Nematic fluctuations close to quantum criticality: a new method for comparing simulations and experiments

The comparison of numerical simulations and spectroscopic results is notoriously difficult because of the analytic continuation in the complex energy plane. In addition to the analytic continuation, life times and mass enhancement factors must be extracted from the experimental spectra using, e.g., Kramers-Kronig transformation with the well-known problems resulting from the extrapolations to low and high energies.
One way out of this dilemma is a transformation of the experimental results from real to imaginary frequencies which provides us with an imaginary-time-ordered correlation function Λ(τ). For determining Λ(τ), the cutoff is much faster. From this transformation, one can extract the quantity βΛ(β/2) with β=1/k_BT. We show in this contribution how this quantity can be derived from the electronic Raman spectra of the iron pnictide Ba(Fe_1-xCo_x)₂As₂ and varies with doping and temperature. We compare the results with those obtained via the Kramers-Kronig formalism at zero frequency. Additionally, we highlight the perspectives of this method with view on quantum criticality and the comparison of experiment and theory.