Novel Fermi Liquid of 2D Polar Molecules

TL;DR

This paper develops a many-body theory for a 2D gas of fermionic polar molecules with dipole moments, revealing a novel Fermi liquid behavior influenced by long-range dipole-dipole interactions and predicting observable effects like zero sound.

Contribution

It introduces a perturbative approach to describe many-body effects in a 2D dipolar Fermi gas beyond mean field, highlighting the role of long-range interactions.

Findings

Thermodynamic quantities expanded up to second order in small parameter

Many-body effects enable zero sound propagation

Predictions are experimentally accessible in ultracold molecule setups

Abstract

We study Fermi liquid properties of a weakly interacting 2D gas of single-component fermionic polar molecules with dipole moments $d$ oriented perpendicularly to the plane of their translational motion. This geometry allows the minimization of inelastic losses due to chemical reactions for reactive molecules and, at the same time, provides a possibility of a clear description of many-body (beyond mean field) effects. The long-range character of the dipole-dipole repulsive interaction between the molecules, which scales as $1/r^3$ at large distances $r$, makes the problem drastically different from the well-known problem of the two-species Fermi gas with repulsive contact interspecies interaction. We solve the low-energy scattering problem and develop a many-body perturbation theory beyond the mean field. The theory relies on the presence of a small parameter $k_Fr_*$, where $k_F$ is the Fermi momentum, and $r_*=md^2/\hbar^2$ is the dipole-dipole length, with $m$ being the molecule mass. We obtain thermodynamic quantities as a series of expansion up to the second order in $k_Fr_*$ and argue that many-body corrections to the ground-state energy can be identified in experiments with ultracold molecules, like it has been recently done for ultracold fermionic atoms. Moreover, we show that only many-body effects provide the existence of zero sound and calculate the sound velocity.