Surmounting the sign problem in non-relativistic calculations: a case study with mass-imbalanced fermions

TL;DR

This paper develops and compares non-perturbative methods, including Complex Langevin and analytic continuation, to overcome the sign problem in calculating properties of mass-imbalanced Fermi systems, revealing new insights into their energy behavior.

Contribution

It introduces and validates two approaches, Complex Langevin and analytic continuation, for studying imbalanced Fermi systems beyond perturbation theory.

Findings

Complex Langevin effectively handles both attractive and repulsive interactions.

Energy levels flatten for strong repulsive couplings, indicating new physical insights.

Methods agree with non-perturbative renormalization group and perturbation theory.

Abstract

The calculation of the ground state and thermodynamics of mass-imbalanced Fermi systems is a challenging many-body problem. Even in one spatial dimension, analytic solutions are limited to special configurations and numerical progress with standard Monte Carlo approaches is hindered by the sign problem. The focus of the present work is on the further development of methods to study imbalanced systems in a fully non-perturbative fashion. We report our calculations of the ground-state energy of mass-imbalanced fermions using two different approaches which are also very popular in the context of the theory of the strong interaction (Quantum Chromodynamics, QCD): (a) the hybrid Monte Carlo algorithm with imaginary mass imbalance, followed by an analytic continuation to the real axis; and (b) the Complex Langevin algorithm. We cover a range of on-site interaction strengths that includes strongly attractive as well as strongly repulsive cases which we verify with non-perturbative renormalization group methods and perturbation theory. Our findings indicate that, for strong repulsive couplings, the energy starts to flatten out, implying interesting consequences for short-range and high-frequency correlation functions. Overall, our results clearly indicate that the Complex Langevin approach is very versatile and works very well for imbalanced Fermi gases with both attractive and repulsive interactions.