# Exact ordering of energy levels for one-dimensional interacting Fermi gases with $SU(n)$ symmetry

## TL;DR

This paper determines the exact ordering of energy levels in one-dimensional $SU(n)$ Fermi gases with hard wall boundaries, extending understanding beyond existing theorems and applicable to both uniform and trapped systems.

## Contribution

It provides a method to order energy levels based on quantum numbers from Bethe ansatz, surpassing the Lieb-Mattis theorem limitations, and applies to non-uniform trapped systems.

## Key findings

- Energy level ordering can be determined by quantum numbers in Bethe ansatz.
- Results extend to non-uniform systems in harmonic traps.
- Effective spin-exchange models describe energy ordering in strong interactions.

## Abstract

Based on the exact solution of one-dimensional Fermi gas systems with $SU(n)$ symmetry in a hard wall, we demonstrate that we are able to sort the ordering of the lowest energy eigenvalues of states with all allowed permutation symmetries, which can be solely marked by certain quantum numbers in the Bethe ansatz equations. Our results give examples beyond the scope of the generalized Lieb-Mattis theorem, which can only compare the ordering of energy levels of states belonging to different symmetry classes if they are comparable according to the pouring principle. In the strongly interacting regime, we show that the ordering of energy levels can be determined by an effective spin-exchange model and extend our results to the non-uniform system trapped in the harmonic potential.