Francis Nier

Hypoelliptic estimates and spectral theory for Fokker-Planck operators and Witten Laplacians

Kohns Proof of the Hypoellipticity of the Hormander Operators.- Compactness Criteria for the Resolvent of Schrodinger Operators.- Global Pseudo-differential Calculus.- Analysis of some Fokker-Planck Operator.- Return to Equillibrium for the Fokker-Planck Operator.- Hypoellipticity and Nilpotent Groups.- Maximal Hypoellipticity for Polynomial of Vector Fields and Spectral Byproducts.- On Fokker-Planck Operators and Nilpotent Techniques.- Maximal Microhypoellipticity for Systems and Applications to Witten Laplacians.- Spectral Properties of the Witten-Laplacians in Connection with Poincare Inequalities for Laplace Integrals.- Semi-classical Analysis for the Schrodinger Operator: Harmonic Approximation.- Decay of Eigenfunctions and Application to the Splitting.- Semi-classical Analysis and Witten Laplacians: Morse Inequalities.- Semi-classical Analysis and Witten Laplacians: Tunneling Effects.- Accurate Asymptotics for the Exponentially Small Eigenvalues of the Witten Laplacian.- Application to the Fokker-Planck Equation.- Epilogue.- Index.

Quantitative analysis of metastability in reversible diffusion processes via a Witten complex approach : the case with boundary

This article is a continuation of previous works by Bovier-Eckhoff-Gayrard-Klein, Bovier-Gayrard-Klein and Helffer-Klein-Nier. It is concerned with the analysis of the exponentially small eigenvalues of a semiclassical Witten Laplacian. We consider here the case of riemanian manifolds with boundary with a Dirichlet realization of the Witten Laplacian. A modified version of this preprint has been published in Memoires de la SMF vol. 105, (2006)

Mean field limit for bosons and propagation of Wigner measures

We consider the N-body Schrodinger dynamics of bosons in the mean field limit with a bounded pair-interaction potential. According to the previous work [Ammari, Z. and Nier, F., “Mean field limit for bosons and infinite dimensional phase-space analysis,” Ann. Henri Poincare 9, 1503 (2008)], the mean field limit is translated into a semiclassical problem with a small parameter e→0, after introducing an e-dependent bosonic quantization. The limits of quantum correlation functions are expressed as a push forward by a nonlinear flow (e.g., Hartree) of the associated Wigner measures. These object and their basic properties were introduced by Ammari and Nier in the infinite dimensional setting. The additional result presented here states that the transport by the nonlinear flow holds for a rather general class of quantum states in their mean field limit.

Optimal non-reversible linear drift for the convergence to equilibrium of a diffusion

We consider non-reversible perturbations of reversible diffusions that do not alter the invariant distribution and we ask whether there exists an optimal perturbation such that the rate of convergence to equilibrium is maximized. We solve this problem for the case of linear drift by proving the existence of such optimal perturbations and by providing an easily implementable algorithm for constructing them. We discuss in particular the role of the prefactor in the exponential convergence estimate. Our rigorous results are illustrated by numerical experiments.

Asymptotic analysis of a scaled wigner equation and quantum scattering

Abstract In this article we show how a scaling on the classical or quantum Liouville equation leads to scattering theory. After a preliminary analysis of classical mechanics, we focus on the quantum case treated via the Wigner function approach. The problem arises from a one-dimensional modelling of electron transport in quantum electronic devices and is more generally related to the study of scattering into cones.

Computing the steady states for an asymptotic model of quantum transport in resonant heterostructures

In this article, we propose a rapid method to compute the steady states, including bifurcation diagrams, of resonant tunneling heterostructures in the far from equilibrium regime. Those calculations are made on a simplified model which takes into account the characteristic quantities which arise from an accurate asymptotic analysis of the nonlinear Schrodinger-Poisson system. After a summary of the existing theoretical results, the asymptotic model is explicitly adapted to physically realistic situations and numerical results are shown in various cases.

Low temperature asymptotics for quasistationary distributions in a bounded domain

We analyze the low temperature asymptotics of the quasi-stationary distribution associated with the overdamped Langevin dynamics (a.k.a. the Einstein-Smoluchowski diffusion equation) in a bounded domain. This analysis is useful to rigorously prove the consistency of an algorithm used in molecular dynamics (the hyperdynamics), in the small temperature regime. More precisely, we show that the algorithm is exact in terms of state-to-state dynamics up to exponentially small factor in the limit of small temperature. The proof is based on the asymptotic spectral analysis of associated Dirichlet and Neumann realizations of Witten Laplacians. In order to cover a reasonably large range of applications, the usual assumptions that the energy landscape is a Morse function has been relaxed as much as possible.

Numerical analysis of the deterministic particle method applied to the Wigner equation

The Wigner equation of quantum mechanics has the form of a kinetic equation with a pseudodifferential operator in a Fourier integral form which requires great care in the numerical approximation. This paper is concerned with the numerical analysis of the weighted particle method, introduced by S. Mas-Gallic and P. A. Raviart, applied to this equation. In particular, we will prove convergence of the method in a physically relevant case, where the Wigner equation models the quantum tunneling of electrons through a potential barrier.

BOSE–EINSTEIN CONDENSATES IN THE LOWEST LANDAU LEVEL: HAMILTONIAN DYNAMICS

In a previous article with A. Aftalion and X. Blanc, it was shown that the hypercontractivity property of the dilation semigroup in spaces of entire functions was a key ingredient in the study of the Lowest Landau Level model for fast rotating Bose–Einstein condensates. That former work was concerned with the stationary constrained variational problem. This article is about the nonlinear Hamiltonian dynamics and the spectral stability of the constrained minima with motivations arising from the description of Tkatchenko modes of Bose–Einstein condensates. Again the hypercontractivity property provides a very strong control of the nonlinear term in the dynamical analysis.

Boundary conditions and subelliptic estimates for geometric Kramers-Fokker-Planck operators on manifolds with boundaries

This article is concerned with maximal accretive realizations of geometric Kramers-Fokker-Planck operators on manifolds with boundaries. A general class of boundary conditions is introduced which ensures the maximal accretivity and some global subelliptic estimates. Those estimates imply nice spectral properties as well as exponential decay properties for the associated semigroup. Admissible boundary conditions cover a wide range of applications for the usual scalar Kramer-Fokker-Planck equation or Bismuts hypoelliptic Laplacian.