# A microscopic derivation of Gibbs measures for nonlinear Schr\"{o}dinger equations with unbounded interaction potentials

## TL;DR

This paper derives the Gibbs measure for nonlinear Schrödinger equations with unbounded interactions from many-body quantum states, extending previous results to higher dimensions and broader potential classes using perturbative and graphical methods.

## Contribution

It provides a microscopic derivation of Gibbs measures for NLS with unbounded potentials in 1D, 2D, and 3D, extending prior work to more general interactions and dimensions.

## Key findings

- Derived Gibbs measures for NLS with unbounded potentials in 1D, 2D, and 3D.
- Extended the class of interaction potentials to include unbounded $L^p$ types.
- Used graphical representation and Borel summation to analyze the expansion.

## Abstract

We study the derivation of the Gibbs measure for the nonlinear Schr\"{o}dinger equation (NLS) from many-body quantum thermal states in the high-temperature limit. In this paper, we consider the nonlocal NLS with defocusing and unbounded $L^p$ interaction potentials on $\mathbb{T}^d$ for $d=1,2,3$. This extends the author's earlier joint work with Fr\"{o}hlich, Knowles, and Schlein, where the regime of defocusing and bounded interaction potentials was considered. When $d=1$, we give an alternative proof of a result previously obtained by Lewin, Nam, and Rougerie. Our proof is based on a perturbative expansion in the interaction. When $d=1$, the thermal state is the grand canonical ensemble. As in the author's earlier joint work with Fr\"{o}hlich, Knowles, and Schlein, when $d=2,3$, the thermal state is a modified grand canonical ensemble, which allows us to estimate the remainder term in the expansion. The terms in the expansion are analysed using a graphical representation and are resummed by using Borel summation. By this method, we are able to prove the result for the optimal range of $p$ and obtain the full range of defocusing interaction potentials which were studied in the classical setting when $d=2,3$ in the work of Bourgain.