175 research outputs found
Non-Gaussianity of resistance fluctuations near electrical breakdown
We study the resistance fluctuation distribution of a thin film near
electrical breakdown. The film is modeled as a stationary resistor networkunder
biased percolation. Depending on the value of the external current,on the
system sizes and on the level of internal disorder, the fluctuation
distribution can exhibit a non-Gaussian behavior. We analyze this
non-Gaussianity in terms of the generalized Gumbel distribution recently
introduced in the context of highly correlated systems near criticality. We
find that when the average fraction of defects approaches the random
percolation threshold, the resistance fluctuation distribution is well
described by the universal behavior of the Bramwell-Holdsworth-Pinton
distribution.Comment: 3 figures, accepted for publication on Semicond Sci Tec
Non-Gaussian Fluctuations in Biased Resistor Networks: Size Effects versus Universal Behavior
We study the distribution of the resistance fluctuations of biased resistor
networks in nonequilibrium steady states. The stationary conditions arise from
the competition between two stochastic and biased processes of breaking and
recovery of the elementary resistors. The fluctuations of the network
resistance are calculated by Monte Carlo simulations which are performed for
different values of the applied current, for networks of different size and
shape and by considering different levels of intrinsic disorder. The
distribution of the resistance fluctuations generally exhibits relevant
deviations from Gaussianity, in particular when the current approaches the
threshold of electrical breakdown. For two-dimensional systems we have shown
that this non-Gaussianity is in general related to finite size effects, thus it
vanishes in the thermodynamic limit, with the remarkable exception of highly
disordered networks. For these systems, close to the critical point of the
conductor-insulator transition, non-Gaussianity persists in the large size
limit and it is well described by the universal Bramwell-Holdsworth-Pinton
distribution. In particular, here we analyze the role of the shape of the
network on the distribution of the resistance fluctuations. Precisely, we
consider quasi-one-dimensional networks elongated along the direction of the
applied current or trasversal to it. A significant anisotropy is found for the
properties of the distribution. These results apply to conducting thin films or
wires with granular structure stressed by high current densities.Comment: 8 pages, 4 figures. Invited talk at the 18-th International
Conference on Noise and Fluctuations, 19-23 September 2005, Salamanc
A network model to investigate structural and electrical properties of proteins
One of the main trend in to date research and development is the
miniaturization of electronic devices. In this perspective, integrated
nanodevices based on proteins or biomolecules are attracting a major interest.
In fact, it has been shown that proteins like bacteriorhodopsin and azurin,
manifest electrical properties which are promising for the development of
active components in the field of molecular electronics. Here we focus on two
relevant kinds of proteins: The bovine rhodopsin, prototype of GPCR protein,
and the enzyme acetylcholinesterase (AChE), whose inhibition is one of the most
qualified treatments of Alzheimer disease. Both these proteins exert their
functioning starting with a conformational change of their native structure.
Our guess is that such a change should be accompanied with a detectable
variation of their electrical properties. To investigate this conjecture, we
present an impedance network model of proteins, able to estimate the different
electrical response associated with the different configurations. The model
resolution of the electrical response is found able to monitor the structure
and the conformational change of the given protein. In this respect, rhodopsin
exhibits a better differential response than AChE. This result gives room to
different interpretations of the degree of conformational change and in
particular supports a recent hypothesis on the existence of a mixed state
already in the native configuration of the protein.Comment: 25 pages, 12 figure
A class of nonlinear wave equations containing the continuous Toda case
We consider a nonlinear field equation which can be derived from a binomial
lattice as a continuous limit. This equation, containing a perturbative
friction-like term and a free parameter , reproduces the Toda case (in
absence of the friction-like term) and other equations of physical interest, by
choosing particular values of . We apply the symmetry and the
approximate symmetry approach, and the prolongation technique. Our main purpose
is to check the limits of validity of different analytical methods in the study
of nonlinear field equations. We show that the equation under investigation
with the friction-like term is characterized by a finite-dimensional Lie
algebra admitting a realization in terms of boson annhilation and creation
operators. In absence of the friction-like term, the equation is linearized and
connected with equations of the Bessel type. Examples of exact solutions are
displayed, and the algebraic structure of the equation is discussed.Comment: Latex file + [equations.sty], 22 p
Vacuum structure for expanding geometry
We consider gravitational wave modes in the FRW metrics in a de Sitter phase
and show that the state space splits into many unitarily inequivalent
representations of the canonical commutation relations. Non-unitary time
evolution is described as a trajectory in the space of the representations. The
generator of time evolution is related to the entropy operator. The
thermodynamic arrow of time is shown to point in the same direction of the
cosmological arrow of time. The vacuum is a two-mode SU(1,1) squeezed state of
thermo field dynamics. The link between expanding geometry, squeezing and
thermal properties is exhibited.Comment: Latex file, epsfig, 1 figure, 21 page
Tuning the Correlation Decay in the Resistance Fluctuations of Multi-Species Networks
A new network model is proposed to describe the resistance noise
in disordered materials for a wide range of values ().
More precisely, we have considered the resistance fluctuations of a thin
resistor with granular structure in different stationary states: from nearly
equilibrium up to far from equilibrium conditions. This system has been
modelled as a network made by different species of resistors, distinguished by
their resistances, temperature coefficients and by the energies associated with
thermally activated processes of breaking and recovery. The correlation
behavior of the resistance fluctuations is analyzed as a function of the
temperature and applied current, in both the frequency and time domains. For
the noise frequency exponent, the model provides at low
currents, in the Ohmic regime, with decreasing inversely with the
temperature, and at high currents, in the non-Ohmic regime.
Since the threshold current associated with the onset of nonlinearity also
depends on the temperature, the proposed model qualitatively accounts for the
complicate behavior of versus temperature and current observed in many
experiments. Correspondingly, in the time domain, the auto-correlation function
of the resistance fluctuations displays a variety of behaviors which are tuned
by the external conditions.Comment: 26 pages, 16 figures, Submitted to JSTAT - Special issue SigmaPhi200
Non-Gaussian Resistance Noise near Electrical Breakdown in Granular Materials
The distribution of resistance fluctuations of conducting thin films with
granular structure near electrical breakdown is studied by numerical
simulations. The film is modeled as a resistor network in a steady state
determined by the competition between two biased processes, breaking and
recovery. Systems of different sizes and with different levels of internal
disorder are considered. Sharp deviations from a Gaussian distribution are
found near breakdown and the effect increases with the degree of internal
disorder. However, we show that in general this non-Gaussianity is related to
the finite size of the system and vanishes in the large size limit.
Nevertheless, near the critical point of the conductor-insulator transition,
deviations from Gaussianity persist when the size is increased and the
distribution of resistance fluctuations is well fitted by the universal
Bramwell-Holdsworth-Pinton distribution.Comment: 8 pages, 6 figures; accepted for publication on Physica
Non-Gaussian Resistance Fluctuations in Disordered Materials
ABSTRACT We study the distribution of resistance fluctuations of conducting thin films with different levels of internal disorder. The film is modeled as a resistor network in a steady state determined by the competition between two biased processes, breaking and recovery of the elementary resistors. The fluctuations of the film resistance are calculated by Monte Carlo simulations which are performed under different bias conditions, from the linear regime up to the threshold for electrical breakdown. Depending on the value of the external current, on the level of disorder and on the size of the system, the distribution of the resistance fluctuations can exhibit significant deviations from Gaussianity. As a general trend, a size dependent, non universal distribution is found for systems with low and intermediate disorder. However, for strongly disordered systems, close to the critical point of the conductor-insulator transition, the non-Gaussianity persists when the size is increased and the distribution of resistance fluctuations is well described by the universal Bramwell-Holdsworth-Pinton distribution
Remarks on flavor-neutrino propagators and oscillation formulae
We examine the general structure of the formulae of neutrino oscillations
proposed by Blasone and Vitiello(BV). Reconstructing their formulae with the
retarded propagators of the flavor neutrino fields for the case of many
flavors, we can get easily the formulae which satisfy the suitable boundary
conditions and are independent of arbitrary mass parameters ,
as is obtained by BV for the case of two flavors. In this two flavor case, our
formulae reduce to those obtained by BV under -invariance condition.
Furthermore, the reconstructed probabilities are shown to coincide with those
derived with recourse to the mass Hilbert space which is
unitarily inequivalent to the flavor Hilbert space . Such a
situation is not found in the corresponding construction a la BV. Then the new
factors in the BV's formulae, which modify the usual oscill ation formulae, are
not the trace of the flavor Hilbert space construction, but come from
Bogolyubov transformation among the operators of spin-1/2 ne utrino with
different masses.Comment: revtex, 16 page
A systematic method for constructing time discretizations of integrable lattice systems: local equations of motion
We propose a new method for discretizing the time variable in integrable
lattice systems while maintaining the locality of the equations of motion. The
method is based on the zero-curvature (Lax pair) representation and the
lowest-order "conservation laws". In contrast to the pioneering work of
Ablowitz and Ladik, our method allows the auxiliary dependent variables
appearing in the stage of time discretization to be expressed locally in terms
of the original dependent variables. The time-discretized lattice systems have
the same set of conserved quantities and the same structures of the solutions
as the continuous-time lattice systems; only the time evolution of the
parameters in the solutions that correspond to the angle variables is
discretized. The effectiveness of our method is illustrated using examples such
as the Toda lattice, the Volterra lattice, the modified Volterra lattice, the
Ablowitz-Ladik lattice (an integrable semi-discrete nonlinear Schroedinger
system), and the lattice Heisenberg ferromagnet model. For the Volterra lattice
and modified Volterra lattice, we also present their ultradiscrete analogues.Comment: 61 pages; (v2)(v3) many minor correction
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