characterized
by the presence of a maximum, for increasing values of the noise
intensity. This is the signature of the non-dynamical stochastic
resonance phenomenon. By using a "hard" threshold model we find that
the maximum of the signal-to-noise ratio occurs in the same range
of noise intensity values for which the behavioral activation shows
a maximum. In the second system, the noise driven translocation of
short polymers in crowded solutions is analyzed. An improved version
of the Rouse model for a flexible polymer has been adopted to mimic
the molecular dynamics, by taking into account both the interactions
between adjacent monomers and introducing a Lennard-Jones potential
between non-adjacent beads. A bending recoil torque has also been
included in our model. The polymer dynamics is simulated in a
two-dimensional domain by numerically solving the Langevin equations
of motion. Thermal fluctuations are taken into account by
introducing a Gaussian uncorrelated noise. The mean first
translocation time of the polymer center of inertia shows a minimum
as a function of the frequency of the oscillating forcing field. In
the third ecosystem, the transient dynamics of the Verhulst model
perturbed by arbitrary non-Gaussian white noise is investigated.
Based on the infinitely divisible distribution of the Lèvy
process we study the nonlinear relaxation of the population density
for three cases of white non-Gaussian noise: (i) shot noise, (ii)
noise with a probability density of increments expressed in terms of
Gamma function, and (iii) Cauchy stable noise. We obtain exact
results for the probability distribution of the population density
in all cases, and for Cauchy stable noise the exact expression of
the nonlinear relaxation time is derived. Moreover starting from an
initial delta function distribution, we find a transition induced by
the multiplicative Lèvy noise, from a trimodal probability
distribution to a bimodal probability distribution in asymptotics.
Finally we find a nonmonotonic behavior of the nonlinear relaxation
time as a function of the Cauchy stable noise intensity.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10447/600792012-01-01T00:00:00ZNonlinear relaxation in quantum and mesoscopic systemshttp://hdl.handle.net/10447/83727Titolo: Nonlinear relaxation in quantum and mesoscopic systems
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10447/837272013-01-01T00:00:00ZMetastability and Relaxation in Quantum and Mesoscopic Systemshttp://hdl.handle.net/10447/83723Titolo: Metastability and Relaxation in Quantum and Mesoscopic Systems
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10447/837232013-01-01T00:00:00ZStochastic resonance effect on the vibratory signals of stink bugshttp://hdl.handle.net/10447/46103Titolo: Stochastic resonance effect on the vibratory signals of stink bugs
Abstract: In this work we investigate the role of the environmental noise in the sexual communication between individuals of the Sicilian green stink bug Nezara Viridula. Our goal is to analyze the spectral features of several types of songs emitted by these insects and to find the amplitude threshold value. Below this value the signal is so weak that no neuronal activation occurs in Nezara Viridula. The presence of activation is revealed by performing directionality tests and observing the behavioral response of the insect in localizing the source of vibratory signals. Afterwards experiments are performed by using a sub-threshold signal added to a white Gaussian noise. The response of the test insect is examined for different level of noise intensity. A behavior corresponding to neuronal activation is observed when a suitable level of white noise is added to the sub-threshold signal. Our experimental measurements show the constructive interplay between the calling songs and the external-added noise. For sub-threshold signals of the calling sequence the response is subject to a non dynamical stochastic resonance phenomenon.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10447/461032007-01-01T00:00:00ZTwo-species model for spatial distributions of sardine and anchovy: A comparison with real datahttp://hdl.handle.net/10447/50190Titolo: Two-species model for spatial distributions of sardine and anchovy: A comparison with real data
Abstract: We present a study of pattern formation in a set
of two coupled equations modeling two competing species. We
consider generalized Lotka-Volterra equations in the presence
of a multiplicative noise which models the interaction between
the species and the environment. The interaction parameter
between the species is a random process which obeys a stochastic
differential equation with a generalized bistable potential in the
presence of a periodic driving term, which accounts for the
environment temperature variation.We find noise-induced spatial
patterns with strong anti-correlation between the two species.
We compare our theoretical results with the experimental data
of the spatial distributions of anchovy and sardine abundances
collected during an oceanographic campaign in the Strait of
Sicily. Preliminary analysis show qualitative agreement between
theoretical and experimental results.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10447/501902010-01-01T00:00:00Z