  Atmospheres of highly irradiated gas giant planets host a large variety of
atomic and ionic species. Here we observe the thermal emission spectra of the
two ultra-hot Jupiters WASP-33b and KELT-20b/MASCARA-2b in the near-infrared
wavelength range with CARMENES. Via high-resolution Doppler spectroscopy, we
searched for neutral silicon (Si) in their dayside atmospheres. We detect the
Si spectral signature of both planets via cross-correlation with model spectra.
Detection levels of 4.8$\mathrm{\sigma}$ and 5.4$\mathrm{\sigma}$,
respectively, are observed when assuming a solar atmospheric composition. This
is the first detection of Si in exoplanet atmospheres. The presence of Si is an
important finding due to its fundamental role in cloud formation and, hence,
for the planetary energy balance. Since the spectral lines are detected in
emission, our results also confirm the presence of an inverted temperature
profile in the dayside atmospheres of both planets.
