Eventi del Dottorato

Dottorato in Fisica

Journal Club Seminari 2021

26/04/2021   ore 14:00
modalita' telematica

ore 14:00Paolo La Francesca
Phase Ordering Dynamics in 2D Ferromagnetic Systems
In the last few years there has been a considerable interest in the theory of phase
ordering dynamics in ferromagnetic systems and in models based on the introduction
of auxiliary fields with Gaussian statistics. The aim of this work was to study the
dynamics of the Landau-Ginzburg model and to numerically test the properties
predicted by the theory on 2D discrete lattices of sizes never considered before.
The occurence of the formation of competing domains of opposite magnetisation with
characteristic linear length obeying to the Lifshitz-Allen-Cahn Law l²(t) ∼ t was
shown. The behaviour of the correlation function G(r,t) at large t was also analysed
and it appeared to be compatible with the scaling hypothesis G(r,t)=F(r/l(t)) except
for the initial region x → 0 with x=r/l(t).
The extent of this non-universality region decreases over time both because the size
of the domains increases, and because the size of the interface regions among the
domains decreases. Furthermore, in the variable x, its linear size must become
negligible in the long term due to the growth of l(t) but it has not been possible,
for now, to provide an estimate of the necessary time for this to happen.
Therefore, excluding the regions in which the correlation function deviates from the
scaling regime, the scaling function was compared with the approximations obtained
by introducing auxiliary fields: the OJK and Mazenko approximations. Specifically,
neither of the two theories has been able to provide an accurate description of the
scaling phenomenon.
The rest of the work focused on comparing the behaviour of the Landau-Ginzburg model
and that of the Monte Carlo simulations for the Ising model. With the exception of
the region r → 0 considered above (for which the Ising model continues to have a
universal behaviour), the correlation functions were found to be compatible between
the two models, giving reason to assume that the scaling phenomena they describe
belong to the same universality class. Consistently, even in this case the
approximations for the scaling function did not work.
ore 14:45Elisabetta Colantoni
Coherent Raman Imaging of Amyloid-? Plaques in Alzheimer’s Diseased Brain
Alzheimer’s disease (AD) is a progressive neurological disease determining both the
reduction of cognitive function and the loss of memory and memories. The actual
therapeutic treatments are not able to heal the AD and to recover cognitive and
memory functions, therefore it is fundamental to get an early and accurate diagnosis
to slow down and contain as much as possible the disease progress. In the studies of
the diseases it has been experienced that the use of biological markers (biomarkers)
allows speeding up the diagnosis with a positive impact on the prognosis too.
This thesis aims to study the Amyloid-β plaques that have been considered as
Alzheimer’s disease biomarker.
During this work the triple-transgenic mouse model of AD (3xTg-AD) was used; the
model exhibit Amyloid-β plaques that are characteristic of the human form of
Alzheimer’s disease. An advanced stage of the disease has been studied: the
late-symptomatic stage present in 72-week-old mice. The study also included the
analysis of a healthy tissue of non-Tg mice. This control tissue has the same age,
equal to 72 weeks, as the AD tissues.
Initially, Fluorescence Micro-Spectroscopy was used to characterize, with two
specific fluorophores, the brain tissue under examination. Trying to minimize any
interferences (ex. samples preparation, labelling) and any physiochemical changes
that can occur inadvertently during the test executions, the complex interactions
between lipids and Amyloid-β plaque were studied employing a label-free vibrational
spectral imaging: Coherent Anti-Stokes Raman Scattering (CARS) is a non-invasive
label-free ultrafast method based on intrinsic vibrational contrast and is
characterized by high spectral resolution, 3D-sectioning capability and high
sensitivity. Vibrational Microscopy, compared to a Fluorescence Microscopy, has the
additional advantage that it can also be used for in-vivo medical applications: the
use of staining or labelling with fluorophores on humans is not preferable in terms
of risk of toxicity. Thanks to this vibrational spectral imaging, the region of the
typical lipid CH groups was probed. 
This study clearly demonstrates that the chemical composition of the plaque is
different from the chemical composition of the brain tissue that surrounds it as
well as from the chemical composition of a non-AD brain.

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