63 research outputs found
Leggett's bound for amorphous solids
We investigate the constraints on the superfluid fraction of an amorphous
solid following from an upper bound derived by Leggett. In order to accomplish
this, we use as input density profiles generated for amorphous solids in a
variety of different manners including by investigating Gaussian fluctuations
around classical results. These rough estimates suggest that, at least at the
level of the upper bound, there is not much difference in terms of
superfluidity between a glass and a crystal characterized by the same Lindemann
ratio. Moreover, we perform Path Integral Monte Carlo simulations of
distinguishable Helium 4 rapidly quenched from the liquid phase to very lower
temperature, at the density of the freezing transition. We find that the system
crystallizes very quickly, without any sign of intermediate glassiness. Overall
our results suggest that the experimental observations of large superfluid
fractions in Helium 4 after a rapid quench correspond to samples evolving far
from equilibrium, instead of being in a stable glass phase. Other scenarios and
comparisons to other results on the super-glass phase are also discussed.Comment: 11 pages, 5 figure
Yielding and plasticity in amorphous solids
The physics of disordered media, from metallic glasses to colloidal
suspensions, granular matter and biological tissues, offers difficult
challenges because it often occurs far from equilibrium, in materials lacking
symmetries and evolving through complex energy landscapes. Here, we review
recent theoretical efforts to provide microscopic insights into the mechanical
properties of amorphous media using approaches from statistical mechanics as
unifying frameworks. We cover both the initial regime corresponding to small
deformations, and the yielding transition marking a change between elastic
response and plastic flow. We discuss the specific features arising for systems
evolving near a jamming transition, and extend our discussion to recent studies
of the rheology of dense biological and active materials.Comment: 20 pages, 7 figure
Body composition assessment: comparison of quantitative values between magnetic resonance imaging and computed tomography.
Background
The primary objective of this study was to compare measurements of skeletal muscle index (SMI), visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) at the level of L3, on subjects who underwent computed tomography (CT) and magnetic resonance imaging (MRI) examinations within a three-month period. The secondary objective was to compare the automatic and semi-automatic quantifications of the same values for CT images.
Methods
Among subjects who underwent CT and MRI at our Institution between 2011 and 2020, exclusion criteria were: presence of extensive artifacts; images not including the whole waist circumference; CT acquired with low-dose technique and lack of non-contrast images. A set of three axial images (CT, MRI T1-weighted and T2-weighted) were used to extract the following measurements with semi-automatic segmentations: SMI [calculated normalizing skeletal muscle area (SMA) by the square height], SAT, VAT. For the CT images only, the same values were also calculated by using automatic segmentation. Statistical analysis was performed comparing quantitative MRI and CT measurements by Pearson correlation analysis and by Bland-Altman agreement analysis.
Results
A total of 123 patients were included. By performing linear regression analysis, CT and MRI measurements of SMI showed a high correlation (r2=0.81 for T1, r2=0.89 for T2), with a mean logarithmic difference between CT and MRI quantitative values of 0.041 for T1-weighted and 0.072 for T2-weighted images. CT and MRI measurements of SAT showed high correlation (r2=0.81 for T1; r2=0.81 for T2), with a mean logarithmic difference between CT and MRI values of 0.0174 for T1-weighted and 0.201 for T2-weighted images. CT and MRI measurements of VAT showed high correlation (r2=0.94 for T1; r2=0.93 for T2), with a mean logarithmic difference of 0.040 for T1-weighted and -0.084 for T2-weighted images. The comparison of values extracted by semi-automatic and automatic segmentations were highly correlated.
Conclusions
Quantification of body composition values at MRI from T1-weighted and T2-weighted images was highly correlated to same values at CT, therefore quantitative values of body composition among patients who underwent either one of the examinations may be compared. CT body composition values extracted by semi-automatic and automatic segmentations showed high correlation
Experimental Determination of Configurational Entropy in a Two-Dimensional Liquid under Random Pinning
A quasi two-dimensional colloidal suspension is studied under the influence
of immobilisation (pinning) of a random fraction of its particles. We introduce
a novel experimental method to perform random pinning and, with the support of
numerical simulation, we find that increasing the pinning concentration
smoothly arrests the system, with a cross-over from a regime of high mobility
and high entropy to a regime of low mobility and low entropy. At the local
level, we study fluctuations in area fraction and concentration of pins and map
them to entropic structural signatures and local mobility, obtaining a measure
for the local entropic fluctuations of the experimental system
Roadmap on machine learning glassy liquids
Unraveling the connections between microscopic structure, emergent physical
properties, and slow dynamics has long been a challenge in the field of the
glass transition. The absence of clear visible structural order in amorphous
configurations complicates the identification of the key features related to
structural relaxation and transport properties. The difficulty in sampling
equilibrated configurations at low temperatures hampers thorough numerical and
theoretical investigations. This roadmap article explores the potential of
machine learning (ML) techniques to face these challenges, building on the
algorithms that have revolutionized computer vision and image recognition. We
present successful ML applications, as well as many open problems for the
future, such as transferability and interpretability of ML approaches. We
highlight new ideas and directions in which ML could provide breakthroughs to
better understand glassy liquids. To foster a collaborative community effort,
the article introduces the "GlassBench" dataset, providing simulation data and
benchmarks for both two-dimensional and three-dimensional glass-formers.
Emphasizing the importance of benchmarks, we identify critical metrics for
comparing the performance of emerging ML methodologies, in line with
benchmarking practices in image and text recognition. The goal of this roadmap
is to provide guidelines for the development of ML techniques in systems
displaying slow dynamics, while inspiring new directions to improve our
understanding of glassy liquids
Can the jamming transition be described using equilibrium statistical mechanics?
When materials such as foams or emulsions are compressed, they display solid
behaviour above the so-called `jamming' transition. Because compression is done
out-of-equilibrium in the absence of thermal fluctuations, jamming appears as a
new kind of a nonequilibrium phase transition. In this proceeding paper, we
suggest that tools from equilibrium statistical mechanics can in fact be used
to describe many specific features of the jamming transition. Our strategy is
to introduce thermal fluctuations and use statistical mechanics to describe the
complex phase behaviour of systems of soft repulsive particles, before sending
temperature to zero at the end of the calculation. We show that currently
available implementations of standard tools such as integral equations,
mode-coupling theory, or replica calculations all break down at low temperature
and large density, but we suggest that new analytical schemes can be developed
to provide a fully microscopic, quantitative description of the jamming
transition.Comment: 8 pages, 6 figs. Talk presented at Statphys24 (July 2010, Cairns,
Australia
Theory of the superglass phase
A superglass is a phase of matter which is characterized at the same time by
superfluidity and a frozen amorphous structure. We introduce a model of
interacting bosons in three dimensions that displays this phase unambiguously
and that can be analyzed exactly or using controlled approximations. Employing
a mapping between quantum Hamiltonians and classical Fokker-Planck operators,
we show that the ground state wavefunction of the quantum model is proportional
to the Boltzmann measure of classical hard spheres. This connection allows us
to obtain quantitative results on static and dynamic quantum correlation
functions. In particular, by translating known results on the glassy dynamics
of Brownian hard spheres we work out the properties of the superglass phase and
of the quantum phase transition between the superfluid and the superglass
phase.Comment: 23 pages, 7 figure
- âŠ