The dynamics of glass forming liquids shows a tremendous slowing down if temperature is decreased: Relaxation times, transport coefficients such as the diffusion constant or the shear viscosity grow by more than 15 orders of magnitude within narrow temperature range. One of the central goals in the field of glass physics is to understand the origin of this dramatic slowing down of the dynamics.There are several scenarios that seem to be able to explain the slowing down of the dynamics and the low temperature state of the glass former. A very popular one is to rationalize the slow dynamics by invoking the existence of a thermodynamic transition, the so-called ideal glass transition from the liquid to an ideal glass states. This ideal glass transition occurs when the configurational entropy, defined as the logarithm of the number of available states, becomes zero. Due to thissingular behavior of the configurational entropy, the relaxation time andtransport coefficients diverge if the transition point is approached [1].However, confirming the existence of the ideal glass transition is a verydifficult task since in practice the system falls out of equilibriumbefore reaching this putative ideal glass transition temperature,thus making the test of this theory most difficult. Furthermore, otherscenarios are also able to explain the slow dynamics without resortingany thermodynamic transition [2].In the present study, we investigate the existence of the idealglass transition by using computer simulation of a canonical binaryLennard-Jones glass former in three dimensions. Massive computationaleffort and efficient sampling algorithm (parallel tempering) allow us toreach equilibrium states that are very deeply supercooled. By measuringthe configurational entropy directly (obtained via thermodynamicintegration), we find that the configuration entropy does not go to zeroat a finite temperature. This implies that the thermodynamic singularityis indeed avoided before reaching a putative ideal glass transitiontemperature predicted by previous studies. Our results indicates thatcontrary to the previous results the system remains in a liquid statedown to zero temperature without showing an ideal glass transition.Furthermore, we analyze the microscopic structure and the potentialenergy landscape of the system to clarify the mechanism that leads tothe avoidance of the ideal glass transition. We find that locallyfavored structures present in the liquid state hardly change in thetemperature range we consider, which rules out another possibility forthe slow dynamics, e.g. the transformation between two distinct liquids,so-called liquid-liquid transition [3]. Instead, we find the potentialenergy landscape contains extensive number of minima even very lowtemperature, which rationalize the avoided singularity.Our results support a theoretical argument that the ideal glasstransition can not exist at bulk system in finite dimensions [5].[1] Adam G and Gibbs JH, J. Chem. Phys., 43, 139 (1965)[2] Chandler D and Garrahan JP, Annu. Rev. Phys. Chem., 61, 191 (2010)[3] Speck T, Royall CP, and Williams SR, arXiv:1409.0751, (2014)[4] Saksaengwijit A and Heuer A, Phys. Rev. Lett., 93, 235701 (2004)[5] Stillinger FH, J. Chem. Phys., 88, 7818 (1988)

In recent years, Raman spectroscopy has become a toolof choice for the structural analysis of glasses, due to its simplicity and thelow cost of acquisition. However, the broad and overlapping peaks observedin the spectra result in a phenomenological and often qualitativeinterpretation. In order to improve the analysis of Raman spectra, and to startto obtain a more quantitative interpretation, one needs to identify the exactcontributions arising from individual structural units.We have used first-principles and combined classical/first-principlesapproaches to create representative atomistic models of some simple binarysoda- and lime-silicates, as well as for some ternary aluminosilicates. Withinthe density functional theory framework, we have calculated the vibrationaldensity of states as well as the IR and Raman spectra of these glass models,and we have found a good agreement with the experimental spectra.The knowledge of the theoretical spectra and the atomic structure has madepossible to identify the signatures of the various constituents of the glasses inthe spectra, as for example the Qi species. The obtained correlations can bethen used to better assign the main bands present in the spectra of morecomplex silicate glasses.

Open channels are peculiar incidents modes that are transmitted by a diffusing medium with high efficiency (up to 100%). Digital holography is used to analyze the correlations of the transmitted field, and to test open channel theory. Quantitative agreement is obtained.

Ideas in theoretical physics of biological systems

Genome processing relies on the intracellular localization and dynamic assembly of higher-order nucleoprotein complexes. In bacteria, the mechanism of assembly for the most widespread partition systems, ParABS, responsible for active DNA segregation remains elusive. We have combined super-resolution, genome-wide, biochemical and modeling approaches to investigate quantitatively the formation of the nucleoprotein complex organized around the centromere-like sequences, parS. We found that the active confinement of nearly all ParB proteins around parS, observed at the single molecule resolution, relies on a network of synergistic interactions involving protein-protein and protein-DNA interactions. Our physico-mathematical modeling of ParB binding pattern revealed that ParB binds stochastically in the vicinity of parS over long distances. Based on our findings, and consistent with previous data, we propose a new model that relies on a nucleation and looping mechanism leading to the formation of a dynamic lattice for the partition complex assembly. We thus provide new bases to model the DNA segregation process. Our original assembly model may also apply to many unrelated proteins that self-assemble in superstructures through nucleation centers.