Phénomène critique | EPFL Graph Search

vignette|Point critique de l'éthane : 1. état subcritique, liquide et gaz ; 2. opalescence critique ; 3. fluide supercritique. En physique, un phénomène critique est un phénomène associé à une transition de phase du deuxième ordre d'un système thermodynamique. Par exemple la transition de phase ferromagnétique et le comportement au voisinage du point critique liquide-gaz. La plupart des phénomènes critiques proviennent d'une divergence de la ou d'un ralentissement de la dynamique. Les phénomènes critiques présentent des relations d'échelle entre différentes grandeurs, une forme d'universalité et un comportement fractal. Le comportement critique diffère de l'approximation du champ moyen car les corrélations deviennent de plus en plus importantes lorsque le système approche du point critique. De nombreuses propriétés du comportement critique d'un système peuvent être décrites dans le cadre du groupe de renormalisation. Divergences au point critique Le modèle d'Ising du fer

Quantum critical scaling for a Heisenberg spin-1/2 chain around saturation

Mingee Chung, Henrik Moodysson Rønnow

We demonstrate quantum critical scaling for an S = 1/2 Heisenberg antiferromagnetic chain compound Cu(C4H4N2)(NO3)(2) in a magnetic field around saturation, by analyzing previously reported magnetization [Y. Kono et al., Phys. Rev. Lett. 114, 037202 (2015)], thermal expansion [J. Rohrkamp et al., J. Phys.: Conf. Ser. 200, 012169 (2010)], and NMR relaxation data [H. Kuhne et al., Phys. Rev. B 80, 045110 (2009)]. The scaling of magnetization is demonstrated through collapsing the data for a range of both temperature and field onto a single curve without making any assumption for a theoretical form. The data collapse is subsequently shown to closely follow the theoretically predicted scaling function without any adjustable parameters. Experimental boundaries for the quantum critical region could be drawn from the variable range beyond which the scaled data deviate from the theoretical function. Similarly to the magnetization, quantum critical scaling of the thermal expansion is also demonstrated. Further, the spin dynamics probed via NMR relaxation rate 1/T-1 close to the saturation is shown to follow the theoretically predicted quantum critical behavior as 1/T-1 proportional to T-0.5 persisting up to temperatures as high as k(B)T similar or equal to J, where J is the exchange coupling constant.

American Physical Society2015

New Insights into the Verwey Transition in Magnetite

Simone Borroni

Magnetite (Fe

_3

_4

) is the first magnetic material discovered by mankind. Despite thousands of years of applications, from the magnetic compass to data storage, and decades of basic research, it remains among the most fascinating materials for physicists. The most important reason for the continuous interest in magnetite is the occurrence of discontinuous changes in its ground state around 120 K, the so-called Verwey transition, characterized by profound modifications in the electronic structure, mutually related to atomic displacements. The remarkable complexity of the transformation process still leaves open fundamental problems in the understanding of the microscopic mechanism of the Verwey transition. Further research efforts are needed to better describe the nature of the elementary excitations at the origin of the initial instability, how it propagates in different degrees of freedom and its relationship to precursor phenomena. This Thesis combines together the potentials of steady-state and time-resolved spectroscopy to study the critical dynamics of the lattice vibrations, and the fluctuations of electronic and structural order across the Verwey transition. Their signatures are identified in the coherent response of the optical functions to ultrashort laser pulses, and the inelastic scattering of light and neutrons, which provide complementary information, with selective sensitivity to different types of elementary excitations. Among the most important results, we manage to observe in the energy and time domain critical modes with different interplay of charge distribution and atomic displacements, and diffusive dynamics, typical of an order-disorder process.

EPFL2018

From adult dentate gyrus neurogenesis to pattern separation

Olivia Gozel

For a few decades, adult dentate gyrus neurogenesis has been widely recognized by the neuroscience community as an intriguing phenomenon. Two observations are particularly puzzling. At the cellular level, the switch from excitation to inhibition of the GABAergic input onto newborn cells has been shown to be crucial for their proper integration into the existing network of dentate gyrus cells. At the behavioral level, adult-born dentate granule cells have been shown to promote pattern separation of similar stimuli in various tasks, while not playing a role in discrimination of distinct stimuli. It is still unclear, however, how these functionalities arise in the network of dentate gyrus cells. Several models of adult dentate gyrus neurogenesis have been designed with various levels of abstraction, and have suggested different roles of newborn cells. Yet, none of these models could explain how newborn cells promote pattern separation of similar stimuli, and not distinct stimuli. Moreover, none of the previous studies modeled the actual integration of adult-born dentate granule cells in the preexisting circuit, but rather initialized their inward connections to random, but fully grown, weights. In my thesis work, I bridge the gap between biological and theoretical knowledge on adult dentate gyrus neurogenesis. I address the puzzling experimental observations and explain for the first time with a model: (i) how newborn cells integrate into the preexisting dentate gyrus network, and (ii) how they promote pattern separation of similar stimuli. More specifically, I propose that the early phase of maturation of newborn cells, when GABAergic input has an excitatory effect, drives the synaptic weights towards the subspace of configurations of familiar stimuli through a cooperative effect. In the late phase of maturation, when GABAergic input switches to inhibitory, the synaptic weights move towards novel features of the presented stimuli through a competitive effect. This theory of newborn cells integration also explains why adult-born dentate granule cells promote better pattern separation of similar stimuli, but not distinct stimuli. Indeed, in the late phase of maturation, newborn cells can only learn novel features that are similar enough to familiar features, because the configuration of their synaptic weights makes them sensitive to familiar features at the end of the early phase of maturation.

EPFL2019