Improved bounds on ultralight scalar dark matter in the radio-frequency
  range
  We present a search for fundamental constant oscillations in the range
$20$~kHz-$100$ MHz, that may arise within models for ultralight dark matter
(UDM). Using two independent, significantly upgraded optical-spectroscopy
apparatus, we achieve up to $\times$1000 greater sensitivity in the search
relative to previous work. We report no observation of UDM and thus constrain
respective couplings to electrons and photons within the investigated UDM
particle mass range $8\cdot 10^{-11}-4\cdot 10^{-7}$ eV. The constraints
significantly exceed previously set bounds, and as we show, may surpass in
future experiments those provided by equivalence-principle experiments in a
specific case regarding the combination of UDM couplings probed by the latter.
