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We introduce an improved approach for obtaining smooth finite-temperaturespectral functions of quantum impurity models using the numericalrenormalization group (NRG) technique. It is based on calculating first theGreen's function on the imaginary-frequency axis, followed by an analyticcontinuation to the real-frequency axis using Pad\'e approximants. Thearbitrariness in choosing a suitable kernel in the conventional broadeningapproach is thereby removed and, furthermore, we find that the Pad\'e method isable to resolve fine details in spectral functions with less artifacts on thescale of omega ~ T. We discuss the convergence properties with respect to theNRG calculation parameters (discretization, truncation cutoff) and the numberof Matsubara points taken into account in the analytic continuation. We testthe technique on the the single-impurity Anderson model and the Hubbard model(within the dynamical mean-field theory). For the Anderson impurity model, wediscuss the shape of the Kondo resonance and its temperature dependence. Forthe Hubbard model, we discuss the inner structure of the Hubbard bands inmetallic and insulating solutions at half-filling, as well as in the doped Mottinsulator. Based on these test cases we conclude that the Pad\'e approximantapproach provides more reliable results for spectral functions at low-frequencyscales of omega < T and that it is capable of resolving sharp spectral featuresalso at high frequencies. It outperforms broadening in most respects.

Pad\'e approximants for improved finite-temperature spectral functions in the numerical renormalization group