  We present systematic measurements of the temperature-dependence of detection
efficiencies in TaN and NbN superconducting nanowire single-photon detectors.
We have observed a clear increase of the cut-off wavelength with decreasing
temperature that we can qualitatively describe with a temperature-dependent
diffusion coefficient of the quasi-particles created after photon absorption.
Furthermore, the detection efficiency at wavelengths shorter than the cut-off
wavelength as well as at longer wavelengths exhibit distinct temperature
dependencies. The underlying causes and possible consequences for microscopic
detection models are discussed.
