Séminaires du mardi 19 février

Séminaire du LPTMS: Isabelle Bouchoule


Generalised hydrodynamics on an atom chip

Isabelle Bouchoule (Institut d'Optique)

Describing the out-of-equilibrium dynamics of many-body quantum systems is a priori a tremendously difficult task. However, a recent theoretical development provides an ab-initio description of the long wavelength dynamics of 1D integrable quantum systems, the so-called Generalised Hydrodynamics (GHD). In contrast to conventional hydrodynamics, GHD does not assume that gas is locally described by the Gibbs ensemble but it keeps track of all conserved quantities of the integrable system. In cold atom experiments, 1D bosonic gases are realised, which are well described by the famous integrable Lieb-Liniger model. Cold atom experiments thus offer an ideal platform to test GHD. We use the atom-chip experiment of LCF, where 1D gases of 87Rb are realized, to test experimentally GHD. Starting from a cold atomic cloud at thermal equilibrium, dynamics is generated by a sudden quench of the longitudinal potential. The measured time evolution of the density profiles are in excellent agreement with predictions from GHD. We also compare our data with predictions from the conventional hydrodynamics method, which assumes locally a thermal equilibrium described by a Gibbs ensemble. Except for the special case of harmonic potentials, we find that conventional hydrodynamics completely fails to reproduce our data. Hydrodynamics even predicts the development of sharp structures leading to a chock phenomena, such a phenomena being absent in the data and in the GHD description.  

Physics-Biology interface seminar: Fernando Luis Barroso Da Silva


Developing and applying fast constant pH methods in biological systems: From biomaterials to virus

Fernando Luis Barroso Da Silva (University of São Paulo, Brazil)


pH is a key parameter for biological and technological processes. Different numerical schemes were developed during the last years for such simulations ranging from Poisson-Boltzmann approaches to explicit solvent based methods. Ideally, the proton equilibria should correctly describe the experimental system without hampering the calculation time. A fast proton titration scheme (FPTS), rooted in the Kirkwood model of impenetrable spheres, where salt is treated at the Debye-Huckel level, was specially developed for proteins and nucleic acids. This method has now been coupled with OPEP5 force field for constant pH molecular dynamics simulations. A benchmark study will be presented. Despite our approximations, both the robustness and its ability to proper describe the system physics by these numerical methods can be confirmed. FPTS was also applied to quantify protein stability and biomolecular interactions. In this talk, I will present results for some protein systems with importance in different applied fields from biomaterials to public health.