Exact first-order density matrix for a d-dimensional harmonically confined Fermi gas at finite temperature

TL;DR

This paper derives an exact finite-temperature first-order density matrix for a harmonically trapped ideal Fermi gas in any dimension, compares it with the Thomas-Fermi approximation, and applies it to calculate exchange energy in 2D quantum dots.

Contribution

It provides the first exact closed-form expression for the finite-temperature first-order density matrix in any dimension for a harmonically confined Fermi gas, extending previous diagonal-only results.

Findings

Thomas-Fermi approximation closely matches the exact density matrix at large N

Derived a closed-form expression for 2D Hartree-Fock exchange energy

Exact results highlight limitations of local-density approximation in 2D

Abstract

We present an exact closed form expression for the {\em finite temperature} first-order density matrix of a harmonically trapped ideal Fermi gas in any dimension. This constitutes a much sought after generalization of the recent results in the literature, where exact expressions have been limited to quantities derived from the {\em diagonal} first-order density matrix. We compare our exact results with the Thomas-Fermi approximation (TFA) and demonstrate numerically that the TFA provides an excellent description of the first-order density matrix in the large-N limit. As an interesting application, we derive a closed form expression for the finite temperature Hartree-Fock exchange energy of a two-dimensional parabolically confined quantum dot. We numerically test this exact result against the 2D TF exchange functional, and comment on the applicability of the local-density approximation (LDA) to the exchange energy of an inhomogeneous 2D Fermi gas.