Maximum entropy analytic continuation for frequency-dependent transport coefficients with non-positive spectral weight

TL;DR

This paper introduces a novel maximum entropy analytic continuation method using auxiliary functions to accurately compute frequency-dependent transport coefficients with non-positive spectral weight, applicable to complex multiorbital systems.

Contribution

It develops a general auxiliary response function approach enabling maximum entropy analytic continuation for transport coefficients with non-positive spectral weights, extending previous methods.

Findings

Successfully applied to the 2D Hubbard model with DMFT and CTQMC.

Extended maximum entropy analytic continuation to complex spectral weights.

Validated the method for thermoelectric response calculations.

Abstract

The computation of transport coefficients, even in linear response, is a major challenge for theoretical methods that rely on analytic continuation of correlations functions obtained numerically in Matsubara space. While maximum entropy methods can be used for certain correlation functions, this is not possible in general, important examples being the Seebeck, Hall, Nernst and Reggi-Leduc coefficients. Indeed, positivity of the spectral weight on the positive real-frequency axis is not guaranteed in these cases. The spectral weight can even be complex in the presence of broken time-reversal symmetry. Various workarounds, such as the neglect of vertex corrections or the study of the infinite frequency or Kelvin limits have been proposed. Here, we show that one can define auxiliary response functions that allow to extract the desired real-frequency susceptibilities from maximum entropy methods in the most general multiorbital cases with no particular symmetry. As a benchmark case, we study the longitudinal thermoelectric response and corresponding Onsager coefficient in the single-band two-dimensional Hubbard model treated with dynamical mean-field theory (DMFT) and continuous-time quantum Monte Carlo (CTQMC). We thereby extend to transport coefficients the maximum entropy analytic continuation with auxiliary functions (MaxEntAux method), developed for the study of the superconducting pairing dynamics of correlated materials.