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Concept

Fluctuation quantique

Résumé

En physique quantique, une fluctuation quantique, ou fluctuation quantique du vide, est le changement temporaire du niveau d'énergie à un certain point de l'espace, expliqué par le principe d'incertitude de Heisenberg qui permet la création spontanée d'une paire virtuelle constituée d'une particule et d'une antiparticule. Pour comprendre ce phénomène, il faut comprendre la nature du vide spatial conformément à la théorie des champs quantiques. Le vide est rempli d’ondes électromagnétiques fluctuantes. Le vide serait comme un océan où les fluctuations seraient représentées par des vaguelettes à sa surface. L'ensemble apparaît à première vue plat, mais de plus près nous voyons des petites fluctuations. Les ondes produites ont toutes les longueurs d'onde possibles. Le vide contient donc une certaine quantité d’énergie due à des photons à une échelle atomique appelée « énergie du point zéro ». Selon la mécanique quantique, l'énergie n'est pas toujours exactement définie, mais elle fluctu

Source officielle

https://fr.wikipedia.org/wiki/Fluctuation_quantique

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Order and dynamics in Kagome like compound Cu2OSO4 and other quantum magnets

Virgile Yves Favre

This thesis presents results of studies of novel compounds modeling complex fundamental physics phenomena. Cu2OSO4 is a copper based magnetic Mott Insulator system, where spin half magnetic moments form a new type of lattice. These intrinsically quantum pins are exhibiting atypical magnetic order and spin dynamics. The recent success in the growth of large single crystals of Cu2OSO4 enabled to perform measurements probing its static and fluctuating properties. The peculiarity of this sample is that its atoms are forming layers, with a geometry close to the intensively studied Kagomé lattice, but with a third of its spins replaced by dimers. This quantum magnetism system has been probed in its bulk, by the means of heat capacity and DC-susceptibility measurements, revealing a transition to a magnetically long range ordered state upon cooling, the details of which are revealed by neutron scattering. Single crystal inelastic neutron scattering shed light on the spin-dynamics in the system, with clear spin waves appearing as fluctuations around the peculiar ground state of the system: a 120 degrees spin configuration where the magnetic moment of the spin-dimer causes the sample to be globally ferrimagnetic. The presented results indicate that Cu2OSO4 represents a new type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored. The magnetic excitations of the compound can be modeled by a yet-to-be-understood internal effective mean-field that no simple magnetic coupling seems to reproduce. K2Ni2(SO4)3 is another compound that allows for the existence of non-trivial topological phases. This thesis presents results of the study of the unusual magnetic behavior of K2Ni2(SO4)3. No clear sign of well-established magnetic long range order has been observed down to dilution temperatures. Neutron scattering reveals the details of the competition between frustration and fluctuations that prevent order from settling in. Low temperature spin excitations take the form of a continuum at 500 mK, but also of broad, energy independent continua at higher temperatures. Bulk and neutron scattering measurements are put in perspective and linked together with a view to building up a better understanding of how quantum spin liquids can be stabilized in general, and in particular in this model compound. Finally, the last contribution of this thesis to the field of condensed matter physics regards the establishment of a state-of-the-art technique to fit heat capacity and unit cell volume of samples to try and make the extraction of magnetic information from specific heat measurements more robust. This newly-developed technique consists in modeling lattice contributions with better accuracy by using data from multiple experimentally accessible quantities to consolidate the fitting scheme. This method has been cautiously applied to several compounds at the forefront of research in experimental physics.

EPFL2021

Chargement

Unconventional mechanisms of magnetic ordering in frustrated models of quantum magnetism

Tommaso Coletta

In this thesis we investigate in the semiclassical approximation how unconventional magnetic ordering, that is magnetic ordering which is not predicted at the classical level, emerges in a variety of frustrated models of quantum magnetism. The first part of this thesis presents our semiclassical investigation of the magnetization processes of the anisotropic triangular lattice Heisenberg antiferromanget and of the J1 − J2 square lattice Heisenberg antiferromanget. Existing analytical, numerical and experimen- tal evidences report that these systems exhibit a magnetization plateau respectively at 1/3 and 1/2 of the saturation value and whose structure can be understood in simple terms as a collinear spin structure. We develop a strategy to estimate the semiclassical energy of collinear spin structures away from their classical stability point. In this framework we are able to predict the stabilization of a collinear up-up-down structure with a magnetization equal to one third of the saturation value in a wide anisotropy range for the triangular lattice Heisen- berg antiferromanget. Similarly we find that the four sublattice up-up-up-down structure with magnetization equal to 1/2 of the saturation value is stabilized deep into the classically striped ordered region of the J1 − J2 square lattice Heisenberg antiferromanget. In the case of the anisotropic triangular lattice Heisenberg antiferromagnet a brief digression is made to the zero field case. The semiclassical approach confirms that quantum fluctuations favor collinear structures over the classical spiral states in either the weakly and strongly coupled chain regimes. The second part of the thesis is devoted to the study of fully frustrated transverse field Ising models. These models have emerged as an alternative tool to access the physics of the quan- tum dimer models (QDM) of Rokhsar and Kivelson at the purely kinetic point. We studied the classical phase diagram of this model for the square and honeycomb lattice geometries. In either case the effect of quantum fluctuations and the relevance to the physics of the QDM is discussed. In the case of the honeycomb lattice geometry we find that the classical phase diagram is rather involved with columnar and plaquette states separated by an infinite series of states of mixed nature. From the study of quantum fluctuations we conjecture that the sqrt(12) × sqrt(12) plaquette phase dominates over the columnar structure down to zero transverse field in the limit S = 1/2. In the case of the square lattice geometry we find that at the classical level the plaquette state is stabilized below the polarized state down to zero transverse field. In agreement with the most recent quantum Monte Carlo results, we show that for S = 1/2 quantum fluctuations favor columnar oder in the whole field range below saturation even though this structure is never classically stable in this parameter range.

EPFL2013

Chargement

Water is one of the most abundant molecules in the universe. It forms a hydrogen bond network with unique structure and dynamics. Hydrogen bonding is highly relevant to many biological processes including membrane formation, protein folding, adsorption, and lubrication. However, the underlying molecular interactions within water and between water and membranes are difficult to study and hence not fully understood. In this thesis, we utilize second harmonic scattering (SHS) to investigate inhomogeneities in bulk water, probe structural changes in lipid bilayer membranes, and detect and characterize protein-membrane interactions. Firstly, we explore the structure of bulk water in comparison to a common model liquid, carbon tetrachloride. Combining SHS measurements and molecular dynamics simulations, we report on the nanoscale inhomogeneities in bulk water that occur on femtosecond timescales. We attribute the rise in the coherent second harmonic (SH) intensity to charge density fluctuations with enhanced nanoscale polarizabilities around transient voids. Even though the transient voids also exist in carbon tetrachloride, they do not show polarizability fluctuations and the SH response of carbon tetrachloride thus corresponds to that of a model isotropic liquid. Then, we study the structure of water around large unilamellar vesicles (LUVs), which serve as a model for cell membranes. We focus on the role of water during the lipid melting phase transition. Using SHS, differential scanning calorimetry, and temperature-dependent zeta-potential measurements, we investigate the structural changes of hydration shells around the vesicle membrane. We use lipids with phosphoserine, phosphocholine, and phosphate headgroups and observe different hydration behavior upon the lipid melting transition depending on the membrane composition. Next, we characterize the interface of membranes composed of lipids containing phosphocholine and phosphate headgroups in solutions with different CaCl2 concentrations. We observe that the SH intensity is decreasing with the addition of CaCl2, which we attribute to the formation of a condensed layer of Na+ counterions on the inner leaflet of the LUVs. Additionally, we extract the vesicle surface potential and second-order surface susceptibility from SHS data for membranes with different content of lipids with phosphate headgroups, and observe composition dependent behaviour which suggests structural membrane reorganization. We continue by exploring protein-membrane interactions using water as a contrast agent. We study a pore-forming toxin, perfringolysin O (PFO), known for selective insertion into cholesterol-rich membranes. Combining SHS measurements and cryo-electron microscopy, we compare the interaction between PFO and LUV membranes with a high and low cholesterol content. By obtaining a sub-picomolar insertion constant of PFO, we show that SHS can be used to determine protein-membrane binding constants in concentration ranges not accessible by conventional methods.Finally, we investigate the interaction of protein aerolysin and its mutants with lipid membranes. We observe changes in surface charge in the cap region of membrane-bound aerolysin at different pH of the surrounding solution. By combining SHS with single-channel measurements, we obtain the membrane binding constant of wild-type aerolysin and its mutants, and we dissect their pore-forming mechanism.

EPFL2022

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EPFL2022

Order and dynamics in Kagome like compound Cu2OSO4 and other quantum magnets

Virgile Yves Favre

EPFL2021

Unconventional mechanisms of magnetic ordering in frustrated models of quantum magnetism

Tommaso Coletta

EPFL2013