  We report new limits on ultralight scalar dark matter (DM) with dilaton-like
couplings to photons that can induce oscillations in the fine-structure
constant alpha. Atomic dysprosium exhibits an electronic structure with two
nearly degenerate levels whose energy splitting is sensitive to changes in
alpha. Spectroscopy data for two isotopes of dysprosium over a two-year span is
analyzed for coherent oscillations with angular frequencies below 1 rad/s. No
signal consistent with a DM coupling is identified, leading to new constraints
on dilaton-like photon couplings over a wide mass range. Under the assumption
that the scalar field comprises all of the DM, our limits on the coupling
exceed those from equivalence-principle tests by up to 4 orders of magnitude
for masses below 3 * 10^-18 eV. Excess oscillatory power, inconsistent with
fine-structure variation, is detected in a control channel, and is likely due
to a systematic effect. Our atomic spectroscopy limits on DM are the first of
their kind, and leave substantial room for improvement with state-of-the-art
atomic clocks.
